Secure Shell - ssh unter CentOS 7.x

openSSH LogoBei Internetdiensten wie eMail oder Web haben sich verschlüsselte Datenübertragungen mit SSL/TLS ohne Eingriffe in das Originalprotokoll durchgesetzt. Bei den klassischen unverschlüsselten Unix-Diensten zum Arbeiten mit entfernten Rechnern oder zur Datenübertragung auf andere Rechner - z.B. telnet, rcp und rsh - erfolgt eine alternative Lösung mittels OpenSSH.

openSSH - Programmsuite

Die für die Secure-Shell benötigten Pakete werden i.d.R. bereits bei der Erstinstallation erfolgreich ins System eingebettet. Bei Centos teilen sich die Programme der Programmsuite auf folgende Pakete auf:

  • openssh : Die OpenSSH-Implementierung der SSH Protokoll-Versionen 2 (und 1)
  • openssh-clients : Die OpenSSH-Client-Anwendungen
  • openssh-server : Der OpenSSH-Server Daemon
  • openssh-askpass : Passphrase-Dialog für OpenSSH und X

openssh

Mittels rpm -qil können wir überprüfen, welche Programme, Konfigurationsdateien und Dokumentationen beim Paket openssh installiert wurden.

 # rpm -qil openssh
Name        : openssh
Version     : 6.6.1p1
Release     : 25.el7_2
Architecture: x86_64
Install Date: Wed 23 Mar 2016 07:14:52 PM CET
Group       : Applications/Internet
Size        : 1450050
License     : BSD
Signature   : RSA/SHA256, Mon 21 Mar 2016 11:22:48 PM CET, Key ID 24c6a8a7f4a80eb5
Source RPM  : openssh-6.6.1p1-25.el7_2.src.rpm
Build Date  : Mon 21 Mar 2016 11:18:48 PM CET
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://www.openssh.com/portable.html
Summary     : An open source implementation of SSH protocol versions 1 and 2
Description :
SSH (Secure SHell) is a program for logging into and executing
commands on a remote machine. SSH is intended to replace rlogin and
rsh, and to provide secure encrypted communications between two
untrusted hosts over an insecure network. X11 connections and
arbitrary TCP/IP ports can also be forwarded over the secure channel.

OpenSSH is OpenBSD's version of the last free version of SSH, bringing
it up to date in terms of security and features.

This package includes the core files necessary for both the OpenSSH
client and server. To make this package useful, you should also
install openssh-clients, openssh-server, or both.
/etc/ssh
/etc/ssh/moduli
/usr/bin/ssh-keygen
/usr/libexec/openssh
/usr/libexec/openssh/ctr-cavstest
/usr/libexec/openssh/ssh-keysign
/usr/share/doc/openssh-6.6.1p1
/usr/share/doc/openssh-6.6.1p1/CREDITS
/usr/share/doc/openssh-6.6.1p1/ChangeLog
/usr/share/doc/openssh-6.6.1p1/INSTALL
/usr/share/doc/openssh-6.6.1p1/OVERVIEW
/usr/share/doc/openssh-6.6.1p1/PROTOCOL
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.agent
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.certkeys
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.chacha20poly1305
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.key
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.krl
/usr/share/doc/openssh-6.6.1p1/PROTOCOL.mux
/usr/share/doc/openssh-6.6.1p1/README
/usr/share/doc/openssh-6.6.1p1/README.dns
/usr/share/doc/openssh-6.6.1p1/README.platform
/usr/share/doc/openssh-6.6.1p1/README.privsep
/usr/share/doc/openssh-6.6.1p1/README.tun
/usr/share/doc/openssh-6.6.1p1/TODO
/usr/share/licenses/openssh-6.6.1p1
/usr/share/licenses/openssh-6.6.1p1/LICENCE
/usr/share/man/man1/ssh-keygen.1.gz
/usr/share/man/man8/ssh-keysign.8.gz

openssh-clients

Beim Paket openssh-clients wird mitgeliefert:

 # rpm -qil openssh-clients
Name        : openssh-clients
Version     : 6.6.1p1
Release     : 25.el7_2
Architecture: x86_64
Install Date: Wed 23 Mar 2016 07:14:59 PM CET
Group       : Applications/Internet
Size        : 2298871
License     : BSD
Signature   : RSA/SHA256, Mon 21 Mar 2016 11:22:58 PM CET, Key ID 24c6a8a7f4a80eb5
Source RPM  : openssh-6.6.1p1-25.el7_2.src.rpm
Build Date  : Mon 21 Mar 2016 11:18:48 PM CET
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://www.openssh.com/portable.html
Summary     : An open source SSH client applications
Description :
OpenSSH is a free version of SSH (Secure SHell), a program for logging
into and executing commands on a remote machine. This package includes
the clients necessary to make encrypted connections to SSH servers.
/etc/ssh/ssh_config
/usr/bin/scp
/usr/bin/sftp
/usr/bin/slogin
/usr/bin/ssh
/usr/bin/ssh-add
/usr/bin/ssh-agent
/usr/bin/ssh-copy-id
/usr/bin/ssh-keyscan
/usr/lib64/fipscheck/ssh.hmac
/usr/libexec/openssh/ssh-pkcs11-helper
/usr/share/man/man1/scp.1.gz
/usr/share/man/man1/sftp.1.gz
/usr/share/man/man1/slogin.1.gz
/usr/share/man/man1/ssh-add.1.gz
/usr/share/man/man1/ssh-agent.1.gz
/usr/share/man/man1/ssh-copy-id.1.gz
/usr/share/man/man1/ssh-keyscan.1.gz
/usr/share/man/man1/ssh.1.gz
/usr/share/man/man5/ssh_config.5.gz
/usr/share/man/man8/ssh-pkcs11-helper.8.gz

openssh-server

Hingegen liefert uns openssh-server folgende Dateien:

 # rpm -qil openssh-server
Version     : 6.6.1p1
Release     : 25.el7_2
Architecture: x86_64
Install Date: Wed 23 Mar 2016 07:14:58 PM CET
Group       : System Environment/Daemons
Size        : 943088
License     : BSD
Signature   : RSA/SHA256, Mon 21 Mar 2016 11:23:11 PM CET, Key ID 24c6a8a7f4a80eb5
Source RPM  : openssh-6.6.1p1-25.el7_2.src.rpm
Build Date  : Mon 21 Mar 2016 11:18:48 PM CET
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://www.openssh.com/portable.html
Summary     : An open source SSH server daemon
Description :
OpenSSH is a free version of SSH (Secure SHell), a program for logging
into and executing commands on a remote machine. This package contains
the secure shell daemon (sshd). The sshd daemon allows SSH clients to
securely connect to your SSH server.
/etc/pam.d/sshd
/etc/ssh/sshd_config
/etc/sysconfig/sshd
/usr/lib/systemd/system/sshd-keygen.service
/usr/lib/systemd/system/sshd.service
/usr/lib/systemd/system/sshd.socket
/usr/lib/systemd/system/sshd@.service
/usr/lib64/fipscheck/sshd.hmac
/usr/libexec/openssh/sftp-server
/usr/sbin/sshd
/usr/sbin/sshd-keygen
/usr/share/man/man5/moduli.5.gz
/usr/share/man/man5/sshd_config.5.gz
/usr/share/man/man8/sftp-server.8.gz
/usr/share/man/man8/sshd.8.gz
/var/empty/sshd

openssh-askpass

Zu guter Letzt sehen wir uns noch das Paket openssh-askpass genauer an:

 # rpm -qil openssh-askpass
Name        : openssh-askpass
Version     : 6.6.1p1
Release     : 25.el7_2
Architecture: x86_64
Install Date: Sat 12 Nov 2016 08:22:40 PM CET
Group       : Applications/Internet
Size        : 15944
License     : BSD
Signature   : RSA/SHA256, Mon 21 Mar 2016 11:22:53 PM CET, Key ID 24c6a8a7f4a80eb5
Source RPM  : openssh-6.6.1p1-25.el7_2.src.rpm
Build Date  : Mon 21 Mar 2016 11:18:48 PM CET
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://www.openssh.com/portable.html
Summary     : A passphrase dialog for OpenSSH and X
Description :
OpenSSH is a free version of SSH (Secure SHell), a program for logging
into and executing commands on a remote machine. This package contains
an X11 passphrase dialog for OpenSSH.
/etc/profile.d/gnome-ssh-askpass.csh
/etc/profile.d/gnome-ssh-askpass.sh
/usr/libexec/openssh/gnome-ssh-askpass
/usr/libexec/openssh/ssh-askpass

Dokumentation

Wichtige Hinweise zur Absicherung von ssh finden sich im Kapitel 4.3.11. Securing SSH aus dem Red Hat Enterprise Linux Security Guide.

manpages

ssh

Die Optionen rund um openssh findet man wie immer, in der manpage zu ssh.

SSH(1)                                 BSD General Commands Manual                                SSH(1)

NAME
     ssh — OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
         [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
         [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option]
         [-p port] [-Q cipher | cipher-auth | mac | kex | key] [-R [bind_address:]port:host:hostport]
         [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] [user@]hostname [command]

DESCRIPTION
     ssh (SSH client) is a program for logging into a remote machine and for executing commands on a
     remote machine.  It is intended to replace rlogin and rsh, and provide secure encrypted communica‐
     tions between two untrusted hosts over an insecure network.  X11 connections and arbitrary TCP
     ports can also be forwarded over the secure channel.

     ssh connects and logs into the specified hostname (with optional user name).  The user must prove
     his/her identity to the remote machine using one of several methods depending on the protocol ver‐
     sion used (see below).

     If command is specified, it is executed on the remote host instead of a login shell.

     The options are as follows:

     -1      Forces ssh to try protocol version 1 only.

     -2      Forces ssh to try protocol version 2 only.

     -4      Forces ssh to use IPv4 addresses only.

     -6      Forces ssh to use IPv6 addresses only.

     -A      Enables forwarding of the authentication agent connection.  This can also be specified on a
             per-host basis in a configuration file.

             Agent forwarding should be enabled with caution.  Users with the ability to bypass file
             permissions on the remote host (for the agent's UNIX-domain socket) can access the local
             agent through the forwarded connection.  An attacker cannot obtain key material from the
             agent, however they can perform operations on the keys that enable them to authenticate
             using the identities loaded into the agent.

     -a      Disables forwarding of the authentication agent connection.

     -b bind_address
             Use bind_address on the local machine as the source address of the connection.  Only useful
             on systems with more than one address.

     -C      Requests compression of all data (including stdin, stdout, stderr, and data for forwarded
             X11 and TCP connections).  The compression algorithm is the same used by gzip(1), and the
             “level” can be controlled by the CompressionLevel option for protocol version 1.  Compres‐
             sion is desirable on modem lines and other slow connections, but will only slow down things
             on fast networks.  The default value can be set on a host-by-host basis in the configura‐
             tion files; see the Compression option.

     -c cipher_spec
             Selects the cipher specification for encrypting the session.

             Protocol version 1 allows specification of a single cipher.  The supported values are
             “3des”, “blowfish”, and “des”.  3des (triple-des) is an encrypt-decrypt-encrypt triple with
             three different keys.  It is believed to be secure.  blowfish is a fast block cipher; it
             appears very secure and is much faster than 3des.  des is only supported in the ssh client
             for interoperability with legacy protocol 1 implementations that do not support the 3des
             cipher.  Its use is strongly discouraged due to cryptographic weaknesses.  The default is
             “3des”.

             For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of
             preference.  See the Ciphers keyword in ssh_config(5) for more information.

     -D [bind_address:]port
             Specifies a local “dynamic” application-level port forwarding.  This works by allocating a
             socket to listen to port on the local side, optionally bound to the specified bind_address.
             Whenever a connection is made to this port, the connection is forwarded over the secure
             channel, and the application protocol is then used to determine where to connect to from
             the remote machine.  Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will
             act as a SOCKS server.  Only root can forward privileged ports.  Dynamic port forwardings
             can also be specified in the configuration file.

             IPv6 addresses can be specified by enclosing the address in square brackets.  Only the
             superuser can forward privileged ports.  By default, the local port is bound in accordance
             with the GatewayPorts setting.  However, an explicit bind_address may be used to bind the
             connection to a specific address.  The bind_address of “localhost” indicates that the lis‐
             tening port be bound for local use only, while an empty address or ‘*’ indicates that the
             port should be available from all interfaces.

     -E log_file
             Append debug logs to log_file instead of standard error.

     -e escape_char
             Sets the escape character for sessions with a pty (default: ‘~’).  The escape character is
             only recognized at the beginning of a line.  The escape character followed by a dot (‘.’)
             closes the connection; followed by control-Z suspends the connection; and followed by
             itself sends the escape character once.  Setting the character to “none” disables any
             escapes and makes the session fully transparent.

     -F configfile
             Specifies an alternative per-user configuration file.  If a configuration file is given on
             the command line, the system-wide configuration file (/etc/ssh/ssh_config) will be ignored.
             The default for the per-user configuration file is ~/.ssh/config.

     -f      Requests ssh to go to background just before command execution.  This is useful if ssh is
             going to ask for passwords or passphrases, but the user wants it in the background.  This
             implies -n.  The recommended way to start X11 programs at a remote site is with something
             like ssh -f host xterm.

             If the ExitOnForwardFailure configuration option is set to “yes”, then a client started
             with -f will wait for all remote port forwards to be successfully established before plac‐
             ing itself in the background.

     -g      Allows remote hosts to connect to local forwarded ports.

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token pro‐
             viding the user's private RSA key.

     -i identity_file
             Selects a file from which the identity (private key) for public key authentication is read.
             The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
             ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.  Identity files may also be
             specified on a per-host basis in the configuration file.  It is possible to have multiple
             -i options (and multiple identities specified in configuration files).  ssh will also try
             to load certificate information from the filename obtained by appending -cert.pub to iden‐
             tity filenames.

     -K      Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to
             the server.

     -k      Disables forwarding (delegation) of GSSAPI credentials to the server.

     -L [bind_address:]port:host:hostport
             Specifies that the given port on the local (client) host is to be forwarded to the given
             host and port on the remote side.  This works by allocating a socket to listen to port on
             the local side, optionally bound to the specified bind_address.  Whenever a connection is
             made to this port, the connection is forwarded over the secure channel, and a connection is
             made to host port hostport from the remote machine.  Port forwardings can also be specified
             in the configuration file.  IPv6 addresses can be specified by enclosing the address in
             square brackets.  Only the superuser can forward privileged ports.  By default, the local
             port is bound in accordance with the GatewayPorts setting.  However, an explicit
             bind_address may be used to bind the connection to a specific address.  The bind_address of
             “localhost” indicates that the listening port be bound for local use only, while an empty
             address or ‘*’ indicates that the port should be available from all interfaces.

     -l login_name
             Specifies the user to log in as on the remote machine.  This also may be specified on a
             per-host basis in the configuration file.

     -M      Places the ssh client into “master” mode for connection sharing.  Multiple -M options
             places ssh into “master” mode with confirmation required before slave connections are
             accepted.  Refer to the description of ControlMaster in ssh_config(5) for details.

     -m mac_spec
             Additionally, for protocol version 2 a comma-separated list of MAC (message authentication
             code) algorithms can be specified in order of preference.  See the MACs keyword for more
             information.

     -N      Do not execute a remote command.  This is useful for just forwarding ports (protocol ver‐
             sion 2 only).

     -n      Redirects stdin from /dev/null (actually, prevents reading from stdin).  This must be used
             when ssh is run in the background.  A common trick is to use this to run X11 programs on a
             remote machine.  For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shad‐
             ows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted
             channel.  The ssh program will be put in the background.  (This does not work if ssh needs
             to ask for a password or passphrase; see also the -f option.)

     -O ctl_cmd
             Control an active connection multiplexing master process.  When the -O option is specified,
             the ctl_cmd argument is interpreted and passed to the master process.  Valid commands are:
             “check” (check that the master process is running), “forward” (request forwardings without
             command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and
             “stop” (request the master to stop accepting further multiplexing requests).

     -o option
             Can be used to give options in the format used in the configuration file.  This is useful
             for specifying options for which there is no separate command-line flag.  For full details
             of the options listed below, and their possible values, see ssh_config(5).

                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIKeyExchange
                   GSSAPIClientIdentity
                   GSSAPIDelegateCredentials
                   GSSAPIRenewalForcesRekey
                   GSSAPITrustDns
                   GSSAPIKexAlgorithms
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   ProxyUseFdpass
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   RequestTTY
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UsePrivilegedPort
                   User
                   UserKnownHostsFile
                   VerifyHostKeyDNS
                   VisualHostKey
                   XAuthLocation

     -p port
             Port to connect to on the remote host.  This can be specified on a per-host basis in the
             configuration file.

     -Q cipher | cipher-auth | mac | kex | key
             Queries ssh for the algorithms supported for the specified version 2.  The available fea‐
             tures are: cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers
             that support authenticated encryption), mac (supported message integrity codes), kex (key
             exchange algorithms), key (key types).

     -q      Quiet mode.  Causes most warning and diagnostic messages to be suppressed.

     -R [bind_address:]port:host:hostport
             Specifies that the given port on the remote (server) host is to be forwarded to the given
             host and port on the local side.  This works by allocating a socket to listen to port on
             the remote side, and whenever a connection is made to this port, the connection is for‐
             warded over the secure channel, and a connection is made to host port hostport from the
             local machine.

             Port forwardings can also be specified in the configuration file.  Privileged ports can be
             forwarded only when logging in as root on the remote machine.  IPv6 addresses can be speci‐
             fied by enclosing the address in square brackets.

             By default, the listening socket on the server will be bound to the loopback interface
             only.  This may be overridden by specifying a bind_address.  An empty bind_address, or the
             address ‘*’, indicates that the remote socket should listen on all interfaces.  Specifying
             a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see
             sshd_config(5)).

             If the port argument is ‘0’, the listen port will be dynamically allocated on the server
             and reported to the client at run time.  When used together with -O forward the allocated
             port will be printed to the standard output.

     -S ctl_path
             Specifies the location of a control socket for connection sharing, or the string “none” to
             disable connection sharing.  Refer to the description of ControlPath and ControlMaster in
             ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote system.  Subsystems are a
             feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for
             other applications (eg. sftp(1)).  The subsystem is specified as the remote command.

     -T      Disable pseudo-tty allocation.

     -t      Force pseudo-tty allocation.  This can be used to execute arbitrary screen-based programs
             on a remote machine, which can be very useful, e.g. when implementing menu services.  Mul‐
             tiple -t options force tty allocation, even if ssh has no local tty.

     -V      Display the version number and exit.

     -v      Verbose mode.  Causes ssh to print debugging messages about its progress.  This is helpful
             in debugging connection, authentication, and configuration problems.  Multiple -v options
             increase the verbosity.  The maximum is 3.

     -W host:port
             Requests that standard input and output on the client be forwarded to host on port over the
             secure channel.  Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings.  Works with
             Protocol version 2 only.

     -w local_tun[:remote_tun]
             Requests tunnel device forwarding with the specified tun(4) devices between the client
             (local_tun) and the server (remote_tun).

             The devices may be specified by numerical ID or the keyword “any”, which uses the next
             available tunnel device.  If remote_tun is not specified, it defaults to “any”.  See also
             the Tunnel and TunnelDevice directives in ssh_config(5).  If the Tunnel directive is unset,
             it is set to the default tunnel mode, which is “point-to-point”.

     -X      Enables X11 forwarding.  This can also be specified on a per-host basis in a configuration
             file.

             X11 forwarding should be enabled with caution.  Users with the ability to bypass file per‐
             missions on the remote host (for the user's X authorization database) can access the local
             X11 display through the forwarded connection.  An attacker may then be able to perform
             activities such as keystroke monitoring.

             For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by
             default.  Please refer to the ssh -Y option and the ForwardX11Trusted directive in
             ssh_config(5) for more information.

     -x      Disables X11 forwarding.

     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not subjected to the X11 SECU‐
             RITY extension controls.

     -y      Send log information using the syslog(3) system module.  By default this information is
             sent to stderr.

     ssh may additionally obtain configuration data from a per-user configuration file and a system-wide
     configuration file.  The file format and configuration options are described in ssh_config(5).

AUTHENTICATION
     The OpenSSH SSH client supports SSH protocols 1 and 2.  The default is to use protocol 2 only,
     though this can be changed via the Protocol option in ssh_config(5) or the -1 and -2 options (see
     above).  Both protocols support similar authentication methods, but protocol 2 is the default since
     it provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES,
     Blowfish, CAST128, or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512,
     umac-64, umac-128, hmac-ripemd160).  Protocol 1 lacks a strong mechanism for ensuring the integrity
     of the connection.

