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    Command:

    ssh_config

    
         /etc/ssh/ssh_config
    
    
    

    DESCRIPTION

         ssh(1) obtains configuration data from the following sources in the fol-
         lowing order:
    
               1.   command-line options
               2.   user's configuration file (~/.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 separated 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 sepa-
         rated by whitespace or optional whitespace and exactly one '='; the lat-
         ter format is useful to avoid the need to quote whitespace when specify-
         ing configuration options using the ssh, scp, and sftp -o option.  Argu-
         ments may optionally be enclosed in double quotes (") in order to repre-
         sent arguments containing spaces.
    
         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.  If more than one pattern is provided,
                 they should be separated by whitespace.  A single '*' as a pat-
                 tern 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 match-
                 ing).
    
                 See PATTERNS for more information on patterns.
    
         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
    
         CheckHostIP
                 If this flag is set to "yes", ssh(1) 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 proto-
                 col version 1.  Currently, "blowfish", "3des", and "des" are sup-
                 ported.  des is only supported in the ssh(1) client for interop-
                 erability with legacy protocol 1 implementations that do not sup-
                 port 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 sup-
                 ported ciphers are "3des-cbc", "aes128-cbc", "aes192-cbc",
                 "aes256-cbc", "aes128-ctr", "aes192-ctr", "aes256-ctr",
                 "arcfour128", "arcfour256", "arcfour", "blowfish-cbc", and
                 "cast128-cbc".  The default is:
    
                    aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                    aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                    aes256-cbc,arcfour
    
         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(1) command line to clear port forwardings set in configura-
                 tion 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.
    
         ConnectTimeout
                 Specifies the timeout (in seconds) used when connecting to the
    
                 Setting this to "ask" will cause ssh to listen for control con-
                 nections, but require confirmation using the SSH_ASKPASS program
                 before they are accepted (see ssh-add(1) for details).  If the
                 ControlPath cannot be opened, ssh will continue without connect-
                 ing to a master instance.
    
                 X11 and ssh-agent(1) forwarding is supported over these multi-
                 plexed connections, however the display and agent forwarded will
                 be the one belonging to the master connection i.e. it is not pos-
                 sible to forward multiple displays or agents.
    
                 Two additional options allow for opportunistic multiplexing: try
                 to use a master connection but fall back to creating a new one if
                 one does not already exist.  These options are: "auto" and
                 "autoask".  The latter requires confirmation like the "ask"
                 option.
    
         ControlPath
                 Specify the path to the control socket used for connection shar-
                 ing as described in the ControlMaster section above or the string
                 "none" to disable connection sharing.  In the path, '%l' will be
                 substituted by the local host name, '%h' will be substituted by
                 the target host name, '%p' the port, and '%r' by the remote login
                 username.  It is recommended that any ControlPath used for oppor-
                 tunistic connection sharing include at least %h, %p, and %r.
                 This ensures that shared connections are uniquely identified.
    
         ControlPersist
                 When used in conjunction with ControlMaster, specifies that the
                 master connection should remain open in the background (waiting
                 for future client connections) after the initial client connec-
                 tion has been closed.  If set to "no", then the master connection
                 will not be placed into the background, and will close as soon as
                 the initial client connection is closed.  If set to "yes", then
                 the master connection will remain in the background indefinitely
                 (until killed or closed via a mechanism such as the ssh(1) "-O
                 exit" option).  If set to a time in seconds, or a time in any of
                 the formats documented in sshd_config(5), then the backgrounded
                 master connection will automatically terminate after it has
                 remained idle (with no client connections) for the specified
                 time.
    
         DynamicForward
                 Specifies that a TCP port on the local machine be forwarded over
                 the secure channel, and the application protocol is then used to
                 determine where to connect to from the remote machine.
    
                 The argument must be [bind_address:]port.  IPv6 addresses can be
                 specified by enclosing addresses in square brackets or by using
                 an alternative syntax: [bind_address/]port.  By default, the
                 local port is bound in accordance with the GatewayPorts setting.
                 ssh-keysign(8) during HostbasedAuthentication.  The argument must
                 be "yes" or "no".  The default is "no".  This option should be
                 placed in the non-hostspecific section.  See ssh-keysign(8) for
                 more information.
    
