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Ben Lindstrom9f049032002-06-21 00:59:05 +00001.\" -*- nroff -*-
2.\"
3.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5.\" All rights reserved
6.\"
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".
12.\"
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.
16.\"
17.\" Redistribution and use in source and binary forms, with or without
18.\" modification, are permitted provided that the following conditions
19.\" are met:
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.
25.\"
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.
36.\"
Damien Millere9d001e2006-01-14 10:10:17 +110037.\" $OpenBSD: ssh_config.5,v 1.74 2006/01/12 22:26:02 jmc Exp $
Ben Lindstrom9f049032002-06-21 00:59:05 +000038.Dd September 25, 1999
39.Dt SSH_CONFIG 5
40.Os
41.Sh NAME
42.Nm ssh_config
43.Nd OpenSSH SSH client configuration files
44.Sh SYNOPSIS
45.Bl -tag -width Ds -compact
Damien Miller167ea5d2005-05-26 12:04:02 +100046.It Pa ~/.ssh/config
Ben Lindstrom9f049032002-06-21 00:59:05 +000047.It Pa /etc/ssh/ssh_config
48.El
49.Sh DESCRIPTION
50.Nm ssh
51obtains configuration data from the following sources in
52the following order:
Ben Lindstrom479b4762002-08-20 19:04:51 +000053.Bl -enum -offset indent -compact
54.It
55command-line options
56.It
57user's configuration file
Damien Miller167ea5d2005-05-26 12:04:02 +100058.Pq Pa ~/.ssh/config
Ben Lindstrom479b4762002-08-20 19:04:51 +000059.It
60system-wide configuration file
61.Pq Pa /etc/ssh/ssh_config
62.El
Ben Lindstrom9f049032002-06-21 00:59:05 +000063.Pp
64For each parameter, the first obtained value
65will be used.
Darren Tucker43d8e282005-02-09 09:51:08 +110066The configuration files contain sections separated by
Ben Lindstrom9f049032002-06-21 00:59:05 +000067.Dq Host
68specifications, and that section is only applied for hosts that
69match one of the patterns given in the specification.
70The matched host name is the one given on the command line.
71.Pp
72Since the first obtained value for each parameter is used, more
73host-specific declarations should be given near the beginning of the
74file, and general defaults at the end.
75.Pp
76The configuration file has the following format:
77.Pp
78Empty lines and lines starting with
79.Ql #
80are comments.
81.Pp
82Otherwise a line is of the format
83.Dq keyword arguments .
84Configuration options may be separated by whitespace or
85optional whitespace and exactly one
86.Ql = ;
87the latter format is useful to avoid the need to quote whitespace
88when specifying configuration options using the
89.Nm ssh ,
90.Nm scp
91and
92.Nm sftp
93.Fl o
94option.
95.Pp
96The possible
97keywords and their meanings are as follows (note that
98keywords are case-insensitive and arguments are case-sensitive):
99.Bl -tag -width Ds
100.It Cm Host
101Restricts the following declarations (up to the next
102.Cm Host
103keyword) to be only for those hosts that match one of the patterns
104given after the keyword.
105.Ql \&*
106and
Damien Millerc2b98272003-09-03 12:13:30 +1000107.Ql \&?
Ben Lindstrom9f049032002-06-21 00:59:05 +0000108can be used as wildcards in the
109patterns.
110A single
111.Ql \&*
112as a pattern can be used to provide global
113defaults for all hosts.
114The host is the
115.Ar hostname
116argument given on the command line (i.e., the name is not converted to
117a canonicalized host name before matching).
Damien Miller20a8f972003-05-18 20:50:30 +1000118.It Cm AddressFamily
Damien Millerfbf486b2003-05-23 18:44:23 +1000119Specifies which address family to use when connecting.
120Valid arguments are
Damien Miller20a8f972003-05-18 20:50:30 +1000121.Dq any ,
122.Dq inet
Darren Tucker79a7acf2005-02-09 09:48:57 +1100123(use IPv4 only) or
Damien Miller20a8f972003-05-18 20:50:30 +1000124.Dq inet6
Darren Tucker79a7acf2005-02-09 09:48:57 +1100125(use IPv6 only).
Ben Lindstrom9f049032002-06-21 00:59:05 +0000126.It Cm BatchMode
127If set to
128.Dq yes ,
129passphrase/password querying will be disabled.
130This option is useful in scripts and other batch jobs where no user
131is present to supply the password.
132The argument must be
133.Dq yes
134or
135.Dq no .
136The default is
137.Dq no .
138.It Cm BindAddress
Darren Tucker89f4d472005-07-14 17:06:21 +1000139Use the specified address on the local machine as the source address of
Darren Tucker6c71d202005-07-14 17:06:50 +1000140the connection.
141Only useful on systems with more than one address.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000142Note that this option does not work if
143.Cm UsePrivilegedPort
144is set to
145.Dq yes .
146.It Cm ChallengeResponseAuthentication
147Specifies whether to use challenge response authentication.
