SSH_CONFIG(5) File Formats Manual SSH_CONFIG(5)

ssh_config —
OpenSSH SSH client configuration files

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

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 separated 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-insensitive and arguments are case-sensitive):

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).
Specifies which address family to use when connecting. Valid arguments are “any”, “inet” (Use IPv4 only) or “inet6” (Use IPv6 only.)
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”.
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”.
Specifies whether to use challenge response authentication. The argument to this keyword must be “yes” or “no”. The default is “yes”.
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”.
Specifies the cipher to use for encrypting the session in protocol 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”.
Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The supported ciphers are “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour”, “blowfish-cbc”, and “cast128-cbc”. The default is 
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
    aes192-cbc,aes256-cbc''
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 argument must be “yes” or “no”. The default is “no”.
Specifies whether to use compression. The argument must be “yes” or “no”. The default is “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. The meaning of the values is the same as in gzip(1). Note that this option applies to protocol version 1 only.
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.
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.
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 SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
Enables the sharing of multiple sessions over a single network connection. When set to “yes” ssh will listen for connections on a control socket specified using the ControlPath argument. Additional sessions can connect to this socket using the same ControlPath with ControlMaster set to “no” (the default.) These sessions will reuse the master instance's network connection rather than initiating new ones.
Specify a the path to the control socket used for connection sharing. See ControlMaster above.
Setting this option to “yes” in the global client configuration file /etc/ssh/ssh_config enables the use of the helper program 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.
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).
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.

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 X11 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 if the ForwardX11Trusted option is also enabled.

If this option is set to “yes” then remote X11 clients will have full access to the original X11 display. If this option is set to “no” then remote X11 clients will be considered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients.

The default is “no”.

See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.

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”.
Specifies a file to use for the global host key database instead of /etc/ssh/ssh_known_hosts.
Specifies whether user authentication based on GSSAPI is allowed. The default is “no”. Note that this option applies to protocol version 2 only.
Forward (delegate) credentials to the server. The default is “no”. Note that this option applies to protocol version 2 only.
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.
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”.
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.
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 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 multiple identity files specified in configuration files; all these identities will be tried in sequence.
Specifies that ssh should only use the authentication identity files configured in the ssh_config files, even if the ssh-agent offers more identities. The argument to this keyword must be “yes” or “no”. This option is intented for situations where ssh-agent offers many different identities. The default is “no”.
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 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, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of verbose output.
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-ripemd160,hmac-sha1-96,hmac-md5-96”.
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.
Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. Default is 3.
Specifies whether to use password authentication. The argument to this keyword must be “yes” or “no”. The default is “yes”.
Specifies the port number to connect on the remote host. Default is 22.
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”.
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.
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). Setting the command to “none” disables this option entirely. Note that CheckHostIP is not available for connects with a proxy command.
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.
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 specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
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.
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.
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 the wildcard characters ‘*’ and ‘?’. Multiple environment variables may be separated by whitespace or spread across multiple SendEnv directives. The default is not to send any environment variables.
Sets a timeout interval in seconds after which if no data has been received from the server, ssh 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.
Sets the number of server alive messages (see above) which may be sent without ssh 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 therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive.

The default value is 3. If, for example, ServerAliveInterval (above) is set to 15, and ServerAliveCountMax is left at the default, if the server becomes unresponsive ssh will disconnect after approximately 45 seconds.

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.
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 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”.
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 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”.

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” for RhostsRSAAuthentication with older servers.
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.
Specifies a file to use for the user host key database instead of $HOME/.ssh/known_hosts.
Specifies whether to verify the remote key using DNS and SSHFP resource records. If this option is set to “yes”, the client will implicitly trust keys that match a secure fingerprint from DNS. Insecure fingerprints will be handled as if this option was set to “ask”. If this option is set to “ask”, information on fingerprint match will be displayed, but the user will still need to confirm new host keys according to the StrictHostKeyChecking option. The argument must be “yes”, “no” or “ask”. The default is “no”. Note that this option applies to protocol version 2 only.
Specifies the full pathname of the xauth(1) program. The default is /usr/X11R6/bin/xauth.

$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. Because of the potential for abuse, this file must have strict permissions: 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.

ssh(1)

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.
September 25, 1999 BSD