man page(1) manual page
Table of Contents
(SSH client) 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 provide secure encrypted communications between
two untrusted hosts over an insecure network. X11 connections, arbitrary
TCP ports and sockets can also be forwarded over the secure channel.
connects and logs into the specified (with optional name). The user must
prove his/her identity to the remote machine using one of several methods
depending on the protocol version used (see below). If is specified, it
is executed on the remote host instead of a login shell. The options are
as follows: Forces to try protocol version 1 only. Forces to try protocol
version 2 only. Forces to use IPv4 addresses only. Forces to use IPv6
addresses only. 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 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. Disables forwarding of the authentication agent connection.
Use on the local machine as the source address of the connection. Only
useful on systems with more than one address. Requests compression of all
data (including stdin, stdout, stderr, and data for forwarded X11, TCP
and connections). The compression algorithm is the same used by and the
can be controlled by the option for protocol version 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 option. Selects the cipher specification
for encrypting the session. Protocol version 1 allows specification of
a single cipher. The supported values are and For protocol version 2,
is a comma-separated list of ciphers listed in order of preference. See
the keyword in for more information. Specifies a local application-level
port forwarding. This works by allocating a socket to listen to on the
local side, optionally bound to the specified 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 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 setting. However, an explicit
may be used to bind the connection to a specific address. The of 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. Append
debug logs to instead of standard error. 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 disables
any escapes and makes the session fully transparent. Specifies an alternative
per-user configuration file. If a configuration file is given on the command
line, the system-wide configuration file will be ignored. The default for
the per-user configuration file is Requests to go to background just
before command execution. This is useful if is going to ask for passwords
or passphrases, but the user wants it in the background. This implies The
recommended way to start X11 programs at a remote site is with something
like If the configuration option is set to then a client started with
will wait for all remote port forwards to be successfully established
before placing itself in the background. 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. Specify the PKCS#11 shared
library should use to communicate with a PKCS#11 token providing the user’s
private RSA key. Selects a file from which the identity (private key) for
public key authentication is read. The default is for protocol version
1, and and for protocol version 2. Identity files may also be specified
on a per-host basis in the configuration file. It is possible to have multiple
options (and multiple identities specified in configuration files). will
also try to load certificate information from the filename obtained by
appending to identity filenames. Enables GSSAPI-based authentication and
forwarding (delegation) of GSSAPI credentials to the server. Disables forwarding
(delegation) of GSSAPI credentials to the server. Specifies that the
given port on the local (client) host is to be forwarded to the given host
and port on the remote side. This works by allocating a socket to listen
to on the local side, optionally bound to the specified Whenever a connection
is made to this port, the connection is forwarded over the secure channel,
and a connection is made to port from the remote machine. 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 setting. However, an explicit may be used to bind the connection to
a specific address. The of 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. 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. Places the client into mode for connection sharing.
Multiple options places into mode with confirmation required before
slave connections are accepted. Refer to the description of in for details.
Additionally, for protocol version 2 a comma-separated list of MAC (message
authentication code) algorithms can be specified in order of preference.
See the keyword for more information. Do not execute a remote command.
This is useful for just forwarding ports (protocol version 2 only). Redirects
stdin from (actually, prevents reading from stdin). This must be used when
is run in the background. A common trick is to use this to run X11 programs
on a remote machine. For example, will start an emacs on shadows.cs.hut.fi,
and the X11 connection will be automatically forwarded over an encrypted
channel. The program will be put in the background. (This does not work
if needs to ask for a password or passphrase; see also the option.) Control
an active connection multiplexing master process. When the option is specified,
the argument is interpreted and passed to the master process. Valid commands
are: (check that the master process is running), (request forwardings
without command execution), (cancel forwardings), (request the master
to exit), and (request the master to stop accepting further multiplexing
requests). 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
Port to connect to on the remote host.
This can be specified on a per-host basis in the configuration file. Queries
for the algorithms supported for the specified version 2. The available
features are: (supported symmetric ciphers), (supported symmetric ciphers
that support authenticated encryption), (supported message integrity codes),
(key exchange algorithms), (key types). Quiet mode. Causes most warning
and diagnostic messages to be suppressed. Specifies that the given
port on the remote (server) host is to be forwarded to the given host and
port on the local side. This works by allocating a socket to listen to
on the remote side, and whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is made
to port from the local machine. 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, the listening socket on the
server will be bound to the loopback interface only. This may be overridden
by specifying a An empty or the address indicates that the remote socket
should listen on all interfaces. Specifying a remote will only succeed
if the server’s option is enabled (see If the argument is the listen
port will be dynamically allocated on the server and reported to the client
at run time. When used together with the allocated port will be printed
to the standard output. Specifies the location of a control socket for
connection sharing, or the string to disable connection sharing. Refer
to the description of and in for details. May be used to request invocation
of a subsystem on the remote system. Subsystems are a feature of the SSH2
protocol which facilitate the use of SSH as a secure transport for other
applications (eg. The subsystem is specified as the remote command. Disable
pseudo-tty allocation. Force pseudo-tty 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 options force tty allocation,
even if has no local tty. Display the version number and exit. Verbose
mode. Causes to print debugging messages about its progress. This is helpful
in debugging connection, authentication, and configuration problems. Multiple
options increase the verbosity. The maximum is 3. Requests that standard
input and output on the client be forwarded to on over the secure channel.
Implies and Works with Protocol version 2 only. Requests tunnel device
forwarding with the specified devices between the client and the server
The devices may be specified by numerical ID or the keyword which uses
the next available tunnel device. If is not specified, it defaults to
See also the and directives in If the directive is unset, it is set
to the default tunnel mode, which is 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. Please refer to the option and the directive in for more
information. Disables X11 forwarding. Enables trusted X11 forwarding. Trusted
X11 forwardings are not subjected to the X11 SECURITY extension controls.
Send log information using the system module. By default this information
is sent to stderr. 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
The OpenSSH
SSH client supports SSH protocols 1 and 2. The default is to use protocol
2 only, though this can be changed via the option in or the and options
(see above). Both protocols support similar authentication methods, but
protocol 2 is the default since it provides additional mechanisms for confidentiality
(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
and integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, umac-128,
hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the integrity
of the connection. The methods available for authentication are: GSSAPI-based
authentication, host-based authentication, public key authentication, challenge-response
authentication, and password authentication. Authentication methods are
tried in the order specified above, though protocol 2 has a configuration
option to change the default order: Host-based authentication works as
follows: If the machine the user logs in from is listed in or on the
remote machine, and the user names are the same on both sides, or if the
files or 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 be able to verify the client’s host key (see the description
of and 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: 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. implements public key authentication protocol automatically, using
one of the DSA, ECDSA, ED25519 or RSA algorithms. Protocol 1 is restricted
to using only RSA keys, but protocol 2 may use any. The HISTORY section
of contains a brief discussion of the DSA and RSA algorithms. The file
lists the public keys that are permitted for logging in. When the user
logs in, the 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 user creates his/her key pair by running This
stores the private key in (protocol 1), (protocol 2 DSA), (protocol
2 ECDSA), (protocol 2 ED25519), or (protocol 2 RSA) and stores the public
key in (protocol 1), (protocol 2 DSA), (protocol 2 ECDSA), (protocol
2 ED25519), or (protocol 2 RSA) in the user’s home directory. The user should
then copy the public key to in his/her home directory on the remote machine.
The file corresponds to the conventional 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 for more information. The most convenient
way to use public key or certificate authentication may be with an authentication
agent. See for more information. Challenge-response authentication works
as follows: The server sends an arbitrary text, and prompts for a response.
Protocol 2 allows multiple challenges and responses; protocol 1 is restricted
to just one challenge/response. Examples of challenge-response authentication
include Authentication (see and PAM (some systems). Finally, if other
authentication methods fail, 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. automatically maintains and checks a database containing identification
for all hosts it has ever been used with. Host keys are stored in in the
user’s home directory. Additionally, the file is automatically checked for
known hosts. Any new hosts are automatically added to the user’s file. If
a host’s identification ever changes, 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 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, or logs into the machine and gives the user
a normal shell on the remote machine. All communication with the remote
command or shell will be automatically encrypted. If a pseudo-terminal has
been allocated (normal login session), the user may use the escape characters
noted below. 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 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, 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 configuration directive or on the command line by the
option. The supported escapes (assuming the default are: Disconnect.
Background List forwarded connections. Background at logout when waiting
for forwarded connection / X11 sessions to terminate. Display a list of
escape characters. Send a BREAK to the remote system (only useful for SSH
protocol version 2 and if the peer supports it). Open command line. Currently
this allows the addition of port forwardings using the and options (see
above). It also allows the cancellation of existing port-forwardings with
for local, for remote and for dynamic port-forwardings. allows
the user to execute a local command if the option is enabled in Basic
help is available, using the option. Request rekeying of the connection
(only useful for SSH protocol version 2 and if the peer supports it). Decrease
the verbosity when errors are being written to stderr. Increase the verbosity
when errors are being written to stderr.