148The argument to this keyword must be
149.Dq yes
150or
151.Dq no .
152The default is
153.Dq yes .
154.It Cm CheckHostIP
155If this flag is set to
156.Dq yes ,
157ssh will additionally check the host IP address in the
158.Pa known_hosts
159file.
160This allows ssh to detect if a host key changed due to DNS spoofing.
161If the option is set to
162.Dq no ,
163the check will not be executed.
164The default is
165.Dq yes .
166.It Cm Cipher
167Specifies the cipher to use for encrypting the session
168in protocol version 1.
169Currently,
170.Dq blowfish ,
171.Dq 3des ,
172and
173.Dq des
174are supported.
175.Ar des
176is only supported in the
177.Nm ssh
178client for interoperability with legacy protocol 1 implementations
179that do not support the
180.Ar 3des
Damien Miller495dca32003-04-01 21:42:14 +1000181cipher.
182Its use is strongly discouraged due to cryptographic weaknesses.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000183The default is
184.Dq 3des .
185.It Cm Ciphers
186Specifies the ciphers allowed for protocol version 2
187in order of preference.
188Multiple ciphers must be comma-separated.
Damien Miller05202ff2004-06-15 10:30:39 +1000189The supported ciphers are
190.Dq 3des-cbc ,
191.Dq aes128-cbc ,
192.Dq aes192-cbc ,
193.Dq aes256-cbc ,
194.Dq aes128-ctr ,
195.Dq aes192-ctr ,
196.Dq aes256-ctr ,
Damien Miller3710f272005-05-26 12:19:17 +1000197.Dq arcfour128 ,
198.Dq arcfour256 ,
Damien Miller05202ff2004-06-15 10:30:39 +1000199.Dq arcfour ,
200.Dq blowfish-cbc ,
201and
202.Dq cast128-cbc .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000203The default is
Ben Lindstrom9f049032002-06-21 00:59:05 +0000204.Bd -literal
Damien Miller3710f272005-05-26 12:19:17 +1000205 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
206 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
207 aes192-ctr,aes256-ctr''
Ben Lindstrom9f049032002-06-21 00:59:05 +0000208.Ed
209.It Cm ClearAllForwardings
210Specifies that all local, remote and dynamic port forwardings
211specified in the configuration files or on the command line be
Damien Miller495dca32003-04-01 21:42:14 +1000212cleared.
213This option is primarily useful when used from the
Ben Lindstrom9f049032002-06-21 00:59:05 +0000214.Nm ssh
215command line to clear port forwardings set in
216configuration files, and is automatically set by
217.Xr scp 1
218and
219.Xr sftp 1 .
220The argument must be
221.Dq yes
222or
223.Dq no .
224The default is
225.Dq no .
226.It Cm Compression
227Specifies whether to use compression.
228The argument must be
229.Dq yes
230or
231.Dq no .
232The default is
233.Dq no .
234.It Cm CompressionLevel
235Specifies the compression level to use if compression is enabled.
236The argument must be an integer from 1 (fast) to 9 (slow, best).
237The default level is 6, which is good for most applications.
238The meaning of the values is the same as in
239.Xr gzip 1 .
240Note that this option applies to protocol version 1 only.
241.It Cm ConnectionAttempts
242Specifies the number of tries (one per second) to make before exiting.
243The argument must be an integer.
244This may be useful in scripts if the connection sometimes fails.
245The default is 1.
Damien Millerb78d5eb2003-05-16 11:39:04 +1000246.It Cm ConnectTimeout
247Specifies the timeout (in seconds) used when connecting to the ssh
Damien Millerfbf486b2003-05-23 18:44:23 +1000248server, instead of using the default system TCP timeout.
249This value is used only when the target is down or really unreachable,
250not when it refuses the connection.
Damien Miller0e220db2004-06-15 10:34:08 +1000251.It Cm ControlMaster
252Enables the sharing of multiple sessions over a single network connection.
253When set to
254.Dq yes
255.Nm ssh
256will listen for connections on a control socket specified using the
257.Cm ControlPath
258argument.
259Additional sessions can connect to this socket using the same
260.Cm ControlPath
261with
262.Cm ControlMaster
263set to
264.Dq no
Damien Miller2234bac2004-06-30 22:38:52 +1000265(the default).
Damien Miller713de762005-11-05 15:13:49 +1100266These sessions will try to reuse the master instance's network connection
Damien Millerb3bfbb72005-11-05 15:11:48 +1100267rather than initiating new ones, but will fall back to connecting normally
268if the control socket does not exist, or is not listening.
269.Pp
Damien Miller23f07702004-06-18 01:19:03 +1000270Setting this to
271.Dq ask
272will cause
273.Nm ssh
274to listen for control connections, but require confirmation using the
275.Ev SSH_ASKPASS
276program before they are accepted (see
277.Xr ssh-add 1
Damien Miller2234bac2004-06-30 22:38:52 +1000278for details).
