ssh_config.5 - third_party/openssh-portable - Git at Google

 .\"  -*- nroff -*-
 .\"
 .\" 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
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
 .\" $OpenBSD: ssh_config.5,v 1.5 2002/08/29 22:54:10 stevesk Exp $
 .Dd September 25, 1999
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
 .Nm ssh_config
 .Nd OpenSSH SSH client configuration files
 .Sh SYNOPSIS
 .Bl -tag -width Ds -compact
 .It Pa $HOME/.ssh/config
 .It Pa /etc/ssh/ssh_config
 .El
 .Sh DESCRIPTION
 .Nm ssh
 obtains configuration data from the following sources in
 the following order:
 .Bl -enum -offset indent -compact
 .It
 command-line options
 .It
 user's configuration file
 .Pq Pa $HOME/.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 bracketed 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.
 .Pp
 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.
 .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.
 .Ql \&*
 and
 .Ql ?
 can be used as wildcards in the
 patterns.
 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).
 .It Cm AFSTokenPassing
 Specifies whether to pass AFS tokens to remote host.
 The argument to this keyword must be
 .Dq yes
 or
 .Dq no .
 This option applies to protocol version 1 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
 Specify the interface to transmit from on machines with multiple
 interfaces or aliased addresses.
 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 ,
 ssh 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
 .Nm ssh
 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 default is
 .Pp
 .Bd -literal
   ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
     aes192-cbc,aes256-cbc''
 .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
 .Nm ssh
 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 DynamicForward
 Specifies that a TCP/IP 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.  The argument must be a port number.
 Currently the SOCKS4 protocol is supported, and
 .Nm ssh
 will act as a SOCKS4 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 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 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 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.
 .It Cm GatewayPorts
 Specifies whether remote hosts are allowed to connect to local
 forwarded ports.
 By default,
 .Nm ssh
 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
 .Nm 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 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:
 .Dq ssh-rsa,ssh-dss .
 .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.
 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 IdentityFile
 Specifies a file from which the user's RSA or DSA authentication identity
 is read. The default is
 .Pa $HOME/.ssh/identity
 for protocol version 1, and
 .Pa $HOME/.ssh/id_rsa
 and
 .Pa $HOME/.ssh/id_dsa
 for protocol version 2.
 Additionally, any identities represented by the authentication agent
 will be used for authentication.
 The file name may use the tilde
 syntax to refer to a user's home directory.
 It is possible to have
 multiple identity files specified in configuration files; all these
 identities will be tried in sequence.
 .It Cm KeepAlive
 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 keepalives), 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 keepalives, the value should be set to
 .Dq no .
 .It Cm KerberosAuthentication
 Specifies whether Kerberos authentication will be used.
 The argument to this keyword must be
 .Dq yes
 or
 .Dq no .
 .It Cm KerberosTgtPassing
 Specifies whether a Kerberos TGT will be forwarded to the server.
 This will only work if the Kerberos server is actually an AFS kaserver.
 The argument to this keyword must be
 .Dq yes
 or
 .Dq no .
 .It Cm LocalForward
 Specifies that a TCP/IP 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 a port number, and the second must be
 .Ar host:port .
 IPv6 addresses can be specified with an alternative syntax:
 .Ar host/port .
 Multiple forwardings may be specified, and additional
 forwardings can be given on the command line.
 Only the superuser can forward privileged ports.
 .It Cm LogLevel
 Gives the verbosity level that is used when logging messages from
 .Nm ssh .
 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
 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
 .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.
 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 Port
 Specifies the port number to connect on the remote host.
 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 for this option is:
 .Dq hostbased,publickey,keyboard-interactive,password .
 .It Cm Protocol
 Specifies the protocol versions
 .Nm ssh
 should support in order of preference.
 The possible values are
 .Dq 1
 and
 .Dq 2 .
 Multiple versions must be comma-separated.
 The default is
 .Dq 2,1 .
 This means that
 .Nm ssh
 tries version 2 and falls back to version 1
 if version 2 is not available.
 .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
 .Pa /bin/sh .
 In the command string,
 .Ql %h
 will be substituted by the host name to
 connect and
 .Ql %p
 by the port.
 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).
 Note that
 .Cm CheckHostIP
 is not available for connects with a proxy command.
 .Pp
 .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 RemoteForward
 Specifies that a TCP/IP 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 a port number, and the second must be
 .Ar host:port .
 IPv6 addresses can be specified with an alternative syntax:
 .Ar host/port .
 Multiple forwardings may be specified, and additional
 forwardings can be given on the command line.
 Only the superuser can forward privileged ports.
 .It Cm RhostsAuthentication
 Specifies whether to try rhosts based authentication.
 Note that this
 declaration only affects the client side and has no effect whatsoever
 on security.
 Most servers do not permit RhostsAuthentication because it
 is not secure (see
 .Cm RhostsRSAAuthentication ) .
 The argument to this keyword must be
 .Dq yes
 or
 .Dq no .
 The default is
 .Dq no .
 This option applies to protocol version 1 only and requires
 .Nm ssh
 to be setuid root and
 .Cm UsePrivilegedPort
 to be set to
 .Dq yes .
 .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
 .Nm ssh
 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 SmartcardDevice
 Specifies which smartcard device to use. The argument to this keyword is
 the device
 .Nm ssh
 should use to communicate with a smartcard used for storing the user's
 private RSA key. By default, no device is specified and smartcard support
 is not activated.
 .It Cm StrictHostKeyChecking
 If this flag is set to
 .Dq yes ,
 .Nm ssh
 will never automatically add host keys to the
 .Pa $HOME/.ssh/known_hosts
 file, and refuses to connect to hosts whose host key has changed.
 This provides maximum protection against trojan horse attacks,
 however, can be annoying when the
 .Pa /etc/ssh/ssh_known_hosts
 file is poorly maintained, or 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 ,
 .Nm 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
 .Nm 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 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
 .Nm ssh
 must be setuid root.
 Note that this option must be set to
 .Dq yes
 if
 .Cm RhostsAuthentication
 and
 .Cm RhostsRSAAuthentication
 authentications are needed 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 $HOME/.ssh/known_hosts .
 .It Cm XAuthLocation
 Specifies the full pathname of the
 .Xr xauth 1
 program.
 The default is
 .Pa /usr/X11R6/bin/xauth .
 .El
 .Sh FILES
 .Bl -tag -width Ds
 .It Pa $HOME/.ssh/config
 This is the per-user configuration file.
 The format of this file is described above.
 This file is used by the
 .Nm ssh
 client.
 This file does not usually contain any sensitive information,
 but the recommended permissions are 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 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.
 .Sh SEE ALSO
 .Xr ssh 1
	.\" -- nroff --
	.\"
	.\" 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
	.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
	.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	.\"
	.\" $OpenBSD: ssh_config.5,v 1.5 2002/08/29 22:54:10 stevesk Exp $
	.Dd September 25, 1999
	.Dt SSH_CONFIG 5
	.Os
	.Sh NAME
	.Nm ssh_config
	.Nd OpenSSH SSH client configuration files
	.Sh SYNOPSIS
	.Bl -tag -width Ds -compact
	.It Pa $HOME/.ssh/config
	.It Pa /etc/ssh/ssh_config
	.El
	.Sh DESCRIPTION
	.Nm ssh
	obtains configuration data from the following sources in
	the following order:
	.Bl -enum -offset indent -compact
	.It
	command-line options
	.It
	user's configuration file
	.Pq Pa $HOME/.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 bracketed 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.
	.Pp
	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.
	.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.
	.Ql \&*
	and
	.Ql ?
	can be used as wildcards in the
	patterns.
	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).
	.It Cm AFSTokenPassing
	Specifies whether to pass AFS tokens to remote host.
	The argument to this keyword must be
	.Dq yes
	or
	.Dq no .
	This option applies to protocol version 1 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
	Specify the interface to transmit from on machines with multiple
	interfaces or aliased addresses.
	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 ,
	ssh 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
	.Nm ssh
	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 default is
	.Pp
	.Bd -literal
	``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
	aes192-cbc,aes256-cbc''
	.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
	.Nm ssh
	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 DynamicForward
	Specifies that a TCP/IP 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. The argument must be a port number.
	Currently the SOCKS4 protocol is supported, and
	.Nm ssh
	will act as a SOCKS4 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 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 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 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.
	.It Cm GatewayPorts
	Specifies whether remote hosts are allowed to connect to local
	forwarded ports.
	By default,
	.Nm ssh
	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
	.Nm 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 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:
	.Dq ssh-rsa,ssh-dss .
	.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.
	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 IdentityFile
	Specifies a file from which the user's RSA or DSA authentication identity
	is read. The default is
	.Pa $HOME/.ssh/identity
	for protocol version 1, and
	.Pa $HOME/.ssh/id_rsa
	and
	.Pa $HOME/.ssh/id_dsa
	for protocol version 2.
	Additionally, any identities represented by the authentication agent
	will be used for authentication.
	The file name may use the tilde
	syntax to refer to a user's home directory.
	It is possible to have
	multiple identity files specified in configuration files; all these
	identities will be tried in sequence.
	.It Cm KeepAlive
	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 keepalives), 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 keepalives, the value should be set to
	.Dq no .
	.It Cm KerberosAuthentication
	Specifies whether Kerberos authentication will be used.
	The argument to this keyword must be
	.Dq yes
	or
	.Dq no .
	.It Cm KerberosTgtPassing
	Specifies whether a Kerberos TGT will be forwarded to the server.
	This will only work if the Kerberos server is actually an AFS kaserver.
	The argument to this keyword must be
	.Dq yes
	or
	.Dq no .
	.It Cm LocalForward
	Specifies that a TCP/IP 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 a port number, and the second must be
	.Ar host:port .
	IPv6 addresses can be specified with an alternative syntax:
	.Ar host/port .
	Multiple forwardings may be specified, and additional
	forwardings can be given on the command line.
	Only the superuser can forward privileged ports.
	.It Cm LogLevel
	Gives the verbosity level that is used when logging messages from
	.Nm ssh .
	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
	.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
	.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.
	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 Port
	Specifies the port number to connect on the remote host.
	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 for this option is:
	.Dq hostbased,publickey,keyboard-interactive,password .
	.It Cm Protocol
	Specifies the protocol versions
	.Nm ssh
	should support in order of preference.
	The possible values are
	.Dq 1
	and
	.Dq 2 .
	Multiple versions must be comma-separated.
	The default is
	.Dq 2,1 .
	This means that
	.Nm ssh
	tries version 2 and falls back to version 1
	if version 2 is not available.
	.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
	.Pa /bin/sh .
	In the command string,
	.Ql %h
	will be substituted by the host name to
	connect and
	.Ql %p
	by the port.
	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).
	Note that
	.Cm CheckHostIP
	is not available for connects with a proxy command.
	.Pp
	.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 RemoteForward
	Specifies that a TCP/IP 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 a port number, and the second must be
	.Ar host:port .
	IPv6 addresses can be specified with an alternative syntax:
	.Ar host/port .
	Multiple forwardings may be specified, and additional
	forwardings can be given on the command line.
	Only the superuser can forward privileged ports.
	.It Cm RhostsAuthentication
	Specifies whether to try rhosts based authentication.
	Note that this
	declaration only affects the client side and has no effect whatsoever
	on security.
	Most servers do not permit RhostsAuthentication because it
	is not secure (see
	.Cm RhostsRSAAuthentication ) .
	The argument to this keyword must be
	.Dq yes
	or
	.Dq no .
	The default is
	.Dq no .
	This option applies to protocol version 1 only and requires
	.Nm ssh
	to be setuid root and
	.Cm UsePrivilegedPort
	to be set to
	.Dq yes .
	.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
	.Nm ssh
	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 SmartcardDevice
	Specifies which smartcard device to use. The argument to this keyword is
	the device
	.Nm ssh
	should use to communicate with a smartcard used for storing the user's
	private RSA key. By default, no device is specified and smartcard support
	is not activated.
	.It Cm StrictHostKeyChecking
	If this flag is set to
	.Dq yes ,
	.Nm ssh
	will never automatically add host keys to the
	.Pa $HOME/.ssh/known_hosts
	file, and refuses to connect to hosts whose host key has changed.
	This provides maximum protection against trojan horse attacks,
	however, can be annoying when the
	.Pa /etc/ssh/ssh_known_hosts
	file is poorly maintained, or 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 ,
	.Nm 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
	.Nm 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 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
	.Nm ssh
	must be setuid root.
	Note that this option must be set to
	.Dq yes
	if
	.Cm RhostsAuthentication
	and
	.Cm RhostsRSAAuthentication
	authentications are needed 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 $HOME/.ssh/known_hosts .
	.It Cm XAuthLocation
	Specifies the full pathname of the
	.Xr xauth 1
	program.
	The default is
	.Pa /usr/X11R6/bin/xauth .
	.El
	.Sh FILES
	.Bl -tag -width Ds
	.It Pa $HOME/.ssh/config
	This is the per-user configuration file.
	The format of this file is described above.
	This file is used by the
	.Nm ssh
	client.
	This file does not usually contain any sensitive information,
	but the recommended permissions are 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 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.
	.Sh SEE ALSO
	.Xr ssh 1