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RedHat 9 (Linux i386) - man page for ssh_config (redhat section 5)

SSH_CONFIG(5)			     BSD File Formats Manual			    SSH_CONFIG(5)

NAME
     ssh_config -- OpenSSH SSH client configuration files

SYNOPSIS
     $HOME/.ssh/config
     /etc/ssh/ssh_config

DESCRIPTION
     ssh obtains configuration data from the following sources in the following order:
	   1.	command-line options
	   2.	user's configuration file ($HOME/.ssh/config)
	   3.	system-wide configuration file (/etc/ssh/ssh_config)

     For each parameter, the first obtained value will be used.  The configuration files contain
     sections bracketed by ``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.

     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.

     The configuration file has the following format:

     Empty lines and lines starting with '#' are comments.

     Otherwise a line is of the format ``keyword arguments''.  Configuration options may be sepa-
     rated by whitespace or optional whitespace and exactly one '='; the latter format is useful
     to avoid the need to quote whitespace when specifying configuration options using the ssh,
     scp and sftp -o option.

     The possible keywords and their meanings are as follows (note that keywords are case-insen-
     sitive and arguments are case-sensitive):

     Host    Restricts the following declarations (up to the next Host keyword) to be only for
	     those hosts that match one of the patterns given after the keyword.  '*' and ''?
	     can be used as wildcards in the patterns.	A single '*' as a pattern can be used to
	     provide global defaults for all hosts.  The host is the hostname argument given on
	     the command line (i.e., the name is not converted to a canonicalized host name
	     before matching).

     AFSTokenPassing
	     Specifies whether to pass AFS tokens to remote host.  The argument to this keyword
	     must be ``yes'' or ``no''.  This option applies to protocol version 1 only.

     BatchMode
	     If set to ``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 pass-
	     word.  The argument must be ``yes'' or ``no''.  The default is ``no''.

     BindAddress
	     Specify the interface to transmit from on machines with multiple interfaces or
	     aliased addresses.  Note that this option does not work if UsePrivilegedPort is set
	     to ``yes''.

     ChallengeResponseAuthentication
	     Specifies whether to use challenge response authentication.  The argument to this
	     keyword must be ``yes'' or ``no''.  The default is ``yes''.

     CheckHostIP
	     If this flag is set to ``yes'', ssh will additionally check the host IP address in
	     the known_hosts file.  This allows ssh to detect if a host key changed due to DNS
	     spoofing.	If the option is set to ``no'', the check will not be executed.  The
	     default is ``yes''.

     Cipher  Specifies the cipher to use for encrypting the session in protocol version 1.  Cur-
	     rently, ``blowfish'', ``3des'', and ``des'' are supported.  des is only supported in
	     the ssh client for interoperability with legacy protocol 1 implementations that do
	     not support the 3des cipher.  Its use is strongly discouraged due to cryptographic
	     weaknesses.  The default is ``3des''.

     Ciphers
	     Specifies the ciphers allowed for protocol version 2 in order of preference.  Multi-
	     ple ciphers must be comma-separated.  The default is

	       ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
		 aes192-cbc,aes256-cbc''

     ClearAllForwardings
	     Specifies that all local, remote and dynamic port forwardings specified in the con-
	     figuration files or on the command line be cleared.  This option is primarily useful
	     when used from the ssh command line to clear port forwardings set in configuration
	     files, and is automatically set by scp(1) and sftp(1).  The argument must be ``yes''
	     or ``no''.  The default is ``no''.

     Compression
	     Specifies whether to use compression.  The argument must be ``yes'' or ``no''.  The
	     default is ``no''.

     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 gzip(1).
	     Note that this option applies to protocol version 1 only.

     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.

     DynamicForward
	     Specifies that a TCP/IP port on the local machine be forwarded over the secure chan-
	     nel, 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 pro-
	     tocol is supported, and 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.

     EscapeChar
	     Sets the escape character (default: '~').	The escape character can also be set on
	     the command line.	The argument should be a single character, '^' followed by a let-
	     ter, or ``none'' to disable the escape character entirely (making the connection
	     transparent for binary data).

     ForwardAgent
	     Specifies whether the connection to the authentication agent (if any) will be for-
	     warded to the remote machine.  The argument must be ``yes'' or ``no''.  The default
	     is ``no''.

	     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.

     ForwardX11
	     Specifies whether X11 connections will be automatically redirected over the secure
	     channel and DISPLAY set.  The argument must be ``yes'' or ``no''.	The default is
	     ``no''.

	     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.

     GatewayPorts
	     Specifies whether remote hosts are allowed to connect to local forwarded ports.  By
	     default, ssh binds local port forwardings to the loopback address.  This prevents
	     other remote hosts from connecting to forwarded ports.  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 ``yes''
	     or ``no''.  The default is ``no''.

     GlobalKnownHostsFile
	     Specifies a file to use for the global host key database instead of
	     /etc/ssh/ssh_known_hosts.

     HostbasedAuthentication
	     Specifies whether to try rhosts based authentication with public key authentication.
	     The argument must be ``yes'' or ``no''.  The default is ``no''.  This option applies
	     to protocol version 2 only and is similar to RhostsRSAAuthentication.

     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: ``ssh-rsa,ssh-dss''.

     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.

     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 HostName specifica-
	     tions).

