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 1.236 2016/07/22 07:00:46 djm Exp $
37 .Dd $Mdocdate: July 22 2016 $
42 .Nd OpenSSH SSH client configuration files
45 .Nm /etc/ssh/ssh_config
48 obtains configuration data from the following sources in
51 .Bl -enum -offset indent -compact
55 user's configuration file
58 system-wide configuration file
59 .Pq Pa /etc/ssh/ssh_config
62 For each parameter, the first obtained value
64 The configuration files contain sections separated by
66 specifications, and that section is only applied for hosts that
67 match one of the patterns given in the specification.
68 The matched host name is usually the one given on the command line
70 .Cm CanonicalizeHostname
71 option for exceptions.)
73 Since the first obtained value for each parameter is used, more
74 host-specific declarations should be given near the beginning of the
75 file, and general defaults at the end.
77 The configuration file has the following format:
79 Empty lines and lines starting with
82 Otherwise a line is of the format
83 .Dq keyword arguments .
84 Configuration options may be separated by whitespace or
85 optional whitespace and exactly one
87 the latter format is useful to avoid the need to quote whitespace
88 when specifying configuration options using the
95 Arguments may optionally be enclosed in double quotes
97 in order to represent arguments containing spaces.
100 keywords and their meanings are as follows (note that
101 keywords are case-insensitive and arguments are case-sensitive):
104 Restricts the following declarations (up to the next
108 keyword) to be only for those hosts that match one of the patterns
109 given after the keyword.
110 If more than one pattern is provided, they should be separated by whitespace.
113 as a pattern can be used to provide global
114 defaults for all hosts.
115 The host is usually the
117 argument given on the command line
119 .Cm CanonicalizeHostname
120 option for exceptions.)
122 A pattern entry may be negated by prefixing it with an exclamation mark
124 If a negated entry is matched, then the
126 entry is ignored, regardless of whether any other patterns on the line
128 Negated matches are therefore useful to provide exceptions for wildcard
133 for more information on patterns.
135 Restricts the following declarations (up to the next
139 keyword) to be used only when the conditions following the
141 keyword are satisfied.
142 Match conditions are specified using one or more criteria
145 which always matches.
146 The available criteria keywords are:
156 criteria must appear alone or immediately after
158 Other criteria may be combined arbitrarily.
164 Criteria may be negated by prepending an exclamation mark
169 keyword matches only when the configuration file is being re-parsed
170 after hostname canonicalization (see the
171 .Cm CanonicalizeHostname
173 This may be useful to specify conditions that work with canonical host
177 keyword executes the specified command under the user's shell.
178 If the command returns a zero exit status then the condition is considered true.
179 Commands containing whitespace characters must be quoted.
180 The following character sequences in the command will be expanded prior to
183 will be substituted by the first component of the local host name,
185 will be substituted by the local host name (including any domain name),
187 will be substituted by the target host name,
189 will be substituted by the original target host name
190 specified on the command-line,
192 the destination port,
194 by the remote login username, and
196 by the username of the user running
199 The other keywords' criteria must be single entries or comma-separated
200 lists and may use the wildcard and negation operators described in the
205 keyword are matched against the target hostname, after any substitution
209 .Cm CanonicalizeHostname
213 keyword matches against the hostname as it was specified on the command-line.
216 keyword matches against the target username on the remote host.
219 keyword matches against the name of the local user running
221 (this keyword may be useful in system-wide
224 .It Cm AddKeysToAgent
225 Specifies whether keys should be automatically added to a running
227 If this option is set to
229 and a key is loaded from a file, the key and its passphrase are added to
230 the agent with the default lifetime, as if by
232 If this option is set to
235 will require confirmation using the
237 program before adding a key (see
240 If this option is set to
242 each use of the key must be confirmed, as if the
244 option was specified to
246 If this option is set to
248 no keys are added to the agent.
258 Specifies which address family to use when connecting.
270 passphrase/password querying will be disabled.
271 This option is useful in scripts and other batch jobs where no user
272 is present to supply the password.
280 Use the specified address on the local machine as the source address of
282 Only useful on systems with more than one address.
283 Note that this option does not work if
284 .Cm UsePrivilegedPort
287 .It Cm CanonicalDomains
289 .Cm CanonicalizeHostname
290 is enabled, this option specifies the list of domain suffixes in which to
291 search for the specified destination host.
292 .It Cm CanonicalizeFallbackLocal
293 Specifies whether to fail with an error when hostname canonicalization fails.
296 will attempt to look up the unqualified hostname using the system resolver's
303 .Cm CanonicalizeHostname
304 is enabled and the target hostname cannot be found in any of the domains
306 .Cm CanonicalDomains .
