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 1.86 2006/02/26 17:17:18 jmc 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
97 keywords and their meanings are as follows (note that
98 keywords are case-insensitive and arguments are case-sensitive):
101 Restricts the following declarations (up to the next
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
107 as a pattern can be used to provide global
108 defaults for all hosts.
111 argument given on the command line (i.e. the name is not converted to
112 a canonicalized host name before matching).
116 for more information on patterns.
118 Specifies which address family to use when connecting.
128 passphrase/password querying will be disabled.
129 This option is useful in scripts and other batch jobs where no user
130 is present to supply the password.
138 Use the specified address on the local machine as the source address of
140 Only useful on systems with more than one address.
141 Note that this option does not work if
142 .Cm UsePrivilegedPort
145 .It Cm ChallengeResponseAuthentication
146 Specifies whether to use challenge-response authentication.
147 The argument to this keyword must be
154 If this flag is set to
157 will additionally check the host IP address in the
160 This allows ssh to detect if a host key changed due to DNS spoofing.
161 If the option is set to
163 the check will not be executed.
167 Specifies the cipher to use for encrypting the session
168 in protocol version 1.
176 is only supported in the
178 client for interoperability with legacy protocol 1 implementations
179 that do not support the
182 Its use is strongly discouraged due to cryptographic weaknesses.
186 Specifies the ciphers allowed for protocol version 2
187 in order of preference.
188 Multiple ciphers must be comma-separated.
189 The supported ciphers are
204 .Bd -literal -offset 3n
205 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
206 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
207 aes192-ctr,aes256-ctr
209 .It Cm ClearAllForwardings
210 Specifies that all local, remote, and dynamic port forwardings
211 specified in the configuration files or on the command line be
213 This option is primarily useful when used from the
215 command line to clear port forwardings set in
216 configuration files, and is automatically set by
227 Specifies whether to use compression.
234 .It Cm CompressionLevel
235 Specifies the compression level to use if compression is enabled.
236 The argument must be an integer from 1 (fast) to 9 (slow, best).
237 The default level is 6, which is good for most applications.
238 The meaning of the values is the same as in
240 Note that this option applies to protocol version 1 only.
241 .It Cm ConnectionAttempts
242 Specifies the number of tries (one per second) to make before exiting.
243 The argument must be an integer.
244 This may be useful in scripts if the connection sometimes fails.
246 .It Cm ConnectTimeout
247 Specifies the timeout (in seconds) used when connecting to the
248 SSH server, instead of using the default system TCP timeout.
249 This value is used only when the target is down or really unreachable,
250 not when it refuses the connection.
252 Enables the sharing of multiple sessions over a single network connection.
256 will listen for connections on a control socket specified using the
259 Additional sessions can connect to this socket using the same
266 These sessions will try to reuse the master instance's network connection
267 rather than initiating new ones, but will fall back to connecting normally
268 if the control socket does not exist, or is not listening.
273 to listen for control connections, but require confirmation using the
275 program before they are accepted (see
281 ssh will continue without connecting to a master instance.
285 forwarding is supported over these multiplexed connections, however the
286 display and agent forwarded will be the one belonging to the master
287 connection i.e. it is not possible to forward multiple displays or agents.
289 Two additional options allow for opportunistic multiplexing: try to use a
290 master connection but fall back to creating a new one if one does not already
296 The latter requires confirmation like the
300 Specify the path to the control socket used for connection sharing as described
303 section above or the string
305 to disable connection sharing.
308 will be substituted by the local host name,
310 will be substituted by the target host name,
314 by the remote login username.
315 It is recommended that any
317 used for opportunistic connection sharing include
318 at least %h, %p, and %r.
319 This ensures that shared connections are uniquely identified.
320 .It Cm DynamicForward
321 Specifies that a TCP port on the local machine be forwarded
322 over the secure channel, and the application
323 protocol is then used to determine where to connect to from the
328 .Oo Ar bind_address : Oc Ar port .
