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
If command is specified, it is executed on the remote host instead of a login shell.
-1 Forces ssh to try protocol version 1 only.
-2 Forces ssh to try protocol version 2 only.
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
-A Enables forwarding of the authentication agent connection. This can also be specified on a per-
host basis in a configuration file.
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions
on the remote host (for the agent's UNIX-domain socket) can access the local agent through the
forwarded connection. An attacker cannot obtain key material from the agent, however they can
perform operations on the keys that enable them to authenticate using the identities loaded into
-a Disables forwarding of the authentication agent connection.
Use bind_address on the local machine as the source address of the connection. Only useful on
systems with more than one address.
-C Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and
TCP connections). The compression algorithm is the same used by gzip(1), and the “level” can be
controlled by the CompressionLevel option for protocol version 1. 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
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher. The supported values are “3des”,
“blowfish”, and “des”. 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different
keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is
much faster than 3des. des is only supported in the ssh client for interoperability with legacy
protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged
due to cryptographic weaknesses. The default is “3des”.
For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of
preference. See the Ciphers keyword in ssh_config(5) for more information.
Specifies a local “dynamic” application-level port forwarding. This works by allocating a socket
to listen to port on the local side, optionally bound to the specified bind_address. Whenever a
connection is made to this port, the connection is forwarded over the secure channel, and the
application protocol is then used to determine where to connect to from the remote machine.
Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only
root can forward privileged ports. Dynamic port forwardings can also be specified in the
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
Sets the escape character for sessions with a pty (default: ‘~’). The escape character is only
recognized at the beginning of a line. The escape character followed by a dot (‘.’) closes the
connection; followed by control-Z suspends the connection; and followed by itself sends the escape
character once. Setting the character to “none” disables any escapes and makes the session fully
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
-g Allows remote hosts to connect to local forwarded ports.
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the
user's private RSA key.
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 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
-k Disables forwarding (delegation) of GSSAPI credentials to the server.
Specifies that the given port on the local (client) host is to be forwarded to the given host and
port on the remote side. This works by allocating a socket to listen to port on the local side,
optionally bound to the specified bind_address. Whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is made to host port hostport
from the remote machine. Port forwardings can also be specified in the configuration file. IPv6
addresses can be specified by enclosing the address in square brackets. Only the superuser can
forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts
setting. However, an explicit bind_address may be used to bind the connection to a specific
address. The bind_address of “localhost” indicates that the listening port be bound for local use
only, while an empty address or ‘*’ indicates that the port should be available from all
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.
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
-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.)
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), “exit” (request the master to exit), and “stop” (request the master to stop accepting
further multiplexing requests).
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).
Port to connect to on the remote host. This can be specified on a per-host basis in the
-q Quiet mode. Causes most warning and diagnostic messages to be suppressed.
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 braces.
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.
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
-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
debugging connection, authentication, and configuration problems. Multiple -v options increase the
verbosity. The maximum is 3.
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 and works with Protocol
version 2 only.
Requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun)
and the server (remote_tun).
The devices may be specified by numerical ID or the keyword “any”, which uses the next available
tunnel device. If remote_tun is not specified, it defaults to “any”. See also the Tunnel and
TunnelDevice directives in ssh_config(5). If the Tunnel directive is unset, it is set to the
default tunnel mode, which is “point-to-point”.
-X Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the ability to bypass file 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
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
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY
-y Send log information using the syslog(3) system module. By default this information is sent to