SSH(1)
OpenSSH remote login client. Static snapshot from Ubuntu 24.04.5 LTS.
SSH(1) General Commands Manual SSH(1)
NAME
ssh — OpenSSH remote login client
SYNOPSIS
ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
[-c cipher_spec] [-D [bind_address:]port] [-E log_file] [-e escape_char]
[-F configfile] [-I pkcs11] [-i identity_file] [-J destination]
[-L address] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option]
[-P tag] [-p port] [-R address] [-S ctl_path] [-W host:port]
[-w local_tun[:remote_tun]] destination [command [argument ...]]
ssh [-Q query_option]
DESCRIPTION
ssh (SSH client) is a program for logging into a remote machine and for exe‐
cuting commands on a remote machine. It is intended to provide secure en‐
crypted communications between two untrusted hosts over an insecure network.
X11 connections, arbitrary TCP ports and Unix-domain sockets can also be for‐
warded over the secure channel.
ssh connects and logs into the specified destination, which may be specified
as either [user@]hostname or a URI of the form ssh://[user@]hostname[:port].
The user must prove their identity to the remote machine using one of several
methods (see below).
If a command is specified, it will be executed on the remote host instead of a
login shell. A complete command line may be specified as command, or it may
have additional arguments. If supplied, the arguments will be appended to the
command, separated by spaces, before it is sent to the server to be executed.
The options are as follows:
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
-A Enables forwarding of connections from an authentication agent such as
ssh-agent(1). This can also be specified on a per-host basis in a
configuration file.
Agent forwarding should be enabled with caution. Users with the abil‐
ity to bypass file permissions on the remote host (for the agent's
Unix-domain socket) can access the local agent through the forwarded
connection. An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent. A safer al‐
ternative may be to use a jump host (see -J).
-a Disables forwarding of the authentication agent connection.
-B bind_interface
Bind to the address of bind_interface before attempting to connect to
the destination host. This is only useful on systems with more than
one address.
-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 com‐
pression algorithm is the same used by gzip(1). Compression is desir‐
able 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
in ssh_config(5).
-c cipher_spec
Selects the cipher specification for encrypting the session.
cipher_spec is a comma-separated list of ciphers listed in order of
preference. See the Ciphers keyword in ssh_config(5) for more infor‐
mation.
-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, op‐
tionally bound to the specified bind_address. Whenever a connection
is made to this port, the connection is forwarded over the secure
channel, and the application protocol is then used to determine where
to connect to from the remote machine. Currently the SOCKS4 and
SOCKS5 protocols are supported, and ssh will act as a SOCKS server.
Only root can forward privileged ports. Dynamic port forwardings can
also be specified in the configuration file.
IPv6 addresses can be specified by enclosing the address in square
brackets. Only the superuser can forward privileged ports. By de‐
fault, 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” in‐
dicates 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 recognized 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 followed by itself
sends the escape character once. Setting the character to “none” dis‐
ables any escapes and makes the session fully transparent.
-F configfile
Specifies an alternative per-user configuration file. If a configura‐
tion 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. If set to “none”, no con‐
figuration files will be read.
-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. Re‐
fer to the description of ForkAfterAuthentication in ssh_config(5) for
details.
-G Causes ssh to print its configuration after evaluating Host and Match
blocks and exit.
-g Allows remote hosts to connect to local forwarded ports. If used on a
multiplexed connection, then this option must be specified on the mas‐
ter process.
-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate with
a PKCS#11 token providing keys for user authentication.
-i identity_file
Selects a file from which the identity (private key) for public key
authentication is read. You can also specify a public key file to use
the corresponding private key that is loaded in ssh-agent(1) when the
private key file is not present locally. The default is
~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk and ~/.ssh/id_dsa. Identity files may also be
specified on a per-host basis in the configuration file. It is possi‐
ble to have multiple -i options (and multiple identities specified in
configuration files). If no certificates have been explicitly speci‐
fied by the CertificateFile directive, ssh will also try to load cer‐
tificate information from the filename obtained by appending -cert.pub
to identity filenames.