     The methods available for authentication are: GSSAPI-based authentication, host-based authentica‐
     tion, public key authentication, challenge-response authentication, and password authentication.
     Authentication methods are tried in the order specified above, though protocol 2 has a configura‐
     tion option to change the default order: PreferredAuthentications.

     Host-based authentication works as follows: If the machine the user logs in from is listed in
     /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote machine, and the user names are the same on
     both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
     machine and contain a line containing the name of the client machine and the name of the user on
     that machine, the user is considered for login.  Additionally, the server must be able to verify
     the client's host key (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
     below) for login to be permitted.  This authentication method closes security holes due to IP
     spoofing, DNS spoofing, and routing spoofing.  [Note to the administrator: /etc/hosts.equiv,
     ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled
     if security is desired.]

     Public key authentication works as follows: The scheme is based on public-key cryptography, using
     cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to
     derive the decryption key from the encryption key.  The idea is that each user creates a pub‐
     lic/private key pair for authentication purposes.  The server knows the public key, and only the
     user knows the private key.  ssh implements public key authentication protocol automatically, using
     one of the DSA, ECDSA, ED25519 or RSA algorithms.  Protocol 1 is restricted to using only RSA keys,
     but protocol 2 may use any.  The HISTORY section of ssl(8) contains a brief discussion of the DSA
     and RSA algorithms.

     The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in.  When the
     user logs in, the ssh program tells the server which key pair it would like to use for authentica‐
     tion.  The client proves that it has access to the private key and the server checks that the cor‐
     responding public key is authorized to accept the account.

     The user creates his/her key pair by running ssh-keygen(1).  This stores the private key in
     ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA),
     ~/.ssh/id_ed25519 (protocol 2 ED25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key
     in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (pro‐
     tocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2 ED25519), or ~/.ssh/id_rsa.pub (protocol 2 RSA)
     in the user's home directory.  The user should then copy the public key to ~/.ssh/authorized_keys
     in his/her home directory on the remote machine.  The authorized_keys file corresponds to the con‐
     ventional ~/.rhosts file, and has one key per line, though the lines can be very long.  After this,
     the user can log in without giving the password.

     A variation on public key authentication is available in the form of certificate authentication:
     instead of a set of public/private keys, signed certificates are used.  This has the advantage that
     a single trusted certification authority can be used in place of many public/private keys.  See the
     CERTIFICATES section of ssh-keygen(1) for more information.

     The most convenient way to use public key or certificate authentication may be with an authentica‐
     tion agent.  See ssh-agent(1) for more information.

     Challenge-response authentication works as follows: The server sends an arbitrary "challenge" text,
     and prompts for a response.  Protocol 2 allows multiple challenges and responses; protocol 1 is
     restricted to just one challenge/response.  Examples of challenge-response authentication include
     BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD systems).

     Finally, if other authentication methods fail, ssh prompts the user for a password.  The password
     is sent to the remote host for checking; however, since all communications are encrypted, the pass‐
     word cannot be seen by someone listening on the network.

     ssh automatically maintains and checks a database containing identification for all hosts it has
     ever been used with.  Host keys are stored in ~/.ssh/known_hosts in the user's home directory.
     Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any new
     hosts are automatically added to the user's file.  If a host's identification ever changes, ssh
     warns about this and disables password authentication to prevent server spoofing or man-in-the-mid‐
     dle attacks, which could otherwise be used to circumvent the encryption.  The StrictHostKeyChecking
     option can be used to control logins to machines whose host key is not known or has changed.

     When the user's identity has been accepted by the server, the server either executes the given com‐
     mand, or logs into the machine and gives the user a normal shell on the remote machine.  All commu‐
     nication with the remote command or shell will be automatically encrypted.

     If a pseudo-terminal has been allocated (normal login session), the user may use the escape charac‐
     ters noted below.

     If no pseudo-tty has been allocated, the session is transparent and can be used to reliably trans‐
     fer binary data.  On most systems, setting the escape character to “none” will also make the ses‐
     sion transparent even if a tty is used.

     The session terminates when the command or shell on the remote machine exits and all X11 and TCP
     connections have been closed.

ESCAPE CHARACTERS
     When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an
     escape character.

     A single tilde character can be sent as ~~ or by following the tilde by a character other than
     those described below.  The escape character must always follow a newline to be interpreted as spe‐
     cial.  The escape character can be changed in configuration files using the EscapeChar configura‐
     tion directive or on the command line by the -e option.

     The supported escapes (assuming the default ‘~’) are:

     ~.      Disconnect.

     ~^Z     Background ssh.

     ~#      List forwarded connections.

     ~&      Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.

     ~?      Display a list of escape characters.

     ~B      Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer
             supports it).

     ~C      Open command line.  Currently this allows the addition of port forwardings using the -L, -R
             and -D options (see above).  It also allows the cancellation of existing port-forwardings
             with -KL[bind_address:]port for local, -KR[bind_address:]port for remote and
             -KD[bind_address:]port for dynamic port-forwardings.  !command allows the user to execute a
             local command if the PermitLocalCommand option is enabled in ssh_config(5).  Basic help is
             available, using the -h option.

     ~R      Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer
             supports it).

     ~V      Decrease the verbosity (LogLevel) when errors are being written to stderr.

     ~v      Increase the verbosity (LogLevel) when errors are being written to stderr.

TCP FORWARDING
     Forwarding of arbitrary TCP connections over the secure channel can be specified either on the com‐
     mand line or in a configuration file.  One possible application of TCP forwarding is a secure con‐
     nection to a mail server; another is going through firewalls.

     In the example below, we look at encrypting communication between an IRC client and server, even
     though the IRC server does not directly support encrypted communications.  This works as follows:
     the user connects to the remote host using ssh, specifying a port to be used to forward connections
     to the remote server.  After that it is possible to start the service which is to be encrypted on
     the client machine, connecting to the same local port, and ssh will encrypt and forward the connec‐
     tion.

     The following example tunnels an IRC session from client machine “127.0.0.1” (localhost) to remote
     server “server.example.com”:

         $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
         $ irc -c '#users' -p 1234 pinky 127.0.0.1

     This tunnels a connection to IRC server “server.example.com”, joining channel “#users”, nickname
     “pinky”, using port 1234.  It doesn't matter which port is used, as long as it's greater than 1023
     (remember, only root can open sockets on privileged ports) and doesn't conflict with any ports
     already in use.  The connection is forwarded to port 6667 on the remote server, since that's the
     standard port for IRC services.

     The -f option backgrounds ssh and the remote command “sleep 10” is specified to allow an amount of
     time (10 seconds, in the example) to start the service which is to be tunnelled.  If no connections
     are made within the time specified, ssh will exit.

X11 FORWARDING
     If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options
     above) and the user is using X11 (the DISPLAY environment variable is set), the connection to the
     X11 display is automatically forwarded to the remote side in such a way that any X11 programs
     started from the shell (or command) will go through the encrypted channel, and the connection to
     the real X server will be made from the local machine.  The user should not manually set DISPLAY.
     Forwarding of X11 connections can be configured on the command line or in configuration files.

     The DISPLAY value set by ssh will point to the server machine, but with a display number greater
     than zero.  This is normal, and happens because ssh creates a “proxy” X server on the server
     machine for forwarding the connections over the encrypted channel.

     ssh will also automatically set up Xauthority data on the server machine.  For this purpose, it
     will generate a random authorization cookie, store it in Xauthority on the server, and verify that
     any forwarded connections carry this cookie and replace it by the real cookie when the connection
     is opened.  The real authentication cookie is never sent to the server machine (and no cookies are
     sent in the plain).

     If the ForwardAgent variable is set to “yes” (or see the description of the -A and -a options
     above) and the user is using an authentication agent, the connection to the agent is automatically
     forwarded to the remote side.

VERIFYING HOST KEYS
     When connecting to a server for the first time, a fingerprint of the server's public key is pre‐
     sented to the user (unless the option StrictHostKeyChecking has been disabled).  Fingerprints can
     be determined using ssh-keygen(1):

           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

     If the fingerprint is already known, it can be matched and the key can be accepted or rejected.
     Because of the difficulty of comparing host keys just by looking at hex strings, there is also sup‐
     port to compare host keys visually, using random art.  By setting the VisualHostKey option to
     “yes”, a small ASCII graphic gets displayed on every login to a server, no matter if the session
     itself is interactive or not.  By learning the pattern a known server produces, a user can easily
     find out that the host key has changed when a completely different pattern is displayed.  Because
     these patterns are not unambiguous however, a pattern that looks similar to the pattern remembered
     only gives a good probability that the host key is the same, not guaranteed proof.

     To get a listing of the fingerprints along with their random art for all known hosts, the following
     command line can be used:

           $ ssh-keygen -lv -f ~/.ssh/known_hosts

     If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints
     verified by DNS.  An additional resource record (RR), SSHFP, is added to a zonefile and the con‐
     necting client is able to match the fingerprint with that of the key presented.

     In this example, we are connecting a client to a server, “host.example.com”.  The SSHFP resource
     records should first be added to the zonefile for host.example.com:

           $ ssh-keygen -r host.example.com.

     The output lines will have to be added to the zonefile.  To check that the zone is answering fin‐
     gerprint queries:

           $ dig -t SSHFP host.example.com

     Finally the client connects:

           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
           [...]
           Matching host key fingerprint found in DNS.
           Are you sure you want to continue connecting (yes/no)?

     See the VerifyHostKeyDNS option in ssh_config(5) for more information.

SSH-BASED VIRTUAL PRIVATE NETWORKS
     ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-
     device, allowing two networks to be joined securely.  The sshd_config(5) configuration option
     PermitTunnel controls whether the server supports this, and at what level (layer 2 or 3 traffic).

     The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24
     using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running
     on the gateway to the remote network, at 192.168.1.15, allows it.

     On the client:

           # ssh -f -w 0:1 192.168.1.15 true
           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
           # route add 10.0.99.0/24 10.1.1.2

     On the server:

           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
           # route add 10.0.50.0/24 10.1.1.1

     Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the
     PermitRootLogin server option.  The following entry would permit connections on tun(4) device 1
     from user “jane” and on tun device 2 from user “john”, if PermitRootLogin is set to
     “forced-commands-only”:

       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

     Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary set‐
     ups, such as for wireless VPNs.  More permanent VPNs are better provided by tools such as
     ipsecctl(8) and isakmpd(8).

ENVIRONMENT
     ssh will normally set the following environment variables:

     DISPLAY               The DISPLAY variable indicates the location of the X11 server.  It is auto‐
                           matically set by ssh to point to a value of the form “hostname:n”, where
                           “hostname” indicates the host where the shell runs, and ‘n’ is an integer ≥
                           1.  ssh uses this special value to forward X11 connections over the secure
                           channel.  The user should normally not set DISPLAY explicitly, as that will
                           render the X11 connection insecure (and will require the user to manually
                           copy any required authorization cookies).

     HOME                  Set to the path of the user's home directory.

     LOGNAME               Synonym for USER; set for compatibility with systems that use this variable.

     MAIL                  Set to the path of the user's mailbox.

     PATH                  Set to the default PATH, as specified when compiling ssh.

     SSH_ASKPASS           If ssh needs a passphrase, it will read the passphrase from the current ter‐
                           minal if it was run from a terminal.  If ssh does not have a terminal associ‐
                           ated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program
                           specified by SSH_ASKPASS and open an X11 window to read the passphrase.  This
                           is particularly useful when calling ssh from a .xsession or related script.
                           (Note that on some machines it may be necessary to redirect the input from
                           /dev/null to make this work.)

     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to communicate with the
                           agent.

     SSH_CONNECTION        Identifies the client and server ends of the connection.  The variable con‐
                           tains four space-separated values: client IP address, client port number,
                           server IP address, and server port number.

     SSH_ORIGINAL_COMMAND  This variable contains the original command line if a forced command is exe‐
                           cuted.  It can be used to extract the original arguments.

     SSH_TTY               This is set to the name of the tty (path to the device) associated with the
                           current shell or command.  If the current session has no tty, this variable
                           is not set.

     TZ                    This variable is set to indicate the present time zone if it was set when the
                           daemon was started (i.e. the daemon passes the value on to new connections).

     USER                  Set to the name of the user logging in.

     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format “VARNAME=value” to the
     environment if the file exists and users are allowed to change their environment.  For more infor‐
     mation, see the PermitUserEnvironment option in sshd_config(5).

ENVIRONMENT
     SSH_USE_STRONG_RNG
             The reseeding of the OpenSSL random generator is usually done from /dev/urandom.  If the
             SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random
             generator is reseeded from /dev/random.  The number of bytes read is defined by the
             SSH_USE_STRONG_RNG value.  Minimum is 14 bytes.  This setting is not recommended on the
             computers without the hardware random generator because insufficient entropy causes the
             connection to be blocked until enough entropy is available.

FILES
     ~/.rhosts
             This file is used for host-based authentication (see above).  On some machines this file
             may need to be world-readable if the user's home directory is on an NFS partition, because
             sshd(8) reads it as root.  Additionally, this file must be owned by the user, and must not
             have write permissions for anyone else.  The recommended permission for most machines is
             read/write for the user, and not accessible by others.

     ~/.shosts
             This file is used in exactly the same way as .rhosts, but allows host-based authentication
             without permitting login with rlogin/rsh.

     ~/.ssh/
             This directory is the default location for all user-specific configuration and authentica‐
             tion information.  There is no general requirement to keep the entire contents of this
             directory secret, but the recommended permissions are read/write/execute for the user, and
             not accessible by others.

     ~/.ssh/authorized_keys
             Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used for logging in as this
             user.  The format of this file is described in the sshd(8) manual page.  This file is not
             highly sensitive, but the recommended permissions are read/write for the user, and not
             accessible by others.

     ~/.ssh/config
             This is the per-user configuration file.  The file format and configuration options are
             described in ssh_config(5).  Because of the potential for abuse, this file must have strict
             permissions: read/write for the user, and not writable by others.

     ~/.ssh/environment
             Contains additional definitions for environment variables; see ENVIRONMENT, above.

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.ssh/id_rsa
             Contains the private key for authentication.  These files contain sensitive data and should
             be readable by the user but not accessible by others (read/write/execute).  ssh will simply
             ignore a private key file if it is accessible by others.  It is possible to specify a
             passphrase when generating the key which will be used to encrypt the sensitive part of this
             file using 3DES.

     ~/.ssh/identity.pub
     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_rsa.pub
             Contains the public key for authentication.  These files are not sensitive and can (but
             need not) be readable by anyone.

     ~/.ssh/known_hosts
             Contains a list of host keys for all hosts the user has logged into that are not already in
             the systemwide list of known host keys.  See sshd(8) for further details of the format of
             this file.

     ~/.ssh/rc
             Commands in this file are executed by ssh when the user logs in, just before the user's
             shell (or command) is started.  See the sshd(8) manual page for more information.

     /etc/hosts.equiv
             This file is for host-based authentication (see above).  It should only be writable by
             root.

     /etc/ssh/shosts.equiv
             This file is used in exactly the same way as hosts.equiv, but allows host-based authentica‐
             tion without permitting login with rlogin/rsh.

     /etc/ssh/ssh_config
             Systemwide configuration file.  The file format and configuration options are described in
             ssh_config(5).

     /etc/ssh/ssh_host_key
     /etc/ssh/ssh_host_dsa_key
     /etc/ssh/ssh_host_ecdsa_key
     /etc/ssh/ssh_host_ed25519_key
     /etc/ssh/ssh_host_rsa_key
             These files contain the private parts of the host keys and are used for host-based authen‐
             tication.  If protocol version 1 is used, ssh must be setuid root, since the host key is
             readable only by root.  For protocol version 2, ssh uses ssh-keysign(8) to access the host
             keys, eliminating the requirement that ssh be setuid root when host-based authentication is
             used.  By default ssh is not setuid root.

     /etc/ssh/ssh_known_hosts
             Systemwide list of known host keys.  This file should be prepared by the system administra‐
             tor to contain the public host keys of all machines in the organization.  It should be
             world-readable.  See sshd(8) for further details of the format of this file.

     /etc/ssh/sshrc
             Commands in this file are executed by ssh when the user logs in, just before the user's
             shell (or command) is started.  See the sshd(8) manual page for more information.

EXIT STATUS
     ssh exits with the exit status of the remote command or with 255 if an error occurred.

IPV6
     IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address
     enclosed in square brackets. Note: The square brackets are metacharacters for the shell and must be
     escaped in shell.

SEE ALSO
     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), tun(4), hosts.equiv(5),
     ssh_config(5), ssh-keysign(8), sshd(8)

STANDARDS
     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January
     2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.

     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
     4255, January 2006.

     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol
     (SSH), RFC 4256, January 2006.

     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335,
     January 2006.

     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes,
     RFC 4344, January 2006.

     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345,
     January 2006.

     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH)
     Transport Layer Protocol, RFC 4419, March 2006.

     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.

     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
     RFC 5656, December 2009.

     A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999,
     International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Camp‐
     bell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added
     newer features and created OpenSSH.  Markus Friedl contributed the support for SSH protocol ver‐
     sions 1.5 and 2.0.

BSD                                         November 12, 2016                                        BSD

sshd_config

 # man sshd_config
NAME
     sshd_config — OpenSSH SSH daemon configuration file

SYNOPSIS
     /etc/ssh/sshd_config

DESCRIPTION
     sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the
     command line).  The file contains keyword-argument pairs, one per line.  Lines starting with ‘#’
     and empty lines are interpreted as comments.  Arguments may optionally be enclosed in double quotes
     (") in order to represent arguments containing spaces.

     The possible keywords and their meanings are as follows (note that keywords are case-insensitive
     and arguments are case-sensitive):

     AcceptEnv
             Specifies what environment variables sent by the client will be copied into the session's
             environ(7).  See SendEnv in ssh_config(5) for how to configure the client.  Note that envi‐
             ronment passing is only supported for protocol 2, and that the TERM environment variable is
             always sent whenever the client requests a pseudo-terminal as it is required by the proto‐
             col.  Variables are specified by name, which may contain the wildcard characters ‘*’ and
             ‘?’.  Multiple environment variables may be separated by whitespace or spread across multi‐
             ple AcceptEnv directives.  Be warned that some environment variables could be used to
             bypass restricted user environments.  For this reason, care should be taken in the use of
             this directive.  The default is not to accept any environment variables.

     AddressFamily
             Specifies which address family should be used by sshd(8).  Valid arguments are “any”,
             “inet” (use IPv4 only), or “inet6” (use IPv6 only).  The default is “any”.

     AllowAgentForwarding
             Specifies whether ssh-agent(1) forwarding is permitted.  The default is “yes”.  Note that
             disabling agent forwarding does not improve security unless users are also denied shell
             access, as they can always install their own forwarders.

     AllowGroups
             This keyword can be followed by a list of group name patterns, separated by spaces.  If
             specified, login is allowed only for users whose primary group or supplementary group list
             matches one of the patterns.  Only group names are valid; a numerical group ID is not rec‐
             ognized.  By default, login is allowed for all groups.  The allow/deny directives are pro‐
             cessed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
             All of the specified user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     AllowTcpForwarding
             Specifies whether TCP forwarding is permitted.  The available options are “yes” or “all” to
             allow TCP forwarding, “no” to prevent all TCP forwarding, “local” to allow local (from the
             perspective of ssh(1)) forwarding only or “remote” to allow remote forwarding only.  The
             default is “yes”.  Note that disabling TCP forwarding does not improve security unless
             users are also denied shell access, as they can always install their own forwarders.

     AllowUsers
             This keyword can be followed by a list of user name patterns, separated by spaces.  If
             specified, login is allowed only for user names that match one of the patterns.  Only user
             names are valid; a numerical user ID is not recognized.  By default, login is allowed for
             all users.  If the pattern takes the form USER@HOST then USER and HOST are separately
             checked, restricting logins to particular users from particular hosts.  The allow/deny
             directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and
             finally AllowGroups.  All of the specified user and group tests must succeed, before user
             is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     AuthenticationMethods
             Specifies the authentication methods that must be successfully completed for a user to be
             granted access.  This option must be followed by one or more comma-separated lists of
             authentication method names.  Successful authentication requires completion of every method
             in at least one of these lists.

             For example, an argument of “publickey,password publickey,keyboard-interactive” would
             require the user to complete public key authentication, followed by either password or key‐
             board interactive authentication.  Only methods that are next in one or more lists are
             offered at each stage, so for this example, it would not be possible to attempt password or
             keyboard-interactive authentication before public key.

