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Damien Miller32aa1441999-10-29 09:15:49 +10001.\" -*- nroff -*-
2.\"
Damien Miller32aa1441999-10-29 09:15:49 +10003.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller32aa1441999-10-29 09:15:49 +10004.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5.\" All rights reserved
6.\"
Damien Millere4340be2000-09-16 13:29:08 +11007.\" 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".
Damien Miller32aa1441999-10-29 09:15:49 +100012.\"
Ben Lindstrom92a2e382001-03-05 06:59:27 +000013.\" 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.
Damien Millere4340be2000-09-16 13:29:08 +110016.\"
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.
Damien Miller32aa1441999-10-29 09:15:49 +100036.\"
Damien Miller208f1ed2006-03-15 11:56:03 +110037.\" $OpenBSD: ssh.1,v 1.257 2006/02/24 20:31:30 jmc Exp $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dd September 25, 1999
39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000043.Nd OpenSSH SSH client (remote login program)
Damien Miller32aa1441999-10-29 09:15:49 +100044.Sh SYNOPSIS
45.Nm ssh
Darren Tuckerba5c5922004-06-18 16:22:39 +100046.Op Fl 1246AaCfgkMNnqsTtVvXxY
Ben Lindstrome0f88042001-04-30 13:06:24 +000047.Op Fl b Ar bind_address
Damien Miller30c3d422000-05-09 11:02:59 +100048.Op Fl c Ar cipher_spec
Darren Tuckerc8d64212005-10-03 18:13:42 +100049.Oo Fl D\ \&
50.Sm off
51.Oo Ar bind_address : Oc
52.Ar port
53.Sm on
54.Oc
Damien Miller32aa1441999-10-29 09:15:49 +100055.Op Fl e Ar escape_char
Ben Lindstrom14f31ab2001-09-12 17:48:04 +000056.Op Fl F Ar configfile
Darren Tucker05d4dfe2005-10-03 18:17:38 +100057.Bk -words
Darren Tucker61776952003-10-02 16:19:47 +100058.Op Fl i Ar identity_file
Darren Tucker05d4dfe2005-10-03 18:17:38 +100059.Ek
Damien Miller36bf7dd2005-03-02 12:02:47 +110060.Oo Fl L\ \&
Damien Miller32aa1441999-10-29 09:15:49 +100061.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +110062.Oo Ar bind_address : Oc
Damien Miller36bf7dd2005-03-02 12:02:47 +110063.Ar port : host : hostport
Damien Miller32aa1441999-10-29 09:15:49 +100064.Sm on
Damien Miller32aa1441999-10-29 09:15:49 +100065.Oc
Darren Tucker05d4dfe2005-10-03 18:17:38 +100066.Bk -words
Darren Tucker61776952003-10-02 16:19:47 +100067.Op Fl l Ar login_name
Darren Tucker05d4dfe2005-10-03 18:17:38 +100068.Ek
Darren Tucker61776952003-10-02 16:19:47 +100069.Op Fl m Ar mac_spec
Darren Tucker7ebfc102004-11-07 20:06:19 +110070.Op Fl O Ar ctl_cmd
Darren Tucker61776952003-10-02 16:19:47 +100071.Op Fl o Ar option
Darren Tucker61776952003-10-02 16:19:47 +100072.Op Fl p Ar port
Damien Miller36bf7dd2005-03-02 12:02:47 +110073.Oo Fl R\ \&
Damien Miller32aa1441999-10-29 09:15:49 +100074.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +110075.Oo Ar bind_address : Oc
Damien Miller36bf7dd2005-03-02 12:02:47 +110076.Ar port : host : hostport
Damien Miller32aa1441999-10-29 09:15:49 +100077.Sm on
Damien Miller32aa1441999-10-29 09:15:49 +100078.Oc
Darren Tucker7ebfc102004-11-07 20:06:19 +110079.Op Fl S Ar ctl_path
Damien Millerf0c8c152005-12-13 19:29:58 +110080.Bk -words
81.Op Fl w Ar tunnel : Ns Ar tunnel
Darren Tucker61776952003-10-02 16:19:47 +100082.Oo Ar user Ns @ Oc Ns Ar hostname
Damien Miller32aa1441999-10-29 09:15:49 +100083.Op Ar command
Damien Millerf0c8c152005-12-13 19:29:58 +110084.Ek
Damien Miller22c77262000-04-13 12:26:34 +100085.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100086.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000087(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110088executing commands on a remote machine.
Darren Tucker61776952003-10-02 16:19:47 +100089It is intended to replace rlogin and rsh,
90and provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110091two untrusted hosts over an insecure network.
Damien Miller8bfaf932006-01-14 10:09:30 +110092X11 connections and arbitrary TCP ports
Darren Tucker61776952003-10-02 16:19:47 +100093can also be forwarded over the secure channel.
Damien Miller32aa1441999-10-29 09:15:49 +100094.Pp
95.Nm
Damien Miller22c77262000-04-13 12:26:34 +100096connects and logs into the specified
Darren Tucker61776952003-10-02 16:19:47 +100097.Ar hostname
98(with optional
99.Ar user
100name).
Damien Miller32aa1441999-10-29 09:15:49 +1000101The user must prove
Damien Millere247cc42000-05-07 12:03:14 +1000102his/her identity to the remote machine using one of several methods
Darren Tucker5434cfe2005-12-20 16:11:35 +1100103depending on the protocol version used (see below).
Damien Millere247cc42000-05-07 12:03:14 +1000104.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000105If
106.Ar command
107is specified,
Darren Tuckerb18c8672005-12-20 16:10:09 +1100108it is executed on the remote host instead of a login shell.
Darren Tucker0b42e6d2004-08-13 21:22:40 +1000109.Pp
Damien Miller0bc1bd82000-11-13 22:57:25 +1100110The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000111.Bl -tag -width Ds
Darren Tucker61776952003-10-02 16:19:47 +1000112.It Fl 1
113Forces
114.Nm
115to try protocol version 1 only.
116.It Fl 2
117Forces
118.Nm
119to try protocol version 2 only.
120.It Fl 4
121Forces
122.Nm
123to use IPv4 addresses only.
124.It Fl 6
125Forces
126.Nm
127to use IPv6 addresses only.
Damien Millerb1715dc2000-05-30 13:44:51 +1000128.It Fl A
129Enables forwarding of the authentication agent connection.
130This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000131.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000132Agent forwarding should be enabled with caution.
133Users with the ability to bypass file permissions on the remote host
134(for the agent's Unix-domain socket)
135can access the local agent through the forwarded connection.
136An attacker cannot obtain key material from the agent,
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000137however they can perform operations on the keys that enable them to
138authenticate using the identities loaded into the agent.
Darren Tucker61776952003-10-02 16:19:47 +1000139.It Fl a
140Disables forwarding of the authentication agent connection.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000141.It Fl b Ar bind_address
Darren Tuckera5cf8552005-07-14 17:04:18 +1000142Use
143.Ar bind_address
144on the local machine as the source address
145of the connection.