330 IPv6 addresses can be specified by enclosing addresses in square brackets or
331 by using an alternative syntax:
332 .Oo Ar bind_address Ns / Oc Ns Ar port .
333 By default, the local port is bound in accordance with the
338 may be used to bind the connection to a specific address.
343 indicates that the listening port be bound for local use only, while an
346 indicates that the port should be available from all interfaces.
348 Currently the SOCKS4 and SOCKS5 protocols are supported, and
350 will act as a SOCKS server.
351 Multiple forwardings may be specified, and
352 additional forwardings can be given on the command line.
353 Only the superuser can forward privileged ports.
354 .It Cm EnableSSHKeysign
355 Setting this option to
357 in the global client configuration file
358 .Pa /etc/ssh/ssh_config
359 enables the use of the helper program
362 .Cm HostbasedAuthentication .
369 This option should be placed in the non-hostspecific section.
372 for more information.
374 Sets the escape character (default:
376 The escape character can also
377 be set on the command line.
378 The argument should be a single character,
380 followed by a letter, or
382 to disable the escape
383 character entirely (making the connection transparent for binary
386 Specifies whether the connection to the authentication agent (if any)
387 will be forwarded to the remote machine.
395 Agent forwarding should be enabled with caution.
396 Users with the ability to bypass file permissions on the remote host
397 (for the agent's Unix-domain socket)
398 can access the local agent through the forwarded connection.
399 An attacker cannot obtain key material from the agent,
400 however they can perform operations on the keys that enable them to
401 authenticate using the identities loaded into the agent.
403 Specifies whether X11 connections will be automatically redirected
404 over the secure channel and
414 X11 forwarding should be enabled with caution.
415 Users with the ability to bypass file permissions on the remote host
416 (for the user's X11 authorization database)
417 can access the local X11 display through the forwarded connection.
418 An attacker may then be able to perform activities such as keystroke monitoring
420 .Cm ForwardX11Trusted
421 option is also enabled.
422 .It Cm ForwardX11Trusted
423 If this option is set to
425 remote X11 clients will have full access to the original X11 display.
427 If this option is set to
429 remote X11 clients will be considered untrusted and prevented
430 from stealing or tampering with data belonging to trusted X11
434 token used for the session will be set to expire after 20 minutes.
435 Remote clients will be refused access after this time.
440 See the X11 SECURITY extension specification for full details on
441 the restrictions imposed on untrusted clients.
443 Specifies whether remote hosts are allowed to connect to local
447 binds local port forwardings to the loopback address.
448 This prevents other remote hosts from connecting to forwarded ports.
450 can be used to specify that ssh
451 should bind local port forwardings to the wildcard address,
452 thus allowing remote hosts to connect to forwarded ports.
459 .It Cm GlobalKnownHostsFile
460 Specifies a file to use for the global
461 host key database instead of
462 .Pa /etc/ssh/ssh_known_hosts .
463 .It Cm GSSAPIAuthentication
464 Specifies whether user authentication based on GSSAPI is allowed.
467 Note that this option applies to protocol version 2 only.
468 .It Cm GSSAPIDelegateCredentials
469 Forward (delegate) credentials to the server.
472 Note that this option applies to protocol version 2 only.
473 .It Cm HashKnownHosts
476 should hash host names and addresses when they are added to
477 .Pa ~/.ssh/known_hosts .
478 These hashed names may be used normally by
482 but they do not reveal identifying information should the file's contents
486 Note that hashing of names and addresses will not be retrospectively applied
487 to existing known hosts files, but these may be manually hashed using
489 .It Cm HostbasedAuthentication
490 Specifies whether to try rhosts based authentication with public key
498 This option applies to protocol version 2 only and
500 .Cm RhostsRSAAuthentication .
501 .It Cm HostKeyAlgorithms
502 Specifies the protocol version 2 host key algorithms
503 that the client wants to use in order of preference.
504 The default for this option is:
505 .Dq ssh-rsa,ssh-dss .
