ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
[-D [bind_address:]port] [-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]
[-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
[-w local_tun[:remote_tun]] [user@]hostname [command]
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
If command is specified, it is executed on the remote host instead of a
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
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
-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
ferent 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 interoperabil-
ity with legacy protocol 1 implementations that do not support
the 3des cipher. Its use is strongly discouraged due to crypto-
graphic 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 configura-
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 '*' indi-
cates that the port should be available from all interfaces.
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
Specifies an alternative per-user configuration file. If a con-
figuration 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
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 server.
-k Disables forwarding (delegation) of GSSAPI credentials to the
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 con-
nection 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 spec-
ified 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. How-
ever, an explicit bind_address may be used to bind the connection
to a specific address. The bind_address of ``localhost'' indi-
cates that the listening port be bound for local use only, while
an empty address or '*' indicates that the port should be avail-
able from all interfaces.
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 shar-
ing. 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
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 for-
warding 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
forwardings without command execution), ``exit'' (request the
master to exit), and ``stop'' (request the master to stop accept-
ing further multiplexing requests).
Can be used to give options in the format used in the configura-
tion 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
Port to connect to on the remote host. This can be specified on
a per-host basis in the configuration file.
-q Quiet mode. Causes most warning and diagnostic messages to be
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 connec-
tion 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 specify-
ing 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 shar-
ing, 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 appli-
-v Verbose mode. Causes ssh to print debugging messages about its
progress. This is helpful in debugging connection, authentica-
tion, and configuration problems. Multiple -v options increase
the verbosity. The maximum is 3.
Requests that standard input and output on the client be for-
warded to host on port over the secure channel. Implies -N, -T,
ExitOnForwardFailure and ClearAllForwardings and works with Pro-
tocol version 2 only.
Requests tunnel device forwarding with the specified tun(4)
devices between the client (local_tun) and the server
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 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
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings 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 configura-
tion file and a system-wide configuration file. The file format and con-
figuration options are described in ssh_config(5).
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
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 authentica-
tion 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 pub-
lic-key cryptography, using cryptosystems 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 authentication purposes. The server knows
the public key, and only the user knows the private key. ssh implements
public key authentication protocol automatically, using one of the DSA,
ECDSA or RSA algorithms. Protocol 1 is restricted to using only RSA
keys, but protocol 2 may use any. The HISTORY section of ssl(8) (on non-
OpenBSD systems, see
tains 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), 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), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home direc-
tory. 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 cer-
tificate 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 informa-
someone listening on the network.
ssh automatically maintains and checks a database containing identifica-
tion 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 iden-
tification 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, 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.
When a pseudo-terminal has been requested, ssh supports a number of func-
tions 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 charac-
ter can be changed in configuration files using the EscapeChar configura-
tion directive or on the command line by the -e option.
The supported escapes (assuming the default '~') are:
~^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).
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 con-
nections 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 mat-
ter 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.
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 pro-
grams 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
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 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 traf-
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.
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
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 perma-
nent VPNs are better provided by tools such as ipsecctl(8) and
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 sys-
tems that use this variable.
MAIL Set to the path of the user's mailbox.
PATH Set to the default PATH, as specified when compil-
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 necessary to redi-
rect 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 con-
nection. The variable contains four space-sepa-
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 connec-
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).
This file is used for host-based authentication (see above). On
some machines this file may need to be world-readable if the
user's home directory is on an NFS partition, because sshd(8)
reads it as root. Additionally, this file must be owned by the
user, and must not have write permissions for anyone else. The
recommended permission for most machines is read/write for the
user, and not accessible by others.
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
This directory is the default location for all user-specific con-
figuration and authentication information. There is no general
requirement to keep the entire contents of this directory secret,
but the recommended permissions are read/write/execute for the
user, and not accessible by others.
Lists the public keys (DSA/ECDSA/RSA) that can be used for log-
ging 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.
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 accessible by others. It may be
group-writable provided that the group in question contains only
Contains additional definitions for environment variables; see
Contains the public key for authentication. These files are not
sensitive and can (but need not) be readable by anyone.
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
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.
This file is for host-based authentication (see above). It
should only be writable by root.
This file is used in exactly the same way as hosts.equiv, but
allows host-based authentication without permitting login with
Systemwide configuration file. The file format and configuration
options are described in ssh_config(5).
These three 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.
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.
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.
ssh exits with the exit status of the remote command or with 255 if an
The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
Generic Message Exchange Authentication for the Secure Shell Protocol
(SSH), RFC 4256, 2006.
The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
Protocol, RFC 4345, 2006.
Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol, RFC 4419, 2006.
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
RFC 5656, 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).
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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD September 19, 2017 BSD
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