             For keyboard interactive authentication it is also possible to restrict authentication to a
             specific device by appending a colon followed by the device identifier “bsdauth”, “pam”, or
             “skey”, depending on the server configuration.  For example, “keyboard-interactive:bsdauth”
             would restrict keyboard interactive authentication to the “bsdauth” device.

             This option is only available for SSH protocol 2 and will yield a fatal error if enabled if
             protocol 1 is also enabled.  Note that each authentication method listed should also be
             explicitly enabled in the configuration.  The default is not to require multiple authenti‐
             cation; successful completion of a single authentication method is sufficient.

     AuthorizedKeysCommand
             Specifies a program to be used to look up the user's public keys.  The program must be
             owned by root and not writable by group or others.  It will be invoked with a single argu‐
             ment of the username being authenticated, and should produce on standard output zero or
             more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)).  If a key supplied
             by AuthorizedKeysCommand does not successfully authenticate and authorize the user then
             public key authentication continues using the usual AuthorizedKeysFile files.  By default,
             no AuthorizedKeysCommand is run.

     AuthorizedKeysCommandUser
             Specifies the user under whose account the AuthorizedKeysCommand is run.  It is recommended
             to use a dedicated user that has no other role on the host than running authorized keys
             commands.

     AuthorizedKeysFile
             Specifies the file that contains the public keys that can be used for user authentication.
             The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
             AuthorizedKeysFile may contain tokens of the form %T which are substituted during connec‐
             tion setup.  The following tokens are defined: %% is replaced by a literal '%', %h is
             replaced by the home directory of the user being authenticated, and %u is replaced by the
             username of that user.  After expansion, AuthorizedKeysFile is taken to be an absolute path
             or one relative to the user's home directory.  Multiple files may be listed, separated by
             whitespace.  The default is “.ssh/authorized_keys .ssh/authorized_keys2”.

     AuthorizedPrincipalsFile
             Specifies a file that lists principal names that are accepted for certificate authentica‐
             tion.  When using certificates signed by a key listed in TrustedUserCAKeys, this file lists
             names, one of which must appear in the certificate for it to be accepted for authentica‐
             tion.  Names are listed one per line preceded by key options (as described in AUTHO‐
             RIZED_KEYS FILE FORMAT in sshd(8)).  Empty lines and comments starting with ‘#’ are
             ignored.

             AuthorizedPrincipalsFile may contain tokens of the form %T which are substituted during
             connection setup.  The following tokens are defined: %% is replaced by a literal '%', %h is
             replaced by the home directory of the user being authenticated, and %u is replaced by the
             username of that user.  After expansion, AuthorizedPrincipalsFile is taken to be an abso‐
             lute path or one relative to the user's home directory.

             The default is “none”, i.e. not to use a principals file – in this case, the username of
             the user must appear in a certificate's principals list for it to be accepted.  Note that
             AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in
             TrustedUserCAKeys and is not consulted for certification authorities trusted via
             ~/.ssh/authorized_keys, though the principals= key option offers a similar facility (see
             sshd(8) for details).

     Banner  The contents of the specified file are sent to the remote user before authentication is
             allowed.  If the argument is “none” then no banner is displayed.  This option is only
             available for protocol version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed (e.g. via PAM or though
             authentication styles supported in login.conf(5)) The default is “yes”.

     ChrootDirectory
             Specifies the pathname of a directory to chroot(2) to after authentication.  All components
             of the pathname must be root-owned directories that are not writable by any other user or
             group.  After the chroot, sshd(8) changes the working directory to the user's home direc‐
             tory.

             The pathname may contain the following tokens that are expanded at runtime once the con‐
             necting user has been authenticated: %% is replaced by a literal '%', %h is replaced by the
             home directory of the user being authenticated, and %u is replaced by the username of that
             user.

             The ChrootDirectory must contain the necessary files and directories to support the user's
             session.  For an interactive session this requires at least a shell, typically sh(1), and
             basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and
             tty(4) devices.  For file transfer sessions using “sftp”, no additional configuration of
             the environment is necessary if the in-process sftp server is used, though sessions which
             use logging do require /dev/log inside the chroot directory (see sftp-server(8) for
             details).

             The default is not to chroot(2).

     Ciphers
             Specifies the ciphers allowed for protocol version 2.  Multiple ciphers must be comma-sepa‐
             rated.  The supported ciphers are:

             “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”,
             “aes256-ctr”, “aes128-gcm@openssh.com”, “aes256-gcm@openssh.com”, “arcfour128”,
             “arcfour256”, “arcfour”, “blowfish-cbc”, “cast128-cbc”, and
             “chacha20-poly1305@openssh.com”.

             The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-gcm@openssh.com,aes256-gcm@openssh.com,
                chacha20-poly1305@openssh.com,
                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                aes256-cbc,arcfour

             The list of available ciphers may also be obtained using the -Q option of ssh(1).

     ClientAliveCountMax
             Sets the number of client alive messages (see below) which may be sent without sshd(8)
             receiving any messages back from the client.  If this threshold is reached while client
             alive messages are being sent, sshd will disconnect the client, terminating the session.
             It is important to note that the use of client alive messages is very different from
             TCPKeepAlive (below).  The client alive messages are sent through the encrypted channel and
             therefore will not be spoofable.  The TCP keepalive option enabled by TCPKeepAlive is
             spoofable.  The client alive mechanism is valuable when the client or server depend on
             knowing when a connection has become inactive.

             The default value is 3.  If ClientAliveInterval (see below) is set to 15, and
             ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected
             after approximately 45 seconds.  This option applies to protocol version 2 only.

     ClientAliveInterval
             Sets a timeout interval in seconds after which if no data has been received from the
             client, sshd(8) will send a message through the encrypted channel to request a response
             from the client.  The default is 0, indicating that these messages will not be sent to the
             client.  This option applies to protocol version 2 only.

     Compression
             Specifies whether compression is allowed, or delayed until the user has authenticated suc‐
             cessfully.  The argument must be “yes”, “delayed”, or “no”.  The default is “delayed”.

     DenyGroups
             This keyword can be followed by a list of group name patterns, separated by spaces.  Login
             is disallowed for users whose primary group or supplementary group list matches one of the
             patterns.  Only group names are valid; a numerical group ID is not recognized.  By default,
             login is allowed for all groups.  The allow/deny directives are processed in the following
             order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All of the specified
             user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     DenyUsers
             This keyword can be followed by a list of user name patterns, separated by spaces.  Login
             is disallowed for user names that match one of the patterns.  Only user names are valid; a
             numerical user ID is not recognized.  By default, login is allowed for all users.  If the
             pattern takes the form USER@HOST then USER and HOST are separately checked, restricting
             logins to particular users from particular hosts.  The allow/deny directives are processed
             in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All of
             the specified user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     ForceCommand
             Forces the execution of the command specified by ForceCommand, ignoring any command sup‐
             plied by the client and ~/.ssh/rc if present.  The command is invoked by using the user's
             login shell with the -c option.  This applies to shell, command, or subsystem execution.
             It is most useful inside a Match block.  The command originally supplied by the client is
             available in the SSH_ORIGINAL_COMMAND environment variable.  Specifying a command of
             “internal-sftp” will force the use of an in-process sftp server that requires no support
             files when used with ChrootDirectory.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports forwarded for the client.
             By default, sshd(8) binds remote port forwardings to the loopback address.  This prevents
             other remote hosts from connecting to forwarded ports.  GatewayPorts can be used to specify
             that sshd should allow remote port forwardings to bind to non-loopback addresses, thus
             allowing other hosts to connect.  The argument may be “no” to force remote port forwardings
             to be available to the local host only, “yes” to force remote port forwardings to bind to
             the wildcard address, or “clientspecified” to allow the client to select the address to
             which the forwarding is bound.  The default is “no”.

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.  The default is “no”.
             Note that this option applies to protocol version 2 only.

     GSSAPIKeyExchange
             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely
             on ssh keys to verify host identity.  The default is “no”.  Note that this option applies
             to protocol version 2 only.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's credentials cache on logout.  The
             default is “yes”.  Note that this option applies to protocol version 2 only.

     GSSAPIEnablek5users
             Specifies whether to look at .k5users file for GSSAPI authentication access control. Fur‐
             ther details are described in ksu(1).  The default is “no”.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI acceptor a client authen‐
             ticates against. If “yes” then the client must authenticate against the host service on the
             current hostname. If “no” then the client may authenticate against any service key stored
             in the machine's default store. This facility is provided to assist with operation on multi
             homed machines.  The default is “yes”.  Note that this option applies only to protocol ver‐
             sion 2 GSSAPI connections, and setting it to “no” may only work with recent Kerberos GSSAPI
             libraries.

     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated following a successful
             connection rekeying. This option can be used to accepted renewed or updated credentials
             from a compatible client. The default is “no”.

     GSSAPIKexAlgorithms
             The list of key exchange algorithms that are accepted by GSSAPI key exchange. Possible val‐
             ues are

                gss-gex-sha1-,
                gss-group1-sha1-,
                gss-group14-sha1-

             The default is “gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-”.  This option only
             applies to protocol version 2 connections using GSSAPI.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public
             key client host authentication is allowed (host-based authentication).  This option is sim‐
             ilar to RhostsRSAAuthentication and applies to protocol version 2 only.  The default is
             “no”.

     HostbasedUsesNameFromPacketOnly
             Specifies whether or not the server will attempt to perform a reverse name lookup when
             matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during
             HostbasedAuthentication.  A setting of “yes” means that sshd(8) uses the name supplied by
             the client rather than attempting to resolve the name from the TCP connection itself.  The
             default is “no”.

     HostCertificate
             Specifies a file containing a public host certificate.  The certificate's public key must
             match a private host key already specified by HostKey.  The default behaviour of sshd(8) is
             not to load any certificates.

     HostKey
             Specifies a file containing a private host key used by SSH.  The default is
             /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_dsa_key,
             /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key
             for protocol version 2.  Note that sshd(8) will refuse to use a file if it is group/world-
             accessible.  It is possible to have multiple host key files.  “rsa1” keys are used for ver‐
             sion 1 and “dsa”, “ecdsa”, “ed25519” or “rsa” are used for version 2 of the SSH protocol.
             It is also possible to specify public host key files instead.  In this case operations on
             the private key will be delegated to an ssh-agent(1).

     HostKeyAgent
             Identifies the UNIX-domain socket used to communicate with an agent that has access to the
             private host keys.  If “SSH_AUTH_SOCK” is specified, the location of the socket will be
             read from the SSH_AUTH_SOCK environment variable.

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or
             HostbasedAuthentication.

             /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.  The default is “yes”.

     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during
             RhostsRSAAuthentication or HostbasedAuthentication.  The default is “no”.

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connection.  Accepted values are
             “af11”, “af12”, “af13”, “af21”, “af22”, “af23”, “af31”, “af32”, “af33”, “af41”, “af42”,
             “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”, “ef”, “lowdelay”,
             “throughput”, “reliability”, or a numeric value.  This option may take one or two argu‐
             ments, separated by whitespace.  If one argument is specified, it is used as the packet
             class unconditionally.  If two values are specified, the first is automatically selected
             for interactive sessions and the second for non-interactive sessions.  The default is
             “lowdelay” for interactive sessions and “throughput” for non-interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to allow keyboard-interactive authentication.  The argument to this key‐
             word must be “yes” or “no”.  The default is to use whatever value
             ChallengeResponseAuthentication is set to (by default “yes”).

     KerberosAuthentication
             Specifies whether the password provided by the user for PasswordAuthentication will be val‐
             idated through the Kerberos KDC.  To use this option, the server needs a Kerberos servtab
             which allows the verification of the KDC's identity.  The default is “no”.

     KerberosGetAFSToken
             If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before
             accessing the user's home directory.  The default is “no”.

     KerberosOrLocalPasswd
             If password authentication through Kerberos fails then the password will be validated via
             any additional local mechanism such as /etc/passwd.  The default is “yes”.

     KerberosTicketCleanup
             Specifies whether to automatically destroy the user's ticket cache file on logout.  The
             default is “yes”.

     KerberosUseKuserok
             Specifies whether to look at .k5login file for user's aliases.  The default is “yes”.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be comma-
             separated.  The default is

                   curve25519-sha256@libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group-exchange-sha1,
                   diffie-hellman-group14-sha1,
                   diffie-hellman-group1-sha1

     KeyRegenerationInterval
             In protocol version 1, the ephemeral server key is automatically regenerated after this
             many seconds (if it has been used).  The purpose of regeneration is to prevent decrypting
             captured sessions by later breaking into the machine and stealing the keys.  The key is
             never stored anywhere.  If the value is 0, the key is never regenerated.  The default is
             3600 (seconds).

     ListenAddress
             Specifies the local addresses sshd(8) should listen on.  The following forms may be used:

                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port

             If port is not specified, sshd will listen on the address and all prior Port options speci‐
             fied.  The default is to listen on all local addresses.  Multiple ListenAddress options are
             permitted.  Additionally, any Port options must precede this option for non-port qualified
             addresses.

     LoginGraceTime
             The server disconnects after this time if the user has not successfully logged in.  If the
             value is 0, there is no time limit.  The default is 120 seconds.

     LogLevel
             Gives the verbosity level that is used when logging messages from sshd(8).  The possible
             values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The
             default is INFO.  DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify higher
             levels of debugging output.  Logging with a DEBUG level violates the privacy of users and
             is not recommended.

     MACs    Specifies the available MAC (message authentication code) algorithms.  The MAC algorithm is
             used in protocol version 2 for data integrity protection.  Multiple algorithms must be
             comma-separated.  The algorithms that contain “-etm” calculate the MAC after encryption
             (encrypt-then-mac).  These are considered safer and their use recommended.  The default is:

                   hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
                   hmac-md5-96-etm@openssh.com,
                   hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
                   hmac-sha1-96,hmac-md5-96

     Match   Introduces a conditional block.  If all of the criteria on the Match line are satisfied,
             the keywords on the following lines override those set in the global section of the config
             file, until either another Match line or the end of the file.  If a keyword appears in mul‐
             tiple Match blocks that are satisified, only the first instance of the keyword is applied.

             The arguments to Match are one or more criteria-pattern pairs or the single token All which
             matches all criteria.  The available criteria are User, Group, Host, LocalAddress,
             LocalPort, and Address.  The match patterns may consist of single entries or comma-sepa‐
             rated lists and may use the wildcard and negation operators described in the PATTERNS sec‐
             tion of ssh_config(5).

             The patterns in an Address criteria may additionally contain addresses to match in CIDR
             address/masklen format, e.g. “192.0.2.0/24” or “3ffe:ffff::/32”.  Note that the mask length
             provided must be consistent with the address - it is an error to specify a mask length that
             is too long for the address or one with bits set in this host portion of the address.  For
             example, “192.0.2.0/33” and “192.0.2.0/8” respectively.

             Only a subset of keywords may be used on the lines following a Match keyword.  Available
             keywords are AcceptEnv, AllowAgentForwarding, AllowGroups, AllowTcpForwarding, AllowUsers,
             AuthenticationMethods, AuthorizedKeysCommand, AuthorizedKeysCommandUser,
             AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
             DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
             HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication, KerberosAuthentication,
             KerberosUseKuserok, MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
             PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication, RSAAuthentication,
             X11DisplayOffset, X11Forwarding and X11UseLocalHost.

     MaxAuthTries
             Specifies the maximum number of authentication attempts permitted per connection.  Once the
             number of failures reaches half this value, additional failures are logged.  The default is
             6.

     MaxSessions
             Specifies the maximum number of open sessions permitted per network connection.  The
             default is 10.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.
             Additional connections will be dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10:30:100.

             Alternatively, random early drop can be enabled by specifying the three colon separated
             values “start:rate:full” (e.g. "10:30:60").  sshd(8) will refuse connection attempts with a
             probability of “rate/100” (30%) if there are currently “start” (10) unauthenticated connec‐
             tions.  The probability increases linearly and all connection attempts are refused if the
             number of unauthenticated connections reaches “full” (60).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The default is “yes”.

     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the server allows login to
             accounts with empty password strings.  The default is “no”.

     PermitOpen
             Specifies the destinations to which TCP port forwarding is permitted.  The forwarding spec‐
             ification must be one of the following forms:

                   PermitOpen host:port
                   PermitOpen IPv4_addr:port
                   PermitOpen [IPv6_addr]:port

             Multiple forwards may be specified by separating them with whitespace.  An argument of
             “any” can be used to remove all restrictions and permit any forwarding requests.  An argu‐
             ment of “none” can be used to prohibit all forwarding requests.  By default all port for‐
             warding requests are permitted.

     PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument must be “yes”,
             “without-password”, “forced-commands-only”, or “no”.  The default is “yes”.

             If this option is set to “without-password”, password authentication is disabled for root.

             If this option is set to “forced-commands-only”, root login with public key authentication
             will be allowed, but only if the command option has been specified (which may be useful for
             taking remote backups even if root login is normally not allowed).  All other authentica‐
             tion methods are disabled for root.

             If this option is set to “no”, root is not allowed to log in.

     PermitTunnel
             Specifies whether tun(4) device forwarding is allowed.  The argument must be “yes”,
             “point-to-point” (layer 3), “ethernet” (layer 2), or “no”.  Specifying “yes” permits both
             “point-to-point” and “ethernet”.  The default is “no”.

     PermitTTY
             Specifies whether pty(4) allocation is permitted.  The default is “yes”.

     PermitUserEnvironment
             Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are
             processed by sshd(8).  The default is “no”.  Enabling environment processing may enable
             users to bypass access restrictions in some configurations using mechanisms such as
             LD_PRELOAD.

     PidFile
             Specifies the file that contains the process ID of the SSH daemon.  The default is
             /var/run/sshd.pid.

     Port    Specifies the port number that sshd(8) listens on.  The default is 22.  Multiple options of
             this type are permitted.  See also ListenAddress.

     PrintLastLog
             Specifies whether sshd(8) should print the date and time of the last user login when a user
             logs in interactively.  The default is “yes”.

     PrintMotd
             Specifies whether sshd(8) should print /etc/motd when a user logs in interactively.  (On
             some systems it is also printed by the shell, /etc/profile, or equivalent.)  The default is
             “yes”.

     Protocol
             Specifies the protocol versions sshd(8) supports.  The possible values are ‘1’ and ‘2’.
             Multiple versions must be comma-separated.  The default is ‘2’.  Note that the order of the
             protocol list does not indicate preference, because the client selects among multiple pro‐
             tocol versions offered by the server.  Specifying “2,1” is identical to “1,2”.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The default is “yes”.  Note that
             this option applies to protocol version 2 only.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted before the session key is
             renegotiated, optionally followed a maximum amount of time that may pass before the session
             key is renegotiated.  The first argument is specified in bytes and may have a suffix of
             ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively.  The default
             is between ‘1G’ and ‘4G’, depending on the cipher.  The optional second value is specified
             in seconds and may use any of the units documented in the TIME FORMATS section.  The
             default value for RekeyLimit is “default none”, which means that rekeying is performed
             after the cipher's default amount of data has been sent or received and no time based
             rekeying is done.  This option applies to protocol version 2 only.

     RevokedKeys
             Specifies revoked public keys.  Keys listed in this file will be refused for public key
             authentication.  Note that if this file is not readable, then public key authentication
             will be refused for all users.  Keys may be specified as a text file, listing one public
             key per line, or as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
             For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA
             host authentication is allowed.  The default is “no”.  This option applies to protocol ver‐
             sion 1 only.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The default is “yes”.  This option
             applies to protocol version 1 only.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1 server key.  The minimum
             value is 512, and the default is 1024.

     ShowPatchLevel
             Specifies whether sshd will display the patch level of the binary in the identification
             string.  The patch level is set at compile-time.  The default is “no”.  This option applies
             to protocol version 1 only.

     StrictModes
             Specifies whether sshd(8) should check file modes and ownership of the user's files and
             home directory before accepting login.  This is normally desirable because novices some‐
             times accidentally leave their directory or files world-writable.  The default is “yes”.
             Note that this does not apply to ChrootDirectory, whose permissions and ownership are
             checked unconditionally.

     Subsystem
             Configures an external subsystem (e.g. file transfer daemon).  Arguments should be a sub‐
             system name and a command (with optional arguments) to execute upon subsystem request.

             The command sftp-server(8) implements the “sftp” file transfer subsystem.

