SSH(1) User Commands 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 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 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 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 `*' indicates
that the port should be available from all interfaces.

-E log_file
Append debug logs to log_file instead of standard error.

-e escape_char
Sets the escape character for sessions with a pty (default: `~').
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot (`.') closes the connection;
followed by control-Z suspends the connection; and followed by
itself sends the escape character once. Setting the character to
"none" disables any escapes and makes the session 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 connection, 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 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 configuration 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 filenames.

-J destination
Connect to the target host by first making an ssh connection to the
jump host described by destination and then establishing a TCP
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 allocating
a socket to listen to either a TCP port on the local side,
optionally bound to the specified bind_address, or to a Unix
socket. Whenever a connection is made to the local port or socket,
the connection is forwarded over the secure channel, and a
connection is made to either host port hostport, or the Unix socket
remote_socket, from the remote machine.

Port forwardings can also be specified in the configuration file.
Only the superuser can forward privileged ports. IPv6 addresses
can be specified by enclosing the address in square brackets.

By default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit bind_address may be
used to bind the connection to a specific address. The
bind_address of "localhost" indicates that the listening port be
bound for local use only, while an empty address or `*' indicates
that the port should be available from all interfaces.

-l login_name
Specifies the user to log in as on the remote machine. This also
may be specified on a per-host basis in the configuration file.

-M Places the ssh client into "master" mode for connection sharing.
Multiple -M options places ssh into "master" mode but with
confirmation required using ssh-askpass(1) before each operation
that changes the multiplexing state (e.g. opening a new session).
Refer to the 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
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 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 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 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
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAcceptedAlgorithms
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
SendEnv
ServerAliveInterval
ServerAliveCountMax
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation

-P tag Specify a tag name that may be used to select configuration in
ssh_config(5). Refer to the Tag and Match keywords in
ssh_config(5) for more information.
-p port
Port to connect to on the remote host. This can be specified on a
per-host basis in the configuration file.

-Q query_option
Queries for the algorithms supported by one of the following
features: cipher (supported symmetric ciphers), cipher-auth
(supported symmetric ciphers that support authenticated
encryption), help (supported query terms for use with the -Q flag),
mac (supported message integrity codes), kex (key exchange
algorithms), key (key types), key-ca-sign (valid CA signature
algorithms for certificates), 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 algorithms).
Alternatively, any keyword from ssh_config(5) or sshd_config(5)
that takes an algorithm list may be used as an alias for the
corresponding query_option.

-q Quiet mode. Causes most warning and diagnostic messages to be
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 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 facilitate the use of SSH as a secure transport
for other applications (e.g. sftp(1)). The subsystem is specified
as the remote command. Refer to the description of SessionType in
ssh_config(5) for details.

-T Disable pseudo-terminal allocation.

-t Force pseudo-terminal allocation. This can be used to execute
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
options.

-w local_tun[:remote_tun]
Requests tunnel device forwarding with the specified tun(4) devices
between the client (local_tun) and the server (remote_tun).

The devices may be specified by numerical ID or the keyword "any",
which uses the next available tunnel device. If remote_tun is not
specified, it defaults to "any". See also the Tunnel and
TunnelDevice directives in ssh_config(5).

If the Tunnel directive is unset, it will be set to the default
tunnel mode, which is "point-to-point". If a different Tunnel
forwarding mode it desired, then it should be specified before -w.

-X Enables X11 forwarding. This can also be specified on a per-host
basis in a configuration file.

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

For this reason, X11 forwarding is subjected to X11 SECURITY
extension restrictions by default. Refer to the ssh -Y option and
the ForwardX11Trusted directive in ssh_config(5) for more
information.

-x Disables X11 forwarding.

-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
subjected to the X11 SECURITY extension controls.

-y Send log information using the syslog(3) system module. By default
this information is sent to stderr.

ssh may additionally obtain configuration data from a per-user
configuration file and a system-wide configuration file. The file format
and configuration options are described in ssh_config(5).

AUTHENTICATION

The OpenSSH SSH client supports SSH protocol 2.

The methods available for authentication are: GSSAPI-based authentication,
host-based authentication, public key authentication, keyboard-interactive
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 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, 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 protocol automatically, using one of the DSA, ECDSA,
Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains a brief
discussion of the DSA and RSA algorithms.

The file ~/.ssh/authorized_keys lists the public keys that are permitted
for logging in. When the user logs in, the ssh program tells the server
which key pair it would like to use for authentication. The client proves
that it has access to the private key and the server checks that the
corresponding public key is authorized to accept the account.

The server may inform the client of errors that prevented public key
authentication from succeeding after authentication completes using a
different method. These may be viewed by increasing the LogLevel to DEBUG
or higher (e.g. by using the -v flag).

The user creates their key pair by running ssh-keygen(1). This stores the
private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
(Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub
(authenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
home directory. The user should then copy the public key to
~/.ssh/authorized_keys in their home directory on the remote machine. The
authorized_keys file corresponds to the conventional ~/.rhosts file, and
has one key per line, though the lines can be very long. After this, the
user can log in without giving the password.

A variation on public key authentication is available in the form of
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/private keys.
See the CERTIFICATES section of ssh-keygen(1) for more information.

