Manual: ssh
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 executing commands on a remote machine. It
is intended to provide secure encrypted communications between two untrusted hosts over an insecure network. X11
connections, arbitrary TCP ports and Unix-domain sockets can also be forwarded over the secure channel.
ssh connects and logs into the specified 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 speci‐
fied on a per-host basis in a configuration file.
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the
remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent, however they can perform operations on the keys that
enable them to authenticate using the identities loaded into 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 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 compression algorithm is the same used by gzip(1). Compression is desirable
on modem lines and other slow connections, but will only slow down things on fast networks. The default
value can be set on a host-by-host basis in the configuration files; see the Compression option in
ssh_config(5).
-c cipher_spec
Selects the cipher specification for encrypting the session. cipher_spec is a comma-separated list of ci‐
phers 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 sup‐
ported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forward‐
ings 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 ‘*’ in‐
dicates 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 char‐
acter to “none” disables any escapes and makes the session fully transparent.
-F configfile
Specifies an alternative per-user configuration file. If a configuration file is given on the command line,
the system-wide configuration file (/etc/ssh/ssh_config) will be ignored. The default for the per-user con‐
figuration 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 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. 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 op‐
tion must be specified on the master process.
-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing 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 pri‐
vate key file is not present locally. The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519 and ~/.ssh/id_ed25519_sk. 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 config‐
uration 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 appending -cert.pub to identity file‐
names.
-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 forwarding to the ultimate destination from there. Multiple jump hops may be speci‐
fied separated by comma characters. IPv6 addresses can be specified by enclosing the address in square
brackets. This is a shortcut to specify a ProxyJump configuration directive. Note that configuration direc‐
tives 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 for‐
warded to the given host and port, or Unix socket, on the remote side. This works by allocating a socket to
listen to either a TCP port on the local side, 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 se‐
cure 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” indi‐
cates 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. Multiple -M options places ssh into
“master” mode but with confirmation required using ssh-askpass(1) before each operation that changes the mul‐
tiplexing state (e.g. opening a new session). Refer to the description of ControlMaster in ssh_config(5) for
details.
-m mac_spec
A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference.
See the MACs keyword 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 common trick is to use this to run X11 programs on a remote machine. For example, ssh -n
shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automati‐
cally forwarded over an encrypted channel. The ssh program will be put in the background. (This does not
work if ssh needs to ask for a password or passphrase; see also the -f option.) 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 argu‐
ment 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 forward‐
ings), “proxy” (connect to a running multiplexing master in proxy mode), “exit” (request the master to exit),
and “stop” (request the master to stop accepting further multiplexing requests).
-o option
Can be used to give options in the format used in the configuration file. This is useful for specifying op‐
tions 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
BindInterface
CASignatureAlgorithms
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChannelTimeout
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectTimeout
ConnectionAttempts
ControlMaster
ControlPath
ControlPersist
DynamicForward
EnableEscapeCommandline
EnableSSHKeysign
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
GatewayPorts
GlobalKnownHostsFile
HashKnownHosts
Host
HostKeyAlgorithms
HostKeyAlias
HostbasedAcceptedAlgorithms
HostbasedAuthentication
Hostname
IPQoS
IdentitiesOnly
IdentityAgent
IdentityFile
IgnoreUnknown
Include
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
LogVerbose
MACs
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
ObscureKeystrokeTiming
PKCS11Provider
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
RevokedHostKeys
SecurityKeyProvider
SendEnv
ServerAliveCountMax
ServerAliveInterval
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
SyslogFacility
TCPKeepAlive
Tag
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 algorithms for certifi‐
cates), key-cert (certificate key types), key-plain (non-certificate key types), key-sig (all key types and
signature algorithms), protocol-version (supported SSH protocol versions), and sig (supported signature algo‐
rithms). 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 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 for‐
warded 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 chan‐
nel, 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 address in
square brackets.
By default, TCP listening sockets on the server will be bound to the loopback interface only. This may be
overridden by specifying a bind_address. An empty bind_address, or the address ‘*’, indicates that the re‐
mote 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.
-S ctl_path
Specifies the location of a control socket for connection sharing, or the string “none” to disable connection
sharing. Refer to the description of ControlPath and ControlMaster in ssh_config(5) for details.
-s May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH
as a secure transport for other applications (e.g. sftp(1)). The subsystem is specified as the remote com‐
mand. 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 allo‐
cation, even if ssh has no local tty.
-V Display the version number and exit.
-v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging con‐
nection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maxi‐
mum 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 configu‐
ration 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 de‐
vice. 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 re‐
mote 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. 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 extension restrictions by default, because
too many programs currently crash in this mode. Set the ForwardX11Trusted option to “no” to restore the up‐
stream 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 con‐
trols.
(Debian-specific: In the default configuration, this option is equivalent to -X, since ForwardX11Trusted de‐
faults to “yes” as described above. Set the ForwardX11Trusted option to “no” to restore the upstream behav‐
iour. 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 au‐
thentication, keyboard-interactive 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 con‐
taining the name of the client machine and the name of the user on that machine, the user is considered for login.
Additionally, the server must be able to verify the client's host key (see the description of
/etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes
security holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv,
~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is de‐
sired.]
Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where
encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key from the
encryption key. The idea is that each user creates a public/private key pair for authentication purposes. The
server knows the public key, and only the user knows the private key. ssh implements public key authentication pro‐
tocol automatically, using one of the ECDSA, Ed25519 or 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 ac‐
count.
The server may inform the client of errors that prevented public key authentication from succeeding after authentica‐
tion 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_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_ecdsa.pub (ECDSA),
~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
thenticator-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 corre‐
sponds 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 au‐
thority can be used in place of many public/private 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 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 re‐
mote host for checking; however, since all communications are encrypted, the password cannot be seen by someone lis‐
tening 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-in‐
teractive 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 ses‐
sions when the client has one. The flags -T and -t can be used to override this behaviour.
If a pseudo-terminal has been allocated, the user may use the escape characters noted below.
If no pseudo-terminal has been allocated, the session is transparent and can be used to reliably transfer binary
data. On most systems, setting the escape character to “none” will also make the session transparent even if a tty
is used.
The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have
been closed.
ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below.
The escape character must always follow a newline to be interpreted as special. The escape character can be changed
in configuration files using the EscapeChar configuration directive or on the command line by the -e option.
The supported escapes (assuming the default ‘~’) are:
~. Disconnect.
~^Z Background 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 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 remote 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). Ba‐
sic 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 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 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 chan‐
nel “#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 en‐
crypted channel, and the connection to the real X server will be made from the local machine. The user should not
manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is
normal, and happens because ssh creates a “proxy” X server on the server machine for forwarding the connections over
the encrypted channel.
ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a ran‐
dom authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this
cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent
to the server machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to “yes” (or see the description of the -A and -a options above) and the user is
using an authentication agent, the connection to the agent is automatically forwarded to the remote side.
VERIFYING HOST KEYS
When connecting to a server for the first time, a 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 accepted or rejected. If only legacy (MD5)
fingerprints for the server are available, the ssh-keygen(1) -E option may be used to downgrade the fingerprint algo‐
rithm to match.
Because of the difficulty of comparing host keys just by looking at fingerprint strings, there is also support to
compare host keys visually, using 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 pat‐
tern 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 fin‐
gerprint 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 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 net‐
work, 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” 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 au‐
thorization 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 win‐
dow 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 connection. The variable contains four space-sep‐
arated values: client IP address, client port number, server IP address, and server port num‐
ber.
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 re‐
quested by the client.
SSH_USER_AUTH Optionally set by sshd(8), this variable may contain a pathname to a file that lists the au‐
thentication 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 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. Addi‐
tionally, this file must be owned by the user, and must not have write permissions for anyone else. The rec‐
ommended 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 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.
~/.ssh/authorized_keys
Lists the public keys (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 permis‐
sions 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 permissions: read/write for
the user, and not writable by others. It may be group-writable provided that the group in question contains
only the user.
~/.ssh/environment
Contains additional definitions for environment variables; see “ENVIRONMENT”, above.
~/.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 sensitive data and should be readable by
the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it
is accessible by others. It is possible to specify a passphrase when generating the key which will be used
to encrypt the sensitive part of this file using AES-128.
~/.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 per‐
mitting login with rlogin/rsh.
/etc/ssh/ssh_config
Systemwide configuration file. The file format and configuration options are described in ssh_config(5).
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and are used for host-based authentication.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be prepared by the system administrator to contain the
public host keys of all machines in the organization. It should be world-readable. See sshd(8) for further
details of the format of this file.
/etc/ssh/sshrc
Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is
started. See the sshd(8) manual page for more 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 December 4, 2024 SSH(1)