openssh/ssh_config.5

.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\"
.\" $OpenBSD: ssh_config.5,v 1.146 2010/12/08 04:02:47 djm Exp $
.Dd $Mdocdate: December 8 2010 $
.Dt SSH_CONFIG 5
.Os
.Sh NAME
.Nm ssh_config
.Nd OpenSSH SSH client configuration files
.Sh SYNOPSIS
.Nm ~/.ssh/config
.Nm /etc/ssh/ssh_config
.Sh DESCRIPTION
.Xr ssh 1
obtains configuration data from the following sources in
the following order:
.Pp
.Bl -enum -offset indent -compact
.It
command-line options
.It
user's configuration file
.Pq Pa ~/.ssh/config
.It
system-wide configuration file
.Pq Pa /etc/ssh/ssh_config
.El
.Pp
For each parameter, the first obtained value
will be used.
The configuration files contain sections separated by
.Dq Host
specifications, and that section is only applied for hosts that
match one of the patterns given in the specification.
The matched host name is the one given on the command line.
.Pp
Since the first obtained value for each parameter is used, more
host-specific declarations should be given near the beginning of the
file, and general defaults at the end.
.Pp
The configuration file has the following format:
.Pp
Empty lines and lines starting with
.Ql #
are comments.
Otherwise a line is of the format
.Dq keyword arguments .
Configuration options may be separated by whitespace or
optional whitespace and exactly one
.Ql = ;
the latter format is useful to avoid the need to quote whitespace
when specifying configuration options using the
.Nm ssh ,
.Nm scp ,
and
.Nm sftp
.Fl o
option.
Arguments may optionally be enclosed in double quotes
.Pq \&"
in order to represent arguments containing spaces.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm Host
Restricts the following declarations (up to the next
.Cm Host
keyword) to be only for those hosts that match one of the patterns
given after the keyword.
If more than one pattern is provided, they should be separated by whitespace.
A single
.Ql *
as a pattern can be used to provide global
defaults for all hosts.
The host is the
.Ar hostname
argument given on the command line (i.e. the name is not converted to
a canonicalized host name before matching).
.Pp
See
.Sx PATTERNS
for more information on patterns.
.It Cm AddressFamily
Specifies which address family to use when connecting.
Valid arguments are
.Dq any ,
.Dq inet
(use IPv4 only), or
.Dq inet6
(use IPv6 only).
.It Cm BatchMode
If set to
.Dq yes ,
passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where no user
is present to supply the password.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm BindAddress
Use the specified address on the local machine as the source address of
the connection.
Only useful on systems with more than one address.
Note that this option does not work if
.Cm UsePrivilegedPort
is set to
.Dq yes .
.It Cm ChallengeResponseAuthentication
Specifies whether to use challenge-response authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm CheckHostIP
If this flag is set to
.Dq yes ,
.Xr ssh 1
will additionally check the host IP address in the
.Pa known_hosts
file.
This allows ssh to detect if a host key changed due to DNS spoofing.
If the option is set to
.Dq no ,
the check will not be executed.
The default is
.Dq yes .
.It Cm Cipher
Specifies the cipher to use for encrypting the session
in protocol version 1.
Currently,
.Dq blowfish ,
.Dq 3des ,
and
.Dq des
are supported.
.Ar des
is only supported in the
.Xr ssh 1
client for interoperability with legacy protocol 1 implementations
that do not support the
.Ar 3des
cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
.Dq 3des .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
The supported ciphers are
.Dq 3des-cbc ,
.Dq aes128-cbc ,
.Dq aes192-cbc ,
.Dq aes256-cbc ,
.Dq aes128-ctr ,
.Dq aes192-ctr ,
.Dq aes256-ctr ,
.Dq arcfour128 ,
.Dq arcfour256 ,
.Dq arcfour ,
.Dq blowfish-cbc ,
and
.Dq cast128-cbc .
The default is:
.Bd -literal -offset 3n
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
.Ed
.It Cm ClearAllForwardings
Specifies that all local, remote, and dynamic port forwardings
specified in the configuration files or on the command line be
cleared.
This option is primarily useful when used from the
.Xr ssh 1
command line to clear port forwardings set in
configuration files, and is automatically set by
.Xr scp 1
and
.Xr sftp 1 .
