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 1.240 2016/10/15 19:56:25 jmc Exp $
38 .Dd $Mdocdate: October 15 2016 $
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 file contains keyword-argument pairs, one per line.
81 and empty lines are interpreted as comments.
82 Arguments may optionally be enclosed in double quotes
84 in order to represent arguments containing spaces.
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
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.
113 The host is usually the
115 argument given on the command line
117 .Cm CanonicalizeHostname
118 keyword for exceptions).
120 A pattern entry may be negated by prefixing it with an exclamation mark
122 If a negated entry is matched, then the
124 entry is ignored, regardless of whether any other patterns on the line
126 Negated matches are therefore useful to provide exceptions for wildcard
131 for more information on patterns.
133 Restricts the following declarations (up to the next
137 keyword) to be used only when the conditions following the
139 keyword are satisfied.
140 Match conditions are specified using one or more criteria
143 which always matches.
144 The available criteria keywords are:
154 criteria must appear alone or immediately after
156 Other criteria may be combined arbitrarily.
162 Criteria may be negated by prepending an exclamation mark
167 keyword matches only when the configuration file is being re-parsed
168 after hostname canonicalization (see the
169 .Cm CanonicalizeHostname
171 This may be useful to specify conditions that work with canonical host
175 keyword executes the specified command under the user's shell.
176 If the command returns a zero exit status then the condition is considered true.
177 Commands containing whitespace characters must be quoted.
180 accept the tokens described in the
184 The other keywords' criteria must be single entries or comma-separated
185 lists and may use the wildcard and negation operators described in the
190 keyword are matched against the target hostname, after any substitution
194 .Cm CanonicalizeHostname
198 keyword matches against the hostname as it was specified on the command-line.
201 keyword matches against the target username on the remote host.
204 keyword matches against the name of the local user running
206 (this keyword may be useful in system-wide
209 .It Cm AddKeysToAgent
210 Specifies whether keys should be automatically added to a running
212 If this option is set to
214 and a key is loaded from a file, the key and its passphrase are added to
215 the agent with the default lifetime, as if by
217 If this option is set to
220 will require confirmation using the
222 program before adding a key (see
225 If this option is set to
227 each use of the key must be confirmed, as if the
229 option was specified to
231 If this option is set to
233 no keys are added to the agent.
242 Specifies which address family to use when connecting.
253 passphrase/password querying will be disabled.
254 This option is useful in scripts and other batch jobs where no user
255 is present to supply the password.
262 Use the specified address on the local machine as the source address of
264 Only useful on systems with more than one address.
265 Note that this option does not work if
266 .Cm UsePrivilegedPort
269 .It Cm CanonicalDomains
271 .Cm CanonicalizeHostname
272 is enabled, this option specifies the list of domain suffixes in which to
273 search for the specified destination host.
274 .It Cm CanonicalizeFallbackLocal
275 Specifies whether to fail with an error when hostname canonicalization fails.
278 will attempt to look up the unqualified hostname using the system resolver's
285 .Cm CanonicalizeHostname
286 is enabled and the target hostname cannot be found in any of the domains
288 .Cm CanonicalDomains .
289 .It Cm CanonicalizeHostname
290 Controls whether explicit hostname canonicalization is performed.
293 is not to perform any name rewriting and let the system resolver handle all
297 then, for connections that do not use a
300 will attempt to canonicalize the hostname specified on the command line
304 .Cm CanonicalizePermittedCNAMEs
307 .Cm CanonicalizeHostname
310 then canonicalization is applied to proxied connections too.
312 If this option is enabled, then the configuration files are processed
313 again using the new target name to pick up any new configuration in matching
318 .It Cm CanonicalizeMaxDots
319 Specifies the maximum number of dot characters in a hostname before
320 canonicalization is disabled.
322 allows a single dot (i.e. hostname.subdomain).
323 .It Cm CanonicalizePermittedCNAMEs
324 Specifies rules to determine whether CNAMEs should be followed when
325 canonicalizing hostnames.
326 The rules consist of one or more arguments of
327 .Ar source_domain_list : Ns Ar target_domain_list ,
329 .Ar source_domain_list
330 is a pattern-list of domains that may follow CNAMEs in canonicalization,
332 .Ar target_domain_list
333 is a pattern-list of domains that they may resolve to.
