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 1.98 2007/01/10 13:23:22 jmc Exp $
38 .Dd September 25, 1999
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 GSSAPI configuration file
60 .Pq Pa $HOME/.ssh/config.gssapi
62 Kerberos configuration file
63 .Pq Pa $HOME/.ssh/config.krb
65 system-wide configuration file
66 .Pq Pa /etc/ssh/ssh_config
69 For each parameter, the first obtained value
71 The configuration files contain sections separated by
73 specifications, and that section is only applied for hosts that
74 match one of the patterns given in the specification.
75 The matched host name is the one given on the command line.
77 Since the first obtained value for each parameter is used, more
78 host-specific declarations should be given near the beginning of the
79 file, and general defaults at the end.
81 The configuration file has the following format:
83 Empty lines and lines starting with
86 Otherwise a line is of the format
87 .Dq keyword arguments .
88 Configuration options may be separated by whitespace or
89 optional whitespace and exactly one
91 the latter format is useful to avoid the need to quote whitespace
92 when specifying configuration options using the
99 Arguments may optionally be enclosed in double quotes
101 in order to represent arguments containing spaces.
104 keywords and their meanings are as follows (note that
105 keywords are case-insensitive and arguments are case-sensitive):
108 Restricts the following declarations (up to the next
110 keyword) to be only for those hosts that match one of the patterns
111 given after the keyword.
114 as a pattern can be used to provide global
115 defaults for all hosts.
118 argument given on the command line (i.e. the name is not converted to
119 a canonicalized host name before matching).
123 for more information on patterns.
125 Specifies which address family to use when connecting.
135 passphrase/password querying will be disabled.
136 This option is useful in scripts and other batch jobs where no user
137 is present to supply the password.
145 Use the specified address on the local machine as the source address of
147 Only useful on systems with more than one address.
148 Note that this option does not work if
149 .Cm UsePrivilegedPort
152 .It Cm ChallengeResponseAuthentication
153 Specifies whether to use challenge-response authentication.
154 The argument to this keyword must be
161 If this flag is set to
164 will additionally check the host IP address in the
167 This allows ssh to detect if a host key changed due to DNS spoofing.
168 If the option is set to
170 the check will not be executed.
174 Specifies the cipher to use for encrypting the session
175 in protocol version 1.
183 is only supported in the
185 client for interoperability with legacy protocol 1 implementations
186 that do not support the
189 Its use is strongly discouraged due to cryptographic weaknesses.
193 Specifies the ciphers allowed for protocol version 2
194 in order of preference.
195 Multiple ciphers must be comma-separated.
196 The supported ciphers are
211 .Bd -literal -offset 3n
212 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
213 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
214 aes192-ctr,aes256-ctr
216 .It Cm ClearAllForwardings
217 Specifies that all local, remote, and dynamic port forwardings
218 specified in the configuration files or on the command line be
220 This option is primarily useful when used from the
222 command line to clear port forwardings set in
223 configuration files, and is automatically set by
234 Specifies whether to use compression.
241 .It Cm CompressionLevel
242 Specifies the compression level to use if compression is enabled.
243 The argument must be an integer from 1 (fast) to 9 (slow, best).
244 The default level is 6, which is good for most applications.
245 The meaning of the values is the same as in
247 Note that this option applies to protocol version 1 only.
248 .It Cm ConnectionAttempts
249 Specifies the number of tries (one per second) to make before exiting.
250 The argument must be an integer.
251 This may be useful in scripts if the connection sometimes fails.
253 .It Cm ConnectTimeout
254 Specifies the timeout (in seconds) used when connecting to the
255 SSH server, instead of using the default system TCP timeout.
256 This value is used only when the target is down or really unreachable,
257 not when it refuses the connection.
259 Enables the sharing of multiple sessions over a single network connection.
263 will listen for connections on a control socket specified using the
266 Additional sessions can connect to this socket using the same
273 These sessions will try to reuse the master instance's network connection
274 rather than initiating new ones, but will fall back to connecting normally
275 if the control socket does not exist, or is not listening.
280 to listen for control connections, but require confirmation using the
282 program before they are accepted (see
288 ssh will continue without connecting to a master instance.
292 forwarding is supported over these multiplexed connections, however the
293 display and agent forwarded will be the one belonging to the master
294 connection i.e. it is not possible to forward multiple displays or agents.
