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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.\"
Darren Tuckere7d4b192006-07-12 22:17:10 +100037.\" $OpenBSD: ssh.1,v 1.263 2006/07/11 18:50:48 markus 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
Damien Miller991dba42006-07-10 20:16:27 +100081.Oo Fl w Ar local_tun Ns
82.Op : Ns Ar remote_tun Oc
Darren Tucker61776952003-10-02 16:19:47 +100083.Oo Ar user Ns @ Oc Ns Ar hostname
Damien Miller32aa1441999-10-29 09:15:49 +100084.Op Ar command
Damien Millerf0c8c152005-12-13 19:29:58 +110085.Ek
Damien Miller22c77262000-04-13 12:26:34 +100086.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100087.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000088(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110089executing commands on a remote machine.
Darren Tucker61776952003-10-02 16:19:47 +100090It is intended to replace rlogin and rsh,
91and provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110092two untrusted hosts over an insecure network.
Damien Miller8bfaf932006-01-14 10:09:30 +110093X11 connections and arbitrary TCP ports
Darren Tucker61776952003-10-02 16:19:47 +100094can also be forwarded over the secure channel.
Damien Miller32aa1441999-10-29 09:15:49 +100095.Pp
96.Nm
Damien Miller22c77262000-04-13 12:26:34 +100097connects and logs into the specified
Darren Tucker61776952003-10-02 16:19:47 +100098.Ar hostname
99(with optional
100.Ar user
101name).
Damien Miller32aa1441999-10-29 09:15:49 +1000102The user must prove
Damien Millere247cc42000-05-07 12:03:14 +1000103his/her identity to the remote machine using one of several methods
Darren Tucker5434cfe2005-12-20 16:11:35 +1100104depending on the protocol version used (see below).
Damien Millere247cc42000-05-07 12:03:14 +1000105.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000106If
107.Ar command
108is specified,
Darren Tuckerb18c8672005-12-20 16:10:09 +1100109it is executed on the remote host instead of a login shell.
Darren Tucker0b42e6d2004-08-13 21:22:40 +1000110.Pp
Damien Miller0bc1bd82000-11-13 22:57:25 +1100111The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000112.Bl -tag -width Ds
Darren Tucker61776952003-10-02 16:19:47 +1000113.It Fl 1
114Forces
115.Nm
116to try protocol version 1 only.
117.It Fl 2
118Forces
119.Nm
120to try protocol version 2 only.
121.It Fl 4
122Forces
123.Nm
124to use IPv4 addresses only.
125.It Fl 6
126Forces
127.Nm
128to use IPv6 addresses only.
Damien Millerb1715dc2000-05-30 13:44:51 +1000129.It Fl A
130Enables forwarding of the authentication agent connection.
131This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000132.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000133Agent forwarding should be enabled with caution.
134Users with the ability to bypass file permissions on the remote host
135(for the agent's Unix-domain socket)
136can access the local agent through the forwarded connection.
137An attacker cannot obtain key material from the agent,
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000138however they can perform operations on the keys that enable them to
139authenticate using the identities loaded into the agent.
Darren Tucker61776952003-10-02 16:19:47 +1000140.It Fl a
141Disables forwarding of the authentication agent connection.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000142.It Fl b Ar bind_address
Darren Tuckera5cf8552005-07-14 17:04:18 +1000143Use
144.Ar bind_address
145on the local machine as the source address
146of the connection.
147Only useful on systems with more than one address.
Darren Tucker61776952003-10-02 16:19:47 +1000148.It Fl C
149Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller8bfaf932006-01-14 10:09:30 +1100150data for forwarded X11 and TCP connections).
Darren Tucker61776952003-10-02 16:19:47 +1000151The compression algorithm is the same used by
152.Xr gzip 1 ,
153and the
154.Dq level
155can be controlled by the
156.Cm CompressionLevel
157option for protocol version 1.
158Compression is desirable on modem lines and other
159slow connections, but will only slow down things on fast networks.
160The default value can be set on a host-by-host basis in the
161configuration files; see the
162.Cm Compression
163option.
Damien Miller05202ff2004-06-15 10:30:39 +1000164.It Fl c Ar cipher_spec
165Selects the cipher specification for encrypting the session.
166.Pp
167Protocol version 1 allows specification of a single cipher.
Darren Tucker1e4308e2005-10-03 18:18:40 +1000168The supported values are
Damien Miller05202ff2004-06-15 10:30:39 +1000169.Dq 3des ,
Darren Tuckerff9f4842005-12-20 16:10:45 +1100170.Dq blowfish ,
Damien Miller05202ff2004-06-15 10:30:39 +1000171and
172.Dq des .
Damien Miller32aa1441999-10-29 09:15:49 +1000173.Ar 3des
174(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
Damien Miller05202ff2004-06-15 10:30:39 +1000175It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000176.Ar blowfish
Darren Tucker61776952003-10-02 16:19:47 +1000177is a fast block cipher; it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100178.Ar 3des .
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000179.Ar des
180is only supported in the
181.Nm
182client for interoperability with legacy protocol 1 implementations
183that do not support the
184.Ar 3des
Damien Miller495dca32003-04-01 21:42:14 +1000185cipher.
186Its use is strongly discouraged due to cryptographic weaknesses.
Damien Miller05202ff2004-06-15 10:30:39 +1000187The default is
188.Dq 3des .
189.Pp
Damien Miller35978212005-12-24 14:56:47 +1100190For protocol version 2,
Damien Miller05202ff2004-06-15 10:30:39 +1000191.Ar cipher_spec
192is a comma-separated list of ciphers
193listed in order of preference.
Damien Miller35978212005-12-24 14:56:47 +1100194The supported ciphers are:
1953des-cbc,
196aes128-cbc,
197aes192-cbc,
198aes256-cbc,
199aes128-ctr,
200aes192-ctr,
201aes256-ctr,
202arcfour128,
203arcfour256,
204arcfour,
205blowfish-cbc,
Damien Miller05202ff2004-06-15 10:30:39 +1000206and
Damien Miller35978212005-12-24 14:56:47 +1100207cast128-cbc.
Darren Tuckerff9f4842005-12-20 16:10:45 +1100208The default is:
209.Bd -literal -offset indent
210aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212aes192-ctr,aes256-ctr
Damien Miller05202ff2004-06-15 10:30:39 +1000213.Ed
Darren Tuckerc8d64212005-10-03 18:13:42 +1000214.It Fl D Xo
215.Sm off
216.Oo Ar bind_address : Oc
217.Ar port
218.Sm on
219.Xc
Darren Tucker61776952003-10-02 16:19:47 +1000220Specifies a local
221.Dq dynamic
222application-level port forwarding.
