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ssh_config


     /etc/ssh/ssh_config


DESCRIPTION

     ssh obtains configuration data from the following sources in the follow­
     ing 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 configu­
     ration 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-spe­
     cific 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 separated by whitespace or optional whitespace and exactly
     one `='; the latter format is useful to avoid the need to quote whites­
     pace when specifying configuration options using the ssh, scp and sftp -o
     option.

     The possible keywords and their meanings are as follows (note that key­
     words are case-insensitive and arguments are case-sensitive):

     Host    Restricts the following declarations (up to the next Host key­
             word) 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).

     AddressFamily
             Specifies which address family to use when connecting.  Valid
             arguments are ``any'', ``inet'' (Use IPv4 only) or ``inet6'' (Use
             IPv6 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 password.  The argument must be
             ``yes'' or ``no''.  The default is ``no''.

     BindAddress
             ``yes''.

     Cipher  Specifies the cipher to use for encrypting the session in proto­
             col version 1.  Currently, ``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.  Multiple 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 configuration 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 argu­
             ment 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.

     ConnectTimeout
             Specifies the timeout (in seconds) used when connecting to the
             ssh server, instead of using the default system TCP timeout.
             This value is used only when the target is down or really
             unreachable, not when it refuses the connection.

     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 and
             can also be set on the command line.  The argument should be a
             single character, `^' followed by a letter, or ``none'' to dis­
             able the escape character entirely (making the connection trans­
             parent for binary data).

     ForwardAgent
             Specifies whether the connection to the authentication agent (if
             any) will be forwarded 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 redi­
             rected 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 con­
             necting 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.

     GSSAPIAuthentication
             Specifies whether authentication based on GSSAPI may be used,
             either using the result of a successful key exchange, or using
             GSSAPI user authentication.  The default is ``yes''.  Note that
             this option applies to protocol version 2 only.

     GSSAPIDelegateCredentials
             Forward (delegate) credentials to the server.  The default is
             ``no''.  Note that this option applies to protocol version 2
             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 connec­
             tions 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 proto­
             col 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 multiple identity files speci­
             fied 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 tem­
             porarily, and some people find it annoying.

             The default is ``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.

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

     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 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 com­
             mand 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, VER­
             BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.  The default is INFO.
             DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
             higher levels of verbose output.


     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 methods.  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 possible 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 com­
             mand 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 some­
             where.  Host key management will be done using the HostName of
             the host being connected (defaulting to the name typed by the
             user).  Setting the command to ``none'' disables this option
             entirely.  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
             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 smart­
             card 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 con­
             nect 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
             connections 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 connec­
             tions.  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'' for RhostsRSAAuthentication
             with older servers.

     User    Specifies the user to log in as.  This can be useful when a dif­
             ferent user name is used on different machines.  This saves the
             trouble of having to remember to give the user name on the com­
             mand line.

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

     VerifyHostKeyDNS
             Specifies whether to verify the remote key using DNS and SSHFP
             resource records.  The default is ``no''.  Note that this option
             applies to protocol version 2 only.

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


FILES

     $HOME/.ssh/config


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 cre­
     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

BSD                           September 25, 1999                           BSD
  

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