Linux command ssh

obtains configuration data from the following sources in the following order:

command-line options

user's configuration file

system-wide configuration file

For each parameter, the first obtained value will be used. The configuration files contain sections separated by

specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is usually the one given on the command line (see the

option for exceptions).

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.

Note that the Debian

package sets several options as standard in

which are not the default in

files are included at the start of the system-wide configuration file, so options set there will override those in

The file contains keyword-argument pairs, one per line. Lines starting with

and empty lines are interpreted as comments. Arguments may optionally be enclosed in double quotes

in order to represent arguments containing spaces. 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

and

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

keyword) 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 pattern can be used to provide global defaults for all hosts. The host is usually the

argument given on the command line (see the

keyword for exceptions).

A pattern entry may be negated by prefixing it with an exclamation mark

If a negated entry is matched, then the

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

for more information on patterns.

Restricts the following declarations (up to the next

keyword) to be used only when the conditions following the

keyword are satisfied. Match conditions are specified using one or more criteria or the single token

which always matches. The available criteria keywords are:

and

The

criteria must appear alone or immediately after

Other criteria may be combined arbitrarily. All criteria but

and

require an argument. Criteria may be negated by prepending an exclamation mark

The

keyword matches only when the configuration file is being re-parsed after hostname canonicalization (see the

option). This may be useful to specify conditions that work with canonical host names only.

The

keyword requests that the configuration be re-parsed (regardless of whether

is enabled), and matches only during this final pass. If

is enabled, then

and

match during the same pass.

The

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. Arguments to

accept the tokens described in the

section.

The other keywords' criteria must be single entries or comma-separated lists and may use the wildcard and negation operators described in the

section. The criteria for the

keyword are matched against the target hostname, after any substitution by the

options. The

keyword matches against the hostname as it was specified on the command-line. The

keyword matches against the target username on the remote host. The

keyword matches against the name of the local user running

(this keyword may be useful in system-wide

files).

Specifies whether keys should be automatically added to a running

If this option is set to

and a key is loaded from a file, the key and its passphrase are added to the agent with the default lifetime, as if by

If this option is set to

will require confirmation using the

program before adding a key (see

for details). If this option is set to

each use of the key must be confirmed, as if the

option was specified to

If this option is set to

no keys are added to the agent. Alternately, this option may be specified as a time interval using the format described in the

section of

to specify the key's lifetime in

after which it will automatically be removed. The argument must be

(the default),

(optionally followed by a time interval),

or a time interval.

Specifies which address family to use when connecting. Valid arguments are

(the default),

(use IPv4 only), or

(use IPv6 only).

If set to

user interaction such as password prompts and host key confirmation requests will be disabled. In addition, the

option will be set to 300 seconds by default (Debian-specific). This option is useful in scripts and other batch jobs where no user is present to interact with

and where it is desirable to detect a broken network swiftly. The argument must be

(the default).

Use the specified address on the local machine as the source address of the connection. Only useful on systems with more than one address.

Use the address of the specified interface on the local machine as the source address of the connection.

When

is enabled, this option specifies the list of domain suffixes in which to search for the specified destination host.

Specifies whether to fail with an error when hostname canonicalization fails. The default,

will attempt to look up the unqualified hostname using the system resolver's search rules. A value of

will cause

to fail instantly if

is enabled and the target hostname cannot be found in any of the domains specified by

Controls whether explicit hostname canonicalization is performed. The default,

is not to perform any name rewriting and let the system resolver handle all hostname lookups. If set to

then, for connections that do not use a

will attempt to canonicalize the hostname specified on the command line using the

suffixes and

rules. If

is set to

then canonicalization is applied to proxied connections too.

If this option is enabled, then the configuration files are processed again using the new target name to pick up any new configuration in matching

and

stanzas.

Specifies the maximum number of dot characters in a hostname before canonicalization is disabled. The default, 1, allows a single dot (i.e. hostname.subdomain).

Specifies rules to determine whether CNAMEs should be followed when canonicalizing hostnames. The rules consist of one or more arguments of

where

is a pattern-list of domains that may follow CNAMEs in canonicalization, and

is a pattern-list of domains that they may resolve to.