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm Compression
Specifies whether to use compression.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm CompressionLevel
Specifies the compression level to use if compression is enabled.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications.
The meaning of the values is the same as in
.Xr gzip 1 .
Note that this option applies to protocol version 1 only.
.It Cm ConnectionAttempts
Specifies the number of tries (one per second) to make before exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
The default is 1.
.It Cm ConnectTimeout
Specifies the timeout (in seconds) used when connecting to the
SSH server, instead of using the default system TCP timeout.
This value is used only when the target is down or really unreachable,
not when it refuses the connection.
.It Cm ControlMaster
Enables the sharing of multiple sessions over a single network connection.
When set to
.Dq yes ,
.Xr ssh 1
will listen for connections on a control socket specified using the
.Cm ControlPath
argument.
Additional sessions can connect to this socket using the same
.Cm ControlPath
with
.Cm ControlMaster
set to
.Dq no
(the default).
These sessions will try to reuse the master instance's network connection
rather than initiating new ones, but will fall back to connecting normally
if the control socket does not exist, or is not listening.
.Pp
Setting this to
.Dq ask
will cause ssh
to listen for control connections, but require confirmation using the
.Ev SSH_ASKPASS
program before they are accepted (see
.Xr ssh-add 1
for details).
If the
.Cm ControlPath
cannot be opened,
ssh will continue without connecting to a master instance.
.Pp
X11 and
.Xr ssh-agent 1
forwarding is supported over these multiplexed connections, however the
display and agent forwarded will be the one belonging to the master
connection i.e. it is not possible to forward multiple displays or agents.
.Pp
Two additional options allow for opportunistic multiplexing: try to use a
master connection but fall back to creating a new one if one does not already
exist.
These options are:
.Dq auto
and
.Dq autoask .
The latter requires confirmation like the
.Dq ask
option.
.It Cm ControlPath
Specify the path to the control socket used for connection sharing as described
in the
.Cm ControlMaster
section above or the string
.Dq none
to disable connection sharing.
In the path,
.Ql %l
will be substituted by the local host name,
.Ql %h
will be substituted by the target host name,
.Ql %p
the port, and
.Ql %r
by the remote login username.
It is recommended that any
.Cm ControlPath
used for opportunistic connection sharing include
at least %h, %p, and %r.
This ensures that shared connections are uniquely identified.
.It Cm ControlPersist
When used in conjunction with
.Cm ControlMaster ,
specifies that the master connection should remain open
in the background (waiting for future client connections)
after the initial client connection has been closed.
If set to
.Dq no ,
then the master connection will not be placed into the background,
and will close as soon as the initial client connection is closed.
If set to
.Dq yes ,
then the master connection will remain in the background indefinitely
(until killed or closed via a mechanism such as the
.Xr ssh 1
.Dq Fl O No exit
option).
If set to a time in seconds, or a time in any of the formats documented in
.Xr sshd_config 5 ,
then the backgrounded master connection will automatically terminate
after it has remained idle (with no client connections) for the
specified time.
.It Cm DynamicForward
Specifies that a TCP port on the local machine be forwarded
over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine.
.Pp
The argument must be
.Sm off
.Oo Ar bind_address : Oc Ar port .
.Sm on
IPv6 addresses can be specified by enclosing addresses in square brackets.
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Dq localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.Pp
Currently the SOCKS4 and SOCKS5 protocols are supported, and
.Xr ssh 1
will act as a SOCKS server.
Multiple forwardings may be specified, and
additional forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.It Cm EnableSSHKeysign
Setting this option to
.Dq yes
in the global client configuration file
.Pa /etc/ssh/ssh_config
enables the use of the helper program
.Xr ssh-keysign 8
during
.Cm HostbasedAuthentication .
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
This option should be placed in the non-hostspecific section.
See
.Xr ssh-keysign 8
for more information.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
The escape character can also
be set on the command line.
The argument should be a single character,
.Ql ^
followed by a letter, or
.Dq none
to disable the escape
character entirely (making the connection transparent for binary
data).
.It Cm ExitOnForwardFailure
Specifies whether
.Xr ssh 1
should terminate the connection if it cannot set up all requested
dynamic, tunnel, local, and remote port forwardings.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm ForwardAgent
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.Pp
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.
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
.Ev DISPLAY
set.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.Pp
X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user's X11 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
if the
.Cm ForwardX11Trusted
option is also enabled.
