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 1.89 2006/03/14 16:32:48 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client configuration files
45 .Bl -tag -width Ds -compact
47 .It Pa /etc/ssh/ssh_config
51 obtains configuration data from the following sources in
54 .Bl -enum -offset indent -compact
58 user's configuration file
61 system-wide configuration file
62 .Pq Pa /etc/ssh/ssh_config
65 For each parameter, the first obtained value
67 The configuration files contain sections separated by
69 specifications, and that section is only applied for hosts that
70 match one of the patterns given in the specification.
71 The matched host name is the one given on the command line.
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
106 keyword) to be only for those hosts that match one of the patterns
107 given after the keyword.
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
389 Specifies whether the connection to the authentication agent (if any)
390 will be forwarded to the remote machine.
398 Agent forwarding should be enabled with caution.
399 Users with the ability to bypass file permissions on the remote host
400 (for the agent's Unix-domain socket)
401 can access the local agent through the forwarded connection.
402 An attacker cannot obtain key material from the agent,
403 however they can perform operations on the keys that enable them to
404 authenticate using the identities loaded into the agent.
406 Specifies whether X11 connections will be automatically redirected
407 over the secure channel and
417 X11 forwarding should be enabled with caution.
418 Users with the ability to bypass file permissions on the remote host
419 (for the user's X11 authorization database)
420 can access the local X11 display through the forwarded connection.
421 An attacker may then be able to perform activities such as keystroke monitoring
423 .Cm ForwardX11Trusted
424 option is also enabled.
425 .It Cm ForwardX11Trusted
426 If this option is set to
428 remote X11 clients will have full access to the original X11 display.
430 If this option is set to
432 remote X11 clients will be considered untrusted and prevented
433 from stealing or tampering with data belonging to trusted X11
437 token used for the session will be set to expire after 20 minutes.
438 Remote clients will be refused access after this time.
443 See the X11 SECURITY extension specification for full details on
444 the restrictions imposed on untrusted clients.
446 Specifies whether remote hosts are allowed to connect to local
450 binds local port forwardings to the loopback address.
451 This prevents other remote hosts from connecting to forwarded ports.
453 can be used to specify that ssh
454 should bind local port forwardings to the wildcard address,
455 thus allowing remote hosts to connect to forwarded ports.
462 .It Cm GlobalKnownHostsFile
463 Specifies a file to use for the global
464 host key database instead of
465 .Pa /etc/ssh/ssh_known_hosts .
466 .It Cm GSSAPIAuthentication
467 Specifies whether user authentication based on GSSAPI is allowed.
470 Note that this option applies to protocol version 2 only.
471 .It Cm GSSAPIDelegateCredentials
472 Forward (delegate) credentials to the server.
475 Note that this option applies to protocol version 2 only.
476 .It Cm HashKnownHosts
479 should hash host names and addresses when they are added to
480 .Pa ~/.ssh/known_hosts .
481 These hashed names may be used normally by
485 but they do not reveal identifying information should the file's contents
489 Note that hashing of names and addresses will not be retrospectively applied
490 to existing known hosts files, but these may be manually hashed using
492 .It Cm HostbasedAuthentication
493 Specifies whether to try rhosts based authentication with public key
501 This option applies to protocol version 2 only and
503 .Cm RhostsRSAAuthentication .
504 .It Cm HostKeyAlgorithms
505 Specifies the protocol version 2 host key algorithms
506 that the client wants to use in order of preference.
507 The default for this option is:
508 .Dq ssh-rsa,ssh-dss .
510 Specifies an alias that should be used instead of the
511 real host name when looking up or saving the host key
512 in the host key database files.
513 This option is useful for tunneling SSH connections
514 or for multiple servers running on a single host.
516 Specifies the real host name to log into.
517 This can be used to specify nicknames or abbreviations for hosts.
518 The default is the name given on the command line.
519 Numeric IP addresses are also permitted (both on the command line and in
522 .It Cm IdentitiesOnly
525 should only use the authentication identity files configured in the
530 offers more identities.
531 The argument to this keyword must be
535 This option is intended for situations where ssh-agent
536 offers many different identities.
540 Specifies a file from which the user's RSA or DSA authentication identity
544 for protocol version 1, and
548 for protocol version 2.
549 Additionally, any identities represented by the authentication agent
550 will be used for authentication.
551 The file name may use the tilde
552 syntax to refer to a user's home directory.
553 It is possible to have
554 multiple identity files specified in configuration files; all these
555 identities will be tried in sequence.
556 .It Cm KbdInteractiveDevices
557 Specifies the list of methods to use in keyboard-interactive authentication.
558 Multiple method names must be comma-separated.
559 The default is to use the server specified list.
560 The methods available vary depending on what the server supports.
561 For an OpenSSH server,
562 it may be zero or more of:
568 Specifies a command to execute on the local machine after successfully
569 connecting to the server.
570 The command string extends to the end of the line, and is executed with
572 This directive is ignored unless
573 .Cm PermitLocalCommand
576 Specifies that a TCP port on the local machine be forwarded over
577 the secure channel to the specified host and port from the remote machine.