336 .Qq *.a.example.com:*.b.example.com,*.c.example.com
337 will allow hostnames matching
339 to be canonicalized to names in the
344 .It Cm CertificateFile
345 Specifies a file from which the user's certificate is read.
346 A corresponding private key must be provided separately in order
347 to use this certificate either
361 may use the tilde syntax to refer to a user's home directory
362 or the tokens described in the
366 It is possible to have multiple certificate files specified in
367 configuration files; these certificates will be tried in sequence.
370 directives will add to the list of certificates used for
372 .It Cm ChallengeResponseAuthentication
373 Specifies whether to use challenge-response authentication.
374 The argument to this keyword must be
383 will additionally check the host IP address in the
386 This allows it to detect if a host key changed due to DNS spoofing
387 and will add addresses of destination hosts to
388 .Pa ~/.ssh/known_hosts
389 in the process, regardless of the setting of
390 .Cm StrictHostKeyChecking .
391 If the option is set to
393 the check will not be executed.
397 Specifies the cipher to use for encrypting the session
398 in protocol version 1.
408 is only supported in the
410 client for interoperability with legacy protocol 1 implementations;
411 its use is strongly discouraged due to cryptographic weaknesses.
413 Specifies the ciphers allowed for protocol version 2
414 in order of preference.
415 Multiple ciphers must be comma-separated.
416 If the specified value begins with a
418 character, then the specified ciphers will be appended to the default set
419 instead of replacing them.
421 The supported ciphers are:
422 .Bd -literal -offset indent
430 aes128-gcm@openssh.com
431 aes256-gcm@openssh.com
437 chacha20-poly1305@openssh.com
441 .Bd -literal -offset indent
442 chacha20-poly1305@openssh.com,
443 aes128-ctr,aes192-ctr,aes256-ctr,
444 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
445 aes128-cbc,aes192-cbc,aes256-cbc
448 The list of available ciphers may also be obtained using
450 .It Cm ClearAllForwardings
451 Specifies that all local, remote, and dynamic port forwardings
452 specified in the configuration files or on the command line be
454 This option is primarily useful when used from the
456 command line to clear port forwardings set in
457 configuration files, and is automatically set by
467 Specifies whether to use compression.
473 .It Cm CompressionLevel
474 Specifies the compression level to use if compression is enabled.
475 The argument must be an integer from 1 (fast) to 9 (slow, best).
476 The default level is 6, which is good for most applications.
477 The meaning of the values is the same as in
479 Note that this option applies to protocol version 1 only.
480 .It Cm ConnectionAttempts
481 Specifies the number of tries (one per second) to make before exiting.
482 The argument must be an integer.
483 This may be useful in scripts if the connection sometimes fails.
485 .It Cm ConnectTimeout
486 Specifies the timeout (in seconds) used when connecting to the
487 SSH server, instead of using the default system TCP timeout.
488 This value is used only when the target is down or really unreachable,
489 not when it refuses the connection.
491 Enables the sharing of multiple sessions over a single network connection.
495 will listen for connections on a control socket specified using the
498 Additional sessions can connect to this socket using the same
505 These sessions will try to reuse the master instance's network connection
506 rather than initiating new ones, but will fall back to connecting normally
507 if the control socket does not exist, or is not listening.
513 to listen for control connections, but require confirmation using
519 will continue without connecting to a master instance.
523 forwarding is supported over these multiplexed connections, however the
524 display and agent forwarded will be the one belonging to the master
525 connection i.e. it is not possible to forward multiple displays or agents.
527 Two additional options allow for opportunistic multiplexing: try to use a
528 master connection but fall back to creating a new one if one does not already
534 The latter requires confirmation like the
538 Specify the path to the control socket used for connection sharing as described
541 section above or the string
543 to disable connection sharing.
546 may use the tilde syntax to refer to a user's home directory
547 or the tokens described in the
550 It is recommended that any
552 used for opportunistic connection sharing include
553 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
554 that is not writable by other users.
555 This ensures that shared connections are uniquely identified.
556 .It Cm ControlPersist
557 When used in conjunction with
559 specifies that the master connection should remain open
560 in the background (waiting for future client connections)
561 after the initial client connection has been closed.
564 then the master connection will not be placed into the background,
565 and will close as soon as the initial client connection is closed.