For example,

will allow hostnames matching

to be canonicalized to names in the

domains.

Specifies which algorithms are allowed for signing of certificates by certificate authorities (CAs). The default is:

ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa

will not accept host certificates signed using algorithms other than those specified.

Specifies a file from which the user's certificate is read. A corresponding private key must be provided separately in order to use this certificate either from an

directive or

flag to

via

or via a

Arguments to

may use the tilde syntax to refer to a user's home directory, the tokens described in the

section and environment variables as described in the

section.

It is possible to have multiple certificate files specified in configuration files; these certificates will be tried in sequence. Multiple

directives will add to the list of certificates used for authentication.

Specifies whether to use challenge-response authentication. The argument to this keyword must be

(the default) or

If set to

(the default),

will additionally check the host IP address in the

file. This allows it to detect if a host key changed due to DNS spoofing and will add addresses of destination hosts to

in the process, regardless of the setting of

If the option is set to

the check will not be executed.

Specifies the ciphers allowed and their order of preference. Multiple ciphers must be comma-separated. If the specified list begins with a

character, then the specified ciphers will be appended to the default set instead of replacing them. If the specified list begins with a

character, then the specified ciphers (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified ciphers will be placed at the head of the default set.

The supported ciphers are:

3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr aes128-gcm@openssh.com aes256-gcm@openssh.com chacha20-poly1305@openssh.com

The default is:

chacha20-poly1305@openssh.com, aes128-ctr,aes192-ctr,aes256-ctr, aes128-gcm@openssh.com,aes256-gcm@openssh.com

The list of available ciphers may also be obtained using

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

command line to clear port forwardings set in configuration files, and is automatically set by

and

The argument must be

(the default).

Specifies whether to use compression. The argument must be

(the default).

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 timeout is applied both to establishing the connection and to performing the initial SSH protocol handshake and key exchange.

Enables the sharing of multiple sessions over a single network connection. When set to

will listen for connections on a control socket specified using the

argument. Additional sessions can connect to this socket using the same

with

set to

(the default). These sessions will try to reuse the master instance's network connection rather than initiating new ones, but will fall back to connecting normally if the control socket does not exist, or is not listening.

Setting this to

will cause

to listen for control connections, but require confirmation using

If the

cannot be opened,

will continue without connecting to a master instance.

X11 and

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:

and

The latter requires confirmation like the

option.

Specify the path to the control socket used for connection sharing as described in the

section above or the string

to disable connection sharing. Arguments to

may use the tilde syntax to refer to a user's home directory, the tokens described in the

section and environment variables as described in the

section. It is recommended that any

used for opportunistic connection sharing include at least %h, %p, and %r (or alternatively %C) and be placed in a directory that is not writable by other users. This ensures that shared connections are uniquely identified.

When used in conjunction with

specifies that the master connection should remain open in the background (waiting for future client connections) after the initial client connection has been closed. If set to

(the default), 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

or 0, then the master connection will remain in the background indefinitely (until killed or closed via a mechanism such as the

If set to a time in seconds, or a time in any of the formats documented in

then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time.

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

IPv6 addresses can be specified by enclosing addresses in square brackets. By default, the local port is bound in accordance with the

setting. However, an explicit

may be used to bind the connection to a specific address. The

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

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.

Setting this option to

in the global client configuration file

enables the use of the helper program

during

The argument must be

(the default). This option should be placed in the non-hostspecific section. See

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

to disable the escape character entirely (making the connection transparent for binary data).

Specifies whether

should terminate the connection if it cannot set up all requested dynamic, tunnel, local, and remote port forwardings, (e.g. if either end is unable to bind and listen on a specified port). Note that

does not apply to connections made over port forwardings and will not, for example, cause

to exit if TCP connections to the ultimate forwarding destination fail. The argument must be

(the default).

Specifies the hash algorithm used when displaying key fingerprints. Valid options are:

and

(the default).

Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument may be

(the default), an explicit path to an agent socket or the name of an environment variable (beginning with

in which to find the path.

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

set. The argument must be

(the default).

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

option is also enabled.

Specify a timeout for untrusted X11 forwarding using the format described in the

section of

X11 connections received by

after this time will be refused. Setting

to zero will disable the timeout and permit X11 forwarding for the life of the connection. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed.

If this option is set to

(the Debian-specific default), remote X11 clients will have full access to the original X11 display.

If this option is set to

(the upstream default), remote X11 clients will be considered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients. Furthermore, the

token used for the session will be set to expire after 20 minutes. Remote clients will be refused access after this time.

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,

binds local port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports.

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

(the default).

Specifies one or more files to use for the global host key database, separated by whitespace. The default is

Specifies whether user authentication based on GSSAPI is allowed. The default is

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.

Forward (delegate) credentials to the server. The default is

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

If set to

then renewal of the client's GSSAPI credentials will force the rekeying of the ssh connection. With a compatible server, this will delegate the renewed credentials to a session on the server.

Checks are made to ensure that credentials are only propagated when the new credentials match the old ones on the originating client and where the receiving server still has the old set in its cache.

The default is

For this to work

needs to be enabled in the server and also used by the client.

If set, specifies the GSSAPI server identity that ssh should expect when connecting to the server. The default is unset, which means that the expected GSSAPI server identity will be determined from the target hostname.

Set to

to indicate that the DNS is trusted to securely canonicalize the name of the host being connected to. If

the hostname entered on the command line will be passed untouched to the GSSAPI library. The default is

The list of key exchange algorithms that are offered for GSSAPI key exchange. Possible values are

gss-gex-sha1-, gss-group1-sha1-, gss-group14-sha1-, gss-group14-sha256-, gss-group16-sha512-, gss-nistp256-sha256-, gss-curve25519-sha256-

The default is

This option only applies to connections using GSSAPI.

Indicates that

should hash host names and addresses when they are added to

These hashed names may be used normally by

and

but they do not visually reveal identifying information if the file's contents are disclosed. The default is

Note that existing names and addresses in known hosts files will not be converted automatically, but may be manually hashed using

Use of this option may break facilities such as tab-completion that rely on being able to read unhashed host names from

Specifies whether to try rhosts based authentication with public key authentication. The argument must be

(the default).

Specifies the key types that will be used for hostbased authentication as a comma-separated list of patterns. Alternately if the specified list begins with a

character, then the specified key types will be appended to the default set instead of replacing them. If the specified list begins with a

character, then the specified key types (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified key types will be placed at the head of the default set. The default for this option is:

ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa

The

option of

may be used to list supported key types.

Specifies the host key algorithms that the client wants to use in order of preference. Alternately if the specified list begins with a

character, then the specified key types will be appended to the default set instead of replacing them. If the specified list begins with a

character, then the specified key types (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified key types will be placed at the head of the default set. The default for this option is:

If hostkeys are known for the destination host then this default is modified to prefer their algorithms.

The list of available key types may also be obtained using

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 and when validating host certificates. 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. Arguments to

accept the tokens described in the

section. Numeric IP addresses are also permitted (both on the command line and in

specifications). The default is the name given on the command line.

Specifies that

should only use the configured authentication identity and certificate files (either the default files, or those explicitly configured in the

files or passed on the

command-line), even if

or a

offers more identities. The argument to this keyword must be

(the default). This option is intended for situations where ssh-agent offers many different identities.

Specifies the

socket used to communicate with the authentication agent.

This option overrides the

environment variable and can be used to select a specific agent. Setting the socket name to

disables the use of an authentication agent. If the string

is specified, the location of the socket will be read from the

environment variable. Otherwise if the specified value begins with a

character, then it will be treated as an environment variable containing the location of the socket.

Arguments to

may use the tilde syntax to refer to a user's home directory, the tokens described in the

section and environment variables as described in the

section.

Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. The default is

and

Additionally, any identities represented by the authentication agent will be used for authentication unless

is set. If no certificates have been explicitly specified by

will try to load certificate information from the filename obtained by appending

to the path of a specified

Arguments to

may use the tilde syntax to refer to a user's home directory or the tokens described in the

section.

It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence. Multiple

directives will add to the list of identities tried (this behaviour differs from that of other configuration directives).

may be used in conjunction with

to select which identities in an agent are offered during authentication.

may also be used in conjunction with

in order to provide any certificate also needed for authentication with the identity.

