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 Lösiung mittels OpenSSH.
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 communications 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 version 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 for‐
warded 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 sys‐
tems 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 con‐
trolled by the CompressionLevel option for protocol version 1. Compression 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 configuration 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 pro‐
tocol 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 prefer‐
ence. 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 connec‐
tion 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 privi‐
leged 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 listening 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 rec‐
ognized at the beginning of a line. The escape character followed by a dot (‘.’) closes the connec‐
tion; followed by control-Z suspends the connection; and followed by itself sends the escape charac‐
ter once. Setting the character to “none” disables any escapes and makes the session fully transpar‐
ent.
-F configfile
Specifies an alternative per-user configuration file. If a configuration file is given on the com‐
mand 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 placing itself in the back‐
ground.
-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 providing 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 identity 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 con‐
nection 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 privi‐
leged ports. By default, the local port is bound in accordance with the GatewayPorts setting. How‐
ever, 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 version 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 shadows.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 speci‐
fying 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 configura‐
tion file.
-Q cipher | cipher-auth | mac | kex | key
Queries ssh for the algorithms supported for the specified version 2. The available features are:
cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenti‐
cated 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 forwarded 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 specified 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. Multiple -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 debug‐
ging connection, authentication, and configuration problems. Multiple -v options increase the ver‐
bosity. 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 tun‐
nel 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 permissions 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 moni‐
toring.
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 SECURITY exten‐
sion 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 configu‐
ration 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 authentication, public
key authentication, challenge-response authentication, and password authentication. Authentication methods
are tried in the order specified above, though protocol 2 has a configuration 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 inse‐
cure and should be disabled if security is desired.]
Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosys‐
tems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryp‐
tion key from the encryption key. The idea is that each user creates a public/private key pair for authenti‐
cation 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 authentication. The client
proves that it has access to the private key and the server checks that the corresponding 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 (protocol 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 conventional ~/.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 cer‐
tification 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 authentication 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 password 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 authen‐
tication to prevent server spoofing or man-in-the-middle 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 command, or
logs into the machine and gives the user a normal shell on the remote machine. All communication 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 characters noted
below.
If no pseudo-tty has been allocated, the session is transparent and can be used to reliably transfer binary
data. On most systems, setting the escape character to “none” will also make the session 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 special. The escape
character can be changed in configuration files using the EscapeChar configuration directive or on the com‐
mand 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 command line
or in a configuration file. One possible application of TCP forwarding is a secure connection 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 connection.
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 connec‐
tion 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 auto‐
matically forwarded to the remote side in such a way that any X11 programs started from the shell (or com‐
mand) 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 connec‐
tions carry this cookie and replace it by the real cookie when the connection is opened. The real authenti‐
cation 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 presented 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 support 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 connecting 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 fingerprint
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 setups, 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 automatically 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 terminal if it
was run from a terminal. If ssh does not have a terminal associated 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 call‐
ing ssh from a .xsession or related script. (Note that on some machines it may be nec‐
essary 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 contains 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 executed. 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 information, 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 acces‐
sible 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 authentication infor‐
mation. 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 read‐
able 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 sys‐
temwide 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 authentication with‐
out 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 authentication.
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 administrator to con‐
tain 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 Campbell, 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 versions 1.5 and 2.0.
BSD November 12, 2016 BSD
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
Konfiguration
ssh Daemon
ssh in der Praxis
Auch wenn das Passwort bei ssh verschlüsselt übertragen wird, lohnt ein Blick auf die Alternative RSA/DSA-Authentifizierung. Bei dieser Variante muss gar kein Passwort über das Netz übertragen werden. Normalerweise muss der User beim Zugriff via ssh auf einen entfernten Rechner sein Passwort eingeben. Zum einen wollen wir aus Sicherheitsgründen darauf verzichten und zum anderen kann dies doch auf Dauer doch als etwas nervig empfunden werden. Einfacher geht dies über asymetrische Schlüssel.
Erzeugung eines Schlüssel
Als erstes erzeugen wir uns einen Schlüssel für die Authentifizierung:
[django@host ~]$ ssh-keygen -b 4096 -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/home/django/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/django/.ssh/id_rsa. Your public key has been saved in /home/django/.ssh/id_rsa.pub. The key fingerprint is: 2b:83:69:f2:76:e8:c9:8b:cf:34:c8:c2:ae:2b:e1:ee django@host.nausch.org
$ ssh-keygen -t ed25519 -o -a 100 -C django@nausch.org -f ~/.ssh/id_ed25519_dmz
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 Dateien:
[django@host .ssh]$ ll insgesamt 24 -rw------- 1 django django 3311 22. Apr 22:11 id_rsa -rw-r--r-- 1 django django 748 22. Apr 22:11 id_rsa.pub
id_rsa enthält den privaten Schlüssel und sollte auf keinen Fall weitergegeben werden und darf auch nur für den Nutzer selbst lesbar sein! id_rsa.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:
[django@host .ssh]$ scp /home/django/.ssh/id_rsa.pub zielhost:/home/django/.ssh/id_rsa.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:
[django@zielhost .ssh]$ cat id_rsa.pub >> authorized_keys
Zu guter Letzt passen wir noch die Berechtigungen an und löschen die nicht mehr benötigte id_rsa.pub
[django@zielhost .ssh]$ chmod 600 authorized_keys [django@zielhost .ssh]$ rm id_rsa.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.
$ ssh-copy-id -i ~/.ssh/id_rsa.pub testhost.intra.nausch.org
Die Angabe ~/.ssh/id_rsa.pub entspricht dabei dem Public-Key und testhost.intra.nausch.org
dem gewünschten Zielhost.
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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