The most convenient way to use public key or certificate authentication may
be with an authentication agent. See ssh-agent(1) 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 remote host for checking; however,
since all communications are encrypted, the password cannot be seen by
someone listening on the network.

ssh automatically maintains and checks a database containing identification
for all hosts it has ever been used with. Host keys are stored in
~/.ssh/known_hosts in the user's home directory. Additionally, the file
/etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any new
hosts are automatically added to the user's file. If a host's
identification ever changes, ssh warns about this and disables password
authentication to prevent server spoofing or man-in-the-middle attacks,
which could otherwise be used to circumvent the encryption. The
StrictHostKeyChecking option can be used to control logins to machines
whose host key is not known or has changed.

When the user's identity has been accepted by the server, the server either
executes the given command in a non-interactive session or, if no command
has been specified, logs into the machine and gives the user a normal shell
as an interactive session. All communication with the remote command or
shell will be automatically encrypted.

If an interactive session is requested, ssh by default will only request a
pseudo-terminal (pty) for interactive sessions when the client has one.
The flags -T and -t can be used to override this behaviour.

If a pseudo-terminal has been allocated, the user may use the escape
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). 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 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 channel "#users", nickname "pinky",
using the standard IRC port, 6667:

$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
$ irc -c '#users' pinky IRC/127.0.0.1

The -f option backgrounds ssh and the remote command "sleep 10" is
specified to allow an amount of time (10 seconds, in the example) to start
the program which is going to use the tunnel. If no connections are made
within the time specified, ssh will exit.

X11 FORWARDING
If the ForwardX11 variable is set to "yes" (or see the description of the
-X, -x, and -Y options above) and the user is using X11 (the DISPLAY
environment variable is set), the connection to the X11 display is
automatically forwarded to the remote side in such a way that any X11
programs started from the shell (or command) will go through the encrypted
channel, and the connection to the real X server will be made from the
local machine. The user should not 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 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 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 /var/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
guaranteed proof.

To get a listing of the fingerprints along with their random art for all
known hosts, the following command line can be used:

$ ssh-keygen -lv -f ~/.ssh/known_hosts

If the fingerprint is unknown, an alternative method of verification is
available: SSH fingerprints verified by DNS. An additional resource record
(RR), SSHFP, is added to a zonefile and the connecting client is able to
match the fingerprint with that of the key presented.

In this example, we are connecting a client to a server,
"host.example.com". The SSHFP resource records should first be added to
the zonefile for host.example.com:

$ ssh-keygen -r host.example.com.

The output lines will have to be added to the zonefile. To check that the
zone is answering fingerprint queries:

$ dig -t SSHFP host.example.com

Finally the client connects:

$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

See the VerifyHostKeyDNS option in ssh_config(5) for more information.

SSH-BASED VIRTUAL PRIVATE NETWORKS
ssh contains support for Virtual Private Network (VPN) tunnelling using the
tun(4) network pseudo-device, allowing two networks to be joined securely.
The sshd_config(5) configuration option PermitTunnel controls whether the
server supports this, and at what level (layer 2 or 3 traffic).

The following example would connect client network 10.0.50.0/24 with remote
network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to
10.1.1.2, provided that the SSH server running on the gateway to the remote
network, at 192.168.1.15, allows it.

On the client:

# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
# route add 10.0.99.0/24 10.1.1.2

On the server:

# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
# route add 10.0.50.0/24 10.1.1.1

Client access may be more finely tuned via the /root/.ssh/authorized_keys
file (see below) and the PermitRootLogin server option. The following
entry would permit connections on tun(4) device 1 from user "jane" and on
tun device 2 from user "john", if PermitRootLogin is set to
"forced-commands-only":

tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

Since an SSH-based setup entails a fair amount of overhead, it may be more
suited to temporary setups, such as for wireless VPNs. More permanent VPNs
are better provided by tools such as ipsecctl(8) and isakmpd(8).

ENVIRONMENT

ssh will normally set the following environment variables:

DISPLAY The DISPLAY variable indicates the location of the
X11 server. It is automatically set by ssh to point
to a value of the form "hostname:n", where "hostname"
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 variable.

MAIL Set to the path of the user's mailbox.

PATH Set to the default PATH, as specified when compiling
ssh.

SSH_ASKPASS If ssh needs a passphrase, it will read the
passphrase from the current terminal if it was run
from a terminal. If ssh does not have a terminal
associated with it but DISPLAY and SSH_ASKPASS are
set, it will execute the program specified by
SSH_ASKPASS and open an X11 window to read the
passphrase. This is particularly useful when calling
ssh from a .xsession or related script. (Note that
on some machines it may be necessary to 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-
separated values: client IP address, client port
number, 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 tunnel forwarding was requested by
the client.

SSH_USER_AUTH Optionally set by sshd(8), this variable may contain
a pathname to a file that lists the authentication
methods successfully used when the session was
established, 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. 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 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.

~/.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 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 configuration
options are described in ssh_config(5).

/var/ssh/ssh_host_key
/var/ssh/ssh_host_dsa_key
/var/ssh/ssh_host_ecdsa_key
/var/ssh/ssh_host_ed25519_key
/var/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-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.

illumos October 11, 2023 illumos