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 port] [-Q query_option] [-R address]
[-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
[command [argument ...]]
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 se‐
cure encrypted communications between two untrusted hosts over an inse‐
cure 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 destination, which may be spec‐
ified 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 in‐
stead of a login shell. A complete command line may be specified as
command, or it may have additional arguments. If supplied, the argu‐
ments 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
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 mater‐
ial 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 alternative 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 con‐
nect 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 ad‐
dress.
-C Requests compression of all data (including stdin, stdout,
stderr, and data for forwarded X11, TCP and Unix‐domain connec‐
tions). The compression algorithm is the same used by gzip(1).
Compression is desirable on modem lines and other slow connec‐
tions, but will only slow down things on fast networks. The de‐
fault value can be set on a host‐by‐host basis in the configura‐
tion 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 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 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 ‘*’ indi‐
cates 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; 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 ses‐
sion fully transparent.
-F configfile
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.
If set to “none”, no configuration 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 im‐
plies -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 it‐
self in the background. Refer 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 spec‐
ified on the master 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 possible to have
multiple -i options (and multiple identities specified in con‐
figuration files). If no certificates have been explicitly
specified by the CertificateFile directive, ssh will also try to
load certificate information from the filename obtained by ap‐
pending ‐cert.pub to identity filenames.
-J destination
Connect to the target host by first making a ssh connection to
the jump host described by destination 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.
Note that configuration directives supplied on the command‐line
generally apply to the destination 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 al‐
locating 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 ‘*’ indi‐
cates that the port should be available from all interfaces.
-l login_name
Specifies the user to log in as on the remote machine. This
also may be specified on a per‐host basis in the configuration
file.
-M Places the ssh client into “master” mode for connection sharing.
Multiple -M options places ssh into “master” mode but with con‐
firmation required using ssh‐askpass(1) before each operation
that changes the multiplexing state (e.g. opening a new ses‐
sion). Refer to the description of ControlMaster in
ssh_config(5) for details.
-m mac_spec
A comma‐separated list of MAC (message authentication code) al‐
gorithms, specified in order of preference. See the MACs key‐
word in ssh_config(5) for more information.
-N Do not execute a remote command. This is useful for just for‐
warding 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
common trick is to use this to run X11 programs on a remote ma‐
chine. 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 pro‐
gram will be put in the background. (This does not work if ssh
needs to ask for a password or passphrase; see also the -f op‐
tion.) 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 master process is running), “forward” (request
forwardings without command execution), “cancel” (cancel for‐
wardings), “exit” (request the master to exit), and “stop” (re‐
quest the master to stop accepting further multiplexing re‐
quests).
-o option
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
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 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 encryp‐
tion), help (supported query terms for use with the -Q flag),
mac (supported message integrity codes), kex (key exchange algo‐
rithms), kex‐gss (GSSAPI key exchange algorithms), key (key
types), key‐cert (certificate key types), key‐plain (non‐cer‐
tificate key types), key‐sig (all key types and signature algo‐
rithms), protocol‐version (supported SSH protocol versions), and
sig (supported signature algorithms). Alternatively, any key‐
word from ssh_config(5) or sshd_config(5) that takes an algo‐
rithm list may be used as an alias for the corresponding
query_option.
-q Quiet mode. Causes most warning and diagnostic messages 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
-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 connec‐
tions 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 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 specify‐
ing a bind_address. An empty bind_address, or the address ‘*’,
indicates that the remote socket should listen on all inter‐
faces. Specifying a remote bind_address will only succeed if
the server’s GatewayPorts option is enabled (see
sshd_config(5)).
If the port argument is ‘0’, the listen port will be dynamically
allocated on the server and reported to the client at run time.
When used together with ‐O forward, the allocated port will be
printed to the standard output.
-S ctl_path
Specifies the location of a control socket for connection 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 facilitate the use of SSH as a secure trans‐
port 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
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, authentica‐
tion, and configuration problems. Multiple -v options increase
the verbosity. The maximum is 3.
-W host:port
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, 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) de‐
vices 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 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 dis‐
play 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 ex‐
tension restrictions by default. Refer to the ssh -Y option and
the ForwardX11Trusted directive in ssh_config(5) for more infor‐
mation.
(Debian‐specific: X11 forwarding is not subjected to X11 SECU‐
RITY extension restrictions by default, because too many pro‐
grams currently crash in this mode. Set the ForwardX11Trusted
option to “no” to restore 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
subjected to the X11 SECURITY extension controls.
(Debian‐specific: In the default configuration, this option is
equivalent to -X, since ForwardX11Trusted defaults to “yes” as
described above. Set the ForwardX11Trusted option to “no” to
restore the upstream behaviour. This may change in future de‐
pending on client‐side improvements.)
-y Send log information using the syslog(3) system module. By de‐
fault this information is sent to stderr.
ssh may additionally obtain configuration data from a per‐user configu‐
ration 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 authentica‐
tion, host‐based authentication, public key authentication, keyboard‐in‐
teractive authentication, 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 machine, 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 login to be
permitted. This authentication method closes security holes due to IP
spoofing, DNS spoofing, and routing spoofing. [Note to the administra‐
tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in gen‐
eral, are inherently insecure and should be disabled if security is de‐
sired.]
Public key authentication works as follows: The scheme is based on pub‐
lic‐key cryptography, using cryptosystems where encryption and decryp‐
tion are done using separate keys, and it is unfeasible to derive the
decryption key from the encryption key. The idea is that each user cre‐
ates a public/private key pair for authentication purposes. The server
knows the public key, and only the user knows the private key. ssh im‐
plements 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) con‐
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 server may inform the client of errors that prevented public key au‐
thentication from succeeding after authentication completes using a dif‐
ferent 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 (authentica‐
tor‐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 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/pri‐
vate keys. See the CERTIFICATES section of ssh‐keygen(1) for more in‐
formation.
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 arbitrary "challenge" text and prompts for a response, possibly mul‐
tiple times. Examples of keyboard‐interactive authentication include
BSD Authentication (see login.conf(5)) and PAM (some non‐OpenBSD sys‐
tems).
Finally, if other authentication methods fail, ssh prompts the user for
a password. The password is sent to the remote host for checking; how‐
ever, since all communications are encrypted, the password cannot be
seen by 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 au‐
thentication 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 ei‐
ther 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 re‐
mote 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
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 char‐
acter can be changed in configuration files using the EscapeChar config‐
uration 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 sup‐
ports it).
~C Open command line. Currently this allows the addition of port
forwardings using the -L, -R and -D options (see above). It
also allows the cancellation of existing port‐forwardings with
-KL[bind_address:]port for local, -KR[bind_address:]port for re‐
mote and -KD[bind_address:]port for dynamic port‐forwardings.
!command allows the user to execute a local command if the
PermitLocalCommand option is enabled in ssh_config(5). Basic
help is available, using the -h option.
~R Request rekeying of the connection (only useful if the peer sup‐
ports 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 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 for an IRC
client, even though the IRC server it connects to does not directly sup‐
port 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 spec‐
ified 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 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 configura‐
tion 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 ma‐
chine. For this purpose, it will generate a random authorization
cookie, store it in Xauthority on the server, and verify that any for‐
warded 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 re‐
mote 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 deter‐
mined using ssh‐keygen(1):
$ ssh‐keygen ‐l ‐f /etc/ssh/ssh_host_rsa_key
If the fingerprint is already known, it can be matched and the key can
be accepted or rejected. If only legacy (MD5) fingerprints for the
server are available, the ssh‐keygen(1) -E option may be used to down‐
grade the fingerprint algorithm to match.
Because of the difficulty of comparing host keys just by looking at fin‐
gerprint 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 se‐
curely. The sshd_config(5) configuration option PermitTunnel controls
whether the server supports this, and at what level (layer 2 or 3 traf‐
fic).
The following example would connect client network 10.0.50.0/24 with re‐
mote 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 gate‐
way 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 perma‐
nent 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 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‐
ing 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
redirect the input from /dev/null to make this
work.)
SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass
program. 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 communicate with the agent.
SSH_CONNECTION Identifies the client and server ends of the con‐
nection. The variable contains four space‐sepa‐
rated values: client IP address, client port num‐
ber, 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 com‐
mand. 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 con‐
tain a pathname to a file that lists the authenti‐
cation methods successfully used when the session
was established, including 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 daemon passes the value on to new con‐
nections).
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 al‐
lowed 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 others.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows
host‐based authentication without permitting login with
rlogin/rsh.
~/.ssh/
This directory is the default location for all user‐specific
configuration and authentication information. There is no gen‐
eral requirement to keep the entire contents of this directory
secret, but the recommended permissions are read/write/execute
for the user, and not accessible by others.
~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be
used for logging in as this user. The format of this file is
described in the sshd(8) manual page. This file is not highly
sensitive, but the recommended permissions are read/write for
the user, and not accessible by others.
~/.ssh/config
This is the per‐user configuration file. The file format and
configuration options are described in ssh_config(5). Because
of the potential for abuse, this file must have strict permis‐
sions: read/write for the user, and not writable by others. It
may be group‐writable provided that the group in question con‐
tains 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 con‐
tain sensitive data and should be readable by the user but not
accessible by others (read/write/execute). ssh will simply ig‐
nore 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
sensitive and can (but need not) be readable by anyone.
~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged
into that are not already in the 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 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 configura‐
tion options are described in ssh_config(5).
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and are
used for host‐based authentication.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be pre‐
pared 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 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 pro‐
tocol versions 1.5 and 2.0.
Debian November 28, 2022 SSH(1)