.It Cm ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding
using the format described in the
.Sx TIME FORMATS
section of
.Xr sshd_config 5 .
X11 connections received by
.Xr ssh 1
after this time will be refused.
The default is to disable untrusted X11 forwarding after twenty minutes has
elapsed.
.It Cm ForwardX11Trusted
If this option is set to
.Dq yes ,
remote X11 clients will have full access to the original X11 display.
.Pp
If this option is set to
.Dq no ,
remote X11 clients will be considered untrusted and prevented
from stealing or tampering with data belonging to trusted X11
clients.
Furthermore, the
.Xr xauth 1
token used for the session will be set to expire after 20 minutes.
Remote clients will be refused access after this time.
.Pp
The default is
.Dq no .
.Pp
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
By default,
.Xr ssh 1
binds local port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that ssh
should bind local port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm GlobalKnownHostsFile
Specifies a file to use for the global
host key database instead of
.Pa /etc/ssh/ssh_known_hosts .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm GSSAPIDelegateCredentials
Forward (delegate) credentials to the server.
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm HashKnownHosts
Indicates that
.Xr ssh 1
should hash host names and addresses when they are added to
.Pa ~/.ssh/known_hosts .
These hashed names may be used normally by
.Xr ssh 1
and
.Xr sshd 8 ,
but they do not reveal identifying information should the file's contents
be disclosed.
The default is
.Dq no .
Note that existing names and addresses in known hosts files
will not be converted automatically,
but may be manually hashed using
.Xr ssh-keygen 1 .
.It Cm HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key
authentication.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
This option applies to protocol version 2 only and
is similar to
.Cm RhostsRSAAuthentication .
.It Cm HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms
that the client wants to use in order of preference.
The default for this option is:
.Bd -literal -offset 3n
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-rsa,ssh-dss
.Ed
.Pp
If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.
.It Cm HostKeyAlias
Specifies an alias that should be used instead of the
real host name when looking up or saving the host key
in the host key database files.
This option is useful for tunneling SSH connections
or for multiple servers running on a single host.
.It Cm HostName
Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
If the hostname contains the character sequence
.Ql %h ,
then this will be replaced with the host name specified on the commandline
(this is useful for manipulating unqualified names).
The default is the name given on the command line.
Numeric IP addresses are also permitted (both on the command line and in
.Cm HostName
specifications).
.It Cm IdentitiesOnly
Specifies that
.Xr ssh 1
should only use the authentication identity files configured in the
.Nm
files,
even if
.Xr ssh-agent 1
offers more identities.
The argument to this keyword must be
.Dq yes
or
.Dq no .
This option is intended for situations where ssh-agent
offers many different identities.
The default is
.Dq no .
.It Cm IdentityFile
Specifies a file from which the user's DSA, ECDSA or DSA authentication
identity is read.
The default is
.Pa ~/.ssh/identity
for protocol version 1, and
.Pa ~/.ssh/id_dsa ,
.Pa ~/.ssh/id_ecdsa
and
.Pa ~/.ssh/id_rsa
for protocol version 2.
Additionally, any identities represented by the authentication agent
will be used for authentication.
.Xr ssh 1
will try to load certificate information from the filename obtained by
appending
.Pa -cert.pub
to the path of a specified
.Cm IdentityFile .
.Pp
The file name may use the tilde
syntax to refer to a user's home directory or one of the following
escape characters:
.Ql %d
(local user's home directory),
.Ql %u
(local user name),
.Ql %l
(local host name),
.Ql %h
(remote host name) or
.Ql %r
(remote user name).
.Pp
It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
.It Cm IPQoS
Specifies the IPv4 type-of-service or DSCP class for connections.
Accepted values are
.Dq af11 ,
.Dq af12 ,
.Dq af13 ,
.Dq af14 ,
.Dq af22 ,
.Dq af23 ,
.Dq af31 ,
.Dq af32 ,
.Dq af33 ,
.Dq af41 ,
.Dq af42 ,
.Dq af43 ,
.Dq cs0 ,
.Dq cs1 ,
.Dq cs2 ,
.Dq cs3 ,
.Dq cs4 ,
.Dq cs5 ,
.Dq cs6 ,
.Dq cs7 ,
.Dq ef ,
.Dq lowdelay ,
.Dq throughput ,
.Dq reliability ,
or a numeric value.