569 then the master connection will remain in the background indefinitely
570 (until killed or closed via a mechanism such as the
572 If set to a time in seconds, or a time in any of the formats documented in
574 then the backgrounded master connection will automatically terminate
575 after it has remained idle (with no client connections) for the
577 .It Cm DynamicForward
578 Specifies that a TCP port on the local machine be forwarded
579 over the secure channel, and the application
580 protocol is then used to determine where to connect to from the
585 .Oo Ar bind_address : Oc Ar port .
587 IPv6 addresses can be specified by enclosing addresses in square brackets.
588 By default, the local port is bound in accordance with the
593 may be used to bind the connection to a specific address.
598 indicates that the listening port be bound for local use only, while an
601 indicates that the port should be available from all interfaces.
603 Currently the SOCKS4 and SOCKS5 protocols are supported, and
605 will act as a SOCKS server.
606 Multiple forwardings may be specified, and
607 additional forwardings can be given on the command line.
608 Only the superuser can forward privileged ports.
609 .It Cm EnableSSHKeysign
610 Setting this option to
612 in the global client configuration file
613 .Pa /etc/ssh/ssh_config
614 enables the use of the helper program
617 .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, (e.g.\&
644 if either end is unable to bind and listen on a specified port).
646 .Cm ExitOnForwardFailure
647 does not apply to connections made over port forwardings and will not,
650 to exit if TCP connections to the ultimate forwarding destination fail.
656 .It Cm FingerprintHash
657 Specifies the hash algorithm used when displaying key fingerprints.
664 Specifies whether the connection to the authentication agent (if any)
665 will be forwarded to the remote machine.
672 Agent forwarding should be enabled with caution.
673 Users with the ability to bypass file permissions on the remote host
674 (for the agent's Unix-domain socket)
675 can access the local agent through the forwarded connection.
676 An attacker cannot obtain key material from the agent,
677 however they can perform operations on the keys that enable them to
678 authenticate using the identities loaded into the agent.
680 Specifies whether X11 connections will be automatically redirected
681 over the secure channel and
690 X11 forwarding should be enabled with caution.
691 Users with the ability to bypass file permissions on the remote host
692 (for the user's X11 authorization database)
693 can access the local X11 display through the forwarded connection.
694 An attacker may then be able to perform activities such as keystroke monitoring
696 .Cm ForwardX11Trusted
697 option is also enabled.
698 .It Cm ForwardX11Timeout
699 Specify a timeout for untrusted X11 forwarding
700 using the format described in the
704 X11 connections received by
706 after this time will be refused.
707 The default is to disable untrusted X11 forwarding after twenty minutes has
709 .It Cm ForwardX11Trusted
710 If this option is set to
712 remote X11 clients will have full access to the original X11 display.
714 If this option is set to
717 remote X11 clients will be considered untrusted and prevented
718 from stealing or tampering with data belonging to trusted X11
722 token used for the session will be set to expire after 20 minutes.
723 Remote clients will be refused access after this time.
725 See the X11 SECURITY extension specification for full details on
726 the restrictions imposed on untrusted clients.
728 Specifies whether remote hosts are allowed to connect to local
732 binds local port forwardings to the loopback address.
733 This prevents other remote hosts from connecting to forwarded ports.
735 can be used to specify that ssh
736 should bind local port forwardings to the wildcard address,
737 thus allowing remote hosts to connect to forwarded ports.
743 .It Cm GlobalKnownHostsFile
744 Specifies one or more files to use for the global
745 host key database, separated by whitespace.
747 .Pa /etc/ssh/ssh_known_hosts ,
748 .Pa /etc/ssh/ssh_known_hosts2 .
749 .It Cm GSSAPIAuthentication
750 Specifies whether user authentication based on GSSAPI is allowed.
753 .It Cm GSSAPIDelegateCredentials
754 Forward (delegate) credentials to the server.
757 .It Cm HashKnownHosts
760 should hash host names and addresses when they are added to
761 .Pa ~/.ssh/known_hosts .
762 These hashed names may be used normally by
766 but they do not reveal identifying information should the file's contents
770 Note that existing names and addresses in known hosts files
771 will not be converted automatically,
772 but may be manually hashed using
774 .It Cm HostbasedAuthentication
775 Specifies whether to try rhosts based authentication with public key
782 .It Cm HostbasedKeyTypes
783 Specifies the key types that will be used for hostbased authentication
784 as a comma-separated pattern list.
785 Alternately if the specified value begins with a
787 character, then the specified key types will be appended to the default set
788 instead of replacing them.
789 The default for this option is:
790 .Bd -literal -offset 3n
791 ecdsa-sha2-nistp256-cert-v01@openssh.com,
792 ecdsa-sha2-nistp384-cert-v01@openssh.com,
793 ecdsa-sha2-nistp521-cert-v01@openssh.com,
794 ssh-ed25519-cert-v01@openssh.com,
795 ssh-rsa-cert-v01@openssh.com,
796 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
804 may be used to list supported key types.
805 .It Cm HostKeyAlgorithms
806 Specifies the host key algorithms
807 that the client wants to use in order of preference.
808 Alternately if the specified value begins with a
810 character, then the specified key types will be appended to the default set
811 instead of replacing them.
812 The default for this option is:
813 .Bd -literal -offset 3n
814 ecdsa-sha2-nistp256-cert-v01@openssh.com,
815 ecdsa-sha2-nistp384-cert-v01@openssh.com,
816 ecdsa-sha2-nistp521-cert-v01@openssh.com,
817 ssh-ed25519-cert-v01@openssh.com,
818 ssh-rsa-cert-v01@openssh.com,
819 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
823 If hostkeys are known for the destination host then this default is modified
824 to prefer their algorithms.
826 The list of available key types may also be obtained using
829 Specifies an alias that should be used instead of the
830 real host name when looking up or saving the host key
831 in the host key database files.
832 This option is useful for tunneling SSH connections
833 or for multiple servers running on a single host.
835 Specifies the real host name to log into.
836 This can be used to specify nicknames or abbreviations for hosts.
839 accept the tokens described in the
842 Numeric IP addresses are also permitted (both on the command line and in
845 The default is the name given on the command line.
846 .It Cm IdentitiesOnly
849 should only use the authentication identity and certificate files explicitly
860 offers more identities.
861 The argument to this keyword must be
866 This option is intended for situations where ssh-agent
867 offers many different identities.
871 socket used to communicate with the authentication agent.
873 This option overrides the
875 environment variable and can be used to select a specific agent.
876 Setting the socket name to
878 disables the use of an authentication agent.
881 is specified, the location of the socket will be read from the
883 environment variable.
887 may use the tilde syntax to refer to a user's home directory
888 or the tokens described in the
892 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
896 for protocol version 1, and
898 .Pa ~/.ssh/id_ecdsa ,
899 .Pa ~/.ssh/id_ed25519
902 for protocol version 2.
903 Additionally, any identities represented by the authentication agent
904 will be used for authentication unless
907 If no certificates have been explicitly specified by
908 .Cm CertificateFile ,
910 will try to load certificate information from the filename obtained by
913 to the path of a specified
918 may use the tilde syntax to refer to a user's home directory
919 or the tokens described in the
923 It is possible to have
924 multiple identity files specified in configuration files; all these
925 identities will be tried in sequence.
928 directives will add to the list of identities tried (this behaviour
929 differs from that of other configuration directives).
932 may be used in conjunction with
934 to select which identities in an agent are offered during authentication.
936 may also be used in conjunction with
938 in order to provide any certificate also needed for authentication with
941 Specifies a pattern-list of unknown options to be ignored if they are
942 encountered in configuration parsing.
943 This may be used to suppress errors if
945 contains options that are unrecognised by
947 It is recommended that
949 be listed early in the configuration file as it will not be applied
950 to unknown options that appear before it.
952 Include the specified configuration file(s).
953 Multiple pathnames may be specified and each pathname may contain
955 wildcards and, for user configurations, shell-like
957 references to user home directories.
958 Files without absolute paths are assumed to be in
960 if included in a user configuration file or
962 if included from the system configuration file.
964 directive may appear inside a
969 to perform conditional inclusion.
971 Specifies the IPv4 type-of-service or DSCP class for connections.
998 This option may take one or two arguments, separated by whitespace.
999 If one argument is specified, it is used as the packet class unconditionally.
1000 If two values are specified, the first is automatically selected for
1001 interactive sessions and the second for non-interactive sessions.
1004 for interactive sessions and
1006 for non-interactive sessions.
1007 .It Cm KbdInteractiveAuthentication
1008 Specifies whether to use keyboard-interactive authentication.
1009 The argument to this keyword must be
1014 .It Cm KbdInteractiveDevices
1015 Specifies the list of methods to use in keyboard-interactive authentication.
1016 Multiple method names must be comma-separated.
1017 The default is to use the server specified list.
1018 The methods available vary depending on what the server supports.
1019 For an OpenSSH server,
1020 it may be zero or more of:
1025 .It Cm KexAlgorithms
1026 Specifies the available KEX (Key Exchange) algorithms.
1027 Multiple algorithms must be comma-separated.
1028 Alternately if the specified value begins with a
1030 character, then the specified methods will be appended to the default set
1031 instead of replacing them.