296 Two additional options allow for opportunistic multiplexing: try to use a
297 master connection but fall back to creating a new one if one does not already
303 The latter requires confirmation like the
307 Specify the path to the control socket used for connection sharing as described
310 section above or the string
312 to disable connection sharing.
315 will be substituted by the local host name,
317 will be substituted by the target host name,
321 by the remote login username.
322 It is recommended that any
324 used for opportunistic connection sharing include
325 at least %h, %p, and %r.
326 This ensures that shared connections are uniquely identified.
327 .It Cm DynamicForward
328 Specifies that a TCP port on the local machine be forwarded
329 over the secure channel, and the application
330 protocol is then used to determine where to connect to from the
335 .Oo Ar bind_address : Oc Ar port .
337 IPv6 addresses can be specified by enclosing addresses in square brackets or
338 by using an alternative syntax:
339 .Oo Ar bind_address Ns / Oc Ns Ar port .
340 By default, the local port is bound in accordance with the
345 may be used to bind the connection to a specific address.
350 indicates that the listening port be bound for local use only, while an
353 indicates that the port should be available from all interfaces.
355 Currently the SOCKS4 and SOCKS5 protocols are supported, and
357 will act as a SOCKS server.
358 Multiple forwardings may be specified, and
359 additional forwardings can be given on the command line.
360 Only the superuser can forward privileged ports.
361 .It Cm EnableSSHKeysign
362 Setting this option to
364 in the global client configuration file
365 .Pa /etc/ssh/ssh_config
366 enables the use of the helper program
369 .Cm HostbasedAuthentication .
376 This option should be placed in the non-hostspecific section.
379 for more information.
381 Sets the escape character (default:
383 The escape character can also
384 be set on the command line.
385 The argument should be a single character,
387 followed by a letter, or
389 to disable the escape
390 character entirely (making the connection transparent for binary
392 .It Cm ExitOnForwardFailure
395 should terminate the connection if it cannot set up all requested
396 dynamic, local, and remote port forwardings.
404 Specifies whether the connection to the authentication agent (if any)
405 will be forwarded to the remote machine.
413 Agent forwarding should be enabled with caution.
414 Users with the ability to bypass file permissions on the remote host
415 (for the agent's Unix-domain socket)
416 can access the local agent through the forwarded connection.
417 An attacker cannot obtain key material from the agent,
418 however they can perform operations on the keys that enable them to
419 authenticate using the identities loaded into the agent.
421 Specifies whether X11 connections will be automatically redirected
422 over the secure channel and
432 X11 forwarding should be enabled with caution.
433 Users with the ability to bypass file permissions on the remote host
434 (for the user's X11 authorization database)
435 can access the local X11 display through the forwarded connection.
436 An attacker may then be able to perform activities such as keystroke monitoring
438 .Cm ForwardX11Trusted
439 option is also enabled.
440 .It Cm ForwardX11Trusted
441 If this option is set to
443 remote X11 clients will have full access to the original X11 display.
445 If this option is set to
447 remote X11 clients will be considered untrusted and prevented
448 from stealing or tampering with data belonging to trusted X11
452 token used for the session will be set to expire after 20 minutes.
453 Remote clients will be refused access after this time.
458 See the X11 SECURITY extension specification for full details on
459 the restrictions imposed on untrusted clients.
461 Specifies whether remote hosts are allowed to connect to local
465 binds local port forwardings to the loopback address.
466 This prevents other remote hosts from connecting to forwarded ports.
468 can be used to specify that ssh
469 should bind local port forwardings to the wildcard address,
470 thus allowing remote hosts to connect to forwarded ports.
477 .It Cm GlobalKnownHostsFile
478 Specifies a file to use for the global
479 host key database instead of
480 .Pa /etc/ssh/ssh_known_hosts .
481 .It Cm GSSAPIAuthentication
482 Specifies whether user authentication based on GSSAPI is allowed.
485 Note that this option applies to protocol version 2 only.
486 .It Cm GSSAPIKeyExchange
487 Specifies whether key exchange based on GSSAPI may be used. When using
488 GSSAPI key exchange the server need not have a host key.
491 Note that this option applies to protocol version 2 only.
492 .It Cm GSSAPIDelegateCredentials
493 Forward (delegate) credentials to the server.
496 Note that this option applies to protocol version 2 only.
497 .It Cm GSSAPITrustDns
500 to indicate that the DNS is trusted to securely canonicalize
501 the name of the host being connected to. If
503 the hostname entered on the
504 command line will be passed untouched to the GSSAPI library.
