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 1.215 2015/08/14 15:32:41 jmc Exp $
38 .Dd $Mdocdate: August 14 2015 $
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 usually the one given on the command line
71 .Cm CanonicalizeHostname
72 option for exceptions.)
74 Since the first obtained value for each parameter is used, more
75 host-specific declarations should be given near the beginning of the
76 file, and general defaults at the end.
78 The configuration file has the following format:
80 Empty lines and lines starting with
83 Otherwise a line is of the format
84 .Dq keyword arguments .
85 Configuration options may be separated by whitespace or
86 optional whitespace and exactly one
88 the latter format is useful to avoid the need to quote whitespace
89 when specifying configuration options using the
96 Arguments may optionally be enclosed in double quotes
98 in order to represent arguments containing spaces.
101 keywords and their meanings are as follows (note that
102 keywords are case-insensitive and arguments are case-sensitive):
105 Restricts the following declarations (up to the next
109 keyword) to be only for those hosts that match one of the patterns
110 given after the keyword.
111 If more than one pattern is provided, they should be separated by whitespace.
114 as a pattern can be used to provide global
115 defaults for all hosts.
116 The host is usually the
118 argument given on the command line
120 .Cm CanonicalizeHostname
121 option for exceptions.)
123 A pattern entry may be negated by prefixing it with an exclamation mark
125 If a negated entry is matched, then the
127 entry is ignored, regardless of whether any other patterns on the line
129 Negated matches are therefore useful to provide exceptions for wildcard
134 for more information on patterns.
136 Restricts the following declarations (up to the next
140 keyword) to be used only when the conditions following the
142 keyword are satisfied.
143 Match conditions are specified using one or more critera
146 which always matches.
147 The available criteria keywords are:
157 criteria must appear alone or immediately after
159 Other criteria may be combined arbitrarily.
165 Criteria may be negated by prepending an exclamation mark
170 keyword matches only when the configuration file is being re-parsed
171 after hostname canonicalization (see the
172 .Cm CanonicalizeHostname
174 This may be useful to specify conditions that work with canonical host
178 keyword executes the specified command under the user's shell.
179 If the command returns a zero exit status then the condition is considered true.
180 Commands containing whitespace characters must be quoted.
181 The following character sequences in the command will be expanded prior to
184 will be substituted by the first component of the local host name,
186 will be substituted by the local host name (including any domain name),
188 will be substituted by the target host name,
190 will be substituted by the original target host name
191 specified on the command-line,
193 the destination port,
195 by the remote login username, and
197 by the username of the user running
200 The other keywords' criteria must be single entries or comma-separated
201 lists and may use the wildcard and negation operators described in the
206 keyword are matched against the target hostname, after any substitution
210 .Cm CanonicalizeHostname
214 keyword matches against the hostname as it was specified on the command-line.
217 keyword matches against the target username on the remote host.
220 keyword matches against the name of the local user running
222 (this keyword may be useful in system-wide
226 Specifies which address family to use when connecting.
236 passphrase/password querying will be disabled.
237 This option is useful in scripts and other batch jobs where no user
238 is present to supply the password.
246 Use the specified address on the local machine as the source address of
248 Only useful on systems with more than one address.
249 Note that this option does not work if
250 .Cm UsePrivilegedPort
253 .It Cm CanonicalDomains
255 .Cm CanonicalizeHostname
256 is enabled, this option specifies the list of domain suffixes in which to
257 search for the specified destination host.
258 .It Cm CanonicalizeFallbackLocal
259 Specifies whether to fail with an error when hostname canonicalization fails.
262 will attempt to look up the unqualified hostname using the system resolver's
269 .Cm CanonicalizeHostname
270 is enabled and the target hostname cannot be found in any of the domains
272 .Cm CanonicalDomains .
273 .It Cm CanonicalizeHostname
274 Controls whether explicit hostname canonicalization is performed.
277 is not to perform any name rewriting and let the system resolver handle all
281 then, for connections that do not use a
284 will attempt to canonicalize the hostname specified on the command line
288 .Cm CanonicalizePermittedCNAMEs
291 .Cm CanonicalizeHostname
294 then canonicalization is applied to proxied connections too.
296 If this option is enabled, then the configuration files are processed
297 again using the new target name to pick up any new configuration in matching
302 .It Cm CanonicalizeMaxDots
303 Specifies the maximum number of dot characters in a hostname before
304 canonicalization is disabled.
307 allows a single dot (i.e. hostname.subdomain).
