SSH(1)SSH(1)NAMEssh - secure shell client (remote login program)
SYNOPSISssh [-l login_name] hostname | user@hostname [ command]
ssh [-afgknqstvxACNTX1246] [-b bind_address] [-m mac_spec]
[-c cipher_spec] [-e escape_char] [-i identity_file]
[-l login_name] [-F configfile] [-o option] [-p port]
[-L [bind_address:]port:host:hostport]
[-R [bind_address:]port:host:hostport]
[-D [bind_address:]port] hostname | user@hostname [command]
DESCRIPTIONssh (Secure Shell) is a program for logging into a remote machine and
for executing commands on a remote machine. It is intended to replace
rlogin and rsh, and to provide secure encrypted communications between
two untrusted hosts over an insecure network. X11 connections and arbi‐
trary TCP/IP ports can also be forwarded over the secure channel.
ssh connects and logs into the specified hostname. The user must prove
his or her identity to the remote machine using one of several methods
depending on the protocol version used:
SSH Protocol Version 1
First, if the machine the user logs in from is listed in
/etc/hosts.equiv or /etc/shosts.equiv on the remote machine, and the
user names are the same on both sides, the user is immediately permit‐
ted to log in. Second, if .rhosts or .shosts exists in the user's home
directory on the remote machine and contains a line containing the name
of the client machine and the name of the user on that machine, the
user is permitted to log in. This form of authentication alone is nor‐
mally not allowed by the server because it is not secure.
The second (and primary) authentication method is the rhosts or
hosts.equiv method combined with RSA-based host authentication. It
means that if the login would be permitted by $HOME/.rhosts,
$HOME/.shosts, /etc/hosts.equiv, or /etc/shosts.equiv, and if addition‐
ally the server can verify the client's host key (see
/etc/ssh_known_hosts in the FILES section), only then is login permit‐
ted. This authentication method closes security holes due to IP spoof‐
ing, DNS spoofing, and routing spoofing.
Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and the
rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.
As a third authentication method, ssh supports RSA-based authentica‐
tion. The scheme is based on public-key cryptography. There are cryp‐
tosystems where encryption and decryption are done using separate keys,
and it is not possible to derive the decryption key from the encryption
key. RSA is one such system. The idea is that each user creates a pub‐
lic/private key pair for authentication purposes. The server knows the
public key, and only the user knows the private key. The file
$HOME/.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 server
checks if this key is permitted, and if so, sends the user (actually
the ssh program running on behalf of the user) a challenge in the form
of a random number, encrypted by the user's public key. The challenge
can only be decrypted using the proper private key. The user's client
then decrypts the challenge using the private key, proving that he or
she knows the private key but without disclosing it to the server.
ssh implements the RSA authentication protocol automatically. The user
creates his or her RSA key pair by running ssh-keygen(1). This stores
the private key in $HOME/.ssh/identity and the public key in
$HOME/.ssh/identity.pub in the user's home directory. The user should
then copy the identity.pub to $HOME/.ssh/authorized_keys in his or her
home directory on the remote machine (the authorized_keys file corre‐
sponds to the conventional $HOME/.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. RSA authentication is much more secure
than rhosts authentication.
The most convenient way to use RSA authentication can be with an
authentication agent. See ssh-agent(1) for more information.
If other authentication methods fail, ssh prompts the user for a pass‐
word. 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 Protocol Version 2
The SSH version 2 protocol supports multiple user authentication meth‐
ods, some of which are similar to those available with the SSH protocol
version 1. These authentication mechanisms are negotiated by the client
and server, with the client trying methods in the order specified in
the PreferredAuthentications client configuration option. The server
decides when enough authentication methods have passed successfully so
as to complete the authentication phase of the protocol.
When a user connects by using protocol version 2, similar authentica‐
tion methods are available. Using the default values for PreferredAu‐
thentications, the client tries to authenticate first by using the
hostbased method. If this method fails, public key authentication is
attempted. Finally, if this method fails, keyboard-interactive and
password authentication are tried.
The public key method is similar to RSA authentication described in the
previous section and allows the RSA or DSA algorithm to be used: The
client uses his or her private key, $HOME/.ssh/id_dsa or
$HOME/.ssh/id_rsa, to sign the session identifier and sends the result
to the server. The server checks whether the matching public key is
listed in $HOME/.ssh/authorized_keys and grants access if both the key
is found and the signature is correct. The session identifier is
derived from a shared Diffie-Hellman value and is only known to the
client and the server.
If public key authentication fails or is not available, a password can
be sent encrypted to the remote host for proving the user's identity,
or an extended prompt/reply protocol can be engaged.