Damien Millerdadfd4d2005-05-26 12:07:13 +1000279If the
280.Cm ControlPath
281can not be opened,
282.Nm ssh
283will continue without connecting to a master instance.
Damien Millerd14b1e72005-06-16 13:19:41 +1000284.Pp
Damien Miller13390022005-07-06 09:44:19 +1000285X11 and
Damien Millerfd94fba2005-07-06 09:44:59 +1000286.Xr ssh-agent 1
Damien Miller13390022005-07-06 09:44:19 +1000287forwarding is supported over these multiplexed connections, however the
Darren Tucker63551872005-12-20 16:14:15 +1100288display and agent forwarded will be the one belonging to the master
Damien Millerfd94fba2005-07-06 09:44:59 +1000289connection i.e. it is not possible to forward multiple displays or agents.
Damien Miller13390022005-07-06 09:44:19 +1000290.Pp
Damien Millerd14b1e72005-06-16 13:19:41 +1000291Two additional options allow for opportunistic multiplexing: try to use a
292master connection but fall back to creating a new one if one does not already
293exist.
294These options are:
295.Dq auto
296and
297.Dq autoask .
298The latter requires confirmation like the
299.Dq ask
300option.
Damien Miller0e220db2004-06-15 10:34:08 +1000301.It Cm ControlPath
Damien Miller6476cad2005-06-16 13:18:34 +1000302Specify the path to the control socket used for connection sharing as described
303in the
Damien Miller0e220db2004-06-15 10:34:08 +1000304.Cm ControlMaster
Damien Miller8f74c8f2005-06-26 08:56:03 +1000305section above or the string
306.Dq none
307to disable connection sharing.
Damien Miller6476cad2005-06-16 13:18:34 +1000308In the path,
309.Ql %h
310will be substituted by the target host name,
311.Ql %p
312the port and
313.Ql %r
314by the remote login username.
Damien Millerd14b1e72005-06-16 13:19:41 +1000315It is recommended that any
316.Cm ControlPath
317used for opportunistic connection sharing include
318all three of these escape sequences.
319This ensures that shared connections are uniquely identified.
Damien Miller2234bac2004-06-30 22:38:52 +1000320.It Cm DynamicForward
Damien Millere9d001e2006-01-14 10:10:17 +1100321Specifies that a TCP port on the local machine be forwarded
Damien Miller2234bac2004-06-30 22:38:52 +1000322over the secure channel, and the application
323protocol is then used to determine where to connect to from the
324remote machine.
Darren Tuckerc8d64212005-10-03 18:13:42 +1000325.Pp
326The argument must be
327.Sm off
328.Oo Ar bind_address : Oc Ar port .
329.Sm on
330IPv6 addresses can be specified by enclosing addresses in square brackets or
331by using an alternative syntax:
332.Oo Ar bind_address Ns / Oc Ns Ar port .
333By default, the local port is bound in accordance with the
334.Cm GatewayPorts
335setting.
336However, an explicit
337.Ar bind_address
338may be used to bind the connection to a specific address.
339The
340.Ar bind_address
341of
342.Dq localhost
343indicates that the listening port be bound for local use only, while an
344empty address or
345.Sq *
346indicates that the port should be available from all interfaces.
347.Pp
Damien Miller2234bac2004-06-30 22:38:52 +1000348Currently the SOCKS4 and SOCKS5 protocols are supported, and
349.Nm ssh
350will act as a SOCKS server.
351Multiple forwardings may be specified, and
352additional forwardings can be given on the command line.
353Only the superuser can forward privileged ports.
Darren Tucker674f71d2003-06-28 12:33:12 +1000354.It Cm EnableSSHKeysign
355Setting this option to
356.Dq yes
357in the global client configuration file
358.Pa /etc/ssh/ssh_config
359enables the use of the helper program
360.Xr ssh-keysign 8
361during
362.Cm HostbasedAuthentication .
363The argument must be
364.Dq yes
365or
366.Dq no .
367The default is
368.Dq no .
Darren Tuckerf132c672003-10-15 15:58:18 +1000369This option should be placed in the non-hostspecific section.
Darren Tucker674f71d2003-06-28 12:33:12 +1000370See
371.Xr ssh-keysign 8
372for more information.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000373.It Cm EscapeChar
374Sets the escape character (default:
375.Ql ~ ) .
376The escape character can also
377be set on the command line.
378The argument should be a single character,
379.Ql ^
380followed by a letter, or
381.Dq none
382to disable the escape
383character entirely (making the connection transparent for binary
384data).
385.It Cm ForwardAgent
386Specifies whether the connection to the authentication agent (if any)
387will be forwarded to the remote machine.
388The argument must be
389.Dq yes
390or
391.Dq no .
392The default is
393.Dq no .
Damien Milleraf653042002-09-04 16:40:37 +1000394.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000395Agent forwarding should be enabled with caution.
396Users with the ability to bypass file permissions on the remote host
397(for the agent's Unix-domain socket)
398can access the local agent through the forwarded connection.