308 .It Cm CanonicalizePermittedCNAMEs
309 Specifies rules to determine whether CNAMEs should be followed when
310 canonicalizing hostnames.
311 The rules consist of one or more arguments of
312 .Ar source_domain_list : Ns Ar target_domain_list ,
314 .Ar source_domain_list
315 is a pattern-list of domains that may follow CNAMEs in canonicalization,
317 .Ar target_domain_list
318 is a pattern-list of domains that they may resolve to.
321 .Dq *.a.example.com:*.b.example.com,*.c.example.com
322 will allow hostnames matching
324 to be canonicalized to names in the
329 .It Cm ChallengeResponseAuthentication
330 Specifies whether to use challenge-response authentication.
331 The argument to this keyword must be
338 If this flag is set to
341 will additionally check the host IP address in the
344 This allows ssh to detect if a host key changed due to DNS spoofing
345 and will add addresses of destination hosts to
346 .Pa ~/.ssh/known_hosts
347 in the process, regardless of the setting of
348 .Cm StrictHostKeyChecking .
349 If the option is set to
351 the check will not be executed.
355 Specifies the cipher to use for encrypting the session
356 in protocol version 1.
364 is only supported in the
366 client for interoperability with legacy protocol 1 implementations
367 that do not support the
370 Its use is strongly discouraged due to cryptographic weaknesses.
374 Specifies the ciphers allowed for protocol version 2
375 in order of preference.
376 Multiple ciphers must be comma-separated.
377 If the specified value begins with a
379 character, then the specified ciphers will be appended to the default set
380 instead of replacing them.
382 The supported ciphers are:
384 .Bl -item -compact -offset indent
400 aes128-gcm@openssh.com
402 aes256-gcm@openssh.com
414 chacha20-poly1305@openssh.com
418 .Bd -literal -offset indent
419 chacha20-poly1305@openssh.com,
420 aes128-ctr,aes192-ctr,aes256-ctr,
421 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
422 arcfour256,arcfour128,
423 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
424 aes192-cbc,aes256-cbc,arcfour
427 The list of available ciphers may also be obtained using the
433 .It Cm ClearAllForwardings
434 Specifies that all local, remote, and dynamic port forwardings
435 specified in the configuration files or on the command line be
437 This option is primarily useful when used from the
439 command line to clear port forwardings set in
440 configuration files, and is automatically set by
451 Specifies whether to use compression.
458 .It Cm CompressionLevel
459 Specifies the compression level to use if compression is enabled.
460 The argument must be an integer from 1 (fast) to 9 (slow, best).
461 The default level is 6, which is good for most applications.
462 The meaning of the values is the same as in
464 Note that this option applies to protocol version 1 only.
465 .It Cm ConnectionAttempts
466 Specifies the number of tries (one per second) to make before exiting.
467 The argument must be an integer.
468 This may be useful in scripts if the connection sometimes fails.
470 .It Cm ConnectTimeout
471 Specifies the timeout (in seconds) used when connecting to the
472 SSH server, instead of using the default system TCP timeout.
473 This value is used only when the target is down or really unreachable,
474 not when it refuses the connection.
476 Enables the sharing of multiple sessions over a single network connection.
480 will listen for connections on a control socket specified using the
483 Additional sessions can connect to this socket using the same
490 These sessions will try to reuse the master instance's network connection
491 rather than initiating new ones, but will fall back to connecting normally
492 if the control socket does not exist, or is not listening.
497 to listen for control connections, but require confirmation using
502 ssh will continue without connecting to a master instance.
506 forwarding is supported over these multiplexed connections, however the
507 display and agent forwarded will be the one belonging to the master
508 connection i.e. it is not possible to forward multiple displays or agents.
510 Two additional options allow for opportunistic multiplexing: try to use a
511 master connection but fall back to creating a new one if one does not already
517 The latter requires confirmation like the
521 Specify the path to the control socket used for connection sharing as described
524 section above or the string
526 to disable connection sharing.
529 will be substituted by the first component of the local host name,
531 will be substituted by the local host name (including any domain name),
533 will be substituted by the target host name,
535 will be substituted by the original target host name
536 specified on the command line,
538 the destination port,
540 by the remote login username,
542 by the username of the user running
545 by a hash of the concatenation: %l%h%p%r.
546 It is recommended that any
548 used for opportunistic connection sharing include
549 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
550 that is not writable by other users.
551 This ensures that shared connections are uniquely identified.
552 .It Cm ControlPersist
553 When used in conjunction with
555 specifies that the master connection should remain open
556 in the background (waiting for future client connections)
557 after the initial client connection has been closed.
560 then the master connection will not be placed into the background,
561 and will close as soon as the initial client connection is closed.
566 then the master connection will remain in the background indefinitely
567 (until killed or closed via a mechanism such as the
571 If set to a time in seconds, or a time in any of the formats documented in
573 then the backgrounded master connection will automatically terminate
574 after it has remained idle (with no client connections) for the
576 .It Cm DynamicForward
577 Specifies that a TCP port on the local machine be forwarded
578 over the secure channel, and the application
579 protocol is then used to determine where to connect to from the
584 .Oo Ar bind_address : Oc Ar port .
586 IPv6 addresses can be specified by enclosing addresses in square brackets.
587 By default, the local port is bound in accordance with the
592 may be used to bind the connection to a specific address.
597 indicates that the listening port be bound for local use only, while an
600 indicates that the port should be available from all interfaces.
602 Currently the SOCKS4 and SOCKS5 protocols are supported, and
604 will act as a SOCKS server.
605 Multiple forwardings may be specified, and
606 additional forwardings can be given on the command line.
607 Only the superuser can forward privileged ports.
608 .It Cm EnableSSHKeysign
609 Setting this option to
611 in the global client configuration file
612 .Pa /etc/ssh/ssh_config
613 enables the use of the helper program
616 .Cm HostbasedAuthentication .
623 This option should be placed in the non-hostspecific section.
626 for more information.
628 Sets the escape character (default:
630 The escape character can also
631 be set on the command line.
632 The argument should be a single character,
634 followed by a letter, or
636 to disable the escape
637 character entirely (making the connection transparent for binary
639 .It Cm ExitOnForwardFailure
642 should terminate the connection if it cannot set up all requested
643 dynamic, tunnel, local, and remote port forwardings.
650 .It Cm FingerprintHash
651 Specifies the hash algorithm used when displaying key fingerprints.
659 Specifies whether the connection to the authentication agent (if any)
660 will be forwarded to the remote machine.
668 Agent forwarding should be enabled with caution.
669 Users with the ability to bypass file permissions on the remote host
670 (for the agent's Unix-domain socket)
671 can access the local agent through the forwarded connection.
672 An attacker cannot obtain key material from the agent,
673 however they can perform operations on the keys that enable them to
674 authenticate using the identities loaded into the agent.
676 Specifies whether X11 connections will be automatically redirected
677 over the secure channel and
687 X11 forwarding should be enabled with caution.
688 Users with the ability to bypass file permissions on the remote host
689 (for the user's X11 authorization database)
690 can access the local X11 display through the forwarded connection.
691 An attacker may then be able to perform activities such as keystroke monitoring
693 .Cm ForwardX11Trusted
694 option is also enabled.
695 .It Cm ForwardX11Timeout
696 Specify a timeout for untrusted X11 forwarding
697 using the format described in the
698 TIME FORMATS section of
700 X11 connections received by
702 after this time will be refused.
703 The default is to disable untrusted X11 forwarding after twenty minutes has
705 .It Cm ForwardX11Trusted
706 If this option is set to
708 remote X11 clients will have full access to the original X11 display.
710 If this option is set to
712 remote X11 clients will be considered untrusted and prevented
713 from stealing or tampering with data belonging to trusted X11
717 token used for the session will be set to expire after 20 minutes.
718 Remote clients will be refused access after this time.
723 See the X11 SECURITY extension specification for full details on
724 the restrictions imposed on untrusted clients.
726 Specifies whether remote hosts are allowed to connect to local
730 binds local port forwardings to the loopback address.
731 This prevents other remote hosts from connecting to forwarded ports.
733 can be used to specify that ssh
734 should bind local port forwardings to the wildcard address,
735 thus allowing remote hosts to connect to forwarded ports.
742 .It Cm GlobalKnownHostsFile
743 Specifies one or more files to use for the global
744 host key database, separated by whitespace.
746 .Pa /etc/ssh/ssh_known_hosts ,
747 .Pa /etc/ssh/ssh_known_hosts2 .
748 .It Cm GSSAPIAuthentication
749 Specifies whether user authentication based on GSSAPI is allowed.
752 Note that this option applies to protocol version 2 only.
753 .It Cm GSSAPIDelegateCredentials
754 Forward (delegate) credentials to the server.
757 Note that this option applies to protocol version 2 only.
758 .It Cm HashKnownHosts
761 should hash host names and addresses when they are added to
762 .Pa ~/.ssh/known_hosts .
