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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 Miller37876e92003-05-15 10:19:46 +100037.\" $OpenBSD: ssh_config.5,v 1.8 2003/05/14 18:16:20 jakob 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
46.It Pa $HOME/.ssh/config
47.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
58.Pq Pa $HOME/.ssh/config
59.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.
66The configuration files contain sections bracketed by
67.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
107.Ql ?
108can 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).
118.It Cm AFSTokenPassing
119Specifies whether to pass AFS tokens to remote host.
120The argument to this keyword must be
121.Dq yes
122or
123.Dq no .
124This option applies to protocol version 1 only.
125.It Cm BatchMode
126If set to
127.Dq yes ,
128passphrase/password querying will be disabled.
129This option is useful in scripts and other batch jobs where no user
130is present to supply the password.
131The argument must be
132.Dq yes
133or
134.Dq no .
135The default is
136.Dq no .
137.It Cm BindAddress
138Specify the interface to transmit from on machines with multiple
139interfaces or aliased addresses.
140Note that this option does not work if
141.Cm UsePrivilegedPort
142is set to
143.Dq yes .
144.It Cm ChallengeResponseAuthentication
145Specifies whether to use challenge response authentication.
146The argument to this keyword must be
147.Dq yes
148or
149.Dq no .
150The default is
151.Dq yes .
152.It Cm CheckHostIP
153If this flag is set to
154.Dq yes ,
155ssh will additionally check the host IP address in the
156.Pa known_hosts
157file.
158This allows ssh to detect if a host key changed due to DNS spoofing.
159If the option is set to
160.Dq no ,
161the check will not be executed.
162The default is
163.Dq yes .
164.It Cm Cipher
165Specifies the cipher to use for encrypting the session
166in protocol version 1.
167Currently,
168.Dq blowfish ,
169.Dq 3des ,
170and
171.Dq des
172are supported.
173.Ar des
174is only supported in the
175.Nm ssh
176client for interoperability with legacy protocol 1 implementations
177that do not support the
178.Ar 3des
Damien Miller495dca32003-04-01 21:42:14 +1000179cipher.
180Its use is strongly discouraged due to cryptographic weaknesses.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000181The default is
182.Dq 3des .
183.It Cm Ciphers
184Specifies the ciphers allowed for protocol version 2
185in order of preference.
186Multiple ciphers must be comma-separated.
187The default is
188.Pp
189.Bd -literal
190 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
191 aes192-cbc,aes256-cbc''
192.Ed
193.It Cm ClearAllForwardings
194Specifies that all local, remote and dynamic port forwardings
195specified in the configuration files or on the command line be
Damien Miller495dca32003-04-01 21:42:14 +1000196cleared.
197This option is primarily useful when used from the
Ben Lindstrom9f049032002-06-21 00:59:05 +0000198.Nm ssh
199command line to clear port forwardings set in
200configuration files, and is automatically set by
201.Xr scp 1
202and
203.Xr sftp 1 .
204The argument must be
205.Dq yes
206or
207.Dq no .
208The default is
209.Dq no .
210.It Cm Compression
211Specifies whether to use compression.
212The argument must be
213.Dq yes
214or
215.Dq no .
216The default is
217.Dq no .
218.It Cm CompressionLevel
219Specifies the compression level to use if compression is enabled.
220The argument must be an integer from 1 (fast) to 9 (slow, best).
221The default level is 6, which is good for most applications.
222The meaning of the values is the same as in
223.Xr gzip 1 .
224Note that this option applies to protocol version 1 only.
225.It Cm ConnectionAttempts
226Specifies the number of tries (one per second) to make before exiting.
227The argument must be an integer.
228This may be useful in scripts if the connection sometimes fails.
229The default is 1.
230.It Cm DynamicForward
231Specifies that a TCP/IP port on the local machine be forwarded
232over the secure channel, and the application
233protocol is then used to determine where to connect to from the
Damien Miller495dca32003-04-01 21:42:14 +1000234remote machine.
235The argument must be a port number.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000236Currently the SOCKS4 protocol is supported, and
237.Nm ssh
238will act as a SOCKS4 server.
239Multiple forwardings may be specified, and
Damien Miller495dca32003-04-01 21:42:14 +1000240additional forwardings can be given on the command line.
241Only the superuser can forward privileged ports.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000242.It Cm EscapeChar
243Sets the escape character (default:
244.Ql ~ ) .
245The escape character can also
246be set on the command line.
247The argument should be a single character,
248.Ql ^
249followed by a letter, or
250.Dq none
251to disable the escape
252character entirely (making the connection transparent for binary
253data).
254.It Cm ForwardAgent
255Specifies whether the connection to the authentication agent (if any)
256will be forwarded to the remote machine.
257The argument must be
258.Dq yes
259or
260.Dq no .
261The default is
262.Dq no .
Damien Milleraf653042002-09-04 16:40:37 +1000263.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000264Agent forwarding should be enabled with caution.
265Users with the ability to bypass file permissions on the remote host
266(for the agent's Unix-domain socket)
267can access the local agent through the forwarded connection.
268An attacker cannot obtain key material from the agent,
Damien Milleraf653042002-09-04 16:40:37 +1000269however they can perform operations on the keys that enable them to
270authenticate using the identities loaded into the agent.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000271.It Cm ForwardX11
272Specifies whether X11 connections will be automatically redirected
273over the secure channel and
274.Ev DISPLAY
275set.
276The argument must be
277.Dq yes
278or
279.Dq no .
280The default is
281.Dq no .
Damien Milleraf653042002-09-04 16:40:37 +1000282.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000283X11 forwarding should be enabled with caution.
284Users with the ability to bypass file permissions on the remote host
285(for the user's X authorization database)
286can access the local X11 display through the forwarded connection.
287An attacker may then be able to perform activities such as keystroke monitoring.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000288.It Cm GatewayPorts
289Specifies whether remote hosts are allowed to connect to local
290forwarded ports.
291By default,
292.Nm ssh
Damien Miller495dca32003-04-01 21:42:14 +1000293binds local port forwardings to the loopback address.
294This prevents other remote hosts from connecting to forwarded ports.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000295.Cm GatewayPorts
296can be used to specify that
297.Nm ssh
298should bind local port forwardings to the wildcard address,
299thus allowing remote hosts to connect to forwarded ports.
300The argument must be
301.Dq yes
302or
303.Dq no .
304The default is
305.Dq no .
306.It Cm GlobalKnownHostsFile
307Specifies a file to use for the global
308host key database instead of
309.Pa /etc/ssh/ssh_known_hosts .
310.It Cm HostbasedAuthentication
311Specifies whether to try rhosts based authentication with public key
312authentication.
313The argument must be
314.Dq yes
315or
316.Dq no .
317The default is
318.Dq no .
319This option applies to protocol version 2 only and
320is similar to
321.Cm RhostsRSAAuthentication .
322.It Cm HostKeyAlgorithms
323Specifies the protocol version 2 host key algorithms
324that the client wants to use in order of preference.
325The default for this option is:
326.Dq ssh-rsa,ssh-dss .
327.It Cm HostKeyAlias
328Specifies an alias that should be used instead of the
329real host name when looking up or saving the host key
330in the host key database files.
331This option is useful for tunneling ssh connections
332or for multiple servers running on a single host.
333.It Cm HostName
334Specifies the real host name to log into.
335This can be used to specify nicknames or abbreviations for hosts.
336Default is the name given on the command line.
337Numeric IP addresses are also permitted (both on the command line and in
338.Cm HostName
339specifications).
340.It Cm IdentityFile
341Specifies a file from which the user's RSA or DSA authentication identity
342is read. The default is
343.Pa $HOME/.ssh/identity
344for protocol version 1, and
345.Pa $HOME/.ssh/id_rsa
346and
347.Pa $HOME/.ssh/id_dsa
348for protocol version 2.
349Additionally, any identities represented by the authentication agent
350will be used for authentication.
351The file name may use the tilde
352syntax to refer to a user's home directory.
353It is possible to have
354multiple identity files specified in configuration files; all these
355identities will be tried in sequence.
356.It Cm KeepAlive
357Specifies whether the system should send TCP keepalive messages to the
358other side.
359If they are sent, death of the connection or crash of one
360of the machines will be properly noticed.
361However, this means that
362connections will die if the route is down temporarily, and some people
363find it annoying.
364.Pp
365The default is
366.Dq yes
367(to send keepalives), and the client will notice
368if the network goes down or the remote host dies.
369This is important in scripts, and many users want it too.
370.Pp
371To disable keepalives, the value should be set to
372.Dq no .
373.It Cm KerberosAuthentication
374Specifies whether Kerberos authentication will be used.
375The argument to this keyword must be
376.Dq yes
377or
378.Dq no .
379.It Cm KerberosTgtPassing
380Specifies whether a Kerberos TGT will be forwarded to the server.
381This will only work if the Kerberos server is actually an AFS kaserver.
382The argument to this keyword must be
383.Dq yes
384or
385.Dq no .
386.It Cm LocalForward
387Specifies that a TCP/IP port on the local machine be forwarded over
388the secure channel to the specified host and port from the remote machine.
389The first argument must be a port number, and the second must be
390.Ar host:port .
391IPv6 addresses can be specified with an alternative syntax:
392.Ar host/port .
393Multiple forwardings may be specified, and additional
394forwardings can be given on the command line.
395Only the superuser can forward privileged ports.
396.It Cm LogLevel
397Gives the verbosity level that is used when logging messages from
398.Nm ssh .
399The possible values are:
400QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
Damien Miller495dca32003-04-01 21:42:14 +1000401The default is INFO.
402DEBUG and DEBUG1 are equivalent.
403DEBUG2 and DEBUG3 each specify higher levels of verbose output.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000404.It Cm MACs
405Specifies the MAC (message authentication code) algorithms
406in order of preference.
407The MAC algorithm is used in protocol version 2
408for data integrity protection.
409Multiple algorithms must be comma-separated.
410The default is
411.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
412.It Cm NoHostAuthenticationForLocalhost
413This option can be used if the home directory is shared across machines.
414In this case localhost will refer to a different machine on each of
415the machines and the user will get many warnings about changed host keys.
416However, this option disables host authentication for localhost.
417The argument to this keyword must be
418.Dq yes
419or
420.Dq no .
421The default is to check the host key for localhost.
422.It Cm NumberOfPasswordPrompts
423Specifies the number of password prompts before giving up.
424The argument to this keyword must be an integer.
425Default is 3.
426.It Cm PasswordAuthentication
427Specifies whether to use password authentication.
428The argument to this keyword must be
429.Dq yes
430or
431.Dq no .
432The default is
433.Dq yes .
434.It Cm Port
435Specifies the port number to connect on the remote host.
436Default is 22.
437.It Cm PreferredAuthentications
438Specifies the order in which the client should try protocol 2
439authentication methods. This allows a client to prefer one method (e.g.
440.Cm keyboard-interactive )
441over another method (e.g.
442.Cm password )
443The default for this option is:
444.Dq hostbased,publickey,keyboard-interactive,password .
445.It Cm Protocol
446Specifies the protocol versions
447.Nm ssh
448should support in order of preference.
449The possible values are
450.Dq 1
451and
452.Dq 2 .
453Multiple versions must be comma-separated.
454The default is
455.Dq 2,1 .
456This means that
457.Nm ssh
458tries version 2 and falls back to version 1
459if version 2 is not available.
460.It Cm ProxyCommand
461Specifies the command to use to connect to the server.
462The command
463string extends to the end of the line, and is executed with
464.Pa /bin/sh .
465In the command string,
466.Ql %h
467will be substituted by the host name to
468connect and
469.Ql %p
470by the port.
471The command can be basically anything,
472and should read from its standard input and write to its standard output.
473It should eventually connect an
474.Xr sshd 8
475server running on some machine, or execute
476.Ic sshd -i
477somewhere.
478Host key management will be done using the
479HostName of the host being connected (defaulting to the name typed by
480the user).
Damien Miller495dca32003-04-01 21:42:14 +1000481Setting the command to
482.Dq none
Damien Miller9f1e33a2003-02-24 11:57:32 +1100483disables this option entirely.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000484Note that
485.Cm CheckHostIP
486is not available for connects with a proxy command.
487.Pp
488.It Cm PubkeyAuthentication
489Specifies whether to try public key authentication.
490The argument to this keyword must be
491.Dq yes
492or
493.Dq no .
494The default is
495.Dq yes .
496This option applies to protocol version 2 only.
497.It Cm RemoteForward
498Specifies that a TCP/IP port on the remote machine be forwarded over
499the secure channel to the specified host and port from the local machine.
500The first argument must be a port number, and the second must be
501.Ar host:port .
502IPv6 addresses can be specified with an alternative syntax:
503.Ar host/port .
504Multiple forwardings may be specified, and additional
505forwardings can be given on the command line.
506Only the superuser can forward privileged ports.
507.It Cm RhostsAuthentication
508Specifies whether to try rhosts based authentication.
509Note that this
510declaration only affects the client side and has no effect whatsoever
511on security.
512Most servers do not permit RhostsAuthentication because it
513is not secure (see
514.Cm RhostsRSAAuthentication ) .
515The argument to this keyword must be
516.Dq yes
517or
518.Dq no .
519The default is
520.Dq no .
Damien Miller9b1dacd2002-09-04 16:47:35 +1000521This option applies to protocol version 1 only and requires
522.Nm ssh
523to be setuid root and
524.Cm UsePrivilegedPort
525to be set to
526.Dq yes .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000527.It Cm RhostsRSAAuthentication
528Specifies whether to try rhosts based authentication with RSA host
529authentication.
530The argument must be
531.Dq yes
532or
533.Dq no .
534The default is
535.Dq no .
536This option applies to protocol version 1 only and requires
537.Nm ssh
538to be setuid root.
539.It Cm RSAAuthentication
540Specifies whether to try RSA authentication.
541The argument to this keyword must be
542.Dq yes
543or
544.Dq no .
545RSA authentication will only be
546attempted if the identity file exists, or an authentication agent is
547running.
548The default is
549.Dq yes .
550Note that this option applies to protocol version 1 only.
551.It Cm SmartcardDevice
552Specifies which smartcard device to use. The argument to this keyword is
553the device
554.Nm ssh
555should use to communicate with a smartcard used for storing the user's
556private RSA key. By default, no device is specified and smartcard support
557is not activated.
558.It Cm StrictHostKeyChecking
559If this flag is set to
560.Dq yes ,
561.Nm ssh
562will never automatically add host keys to the
563.Pa $HOME/.ssh/known_hosts
564file, and refuses to connect to hosts whose host key has changed.
565This provides maximum protection against trojan horse attacks,
566however, can be annoying when the
567.Pa /etc/ssh/ssh_known_hosts
568file is poorly maintained, or connections to new hosts are
569frequently made.
570This option forces the user to manually
571add all new hosts.
572If this flag is set to
573.Dq no ,
574.Nm ssh
575will automatically add new host keys to the
576user known hosts files.
577If this flag is set to
578.Dq ask ,
579new host keys
580will be added to the user known host files only after the user
581has confirmed that is what they really want to do, and
582.Nm ssh
583will refuse to connect to hosts whose host key has changed.
584The host keys of
585known hosts will be verified automatically in all cases.
586The argument must be
587.Dq yes ,
588.Dq no
589or
590.Dq ask .
591The default is
592.Dq ask .
593.It Cm UsePrivilegedPort
594Specifies whether to use a privileged port for outgoing connections.
595The argument must be
596.Dq yes
597or
598.Dq no .
599The default is
600.Dq no .
Damien Miller9b1dacd2002-09-04 16:47:35 +1000601If set to
602.Dq yes
603.Nm ssh
604must be setuid root.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000605Note that this option must be set to
606.Dq yes
607if
608.Cm RhostsAuthentication
609and
610.Cm RhostsRSAAuthentication
611authentications are needed with older servers.
612.It Cm User
613Specifies the user to log in as.
614This can be useful when a different user name is used on different machines.
615This saves the trouble of
616having to remember to give the user name on the command line.
617.It Cm UserKnownHostsFile
618Specifies a file to use for the user
619host key database instead of
620.Pa $HOME/.ssh/known_hosts .
Damien Miller37876e92003-05-15 10:19:46 +1000621.It Cm VerifyHostKeyDNS
622Specifies whether to verify the remote key using DNS and SSHFP resource
623records.
624The default is
625.Dq no .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000626.It Cm XAuthLocation
Damien Miller05913ba2002-09-04 16:51:03 +1000627Specifies the full pathname of the
Ben Lindstrom9f049032002-06-21 00:59:05 +0000628.Xr xauth 1
629program.
630The default is
631.Pa /usr/X11R6/bin/xauth .
632.El
633.Sh FILES
634.Bl -tag -width Ds
635.It Pa $HOME/.ssh/config
636This is the per-user configuration file.
637The format of this file is described above.
638This file is used by the
639.Nm ssh
640client.
641This file does not usually contain any sensitive information,
642but the recommended permissions are read/write for the user, and not
643accessible by others.
644.It Pa /etc/ssh/ssh_config
645Systemwide configuration file.
646This file provides defaults for those
647values that are not specified in the user's configuration file, and
648for those users who do not have a configuration file.
649This file must be world-readable.
650.El
651.Sh AUTHORS
652OpenSSH is a derivative of the original and free
653ssh 1.2.12 release by Tatu Ylonen.
654Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
655Theo de Raadt and Dug Song
656removed many bugs, re-added newer features and
657created OpenSSH.
658Markus Friedl contributed the support for SSH
659protocol versions 1.5 and 2.0.
660.Sh SEE ALSO
661.Xr ssh 1