399An attacker cannot obtain key material from the agent,
Damien Milleraf653042002-09-04 16:40:37 +1000400however they can perform operations on the keys that enable them to
401authenticate using the identities loaded into the agent.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000402.It Cm ForwardX11
403Specifies whether X11 connections will be automatically redirected
404over the secure channel and
405.Ev DISPLAY
406set.
407The argument must be
408.Dq yes
409or
410.Dq no .
411The default is
412.Dq no .
Damien Milleraf653042002-09-04 16:40:37 +1000413.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000414X11 forwarding should be enabled with caution.
415Users with the ability to bypass file permissions on the remote host
Darren Tucker0a118da2003-10-15 15:54:32 +1000416(for the user's X11 authorization database)
Damien Miller495dca32003-04-01 21:42:14 +1000417can access the local X11 display through the forwarded connection.
Darren Tucker0a118da2003-10-15 15:54:32 +1000418An attacker may then be able to perform activities such as keystroke monitoring
419if the
420.Cm ForwardX11Trusted
421option is also enabled.
422.It Cm ForwardX11Trusted
Darren Tuckerdcf6ec42004-05-13 13:03:56 +1000423If this option is set to
Darren Tucker0a118da2003-10-15 15:54:32 +1000424.Dq yes
425then remote X11 clients will have full access to the original X11 display.
Damien Miller1717fd42005-03-01 21:17:31 +1100426.Pp
Darren Tucker0a118da2003-10-15 15:54:32 +1000427If this option is set to
428.Dq no
429then remote X11 clients will be considered untrusted and prevented
430from stealing or tampering with data belonging to trusted X11
431clients.
Damien Miller1717fd42005-03-01 21:17:31 +1100432Furthermore, the
433.Xr xauth 1
434token used for the session will be set to expire after 20 minutes.
435Remote clients will be refused access after this time.
Darren Tucker0a118da2003-10-15 15:54:32 +1000436.Pp
437The default is
438.Dq no .
439.Pp
440See the X11 SECURITY extension specification for full details on
441the restrictions imposed on untrusted clients.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000442.It Cm GatewayPorts
443Specifies whether remote hosts are allowed to connect to local
444forwarded ports.
445By default,
446.Nm ssh
Damien Miller495dca32003-04-01 21:42:14 +1000447binds local port forwardings to the loopback address.
448This prevents other remote hosts from connecting to forwarded ports.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000449.Cm GatewayPorts
450can be used to specify that
451.Nm ssh
452should bind local port forwardings to the wildcard address,
453thus allowing remote hosts to connect to forwarded ports.
454The argument must be
455.Dq yes
456or
457.Dq no .
458The default is
459.Dq no .
460.It Cm GlobalKnownHostsFile
461Specifies a file to use for the global
462host key database instead of
463.Pa /etc/ssh/ssh_known_hosts .
Darren Tucker0efd1552003-08-26 11:49:55 +1000464.It Cm GSSAPIAuthentication
Damien Millerbaafb982003-12-17 16:32:23 +1100465Specifies whether user authentication based on GSSAPI is allowed.
Damien Millerc2b98272003-09-03 12:13:30 +1000466The default is
Darren Tuckera044f472003-10-15 15:52:03 +1000467.Dq no .
Darren Tucker0efd1552003-08-26 11:49:55 +1000468Note that this option applies to protocol version 2 only.
469.It Cm GSSAPIDelegateCredentials
470Forward (delegate) credentials to the server.
471The default is
472.Dq no .
473Note that this option applies to protocol version 2 only.
Damien Millere1776152005-03-01 21:47:37 +1100474.It Cm HashKnownHosts
475Indicates that
476.Nm ssh
477should hash host names and addresses when they are added to
Damien Miller167ea5d2005-05-26 12:04:02 +1000478.Pa ~/.ssh/known_hosts .
Damien Millere1776152005-03-01 21:47:37 +1100479These hashed names may be used normally by
480.Nm ssh
481and
482.Nm sshd ,
483but they do not reveal identifying information should the file's contents
484be disclosed.
485The default is
486.Dq no .
Damien Millerf8c55462005-03-02 12:03:05 +1100487Note that hashing of names and addresses will not be retrospectively applied
Damien Miller4b42d7f2005-03-01 21:48:35 +1100488to existing known hosts files, but these may be manually hashed using
489.Xr ssh-keygen 1 .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000490.It Cm HostbasedAuthentication
491Specifies whether to try rhosts based authentication with public key
492authentication.
493The argument must be
494.Dq yes
495or
496.Dq no .
497The default is
498.Dq no .
499This option applies to protocol version 2 only and
500is similar to
501.Cm RhostsRSAAuthentication .
502.It Cm HostKeyAlgorithms
503Specifies the protocol version 2 host key algorithms
504that the client wants to use in order of preference.
505The default for this option is:
506.Dq ssh-rsa,ssh-dss .
507.It Cm HostKeyAlias
508Specifies an alias that should be used instead of the
509real host name when looking up or saving the host key
510in the host key database files.