This option may take one or two arguments, separated by whitespace.
If one argument is specified, it is used as the packet class unconditionally.
If two values are specified, the first is automatically selected for
interactive sessions and the second for non-interactive sessions.
The default is
.Dq lowdelay
for interactive sessions and
.Dq throughput
for non-interactive sessions.
.It Cm KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm KbdInteractiveDevices
Specifies the list of methods to use in keyboard-interactive authentication.
Multiple method names must be comma-separated.
The default is to use the server specified list.
The methods available vary depending on what the server supports.
For an OpenSSH server,
it may be zero or more of:
.Dq bsdauth ,
.Dq pam ,
and
.Dq skey .
.It Cm KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
The default is:
.Bd -literal -offset indent
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group14-sha1,
diffie-hellman-group1-sha1
.Ed
.It Cm LocalCommand
Specifies a command to execute on the local machine after successfully
connecting to the server.
The command string extends to the end of the line, and is executed with
the user's shell.
The following escape character substitutions will be performed:
.Ql %d
(local user's home directory),
.Ql %h
(remote host name),
.Ql %l
(local host name),
.Ql %n
(host name as provided on the command line),
.Ql %p
(remote port),
.Ql %r
(remote user name) or
.Ql %u
(local user name).
.Pp
The command is run synchronously and does not have access to the
session of the
.Xr ssh 1
that spawned it.
It should not be used for interactive commands.
.Pp
This directive is ignored unless
.Cm PermitLocalCommand
has been enabled.
.It Cm LocalForward
Specifies that a TCP port on the local machine be forwarded over
the secure channel to the specified host and port from the remote machine.
The first argument must be
.Sm off
.Oo Ar bind_address : Oc Ar port
.Sm on
and the second argument must be
.Ar host : Ns Ar hostport .
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional forwardings can be
given on the command line.
Only the superuser can forward privileged ports.
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Dq localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Xr ssh 1 .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO.
DEBUG and DEBUG1 are equivalent.
DEBUG2 and DEBUG3 each specify higher levels of verbose output.
.It Cm MACs
Specifies the MAC (message authentication code) algorithms
in order of preference.
The MAC algorithm is used in protocol version 2
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is:
.Bd -literal -offset indent
hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
.Ed
.It Cm NoHostAuthenticationForLocalhost
This option can be used if the home directory is shared across machines.
In this case localhost will refer to a different machine on each of
the machines and the user will get many warnings about changed host keys.
However, this option disables host authentication for localhost.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is to check the host key for localhost.
.It Cm NumberOfPasswordPrompts
Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
The default is 3.
.It Cm PasswordAuthentication
Specifies whether to use password authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm PermitLocalCommand
Allow local command execution via the
.Ic LocalCommand
option or using the
.Ic !\& Ns Ar command
escape sequence in
.Xr ssh 1 .
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm PKCS11Provider
Specifies which PKCS#11 provider to use.
The argument to this keyword is the PKCS#11 shared library
.Xr ssh 1
should use to communicate with a PKCS#11 token providing the user's
private RSA key.
.It Cm Port
Specifies the port number to connect on the remote host.
The default is 22.
.It Cm PreferredAuthentications
Specifies the order in which the client should try protocol 2
authentication methods.
This allows a client to prefer one method (e.g.\&
.Cm keyboard-interactive )
over another method (e.g.\&
.Cm password ) .
The default is:
.Bd -literal -offset indent
gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password
.Ed
.It Cm Protocol
Specifies the protocol versions
.Xr ssh 1
should support in order of preference.
The possible values are
.Sq 1
and
.Sq 2 .
Multiple versions must be comma-separated.
When this option is set to
.Dq 2,1
.Nm ssh
will try version 2 and fall back to version 1
if version 2 is not available.
The default is
.Sq 2 .
.It Cm ProxyCommand
Specifies the command to use to connect to the server.
The command
string extends to the end of the line, and is executed with
the user's shell.
In the command string, any occurrence of
.Ql %h
will be substituted by the host name to
connect,
.Ql %p
by the port, and
.Ql %r
by the remote user name.
The command can be basically anything,
and should read from its standard input and write to its standard output.
It should eventually connect an
.Xr sshd 8
server running on some machine, or execute
.Ic sshd -i
somewhere.
