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 1.228 2016/02/20 23:01:46 sobrado Exp $
38 .Dd $Mdocdate: February 20 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 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 criteria
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
225 .It Cm AddKeysToAgent
226 Specifies whether keys should be automatically added to a running
228 If this option is set to
230 and a key is loaded from a file, the key and its passphrase are added to
231 the agent with the default lifetime, as if by
233 If this option is set to
236 will require confirmation using the
238 program before adding a key (see
241 If this option is set to
243 each use of the key must be confirmed, as if the
245 option was specified to
247 If this option is set to
249 no keys are added to the agent.
259 Specifies which address family to use when connecting.
271 passphrase/password querying will be disabled.
272 This option is useful in scripts and other batch jobs where no user
273 is present to supply the password.
281 Use the specified address on the local machine as the source address of
283 Only useful on systems with more than one address.
284 Note that this option does not work if
285 .Cm UsePrivilegedPort
288 .It Cm CanonicalDomains
290 .Cm CanonicalizeHostname
291 is enabled, this option specifies the list of domain suffixes in which to
292 search for the specified destination host.
293 .It Cm CanonicalizeFallbackLocal
294 Specifies whether to fail with an error when hostname canonicalization fails.
297 will attempt to look up the unqualified hostname using the system resolver's
304 .Cm CanonicalizeHostname
305 is enabled and the target hostname cannot be found in any of the domains
307 .Cm CanonicalDomains .
308 .It Cm CanonicalizeHostname
309 Controls whether explicit hostname canonicalization is performed.
312 is not to perform any name rewriting and let the system resolver handle all
316 then, for connections that do not use a
319 will attempt to canonicalize the hostname specified on the command line
323 .Cm CanonicalizePermittedCNAMEs
326 .Cm CanonicalizeHostname
329 then canonicalization is applied to proxied connections too.
331 If this option is enabled, then the configuration files are processed
332 again using the new target name to pick up any new configuration in matching
337 .It Cm CanonicalizeMaxDots
338 Specifies the maximum number of dot characters in a hostname before
339 canonicalization is disabled.
342 allows a single dot (i.e. hostname.subdomain).
343 .It Cm CanonicalizePermittedCNAMEs
344 Specifies rules to determine whether CNAMEs should be followed when
345 canonicalizing hostnames.
346 The rules consist of one or more arguments of
347 .Ar source_domain_list : Ns Ar target_domain_list ,
349 .Ar source_domain_list
350 is a pattern-list of domains that may follow CNAMEs in canonicalization,
352 .Ar target_domain_list
353 is a pattern-list of domains that they may resolve to.
356 .Dq *.a.example.com:*.b.example.com,*.c.example.com
357 will allow hostnames matching
359 to be canonicalized to names in the
364 .It Cm CertificateFile
365 Specifies a file from which the user's certificate is read.
366 A corresponding private key must be provided separately in order
367 to use this certificate either
379 The file name may use the tilde
380 syntax to refer to a user's home directory or one of the following
383 (local user's home directory),
389 (remote host name) or
393 It is possible to have multiple certificate files specified in
394 configuration files; these certificates will be tried in sequence.
397 directives will add to the list of certificates used for
399 .It Cm ChallengeResponseAuthentication
400 Specifies whether to use challenge-response authentication.
401 The argument to this keyword must be
408 If this flag is set to
411 will additionally check the host IP address in the
414 This allows ssh to detect if a host key changed due to DNS spoofing
415 and will add addresses of destination hosts to
416 .Pa ~/.ssh/known_hosts
417 in the process, regardless of the setting of
418 .Cm StrictHostKeyChecking .
419 If the option is set to
421 the check will not be executed.
425 Specifies the cipher to use for encrypting the session
426 in protocol version 1.
434 is only supported in the
436 client for interoperability with legacy protocol 1 implementations
437 that do not support the
440 Its use is strongly discouraged due to cryptographic weaknesses.
444 Specifies the ciphers allowed for protocol version 2
445 in order of preference.
446 Multiple ciphers must be comma-separated.
447 If the specified value begins with a
449 character, then the specified ciphers will be appended to the default set
450 instead of replacing them.
