2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: ssh_config.5,v 1.184 2014/01/19 04:48:08 djm Exp $
43 .Nd OpenSSH SSH client configuration files
46 .Nm /etc/ssh/ssh_config
49 obtains configuration data from the following sources in
52 .Bl -enum -offset indent -compact
56 user's configuration file
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
63 For each parameter, the first obtained value
65 The configuration files contain sections separated by
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
75 The configuration file has the following format:
77 Empty lines and lines starting with
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
93 Arguments may optionally be enclosed in double quotes
95 in order to represent arguments containing spaces.
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
102 Restricts the following declarations (up to the next
106 keyword) to be only for those hosts that match one of the patterns
107 given after the keyword.
108 If more than one pattern is provided, they should be separated by whitespace.
111 as a pattern can be used to provide global
112 defaults for all hosts.
115 argument given on the command line (i.e. the name is not converted to
116 a canonicalized host name before matching).
118 A pattern entry may be negated by prefixing it with an exclamation mark
120 If a negated entry is matched, then the
122 entry is ignored, regardless of whether any other patterns on the line
124 Negated matches are therefore useful to provide exceptions for wildcard
129 for more information on patterns.
131 Restricts the following declarations (up to the next
135 keyword) to be used only when the conditions following the
137 keyword are satisfied.
138 Match conditions are specified using one or more keyword/criteria pairs
141 which matches all criteria.
142 The available keywords are:
152 keyword executes the specified command under the user's shell.
153 If the command returns a zero exit status then the condition is considered true.
154 Commands containing whitespace characters must be quoted.
155 The following character sequences in the command will be expanded prior to
158 will be substituted by the first component of the local host name,
160 will be substituted by the local host name (including any domain name),
162 will be substituted by the target host name,
164 will be substituted by the original target host name
165 specified on the command-line,
167 the destination port,
169 by the remote login username, and
171 by the username of the user running
174 The other keywords' criteria must be single entries or comma-separated
175 lists and may use the wildcard and negation operators described in the
180 keyword are matched against the target hostname, after any substitution
186 keyword matches against the hostname as it was specified on the command-line.
189 keyword matches against the target username on the remote host.
192 keyword matches against the name of the local user running
194 (this keyword may be useful in system-wide
198 Specifies which address family to use when connecting.
208 passphrase/password querying will be disabled.
209 This option is useful in scripts and other batch jobs where no user
210 is present to supply the password.
218 Use the specified address on the local machine as the source address of
220 Only useful on systems with more than one address.
221 Note that this option does not work if
222 .Cm UsePrivilegedPort
225 .It Cm CanonicalDomains
227 .Cm CanonicalizeHostname
228 is enabled, this option specifies the list of domain suffixes in which to
229 search for the specified destination host.
230 .It Cm CanonicalizeFallbackLocal
231 Specifies whether to fail with an error when hostname canonicalization fails.
234 will attempt to look up the unqualified hostname using the system resolver's
241 .Cm CanonicalizeHostname
242 is enabled and the target hostname cannot be found in any of the domains
244 .Cm CanonicalDomains .
245 .It Cm CanonicalizeHostname
246 Controls whether explicit hostname canonicalization is performed.
249 is not to perform any name rewriting and let the system resolver handle all
253 then, for connections that do not use a
256 will attempt to canonicalize the hostname specified on the command line
260 .Cm CanonicalizePermittedCNAMEs
263 .Cm CanonicalizeHostname
266 then canonicalization is applied to proxied connections too.
267 .It Cm CanonicalizeMaxDots
268 Specifies the maximum number of dot characters in a hostname before
269 canonicalization is disabled.
272 allows a single dot (i.e. hostname.subdomain).
273 .It Cm CanonicalizePermittedCNAMEs
274 Specifies rules to determine whether CNAMEs should be followed when
275 canonicalizing hostnames.
276 The rules consist of one or more arguments of
277 .Ar source_domain_list : Ns Ar target_domain_list ,
279 .Ar source_domain_list
280 is a pattern-list of domains that may follow CNAMEs in canonicalization,
282 .Ar target_domain_list
283 is a pattern-list of domains that they may resolve to.