Additionally, ssh supports hostbased or challenge response authentica‐
tion.
Protocol 2 provides additional mechanisms for confidentiality (the
traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and
integrity (hmac-sha1, hmac-md5). Protocol 1 lacks a strong mechanism
for ensuring the integrity of the connection.
Login Session and Remote Execution
When the user's identity has been accepted by the server, the server
either executes the specified command, or logs into the machine and
gives the user a normal shell on the remote machine. All communication
with the remote command or shell is automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the
user can use the escape characters noted below. If a pseudo-terminal
has been allocated (normal login session), the user can disconnect with
~., and suspend ssh with ~^Z. All forwarded connections can be listed
with ~#. If the session blocks waiting for forwarded X11 or TCP/IP con‐
nections to terminate, ssh can be backgrounded with ~&, although this
should not be used while the user shell is active, as it can cause the
shell to hang. All available escapes can be listed with ~?.
A single tilde character can be sent as ~~, or by following the tilde
with a character other than those described above. The escape character
must always follow a newline to be interpreted as special. The escape
character can be changed in configuration files or on the command line.
If no pseudo tty 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" also makes 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/IP connections have been closed. The exit
status of the remote program is returned as the exit status of ssh.
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
with 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 for SSH protocol
version 2 and if the peer supports it.
~C
Open command line. Only useful for adding port forwardings using
the -L and -R options).
~R
Request rekeying of the connection. Only useful for SSH protocol
version 2 and if the peer supports it.
X11 and TCP Forwarding
If the ForwardX11 variable is set to ``yes'' (or, see the description
of the -X and -x options described later) 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) goes through
the encrypted channel, and the connection to the real X server is made
from the local machine. The user should not manually set DISPLAY. For‐
warding of X11 connections can be configured on the command line or in
configuration files.
The DISPLAY value set by ssh points to the server machine, but with a
display number greater than zero. This is normal behavior, because ssh
creates a "proxy" X11 server on the server machine for forwarding the
connections over the encrypted channel.
ssh also automatically sets up Xauthority data on the server machine.
For this purpose, it generates 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 described later) and the user is using an
authentication agent, the connection to the agent is automatically for‐
warded to the remote side.
Forwarding of arbitrary TCP/IP connections over the secure channel can
be specified either on the command line or in a configuration file. One
possible application of TCP/IP forwarding is a secure connection to an
electronic purse. Another possible application is firewall traversal.
Server Authentication
ssh automatically maintains and checks a database containing identifi‐
cations for all hosts it has ever been used with. Host keys are stored
in $HOME/.ssh/known_hosts in the user's home directory. Additionally,
the file /etc/ssh_known_hosts is automatically checked for known hosts.
The behavior of ssh with respect to unknown host keys is controlled by
the StrictHostKeyChecking parameter. If a host's identification ever
changes, ssh warns about this and disables password authentication to
prevent a trojan horse from getting the user's password. Another pur‐
pose of this mechanism is to prevent attacks by intermediaries which
could otherwise be used to circumvent the encryption. The StrictHostK‐
eyChecking option can be used to prevent logins to machines whose host
key is not known or has changed.
However, when using key exchange protected by GSS-API, the server can
advertise a host key. The client automatically adds this host key to
its known hosts file, $HOME/.ssh/known_hosts, regardless of the setting
of the StrictHostKeyChecking option, unless the advertised host key
collides with an existing known hosts entry.
When the user's GSS-API credentials expire, the client continues to be
able to rekey the session using the server's public host key to protect
the key exchanges.
GSS-API User and Server Authentication
ssh uses the user's GSS-API credentials to authenticate the client to
the server wherever possible, if GssKeyEx and/or GssAuthentication are
set.
With GssKeyEx, one can have an SSHv2 server that has no host public
keys, so that only GssKeyEx can be used. With such servers, rekeying
fails if the client's credentials are expired.
GSS-API user authentication has the disadvantage that it does not obvi‐
ate the need for SSH host keys, but its failure does not impact rekey‐
ing. ssh can try other authentication methods (such as public key,
password, and so on) if GSS-API authentication fails.
Delegation of GSS-API credentials can be quite useful, but is not with‐
out danger. As with passwords, users should not delegate GSS creden‐
tials to untrusted servers, since a compromised server can use a user's
delegated GSS credentials to impersonate the user.
GSS-API user authorization is covered in gss_auth_rules(5).
Rekeying can be used to redelegate credentials when GssKeyEx is "yes".
(See ~R under Escape Characters above.)
OPTIONS
The following options are supported:
-1
Forces ssh to try protocol version 1 only.
-2
Forces ssh to try protocol version 2 only.
-4
Forces ssh to use IPv4 addresses only.
-6
Forces ssh to use IPv6 addresses only.
-a
Disables forwarding of the authentication agent connection.
-A
Enables forwarding of the authentication agent connection. 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, the attacker can perform operations on the keys
that enable the attacker to authenticate using the identities
loaded into the agent.
-b bind_address
Specifies the interface to transmit from on machines with multiple
interfaces or aliased addresses.
-c cipher_spec
Selects the cipher specification for encrypting the session.
For protocol version 1, cipher_spec is a single cipher. See the
Cipher option in ssh_config(4) for more information.
For protocol version 2, cipher_spec is a comma-separated list of
ciphers listed in order of preference. See the Ciphers option in
ssh_config(4) for more information.
-C
Requests compression of all data (including stdin, stdout, stderr,
and data for forwarded X11 and TCP/IP connections). The compression
algorithm is the same used by gzip(1). The gzip man page is avail‐
able in the SUNWsfman package. The "level" can be controlled by the
CompressionLevel option (see ssh_config(4)). Compression is desir‐
able on modem lines and other slow connections, but only slows 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(4).
-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 connec‐
tion is made to this port, the connection is forwarded over the
secure channel. 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 acts as a SOCKS server.
Only a user with enough privileges can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration
file.
IPv6 addresses can be specified with an alternative syntax:
[bind_address/]port or by enclosing the address in square brackets.
By default, the local port is bound in accordance with the Gateway‐
Ports setting. However, an explicit bind_address can 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 ch | ^ch | none
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.
If followed by CTRL-z, the escape character suspends the connec‐
tion. If followed by itself, the escape character sends itself
once. Setting the character to none disables any escapes and makes
the session fully transparent.
-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 the -n
option. The recommended way to start X11 programs at a remote site
is with something like ssh-f host xterm.
-F configfile
Specifies an alternative per-user configuration file. If a configu‐
ration file is specified on the command line, the system-wide con‐
figuration file, /etc/ssh_config, is ignored. The default for the
per-user configuration file is $HOME/.ssh/config.
-g
Allows remote hosts to connect to local forwarded ports.
-i identity_file
Selects a file from which the identity (private key) for RSA or DSA
authentication is read. The default is $HOME/.ssh/identity for pro‐
tocol version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for
protocol version 2. Identity files can also be specified on a per-
host basis in the configuration file. It is possible to have multi‐
ple -i options (and multiple identities specified in configuration
files).
-l login_name
Specifies the user to log in as on the remote machine. This also
can be specified on a per-host basis in the configuration file.
-L [bind_address:]port:host:hostport
Specifies that the specified port on the local (client) host is to
be forwarded to the specified host and port on the remote side.
This works by allocating a socket to listen to the port on the
local side, optionally bound to the specified bind_address. Then,
whenever a connection is made to this port, the connection is for‐
warded over the secure channel and a connection is made to host
port hostport from the remote machine. Port forwardings can also be
specified in the configuration file. Only a user with enough privi‐
leges can forward privileged ports. IPv6 addresses can be specified
with an alternative syntax: [bind_address/]port/host/h ostport or
by enclosing the address in square brackets.
By default, the local port is bound in accordance with the Gateway‐
Ports setting. However, an explicit bind_address can 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.
-m mac_spec
Additionally, for protocol version 2 a comma-separated list of MAC
(message authentication code) algorithms can be specified in order
of preference. See the MACs keyword for more information.
-n
Redirects stdin from /dev/null (actually, prevents reading from
stdin). This must be used when ssh is run in the background. A com‐
mon trick is to use this to run X11 programs on a remote machine.
For example,
ssh-n shadows.cs.hut.fi emacs &
starts an emacs on shadows.cs.hut.fi, and the X11 connection is
automatically forwarded over an encrypted channel. The ssh program
is put in the background. This does not work if ssh needs to ask
for a password or passphrase. See also the -f option.
-N
Does not execute a remote command. This is useful if you just want
to forward ports (protocol version 2 only).
-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. The option has the same format as a
line in the configuration file.
-p port
Specifies the port to connect to on the remote host. This can be
specified on a per-host basis in the configuration file.
-P
Obsoleted option. SSHv1 connections from privileged ports are not
supported.
-q
Quiet mode. Causes all warning and diagnostic messages to be sup‐
pressed. Only fatal errors are displayed.