452 The supported ciphers are:
454 .Bl -item -compact -offset indent
470 aes128-gcm@openssh.com
472 aes256-gcm@openssh.com
484 chacha20-poly1305@openssh.com
488 .Bd -literal -offset indent
489 chacha20-poly1305@openssh.com,
490 aes128-ctr,aes192-ctr,aes256-ctr,
491 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
492 aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc
495 The list of available ciphers may also be obtained using the
501 .It Cm ClearAllForwardings
502 Specifies that all local, remote, and dynamic port forwardings
503 specified in the configuration files or on the command line be
505 This option is primarily useful when used from the
507 command line to clear port forwardings set in
508 configuration files, and is automatically set by
519 Specifies whether to use compression.
526 .It Cm CompressionLevel
527 Specifies the compression level to use if compression is enabled.
528 The argument must be an integer from 1 (fast) to 9 (slow, best).
529 The default level is 6, which is good for most applications.
530 The meaning of the values is the same as in
532 Note that this option applies to protocol version 1 only.
533 .It Cm ConnectionAttempts
534 Specifies the number of tries (one per second) to make before exiting.
535 The argument must be an integer.
536 This may be useful in scripts if the connection sometimes fails.
538 .It Cm ConnectTimeout
539 Specifies the timeout (in seconds) used when connecting to the
540 SSH server, instead of using the default system TCP timeout.
541 This value is used only when the target is down or really unreachable,
542 not when it refuses the connection.
544 Enables the sharing of multiple sessions over a single network connection.
548 will listen for connections on a control socket specified using the
551 Additional sessions can connect to this socket using the same
558 These sessions will try to reuse the master instance's network connection
559 rather than initiating new ones, but will fall back to connecting normally
560 if the control socket does not exist, or is not listening.
565 to listen for control connections, but require confirmation using
570 ssh will continue without connecting to a master instance.
574 forwarding is supported over these multiplexed connections, however the
575 display and agent forwarded will be the one belonging to the master
576 connection i.e. it is not possible to forward multiple displays or agents.
578 Two additional options allow for opportunistic multiplexing: try to use a
579 master connection but fall back to creating a new one if one does not already
585 The latter requires confirmation like the
589 Specify the path to the control socket used for connection sharing as described
592 section above or the string
594 to disable connection sharing.
597 will be substituted by the first component of the local host name,
599 will be substituted by the local host name (including any domain name),
601 will be substituted by the target host name,
603 will be substituted by the original target host name
604 specified on the command line,
606 the destination port,
608 by the remote login username,
612 by the numeric user ID (uid) of the user running
616 by a hash of the concatenation: %l%h%p%r.
617 It is recommended that any
619 used for opportunistic connection sharing include
620 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
621 that is not writable by other users.
622 This ensures that shared connections are uniquely identified.
623 .It Cm ControlPersist
624 When used in conjunction with
626 specifies that the master connection should remain open
627 in the background (waiting for future client connections)
628 after the initial client connection has been closed.
631 then the master connection will not be placed into the background,
632 and will close as soon as the initial client connection is closed.
637 then the master connection will remain in the background indefinitely
638 (until killed or closed via a mechanism such as the
642 If set to a time in seconds, or a time in any of the formats documented in
644 then the backgrounded master connection will automatically terminate
645 after it has remained idle (with no client connections) for the
647 .It Cm DynamicForward
648 Specifies that a TCP port on the local machine be forwarded
649 over the secure channel, and the application
650 protocol is then used to determine where to connect to from the
655 .Oo Ar bind_address : Oc Ar port .
657 IPv6 addresses can be specified by enclosing addresses in square brackets.
658 By default, the local port is bound in accordance with the
663 may be used to bind the connection to a specific address.
668 indicates that the listening port be bound for local use only, while an
671 indicates that the port should be available from all interfaces.
673 Currently the SOCKS4 and SOCKS5 protocols are supported, and
675 will act as a SOCKS server.
676 Multiple forwardings may be specified, and
677 additional forwardings can be given on the command line.
678 Only the superuser can forward privileged ports.
679 .It Cm EnableSSHKeysign
680 Setting this option to
682 in the global client configuration file
683 .Pa /etc/ssh/ssh_config
684 enables the use of the helper program
687 .Cm HostbasedAuthentication .