507 Specifies an alias that should be used instead of the
508 real host name when looking up or saving the host key
509 in the host key database files.
510 This option is useful for tunneling SSH connections
511 or for multiple servers running on a single host.
513 Specifies the real host name to log into.
514 This can be used to specify nicknames or abbreviations for hosts.
515 The default is the name given on the command line.
516 Numeric IP addresses are also permitted (both on the command line and in
519 .It Cm IdentitiesOnly
522 should only use the authentication identity files configured in the
527 offers more identities.
528 The argument to this keyword must be
532 This option is intended for situations where ssh-agent
533 offers many different identities.
537 Specifies a file from which the user's RSA or DSA authentication identity
541 for protocol version 1, and
545 for protocol version 2.
546 Additionally, any identities represented by the authentication agent
547 will be used for authentication.
548 The file name may use the tilde
549 syntax to refer to a user's home directory.
550 It is possible to have
551 multiple identity files specified in configuration files; all these
552 identities will be tried in sequence.
553 .It Cm KbdInteractiveDevices
554 Specifies the list of methods to use in keyboard-interactive authentication.
555 Multiple method names must be comma-separated.
556 The default is to use the server specified list.
557 The methods available vary depending on what the server supports.
558 For an OpenSSH server,
559 it may be zero or more of:
565 Specifies a command to execute on the local machine after successfully
566 connecting to the server.
567 The command string extends to the end of the line, and is executed with
569 This directive is ignored unless
570 .Cm PermitLocalCommand
573 Specifies that a TCP port on the local machine be forwarded over
574 the secure channel to the specified host and port from the remote machine.
575 The first argument must be
577 .Oo Ar bind_address : Oc Ar port
579 and the second argument must be
580 .Ar host : Ns Ar hostport .
581 IPv6 addresses can be specified by enclosing addresses in square brackets or
582 by using an alternative syntax:
583 .Oo Ar bind_address Ns / Oc Ns Ar port
585 .Ar host Ns / Ns Ar hostport .
586 Multiple forwardings may be specified, and additional forwardings can be
587 given on the command line.
588 Only the superuser can forward privileged ports.
589 By default, the local port is bound in accordance with the
594 may be used to bind the connection to a specific address.
599 indicates that the listening port be bound for local use only, while an
602 indicates that the port should be available from all interfaces.
604 Gives the verbosity level that is used when logging messages from
606 The possible values are:
607 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
609 DEBUG and DEBUG1 are equivalent.
610 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
612 Specifies the MAC (message authentication code) algorithms
613 in order of preference.
614 The MAC algorithm is used in protocol version 2
615 for data integrity protection.
616 Multiple algorithms must be comma-separated.
618 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
619 .It Cm NoHostAuthenticationForLocalhost
620 This option can be used if the home directory is shared across machines.
621 In this case localhost will refer to a different machine on each of
622 the machines and the user will get many warnings about changed host keys.
623 However, this option disables host authentication for localhost.
624 The argument to this keyword must be
628 The default is to check the host key for localhost.
629 .It Cm NumberOfPasswordPrompts
630 Specifies the number of password prompts before giving up.
631 The argument to this keyword must be an integer.
633 .It Cm PasswordAuthentication
634 Specifies whether to use password authentication.
635 The argument to this keyword must be
641 .It Cm PermitLocalCommand
642 Allow local command execution via the
645 .Ic !\& Ns Ar command
655 Specifies the port number to connect on the remote host.
657 .It Cm PreferredAuthentications
658 Specifies the order in which the client should try protocol 2
659 authentication methods.
660 This allows a client to prefer one method (e.g.\&
661 .Cm keyboard-interactive )
662 over another method (e.g.\&
664 The default for this option is:
665 .Dq hostbased,publickey,keyboard-interactive,password .
667 Specifies the protocol versions
669 should support in order of preference.
670 The possible values are
674 Multiple versions must be comma-separated.
678 tries version 2 and falls back to version 1
679 if version 2 is not available.