1033 .Bd -literal -offset indent
1034 curve25519-sha256,curve25519-sha256@libssh.org,
1035 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1036 diffie-hellman-group-exchange-sha256,
1037 diffie-hellman-group-exchange-sha1,
1038 diffie-hellman-group14-sha1
1041 The list of available key exchange algorithms may also be obtained using
1044 Specifies a command to execute on the local machine after successfully
1045 connecting to the server.
1046 The command string extends to the end of the line, and is executed with
1050 accept the tokens described in the
1054 The command is run synchronously and does not have access to the
1058 It should not be used for interactive commands.
1060 This directive is ignored unless
1061 .Cm PermitLocalCommand
1064 Specifies that a TCP port on the local machine be forwarded over
1065 the secure channel to the specified host and port from the remote machine.
1066 The first argument must be
1068 .Oo Ar bind_address : Oc Ar port
1070 and the second argument must be
1071 .Ar host : Ns Ar hostport .
1072 IPv6 addresses can be specified by enclosing addresses in square brackets.
1073 Multiple forwardings may be specified, and additional forwardings can be
1074 given on the command line.
1075 Only the superuser can forward privileged ports.
1076 By default, the local port is bound in accordance with the
1079 However, an explicit
1081 may be used to bind the connection to a specific address.
1086 indicates that the listening port be bound for local use only, while an
1089 indicates that the port should be available from all interfaces.
1091 Gives the verbosity level that is used when logging messages from
1093 The possible values are:
1094 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1095 The default is INFO.
1096 DEBUG and DEBUG1 are equivalent.
1097 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1099 Specifies the MAC (message authentication code) algorithms
1100 in order of preference.
1101 The MAC algorithm is used for data integrity protection.
1102 Multiple algorithms must be comma-separated.
1103 If the specified value begins with a
1105 character, then the specified algorithms will be appended to the default set
1106 instead of replacing them.
1108 The algorithms that contain
1110 calculate the MAC after encryption (encrypt-then-mac).
1111 These are considered safer and their use recommended.
1114 .Bd -literal -offset indent
1115 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1116 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1117 hmac-sha1-etm@openssh.com,
1118 umac-64@openssh.com,umac-128@openssh.com,
1119 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1122 The list of available MAC algorithms may also be obtained using
1124 .It Cm NoHostAuthenticationForLocalhost
1125 This option can be used if the home directory is shared across machines.
1126 In this case localhost will refer to a different machine on each of
1127 the machines and the user will get many warnings about changed host keys.
1128 However, this option disables host authentication for localhost.
1129 The argument to this keyword must be
1134 .It Cm NumberOfPasswordPrompts
1135 Specifies the number of password prompts before giving up.
1136 The argument to this keyword must be an integer.
1138 .It Cm PasswordAuthentication
1139 Specifies whether to use password authentication.
1140 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
1157 .It Cm PKCS11Provider
1158 Specifies which PKCS#11 provider to use.
1159 The argument to this keyword is the PKCS#11 shared library
1161 should use to communicate with a PKCS#11 token providing the user's
1164 Specifies the port number to connect on the remote host.
1166 .It Cm PreferredAuthentications
1167 Specifies the order in which the client should try authentication methods.
1168 This allows a client to prefer one method (e.g.\&
1169 .Cm keyboard-interactive )
1170 over another method (e.g.\&
1173 .Bd -literal -offset indent
1174 gssapi-with-mic,hostbased,publickey,
1175 keyboard-interactive,password
1178 Specifies the protocol versions
1180 should support in order of preference.
1181 The possible values are 1 and 2.
1182 Multiple versions must be comma-separated.
1183 When this option is set to
1186 will try version 2 and fall back to version 1
1187 if version 2 is not available.
1188 The default is version 2.
1189 Protocol 1 suffers from a number of cryptographic weaknesses and should
1191 It is only offered to support legacy devices.
1193 Specifies the command to use to connect to the server.
1195 string extends to the end of the line, and is executed
1196 using the user's shell
1198 directive to avoid a lingering shell process.
1202 accept the tokens described in the
1205 The command can be basically anything,
1206 and should read from its standard input and write to its standard output.
1207 It should eventually connect an
1209 server running on some machine, or execute
1212 Host key management will be done using the
1213 HostName of the host being connected (defaulting to the name typed by
1215 Setting the command to
1217 disables this option entirely.
1220 is not available for connects with a proxy command.
1222 This directive is useful in conjunction with
1224 and its proxy support.
1225 For example, the following directive would connect via an HTTP proxy at
1227 .Bd -literal -offset 3n
1228 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1231 Specifies one or more jump proxies as
1239 Multiple proxies may be separated by comma characters and will be visited
1241 Setting this option will cause
1243 to connect to the target host by first making a
1245 connection to the specified
1247 host and then establishing a
1248 TCP forwarding to the ultimate target from there.
1250 Note that this option will compete with the
1252 option - whichever is specified first will prevent later instances of the
1253 other from taking effect.
1254 .It Cm ProxyUseFdpass
1257 will pass a connected file descriptor back to
1259 instead of continuing to execute and pass data.
1262 .It Cm PubkeyAcceptedKeyTypes
1263 Specifies the key types that will be used for public key authentication
1264 as a comma-separated pattern list.
1265 Alternately if the specified value begins with a
1267 character, then the key types after it will be appended to the default
1268 instead of replacing it.
1269 The default for this option is:
1270 .Bd -literal -offset 3n
1271 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1272 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1273 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1274 ssh-ed25519-cert-v01@openssh.com,
1275 ssh-rsa-cert-v01@openssh.com,
1276 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1280 The list of available key types may also be obtained using
1282 .It Cm PubkeyAuthentication
1283 Specifies whether to try public key authentication.
1284 The argument to this keyword must be
1290 Specifies the maximum amount of data that may be transmitted before the
1291 session key is renegotiated, optionally followed a maximum amount of
1292 time that may pass before the session key is renegotiated.
1293 The first argument is specified in bytes and may have a suffix of
1298 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1299 The default is between
1303 depending on the cipher.
1304 The optional second value is specified in seconds and may use any of the
1305 units documented in the
1309 The default value for
1313 which means that rekeying is performed after the cipher's default amount
1314 of data has been sent or received and no time based rekeying is done.
1315 .It Cm RemoteForward
1316 Specifies that a TCP port on the remote machine be forwarded over
1317 the secure channel to the specified host and port from the local machine.
1318 The first argument must be
1320 .Oo Ar bind_address : Oc Ar port
1322 and the second argument must be
1323 .Ar host : Ns Ar hostport .
1324 IPv6 addresses can be specified by enclosing addresses in square brackets.
1325 Multiple forwardings may be specified, and additional
1326 forwardings can be given on the command line.
1327 Privileged ports can be forwarded only when
1328 logging in as root on the remote machine.
1333 the listen port will be dynamically allocated on the server and reported
1334 to the client at run time.
1338 is not specified, the default is to only bind to loopback addresses.
1343 or an empty string, then the forwarding is requested to listen on all
1347 will only succeed if the server's
1349 option is enabled (see
1350 .Xr sshd_config 5 ) .
1352 Specifies whether to request a pseudo-tty for the session.
1353 The argument may be one of:
1355 (never request a TTY),
1357 (always request a TTY when standard input is a TTY),
1359 (always request a TTY) or
1361 (request a TTY when opening a login session).
1362 This option mirrors the
1368 .It Cm RevokedHostKeys
1369 Specifies revoked host public keys.
1370 Keys listed in this file will be refused for host authentication.
1371 Note that if this file does not exist or is not readable,
1372 then host authentication will be refused for all hosts.
1373 Keys may be specified as a text file, listing one public key per line, or as
1374 an OpenSSH Key Revocation List (KRL) as generated by
1376 For more information on KRLs, see the KEY REVOCATION LISTS section in
1378 .It Cm RhostsRSAAuthentication
1379 Specifies whether to try rhosts based authentication with RSA host
1381 The argument must be
1386 This option applies to protocol version 1 only and requires
1389 .It Cm RSAAuthentication
1390 Specifies whether to try RSA authentication.
1391 The argument to this keyword must be
1396 RSA authentication will only be
1397 attempted if the identity file exists, or an authentication agent is
1399 Note that this option applies to protocol version 1 only.
1401 Specifies what variables from the local
1403 should be sent to the server.
