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SSH(1) BSD 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 secure encrypted communications between two un‐
trusted 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 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 lo‐
cal 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 au‐
thenticate 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 for‐
warded 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.

-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 allocat‐
ing 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 accor‐
dance 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 fol‐
lowed 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 configuration file is given
on the command line, the system-wide configuration file (/etc/ssh/ssh_config) will be
ignored. The default for the per-user configuration file is ~/.ssh/config. 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 con‐
nection, then this option 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 private key file is not present locally. The de‐
fault 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 configuration files). If no certificates have been
explicitly specified 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 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 di‐
rectives 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 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. How‐
ever, an explicit bind_address may be used to bind the connection to a specific ad‐
dress. 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 avail‐
able 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) be‐
fore each operation that changes the multiplexing 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 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 pro‐
grams on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an
emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over
an encrypted channel. The ssh program will be put in the background. (This does not
work if ssh needs to ask for a password or passphrase; see also the -f option.) 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 speci‐
fied, the ctl_cmd argument is interpreted and passed to the master process. Valid com‐
mands are: “check” (check that the master process is running), “forward” (request for‐
wardings without command execution), “cancel” (cancel forwardings), “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 use‐
ful 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
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
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 (sup‐
ported symmetric ciphers), cipher-auth (supported symmetric ciphers that support au‐
thenticated 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-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 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 con‐
nection 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 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 remote socket should listen on all interfaces.
Specifying a remote bind_address will only succeed if the server's GatewayPorts option
is enabled (see sshd_config(5)).

If the port argument is ‘0’, the listen port will be dynamically allocated on the
server and reported to the client at run time. When used together with -O forward the
allocated port will be printed to the standard output.

-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 fa‐
cilitate 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 arbitrary screen-based
programs on a remote machine, which can be very useful, e.g. when implementing menu
services. Multiple -t options force tty allocation, even if ssh has no local tty.

-V Display the version number and exit.

-v Verbose mode. Causes ssh to print debugging messages about its progress. This is
helpful in debugging connection, authentication, and configuration problems. Multiple
-v options increase the verbosity. The maximum is 3.

-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 op‐
tions.

-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 basis in a configura‐
tion file.

X11 forwarding should be enabled with caution. Users with the ability to bypass file
permissions on the remote host (for the user's X authorization database) can access the
local X11 display through the forwarded connection. An attacker may then be able to
perform activities such as keystroke monitoring.

For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by
default. 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 restric‐
tions by default, because too many programs 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 op‐
tion to “no” to restore the upstream 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 authenti‐
cation, public key authentication, keyboard-interactive authentication, and password authenti‐
cation. 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. Ad‐
ditionally, 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 au‐
thentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoof‐
ing. [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 desired.]

Public key authentication works as follows: The scheme is based on public-key cryptography, us‐
ing cryptosystems where encryption and decryption are done using separate keys, and it is un‐
feasible 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 implements public key authentication protocol au‐
tomatically, 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 au‐
thentication. 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 authentication from suc‐
ceeding after authentication completes using a different method. These may be viewed by in‐
creasing 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 con‐
ventional ~/.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 authentica‐
tion: instead of a set of public/private keys, signed certificates are used. This has the ad‐
vantage 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 information.

The most convenient way to use public key or certificate authentication may be with an authen‐
tication 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 au‐
thentication 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 pass‐
word 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 direc‐
tory. 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 encryp‐
tion. 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 ma‐
chine 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 characters noted below.

If no pseudo-terminal has been allocated, the session is transparent and can be used to reli‐
ably 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 con‐
figuration 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 termi‐
nate.

~? 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).
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 application of TCP forwarding is a se‐
cure 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 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 op‐
tions above) and the user is using X11 (the DISPLAY environment variable is set), the connec‐
tion 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 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 random authorization cookie, store it in Xauthority on the server, and verify
that any forwarded connections carry this cookie and replace it by the real cookie when the
connection is opened. The real authentication cookie is never sent to the server machine (and
no cookies are sent in the plain).

If the 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 automati‐
cally 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). Finger‐
prints 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 algorithm 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 pattern remembered only gives a good probability that the host key is the same, not guaran‐
teed proof.

To get a listing of the fingerprints along with their random art for all known hosts, the fol‐
lowing 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 finger‐
prints 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 re‐
source 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 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) de‐
vice 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 au‐
tomatically 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 systems that use this vari‐
able.

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 nec‐
essary 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 vari‐
able 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-separated 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 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 tun‐
nel 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, 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 allowed to change their environment. For more in‐
formation, 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 authentica‐
tion without permitting login with rlogin/rsh.

~/.ssh/
This directory is the default location for all user-specific configuration and authen‐
tication 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 (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 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_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 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 sensi‐
tive 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 al‐
ready 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 authen‐
tication without permitting 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_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 au‐
thentication.

/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be prepared by the system admin‐
istrator 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.

BSD February 6, 2022 BSD