511This option is useful for tunneling ssh connections
512or for multiple servers running on a single host.
513.It Cm HostName
514Specifies the real host name to log into.
515This can be used to specify nicknames or abbreviations for hosts.
516Default is the name given on the command line.
517Numeric IP addresses are also permitted (both on the command line and in
518.Cm HostName
519specifications).
Damien Millerbd394c32004-03-08 23:12:36 +1100520.It Cm IdentitiesOnly
521Specifies that
522.Nm ssh
523should only use the authentication identity files configured in the
Damien Miller1a812582004-04-20 20:13:32 +1000524.Nm
Damien Millerbd394c32004-03-08 23:12:36 +1100525files,
526even if the
527.Nm ssh-agent
528offers more identities.
529The argument to this keyword must be
530.Dq yes
531or
532.Dq no .
Darren Tucker7eba8202005-12-20 16:15:14 +1100533This option is intended for situations where
Damien Millerbd394c32004-03-08 23:12:36 +1100534.Nm ssh-agent
535offers many different identities.
536The default is
537.Dq no .
Damien Miller957d4e42005-12-13 19:30:45 +1100538.It Cm IdentityFile
539Specifies a file from which the user's RSA or DSA authentication identity
540is read.
541The default is
542.Pa ~/.ssh/identity
543for protocol version 1, and
544.Pa ~/.ssh/id_rsa
545and
546.Pa ~/.ssh/id_dsa
547for protocol version 2.
548Additionally, any identities represented by the authentication agent
549will be used for authentication.
550The file name may use the tilde
551syntax to refer to a user's home directory.
552It is possible to have
553multiple identity files specified in configuration files; all these
554identities will be tried in sequence.
Darren Tucker636ca902004-11-05 20:22:00 +1100555.It Cm KbdInteractiveDevices
556Specifies the list of methods to use in keyboard-interactive authentication.
557Multiple method names must be comma-separated.
558The default is to use the server specified list.
Damien Millerd27b9472005-12-13 19:29:02 +1100559.It Cm LocalCommand
560Specifies a command to execute on the local machine after successfully
561connecting to the server.
562The command string extends to the end of the line, and is executed with
563.Pa /bin/sh .
564This directive is ignored unless
565.Cm PermitLocalCommand
566has been enabled.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000567.It Cm LocalForward
Damien Millere9d001e2006-01-14 10:10:17 +1100568Specifies that a TCP port on the local machine be forwarded over
Ben Lindstrom9f049032002-06-21 00:59:05 +0000569the secure channel to the specified host and port from the remote machine.
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000570The first argument must be
Damien Millerf91ee4c2005-03-01 21:24:33 +1100571.Sm off
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000572.Oo Ar bind_address : Oc Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100573.Sm on
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000574and the second argument must be
575.Ar host : Ns Ar hostport .
Damien Millerf8c55462005-03-02 12:03:05 +1100576IPv6 addresses can be specified by enclosing addresses in square brackets or
Damien Millerf91ee4c2005-03-01 21:24:33 +1100577by using an alternative syntax:
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000578.Oo Ar bind_address Ns / Oc Ns Ar port
579and
580.Ar host Ns / Ns Ar hostport .
Damien Millerf8c55462005-03-02 12:03:05 +1100581Multiple forwardings may be specified, and additional forwardings can be
Damien Millerf91ee4c2005-03-01 21:24:33 +1100582given on the command line.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000583Only the superuser can forward privileged ports.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100584By default, the local port is bound in accordance with the
585.Cm GatewayPorts
586setting.
587However, an explicit
588.Ar bind_address
589may be used to bind the connection to a specific address.
590The
591.Ar bind_address
592of
593.Dq localhost
Damien Millerf8c55462005-03-02 12:03:05 +1100594indicates that the listening port be bound for local use only, while an
595empty address or
596.Sq *
Damien Millerf91ee4c2005-03-01 21:24:33 +1100597indicates that the port should be available from all interfaces.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000598.It Cm LogLevel
599Gives the verbosity level that is used when logging messages from
600.Nm ssh .
601The possible values are:
602QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
Damien Miller495dca32003-04-01 21:42:14 +1000603The default is INFO.
604DEBUG and DEBUG1 are equivalent.
605DEBUG2 and DEBUG3 each specify higher levels of verbose output.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000606.It Cm MACs
607Specifies the MAC (message authentication code) algorithms
608in order of preference.
609The MAC algorithm is used in protocol version 2
610for data integrity protection.
611Multiple algorithms must be comma-separated.
612The default is
613.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
614.It Cm NoHostAuthenticationForLocalhost
615This option can be used if the home directory is shared across machines.
616In this case localhost will refer to a different machine on each of
617the machines and the user will get many warnings about changed host keys.
618However, this option disables host authentication for localhost.
619The argument to this keyword must be
620.Dq yes
621or
622.Dq no .
623The default is to check the host key for localhost.
624.It Cm NumberOfPasswordPrompts
625Specifies the number of password prompts before giving up.
626The argument to this keyword must be an integer.