146Only useful on systems with more than one address.
Darren Tucker61776952003-10-02 16:19:47 +1000147.It Fl C
148Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller8bfaf932006-01-14 10:09:30 +1100149data for forwarded X11 and TCP connections).
Darren Tucker61776952003-10-02 16:19:47 +1000150The compression algorithm is the same used by
151.Xr gzip 1 ,
152and the
153.Dq level
154can be controlled by the
155.Cm CompressionLevel
156option for protocol version 1.
157Compression is desirable on modem lines and other
158slow connections, but will only slow down things on fast networks.
159The default value can be set on a host-by-host basis in the
160configuration files; see the
161.Cm Compression
162option.
Damien Miller05202ff2004-06-15 10:30:39 +1000163.It Fl c Ar cipher_spec
164Selects the cipher specification for encrypting the session.
165.Pp
166Protocol version 1 allows specification of a single cipher.
Darren Tucker1e4308e2005-10-03 18:18:40 +1000167The supported values are
Damien Miller05202ff2004-06-15 10:30:39 +1000168.Dq 3des ,
Darren Tuckerff9f4842005-12-20 16:10:45 +1100169.Dq blowfish ,
Damien Miller05202ff2004-06-15 10:30:39 +1000170and
171.Dq des .
Damien Miller32aa1441999-10-29 09:15:49 +1000172.Ar 3des
173(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
Damien Miller05202ff2004-06-15 10:30:39 +1000174It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000175.Ar blowfish
Darren Tucker61776952003-10-02 16:19:47 +1000176is a fast block cipher; it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100177.Ar 3des .
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000178.Ar des
179is only supported in the
180.Nm
181client for interoperability with legacy protocol 1 implementations
182that do not support the
183.Ar 3des
Damien Miller495dca32003-04-01 21:42:14 +1000184cipher.
185Its use is strongly discouraged due to cryptographic weaknesses.
Damien Miller05202ff2004-06-15 10:30:39 +1000186The default is
187.Dq 3des .
188.Pp
Damien Miller35978212005-12-24 14:56:47 +1100189For protocol version 2,
Damien Miller05202ff2004-06-15 10:30:39 +1000190.Ar cipher_spec
191is a comma-separated list of ciphers
192listed in order of preference.
Damien Miller35978212005-12-24 14:56:47 +1100193The supported ciphers are:
1943des-cbc,
195aes128-cbc,
196aes192-cbc,
197aes256-cbc,
198aes128-ctr,
199aes192-ctr,
200aes256-ctr,
201arcfour128,
202arcfour256,
203arcfour,
204blowfish-cbc,
Damien Miller05202ff2004-06-15 10:30:39 +1000205and
Damien Miller35978212005-12-24 14:56:47 +1100206cast128-cbc.
Darren Tuckerff9f4842005-12-20 16:10:45 +1100207The default is:
208.Bd -literal -offset indent
209aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
210arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
211aes192-ctr,aes256-ctr
Damien Miller05202ff2004-06-15 10:30:39 +1000212.Ed
Darren Tuckerc8d64212005-10-03 18:13:42 +1000213.It Fl D Xo
214.Sm off
215.Oo Ar bind_address : Oc
216.Ar port
217.Sm on
218.Xc
Darren Tucker61776952003-10-02 16:19:47 +1000219Specifies a local
220.Dq dynamic
221application-level port forwarding.
222This works by allocating a socket to listen to
223.Ar port
Darren Tuckerc8d64212005-10-03 18:13:42 +1000224on the local side, optionally bound to the specified
225.Ar bind_address .
226Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000227connection is forwarded over the secure channel, and the application
228protocol is then used to determine where to connect to from the
229remote machine.
230Currently the SOCKS4 and SOCKS5 protocols are supported, and
231.Nm
232will act as a SOCKS server.
233Only root can forward privileged ports.
234Dynamic port forwardings can also be specified in the configuration file.
Darren Tuckerc8d64212005-10-03 18:13:42 +1000235.Pp
236IPv6 addresses can be specified with an alternative syntax:
237.Sm off
238.Xo
239.Op Ar bind_address No /
240.Ar port
241.Xc
242.Sm on
243or by enclosing the address in square brackets.
244Only the superuser can forward privileged ports.
245By default, the local port is bound in accordance with the
246.Cm GatewayPorts
247setting.
248However, an explicit
249.Ar bind_address
250may be used to bind the connection to a specific address.
251The
252.Ar bind_address
253of
254.Dq localhost
255indicates that the listening port be bound for local use only, while an
256empty address or
257.Sq *
258indicates that the port should be available from all interfaces.
Damien Miller2142ba02005-12-24 14:56:29 +1100259.It Fl e Ar escape_char
Damien Miller32aa1441999-10-29 09:15:49 +1000260Sets the escape character for sessions with a pty (default:
261.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100262The escape character is only recognized at the beginning of a line.
263The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000264.Pq Ql \&.
Darren Tucker61776952003-10-02 16:19:47 +1000265closes the connection;
266followed by control-Z suspends the connection;
267and followed by itself sends the escape character once.
Damien Miller7684ee12000-03-17 23:40:15 +1100268Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000269.Dq none
270disables any escapes and makes the session fully transparent.
Darren Tucker61776952003-10-02 16:19:47 +1000271.It Fl F Ar configfile
272Specifies an alternative per-user configuration file.
273If a configuration file is given on the command line,
274the system-wide configuration file
275.Pq Pa /etc/ssh/ssh_config
276will be ignored.
277The default for the per-user configuration file is
Damien Miller167ea5d2005-05-26 12:04:02 +1000278.Pa ~/.ssh/config .
Damien Miller32aa1441999-10-29 09:15:49 +1000279.It Fl f
280Requests
281.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100282to go to background just before command execution.
283This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000284.Nm
285is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100286wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000287This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000288.Fl n .
289The recommended way to start X11 programs at a remote site is with
290something like
291.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100292.It Fl g
293Allows remote hosts to connect to local forwarded ports.
Darren Tucker61776952003-10-02 16:19:47 +1000294.It Fl I Ar smartcard_device
Damien Miller2142ba02005-12-24 14:56:29 +1100295Specify the device
Darren Tucker61776952003-10-02 16:19:47 +1000296.Nm
297should use to communicate with a smartcard used for storing the user's
298private RSA key.
Damien Miller2142ba02005-12-24 14:56:29 +1100299This option is only available if support for smartcard devices
300is compiled in (default is no support).
Damien Miller32aa1441999-10-29 09:15:49 +1000301.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100302Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100303RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100304The default is
Damien Miller167ea5d2005-05-26 12:04:02 +1000305.Pa ~/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100306for protocol version 1, and
Damien Miller167ea5d2005-05-26 12:04:02 +1000307.Pa ~/.ssh/id_rsa
Damien Millerfae23862002-01-22 23:32:26 +1100308and
Damien Miller167ea5d2005-05-26 12:04:02 +1000309.Pa ~/.ssh/id_dsa
Damien Millerfae23862002-01-22 23:32:26 +1100310for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100311Identity files may also be specified on
312a per-host basis in the configuration file.
313It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000314.Fl i
315options (and multiple identities specified in
316configuration files).
Damien Miller32aa1441999-10-29 09:15:49 +1000317.It Fl k
Damien Millere0113cc2003-11-24 13:10:09 +1100318Disables forwarding (delegation) of GSSAPI credentials to the server.
Darren Tucker61776952003-10-02 16:19:47 +1000319.It Fl L Xo
320.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100321.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000322.Ar port : host : hostport
323.Sm on
324.Xc
325Specifies that the given port on the local (client) host is to be
326forwarded to the given host and port on the remote side.
327This works by allocating a socket to listen to
328.Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100329on the local side, optionally bound to the specified
330.Ar bind_address .
331Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000332connection is forwarded over the secure channel, and a connection is
333made to
334.Ar host
335port
336.Ar hostport
337from the remote machine.
338Port forwardings can also be specified in the configuration file.
Darren Tucker61776952003-10-02 16:19:47 +1000339IPv6 addresses can be specified with an alternative syntax:
340.Sm off
341.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100342.Op Ar bind_address No /
Darren Tucker61776952003-10-02 16:19:47 +1000343.Ar port No / Ar host No /
Damien Millerf91ee4c2005-03-01 21:24:33 +1100344.Ar hostport
Darren Tucker61776952003-10-02 16:19:47 +1000345.Xc
346.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100347or by enclosing the address in square brackets.
348Only the superuser can forward privileged ports.
349By default, the local port is bound in accordance with the
350.Cm GatewayPorts
351setting.
352However, an explicit
353.Ar bind_address
354may be used to bind the connection to a specific address.
355The
356.Ar bind_address
357of
358.Dq localhost
Damien Miller36bf7dd2005-03-02 12:02:47 +1100359indicates that the listening port be bound for local use only, while an
360empty address or
361.Sq *
Damien Millerf91ee4c2005-03-01 21:24:33 +1100362indicates that the port should be available from all interfaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000363.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100364Specifies the user to log in as on the remote machine.
365This also may be specified on a per-host basis in the configuration file.
Damien Miller0e220db2004-06-15 10:34:08 +1000366.It Fl M
367Places the
368.Nm
369client into
370.Dq master
371mode for connection sharing.
Damien Miller88b25522005-12-31 16:23:15 +1100372Multiple
373.Fl M
374options places
375.Nm
376into
377.Dq master
378mode with confirmation required before slave connections are accepted.
Damien Miller0e220db2004-06-15 10:34:08 +1000379Refer to the description of
380.Cm ControlMaster
381in
382.Xr ssh_config 5
383for details.
Damien Miller5d1eceb2004-06-30 22:37:57 +1000384.It Fl m Ar mac_spec
385Additionally, for protocol version 2 a comma-separated list of MAC
386(message authentication code) algorithms can
387be specified in order of preference.
388See the
389.Cm MACs
390keyword for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000391.It Fl N
392Do not execute a remote command.
393This is useful for just forwarding ports
394(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000395.It Fl n
396Redirects stdin from
397.Pa /dev/null
398(actually, prevents reading from stdin).
399This must be used when
400.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100401is run in the background.
402A common trick is to use this to run X11 programs on a remote machine.
403For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000404.Ic ssh -n shadows.cs.hut.fi emacs &
405will start an emacs on shadows.cs.hut.fi, and the X11
406connection will be automatically forwarded over an encrypted channel.
407The
408.Nm
409program will be put in the background.
410(This does not work if
411.Nm
412needs to ask for a password or passphrase; see also the
413.Fl f
414option.)
Darren Tuckere04644c2004-12-03 14:08:45 +1100415.It Fl O Ar ctl_cmd
416Control an active connection multiplexing master process.
417When the
418.Fl O
419option is specified, the
420.Ar ctl_cmd
421argument is interpreted and passed to the master process.
422Valid commands are:
423.Dq check
424(check that the master process is running) and
425.Dq exit
426(request the master to exit).
Damien Miller32aa1441999-10-29 09:15:49 +1000427.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000428Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000429This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100430command-line flag.
Darren Tucker61776952003-10-02 16:19:47 +1000431For full details of the options listed below, and their possible values, see
432.Xr ssh_config 5 .
433.Pp
434.Bl -tag -width Ds -offset indent -compact
435.It AddressFamily
436.It BatchMode
437.It BindAddress
438.It ChallengeResponseAuthentication
439.It CheckHostIP
440.It Cipher
441.It Ciphers
442.It ClearAllForwardings
443.It Compression
444.It CompressionLevel
445.It ConnectionAttempts
Darren Tuckeredae0ec2004-05-02 22:15:52 +1000446.It ConnectTimeout
Damien Miller0e220db2004-06-15 10:34:08 +1000447.It ControlMaster
448.It ControlPath
Darren Tucker61776952003-10-02 16:19:47 +1000449.It DynamicForward
Darren Tucker61776952003-10-02 16:19:47 +1000450.It EscapeChar
451.It ForwardAgent
452.It ForwardX11
Darren Tucker0a118da2003-10-15 15:54:32 +1000453.It ForwardX11Trusted
Darren Tucker61776952003-10-02 16:19:47 +1000454.It GatewayPorts
455.It GlobalKnownHostsFile
456.It GSSAPIAuthentication
457.It GSSAPIDelegateCredentials
Damien Millere1776152005-03-01 21:47:37 +1100458.It HashKnownHosts
Darren Tucker61776952003-10-02 16:19:47 +1000459.It Host
460.It HostbasedAuthentication
461.It HostKeyAlgorithms
462.It HostKeyAlias
463.It HostName
464.It IdentityFile
Damien Millerbd394c32004-03-08 23:12:36 +1100465.It IdentitiesOnly
Darren Tucker636ca902004-11-05 20:22:00 +1100466.It KbdInteractiveDevices
Damien Millerd27b9472005-12-13 19:29:02 +1100467.It LocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000468.It LocalForward
469.It LogLevel
470.It MACs
471.It NoHostAuthenticationForLocalhost
472.It NumberOfPasswordPrompts
473.It PasswordAuthentication
Damien Millerd27b9472005-12-13 19:29:02 +1100474.It PermitLocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000475.It Port
476.It PreferredAuthentications
477.It Protocol
478.It ProxyCommand
479.It PubkeyAuthentication
Darren Tucker62388b22006-01-20 11:31:47 +1100480.It RekeyLimit
Darren Tucker61776952003-10-02 16:19:47 +1000481.It RemoteForward
482.It RhostsRSAAuthentication
483.It RSAAuthentication
Darren Tucker7a6c0662004-05-02 22:14:03 +1000484.It SendEnv
Damien Miller509b0102003-12-17 16:33:10 +1100485.It ServerAliveInterval
486.It ServerAliveCountMax
Darren Tucker61776952003-10-02 16:19:47 +1000487.It SmartcardDevice
488.It StrictHostKeyChecking
Damien Miller12c150e2003-12-17 16:31:10 +1100489.It TCPKeepAlive
Damien Millerd27b9472005-12-13 19:29:02 +1100490.It Tunnel
491.It TunnelDevice
Darren Tucker61776952003-10-02 16:19:47 +1000492.It UsePrivilegedPort
493.It User
494.It UserKnownHostsFile
495.It VerifyHostKeyDNS
496.It XAuthLocation
497.El
Damien Miller32aa1441999-10-29 09:15:49 +1000498.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100499Port to connect to on the remote host.
500This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000501per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000502.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100503Quiet mode.
504Causes all warning and diagnostic messages to be suppressed.
Darren Tucker61776952003-10-02 16:19:47 +1000505.It Fl R Xo
506.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100507.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000508.Ar port : host : hostport
509.Sm on
510.Xc
Damien Miller32aa1441999-10-29 09:15:49 +1000511Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100512forwarded to the given host and port on the local side.
513This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000514.Ar port
515on the remote side, and whenever a connection is made to this port, the
516connection is forwarded over the secure channel, and a connection is
517made to
Damien Miller34132e52000-01-14 15:45:46 +1100518.Ar host
519port
520.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100521from the local machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100522.Pp
Damien Miller7684ee12000-03-17 23:40:15 +1100523Port forwardings can also be specified in the configuration file.
524Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000525logging in as root on the remote machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100526IPv6 addresses can be specified by enclosing the address in square braces or
527using an alternative syntax:
Damien Millerf91ee4c2005-03-01 21:24:33 +1100528.Sm off
Damien Miller36bf7dd2005-03-02 12:02:47 +1100529.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100530.Op Ar bind_address No /
Damien Miller36bf7dd2005-03-02 12:02:47 +1100531.Ar host No / Ar port No /
532.Ar hostport
Damien Millerf91ee4c2005-03-01 21:24:33 +1100533.Xc .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100534.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100535.Pp
536By default, the listening socket on the server will be bound to the loopback
537interface only.
538This may be overriden by specifying a
539.Ar bind_address .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100540An empty
541.Ar bind_address ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100542or the address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100543.Ql * ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100544indicates that the remote socket should listen on all interfaces.
545Specifying a remote
546.Ar bind_address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100547will only succeed if the server's
548.Cm GatewayPorts
Damien Millerf91ee4c2005-03-01 21:24:33 +1100549option is enabled (see
Damien Miller36bf7dd2005-03-02 12:02:47 +1100550.Xr sshd_config 5 ) .
Darren Tucker7ebfc102004-11-07 20:06:19 +1100551.It Fl S Ar ctl_path
Damien Miller5d1eceb2004-06-30 22:37:57 +1000552Specifies the location of a control socket for connection sharing.
Damien Miller0e220db2004-06-15 10:34:08 +1000553Refer to the description of
Darren Tuckerba5c5922004-06-18 16:22:39 +1000554.Cm ControlPath
555and
Damien Miller0e220db2004-06-15 10:34:08 +1000556.Cm ControlMaster
557in
558.Xr ssh_config 5
559for details.
Darren Tucker61776952003-10-02 16:19:47 +1000560.It Fl s
561May be used to request invocation of a subsystem on the remote system.
562Subsystems are a feature of the SSH2 protocol which facilitate the use
563of SSH as a secure transport for other applications (eg.\&
564.Xr sftp 1 ) .
565The subsystem is specified as the remote command.
566.It Fl T
567Disable pseudo-tty allocation.
568.It Fl t
569Force pseudo-tty allocation.
570This can be used to execute arbitrary
571screen-based programs on a remote machine, which can be very useful,
Damien Miller208f1ed2006-03-15 11:56:03 +1100572e.g. when implementing menu services.
Darren Tucker61776952003-10-02 16:19:47 +1000573Multiple
574.Fl t
575options force tty allocation, even if
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000576.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000577has no local tty.
578.It Fl V
579Display the version number and exit.
580.It Fl v
581Verbose mode.
582Causes
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000583.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000584to print debugging messages about its progress.
585This is helpful in
586debugging connection, authentication, and configuration problems.
587Multiple
588.Fl v
589options increase the verbosity.
590The maximum is 3.
Damien Millerf0c8c152005-12-13 19:29:58 +1100591.It Fl w Ar tunnel : Ns Ar tunnel
Damien Millerd27b9472005-12-13 19:29:02 +1100592Requests a
593.Xr tun 4
Damien Millercf1e3422005-12-24 14:56:04 +1100594device on the client
595(first
596.Ar tunnel
597arg)
598and server
599(second
600.Ar tunnel
601arg).
602The devices may be specified by numerical ID or the keyword
603.Dq any ,
604which uses the next available tunnel device.
605See also the
Damien Millerd27b9472005-12-13 19:29:02 +1100606.Cm Tunnel
607directive in
608.Xr ssh_config 5 .
Darren Tucker61776952003-10-02 16:19:47 +1000609.It Fl X
610Enables X11 forwarding.
611This can also be specified on a per-host basis in a configuration file.
612.Pp
613X11 forwarding should be enabled with caution.
614Users with the ability to bypass file permissions on the remote host
615(for the user's X authorization database)
616can access the local X11 display through the forwarded connection.
617An attacker may then be able to perform activities such as keystroke monitoring.
Damien Miller947219e2005-03-02 13:22:30 +1100618.Pp
Damien Millerb022b232005-03-05 11:22:36 +1100619For this reason, X11 forwarding is subjected to X11 SECURITY extension
Damien Miller947219e2005-03-02 13:22:30 +1100620restrictions by default.
621Please refer to the
622.Nm
623.Fl Y
624option and the
625.Cm ForwardX11Trusted
626directive in
627.Xr ssh_config 5
628for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000629.It Fl x
630Disables X11 forwarding.
Darren Tucker0a118da2003-10-15 15:54:32 +1000631.It Fl Y
632Enables trusted X11 forwarding.
Damien Miller947219e2005-03-02 13:22:30 +1100633Trusted X11 forwardings are not subjected to the X11 SECURITY extension
634controls.
Damien Miller32aa1441999-10-29 09:15:49 +1000635.El
Damien Millere9b333a2005-12-24 14:53:04 +1100636.Pp
637.Nm
638may additionally obtain configuration data from
639a per-user configuration file and a system-wide configuration file.
640The file format and configuration options are described in
641.Xr ssh_config 5 .
642.Pp
643.Nm
644exits with the exit status of the remote command or with 255
645if an error occurred.
Damien Millerc93a8132005-12-24 14:52:13 +1100646.Sh AUTHENTICATION
Darren Tucker94299ec2006-01-20 11:30:14 +1100647The OpenSSH SSH client supports SSH protocols 1 and 2.
Damien Millerc93a8132005-12-24 14:52:13 +1100648Protocol 2 is the default, with
649.Nm
650falling back to protocol 1 if it detects protocol 2 is unsupported.
651These settings may be altered using the
652.Cm Protocol
653option in
654.Xr ssh_config 5 ,
655or enforced using the
656.Fl 1
657and
658.Fl 2
659options (see above).