-J destination
Connect to the target host by first making an ssh connection to the
jump host described by destination and then establishing a TCP for‐
warding 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. Note that configuration
directives supplied on the command-line generally apply to the desti‐
nation host and not any specified jump hosts. Use ~/.ssh/config to
specify configuration for jump hosts.
-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 Unix socket on the
local (client) host are to be forwarded 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, optionally 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 configuration 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 connection sharing. Mul‐
tiple -M options places ssh into “master” mode but with confirmation
required using ssh-askpass(1) before each operation that changes the
multiplexing state (e.g. opening a new session). Refer to the de‐
scription of ControlMaster in ssh_config(5) for details.
-m mac_spec
A comma-separated list of MAC (message authentication code) algo‐
rithms, specified in order of preference. See the MACs keyword in
ssh_config(5) for more information.
-N Do not execute a remote command. This is useful for just forwarding
ports. Refer to the description of SessionType in ssh_config(5) for
details.
-n Redirects stdin from /dev/null (actually, prevents reading from
stdin). This must be used when ssh is run in the background. A com‐
mon trick is to use this to run X11 programs on a remote machine. For
example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shad‐
ows.cs.hut.fi, and the X11 connection will be automatically forwarded
over an encrypted channel. The ssh program will be put in the back‐
ground. (This does not work if ssh needs to ask for a password or
passphrase; see also the -f option.) Refer to the description of
StdinNull in ssh_config(5) for details.
-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 mas‐
ter process is running), “forward” (request forwardings without com‐
mand execution), “cancel” (cancel forwardings), “exit” (request the
master to exit), and “stop” (request the master to stop accepting fur‐
ther 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
CASignatureAlgorithms
CertificateFile
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EnableEscapeCommandline
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostbasedAcceptedAlgorithms
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
SendEnv
ServerAliveInterval
ServerAliveCountMax
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation
-P tag Specify a tag name that may be used to select configuration in
ssh_config(5). Refer to the Tag and Match keywords in ssh_config(5)
for more information.
-p port
Port to connect to on the remote host. This can be specified on a
per-host basis in the configuration file.
-Q query_option
Queries for the algorithms supported by one of the following features:
cipher (supported symmetric ciphers), cipher-auth (supported symmetric
ciphers that support authenticated encryption), help (supported query
terms for use with the -Q flag), mac (supported message integrity
codes), kex (key exchange algorithms), kex-gss (GSSAPI key exchange
algorithms), key (key types), key-ca-sign (valid CA signature algo‐
rithms for certificates), key-cert (certificate key types), key-plain
(non-certificate key types), key-sig (all key types and signature al‐
gorithms), protocol-version (supported SSH protocol versions), and sig
(supported signature algorithms). Alternatively, any keyword from
ssh_config(5) or sshd_config(5) that takes an algorithm list may be
used as an alias for the corresponding query_option.
-q Quiet mode. Causes most warning and diagnostic messages to be sup‐
pressed.
-R [bind_address:]port:host:hostport
-R [bind_address:]port:local_socket
-R remote_socket:host:hostport
-R remote_socket:local_socket
-R [bind_address:]port
Specifies that connections to the given TCP port or Unix socket on the
remote (server) host are to be forwarded to 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 from the local machine to either an
explicit destination specified by host port hostport, or local_socket,
or, if no explicit destination was specified, ssh will act as a SOCKS
4/5 proxy and forward connections to the destinations requested by the
remote SOCKS client.
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 ad‐
dress 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. Specifying a
remote bind_address will only succeed if the server's GatewayPorts op‐
tion is enabled (see sshd_config(5)).
If the port argument is ‘0’, the listen port will be dynamically allo‐
cated 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 de‐
scription of ControlPath and ControlMaster in ssh_config(5) for de‐
tails.
-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. Refer to the description of SessionType in ssh_config(5) for
details.