627Default is 3.
628.It Cm PasswordAuthentication
629Specifies whether to use password authentication.
630The argument to this keyword must be
631.Dq yes
632or
633.Dq no .
634The default is
635.Dq yes .
Damien Millerd27b9472005-12-13 19:29:02 +1100636.It Cm PermitLocalCommand
637Allow local command execution via the
638.Ic LocalCommand
639option or using the
Damien Miller4b2319f2005-12-13 19:30:27 +1100640.Ic !\& Ns Ar command
Damien Millerd27b9472005-12-13 19:29:02 +1100641escape sequence in
642.Xr ssh 1 .
643The argument must be
644.Dq yes
645or
646.Dq no .
647The default is
648.Dq no .
Damien Miller957d4e42005-12-13 19:30:45 +1100649.It Cm Port
650Specifies the port number to connect on the remote host.
651Default is 22.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000652.It Cm PreferredAuthentications
653Specifies the order in which the client should try protocol 2
Damien Millerfbf486b2003-05-23 18:44:23 +1000654authentication methods.
Darren Tucker1adc2bd2005-03-14 23:14:20 +1100655This allows a client to prefer one method (e.g.\&
Ben Lindstrom9f049032002-06-21 00:59:05 +0000656.Cm keyboard-interactive )
Darren Tucker1adc2bd2005-03-14 23:14:20 +1100657over another method (e.g.\&
Ben Lindstrom9f049032002-06-21 00:59:05 +0000658.Cm password )
659The default for this option is:
660.Dq hostbased,publickey,keyboard-interactive,password .
661.It Cm Protocol
662Specifies the protocol versions
663.Nm ssh
664should support in order of preference.
665The possible values are
666.Dq 1
667and
668.Dq 2 .
669Multiple versions must be comma-separated.
670The default is
671.Dq 2,1 .
672This means that
673.Nm ssh
674tries version 2 and falls back to version 1
675if version 2 is not available.
676.It Cm ProxyCommand
677Specifies the command to use to connect to the server.
678The command
679string extends to the end of the line, and is executed with
680.Pa /bin/sh .
681In the command string,
682.Ql %h
683will be substituted by the host name to
684connect and
685.Ql %p
686by the port.
687The command can be basically anything,
688and should read from its standard input and write to its standard output.
689It should eventually connect an
690.Xr sshd 8
691server running on some machine, or execute
692.Ic sshd -i
693somewhere.
694Host key management will be done using the
695HostName of the host being connected (defaulting to the name typed by
696the user).
Damien Miller495dca32003-04-01 21:42:14 +1000697Setting the command to
698.Dq none
Damien Miller9f1e33a2003-02-24 11:57:32 +1100699disables this option entirely.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000700Note that
701.Cm CheckHostIP
702is not available for connects with a proxy command.
703.Pp
Damien Millerebcfedc2005-05-26 12:13:56 +1000704This directive is useful in conjunction with
705.Xr nc 1
706and its proxy support.
Damien Millerdfec2942005-05-26 12:14:32 +1000707For example, the following directive would connect via an HTTP proxy at
Damien Millerebcfedc2005-05-26 12:13:56 +1000708192.0.2.0:
709.Bd -literal -offset 3n
710ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
711.Ed
Ben Lindstrom9f049032002-06-21 00:59:05 +0000712.It Cm PubkeyAuthentication
713Specifies whether to try public key authentication.
714The argument to this keyword must be
715.Dq yes
716or
717.Dq no .
718The default is
719.Dq yes .
720This option applies to protocol version 2 only.
721.It Cm RemoteForward
Damien Millere9d001e2006-01-14 10:10:17 +1100722Specifies that a TCP port on the remote machine be forwarded over
Ben Lindstrom9f049032002-06-21 00:59:05 +0000723the secure channel to the specified host and port from the local machine.
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000724The first argument must be
Damien Millerf91ee4c2005-03-01 21:24:33 +1100725.Sm off
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000726.Oo Ar bind_address : Oc Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100727.Sm on
Darren Tucker5ede2ad2005-03-31 21:31:10 +1000728and the second argument must be
729.Ar host : Ns Ar hostport .
730IPv6 addresses can be specified by enclosing addresses in square brackets
731or by using an alternative syntax:
732.Oo Ar bind_address Ns / Oc Ns Ar port
733and
734.Ar host Ns / Ns Ar hostport .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000735Multiple forwardings may be specified, and additional
736forwardings can be given on the command line.
737Only the superuser can forward privileged ports.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100738.Pp
739If the
740.Ar bind_address
741is not specified, the default is to only bind to loopback addresses.
742If the
743.Ar bind_address
744is
745.Ql *
746or an empty string, then the forwarding is requested to listen on all
747interfaces.
748Specifying a remote
749.Ar bind_address
Damien Millerf8c55462005-03-02 12:03:05 +1100750will only succeed if the server's
751.Cm GatewayPorts
Damien Millerf91ee4c2005-03-01 21:24:33 +1100752option is enabled (see
Damien Millerf8c55462005-03-02 12:03:05 +1100753.Xr sshd_config 5 ) .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000754.It Cm RhostsRSAAuthentication
755Specifies whether to try rhosts based authentication with RSA host
756authentication.
757The argument must be
758.Dq yes
759or
760.Dq no .
761The default is
762.Dq no .
763This option applies to protocol version 1 only and requires
764.Nm ssh
765to be setuid root.
766.It Cm RSAAuthentication
767Specifies whether to try RSA authentication.
768The argument to this keyword must be
769.Dq yes
770or
771.Dq no .
772RSA authentication will only be
773attempted if the identity file exists, or an authentication agent is
774running.
775The default is
776.Dq yes .
777Note that this option applies to protocol version 1 only.
Darren Tucker46bc0752004-05-02 22:11:30 +1000778.It Cm SendEnv
779Specifies what variables from the local
780.Xr environ 7
781should be sent to the server.
782Note that environment passing is only supported for protocol 2, the
Darren Tucker1e0c9bf2004-05-02 22:12:48 +1000783server must also support it, and the server must be configured to
784accept these environment variables.
Darren Tucker46bc0752004-05-02 22:11:30 +1000785Refer to
786.Cm AcceptEnv
787in
788.Xr sshd_config 5
789for how to configure the server.
790Variables are specified by name, which may contain the wildcard characters
791.Ql \&*
792and
793.Ql \&? .
Darren Tucker1e0c9bf2004-05-02 22:12:48 +1000794Multiple environment variables may be separated by whitespace or spread
Darren Tucker46bc0752004-05-02 22:11:30 +1000795across multiple
796.Cm SendEnv
797directives.
798The default is not to send any environment variables.
Damien Miller509b0102003-12-17 16:33:10 +1100799.It Cm ServerAliveCountMax
Damien Millerb7977702006-01-03 18:47:31 +1100800Sets the number of server alive messages (see below) which may be
Damien Miller509b0102003-12-17 16:33:10 +1100801sent without
802.Nm ssh
803receiving any messages back from the server.
804If this threshold is reached while server alive messages are being sent,
805.Nm ssh
806will disconnect from the server, terminating the session.
807It is important to note that the use of server alive messages is very
808different from
809.Cm TCPKeepAlive
810(below).
811The server alive messages are sent through the encrypted channel
812and therefore will not be spoofable.
813The TCP keepalive option enabled by
814.Cm TCPKeepAlive
815is spoofable.
816The server alive mechanism is valuable when the client or
817server depend on knowing when a connection has become inactive.
818.Pp
819The default value is 3.
820If, for example,
821.Cm ServerAliveInterval
Damien Millerb7977702006-01-03 18:47:31 +1100822(see below) is set to 15, and
Damien Miller509b0102003-12-17 16:33:10 +1100823.Cm ServerAliveCountMax
824is left at the default, if the server becomes unresponsive ssh
825will disconnect after approximately 45 seconds.
Damien Miller957d4e42005-12-13 19:30:45 +1100826.It Cm ServerAliveInterval
827Sets a timeout interval in seconds after which if no data has been received
828from the server,
829.Nm ssh
830will send a message through the encrypted
831channel to request a response from the server.
832The default
833is 0, indicating that these messages will not be sent to the server.
834This option applies to protocol version 2 only.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000835.It Cm SmartcardDevice
Damien Millerfbf486b2003-05-23 18:44:23 +1000836Specifies which smartcard device to use.
837The argument to this keyword is the device
Ben Lindstrom9f049032002-06-21 00:59:05 +0000838.Nm ssh
839should use to communicate with a smartcard used for storing the user's
Damien Millerfbf486b2003-05-23 18:44:23 +1000840private RSA key.
841By default, no device is specified and smartcard support is not activated.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000842.It Cm StrictHostKeyChecking
843If this flag is set to
844.Dq yes ,
845.Nm ssh
846will never automatically add host keys to the
Damien Miller167ea5d2005-05-26 12:04:02 +1000847.Pa ~/.ssh/known_hosts
Ben Lindstrom9f049032002-06-21 00:59:05 +0000848file, and refuses to connect to hosts whose host key has changed.
849This provides maximum protection against trojan horse attacks,
850however, can be annoying when the
851.Pa /etc/ssh/ssh_known_hosts
852file is poorly maintained, or connections to new hosts are
853frequently made.
854This option forces the user to manually
855add all new hosts.
856If this flag is set to
857.Dq no ,
858.Nm ssh
859will automatically add new host keys to the
860user known hosts files.
861If this flag is set to
862.Dq ask ,
863new host keys
864will be added to the user known host files only after the user
865has confirmed that is what they really want to do, and
866.Nm ssh
867will refuse to connect to hosts whose host key has changed.
868The host keys of
869known hosts will be verified automatically in all cases.
870The argument must be
871.Dq yes ,
872.Dq no
873or
874.Dq ask .
875The default is
876.Dq ask .
