3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh_config.5,v 1.111 2008/06/26 11:46:31 grunk Exp $
44 .Nd OpenSSH SSH client configuration files
47 .Nm /etc/ssh/ssh_config
50 obtains configuration data from the following sources in
53 .Bl -enum -offset indent -compact
57 user's configuration file
60 system-wide configuration file
61 .Pq Pa /etc/ssh/ssh_config
64 For each parameter, the first obtained value
66 The configuration files contain sections separated by
68 specifications, and that section is only applied for hosts that
69 match one of the patterns given in the specification.
70 The matched host name is the one given on the command line.
72 Since the first obtained value for each parameter is used, more
73 host-specific declarations should be given near the beginning of the
74 file, and general defaults at the end.
76 The configuration file has the following format:
78 Empty lines and lines starting with
81 Otherwise a line is of the format
82 .Dq keyword arguments .
83 Configuration options may be separated by whitespace or
84 optional whitespace and exactly one
86 the latter format is useful to avoid the need to quote whitespace
87 when specifying configuration options using the
94 Arguments may optionally be enclosed in double quotes
96 in order to represent arguments containing spaces.
99 keywords and their meanings are as follows (note that
100 keywords are case-insensitive and arguments are case-sensitive):
103 Restricts the following declarations (up to the next
105 keyword) to be only for those hosts that match one of the patterns
106 given after the keyword.
107 If more than one pattern is provided, they should be separated by whitepsace.
110 as a pattern can be used to provide global
111 defaults for all hosts.
114 argument given on the command line (i.e. the name is not converted to
115 a canonicalized host name before matching).
119 for more information on patterns.
121 Specifies which address family to use when connecting.
131 passphrase/password querying will be disabled.
132 This option is useful in scripts and other batch jobs where no user
133 is present to supply the password.
141 Use the specified address on the local machine as the source address of
143 Only useful on systems with more than one address.
144 Note that this option does not work if
145 .Cm UsePrivilegedPort
148 .It Cm ChallengeResponseAuthentication
149 Specifies whether to use challenge-response authentication.
150 The argument to this keyword must be
157 If this flag is set to
160 will additionally check the host IP address in the
163 This allows ssh to detect if a host key changed due to DNS spoofing.
164 If the option is set to
166 the check will not be executed.
170 Specifies the cipher to use for encrypting the session
171 in protocol version 1.
179 is only supported in the
181 client for interoperability with legacy protocol 1 implementations
182 that do not support the
185 Its use is strongly discouraged due to cryptographic weaknesses.
189 Specifies the ciphers allowed for protocol version 2
190 in order of preference.
191 Multiple ciphers must be comma-separated.
192 The supported ciphers are
207 .Bd -literal -offset 3n
208 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
209 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
210 aes192-ctr,aes256-ctr
212 .It Cm ClearAllForwardings
213 Specifies that all local, remote, and dynamic port forwardings
214 specified in the configuration files or on the command line be
216 This option is primarily useful when used from the
218 command line to clear port forwardings set in
219 configuration files, and is automatically set by
230 Specifies whether to use compression.
237 .It Cm CompressionLevel
238 Specifies the compression level to use if compression is enabled.
239 The argument must be an integer from 1 (fast) to 9 (slow, best).
240 The default level is 6, which is good for most applications.
241 The meaning of the values is the same as in
243 Note that this option applies to protocol version 1 only.
244 .It Cm ConnectionAttempts
245 Specifies the number of tries (one per second) to make before exiting.
246 The argument must be an integer.
247 This may be useful in scripts if the connection sometimes fails.
249 .It Cm ConnectTimeout
250 Specifies the timeout (in seconds) used when connecting to the
251 SSH server, instead of using the default system TCP timeout.
252 This value is used only when the target is down or really unreachable,
253 not when it refuses the connection.
255 Enables the sharing of multiple sessions over a single network connection.
259 will listen for connections on a control socket specified using the
262 Additional sessions can connect to this socket using the same
269 These sessions will try to reuse the master instance's network connection
270 rather than initiating new ones, but will fall back to connecting normally
271 if the control socket does not exist, or is not listening.
276 to listen for control connections, but require confirmation using the
278 program before they are accepted (see
284 ssh will continue without connecting to a master instance.
288 forwarding is supported over these multiplexed connections, however the
289 display and agent forwarded will be the one belonging to the master
290 connection i.e. it is not possible to forward multiple displays or agents.