             Alternately the name “internal-sftp” implements an in-process “sftp” server.  This may sim‐
             plify configurations using ChrootDirectory to force a different filesystem root on clients.

             By default no subsystems are defined.  Note that this option applies to protocol version 2
             only.

     SyslogFacility
             Gives the facility code that is used when logging messages from sshd(8).  The possible val‐
             ues are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,
             LOCAL6, LOCAL7.  The default is AUTH.

     TCPKeepAlive
             Specifies whether the system should send TCP keepalive messages to the other side.  If they
             are sent, death of the connection or crash of one of the machines will be properly noticed.
             However, this means that connections will die if the route is down temporarily, and some
             people find it annoying.  On the other hand, if TCP keepalives are not sent, sessions may
             hang indefinitely on the server, leaving “ghost” users and consuming server resources.

             The default is “yes” (to send TCP keepalive messages), and the server will notice if the
             network goes down or the client host crashes.  This avoids infinitely hanging sessions.

             To disable TCP keepalive messages, the value should be set to “no”.

     TrustedUserCAKeys
             Specifies a file containing public keys of certificate authorities that are trusted to sign
             user certificates for authentication.  Keys are listed one per line; empty lines and com‐
             ments starting with ‘#’ are allowed.  If a certificate is presented for authentication and
             has its signing CA key listed in this file, then it may be used for authentication for any
             user listed in the certificate's principals list.  Note that certificates that lack a list
             of principals will not be permitted for authentication using TrustedUserCAKeys.  For more
             details on certificates, see the CERTIFICATES section in ssh-keygen(1).

     UseDNS  Specifies whether sshd(8) should look up the remote host name and check that the resolved
             host name for the remote IP address maps back to the very same IP address.  The default is
             “yes”.

     UseLogin
             Specifies whether login(1) is used for interactive login sessions.  The default is “no”.
             Note that login(1) is never used for remote command execution.  Note also, that if this is
             enabled, X11Forwarding will be disabled because login(1) does not know how to handle
             xauth(1) cookies.  If UsePrivilegeSeparation is specified, it will be disabled after
             authentication.

     UsePAM  Enables the Pluggable Authentication Module interface.  If set to “yes” this will enable
             PAM authentication using ChallengeResponseAuthentication and PasswordAuthentication in
             addition to PAM account and session module processing for all authentication types.

             Because PAM challenge-response authentication usually serves an equivalent role to password
             authentication, you should disable either PasswordAuthentication or
             ChallengeResponseAuthentication.

             If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user.  The default
             is “no”.

     UsePrivilegeSeparation
             Specifies whether sshd(8) separates privileges by creating an unprivileged child process to
             deal with incoming network traffic.  After successful authentication, another process will
             be created that has the privilege of the authenticated user.  The goal of privilege separa‐
             tion is to prevent privilege escalation by containing any corruption within the unprivi‐
             leged processes.  The default is “yes”.  If UsePrivilegeSeparation is set to “sandbox” then
             the pre-authentication unprivileged process is subject to additional restrictions.

     VersionAddendum
             Optionally specifies additional text to append to the SSH protocol banner sent by the
             server upon connection.  The default is “none”.

     X11DisplayOffset
             Specifies the first display number available for sshd(8)'s X11 forwarding.  This prevents
             sshd from interfering with real X11 servers.  The default is 10.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The argument must be “yes” or “no”.  The
             default is “no”.

             When X11 forwarding is enabled, there may be additional exposure to the server and to
             client displays if the sshd(8) proxy display is configured to listen on the wildcard
             address (see X11UseLocalhost below), though this is not the default.  Additionally, the
             authentication spoofing and authentication data verification and substitution occur on the
             client side.  The security risk of using X11 forwarding is that the client's X11 display
             server may be exposed to attack when the SSH client requests forwarding (see the warnings
             for ForwardX11 in ssh_config(5)).  A system administrator may have a stance in which they
             want to protect clients that may expose themselves to attack by unwittingly requesting X11
             forwarding, which can warrant a “no” setting.

             Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as
             users can always install their own forwarders.  X11 forwarding is automatically disabled if
             UseLogin is enabled.

     X11UseLocalhost
             Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or
             to the wildcard address.  By default, sshd binds the forwarding server to the loopback
             address and sets the hostname part of the DISPLAY environment variable to “localhost”.
             This prevents remote hosts from connecting to the proxy display.  However, some older X11
             clients may not function with this configuration.  X11UseLocalhost may be set to “no” to
             specify that the forwarding server should be bound to the wildcard address.  The argument
             must be “yes” or “no”.  The default is “yes”.

     XAuthLocation
             Specifies the full pathname of the xauth(1) program.  The default is /usr/bin/xauth.

TIME FORMATS
     sshd(8) command-line arguments and configuration file options that specify time may be expressed
     using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier
     is one of the following:

           ⟨none⟩  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

     Each member of the sequence is added together to calculate the total time value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd(8).  This file should be writable by root only, but it
             is recommended (though not necessary) that it be world-readable.

SEE ALSO
     sshd(8)

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Camp‐
     bell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added
     newer features and created OpenSSH.  Markus Friedl contributed the support for SSH protocol ver‐
     sions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support for privilege separation.

BSD                                         November 13, 2016                                        BSD

ssh_config

 # man ssh_config 
SSHD_CONFIG(5)                           BSD File Formats Manual                          SSHD_CONFIG(5)

NAME
     sshd_config — OpenSSH SSH daemon configuration file

SYNOPSIS
     /etc/ssh/sshd_config

DESCRIPTION
     sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the
     command line).  The file contains keyword-argument pairs, one per line.  Lines starting with ‘#’
     and empty lines are interpreted as comments.  Arguments may optionally be enclosed in double quotes
     (") in order to represent arguments containing spaces.

     The possible keywords and their meanings are as follows (note that keywords are case-insensitive
     and arguments are case-sensitive):

     AcceptEnv
             Specifies what environment variables sent by the client will be copied into the session's
             environ(7).  See SendEnv in ssh_config(5) for how to configure the client.  Note that envi‐
             ronment passing is only supported for protocol 2, and that the TERM environment variable is
             always sent whenever the client requests a pseudo-terminal as it is required by the proto‐
             col.  Variables are specified by name, which may contain the wildcard characters ‘*’ and
             ‘?’.  Multiple environment variables may be separated by whitespace or spread across multi‐
             ple AcceptEnv directives.  Be warned that some environment variables could be used to
             bypass restricted user environments.  For this reason, care should be taken in the use of
             this directive.  The default is not to accept any environment variables.

     AddressFamily
             Specifies which address family should be used by sshd(8).  Valid arguments are “any”,
             “inet” (use IPv4 only), or “inet6” (use IPv6 only).  The default is “any”.

     AllowAgentForwarding
             Specifies whether ssh-agent(1) forwarding is permitted.  The default is “yes”.  Note that
             disabling agent forwarding does not improve security unless users are also denied shell
             access, as they can always install their own forwarders.

     AllowGroups
             This keyword can be followed by a list of group name patterns, separated by spaces.  If
             specified, login is allowed only for users whose primary group or supplementary group list
             matches one of the patterns.  Only group names are valid; a numerical group ID is not rec‐
             ognized.  By default, login is allowed for all groups.  The allow/deny directives are pro‐
             cessed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
             All of the specified user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     AllowTcpForwarding
             Specifies whether TCP forwarding is permitted.  The available options are “yes” or “all” to
             allow TCP forwarding, “no” to prevent all TCP forwarding, “local” to allow local (from the
             perspective of ssh(1)) forwarding only or “remote” to allow remote forwarding only.  The
             default is “yes”.  Note that disabling TCP forwarding does not improve security unless
             users are also denied shell access, as they can always install their own forwarders.

     AllowUsers
             This keyword can be followed by a list of user name patterns, separated by spaces.  If
             specified, login is allowed only for user names that match one of the patterns.  Only user
             names are valid; a numerical user ID is not recognized.  By default, login is allowed for
             all users.  If the pattern takes the form USER@HOST then USER and HOST are separately
             checked, restricting logins to particular users from particular hosts.  The allow/deny
             directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and
             finally AllowGroups.  All of the specified user and group tests must succeed, before user
             is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     AuthenticationMethods
             Specifies the authentication methods that must be successfully completed for a user to be
             granted access.  This option must be followed by one or more comma-separated lists of
             authentication method names.  Successful authentication requires completion of every method
             in at least one of these lists.

             For example, an argument of “publickey,password publickey,keyboard-interactive” would
             require the user to complete public key authentication, followed by either password or key‐
             board interactive authentication.  Only methods that are next in one or more lists are
             offered at each stage, so for this example, it would not be possible to attempt password or
             keyboard-interactive authentication before public key.

             For keyboard interactive authentication it is also possible to restrict authentication to a
             specific device by appending a colon followed by the device identifier “bsdauth”, “pam”, or
             “skey”, depending on the server configuration.  For example, “keyboard-interactive:bsdauth”
             would restrict keyboard interactive authentication to the “bsdauth” device.

             This option is only available for SSH protocol 2 and will yield a fatal error if enabled if
             protocol 1 is also enabled.  Note that each authentication method listed should also be
             explicitly enabled in the configuration.  The default is not to require multiple authenti‐
             cation; successful completion of a single authentication method is sufficient.

     AuthorizedKeysCommand
             Specifies a program to be used to look up the user's public keys.  The program must be
             owned by root and not writable by group or others.  It will be invoked with a single argu‐
             ment of the username being authenticated, and should produce on standard output zero or
             more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)).  If a key supplied
             by AuthorizedKeysCommand does not successfully authenticate and authorize the user then
             public key authentication continues using the usual AuthorizedKeysFile files.  By default,
             no AuthorizedKeysCommand is run.

     AuthorizedKeysCommandUser
             Specifies the user under whose account the AuthorizedKeysCommand is run.  It is recommended
             to use a dedicated user that has no other role on the host than running authorized keys
             commands.

     AuthorizedKeysFile
             Specifies the file that contains the public keys that can be used for user authentication.
             The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
             AuthorizedKeysFile may contain tokens of the form %T which are substituted during connec‐
             tion setup.  The following tokens are defined: %% is replaced by a literal '%', %h is
             replaced by the home directory of the user being authenticated, and %u is replaced by the
             username of that user.  After expansion, AuthorizedKeysFile is taken to be an absolute path
             or one relative to the user's home directory.  Multiple files may be listed, separated by
             whitespace.  The default is “.ssh/authorized_keys .ssh/authorized_keys2”.

     AuthorizedPrincipalsFile
             Specifies a file that lists principal names that are accepted for certificate authentica‐
             tion.  When using certificates signed by a key listed in TrustedUserCAKeys, this file lists
             names, one of which must appear in the certificate for it to be accepted for authentica‐
             tion.  Names are listed one per line preceded by key options (as described in AUTHO‐
             RIZED_KEYS FILE FORMAT in sshd(8)).  Empty lines and comments starting with ‘#’ are
             ignored.

             AuthorizedPrincipalsFile may contain tokens of the form %T which are substituted during
             connection setup.  The following tokens are defined: %% is replaced by a literal '%', %h is
             replaced by the home directory of the user being authenticated, and %u is replaced by the
             username of that user.  After expansion, AuthorizedPrincipalsFile is taken to be an abso‐
             lute path or one relative to the user's home directory.

             The default is “none”, i.e. not to use a principals file – in this case, the username of
             the user must appear in a certificate's principals list for it to be accepted.  Note that
             AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in
             TrustedUserCAKeys and is not consulted for certification authorities trusted via
             ~/.ssh/authorized_keys, though the principals= key option offers a similar facility (see
             sshd(8) for details).

     Banner  The contents of the specified file are sent to the remote user before authentication is
             allowed.  If the argument is “none” then no banner is displayed.  This option is only
             available for protocol version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed (e.g. via PAM or though
             authentication styles supported in login.conf(5)) The default is “yes”.

     ChrootDirectory
             Specifies the pathname of a directory to chroot(2) to after authentication.  All components
             of the pathname must be root-owned directories that are not writable by any other user or
             group.  After the chroot, sshd(8) changes the working directory to the user's home direc‐
             tory.

             The pathname may contain the following tokens that are expanded at runtime once the con‐
             necting user has been authenticated: %% is replaced by a literal '%', %h is replaced by the
             home directory of the user being authenticated, and %u is replaced by the username of that
             user.

             The ChrootDirectory must contain the necessary files and directories to support the user's
             session.  For an interactive session this requires at least a shell, typically sh(1), and
             basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and
             tty(4) devices.  For file transfer sessions using “sftp”, no additional configuration of
             the environment is necessary if the in-process sftp server is used, though sessions which
             use logging do require /dev/log inside the chroot directory (see sftp-server(8) for
             details).

             The default is not to chroot(2).

     Ciphers
             Specifies the ciphers allowed for protocol version 2.  Multiple ciphers must be comma-sepa‐
             rated.  The supported ciphers are:

             “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”,
             “aes256-ctr”, “aes128-gcm@openssh.com”, “aes256-gcm@openssh.com”, “arcfour128”,
             “arcfour256”, “arcfour”, “blowfish-cbc”, “cast128-cbc”, and
             “chacha20-poly1305@openssh.com”.

             The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-gcm@openssh.com,aes256-gcm@openssh.com,
                chacha20-poly1305@openssh.com,
                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                aes256-cbc,arcfour

             The list of available ciphers may also be obtained using the -Q option of ssh(1).

     ClientAliveCountMax
             Sets the number of client alive messages (see below) which may be sent without sshd(8)
             receiving any messages back from the client.  If this threshold is reached while client
             alive messages are being sent, sshd will disconnect the client, terminating the session.
             It is important to note that the use of client alive messages is very different from
             TCPKeepAlive (below).  The client alive messages are sent through the encrypted channel and
             therefore will not be spoofable.  The TCP keepalive option enabled by TCPKeepAlive is
             spoofable.  The client alive mechanism is valuable when the client or server depend on
             knowing when a connection has become inactive.

             The default value is 3.  If ClientAliveInterval (see below) is set to 15, and
             ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected
             after approximately 45 seconds.  This option applies to protocol version 2 only.

     ClientAliveInterval
             Sets a timeout interval in seconds after which if no data has been received from the
             client, sshd(8) will send a message through the encrypted channel to request a response
             from the client.  The default is 0, indicating that these messages will not be sent to the
             client.  This option applies to protocol version 2 only.

     Compression
             Specifies whether compression is allowed, or delayed until the user has authenticated suc‐
             cessfully.  The argument must be “yes”, “delayed”, or “no”.  The default is “delayed”.

     DenyGroups
             This keyword can be followed by a list of group name patterns, separated by spaces.  Login
             is disallowed for users whose primary group or supplementary group list matches one of the
             patterns.  Only group names are valid; a numerical group ID is not recognized.  By default,
             login is allowed for all groups.  The allow/deny directives are processed in the following
             order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All of the specified
             user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     DenyUsers
             This keyword can be followed by a list of user name patterns, separated by spaces.  Login
             is disallowed for user names that match one of the patterns.  Only user names are valid; a
             numerical user ID is not recognized.  By default, login is allowed for all users.  If the
             pattern takes the form USER@HOST then USER and HOST are separately checked, restricting
             logins to particular users from particular hosts.  The allow/deny directives are processed
             in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All of
             the specified user and group tests must succeed, before user is allowed to log in.

             See PATTERNS in ssh_config(5) for more information on patterns.

     ForceCommand
             Forces the execution of the command specified by ForceCommand, ignoring any command sup‐
             plied by the client and ~/.ssh/rc if present.  The command is invoked by using the user's
             login shell with the -c option.  This applies to shell, command, or subsystem execution.
             It is most useful inside a Match block.  The command originally supplied by the client is
             available in the SSH_ORIGINAL_COMMAND environment variable.  Specifying a command of
             “internal-sftp” will force the use of an in-process sftp server that requires no support
             files when used with ChrootDirectory.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports forwarded for the client.
             By default, sshd(8) binds remote port forwardings to the loopback address.  This prevents
             other remote hosts from connecting to forwarded ports.  GatewayPorts can be used to specify
             that sshd should allow remote port forwardings to bind to non-loopback addresses, thus
             allowing other hosts to connect.  The argument may be “no” to force remote port forwardings
             to be available to the local host only, “yes” to force remote port forwardings to bind to
             the wildcard address, or “clientspecified” to allow the client to select the address to
             which the forwarding is bound.  The default is “no”.

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.  The default is “no”.
             Note that this option applies to protocol version 2 only.

     GSSAPIKeyExchange
 Manual page sshd_config(5) line 201 (press h for help or q to quit)
             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely
             on ssh keys to verify host identity.  The default is “no”.  Note that this option applies
             to protocol version 2 only.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's credentials cache on logout.  The
             default is “yes”.  Note that this option applies to protocol version 2 only.

     GSSAPIEnablek5users
             Specifies whether to look at .k5users file for GSSAPI authentication access control. Fur‐
             ther details are described in ksu(1).  The default is “no”.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI acceptor a client authen‐
             ticates against. If “yes” then the client must authenticate against the host service on the
             current hostname. If “no” then the client may authenticate against any service key stored
             in the machine's default store. This facility is provided to assist with operation on multi
             homed machines.  The default is “yes”.  Note that this option applies only to protocol ver‐
             sion 2 GSSAPI connections, and setting it to “no” may only work with recent Kerberos GSSAPI
             libraries.

     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated following a successful
             connection rekeying. This option can be used to accepted renewed or updated credentials
             from a compatible client. The default is “no”.

     GSSAPIKexAlgorithms
             The list of key exchange algorithms that are accepted by GSSAPI key exchange. Possible val‐
             ues are

                gss-gex-sha1-,
                gss-group1-sha1-,
                gss-group14-sha1-

             The default is “gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-”.  This option only
             applies to protocol version 2 connections using GSSAPI.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public
             key client host authentication is allowed (host-based authentication).  This option is sim‐
             ilar to RhostsRSAAuthentication and applies to protocol version 2 only.  The default is
             “no”.

     HostbasedUsesNameFromPacketOnly
             Specifies whether or not the server will attempt to perform a reverse name lookup when
             matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during
             HostbasedAuthentication.  A setting of “yes” means that sshd(8) uses the name supplied by
             the client rather than attempting to resolve the name from the TCP connection itself.  The
             default is “no”.

     HostCertificate
             Specifies a file containing a public host certificate.  The certificate's public key must
             match a private host key already specified by HostKey.  The default behaviour of sshd(8) is
             not to load any certificates.

     HostKey
             Specifies a file containing a private host key used by SSH.  The default is
             /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_dsa_key,
             /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key
             for protocol version 2.  Note that sshd(8) will refuse to use a file if it is group/world-
             accessible.  It is possible to have multiple host key files.  “rsa1” keys are used for ver‐
             sion 1 and “dsa”, “ecdsa”, “ed25519” or “rsa” are used for version 2 of the SSH protocol.
             It is also possible to specify public host key files instead.  In this case operations on
             the private key will be delegated to an ssh-agent(1).

     HostKeyAgent
             Identifies the UNIX-domain socket used to communicate with an agent that has access to the
             private host keys.  If “SSH_AUTH_SOCK” is specified, the location of the socket will be
             read from the SSH_AUTH_SOCK environment variable.

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or
             HostbasedAuthentication.

             /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.  The default is “yes”.

     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during
             RhostsRSAAuthentication or HostbasedAuthentication.  The default is “no”.

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connection.  Accepted values are
             “af11”, “af12”, “af13”, “af21”, “af22”, “af23”, “af31”, “af32”, “af33”, “af41”, “af42”,
             “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”, “ef”, “lowdelay”,
             “throughput”, “reliability”, or a numeric value.  This option may take one or two argu‐
             ments, separated by whitespace.  If one argument is specified, it is used as the packet
             class unconditionally.  If two values are specified, the first is automatically selected
             for interactive sessions and the second for non-interactive sessions.  The default is
             “lowdelay” for interactive sessions and “throughput” for non-interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to allow keyboard-interactive authentication.  The argument to this key‐
             word must be “yes” or “no”.  The default is to use whatever value
             ChallengeResponseAuthentication is set to (by default “yes”).

     KerberosAuthentication
             Specifies whether the password provided by the user for PasswordAuthentication will be val‐
             idated through the Kerberos KDC.  To use this option, the server needs a Kerberos servtab
             which allows the verification of the KDC's identity.  The default is “no”.

     KerberosGetAFSToken
             If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before
             accessing the user's home directory.  The default is “no”.

     KerberosOrLocalPasswd
             If password authentication through Kerberos fails then the password will be validated via
             any additional local mechanism such as /etc/passwd.  The default is “yes”.

