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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\" $OpenBSD: ssh_config.5,v 1.29 2004/03/05 10:53:58 markus Exp $
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.Dd September 25, 1999
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.Nd OpenSSH SSH client configuration files
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.Bl -tag -width Ds -compact
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.It Pa $HOME/.ssh/config
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.It Pa /etc/ssh/ssh_config
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obtains configuration data from the following sources in
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.Bl -enum -offset indent -compact
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user's configuration file
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.Pq Pa $HOME/.ssh/config
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system-wide configuration file
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.Pq Pa /etc/ssh/ssh_config
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For each parameter, the first obtained value
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The configuration files contain sections bracketed by
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specifications, and that section is only applied for hosts that
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match one of the patterns given in the specification.
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The matched host name is the one given on the command line.
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Since the first obtained value for each parameter is used, more
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host-specific declarations should be given near the beginning of the
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file, and general defaults at the end.
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The configuration file has the following format:
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Empty lines and lines starting with
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Otherwise a line is of the format
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.Dq keyword arguments .
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Configuration options may be separated by whitespace or
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optional whitespace and exactly one
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the latter format is useful to avoid the need to quote whitespace
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when specifying configuration options using the
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keywords and their meanings are as follows (note that
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keywords are case-insensitive and arguments are case-sensitive):
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Restricts the following declarations (up to the next
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keyword) to be only for those hosts that match one of the patterns
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given after the keyword.
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can be used as wildcards in the
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as a pattern can be used to provide global
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defaults for all hosts.
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argument given on the command line (i.e., the name is not converted to
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a canonicalized host name before matching).
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Specifies which address family to use when connecting.
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passphrase/password querying will be disabled.
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This option is useful in scripts and other batch jobs where no user
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is present to supply the password.
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Specify the interface to transmit from on machines with multiple
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interfaces or aliased addresses.
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Note that this option does not work if
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.Cm UsePrivilegedPort
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.It Cm ChallengeResponseAuthentication
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Specifies whether to use challenge response authentication.
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The argument to this keyword must be
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If this flag is set to
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ssh will additionally check the host IP address in the
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This allows ssh to detect if a host key changed due to DNS spoofing.
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If the option is set to
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the check will not be executed.
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Specifies the cipher to use for encrypting the session
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in protocol version 1.
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is only supported in the
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client for interoperability with legacy protocol 1 implementations
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that do not support the
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Its use is strongly discouraged due to cryptographic weaknesses.
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Specifies the ciphers allowed for protocol version 2
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in order of preference.
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Multiple ciphers must be comma-separated.
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``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
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aes192-cbc,aes256-cbc''
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.It Cm ClearAllForwardings
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Specifies that all local, remote and dynamic port forwardings
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specified in the configuration files or on the command line be
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This option is primarily useful when used from the
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command line to clear port forwardings set in
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configuration files, and is automatically set by
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Specifies whether to use compression.
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.It Cm CompressionLevel
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Specifies the compression level to use if compression is enabled.
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The argument must be an integer from 1 (fast) to 9 (slow, best).
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The default level is 6, which is good for most applications.
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The meaning of the values is the same as in
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Note that this option applies to protocol version 1 only.
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.It Cm ConnectionAttempts
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Specifies the number of tries (one per second) to make before exiting.
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The argument must be an integer.
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This may be useful in scripts if the connection sometimes fails.
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.It Cm ConnectTimeout
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Specifies the timeout (in seconds) used when connecting to the ssh
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server, instead of using the default system TCP timeout.
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This value is used only when the target is down or really unreachable,
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not when it refuses the connection.
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.It Cm DynamicForward
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Specifies that a TCP/IP port on the local machine be forwarded
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over the secure channel, and the application
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protocol is then used to determine where to connect to from the
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The argument must be a port number.
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Currently the SOCKS4 and SOCKS5 protocols are supported, and
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will act as a SOCKS server.
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Multiple forwardings may be specified, and
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additional forwardings can be given on the command line.
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Only the superuser can forward privileged ports.
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.It Cm EnableSSHKeysign
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Setting this option to
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in the global client configuration file
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.Pa /etc/ssh/ssh_config
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enables the use of the helper program
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.Cm HostbasedAuthentication .
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This option should be placed in the non-hostspecific section.
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for more information.
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Sets the escape character (default:
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The escape character can also
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be set on the command line.
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The argument should be a single character,
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followed by a letter, or
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to disable the escape
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character entirely (making the connection transparent for binary
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Specifies whether the connection to the authentication agent (if any)
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will be forwarded to the remote machine.
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Agent forwarding should be enabled with caution.
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Users with the ability to bypass file permissions on the remote host
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(for the agent's Unix-domain socket)
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can access the local agent through the forwarded connection.
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An attacker cannot obtain key material from the agent,
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however they can perform operations on the keys that enable them to
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authenticate using the identities loaded into the agent.
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Specifies whether X11 connections will be automatically redirected
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over the secure channel and
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X11 forwarding should be enabled with caution.
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Users with the ability to bypass file permissions on the remote host
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(for the user's X11 authorization database)
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can access the local X11 display through the forwarded connection.
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An attacker may then be able to perform activities such as keystroke monitoring
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.Cm ForwardX11Trusted
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option is also enabled.
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.It Cm ForwardX11Trusted
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If the this option is set to
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then remote X11 clients will have full access to the original X11 display.
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If this option is set to
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then remote X11 clients will be considered untrusted and prevented
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from stealing or tampering with data belonging to trusted X11
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See the X11 SECURITY extension specification for full details on
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the restrictions imposed on untrusted clients.
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Specifies whether remote hosts are allowed to connect to local
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binds local port forwardings to the loopback address.
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This prevents other remote hosts from connecting to forwarded ports.
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can be used to specify that
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should bind local port forwardings to the wildcard address,
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thus allowing remote hosts to connect to forwarded ports.
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.It Cm GlobalKnownHostsFile
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Specifies a file to use for the global
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host key database instead of
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.Pa /etc/ssh/ssh_known_hosts .
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.It Cm GSSAPIAuthentication
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Specifies whether user authentication based on GSSAPI is allowed.
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Note that this option applies to protocol version 2 only.
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.It Cm GSSAPIDelegateCredentials
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Forward (delegate) credentials to the server.
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Note that this option applies to protocol version 2 only.
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.It Cm HostbasedAuthentication
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Specifies whether to try rhosts based authentication with public key
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This option applies to protocol version 2 only and
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.Cm RhostsRSAAuthentication .
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.It Cm HostKeyAlgorithms
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Specifies the protocol version 2 host key algorithms
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that the client wants to use in order of preference.
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The default for this option is:
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.Dq ssh-rsa,ssh-dss .
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Specifies an alias that should be used instead of the
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real host name when looking up or saving the host key
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in the host key database files.
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This option is useful for tunneling ssh connections
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or for multiple servers running on a single host.
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Specifies the real host name to log into.
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This can be used to specify nicknames or abbreviations for hosts.
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Default is the name given on the command line.
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Numeric IP addresses are also permitted (both on the command line and in
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Specifies a file from which the user's RSA or DSA authentication identity
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.Pa $HOME/.ssh/identity
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for protocol version 1, and
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.Pa $HOME/.ssh/id_rsa
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.Pa $HOME/.ssh/id_dsa
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for protocol version 2.
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Additionally, any identities represented by the authentication agent
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will be used for authentication.
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The file name may use the tilde
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syntax to refer to a user's home directory.
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It is possible to have
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multiple identity files specified in configuration files; all these
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identities will be tried in sequence.
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.It Cm IdentitiesOnly
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should only use the authentication identity files configured in the
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offers more identities.
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The argument to this keyword must be
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This option is intented for situations where
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offers many different identities.
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Specifies that a TCP/IP port on the local machine be forwarded over
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the secure channel to the specified host and port from the remote machine.
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The first argument must be a port number, and the second must be
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IPv6 addresses can be specified with an alternative syntax:
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Multiple forwardings may be specified, and additional
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forwardings can be given on the command line.
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Only the superuser can forward privileged ports.
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Gives the verbosity level that is used when logging messages from
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The possible values are:
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QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
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DEBUG and DEBUG1 are equivalent.
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DEBUG2 and DEBUG3 each specify higher levels of verbose output.
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Specifies the MAC (message authentication code) algorithms
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in order of preference.
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The MAC algorithm is used in protocol version 2
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for data integrity protection.
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Multiple algorithms must be comma-separated.
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.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
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.It Cm NoHostAuthenticationForLocalhost
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This option can be used if the home directory is shared across machines.
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In this case localhost will refer to a different machine on each of
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the machines and the user will get many warnings about changed host keys.
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However, this option disables host authentication for localhost.
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The argument to this keyword must be
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The default is to check the host key for localhost.
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.It Cm NumberOfPasswordPrompts
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Specifies the number of password prompts before giving up.
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The argument to this keyword must be an integer.
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.It Cm PasswordAuthentication
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Specifies whether to use password authentication.
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The argument to this keyword must be
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Specifies the port number to connect on the remote host.
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.It Cm PreferredAuthentications
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Specifies the order in which the client should try protocol 2
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authentication methods.
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This allows a client to prefer one method (e.g.
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.Cm keyboard-interactive )
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over another method (e.g.
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The default for this option is:
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.Dq hostbased,publickey,keyboard-interactive,password .
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Specifies the protocol versions
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should support in order of preference.
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The possible values are
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Multiple versions must be comma-separated.
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tries version 2 and falls back to version 1
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if version 2 is not available.
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Specifies the command to use to connect to the server.
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string extends to the end of the line, and is executed with
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In the command string,
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will be substituted by the host name to
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The command can be basically anything,
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and should read from its standard input and write to its standard output.
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It should eventually connect an
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server running on some machine, or execute
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Host key management will be done using the
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HostName of the host being connected (defaulting to the name typed by
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Setting the command to
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disables this option entirely.
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is not available for connects with a proxy command.
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.It Cm PubkeyAuthentication
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Specifies whether to try public key authentication.
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The argument to this keyword must be
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This option applies to protocol version 2 only.
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Specifies that a TCP/IP port on the remote machine be forwarded over
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the secure channel to the specified host and port from the local machine.
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The first argument must be a port number, and the second must be
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IPv6 addresses can be specified with an alternative syntax:
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Multiple forwardings may be specified, and additional
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forwardings can be given on the command line.
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Only the superuser can forward privileged ports.
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.It Cm RhostsRSAAuthentication
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Specifies whether to try rhosts based authentication with RSA host
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This option applies to protocol version 1 only and requires
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.It Cm RSAAuthentication
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Specifies whether to try RSA authentication.
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The argument to this keyword must be
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RSA authentication will only be
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attempted if the identity file exists, or an authentication agent is
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Note that this option applies to protocol version 1 only.
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.It Cm ServerAliveInterval
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Sets a timeout interval in seconds after which if no data has been received
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will send a message through the encrypted
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channel to request a response from the server.
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is 0, indicating that these messages will not be sent to the server.
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This option applies to protocol version 2 only.
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.It Cm ServerAliveCountMax
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Sets the number of server alive messages (see above) which may be
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receiving any messages back from the server.
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If this threshold is reached while server alive messages are being sent,
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will disconnect from the server, terminating the session.
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It is important to note that the use of server alive messages is very
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The server alive messages are sent through the encrypted channel
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and therefore will not be spoofable.
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The TCP keepalive option enabled by
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The server alive mechanism is valuable when the client or
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server depend on knowing when a connection has become inactive.
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The default value is 3.
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.Cm ServerAliveInterval
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(above) is set to 15, and
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.Cm ServerAliveCountMax
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is left at the default, if the server becomes unresponsive ssh
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will disconnect after approximately 45 seconds.
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.It Cm SmartcardDevice
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Specifies which smartcard device to use.
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The argument to this keyword is the device
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should use to communicate with a smartcard used for storing the user's
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By default, no device is specified and smartcard support is not activated.
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.It Cm StrictHostKeyChecking
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If this flag is set to
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will never automatically add host keys to the
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.Pa $HOME/.ssh/known_hosts
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file, and refuses to connect to hosts whose host key has changed.
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This provides maximum protection against trojan horse attacks,
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however, can be annoying when the
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.Pa /etc/ssh/ssh_known_hosts
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file is poorly maintained, or connections to new hosts are
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This option forces the user to manually
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If this flag is set to
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will automatically add new host keys to the
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user known hosts files.
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If this flag is set to
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will be added to the user known host files only after the user
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has confirmed that is what they really want to do, and
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will refuse to connect to hosts whose host key has changed.
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known hosts will be verified automatically in all cases.
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Specifies whether the system should send TCP keepalive messages to the
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If they are sent, death of the connection or crash of one
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of the machines will be properly noticed.
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However, this means that
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connections will die if the route is down temporarily, and some people
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(to send TCP keepalive messages), and the client will notice
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if the network goes down or the remote host dies.
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This is important in scripts, and many users want it too.
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To disable TCP keepalive messages, the value should be set to
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.It Cm UsePrivilegedPort
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Specifies whether to use a privileged port for outgoing connections.
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Note that this option must be set to
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.Cm RhostsRSAAuthentication
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Specifies the user to log in as.
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This can be useful when a different user name is used on different machines.
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This saves the trouble of
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having to remember to give the user name on the command line.
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.It Cm UserKnownHostsFile
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Specifies a file to use for the user
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host key database instead of
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.Pa $HOME/.ssh/known_hosts .
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.It Cm VerifyHostKeyDNS
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Specifies whether to verify the remote key using DNS and SSHFP resource
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If this option is set to
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the client will implicitly trust keys that match a secure fingerprint
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Insecure fingerprints will be handled as if this option was set to
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If this option is set to
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information on fingerprint match will be displayed, but the user will still
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need to confirm new host keys according to the
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.Cm StrictHostKeyChecking
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Note that this option applies to protocol version 2 only.
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Specifies the full pathname of the
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.Pa /usr/X11R6/bin/xauth .
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.It Pa $HOME/.ssh/config
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This is the per-user configuration file.
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The format of this file is described above.
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This file is used by the
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This file does not usually contain any sensitive information,
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but the recommended permissions are read/write for the user, and not
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accessible by others.
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.It Pa /etc/ssh/ssh_config
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Systemwide configuration file.
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This file provides defaults for those
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values that are not specified in the user's configuration file, and
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for those users who do not have a configuration file.
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This file must be world-readable.
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OpenSSH is a derivative of the original and free
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ssh 1.2.12 release by Tatu Ylonen.
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Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
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Theo de Raadt and Dug Song
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removed many bugs, re-added newer features and
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Markus Friedl contributed the support for SSH
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protocol versions 1.5 and 2.0.
added
removed
ssh_config.5
