The first version of ssh_config, above, is the host view of 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/config), and 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 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.
In the configuration file, empty lines and lines starting with a hash mark (#) are comments. Otherwise, a line is of the format: keyword arguments. Interpretation of the file is case-sensitive. The possible keywords and their meanings are as follows:
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. Asterisk (*) and question mark (?) 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 (that is, the name is not converted to a canonicalized host name before matching).
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.
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.
Specifies the cipher to use for encrypting the session in protocol version 1; blowfish and 3des are the only valid values. Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The default is 3des-cbc,blowfish-cbc,aes-128-cbc.
Specifies whether to use compression. The argument must be yes or no.
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..
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.
Specifies whether to try DSA authentication. The argument to this keyword must be yes or no. DSA authentication is attempted only if a DSA identity file exists. Note that this option applies to protocol version 2 only.
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).
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.
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.
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.
Specifies whether remote hosts are allowed to connect to local forwarded ports. The argument must be yes or no. The default is no.
Specifies a file to use instead of /etc/ssh_known_hosts.
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).
Specifies the file from which the user's RSA authentication identity is read. The default is $HOME/.ssh/identity in the user's home directory. Additionally, any identities represented by the authentication 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 of these identities will be tried in sequence.
Specifies the file from which the user's DSA authentication identity is read. The default is $HOME/.ssh/id_dsa in the user's home directory. 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 of these identities will be tried in sequence.
Specifies whether the system should send 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.
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. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
Gives the verbosity level that is used when logging messages from ssh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. The default is INFO.
Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. The default is 3.
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.
Specifies the port number to connect on the remote host. The default is 22.
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 1,2. This means that ssh tries version 1 and falls back to version 2 if version 1 is not available.
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.
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. You can specify multiple forwardings and give additional forwardings on the command line. Only the superuser can forward privileged ports.
Specifies whether to try rhosts-based authentication. Note that this declaration affects only the client side and has no effect whatsoever 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 keyword must be yes or no.
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.
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 inconvenience if you do not have good /etc/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.
Specifies whether to use a privileged port for outgoing connections. The argument must be yes or no. The default is yes. Note that setting this option to no turns off RhostsAuthentication and RhostsRSAAuthentication.
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.
Specifies a file to use instead of $HOME/.ssh/known_hosts.
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.
Specifies the location of the xauth(1) program. The default is /usr/openwin/bin/xauth.