763 These hashed names may be used normally by
767 but they do not reveal identifying information should the file's contents
771 Note that existing names and addresses in known hosts files
772 will not be converted automatically,
773 but may be manually hashed using
775 .It Cm HostbasedAuthentication
776 Specifies whether to try rhosts based authentication with public key
784 This option applies to protocol version 2 only and
786 .Cm RhostsRSAAuthentication .
787 .It Cm HostbasedKeyTypes
788 Specifies the key types that will be used for hostbased authentication
789 as a comma-separated pattern list.
790 Alternately if the specified value begins with a
792 character, then the specified key types will be appended to the default set
793 instead of replacing them.
794 The default for this option is:
795 .Bd -literal -offset 3n
796 ecdsa-sha2-nistp256-cert-v01@openssh.com,
797 ecdsa-sha2-nistp384-cert-v01@openssh.com,
798 ecdsa-sha2-nistp521-cert-v01@openssh.com,
799 ssh-ed25519-cert-v01@openssh.com,
800 ssh-rsa-cert-v01@openssh.com,
801 ssh-dss-cert-v01@openssh.com,
802 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,
803 ecdsa-sha2-nistp521,ssh-ed25519,
811 may be used to list supported key types.
812 .It Cm HostKeyAlgorithms
813 Specifies the protocol version 2 host key algorithms
814 that the client wants to use in order of preference.
815 Alternately if the specified value begins with a
817 character, then the specified key types will be appended to the default set
818 instead of replacing them.
819 The default for this option is:
820 .Bd -literal -offset 3n
821 ecdsa-sha2-nistp256-cert-v01@openssh.com,
822 ecdsa-sha2-nistp384-cert-v01@openssh.com,
823 ecdsa-sha2-nistp521-cert-v01@openssh.com,
824 ssh-ed25519-cert-v01@openssh.com,
825 ssh-rsa-cert-v01@openssh.com,
826 ssh-dss-cert-v01@openssh.com,
827 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,
828 ecdsa-sha2-nistp521,ssh-ed25519,
832 If hostkeys are known for the destination host then this default is modified
833 to prefer their algorithms.
835 The list of available key types may also be obtained using the
842 Specifies an alias that should be used instead of the
843 real host name when looking up or saving the host key
844 in the host key database files.
845 This option is useful for tunneling SSH connections
846 or for multiple servers running on a single host.
848 Specifies the real host name to log into.
849 This can be used to specify nicknames or abbreviations for hosts.
850 If the hostname contains the character sequence
852 then this will be replaced with the host name specified on the command line
853 (this is useful for manipulating unqualified names).
854 The character sequence
856 will be replaced by a single
858 character, which may be used when specifying IPv6 link-local addresses.
860 The default is the name given on the command line.
861 Numeric IP addresses are also permitted (both on the command line and in
864 .It Cm IdentitiesOnly
867 should only use the authentication identity files configured in the
874 offers more identities.
875 The argument to this keyword must be
879 This option is intended for situations where ssh-agent
880 offers many different identities.
884 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
888 for protocol version 1, and
890 .Pa ~/.ssh/id_ecdsa ,
891 .Pa ~/.ssh/id_ed25519
894 for protocol version 2.
895 Additionally, any identities represented by the authentication agent
896 will be used for authentication unless
900 will try to load certificate information from the filename obtained by
903 to the path of a specified
906 The file name may use the tilde
907 syntax to refer to a user's home directory or one of the following
910 (local user's home directory),
916 (remote host name) or
920 It is possible to have
921 multiple identity files specified in configuration files; all these
922 identities will be tried in sequence.
925 directives will add to the list of identities tried (this behaviour
926 differs from that of other configuration directives).
929 may be used in conjunction with
931 to select which identities in an agent are offered during authentication.
933 Specifies a pattern-list of unknown options to be ignored if they are
934 encountered in configuration parsing.
935 This may be used to suppress errors if
937 contains options that are unrecognised by
939 It is recommended that
941 be listed early in the configuration file as it will not be applied
942 to unknown options that appear before it.
944 Specifies the IPv4 type-of-service or DSCP class for connections.
971 This option may take one or two arguments, separated by whitespace.
972 If one argument is specified, it is used as the packet class unconditionally.
973 If two values are specified, the first is automatically selected for
974 interactive sessions and the second for non-interactive sessions.
977 for interactive sessions and
979 for non-interactive sessions.