     IdentityFile
	     Specifies a file from which the user's RSA or DSA authentication identity is read.
	     The default is $HOME/.ssh/identity for protocol version 1, and $HOME/.ssh/id_rsa and
	     $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 multi-
	     ple identity files specified in configuration files; all these identities will be
	     tried in sequence.

     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.

	     The default is ``yes'' (to send keepalives), and the client will notice if the net-
	     work goes down or the remote host dies.  This is important in scripts, and many
	     users want it too.

	     To disable keepalives, the value should be set to ``no''.

     KerberosAuthentication
	     Specifies whether Kerberos authentication will be used.  The argument to this key-
	     word must be ``yes'' or ``no''.

     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 key-
	     word must be ``yes'' or ``no''.

     LocalForward
	     Specifies that a TCP/IP port on the local machine be forwarded over the secure chan-
	     nel to the specified host and port from the remote machine.  The first argument must
	     be a port number, and the second must be host:port.  IPv6 addresses can be specified
	     with an alternative syntax: host/port.  Multiple forwardings may be specified, and
	     additional forwardings can be given on the command line.  Only the superuser can
	     forward privileged ports.

     LogLevel
	     Gives the verbosity level that is used when logging messages from 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 spec-
	     ify higher levels of verbose output.

     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.  Mul-
	     tiple algorithms must be comma-separated.	The default is
	     ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.

     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 ``yes'' or
	     ``no''.  The default is to check the host key for localhost.

     NumberOfPasswordPrompts
	     Specifies the number of password prompts before giving up.  The argument to this
	     keyword must be an integer.  Default is 3.

     PasswordAuthentication
	     Specifies whether to use password authentication.	The argument to this keyword must
	     be ``yes'' or ``no''.  The default is ``yes''.

     Port    Specifies the port number to connect on the remote host.  Default is 22.

     PreferredAuthentications
	     Specifies the order in which the client should try protocol 2 authentication meth-
	     ods. This allows a client to prefer one method (e.g.  keyboard-interactive) over
	     another method (e.g.  password) The default for this option is:
	     ``hostbased,publickey,keyboard-interactive,password''.

     Protocol
	     Specifies the protocol versions ssh should support in order of preference.  The pos-
	     sible values are ``1'' and ``2''.	Multiple versions must be comma-separated.  The
	     default is ``2,1''.  This means that ssh tries version 2 and falls back to version 1
	     if version 2 is not available.

     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 /bin/sh.  In the command string, '%h'
	     will be substituted by the host name to connect and '%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 sshd(8) server running on some
	     machine, or execute 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 CheckHostIP is not available for connects with a proxy command.

     PubkeyAuthentication
	     Specifies whether to try public key authentication.  The argument to this keyword
	     must be ``yes'' or ``no''.  The default is ``yes''.  This option applies to protocol
	     version 2 only.

     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 host:port.  IPv6 addresses can be
	     specified with an alternative syntax: host/port.  Multiple forwardings may be speci-
	     fied, and additional forwardings can be given on the command line.  Only the supe-
	     ruser can forward privileged ports.

     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
	     RhostsRSAAuthentication).	The argument to this keyword must be ``yes'' or ``no''.
	     The default is ``no''.  This option applies to protocol version 1 only and requires
	     ssh to be setuid root and UsePrivilegedPort to be set to ``yes''.

     RhostsRSAAuthentication
	     Specifies whether to try rhosts based authentication with RSA host authentication.
	     The argument must be ``yes'' or ``no''.  The default is ``no''.  This option applies
	     to protocol version 1 only and requires ssh to be setuid root.

     RSAAuthentication
	     Specifies whether to try RSA authentication.  The argument to this keyword must be
	     ``yes'' or ``no''.  RSA authentication will only be attempted if the identity file
	     exists, or an authentication agent is running.  The default is ``yes''.  Note that
	     this option applies to protocol version 1 only.

     SmartcardDevice
	     Specifies which smartcard device to use. The argument to this keyword is the device
	     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.

     StrictHostKeyChecking
	     If this flag is set to ``yes'', ssh will never automatically add host keys to the
	     $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 /etc/ssh/ssh_known_hosts file is poorly maintained, or con-
	     nections to new hosts are frequently made.  This option forces the user to manually
	     add all new hosts.  If this flag is set to ``no'', ssh will automatically add new
	     host keys to the user known hosts files.  If this flag is set to ``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 ``yes'', ``no'' or ``ask''.  The default is
	     ``ask''.

     UsePrivilegedPort
	     Specifies whether to use a privileged port for outgoing connections.  The argument
	     must be ``yes'' or ``no''.  The default is ``no''.  If set to ``yes'' ssh must be
	     setuid root.  Note that this option must be set to ``yes'' if RhostsAuthentication
	     and RhostsRSAAuthentication authentications are needed with older servers.

     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.

     UserKnownHostsFile
	     Specifies a file to use for the user host key database instead of
	     $HOME/.ssh/known_hosts.

     XAuthLocation
	     Specifies the full pathname of the xauth(1) program.  The default is
	     /usr/X11R6/bin/xauth.

FILES
     $HOME/.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.  This file does not usually contain any
	     sensitive information, but the recommended permissions are read/write for the user,
	     and not accessible by others.

     /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.

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.

SEE ALSO
     ssh(1)

BSD					September 25, 1999				      BSD


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