1404 The server must also support it, and the server must be configured to
1405 accept these environment variables.
1408 environment variable is always sent whenever a
1409 pseudo-terminal is requested as it is required by the protocol.
1414 for how to configure the server.
1415 Variables are specified by name, which may contain wildcard characters.
1416 Multiple environment variables may be separated by whitespace or spread
1420 The default is not to send any environment variables.
1424 for more information on patterns.
1425 .It Cm ServerAliveCountMax
1426 Sets the number of server alive messages (see below) which may be
1429 receiving any messages back from the server.
1430 If this threshold is reached while server alive messages are being sent,
1431 ssh will disconnect from the server, terminating the session.
1432 It is important to note that the use of server alive messages is very
1436 The server alive messages are sent through the encrypted channel
1437 and therefore will not be spoofable.
1438 The TCP keepalive option enabled by
1441 The server alive mechanism is valuable when the client or
1442 server depend on knowing when a connection has become inactive.
1444 The default value is 3.
1446 .Cm ServerAliveInterval
1447 (see below) is set to 15 and
1448 .Cm ServerAliveCountMax
1449 is left at the default, if the server becomes unresponsive,
1450 ssh will disconnect after approximately 45 seconds.
1451 .It Cm ServerAliveInterval
1452 Sets a timeout interval in seconds after which if no data has been received
1455 will send a message through the encrypted
1456 channel to request a response from the server.
1458 is 0, indicating that these messages will not be sent to the server.
1459 .It Cm StreamLocalBindMask
1460 Sets the octal file creation mode mask
1462 used when creating a Unix-domain socket file for local or remote
1464 This option is only used for port forwarding to a Unix-domain socket file.
1466 The default value is 0177, which creates a Unix-domain socket file that is
1467 readable and writable only by the owner.
1468 Note that not all operating systems honor the file mode on Unix-domain
1470 .It Cm StreamLocalBindUnlink
1471 Specifies whether to remove an existing Unix-domain socket file for local
1472 or remote port forwarding before creating a new one.
1473 If the socket file already exists and
1474 .Cm StreamLocalBindUnlink
1477 will be unable to forward the port to the Unix-domain socket file.
1478 This option is only used for port forwarding to a Unix-domain socket file.
1480 The argument must be
1485 .It Cm StrictHostKeyChecking
1486 If this flag is set to
1489 will never automatically add host keys to the
1490 .Pa ~/.ssh/known_hosts
1491 file, and refuses to connect to hosts whose host key has changed.
1492 This provides maximum protection against trojan horse attacks,
1493 though it can be annoying when the
1494 .Pa /etc/ssh/ssh_known_hosts
1495 file is poorly maintained or when connections to new hosts are
1497 This option forces the user to manually
1499 If this flag is set to
1501 ssh will automatically add new host keys to the
1502 user known hosts files.
1503 If this flag is set to
1507 will be added to the user known host files only after the user
1508 has confirmed that is what they really want to do, and
1509 ssh will refuse to connect to hosts whose host key has changed.
1511 known hosts will be verified automatically in all cases.
1513 Specifies whether the system should send TCP keepalive messages to the
1515 If they are sent, death of the connection or crash of one
1516 of the machines will be properly noticed.
1517 However, this means that
1518 connections will die if the route is down temporarily, and some people
1523 (to send TCP keepalive messages), and the client will notice
1524 if the network goes down or the remote host dies.
1525 This is important in scripts, and many users want it too.
1527 To disable TCP keepalive messages, the value should be set to
1532 device forwarding between the client and the server.
1533 The argument must be
1544 requests the default tunnel mode, which is
1545 .Cm point-to-point .
1549 devices to open on the client
1554 The argument must be
1556 .Ar local_tun Op : Ar remote_tun .
1558 The devices may be specified by numerical ID or the keyword
1560 which uses the next available tunnel device.
1563 is not specified, it defaults to
1567 .It Cm UpdateHostKeys
1570 should accept notifications of additional hostkeys from the server sent
1571 after authentication has completed and add them to
1572 .Cm UserKnownHostsFile .
1573 The argument must be
1578 Enabling this option allows learning alternate hostkeys for a server
1579 and supports graceful key rotation by allowing a server to send replacement
1580 public keys before old ones are removed.
1581 Additional hostkeys are only accepted if the key used to authenticate the
1582 host was already trusted or explicitly accepted by the user.
1587 then the user is asked to confirm the modifications to the known_hosts file.