578 The first argument must be
580 .Oo Ar bind_address : Oc Ar port
582 and the second argument must be
583 .Ar host : Ns Ar hostport .
584 IPv6 addresses can be specified by enclosing addresses in square brackets or
585 by using an alternative syntax:
586 .Oo Ar bind_address Ns / Oc Ns Ar port
588 .Ar host Ns / Ns Ar hostport .
589 Multiple forwardings may be specified, and additional forwardings can be
590 given on the command line.
591 Only the superuser can forward privileged ports.
592 By default, the local port is bound in accordance with the
597 may be used to bind the connection to a specific address.
602 indicates that the listening port be bound for local use only, while an
605 indicates that the port should be available from all interfaces.
607 Gives the verbosity level that is used when logging messages from
609 The possible values are:
610 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
612 DEBUG and DEBUG1 are equivalent.
613 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
615 Specifies the MAC (message authentication code) algorithms
616 in order of preference.
617 The MAC algorithm is used in protocol version 2
618 for data integrity protection.
619 Multiple algorithms must be comma-separated.
621 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
622 .It Cm NoHostAuthenticationForLocalhost
623 This option can be used if the home directory is shared across machines.
624 In this case localhost will refer to a different machine on each of
625 the machines and the user will get many warnings about changed host keys.
626 However, this option disables host authentication for localhost.
627 The argument to this keyword must be
631 The default is to check the host key for localhost.
632 .It Cm NumberOfPasswordPrompts
633 Specifies the number of password prompts before giving up.
634 The argument to this keyword must be an integer.
636 .It Cm PasswordAuthentication
637 Specifies whether to use password authentication.
638 The argument to this keyword must be
644 .It Cm PermitLocalCommand
645 Allow local command execution via the
648 .Ic !\& Ns Ar command
658 Specifies the port number to connect on the remote host.
660 .It Cm PreferredAuthentications
661 Specifies the order in which the client should try protocol 2
662 authentication methods.
663 This allows a client to prefer one method (e.g.\&
664 .Cm keyboard-interactive )
665 over another method (e.g.\&
667 The default for this option is:
668 .Dq hostbased,publickey,keyboard-interactive,password .
670 Specifies the protocol versions
672 should support in order of preference.
673 The possible values are
677 Multiple versions must be comma-separated.
681 tries version 2 and falls back to version 1
682 if version 2 is not available.
684 Specifies the command to use to connect to the server.
686 string extends to the end of the line, and is executed with
688 In the command string,
690 will be substituted by the host name to
694 The command can be basically anything,
695 and should read from its standard input and write to its standard output.
696 It should eventually connect an
698 server running on some machine, or execute
701 Host key management will be done using the
702 HostName of the host being connected (defaulting to the name typed by
704 Setting the command to
706 disables this option entirely.
709 is not available for connects with a proxy command.
711 This directive is useful in conjunction with
713 and its proxy support.
714 For example, the following directive would connect via an HTTP proxy at
716 .Bd -literal -offset 3n
717 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
719 .It Cm PubkeyAuthentication
720 Specifies whether to try public key authentication.
721 The argument to this keyword must be
727 This option applies to protocol version 2 only.
729 Specifies the maximum amount of data that may be transmitted before the
730 session key is renegotiated.
731 The argument is the number of bytes, with an optional suffix of
736 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
737 The default is between
741 depending on the cipher.
742 This option applies to protocol version 2 only.
744 Specifies that a TCP port on the remote machine be forwarded over
745 the secure channel to the specified host and port from the local machine.
746 The first argument must be
748 .Oo Ar bind_address : Oc Ar port
750 and the second argument must be
751 .Ar host : Ns Ar hostport .
752 IPv6 addresses can be specified by enclosing addresses in square brackets
753 or by using an alternative syntax:
754 .Oo Ar bind_address Ns / Oc Ns Ar port
756 .Ar host Ns / Ns Ar hostport .
757 Multiple forwardings may be specified, and additional
758 forwardings can be given on the command line.
759 Only the superuser can forward privileged ports.
763 is not specified, the default is to only bind to loopback addresses.
768 or an empty string, then the forwarding is requested to listen on all
772 will only succeed if the server's
774 option is enabled (see
775 .Xr sshd_config 5 ) .
776 .It Cm RhostsRSAAuthentication
777 Specifies whether to try rhosts based authentication with RSA host
785 This option applies to protocol version 1 only and requires
788 .It Cm RSAAuthentication
789 Specifies whether to try RSA authentication.
790 The argument to this keyword must be
794 RSA authentication will only be
795 attempted if the identity file exists, or an authentication agent is
799 Note that this option applies to protocol version 1 only.
801 Specifies what variables from the local
803 should be sent to the server.
804 Note that environment passing is only supported for protocol 2.
805 The server must also support it, and the server must be configured to
806 accept these environment variables.
811 for how to configure the server.
812 Variables are specified by name, which may contain wildcard characters.
813 Multiple environment variables may be separated by whitespace or spread
817 The default is not to send any environment variables.
821 for more information on patterns.
822 .It Cm ServerAliveCountMax
823 Sets the number of server alive messages (see below) which may be
826 receiving any messages back from the server.
827 If this threshold is reached while server alive messages are being sent,
828 ssh will disconnect from the server, terminating the session.
829 It is important to note that the use of server alive messages is very
833 The server alive messages are sent through the encrypted channel
834 and therefore will not be spoofable.
835 The TCP keepalive option enabled by
838 The server alive mechanism is valuable when the client or
839 server depend on knowing when a connection has become inactive.
841 The default value is 3.
843 .Cm ServerAliveInterval
844 (see below) is set to 15 and
845 .Cm ServerAliveCountMax
846 is left at the default, if the server becomes unresponsive,
847 ssh will disconnect after approximately 45 seconds.
848 This option applies to protocol version 2 only.
849 .It Cm ServerAliveInterval
850 Sets a timeout interval in seconds after which if no data has been received
853 will send a message through the encrypted
854 channel to request a response from the server.
856 is 0, indicating that these messages will not be sent to the server.
857 This option applies to protocol version 2 only.
858 .It Cm SmartcardDevice
859 Specifies which smartcard device to use.
860 The argument to this keyword is the device
862 should use to communicate with a smartcard used for storing the user's
864 By default, no device is specified and smartcard support is not activated.
865 .It Cm StrictHostKeyChecking
866 If this flag is set to
869 will never automatically add host keys to the
870 .Pa ~/.ssh/known_hosts
871 file, and refuses to connect to hosts whose host key has changed.
872 This provides maximum protection against trojan horse attacks,
873 though it can be annoying when the
874 .Pa /etc/ssh/ssh_known_hosts
875 file is poorly maintained or when connections to new hosts are
877 This option forces the user to manually
879 If this flag is set to
881 ssh will automatically add new host keys to the
882 user known hosts files.
883 If this flag is set to
886 will be added to the user known host files only after the user
887 has confirmed that is what they really want to do, and
888 ssh will refuse to connect to hosts whose host key has changed.
890 known hosts will be verified automatically in all cases.
899 Specifies whether the system should send TCP keepalive messages to the
901 If they are sent, death of the connection or crash of one
902 of the machines will be properly noticed.
903 However, this means that
904 connections will die if the route is down temporarily, and some people
909 (to send TCP keepalive messages), and the client will notice
910 if the network goes down or the remote host dies.
911 This is important in scripts, and many users want it too.
913 To disable TCP keepalive messages, the value should be set to
918 device forwarding between the client and the server.
919 This option also allows requesting layer 2 (ethernet)
920 instead of layer 3 (point-to-point) tunneling from the server.
932 device on the client.
933 Without this option, the next available device will be used.
934 .It Cm UsePrivilegedPort
935 Specifies whether to use a privileged port for outgoing connections.
946 Note that this option must be set to
949 .Cm RhostsRSAAuthentication
952 Specifies the user to log in as.
953 This can be useful when a different user name is used on different machines.
954 This saves the trouble of
955 having to remember to give the user name on the command line.
956 .It Cm UserKnownHostsFile
957 Specifies a file to use for the user
958 host key database instead of
959 .Pa ~/.ssh/known_hosts .
960 .It Cm VerifyHostKeyDNS
961 Specifies whether to verify the remote key using DNS and SSHFP resource
963 If this option is set to
965 the client will implicitly trust keys that match a secure fingerprint
967 Insecure fingerprints will be handled as if this option was set to
969 If this option is set to
971 information on fingerprint match will be displayed, but the user will still
972 need to confirm new host keys according to the
973 .Cm StrictHostKeyChecking
982 Note that this option applies to protocol version 2 only.
985 .Sx VERIFYING HOST KEYS
989 Specifies the full pathname of the
993 .Pa /usr/X11R6/bin/xauth .
998 consists of zero or more non-whitespace characters,
1000 (a wildcard that matches zero or more characters),
1003 (a wildcard that matches exactly one character).
1004 For example, to specify a set of declarations for any host in the
1007 the following pattern could be used:
1011 The following pattern
1012 would match any host in the 192.168.0.[0-9] network range:
1014 .Dl Host 192.168.0.?
1018 is a comma-separated list of patterns.
1019 Patterns within pattern-lists may be negated
1020 by preceding them with an exclamation mark
1023 to allow a key to be used from anywhere within an organisation
1027 the following entry (in authorized_keys) could be used:
1029 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1032 .It Pa ~/.ssh/config
1033 This is the per-user configuration file.
1034 The format of this file is described above.
1035 This file is used by the SSH client.
1036 Because of the potential for abuse, this file must have strict permissions:
1037 read/write for the user, and not accessible by others.
1038 .It Pa /etc/ssh/ssh_config
1039 Systemwide configuration file.
1040 This file provides defaults for those
1041 values that are not specified in the user's configuration file, and
1042 for those users who do not have a configuration file.
1043 This file must be world-readable.
1048 OpenSSH is a derivative of the original and free
1049 ssh 1.2.12 release by Tatu Ylonen.
1050 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1051 Theo de Raadt and Dug Song
1052 removed many bugs, re-added newer features and
1054 Markus Friedl contributed the support for SSH
1055 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob