| .\" |
| .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| .\" All rights reserved |
| .\" |
| .\" As far as I am concerned, the code I have written for this software |
| .\" can be used freely for any purpose. Any derived versions of this |
| .\" software must be clearly marked as such, and if the derived work is |
| .\" incompatible with the protocol description in the RFC file, it must be |
| .\" called by a name other than "ssh" or "Secure Shell". |
| .\" |
| .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| .\" |
| .\" Redistribution and use in source and binary forms, with or without |
| .\" modification, are permitted provided that the following conditions |
| .\" are met: |
| .\" 1. Redistributions of source code must retain the above copyright |
| .\" notice, this list of conditions and the following disclaimer. |
| .\" 2. Redistributions in binary form must reproduce the above copyright |
| .\" notice, this list of conditions and the following disclaimer in the |
| .\" documentation and/or other materials provided with the distribution. |
| .\" |
| .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| .\" |
| .\" $OpenBSD: ssh_config.5,v 1.239 2016/09/28 17:59:22 jmc Exp $ |
| .Dd $Mdocdate: September 28 2016 $ |
| .Dt SSH_CONFIG 5 |
| .Os |
| .Sh NAME |
| .Nm ssh_config |
| .Nd OpenSSH SSH client configuration files |
| .Sh SYNOPSIS |
| .Nm ~/.ssh/config |
| .Nm /etc/ssh/ssh_config |
| .Sh DESCRIPTION |
| .Xr ssh 1 |
| obtains configuration data from the following sources in |
| the following order: |
| .Pp |
| .Bl -enum -offset indent -compact |
| .It |
| command-line options |
| .It |
| user's configuration file |
| .Pq Pa ~/.ssh/config |
| .It |
| system-wide configuration file |
| .Pq Pa /etc/ssh/ssh_config |
| .El |
| .Pp |
| For each parameter, the first obtained value |
| will be used. |
| The configuration files contain sections separated by |
| .Dq 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 usually the one given on the command line |
| (see the |
| .Cm CanonicalizeHostname |
| option for exceptions.) |
| .Pp |
| 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. |
| .Pp |
| The configuration file has the following format: |
| .Pp |
| Empty lines and lines starting with |
| .Ql # |
| are comments. |
| Otherwise a line is of the format |
| .Dq keyword arguments . |
| Configuration options may be separated by whitespace or |
| optional whitespace and exactly one |
| .Ql = ; |
| the latter format is useful to avoid the need to quote whitespace |
| when specifying configuration options using the |
| .Nm ssh , |
| .Nm scp , |
| and |
| .Nm sftp |
| .Fl o |
| option. |
| Arguments may optionally be enclosed in double quotes |
| .Pq \&" |
| in order to represent arguments containing spaces. |
| .Pp |
| The possible |
| keywords and their meanings are as follows (note that |
| keywords are case-insensitive and arguments are case-sensitive): |
| .Bl -tag -width Ds |
| .It Cm Host |
| Restricts the following declarations (up to the next |
| .Cm Host |
| or |
| .Cm Match |
| 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 |
| .Ql * |
| as a pattern can be used to provide global |
| defaults for all hosts. |
| The host is usually the |
| .Ar hostname |
| argument given on the command line |
| (see the |
| .Cm CanonicalizeHostname |
| option for exceptions.) |
| .Pp |
| A pattern entry may be negated by prefixing it with an exclamation mark |
| .Pq Sq !\& . |
| If a negated entry is matched, then the |
| .Cm 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. |
| .Pp |
| See |
| .Sx PATTERNS |
| for more information on patterns. |
| .It Cm Match |
| Restricts the following declarations (up to the next |
| .Cm Host |
| or |
| .Cm Match |
| keyword) to be used only when the conditions following the |
| .Cm Match |
| keyword are satisfied. |
| Match conditions are specified using one or more criteria |
| or the single token |
| .Cm all |
| which always matches. |
| The available criteria keywords are: |
| .Cm canonical , |
| .Cm exec , |
| .Cm host , |
| .Cm originalhost , |
| .Cm user , |
| and |
| .Cm localuser . |
| The |
| .Cm all |
| criteria must appear alone or immediately after |
| .Cm canonical . |
| Other criteria may be combined arbitrarily. |
| All criteria but |
| .Cm all |
| and |
| .Cm canonical |
| require an argument. |
| Criteria may be negated by prepending an exclamation mark |
| .Pq Sq !\& . |
| .Pp |
| The |
| .Cm canonical |
| keyword matches only when the configuration file is being re-parsed |
| after hostname canonicalization (see the |
| .Cm CanonicalizeHostname |
| option.) |
| This may be useful to specify conditions that work with canonical host |
| names only. |
| The |
| .Cm 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. |
| Arguments to |
| .Cm exec |
| accept the tokens described in the |
| .Sx TOKENS |
| section. |
| .Pp |
| 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 |
| .Cm host |
| keyword are matched against the target hostname, after any substitution |
| by the |
| .Cm Hostname |
| or |
| .Cm CanonicalizeHostname |
| options. |
| The |
| .Cm originalhost |
| keyword matches against the hostname as it was specified on the command-line. |
| The |
| .Cm user |
| keyword matches against the target username on the remote host. |
| The |
| .Cm localuser |
| keyword matches against the name of the local user running |
| .Xr ssh 1 |
| (this keyword may be useful in system-wide |
| .Nm |
| files). |
| .It Cm AddKeysToAgent |
| Specifies whether keys should be automatically added to a running |
| .Xr ssh-agent 1 . |
| If this option is set to |
| .Dq yes |
| 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 |