223This works by allocating a socket to listen to
224.Ar port
Darren Tuckerc8d64212005-10-03 18:13:42 +1000225on the local side, optionally bound to the specified
226.Ar bind_address .
227Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000228connection is forwarded over the secure channel, and the application
229protocol is then used to determine where to connect to from the
230remote machine.
231Currently the SOCKS4 and SOCKS5 protocols are supported, and
232.Nm
233will act as a SOCKS server.
234Only root can forward privileged ports.
235Dynamic port forwardings can also be specified in the configuration file.
Darren Tuckerc8d64212005-10-03 18:13:42 +1000236.Pp
237IPv6 addresses can be specified with an alternative syntax:
238.Sm off
239.Xo
240.Op Ar bind_address No /
241.Ar port
242.Xc
243.Sm on
244or by enclosing the address in square brackets.
245Only the superuser can forward privileged ports.
246By default, the local port is bound in accordance with the
247.Cm GatewayPorts
248setting.
249However, an explicit
250.Ar bind_address
251may be used to bind the connection to a specific address.
252The
253.Ar bind_address
254of
255.Dq localhost
256indicates that the listening port be bound for local use only, while an
257empty address or
258.Sq *
259indicates that the port should be available from all interfaces.
Damien Miller2142ba02005-12-24 14:56:29 +1100260.It Fl e Ar escape_char
Damien Miller32aa1441999-10-29 09:15:49 +1000261Sets the escape character for sessions with a pty (default:
262.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100263The escape character is only recognized at the beginning of a line.
264The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000265.Pq Ql \&.
Darren Tucker61776952003-10-02 16:19:47 +1000266closes the connection;
267followed by control-Z suspends the connection;
268and followed by itself sends the escape character once.
Damien Miller7684ee12000-03-17 23:40:15 +1100269Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000270.Dq none
271disables any escapes and makes the session fully transparent.
Darren Tucker61776952003-10-02 16:19:47 +1000272.It Fl F Ar configfile
273Specifies an alternative per-user configuration file.
274If a configuration file is given on the command line,
275the system-wide configuration file
276.Pq Pa /etc/ssh/ssh_config
277will be ignored.
278The default for the per-user configuration file is
Damien Miller167ea5d2005-05-26 12:04:02 +1000279.Pa ~/.ssh/config .
Damien Miller32aa1441999-10-29 09:15:49 +1000280.It Fl f
281Requests
282.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100283to go to background just before command execution.
284This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000285.Nm
286is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100287wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000288This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000289.Fl n .
290The recommended way to start X11 programs at a remote site is with
291something like
292.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100293.It Fl g
294Allows remote hosts to connect to local forwarded ports.
Darren Tucker61776952003-10-02 16:19:47 +1000295.It Fl I Ar smartcard_device
Damien Miller2142ba02005-12-24 14:56:29 +1100296Specify the device
Darren Tucker61776952003-10-02 16:19:47 +1000297.Nm
298should use to communicate with a smartcard used for storing the user's
299private RSA key.
Damien Miller2142ba02005-12-24 14:56:29 +1100300This option is only available if support for smartcard devices
301is compiled in (default is no support).
Damien Miller32aa1441999-10-29 09:15:49 +1000302.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100303Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100304RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100305The default is
Damien Miller167ea5d2005-05-26 12:04:02 +1000306.Pa ~/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100307for protocol version 1, and
Damien Miller167ea5d2005-05-26 12:04:02 +1000308.Pa ~/.ssh/id_rsa
Damien Millerfae23862002-01-22 23:32:26 +1100309and
Damien Miller167ea5d2005-05-26 12:04:02 +1000310.Pa ~/.ssh/id_dsa
Damien Millerfae23862002-01-22 23:32:26 +1100311for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100312Identity files may also be specified on
313a per-host basis in the configuration file.
314It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000315.Fl i
316options (and multiple identities specified in
317configuration files).
Damien Miller32aa1441999-10-29 09:15:49 +1000318.It Fl k
Damien Millere0113cc2003-11-24 13:10:09 +1100319Disables forwarding (delegation) of GSSAPI credentials to the server.
Darren Tucker61776952003-10-02 16:19:47 +1000320.It Fl L Xo
321.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100322.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000323.Ar port : host : hostport
324.Sm on
325.Xc
326Specifies that the given port on the local (client) host is to be
327forwarded to the given host and port on the remote side.
328This works by allocating a socket to listen to
329.Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100330on the local side, optionally bound to the specified
331.Ar bind_address .
332Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000333connection is forwarded over the secure channel, and a connection is
334made to
335.Ar host
336port
337.Ar hostport
338from the remote machine.
339Port forwardings can also be specified in the configuration file.
Darren Tucker61776952003-10-02 16:19:47 +1000340IPv6 addresses can be specified with an alternative syntax:
341.Sm off
342.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100343.Op Ar bind_address No /
Darren Tucker61776952003-10-02 16:19:47 +1000344.Ar port No / Ar host No /
Damien Millerf91ee4c2005-03-01 21:24:33 +1100345.Ar hostport
Darren Tucker61776952003-10-02 16:19:47 +1000346.Xc
347.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100348or by enclosing the address in square brackets.
349Only the superuser can forward privileged ports.
350By default, the local port is bound in accordance with the
351.Cm GatewayPorts
352setting.
353However, an explicit
354.Ar bind_address
355may be used to bind the connection to a specific address.
356The
357.Ar bind_address
358of
359.Dq localhost
Damien Miller36bf7dd2005-03-02 12:02:47 +1100360indicates that the listening port be bound for local use only, while an
361empty address or
362.Sq *
Damien Millerf91ee4c2005-03-01 21:24:33 +1100363indicates that the port should be available from all interfaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000364.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100365Specifies the user to log in as on the remote machine.
366This also may be specified on a per-host basis in the configuration file.
Damien Miller0e220db2004-06-15 10:34:08 +1000367.It Fl M
368Places the
369.Nm
370client into
371.Dq master
372mode for connection sharing.
Damien Miller88b25522005-12-31 16:23:15 +1100373Multiple
374.Fl M
375options places
376.Nm
377into
378.Dq master
379mode with confirmation required before slave connections are accepted.
Damien Miller0e220db2004-06-15 10:34:08 +1000380Refer to the description of
381.Cm ControlMaster
382in
383.Xr ssh_config 5
384for details.
Damien Miller5d1eceb2004-06-30 22:37:57 +1000385.It Fl m Ar mac_spec
386Additionally, for protocol version 2 a comma-separated list of MAC
387(message authentication code) algorithms can
388be specified in order of preference.
389See the
390.Cm MACs
391keyword for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000392.It Fl N
393Do not execute a remote command.
394This is useful for just forwarding ports
395(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000396.It Fl n
397Redirects stdin from
398.Pa /dev/null
399(actually, prevents reading from stdin).