Damien Miller12c150e2003-12-17 16:31:10 +1100877.It Cm TCPKeepAlive
878Specifies whether the system should send TCP keepalive messages to the
879other side.
880If they are sent, death of the connection or crash of one
881of the machines will be properly noticed.
882However, this means that
883connections will die if the route is down temporarily, and some people
884find it annoying.
885.Pp
886The default is
887.Dq yes
888(to send TCP keepalive messages), and the client will notice
889if the network goes down or the remote host dies.
890This is important in scripts, and many users want it too.
891.Pp
892To disable TCP keepalive messages, the value should be set to
893.Dq no .
Damien Millerd27b9472005-12-13 19:29:02 +1100894.It Cm Tunnel
895Request starting
896.Xr tun 4
Damien Miller7746c392005-12-13 19:33:37 +1100897device forwarding between the client and the server.
898This option also allows requesting layer 2 (ethernet)
899instead of layer 3 (point-to-point) tunneling from the server.
Damien Millerd27b9472005-12-13 19:29:02 +1100900The argument must be
Damien Miller7b58e802005-12-13 19:33:19 +1100901.Dq yes ,
902.Dq point-to-point ,
903.Dq ethernet
Damien Millerd27b9472005-12-13 19:29:02 +1100904or
905.Dq no .
906The default is
907.Dq no .
908.It Cm TunnelDevice
909Force a specified
910.Xr tun 4
911device on the client.
912Without this option, the next available device will be used.
Damien Millere8cd7412005-12-24 14:55:47 +1100913.It Cm UsePrivilegedPort
914Specifies whether to use a privileged port for outgoing connections.
915The argument must be
916.Dq yes
917or
918.Dq no .
919The default is
920.Dq no .
921If set to
922.Dq yes
923.Nm ssh
924must be setuid root.
925Note that this option must be set to
926.Dq yes
927for
928.Cm RhostsRSAAuthentication
929with older servers.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000930.It Cm User
931Specifies the user to log in as.
932This can be useful when a different user name is used on different machines.
933This saves the trouble of
934having to remember to give the user name on the command line.
935.It Cm UserKnownHostsFile
936Specifies a file to use for the user
937host key database instead of
Damien Miller167ea5d2005-05-26 12:04:02 +1000938.Pa ~/.ssh/known_hosts .
Damien Miller37876e92003-05-15 10:19:46 +1000939.It Cm VerifyHostKeyDNS
940Specifies whether to verify the remote key using DNS and SSHFP resource
941records.
Damien Miller150b5572003-11-17 21:19:29 +1100942If this option is set to
943.Dq yes ,
Damien Millerfe448472003-11-17 21:19:49 +1100944the client will implicitly trust keys that match a secure fingerprint
Damien Miller150b5572003-11-17 21:19:29 +1100945from DNS.
946Insecure fingerprints will be handled as if this option was set to
947.Dq ask .
948If this option is set to
949.Dq ask ,
950information on fingerprint match will be displayed, but the user will still
951need to confirm new host keys according to the
952.Cm StrictHostKeyChecking
953option.
954The argument must be
955.Dq yes ,
956.Dq no
Damien Millerfe448472003-11-17 21:19:49 +1100957or
958.Dq ask .
Damien Miller37876e92003-05-15 10:19:46 +1000959The default is
960.Dq no .
Damien Millereacbb4f2003-06-02 19:10:41 +1000961Note that this option applies to protocol version 2 only.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000962.It Cm XAuthLocation
Damien Miller05913ba2002-09-04 16:51:03 +1000963Specifies the full pathname of the
Ben Lindstrom9f049032002-06-21 00:59:05 +0000964.Xr xauth 1
965program.
966The default is
967.Pa /usr/X11R6/bin/xauth .
968.El
969.Sh FILES
970.Bl -tag -width Ds
Damien Miller167ea5d2005-05-26 12:04:02 +1000971.It Pa ~/.ssh/config
Ben Lindstrom9f049032002-06-21 00:59:05 +0000972This is the per-user configuration file.
973The format of this file is described above.
974This file is used by the
975.Nm ssh
976client.
Damien Millerc970cb92004-04-20 20:12:53 +1000977Because of the potential for abuse, this file must have strict permissions:
978read/write for the user, and not accessible by others.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000979.It Pa /etc/ssh/ssh_config
980Systemwide configuration file.
981This file provides defaults for those
982values that are not specified in the user's configuration file, and
983for those users who do not have a configuration file.
984This file must be world-readable.
985.El
Damien Millerf1ce5052003-06-11 22:04:39 +1000986.Sh SEE ALSO
987.Xr ssh 1
Ben Lindstrom9f049032002-06-21 00:59:05 +0000988.Sh AUTHORS
989OpenSSH is a derivative of the original and free
990ssh 1.2.12 release by Tatu Ylonen.
991Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
992Theo de Raadt and Dug Song
993removed many bugs, re-added newer features and
994created OpenSSH.
995Markus Friedl contributed the support for SSH
996protocol versions 1.5 and 2.0.