| .Xr ssh-add 1 . |
| If this option is set to |
| .Dq ask , |
| .Nm ssh |
| will require confirmation using the |
| .Ev SSH_ASKPASS |
| program before adding a key (see |
| .Xr ssh-add 1 |
| for details). |
| If this option is set to |
| .Dq confirm , |
| each use of the key must be confirmed, as if the |
| .Fl c |
| option was specified to |
| .Xr ssh-add 1 . |
| If this option is set to |
| .Dq no , |
| no keys are added to the agent. |
| The argument must be |
| .Dq yes , |
| .Dq confirm , |
| .Dq ask , |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm AddressFamily |
| Specifies which address family to use when connecting. |
| Valid arguments are |
| .Dq any , |
| .Dq inet |
| (use IPv4 only), or |
| .Dq inet6 |
| (use IPv6 only). |
| The default is |
| .Dq any . |
| .It Cm BatchMode |
| If set to |
| .Dq 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 |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm BindAddress |
| Use the specified address on the local machine as the source address of |
| the connection. |
| Only useful on systems with more than one address. |
| Note that this option does not work if |
| .Cm UsePrivilegedPort |
| is set to |
| .Dq yes . |
| .It Cm CanonicalDomains |
| When |
| .Cm CanonicalizeHostname |
| is enabled, this option specifies the list of domain suffixes in which to |
| search for the specified destination host. |
| .It Cm CanonicalizeFallbackLocal |
| Specifies whether to fail with an error when hostname canonicalization fails. |
| The default, |
| .Dq yes , |
| will attempt to look up the unqualified hostname using the system resolver's |
| search rules. |
| A value of |
| .Dq no |
| will cause |
| .Xr ssh 1 |
| to fail instantly if |
| .Cm CanonicalizeHostname |
| is enabled and the target hostname cannot be found in any of the domains |
| specified by |
| .Cm CanonicalDomains . |
| .It Cm CanonicalizeHostname |
| Controls whether explicit hostname canonicalization is performed. |
| The default, |
| .Dq no , |
| is not to perform any name rewriting and let the system resolver handle all |
| hostname lookups. |
| If set to |
| .Dq yes |
| then, for connections that do not use a |
| .Cm ProxyCommand , |
| .Xr ssh 1 |
| will attempt to canonicalize the hostname specified on the command line |
| using the |
| .Cm CanonicalDomains |
| suffixes and |
| .Cm CanonicalizePermittedCNAMEs |
| rules. |
| If |
| .Cm CanonicalizeHostname |
| is set to |
| .Dq always , |
| then canonicalization is applied to proxied connections too. |
| .Pp |
| 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 |
| .Cm Host |
| and |
| .Cm Match |
| stanzas. |
| .It Cm CanonicalizeMaxDots |
| Specifies the maximum number of dot characters in a hostname before |
| canonicalization is disabled. |
| The default, |
| .Dq 1 , |
| allows a single dot (i.e. hostname.subdomain). |
| .It Cm CanonicalizePermittedCNAMEs |
| Specifies rules to determine whether CNAMEs should be followed when |
| canonicalizing hostnames. |
| The rules consist of one or more arguments of |
| .Ar source_domain_list : Ns Ar target_domain_list , |
| where |
| .Ar source_domain_list |
| is a pattern-list of domains that may follow CNAMEs in canonicalization, |
| and |
| .Ar target_domain_list |
| is a pattern-list of domains that they may resolve to. |
| .Pp |
| For example, |
| .Dq *.a.example.com:*.b.example.com,*.c.example.com |
| will allow hostnames matching |
| .Dq *.a.example.com |
| to be canonicalized to names in the |
| .Dq *.b.example.com |
| or |
| .Dq *.c.example.com |
| domains. |
| .It Cm CertificateFile |
| 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 |
| .Cm IdentityFile |
| directive or |
| .Fl i |
| flag to |
| .Xr ssh 1 , |
| via |
| .Xr ssh-agent 1 , |
| or via a |
| .Cm PKCS11Provider . |
| .Pp |
| Arguments to |
| .Cm CertificateFile |
| may use the tilde syntax to refer to a user's home directory |
| or the tokens described in the |
| .Sx TOKENS |
| section. |
| .Pp |
| It is possible to have multiple certificate files specified in |
| configuration files; these certificates will be tried in sequence. |
| Multiple |
| .Cm CertificateFile |
| directives will add to the list of certificates used for |
| authentication. |
| .It Cm ChallengeResponseAuthentication |
| Specifies whether to use challenge-response authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm CheckHostIP |
| If this flag is set to |
| .Dq yes , |
| .Xr ssh 1 |
| will additionally check the host IP address in the |
| .Pa known_hosts |
| file. |
| This allows ssh to detect if a host key changed due to DNS spoofing |
| and will add addresses of destination hosts to |
| .Pa ~/.ssh/known_hosts |
| in the process, regardless of the setting of |
| .Cm StrictHostKeyChecking . |
| If the option is set to |
| .Dq no , |
| the check will not be executed. |
| The default is |
| .Dq yes . |
| .It Cm Cipher |
| Specifies the cipher to use for encrypting the session |
| in protocol version 1. |
| Currently, |
| .Dq blowfish , |
| .Dq 3des , |
| and |
| .Dq des |
| are supported. |
| .Ar des |
| is only supported in the |
| .Xr ssh 1 |
| client for interoperability with legacy protocol 1 implementations |
| that do not support the |
| .Ar 3des |
| cipher. |
| Its use is strongly discouraged due to cryptographic weaknesses. |
| The default is |
| .Dq 3des . |
| .It Cm Ciphers |
| Specifies the ciphers allowed for protocol version 2 |
| in order of preference. |
| Multiple ciphers must be comma-separated. |
| If the specified value begins with a |
| .Sq + |
| character, then the specified ciphers will be appended to the default set |
| instead of replacing them. |
| .Pp |
| The supported ciphers are: |
| .Pp |
| .Bl -item -compact -offset indent |