400This must be used when
401.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100402is run in the background.
403A common trick is to use this to run X11 programs on a remote machine.
404For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000405.Ic ssh -n shadows.cs.hut.fi emacs &
406will start an emacs on shadows.cs.hut.fi, and the X11
407connection will be automatically forwarded over an encrypted channel.
408The
409.Nm
410program will be put in the background.
411(This does not work if
412.Nm
413needs to ask for a password or passphrase; see also the
414.Fl f
415option.)
Darren Tuckere04644c2004-12-03 14:08:45 +1100416.It Fl O Ar ctl_cmd
417Control an active connection multiplexing master process.
418When the
419.Fl O
420option is specified, the
421.Ar ctl_cmd
422argument is interpreted and passed to the master process.
423Valid commands are:
424.Dq check
425(check that the master process is running) and
426.Dq exit
427(request the master to exit).
Damien Miller32aa1441999-10-29 09:15:49 +1000428.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000429Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000430This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100431command-line flag.
Darren Tucker61776952003-10-02 16:19:47 +1000432For full details of the options listed below, and their possible values, see
433.Xr ssh_config 5 .
434.Pp
435.Bl -tag -width Ds -offset indent -compact
436.It AddressFamily
437.It BatchMode
438.It BindAddress
439.It ChallengeResponseAuthentication
440.It CheckHostIP
441.It Cipher
442.It Ciphers
443.It ClearAllForwardings
444.It Compression
445.It CompressionLevel
446.It ConnectionAttempts
Darren Tuckeredae0ec2004-05-02 22:15:52 +1000447.It ConnectTimeout
Damien Miller0e220db2004-06-15 10:34:08 +1000448.It ControlMaster
449.It ControlPath
Darren Tucker61776952003-10-02 16:19:47 +1000450.It DynamicForward
Darren Tucker61776952003-10-02 16:19:47 +1000451.It EscapeChar
Darren Tuckere7d4b192006-07-12 22:17:10 +1000452.It ExitOnForwardFailure
Darren Tucker61776952003-10-02 16:19:47 +1000453.It ForwardAgent
454.It ForwardX11
Darren Tucker0a118da2003-10-15 15:54:32 +1000455.It ForwardX11Trusted
Darren Tucker61776952003-10-02 16:19:47 +1000456.It GatewayPorts
457.It GlobalKnownHostsFile
458.It GSSAPIAuthentication
459.It GSSAPIDelegateCredentials
Damien Millere1776152005-03-01 21:47:37 +1100460.It HashKnownHosts
Darren Tucker61776952003-10-02 16:19:47 +1000461.It Host
462.It HostbasedAuthentication
463.It HostKeyAlgorithms
464.It HostKeyAlias
465.It HostName
466.It IdentityFile
Damien Millerbd394c32004-03-08 23:12:36 +1100467.It IdentitiesOnly
Darren Tucker636ca902004-11-05 20:22:00 +1100468.It KbdInteractiveDevices
Damien Millerd27b9472005-12-13 19:29:02 +1100469.It LocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000470.It LocalForward
471.It LogLevel
472.It MACs
473.It NoHostAuthenticationForLocalhost
474.It NumberOfPasswordPrompts
475.It PasswordAuthentication
Damien Millerd27b9472005-12-13 19:29:02 +1100476.It PermitLocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000477.It Port
478.It PreferredAuthentications
479.It Protocol
480.It ProxyCommand
481.It PubkeyAuthentication
Darren Tucker62388b22006-01-20 11:31:47 +1100482.It RekeyLimit
Darren Tucker61776952003-10-02 16:19:47 +1000483.It RemoteForward
484.It RhostsRSAAuthentication
485.It RSAAuthentication
Darren Tucker7a6c0662004-05-02 22:14:03 +1000486.It SendEnv
Damien Miller509b0102003-12-17 16:33:10 +1100487.It ServerAliveInterval
488.It ServerAliveCountMax
Darren Tucker61776952003-10-02 16:19:47 +1000489.It SmartcardDevice
490.It StrictHostKeyChecking
Damien Miller12c150e2003-12-17 16:31:10 +1100491.It TCPKeepAlive
Damien Millerd27b9472005-12-13 19:29:02 +1100492.It Tunnel
493.It TunnelDevice
Darren Tucker61776952003-10-02 16:19:47 +1000494.It UsePrivilegedPort
495.It User
496.It UserKnownHostsFile
497.It VerifyHostKeyDNS
498.It XAuthLocation
499.El
Damien Miller32aa1441999-10-29 09:15:49 +1000500.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100501Port to connect to on the remote host.
502This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000503per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000504.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100505Quiet mode.
506Causes all warning and diagnostic messages to be suppressed.
Darren Tucker61776952003-10-02 16:19:47 +1000507.It Fl R Xo
508.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100509.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000510.Ar port : host : hostport
511.Sm on
512.Xc
Damien Miller32aa1441999-10-29 09:15:49 +1000513Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100514forwarded to the given host and port on the local side.
515This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000516.Ar port
517on the remote side, and whenever a connection is made to this port, the
518connection is forwarded over the secure channel, and a connection is
519made to
Damien Miller34132e52000-01-14 15:45:46 +1100520.Ar host
521port
522.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100523from the local machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100524.Pp
Damien Miller7684ee12000-03-17 23:40:15 +1100525Port forwardings can also be specified in the configuration file.
526Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000527logging in as root on the remote machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100528IPv6 addresses can be specified by enclosing the address in square braces or
529using an alternative syntax:
Damien Millerf91ee4c2005-03-01 21:24:33 +1100530.Sm off
Damien Miller36bf7dd2005-03-02 12:02:47 +1100531.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100532.Op Ar bind_address No /
Damien Miller36bf7dd2005-03-02 12:02:47 +1100533.Ar host No / Ar port No /
534.Ar hostport
Damien Millerf91ee4c2005-03-01 21:24:33 +1100535.Xc .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100536.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100537.Pp
538By default, the listening socket on the server will be bound to the loopback
539interface only.
540This may be overriden by specifying a
541.Ar bind_address .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100542An empty
543.Ar bind_address ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100544or the address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100545.Ql * ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100546indicates that the remote socket should listen on all interfaces.
547Specifying a remote
548.Ar bind_address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100549will only succeed if the server's
550.Cm GatewayPorts
Damien Millerf91ee4c2005-03-01 21:24:33 +1100551option is enabled (see
Damien Miller36bf7dd2005-03-02 12:02:47 +1100552.Xr sshd_config 5 ) .
Darren Tucker7ebfc102004-11-07 20:06:19 +1100553.It Fl S Ar ctl_path
Damien Miller5d1eceb2004-06-30 22:37:57 +1000554Specifies the location of a control socket for connection sharing.
