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SSH(1) General Commands Manual SSH(1)
ssh — OpenSSH remote login client
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]
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).
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/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.
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.
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.
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.
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 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 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).
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).
~/.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/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).
/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 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.
ssh exits with the exit status of the remote command or with 255
if an error occurred.
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)
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).
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.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report
for this manual page, see ⟨http://www.openssh.com/report.html⟩.
This page was obtained from the tarball openssh-9.7p1.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2024-06-14. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
GNU October 11, 2023 SSH(1)
Pages that refer to this page: stap-jupyter(1), systemd-ssh-proxy(1), systemd-stdio-bridge(1), tar(1), sd_bus_default(3), environment.d(5), proc_sys_vm(5), [email protected](5), pty(7), systemd-ssh-generator(8)
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