292 Two additional options allow for opportunistic multiplexing: try to use a
293 master connection but fall back to creating a new one if one does not already
299 The latter requires confirmation like the
303 Specify the path to the control socket used for connection sharing as described
306 section above or the string
308 to disable connection sharing.
311 will be substituted by the local host name,
313 will be substituted by the target host name,
317 by the remote login username.
318 It is recommended that any
320 used for opportunistic connection sharing include
321 at least %h, %p, and %r.
322 This ensures that shared connections are uniquely identified.
323 .It Cm DynamicForward
324 Specifies that a TCP port on the local machine be forwarded
325 over the secure channel, and the application
326 protocol is then used to determine where to connect to from the
331 .Oo Ar bind_address : Oc Ar port .
333 IPv6 addresses can be specified by enclosing addresses in square brackets or
334 by using an alternative syntax:
335 .Oo Ar bind_address Ns / Oc Ns Ar port .
336 By default, the local port is bound in accordance with the
341 may be used to bind the connection to a specific address.
346 indicates that the listening port be bound for local use only, while an
349 indicates that the port should be available from all interfaces.
351 Currently the SOCKS4 and SOCKS5 protocols are supported, and
353 will act as a SOCKS server.
354 Multiple forwardings may be specified, and
355 additional forwardings can be given on the command line.
356 Only the superuser can forward privileged ports.
357 .It Cm EnableSSHKeysign
358 Setting this option to
360 in the global client configuration file
361 .Pa /etc/ssh/ssh_config
362 enables the use of the helper program
365 .Cm HostbasedAuthentication .
372 This option should be placed in the non-hostspecific section.
375 for more information.
377 Sets the escape character (default:
379 The escape character can also
380 be set on the command line.
381 The argument should be a single character,
383 followed by a letter, or
385 to disable the escape
386 character entirely (making the connection transparent for binary
388 .It Cm ExitOnForwardFailure
391 should terminate the connection if it cannot set up all requested
392 dynamic, tunnel, local, and remote port forwardings.
400 Specifies whether the connection to the authentication agent (if any)
401 will be forwarded to the remote machine.
409 Agent forwarding should be enabled with caution.
410 Users with the ability to bypass file permissions on the remote host
411 (for the agent's Unix-domain socket)
412 can access the local agent through the forwarded connection.
413 An attacker cannot obtain key material from the agent,
414 however they can perform operations on the keys that enable them to
415 authenticate using the identities loaded into the agent.
417 Specifies whether X11 connections will be automatically redirected
418 over the secure channel and
428 X11 forwarding should be enabled with caution.
429 Users with the ability to bypass file permissions on the remote host
430 (for the user's X11 authorization database)
431 can access the local X11 display through the forwarded connection.
432 An attacker may then be able to perform activities such as keystroke monitoring
434 .Cm ForwardX11Trusted
435 option is also enabled.
436 .It Cm ForwardX11Trusted
437 If this option is set to
439 remote X11 clients will have full access to the original X11 display.
441 If this option is set to
443 remote X11 clients will be considered untrusted and prevented
444 from stealing or tampering with data belonging to trusted X11
448 token used for the session will be set to expire after 20 minutes.
449 Remote clients will be refused access after this time.
454 See the X11 SECURITY extension specification for full details on
455 the restrictions imposed on untrusted clients.
457 Specifies whether remote hosts are allowed to connect to local
461 binds local port forwardings to the loopback address.
462 This prevents other remote hosts from connecting to forwarded ports.
464 can be used to specify that ssh
465 should bind local port forwardings to the wildcard address,
466 thus allowing remote hosts to connect to forwarded ports.
473 .It Cm GlobalKnownHostsFile
474 Specifies a file to use for the global
475 host key database instead of
476 .Pa /etc/ssh/ssh_known_hosts .
477 .It Cm GSSAPIAuthentication
478 Specifies whether user authentication based on GSSAPI is allowed.
481 Note that this option applies to protocol version 2 only.
482 .It Cm GSSAPIDelegateCredentials
483 Forward (delegate) credentials to the server.
486 Note that this option applies to protocol version 2 only.
487 .It Cm HashKnownHosts
490 should hash host names and addresses when they are added to
491 .Pa ~/.ssh/known_hosts .
492 These hashed names may be used normally by
496 but they do not reveal identifying information should the file's contents
500 Note that existing names and addresses in known hosts files
501 will not be converted automatically,
502 but may be manually hashed using
504 .It Cm HostbasedAuthentication
505 Specifies whether to try rhosts based authentication with public key
513 This option applies to protocol version 2 only and
515 .Cm RhostsRSAAuthentication .