1588 Confirmation is currently incompatible with
1589 .Cm ControlPersist ,
1590 and will be disabled if it is enabled.
1594 from OpenSSH 6.8 and greater support the
1595 .Qq hostkeys@openssh.com
1596 protocol extension used to inform the client of all the server's hostkeys.
1597 .It Cm UsePrivilegedPort
1598 Specifies whether to use a privileged port for outgoing connections.
1599 The argument must be
1607 must be setuid root.
1608 Note that this option must be set to
1611 .Cm RhostsRSAAuthentication
1614 Specifies the user to log in as.
1615 This can be useful when a different user name is used on different machines.
1616 This saves the trouble of
1617 having to remember to give the user name on the command line.
1618 .It Cm UserKnownHostsFile
1619 Specifies one or more files to use for the user
1620 host key database, separated by whitespace.
1622 .Pa ~/.ssh/known_hosts ,
1623 .Pa ~/.ssh/known_hosts2 .
1624 .It Cm VerifyHostKeyDNS
1625 Specifies whether to verify the remote key using DNS and SSHFP resource
1627 If this option is set to
1629 the client will implicitly trust keys that match a secure fingerprint
1631 Insecure fingerprints will be handled as if this option was set to
1633 If this option is set to
1635 information on fingerprint match will be displayed, but the user will still
1636 need to confirm new host keys according to the
1637 .Cm StrictHostKeyChecking
1641 if compiled with LDNS and
1646 .Sx VERIFYING HOST KEYS
1649 .It Cm VersionAddendum
1650 Specifies a string to append to the regular version string to identify
1651 OS- or site-specific modifications.
1653 .Dq FreeBSD-20170305 .
1656 may be used to disable this.
1657 .It Cm VisualHostKey
1658 If this flag is set to
1660 an ASCII art representation of the remote host key fingerprint is
1661 printed in addition to the fingerprint string at login and
1662 for unknown host keys.
1663 If this flag is set to
1666 no fingerprint strings are printed at login and
1667 only the fingerprint string will be printed for unknown host keys.
1668 .It Cm XAuthLocation
1669 Specifies the full pathname of the
1673 .Pa /usr/local/bin/xauth .
1678 consists of zero or more non-whitespace characters,
1680 (a wildcard that matches zero or more characters),
1683 (a wildcard that matches exactly one character).
1684 For example, to specify a set of declarations for any host in the
1687 the following pattern could be used:
1691 The following pattern
1692 would match any host in the 192.168.0.[0-9] network range:
1694 .Dl Host 192.168.0.?
1698 is a comma-separated list of patterns.
1699 Patterns within pattern-lists may be negated
1700 by preceding them with an exclamation mark
1703 to allow a key to be used from anywhere within an organization
1707 the following entry (in authorized_keys) could be used:
1709 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1711 Arguments to some keywords can make use of tokens,
1712 which are expanded at runtime:
1714 .Bl -tag -width XXXX -offset indent -compact
1719 Shorthand for %l%h%p%r.
1721 Local user's home directory.
1723 The remote hostname.
1729 The local hostname, including the domain name.
1731 The original remote hostname, as given on the command line.
1735 The remote username.
1741 accepts the tokens %%, %h, %L, %l, %n, %p, %r, and %u.
1744 accepts the tokens %%, %d, %h, %l, %r, and %u.
1747 accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u.
1750 accepts the tokens %% and %h.
1755 accept the tokens %%, %d, %h, %l, %r, and %u.
1758 accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u.
1761 accepts the tokens %%, %h, %p, and %r.
1764 .It Pa ~/.ssh/config
1765 This is the per-user configuration file.
1766 The format of this file is described above.
1767 This file is used by the SSH client.
1768 Because of the potential for abuse, this file must have strict permissions:
1769 read/write for the user, and not accessible by others.
1770 .It Pa /etc/ssh/ssh_config
1771 Systemwide configuration file.
1772 This file provides defaults for those
1773 values that are not specified in the user's configuration file, and
1774 for those users who do not have a configuration file.
1775 This file must be world-readable.
1781 OpenSSH is a derivative of the original and free
1782 ssh 1.2.12 release by
1784 .An Aaron Campbell , Bob Beck , Markus Friedl ,
1785 .An Niels Provos , Theo de Raadt
1788 removed many bugs, re-added newer features and
1791 contributed the support for SSH protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob