Dies ist eine alte Version des Dokuments!
Secure Shell - ssh
Bei 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.
Die Optionen rund um opennssh findet amn 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
ssh in der Praxis
Auch wenn Passworte bei ssh verschlüsselt übertragen werden, wollen wir zwei wesentliche Aspekte bei Verwendung der ssh berücksichtigen:
- 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! - 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.
# 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
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 Schlüsselpäärchens
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 bei einer NSA oder BND, noch solhcen Schlüssel einsetzen kann und mag, muss natürlich jeder Admin für sich sekbst entscheiden. Auf solche Schlüssel muss man aber nicht mehr zwingend zurückgreifen, stehen doch aktuellere und zeitgemäße Cipher, MACs, Schlüssel Typen und Key Exchange Algorithmen zur Verfügung. Als Alternative zu einem RSA-Keys wollen wir nun nun einen ed25519 Schlüssels erzeugen. Ed25519 ist ein Elliptic Curve Signature Schema, welches beste Sicherheit bei vertretbaren Aufwand verspricht, als ECDSA oder DSA dies versprechen. 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.
Zielverzeichnis anlegen und öffentlichen Schlüssel kopieren
Auf dem Zielrechner legen wir nun das Verzeichnis .ssh an und schützen es entsprechend.
[django@zielhost django]$ mkdir .ssh [django@zielhost django]$ chmod 700 .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
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.
authorized_keys vs. authorized_keys2
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.
ssh-Daemon
ssh-Daemon einrichten/anpassen
Folgende Zeilen müssen wir in der Datei /etc/ssh/sshd_config anpassen:
# vim /etc/ssh/sshd_config
PermitRootLogin yes RSAAuthentication yes PubkeyAuthentication yes AuthorizedKeysFile .ssh/authorized_keys
Anschließend starten wir mit
# service sshd restart
den ssh-Daemon neu und melden uns mit ssh zielhost am entfernten Rechner an.
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:
# service sshd restart
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:
# chkconfig sshd on
Ein Überprüfung ob der Dienst (Daemon) sshd wirklich bei jedem Systemstart automatisch mit gestartet wird, kann durch folgenden Befehl erreicht werden:
# chkconfig --list | grep sshd sshd 0:Aus 1:Aus 2:Ein 3:Ein 4:Ein 5:Ein 6:Aus
Wichtig ist jeweils der Schalter on bei den Runleveln - 2 3 4 5.
sshd-Agent nutzen
Damit man nicht bei jedem anmelden am entfernten Rechner, die Passphrase erneut eingeben müssen, nutzen wir nun den ssh-agent.
Im Homeverzeichnis des remote-clients legen wir ein Verzeichnis autostart an:
$ mkir /home/django/.config/autostart
Dort legen wir die Datei ssh-add.desktop mit folgendem Inhalt an:
[Desktop Entry] Name=No name Encoding=UTF-8 Version=1.0 Exec=ssh-add X-GNOME-Autostart-enabled=true
Somit wird beim nächsten Amelden am X-Gnome-Desktop die passphrase einmalig abgefragt und beim Anmelden am Remotesystem mit übertragen.
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 Adminsitration dort hin möchte oder gar Dateien via scp kopieren möchte.
Schauenm 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
<uml width=775 title=„Grafische System-Übersicht“>
state Firewall_A {
Firewall_A : ----------- Firewall_A : A-Firewall Firewall_A : -----------
}
state Internet { state fremdgehostetes_System
fremdgehostetes_System : Host beim Housing-Provider fremdgehostetes_System : Hostname <was-das-auch-immer-für-ein geiler-FQDN-sein-mag> fremdgehostetes_System : IP-Adresse: aa-bb-cc-dd
}
state DMZ {
state bredmz {
bredmz : EDMZ-Netzwerkswitch (bredmz)
bredmz : Netz:10.0.0.0/24
}
state bridmz {
bridmz : EDMZ-Netzwerkswitch (bridmz)
bridmz : Netz:10.10.0.0/24
}
state edmz_switch {
edmz_switch : EDMZ-Switch
edmz_switch : Netgear Typ: x
}
state FWB {
FWB : FQDN: vml000010.dmz.nausch.org
FWB : ------------------------------------
FWB : Services: iptables
}
state FWC {
FWC : FQDN: vml000020.dmz.nausch.org
FWC : ------------------------------------
FWC : Services: iptables
}
state DMZ_HOSTs {
state IDMZ_Repository_Host {
IDMZ_Repository_Host : FQDN: vml000050.dmz.nausch.org
IDMZ_Repository_Host : CNAME : syslog, tftp, install
IDMZ_Repository_Host : IP (IDMZ) : eth0 - 52:54:00:19:08:67 - 10.0.0.50
IDMZ_Repository_Host : Services : syslog, httpd, rpm-repository, tftp/pxe
}
state EDMZ_Repository_Host {
EDMZ_Repository_Host : FQDN: vml100080.dmz.nausch.org
EDMZ_Repository_Host : CNAME : mail
EDMZ_Repository_Host : IP (EDMZ) : eth0 - 52:54:00:14:12:71 - 10.0.0.50
EDMZ_Repository_Host : Services : syslog, httpd, rpm-repository, tftp/pxe
}
} }
state Intranet {
state Switch {
Switch : Physikalischer Netzwerk-Switch
Switch : Netz 10.10.10.0/26
}
state Workstation {
Workstation : Gerät: Djangos Admin-Workstation
Workstation : Hostname: pml010040
Workstation : CNAME: office-work
Workstation : MAC:
Workstation : IP:10.10.10.40
}
}
FWB -down-> edmz_switch
bredmz -down-> FWB FWC -up-> bredmz FWC -up-> bridmz bredmz -up-> EDMZ_Repository_Host
bridmz -up-> IDMZ_Repository_Host
Switch -up-> FWC
Workstation -up-> Switch
edmz_switch --> Firewall_A Firewall_A -right-> fremdgehostetes_System
</uml>
Lösung
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 und netcat
Die Komfortabelste Variante ist nun die Nutzung der Option ProxyCommand. Hierzu legen wir uns einmalig eine entsprechende Konfigurationsdatei auf unserer Administrations-Workstation mit nachfolgendem Inhalt an.
$ 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 # Zweiter Sprunghost (fwb) - nur über fwc erreichbar # Host --> fwc --> fwb Host fwb Hostname firewall-b.edmz.nausch.org ProxyCommand ssh fwc nc -w 120 %h %p # Dritter Sprunghost (fwa) - nur über fwb erreichbar # Host --> fwc --> fwb --> fwa Host fwa Hostname firewall-a.nausch.org ProxyCommand ssh fwb nc -w 120 %h %p # externer Server im Internet nur über externe Firewall "A" erreichbar # also: Host --> fwc --> fwb --> fwa --> daxie Host daxie Hostname <was-das-auch-immer-für-ein geiler-FQDN-sein-mag> ProxyCommand ssh -l root -i ~/.ssh/id_rsa_daxie -2 -4 fwa nc -w 120 %h %p
Anschließend passen wir noch die Dateiberechtigungen an, damit ssh später nicht mäkelt.
$ chmod 600 ~/.ssh/config
Auf den Sprunghosts wird das Paket netcat benötigt. Wenn dies noch nicht bei der Grundinstallation unseres Systems bereits installiert wurde, werden wir dies nun noch nachholen.
# yum install nc -y
Test
Nun können wir ganz einfach direkt einen Tunnel zu unserem Zielhost aufspannen, genauso also würden wir den Zielhostz 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 daxie:/tmp/
$ scp daxie:/home/baby/Photos/Bild_001.png .
Links
~~DISCUSSION~~
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