Specifies a pattern-list of unknown options to be ignored if they are encountered in configuration parsing. This may be used to suppress errors if

contains options that are unrecognised by

It is recommended that

be listed early in the configuration file as it will not be applied to unknown options that appear before it.

Include the specified configuration file(s). Multiple pathnames may be specified and each pathname may contain

wildcards and, for user configurations, shell-like

references to user home directories. Wildcards will be expanded and processed in lexical order. Files without absolute paths are assumed to be in

if included in a user configuration file or

if included from the system configuration file.

directive may appear inside a

block to perform conditional inclusion.

Specifies the IPv4 type-of-service or DSCP class for connections. Accepted values are

a numeric value, or

to use the operating system default. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The default is

for interactive sessions and

for non-interactive sessions.

Specifies whether to use keyboard-interactive authentication. The argument to this keyword must be

(the default) or

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:

and

Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. If the specified list begins with a

character, then the specified methods will be appended to the default set instead of replacing them. If the specified list begins with a

character, then the specified methods (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified methods will be placed at the head of the default set. The default is:

curve25519-sha256,curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256

The list of available key exchange algorithms may also be obtained using

Specifies a command to execute on the local machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to

accept the tokens described in the

section.

The command is run synchronously and does not have access to the session of the

that spawned it. It should not be used for interactive commands.

This directive is ignored unless

has been enabled.

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 specifies the listener and may be

or a Unix domain socket path. The second argument is the destination and may be

or a Unix domain socket path if the remote host supports it.

IPv6 addresses can be specified by enclosing addresses in square brackets. 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 accordance with the

setting. However, an explicit

may be used to bind the connection to a specific address. The

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. Unix domain socket paths may use the tokens described in the

section and environment variables as described in the

section.

Gives the verbosity level that is used when logging messages from

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 for data integrity protection. Multiple algorithms must be comma-separated. If the specified list begins with a

character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a

character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified algorithms will be placed at the head of the default set.

The algorithms that contain

calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended.

The default is:

umac-64-etm@openssh.com,umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com,umac-128@openssh.com, hmac-sha2-256,hmac-sha2-512,hmac-sha1

The list of available MAC algorithms may also be obtained using

Disable host authentication for localhost (loopback addresses). The argument to this keyword must be

(the default).

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

(the default) or

Allow local command execution via the

option or using the

escape sequence in

The argument must be

(the default).

Specifies which PKCS#11 provider to use or

to indicate that no provider should be used (the default). The argument to this keyword is a path to the PKCS#11 shared library

should use to communicate with a PKCS#11 token providing keys for user authentication.

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

Specifies the order in which the client should try authentication methods. This allows a client to prefer one method (e.g.

over another method (e.g.

The default is:

gssapi-with-mic,hostbased,publickey, keyboard-interactive,password

Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed using the user's shell

directive to avoid a lingering shell process.

Arguments to

accept the tokens described in the

section. The command can be basically anything, and should read from its standard input and write to its standard output. It should eventually connect an

server running on some machine, or execute

somewhere. Host key management will be done using the

of the host being connected (defaulting to the name typed by the user). Setting the command to

disables this option entirely. Note that

is not available for connects with a proxy command.

This directive is useful in conjunction with

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

Specifies one or more jump proxies as either

or an ssh URI

Multiple proxies may be separated by comma characters and will be visited sequentially. Setting this option will cause

to connect to the target host by first making a

connection to the specified

host and then establishing a TCP forwarding to the ultimate target from there.

Note that this option will compete with the

option - whichever is specified first will prevent later instances of the other from taking effect.

Note also that the configuration for the destination host (either supplied via the command-line or the configuration file) is not generally applied to jump hosts.

should be used if specific configuration is required for jump hosts.