307 .It Cm CanonicalizeHostname
308 Controls whether explicit hostname canonicalization is performed.
311 is not to perform any name rewriting and let the system resolver handle all
315 then, for connections that do not use a
318 will attempt to canonicalize the hostname specified on the command line
322 .Cm CanonicalizePermittedCNAMEs
325 .Cm CanonicalizeHostname
328 then canonicalization is applied to proxied connections too.
330 If this option is enabled, then the configuration files are processed
331 again using the new target name to pick up any new configuration in matching
336 .It Cm CanonicalizeMaxDots
337 Specifies the maximum number of dot characters in a hostname before
338 canonicalization is disabled.
341 allows a single dot (i.e. hostname.subdomain).
342 .It Cm CanonicalizePermittedCNAMEs
343 Specifies rules to determine whether CNAMEs should be followed when
344 canonicalizing hostnames.
345 The rules consist of one or more arguments of
346 .Ar source_domain_list : Ns Ar target_domain_list ,
348 .Ar source_domain_list
349 is a pattern-list of domains that may follow CNAMEs in canonicalization,
351 .Ar target_domain_list
352 is a pattern-list of domains that they may resolve to.
355 .Dq *.a.example.com:*.b.example.com,*.c.example.com
356 will allow hostnames matching
358 to be canonicalized to names in the
363 .It Cm CertificateFile
364 Specifies a file from which the user's certificate is read.
365 A corresponding private key must be provided separately in order
366 to use this certificate either
378 The file name may use the tilde
379 syntax to refer to a user's home directory or one of the following
382 (local user's home directory),
388 (remote host name) or
392 It is possible to have multiple certificate files specified in
393 configuration files; these certificates will be tried in sequence.
396 directives will add to the list of certificates used for
398 .It Cm ChallengeResponseAuthentication
399 Specifies whether to use challenge-response authentication.
400 The argument to this keyword must be
407 If this flag is set to
410 will additionally check the host IP address in the
413 This allows ssh to detect if a host key changed due to DNS spoofing
414 and will add addresses of destination hosts to
415 .Pa ~/.ssh/known_hosts
416 in the process, regardless of the setting of
417 .Cm StrictHostKeyChecking .
418 If the option is set to
420 the check will not be executed.
424 Specifies the cipher to use for encrypting the session
425 in protocol version 1.
433 is only supported in the
435 client for interoperability with legacy protocol 1 implementations
436 that do not support the
439 Its use is strongly discouraged due to cryptographic weaknesses.
443 Specifies the ciphers allowed for protocol version 2
444 in order of preference.
445 Multiple ciphers must be comma-separated.
446 If the specified value begins with a
448 character, then the specified ciphers will be appended to the default set
449 instead of replacing them.
451 The supported ciphers are:
453 .Bl -item -compact -offset indent
469 aes128-gcm@openssh.com
471 aes256-gcm@openssh.com
483 chacha20-poly1305@openssh.com
487 .Bd -literal -offset indent
488 chacha20-poly1305@openssh.com,
489 aes128-ctr,aes192-ctr,aes256-ctr,
490 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
491 aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc
494 The list of available ciphers may also be obtained using the
500 .It Cm ClearAllForwardings
501 Specifies that all local, remote, and dynamic port forwardings
502 specified in the configuration files or on the command line be
504 This option is primarily useful when used from the
506 command line to clear port forwardings set in
507 configuration files, and is automatically set by
518 Specifies whether to use compression.
525 .It Cm CompressionLevel
526 Specifies the compression level to use if compression is enabled.
527 The argument must be an integer from 1 (fast) to 9 (slow, best).
528 The default level is 6, which is good for most applications.
529 The meaning of the values is the same as in
531 Note that this option applies to protocol version 1 only.
532 .It Cm ConnectionAttempts
533 Specifies the number of tries (one per second) to make before exiting.
534 The argument must be an integer.
535 This may be useful in scripts if the connection sometimes fails.
537 .It Cm ConnectTimeout
538 Specifies the timeout (in seconds) used when connecting to the
539 SSH server, instead of using the default system TCP timeout.
540 This value is used only when the target is down or really unreachable,
541 not when it refuses the connection.
543 Enables the sharing of multiple sessions over a single network connection.
547 will listen for connections on a control socket specified using the
550 Additional sessions can connect to this socket using the same
557 These sessions will try to reuse the master instance's network connection
558 rather than initiating new ones, but will fall back to connecting normally
559 if the control socket does not exist, or is not listening.
564 to listen for control connections, but require confirmation using
569 ssh will continue without connecting to a master instance.
573 forwarding is supported over these multiplexed connections, however the
574 display and agent forwarded will be the one belonging to the master
575 connection i.e. it is not possible to forward multiple displays or agents.