516 .It Cm HostKeyAlgorithms
517 Specifies the protocol version 2 host key algorithms
518 that the client wants to use in order of preference.
519 The default for this option is:
520 .Dq ssh-dss,ssh-rsa .
522 Specifies an alias that should be used instead of the
523 real host name when looking up or saving the host key
524 in the host key database files.
525 This option is useful for tunneling SSH connections
526 or for multiple servers running on a single host.
528 Specifies the real host name to log into.
529 This can be used to specify nicknames or abbreviations for hosts.
530 The default is the name given on the command line.
531 Numeric IP addresses are also permitted (both on the command line and in
534 .It Cm IdentitiesOnly
537 should only use the authentication identity files configured in the
542 offers more identities.
543 The argument to this keyword must be
547 This option is intended for situations where ssh-agent
548 offers many different identities.
552 Specifies a file from which the user's RSA or DSA authentication identity
556 for protocol version 1, and
560 for protocol version 2.
561 Additionally, any identities represented by the authentication agent
562 will be used for authentication.
564 The file name may use the tilde
565 syntax to refer to a user's home directory or one of the following
568 (local user's home directory),
574 (remote host name) or
578 It is possible to have
579 multiple identity files specified in configuration files; all these
580 identities will be tried in sequence.
581 .It Cm KbdInteractiveAuthentication
582 Specifies whether to use keyboard-interactive authentication.
583 The argument to this keyword must be
589 .It Cm KbdInteractiveDevices
590 Specifies the list of methods to use in keyboard-interactive authentication.
591 Multiple method names must be comma-separated.
592 The default is to use the server specified list.
593 The methods available vary depending on what the server supports.
594 For an OpenSSH server,
595 it may be zero or more of:
601 Specifies a command to execute on the local machine after successfully
602 connecting to the server.
603 The command string extends to the end of the line, and is executed with
605 The following escape character substitutions will be performed:
607 (local user's home directory),
613 (host name as provided on the command line),
617 (remote user name) or
620 This directive is ignored unless
621 .Cm PermitLocalCommand
624 Specifies that a TCP port on the local machine be forwarded over
625 the secure channel to the specified host and port from the remote machine.
626 The first argument must be
628 .Oo Ar bind_address : Oc Ar port
630 and the second argument must be
631 .Ar host : Ns Ar hostport .
632 IPv6 addresses can be specified by enclosing addresses in square brackets or
633 by using an alternative syntax:
634 .Oo Ar bind_address Ns / Oc Ns Ar port
636 .Ar host Ns / Ns Ar hostport .
637 Multiple forwardings may be specified, and additional forwardings can be
638 given on the command line.
639 Only the superuser can forward privileged ports.
640 By default, the local port is bound in accordance with the
645 may be used to bind the connection to a specific address.
650 indicates that the listening port be bound for local use only, while an
653 indicates that the port should be available from all interfaces.
655 Gives the verbosity level that is used when logging messages from
657 The possible values are:
658 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
660 DEBUG and DEBUG1 are equivalent.
661 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
663 Specifies the MAC (message authentication code) algorithms
664 in order of preference.
665 The MAC algorithm is used in protocol version 2
666 for data integrity protection.
667 Multiple algorithms must be comma-separated.
669 .Bd -literal -offset indent
670 hmac-md5,hmac-sha1,umac-64@openssh.com,
671 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
673 .It Cm NoHostAuthenticationForLocalhost
674 This option can be used if the home directory is shared across machines.
675 In this case localhost will refer to a different machine on each of
676 the machines and the user will get many warnings about changed host keys.
677 However, this option disables host authentication for localhost.
678 The argument to this keyword must be
682 The default is to check the host key for localhost.
683 .It Cm NumberOfPasswordPrompts
684 Specifies the number of password prompts before giving up.
685 The argument to this keyword must be an integer.
687 .It Cm PasswordAuthentication
688 Specifies whether to use password authentication.
689 The argument to this keyword must be
695 .It Cm PermitLocalCommand
696 Allow local command execution via the
699 .Ic !\& Ns Ar command
709 Specifies the port number to connect on the remote host.
711 .It Cm PreferredAuthentications
712 Specifies the order in which the client should try protocol 2
713 authentication methods.
714 This allows a client to prefer one method (e.g.\&
715 .Cm keyboard-interactive )
716 over another method (e.g.\&
718 The default for this option is:
719 .Do gssapi-with-mic ,
722 keyboard-interactive,
726 Specifies the protocol versions
728 should support in order of preference.
729 The possible values are
733 Multiple versions must be comma-separated.
737 tries version 2 and falls back to version 1
738 if version 2 is not available.
740 Specifies the command to use to connect to the server.
742 string extends to the end of the line, and is executed with
744 In the command string,
746 will be substituted by the host name to
750 The command can be basically anything,
751 and should read from its standard input and write to its standard output.
752 It should eventually connect an
754 server running on some machine, or execute
757 Host key management will be done using the
758 HostName of the host being connected (defaulting to the name typed by
760 Setting the command to
762 disables this option entirely.
765 is not available for connects with a proxy command.
767 This directive is useful in conjunction with
769 and its proxy support.
770 For example, the following directive would connect via an HTTP proxy at
772 .Bd -literal -offset 3n
773 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
775 .It Cm PubkeyAuthentication
776 Specifies whether to try public key authentication.
777 The argument to this keyword must be
783 This option applies to protocol version 2 only.
785 Specifies the maximum amount of data that may be transmitted before the
786 session key is renegotiated.
787 The argument is the number of bytes, with an optional suffix of
792 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
793 The default is between
797 depending on the cipher.
798 This option applies to protocol version 2 only.
800 Specifies that a TCP port on the remote machine be forwarded over
801 the secure channel to the specified host and port from the local machine.
802 The first argument must be
804 .Oo Ar bind_address : Oc Ar port
806 and the second argument must be
807 .Ar host : Ns Ar hostport .
808 IPv6 addresses can be specified by enclosing addresses in square brackets
809 or by using an alternative syntax:
810 .Oo Ar bind_address Ns / Oc Ns Ar port
812 .Ar host Ns / Ns Ar hostport .
813 Multiple forwardings may be specified, and additional
814 forwardings can be given on the command line.
815 Only the superuser can forward privileged ports.
819 is not specified, the default is to only bind to loopback addresses.
824 or an empty string, then the forwarding is requested to listen on all
828 will only succeed if the server's
830 option is enabled (see
831 .Xr sshd_config 5 ) .
832 .It Cm RhostsRSAAuthentication
833 Specifies whether to try rhosts based authentication with RSA host
841 This option applies to protocol version 1 only and requires
844 .It Cm RSAAuthentication
845 Specifies whether to try RSA authentication.
846 The argument to this keyword must be
850 RSA authentication will only be
851 attempted if the identity file exists, or an authentication agent is
855 Note that this option applies to protocol version 1 only.
857 Specifies what variables from the local
859 should be sent to the server.
860 Note that environment passing is only supported for protocol 2.
861 The server must also support it, and the server must be configured to
862 accept these environment variables.
867 for how to configure the server.
868 Variables are specified by name, which may contain wildcard characters.
869 Multiple environment variables may be separated by whitespace or spread
873 The default is not to send any environment variables.
877 for more information on patterns.
878 .It Cm ServerAliveCountMax
879 Sets the number of server alive messages (see below) which may be
882 receiving any messages back from the server.
883 If this threshold is reached while server alive messages are being sent,
884 ssh will disconnect from the server, terminating the session.
885 It is important to note that the use of server alive messages is very
889 The server alive messages are sent through the encrypted channel
890 and therefore will not be spoofable.
891 The TCP keepalive option enabled by
894 The server alive mechanism is valuable when the client or
895 server depend on knowing when a connection has become inactive.
897 The default value is 3.
899 .Cm ServerAliveInterval
900 (see below) is set to 15 and
901 .Cm ServerAliveCountMax
902 is left at the default, if the server becomes unresponsive,
903 ssh will disconnect after approximately 45 seconds.
904 This option applies to protocol version 2 only.
905 .It Cm ServerAliveInterval
906 Sets a timeout interval in seconds after which if no data has been received
909 will send a message through the encrypted
910 channel to request a response from the server.
912 is 0, indicating that these messages will not be sent to the server.
913 This option applies to protocol version 2 only.
914 .It Cm SmartcardDevice
915 Specifies which smartcard device to use.
916 The argument to this keyword is the device
918 should use to communicate with a smartcard used for storing the user's
920 By default, no device is specified and smartcard support is not activated.
921 .It Cm StrictHostKeyChecking
922 If this flag is set to
925 will never automatically add host keys to the
926 .Pa ~/.ssh/known_hosts
927 file, and refuses to connect to hosts whose host key has changed.
928 This provides maximum protection against trojan horse attacks,
929 though it can be annoying when the
930 .Pa /etc/ssh/ssh_known_hosts
931 file is poorly maintained or when connections to new hosts are
933 This option forces the user to manually
935 If this flag is set to
937 ssh will automatically add new host keys to the
938 user known hosts files.
939 If this flag is set to
942 will be added to the user known host files only after the user
943 has confirmed that is what they really want to do, and
944 ssh will refuse to connect to hosts whose host key has changed.
946 known hosts will be verified automatically in all cases.
955 Specifies whether the system should send TCP keepalive messages to the
957 If they are sent, death of the connection or crash of one
958 of the machines will be properly noticed.
959 However, this means that
960 connections will die if the route is down temporarily, and some people
965 (to send TCP keepalive messages), and the client will notice
966 if the network goes down or the remote host dies.
967 This is important in scripts, and many users want it too.
969 To disable TCP keepalive messages, the value should be set to
974 device forwarding between the client and the server.
985 requests the default tunnel mode, which is
992 devices to open on the client
999 .Ar local_tun Op : Ar remote_tun .
1001 The devices may be specified by numerical ID or the keyword
1003 which uses the next available tunnel device.
1006 is not specified, it defaults to
1010 .It Cm UsePrivilegedPort
1011 Specifies whether to use a privileged port for outgoing connections.
1012 The argument must be
1021 must be setuid root.
1022 Note that this option must be set to
1025 .Cm RhostsRSAAuthentication
1028 Specifies the user to log in as.
1029 This can be useful when a different user name is used on different machines.
1030 This saves the trouble of
1031 having to remember to give the user name on the command line.
1032 .It Cm UserKnownHostsFile
1033 Specifies a file to use for the user
1034 host key database instead of
1035 .Pa ~/.ssh/known_hosts .
1036 .It Cm VerifyHostKeyDNS
1037 Specifies whether to verify the remote key using DNS and SSHFP resource
1039 If this option is set to
1041 the client will implicitly trust keys that match a secure fingerprint
1043 Insecure fingerprints will be handled as if this option was set to
1045 If this option is set to
1047 information on fingerprint match will be displayed, but the user will still
1048 need to confirm new host keys according to the
1049 .Cm StrictHostKeyChecking
1051 The argument must be
1058 Note that this option applies to protocol version 2 only.
1061 .Sx VERIFYING HOST KEYS
1064 .It Cm VersionAddendum
1065 Specifies a string to append to the regular version string to identify
1066 OS- or site-specific modifications.
1068 .Dq FreeBSD-20080901 .
1069 .It Cm VisualHostKey
1070 If this flag is set to
1072 an ASCII art representation of the remote host key fingerprint is
1073 printed additionally to the hex fingerprint string.
1074 If this flag is set to
1076 only the hex fingerprint string will be printed.
1079 .It Cm XAuthLocation
1080 Specifies the full pathname of the
1084 .Pa /usr/local/bin/xauth .
1089 consists of zero or more non-whitespace characters,
1091 (a wildcard that matches zero or more characters),
1094 (a wildcard that matches exactly one character).
1095 For example, to specify a set of declarations for any host in the
1098 the following pattern could be used:
1102 The following pattern
1103 would match any host in the 192.168.0.[0-9] network range:
1105 .Dl Host 192.168.0.?
1109 is a comma-separated list of patterns.
1110 Patterns within pattern-lists may be negated
1111 by preceding them with an exclamation mark
1114 to allow a key to be used from anywhere within an organisation
1118 the following entry (in authorized_keys) could be used:
1120 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1123 .It Pa ~/.ssh/config
1124 This is the per-user configuration file.
1125 The format of this file is described above.
1126 This file is used by the SSH client.
1127 Because of the potential for abuse, this file must have strict permissions:
1128 read/write for the user, and not accessible by others.
1129 .It Pa /etc/ssh/ssh_config
1130 Systemwide configuration file.
1131 This file provides defaults for those
1132 values that are not specified in the user's configuration file, and
1133 for those users who do not have a configuration file.
1134 This file must be world-readable.
1139 OpenSSH is a derivative of the original and free
1140 ssh 1.2.12 release by Tatu Ylonen.
1141 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1142 Theo de Raadt and Dug Song
1143 removed many bugs, re-added newer features and
1145 Markus Friedl contributed the support for SSH
1146 protocol versions 1.5 and 2.0.
projects / FreeBSD / releng / 7.2.git / blob