Specifies that

will pass a connected file descriptor back to

instead of continuing to execute and pass data. The default is

Specifies the key types that will be used for public key authentication as a comma-separated list of patterns. If the specified list begins with a

character, then the key types after it will be appended to the default instead of replacing it. If the specified list begins with a

character, then the specified key types (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a

character, then the specified key types will be placed at the head of the default set. The default for this option is:

The list of available key types may also be obtained using

Specifies whether to try public key authentication. The argument to this keyword must be

(the default) or

Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of time that may pass before the session key is renegotiated. The first argument is specified in bytes and may have a suffix of

to indicate Kilobytes, Megabytes, or Gigabytes, respectively. The default is between

and

depending on the cipher. The optional second value is specified in seconds and may use any of the units documented in the TIME FORMATS section of

The default value for

which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done.

Specifies a command to execute on the remote machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to

accept the tokens described in the

section.

Specifies that a TCP port on the remote machine be forwarded over the secure channel. The remote port may either be forwarded to a specified host and port from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote client to connect to arbitrary destinations from the local machine. The first argument is the listening specification and may be

or, if the remote host supports it, a Unix domain socket path. If forwarding to a specific destination then the second argument must be

or a Unix domain socket path, otherwise if no destination argument is specified then the remote forwarding will be established as a SOCKS proxy.

IPv6 addresses can be specified by enclosing addresses in square brackets. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Privileged ports can be forwarded only when logging in as root on the remote machine. Unix domain socket paths may use the tokens described in the

section and environment variables as described in the

section.

If the

argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time.

If the

is not specified, the default is to only bind to loopback addresses. If the

or an empty string, then the forwarding is requested to listen on all interfaces. Specifying a remote

will only succeed if the server's

option is enabled (see

Specifies whether to request a pseudo-tty for the session. The argument may be one of:

(never request a TTY),

(always request a TTY when standard input is a TTY),

(always request a TTY) or

(request a TTY when opening a login session). This option mirrors the

and

flags for

Specifies revoked host public keys. Keys listed in this file will be refused for host authentication. Note that if this file does not exist or is not readable, then host authentication will be refused for all hosts. Keys may be specified as a text file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by

For more information on KRLs, see the KEY REVOCATION LISTS section in

Specifies a path to a library that will be used when loading any FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support.

If the specified value begins with a

character, then it will be treated as an environment variable containing the path to the library.

Specifies what variables from the local

should be sent to the server. The server must also support it, and the server must be configured to accept these environment variables. Note that the

environment variable is always sent whenever a pseudo-terminal is requested as it is required by the protocol. Refer to

for how to configure the server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be separated by whitespace or spread across multiple

directives.

See

for more information on patterns.

It is possible to clear previously set

variable names by prefixing patterns with

The default is not to send any environment variables.

Sets the number of server alive messages (see below) which may be sent without

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

(below). The server alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by

is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive.

The default value is 3. If, for example,

(see below) is set to 15 and

is left at the default, if the server becomes unresponsive, ssh will disconnect after approximately 45 seconds.

Sets a timeout interval in seconds after which if no data has been received from the server,

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, or 300 if the

option is set (Debian-specific).

and

are Debian-specific compatibility aliases for this option.

Directly specify one or more environment variables and their contents to be sent to the server. Similarly to

the server must be prepared to accept the environment variable.

Sets the octal file creation mode mask

used when creating a Unix-domain socket file for local or remote port forwarding. This option is only used for port forwarding to a Unix-domain socket file.

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.

Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and

is not enabled,

will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file.

The argument must be

(the default).

If this flag is set to

will never automatically add host keys to the

file, and refuses to connect to hosts whose host key has changed. This provides maximum protection against man-in-the-middle (MITM) attacks, though it can be annoying when the

file is poorly maintained or when connections to new hosts are frequently made. This option forces the user to manually add all new hosts.

If this flag is set to

then ssh will automatically add new host keys to the user known hosts files, but will not permit connections to hosts with changed host keys. If this flag is set to

ssh will automatically add new host keys to the user known hosts files and allow connections to hosts with changed hostkeys to proceed, subject to some restrictions. If this flag is set to

(the default), 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.

Gives the facility code that is used when logging messages from

The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is USER.

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. This option only uses TCP keepalives (as opposed to using ssh level keepalives), so takes a long time to notice when the connection dies. As such, you probably want the

option as well. However, this means that connections will die if the route is down temporarily, and some people find it annoying.

The default is

(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