         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 letter, or "none" to disable
                 the escape character entirely (making the connection transparent
                 for binary data).
    
         ExitOnForwardFailure
                 Specifies whether ssh(1) should terminate the connection if it
                 cannot set up all requested dynamic, tunnel, local, and remote
                 port forwardings.  The argument must be "yes" or "no".  The
                 default is "no".
    
         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 X11 authorization database) can access the local X11 dis-
                 play through the forwarded connection.  An attacker may then be
                 able to perform activities such as keystroke monitoring if the
                 ForwardX11Trusted option is also enabled.
    
         ForwardX11Trusted
                 If this option is set to "yes", remote X11 clients will have full
                 access to the original X11 display.
    
                 If this option is set to "no", remote X11 clients will be consid-
                 ered untrusted and prevented from stealing or tampering with data
                 belonging to trusted X11 clients.  Furthermore, the xauth(1)
                 token used for the session will be set to expire after 20 min-
                 utes.  Remote clients will be refused access after this time.
    
         GlobalKnownHostsFile
                 Specifies a file to use for the global host key database instead
                 of /etc/ssh/ssh_known_hosts.
    
         GSSAPIAuthentication
                 Specifies whether user authentication based on GSSAPI is allowed.
                 The default is "no".  Note that this option applies to protocol
                 version 2 only.
    
         GSSAPIKeyExchange
                 Specifies whether key exchange based on GSSAPI may be used. When
                 using GSSAPI key exchange the server need not have a host key.
                 The default is "no".  Note that this option applies to protocol
                 version 2 only.
    
         GSSAPIClientIdentity
                 If set, specifies the GSSAPI client identity that ssh should use
                 when connecting to the server. The default is unset, which means
                 that the default identity will be used.
    
         GSSAPIDelegateCredentials
                 Forward (delegate) credentials to the server.  The default is
                 "no".  Note that this option applies to protocol version 2 con-
                 nections using GSSAPI.
    
         GSSAPIRenewalForcesRekey
                 If set to "yes" then renewal of the client's GSSAPI credentials
                 will force the rekeying of the ssh connection. With a compatible
                 server, this can delegate the renewed credentials to a session on
                 the server.  The default is "no".
    
         GSSAPITrustDns
                 Set to "yes to indicate that the DNS is trusted to securely
                 canonicalize" the name of the host being connected to. If "no,
                 the hostname entered on the" command line will be passed
                 untouched to the GSSAPI library.  The default is "no".  This
                 option only applies to protocol version 2 connections using GSS-
                 API.
    
         HashKnownHosts
                 Indicates that ssh(1) should hash host names and addresses when
                 they are added to ~/.ssh/known_hosts.  These hashed names may be
                 used normally by ssh(1) and sshd(8), but they do not reveal iden-
                 tifying information should the file's contents be disclosed.  The
                 default is "no".  Note that existing names and addresses in known
                 hosts files will not be converted automatically, but may be manu-
                 ally hashed using ssh-keygen(1).
    
         HostbasedAuthentication
                 Specifies whether to try rhosts based authentication with public
                 key authentication.  The argument must be "yes" or "no".  The
                 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.  The 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).
    
         IdentitiesOnly
                 Specifies that ssh(1) should only use the authentication identity
                 files configured in the ssh_config files, even if ssh-agent(1)
                 offers more identities.  The argument to this keyword must be
                 "yes" or "no".  This option is intended for situations where ssh-
                 agent offers many different identities.  The default is "no".
    
         IdentityFile
                 Specifies a file from which the user's DSA, ECDSA or DSA authen-
                 tication identity is read.  The default is ~/.ssh/identity for
                 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
                 ~/.ssh/id_rsa for protocol version 2.  Additionally, any identi-
                 ties represented by the authentication agent will be used for
                 authentication.  ssh(1) will try to load certificate information
                 from the filename obtained by appending -cert.pub to the path of
                 a specified IdentityFile.
    
                 The file name may use the tilde syntax to refer to a user's home
                 directory or one of the following escape characters: '%d' (local
                 user's home directory), '%u' (local user name), '%l' (local host
                 name), '%h' (remote host name) or '%r' (remote user name).
    
                 It is possible to have multiple identity files specified in con-
                 figuration files; all these identities will be tried in sequence.
    