577 Two additional options allow for opportunistic multiplexing: try to use a
578 master connection but fall back to creating a new one if one does not already
584 The latter requires confirmation like the
588 Specify the path to the control socket used for connection sharing as described
591 section above or the string
593 to disable connection sharing.
596 will be substituted by the first component of the local host name,
598 will be substituted by the local host name (including any domain name),
600 will be substituted by the target host name,
602 will be substituted by the original target host name
603 specified on the command line,
605 the destination port,
607 by the remote login username,
611 by the numeric user ID (uid) of the user running
615 by a hash of the concatenation: %l%h%p%r.
616 It is recommended that any
618 used for opportunistic connection sharing include
619 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
620 that is not writable by other users.
621 This ensures that shared connections are uniquely identified.
622 .It Cm ControlPersist
623 When used in conjunction with
625 specifies that the master connection should remain open
626 in the background (waiting for future client connections)
627 after the initial client connection has been closed.
630 then the master connection will not be placed into the background,
631 and will close as soon as the initial client connection is closed.
636 then the master connection will remain in the background indefinitely
637 (until killed or closed via a mechanism such as the
641 If set to a time in seconds, or a time in any of the formats documented in
643 then the backgrounded master connection will automatically terminate
644 after it has remained idle (with no client connections) for the
646 .It Cm DynamicForward
647 Specifies that a TCP port on the local machine be forwarded
648 over the secure channel, and the application
649 protocol is then used to determine where to connect to from the
654 .Oo Ar bind_address : Oc Ar port .
656 IPv6 addresses can be specified by enclosing addresses in square brackets.
657 By default, the local port is bound in accordance with the
662 may be used to bind the connection to a specific address.
667 indicates that the listening port be bound for local use only, while an
670 indicates that the port should be available from all interfaces.
672 Currently the SOCKS4 and SOCKS5 protocols are supported, and
674 will act as a SOCKS server.
675 Multiple forwardings may be specified, and
676 additional forwardings can be given on the command line.
677 Only the superuser can forward privileged ports.
678 .It Cm EnableSSHKeysign
679 Setting this option to
681 in the global client configuration file
682 .Pa /etc/ssh/ssh_config
683 enables the use of the helper program
686 .Cm HostbasedAuthentication .
693 This option should be placed in the non-hostspecific section.
696 for more information.
698 Sets the escape character (default:
700 The escape character can also
701 be set on the command line.
702 The argument should be a single character,
704 followed by a letter, or
706 to disable the escape
707 character entirely (making the connection transparent for binary
709 .It Cm ExitOnForwardFailure
712 should terminate the connection if it cannot set up all requested
713 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
714 if either end is unable to bind and listen on a specified port).
716 .Cm ExitOnForwardFailure
717 does not apply to connections made over port forwardings and will not,
720 to exit if TCP connections to the ultimate forwarding destination fail.
727 .It Cm FingerprintHash
728 Specifies the hash algorithm used when displaying key fingerprints.
736 Specifies whether the connection to the authentication agent (if any)
737 will be forwarded to the remote machine.
745 Agent forwarding should be enabled with caution.
746 Users with the ability to bypass file permissions on the remote host
747 (for the agent's Unix-domain socket)
748 can access the local agent through the forwarded connection.
749 An attacker cannot obtain key material from the agent,
750 however they can perform operations on the keys that enable them to
751 authenticate using the identities loaded into the agent.
753 Specifies whether X11 connections will be automatically redirected
754 over the secure channel and
764 X11 forwarding should be enabled with caution.
765 Users with the ability to bypass file permissions on the remote host
766 (for the user's X11 authorization database)
767 can access the local X11 display through the forwarded connection.
768 An attacker may then be able to perform activities such as keystroke monitoring
770 .Cm ForwardX11Trusted
771 option is also enabled.
772 .It Cm ForwardX11Timeout
773 Specify a timeout for untrusted X11 forwarding
774 using the format described in the
775 TIME FORMATS section of
777 X11 connections received by
779 after this time will be refused.
780 The default is to disable untrusted X11 forwarding after twenty minutes has
782 .It Cm ForwardX11Trusted
783 If this option is set to
785 remote X11 clients will have full access to the original X11 display.
787 If this option is set to
789 remote X11 clients will be considered untrusted and prevented
790 from stealing or tampering with data belonging to trusted X11
794 token used for the session will be set to expire after 20 minutes.
795 Remote clients will be refused access after this time.
800 See the X11 SECURITY extension specification for full details on
801 the restrictions imposed on untrusted clients.
803 Specifies whether remote hosts are allowed to connect to local
807 binds local port forwardings to the loopback address.
808 This prevents other remote hosts from connecting to forwarded ports.
810 can be used to specify that ssh
811 should bind local port forwardings to the wildcard address,
812 thus allowing remote hosts to connect to forwarded ports.
819 .It Cm GlobalKnownHostsFile
820 Specifies one or more files to use for the global
821 host key database, separated by whitespace.
823 .Pa /etc/ssh/ssh_known_hosts ,
824 .Pa /etc/ssh/ssh_known_hosts2 .
825 .It Cm GSSAPIAuthentication
826 Specifies whether user authentication based on GSSAPI is allowed.
829 .It Cm GSSAPIDelegateCredentials
830 Forward (delegate) credentials to the server.
833 .It Cm HashKnownHosts
836 should hash host names and addresses when they are added to
837 .Pa ~/.ssh/known_hosts .
838 These hashed names may be used normally by
842 but they do not reveal identifying information should the file's contents
846 Note that existing names and addresses in known hosts files
847 will not be converted automatically,
848 but may be manually hashed using
850 .It Cm HostbasedAuthentication
851 Specifies whether to try rhosts based authentication with public key
859 .It Cm HostbasedKeyTypes
860 Specifies the key types that will be used for hostbased authentication
861 as a comma-separated pattern list.
862 Alternately if the specified value begins with a
864 character, then the specified key types will be appended to the default set
865 instead of replacing them.
866 The default for this option is:
867 .Bd -literal -offset 3n
868 ecdsa-sha2-nistp256-cert-v01@openssh.com,
869 ecdsa-sha2-nistp384-cert-v01@openssh.com,
870 ecdsa-sha2-nistp521-cert-v01@openssh.com,
871 ssh-ed25519-cert-v01@openssh.com,
872 ssh-rsa-cert-v01@openssh.com,
873 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
881 may be used to list supported key types.
882 .It Cm HostKeyAlgorithms
883 Specifies the host key algorithms
884 that the client wants to use in order of preference.
885 Alternately if the specified value begins with a
887 character, then the specified key types will be appended to the default set
888 instead of replacing them.
889 The default for this option is:
890 .Bd -literal -offset 3n
891 ecdsa-sha2-nistp256-cert-v01@openssh.com,
892 ecdsa-sha2-nistp384-cert-v01@openssh.com,
893 ecdsa-sha2-nistp521-cert-v01@openssh.com,
894 ssh-ed25519-cert-v01@openssh.com,
895 ssh-rsa-cert-v01@openssh.com,
896 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
900 If hostkeys are known for the destination host then this default is modified
901 to prefer their algorithms.
903 The list of available key types may also be obtained using the
910 Specifies an alias that should be used instead of the
911 real host name when looking up or saving the host key
912 in the host key database files.
913 This option is useful for tunneling SSH connections
914 or for multiple servers running on a single host.
916 Specifies the real host name to log into.
917 This can be used to specify nicknames or abbreviations for hosts.
918 If the hostname contains the character sequence
920 then this will be replaced with the host name specified on the command line
921 (this is useful for manipulating unqualified names).
922 The character sequence
924 will be replaced by a single
926 character, which may be used when specifying IPv6 link-local addresses.
928 The default is the name given on the command line.
929 Numeric IP addresses are also permitted (both on the command line and in
932 .It Cm IdentitiesOnly
935 should only use the authentication identity and certificate files explicitly
946 offers more identities.
947 The argument to this keyword must be
951 This option is intended for situations where ssh-agent
952 offers many different identities.
958 socket used to communicate with the authentication agent.
960 This option overrides the
962 environment variable and can be used to select a specific agent.
963 Setting the socket name to
965 disables the use of an authentication agent.
968 is specified, the location of the socket will be read from the
970 environment variable.
972 The socket name may use the tilde
973 syntax to refer to a user's home directory or one of the following
976 (local user's home directory),
982 (remote host name) or
986 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
990 for protocol version 1, and
992 .Pa ~/.ssh/id_ecdsa ,
993 .Pa ~/.ssh/id_ed25519
996 for protocol version 2.
997 Additionally, any identities represented by the authentication agent
998 will be used for authentication unless
1001 If no certificates have been explicitly specified by
1002 .Cm CertificateFile ,
1004 will try to load certificate information from the filename obtained by
1007 to the path of a specified
1010 The file name may use the tilde
1011 syntax to refer to a user's home directory or one of the following
1014 (local user's home directory),
1020 (remote host name) or
1024 It is possible to have
1025 multiple identity files specified in configuration files; all these
1026 identities will be tried in sequence.
1029 directives will add to the list of identities tried (this behaviour
1030 differs from that of other configuration directives).
1033 may be used in conjunction with
1035 to select which identities in an agent are offered during authentication.
1037 may also be used in conjunction with
1039 in order to provide any certificate also needed for authentication with
1041 .It Cm IgnoreUnknown
1042 Specifies a pattern-list of unknown options to be ignored if they are
1043 encountered in configuration parsing.
1044 This may be used to suppress errors if
1046 contains options that are unrecognised by
1048 It is recommended that
1050 be listed early in the configuration file as it will not be applied
1051 to unknown options that appear before it.
1053 Include the specified configuration file(s).
1054 Multiple pathnames may be specified and each pathname may contain
1056 wildcards and, for user configurations, shell-like
1058 references to user home directories.
1059 Files without absolute paths are assumed to be in
1061 if included in a user configuration file or
1063 if included from the system configuration file.
1065 directive may appear inside a
1070 to perform conditional inclusion.
1072 Specifies the IPv4 type-of-service or DSCP class for connections.
1099 This option may take one or two arguments, separated by whitespace.
1100 If one argument is specified, it is used as the packet class unconditionally.
1101 If two values are specified, the first is automatically selected for
1102 interactive sessions and the second for non-interactive sessions.
1105 for interactive sessions and
1107 for non-interactive sessions.
1108 .It Cm KbdInteractiveAuthentication
1109 Specifies whether to use keyboard-interactive authentication.
1110 The argument to this keyword must be
1116 .It Cm KbdInteractiveDevices
1117 Specifies the list of methods to use in keyboard-interactive authentication.
1118 Multiple method names must be comma-separated.
1119 The default is to use the server specified list.
1120 The methods available vary depending on what the server supports.
1121 For an OpenSSH server,
1122 it may be zero or more of:
1127 .It Cm KexAlgorithms
1128 Specifies the available KEX (Key Exchange) algorithms.
1129 Multiple algorithms must be comma-separated.
1130 Alternately if the specified value begins with a
1132 character, then the specified methods will be appended to the default set
1133 instead of replacing them.
1135 .Bd -literal -offset indent
1136 curve25519-sha256@libssh.org,
1137 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1138 diffie-hellman-group-exchange-sha256,
1139 diffie-hellman-group-exchange-sha1,
1140 diffie-hellman-group14-sha1
1143 The list of available key exchange algorithms may also be obtained using the
1150 Specifies a command to execute on the local machine after successfully
1151 connecting to the server.
1152 The command string extends to the end of the line, and is executed with
1154 The following escape character substitutions will be performed:
1156 (local user's home directory),
1162 (host name as provided on the command line),
1166 (remote user name) or
1168 (local user name) or
1170 by a hash of the concatenation: %l%h%p%r.
1172 The command is run synchronously and does not have access to the
1176 It should not be used for interactive commands.
1178 This directive is ignored unless
1179 .Cm PermitLocalCommand
1182 Specifies that a TCP port on the local machine be forwarded over
1183 the secure channel to the specified host and port from the remote machine.
1184 The first argument must be
1186 .Oo Ar bind_address : Oc Ar port
1188 and the second argument must be
1189 .Ar host : Ns Ar hostport .
1190 IPv6 addresses can be specified by enclosing addresses in square brackets.
1191 Multiple forwardings may be specified, and additional forwardings can be
1192 given on the command line.
1193 Only the superuser can forward privileged ports.
1194 By default, the local port is bound in accordance with the
1197 However, an explicit
1199 may be used to bind the connection to a specific address.
1204 indicates that the listening port be bound for local use only, while an
1207 indicates that the port should be available from all interfaces.
1209 Gives the verbosity level that is used when logging messages from
1211 The possible values are:
1212 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1213 The default is INFO.
1214 DEBUG and DEBUG1 are equivalent.
1215 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1217 Specifies the MAC (message authentication code) algorithms
1218 in order of preference.
1219 The MAC algorithm is used for data integrity protection.
1220 Multiple algorithms must be comma-separated.
1221 If the specified value begins with a
1223 character, then the specified algorithms will be appended to the default set
1224 instead of replacing them.
1226 The algorithms that contain
1228 calculate the MAC after encryption (encrypt-then-mac).
1229 These are considered safer and their use recommended.
1232 .Bd -literal -offset indent
1233 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1234 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1235 hmac-sha1-etm@openssh.com,
1236 umac-64@openssh.com,umac-128@openssh.com,
1237 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1240 The list of available MAC algorithms may also be obtained using the
1246 .It Cm NoHostAuthenticationForLocalhost
1247 This option can be used if the home directory is shared across machines.
1248 In this case localhost will refer to a different machine on each of
1249 the machines and the user will get many warnings about changed host keys.
1250 However, this option disables host authentication for localhost.
1251 The argument to this keyword must be
1255 The default is to check the host key for localhost.
1256 .It Cm NumberOfPasswordPrompts
1257 Specifies the number of password prompts before giving up.
1258 The argument to this keyword must be an integer.
1260 .It Cm PasswordAuthentication
1261 Specifies whether to use password authentication.
1262 The argument to this keyword must be
1268 .It Cm PermitLocalCommand
1269 Allow local command execution via the
1272 .Ic !\& Ns Ar command
1275 The argument must be
1281 .It Cm PKCS11Provider
1282 Specifies which PKCS#11 provider to use.
1283 The argument to this keyword is the PKCS#11 shared library
1285 should use to communicate with a PKCS#11 token providing the user's
1288 Specifies the port number to connect on the remote host.
1290 .It Cm PreferredAuthentications
1291 Specifies the order in which the client should try authentication methods.
1292 This allows a client to prefer one method (e.g.\&
1293 .Cm keyboard-interactive )
1294 over another method (e.g.\&
1297 .Bd -literal -offset indent
1298 gssapi-with-mic,hostbased,publickey,
1299 keyboard-interactive,password
1302 Specifies the protocol versions
1304 should support in order of preference.
1305 The possible values are
1309 Multiple versions must be comma-separated.
1310 When this option is set to
1313 will try version 2 and fall back to version 1
1314 if version 2 is not available.
1317 Protocol 1 suffers from a number of cryptographic weaknesses and should
1319 It is only offered to support legacy devices.
1321 Specifies the command to use to connect to the server.
1323 string extends to the end of the line, and is executed
1324 using the user's shell
1326 directive to avoid a lingering shell process.
1328 In the command string, any occurrence of
1330 will be substituted by the host name to
1335 by the remote user name.
1336 The command can be basically anything,
1337 and should read from its standard input and write to its standard output.
1338 It should eventually connect an
1340 server running on some machine, or execute
1343 Host key management will be done using the
1344 HostName of the host being connected (defaulting to the name typed by
1346 Setting the command to
1348 disables this option entirely.
1351 is not available for connects with a proxy command.
1353 This directive is useful in conjunction with
1355 and its proxy support.
1356 For example, the following directive would connect via an HTTP proxy at
1358 .Bd -literal -offset 3n
1359 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1362 Specifies one or more jump proxies as
1370 Multiple proxies may be separated by comma characters and will be visited
1372 Setting this option will cause
1374 to connect to the target host by first making a
1376 connection to the specified
1378 host and then establishing a
1379 TCP forwarding to the ultimate target from there.
1381 Note that this option will compete with the
1383 option - whichever is specified first will prevent later instances of the
1384 other from taking effect.
1385 .It Cm ProxyUseFdpass
1388 will pass a connected file descriptor back to
1390 instead of continuing to execute and pass data.
1393 .It Cm PubkeyAcceptedKeyTypes
1394 Specifies the key types that will be used for public key authentication
1395 as a comma-separated pattern list.
1396 Alternately if the specified value begins with a
1398 character, then the key types after it will be appended to the default
1399 instead of replacing it.
1400 The default for this option is:
1401 .Bd -literal -offset 3n
1402 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1403 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1404 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1405 ssh-ed25519-cert-v01@openssh.com,
1406 ssh-rsa-cert-v01@openssh.com,
1407 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1415 may be used to list supported key types.
1416 .It Cm PubkeyAuthentication
1417 Specifies whether to try public key authentication.
1418 The argument to this keyword must be
1425 Specifies the maximum amount of data that may be transmitted before the
1426 session key is renegotiated, optionally followed a maximum amount of
1427 time that may pass before the session key is renegotiated.
1428 The first argument is specified in bytes and may have a suffix of
1433 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1434 The default is between
1438 depending on the cipher.
1439 The optional second value is specified in seconds and may use any of the
1440 units documented in the
1441 TIME FORMATS section of
1443 The default value for
1447 which means that rekeying is performed after the cipher's default amount
1448 of data has been sent or received and no time based rekeying is done.
1449 .It Cm RemoteForward
1450 Specifies that a TCP port on the remote machine be forwarded over
1451 the secure channel to the specified host and port from the local machine.
1452 The first argument must be
1454 .Oo Ar bind_address : Oc Ar port
1456 and the second argument must be
1457 .Ar host : Ns Ar hostport .
1458 IPv6 addresses can be specified by enclosing addresses in square brackets.
1459 Multiple forwardings may be specified, and additional
1460 forwardings can be given on the command line.
1461 Privileged ports can be forwarded only when
1462 logging in as root on the remote machine.
1468 the listen port will be dynamically allocated on the server and reported
1469 to the client at run time.
1473 is not specified, the default is to only bind to loopback addresses.
1478 or an empty string, then the forwarding is requested to listen on all
1482 will only succeed if the server's
1484 option is enabled (see
1485 .Xr sshd_config 5 ) .
1487 Specifies whether to request a pseudo-tty for the session.
1488 The argument may be one of:
1490 (never request a TTY),
1492 (always request a TTY when standard input is a TTY),
1494 (always request a TTY) or
1496 (request a TTY when opening a login session).
1497 This option mirrors the
1503 .It Cm RevokedHostKeys
1504 Specifies revoked host public keys.
1505 Keys listed in this file will be refused for host authentication.
1506 Note that if this file does not exist or is not readable,
1507 then host authentication will be refused for all hosts.
1508 Keys may be specified as a text file, listing one public key per line, or as
1509 an OpenSSH Key Revocation List (KRL) as generated by
1511 For more information on KRLs, see the KEY REVOCATION LISTS section in
1513 .It Cm RhostsRSAAuthentication
1514 Specifies whether to try rhosts based authentication with RSA host
1516 The argument must be
1522 This option applies to protocol version 1 only and requires
1525 .It Cm RSAAuthentication
1526 Specifies whether to try RSA authentication.
1527 The argument to this keyword must be
1531 RSA authentication will only be
1532 attempted if the identity file exists, or an authentication agent is
1536 Note that this option applies to protocol version 1 only.
1538 Specifies what variables from the local
1540 should be sent to the server.
1541 The server must also support it, and the server must be configured to
1542 accept these environment variables.
1545 environment variable is always sent whenever a
1546 pseudo-terminal is requested as it is required by the protocol.
1551 for how to configure the server.
1552 Variables are specified by name, which may contain wildcard characters.
1553 Multiple environment variables may be separated by whitespace or spread
1557 The default is not to send any environment variables.
1561 for more information on patterns.
1562 .It Cm ServerAliveCountMax
1563 Sets the number of server alive messages (see below) which may be
1566 receiving any messages back from the server.
1567 If this threshold is reached while server alive messages are being sent,
1568 ssh will disconnect from the server, terminating the session.
1569 It is important to note that the use of server alive messages is very
1573 The server alive messages are sent through the encrypted channel
1574 and therefore will not be spoofable.
1575 The TCP keepalive option enabled by
1578 The server alive mechanism is valuable when the client or
1579 server depend on knowing when a connection has become inactive.
1581 The default value is 3.
1583 .Cm ServerAliveInterval
1584 (see below) is set to 15 and
1585 .Cm ServerAliveCountMax
1586 is left at the default, if the server becomes unresponsive,
1587 ssh will disconnect after approximately 45 seconds.
1588 .It Cm ServerAliveInterval
1589 Sets a timeout interval in seconds after which if no data has been received
1592 will send a message through the encrypted
1593 channel to request a response from the server.
1595 is 0, indicating that these messages will not be sent to the server.
1596 .It Cm StreamLocalBindMask
1597 Sets the octal file creation mode mask
1599 used when creating a Unix-domain socket file for local or remote
1601 This option is only used for port forwarding to a Unix-domain socket file.
1603 The default value is 0177, which creates a Unix-domain socket file that is
1604 readable and writable only by the owner.
1605 Note that not all operating systems honor the file mode on Unix-domain
1607 .It Cm StreamLocalBindUnlink
1608 Specifies whether to remove an existing Unix-domain socket file for local
1609 or remote port forwarding before creating a new one.
1610 If the socket file already exists and
1611 .Cm StreamLocalBindUnlink
1614 will be unable to forward the port to the Unix-domain socket file.
1615 This option is only used for port forwarding to a Unix-domain socket file.
1617 The argument must be
1623 .It Cm StrictHostKeyChecking
1624 If this flag is set to
1627 will never automatically add host keys to the
1628 .Pa ~/.ssh/known_hosts
1629 file, and refuses to connect to hosts whose host key has changed.
1630 This provides maximum protection against trojan horse attacks,
1631 though it can be annoying when the
1632 .Pa /etc/ssh/ssh_known_hosts
1633 file is poorly maintained or when connections to new hosts are
1635 This option forces the user to manually
1637 If this flag is set to
1639 ssh will automatically add new host keys to the
1640 user known hosts files.
1641 If this flag is set to
1644 will be added to the user known host files only after the user
1645 has confirmed that is what they really want to do, and
1646 ssh will refuse to connect to hosts whose host key has changed.
1648 known hosts will be verified automatically in all cases.
1649 The argument must be
1657 Specifies whether the system should send TCP keepalive messages to the
1659 If they are sent, death of the connection or crash of one
1660 of the machines will be properly noticed.
1661 However, this means that
1662 connections will die if the route is down temporarily, and some people
1667 (to send TCP keepalive messages), and the client will notice
1668 if the network goes down or the remote host dies.
1669 This is important in scripts, and many users want it too.
1671 To disable TCP keepalive messages, the value should be set to
1676 device forwarding between the client and the server.
1677 The argument must be
1687 requests the default tunnel mode, which is
1688 .Dq point-to-point .
1694 devices to open on the client
1699 The argument must be
1701 .Ar local_tun Op : Ar remote_tun .
1703 The devices may be specified by numerical ID or the keyword
1705 which uses the next available tunnel device.
1708 is not specified, it defaults to
1712 .It Cm UpdateHostKeys
1715 should accept notifications of additional hostkeys from the server sent
1716 after authentication has completed and add them to
1717 .Cm UserKnownHostsFile .
1718 The argument must be
1723 Enabling this option allows learning alternate hostkeys for a server
1724 and supports graceful key rotation by allowing a server to send replacement
1725 public keys before old ones are removed.
1726 Additional hostkeys are only accepted if the key used to authenticate the
1727 host was already trusted or explicitly accepted by the user.
1732 then the user is asked to confirm the modifications to the known_hosts file.
1733 Confirmation is currently incompatible with
1734 .Cm ControlPersist ,
1735 and will be disabled if it is enabled.
1739 from OpenSSH 6.8 and greater support the
1740 .Dq hostkeys@openssh.com
1741 protocol extension used to inform the client of all the server's hostkeys.
1742 .It Cm UsePrivilegedPort
1743 Specifies whether to use a privileged port for outgoing connections.
1744 The argument must be
1753 must be setuid root.
1754 Note that this option must be set to
1757 .Cm RhostsRSAAuthentication
1760 Specifies the user to log in as.
1761 This can be useful when a different user name is used on different machines.
1762 This saves the trouble of
1763 having to remember to give the user name on the command line.
1764 .It Cm UserKnownHostsFile
1765 Specifies one or more files to use for the user
1766 host key database, separated by whitespace.
1768 .Pa ~/.ssh/known_hosts ,
1769 .Pa ~/.ssh/known_hosts2 .
1770 .It Cm VerifyHostKeyDNS
1771 Specifies whether to verify the remote key using DNS and SSHFP resource
1773 If this option is set to
1775 the client will implicitly trust keys that match a secure fingerprint
1777 Insecure fingerprints will be handled as if this option was set to
1779 If this option is set to
1781 information on fingerprint match will be displayed, but the user will still
1782 need to confirm new host keys according to the
1783 .Cm StrictHostKeyChecking
1785 The argument must be
1793 See also VERIFYING HOST KEYS in
1795 .It Cm VisualHostKey
1796 If this flag is set to
1798 an ASCII art representation of the remote host key fingerprint is
1799 printed in addition to the fingerprint string at login and
1800 for unknown host keys.
1801 If this flag is set to
1803 no fingerprint strings are printed at login and
1804 only the fingerprint string will be printed for unknown host keys.
1807 .It Cm XAuthLocation
1808 Specifies the full pathname of the
1812 .Pa /usr/local/bin/xauth .
1817 consists of zero or more non-whitespace characters,
1819 (a wildcard that matches zero or more characters),
1822 (a wildcard that matches exactly one character).
1823 For example, to specify a set of declarations for any host in the
1826 the following pattern could be used:
1830 The following pattern
1831 would match any host in the 192.168.0.[0-9] network range:
1833 .Dl Host 192.168.0.?
1837 is a comma-separated list of patterns.
1838 Patterns within pattern-lists may be negated
1839 by preceding them with an exclamation mark
1842 to allow a key to be used from anywhere within an organization
1846 the following entry (in authorized_keys) could be used:
1848 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1851 .It Pa ~/.ssh/config
1852 This is the per-user configuration file.
1853 The format of this file is described above.
1854 This file is used by the SSH client.
1855 Because of the potential for abuse, this file must have strict permissions:
1856 read/write for the user, and not accessible by others.
1857 .It Pa /etc/ssh/ssh_config
1858 Systemwide configuration file.
1859 This file provides defaults for those
1860 values that are not specified in the user's configuration file, and
1861 for those users who do not have a configuration file.
1862 This file must be world-readable.
1867 OpenSSH is a derivative of the original and free
1868 ssh 1.2.12 release by Tatu Ylonen.
1869 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1870 Theo de Raadt and Dug Song
1871 removed many bugs, re-added newer features and
1873 Markus Friedl contributed the support for SSH
1874 protocol versions 1.5 and 2.0.
projects / dragonfly.git / blob