ssh_config(4)															     ssh_config(4)

NAME

       ssh_config - ssh configuration file

SYNOPSIS

       /etc/ssh/ssh_config

       $HOME/.ssh/config

       The first ssh_config path, above, provides the system-wide defaults for ssh(1). The second version is user-specific defaults for ssh.

       ssh obtains configuration data from the following sources (in this order): command line options, user's configuration file ($HOME/.ssh/con-
       fig), and system-wide configuration file (/etc/ssh/ssh_config). For each parameter, the first obtained value will be used.  The	configura-
       tion  files  contain  sections  bracketed by Host specifications, and that section is applied only 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, host-specific declarations should be given near the beginning of the	file,  and
       general defaults at the end.

       For  each  parameter, the first obtained value will be used. The configuration files contain sections bracketed by "Host" specifications. A
       given section is applied only for hosts that match one of the patterns given in the specification. The matched host name is the	one  given
       on the command line.

       Because 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 and syntax:

	 o  Empty lines and lines starting with # are comments.

	 o  Non-commented lines are of the form:

	    keyword arguments

	 o  Configuration options can be separated by whitespace or optional whitespace and exactly one equal sign. The latter format  allows  you
	    to avoid the need to quote whitespace when specifying configuration options using the -o option to ssh, scp, and sftp.

       The possible keywords and their meanings are listed below. Note that keywords are case-insensitive 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. An asterisk (*) and a question mark (?) can be used as wildcards in the patterns. A single asterisk as a pattern can
	   be  used  to  provide global defaults for all hosts. The host is the host name argument given on the command line (that is, the name is
	   not converted to a canonicalized host name before matching).

       BatchMode

	   The argument must be yes or no. If set to yes, passphrase/password querying will be disabled. This option  is  useful  in  scripts  and
	   other batch jobs where you have no user to supply the password.

       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.

       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.

       Cipher

	   Specifies the cipher to use for encrypting the session in protocol version 1; blowfish and 3des are the only valid values.

       Ciphers

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

	   aes128-ctr,aes128-cbc,arcfour,3des-cbc,blowfish-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  auto-
	   matically 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. Defaults to 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. kNote that this option applies to protocol version 1 only.

       ConnectionAttempts

	   Specifies the number of tries (one per second) to make before falling back to rsh or exiting. The argument must be an integer. This can
	   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 channel. The application protocol is then used to deter-
	   mine where to connect to from the remote machine. The argument must be a port number. Currently the SOCKS4 protocol is  supported,  and
	   ssh will act as a SOCKS4 server. Multiple forwardings can be specified and additional forwardings can be specified on the command line.
	   Only the superuser can forward privileged ports.

       EscapeChar

	   Sets the escape character. The default is tilde (~). 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).

       FallBackToRsh

	   Specifies that if connecting with ssh fails due to a connection refused error (there is no  sshd(8)	listening  on  the  remote  host),
	   rsh(1) should automatically be used instead (after a suitable warning about the session being unencrypted). The argument must be yes or
	   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 he can perform operations on the keys that enable him 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 might 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 instead of /etc/ssh/ssh_known_hosts.

       GSSAPIAuthentication

	   Enables/disables GSS-API user authentication. The default is yes.

       GSSAPIKeyExchange

	   Enables/disables GSS-API-authenticated key exchanges. The default is yes.

	   This  option  is  intended  primarily  to  allow  users to disable the use of GSS-API key exchange for SSHv2 when it would otherwise be
	   selected and then fail (due to server misconfiguration, for example). SSHv2 key exchange failure always results in disconnection.

	   This option also enables the use of the GSS-API to authenticate the user to the server after the key exchange. Note	that  GSS-API  key
	   exchange can succeed but the subsequent authentication using the GSS-API fail if the server does not authorize the user's GSS principal
	   name to the target user account.

       GSSAPIDelegateCredentials

	   Enables/disables GSS-API credential forwarding. The default is no.

       GSSAPIStoreDelegatedCredentials

	   Controls whether sshd(1M) stores any GSS-API credentials delegated by clients.

       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 specifications).