Damien Miller0e220db2004-06-15 10:34:08 +1000555Refer to the description of
Darren Tuckerba5c5922004-06-18 16:22:39 +1000556.Cm ControlPath
557and
Damien Miller0e220db2004-06-15 10:34:08 +1000558.Cm ControlMaster
559in
560.Xr ssh_config 5
561for details.
Darren Tucker61776952003-10-02 16:19:47 +1000562.It Fl s
563May be used to request invocation of a subsystem on the remote system.
564Subsystems are a feature of the SSH2 protocol which facilitate the use
565of SSH as a secure transport for other applications (eg.\&
566.Xr sftp 1 ) .
567The subsystem is specified as the remote command.
568.It Fl T
569Disable pseudo-tty allocation.
570.It Fl t
571Force pseudo-tty allocation.
572This can be used to execute arbitrary
573screen-based programs on a remote machine, which can be very useful,
Damien Miller208f1ed2006-03-15 11:56:03 +1100574e.g. when implementing menu services.
Darren Tucker61776952003-10-02 16:19:47 +1000575Multiple
576.Fl t
577options force tty allocation, even if
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000578.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000579has no local tty.
580.It Fl V
581Display the version number and exit.
582.It Fl v
583Verbose mode.
584Causes
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000585.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000586to print debugging messages about its progress.
587This is helpful in
588debugging connection, authentication, and configuration problems.
589Multiple
590.Fl v
591options increase the verbosity.
592The maximum is 3.
Damien Miller991dba42006-07-10 20:16:27 +1000593.It Fl w Xo
594.Ar local_tun Ns Op : Ns Ar remote_tun
595.Xc
596Requests
597tunnel
598device forwarding with the specified
Damien Millerd27b9472005-12-13 19:29:02 +1100599.Xr tun 4
Damien Miller991dba42006-07-10 20:16:27 +1000600devices between the client
601.Pq Ar local_tun
602and the server
603.Pq Ar remote_tun .
604.Pp
Damien Millercf1e3422005-12-24 14:56:04 +1100605The devices may be specified by numerical ID or the keyword
606.Dq any ,
607which uses the next available tunnel device.
Damien Miller991dba42006-07-10 20:16:27 +1000608If
609.Ar remote_tun
610is not specified, it defaults to
611.Dq any .
Damien Millercf1e3422005-12-24 14:56:04 +1100612See also the
Damien Millerd27b9472005-12-13 19:29:02 +1100613.Cm Tunnel
Damien Miller991dba42006-07-10 20:16:27 +1000614and
615.Cm TunnelDevice
616directives in
Damien Millerd27b9472005-12-13 19:29:02 +1100617.Xr ssh_config 5 .
Damien Miller991dba42006-07-10 20:16:27 +1000618If the
619.Cm Tunnel
620directive is unset, it is set to the default tunnel mode, which is
621.Dq point-to-point .
Darren Tucker61776952003-10-02 16:19:47 +1000622.It Fl X
623Enables X11 forwarding.
624This can also be specified on a per-host basis in a configuration file.
625.Pp
626X11 forwarding should be enabled with caution.
627Users with the ability to bypass file permissions on the remote host
628(for the user's X authorization database)
629can access the local X11 display through the forwarded connection.
630An attacker may then be able to perform activities such as keystroke monitoring.
Damien Miller947219e2005-03-02 13:22:30 +1100631.Pp
Damien Millerb022b232005-03-05 11:22:36 +1100632For this reason, X11 forwarding is subjected to X11 SECURITY extension
Damien Miller947219e2005-03-02 13:22:30 +1100633restrictions by default.
634Please refer to the
635.Nm
636.Fl Y
637option and the
638.Cm ForwardX11Trusted
639directive in
640.Xr ssh_config 5
641for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000642.It Fl x
643Disables X11 forwarding.
Darren Tucker0a118da2003-10-15 15:54:32 +1000644.It Fl Y
645Enables trusted X11 forwarding.
Damien Miller947219e2005-03-02 13:22:30 +1100646Trusted X11 forwardings are not subjected to the X11 SECURITY extension
647controls.
Damien Miller32aa1441999-10-29 09:15:49 +1000648.El
Damien Millere9b333a2005-12-24 14:53:04 +1100649.Pp
650.Nm
651may additionally obtain configuration data from
652a per-user configuration file and a system-wide configuration file.
653The file format and configuration options are described in
654.Xr ssh_config 5 .
655.Pp
656.Nm
657exits with the exit status of the remote command or with 255
658if an error occurred.
Damien Millerc93a8132005-12-24 14:52:13 +1100659.Sh AUTHENTICATION
Darren Tucker94299ec2006-01-20 11:30:14 +1100660The OpenSSH SSH client supports SSH protocols 1 and 2.
Damien Millerc93a8132005-12-24 14:52:13 +1100661Protocol 2 is the default, with
662.Nm
663falling back to protocol 1 if it detects protocol 2 is unsupported.
664These settings may be altered using the
665.Cm Protocol
666option in
667.Xr ssh_config 5 ,
668or enforced using the
669.Fl 1
670and
671.Fl 2
672options (see above).
673Both protocols support similar authentication methods,
674but protocol 2 is preferred since
675it provides additional mechanisms for confidentiality
676(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
677and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
678Protocol 1 lacks a strong mechanism for ensuring the
679integrity of the connection.
680.Pp
681The methods available for authentication are:
Damien Millerfbc94c82006-06-13 13:03:16 +1000682GSSAPI-based authentication,
Damien Millerc93a8132005-12-24 14:52:13 +1100683host-based authentication,
684public key authentication,
685challenge-response authentication,
686and password authentication.
687Authentication methods are tried in the order specified above,
688though protocol 2 has a configuration option to change the default order:
689.Cm PreferredAuthentications .
690.Pp
691Host-based authentication works as follows:
Darren Tuckerd3877b92005-12-20 16:09:36 +1100692If the machine the user logs in from is listed in
693.Pa /etc/hosts.equiv
694or
695.Pa /etc/shosts.equiv
696on the remote machine, and the user names are
697the same on both sides, or if the files
698.Pa ~/.rhosts
699or
700.Pa ~/.shosts
701exist in the user's home directory on the
702remote machine and contain a line containing the name of the client
703machine and the name of the user on that machine, the user is
Damien Millerc93a8132005-12-24 14:52:13 +1100704considered for login.
705Additionally, the server
706.Em must
707be able to verify the client's
708host key (see the description of
Darren Tuckerd3877b92005-12-20 16:09:36 +1100709.Pa /etc/ssh/ssh_known_hosts
710and
Damien Millerc93a8132005-12-24 14:52:13 +1100711.Pa ~/.ssh/known_hosts ,
712below)
713for login to be permitted.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100714This authentication method closes security holes due to IP
Damien Millerc93a8132005-12-24 14:52:13 +1100715spoofing, DNS spoofing, and routing spoofing.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100716[Note to the administrator:
717.Pa /etc/hosts.equiv ,
718.Pa ~/.rhosts ,
719and the rlogin/rsh protocol in general, are inherently insecure and should be
720disabled if security is desired.]
721.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100722Public key authentication works as follows:
723The scheme is based on public-key cryptography,
724using cryptosystems
725where encryption and decryption are done using separate keys,
726and it is unfeasible to derive the decryption key from the encryption key.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100727The idea is that each user creates a public/private
728key pair for authentication purposes.
729The server knows the public key, and only the user knows the private key.
Damien Millerc93a8132005-12-24 14:52:13 +1100730.Nm
731implements public key authentication protocol automatically,
732using either the RSA or DSA algorithms.
733Protocol 1 is restricted to using only RSA keys,
734but protocol 2 may use either.
735The
736.Sx HISTORY
737section of
738.Xr ssl 8
739contains a brief discussion of the two algorithms.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100740.Pp
741The file
742.Pa ~/.ssh/authorized_keys
743lists the public keys that are permitted for logging in.
744When the user logs in, the
745.Nm
746program tells the server which key pair it would like to use for
747authentication.
Damien Millerc93a8132005-12-24 14:52:13 +1100748The client proves that it has access to the private key
749and the server checks that the corresponding public key
750is authorized to accept the account.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100751.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100752The user creates his/her key pair by running
Darren Tuckerd3877b92005-12-20 16:09:36 +1100753.Xr ssh-keygen 1 .
754This stores the private key in
755.Pa ~/.ssh/identity
Damien Millerc93a8132005-12-24 14:52:13 +1100756(protocol 1),
757.Pa ~/.ssh/id_dsa
758(protocol 2 DSA),
759or
760.Pa ~/.ssh/id_rsa
761(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100762and stores the public key in
763.Pa ~/.ssh/identity.pub
Damien Millerc93a8132005-12-24 14:52:13 +1100764(protocol 1),
765.Pa ~/.ssh/id_dsa.pub
766(protocol 2 DSA),
767or
768.Pa ~/.ssh/id_rsa.pub
769(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100770in the user's home directory.
Damien Millerc93a8132005-12-24 14:52:13 +1100771The user should then copy the public key
Darren Tuckerd3877b92005-12-20 16:09:36 +1100772to
773.Pa ~/.ssh/authorized_keys
Damien Millerc93a8132005-12-24 14:52:13 +1100774in his/her home directory on the remote machine.
775The
Darren Tuckerd3877b92005-12-20 16:09:36 +1100776.Pa authorized_keys
777file corresponds to the conventional
778.Pa ~/.rhosts
779file, and has one key
Damien Millerc93a8132005-12-24 14:52:13 +1100780per line, though the lines can be very long.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100781After this, the user can log in without giving the password.
782.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100783The most convenient way to use public key authentication may be with an
Darren Tuckerd3877b92005-12-20 16:09:36 +1100784authentication agent.
785See
786.Xr ssh-agent 1
787for more information.
788.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100789Challenge-response authentication works as follows:
790The server sends an arbitrary
791.Qq challenge
792text, and prompts for a response.
793Protocol 2 allows multiple challenges and responses;
794protocol 1 is restricted to just one challenge/response.
795Examples of challenge-response authentication include
796BSD Authentication (see
797.Xr login.conf 5 )
798and PAM (some non-OpenBSD systems).
799.Pp
800Finally, if other authentication methods fail,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100801.Nm
802prompts the user for a password.
803The password is sent to the remote
804host for checking; however, since all communications are encrypted,
805the password cannot be seen by someone listening on the network.
Damien Miller48c94ab2006-01-02 23:38:00 +1100806.Pp
807.Nm
808automatically maintains and checks a database containing
809identification for all hosts it has ever been used with.
810Host keys are stored in
811.Pa ~/.ssh/known_hosts
812in the user's home directory.
813Additionally, the file
814.Pa /etc/ssh/ssh_known_hosts
815is automatically checked for known hosts.
816Any new hosts are automatically added to the user's file.
817If a host's identification ever changes,
818.Nm
819warns about this and disables password authentication to prevent
820server spoofing or man-in-the-middle attacks,
821which could otherwise be used to circumvent the encryption.
822The
823.Cm StrictHostKeyChecking
824option can be used to control logins to machines whose
825host key is not known or has changed.
826.Pp
Darren Tuckerd3877b92005-12-20 16:09:36 +1100827When the user's identity has been accepted by the server, the server
828either executes the given command, or logs into the machine and gives
829the user a normal shell on the remote machine.
830All communication with
831the remote command or shell will be automatically encrypted.
832.Pp
833If a pseudo-terminal has been allocated (normal login session), the
834user may use the escape characters noted below.
835.Pp
836If no pseudo-tty has been allocated,
837the session is transparent and can be used to reliably transfer binary data.
838On most systems, setting the escape character to
839.Dq none
840will also make the session transparent even if a tty is used.
841.Pp
842The session terminates when the command or shell on the remote
Damien Miller8bfaf932006-01-14 10:09:30 +1100843machine exits and all X11 and TCP connections have been closed.
Damien Miller52d20612005-12-24 14:52:36 +1100844.Sh ESCAPE CHARACTERS
Darren Tuckerd3877b92005-12-20 16:09:36 +1100845When a pseudo-terminal has been requested,
846.Nm
847supports a number of functions through the use of an escape character.
848.Pp
849A single tilde character can be sent as
850.Ic ~~
851or by following the tilde by a character other than those described below.
852The escape character must always follow a newline to be interpreted as
853special.
854The escape character can be changed in configuration files using the
855.Cm EscapeChar
856configuration directive or on the command line by the
857.Fl e
858option.
859.Pp
860The supported escapes (assuming the default
861.Ql ~ )
862are:
863.Bl -tag -width Ds
864.It Cm ~.
865Disconnect.
866.It Cm ~^Z
867Background
Damien Miller1164c292006-01-02 23:38:37 +1100868.Nm .
Darren Tuckerd3877b92005-12-20 16:09:36 +1100869.It Cm ~#
870List forwarded connections.
871.It Cm ~&
872Background
873.Nm
874at logout when waiting for forwarded connection / X11 sessions to terminate.
875.It Cm ~?
876Display a list of escape characters.
877.It Cm ~B
878Send a BREAK to the remote system
879(only useful for SSH protocol version 2 and if the peer supports it).
880.It Cm ~C
881Open command line.
882Currently this allows the addition of port forwardings using the
883.Fl L
884and
885.Fl R
Damien Miller329cb012005-12-24 14:53:23 +1100886options (see above).
Darren Tuckerd3877b92005-12-20 16:09:36 +1100887It also allows the cancellation of existing remote port-forwardings
888using
Damien Miller57e8ad32006-07-10 20:20:52 +1000889.Sm off
890.Fl KR Oo Ar bind_address : Oc Ar port .
891.Sm on
Darren Tuckerd3877b92005-12-20 16:09:36 +1100892.Ic !\& Ns Ar command
893allows the user to execute a local command if the
894.Ic PermitLocalCommand
895option is enabled in
896.Xr ssh_config 5 .
897Basic help is available, using the
898.Fl h
899option.
900.It Cm ~R
901Request rekeying of the connection
902(only useful for SSH protocol version 2 and if the peer supports it).
903.El
Damien Millerf3177182006-01-14 10:09:13 +1100904.Sh TCP FORWARDING
905Forwarding of arbitrary TCP connections over the secure channel can
906be specified either on the command line or in a configuration file.
907One possible application of TCP forwarding is a secure connection to a
908mail server; another is going through firewalls.
909.Pp
910In the example below, we look at encrypting communication between
911an IRC client and server, even though the IRC server does not directly
912support encrypted communications.
913This works as follows:
914the user connects to the remote host using
915.Nm ,
916specifying a port to be used to forward connections
917to the remote server.
918After that it is possible to start the service which is to be encrypted
919on the client machine,
920connecting to the same local port,
921and
922.Nm
923will encrypt and forward the connection.
924.Pp
925The following example tunnels an IRC session from client machine
926.Dq 127.0.0.1
927(localhost)
928to remote server
929.Dq server.example.com :
930.Bd -literal -offset 4n
931$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
932$ irc -c '#users' -p 1234 pinky 127.0.0.1
933.Ed
934.Pp
935This tunnels a connection to IRC server
936.Dq server.example.com ,
937joining channel
938.Dq #users ,
939nickname
940.Dq pinky ,
941using port 1234.
942It doesn't matter which port is used,
943as long as it's greater than 1023
944(remember, only root can open sockets on privileged ports)
945and doesn't conflict with any ports already in use.
946The connection is forwarded to port 6667 on the remote server,
947since that's the standard port for IRC services.
948.Pp
949The
950.Fl f
951option backgrounds
952.Nm
953and the remote command
954.Dq sleep 10
955is specified to allow an amount of time
956(10 seconds, in the example)
957to start the service which is to be tunnelled.
958If no connections are made within the time specified,
959.Nm
960will exit.
Damien Millerf3177182006-01-14 10:09:13 +1100961.Sh X11 FORWARDING
Darren Tuckerd3877b92005-12-20 16:09:36 +1100962If the
963.Cm ForwardX11
964variable is set to
965.Dq yes
966(or see the description of the
Damien Millerd7f308f2005-12-24 14:55:16 +1100967.Fl X ,
968.Fl x ,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100969and
Damien Millerd7f308f2005-12-24 14:55:16 +1100970.Fl Y
Damien Miller9a765b22005-12-24 14:53:44 +1100971options above)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100972and the user is using X11 (the
973.Ev DISPLAY
974environment variable is set), the connection to the X11 display is
975automatically forwarded to the remote side in such a way that any X11
976programs started from the shell (or command) will go through the
977encrypted channel, and the connection to the real X server will be made
978from the local machine.
979The user should not manually set
980.Ev DISPLAY .
981Forwarding of X11 connections can be
982configured on the command line or in configuration files.
983.Pp
984The
985.Ev DISPLAY
986value set by
987.Nm
988will point to the server machine, but with a display number greater than zero.
989This is normal, and happens because
990.Nm
991creates a
992.Dq proxy
993X server on the server machine for forwarding the
994connections over the encrypted channel.
995.Pp
996.Nm
997will also automatically set up Xauthority data on the server machine.
998For this purpose, it will generate a random authorization cookie,
999store it in Xauthority on the server, and verify that any forwarded
1000connections carry this cookie and replace it by the real cookie when
1001the connection is opened.
1002The real authentication cookie is never
1003sent to the server machine (and no cookies are sent in the plain).
1004.Pp
1005If the
1006.Cm ForwardAgent
1007variable is set to
1008.Dq yes
1009(or see the description of the
1010.Fl A
1011and
1012.Fl a
Damien Miller9a765b22005-12-24 14:53:44 +11001013options above) and
Darren Tuckerd3877b92005-12-20 16:09:36 +11001014the user is using an authentication agent, the connection to the agent
1015is automatically forwarded to the remote side.
Damien Millerbbc59092006-01-31 21:46:51 +11001016.Sh VERIFYING HOST KEYS
1017When connecting to a server for the first time,
1018a fingerprint of the server's public key is presented to the user
1019(unless the option
1020.Cm StrictHostKeyChecking
1021has been disabled).
1022Fingerprints can be determined using
1023.Xr ssh-keygen 1 :
1024.Pp
1025.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1026.Pp
1027If the fingerprint is already known,
1028it can be matched and verified,
1029and the key can be accepted.
1030If the fingerprint is unknown,
1031an alternative method of verification is available:
1032SSH fingerprints verified by DNS.
1033An additional resource record (RR),
1034SSHFP,
1035is added to a zonefile
1036and the connecting client is able to match the fingerprint
1037with that of the key presented.
1038.Pp
1039In this example, we are connecting a client to a server,
1040.Dq host.example.com .
1041The SSHFP resource records should first be added to the zonefile for
1042host.example.com:
1043.Bd -literal -offset indent
Damien Millera1b3d632006-03-26 00:07:02 +11001044$ ssh-keygen -r host.example.com.
Damien Millerbbc59092006-01-31 21:46:51 +11001045.Ed
1046.Pp
1047The output lines will have to be added to the zonefile.
1048To check that the zone is answering fingerprint queries:
1049.Pp
1050.Dl $ dig -t SSHFP host.example.com
1051.Pp
1052Finally the client connects:
1053.Bd -literal -offset indent
1054$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1055[...]
1056Matching host key fingerprint found in DNS.
1057Are you sure you want to continue connecting (yes/no)?
1058.Ed
1059.Pp
1060See the
1061.Cm VerifyHostKeyDNS
1062option in
1063.Xr ssh_config 5
1064for more information.
Darren Tucker248dd132006-01-20 11:30:58 +11001065.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1066.Nm
1067contains support for Virtual Private Network (VPN) tunnelling
1068using the
1069.Xr tun 4
1070network pseudo-device,
1071allowing two networks to be joined securely.
1072The
1073.Xr sshd_config 5
1074configuration option
1075.Cm PermitTunnel
1076controls whether the server supports this,
1077and at what level (layer 2 or 3 traffic).