| .It |
| 3des-cbc |
| .It |
| aes128-cbc |
| .It |
| aes192-cbc |
| .It |
| aes256-cbc |
| .It |
| aes128-ctr |
| .It |
| aes192-ctr |
| .It |
| aes256-ctr |
| .It |
| aes128-gcm@openssh.com |
| .It |
| aes256-gcm@openssh.com |
| .It |
| arcfour |
| .It |
| arcfour128 |
| .It |
| arcfour256 |
| .It |
| blowfish-cbc |
| .It |
| cast128-cbc |
| .It |
| chacha20-poly1305@openssh.com |
| .El |
| .Pp |
| The default is: |
| .Bd -literal -offset indent |
| chacha20-poly1305@openssh.com, |
| aes128-ctr,aes192-ctr,aes256-ctr, |
| aes128-gcm@openssh.com,aes256-gcm@openssh.com, |
| aes128-cbc,aes192-cbc,aes256-cbc |
| .Ed |
| .Pp |
| The list of available ciphers may also be obtained using the |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| with an argument of |
| .Dq cipher . |
| .It Cm 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 |
| .Xr ssh 1 |
| command line to clear port forwardings set in |
| configuration files, and is automatically set by |
| .Xr scp 1 |
| and |
| .Xr sftp 1 . |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm Compression |
| Specifies whether to use compression. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm 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. |
| The meaning of the values is the same as in |
| .Xr gzip 1 . |
| Note that this option applies to protocol version 1 only. |
| .It Cm ConnectionAttempts |
| 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. |
| .It Cm ConnectTimeout |
| 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. |
| .It Cm ControlMaster |
| Enables the sharing of multiple sessions over a single network connection. |
| When set to |
| .Dq yes , |
| .Xr ssh 1 |
| will listen for connections on a control socket specified using the |
| .Cm ControlPath |
| argument. |
| Additional sessions can connect to this socket using the same |
| .Cm ControlPath |
| with |
| .Cm ControlMaster |
| set to |
| .Dq no |
| (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. |
| .Pp |
| Setting this to |
| .Dq ask |
| will cause ssh |
| to listen for control connections, but require confirmation using |
| .Xr ssh-askpass 1 . |
| If the |
| .Cm ControlPath |
| cannot be opened, |
| ssh will continue without connecting to a master instance. |
| .Pp |
| X11 and |
| .Xr ssh-agent 1 |
| 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. |
| .Pp |
| 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: |
| .Dq auto |
| and |
| .Dq autoask . |
| The latter requires confirmation like the |
| .Dq ask |
| option. |
| .It Cm ControlPath |
| Specify the path to the control socket used for connection sharing as described |
| in the |
| .Cm ControlMaster |
| section above or the string |
| .Dq none |
| to disable connection sharing. |
| Arguments to |
| .Cm ControlPath |
| may use the tilde syntax to refer to a user's home directory |
| or the tokens described in the |
| .Sx TOKENS |
| section. |
| It is recommended that any |
| .Cm ControlPath |
| 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. |
| .It Cm ControlPersist |
| When used in conjunction with |
| .Cm ControlMaster , |
| 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 |
| .Dq no , |
| 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 |
| .Dq yes |
| or |
| .Dq 0 , |
| then the master connection will remain in the background indefinitely |
| (until killed or closed via a mechanism such as the |
| .Xr ssh 1 |
| .Dq Fl O No exit |
| option). |
| If set to a time in seconds, or a time in any of the formats documented in |
| .Xr sshd_config 5 , |
| then the backgrounded master connection will automatically terminate |
| after it has remained idle (with no client connections) for the |
| specified time. |
| .It Cm DynamicForward |
| 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. |
| .Pp |
| The argument must be |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port . |
| .Sm on |
| IPv6 addresses can be specified by enclosing addresses in square brackets. |
| By default, the local port is bound in accordance with the |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .Pp |
| Currently the SOCKS4 and SOCKS5 protocols are supported, and |
| .Xr 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. |
| .It Cm EnableSSHKeysign |
| Setting this option to |
| .Dq yes |
| in the global client configuration file |
| .Pa /etc/ssh/ssh_config |
| enables the use of the helper program |
| .Xr ssh-keysign 8 |
| during |
| .Cm HostbasedAuthentication . |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option should be placed in the non-hostspecific section. |
| See |
| .Xr ssh-keysign 8 |
| for more information. |
| .It Cm EscapeChar |
| Sets the escape character (default: |
| .Ql ~ ) . |
| The escape character can also |
| be set on the command line. |
| The argument should be a single character, |
| .Ql ^ |
| followed by a letter, or |
| .Dq none |
| to disable the escape |
| character entirely (making the connection transparent for binary |
| data). |
| .It Cm ExitOnForwardFailure |
| Specifies whether |
| .Xr ssh 1 |
| 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 |
| .Cm ExitOnForwardFailure |
| does not apply to connections made over port forwardings and will not, |
| for example, cause |
| .Xr ssh 1 |
| to exit if TCP connections to the ultimate forwarding destination fail. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm FingerprintHash |
| Specifies the hash algorithm used when displaying key fingerprints. |
| Valid options are: |
| .Dq md5 |
| and |
| .Dq sha256 . |
| The default is |
| .Dq sha256 . |
| .It Cm ForwardAgent |
| Specifies whether the connection to the authentication agent (if any) |
