Section: File Formats (5)
<OL> <LI>command-line options</LI> <LI>user's configuration file (~/.ssh/config )</LI> <LI>system-wide configuration file (/etc/ssh/ssh_config )</LI> </OL>
For each parameter, the first obtained value will be used. The configuration 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-specific declarations should be given near the beginning of the file, and general defaults at the end.
<UL> <LI>SendEnv LANG LC_*</LI> <LI>HashKnownHosts yes</LI> <LI>GSSAPIAuthentication yes</LI> </UL>
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. Arguments may optionally be enclosed in double quotes () in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):
A pattern entry may be negated by prefixing it with an exclamation mark (`!' ) If a negated entry is matched, then the Host entry is ignored, regardless of whether any other patterns on the line match. Negated matches are therefore useful to provide exceptions for wildcard matches.
See Sx PATTERNS for more information on patterns.
The exec keyword executes the specified command under the user's shell. If the command returns a zero exit status then the condition is considered true. Commands containing whitespace characters must be quoted. The following character sequences in the command will be expanded prior to execution: `%L' will be substituted by the first component of the local host name, `%l' will be substituted by the local host name (including any domain name), `%h' will be substituted by the target host name, `%n' will be substituted by the original target host name specified on the command-line, `%p' the destination port, `%r' by the remote login username, and `%u' by the username of the user running ssh?(1).
The other keywords' criteria must be single entries or comma-separated lists and may use the wildcard and negation operators described in the Sx PATTERNS section. The criteria for the host keyword are matched against the target hostname, after any substitution by the Hostname option. The originalhost keyword matches against the hostname as it was specified on the command-line. The user keyword matches against the target username on the remote host. The localuser keyword matches against the name of the local user running ssh?(1) (this keyword may be useful in system-wide ~/.ssh/config files).
If this option is enabled and canonicalisation results in the target hostname changing, then the configuration files are processed again using the new target name to pick up any new configuration in matching Host stanzas.
For example, ``*.a.example.com:*.b.example.com,*.c.example.com will allow hostnames matching ``*.a.example.com to be canonicalized to names in the ``*.b.example.com or ``*.c.example.com domains.
The default is:
aes128-ctr,aes192-ctr,aes256-ctr, firstname.lastname@example.org,email@example.com, firstname.lastname@example.org, arcfour256,arcfour128, aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc, aes192-cbc,aes256-cbc,arcfour
Setting this to ``ask'' will cause ssh to listen for control connections, but require confirmation using the SSH_ASKPASS program before they are accepted (see ssh-add1 for details). If the ControlPath cannot be opened, ssh will continue without connecting to a master instance.
X11 and ssh-agent1 forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to the master connection i.e. it is not possible 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.
The argument must be [bind_address : port ] IPv6 addresses can be specified by enclosing addresses in square brackets. By default, the local port is bound in accordance 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 listening port be bound for local use only, while an empty address or `*' indicates that the port should be available from all interfaces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh?(1) 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.
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.
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 ``no'' remote X11 clients will be considered 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 minutes. Remote clients will be refused access after this time.
The default is ``yes'' (Debian-specific).
See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.
email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com,firstname.lastname@example.org, email@example.com,firstname.lastname@example.org, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa,ssh-dss
If hostkeys are known for the destination host then this default is modified to prefer their algorithms.
The default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).
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 configuration files; all these identities will be tried in sequence. Multiple IdentityFile directives will add to the list of identities tried (this behaviour differs from that of other configuration directives).
IdentityFile may be used in conjunction with IdentitiesOnly to select which identities in an agent are offered during authentication.
email@example.com, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1, diffie-hellman-group1-sha1
@] </BLOCKQUOTE> :
This directive is ignored unless PermitLocalCommand has been enabled.
firstname.lastname@example.org,email@example.com, firstname.lastname@example.org,email@example.com, firstname.lastname@example.org,email@example.com, hmac-sha2-256,hmac-sha2-512, firstname.lastname@example.org,email@example.com, firstname.lastname@example.org, email@example.com,firstname.lastname@example.org, hmac-md5,hmac-sha1,hmac-ripemd160, hmac-sha1-96,hmac-md5-96
@] </BLOCKQUOTE> :
In the command string, any occurrence of `%h' will be substituted by the host name to connect, `%p' by the port, and `%r' by the remote user name. 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.
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p:
If the port argument is `0' , the listen port will be dynamically allocated on the server and reported to the client at run time.
If the bind_address is not specified, the default is to only bind to loopback addresses. If the bind_address is `*' or an empty string, then the forwarding is requested to listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config5).
See Sx PATTERNS for more information on patterns.
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; in protocol version 1 there is no mechanism to request a response from the server to the server alive messages, so disconnection is the responsibility of the TCP stack.
The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
The argument must be ``yes or ``no The default is ``no''
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''
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''
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 declarations for any host in the ``.co.uk'' set of domains, the following pattern could be used:
The following pattern would match any host in the 192.168.0.[0-9] network range:
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 organization except from the ``dialup'' pool, the following entry (in authorized_keys) could be used:
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.
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