     KerberosTicketCleanup
             Specifies whether to automatically destroy the user's ticket cache file on logout.  The
             default is “yes”.

     KerberosUseKuserok
             Specifies whether to look at .k5login file for user's aliases.  The default is “yes”.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be comma-
             separated.  The default is

                   curve25519-sha256@libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group-exchange-sha1,
                   diffie-hellman-group14-sha1,
                   diffie-hellman-group1-sha1

     KeyRegenerationInterval
             In protocol version 1, the ephemeral server key is automatically regenerated after this
             many seconds (if it has been used).  The purpose of regeneration is to prevent decrypting
             captured sessions by later breaking into the machine and stealing the keys.  The key is
             never stored anywhere.  If the value is 0, the key is never regenerated.  The default is
             3600 (seconds).

     ListenAddress
             Specifies the local addresses sshd(8) should listen on.  The following forms may be used:

                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port

             If port is not specified, sshd will listen on the address and all prior Port options speci‐
             fied.  The default is to listen on all local addresses.  Multiple ListenAddress options are
             permitted.  Additionally, any Port options must precede this option for non-port qualified
             addresses.

     LoginGraceTime
             The server disconnects after this time if the user has not successfully logged in.  If the
             value is 0, there is no time limit.  The default is 120 seconds.

     LogLevel
             Gives the verbosity level that is used when logging messages from sshd(8).  The possible
             values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The
             default is INFO.  DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify higher
             levels of debugging output.  Logging with a DEBUG level violates the privacy of users and
             is not recommended.

     MACs    Specifies the available MAC (message authentication code) algorithms.  The MAC algorithm is
             used in protocol version 2 for data integrity protection.  Multiple algorithms must be
             comma-separated.  The algorithms that contain “-etm” calculate the MAC after encryption
             (encrypt-then-mac).  These are considered safer and their use recommended.  The default is:

                   hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
                   hmac-md5-96-etm@openssh.com,
                   hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
                   hmac-sha1-96,hmac-md5-96

     Match   Introduces a conditional block.  If all of the criteria on the Match line are satisfied,
             the keywords on the following lines override those set in the global section of the config
             file, until either another Match line or the end of the file.  If a keyword appears in mul‐
             tiple Match blocks that are satisified, only the first instance of the keyword is applied.

             The arguments to Match are one or more criteria-pattern pairs or the single token All which
             matches all criteria.  The available criteria are User, Group, Host, LocalAddress,
             LocalPort, and Address.  The match patterns may consist of single entries or comma-sepa‐
             rated lists and may use the wildcard and negation operators described in the PATTERNS sec‐
             tion of ssh_config(5).

             The patterns in an Address criteria may additionally contain addresses to match in CIDR
             address/masklen format, e.g. “192.0.2.0/24” or “3ffe:ffff::/32”.  Note that the mask length
             provided must be consistent with the address - it is an error to specify a mask length that
             is too long for the address or one with bits set in this host portion of the address.  For
             example, “192.0.2.0/33” and “192.0.2.0/8” respectively.

             Only a subset of keywords may be used on the lines following a Match keyword.  Available
             keywords are AcceptEnv, AllowAgentForwarding, AllowGroups, AllowTcpForwarding, AllowUsers,
             AuthenticationMethods, AuthorizedKeysCommand, AuthorizedKeysCommandUser,
             AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
             DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
             HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication, KerberosAuthentication,
             KerberosUseKuserok, MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
             PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication, RSAAuthentication,
             X11DisplayOffset, X11Forwarding and X11UseLocalHost.

     MaxAuthTries
             Specifies the maximum number of authentication attempts permitted per connection.  Once the
             number of failures reaches half this value, additional failures are logged.  The default is
             6.

     MaxSessions
             Specifies the maximum number of open sessions permitted per network connection.  The
             default is 10.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.
             Additional connections will be dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10:30:100.

             Alternatively, random early drop can be enabled by specifying the three colon separated
             values “start:rate:full” (e.g. "10:30:60").  sshd(8) will refuse connection attempts with a
             probability of “rate/100” (30%) if there are currently “start” (10) unauthenticated connec‐
             tions.  The probability increases linearly and all connection attempts are refused if the
             number of unauthenticated connections reaches “full” (60).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The default is “yes”.

     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the server allows login to
             accounts with empty password strings.  The default is “no”.

     PermitOpen
             Specifies the destinations to which TCP port forwarding is permitted.  The forwarding spec‐
             ification must be one of the following forms:

                   PermitOpen host:port
                   PermitOpen IPv4_addr:port
                   PermitOpen [IPv6_addr]:port

             Multiple forwards may be specified by separating them with whitespace.  An argument of
             “any” can be used to remove all restrictions and permit any forwarding requests.  An argu‐
             ment of “none” can be used to prohibit all forwarding requests.  By default all port for‐
             warding requests are permitted.

     PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument must be “yes”,
             “without-password”, “forced-commands-only”, or “no”.  The default is “yes”.

             If this option is set to “without-password”, password authentication is disabled for root.

             If this option is set to “forced-commands-only”, root login with public key authentication
             will be allowed, but only if the command option has been specified (which may be useful for
             taking remote backups even if root login is normally not allowed).  All other authentica‐
             tion methods are disabled for root.

             If this option is set to “no”, root is not allowed to log in.

     PermitTunnel
             Specifies whether tun(4) device forwarding is allowed.  The argument must be “yes”,
             “point-to-point” (layer 3), “ethernet” (layer 2), or “no”.  Specifying “yes” permits both
             “point-to-point” and “ethernet”.  The default is “no”.

     PermitTTY
             Specifies whether pty(4) allocation is permitted.  The default is “yes”.

     PermitUserEnvironment
             Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are
             processed by sshd(8).  The default is “no”.  Enabling environment processing may enable
             users to bypass access restrictions in some configurations using mechanisms such as
             LD_PRELOAD.

     PidFile
             Specifies the file that contains the process ID of the SSH daemon.  The default is
             /var/run/sshd.pid.

     Port    Specifies the port number that sshd(8) listens on.  The default is 22.  Multiple options of
             this type are permitted.  See also ListenAddress.

     PrintLastLog
             Specifies whether sshd(8) should print the date and time of the last user login when a user
             logs in interactively.  The default is “yes”.

     PrintMotd
             Specifies whether sshd(8) should print /etc/motd when a user logs in interactively.  (On
             some systems it is also printed by the shell, /etc/profile, or equivalent.)  The default is
             “yes”.

     Protocol
             Specifies the protocol versions sshd(8) supports.  The possible values are ‘1’ and ‘2’.
             Multiple versions must be comma-separated.  The default is ‘2’.  Note that the order of the
             protocol list does not indicate preference, because the client selects among multiple pro‐
             tocol versions offered by the server.  Specifying “2,1” is identical to “1,2”.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The default is “yes”.  Note that
             this option applies to protocol version 2 only.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted before the session key is
             renegotiated, optionally followed a maximum amount of time that may pass before the session
             key is renegotiated.  The first argument is specified in bytes and may have a suffix of
             ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively.  The default
             is between ‘1G’ and ‘4G’, depending on the cipher.  The optional second value is specified
             in seconds and may use any of the units documented in the TIME FORMATS section.  The
             default value for RekeyLimit is “default none”, which means that rekeying is performed
             after the cipher's default amount of data has been sent or received and no time based
             rekeying is done.  This option applies to protocol version 2 only.

     RevokedKeys
             Specifies revoked public keys.  Keys listed in this file will be refused for public key
             authentication.  Note that if this file is not readable, then public key authentication
             will be refused for all users.  Keys may be specified as a text file, listing one public
             key per line, or as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
             For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA
             host authentication is allowed.  The default is “no”.  This option applies to protocol ver‐
             sion 1 only.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The default is “yes”.  This option
             applies to protocol version 1 only.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1 server key.  The minimum
             value is 512, and the default is 1024.

     ShowPatchLevel
             Specifies whether sshd will display the patch level of the binary in the identification
             string.  The patch level is set at compile-time.  The default is “no”.  This option applies
             to protocol version 1 only.

     StrictModes
             Specifies whether sshd(8) should check file modes and ownership of the user's files and
             home directory before accepting login.  This is normally desirable because novices some‐
             times accidentally leave their directory or files world-writable.  The default is “yes”.
             Note that this does not apply to ChrootDirectory, whose permissions and ownership are
             checked unconditionally.

     Subsystem
             Configures an external subsystem (e.g. file transfer daemon).  Arguments should be a sub‐
             system name and a command (with optional arguments) to execute upon subsystem request.

             The command sftp-server(8) implements the “sftp” file transfer subsystem.

             Alternately the name “internal-sftp” implements an in-process “sftp” server.  This may sim‐
             plify configurations using ChrootDirectory to force a different filesystem root on clients.

             By default no subsystems are defined.  Note that this option applies to protocol version 2
             only.

     SyslogFacility
             Gives the facility code that is used when logging messages from sshd(8).  The possible val‐
             ues are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,
             LOCAL6, LOCAL7.  The default is AUTH.

     TCPKeepAlive
             Specifies whether the system should send TCP keepalive messages to the other side.  If they
             are sent, death of the connection or crash of one of the machines will be properly noticed.
             However, this means that connections will die if the route is down temporarily, and some
             people find it annoying.  On the other hand, if TCP keepalives are not sent, sessions may
             hang indefinitely on the server, leaving “ghost” users and consuming server resources.

             The default is “yes” (to send TCP keepalive messages), and the server will notice if the
             network goes down or the client host crashes.  This avoids infinitely hanging sessions.

             To disable TCP keepalive messages, the value should be set to “no”.

     TrustedUserCAKeys
             Specifies a file containing public keys of certificate authorities that are trusted to sign
             user certificates for authentication.  Keys are listed one per line; empty lines and com‐
             ments starting with ‘#’ are allowed.  If a certificate is presented for authentication and
             has its signing CA key listed in this file, then it may be used for authentication for any
             user listed in the certificate's principals list.  Note that certificates that lack a list
             of principals will not be permitted for authentication using TrustedUserCAKeys.  For more
             details on certificates, see the CERTIFICATES section in ssh-keygen(1).

     UseDNS  Specifies whether sshd(8) should look up the remote host name and check that the resolved
             host name for the remote IP address maps back to the very same IP address.  The default is
             “yes”.

     UseLogin
             Specifies whether login(1) is used for interactive login sessions.  The default is “no”.
             Note that login(1) is never used for remote command execution.  Note also, that if this is
             enabled, X11Forwarding will be disabled because login(1) does not know how to handle
             xauth(1) cookies.  If UsePrivilegeSeparation is specified, it will be disabled after
             authentication.

     UsePAM  Enables the Pluggable Authentication Module interface.  If set to “yes” this will enable
             PAM authentication using ChallengeResponseAuthentication and PasswordAuthentication in
             addition to PAM account and session module processing for all authentication types.

             Because PAM challenge-response authentication usually serves an equivalent role to password
             authentication, you should disable either PasswordAuthentication or
             ChallengeResponseAuthentication.

             If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user.  The default
             is “no”.

     UsePrivilegeSeparation
             Specifies whether sshd(8) separates privileges by creating an unprivileged child process to
             deal with incoming network traffic.  After successful authentication, another process will
             be created that has the privilege of the authenticated user.  The goal of privilege separa‐
             tion is to prevent privilege escalation by containing any corruption within the unprivi‐
             leged processes.  The default is “yes”.  If UsePrivilegeSeparation is set to “sandbox” then
             the pre-authentication unprivileged process is subject to additional restrictions.

     VersionAddendum
             Optionally specifies additional text to append to the SSH protocol banner sent by the
             server upon connection.  The default is “none”.

     X11DisplayOffset
             Specifies the first display number available for sshd(8)'s X11 forwarding.  This prevents
             sshd from interfering with real X11 servers.  The default is 10.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The argument must be “yes” or “no”.  The
             default is “no”.

             When X11 forwarding is enabled, there may be additional exposure to the server and to
             client displays if the sshd(8) proxy display is configured to listen on the wildcard
             address (see X11UseLocalhost below), though this is not the default.  Additionally, the
             authentication spoofing and authentication data verification and substitution occur on the
             client side.  The security risk of using X11 forwarding is that the client's X11 display
             server may be exposed to attack when the SSH client requests forwarding (see the warnings
             for ForwardX11 in ssh_config(5)).  A system administrator may have a stance in which they
             want to protect clients that may expose themselves to attack by unwittingly requesting X11
             forwarding, which can warrant a “no” setting.

             Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as
             users can always install their own forwarders.  X11 forwarding is automatically disabled if
             UseLogin is enabled.

     X11UseLocalhost
             Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or
             to the wildcard address.  By default, sshd binds the forwarding server to the loopback
             address and sets the hostname part of the DISPLAY environment variable to “localhost”.
             This prevents remote hosts from connecting to the proxy display.  However, some older X11
             clients may not function with this configuration.  X11UseLocalhost may be set to “no” to
             specify that the forwarding server should be bound to the wildcard address.  The argument
             must be “yes” or “no”.  The default is “yes”.

     XAuthLocation
             Specifies the full pathname of the xauth(1) program.  The default is /usr/bin/xauth.

TIME FORMATS
     sshd(8) command-line arguments and configuration file options that specify time may be expressed
     using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier
     is one of the following:

           ⟨none⟩  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

     Each member of the sequence is added together to calculate the total time value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd(8).  This file should be writable by root only, but it
             is recommended (though not necessary) that it be world-readable.

SEE ALSO
     sshd(8)

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Camp‐
     bell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added
     newer features and created OpenSSH.  Markus Friedl contributed the support for SSH protocol ver‐
     sions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support for privilege separation.

BSD                                         November 13, 2016                                        BSD

ssh-keygen

 # man ssh-keygen
SSH-KEYGEN(1)                          BSD General Commands Manual                         SSH-KEYGEN(1)

NAME
     ssh-keygen — authentication key generation, management and conversion

SYNOPSIS
     ssh-keygen [-q] [-b bits] [-t type] [-N new_passphrase] [-C comment] [-f output_keyfile]
     ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
     ssh-keygen -i [-m key_format] [-f input_keyfile]
     ssh-keygen -e [-m key_format] [-f input_keyfile]
     ssh-keygen -y [-f input_keyfile]
     ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
     ssh-keygen -l [-f input_keyfile]
     ssh-keygen -B [-f input_keyfile]
     ssh-keygen -D pkcs11
     ssh-keygen -F hostname [-f known_hosts_file] [-l]
     ssh-keygen -H [-f known_hosts_file]
     ssh-keygen -R hostname [-f known_hosts_file]
     ssh-keygen -r hostname [-f input_keyfile] [-g]
     ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
     ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines] [-j start_line]
                [-K checkpt] [-W generator]
     ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals] [-O option]
                [-V validity_interval] [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]
     ssh-keygen -A
     ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file ...
     ssh-keygen -Q -f krl_file file ...

DESCRIPTION
     ssh-keygen generates, manages and converts authentication keys for ssh(1).  ssh-keygen can create
     RSA keys for use by SSH protocol version 1 and DSA, ECDSA, ED25519 or RSA keys for use by SSH pro‐
     tocol version 2.  The type of key to be generated is specified with the -t option.  If invoked
     without any arguments, ssh-keygen will generate an RSA key for use in SSH protocol 2 connections.

     ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH-GEX).  See
     the MODULI GENERATION section for details.

     Finally, ssh-keygen can be used to generate and update Key Revocation Lists, and to test whether
     given keys have been revoked by one.  See the KEY REVOCATION LISTS section for details.

     Normally each user wishing to use SSH with public key authentication runs this once to create the
     authentication key in ~/.ssh/identity, ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 or
     ~/.ssh/id_rsa.  Additionally, the system administrator may use this to generate host keys, as seen
     in /etc/rc.

     Normally this program generates the key and asks for a file in which to store the private key.  The
     public key is stored in a file with the same name but “.pub” appended.  The program also asks for a
     passphrase.  The passphrase may be empty to indicate no passphrase (host keys must have an empty
     passphrase), or it may be a string of arbitrary length.  A passphrase is similar to a password,
     except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string
     of characters you want.  Good passphrases are 10-30 characters long, are not simple sentences or
     otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides
     very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanu‐
     meric characters.  The passphrase can be changed later by using the -p option.

     There is no way to recover a lost passphrase.  If the passphrase is lost or forgotten, a new key
     must be generated and the corresponding public key copied to other machines.

     For RSA1 keys, there is also a comment field in the key file that is only for convenience to the
     user to help identify the key.  The comment can tell what the key is for, or whatever is useful.
     The comment is initialized to “user@host” when the key is created, but can be changed using the -c
     option.

     After a key is generated, instructions below detail where the keys should be placed to be acti‐
     vated.

     The options are as follows:

     -A      For each of the key types (rsa1, rsa, dsa, ecdsa and ed25519) for which host keys do not
             exist, generate the host keys with the default key file path, an empty passphrase, default
             bits for the key type, and default comment.  This is used by /etc/rc to generate new host
             keys.

     -a rounds
             When saving a new-format private key (i.e. an ed25519 key or any SSH protocol 2 key when
             the -o flag is set), this option specifies the number of KDF (key derivation function)
             rounds used.  Higher numbers result in slower passphrase verification and increased resis‐
             tance to brute-force password cracking (should the keys be stolen).

             When screening DH-GEX candidates ( using the -T command).  This option specifies the number
             of primality tests to perform.

     -B      Show the bubblebabble digest of specified private or public key file.

     -b bits
             Specifies the number of bits in the key to create.  For RSA keys, the minimum size is 768
             bits and the default is 2048 bits.  Generally, 2048 bits is considered sufficient.  DSA
             keys must be exactly 1024 bits as specified by FIPS 186-2.  For ECDSA keys, the -b flag
             determines the key length by selecting from one of three elliptic curve sizes: 256, 384 or
             521 bits.  Attempting to use bit lengths other than these three values for ECDSA keys will
             fail.  ED25519 keys have a fixed length and the -b flag will be ignored.

     -C comment
             Provides a new comment.

     -c      Requests changing the comment in the private and public key files.  This operation is only
             supported for RSA1 keys.  The program will prompt for the file containing the private keys,
             for the passphrase if the key has one, and for the new comment.

     -D pkcs11
             Download the RSA public keys provided by the PKCS#11 shared library pkcs11.  When used in
             combination with -s, this option indicates that a CA key resides in a PKCS#11 token (see
             the CERTIFICATES section for details).

     -e      This option will read a private or public OpenSSH key file and print to stdout the key in
             one of the formats specified by the -m option.  The default export format is “RFC4716”.
             This option allows exporting OpenSSH keys for use by other programs, including several com‐
             mercial SSH implementations.

     -F hostname
             Search for the specified hostname in a known_hosts file, listing any occurrences found.
             This option is useful to find hashed host names or addresses and may also be used in con‐
             junction with the -H option to print found keys in a hashed format.

     -f filename
             Specifies the filename of the key file.

     -G output_file
             Generate candidate primes for DH-GEX.  These primes must be screened for safety (using the
             -T option) before use.

     -g      Use generic DNS format when printing fingerprint resource records using the -r command.

     -H      Hash a known_hosts file.  This replaces all hostnames and addresses with hashed representa‐
             tions within the specified file; the original content is moved to a file with a .old suf‐
             fix.  These hashes may be used normally by ssh and sshd, but they do not reveal identifying
             information should the file's contents be disclosed.  This option will not modify existing
             hashed hostnames and is therefore safe to use on files that mix hashed and non-hashed
             names.

     -h      When signing a key, create a host certificate instead of a user certificate.  Please see
             the CERTIFICATES section for details.

     -I certificate_identity
             Specify the key identity when signing a public key.  Please see the CERTIFICATES section
             for details.

     -i      This option will read an unencrypted private (or public) key file in the format specified
             by the -m option and print an OpenSSH compatible private (or public) key to stdout.

     -J num_lines
             Exit after screening the specified number of lines while performing DH candidate screening
             using the -T option.

     -j start_line
             Start screening at the specified line number while performing DH candidate screening using
             the -T option.

     -K checkpt
             Write the last line processed to the file checkpt while performing DH candidate screening
             using the -T option.  This will be used to skip lines in the input file that have already
             been processed if the job is restarted.  This option allows importing keys from other soft‐
             ware, including several commercial SSH implementations.  The default import format is
             “RFC4716”.

     -k      Generate a KRL file.  In this mode, ssh-keygen will generate a KRL file at the location
             specified via the -f flag that revokes every key or certificate presented on the command
             line.  Keys/certificates to be revoked may be specified by public key file or using the
             format described in the KEY REVOCATION LISTS section.

     -L      Prints the contents of a certificate.

     -l      Show fingerprint of specified public key file.  Private RSA1 keys are also supported.  For
             RSA and DSA keys ssh-keygen tries to find the matching public key file and prints its fin‐
             gerprint.  If combined with -v, an ASCII art representation of the key is supplied with the
             fingerprint.

     -M memory
             Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-
             GEX.

     -m key_format
             Specify a key format for the -i (import) or -e (export) conversion options.  The supported
             key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8” (PEM PKCS8 public
             key) or “PEM” (PEM public key).  The default conversion format is “RFC4716”.

     -N new_passphrase
             Provides the new passphrase.

     -n principals
             Specify one or more principals (user or host names) to be included in a certificate when
             signing a key.  Multiple principals may be specified, separated by commas.  Please see the
             CERTIFICATES section for details.

     -O option
             Specify a certificate option when signing a key.  This option may be specified multiple
             times.  Please see the CERTIFICATES section for details.  The options that are valid for
             user certificates are:

             clear   Clear all enabled permissions.  This is useful for clearing the default set of per‐
                     missions so permissions may be added individually.

             force-command=command
                     Forces the execution of command instead of any shell or command specified by the
                     user when the certificate is used for authentication.

             no-agent-forwarding
                     Disable ssh-agent(1) forwarding (permitted by default).

             no-port-forwarding
                     Disable port forwarding (permitted by default).

             no-pty  Disable PTY allocation (permitted by default).

             no-user-rc
                     Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

             no-x11-forwarding
                     Disable X11 forwarding (permitted by default).

             permit-agent-forwarding
                     Allows ssh-agent(1) forwarding.

             permit-port-forwarding
                     Allows port forwarding.

             permit-pty
                     Allows PTY allocation.

             permit-user-rc
                     Allows execution of ~/.ssh/rc by sshd(8).

             permit-x11-forwarding
                     Allows X11 forwarding.

             source-address=address_list
                     Restrict the source addresses from which the certificate is considered valid.  The
                     address_list is a comma-separated list of one or more address/netmask pairs in CIDR
                     format.

             At present, no options are valid for host keys.

     -o      Causes ssh-keygen to save SSH protocol 2 private keys using the new OpenSSH format rather
             than the more compatible PEM format.  The new format has increased resistance to brute-
             force password cracking but is not supported by versions of OpenSSH prior to 6.5.  Ed25519
             keys always use the new private key format.

     -P passphrase
             Provides the (old) passphrase.

     -p      Requests changing the passphrase of a private key file instead of creating a new private
             key.  The program will prompt for the file containing the private key, for the old
             passphrase, and twice for the new passphrase.

     -Q      Test whether keys have been revoked in a KRL.

     -q      Silence ssh-keygen.

     -R hostname
             Removes all keys belonging to hostname from a known_hosts file.  This option is useful to
             delete hashed hosts (see the -H option above).

     -r hostname
             Print the SSHFP fingerprint resource record named hostname for the specified public key
             file.

     -S start
             Specify start point (in hex) when generating candidate moduli for DH-GEX.

     -s ca_key
             Certify (sign) a public key using the specified CA key.  Please see the CERTIFICATES sec‐
             tion for details.

             When generating a KRL, -s specifies a path to a CA public key file used to revoke certifi‐
             cates directly by key ID or serial number.  See the KEY REVOCATION LISTS section for
             details.

     -T output_file
             Test DH group exchange candidate primes (generated using the -G option) for safety.

     -t type
             Specifies the type of key to create.  The possible values are “rsa1” for protocol version 1
             and “dsa”, “ecdsa”, “ed25519”, or “rsa” for protocol version 2.

     -u      Update a KRL.  When specified with -k, keys listed via the command line are added to the
             existing KRL rather than a new KRL being created.

     -V validity_interval
             Specify a validity interval when signing a certificate.  A validity interval may consist of
             a single time, indicating that the certificate is valid beginning now and expiring at that
             time, or may consist of two times separated by a colon to indicate an explicit time inter‐
             val.  The start time may be specified as a date in YYYYMMDD format, a time in YYYYMMDDHH‐
             MMSS format or a relative time (to the current time) consisting of a minus sign followed by
             a relative time in the format described in the TIME FORMATS section of sshd_config(5).  The
             end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
             starting with a plus character.

             For example: “+52w1d” (valid from now to 52 weeks and one day from now), “-4w:+4w” (valid
             from four weeks ago to four weeks from now), “20100101123000:20110101123000” (valid from
             12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011), “-1d:20110101” (valid from
             yesterday to midnight, January 1st, 2011).

     -v      Verbose mode.  Causes ssh-keygen to print debugging messages about its progress.  This is
             helpful for debugging moduli generation.  Multiple -v options increase the verbosity.  The
             maximum is 3.

     -W generator
             Specify desired generator when testing candidate moduli for DH-GEX.

     -y      This option will read a private OpenSSH format file and print an OpenSSH public key to std‐
             out.

     -z serial_number
             Specifies a serial number to be embedded in the certificate to distinguish this certificate
             from others from the same CA.  The default serial number is zero.

             When generating a KRL, the -z flag is used to specify a KRL version number.

MODULI GENERATION
     ssh-keygen may be used to generate groups for the Diffie-Hellman Group Exchange (DH-GEX) protocol.
     Generating these groups is a two-step process: first, candidate primes are generated using a fast,
     but memory intensive process.  These candidate primes are then tested for suitability (a CPU-inten‐
     sive process).

     Generation of primes is performed using the -G option.  The desired length of the primes may be
     specified by the -b option.  For example:

           # ssh-keygen -G moduli-2048.candidates -b 2048

     By default, the search for primes begins at a random point in the desired length range.  This may
     be overridden using the -S option, which specifies a different start point (in hex).

     Once a set of candidates have been generated, they must be screened for suitability.  This may be
     performed using the -T option.  In this mode ssh-keygen will read candidates from standard input
     (or a file specified using the -f option).  For example:

           # ssh-keygen -T moduli-2048 -f moduli-2048.candidates

     By default, each candidate will be subjected to 100 primality tests.  This may be overridden using
     the -a option.  The DH generator value will be chosen automatically for the prime under considera‐
     tion.  If a specific generator is desired, it may be requested using the -W option.  Valid genera‐
     tor values are 2, 3, and 5.

     Screened DH groups may be installed in /etc/ssh/moduli.  It is important that this file contains
     moduli of a range of bit lengths and that both ends of a connection share common moduli.

CERTIFICATES
     ssh-keygen supports signing of keys to produce certificates that may be used for user or host
     authentication.  Certificates consist of a public key, some identity information, zero or more
     principal (user or host) names and a set of options that are signed by a Certification Authority
     (CA) key.  Clients or servers may then trust only the CA key and verify its signature on a certifi‐
     cate rather than trusting many user/host keys.  Note that OpenSSH certificates are a different, and
     much simpler, format to the X.509 certificates used in ssl(8).

     ssh-keygen supports two types of certificates: user and host.  User certificates authenticate users
     to servers, whereas host certificates authenticate server hosts to users.  To generate a user cer‐
     tificate:

           $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub

     The resultant certificate will be placed in /path/to/user_key-cert.pub.  A host certificate
     requires the -h option:

           $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub

     The host certificate will be output to /path/to/host_key-cert.pub.

     It is possible to sign using a CA key stored in a PKCS#11 token by providing the token library
     using -D and identifying the CA key by providing its public half as an argument to -s:

           $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id host_key.pub

     In all cases, key_id is a "key identifier" that is logged by the server when the certificate is
     used for authentication.

     Certificates may be limited to be valid for a set of principal (user/host) names.  By default, gen‐
     erated certificates are valid for all users or hosts.  To generate a certificate for a specified
     set of principals:

           $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
           $ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub

     Additional limitations on the validity and use of user certificates may be specified through cer‐
     tificate options.  A certificate option may disable features of the SSH session, may be valid only
     when presented from particular source addresses or may force the use of a specific command.  For a
     list of valid certificate options, see the documentation for the -O option above.

     Finally, certificates may be defined with a validity lifetime.  The -V option allows specification
     of certificate start and end times.  A certificate that is presented at a time outside this range
     will not be considered valid.  By default, certificates are valid from UNIX Epoch to the distant
     future.

     For certificates to be used for user or host authentication, the CA public key must be trusted by
     sshd(8) or ssh(1).  Please refer to those manual pages for details.

KEY REVOCATION LISTS
     ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).  These binary files spec‐
     ify keys or certificates to be revoked using a compact format, taking as little as one bit per cer‐
     tificate if they are being revoked by serial number.

     KRLs may be generated using the -k flag.  This option reads one or more files from the command line
     and generates a new KRL.  The files may either contain a KRL specification (see below) or public
     keys, listed one per line.  Plain public keys are revoked by listing their hash or contents in the
     KRL and certificates revoked by serial number or key ID (if the serial is zero or not available).

     Revoking keys using a KRL specification offers explicit control over the types of record used to
     revoke keys and may be used to directly revoke certificates by serial number or key ID without hav‐
     ing the complete original certificate on hand.  A KRL specification consists of lines containing
     one of the following directives followed by a colon and some directive-specific information.

     serial: serial_number[-serial_number]
             Revokes a certificate with the specified serial number.  Serial numbers are 64-bit values,
             not including zero and may be expressed in decimal, hex or octal.  If two serial numbers
             are specified separated by a hyphen, then the range of serial numbers including and between
             each is revoked.  The CA key must have been specified on the ssh-keygen command line using
             the -s option.

     id: key_id
             Revokes a certificate with the specified key ID string.  The CA key must have been speci‐
             fied on the ssh-keygen command line using the -s option.

     key: public_key
             Revokes the specified key.  If a certificate is listed, then it is revoked as a plain pub‐
             lic key.

     sha1: public_key
             Revokes the specified key by its SHA1 hash.

     KRLs may be updated using the -u flag in addition to -k.  When this option is specified, keys
     listed via the command line are merged into the KRL, adding to those already there.

     It is also possible, given a KRL, to test whether it revokes a particular key (or keys).  The -Q
     flag will query an existing KRL, testing each key specified on the commandline.  If any key listed
     on the command line has been revoked (or an error encountered) then ssh-keygen will exit with a
     non-zero exit status.  A zero exit status will only be returned if no key was revoked.

FILES
     ~/.ssh/identity
             Contains the protocol version 1 RSA authentication identity of the user.  This file should
             not be readable by anyone but the user.  It is possible to specify a passphrase when gener‐
             ating the key; that passphrase will be used to encrypt the private part of this file using
             3DES.  This file is not automatically accessed by ssh-keygen but it is offered as the
             default file for the private key.  ssh(1) will read this file when a login attempt is made.

     ~/.ssh/identity.pub
             Contains the protocol version 1 RSA public key for authentication.  The contents of this
             file should be added to ~/.ssh/authorized_keys on all machines where the user wishes to log
             in using RSA authentication.  There is no need to keep the contents of this file secret.

     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.ssh/id_rsa
             Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA authentication identity of the
             user.  This file should not be readable by anyone but the user.  It is possible to specify
             a passphrase when generating the key; that passphrase will be used to encrypt the private
             part of this file using 128-bit AES.  This file is not automatically accessed by ssh-keygen
             but it is offered as the default file for the private key.  ssh(1) will read this file when
             a login attempt is made.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_rsa.pub
             Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA public key for authentication.
             The contents of this file should be added to ~/.ssh/authorized_keys on all machines where
             the user wishes to log in using public key authentication.  There is no need to keep the
             contents of this file secret.
             default file for the private key.  ssh(1) will read this file when a login attempt is made.

     ~/.ssh/identity.pub
             Contains the protocol version 1 RSA public key for authentication.  The contents of this
             file should be added to ~/.ssh/authorized_keys on all machines where the user wishes to log
             in using RSA authentication.  There is no need to keep the contents of this file secret.

     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.ssh/id_rsa
             Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA authentication identity of the
             user.  This file should not be readable by anyone but the user.  It is possible to specify
             a passphrase when generating the key; that passphrase will be used to encrypt the private
             part of this file using 128-bit AES.  This file is not automatically accessed by ssh-keygen
             but it is offered as the default file for the private key.  ssh(1) will read this file when
             a login attempt is made.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_rsa.pub
             Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA public key for authentication.
             The contents of this file should be added to ~/.ssh/authorized_keys on all machines where
             the user wishes to log in using public key authentication.  There is no need to keep the
             contents of this file secret.

     /etc/ssh/moduli
             Contains Diffie-Hellman groups used for DH-GEX.  The file format is described in moduli(5).

ENVIRONMENT
     SSH_USE_STRONG_RNG
             The reseeding of the OpenSSL random generator is usually done from /dev/urandom.  If the
             SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random
             generator is reseeded from /dev/random.  The number of bytes read is defined by the
             SSH_USE_STRONG_RNG value.  Minimum is 14 bytes.  This setting is not recommended on the
             computers without the hardware random generator because insufficient entropy causes the
             connection to be blocked until enough entropy is available.

SEE ALSO
     ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

     The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Camp‐
     bell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added
     newer features and created OpenSSH.  Markus Friedl contributed the support for SSH protocol ver‐
     sions 1.5 and 2.0.

BSD                                         November 13, 2016                                        BSD

ssh in der Praxis (Teil 1)

Auch wenn Passworte bei ssh verschlüsselt übertragen werden, wollen wir zwei wesentliche Aspekte bei Verwendung der ssh berücksichtigen:

  1. Der Benutzer root soll sich bei unseren Systemen nicht mehr remote anmelden dürfen. Lediglich ein oder die berechtigten Nutzern erhalten die Erlaubnis, von entfernter Stelle sich anzumelden. Via su - kann dann der berechtigte Administrator, root-Rechte erhalten!
  2. Wir werden Key-basierte Anmeldungen verwenden und keine Anmeldungen mit Passwort zulassen. Somit laufen wir nicht in Gefahr, Zugänge durch Trivialpassworte angreifbar zu machen. Stattdessen werden wir uns für unsere Administratoren und berechtigten Nutzern, ein Schlüsselpaar bestehend aus privaten und öffentlichen Schlüssel erzeugen. Bei der Erzeugung dieses Schlüsselpaares werden wir eine Schlüsselpasswort (passphrase) angeben, welches Zur Nutzung des Schlüssel abgefragt wird.

Zum Erstellen eines Schlüsselpaares nutzen wir das Programm ssh-keygen. Einen Überberlick über die möglichen Optionen erhalten wir beim Abruf der zugehörigen manpage.

Bevor wir uns die Entscheidung treffen können, welchen Schlüssel-Typ wir erzeugen wollen, müssen wir überlegen, von welchem System wir aus auf unseren Linux/CentOS-Host zugreifen wollen. Sind wir in der misslichen Lage und müssen von einem Windows-Rechner aus auf unseren Linux-Host zugreifen, müssen wir sicherheitstechnische Abstrichen machen, da putty z.B. nicht alle aktuiellen Cipher, MAC und Schlüsselaustauschmechanismen zu Verfügung stellt, die uns z.B der CentOS 7 Zielserver ggf. anbietet. Ähnliches gilt, wenn wir z.B. von einem CentOS 6 System auf eine aktuelles CentOS 7 System via ssh zugreifen wollen.

Hier empfiehlt es sich auf den beteiligten System zu überprüfen, welche Cipher, MACs, Schlüssel Typen und Key Exchange Algorithmen unterstützt werden. Zum Abfragen können wir den Befehl ssh mit der Option -Q verwenden.

Liste der unterstützten Cipher

 # ssh -Q cipher
3des-cbc
blowfish-cbc
cast128-cbc
arcfour
arcfour128
arcfour256
aes128-cbc
aes192-cbc
aes256-cbc
rijndael-cbc@lysator.liu.se
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com

Liste der unterstützten MACs

 # ssh -Q mac
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-ripemd160@openssh.com
umac-64@openssh.com
umac-128@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com

Liste der unterstützten Schlüssel Typen

 # ssh -Q key
ssh-rsa
ssh-dss
ssh-ed25519
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
ssh-rsa-cert-v01@openssh.com
ssh-dss-cert-v01@openssh.com
ecdsa-sha2-nistp256-cert-v01@openssh.com
ecdsa-sha2-nistp384-cert-v01@openssh.com
ecdsa-sha2-nistp521-cert-v01@openssh.com
ssh-rsa-cert-v00@openssh.com
ssh-dss-cert-v00@openssh.com
ssh-ed25519-cert-v01@openssh.com
null

Liste alller unterstützten Key Exchange Algorithmen

 # ssh -Q kex
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
diffie-hellman-group1-sha1
curve25519-sha256@libssh.org
gss-gex-sha1-
gss-group1-sha1-
gss-group14-sha1-

Erzeugung eines SSH-Schlüssel(paars)

RSA Key

Im ersten Beispiel erzeugen wir uns einen 4096er RSA-Schlüssel für die Authentifizierung:

  $ ssh-keygen -b 4096 -t rsa -C django@nausch.org -f ~/.ssh/id_rsa4096_dmz
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/django/.ssh/id_rsa4096_dmz.
Your public key has been saved in /home/django/.ssh/id_rsa4096_dmz.pub.
The key fingerprint is:
44:8b:1a:4b:87:95:3a:23:af:65:b7:e6:1a:bf:98:3d django@nausch.org
The key's randomart image is:
+--[ RSA 4096]----+
|      ...        |
|     o.o .       |
|    +.o o        |
|  ..+= .         |
|   ooo  S        |
|    + .          |
|   +.. .         |
|  .  *E          |
|    ++=o         |
+-----------------+

Die passphrase die man hier angibt, wird später beim Anmelden auf dem entfernten Rechner abgefragt, oder vom ssh-agent bei der Anmeldung mitübergeben.

Nun liegen in dem Verzeichnis /home/django/.ssh zwei weitere Dateien:

# ll ~/.ssh/id_rsa*
-rw-------. 1 django django 3326 13. Nov 15:27 /home/django/.ssh/id_rsa4096_dmz
-rw-r--r--. 1 django django  743 13. Nov 15:27 /home/django/.ssh/id_rsa4096_dmz.pub

id_rsa4096_dmz enthält den privaten Schlüssel und sollte auf keinen Fall weitergegeben werden und darf auch nur für den Nutzer selbst lesbar sein! id_rsa4096_dmz.pub, der öffentliche Schlüssel, dagegen muss auf den Zielrechner kopiert werden.

ED25519 Key

Ob man in Zeiten von Überwachungsphantasten in Unternehmen und vor allem auch bei einer NSA oder BND, noch solchen RSA-Schlüssel einsetzen kann und mag, muss natürlich jeder Admin für sich selbst entscheiden.

Der Sicherheitsguru Bruce Schneier hat in seinem Blog hierzu eine eindeutige Aussage getätigt:

„On the crypto bits in your guardian piece, I found especially interesting that you suggest classic discrete log crypto over ecc. I want to ask if you could elaborate more on that.“ I no longer trust the constants. I believe the NSA has manipulated them through their relationships with industry.

Auf diese Schlüssel muss man aber nicht mehr zwingend zurückgreifen, stehen doch aktuellere und zeitgemäßere Cipher, MACs, Schlüssel Typen und Key Exchange Algorithmen zur Verfügung. Als Alternative zu einem RSA-Keys wollen wir nun nun einen ed25519 Schlüssel erzeugen. Ed25519 ist ein Elliptic Curve Signature Schema, welches beste Sicherheit bei vertretbaren Aufwand verspricht, als ECDSA oder DSA dies der Fall ist. Zur Auswahl sicherer kryptografischer Kurven bei der Elliptic-Curve Cryptography findet man auf der Seite hier hilfreiche Erklärungen und eine Gegenüberstellung der möglichen verschiedenen Alternativen.

 $ ssh-keygen -t ed25519 -o -a 100 -C django@nausch.org -f ~/.ssh/id_ed25519_dmz
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/django/.ssh/id_ed25519_dmz.
Your public key has been saved in /home/django/.ssh/id_ed25519_dmz.pub.
The key fingerprint is:
a3:03:59:5c:1b:d3:60:2a:93:77:2a:9f:9d:fc:e8:68 django@nausch.org
The key's randomart image is:
+--[ED25519  256--+
|        *o       |
|     o + +.      |
|    + = o        |
|     * o         |
|    + . S        |
|     + = o       |
|      = +        |
|      Eo o       |
|     ...o .      |
+-----------------+

Die passphrase die man hier angibt, wird später beim Anmelden auf dem entfernten Rechner abgefragt, oder vom ssh-agent bei der Anmeldung mitübergeben.

Nun liegen in dem Verzeichnis /home/django/.ssh zwei weitere Dateien:

# ll ~/.ssh/*ed25519*
-rw-------. 1 django django 464  2. Nov 21:43 /home/django/.ssh/id_ed25519_dmz
-rw-r--r--. 1 django django  99  2. Nov 21:43 /home/django/.ssh/id_ed25519_dmz.pub

id_ed25519_dmz enthält den privaten Schlüssel und sollte auf keinen Fall weitergegeben werden und darf auch nur für den Nutzer selbst lesbar sein! id_ed25519_dmz.pub, der öffentliche Schlüssel, dagegen muss auf den Zielrechner kopiert werden.

Passphrase eines SSH-Keys ändern

Will man die passphrase eines bestehenden SSH-Keys ändern benutzt man wie schon bei der Erstellung des Schlüssel(paares) den Befehl ssh-keygen.

 $ ssh-keygen -p -f ~/.ssh/id_ed25519_dmz
 Enter old passphrase: 
 Enter new passphrase (empty for no passphrase): 
 Enter same passphrase again: 
Your identification has been saved with the new passphrase.</code>

Zielverzeichnis anlegen und öffentlichen Schlüssel kopieren

Auf dem Zielrechner legen wir nun das Verzeichnis .ssh an und schützen es entsprechend.

[django@zielhost django]$ (umask 077 ; mkdir -p $HOME/.ssh)

Den öffentlichen Schlüssel kopieren wir dann wie folgt auf das Zielsystem; hatten wir uns einen RSA-key erstellt verwenden wir folgenden Aufruf:

 $  scp /home/django/.ssh/id_rsa4096_dmz.pub zielhost:/home/django/.ssh/key.pub

bzw. bei einem ed25519 Schlüssel:

 $  scp /home/django/.ssh/id_ed25519_dmz.pub zielhost:/home/django/.ssh/key.pub


Bei der Einführung von SSH Version 2 kam die Datei authorized_keys2 zum Einsatz. Seit OpenSSH 3.0 wird nun wiederum neben der authorized_keys2 wieder die authorized_keys verwendet. In unserem Fall nutzen wir in unserem obigen Beispiel daher nur noch die Schlüsseldatei authorized_keys.

Anschliessend wird der Schlüssel in die Datei authorized_keys kopiert. Diese Datei kann mehrere Schlüssel enthalten, daher ist das doppelte Umleitungszeichen wichtig, um eine evt. existierende Datei nicht versehentlich zu überschreiben. Somit wird der neue Schlüssel in die Datei hinzugefügt:

 $ cat key.pub >> authorized_keys

Zu guter Letzt passen wir noch die Berechtigungen an und löschen die nicht mehr benötigte id_rsa.pub

 $ chmod 600 authorized_keys
 $ rm key.pub

Das Kopieren des Public-Keys auf unseren Zielhost mit Anpassen der Dateiberechtigungen kann man natürlich auch einfacher vornehmen. Man benutzt hierzu einfach den Befehl ssh-copy-id aus dem Paket openssh-clients.

  • RSA-Key
     $ ssh-copy-id -i ~/.ssh/id_rsa.pub testhost.intra.nausch.org
  • ed25519-Key
     $ ssh-copy-id -i ~/.ssh/id_ed25519_dmz testhost.intra.nausch.org

Mit der Angabe ~/.ssh/id_rsa4096_dmz bzw. ~/.ssh/id_ed25519_dmz kopiert dann der Befehl ssh-copy-id den zugehörigen öffentlichen Schlüssel auf den Zielhost testhost.intra.nausch.org.

Wichtig ist jedoch immer:

Der User muß selbst darauf achten, dass sein privater Schlüssel nicht in fremde Hände gelangt! Will man noch sicherer gehen, vergibt man, wie Eingangs bereits erwähnt, bei der Erzeugung des Schlüssels eine Passphrase. Diese muss er User dann aber bei jedem neuen Verbindungsaufbau angeben!

Möchte man seine Schlüssel sicher ausserhalb seines Rechners/Dateisystems verwahren, bietet sich die Nutzung eines externen Krypto-Devices wie z.B. einem Nitrokey Start an. Die Nutzung eines ED25519-Schlüssels ist in diesem Artikel ausführlich beschrieben.

ssh-Daemon

ssh-Daemon einrichten/anpassen

Bevor wir unseren ssh-Damon nun individuell konfigurieren, werden wir die originale Konfigurationsdatei des Servers erst einmal sichern.

 # cp -a /etc/ssh/sshd_config /etc/ssh/sshd_config.orig

Anschliessend tragen wir die Optionen wie in der nachfolgenden Musterkonfigurationsdatei nach.

 # vim /etc/ssh/sshd_config
/etc/ssh/sshd_config
#       $OpenBSD: sshd_config,v 1.93 2014/01/10 05:59:19 djm Exp $
 
# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.
 
# This sshd was compiled with PATH=/usr/local/bin:/usr/bin
 
# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.
 
# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
 
# Specifies which address family should be used by sshd(8). Valid arguments
# are ''any'', ''inet'' (use IPv4 only), or ''inet6'' (use IPv6 only). 
AddressFamily any
 
# Specifies the local addresses sshd(8) should listen on. The following 
# forms may be used:
#                   ListenAddress host|IPv4_addr|IPv6_addr
#                   ListenAddress host|IPv4_addr:port
#                   ListenAddress [host|IPv6_addr]:port
# If port is not specified, sshd will listen on the address and all prior 
# Port options specified. The default is to listen on all local addresses. 
# Multiple ListenAddress options are permitted. Additionally, any Port 
# options must precede this option for non-port qualified addresses. 
ListenAddress 0.0.0.0:10022
 
# Specifies the protocol versions sshd(8) supports. The possible values are 
# '1' and '2'. Multiple versions must be comma-separated. The default is 
# ''2,1''. Note that the order of the protocol list does not indicate 
# preference, because the client selects among multiple protocol versions 
# offered by the server. Specifying ''2,1'' is identical to ''1,2''.  
Protocol 2
 
# Specifies a file containing a private host key used by SSH. The default 
# is /etc/ssh/ssh_host_key for protocol version 1, and 
# /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for protocol 
# version 2. Note that sshd(8) will refuse to use a file if it is 
# group/world-accessible. It is possible to have multiple host key files.
# ''rsa1'' keys are used for version 1 and ''dsa'' or ''rsa'' are used for
# version 2 of the SSH protocol. 
HostKey /etc/ssh/ssh_host_ed25519_key
 
# Specifies the ciphers allowed for protocol version 2. Multiple ciphers 
# must be comma-separated. The supported ciphers are ''3des-cbc'', 
# ''aes128-cbc'', ''aes192-cbc'', ''aes256-cbc'', ''aes128-ctr'', 
# ''aes192-ctr'', ''aes256-ctr'', ''arcfour128'', ''arcfour256'', 
# ''arcfour'', ''blowfish-cbc'', and ''cast128-cbc''.
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
 
# MACs' Specifies the available MAC (message authentication code) 
# algorithms. The MAC algorithm is used in protocol version 2 for data 
# integrity protection. Multiple algorithms must be comma-separated.
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
 
# Specifies the available KEX (Key Exchange) algorithms. Multiple 
# algorithms must be comma-separated. For ineroperability with Eclipse 
# and WinSCP): 
# KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
# If needed, open /etc/ssh/moduli if exists, and delete lines where the 
# 5th column is less than 2000.
#   awk '$5 > 2000' /etc/ssh/moduli > "${HOME}/moduli"
#   wc -l "${HOME}/moduli"
# make sure there is something left
#   mv "${HOME}/moduli" /etc/ssh/moduli
#
# CentOS 6
# KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
# CentOS 7 / Fedora >21 "only"
KexAlgorithms curve25519-sha256@libssh.org
 
# Logging
# Gives the facility code that is used when logging messages from sshd(8). 
# The possible values are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, 
# LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 
SyslogFacility AUTHPRIV
 
# Gives the verbosity level that is used when logging messages from sshd(8).
# The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, 
# DEBUG1, DEBUG2, and DEBUG3. The default is INFO. DEBUG and DEBUG1 are 
# equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging 
# output. Logging with a DEBUG level violates the privacy of users and is 
# not recommended.
# LogLevel VERBOSE logs user's key fingerprint on login. Needed to have a 
# clear audit track of which key was using to log in.
LogLevel VERBOSE
 
# Configures an external subsystem (e.g. file transfer daemon). Arguments 
# should be a subsystem name and a command (with optional arguments) to 
# execute upon subsystem request. Log sftp level file access 
# (read/write/etc.) that would not be easily logged otherwise.
Subsystem sftp /usr/lib/ssh/sftp-server -f AUTHPRIV -l INFO
 
# Authentication:
# The server disconnects after this time if the user has not successfully 
# logged in. If the value is 0, there is no time limit.
LoginGraceTime 0
 
# Specifies whether root can log in using ssh(1). The argument must be 
# ''yes'', ''without-password'', ''forced-commands-only'', or ''no''. 
# The default is ''yes''. If this option is set to ''without-password'', 
# password authentication is disabled for root. If this option is set to
# ''forced-commands-only'', root login with public key authentication will 
# be allowed, but only if the command option has been specified (which 
# may be useful for taking remote backups even if root login is normally 
# not allowed). All other authentication methods are disabled for root.
# If this option is set to ''no'', root is not allowed to log in.  
PermitRootLogin no
 
# This keyword can be followed by a list of user name patterns, separated 
# by spaces. If specified, login is allowed only for user names that match 
# one of the patterns. Only user names are valid; a numerical user ID is 
# not recognized. By default, login is allowed for all users. If the pattern
# takes the form USER@HOST then USER and HOST are separately checked, 
# restricting logins to particular users from particular hosts. The 
# allow/deny directives are processed in the following order: 
# DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 
AllowUsers django
 
# Specifies whether sshd(8) should check file modes and ownership of the 
# user's files and home directory before accepting login. This is normally 
# desirable because novices sometimes accidentally leave their directory 
# or files world-writable.
StrictModes yes
 
# Specifies the maximum number of authentication attempts permitted per 
# connection. Once the number of failures reaches half this value, 
# additional failures are logged.
MaxAuthTries 2
 
# Specifies the maximum number of open sessions permitted per network 
# connection.
MaxSessions 10
 
# Specifies the file that contains the public keys that can be used for 
# user authentication. AuthorizedKeysFile may contain tokens of the form
# %T which are substituted during connection setup. The following tokens
# are defined: %% is replaced by a literal '%', %h is replaced by the 
# home directory of the user being authenticated, and %u is replaced by
# the username of that user. After expansion, AuthorizedKeysFile is
# taken to be an absolute path or one relative to the user's home directory.
AuthorizedKeysFile      .ssh/authorized_keys
 
# Specifies whether public key authentication is allowed. The default is 
# ''yes''. Note that this option applies to protocol version 2 only.
PubkeyAuthentication yes
 
# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
# Change to yes if you don't trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and HostbasedAuthentication
RhostsRSAAuthentication no
# Specifies whether rhosts or /etc/hosts.equiv authentication together 
# with successful public key client host authentication is allowed 
# (host-based authentication). This option is similar to 
# RhostsRSAAuthentication and applies to protocol version 2 only.
HostbasedAuthentication no
 
# Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts 
# during RhostsRSAAuthentication or HostbasedAuthentication.
IgnoreUserKnownHosts no
 
# Specifies that .rhosts and .shosts files will not be used in 
# RhostsRSAAuthentication or HostbasedAuthentication.
# /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.
IgnoreRhosts yes
 
# Specifies whether password authentication is allowed. To disable tunneled 
# clear text passwords, change to no here!
PasswordAuthentication yes
 
# When password authentication is allowed, it specifies whether the server 
# allows login to accounts with empty password strings. The default is ''no''.  
PermitEmptyPasswords no
 
# Specifies whether challenge-response authentication is allowed 
# (e.g. via PAM or though authentication styles supported in login.conf(5))
# Change to no to disable s/key passwords
ChallengeResponseAuthentication no
 
# Specifies whether user authentication based on GSSAPI is allowed.
GSSAPIAuthentication no
 
# Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key 
# exchange doesn't rely on ssh keys to verify host identity. 
GSSAPIKeyExchange no
 
# Specifies whether to automatically destroy the user's credentials cache 
# on logout. 
GSSAPICleanupCredentials yes
 
# Determines whether to be strict about the identity of the GSSAPI acceptor 
# a client authenticates against. If ''yes'' then the client must authenticate
# against the host service on the current hostname. If ''no'' then the client 
# may authenticate against any service key stored in the machine's default
# store. This facility is provided to assist with operation on multi homed 
# machines. The default is ''yes''. Note that this option applies only to 
# protocol version 2 GSSAPI connections, and setting it to ''no'' may only 
# work with recent Kerberos GSSAPI libraries. 
GSSAPIStrictAcceptorCheck yes
 
# Controls whether the user's GSSAPI credentials should be updated following 
# a successful connection rekeying. This option can be used to accepted 
# renewed or updated credentials from a compatible client.
GSSAPIStoreCredentialsOnRekey no
 
# Specifies whether ssh-agent(1) forwarding is permitted. The default is 
# ''yes''. Note that disabling agent forwarding does not improve security 
# unless users are also denied shell access, as they can always install 
# their own forwarders.
AllowAgentForwarding yes
 
# Specifies whether TCP forwarding is permitted. The default is ''yes''. 
# Note that disabling TCP forwarding does not improve security unless users
# are also denied shell access, as they can always install their own 
# forwarders. 
AllowTcpForwarding yes
 
# Specifies whether remote hosts are allowed to connect to ports forwarded
# for the client. By default, sshd(8) binds remote port forwardings to the
# loopback address. This prevents other remote hosts from connecting to 
# forwarded ports. GatewayPorts can be used to specify that sshd should 
# allow remote port forwardings to bind to non-loopback addresses, thus 
# allowing other hosts to connect. The argument may be ''no'' to force 
# remote port forwardings to be available to the local host only, ''yes''
# to force remote port forwardings to bind to the wildcard address, or 
# ''clientspecified'' to allow the client to select the address to which 
# the forwarding is bound. The default is ''no''. 
GatewayPorts no
 
# Specifies whether X11 forwarding is permitted. The argument must be 
# ''yes'' or ''no''. The default is ''no''.
# When X11 forwarding is enabled, there may be additional exposure to the
# server and to client displays if the sshd(8) proxy display is configured
# to listen on the wildcard address (see X11UseLocalhost below), though this
# is not the default. Additionally, the authentication spoofing and 
# authentication data verification and substitution occur on the client side.
# The security risk of using X11 forwarding is that the client's X11 display
# server may be exposed to attack when the SSH client requests forwarding 
# (see the warnings for ForwardX11 in ssh_config(5)). A system administrator
# may have a stance in which they want to protect clients that may expose
# themselves to attack by unwittingly requesting X11 forwarding, which can 
# warrant a ''no'' setting. Note that disabling X11 forwarding does not 
# prevent users from forwarding X11 traffic, as users can always install 
# their own forwarders. X11 forwarding is automatically disabled if UseLogin
# is enabled. 
X11Forwarding yes
 
# Specifies the first display number available for sshd(8)'s X11 forwarding.
# This prevents sshd from interfering with real X11 servers. 
# The default is 10. 
X11DisplayOffset 10
 
# Specifies whether sshd(8) should bind the X11 forwarding server to the 
# loopback address or to the wildcard address. By default, sshd binds the
# forwarding server to the loopback address and sets the hostname part of
# the DISPLAY environment variable to ''localhost''. This prevents remote
# hosts from connecting to the proxy display. However, some older X11 clients
# may not function with this configuration. X11UseLocalhost may be set to 
# ''no'' to specify that the forwarding server should be bound to the 
# wildcard address. The argument must be ''yes'' or ''no''. The default is 
# ''yes''. 
X11UseLocalhost yes
 
# Specifies whether sshd(8) should print /etc/motd when a user logs in 
# interactively. (On some systems it is also printed by the shell, 
# /etc/profile, or equivalent.) The default is ''yes''.
PrintMotd yes
 
# Specifies whether sshd(8) should print the date and time of the last user
# login when a user logs in interactively. The default is ''yes''. 
PrintLastLog yes
 
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
# WARNING: 'UsePAM no' is not supported in Red Hat Enterprise Linux 
# and may cause several problems.
UsePAM yes
 
# Specifies whether sshd(8) separates privileges by creating an unprivileged
# child process to deal with incoming network traffic. After successful 
# authentication, another process will be created that has the privilege of
# the authenticated user. The goal of privilege separation is to prevent 
# privilege escalation by containing any corruption within the unprivileged 
# processes. 
UsePrivilegeSeparation sandbox
 
# Sets a timeout interval in seconds after which if no data has been
# received from the client, sshd(8) will send a message through the 
# encrypted channel to request a response from the client. The default is 0,
# indicating that these messages will not be sent to the client. This option
# applies to protocol version 2 only. 
ClientAliveInterval 900
 
 
# Sets the number of client alive messages (see below) which may be sent 
# without sshd(8) receiving any messages back from the client. If this 
# threshold is reached while client alive messages are being sent, sshd will
# disconnect the client, terminating the session. It is important to note 
# that the use of client alive messages is very different from TCPKeepAlive
# (below). The client alive messages are sent through the encrypted channel
# and therefore will not be spoofable. The TCP keepalive option enabled by 
# TCPKeepAlive is spoofable. The client alive mechanism is valuable when the
# client or server depend on knowing when a connection has become inactive.
# The default value is 3. If ClientAliveInterval (see below) is set to 15,
# and ClientAliveCountMax is left at the default, unresponsive SSH clients 
# will be disconnected after approximately 45 seconds. This option applies 
# to protocol version 2 only. 
ClientAliveCountMax 0
 
# Specifies whether the system should send TCP keepalive messages to the 
# other side. If they are sent, death of the connection or crash of one of
# the machines will be properly noticed. However, this means that 
# connections will die if the route is down temporarily, and some people 
# find it annoying. On the other hand, if TCP keepalives are not sent, 
# sessions may hang indefinitely on the server, leaving ''ghost'' users 
# and consuming server resources. The default is ''yes'' (to send TCP 
# keepalive messages), and the server will notice if the network goes down
# or the client host crashes. This avoids infinitely hanging sessions.
# To disable TCP keepalive messages, the value should be set to ''no''. 
TCPKeepAlive yes
 
# Specifies whether sshd(8) should look up the remote host name and check
# that the resolved host name for the remote IP address maps back to the 
# very same IP address. 
UseDNS yes
 
# Specifies the file that contains the process ID of the SSH daemon. 
# The default is /var/run/sshd.pid. 
PidFile /var/run/sshd.pid
 
# Specifies the maximum number of concurrent unauthenticated connections
# to the SSH daemon. Additional connections will be dropped until 
# authentication succeeds or the LoginGraceTime expires for a connection.
# The default is 10. 
# Alternatively, random early drop can be enabled by specifying the three
# colon separated values ''start:rate:full'' (e.g. "10:30:60"). sshd(8) 
# will refuse connection attempts with a probability of ''rate/100'' (30%)
# if there are currently ''start'' (10) unauthenticated connections. The 
# probability increases linearly and all connection attempts are refused 
# if the number of unauthenticated connections reaches ''full'' (60). 
MaxStartups 10:30:100
 
# Specifies whether tun(4) device forwarding is allowed. The argument must 
# be ''yes'', ''point-to-point'' (layer 3), ''ethernet'' (layer 2), or 
# ''no''. Specifying ''yes'' permits both ''point-to-point'' and 
# ''ethernet''. The default is ''no''.
PermitTunnel no
 
# Specifies a path to chroot(2) to after authentication. This path, and all
# its components, must be root-owned directories that are not writable by 
# any other user or group. After the chroot, sshd(8) changes the working 
# directory to the user's home directory.
# The path may contain the following tokens that are expanded at runtime 
# once the connecting user has been authenticated: %% is replaced by a 
# literal '%', %h is replaced by the home directory of the user being 
# authenticated, and %u is replaced by the username of that user.
# The ChrootDirectory must contain the necessary files and directories to 
# support the user's session. For an interactive session this requires at
# least a shell, typically sh(1), and basic /dev nodes such as null(4), 
# zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) devices.
# For file transfer sessions using ''sftp'', no additional configuration 
# of the environment is necessary if the in-process sftp server is used, 
# though sessions which use logging do require /dev/log inside the chroot
# directory (see sftp-server(8) for details).
ChrootDirectory none
 
# The contents of the specified file are sent to the remote user before 
# authentication is allowed. 
Banner /etc/issue.net
 
# pecifies whether ~/.ssh/environment and environment= options in 
# ~/.ssh/authorized_keys are processed by sshd(8). Enabling environment 
# processing may enable users to bypass access restrictions in some 
# configurations using mechanisms such as LD_PRELOAD.
PermitUserEnvironment no
 
# Accept locale-related environment variables
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS
 
# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
#       AllowTcpForwarding no
#       PermitTTY no
#       ForceCommand cvs server

Im Wesentlichen werden wir hier zu Absicherung bzw. Härtung des SSH-Daemon folgende Parameter besondere Beachtung schenken.