| will be forwarded to the remote machine. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .Pp |
| 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. |
| .It Cm ForwardX11 |
| Specifies whether X11 connections will be automatically redirected |
| over the secure channel and |
| .Ev DISPLAY |
| set. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .Pp |
| 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 |
| .Cm ForwardX11Trusted |
| option is also enabled. |
| .It Cm ForwardX11Timeout |
| Specify a timeout for untrusted X11 forwarding |
| using the format described in the |
| TIME FORMATS section of |
| .Xr sshd_config 5 . |
| X11 connections received by |
| .Xr ssh 1 |
| after this time will be refused. |
| The default is to disable untrusted X11 forwarding after twenty minutes has |
| elapsed. |
| .It Cm ForwardX11Trusted |
| If this option is set to |
| .Dq yes , |
| remote X11 clients will have full access to the original X11 display. |
| .Pp |
| If this option is set to |
| .Dq no , |
| remote X11 clients will be considered untrusted and prevented |
| from stealing or tampering with data belonging to trusted X11 |
| clients. |
| Furthermore, the |
| .Xr xauth 1 |
| token used for the session will be set to expire after 20 minutes. |
| Remote clients will be refused access after this time. |
| .Pp |
| The default is |
| .Dq no . |
| .Pp |
| See the X11 SECURITY extension specification for full details on |
| the restrictions imposed on untrusted clients. |
| .It Cm GatewayPorts |
| Specifies whether remote hosts are allowed to connect to local |
| forwarded ports. |
| By default, |
| .Xr ssh 1 |
| binds local port forwardings to the loopback address. |
| This prevents other remote hosts from connecting to forwarded ports. |
| .Cm 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 |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm GlobalKnownHostsFile |
| Specifies one or more files to use for the global |
| host key database, separated by whitespace. |
| The default is |
| .Pa /etc/ssh/ssh_known_hosts , |
| .Pa /etc/ssh/ssh_known_hosts2 . |
| .It Cm GSSAPIAuthentication |
| Specifies whether user authentication based on GSSAPI is allowed. |
| The default is |
| .Dq no . |
| .It Cm GSSAPIDelegateCredentials |
| Forward (delegate) credentials to the server. |
| The default is |
| .Dq no . |
| .It Cm HashKnownHosts |
| Indicates that |
| .Xr ssh 1 |
| should hash host names and addresses when they are added to |
| .Pa ~/.ssh/known_hosts . |
| These hashed names may be used normally by |
| .Xr ssh 1 |
| and |
| .Xr sshd 8 , |
| but they do not reveal identifying information should the file's contents |
| be disclosed. |
| The default is |
| .Dq no . |
| Note that existing names and addresses in known hosts files |
| will not be converted automatically, |
| but may be manually hashed using |
| .Xr ssh-keygen 1 . |
| .It Cm HostbasedAuthentication |
| Specifies whether to try rhosts based authentication with public key |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm HostbasedKeyTypes |
| Specifies the key types that will be used for hostbased authentication |
| as a comma-separated pattern list. |
| Alternately if the specified value begins with a |
| .Sq + |
| character, then the specified key types will be appended to the default set |
| instead of replacing them. |
| The default for this option is: |
| .Bd -literal -offset 3n |
| ecdsa-sha2-nistp256-cert-v01@openssh.com, |
| ecdsa-sha2-nistp384-cert-v01@openssh.com, |
| ecdsa-sha2-nistp521-cert-v01@openssh.com, |
| ssh-ed25519-cert-v01@openssh.com, |
| ssh-rsa-cert-v01@openssh.com, |
| ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, |
| ssh-ed25519,ssh-rsa |
| .Ed |
| .Pp |
| The |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| may be used to list supported key types. |
| .It Cm HostKeyAlgorithms |
| Specifies the host key algorithms |
| that the client wants to use in order of preference. |
| Alternately if the specified value begins with a |
| .Sq + |
| character, then the specified key types will be appended to the default set |
| instead of replacing them. |
| The default for this option is: |
| .Bd -literal -offset 3n |
| ecdsa-sha2-nistp256-cert-v01@openssh.com, |
| ecdsa-sha2-nistp384-cert-v01@openssh.com, |
| ecdsa-sha2-nistp521-cert-v01@openssh.com, |
| ssh-ed25519-cert-v01@openssh.com, |
| ssh-rsa-cert-v01@openssh.com, |
| ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, |
| ssh-ed25519,ssh-rsa |
| .Ed |
| .Pp |
| If hostkeys are known for the destination host then this default is modified |
| to prefer their algorithms. |
| .Pp |
| The list of available key types may also be obtained using the |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| with an argument of |
| .Dq key . |
| .It Cm 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. |
| .It Cm HostName |
| Specifies the real host name to log into. |
| This can be used to specify nicknames or abbreviations for hosts. |
| Arguments to |
| .Cm HostName |
| accept the tokens described in the |
| .Sx TOKENS |
| section. |
| Numeric IP addresses are also permitted (both on the command line and in |
| .Cm HostName |
| specifications). |
| The default is the name given on the command line. |
| .It Cm IdentitiesOnly |
| Specifies that |
| .Xr ssh 1 |
| should only use the authentication identity and certificate files explicitly |
| configured in the |
| .Nm |
| files |
| or passed on the |
| .Xr ssh 1 |
| command-line, |
| even if |
| .Xr ssh-agent 1 |
| or a |
| .Cm PKCS11Provider |
| offers more identities. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| This option is intended for situations where ssh-agent |
| offers many different identities. |
| The default is |