286 .Dq *.a.example.com:*.b.example.com,*.c.example.com
287 will allow hostnames matching
289 to be canonicalized to names in the
294 .It Cm ChallengeResponseAuthentication
295 Specifies whether to use challenge-response authentication.
296 The argument to this keyword must be
303 If this flag is set to
306 will additionally check the host IP address in the
309 This allows ssh to detect if a host key changed due to DNS spoofing.
310 If the option is set to
312 the check will not be executed.
316 Specifies the cipher to use for encrypting the session
317 in protocol version 1.
325 is only supported in the
327 client for interoperability with legacy protocol 1 implementations
328 that do not support the
331 Its use is strongly discouraged due to cryptographic weaknesses.
335 Specifies the ciphers allowed for protocol version 2
336 in order of preference.
337 Multiple ciphers must be comma-separated.
338 The supported ciphers are:
347 .Dq aes128-gcm@openssh.com ,
348 .Dq aes256-gcm@openssh.com ,
355 .Dq chacha20-poly1305@openssh.com .
358 .Bd -literal -offset 3n
359 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
360 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
361 chacha20-poly1305@openssh.com,
362 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
366 The list of available ciphers may also be obtained using the
370 .It Cm ClearAllForwardings
371 Specifies that all local, remote, and dynamic port forwardings
372 specified in the configuration files or on the command line be
374 This option is primarily useful when used from the
376 command line to clear port forwardings set in
377 configuration files, and is automatically set by
388 Specifies whether to use compression.
395 .It Cm CompressionLevel
396 Specifies the compression level to use if compression is enabled.
397 The argument must be an integer from 1 (fast) to 9 (slow, best).
398 The default level is 6, which is good for most applications.
399 The meaning of the values is the same as in
401 Note that this option applies to protocol version 1 only.
402 .It Cm ConnectionAttempts
403 Specifies the number of tries (one per second) to make before exiting.
404 The argument must be an integer.
405 This may be useful in scripts if the connection sometimes fails.
407 .It Cm ConnectTimeout
408 Specifies the timeout (in seconds) used when connecting to the
409 SSH server, instead of using the default system TCP timeout.
410 This value is used only when the target is down or really unreachable,
411 not when it refuses the connection.
413 Enables the sharing of multiple sessions over a single network connection.
417 will listen for connections on a control socket specified using the
420 Additional sessions can connect to this socket using the same
427 These sessions will try to reuse the master instance's network connection
428 rather than initiating new ones, but will fall back to connecting normally
429 if the control socket does not exist, or is not listening.
434 to listen for control connections, but require confirmation using the
436 program before they are accepted (see
442 ssh will continue without connecting to a master instance.
446 forwarding is supported over these multiplexed connections, however the
447 display and agent forwarded will be the one belonging to the master
448 connection i.e. it is not possible to forward multiple displays or agents.
450 Two additional options allow for opportunistic multiplexing: try to use a
451 master connection but fall back to creating a new one if one does not already
457 The latter requires confirmation like the
461 Specify the path to the control socket used for connection sharing as described
464 section above or the string
466 to disable connection sharing.
469 will be substituted by the first component of the local host name,
471 will be substituted by the local host name (including any domain name),
473 will be substituted by the target host name,
475 will be substituted by the original target host name
476 specified on the command line,
478 the destination port,
480 by the remote login username, and
482 by the username of the user running
484 It is recommended that any
486 used for opportunistic connection sharing include
487 at least %h, %p, and %r.
488 This ensures that shared connections are uniquely identified.
489 .It Cm ControlPersist
490 When used in conjunction with
492 specifies that the master connection should remain open
493 in the background (waiting for future client connections)
494 after the initial client connection has been closed.
497 then the master connection will not be placed into the background,
498 and will close as soon as the initial client connection is closed.
501 then the master connection will remain in the background indefinitely
502 (until killed or closed via a mechanism such as the
506 If set to a time in seconds, or a time in any of the formats documented in
508 then the backgrounded master connection will automatically terminate
509 after it has remained idle (with no client connections) for the
511 .It Cm DynamicForward
512 Specifies that a TCP port on the local machine be forwarded
513 over the secure channel, and the application
514 protocol is then used to determine where to connect to from the
519 .Oo Ar bind_address : Oc Ar port .