Forwarding of arbitrary
TCP connections over the 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 between
an IRC client and server, even though the IRC server does not directly
support encrypted communications. This works as follows: the user connects
to the remote host using specifying a port to be used to forward connections
to the remote server. After that it is possible to start the service which
is to be encrypted on the client machine, connecting to the same local
port, and will encrypt and forward the connection. The following example
tunnels an IRC session from client machine (localhost) to remote server
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 $ irc -c ’#users’
-p 1234 pinky 127.0.0.1 This tunnels a connection to IRC server joining
channel nickname using port 1234. It doesn’t matter which port is used,
as long as it’s greater than 1023 (remember, only root can open sockets
on privileged ports) and doesn’t conflict with any ports already in use.
The connection is forwarded to port 6667 on the remote server, since that’s
the standard port for IRC services. The option backgrounds and the remote
command is specified to allow an amount of time (10 seconds, in the example)
to start the service which is to be tunnelled. If no connections are made
within the time specified, will exit.
If the variable is
set to (or see the description of the and options above) and the user
is using X11 (the 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 Forwarding
of X11 connections can be configured on the command line or in configuration
files. The value set by will point to the server machine, but with a
display number greater than zero. This is normal, and happens because creates
a X server on the server machine for forwarding the connections over the
encrypted channel. 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 variable
is set to (or see the description of the and 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 has been disabled). Fingerprints can be
determined using If the fingerprint is already known, it can be matched
and the key can be accepted or rejected. Because of the difficulty of comparing
host keys just by looking at hex strings, there is also support to compare
host keys visually, using By setting the option to 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: 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, 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: 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 option in for more
information.
contains support for Virtual
Private Network (VPN) tunnelling using the network pseudo-device, allowing
two networks to be joined securely. The configuration option 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 file
(see below) and the server option. The following entry would permit connections
on device 1 from user and on tun device 2 from user if is set to
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 and
will normally set the following environment variables:
The variable indicates the location of the X11 server. It is automatically
set by to point to a value of the form where indicates the host where
the shell runs, and is an integer Ge 1. uses this special value to forward
X11 connections over the secure channel. The user should normally not set
explicitly, as that will render the X11 connection insecure (and will
require the user to manually copy any required authorization cookies).
Set to the path of the user’s home directory. Synonym for set for compatibility
with systems that use this variable. Set to the path of the user’s mailbox.
Set to the default as specified when compiling If needs a passphrase,
it will read the passphrase from the current terminal if it was run from
a terminal. If does not have a terminal associated with it but and are
set, it will execute the program specified by and open an X11 window to
read the passphrase. This is particularly useful when calling from a or
related script. (Note that on some machines it may be necessary to redirect
the input from to make this work.) Identifies the path of a socket used
to communicate with the agent. 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.
This variable contains the original command line if a forced command is
executed. It can be used to extract the original arguments. 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. 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).
Set to the name of the user logging in. Additionally, reads and adds
lines of the format to the environment if the file exists and users are
allowed to change their environment. For more information, see the option
in
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 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. This file is used in exactly
the same way as but allows host-based authentication without permitting
login with rlogin/rsh. 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. 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 manual page. This file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by others.
This is the per-user configuration file. The file format and configuration
options are described in Because of the potential for abuse, this file
must have strict permissions: read/write for the user, and not writable
by others. Contains additional definitions for environment variables;
see above. 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). 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 3DES. Contains the public key for authentication. These
files are not sensitive and can (but need not) be readable by anyone.
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 for further
details of the format of this file. Commands in this file are executed
by when the user logs in, just before the user’s shell (or command) is
started. See the manual page for more information. This file is for host-based
authentication (see above). It should only be writable by root. This file
is used in exactly the same way as but allows host-based authentication
without permitting login with rlogin/rsh. Systemwide configuration file.
The file format and configuration options are described in These
files contain the private parts of the host keys and are used for host-based
authentication. If protocol version 1 is used, must be setuid root, since
the host key is readable only by root. For protocol version 2, uses to
access the host keys, eliminating the requirement that be setuid root
when host-based authentication is used. By default is not setuid root.
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 for further details of
the format of this file. Commands in this file are executed by when the
user logs in, just before the user’s shell (or command) is started. See the
manual page for more information.
exits with the exit status
of the remote command or with 255 if an error occurred.
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.
Table of Contents
![[Go to CFHT Home Page]](/images/cfht-icon.gif){kind=icon}
Man Pages 
Back to Software Index 
Manpage Top Level