980 .It Cm KbdInteractiveAuthentication
981 Specifies whether to use keyboard-interactive authentication.
982 The argument to this keyword must be
988 .It Cm KbdInteractiveDevices
989 Specifies the list of methods to use in keyboard-interactive authentication.
990 Multiple method names must be comma-separated.
991 The default is to use the server specified list.
992 The methods available vary depending on what the server supports.
993 For an OpenSSH server,
994 it may be zero or more of:
1000 Specifies the available KEX (Key Exchange) algorithms.
1001 Multiple algorithms must be comma-separated.
1002 Alternately if the specified value begins with a
1004 character, then the specified methods will be appended to the default set
1005 instead of replacing them.
1007 .Bd -literal -offset indent
1008 curve25519-sha256@libssh.org,
1009 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1010 diffie-hellman-group-exchange-sha256,
1011 diffie-hellman-group-exchange-sha1,
1012 diffie-hellman-group14-sha1
1015 The list of available key exchange algorithms may also be obtained using the
1022 Specifies a command to execute on the local machine after successfully
1023 connecting to the server.
1024 The command string extends to the end of the line, and is executed with
1026 The following escape character substitutions will be performed:
1028 (local user's home directory),
1034 (host name as provided on the command line),
1038 (remote user name) or
1040 (local user name) or
1042 by a hash of the concatenation: %l%h%p%r.
1044 The command is run synchronously and does not have access to the
1048 It should not be used for interactive commands.
1050 This directive is ignored unless
1051 .Cm PermitLocalCommand
1054 Specifies that a TCP port on the local machine be forwarded over
1055 the secure channel to the specified host and port from the remote machine.
1056 The first argument must be
1058 .Oo Ar bind_address : Oc Ar port
1060 and the second argument must be
1061 .Ar host : Ns Ar hostport .
1062 IPv6 addresses can be specified by enclosing addresses in square brackets.
1063 Multiple forwardings may be specified, and additional forwardings can be
1064 given on the command line.
1065 Only the superuser can forward privileged ports.
1066 By default, the local port is bound in accordance with the
1069 However, an explicit
1071 may be used to bind the connection to a specific address.
1076 indicates that the listening port be bound for local use only, while an
1079 indicates that the port should be available from all interfaces.
1081 Gives the verbosity level that is used when logging messages from
1083 The possible values are:
1084 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1085 The default is INFO.
1086 DEBUG and DEBUG1 are equivalent.
1087 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1089 Specifies the MAC (message authentication code) algorithms
1090 in order of preference.
1091 The MAC algorithm is used in protocol version 2
1092 for data integrity protection.
1093 Multiple algorithms must be comma-separated.
1094 If the specified value begins with a
1096 character, then the specified algorithms will be appended to the default set
1097 instead of replacing them.
1099 The algorithms that contain
1101 calculate the MAC after encryption (encrypt-then-mac).
1102 These are considered safer and their use recommended.
1105 .Bd -literal -offset indent
1106 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1107 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1108 umac-64@openssh.com,umac-128@openssh.com,
1109 hmac-sha2-256,hmac-sha2-512,
1110 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1111 hmac-ripemd160-etm@openssh.com,
1112 hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1113 hmac-md5,hmac-sha1,hmac-ripemd160,
1114 hmac-sha1-96,hmac-md5-96
1117 The list of available MAC algorithms may also be obtained using the
1123 .It Cm NoHostAuthenticationForLocalhost
1124 This option can be used if the home directory is shared across machines.
1125 In this case localhost will refer to a different machine on each of
1126 the machines and the user will get many warnings about changed host keys.
1127 However, this option disables host authentication for localhost.
1128 The argument to this keyword must be
1132 The default is to check the host key for localhost.
1133 .It Cm NumberOfPasswordPrompts
1134 Specifies the number of password prompts before giving up.
1135 The argument to this keyword must be an integer.
1137 .It Cm PasswordAuthentication
1138 Specifies whether to use password authentication.
1139 The argument to this keyword must be
1145 .It Cm PermitLocalCommand
1146 Allow local command execution via the
1149 .Ic !\& Ns Ar command
1152 The argument must be
1158 .It Cm PKCS11Provider
1159 Specifies which PKCS#11 provider to use.
1160 The argument to this keyword is the PKCS#11 shared library
1162 should use to communicate with a PKCS#11 token providing the user's
1165 Specifies the port number to connect on the remote host.
1167 .It Cm PreferredAuthentications
1168 Specifies the order in which the client should try protocol 2
1169 authentication methods.