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal allocation. This can be used to execute arbi‐
trary 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 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, though these can be
overridden in the configuration 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 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 will be set to the default tunnel
mode, which is “point-to-point”. If a different Tunnel forwarding
mode it desired, then it should be specified before -w.
-X Enables X11 forwarding. This can also be specified on a per-host ba‐
sis 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 autho‐
rization database) can access the local X11 display through the for‐
warded 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. Refer to the ssh -Y option and the
ForwardX11Trusted directive in ssh_config(5) for more information.
(Debian-specific: X11 forwarding is not subjected to X11 SECURITY ex‐
tension restrictions by default, because too many programs currently
crash in this mode. Set the ForwardX11Trusted option to “no” to re‐
store the upstream behaviour. This may change in future depending on
client-side improvements.)
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not sub‐
jected to the X11 SECURITY extension controls.
(Debian-specific: In the default configuration, this option is equiva‐
lent to -X, since ForwardX11Trusted defaults to “yes” as described
above. Set the ForwardX11Trusted option to “no” to restore the up‐
stream behaviour. This may change in future depending on client-side
improvements.)
-y Send log information using the syslog(3) system module. By default
this information is sent to stderr.
ssh may additionally obtain configuration data from a per-user configuration
file and a system-wide configuration file. The file format and configuration
options are described in ssh_config(5).
AUTHENTICATION
The OpenSSH SSH client supports SSH protocol 2.
The methods available for authentication are: GSSAPI-based authentication,
host-based authentication, public key authentication, keyboard-interactive au‐
thentication, and password 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 ma‐
chine, the user is non-root 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 lo‐
gin 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 in‐
herently 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 us‐
ing 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 protocol
automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms. The
HISTORY section of ssl(8) (on non-OpenBSD systems, see
http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) 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 server may inform the client of errors that prevented public key authenti‐
cation from succeeding after authentication completes using a different
method. These may be viewed by increasing the LogLevel to DEBUG or higher
(e.g. by using the -v flag).
The user creates their key pair by running ssh-keygen(1). This stores the
private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519 (Ed25519),
~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or ~/.ssh/id_rsa (RSA)
and stores the public key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub
(ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA),
~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (authenticator-
hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's home directory. The
user should then copy the public key to ~/.ssh/authorized_keys in their 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 certifi‐
cate authentication: instead of a set of public/private keys, signed certifi‐
cates are used. This has the advantage that a single trusted certification
authority can be used in place of many public/private keys. See the CERTIFI‐
CATES 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.
Keyboard-interactive authentication works as follows: The server sends an ar‐
bitrary "challenge" text and prompts for a response, possibly multiple times.
Examples of keyboard-interactive 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 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 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 charac‐
ters 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 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 if the peer supports
it).
~C Open command line. Currently this allows the addition of port for‐
wardings 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 al‐
lows the user to execute a local command if the PermitLocalCommand op‐
tion is enabled in ssh_config(5). Basic help is available, using the
-h 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 a secure channel can be specified
either on the command line or in a configuration file. One possible applica‐
tion of TCP forwarding is a secure connection to a mail server; another is go‐
ing through firewalls.
In the example below, we look at encrypting communication for an IRC client,
even though the IRC server it connects to does not directly support encrypted
communication. This works as follows: the user connects to the remote host
using ssh, specifying the ports to be used to forward the connection. After
that it is possible to start the program locally, and ssh will encrypt and
forward the connection to the remote server.
The following example tunnels an IRC session from the client to an IRC server
at “server.example.com”, joining channel “#users”, nickname “pinky”, using the
standard IRC port, 6667:
$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
$ irc -c '#users' pinky IRC/127.0.0.1
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 program
which is going to use the tunnel. If no connections are made within the time
specified, ssh will exit.
X11 FORWARDING
If the ForwardX11 variable is set to “yes” (or see the description of the -X,
-x, and -Y options above) and the user is using X11 (the DISPLAY environment
variable is set), the connection to the X11 display is automatically forwarded
to the remote side in such a way that any X11 programs started from the shell
(or command) will go through the encrypted channel, and the connection to the
real X server will be made from the local machine. The user should not manu‐
ally set DISPLAY. Forwarding of X11 connections can be configured on the com‐
mand line or in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with a dis‐
play number greater than zero. This is normal, and happens because ssh cre‐
ates 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 Xau‐
thority 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 con‐
nection 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 ac‐
cepted or rejected. If only legacy (MD5) fingerprints for the server are
available, the ssh-keygen(1) -E option may be used to downgrade the finger‐
print algorithm to match.
Because of the difficulty of comparing host keys just by looking at finger‐
print 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 pro‐
duces, a user can easily find out that the host key has changed when a com‐
pletely different pattern is displayed. Because these patterns are not unam‐
biguous 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 avail‐
able: 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.ex‐
ample.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” indi‐
cates the host where the shell runs, and ‘n’ is an inte‐
ger ≥ 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 calling 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_ASKPASS_REQUIRE Allows further control over the use of an askpass pro‐
gram. If this variable is set to “never” then ssh will
never attempt to use one. If it is set to “prefer”,
then ssh will prefer to use the askpass program instead
of the TTY when requesting passwords. Finally, if the
variable is set to “force”, then the askpass program
will be used for all passphrase input regardless of
whether DISPLAY is set.
SSH_AUTH_SOCK Identifies the path of a Unix-domain socket used to com‐
municate 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 ad‐
dress, 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.
SSH_TUNNEL Optionally set by sshd(8) to contain the interface names
assigned if tunnel forwarding was requested by the
client.
SSH_USER_AUTH Optionally set by sshd(8), this variable may contain a
pathname to a file that lists the authentication methods
successfully used when the session was established, in‐
cluding any public keys that were used.
TZ This variable is set to indicate the present time zone
if it was set when the daemon was started (i.e. the dae‐
mon 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).
FILES
~/.rhosts
This file is used for host-based authentication (see above). On some
machines this file may need to be world-readable if the user's home
directory is on an NFS partition, because sshd(8) reads it as root.
Additionally, this file must be owned by the user, and must not have
write permissions for anyone else. The recommended permission for
most machines is read/write for the user, and not accessible by oth‐
ers.
~/.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 configu‐
ration and authentication information. There is no general require‐
ment 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 rec‐
ommended 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 configu‐
ration options are described in ssh_config(5). Because of the poten‐
tial for abuse, this file must have strict permissions: read/write for
the user, and not writable by others. It may be group-writable pro‐
vided that the group in question contains only the user.
~/.ssh/environment
Contains additional definitions for environment variables; see
“ENVIRONMENT”, above.
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ecdsa_sk
~/.ssh/id_ed25519
~/.ssh/id_ed25519_sk
~/.ssh/id_rsa
Contains the private key for authentication. These files contain sen‐
sitive data and should be readable by the user but not accessible by
others (read/write/execute). ssh will simply ignore a private key
file if it is accessible by others. It is possible to specify a
passphrase when generating the key which will be used to encrypt the
sensitive part of this file using AES-128.
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ecdsa_sk.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_ed25519_sk.pub
~/.ssh/id_rsa.pub
Contains the public key for authentication. These files are not sen‐
sitive and can (but need not) be readable by anyone.
~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged into
that are not already in the systemwide list of known host keys. See
sshd(8) for further details of the format of this file.
~/.ssh/rc
Commands in this file are executed by ssh when the user logs in, just
before the user's shell (or command) is started. See the sshd(8) man‐
ual 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 without permitting login with rlogin/rsh.
/etc/ssh/ssh_config
Systemwide configuration file. The file format and configuration op‐
tions 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.
/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 ma‐
chines 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) man‐
ual page for more information.
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-argv0(1), ssh-keygen(1),
ssh-keyscan(1), tun(4), 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.
Debian October 11, 2023 SSH(1)
Generated 2026-11-02 from the manual installed on node.spa-dev-dh.xyz. Original copyright and license terms apply.