Host key management will be done using the
HostName of the host being connected (defaulting to the name typed by
the user).
Setting the command to
.Dq none
disables this option entirely.
Note that
.Cm CheckHostIP
is not available for connects with a proxy command.
.Pp
This directive is useful in conjunction with
.Xr nc 1
and its proxy support.
For example, the following directive would connect via an HTTP proxy at
192.0.2.0:
.Bd -literal -offset 3n
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
.Ed
.It Cm PubkeyAuthentication
Specifies whether to try public key authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
This option applies to protocol version 2 only.
.It Cm RekeyLimit
Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated.
The argument is the number of bytes, with an optional suffix of
.Sq K ,
.Sq M ,
or
.Sq G
to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
The default is between
.Sq 1G
and
.Sq 4G ,
depending on the cipher.
This option applies to protocol version 2 only.
.It Cm RemoteForward
Specifies that a TCP port on the remote machine be forwarded over
the secure channel to the specified host and port from the local machine.
The first argument must be
.Sm off
.Oo Ar bind_address : Oc Ar port
.Sm on
and the second argument must be
.Ar host : Ns Ar hostport .
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Privileged ports can be forwarded only when
logging in as root on the remote machine.
.Pp
If the
.Ar port
argument is
.Ql 0 ,
the listen port will be dynamically allocated on the server and reported
to the client at run time.
.Pp
If the
.Ar bind_address
is not specified, the default is to only bind to loopback addresses.
If the
.Ar bind_address
is
.Ql *
or an empty string, then the forwarding is requested to listen on all
interfaces.
Specifying a remote
.Ar bind_address
will only succeed if the server's
.Cm GatewayPorts
option is enabled (see
.Xr sshd_config 5 ) .
.It Cm RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host
authentication.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
This option applies to protocol version 1 only and requires
.Xr ssh 1
to be setuid root.
.It Cm RSAAuthentication
Specifies whether to try RSA authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
The default is
.Dq yes .
Note that this option applies to protocol version 1 only.
.It Cm SendEnv
Specifies what variables from the local
.Xr environ 7
should be sent to the server.
Note that environment passing is only supported for protocol 2.
The server must also support it, and the server must be configured to
accept these environment variables.
Refer to
.Cm AcceptEnv
in
.Xr sshd_config 5
for how to configure the server.
Variables are specified by name, which may contain wildcard characters.
Multiple environment variables may be separated by whitespace or spread
across multiple
.Cm SendEnv
directives.
The default is not to send any environment variables.
.Pp
See
.Sx PATTERNS
for more information on patterns.
.It Cm ServerAliveCountMax
Sets the number of server alive messages (see below) which may be
sent without
.Xr ssh 1
receiving any messages back from the server.
If this threshold is reached while server alive messages are being sent,
ssh will disconnect from the server, terminating the session.
It is important to note that the use of server alive messages is very
different from
.Cm TCPKeepAlive
(below).
The server alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
.Cm TCPKeepAlive
is spoofable.
The server alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.
.Pp
The default value is 3.
If, for example,
.Cm ServerAliveInterval
(see below) is set to 15 and
.Cm ServerAliveCountMax
is left at the default, if the server becomes unresponsive,
ssh will disconnect after approximately 45 seconds.
This option applies to protocol version 2 only.
.It Cm ServerAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the server,
.Xr ssh 1
will send a message through the encrypted
channel to request a response from the server.
The default
is 0, indicating that these messages will not be sent to the server.
This option applies to protocol version 2 only.
.It Cm StrictHostKeyChecking
If this flag is set to
.Dq yes ,
.Xr ssh 1
will never automatically add host keys to the
.Pa ~/.ssh/known_hosts
file, and refuses to connect to hosts whose host key has changed.
This provides maximum protection against trojan horse attacks,
though it can be annoying when the
.Pa /etc/ssh/ssh_known_hosts
file is poorly maintained or when connections to new hosts are
frequently made.
This option forces the user to manually
add all new hosts.
If this flag is set to
.Dq no ,
ssh will automatically add new host keys to the
user known hosts files.
If this flag is set to
.Dq ask ,
new host keys
will be added to the user known host files only after the user
has confirmed that is what they really want to do, and
ssh will refuse to connect to hosts whose host key has changed.
