SSH(1) USER COMMANDS SSH(1)
NAME
ssh - OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c
cipher_spec] [-D[bind_address :]port] [-E log_file] [-e
escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
[-J[user@]host[:port] [-L address] [-l login_name] [-m
mac_spec] [-O ctl_cmd] [-o option] [-p port] [-Q
query_option] [-R address] [-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 provide secure encrypted communications
between two untrusted hosts over an insecure network. X11
connections, arbitrary TCP ports and UNIX-domain sockets 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 (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 connec-
tion. 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 connec-
tion. An attacker cannot obtain key material from the
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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 connec-
tion.
-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, TCP and
UNIX-domain 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'', ``blowf-
ish'', and ``des''. 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 for-
wardings can also be specified in the configuration
file.
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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 recog-
nized at the beginning of a line. The escape character
followed by a dot (`.') closes the connection; fol-
lowed by control-Z suspends the connection; and fol-
lowed by itself sends the escape character once. Set-
ting 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 placing itself in the background.
-G Causes ssh to print its configuration after evaluating
Host and Match blocks and exit.
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-g Allows remote hosts to connect to local forwarded
ports. If used on a multiplexed connection, then this
option must be specified on the master process.
-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 confi-
guration file. It is possible to have multiple -i
options (and multiple identities specified in confi-
guration files). If no certificates have been expli-
citly specified by the CertificateFile directive, ssh
will also try to load certificate information from the
filename obtained by appending -cert.pub to identity
filenames.
-J [user@] host [:port]
Connect to the target host by first making a ssh con-
nection to the jump host and then establishing a TCP
forwarding to the ultimate destination from there.
Multiple jump hops may be specified separated by comma
characters. This is a shortcut to specify a ProxyJump
configuration directive.
-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
-L [bind_address:] port:remote_socket
-L local_socket:host:hostport
-L local_socket:remote_socket
Specifies that connections to the given TCP port or
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Unix socket on the local (client) host are to be for-
warded to the given host and port, or Unix socket, on
the remote side. This works by allocating a socket to
listen to either a TCP port on the local side, option-
ally bound to the specified bind_address, or to a Unix
socket. Whenever a connection is made to the local
port or socket, the connection is forwarded over the
secure channel, and a connection is made to either host
port hostport, or the Unix socket remote_socket, from
the remote machine.
Port forwardings can also be specified in the confi-
guration file. Only the superuser can forward
privileged ports. IPv6 addresses can be specified by
enclosing the address in square brackets.
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 connec-
tion 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
A comma-separated list of MAC (message authentication
code) algorithms, 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.
-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
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-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 pro-
cess. When the -O option is specified, the ctl_cmd
argument is interpreted and passed to the master pro-
cess. Valid commands are: ``check'' (check that the
master process is running), ``forward'' (request for-
wardings 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).
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
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Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
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HostKeyAlgorithms
HostKeyAlias
HostName
IdentitiesOnly
IdentityAgent
IdentityFile
Include
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
Protocol
ProxyCommand
ProxyJump
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ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteForward
RequestTTY
RhostsRSAAuthentication
RSAAuthentication
SendEnv
ServerAliveInterval
ServerAliveCountMax
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
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
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file.
-Q query_option
Queries ssh for the algorithms supported for the speci-
fied version 2. The available features are: cipher
(supported symmetric ciphers), cipher-auth (supported
symmetric ciphers that support authenticated encryp-
tion), mac (supported message integrity codes), kex
(key exchange algorithms), key (key types), key-cert
(certificate key types), key-plain (non-certificate key
types), and protocol-version (supported SSH protocol
versions).
-q Quiet mode. Causes most warning and diagnostic mes-
sages to be suppressed.
-R [bind_address:] port:host:hostport
-R [bind_address:] port:local_socket
-R remote_socket:host:hostport
-R remote_socket:local_socket
Specifies that connections to the given TCP port or
Unix socket on the remote (server) host are to be for-
warded to the given host and port, or Unix socket, on
the local side. This works by allocating a socket to
listen to either a TCP port or to a Unix socket on the
remote side. Whenever a connection is made to this
port or Unix socket, the connection is forwarded over
the secure channel, and a connection is made to either
host port hostport, or local_socket, from the local
machine.
Port forwardings can also be specified in the confi-
guration 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, TCP listening sockets 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. Speci-
fying a remote bind_address will only succeed if the
server's GatewayPorts option is enabled (see
sshd_config(5)) .
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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 out-
put.
-S ctl_path
Specifies the location of a control socket for connec-
tion sharing, or the string ``none'' to disable connec-
tion 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 facilitate the use of SSH as
a secure transport for other applications (e.g.
sftp(1)) . The subsystem is specified as the remote
command.
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal allocation. This can be used to
execute arbitrary screen-based programs on a remote
machine, which can be very useful, e.g. when implement-
ing menu services. Multiple -t options force tty allo-
cation, 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 con-
nection, authentication, and configuration problems.
Multiple -v options increase the verbosity. The max-
imum 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 ClearAllFor-
wardings, though these can be overridden in the confi-
guration file or using -o command line options.
-w local_tun[:remote_tun]
Requests tunnel device forwarding with the specified
tun(4) devices between the client (local_tun) and the
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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 direc-
tives 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 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 forward-
ings are not subjected to the X11 SECURITY 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 confi-
guration 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). Protocol 1 should not be used
and is only offered to support legacy devices. It suffers
from a number of cryptographic weaknesses and doesn't sup-
port many of the advanced features available for protocol 2.