| .Dq no . |
| .It Cm IdentityAgent |
| Specifies the |
| .Ux Ns -domain |
| socket used to communicate with the authentication agent. |
| .Pp |
| This option overrides the |
| .Dq SSH_AUTH_SOCK |
| environment variable and can be used to select a specific agent. |
| Setting the socket name to |
| .Dq none |
| disables the use of an authentication agent. |
| If the string |
| .Dq SSH_AUTH_SOCK |
| is specified, the location of the socket will be read from the |
| .Ev SSH_AUTH_SOCK |
| environment variable. |
| .Pp |
| Arguments to |
| .Cm IdentityAgent |
| may use the tilde syntax to refer to a user's home directory |
| or the tokens described in the |
| .Sx TOKENS |
| section. |
| .It Cm IdentityFile |
| Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication |
| identity is read. |
| The default is |
| .Pa ~/.ssh/identity |
| for protocol version 1, and |
| .Pa ~/.ssh/id_dsa , |
| .Pa ~/.ssh/id_ecdsa , |
| .Pa ~/.ssh/id_ed25519 |
| and |
| .Pa ~/.ssh/id_rsa |
| for protocol version 2. |
| Additionally, any identities represented by the authentication agent |
| will be used for authentication unless |
| .Cm IdentitiesOnly |
| is set. |
| If no certificates have been explicitly specified by |
| .Cm CertificateFile , |
| .Xr ssh 1 |
| will try to load certificate information from the filename obtained by |
| appending |
| .Pa -cert.pub |
| to the path of a specified |
| .Cm IdentityFile . |
| .Pp |
| Arguments to |
| .Cm IdentityFile |
| may use the tilde syntax to refer to a user's home directory |
| or the tokens described in the |
| .Sx TOKENS |
| section. |
| .Pp |
| It is possible to have |
| multiple identity files specified in configuration files; all these |
| identities will be tried in sequence. |
| Multiple |
| .Cm IdentityFile |
| directives will add to the list of identities tried (this behaviour |
| differs from that of other configuration directives). |
| .Pp |
| .Cm IdentityFile |
| may be used in conjunction with |
| .Cm IdentitiesOnly |
| to select which identities in an agent are offered during authentication. |
| .Cm IdentityFile |
| may also be used in conjunction with |
| .Cm CertificateFile |
| in order to provide any certificate also needed for authentication with |
| the identity. |
| .It Cm IgnoreUnknown |
| 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 |
| .Nm |
| contains options that are unrecognised by |
| .Xr ssh 1 . |
| It is recommended that |
| .Cm IgnoreUnknown |
| be listed early in the configuration file as it will not be applied |
| to unknown options that appear before it. |
| .It Cm Include |
| Include the specified configuration file(s). |
| Multiple pathnames may be specified and each pathname may contain |
| .Xr glob 3 |
| wildcards and, for user configurations, shell-like |
| .Dq ~ |
| references to user home directories. |
| Files without absolute paths are assumed to be in |
| .Pa ~/.ssh |
| if included in a user configuration file or |
| .Pa /etc/ssh |
| if included from the system configuration file. |
| .Cm Include |
| directive may appear inside a |
| .Cm Match |
| or |
| .Cm Host |
| block |
| to perform conditional inclusion. |
| .It Cm IPQoS |
| Specifies the IPv4 type-of-service or DSCP class for connections. |
| Accepted values are |
| .Dq af11 , |
| .Dq af12 , |
| .Dq af13 , |
| .Dq af21 , |
| .Dq af22 , |
| .Dq af23 , |
| .Dq af31 , |
| .Dq af32 , |
| .Dq af33 , |
| .Dq af41 , |
| .Dq af42 , |
| .Dq af43 , |
| .Dq cs0 , |
| .Dq cs1 , |
| .Dq cs2 , |
| .Dq cs3 , |
| .Dq cs4 , |
| .Dq cs5 , |
| .Dq cs6 , |
| .Dq cs7 , |
| .Dq ef , |
| .Dq lowdelay , |
| .Dq throughput , |
| .Dq reliability , |
| or a numeric value. |
| 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 |
| .Dq lowdelay |
| for interactive sessions and |
| .Dq throughput |
| for non-interactive sessions. |
| .It Cm KbdInteractiveAuthentication |
| Specifies whether to use keyboard-interactive authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm KbdInteractiveDevices |
| 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: |
| .Dq bsdauth , |
| .Dq pam , |
| and |
| .Dq skey . |
| .It Cm KexAlgorithms |
| Specifies the available KEX (Key Exchange) algorithms. |
| Multiple algorithms must be comma-separated. |
| Alternately if the specified value begins with a |
| .Sq + |
| character, then the specified methods will be appended to the default set |
| instead of replacing them. |
| The default is: |
| .Bd -literal -offset indent |
| curve25519-sha256,curve25519-sha256@libssh.org, |
| ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, |
| diffie-hellman-group-exchange-sha256, |
| diffie-hellman-group-exchange-sha1, |
| diffie-hellman-group14-sha1 |
| .Ed |
| .Pp |
| The list of available key exchange algorithms may also be obtained using the |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| with an argument of |
| .Dq kex . |
| .It Cm LocalCommand |
| 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 |
| .Cm LocalCommand |
| accept the tokens described in the |
| .Sx TOKENS |
| section. |
| .Pp |
| The command is run synchronously and does not have access to the |
| session of the |
| .Xr ssh 1 |
| that spawned it. |
| It should not be used for interactive commands. |
| .Pp |
| This directive is ignored unless |
| .Cm PermitLocalCommand |
| has been enabled. |
| .It Cm LocalForward |
| 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 must be |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port |
| .Sm on |
| and the second argument must be |
| .Ar host : Ns Ar hostport . |
| 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 |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .It Cm LogLevel |
| Gives the verbosity level that is used when logging messages from |