The host keys of
known hosts will be verified automatically in all cases.
The argument must be
.Dq yes ,
.Dq no ,
or
.Dq ask .
The default is
.Dq ask .
.It Cm TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
.Pp
The default is
.Dq yes
(to send TCP keepalive messages), and the client will notice
if the network goes down or the remote host dies.
This is important in scripts, and many users want it too.
.Pp
To disable TCP keepalive messages, the value should be set to
.Dq no .
.It Cm Tunnel
Request
.Xr tun 4
device forwarding between the client and the server.
The argument must be
.Dq yes ,
.Dq point-to-point
(layer 3),
.Dq ethernet
(layer 2),
or
.Dq no .
Specifying
.Dq yes
requests the default tunnel mode, which is
.Dq point-to-point .
The default is
.Dq no .
.It Cm TunnelDevice
Specifies the
.Xr tun 4
devices to open on the client
.Pq Ar local_tun
and the server
.Pq Ar remote_tun .
.Pp
The argument must be
.Sm off
.Ar local_tun Op : Ar remote_tun .
.Sm on
The devices may be specified by numerical ID or the keyword
.Dq any ,
which uses the next available tunnel device.
If
.Ar remote_tun
is not specified, it defaults to
.Dq any .
The default is
.Dq any:any .
.It Cm UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
If set to
.Dq yes ,
.Xr ssh 1
must be setuid root.
Note that this option must be set to
.Dq yes
for
.Cm RhostsRSAAuthentication
with older servers.
.It Cm User
Specifies the user to log in as.
This can be useful when a different user name is used on different machines.
This saves the trouble of
having to remember to give the user name on the command line.
.It Cm UserKnownHostsFile
Specifies a file to use for the user
host key database instead of
.Pa ~/.ssh/known_hosts .
.It Cm VerifyHostKeyDNS
Specifies whether to verify the remote key using DNS and SSHFP resource
records.
If this option is set to
.Dq yes ,
the client will implicitly trust keys that match a secure fingerprint
from DNS.
Insecure fingerprints will be handled as if this option was set to
.Dq ask .
If this option is set to
.Dq ask ,
information on fingerprint match will be displayed, but the user will still
need to confirm new host keys according to the
.Cm StrictHostKeyChecking
option.
The argument must be
.Dq yes ,
.Dq no ,
or
.Dq ask .
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.Pp
See also
.Sx VERIFYING HOST KEYS
in
.Xr ssh 1 .
.It Cm VisualHostKey
If this flag is set to
.Dq yes ,
an ASCII art representation of the remote host key fingerprint is
printed in addition to the hex fingerprint string at login and
for unknown host keys.
If this flag is set to
.Dq no ,
no fingerprint strings are printed at login and
only the hex fingerprint string will be printed for unknown host keys.
The default is
.Dq no .
.It Cm XAuthLocation
Specifies the full pathname of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Sh PATTERNS
A
.Em pattern
consists of zero or more non-whitespace characters,
.Sq *
(a wildcard that matches zero or more characters),
or
.Sq ?\&
(a wildcard that matches exactly one character).
For example, to specify a set of declarations for any host in the
.Dq .co.uk
set of domains,
the following pattern could be used:
.Pp
.Dl Host *.co.uk
.Pp
The following pattern
would match any host in the 192.168.0.[0-9] network range:
.Pp
.Dl Host 192.168.0.?
.Pp
A
.Em pattern-list
is a comma-separated list of patterns.
Patterns within pattern-lists may be negated
by preceding them with an exclamation mark
.Pq Sq !\& .
For example,
to allow a key to be used from anywhere within an organisation
except from the
.Dq dialup
pool,
the following entry (in authorized_keys) could be used:
.Pp
.Dl from=\&"!*.dialup.example.com,*.example.com\&"
.Sh FILES
.Bl -tag -width Ds
.It Pa ~/.ssh/config
This is the per-user configuration file.
The format of this file is described above.
This file is used by the SSH client.
Because of the potential for abuse, this file must have strict permissions:
read/write for the user, and not accessible by others.
.It Pa /etc/ssh/ssh_config
Systemwide configuration file.
This file provides defaults for those
values that are not specified in the user's configuration file, and
for those users who do not have a configuration file.
This file must be world-readable.
.El
.Sh SEE ALSO
.Xr ssh 1
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.