1170 This allows a client to prefer one method (e.g.\&
1171 .Cm keyboard-interactive )
1172 over another method (e.g.\&
1175 .Bd -literal -offset indent
1176 gssapi-with-mic,hostbased,publickey,
1177 keyboard-interactive,password
1180 Specifies the protocol versions
1182 should support in order of preference.
1183 The possible values are
1187 Multiple versions must be comma-separated.
1188 When this option is set to
1191 will try version 2 and fall back to version 1
1192 if version 2 is not available.
1196 Specifies the command to use to connect to the server.
1198 string extends to the end of the line, and is executed
1199 using the user's shell
1201 directive to avoid a lingering shell process.
1203 In the command string, any occurrence of
1205 will be substituted by the host name to
1210 by the remote user name.
1211 The command can be basically anything,
1212 and should read from its standard input and write to its standard output.
1213 It should eventually connect an
1215 server running on some machine, or execute
1218 Host key management will be done using the
1219 HostName of the host being connected (defaulting to the name typed by
1221 Setting the command to
1223 disables this option entirely.
1226 is not available for connects with a proxy command.
1228 This directive is useful in conjunction with
1230 and its proxy support.
1231 For example, the following directive would connect via an HTTP proxy at
1233 .Bd -literal -offset 3n
1234 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1236 .It Cm ProxyUseFdpass
1239 will pass a connected file descriptor back to
1241 instead of continuing to execute and pass data.
1244 .It Cm PubkeyAcceptedKeyTypes
1245 Specifies the key types that will be used for public key authentication
1246 as a comma-separated pattern list.
1247 Alternately if the specified value begins with a
1249 character, then the key types after it will be appended to the default
1250 instead of replacing it.
1251 The default for this option is:
1252 .Bd -literal -offset 3n
1253 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1254 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1255 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1256 ssh-ed25519-cert-v01@openssh.com,
1257 ssh-rsa-cert-v01@openssh.com,
1258 ssh-dss-cert-v01@openssh.com,
1259 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,
1260 ecdsa-sha2-nistp521,ssh-ed25519,
1268 may be used to list supported key types.
1269 .It Cm PubkeyAuthentication
1270 Specifies whether to try public key authentication.
1271 The argument to this keyword must be
1277 This option applies to protocol version 2 only.
1279 Specifies the maximum amount of data that may be transmitted before the
1280 session key is renegotiated, optionally followed a maximum amount of
1281 time that may pass before the session key is renegotiated.
1282 The first argument is specified in bytes and may have a suffix of
1287 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1288 The default is between
1292 depending on the cipher.
1293 The optional second value is specified in seconds and may use any of the
1294 units documented in the
1295 TIME FORMATS section of
1297 The default value for
1301 which means that rekeying is performed after the cipher's default amount
1302 of data has been sent or received and no time based rekeying is done.
1303 This option applies to protocol version 2 only.
1304 .It Cm RemoteForward
1305 Specifies that a TCP port on the remote machine be forwarded over
1306 the secure channel to the specified host and port from the local machine.
1307 The first argument must be
1309 .Oo Ar bind_address : Oc Ar port
1311 and the second argument must be
1312 .Ar host : Ns Ar hostport .
1313 IPv6 addresses can be specified by enclosing addresses in square brackets.
1314 Multiple forwardings may be specified, and additional
1315 forwardings can be given on the command line.
1316 Privileged ports can be forwarded only when
1317 logging in as root on the remote machine.
1323 the listen port will be dynamically allocated on the server and reported
1324 to the client at run time.
1328 is not specified, the default is to only bind to loopback addresses.
1333 or an empty string, then the forwarding is requested to listen on all
1337 will only succeed if the server's
1339 option is enabled (see
1340 .Xr sshd_config 5 ) .
1342 Specifies whether to request a pseudo-tty for the session.
1343 The argument may be one of:
1345 (never request a TTY),
1347 (always request a TTY when standard input is a TTY),
1349 (always request a TTY) or
1351 (request a TTY when opening a login session).
1352 This option mirrors the
1358 .It Cm RevokedHostKeys
1359 Specifies revoked host public keys.
1360 Keys listed in this file will be refused for host authentication.
1361 Note that if this file does not exist or is not readable,
1362 then host authentication will be refused for all hosts.