| .Xr ssh 1 . |
| 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. |
| .It Cm MACs |
| 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 value begins with a |
| .Sq + |
| character, then the specified algorithms will be appended to the default set |
| instead of replacing them. |
| .Pp |
| The algorithms that contain |
| .Dq -etm |
| calculate the MAC after encryption (encrypt-then-mac). |
| These are considered safer and their use recommended. |
| .Pp |
| The default is: |
| .Bd -literal -offset indent |
| 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 |
| .Ed |
| .Pp |
| The list of available MAC algorithms may also be obtained using the |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| with an argument of |
| .Dq mac . |
| .It Cm 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 |
| .Dq yes |
| or |
| .Dq no . |
| The default is to check the host key for localhost. |
| .It Cm NumberOfPasswordPrompts |
| Specifies the number of password prompts before giving up. |
| The argument to this keyword must be an integer. |
| The default is 3. |
| .It Cm PasswordAuthentication |
| Specifies whether to use password authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm PermitLocalCommand |
| Allow local command execution via the |
| .Ic LocalCommand |
| option or using the |
| .Ic !\& Ns Ar command |
| escape sequence in |
| .Xr ssh 1 . |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm PKCS11Provider |
| Specifies which PKCS#11 provider to use. |
| The argument to this keyword is the PKCS#11 shared library |
| .Xr ssh 1 |
| should use to communicate with a PKCS#11 token providing the user's |
| private RSA key. |
| .It Cm Port |
| Specifies the port number to connect on the remote host. |
| The default is 22. |
| .It Cm PreferredAuthentications |
| Specifies the order in which the client should try authentication methods. |
| This allows a client to prefer one method (e.g.\& |
| .Cm keyboard-interactive ) |
| over another method (e.g.\& |
| .Cm password ) . |
| The default is: |
| .Bd -literal -offset indent |
| gssapi-with-mic,hostbased,publickey, |
| keyboard-interactive,password |
| .Ed |
| .It Cm Protocol |
| Specifies the protocol versions |
| .Xr ssh 1 |
| should support in order of preference. |
| The possible values are |
| .Sq 1 |
| and |
| .Sq 2 . |
| Multiple versions must be comma-separated. |
| When this option is set to |
| .Dq 2,1 |
| .Nm ssh |
| will try version 2 and fall back to version 1 |
| if version 2 is not available. |
| The default is |
| .Sq 2 . |
| Protocol 1 suffers from a number of cryptographic weaknesses and should |
| not be used. |
| It is only offered to support legacy devices. |
| .It Cm ProxyCommand |
| 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 |
| .Ql exec |
| directive to avoid a lingering shell process. |
| .Pp |
| Arguments to |
| .Cm ProxyCommand |
| accept the tokens described in the |
| .Sx TOKENS |
| 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 |
| .Xr sshd 8 |
| server running on some machine, or execute |
| .Ic 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 |
| .Dq none |
| disables this option entirely. |
| Note that |
| .Cm CheckHostIP |
| is not available for connects with a proxy command. |
| .Pp |
| This directive is useful in conjunction with |
| .Xr nc 1 |
| and its proxy support. |
| For example, the following directive would connect via an HTTP proxy at |
| 192.0.2.0: |
| .Bd -literal -offset 3n |
| ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p |
| .Ed |
| .It Cm ProxyJump |
| Specifies one or more jump proxies as |
| .Xo |
| .Sm off |
| .Op Ar user No @ |
| .Ar host |
| .Op : Ns Ar port |
| .Sm on |
| .Xc . |
| Multiple proxies may be separated by comma characters and will be visited |
| sequentially. |
| Setting this option will cause |
| .Xr ssh 1 |
| to connect to the target host by first making a |
| .Xr ssh 1 |
| connection to the specified |
| .Cm ProxyJump |
| host and then establishing a |
| TCP forwarding to the ultimate target from there. |
| .Pp |
| Note that this option will compete with the |
| .Cm ProxyCommand |
| option - whichever is specified first will prevent later instances of the |
| other from taking effect. |
| .It Cm ProxyUseFdpass |
| Specifies that |
| .Cm ProxyCommand |
| will pass a connected file descriptor back to |
| .Xr ssh 1 |
| instead of continuing to execute and pass data. |
| The default is |
| .Dq no . |
| .It Cm PubkeyAcceptedKeyTypes |
| Specifies the key types that will be used for public key authentication |
| as a comma-separated pattern list. |
| Alternately if the specified value begins with a |
| .Sq + |
| character, then the key types after it will be appended to the default |
| instead of replacing it. |
| The default for this option is: |
| .Bd -literal -offset 3n |
| ecdsa-sha2-nistp256-cert-v01@openssh.com, |
| ecdsa-sha2-nistp384-cert-v01@openssh.com, |
| ecdsa-sha2-nistp521-cert-v01@openssh.com, |
| ssh-ed25519-cert-v01@openssh.com, |
| ssh-rsa-cert-v01@openssh.com, |
| ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, |
| ssh-ed25519,ssh-rsa |
| .Ed |
| .Pp |
| The |
| .Fl Q |
| option of |
| .Xr ssh 1 |
| may be used to list supported key types. |
| .It Cm PubkeyAuthentication |
| Specifies whether to try public key authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm RekeyLimit |
| 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 |
| .Sq K , |
| .Sq M , |
| or |
| .Sq G |
| to indicate Kilobytes, Megabytes, or Gigabytes, respectively. |
| The default is between |
| .Sq 1G |
| and |
| .Sq 4G , |
| 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 |
| .Xr sshd_config 5 . |
| The default value for |