521 IPv6 addresses can be specified by enclosing addresses in square brackets.
522 By default, the local port is bound in accordance with the
527 may be used to bind the connection to a specific address.
532 indicates that the listening port be bound for local use only, while an
535 indicates that the port should be available from all interfaces.
537 Currently the SOCKS4 and SOCKS5 protocols are supported, and
539 will act as a SOCKS server.
540 Multiple forwardings may be specified, and
541 additional forwardings can be given on the command line.
542 Only the superuser can forward privileged ports.
543 .It Cm EnableSSHKeysign
544 Setting this option to
546 in the global client configuration file
547 .Pa /etc/ssh/ssh_config
548 enables the use of the helper program
551 .Cm HostbasedAuthentication .
558 This option should be placed in the non-hostspecific section.
561 for more information.
563 Sets the escape character (default:
565 The escape character can also
566 be set on the command line.
567 The argument should be a single character,
569 followed by a letter, or
571 to disable the escape
572 character entirely (making the connection transparent for binary
574 .It Cm ExitOnForwardFailure
577 should terminate the connection if it cannot set up all requested
578 dynamic, tunnel, local, and remote port forwardings.
586 Specifies whether the connection to the authentication agent (if any)
587 will be forwarded to the remote machine.
595 Agent forwarding should be enabled with caution.
596 Users with the ability to bypass file permissions on the remote host
597 (for the agent's Unix-domain socket)
598 can access the local agent through the forwarded connection.
599 An attacker cannot obtain key material from the agent,
600 however they can perform operations on the keys that enable them to
601 authenticate using the identities loaded into the agent.
603 Specifies whether X11 connections will be automatically redirected
604 over the secure channel and
614 X11 forwarding should be enabled with caution.
615 Users with the ability to bypass file permissions on the remote host
616 (for the user's X11 authorization database)
617 can access the local X11 display through the forwarded connection.
618 An attacker may then be able to perform activities such as keystroke monitoring
620 .Cm ForwardX11Trusted
621 option is also enabled.
622 .It Cm ForwardX11Timeout
623 Specify a timeout for untrusted X11 forwarding
624 using the format described in the
625 TIME FORMATS section of
627 X11 connections received by
629 after this time will be refused.
630 The default is to disable untrusted X11 forwarding after twenty minutes has
632 .It Cm ForwardX11Trusted
633 If this option is set to
635 remote X11 clients will have full access to the original X11 display.
637 If this option is set to
639 remote X11 clients will be considered untrusted and prevented
640 from stealing or tampering with data belonging to trusted X11
644 token used for the session will be set to expire after 20 minutes.
645 Remote clients will be refused access after this time.
650 See the X11 SECURITY extension specification for full details on
651 the restrictions imposed on untrusted clients.
653 Specifies whether remote hosts are allowed to connect to local
657 binds local port forwardings to the loopback address.
658 This prevents other remote hosts from connecting to forwarded ports.
660 can be used to specify that ssh
661 should bind local port forwardings to the wildcard address,
662 thus allowing remote hosts to connect to forwarded ports.
669 .It Cm GlobalKnownHostsFile
670 Specifies one or more files to use for the global
671 host key database, separated by whitespace.
673 .Pa /etc/ssh/ssh_known_hosts ,
674 .Pa /etc/ssh/ssh_known_hosts2 .
675 .It Cm GSSAPIAuthentication
676 Specifies whether user authentication based on GSSAPI is allowed.
679 Note that this option applies to protocol version 2 only.
680 .It Cm GSSAPIDelegateCredentials
681 Forward (delegate) credentials to the server.
684 Note that this option applies to protocol version 2 only.
685 .It Cm HashKnownHosts
688 should hash host names and addresses when they are added to
689 .Pa ~/.ssh/known_hosts .
690 These hashed names may be used normally by
694 but they do not reveal identifying information should the file's contents
698 Note that existing names and addresses in known hosts files
699 will not be converted automatically,
700 but may be manually hashed using
702 .It Cm HostbasedAuthentication
703 Specifies whether to try rhosts based authentication with public key
711 This option applies to protocol version 2 only and
713 .Cm RhostsRSAAuthentication .
714 .It Cm HostKeyAlgorithms
715 Specifies the protocol version 2 host key algorithms
716 that the client wants to use in order of preference.