660Both protocols support similar authentication methods,
661but protocol 2 is preferred since
662it provides additional mechanisms for confidentiality
663(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
664and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
665Protocol 1 lacks a strong mechanism for ensuring the
666integrity of the connection.
667.Pp
668The methods available for authentication are:
669host-based authentication,
670public key authentication,
671challenge-response authentication,
672and password authentication.
673Authentication methods are tried in the order specified above,
674though protocol 2 has a configuration option to change the default order:
675.Cm PreferredAuthentications .
676.Pp
677Host-based authentication works as follows:
Darren Tuckerd3877b92005-12-20 16:09:36 +1100678If the machine the user logs in from is listed in
679.Pa /etc/hosts.equiv
680or
681.Pa /etc/shosts.equiv
682on the remote machine, and the user names are
683the same on both sides, or if the files
684.Pa ~/.rhosts
685or
686.Pa ~/.shosts
687exist in the user's home directory on the
688remote machine and contain a line containing the name of the client
689machine and the name of the user on that machine, the user is
Damien Millerc93a8132005-12-24 14:52:13 +1100690considered for login.
691Additionally, the server
692.Em must
693be able to verify the client's
694host key (see the description of
Darren Tuckerd3877b92005-12-20 16:09:36 +1100695.Pa /etc/ssh/ssh_known_hosts
696and
Damien Millerc93a8132005-12-24 14:52:13 +1100697.Pa ~/.ssh/known_hosts ,
698below)
699for login to be permitted.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100700This authentication method closes security holes due to IP
Damien Millerc93a8132005-12-24 14:52:13 +1100701spoofing, DNS spoofing, and routing spoofing.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100702[Note to the administrator:
703.Pa /etc/hosts.equiv ,
704.Pa ~/.rhosts ,
705and the rlogin/rsh protocol in general, are inherently insecure and should be
706disabled if security is desired.]
707.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100708Public key authentication works as follows:
709The scheme is based on public-key cryptography,
710using cryptosystems
711where encryption and decryption are done using separate keys,
712and it is unfeasible to derive the decryption key from the encryption key.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100713The idea is that each user creates a public/private
714key pair for authentication purposes.
715The server knows the public key, and only the user knows the private key.
Damien Millerc93a8132005-12-24 14:52:13 +1100716.Nm
717implements public key authentication protocol automatically,
718using either the RSA or DSA algorithms.
719Protocol 1 is restricted to using only RSA keys,
720but protocol 2 may use either.
721The
722.Sx HISTORY
723section of
724.Xr ssl 8
725contains a brief discussion of the two algorithms.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100726.Pp
727The file
728.Pa ~/.ssh/authorized_keys
729lists the public keys that are permitted for logging in.
730When the user logs in, the
731.Nm
732program tells the server which key pair it would like to use for
733authentication.
Damien Millerc93a8132005-12-24 14:52:13 +1100734The client proves that it has access to the private key
735and the server checks that the corresponding public key
736is authorized to accept the account.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100737.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100738The user creates his/her key pair by running
Darren Tuckerd3877b92005-12-20 16:09:36 +1100739.Xr ssh-keygen 1 .
740This stores the private key in
741.Pa ~/.ssh/identity
Damien Millerc93a8132005-12-24 14:52:13 +1100742(protocol 1),
743.Pa ~/.ssh/id_dsa
744(protocol 2 DSA),
745or
746.Pa ~/.ssh/id_rsa
747(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100748and stores the public key in
749.Pa ~/.ssh/identity.pub
Damien Millerc93a8132005-12-24 14:52:13 +1100750(protocol 1),
751.Pa ~/.ssh/id_dsa.pub
752(protocol 2 DSA),
753or
754.Pa ~/.ssh/id_rsa.pub
755(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100756in the user's home directory.
Damien Millerc93a8132005-12-24 14:52:13 +1100757The user should then copy the public key
Darren Tuckerd3877b92005-12-20 16:09:36 +1100758to
759.Pa ~/.ssh/authorized_keys
Damien Millerc93a8132005-12-24 14:52:13 +1100760in his/her home directory on the remote machine.
761The
Darren Tuckerd3877b92005-12-20 16:09:36 +1100762.Pa authorized_keys
763file corresponds to the conventional
764.Pa ~/.rhosts
765file, and has one key
Damien Millerc93a8132005-12-24 14:52:13 +1100766per line, though the lines can be very long.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100767After this, the user can log in without giving the password.
768.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100769The most convenient way to use public key authentication may be with an
Darren Tuckerd3877b92005-12-20 16:09:36 +1100770authentication agent.
771See
772.Xr ssh-agent 1
773for more information.
774.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100775Challenge-response authentication works as follows:
776The server sends an arbitrary
777.Qq challenge
778text, and prompts for a response.
779Protocol 2 allows multiple challenges and responses;
780protocol 1 is restricted to just one challenge/response.
781Examples of challenge-response authentication include
782BSD Authentication (see
783.Xr login.conf 5 )
784and PAM (some non-OpenBSD systems).
785.Pp
786Finally, if other authentication methods fail,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100787.Nm
788prompts the user for a password.
789The password is sent to the remote
790host for checking; however, since all communications are encrypted,
791the password cannot be seen by someone listening on the network.
Damien Miller48c94ab2006-01-02 23:38:00 +1100792.Pp
793.Nm
794automatically maintains and checks a database containing
795identification for all hosts it has ever been used with.
796Host keys are stored in
797.Pa ~/.ssh/known_hosts
798in the user's home directory.
799Additionally, the file
800.Pa /etc/ssh/ssh_known_hosts
801is automatically checked for known hosts.
802Any new hosts are automatically added to the user's file.
803If a host's identification ever changes,
804.Nm
805warns about this and disables password authentication to prevent
806server spoofing or man-in-the-middle attacks,
807which could otherwise be used to circumvent the encryption.
808The
809.Cm StrictHostKeyChecking
810option can be used to control logins to machines whose
811host key is not known or has changed.
812.Pp
Darren Tuckerd3877b92005-12-20 16:09:36 +1100813When the user's identity has been accepted by the server, the server
814either executes the given command, or logs into the machine and gives
815the user a normal shell on the remote machine.
816All communication with
817the remote command or shell will be automatically encrypted.
818.Pp
819If a pseudo-terminal has been allocated (normal login session), the
820user may use the escape characters noted below.
821.Pp
822If no pseudo-tty has been allocated,
823the session is transparent and can be used to reliably transfer binary data.
824On most systems, setting the escape character to
825.Dq none
826will also make the session transparent even if a tty is used.
827.Pp
828The session terminates when the command or shell on the remote
Damien Miller8bfaf932006-01-14 10:09:30 +1100829machine exits and all X11 and TCP connections have been closed.
Damien Miller52d20612005-12-24 14:52:36 +1100830.Sh ESCAPE CHARACTERS
Darren Tuckerd3877b92005-12-20 16:09:36 +1100831When a pseudo-terminal has been requested,
832.Nm
833supports a number of functions through the use of an escape character.