1078.Pp
1079The following example would connect client network 10.0.50.0/24
1080with remote network 10.0.99.0/24, provided that the SSH server
1081running on the gateway to the remote network,
1082at 192.168.1.15, allows it:
1083.Bd -literal -offset indent
1084# ssh -f -w 0:1 192.168.1.15 true
1085# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
1086.Ed
1087.Pp
1088Client access may be more finely tuned via the
1089.Pa /root/.ssh/authorized_keys
1090file (see below) and the
1091.Cm PermitRootLogin
1092server option.
Damien Millere93eaaa2006-03-15 11:05:59 +11001093The following entry would permit connections on
Darren Tucker248dd132006-01-20 11:30:58 +11001094.Xr tun 4
Damien Millere93eaaa2006-03-15 11:05:59 +11001095device 1 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001096.Dq jane
Damien Millere93eaaa2006-03-15 11:05:59 +11001097and on tun device 2 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001098.Dq john ,
1099if
1100.Cm PermitRootLogin
1101is set to
1102.Dq forced-commands-only :
1103.Bd -literal -offset 2n
1104tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
Damien Miller9f67a212006-03-15 11:05:35 +11001105tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Darren Tucker248dd132006-01-20 11:30:58 +11001106.Ed
1107.Pp
1108Since a SSH-based setup entails a fair amount of overhead,
1109it may be more suited to temporary setups,
1110such as for wireless VPNs.
1111More permanent VPNs are better provided by tools such as
1112.Xr ipsecctl 8
1113and
1114.Xr isakmpd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +10001115.Sh ENVIRONMENT
1116.Nm
1117will normally set the following environment variables:
Damien Miller6aa22902006-01-06 14:48:34 +11001118.Bl -tag -width "SSH_ORIGINAL_COMMAND"
Damien Miller32aa1441999-10-29 09:15:49 +10001119.It Ev DISPLAY
1120The
1121.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +11001122variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +10001123It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +10001124.Nm
1125to point to a value of the form
Damien Miller14af93e2006-01-02 23:38:21 +11001126.Dq hostname:n ,
1127where
1128.Dq hostname
1129indicates the host where the shell runs, and
1130.Sq n
1131is an integer \*(Ge 1.
Damien Miller7684ee12000-03-17 23:40:15 +11001132.Nm
1133uses this special value to forward X11 connections over the secure
1134channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001135The user should normally not set
1136.Ev DISPLAY
1137explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +10001138will render the X11 connection insecure (and will require the user to
1139manually copy any required authorization cookies).
1140.It Ev HOME
1141Set to the path of the user's home directory.
1142.It Ev LOGNAME
1143Synonym for
1144.Ev USER ;
1145set for compatibility with systems that use this variable.
1146.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +00001147Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +11001148.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +10001149Set to the default
1150.Ev PATH ,
1151as specified when compiling
Damien Miller1164c292006-01-02 23:38:37 +11001152.Nm .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001153.It Ev SSH_ASKPASS
1154If
1155.Nm
1156needs a passphrase, it will read the passphrase from the current
1157terminal if it was run from a terminal.
1158If
1159.Nm
1160does not have a terminal associated with it but
1161.Ev DISPLAY
1162and
1163.Ev SSH_ASKPASS
1164are set, it will execute the program specified by
1165.Ev SSH_ASKPASS
1166and open an X11 window to read the passphrase.
1167This is particularly useful when calling
1168.Nm
1169from a
Darren Tucker4e4fe002004-11-05 20:01:03 +11001170.Pa .xsession
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001171or related script.
1172(Note that on some machines it
1173may be necessary to redirect the input from
1174.Pa /dev/null
1175to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +10001176.It Ev SSH_AUTH_SOCK
Damien Miller14af93e2006-01-02 23:38:21 +11001177Identifies the path of a
1178.Ux Ns -domain
1179socket used to communicate with the agent.
Damien Millerf37e2462002-09-19 11:47:55 +10001180.It Ev SSH_CONNECTION
1181Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +11001182The variable contains
Damien Miller14af93e2006-01-02 23:38:21 +11001183four space-separated values: client IP address, client port number,
1184server IP address, and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001185.It Ev SSH_ORIGINAL_COMMAND
Damien Miller14af93e2006-01-02 23:38:21 +11001186This variable contains the original command line if a forced command
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001187is executed.
1188It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +10001189.It Ev SSH_TTY
1190This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +11001191with the current shell or command.
1192If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +10001193this variable is not set.
1194.It Ev TZ
Damien Miller6dbdb6a2005-12-13 19:25:43 +11001195This variable is set to indicate the present time zone if it
Damien Miller208f1ed2006-03-15 11:56:03 +11001196was set when the daemon was started (i.e. the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +10001197on to new connections).
1198.It Ev USER
1199Set to the name of the user logging in.
1200.El
1201.Pp
Damien Miller22c77262000-04-13 12:26:34 +10001202Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +10001203.Nm
Damien Miller22c77262000-04-13 12:26:34 +10001204reads
Damien Miller167ea5d2005-05-26 12:04:02 +10001205.Pa ~/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +10001206and adds lines of the format
1207.Dq VARNAME=value
Damien Miller14af93e2006-01-02 23:38:21 +11001208to the environment if the file exists and users are allowed to
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001209change their environment.
Darren Tucker61776952003-10-02 16:19:47 +10001210For more information, see the
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001211.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +00001212option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001213.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +10001214.Sh FILES
Damien Miller7655f5c2006-01-06 14:48:18 +11001215.Bl -tag -width Ds -compact
Damien Miller7655f5c2006-01-06 14:48:18 +11001216.It ~/.rhosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001217This file is used for host-based authentication (see above).
Ben Lindstromebd888d2001-03-05 05:49:29 +00001218On some machines this file may need to be
Damien Miller1bcdb502006-01-06 14:49:38 +11001219world-readable if the user's home directory is on an NFS partition,
Damien Miller32aa1441999-10-29 09:15:49 +10001220because
1221.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001222reads it as root.
1223Additionally, this file must be owned by the user,
1224and must not have write permissions for anyone else.
1225The recommended
Damien Miller32aa1441999-10-29 09:15:49 +10001226permission for most machines is read/write for the user, and not
1227accessible by others.
1228.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001229.It ~/.shosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001230This file is used in exactly the same way as
1231.Pa .rhosts ,
1232but allows host-based authentication without permitting login with
1233rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001234.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001235.It ~/.ssh/authorized_keys
1236Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1237The format of this file is described in the
1238.Xr sshd 8
1239manual page.
Damien Millerfb8ea742006-01-06 14:48:52 +11001240This file is not highly sensitive, but the recommended
1241permissions are read/write for the user, and not accessible by others.
1242.Pp
1243.It ~/.ssh/config
1244This is the per-user configuration file.
1245The file format and configuration options are described in
1246.Xr ssh_config 5 .
1247Because of the potential for abuse, this file must have strict permissions:
1248read/write for the user, and not accessible by others.
1249.Pp
1250.It ~/.ssh/environment
Damien Miller4c102ee2006-01-06 14:49:17 +11001251Contains additional definitions for environment variables; see
1252.Sx ENVIRONMENT ,
Damien Millerfb8ea742006-01-06 14:48:52 +11001253above.
1254.Pp
1255.It ~/.ssh/identity
1256.It ~/.ssh/id_dsa
1257.It ~/.ssh/id_rsa
1258Contains the private key for authentication.
1259These files
1260contain sensitive data and should be readable by the user but not
1261accessible by others (read/write/execute).
1262.Nm
1263will simply ignore a private key file if it is accessible by others.
1264It is possible to specify a passphrase when
1265generating the key which will be used to encrypt the
1266sensitive part of this file using 3DES.
1267.Pp
1268.It ~/.ssh/identity.pub
1269.It ~/.ssh/id_dsa.pub
1270.It ~/.ssh/id_rsa.pub
1271Contains the public key for authentication.
1272These files are not
1273sensitive and can (but need not) be readable by anyone.
Damien Millerfb8ea742006-01-06 14:48:52 +11001274.Pp
1275.It ~/.ssh/known_hosts
Damien Millere87eb4c2006-01-14 10:08:36 +11001276Contains a list of host keys for all hosts the user has logged into
1277that are not already in the systemwide list of known host keys.
Damien Millerfb8ea742006-01-06 14:48:52 +11001278See
Damien Millere87eb4c2006-01-14 10:08:36 +11001279.Xr sshd 8
1280for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001281.Pp
1282.It ~/.ssh/rc
1283Commands in this file are executed by
1284.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001285when the user logs in, just before the user's shell (or command) is
Damien Millerfb8ea742006-01-06 14:48:52 +11001286started.
1287See the
1288.Xr sshd 8
1289manual page for more information.
1290.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001291.It /etc/hosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001292This file is for host-based authentication (see above).
1293It should only be writable by root.
Damien Miller7655f5c2006-01-06 14:48:18 +11001294.Pp
1295.It /etc/shosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001296This file is used in exactly the same way as
1297.Pa hosts.equiv ,
1298but allows host-based authentication without permitting login with
1299rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001300.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001301.It Pa /etc/ssh/ssh_config
1302Systemwide configuration file.
1303The file format and configuration options are described in
1304.Xr ssh_config 5 .
1305.Pp
1306.It /etc/ssh/ssh_host_key
1307.It /etc/ssh/ssh_host_dsa_key
1308.It /etc/ssh/ssh_host_rsa_key
1309These three files contain the private parts of the host keys
Damien Miller7e76e1f2006-01-14 10:08:57 +11001310and are used for host-based authentication.
1311If protocol version 1 is used,
Damien Millerfb8ea742006-01-06 14:48:52 +11001312.Nm
1313must be setuid root, since the host key is readable only by root.
1314For protocol version 2,
1315.Nm
1316uses
1317.Xr ssh-keysign 8
Damien Miller7e76e1f2006-01-14 10:08:57 +11001318to access the host keys,
1319eliminating the requirement that
Damien Millerfb8ea742006-01-06 14:48:52 +11001320.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001321be setuid root when host-based authentication is used.
Damien Millerfb8ea742006-01-06 14:48:52 +11001322By default
1323.Nm
1324is not setuid root.
1325.Pp
1326.It /etc/ssh/ssh_known_hosts
1327Systemwide list of known host keys.
1328This file should be prepared by the
1329system administrator to contain the public host keys of all machines in the
1330organization.
Damien Millere87eb4c2006-01-14 10:08:36 +11001331It should be world-readable.
1332See
Damien Millerfb8ea742006-01-06 14:48:52 +11001333.Xr sshd 8
Damien Millere87eb4c2006-01-14 10:08:36 +11001334for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001335.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001336.It /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +10001337Commands in this file are executed by
1338.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001339when the user logs in, just before the user's shell (or command) is started.
Damien Miller32aa1441999-10-29 09:15:49 +10001340See the
1341.Xr sshd 8
1342manual page for more information.
Damien Miller7b28dc52000-09-05 13:34:53 +11001343.El
Damien Miller32aa1441999-10-29 09:15:49 +10001344.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +10001345.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +11001346.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +10001347.Xr ssh-add 1 ,
1348.Xr ssh-agent 1 ,
1349.Xr ssh-keygen 1 ,
Damien Miller128a0f12006-01-06 14:50:11 +11001350.Xr ssh-keyscan 1 ,
Darren Tucker248dd132006-01-20 11:30:58 +11001351.Xr tun 4 ,
Darren Tucker61776952003-10-02 16:19:47 +10001352.Xr hosts.equiv 5 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +00001353.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +00001354.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +00001355.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +00001356.Rs
Damien Miller39a93a32006-03-15 11:34:45 +11001357.%R RFC 4250
1358.%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1359.%D 2006
1360.Re
1361.Rs
1362.%R RFC 4251
1363.%T "The Secure Shell (SSH) Protocol Architecture"
1364.%D 2006
1365.Re
1366.Rs
1367.%R RFC 4252
1368.%T "The Secure Shell (SSH) Authentication Protocol"
1369.%D 2006
1370.Re
1371.Rs
1372.%R RFC 4253
1373.%T "The Secure Shell (SSH) Transport Layer Protocol"
1374.%D 2006
1375.Re
1376.Rs
1377.%R RFC 4254
1378.%T "The Secure Shell (SSH) Connection Protocol"
1379.%D 2006
1380.Re
1381.Rs
1382.%R RFC 4255
1383.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1384.%D 2006
1385.Re
1386.Rs
1387.%R RFC 4256
1388.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1389.%D 2006
1390.Re
1391.Rs
1392.%R RFC 4335
1393.%T "The Secure Shell (SSH) Session Channel Break Extension"
1394.%D 2006
1395.Re
1396.Rs
1397.%R RFC 4344
1398.%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1399.%D 2006
1400.Re
1401.Rs
1402.%R RFC 4345
1403.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1404.%D 2006
Ben Lindstrom160ec622001-04-22 17:17:46 +00001405.Re
Damien Miller3e96d742006-03-25 23:39:29 +11001406.Rs
1407.%R RFC 4419
1408.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1409.%D 2006
1410.Re
Damien Millerf1ce5052003-06-11 22:04:39 +10001411.Sh AUTHORS
1412OpenSSH is a derivative of the original and free
1413ssh 1.2.12 release by Tatu Ylonen.
1414Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1415Theo de Raadt and Dug Song
1416removed many bugs, re-added newer features and
1417created OpenSSH.
1418Markus Friedl contributed the support for SSH
1419protocol versions 1.5 and 2.0.