1363 Keys may be specified as a text file, listing one public key per line, or as
1364 an OpenSSH Key Revocation List (KRL) as generated by
1366 For more information on KRLs, see the KEY REVOCATION LISTS section in
1368 .It Cm RhostsRSAAuthentication
1369 Specifies whether to try rhosts based authentication with RSA host
1371 The argument must be
1377 This option applies to protocol version 1 only and requires
1380 .It Cm RSAAuthentication
1381 Specifies whether to try RSA authentication.
1382 The argument to this keyword must be
1386 RSA authentication will only be
1387 attempted if the identity file exists, or an authentication agent is
1391 Note that this option applies to protocol version 1 only.
1393 Specifies what variables from the local
1395 should be sent to the server.
1396 Note that environment passing is only supported for protocol 2.
1397 The server must also support it, and the server must be configured to
1398 accept these environment variables.
1401 environment variable is always sent whenever a
1402 pseudo-terminal is requested as it is required by the protocol.
1407 for how to configure the server.
1408 Variables are specified by name, which may contain wildcard characters.
1409 Multiple environment variables may be separated by whitespace or spread
1413 The default is not to send any environment variables.
1417 for more information on patterns.
1418 .It Cm ServerAliveCountMax
1419 Sets the number of server alive messages (see below) which may be
1422 receiving any messages back from the server.
1423 If this threshold is reached while server alive messages are being sent,
1424 ssh will disconnect from the server, terminating the session.
1425 It is important to note that the use of server alive messages is very
1429 The server alive messages are sent through the encrypted channel
1430 and therefore will not be spoofable.
1431 The TCP keepalive option enabled by
1434 The server alive mechanism is valuable when the client or
1435 server depend on knowing when a connection has become inactive.
1437 The default value is 3.
1439 .Cm ServerAliveInterval
1440 (see below) is set to 15 and
1441 .Cm ServerAliveCountMax
1442 is left at the default, if the server becomes unresponsive,
1443 ssh will disconnect after approximately 45 seconds.
1444 This option applies to protocol version 2 only.
1445 .It Cm ServerAliveInterval
1446 Sets a timeout interval in seconds after which if no data has been received
1449 will send a message through the encrypted
1450 channel to request a response from the server.
1452 is 0, indicating that these messages will not be sent to the server.
1453 This option applies to protocol version 2 only.
1454 .It Cm StreamLocalBindMask
1455 Sets the octal file creation mode mask
1457 used when creating a Unix-domain socket file for local or remote
1459 This option is only used for port forwarding to a Unix-domain socket file.
1461 The default value is 0177, which creates a Unix-domain socket file that is
1462 readable and writable only by the owner.
1463 Note that not all operating systems honor the file mode on Unix-domain
1465 .It Cm StreamLocalBindUnlink
1466 Specifies whether to remove an existing Unix-domain socket file for local
1467 or remote port forwarding before creating a new one.
1468 If the socket file already exists and
1469 .Cm StreamLocalBindUnlink
1472 will be unable to forward the port to the Unix-domain socket file.
1473 This option is only used for port forwarding to a Unix-domain socket file.
1475 The argument must be
1481 .It Cm StrictHostKeyChecking
1482 If this flag is set to
1485 will never automatically add host keys to the
1486 .Pa ~/.ssh/known_hosts
1487 file, and refuses to connect to hosts whose host key has changed.
1488 This provides maximum protection against trojan horse attacks,
1489 though it can be annoying when the
1490 .Pa /etc/ssh/ssh_known_hosts
1491 file is poorly maintained or when connections to new hosts are
1493 This option forces the user to manually
1495 If this flag is set to
1497 ssh will automatically add new host keys to the
1498 user known hosts files.
1499 If this flag is set to
1502 will be added to the user known host files only after the user
1503 has confirmed that is what they really want to do, and
1504 ssh will refuse to connect to hosts whose host key has changed.
1506 known hosts will be verified automatically in all cases.
1507 The argument must be
1515 Specifies whether the system should send TCP keepalive messages to the
1517 If they are sent, death of the connection or crash of one
1518 of the machines will be properly noticed.
1519 However, this means that
1520 connections will die if the route is down temporarily, and some people
1525 (to send TCP keepalive messages), and the client will notice
1526 if the network goes down or the remote host dies.
1527 This is important in scripts, and many users want it too.
1529 To disable TCP keepalive messages, the value should be set to
1534 device forwarding between the client and the server.
1535 The argument must be
1545 requests the default tunnel mode, which is
1546 .Dq point-to-point .
1552 devices to open on the client
1557 The argument must be
1559 .Ar local_tun Op : Ar remote_tun .