834.Pp
835A single tilde character can be sent as
836.Ic ~~
837or by following the tilde by a character other than those described below.
838The escape character must always follow a newline to be interpreted as
839special.
840The escape character can be changed in configuration files using the
841.Cm EscapeChar
842configuration directive or on the command line by the
843.Fl e
844option.
845.Pp
846The supported escapes (assuming the default
847.Ql ~ )
848are:
849.Bl -tag -width Ds
850.It Cm ~.
851Disconnect.
852.It Cm ~^Z
853Background
Damien Miller1164c292006-01-02 23:38:37 +1100854.Nm .
Darren Tuckerd3877b92005-12-20 16:09:36 +1100855.It Cm ~#
856List forwarded connections.
857.It Cm ~&
858Background
859.Nm
860at logout when waiting for forwarded connection / X11 sessions to terminate.
861.It Cm ~?
862Display a list of escape characters.
863.It Cm ~B
864Send a BREAK to the remote system
865(only useful for SSH protocol version 2 and if the peer supports it).
866.It Cm ~C
867Open command line.
868Currently this allows the addition of port forwardings using the
869.Fl L
870and
871.Fl R
Damien Miller329cb012005-12-24 14:53:23 +1100872options (see above).
Darren Tuckerd3877b92005-12-20 16:09:36 +1100873It also allows the cancellation of existing remote port-forwardings
874using
875.Fl KR Ar hostport .
876.Ic !\& Ns Ar command
877allows the user to execute a local command if the
878.Ic PermitLocalCommand
879option is enabled in
880.Xr ssh_config 5 .
881Basic help is available, using the
882.Fl h
883option.
884.It Cm ~R
885Request rekeying of the connection
886(only useful for SSH protocol version 2 and if the peer supports it).
887.El
Damien Millerf3177182006-01-14 10:09:13 +1100888.Sh TCP FORWARDING
889Forwarding of arbitrary TCP connections over the secure channel can
890be specified either on the command line or in a configuration file.
891One possible application of TCP forwarding is a secure connection to a
892mail server; another is going through firewalls.
893.Pp
894In the example below, we look at encrypting communication between
895an IRC client and server, even though the IRC server does not directly
896support encrypted communications.
897This works as follows:
898the user connects to the remote host using
899.Nm ,
900specifying a port to be used to forward connections
901to the remote server.
902After that it is possible to start the service which is to be encrypted
903on the client machine,
904connecting to the same local port,
905and
906.Nm
907will encrypt and forward the connection.
908.Pp
909The following example tunnels an IRC session from client machine
910.Dq 127.0.0.1
911(localhost)
912to remote server
913.Dq server.example.com :
914.Bd -literal -offset 4n
915$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
916$ irc -c '#users' -p 1234 pinky 127.0.0.1
917.Ed
918.Pp
919This tunnels a connection to IRC server
920.Dq server.example.com ,
921joining channel
922.Dq #users ,
923nickname
924.Dq pinky ,
925using port 1234.
926It doesn't matter which port is used,
927as long as it's greater than 1023
928(remember, only root can open sockets on privileged ports)
929and doesn't conflict with any ports already in use.
930The connection is forwarded to port 6667 on the remote server,
931since that's the standard port for IRC services.
932.Pp
933The
934.Fl f
935option backgrounds
936.Nm
937and the remote command
938.Dq sleep 10
939is specified to allow an amount of time
940(10 seconds, in the example)
941to start the service which is to be tunnelled.
942If no connections are made within the time specified,
943.Nm
944will exit.
Damien Millerf3177182006-01-14 10:09:13 +1100945.Sh X11 FORWARDING
Darren Tuckerd3877b92005-12-20 16:09:36 +1100946If the
947.Cm ForwardX11
948variable is set to
949.Dq yes
950(or see the description of the
Damien Millerd7f308f2005-12-24 14:55:16 +1100951.Fl X ,
952.Fl x ,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100953and
Damien Millerd7f308f2005-12-24 14:55:16 +1100954.Fl Y
Damien Miller9a765b22005-12-24 14:53:44 +1100955options above)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100956and the user is using X11 (the
957.Ev DISPLAY
958environment variable is set), the connection to the X11 display is
959automatically forwarded to the remote side in such a way that any X11
960programs started from the shell (or command) will go through the
961encrypted channel, and the connection to the real X server will be made
962from the local machine.
963The user should not manually set
964.Ev DISPLAY .
965Forwarding of X11 connections can be
966configured on the command line or in configuration files.
967.Pp
968The
969.Ev DISPLAY
970value set by
971.Nm
972will point to the server machine, but with a display number greater than zero.
973This is normal, and happens because
974.Nm
975creates a
976.Dq proxy
977X server on the server machine for forwarding the
978connections over the encrypted channel.
979.Pp
980.Nm
981will also automatically set up Xauthority data on the server machine.
982For this purpose, it will generate a random authorization cookie,
983store it in Xauthority on the server, and verify that any forwarded
984connections carry this cookie and replace it by the real cookie when
985the connection is opened.
986The real authentication cookie is never
987sent to the server machine (and no cookies are sent in the plain).
988.Pp
989If the
990.Cm ForwardAgent
991variable is set to
992.Dq yes
993(or see the description of the
994.Fl A
995and
996.Fl a
Damien Miller9a765b22005-12-24 14:53:44 +1100997options above) and
Darren Tuckerd3877b92005-12-20 16:09:36 +1100998the user is using an authentication agent, the connection to the agent
999is automatically forwarded to the remote side.
Damien Millerbbc59092006-01-31 21:46:51 +11001000.Sh VERIFYING HOST KEYS
1001When connecting to a server for the first time,
1002a fingerprint of the server's public key is presented to the user
1003(unless the option
1004.Cm StrictHostKeyChecking
1005has been disabled).
1006Fingerprints can be determined using
1007.Xr ssh-keygen 1 :
1008.Pp
1009.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1010.Pp
1011If the fingerprint is already known,
1012it can be matched and verified,
1013and the key can be accepted.
1014If the fingerprint is unknown,
1015an alternative method of verification is available:
1016SSH fingerprints verified by DNS.
1017An additional resource record (RR),
1018SSHFP,
1019is added to a zonefile
1020and the connecting client is able to match the fingerprint
1021with that of the key presented.
1022.Pp
1023In this example, we are connecting a client to a server,
1024.Dq host.example.com .
1025The SSHFP resource records should first be added to the zonefile for
1026host.example.com:
1027.Bd -literal -offset indent
1028$ ssh-keygen -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com.
1029$ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -r host.example.com.
1030.Ed
1031.Pp
1032The output lines will have to be added to the zonefile.
1033To check that the zone is answering fingerprint queries:
1034.Pp
1035.Dl $ dig -t SSHFP host.example.com
1036.Pp
1037Finally the client connects:
1038.Bd -literal -offset indent
1039$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1040[...]
1041Matching host key fingerprint found in DNS.
1042Are you sure you want to continue connecting (yes/no)?
1043.Ed
1044.Pp
1045See the
1046.Cm VerifyHostKeyDNS
1047option in
1048.Xr ssh_config 5
1049for more information.
Darren Tucker248dd132006-01-20 11:30:58 +11001050.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1051.Nm
1052contains support for Virtual Private Network (VPN) tunnelling
1053using the
1054.Xr tun 4
1055network pseudo-device,
1056allowing two networks to be joined securely.
1057The
1058.Xr sshd_config 5
1059configuration option
1060.Cm PermitTunnel
1061controls whether the server supports this,
1062and at what level (layer 2 or 3 traffic).
1063.Pp
1064The following example would connect client network 10.0.50.0/24
1065with remote network 10.0.99.0/24, provided that the SSH server
1066running on the gateway to the remote network,
1067at 192.168.1.15, allows it:
1068.Bd -literal -offset indent
1069# ssh -f -w 0:1 192.168.1.15 true
1070# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
1071.Ed
1072.Pp
1073Client access may be more finely tuned via the
1074.Pa /root/.ssh/authorized_keys
1075file (see below) and the
1076.Cm PermitRootLogin
1077server option.
Damien Millere93eaaa2006-03-15 11:05:59 +11001078The following entry would permit connections on
Darren Tucker248dd132006-01-20 11:30:58 +11001079.Xr tun 4
Damien Millere93eaaa2006-03-15 11:05:59 +11001080device 1 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001081.Dq jane
Damien Millere93eaaa2006-03-15 11:05:59 +11001082and on tun device 2 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001083.Dq john ,
1084if
1085.Cm PermitRootLogin
1086is set to
1087.Dq forced-commands-only :
1088.Bd -literal -offset 2n
1089tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
Damien Miller9f67a212006-03-15 11:05:35 +11001090tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Darren Tucker248dd132006-01-20 11:30:58 +11001091.Ed
1092.Pp
1093Since a SSH-based setup entails a fair amount of overhead,
1094it may be more suited to temporary setups,
1095such as for wireless VPNs.
1096More permanent VPNs are better provided by tools such as
1097.Xr ipsecctl 8
1098and
1099.Xr isakmpd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +10001100.Sh ENVIRONMENT
1101.Nm
1102will normally set the following environment variables:
Damien Miller6aa22902006-01-06 14:48:34 +11001103.Bl -tag -width "SSH_ORIGINAL_COMMAND"
Damien Miller32aa1441999-10-29 09:15:49 +10001104.It Ev DISPLAY
1105The
1106.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +11001107variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +10001108It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +10001109.Nm
1110to point to a value of the form
Damien Miller14af93e2006-01-02 23:38:21 +11001111.Dq hostname:n ,
1112where
1113.Dq hostname
1114indicates the host where the shell runs, and
1115.Sq n
1116is an integer \*(Ge 1.
Damien Miller7684ee12000-03-17 23:40:15 +11001117.Nm
1118uses this special value to forward X11 connections over the secure
1119channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001120The user should normally not set
1121.Ev DISPLAY
1122explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +10001123will render the X11 connection insecure (and will require the user to
1124manually copy any required authorization cookies).
1125.It Ev HOME
1126Set to the path of the user's home directory.
1127.It Ev LOGNAME
1128Synonym for
1129.Ev USER ;
1130set for compatibility with systems that use this variable.
1131.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +00001132Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +11001133.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +10001134Set to the default
1135.Ev PATH ,
1136as specified when compiling
Damien Miller1164c292006-01-02 23:38:37 +11001137.Nm .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001138.It Ev SSH_ASKPASS
1139If
1140.Nm
1141needs a passphrase, it will read the passphrase from the current
1142terminal if it was run from a terminal.
1143If
1144.Nm
1145does not have a terminal associated with it but
1146.Ev DISPLAY
1147and
1148.Ev SSH_ASKPASS
1149are set, it will execute the program specified by
1150.Ev SSH_ASKPASS
1151and open an X11 window to read the passphrase.
1152This is particularly useful when calling
1153.Nm
1154from a
Darren Tucker4e4fe002004-11-05 20:01:03 +11001155.Pa .xsession
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001156or related script.
1157(Note that on some machines it
1158may be necessary to redirect the input from
1159.Pa /dev/null
1160to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +10001161.It Ev SSH_AUTH_SOCK
Damien Miller14af93e2006-01-02 23:38:21 +11001162Identifies the path of a
1163.Ux Ns -domain
1164socket used to communicate with the agent.
Damien Millerf37e2462002-09-19 11:47:55 +10001165.It Ev SSH_CONNECTION
1166Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +11001167The variable contains
Damien Miller14af93e2006-01-02 23:38:21 +11001168four space-separated values: client IP address, client port number,
1169server IP address, and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001170.It Ev SSH_ORIGINAL_COMMAND
Damien Miller14af93e2006-01-02 23:38:21 +11001171This variable contains the original command line if a forced command
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001172is executed.
1173It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +10001174.It Ev SSH_TTY
1175This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +11001176with the current shell or command.
1177If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +10001178this variable is not set.
1179.It Ev TZ
Damien Miller6dbdb6a2005-12-13 19:25:43 +11001180This variable is set to indicate the present time zone if it
Damien Miller208f1ed2006-03-15 11:56:03 +11001181was set when the daemon was started (i.e. the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +10001182on to new connections).
1183.It Ev USER
1184Set to the name of the user logging in.
1185.El
1186.Pp
Damien Miller22c77262000-04-13 12:26:34 +10001187Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +10001188.Nm
Damien Miller22c77262000-04-13 12:26:34 +10001189reads
Damien Miller167ea5d2005-05-26 12:04:02 +10001190.Pa ~/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +10001191and adds lines of the format
1192.Dq VARNAME=value
Damien Miller14af93e2006-01-02 23:38:21 +11001193to the environment if the file exists and users are allowed to
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001194change their environment.
Darren Tucker61776952003-10-02 16:19:47 +10001195For more information, see the
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001196.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +00001197option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001198.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +10001199.Sh FILES
Damien Miller7655f5c2006-01-06 14:48:18 +11001200.Bl -tag -width Ds -compact
Damien Miller7655f5c2006-01-06 14:48:18 +11001201.It ~/.rhosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001202This file is used for host-based authentication (see above).
Ben Lindstromebd888d2001-03-05 05:49:29 +00001203On some machines this file may need to be
Damien Miller1bcdb502006-01-06 14:49:38 +11001204world-readable if the user's home directory is on an NFS partition,
Damien Miller32aa1441999-10-29 09:15:49 +10001205because
1206.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001207reads it as root.
1208Additionally, this file must be owned by the user,
1209and must not have write permissions for anyone else.
1210The recommended
Damien Miller32aa1441999-10-29 09:15:49 +10001211permission for most machines is read/write for the user, and not
1212accessible by others.
1213.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001214.It ~/.shosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001215This file is used in exactly the same way as
1216.Pa .rhosts ,
1217but allows host-based authentication without permitting login with
1218rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001219.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001220.It ~/.ssh/authorized_keys
1221Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1222The format of this file is described in the
1223.Xr sshd 8
1224manual page.