         KbdInteractiveAuthentication
                 Specifies whether to use keyboard-interactive authentication.
                 The argument to this keyword must be "yes" or "no".  The default
                 is "yes".
    
         KbdInteractiveDevices
                 Specifies the list of methods to use in keyboard-interactive
                 authentication.  Multiple method names must be comma-separated.
                 The default is to use the server specified list.  The methods
                 available vary depending on what the server supports.  For an
                 OpenSSH server, it may be zero or more of: "bsdauth", "pam", and
                 "skey".
    
         KexAlgorithms
                 Specifies the available KEX (Key Exchange) algorithms.  Multiple
                 algorithms must be comma-separated.  The default is
    
         LocalForward
                 Specifies that a TCP port on the local machine be forwarded over
                 the secure channel to the specified host and port from the remote
                 machine.  The first argument must be [bind_address:]port and the
                 second argument must be host:hostport.  IPv6 addresses can be
                 specified by enclosing addresses in square brackets or by using
                 an alternative syntax: [bind_address/]port and host/hostport.
                 Multiple forwardings may be specified, and additional forwardings
                 can be given on the command line.  Only the superuser can forward
                 privileged ports.  By default, the local port is bound in accor-
                 dance with the GatewayPorts setting.  However, an explicit
                 bind_address may be used to bind the connection to a specific
                 address.  The bind_address of "localhost" indicates that the lis-
                 tening port be bound for local use only, while an empty address
                 or '*' indicates that the port should be available from all
                 interfaces.
    
         LogLevel
                 Gives the verbosity level that is used when logging messages from
                 ssh(1).  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.
    
         MACs    Specifies the MAC (message authentication code) algorithms in
                 order of preference.  The MAC algorithm is used in protocol ver-
                 sion 2 for data integrity protection.  Multiple algorithms must
                 be comma-separated.  The default is:
    
                       hmac-md5,hmac-sha1,umac-64@openssh.com,
                       hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
                       hmac-sha2-256,hmac-sha2-512
    
         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 warn-
                 ings 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.  The default is 3.
    
         PasswordAuthentication
                 Specifies whether to use password authentication.  The argument
                 to this keyword must be "yes" or "no".  The default is "yes".
    
         PermitLocalCommand
         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: "gssapi-with-mic,
                 hostbased, publickey, keyboard-interactive, password".
    
         Protocol
                 Specifies the protocol versions ssh(1) should support in order of
                 preference.  The possible values are '1' and '2'.  Multiple ver-
                 sions 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
                 the user's shell.  In the command string, '%h' will be substi-
                 tuted by the host name to connect and '%p' by the port.  The com-
                 mand 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).  Setting the command to "none" disables this
                 option entirely.  Note that CheckHostIP is not available for con-
                 nects with a proxy command.
    
                 This directive is useful in conjunction with nc(1) and its proxy
                 support.  For example, the following directive would connect via
                 an HTTP proxy at 192.0.2.0:
    
                    ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
    
         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.
    
         RekeyLimit
                 Specifies the maximum amount of data that may be transmitted
                 before the session key is renegotiated.  The argument is the num-
                 ber of bytes, with an optional suffix of 'K', 'M', or 'G' to
                 indicate Kilobytes, Megabytes, or Gigabytes, respectively.  The
                 default is between '1G' and '4G', depending on the cipher.  This
                 option applies to protocol version 2 only.
    
         RemoteForward
                 Specifies that a TCP port on the remote machine be forwarded over
                 the secure channel to the specified host and port from the local
                 machine.  The first argument must be [bind_address:]port and the
                 second argument must be host:hostport.  IPv6 addresses can be
    
         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(1) 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.
    
         SendEnv
                 Specifies what variables from the local environ(7) should be sent
                 to the server.  Note that environment passing is only supported
                 for protocol 2.  The server must also support it, and the server
                 must be configured to accept these environment variables.  Refer
                 to AcceptEnv in sshd_config(5) for how to configure the server.
                 Variables are specified by name, which may contain wildcard char-
                 acters.  Multiple environment variables may be separated by
                 whitespace or spread across multiple SendEnv directives.  The
                 default is not to send any environment variables.
    
                 See PATTERNS for more information on patterns.
    
         ServerAliveCountMax
                 Sets the number of server alive messages (see below) which may be
                 sent without ssh(1) receiving any messages back from the server.
                 If this threshold is reached while server alive messages are
                 being sent, ssh will disconnect from the server, terminating the
                 session.  It is important to note that the use of server alive
                 messages is very different from TCPKeepAlive (below).  The server
                 alive messages are sent through the encrypted channel and there-
                 fore will not be spoofable.  The TCP keepalive option enabled by
                 TCPKeepAlive is spoofable.  The server alive mechanism is valu-
                 able when the client or server depend on knowing when a connec-
                 tion has become inactive.
    
                 The default value is 3.  If, for example, ServerAliveInterval
                 (see below) is set to 15 and ServerAliveCountMax is left at the
                 default, if the server becomes unresponsive, ssh will disconnect
                 after approximately 45 seconds.  This option applies to protocol
                 version 2 only.
    
         ServerAliveInterval
                 Sets a timeout interval in seconds after which if no data has
                 been received from the server, ssh(1) will send a message through
                 the encrypted channel to request a response from the server.  The
                 default is 0, indicating that these messages will not be sent to
                 the server.  This option applies to protocol version 2 only.
                 keys of known hosts will be verified automatically in all cases.
                 The argument must be "yes", "no", or "ask".  The default is
                 "ask".
    
         TCPKeepAlive
                 Specifies whether the system should send TCP keepalive messages
                 to the other side.  If they are sent, death of the connection or
                 crash of one of the machines will be properly noticed.  However,
                 this means that connections will die if the route is down tem-
                 porarily, and some people find it annoying.
    
                 The default is "yes" (to send TCP keepalive messages), and the
                 client will notice if the network goes down or the remote host
                 dies.  This is important in scripts, and many users want it too.
    
                 To disable TCP keepalive messages, the value should be set to
                 "no".
    
         Tunnel  Request tun(4) device forwarding between the client and the
                 server.  The argument must be "yes", "point-to-point" (layer 3),
                 "ethernet" (layer 2), or "no".  Specifying "yes" requests the
                 default tunnel mode, which is "point-to-point".  The default is
                 "no".
    
         TunnelDevice
                 Specifies the tun(4) devices to open on the client (local_tun)
                 and the server (remote_tun).
    
                 The argument must be local_tun[:remote_tun].  The devices may be
                 specified by numerical ID or the keyword "any", which uses the
                 next available tunnel device.  If remote_tun is not specified, it
                 defaults to "any".  The default is "any:any".
    
         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(1) 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
                 ~/.ssh/known_hosts.
    
         VerifyHostKeyDNS
                 Specifies whether to verify the remote key using DNS and SSHFP
                 resource records.  If this option is set to "yes", the client
                 fingerprint string at login and for unknown host keys.  If this
                 flag is set to "no", no fingerprint strings are printed at login
                 and only the hex fingerprint string will be printed for unknown
                 host keys.  The default is "no".
    
         XAuthLocation
                 Specifies the full pathname of the xauth(1) program.  The default
                 is /usr/bin/xauth.
    
    
    

    PATTERNS

         A pattern consists of zero or more non-whitespace characters, '*' (a
         wildcard that matches zero or more characters), or '?' (a wildcard that
         matches exactly one character).  For example, to specify a set of decla-
         rations for any host in the ".co.uk" set of domains, the following pat-
         tern could be used:
    
               Host *.co.uk
    
         The following pattern would match any host in the 192.168.0.[0-9] network
         range:
    
               Host 192.168.0.?
    
         A pattern-list is a comma-separated list of patterns.  Patterns within
         pattern-lists may be negated by preceding them with an exclamation mark
         ('!').  For example, to allow a key to be used from anywhere within an
         organisation except from the "dialup" pool, the following entry (in
         authorized_keys) could be used:
    
               from="!*.dialup.example.com,*.example.com"
    
    
    

    FILES

         ~/.ssh/config
                 This is the per-user configuration file.  The format of this file
                 is described above.  This file is used by the SSH client.
                 Because of the potential for abuse, this file must have strict
                 permissions: read/write for the user, and not accessible by oth-
                 ers.
    
         /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.
    
    
    

    SEE ALSO

         ssh(1)
    
    
    

    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-
    
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