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The methods available for authentication are: GSSAPI-based
authentication, host-based authentication, public key
authentication, challenge-response authentication, and pass-
word authentication. Authentication methods are tried in
the order specified above, though PreferredAuthentications
can be used to change the default order.
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
public/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 pro-
tocol automatically, using one of the DSA, ECDSA, Ed25519 or
RSA algorithms. 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 (DSA), ~/.ssh/id_ecdsa (ECDSA),
~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and
stores the public key in ~/.ssh/identity.pub (protocol 1),
~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA)
in the user's home directory. The user should then copy the
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public key to ~/.ssh/authorized_keys in his/her home direc-
tory 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 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 authentication agent. See
ssh-agent(1) and (optionally) the AddKeysToAgent directive
in ssh_config(5) for more information.
Challenge-response authentication works as follows: The
server sends an arbitrary Qq challenge text, and prompts for
a response. Examples of challenge-response authentication
include Bx 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 authentication 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 in a non-
interactive session or, if no command has been specified,
logs into the machine and gives the user a normal shell as
an interactive session. All communication with the remote
command or shell will be automatically encrypted.
If an interactive session is requested ssh by default will
only request a pseudo-terminal (pty) for interactive
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sessions when the client has one. The flags -T and -t can
be used to override this behaviour.
If a pseudo-terminal has been allocated the user may use the
escape characters noted below.
If no pseudo-terminal 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 command line by the -e option.
The supported escapes (assuming the default `~' ) are:
~. Disconnect.
~^Z Background
~# 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 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 exist-
ing port-forwardings with -KL[bind_address:]port for
local, -KR[bind_address:]port for remote and
-KD[bind_address:]port for dynamic port-forwardings. !
Ns 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
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option.
~R Request rekeying of the connection (only useful 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 for-
warding 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 , 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 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
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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 finger-
print 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):
Dl $ 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. If only legacy (MD5)
fingerprints for the server are available, the ssh-keygen(1)
-E option may be used to downgrade the fingerprint algorithm
to match.
Because of the difficulty of comparing host keys just by
looking at fingerprint strings, there is also support to
compare host keys visually, using randomart. By setting the
VisualHostKey option to ``yes'', a small ASCII graphic gets
displayed on every login to a server, no matter if the ses-
sion 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
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however, a pattern that looks similar to the pattern remem-
bered 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:
Dl $ 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:
Dl $ 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) tun-
nelling using the tun(4) network pseudo-device, allowing two
networks to be joined securely. The sshd_config(5) confi-
guration 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, pro-
vided that the SSH server running on the gateway to the
remote network, at 192.168.1.15, allows it.
On the client:
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SSH(1) USER COMMANDS SSH(1)
# 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 Permi-
tRootLogin 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 wire-
less 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_ASKPASS
If ssh needs a passphrase, it will read the passphrase
from the current terminal if it was run from a
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SSH(1) USER COMMANDS SSH(1)
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 particu-
larly 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 connec-
tion. 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 environ-
ment if the file exists and users are allowed to change
their environment. For more information, see the Per-
mitUserEnvironment option in sshd_config(5).
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. Addi-
tionally, this file must be owned by the user, and must
not have write permissions for anyone else. The recom-
mended permission for most machines is read/write for
the user, and not accessible by others.
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SSH(1) USER COMMANDS SSH(1)
~/.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 information.
There is no general requirement to keep the entire con-
tents 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 acces-
sible by others.
~/.ssh/config
This is the per-user configuration file. The file for-
mat 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 vari-
ables; 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
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SSH(1) USER COMMANDS SSH(1)
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 read-
able 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 informa-
tion.
/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
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
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SSH(1) USER COMMANDS SSH(1)
/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.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should
be prepared by the system administrator 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 informa-
tion.
EXIT STATUS
ssh exits with the exit status of the remote command or with
255 if an error occurred.
SEE ALSO
scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8),
sshd(8)
STANDARDS
C. Lonvick and S. Lehtinen, The Secure Shell (SSH) Protocol
Assigned Numbers, RFC 4250, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Protocol
Architecture, RFC 4251, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Authentica-
tion Protocol, RFC 4252, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Transport
Layer Protocol, RFC 4253, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Connection
Protocol, RFC 4254, January 2006.
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SSH(1) USER COMMANDS SSH(1)
W. Griffin and J. Schlyter, Using DNS to Securely Publish
Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
M. Forssen and F. Cusack, Generic Message Exchange Authenti-
cation for the Secure Shell Protocol (SSH), RFC 4256, Janu-
ary 2006.
P. Remaker and J. Galbraith, The Secure Shell (SSH) Session
Channel Break Extension, RFC 4335, January 2006.
C. Namprempre, T. Kohno and M. Bellare, 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.
W. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group
Exchange for the Secure Shell (SSH) Transport Layer Proto-
col, RFC 4419, March 2006.
R. Thayer and J. Galbraith, The Secure Shell (SSH) Public
Key File Format, RFC 4716, November 2006.
J. Green and D. Stebila, Elliptic Curve Algorithm Integra-
tion in the Secure Shell Transport Layer, RFC 5656, December
2009.
D. Song and A. Perrig, 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 ver-
sions 1.5 and 2.0.
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