681 Specifies the command to use to connect to the server.
683 string extends to the end of the line, and is executed with
685 In the command string,
687 will be substituted by the host name to
691 The command can be basically anything,
692 and should read from its standard input and write to its standard output.
693 It should eventually connect an
695 server running on some machine, or execute
698 Host key management will be done using the
699 HostName of the host being connected (defaulting to the name typed by
701 Setting the command to
703 disables this option entirely.
706 is not available for connects with a proxy command.
708 This directive is useful in conjunction with
710 and its proxy support.
711 For example, the following directive would connect via an HTTP proxy at
713 .Bd -literal -offset 3n
714 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
716 .It Cm PubkeyAuthentication
717 Specifies whether to try public key authentication.
718 The argument to this keyword must be
724 This option applies to protocol version 2 only.
726 Specifies the maximum amount of data that may be transmitted before the
727 session key is renegotiated.
728 The argument is the number of bytes, with an optional suffix of
733 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
734 The default is between
738 depending on the cipher.
739 This option applies to protocol version 2 only.
741 Specifies that a TCP port on the remote machine be forwarded over
742 the secure channel to the specified host and port from the local machine.
743 The first argument must be
745 .Oo Ar bind_address : Oc Ar port
747 and the second argument must be
748 .Ar host : Ns Ar hostport .
749 IPv6 addresses can be specified by enclosing addresses in square brackets
750 or by using an alternative syntax:
751 .Oo Ar bind_address Ns / Oc Ns Ar port
753 .Ar host Ns / Ns Ar hostport .
754 Multiple forwardings may be specified, and additional
755 forwardings can be given on the command line.
756 Only the superuser can forward privileged ports.
760 is not specified, the default is to only bind to loopback addresses.
765 or an empty string, then the forwarding is requested to listen on all
769 will only succeed if the server's
771 option is enabled (see
772 .Xr sshd_config 5 ) .
773 .It Cm RhostsRSAAuthentication
774 Specifies whether to try rhosts based authentication with RSA host
782 This option applies to protocol version 1 only and requires
785 .It Cm RSAAuthentication
786 Specifies whether to try RSA authentication.
787 The argument to this keyword must be
791 RSA authentication will only be
792 attempted if the identity file exists, or an authentication agent is
796 Note that this option applies to protocol version 1 only.
798 Specifies what variables from the local
800 should be sent to the server.
801 Note that environment passing is only supported for protocol 2.
802 The server must also support it, and the server must be configured to
803 accept these environment variables.
808 for how to configure the server.
809 Variables are specified by name, which may contain wildcard characters.
810 Multiple environment variables may be separated by whitespace or spread
814 The default is not to send any environment variables.
818 for more information on patterns.
819 .It Cm ServerAliveCountMax
820 Sets the number of server alive messages (see below) which may be
823 receiving any messages back from the server.
824 If this threshold is reached while server alive messages are being sent,
825 ssh will disconnect from the server, terminating the session.
826 It is important to note that the use of server alive messages is very
830 The server alive messages are sent through the encrypted channel
831 and therefore will not be spoofable.
832 The TCP keepalive option enabled by
835 The server alive mechanism is valuable when the client or
836 server depend on knowing when a connection has become inactive.
838 The default value is 3.
840 .Cm ServerAliveInterval
841 (see below) is set to 15 and
842 .Cm ServerAliveCountMax
843 is left at the default, if the server becomes unresponsive,
844 ssh will disconnect after approximately 45 seconds.
845 .It Cm ServerAliveInterval
846 Sets a timeout interval in seconds after which if no data has been received
849 will send a message through the encrypted
850 channel to request a response from the server.
852 is 0, indicating that these messages will not be sent to the server.