| .Cm RekeyLimit |
| is |
| .Dq default none , |
| 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. |
| .It Cm RemoteForward |
| Specifies that a TCP 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 |
| .Sm off |
| .Oo Ar bind_address : Oc Ar port |
| .Sm on |
| and the second argument must be |
| .Ar host : Ns Ar hostport . |
| 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. |
| .Pp |
| If the |
| .Ar port |
| argument is |
| .Ql 0 , |
| the listen port will be dynamically allocated on the server and reported |
| to the client at run time. |
| .Pp |
| If the |
| .Ar bind_address |
| is not specified, the default is to only bind to loopback addresses. |
| If the |
| .Ar bind_address |
| is |
| .Ql * |
| or an empty string, then the forwarding is requested to listen on all |
| interfaces. |
| Specifying a remote |
| .Ar bind_address |
| will only succeed if the server's |
| .Cm GatewayPorts |
| option is enabled (see |
| .Xr sshd_config 5 ) . |
| .It Cm RequestTTY |
| Specifies whether to request a pseudo-tty for the session. |
| The argument may be one of: |
| .Dq no |
| (never request a TTY), |
| .Dq yes |
| (always request a TTY when standard input is a TTY), |
| .Dq force |
| (always request a TTY) or |
| .Dq auto |
| (request a TTY when opening a login session). |
| This option mirrors the |
| .Fl t |
| and |
| .Fl T |
| flags for |
| .Xr ssh 1 . |
| .It Cm RevokedHostKeys |
| 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 |
| .Xr ssh-keygen 1 . |
| For more information on KRLs, see the KEY REVOCATION LISTS section in |
| .Xr ssh-keygen 1 . |
| .It Cm RhostsRSAAuthentication |
| Specifies whether to try rhosts based authentication with RSA host |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option applies to protocol version 1 only and requires |
| .Xr ssh 1 |
| to be setuid root. |
| .It Cm RSAAuthentication |
| Specifies whether to try RSA authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| RSA authentication will only be |
| attempted if the identity file exists, or an authentication agent is |
| running. |
| The default is |
| .Dq yes . |
| Note that this option applies to protocol version 1 only. |
| .It Cm SendEnv |
| Specifies what variables from the local |
| .Xr environ 7 |
| 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 |
| .Ev TERM |
| environment variable is always sent whenever a |
| pseudo-terminal is requested as it is required by the protocol. |
| Refer to |
| .Cm AcceptEnv |
| in |
| .Xr sshd_config 5 |
| 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 |
| .Cm SendEnv |
| directives. |
| The default is not to send any environment variables. |
| .Pp |
| See |
| .Sx PATTERNS |
| for more information on patterns. |
| .It Cm ServerAliveCountMax |
| Sets the number of server alive messages (see below) which may be |
| sent without |
| .Xr ssh 1 |
| 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 |
| .Cm TCPKeepAlive |
| (below). |
| The server alive messages are sent through the encrypted channel |
| and therefore will not be spoofable. |
| The TCP keepalive option enabled by |
| .Cm TCPKeepAlive |
| is spoofable. |
| The server alive mechanism is valuable when the client or |
| server depend on knowing when a connection has become inactive. |
| .Pp |
| The default value is 3. |
| If, for example, |
| .Cm ServerAliveInterval |
| (see below) is set to 15 and |
| .Cm ServerAliveCountMax |
| is left at the default, if the server becomes unresponsive, |
| ssh will disconnect after approximately 45 seconds. |
| .It Cm ServerAliveInterval |
| Sets a timeout interval in seconds after which if no data has been received |
| from the server, |
| .Xr ssh 1 |
| 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. |
| .It Cm StreamLocalBindMask |
| Sets the octal file creation mode mask |
| .Pq umask |
| 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. |
| .Pp |
| 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. |
| .It Cm StreamLocalBindUnlink |
| 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 |
| .Cm StreamLocalBindUnlink |
| is not enabled, |
| .Nm ssh |
| 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. |
| .Pp |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm StrictHostKeyChecking |
| If this flag is set to |
| .Dq yes , |
| .Xr ssh 1 |
| will never automatically add host keys to the |
| .Pa ~/.ssh/known_hosts |
| file, and refuses to connect to hosts whose host key has changed. |
| This provides maximum protection against trojan horse attacks, |
| though it can be annoying when the |
| .Pa /etc/ssh/ssh_known_hosts |
| 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 |
| .Dq no , |
| ssh will automatically add new host keys to the |
| user known hosts files. |
| If this flag is set to |
| .Dq 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 |
| .Dq yes , |
| .Dq no , |
| or |
| .Dq ask . |
| The default is |
| .Dq ask . |
| .It Cm TCPKeepAlive |
| 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. |
| .Pp |
| The default is |
| .Dq 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. |
| .Pp |
| To disable TCP keepalive messages, the value should be set to |
| .Dq no . |
| .It Cm Tunnel |
| Request |
| .Xr tun 4 |
| device forwarding between the client and the server. |
| The argument must be |
| .Dq yes , |
| .Dq point-to-point |
| (layer 3), |
| .Dq ethernet |
| (layer 2), |
| or |
| .Dq no . |
| Specifying |
| .Dq yes |
| requests the default tunnel mode, which is |
| .Dq point-to-point . |
| The default is |
| .Dq no . |
| .It Cm TunnelDevice |
| Specifies the |
| .Xr tun 4 |
| devices to open on the client |
| .Pq Ar local_tun |
| and the server |
| .Pq Ar remote_tun . |
| .Pp |
| The argument must be |
| .Sm off |
| .Ar local_tun Op : Ar remote_tun . |
| .Sm on |
| The devices may be specified by numerical ID or the keyword |
| .Dq any , |
| which uses the next available tunnel device. |
| If |
| .Ar remote_tun |
| is not specified, it defaults to |
| .Dq any . |
| The default is |
| .Dq any:any . |
| .It Cm UpdateHostKeys |
| Specifies whether |
| .Xr ssh 1 |
| should accept notifications of additional hostkeys from the server sent |
| after authentication has completed and add them to |
| .Cm UserKnownHostsFile . |
| The argument must be |
| .Dq yes , |
| .Dq no |
| (the default) or |
| .Dq ask . |
| Enabling this option allows learning alternate hostkeys for a server |
| and supports graceful key rotation by allowing a server to send replacement |
| public keys before old ones are removed. |
| Additional hostkeys are only accepted if the key used to authenticate the |
| host was already trusted or explicitly accepted by the user. |
| If |
| .Cm UpdateHostKeys |
| is set to |
| .Dq ask , |
| then the user is asked to confirm the modifications to the known_hosts file. |
| Confirmation is currently incompatible with |
| .Cm ControlPersist , |
| and will be disabled if it is enabled. |
| .Pp |
| Presently, only |
| .Xr sshd 8 |
| from OpenSSH 6.8 and greater support the |
| .Dq hostkeys@openssh.com |
| protocol extension used to inform the client of all the server's hostkeys. |
| .It Cm UsePrivilegedPort |
| Specifies whether to use a privileged port for outgoing connections. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| If set to |
| .Dq yes , |
| .Xr ssh 1 |
| must be setuid root. |
| Note that this option must be set to |
| .Dq yes |
| for |
| .Cm RhostsRSAAuthentication |
| with older servers. |
| .It Cm User |
| 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. |
| .It Cm UserKnownHostsFile |
| Specifies one or more files to use for the user |
| host key database, separated by whitespace. |
| The default is |
| .Pa ~/.ssh/known_hosts , |
| .Pa ~/.ssh/known_hosts2 . |
| .It Cm VerifyHostKeyDNS |
| Specifies whether to verify the remote key using DNS and SSHFP resource |
| records. |
| If this option is set to |
| .Dq 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 |
| .Dq ask . |
| If this option is set to |
| .Dq ask , |
| information on fingerprint match will be displayed, but the user will still |
| need to confirm new host keys according to the |
| .Cm StrictHostKeyChecking |
| option. |
| The argument must be |
| .Dq yes , |
| .Dq no , |
| or |
| .Dq ask . |
| The default is |
| .Dq no . |
| .Pp |
| See also VERIFYING HOST KEYS in |
| .Xr ssh 1 . |
| .It Cm VisualHostKey |
| If this flag is set to |
| .Dq yes , |
| an ASCII art representation of the remote host key fingerprint is |
| printed in addition to the fingerprint string at login and |
| for unknown host keys. |
| If this flag is set to |
| .Dq no , |
| no fingerprint strings are printed at login and |
| only the fingerprint string will be printed for unknown host keys. |
| The default is |
| .Dq no . |
| .It Cm XAuthLocation |
| Specifies the full pathname of the |
| .Xr xauth 1 |
| program. |
| The default is |
| .Pa /usr/X11R6/bin/xauth . |
| .El |
| .Sh PATTERNS |
| A |
| .Em pattern |
| consists of zero or more non-whitespace characters, |
| .Sq * |
| (a wildcard that matches zero or more characters), |
| or |
| .Sq ?\& |
| (a wildcard that matches exactly one character). |
| For example, to specify a set of declarations for any host in the |
| .Dq .co.uk |
| set of domains, |
| the following pattern could be used: |
| .Pp |
| .Dl Host *.co.uk |
| .Pp |
| The following pattern |
| would match any host in the 192.168.0.[0-9] network range: |
| .Pp |
| .Dl Host 192.168.0.? |
| .Pp |
| A |
| .Em pattern-list |
| is a comma-separated list of patterns. |
| Patterns within pattern-lists may be negated |
| by preceding them with an exclamation mark |
| .Pq Sq !\& . |
| For example, |
| to allow a key to be used from anywhere within an organization |
| except from the |
| .Dq dialup |
| pool, |
| the following entry (in authorized_keys) could be used: |
| .Pp |
| .Dl from=\&"!*.dialup.example.com,*.example.com\&" |
| .Sh TOKENS |
| Arguments to some keywords can make use of tokens, |
| which are expanded at runtime: |
| .Pp |
| .Bl -tag -width XXXX -offset indent -compact |
| .It %% |
| A literal |
| .Sq % . |
| .It \&%C |
| Shorthand for %l%h%p%r. |
| .It %d |
| Local user's home directory. |
| .It %h |
| The remote hostname. |
| .It %i |
| The local user ID. |
| .It %L |
| The local hostname. |
| .It %l |
| The local hostname, including the domain name. |
| .It %n |
| The original remote hostname, as given on the command line. |
| .It %p |
| The remote port. |
| .It %r |
| The remote username. |
| .It %u |
| The local username. |
| .El |
| .Pp |
| .Cm Match exec |
| accepts the tokens %%, %h, %L, %l, %n, %p, %r, and %u. |
| .Pp |
| .Cm CertificateFile |
| accepts the tokens %%, %d, %h, %l, %r, and %u. |
| .Pp |
| .Cm ControlPath |
| accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. |
| .Pp |
| .Cm HostName |
| accepts the tokens %% and %h. |
| .Pp |
| .Cm IdentityAgent |
| and |
| .Cm IdentityFile |
| accept the tokens %%, %d, %h, %l, %r, and %u. |
| .Pp |
| .Cm LocalCommand |
| accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u. |
| .Pp |
| .Cm ProxyCommand |
| accepts the tokens %%, %h, %p, and %r. |
| .Sh FILES |
| .Bl -tag -width Ds |
| .It Pa ~/.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. |
| .It Pa /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. |
| .El |
| .Sh SEE ALSO |
| .Xr ssh 1 |
| .Sh AUTHORS |
| 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. |