  • AddressFamily: Abhängig von den verwendeten Adressklassen, setzen wir hier entweder any für IPv4 und IPv6 oder z.B. inet für IPv4 only.
  • ListenAddress: Hier legen wir fest auf welcher IP-Adresse und welchem Port der SSH-Daemon lauschen soll. Bei Systemen, die direkt aus fremden Netzen erreichbar sind, hat es sich bewährt, den SSH-Daemon nicht auf dem Standardport 22 lauschen zu lassen. Einfache script-kiddies lassen sich so z.B. schon ein wenig aussperren. Eine Garantie, dass derartige Zugriffe mit einer Portverlegung ausbleiben, gibt es natürlich nicht.
  • HostKey: Hier geben wir nun an, dass nur noch die besten und vertrauenswürdigen Host-Schlüssel zur Anwendung kommen sollen.
  • Ciphers, MACs und KexAlgorithms: Hier gilt auch das vorgenannte, wie beim Parameter HosKeys. Natürlich muss man hier Rücksicht auf ggf. vorhandene puuty-User oder Nutzer von älteren Linux-Distributionen Rücksicht nehmen. Oft muss bei der Qualität und Gülte dann der ein oder andere Abstrich dennoch gemacht werden, um solche Nutzer nicht auszuschließen.
  • PermitRootLogin: Hier unterbinden wir root-logins mit einem no!
  • AllowUsers: Nur die berechtigten Benutzer, die sich auf dem Server anmelden dürfen, werden hier angegeben!
  • MaxAuthTries: Hier legen wir ein vertretbare Anzahl Anmeldeversuche fest. Da wir bei den Clientkonfigurationen jeweils den richtigen Schlüssel zum richtigen Zielhost festlegen, können wir auch bei Nutzung vieler unterschiedlicher Schlüssel im Verzeichnis ~/.ssh, den MaxAuthTries auf 2 setzen!
  • PasswordAuthentication: Da wir keine Passwortgestützten Logins der Admin zulassen wollen, setzen wir diesen Parameter auf no.

Alle weiteren Parameter sind in unserer Musterkonfigurationsdatei hin- und ausreichend beschrieben. Bei Bedarf passen wir diese unseren Anforderungen nach noch an!

Bevor wir unsere Konfiguration scharf schalten überprüfen wir noch, ob sich nicht ein syntaktischer Fehler eingeschlichen hat.

 # sshd -t
/etc/ssh/sshd_config: line 101: Bad configuration option: oginGraceTime
/etc/ssh/sshd_config: terminating, 1 bad configuration options

Wir berichtigen also den Schreibfehler und überprüfen erneut die Änderung.

 # sshd -t

Erfolgt keine Fehlermeldung, bedeutet dies, dass alles in Ordnung ist und wir nun unseren Daemon einmal durchstarten können.

# systemctl restart sshd.service

Bei Bedarf können wir auch den Status unseres SSH-Daemon wie folgt abfragen:

 # systemctl status sshd.service 

 sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Sun 2016-11-20 14:34:10 CET; 3min 50s ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 27346 (sshd)
   CGroup: /system.slice/sshd.service
           └─27346 /usr/sbin/sshd -D

Nov 20 14:34:10 vml000017.dmz.nausch.org systemd[1]: Started OpenSSH server daemon.
Nov 20 14:34:10 vml000017.dmz.nausch.org systemd[1]: Starting OpenSSH server daemon...
Nov 20 14:34:10 vml000017.dmz.nausch.org sshd[27346]: Set /proc/self/oom_score_adj from 0 to -1000
Nov 20 14:34:10 vml000017.dmz.nausch.org sshd[27346]: Server listening on 0.0.0.0 port 22.

Finale sshd-Änderungen

Das war's eigentlich schon. Im Moment kann sich der user mittels rsa-key oder seinem Passwort anmelden -es funktionieren beide Verfahren. Das kann während der Umstiegphase von Passwörtern auf Schlüssel wichtig sein, um sich z.B. nicht versehentlich selbst auszusperren. Schlägt die Anmeldung mit dem fehl, tritt wieder die Passwortauthentifizierung in Kraft.

Wenn jedoch alles wunschgemäß funktioniert sollte man in der /etc/ssh/sshd_config des Zielsystems folgenden Eintrag freischalten:

 # vim /etc/ssh/sshd_config
PasswordAuthentication no

Anschließend den daemon wieder mittels:

# systemctl restart sshd.service

neu starten.

Somit sind dann nur noch Userlogins zugelassen, die einen Publikkey auf dem Zielsystem besitzen.

Wichtig ist jedoch immer:

Der User muß selbst darauf achten, dass sein privater Schlüssel nicht in fremde Hände gelangt! Will man noch sicherer gehen, vergibt man, wie Eingangs bereits erwähnt, bei der Erzeugung des Schlüssels eine Passphrase. Diese muss er User dann aber bei jedem neuen Verbindungsaufbau angeben!

ssh-Daemon automatisch starten

Damit der ssh-Daemon sshd automatisch bei jedem Systemstart startet, kann die Einrichtung eines Start-Scripts über folgenden Befehl erreicht werden:

# systemctl enable sshd.service

Ein Überprüfung ob der Dienst (Daemon) sshd wirklich bei jedem Systemstart automatisch mit gestartet wird, kann durch folgenden Befehl erreicht werden:

# systemctl is-enabled sshd.service
enabled

ssh in der Praxis (Teil 2)

Verbindungsaufbau via SSH

Nachdem wir den Login mit Passwort deaktiviert haben, werden wir nun beim Verbindungsaufbau nach der Passphrase des zur Anwendung kommenden Schlüssels gefragt.

 $ ssh www
##############################################################################
#                                                                            #
#                        This is server of nausch.org.                       #
#                                                                            #
#             Unauthorized access to this system is prohibited !             #
#                                                                            #
#    This system is actively monitored and all connections may be logged.    #
#         By accessing this system, you consent to this monitoring.          #
#                                                                            #
##############################################################################
Enter passphrase for key '/home/django/.ssh/id_ed25519':

Nach Eingabe der Passphrase, die wir beim Anlegen des Schlüssels angegeben hatten, wird der Schlüssel freigeschaltet und wir erhalten den Zugriff auf unser Zielsystem.

Last login: Sat Nov 19 21:31:52 2016 from ip-54-229-238-178.static.contabo.net
##############################################################################
#                                                                            #
#                        This is server of nausch.org.                       #
#                                                                            #
#                            kd-141271-b357-fr138d                           #
#                                                                            #
#             Unauthorized access to this system is prohibited !             #
#                                                                            #
#    This system is actively monitored and all connections may be logged.    #
#         By accessing this system, you consent to this monitoring.          #
#                                                                            #
##############################################################################

[django@kd-141271-b357-fr138d ~]$

Client Konfiguration

Bei der täglichen Arbeit werden es mit meist sehr unterschiedlichen Servern/Zielsystemen zu tun haben. Zum einen unterscheiden sich diese in Ihrer Aufgabe und Funktion, wie z.B. einem Applikationsserver wie einem Mailserver oder einem Firewallsystem. Zum anderen kann es sich bei den Zielsystemen um unterschiedlichste Distributionen und Versionsständen handeln. Dies bringt dann nicht unerhebliche Einschränkungen bei den zur Verfügung stehenden Ciphern, MACs, den Schlüssel Typen sowie den Key Exchange Algorithmen mit sich.

Um nicht beim Aufruf von ssh nun jedesmal alle benötigten Parameter extra angeben zu müssen, werden wir uns für jeden der zu betreuenden Systeme eine passende Clientkonfiguration anlegen. Nicht nur die erneute Eingabe der benötigten Parameter fällt weg, sondern wir können auch später sehr einfach mit der bash-completion mittels der TAB-Taste dann komfortabel aus den angelegten Zielhosts auswählen!

Wir legen uns hierzu einmalig eine entsprechende Konfigurationsdatei conf im Verzeichnis ~/.ssh/ auf unserer Administrations-Workstation mit nachfolgendem Inhalt an.

 $ vim ~/.ssh/config
~/.ssh/config
# Clientkonfigurationsbeispiel für unterschiedliche Zielsysteme 
# Django : 2016-11-18
Host 04x1
     Hostname kd-141271-b357-fr138d.dmz.nausch.org
     Port 9922
     User django
     ForwardX11 yes
     ForwardAgent yes
     Protocol 2
     IdentityFile ~/.ssh/id_ed25519_dmz
 
Host nss
     Hostname d34db33f1.idmz.nausch.org
     Port 22
     User django
     Compression yes
     Protocol 2
     IdentityFile ~/.ssh/id_rsa4096_dmz

Da wir bei der Konfiguration unseres SSH-Daemons die Option StrictModes yes gesetzt hatten, damit fehlerhafte Datei und Verzeichnisrechte erkannt werden, passen wir noch die Dateirechte entsprechend an.

 $ chmod 600 ~/.ssh/config

Beim Verbindungsaufbau brauchen wir nunmehr nur noch den definierten „Pseudo“Host-Namen (dieser Hostname muss nicht unbedingt ein realer Hostname sein, der über DNS auflösbar ist) angeben!

Im folgenden Beispiel würde eien Verbindungs zum Zielhost kd-141271-b357-fr138d.dmz.nausch.org aufgebeut werden.

 $ ssh 04x1

Ohne unsere Clientkonfigurationsdatei müssten wir hingegen folgenden Aufruf verwenden.

 $ ssh -p 9922 -u django -Y -A -2 -i ~/.ssh/id_ed25519_dmz kd-141271-b357-fr138d.dmz.nausch.org

SSH Authentication Agent

Um nicht nun jedes mal beim Aufbau einer ssh-Verbindung nach der Passphrase gefragt zu werden, können wir den Authentication Agent verwenden. Dieser Agent hält den freigeschalteten privaten Schlüssel und über gibt diesen beim Verbindungsaufbau dem SSH-Client.

RSA-Schlüssel

Im Falle eines RSA-Schlüssels erfolgt das Laden eines privaten RSA-Schlüssel gemäß nachfolgendem Beispiel.

 $ ssh-add ~/.ssh/id_rsa4096_dmz
Enter passphrase for /home/django/.ssh/id_rsa4096_dmz: 
Identity added: /home/django/.ssh/id_rsa4096_dmz (/home/django/.ssh/id_rsa4096_dmz)

Wichtig:

Vor allem bei Verwendung der Option -A (Agent Forwarding) beim SSH-Client -Aufruf, sollte der geladene private Schlüssel mit einer Verfallszeit geladen werden! Nach Ablauf der definierten Zeit, wird der private Schlüssel dann automatisch aus dem Speicher gelöscht.

In der manpage des SSH-Daemon findet sich hierzu eine entsprechende Warnung:

Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.

Wir werden also den privaten RSA-Schlüssel besser z.B. mit einer Verfallszeit von 30 Minuten wie folgt laden.

 $ ssh-add -t 1800 ~/.ssh/id_rsa4096_dmz

ED25519 Schlüssel

Versuchen wir unseren privaten ED25519-Schlüssel entsprechend dem vorgenannten Beispiel des RSA-Schlüssels zu laden, erhalten wir aktuell1) jedoch eine Fehlermeldung:
Error reading response length from authentication socket.

 $ ssh-add -t 1800 .ssh/id_ed25519_dmz
Enter passphrase for .ssh/id_ed25519_dmz: 
Error reading response length from authentication socket.
Could not add identity: .ssh/id_ed25519_dmz

Um nun die als weitaus sicherer geltenden ED25519-Schlüssel laden zu können, müssen wir uns zur Zeit mit einem kleinen Trick behelfen. Wir laden den Schlüssel mit einem vorangestellen eval 'ssh-agent' .

 $ eval `ssh-agent` && ssh-add -t 1800 ~/.ssh/id_ed25519_dmz
Agent pid 27998
Enter passphrase for /home/django/.ssh/id_ed25519_dmz: 
Identity added: /home/django/.ssh/id_ed25519_dmz (/home/django/.ssh/id_ed25519_dmz)
Lifetime set to 1800 seconds

WICHTIG: Nun wird aber bei jedem Laden des/der Schlüssel ein neuer SSH-Agent gestartet, was natürlich bei längerer Laufzeit einer Session weder wünschenswert noch zu empfehlen ist!

Wir werden daher in der .bashrc unseres Homeverzeichnisses folgende Erweiterung anfügen:

 # vim ~/.bashrc
~/.bashrc
...
 
SSH_ENV="$HOME/.ssh/environment"
 
function start_agent {
    echo "Initialising new SSH agent..."
    (umask 066; /usr/bin/ssh-agent > "${SSH_ENV}")
    . "${SSH_ENV}" > /dev/null
    /usr/bin/ssh-add;
}
 
# Source SSH settings, if applicable
 
if [ -f "${SSH_ENV}" ]; then
    . "${SSH_ENV}" > /dev/null
    ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
        start_agent;
    }
else
    start_agent;
fi

Nun können wir wie gewohnt die neuen als sicherer geltenden ED25519 Schlüssel genau so einfach laden, wie die altbekannten RSA-Schlüssel!

SSH ProxyCommand - Transparente multi-jump SSH

Oft steht man vor einem Problem, dass man ein Host nicht direkt via ssh erreichbar ist, man aber dennoch zur Adminstration dort hin möchte oder gar Dateien via scp kopieren möchte.

Schauen wir uns hierzu einfach mal nachstehende Skizze an.

Von der Admin-Workstation aus, wollen wir nun nicht nur zum nächstgelegenen Host springen, sondern auch zum übernächsten oder gar zu einem Host im Internet, den wir aber aus Sicherheitsgründen nicht direkt erreichen dürfen und auch können.

System-Skizze

PlantUML Graph

Lösungsbeispiele

manuelle Sprünge

Die vermutlich naheliegendste Variante ist vermutlich der jeweils manuelle Spring zum nächsten Host. Damit wir in unserem Beispiel die A-Firewall erreichen können springen wir nacheinander zum jeweilig nächsten Host. Mit Hilfe der Option -A nutzen wir dabei die SSH-Key-Forwarding-Option des SSH-Agenten.

 $ ssh -A firewall-c.idmz.nausch.org
 $ ssh -A firewall-b.edmz.nausch.org
 $ ssh -A firewall-a.nausch.org

Na ja, komfortabel ist das nicht gerade und beim Kopieren von Daten von Ende zu Ende nervt das doch gewaltig, oder?

verkette Sprünge

O.K. wird sich da der ein oder andere sagen, dann verkette ich die Sprünge doch einfach.

 $ ssh -A -t firewall-c.idmz.nausch.org ssh -A -t firewall-b.edmz.nausch.org ssh -A firewall-a.nausch.org

Oder ich schreib mir jeweils kleine bash-scripte zum Springen

 $ vim ~/bin/fwa-jump
~/bin/fwa-jump
# /bin/bash
ssh -t -A -Y firewall-c.idmz.nausch.org 'ssh -Y -A -l swat maill.idmz.nausch.org'

Na ja, das Kopieren geht immer noch nur von Host zu Host, oder eben mit einer verketteten Befehlsfolge oder eigenen Bash-Scripten.

ssh mit ProxyCommand

Die Komfortabelste Variante ist nun die Nutzung der Option ProxyCommand. Hierzu erweitern wir die bereits bei der Client Konfiguration angelegte Datei ~/.ssh/config auf unserer Administrations-Workstation mit nachfolgendem Inhalt.

 $ vim ~/.ssh/config
~/.ssh/config
# Django : 2012-06-13
# ssh-jumps über mehrere Sprunghosts
 
# Erster Sprunghost (fwc) - direkt erreichbar
# Host -->  fwc
Host fwc
    Hostname firewall-c.idmz.nausch.org
    Port 22
    Protocol 2
    user admin
    IdentityFile ~/.ssh/id_ed25519_idmz 
 
# Zweiter Sprunghost (fwb) - nur über fwc erreichbar
# Host -->  fwc --> fwb
Host fwb
    Hostname firewall-b.edmz.nausch.org
    Port 22
    Protocol 2
    user admin
    IdentityFile ~/.ssh/id_ed25519_edmz
    ProxyCommand  ssh -A -q -W %h:%p fwc
 
# Dritter Sprunghost (fwa) - nur über fwb erreichbar
# Host --> fwc --> fwb --> fwa 
Host fwa
    Hostname firewall-a.nausch.org
    Port 22222
    Protocol 2
    user admin
    IdentityFile ~/.ssh/id_ed25519_edmz
    ProxyCommand  ssh -A -q -W %h:%p fwc
 
# externer Server im Internet nur über externe Firewall "A" erreichbar
# also: Host --> fwc --> fwb --> fwa --> daxie
Host s1u7
    Hostname <was-das-auch-immer-für-ein geiler-FQDN-sein-mag>
    Port 42422
    Protocol 2
    user n3rd
    IdentityFile ~/.ssh/id_ed25519_n3rd
    ProxyCommand  ssh -A -q -W %h:%p fwa

Test

Nun können wir ganz einfach direkt einen Tunnel zu unserem Zielhost aufspannen, genauso also würden wir den Zielhost direkt „sehen“.

 $ ssh fwa

Auch können wir nun ohne großem Heckmeck Dateien von einem Ende zum anderen Ende kopieren.

 $ scp ~/Downloads/enigmail-1.4-sm+tb.xpi 51u7:/tmp/
 $ scp 51u7:/home/babe/51lv14/04x.png .

Fehlersuche

Klappt aus unerfindlichen Gründen eine Verbindungsaufbau zu einem Zielsystem nicht, können wir mit der Option -v beim Aufruf von ssh die Ausgabe im Modus verbose erzwingen. Dort finden wir dann in der Regel entsprechende Hinweise, warum ein Verbindungsaufbau fehl schlug.

 $ ssh -v centos7-testhost
OpenSSH_6.6.1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /home/django/.ssh/config
debug1: /home/django/.ssh/config line 2: Applying options for nausch
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Hostname has changed; re-reading configuration
debug1: Reading configuration data /home/django/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Connecting to 192.168.1.173 [192.168.1.173] port 22.
debug1: Connection established.
debug1: identity file /home/django/.ssh/id_ed25519_dmz type 4
debug1: identity file /home/django/.ssh/id_ed25519_dmz-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1
debug1: match: OpenSSH_6.6.1 pat OpenSSH_6.6.1* compat 0x04000000
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes256-ctr hmac-sha2-256-etm@openssh.com none
debug1: kex: client->server aes256-ctr hmac-sha2-256-etm@openssh.com none
debug1: kex: curve25519-sha256@libssh.org need=32 dh_need=32
debug1: kex: curve25519-sha256@libssh.org need=32 dh_need=32
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ED25519 53:69:6c:76:69:61:44:61:78:69:73:74:67:65:69:6c
debug1: Host '[192.168.1.173]:22' is known and matches the ED25519 host key.
debug1: Found key in /home/django/.ssh/known_hosts:1412
debug1: ssh_ed25519_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
##############################################################################
#                                                                            #
#                       This is a private home server.                       #
#                                                                            #
#             Unauthorized access to this system is prohibited !             #
#                                                                            #
#    This system is actively monitored and all connections may be logged.    #
#         By accessing this system, you consent to this monitoring.          #
#                                                                            #
##############################################################################
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering ED25519 public key: /home/django/.ssh/id_ed25519_dmz
debug1: Server accepts key: pkalg ssh-ed25519 blen 51
debug1: key_parse_private_pem: PEM_read_PrivateKey failed
debug1: read PEM private key done: type <unknown>
Enter passphrase for key '/home/django/.ssh/id_ed25519_dmz': 
debug1: No more authentication methods to try.
Permission denied (publickey).

Im vorliegendem Beispiel wurde im Vorfeld der benötigte Schlüssel nicht geladen. Beim Verbindungsaufbau wurde dazu die Passphrase nicht richtig eingegeben und somit der Zugriff auf den publickey verweigert, was letztendlich zum Abbruch der Verbindung führte.

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