       IdentityFile

	   Specifies a file from which the user's RSA or DSA authentication identity is read. The default is $HOME/.ssh/identity for protocol ver-
	   sion  1 and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for protocol version 2. Additionally, any identities represented by the authentica-
	   tion agent will be used for authentication. The file name can use the tilde syntax to refer to a user's home directory. It is  possible
	   to have multiple identity files specified in configuration files; all these identities will be tried in sequence.

       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 die if the route is down temporarily,	which  can
	   be a source of annoyance.

	   The	default  is  yes  (to  send  keepalives), which means the client notices 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  in  both  the  server  and  the
	   client configuration files.

       LocalForward

	   Specifies  that  a  TCP/IP port on the local machine be forwarded over the secure channel to a given host: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  can 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: FATAL,  ERROR,  INFO,  VERBOSE,  DEBUG,
	   DEBUG1,  DEBUG2, and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of ver-
	   bose 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. Multiple algorithms must be comma-separated. The default is hmac-md5,hmac-sha1,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. The default is 3.

       PasswordAuthentication

	   Specifies whether to use password authentication. The argument to this keyword must be yes or no. Note that this option applies to both
	   protocol versions 1 and 2. The default is yes.

       Port

	   Specifies the port number to connect on the remote host. The default is 22.

       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.

       PreferredAuthentications

	   Specifies  the  order  in which the client should try protocol 2 authentication methods. This allows a client to prefer one method (for
	   example, keyboard-interactive) over another method (for example, password). The default for this  option  is:  hostbased,publickey,key-
	   board-interactive,password.

       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 is substituted by the host name to connect and %p by the port. The string	can  be  any  valid  command,  and
	   should  read  from its standard input and write to its standard output. It should eventually connect an sshd(1M) 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 a given host: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. You can specify multiple forwardings and give additional forwardings on the command line. Only  the  superuser  can  forward
	   privileged ports.

       RhostsAuthentication

	   Specifies whether to try rhosts-based authentication. Note that this declaration affects only the client side and has no effect whatso-
	   ever on security. Disabling rhosts authentication can reduce authentication time on slow connections when rhosts authentication is  not
	   used. Most servers do not permit RhostsAuthentication because it is not secure (see RhostsRSAAuthentication). The argument to this key-
	   word must be yes or no. This option applies only to the protocol version 1 and requires that ssh be setuid root and that UsePrivileged-
	   Port be set to yes.

       RhostsRSAAuthentication

	   Specifies  whether  to try rhosts-based authentication with RSA host authentication. This is the primary authentication method for most
	   sites. The argument must be yes or no. This option applies only to the protocol version 1 and requires that ssh be setuid root and that
	   UsePrivilegedPort be set to yes.

       StrictHostKeyChecking

	   If  this  flag is set to yes, ssh will never automatically add host keys to the $HOME/.ssh/known_hosts file, and will refuse to connect
	   hosts whose host key has changed.  This provides maximum protection against trojan horse attacks. However, it can be a source of incon-
	   venience if you do not have good /etc/ssh/ssh_known_hosts files installed and frequently connect new hosts. This option forces the user
	   to manually add any new hosts. Normally this option is disabled, and new hosts will automatically be added to the known host files. The
	   host keys of known hosts will be verified automatically in either case. The argument must be yes or 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 yes. Note that set-
	   ting this option to no turns off RhostsAuthentication and RhostsRSAAuthentication. If set to yes ssh must be setuid root.  Defaults	to
	   no.

       User

	   Specifies  the user to log in as. This can be useful if you have different user names on different machines. This saves you the trouble
	   of having to remember to enter the user name on the command line.

       UserKnownHostsFile

	   Specifies a file to use instead of $HOME/.ssh/known_hosts.

       UseRsh

	   Specifies that rlogin or rsh should be used for this host. It is possible that the host does not support the ssh protocol. This  causes
	   ssh to immediately execute rsh(1). All other options (except HostName) are ignored if this has been specified. The argument must be yes
	   or no.

       XAuthLocation

	   Specifies the location of the xauth(1) program. The default is /usr/openwin/bin/xauth.

       ssh(1), ssh-http-proxy-connect(1), ssh-socks5-proxy-connect(1), sshd(1M), sshd_config(4), SEAM(5)

								    6 Oct 2005							     ssh_config(4)