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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]
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 pro‐
vide 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 his/her identity
to the remote machine using one of several methods (see below).
If a command is specified, it is executed on the remote host instead
of a login shell.
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 alter‐
native 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 con‐
nections). 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 forward‐
ing. 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 con‐
nect 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 spe‐
cific 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 begin‐
ning 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 execu‐
tion. 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.
-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 commu‐
nicate with a PKCS#11 token providing keys for user authen‐
tication.
-i identity_file
Selects a file from which the identity (private key) for
public key authentication is read. The default is
~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa.
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 a ssh connection
to the jump host described by destination and then estab‐
lishing 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 destina‐
tion host and not any specified jump hosts. Use
~/.ssh/config to specify configuration for jump hosts.
-K Enables GSSAPI-based authentication and forwarding (delega‐
tion) 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 for‐
warded 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 configura‐
tion 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 for more information.
-N Do not execute a remote command. This is useful for just
forwarding ports.
-n Redirects stdin from /dev/null (actually, prevents reading
from stdin). This must be used when ssh is run in the back‐
ground. 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 back‐
ground. (This does not work if ssh needs to ask for a pass‐
word or passphrase; see also the -f option.)
-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 com‐
mands are: “check” (check that the master process is run‐
ning), “forward” (request forwardings without command execu‐
tion), “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 con‐
figuration 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 val‐
ues, see ssh_config(5).
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CASignatureAlgorithms
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
SendEnv
ServerAliveInterval
ServerAliveCountMax
SetEnv
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 speci‐
fied on a per-host basis in the configuration file.
-Q query_option
Queries ssh for the algorithms supported for the specified
version 2. The available features are: 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 mes‐
sage 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 algo‐
rithms), 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 corre‐
sponding 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 connec‐
tion is forwarded over the secure channel, and a connection
is made from the local machine to either an explicit desti‐
nation 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 overrid‐
den 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 dynami‐
cally 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.
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal allocation. This can be used to exe‐
cute arbitrary screen-based programs on a remote machine,
which can be very useful, e.g. when implementing menu ser‐
vices. 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 dif‐
ferent 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 key‐
stroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY
extension restrictions by default. Please 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 con‐
figuration file and a system-wide configuration file. The file for‐
mat 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 authenti‐
cation, host-based authentication, public key authentication, chal‐
lenge-response authentication, and password authentication. Authen‐
tication 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. Addition‐
ally, 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 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, 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 pur‐
poses. 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 algo‐
rithms. 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 per‐
mitted 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 his/her 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 (authenti‐
cator-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 his/her 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 pass‐
word.
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 pub‐
lic/private keys. See the CERTIFICATES section of ssh-keygen(1) for
more information.
The most convenient way to use public key or certificate authentica‐
tion may be with an authentication agent. See ssh-agent(1) and
(optionally) the AddKeysToAgent directive in ssh_config(5) for more
information.
Challenge-response authentication works as follows: The server sends
an arbitrary "challenge" text, and prompts for a response. Examples
of challenge-response 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 check‐
ing; 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 identi‐
fication for all hosts it has ever been used with. Host keys are
stored in ~/.ssh/known_hosts in the user's home directory. Addi‐
tionally, 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 communica‐
tion 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 charac‐
ter 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 connec‐
tion / 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”, nick‐
name “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 connec‐
tions 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 dis‐
play 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 config‐
ured 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 hap‐
pens 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 authentica‐
tion 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 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 vis‐
ually, 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 pat‐
tern is displayed. Because these patterns are not unambiguous how‐
ever, 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 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 connect‐
ing 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 informa‐
tion.
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 inte‐
ger ≥ 1. ssh uses this special value to for‐
ward X11 connections over the secure channel.
The user should normally not set DISPLAY
explicitly, as that will render the X11 con‐
nection insecure (and will require the user to
manually copy any required authorization cook‐
ies).
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 pro‐
gram specified by SSH_ASKPASS and open an X11
window to read the passphrase. This is par‐
ticularly 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_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 informa‐
tion, 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 oth‐
ers.
~/.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 oth‐
ers. 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 configu‐
ration options are described in ssh_config(5).
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and
are used for host-based authentication.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be
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 for‐
mat 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.
IPV6
IPv6 address can be used everywhere where IPv4 address. In all
entries must be the IPv6 address enclosed in square brackets. Note:
The square brackets are metacharacters for the shell and must be
escaped in shell.
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.
BSD April 17, 2020 BSD