1561 The devices may be specified by numerical ID or the keyword
1563 which uses the next available tunnel device.
1566 is not specified, it defaults to
1570 .It Cm UpdateHostKeys
1573 should accept notifications of additional hostkeys from the server sent
1574 after authentication has completed and add them to
1575 .Cm UserKnownHostsFile .
1576 The argument must be
1581 Enabling this option allows learning alternate hostkeys for a server
1582 and supports graceful key rotation by allowing a server to send replacement
1583 public keys before old ones are removed.
1584 Additional hostkeys are only accepted if the key used to authenticate the
1585 host was already trusted or explicity accepted by the user.
1590 then the user is asked to confirm the modifications to the known_hosts file.
1591 Confirmation is currently incompatible with
1592 .Cm ControlPersist ,
1593 and will be disabled if it is enabled.
1597 from OpenSSH 6.8 and greater support the
1598 .Dq hostkeys@openssh.com
1599 protocol extension used to inform the client of all the server's hostkeys.
1600 .It Cm UsePrivilegedPort
1601 Specifies whether to use a privileged port for outgoing connections.
1602 The argument must be
1611 must be setuid root.
1612 Note that this option must be set to
1615 .Cm RhostsRSAAuthentication
1618 Specifies the user to log in as.
1619 This can be useful when a different user name is used on different machines.
1620 This saves the trouble of
1621 having to remember to give the user name on the command line.
1622 .It Cm UserKnownHostsFile
1623 Specifies one or more files to use for the user
1624 host key database, separated by whitespace.
1626 .Pa ~/.ssh/known_hosts ,
1627 .Pa ~/.ssh/known_hosts2 .
1628 .It Cm VerifyHostKeyDNS
1629 Specifies whether to verify the remote key using DNS and SSHFP resource
1631 If this option is set to
1633 the client will implicitly trust keys that match a secure fingerprint
1635 Insecure fingerprints will be handled as if this option was set to
1637 If this option is set to
1639 information on fingerprint match will be displayed, but the user will still
1640 need to confirm new host keys according to the
1641 .Cm StrictHostKeyChecking
1643 The argument must be
1650 if compiled with LDNS and
1653 Note that this option applies to protocol version 2 only.
1655 See also VERIFYING HOST KEYS in
1657 .It Cm VersionAddendum
1658 Specifies a string to append to the regular version string to identify
1659 OS- or site-specific modifications.
1661 .Dq FreeBSD-20160125 .
1664 may be used to disable this.
1665 .It Cm VisualHostKey
1666 If this flag is set to
1668 an ASCII art representation of the remote host key fingerprint is
1669 printed in addition to the fingerprint string at login and
1670 for unknown host keys.
1671 If this flag is set to
1673 no fingerprint strings are printed at login and
1674 only the fingerprint string will be printed for unknown host keys.
1677 .It Cm XAuthLocation
1678 Specifies the full pathname of the
1682 .Pa /usr/local/bin/xauth .
1687 consists of zero or more non-whitespace characters,
1689 (a wildcard that matches zero or more characters),
1692 (a wildcard that matches exactly one character).
1693 For example, to specify a set of declarations for any host in the
1696 the following pattern could be used:
1700 The following pattern
1701 would match any host in the 192.168.0.[0-9] network range:
1703 .Dl Host 192.168.0.?
1707 is a comma-separated list of patterns.
1708 Patterns within pattern-lists may be negated
1709 by preceding them with an exclamation mark
1712 to allow a key to be used from anywhere within an organization
1716 the following entry (in authorized_keys) could be used:
1718 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1721 .It Pa ~/.ssh/config
1722 This is the per-user configuration file.
1723 The format of this file is described above.
1724 This file is used by the SSH client.
1725 Because of the potential for abuse, this file must have strict permissions:
1726 read/write for the user, and not accessible by others.
1727 .It Pa /etc/ssh/ssh_config
1728 Systemwide configuration file.
1729 This file provides defaults for those
1730 values that are not specified in the user's configuration file, and
1731 for those users who do not have a configuration file.
1732 This file must be world-readable.
1737 OpenSSH is a derivative of the original and free
1738 ssh 1.2.12 release by Tatu Ylonen.
1739 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1740 Theo de Raadt and Dug Song
1741 removed many bugs, re-added newer features and
1743 Markus Friedl contributed the support for SSH
1744 protocol versions 1.5 and 2.0.
projects / FreeBSD / releng / 10.3.git / blob