-R [bind_address:]port:host:hostport
Specifies that the specified port on the remote (server) host is to
be forwarded to the specified host and port on the local side. This
works by allocating a socket to listen to the port on the remote
side. Then, whenever a connection is made to this port, the connec‐
tion is forwarded over the secure channel and a connection is made
to host port hostport from the local machine. Port forwardings can
also be specified in the configuration file. Privileged ports can
be forwarded only when logging in on the remote machine as a user
with enough privileges.
IPv6 addresses can be specified by enclosing the address in square
braces or using an alternative syntax: [bind_address/]host/
port/hostport.
By default, the listening socket on the server is bound to the
loopback interface only. This can 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 only succeeds if the server's GatewayPorts
option is enabled. See sshd_config(4).
-s
Can be used to request invocation of a subsystem on the remote sys‐
tem. Subsystems are a feature of the SSH2 protocol which facili‐
tate the use of SSH as a secure transport for other applications,
for example, sftp. The subsystem is specified as the remote com‐
mand.
-t
Forces pseudo-tty allocation. This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very use‐
ful, for example, when implementing menu services. Multiple -t
options force allocation, even if ssh has no local tty.
-T
Disables pseudo-tty allocation (protocol version 2 only).
-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 ver‐
bosity. Maximum is 3.
-x
Disables X11 forwarding.
-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 abil‐
ity 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 can then be able to perform
activities such as keystroke monitoring.
For this reason, X11 forwarding might be subjected to X11 SECURITY
extension restrictions. Refer to the ForwardX11Trusted directive in
ssh_config(4) for more information.
If X11 forwarding is enabled, remote X11 clients is trusted by
default. This means that they have full access to the original X11
display.
ENVIRONMENT VARIABLESssh normally sets the following environment variables:
DISPLAY
The DISPLAY variable must be set for X11 display forwarding to
work.
SSH_ASKPASS
If ssh needs a passphrase, it reads the passphrase from the current
terminal if it was run from a terminal. If ssh does not have a ter‐
minal associated with it but DISPLAY and SSH_ASKPASS are set, it
executes the program specified by SSH_ASKPASS and opens an X11 win‐
dow to read the passphrase. This is particularly useful when call‐
ing ssh from a .Xsession or related script. On some machines it
might be necessary to redirect the input from /dev/null to make
this work. The system is shipped with /usr/lib/ssh/ssh-askpass
which is the default value for SSH_ASKPASS
SSH_AUTH_SOCK
Indicates the path of a unix-domain socket used to communicate with
the agent.
SSH_LANGS
A comma-separated list of IETF language tags (see RFC3066) indicat‐
ing the languages that the user can read and write. Used for nego‐
tiation of the locale on the server.
LANG, LC_ALL, LC_COLLATE, LC_CTYPE,
LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME
The values of these environment variables can be set in remote ses‐
sions according to the locale settings on the client side and
availability of support for those locales on the server side. Envi‐
ronment Variable Passing (see RFC 4254) is used for passing them
over to the server side.
See the ENVIRONMENT VARIABLES section in the sshd(1M) man page for more
information on how locale setting can be further changed depending on
server side configuration.
EXIT STATUS
The status of the remote program is returned as the exit status of ssh.
255 is returned if an error occurred at anytime during the ssh connec‐
tion, including the initial key exchange.
FILES
$HOME/.ssh/known_hosts
Records host keys for all hosts the user has
logged into that are not in
/etc/ssh/ssh_known_hosts. See sshd(1M).
$HOME/.ssh/identity
$HOME/.ssh/id_dsa
$HOME/.ssh/id_ssa
Contains the authentication identity of the
user. These files are for protocol 1 RSA,
protocol 2 DSA, and protocol 2 RSA, respec‐
tively. These files contain sensitive data
and should be readable by the user but not
accessible by others (read/write/execute).
ssh ignores a private key file if it is
accessible by others. It is possible to spec‐
ify a passphrase when generating the key. The
passphrase is used to encrypt the sensitive
part of this file using 3DES.
/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 sshd(1M) for
more information.
$HOME/.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 sshd(1M) for
more information.
$HOME/.ssh/environment
Contains additional definitions for environ‐
ment variables. See ENVIRONMENT VARIABLES.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ See below. │
└────────────────────┴─────────────────┘
The command line syntax is Committed. The remote locale selection
through passing LC_* environment variables is Uncommitted.
SEE ALSOrlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
ssh-http-proxy-connect(1), ssh-socks5-proxy-connect(1), telnet(1),
sshd(1M), ssh_config(4), sshd_config(4), attributes(5),
gss_auth_rules(5), kerberos(5), privileges(5)
RFC 1928
RFC 4254
May 20, 2009 SSH(1)