507 This option only applies to protocol version 2 connections using GSSAPI.
508 .It Cm HashKnownHosts
511 should hash host names and addresses when they are added to
512 .Pa ~/.ssh/known_hosts .
513 These hashed names may be used normally by
517 but they do not reveal identifying information should the file's contents
521 Note that existing names and addresses in known hosts files
522 will not be converted automatically,
523 but may be manually hashed using
525 .It Cm HostbasedAuthentication
526 Specifies whether to try rhosts based authentication with public key
534 This option applies to protocol version 2 only and
536 .Cm RhostsRSAAuthentication .
537 .It Cm HostKeyAlgorithms
538 Specifies the protocol version 2 host key algorithms
539 that the client wants to use in order of preference.
540 The default for this option is:
541 .Dq ssh-rsa,ssh-dss .
543 Specifies an alias that should be used instead of the
544 real host name when looking up or saving the host key
545 in the host key database files.
546 This option is useful for tunneling SSH connections
547 or for multiple servers running on a single host.
549 Specifies the real host name to log into.
550 This can be used to specify nicknames or abbreviations for hosts.
551 The default is the name given on the command line.
552 Numeric IP addresses are also permitted (both on the command line and in
555 .It Cm IdentitiesOnly
558 should only use the authentication identity files configured in the
563 offers more identities.
564 The argument to this keyword must be
568 This option is intended for situations where ssh-agent
569 offers many different identities.
573 Specifies a file from which the user's RSA or DSA authentication identity
577 for protocol version 1, and
581 for protocol version 2.
582 Additionally, any identities represented by the authentication agent
583 will be used for authentication.
585 The file name may use the tilde
586 syntax to refer to a user's home directory or one of the following
589 (local user's home directory),
595 (remote host name) or
599 It is possible to have
600 multiple identity files specified in configuration files; all these
601 identities will be tried in sequence.
602 .It Cm KbdInteractiveDevices
603 Specifies the list of methods to use in keyboard-interactive authentication.
604 Multiple method names must be comma-separated.
605 The default is to use the server specified list.
606 The methods available vary depending on what the server supports.
607 For an OpenSSH server,
608 it may be zero or more of:
614 Specifies a command to execute on the local machine after successfully
615 connecting to the server.
616 The command string extends to the end of the line, and is executed with
618 This directive is ignored unless
619 .Cm PermitLocalCommand
622 Specifies that a TCP port on the local machine be forwarded over
623 the secure channel to the specified host and port from the remote machine.
624 The first argument must be
626 .Oo Ar bind_address : Oc Ar port
628 and the second argument must be
629 .Ar host : Ns Ar hostport .
630 IPv6 addresses can be specified by enclosing addresses in square brackets or
631 by using an alternative syntax:
632 .Oo Ar bind_address Ns / Oc Ns Ar port
634 .Ar host Ns / Ns Ar hostport .
635 Multiple forwardings may be specified, and additional forwardings can be
636 given on the command line.
637 Only the superuser can forward privileged ports.
638 By default, the local port is bound in accordance with the
643 may be used to bind the connection to a specific address.
648 indicates that the listening port be bound for local use only, while an
651 indicates that the port should be available from all interfaces.
653 Gives the verbosity level that is used when logging messages from
655 The possible values are:
656 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
658 DEBUG and DEBUG1 are equivalent.
659 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
661 Specifies the MAC (message authentication code) algorithms
662 in order of preference.
663 The MAC algorithm is used in protocol version 2
664 for data integrity protection.
665 Multiple algorithms must be comma-separated.
667 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
668 .It Cm NoHostAuthenticationForLocalhost
669 This option can be used if the home directory is shared across machines.
670 In this case localhost will refer to a different machine on each of
671 the machines and the user will get many warnings about changed host keys.
672 However, this option disables host authentication for localhost.
673 The argument to this keyword must be
677 The default is to check the host key for localhost.
678 .It Cm NumberOfPasswordPrompts
679 Specifies the number of password prompts before giving up.
680 The argument to this keyword must be an integer.
682 .It Cm PasswordAuthentication
683 Specifies whether to use password authentication.
684 The argument to this keyword must be
690 .It Cm PermitLocalCommand
691 Allow local command execution via the
694 .Ic !\& Ns Ar command
704 Specifies the port number to connect on the remote host.
706 .It Cm PreferredAuthentications
707 Specifies the order in which the client should try protocol 2
708 authentication methods.