694 This option should be placed in the non-hostspecific section.
697 for more information.
699 Sets the escape character (default:
701 The escape character can also
702 be set on the command line.
703 The argument should be a single character,
705 followed by a letter, or
707 to disable the escape
708 character entirely (making the connection transparent for binary
710 .It Cm ExitOnForwardFailure
713 should terminate the connection if it cannot set up all requested
714 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
715 if either end is unable to bind and listen on a specified port).
717 .Cm ExitOnForwardFailure
718 does not apply to connections made over port forwardings and will not,
721 to exit if TCP connections to the ultimate forwarding destination fail.
728 .It Cm FingerprintHash
729 Specifies the hash algorithm used when displaying key fingerprints.
737 Specifies whether the connection to the authentication agent (if any)
738 will be forwarded to the remote machine.
746 Agent forwarding should be enabled with caution.
747 Users with the ability to bypass file permissions on the remote host
748 (for the agent's Unix-domain socket)
749 can access the local agent through the forwarded connection.
750 An attacker cannot obtain key material from the agent,
751 however they can perform operations on the keys that enable them to
752 authenticate using the identities loaded into the agent.
754 Specifies whether X11 connections will be automatically redirected
755 over the secure channel and
765 X11 forwarding should be enabled with caution.
766 Users with the ability to bypass file permissions on the remote host
767 (for the user's X11 authorization database)
768 can access the local X11 display through the forwarded connection.
769 An attacker may then be able to perform activities such as keystroke monitoring
771 .Cm ForwardX11Trusted
772 option is also enabled.
773 .It Cm ForwardX11Timeout
774 Specify a timeout for untrusted X11 forwarding
775 using the format described in the
776 TIME FORMATS section of
778 X11 connections received by
780 after this time will be refused.
781 The default is to disable untrusted X11 forwarding after twenty minutes has
783 .It Cm ForwardX11Trusted
784 If this option is set to
786 remote X11 clients will have full access to the original X11 display.
788 If this option is set to
790 remote X11 clients will be considered untrusted and prevented
791 from stealing or tampering with data belonging to trusted X11
795 token used for the session will be set to expire after 20 minutes.
796 Remote clients will be refused access after this time.
801 See the X11 SECURITY extension specification for full details on
802 the restrictions imposed on untrusted clients.
804 Specifies whether remote hosts are allowed to connect to local
808 binds local port forwardings to the loopback address.
809 This prevents other remote hosts from connecting to forwarded ports.
811 can be used to specify that ssh
812 should bind local port forwardings to the wildcard address,
813 thus allowing remote hosts to connect to forwarded ports.
820 .It Cm GlobalKnownHostsFile
821 Specifies one or more files to use for the global
822 host key database, separated by whitespace.
824 .Pa /etc/ssh/ssh_known_hosts ,
825 .Pa /etc/ssh/ssh_known_hosts2 .
826 .It Cm GSSAPIAuthentication
827 Specifies whether user authentication based on GSSAPI is allowed.
830 .It Cm GSSAPIDelegateCredentials
831 Forward (delegate) credentials to the server.
834 .It Cm HashKnownHosts
837 should hash host names and addresses when they are added to
838 .Pa ~/.ssh/known_hosts .
839 These hashed names may be used normally by
843 but they do not reveal identifying information should the file's contents
847 Note that existing names and addresses in known hosts files
848 will not be converted automatically,
849 but may be manually hashed using
851 .It Cm HostbasedAuthentication
852 Specifies whether to try rhosts based authentication with public key
860 .It Cm HostbasedKeyTypes
861 Specifies the key types that will be used for hostbased authentication
862 as a comma-separated pattern list.
863 Alternately if the specified value begins with a
865 character, then the specified key types will be appended to the default set
866 instead of replacing them.
867 The default for this option is:
868 .Bd -literal -offset 3n
869 ecdsa-sha2-nistp256-cert-v01@openssh.com,
870 ecdsa-sha2-nistp384-cert-v01@openssh.com,
871 ecdsa-sha2-nistp521-cert-v01@openssh.com,
872 ssh-ed25519-cert-v01@openssh.com,
873 ssh-rsa-cert-v01@openssh.com,
874 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
882 may be used to list supported key types.