Damien Millerfb8ea742006-01-06 14:48:52 +11001225This file is not highly sensitive, but the recommended
1226permissions are read/write for the user, and not accessible by others.
1227.Pp
1228.It ~/.ssh/config
1229This is the per-user configuration file.
1230The file format and configuration options are described in
1231.Xr ssh_config 5 .
1232Because of the potential for abuse, this file must have strict permissions:
1233read/write for the user, and not accessible by others.
1234.Pp
1235.It ~/.ssh/environment
Damien Miller4c102ee2006-01-06 14:49:17 +11001236Contains additional definitions for environment variables; see
1237.Sx ENVIRONMENT ,
Damien Millerfb8ea742006-01-06 14:48:52 +11001238above.
1239.Pp
1240.It ~/.ssh/identity
1241.It ~/.ssh/id_dsa
1242.It ~/.ssh/id_rsa
1243Contains the private key for authentication.
1244These files
1245contain sensitive data and should be readable by the user but not
1246accessible by others (read/write/execute).
1247.Nm
1248will simply ignore a private key file if it is accessible by others.
1249It is possible to specify a passphrase when
1250generating the key which will be used to encrypt the
1251sensitive part of this file using 3DES.
1252.Pp
1253.It ~/.ssh/identity.pub
1254.It ~/.ssh/id_dsa.pub
1255.It ~/.ssh/id_rsa.pub
1256Contains the public key for authentication.
1257These files are not
1258sensitive and can (but need not) be readable by anyone.
Damien Millerfb8ea742006-01-06 14:48:52 +11001259.Pp
1260.It ~/.ssh/known_hosts
Damien Millere87eb4c2006-01-14 10:08:36 +11001261Contains a list of host keys for all hosts the user has logged into
1262that are not already in the systemwide list of known host keys.
Damien Millerfb8ea742006-01-06 14:48:52 +11001263See
Damien Millere87eb4c2006-01-14 10:08:36 +11001264.Xr sshd 8
1265for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001266.Pp
1267.It ~/.ssh/rc
1268Commands in this file are executed by
1269.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001270when the user logs in, just before the user's shell (or command) is
Damien Millerfb8ea742006-01-06 14:48:52 +11001271started.
1272See the
1273.Xr sshd 8
1274manual page for more information.
1275.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001276.It /etc/hosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001277This file is for host-based authentication (see above).
1278It should only be writable by root.
Damien Miller7655f5c2006-01-06 14:48:18 +11001279.Pp
1280.It /etc/shosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001281This file is used in exactly the same way as
1282.Pa hosts.equiv ,
1283but allows host-based authentication without permitting login with
1284rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001285.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001286.It Pa /etc/ssh/ssh_config
1287Systemwide configuration file.
1288The file format and configuration options are described in
1289.Xr ssh_config 5 .
1290.Pp
1291.It /etc/ssh/ssh_host_key
1292.It /etc/ssh/ssh_host_dsa_key
1293.It /etc/ssh/ssh_host_rsa_key
1294These three files contain the private parts of the host keys
Damien Miller7e76e1f2006-01-14 10:08:57 +11001295and are used for host-based authentication.
1296If protocol version 1 is used,
Damien Millerfb8ea742006-01-06 14:48:52 +11001297.Nm
1298must be setuid root, since the host key is readable only by root.
1299For protocol version 2,
1300.Nm
1301uses
1302.Xr ssh-keysign 8
Damien Miller7e76e1f2006-01-14 10:08:57 +11001303to access the host keys,
1304eliminating the requirement that
Damien Millerfb8ea742006-01-06 14:48:52 +11001305.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001306be setuid root when host-based authentication is used.
Damien Millerfb8ea742006-01-06 14:48:52 +11001307By default
1308.Nm
1309is not setuid root.
1310.Pp
1311.It /etc/ssh/ssh_known_hosts
1312Systemwide list of known host keys.
1313This file should be prepared by the
1314system administrator to contain the public host keys of all machines in the
1315organization.
Damien Millere87eb4c2006-01-14 10:08:36 +11001316It should be world-readable.
1317See
Damien Millerfb8ea742006-01-06 14:48:52 +11001318.Xr sshd 8
Damien Millere87eb4c2006-01-14 10:08:36 +11001319for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001320.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001321.It /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +10001322Commands in this file are executed by
1323.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001324when the user logs in, just before the user's shell (or command) is started.
Damien Miller32aa1441999-10-29 09:15:49 +10001325See the
1326.Xr sshd 8
1327manual page for more information.
Damien Miller7b28dc52000-09-05 13:34:53 +11001328.El
Damien Miller32aa1441999-10-29 09:15:49 +10001329.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +10001330.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +11001331.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +10001332.Xr ssh-add 1 ,
1333.Xr ssh-agent 1 ,
1334.Xr ssh-keygen 1 ,
Damien Miller128a0f12006-01-06 14:50:11 +11001335.Xr ssh-keyscan 1 ,
Darren Tucker248dd132006-01-20 11:30:58 +11001336.Xr tun 4 ,
Darren Tucker61776952003-10-02 16:19:47 +10001337.Xr hosts.equiv 5 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +00001338.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +00001339.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +00001340.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +00001341.Rs
Damien Miller39a93a32006-03-15 11:34:45 +11001342.%R RFC 4250
1343.%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1344.%D 2006
1345.Re
1346.Rs
1347.%R RFC 4251
1348.%T "The Secure Shell (SSH) Protocol Architecture"
1349.%D 2006
1350.Re
1351.Rs
1352.%R RFC 4252
1353.%T "The Secure Shell (SSH) Authentication Protocol"
1354.%D 2006
1355.Re
1356.Rs
1357.%R RFC 4253
1358.%T "The Secure Shell (SSH) Transport Layer Protocol"
1359.%D 2006
1360.Re
1361.Rs
1362.%R RFC 4254
1363.%T "The Secure Shell (SSH) Connection Protocol"
1364.%D 2006
1365.Re
1366.Rs
1367.%R RFC 4255
1368.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1369.%D 2006
1370.Re
1371.Rs
1372.%R RFC 4256
1373.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1374.%D 2006
1375.Re
1376.Rs
1377.%R RFC 4335
1378.%T "The Secure Shell (SSH) Session Channel Break Extension"
1379.%D 2006
1380.Re
1381.Rs
1382.%R RFC 4344
1383.%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1384.%D 2006
1385.Re
1386.Rs
1387.%R RFC 4345
1388.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1389.%D 2006
Ben Lindstrom160ec622001-04-22 17:17:46 +00001390.Re
Damien Millerf1ce5052003-06-11 22:04:39 +10001391.Sh AUTHORS
1392OpenSSH is a derivative of the original and free
1393ssh 1.2.12 release by Tatu Ylonen.
1394Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1395Theo de Raadt and Dug Song
1396removed many bugs, re-added newer features and
1397created OpenSSH.
1398Markus Friedl contributed the support for SSH
1399protocol versions 1.5 and 2.0.