709 This allows a client to prefer one method (e.g.\&
710 .Cm keyboard-interactive )
711 over another method (e.g.\&
713 The default for this option is:
719 keyboard-interactive,
723 Specifies the protocol versions
725 should support in order of preference.
726 The possible values are
730 Multiple versions must be comma-separated.
734 tries version 2 and falls back to version 1
735 if version 2 is not available.
737 Specifies the command to use to connect to the server.
739 string extends to the end of the line, and is executed with
741 In the command string,
743 will be substituted by the host name to
747 The command can be basically anything,
748 and should read from its standard input and write to its standard output.
749 It should eventually connect an
751 server running on some machine, or execute
754 Host key management will be done using the
755 HostName of the host being connected (defaulting to the name typed by
757 Setting the command to
759 disables this option entirely.
762 is not available for connects with a proxy command.
764 This directive is useful in conjunction with
766 and its proxy support.
767 For example, the following directive would connect via an HTTP proxy at
769 .Bd -literal -offset 3n
770 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
772 .It Cm PubkeyAuthentication
773 Specifies whether to try public key authentication.
774 The argument to this keyword must be
780 This option applies to protocol version 2 only.
782 Specifies the maximum amount of data that may be transmitted before the
783 session key is renegotiated.
784 The argument is the number of bytes, with an optional suffix of
789 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
790 The default is between
794 depending on the cipher.
795 This option applies to protocol version 2 only.
797 Specifies that a TCP port on the remote machine be forwarded over
798 the secure channel to the specified host and port from the local machine.
799 The first argument must be
801 .Oo Ar bind_address : Oc Ar port
803 and the second argument must be
804 .Ar host : Ns Ar hostport .
805 IPv6 addresses can be specified by enclosing addresses in square brackets
806 or by using an alternative syntax:
807 .Oo Ar bind_address Ns / Oc Ns Ar port
809 .Ar host Ns / Ns Ar hostport .
810 Multiple forwardings may be specified, and additional
811 forwardings can be given on the command line.
812 Only the superuser can forward privileged ports.
816 is not specified, the default is to only bind to loopback addresses.
821 or an empty string, then the forwarding is requested to listen on all
825 will only succeed if the server's
827 option is enabled (see
828 .Xr sshd_config 5 ) .
829 .It Cm RhostsRSAAuthentication
830 Specifies whether to try rhosts based authentication with RSA host
838 This option applies to protocol version 1 only and requires
841 .It Cm RSAAuthentication
842 Specifies whether to try RSA authentication.
843 The argument to this keyword must be
847 RSA authentication will only be
848 attempted if the identity file exists, or an authentication agent is
852 Note that this option applies to protocol version 1 only.
854 Specifies what variables from the local
856 should be sent to the server.
857 Note that environment passing is only supported for protocol 2.
858 The server must also support it, and the server must be configured to
859 accept these environment variables.
864 for how to configure the server.
865 Variables are specified by name, which may contain wildcard characters.
866 Multiple environment variables may be separated by whitespace or spread
870 The default is not to send any environment variables.
874 for more information on patterns.
875 .It Cm ServerAliveCountMax
876 Sets the number of server alive messages (see below) which may be
879 receiving any messages back from the server.
880 If this threshold is reached while server alive messages are being sent,
881 ssh will disconnect from the server, terminating the session.
882 It is important to note that the use of server alive messages is very
886 The server alive messages are sent through the encrypted channel
887 and therefore will not be spoofable.
888 The TCP keepalive option enabled by
891 The server alive mechanism is valuable when the client or
892 server depend on knowing when a connection has become inactive.
894 The default value is 3.
896 .Cm ServerAliveInterval
897 (see below) is set to 15 and
898 .Cm ServerAliveCountMax
899 is left at the default, if the server becomes unresponsive,
900 ssh will disconnect after approximately 45 seconds.
901 This option applies to protocol version 2 only.
902 .It Cm ServerAliveInterval
903 Sets a timeout interval in seconds after which if no data has been received
906 will send a message through the encrypted
907 channel to request a response from the server.
909 is 0, indicating that these messages will not be sent to the server.
910 This option applies to protocol version 2 only.
911 .It Cm SmartcardDevice
912 Specifies which smartcard device to use.
913 The argument to this keyword is the device
915 should use to communicate with a smartcard used for storing the user's
917 By default, no device is specified and smartcard support is not activated.
918 .It Cm StrictHostKeyChecking
919 If this flag is set to
922 will never automatically add host keys to the
923 .Pa ~/.ssh/known_hosts
924 file, and refuses to connect to hosts whose host key has changed.
925 This provides maximum protection against trojan horse attacks,
926 though it can be annoying when the
927 .Pa /etc/ssh/ssh_known_hosts
928 file is poorly maintained or when connections to new hosts are
930 This option forces the user to manually
932 If this flag is set to
934 ssh will automatically add new host keys to the
935 user known hosts files.
936 If this flag is set to
939 will be added to the user known host files only after the user
940 has confirmed that is what they really want to do, and
941 ssh will refuse to connect to hosts whose host key has changed.
943 known hosts will be verified automatically in all cases.
952 Specifies whether the system should send TCP keepalive messages to the
954 If they are sent, death of the connection or crash of one
955 of the machines will be properly noticed.
956 However, this means that
957 connections will die if the route is down temporarily, and some people
962 (to send TCP keepalive messages), and the client will notice
963 if the network goes down or the remote host dies.
964 This is important in scripts, and many users want it too.
966 To disable TCP keepalive messages, the value should be set to
971 device forwarding between the client and the server.
982 requests the default tunnel mode, which is
989 devices to open on the client
996 .Ar local_tun Op : Ar remote_tun .
998 The devices may be specified by numerical ID or the keyword
1000 which uses the next available tunnel device.
1003 is not specified, it defaults to
1007 .It Cm UsePrivilegedPort
1008 Specifies whether to use a privileged port for outgoing connections.
1009 The argument must be
1018 must be setuid root.
1019 Note that this option must be set to
1022 .Cm RhostsRSAAuthentication
1025 Specifies the user to log in as.
1026 This can be useful when a different user name is used on different machines.
1027 This saves the trouble of
1028 having to remember to give the user name on the command line.
1029 .It Cm UserKnownHostsFile
1030 Specifies a file to use for the user
1031 host key database instead of
1032 .Pa ~/.ssh/known_hosts .
1033 .It Cm VerifyHostKeyDNS
1034 Specifies whether to verify the remote key using DNS and SSHFP resource
1036 If this option is set to
1038 the client will implicitly trust keys that match a secure fingerprint
1040 Insecure fingerprints will be handled as if this option was set to
1042 If this option is set to
1044 information on fingerprint match will be displayed, but the user will still
1045 need to confirm new host keys according to the
1046 .Cm StrictHostKeyChecking
1048 The argument must be
1055 Note that this option applies to protocol version 2 only.
1058 .Sx VERIFYING HOST KEYS
1061 .It Cm XAuthLocation
1062 Specifies the full pathname of the
1066 .Pa /usr/X11R6/bin/xauth .
1071 consists of zero or more non-whitespace characters,
1073 (a wildcard that matches zero or more characters),
1076 (a wildcard that matches exactly one character).
1077 For example, to specify a set of declarations for any host in the
1080 the following pattern could be used:
1084 The following pattern
1085 would match any host in the 192.168.0.[0-9] network range:
1087 .Dl Host 192.168.0.?
1091 is a comma-separated list of patterns.
1092 Patterns within pattern-lists may be negated
1093 by preceding them with an exclamation mark
1096 to allow a key to be used from anywhere within an organisation
1100 the following entry (in authorized_keys) could be used:
1102 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1105 .It Pa ~/.ssh/config
1106 This is the per-user configuration file.
1107 The format of this file is described above.
1108 This file is used by the SSH client.
1109 Because of the potential for abuse, this file must have strict permissions:
1110 read/write for the user, and not accessible by others.
1111 .It Pa /etc/ssh/ssh_config
1112 Systemwide configuration file.
1113 This file provides defaults for those
1114 values that are not specified in the user's configuration file, and
1115 for those users who do not have a configuration file.
1116 This file must be world-readable.
1121 OpenSSH is a derivative of the original and free
1122 ssh 1.2.12 release by Tatu Ylonen.
1123 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1124 Theo de Raadt and Dug Song
1125 removed many bugs, re-added newer features and
1127 Markus Friedl contributed the support for SSH
1128 protocol versions 1.5 and 2.0.
andersk / gssapi-openssh.git / blob