883 .It Cm HostKeyAlgorithms
884 Specifies the host key algorithms
885 that the client wants to use in order of preference.
886 Alternately if the specified value begins with a
888 character, then the specified key types will be appended to the default set
889 instead of replacing them.
890 The default for this option is:
891 .Bd -literal -offset 3n
892 ecdsa-sha2-nistp256-cert-v01@openssh.com,
893 ecdsa-sha2-nistp384-cert-v01@openssh.com,
894 ecdsa-sha2-nistp521-cert-v01@openssh.com,
895 ssh-ed25519-cert-v01@openssh.com,
896 ssh-rsa-cert-v01@openssh.com,
897 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
901 If hostkeys are known for the destination host then this default is modified
902 to prefer their algorithms.
904 The list of available key types may also be obtained using the
911 Specifies an alias that should be used instead of the
912 real host name when looking up or saving the host key
913 in the host key database files.
914 This option is useful for tunneling SSH connections
915 or for multiple servers running on a single host.
917 Specifies the real host name to log into.
918 This can be used to specify nicknames or abbreviations for hosts.
919 If the hostname contains the character sequence
921 then this will be replaced with the host name specified on the command line
922 (this is useful for manipulating unqualified names).
923 The character sequence
925 will be replaced by a single
927 character, which may be used when specifying IPv6 link-local addresses.
929 The default is the name given on the command line.
930 Numeric IP addresses are also permitted (both on the command line and in
933 .It Cm IdentitiesOnly
936 should only use the authentication identity and certificate files explicitly
947 offers more identities.
948 The argument to this keyword must be
952 This option is intended for situations where ssh-agent
953 offers many different identities.
957 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
961 for protocol version 1, and
963 .Pa ~/.ssh/id_ecdsa ,
964 .Pa ~/.ssh/id_ed25519
967 for protocol version 2.
968 Additionally, any identities represented by the authentication agent
969 will be used for authentication unless
972 If no certificates have been explicitly specified by
973 .Cm CertificateFile ,
975 will try to load certificate information from the filename obtained by
978 to the path of a specified
981 The file name may use the tilde
982 syntax to refer to a user's home directory or one of the following
985 (local user's home directory),
991 (remote host name) or
995 It is possible to have
996 multiple identity files specified in configuration files; all these
997 identities will be tried in sequence.
1000 directives will add to the list of identities tried (this behaviour
1001 differs from that of other configuration directives).
1004 may be used in conjunction with
1006 to select which identities in an agent are offered during authentication.
1008 may also be used in conjunction with
1010 in order to provide any certificate also needed for authentication with
1012 .It Cm IgnoreUnknown
1013 Specifies a pattern-list of unknown options to be ignored if they are
1014 encountered in configuration parsing.
1015 This may be used to suppress errors if
1017 contains options that are unrecognised by
1019 It is recommended that
1021 be listed early in the configuration file as it will not be applied
1022 to unknown options that appear before it.
1024 Specifies the IPv4 type-of-service or DSCP class for connections.
1051 This option may take one or two arguments, separated by whitespace.
1052 If one argument is specified, it is used as the packet class unconditionally.
1053 If two values are specified, the first is automatically selected for
1054 interactive sessions and the second for non-interactive sessions.
1057 for interactive sessions and
1059 for non-interactive sessions.
1060 .It Cm KbdInteractiveAuthentication
1061 Specifies whether to use keyboard-interactive authentication.
1062 The argument to this keyword must be
1068 .It Cm KbdInteractiveDevices
1069 Specifies the list of methods to use in keyboard-interactive authentication.
1070 Multiple method names must be comma-separated.
1071 The default is to use the server specified list.
1072 The methods available vary depending on what the server supports.
1073 For an OpenSSH server,
1074 it may be zero or more of:
1079 .It Cm KexAlgorithms
1080 Specifies the available KEX (Key Exchange) algorithms.
1081 Multiple algorithms must be comma-separated.
1082 Alternately if the specified value begins with a
1084 character, then the specified methods will be appended to the default set
1085 instead of replacing them.
1087 .Bd -literal -offset indent
1088 curve25519-sha256@libssh.org,
1089 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1090 diffie-hellman-group-exchange-sha256,
1091 diffie-hellman-group-exchange-sha1,
1092 diffie-hellman-group14-sha1
1095 The list of available key exchange algorithms may also be obtained using the
1102 Specifies a command to execute on the local machine after successfully
1103 connecting to the server.
1104 The command string extends to the end of the line, and is executed with
1106 The following escape character substitutions will be performed:
1108 (local user's home directory),
1114 (host name as provided on the command line),
1118 (remote user name) or
1120 (local user name) or
1122 by a hash of the concatenation: %l%h%p%r.
1124 The command is run synchronously and does not have access to the
1128 It should not be used for interactive commands.
1130 This directive is ignored unless
1131 .Cm PermitLocalCommand
1134 Specifies that a TCP port on the local machine be forwarded over
1135 the secure channel to the specified host and port from the remote machine.
1136 The first argument must be
1138 .Oo Ar bind_address : Oc Ar port
1140 and the second argument must be
1141 .Ar host : Ns Ar hostport .
1142 IPv6 addresses can be specified by enclosing addresses in square brackets.
1143 Multiple forwardings may be specified, and additional forwardings can be
1144 given on the command line.
1145 Only the superuser can forward privileged ports.
1146 By default, the local port is bound in accordance with the
1149 However, an explicit
1151 may be used to bind the connection to a specific address.
1156 indicates that the listening port be bound for local use only, while an
1159 indicates that the port should be available from all interfaces.
1161 Gives the verbosity level that is used when logging messages from
1163 The possible values are:
1164 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1165 The default is INFO.
1166 DEBUG and DEBUG1 are equivalent.
1167 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1169 Specifies the MAC (message authentication code) algorithms
1170 in order of preference.
1171 The MAC algorithm is used for data integrity protection.
1172 Multiple algorithms must be comma-separated.
1173 If the specified value begins with a
1175 character, then the specified algorithms will be appended to the default set
1176 instead of replacing them.
1178 The algorithms that contain
1180 calculate the MAC after encryption (encrypt-then-mac).
1181 These are considered safer and their use recommended.
1184 .Bd -literal -offset indent
1185 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1186 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1187 hmac-sha1-etm@openssh.com,
1188 umac-64@openssh.com,umac-128@openssh.com,
1189 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1192 The list of available MAC algorithms may also be obtained using the
1198 .It Cm NoHostAuthenticationForLocalhost
1199 This option can be used if the home directory is shared across machines.
1200 In this case localhost will refer to a different machine on each of
1201 the machines and the user will get many warnings about changed host keys.
1202 However, this option disables host authentication for localhost.
1203 The argument to this keyword must be
1207 The default is to check the host key for localhost.
1208 .It Cm NumberOfPasswordPrompts
1209 Specifies the number of password prompts before giving up.
1210 The argument to this keyword must be an integer.
1212 .It Cm PasswordAuthentication
1213 Specifies whether to use password authentication.
1214 The argument to this keyword must be
1220 .It Cm PermitLocalCommand
1221 Allow local command execution via the
1224 .Ic !\& Ns Ar command
1227 The argument must be
1233 .It Cm PKCS11Provider
1234 Specifies which PKCS#11 provider to use.
1235 The argument to this keyword is the PKCS#11 shared library
1237 should use to communicate with a PKCS#11 token providing the user's
1240 Specifies the port number to connect on the remote host.
1242 .It Cm PreferredAuthentications
1243 Specifies the order in which the client should try authentication methods.
1244 This allows a client to prefer one method (e.g.\&
1245 .Cm keyboard-interactive )
1246 over another method (e.g.\&
1249 .Bd -literal -offset indent
1250 gssapi-with-mic,hostbased,publickey,
1251 keyboard-interactive,password
1254 Specifies the protocol versions
1256 should support in order of preference.
1257 The possible values are
1261 Multiple versions must be comma-separated.
1262 When this option is set to
1265 will try version 2 and fall back to version 1
1266 if version 2 is not available.
1269 Protocol 1 suffers from a number of cryptographic weaknesses and should
1271 It is only offered to support legacy devices.
1273 Specifies the command to use to connect to the server.
1275 string extends to the end of the line, and is executed
1276 using the user's shell
1278 directive to avoid a lingering shell process.
1280 In the command string, any occurrence of
1282 will be substituted by the host name to
1287 by the remote user name.
1288 The command can be basically anything,
1289 and should read from its standard input and write to its standard output.
1290 It should eventually connect an
1292 server running on some machine, or execute
1295 Host key management will be done using the
1296 HostName of the host being connected (defaulting to the name typed by
1298 Setting the command to
1300 disables this option entirely.
1303 is not available for connects with a proxy command.
1305 This directive is useful in conjunction with
1307 and its proxy support.
1308 For example, the following directive would connect via an HTTP proxy at
1310 .Bd -literal -offset 3n
1311 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1313 .It Cm ProxyUseFdpass
1316 will pass a connected file descriptor back to
1318 instead of continuing to execute and pass data.
1321 .It Cm PubkeyAcceptedKeyTypes
1322 Specifies the key types that will be used for public key authentication
1323 as a comma-separated pattern list.
1324 Alternately if the specified value begins with a
1326 character, then the key types after it will be appended to the default
1327 instead of replacing it.
1328 The default for this option is:
1329 .Bd -literal -offset 3n
1330 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1331 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1332 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1333 ssh-ed25519-cert-v01@openssh.com,
1334 ssh-rsa-cert-v01@openssh.com,
1335 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1343 may be used to list supported key types.
1344 .It Cm PubkeyAuthentication
1345 Specifies whether to try public key authentication.
1346 The argument to this keyword must be
1353 Specifies the maximum amount of data that may be transmitted before the
1354 session key is renegotiated, optionally followed a maximum amount of
1355 time that may pass before the session key is renegotiated.
1356 The first argument is specified in bytes and may have a suffix of
1361 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1362 The default is between
1366 depending on the cipher.
1367 The optional second value is specified in seconds and may use any of the
1368 units documented in the
1369 TIME FORMATS section of
1371 The default value for
1375 which means that rekeying is performed after the cipher's default amount
1376 of data has been sent or received and no time based rekeying is done.
1377 .It Cm RemoteForward
1378 Specifies that a TCP port on the remote machine be forwarded over
1379 the secure channel to the specified host and port from the local machine.
1380 The first argument must be
1382 .Oo Ar bind_address : Oc Ar port
1384 and the second argument must be
1385 .Ar host : Ns Ar hostport .
1386 IPv6 addresses can be specified by enclosing addresses in square brackets.
1387 Multiple forwardings may be specified, and additional
1388 forwardings can be given on the command line.
1389 Privileged ports can be forwarded only when
1390 logging in as root on the remote machine.
1396 the listen port will be dynamically allocated on the server and reported
1397 to the client at run time.
1401 is not specified, the default is to only bind to loopback addresses.
1406 or an empty string, then the forwarding is requested to listen on all
1410 will only succeed if the server's
1412 option is enabled (see
1413 .Xr sshd_config 5 ) .
1415 Specifies whether to request a pseudo-tty for the session.
1416 The argument may be one of:
1418 (never request a TTY),
1420 (always request a TTY when standard input is a TTY),
1422 (always request a TTY) or
1424 (request a TTY when opening a login session).
1425 This option mirrors the
1431 .It Cm RevokedHostKeys
1432 Specifies revoked host public keys.
1433 Keys listed in this file will be refused for host authentication.
1434 Note that if this file does not exist or is not readable,
1435 then host authentication will be refused for all hosts.
1436 Keys may be specified as a text file, listing one public key per line, or as
1437 an OpenSSH Key Revocation List (KRL) as generated by
1439 For more information on KRLs, see the KEY REVOCATION LISTS section in
1441 .It Cm RhostsRSAAuthentication
1442 Specifies whether to try rhosts based authentication with RSA host
1444 The argument must be
1450 This option applies to protocol version 1 only and requires
1453 .It Cm RSAAuthentication
1454 Specifies whether to try RSA authentication.
1455 The argument to this keyword must be
1459 RSA authentication will only be
1460 attempted if the identity file exists, or an authentication agent is
1464 Note that this option applies to protocol version 1 only.
1466 Specifies what variables from the local
1468 should be sent to the server.
1469 The server must also support it, and the server must be configured to
1470 accept these environment variables.
1473 environment variable is always sent whenever a
1474 pseudo-terminal is requested as it is required by the protocol.
1479 for how to configure the server.
1480 Variables are specified by name, which may contain wildcard characters.
1481 Multiple environment variables may be separated by whitespace or spread
1485 The default is not to send any environment variables.
1489 for more information on patterns.
1490 .It Cm ServerAliveCountMax
1491 Sets the number of server alive messages (see below) which may be
1494 receiving any messages back from the server.
1495 If this threshold is reached while server alive messages are being sent,
1496 ssh will disconnect from the server, terminating the session.
1497 It is important to note that the use of server alive messages is very
1501 The server alive messages are sent through the encrypted channel
1502 and therefore will not be spoofable.
1503 The TCP keepalive option enabled by
1506 The server alive mechanism is valuable when the client or
1507 server depend on knowing when a connection has become inactive.
1509 The default value is 3.
1511 .Cm ServerAliveInterval
1512 (see below) is set to 15 and
1513 .Cm ServerAliveCountMax
1514 is left at the default, if the server becomes unresponsive,
1515 ssh will disconnect after approximately 45 seconds.
1516 .It Cm ServerAliveInterval
1517 Sets a timeout interval in seconds after which if no data has been received
1520 will send a message through the encrypted
1521 channel to request a response from the server.
1523 is 0, indicating that these messages will not be sent to the server.
1524 .It Cm StreamLocalBindMask
1525 Sets the octal file creation mode mask
1527 used when creating a Unix-domain socket file for local or remote
1529 This option is only used for port forwarding to a Unix-domain socket file.
1531 The default value is 0177, which creates a Unix-domain socket file that is
1532 readable and writable only by the owner.
1533 Note that not all operating systems honor the file mode on Unix-domain
1535 .It Cm StreamLocalBindUnlink
1536 Specifies whether to remove an existing Unix-domain socket file for local
1537 or remote port forwarding before creating a new one.
1538 If the socket file already exists and
1539 .Cm StreamLocalBindUnlink
1542 will be unable to forward the port to the Unix-domain socket file.
1543 This option is only used for port forwarding to a Unix-domain socket file.
1545 The argument must be
1551 .It Cm StrictHostKeyChecking
1552 If this flag is set to
1555 will never automatically add host keys to the
1556 .Pa ~/.ssh/known_hosts
1557 file, and refuses to connect to hosts whose host key has changed.
1558 This provides maximum protection against trojan horse attacks,
1559 though it can be annoying when the
1560 .Pa /etc/ssh/ssh_known_hosts
1561 file is poorly maintained or when connections to new hosts are
1563 This option forces the user to manually
1565 If this flag is set to
1567 ssh will automatically add new host keys to the
1568 user known hosts files.
1569 If this flag is set to
1572 will be added to the user known host files only after the user
1573 has confirmed that is what they really want to do, and
1574 ssh will refuse to connect to hosts whose host key has changed.
1576 known hosts will be verified automatically in all cases.
1577 The argument must be
1585 Specifies whether the system should send TCP keepalive messages to the
1587 If they are sent, death of the connection or crash of one
1588 of the machines will be properly noticed.
1589 However, this means that
1590 connections will die if the route is down temporarily, and some people
1595 (to send TCP keepalive messages), and the client will notice
1596 if the network goes down or the remote host dies.
1597 This is important in scripts, and many users want it too.
1599 To disable TCP keepalive messages, the value should be set to
1604 device forwarding between the client and the server.
1605 The argument must be
1615 requests the default tunnel mode, which is
1616 .Dq point-to-point .
1622 devices to open on the client
1627 The argument must be
1629 .Ar local_tun Op : Ar remote_tun .
1631 The devices may be specified by numerical ID or the keyword
1633 which uses the next available tunnel device.
1636 is not specified, it defaults to
1640 .It Cm UpdateHostKeys
1643 should accept notifications of additional hostkeys from the server sent
1644 after authentication has completed and add them to
1645 .Cm UserKnownHostsFile .
1646 The argument must be
1651 Enabling this option allows learning alternate hostkeys for a server
1652 and supports graceful key rotation by allowing a server to send replacement
1653 public keys before old ones are removed.
1654 Additional hostkeys are only accepted if the key used to authenticate the
1655 host was already trusted or explicitly accepted by the user.
1660 then the user is asked to confirm the modifications to the known_hosts file.
1661 Confirmation is currently incompatible with
1662 .Cm ControlPersist ,
1663 and will be disabled if it is enabled.
1667 from OpenSSH 6.8 and greater support the
1668 .Dq hostkeys@openssh.com
1669 protocol extension used to inform the client of all the server's hostkeys.
1670 .It Cm UsePrivilegedPort
1671 Specifies whether to use a privileged port for outgoing connections.
1672 The argument must be
1681 must be setuid root.
1682 Note that this option must be set to
1685 .Cm RhostsRSAAuthentication
1688 Specifies the user to log in as.
1689 This can be useful when a different user name is used on different machines.
1690 This saves the trouble of
1691 having to remember to give the user name on the command line.
1692 .It Cm UserKnownHostsFile
1693 Specifies one or more files to use for the user
1694 host key database, separated by whitespace.
1696 .Pa ~/.ssh/known_hosts ,
1697 .Pa ~/.ssh/known_hosts2 .
1698 .It Cm VerifyHostKeyDNS
1699 Specifies whether to verify the remote key using DNS and SSHFP resource
1701 If this option is set to
1703 the client will implicitly trust keys that match a secure fingerprint
1705 Insecure fingerprints will be handled as if this option was set to
1707 If this option is set to
1709 information on fingerprint match will be displayed, but the user will still
1710 need to confirm new host keys according to the
1711 .Cm StrictHostKeyChecking
1713 The argument must be
1720 if compiled with LDNS and
1724 See also VERIFYING HOST KEYS in
1726 .It Cm VersionAddendum
1727 Specifies a string to append to the regular version string to identify
1728 OS- or site-specific modifications.
1730 .Dq FreeBSD-20160310 .
1733 may be used to disable this.
1734 .It Cm VisualHostKey
1735 If this flag is set to
1737 an ASCII art representation of the remote host key fingerprint is
1738 printed in addition to the fingerprint string at login and
1739 for unknown host keys.
1740 If this flag is set to
1742 no fingerprint strings are printed at login and
1743 only the fingerprint string will be printed for unknown host keys.
1746 .It Cm XAuthLocation
1747 Specifies the full pathname of the
1751 .Pa /usr/local/bin/xauth .
1756 consists of zero or more non-whitespace characters,
1758 (a wildcard that matches zero or more characters),
1761 (a wildcard that matches exactly one character).
1762 For example, to specify a set of declarations for any host in the
1765 the following pattern could be used:
1769 The following pattern
1770 would match any host in the 192.168.0.[0-9] network range:
1772 .Dl Host 192.168.0.?
1776 is a comma-separated list of patterns.
1777 Patterns within pattern-lists may be negated
1778 by preceding them with an exclamation mark
1781 to allow a key to be used from anywhere within an organization
1785 the following entry (in authorized_keys) could be used:
1787 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1790 .It Pa ~/.ssh/config
1791 This is the per-user configuration file.
1792 The format of this file is described above.
1793 This file is used by the SSH client.
1794 Because of the potential for abuse, this file must have strict permissions:
1795 read/write for the user, and not accessible by others.
1796 .It Pa /etc/ssh/ssh_config
1797 Systemwide configuration file.
1798 This file provides defaults for those
1799 values that are not specified in the user's configuration file, and
1800 for those users who do not have a configuration file.
1801 This file must be world-readable.
1806 OpenSSH is a derivative of the original and free
1807 ssh 1.2.12 release by Tatu Ylonen.
1808 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1809 Theo de Raadt and Dug Song
1810 removed many bugs, re-added newer features and
1812 Markus Friedl contributed the support for SSH
1813 protocol versions 1.5 and 2.0.
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