717 The default for this option is:
718 .Bd -literal -offset 3n
719 ecdsa-sha2-nistp256-cert-v01@openssh.com,
720 ecdsa-sha2-nistp384-cert-v01@openssh.com,
721 ecdsa-sha2-nistp521-cert-v01@openssh.com,
722 ssh-ed25519-cert-v01@openssh.com,
723 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
724 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
725 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
726 ssh-ed25519,ssh-rsa,ssh-dss
729 If hostkeys are known for the destination host then this default is modified
730 to prefer their algorithms.
732 Specifies an alias that should be used instead of the
733 real host name when looking up or saving the host key
734 in the host key database files.
735 This option is useful for tunneling SSH connections
736 or for multiple servers running on a single host.
738 Specifies the real host name to log into.
739 This can be used to specify nicknames or abbreviations for hosts.
740 If the hostname contains the character sequence
742 then this will be replaced with the host name specified on the command line
743 (this is useful for manipulating unqualified names).
744 The default is the name given on the command line.
745 Numeric IP addresses are also permitted (both on the command line and in
748 .It Cm IdentitiesOnly
751 should only use the authentication identity files configured in the
758 offers more identities.
759 The argument to this keyword must be
763 This option is intended for situations where ssh-agent
764 offers many different identities.
768 Specifies a file from which the user's DSA, ECDSA, ED25519 or RSA authentication
772 for protocol version 1, and
774 .Pa ~/.ssh/id_ecdsa ,
775 .Pa ~/.ssh/id_ed25519
778 for protocol version 2.
779 Additionally, any identities represented by the authentication agent
780 will be used for authentication unless
784 will try to load certificate information from the filename obtained by
787 to the path of a specified
790 The file name may use the tilde
791 syntax to refer to a user's home directory or one of the following
794 (local user's home directory),
800 (remote host name) or
804 It is possible to have
805 multiple identity files specified in configuration files; all these
806 identities will be tried in sequence.
809 directives will add to the list of identities tried (this behaviour
810 differs from that of other configuration directives).
813 may be used in conjunction with
815 to select which identities in an agent are offered during authentication.
817 Specifies a pattern-list of unknown options to be ignored if they are
818 encountered in configuration parsing.
819 This may be used to suppress errors if
821 contains options that are unrecognised by
823 It is recommended that
825 be listed early in the configuration file as it will not be applied
826 to unknown options that appear before it.
828 Specifies the IPv4 type-of-service or DSCP class for connections.
855 This option may take one or two arguments, separated by whitespace.
856 If one argument is specified, it is used as the packet class unconditionally.
857 If two values are specified, the first is automatically selected for
858 interactive sessions and the second for non-interactive sessions.
861 for interactive sessions and
863 for non-interactive sessions.
864 .It Cm KbdInteractiveAuthentication
865 Specifies whether to use keyboard-interactive authentication.
866 The argument to this keyword must be
872 .It Cm KbdInteractiveDevices
873 Specifies the list of methods to use in keyboard-interactive authentication.
874 Multiple method names must be comma-separated.
875 The default is to use the server specified list.
876 The methods available vary depending on what the server supports.
877 For an OpenSSH server,
878 it may be zero or more of:
884 Specifies the available KEX (Key Exchange) algorithms.
885 Multiple algorithms must be comma-separated.
887 .Bd -literal -offset indent
888 curve25519-sha256@libssh.org,
889 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
890 diffie-hellman-group-exchange-sha256,
891 diffie-hellman-group-exchange-sha1,
892 diffie-hellman-group14-sha1,
893 diffie-hellman-group1-sha1
896 Specifies a command to execute on the local machine after successfully
897 connecting to the server.
898 The command string extends to the end of the line, and is executed with
900 The following escape character substitutions will be performed:
902 (local user's home directory),
908 (host name as provided on the command line),
912 (remote user name) or
916 The command is run synchronously and does not have access to the
920 It should not be used for interactive commands.
922 This directive is ignored unless
923 .Cm PermitLocalCommand
926 Specifies that a TCP port on the local machine be forwarded over
927 the secure channel to the specified host and port from the remote machine.
928 The first argument must be
930 .Oo Ar bind_address : Oc Ar port
932 and the second argument must be
933 .Ar host : Ns Ar hostport .
934 IPv6 addresses can be specified by enclosing addresses in square brackets.
935 Multiple forwardings may be specified, and additional forwardings can be
936 given on the command line.
937 Only the superuser can forward privileged ports.
938 By default, the local port is bound in accordance with the
943 may be used to bind the connection to a specific address.
948 indicates that the listening port be bound for local use only, while an
951 indicates that the port should be available from all interfaces.
953 Gives the verbosity level that is used when logging messages from
955 The possible values are:
956 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
958 DEBUG and DEBUG1 are equivalent.
959 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
961 Specifies the MAC (message authentication code) algorithms
962 in order of preference.
963 The MAC algorithm is used in protocol version 2
964 for data integrity protection.
965 Multiple algorithms must be comma-separated.
966 The algorithms that contain
968 calculate the MAC after encryption (encrypt-then-mac).
969 These are considered safer and their use recommended.
971 .Bd -literal -offset indent
972 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
973 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
974 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
975 hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
976 hmac-md5-96-etm@openssh.com,
977 hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
978 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
979 hmac-sha1-96,hmac-md5-96
981 .It Cm NoHostAuthenticationForLocalhost
982 This option can be used if the home directory is shared across machines.
983 In this case localhost will refer to a different machine on each of
984 the machines and the user will get many warnings about changed host keys.
985 However, this option disables host authentication for localhost.
986 The argument to this keyword must be
990 The default is to check the host key for localhost.
991 .It Cm NumberOfPasswordPrompts
992 Specifies the number of password prompts before giving up.
993 The argument to this keyword must be an integer.
995 .It Cm PasswordAuthentication
996 Specifies whether to use password authentication.
997 The argument to this keyword must be
1003 .It Cm PermitLocalCommand
1004 Allow local command execution via the
1007 .Ic !\& Ns Ar command
1010 The argument must be
1016 .It Cm PKCS11Provider
1017 Specifies which PKCS#11 provider to use.
1018 The argument to this keyword is the PKCS#11 shared library
1020 should use to communicate with a PKCS#11 token providing the user's
1023 Specifies the port number to connect on the remote host.
1025 .It Cm PreferredAuthentications
1026 Specifies the order in which the client should try protocol 2
1027 authentication methods.
1028 This allows a client to prefer one method (e.g.\&
1029 .Cm keyboard-interactive )
1030 over another method (e.g.\&
1033 .Bd -literal -offset indent
1034 gssapi-with-mic,hostbased,publickey,
1035 keyboard-interactive,password
1038 Specifies the protocol versions
1040 should support in order of preference.
1041 The possible values are
1045 Multiple versions must be comma-separated.
1046 When this option is set to
1049 will try version 2 and fall back to version 1
1050 if version 2 is not available.
1054 Specifies the command to use to connect to the server.
1056 string extends to the end of the line, and is executed with
1058 In the command string, any occurrence of
1060 will be substituted by the host name to
1065 by the remote user name.
1066 The command can be basically anything,
1067 and should read from its standard input and write to its standard output.
1068 It should eventually connect an
1070 server running on some machine, or execute
1073 Host key management will be done using the
1074 HostName of the host being connected (defaulting to the name typed by
1076 Setting the command to
1078 disables this option entirely.
1081 is not available for connects with a proxy command.
1083 This directive is useful in conjunction with
1085 and its proxy support.
1086 For example, the following directive would connect via an HTTP proxy at
1088 .Bd -literal -offset 3n
1089 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1091 .It Cm ProxyUseFdpass
1094 will pass a connected file descriptor back to
1096 instead of continuing to execute and pass data.
1099 .It Cm PubkeyAuthentication
1100 Specifies whether to try public key authentication.
1101 The argument to this keyword must be
1107 This option applies to protocol version 2 only.
1109 Specifies the maximum amount of data that may be transmitted before the
1110 session key is renegotiated, optionally followed a maximum amount of
1111 time that may pass before the session key is renegotiated.
1112 The first argument is specified in bytes and may have a suffix of
1117 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1118 The default is between
1122 depending on the cipher.
1123 The optional second value is specified in seconds and may use any of the
1124 units documented in the
1125 TIME FORMATS section of
1127 The default value for
1131 which means that rekeying is performed after the cipher's default amount
1132 of data has been sent or received and no time based rekeying is done.
1133 This option applies to protocol version 2 only.
1134 .It Cm RemoteForward
1135 Specifies that a TCP port on the remote machine be forwarded over
1136 the secure channel to the specified host and port from the local machine.
1137 The first argument must be
1139 .Oo Ar bind_address : Oc Ar port
1141 and the second argument must be
1142 .Ar host : Ns Ar hostport .
1143 IPv6 addresses can be specified by enclosing addresses in square brackets.
1144 Multiple forwardings may be specified, and additional
1145 forwardings can be given on the command line.
1146 Privileged ports can be forwarded only when
1147 logging in as root on the remote machine.
1153 the listen port will be dynamically allocated on the server and reported
1154 to the client at run time.
1158 is not specified, the default is to only bind to loopback addresses.
1163 or an empty string, then the forwarding is requested to listen on all
1167 will only succeed if the server's
1169 option is enabled (see
1170 .Xr sshd_config 5 ) .
1172 Specifies whether to request a pseudo-tty for the session.
1173 The argument may be one of:
1175 (never request a TTY),
1177 (always request a TTY when standard input is a TTY),
1179 (always request a TTY) or
1181 (request a TTY when opening a login session).
1182 This option mirrors the
1188 .It Cm RhostsRSAAuthentication
1189 Specifies whether to try rhosts based authentication with RSA host
1191 The argument must be
1197 This option applies to protocol version 1 only and requires
1200 .It Cm RSAAuthentication
1201 Specifies whether to try RSA authentication.
1202 The argument to this keyword must be
1206 RSA authentication will only be
1207 attempted if the identity file exists, or an authentication agent is
1211 Note that this option applies to protocol version 1 only.
1213 Specifies what variables from the local
1215 should be sent to the server.
1216 Note that environment passing is only supported for protocol 2.
1217 The server must also support it, and the server must be configured to
1218 accept these environment variables.
1223 for how to configure the server.
1224 Variables are specified by name, which may contain wildcard characters.
1225 Multiple environment variables may be separated by whitespace or spread
1229 The default is not to send any environment variables.
1233 for more information on patterns.
1234 .It Cm ServerAliveCountMax
1235 Sets the number of server alive messages (see below) which may be
1238 receiving any messages back from the server.
1239 If this threshold is reached while server alive messages are being sent,
1240 ssh will disconnect from the server, terminating the session.
1241 It is important to note that the use of server alive messages is very
1245 The server alive messages are sent through the encrypted channel
1246 and therefore will not be spoofable.
1247 The TCP keepalive option enabled by
1250 The server alive mechanism is valuable when the client or
1251 server depend on knowing when a connection has become inactive.
1253 The default value is 3.
1255 .Cm ServerAliveInterval
1256 (see below) is set to 15 and
1257 .Cm ServerAliveCountMax
1258 is left at the default, if the server becomes unresponsive,
1259 ssh will disconnect after approximately 45 seconds.
1260 This option applies to protocol version 2 only.
1261 .It Cm ServerAliveInterval
1262 Sets a timeout interval in seconds after which if no data has been received
1265 will send a message through the encrypted
1266 channel to request a response from the server.
1268 is 0, indicating that these messages will not be sent to the server.
1269 This option applies to protocol version 2 only.
1270 .It Cm StrictHostKeyChecking
1271 If this flag is set to
1274 will never automatically add host keys to the
1275 .Pa ~/.ssh/known_hosts
1276 file, and refuses to connect to hosts whose host key has changed.
1277 This provides maximum protection against trojan horse attacks,
1278 though it can be annoying when the
1279 .Pa /etc/ssh/ssh_known_hosts
1280 file is poorly maintained or when connections to new hosts are
1282 This option forces the user to manually
1284 If this flag is set to
1286 ssh will automatically add new host keys to the
1287 user known hosts files.
1288 If this flag is set to
1291 will be added to the user known host files only after the user
1292 has confirmed that is what they really want to do, and
1293 ssh will refuse to connect to hosts whose host key has changed.
1295 known hosts will be verified automatically in all cases.
1296 The argument must be
1304 Specifies whether the system should send TCP keepalive messages to the
1306 If they are sent, death of the connection or crash of one
1307 of the machines will be properly noticed.
1308 However, this means that
1309 connections will die if the route is down temporarily, and some people
1314 (to send TCP keepalive messages), and the client will notice
1315 if the network goes down or the remote host dies.
1316 This is important in scripts, and many users want it too.
1318 To disable TCP keepalive messages, the value should be set to
1323 device forwarding between the client and the server.
1324 The argument must be
1334 requests the default tunnel mode, which is
1335 .Dq point-to-point .
1341 devices to open on the client
1346 The argument must be
1348 .Ar local_tun Op : Ar remote_tun .
1350 The devices may be specified by numerical ID or the keyword
1352 which uses the next available tunnel device.
1355 is not specified, it defaults to
1359 .It Cm UsePrivilegedPort
1360 Specifies whether to use a privileged port for outgoing connections.
1361 The argument must be
1370 must be setuid root.
1371 Note that this option must be set to
1374 .Cm RhostsRSAAuthentication
1377 Specifies the user to log in as.
1378 This can be useful when a different user name is used on different machines.
1379 This saves the trouble of
1380 having to remember to give the user name on the command line.
1381 .It Cm UserKnownHostsFile
1382 Specifies one or more files to use for the user
1383 host key database, separated by whitespace.
1385 .Pa ~/.ssh/known_hosts ,
1386 .Pa ~/.ssh/known_hosts2 .
1387 .It Cm VerifyHostKeyDNS
1388 Specifies whether to verify the remote key using DNS and SSHFP resource
1390 If this option is set to
1392 the client will implicitly trust keys that match a secure fingerprint
1394 Insecure fingerprints will be handled as if this option was set to
1396 If this option is set to
1398 information on fingerprint match will be displayed, but the user will still
1399 need to confirm new host keys according to the
1400 .Cm StrictHostKeyChecking
1402 The argument must be
1409 if compiled with LDNS and
1412 Note that this option applies to protocol version 2 only.
1414 See also VERIFYING HOST KEYS in
1416 .It Cm VersionAddendum
1417 Specifies a string to append to the regular version string to identify
1418 OS- or site-specific modifications.
1420 .Dq FreeBSD-20140130 .
1421 .It Cm VisualHostKey
1422 If this flag is set to
1424 an ASCII art representation of the remote host key fingerprint is
1425 printed in addition to the hex fingerprint string at login and
1426 for unknown host keys.
1427 If this flag is set to
1429 no fingerprint strings are printed at login and
1430 only the hex fingerprint string will be printed for unknown host keys.
1433 .It Cm XAuthLocation
1434 Specifies the full pathname of the
1438 .Pa /usr/local/bin/xauth .
1443 consists of zero or more non-whitespace characters,
1445 (a wildcard that matches zero or more characters),
1448 (a wildcard that matches exactly one character).
1449 For example, to specify a set of declarations for any host in the
1452 the following pattern could be used:
1456 The following pattern
1457 would match any host in the 192.168.0.[0-9] network range:
1459 .Dl Host 192.168.0.?
1463 is a comma-separated list of patterns.
1464 Patterns within pattern-lists may be negated
1465 by preceding them with an exclamation mark
1468 to allow a key to be used from anywhere within an organization
1472 the following entry (in authorized_keys) could be used:
1474 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1477 .It Pa ~/.ssh/config
1478 This is the per-user configuration file.
1479 The format of this file is described above.
1480 This file is used by the SSH client.
1481 Because of the potential for abuse, this file must have strict permissions:
1482 read/write for the user, and not accessible by others.
1483 .It Pa /etc/ssh/ssh_config
1484 Systemwide configuration file.
1485 This file provides defaults for those
1486 values that are not specified in the user's configuration file, and
1487 for those users who do not have a configuration file.
1488 This file must be world-readable.
1493 OpenSSH is a derivative of the original and free
1494 ssh 1.2.12 release by Tatu Ylonen.
1495 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1496 Theo de Raadt and Dug Song
1497 removed many bugs, re-added newer features and
1499 Markus Friedl contributed the support for SSH
1500 protocol versions 1.5 and 2.0.