853 This option applies to protocol version 2 only.
854 .It Cm SmartcardDevice
855 Specifies which smartcard device to use.
856 The argument to this keyword is the device
858 should use to communicate with a smartcard used for storing the user's
860 By default, no device is specified and smartcard support is not activated.
861 .It Cm StrictHostKeyChecking
862 If this flag is set to
865 will never automatically add host keys to the
866 .Pa ~/.ssh/known_hosts
867 file, and refuses to connect to hosts whose host key has changed.
868 This provides maximum protection against trojan horse attacks,
869 though it can be annoying when the
870 .Pa /etc/ssh/ssh_known_hosts
871 file is poorly maintained or when connections to new hosts are
873 This option forces the user to manually
875 If this flag is set to
877 ssh will automatically add new host keys to the
878 user known hosts files.
879 If this flag is set to
882 will be added to the user known host files only after the user
883 has confirmed that is what they really want to do, and
884 ssh will refuse to connect to hosts whose host key has changed.
886 known hosts will be verified automatically in all cases.
895 Specifies whether the system should send TCP keepalive messages to the
897 If they are sent, death of the connection or crash of one
898 of the machines will be properly noticed.
899 However, this means that
900 connections will die if the route is down temporarily, and some people
905 (to send TCP keepalive messages), and the client will notice
906 if the network goes down or the remote host dies.
907 This is important in scripts, and many users want it too.
909 To disable TCP keepalive messages, the value should be set to
914 device forwarding between the client and the server.
915 This option also allows requesting layer 2 (ethernet)
916 instead of layer 3 (point-to-point) tunneling from the server.
928 device on the client.
929 Without this option, the next available device will be used.
930 .It Cm UsePrivilegedPort
931 Specifies whether to use a privileged port for outgoing connections.
942 Note that this option must be set to
945 .Cm RhostsRSAAuthentication
948 Specifies the user to log in as.
949 This can be useful when a different user name is used on different machines.
950 This saves the trouble of
951 having to remember to give the user name on the command line.
952 .It Cm UserKnownHostsFile
953 Specifies a file to use for the user
954 host key database instead of
955 .Pa ~/.ssh/known_hosts .
956 .It Cm VerifyHostKeyDNS
957 Specifies whether to verify the remote key using DNS and SSHFP resource
959 If this option is set to
961 the client will implicitly trust keys that match a secure fingerprint
963 Insecure fingerprints will be handled as if this option was set to
965 If this option is set to
967 information on fingerprint match will be displayed, but the user will still
968 need to confirm new host keys according to the
969 .Cm StrictHostKeyChecking
978 Note that this option applies to protocol version 2 only.
981 .Sx VERIFYING HOST KEYS
985 Specifies the full pathname of the
989 .Pa /usr/X11R6/bin/xauth .
994 consists of zero or more non-whitespace characters,
996 (a wildcard that matches zero or more characters),
999 (a wildcard that matches exactly one character).
1000 For example, to specify a set of declarations for any host in the
1003 the following pattern could be used:
1007 The following pattern
1008 would match any host in the 192.168.0.[0-9] network range:
1010 .Dl Host 192.168.0.?
1014 is a comma-separated list of patterns.
1015 Patterns within pattern-lists may be negated
1016 by preceding them with an exclamation mark
1019 to allow a key to be used from anywhere within an organisation
1023 the following entry (in authorized_keys) could be used:
1025 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1028 .It Pa ~/.ssh/config
1029 This is the per-user configuration file.
1030 The format of this file is described above.
1031 This file is used by the SSH client.
1032 Because of the potential for abuse, this file must have strict permissions:
1033 read/write for the user, and not accessible by others.
1034 .It Pa /etc/ssh/ssh_config
1035 Systemwide configuration file.
1036 This file provides defaults for those
1037 values that are not specified in the user's configuration file, and
1038 for those users who do not have a configuration file.
1039 This file must be world-readable.
1044 OpenSSH is a derivative of the original and free
1045 ssh 1.2.12 release by Tatu Ylonen.
1046 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1047 Theo de Raadt and Dug Song
1048 removed many bugs, re-added newer features and
1050 Markus Friedl contributed the support for SSH
1051 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob