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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 Tucker9a2a6092008-07-04 12:53:50 +100037.\" $OpenBSD: ssh.1,v 1.277 2008/07/02 13:47:39 djm Exp $
38.Dd $Mdocdate: July 2 2008 $
Damien Miller32aa1441999-10-29 09:15:49 +100039.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 Tucker0409e152007-06-13 00:00:58 +100046.Op Fl 1246AaCfgKkMNnqsTtVvXxY
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 .
Darren Tucker9a2a6092008-07-04 12:53:50 +1000293.Pp
294If the
295.Cm ExitOnForwardFailure
296configuration option is set to
297.Dq yes ,
298then a client started with
299.Fl f
300will wait for all remote port forwards to be successfully established
301before placing itself in the background.
Damien Miller396691a2000-01-20 22:44:08 +1100302.It Fl g
303Allows remote hosts to connect to local forwarded ports.
Darren Tucker61776952003-10-02 16:19:47 +1000304.It Fl I Ar smartcard_device
Damien Miller2142ba02005-12-24 14:56:29 +1100305Specify the device
Darren Tucker61776952003-10-02 16:19:47 +1000306.Nm
307should use to communicate with a smartcard used for storing the user's
308private RSA key.
Damien Miller2142ba02005-12-24 14:56:29 +1100309This option is only available if support for smartcard devices
310is compiled in (default is no support).
Damien Miller32aa1441999-10-29 09:15:49 +1000311.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100312Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100313RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100314The default is
Damien Miller167ea5d2005-05-26 12:04:02 +1000315.Pa ~/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100316for protocol version 1, and
Damien Miller167ea5d2005-05-26 12:04:02 +1000317.Pa ~/.ssh/id_rsa
Damien Millerfae23862002-01-22 23:32:26 +1100318and
Damien Miller167ea5d2005-05-26 12:04:02 +1000319.Pa ~/.ssh/id_dsa
Damien Millerfae23862002-01-22 23:32:26 +1100320for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100321Identity files may also be specified on
322a per-host basis in the configuration file.
323It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000324.Fl i
325options (and multiple identities specified in
326configuration files).
Darren Tucker415bddc2007-06-12 23:43:16 +1000327.It Fl K
328Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
329credentials to the server.
Damien Miller32aa1441999-10-29 09:15:49 +1000330.It Fl k
Damien Millere0113cc2003-11-24 13:10:09 +1100331Disables forwarding (delegation) of GSSAPI credentials to the server.
Darren Tucker61776952003-10-02 16:19:47 +1000332.It Fl L Xo
333.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100334.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000335.Ar port : host : hostport
336.Sm on
337.Xc
338Specifies that the given port on the local (client) host is to be
339forwarded to the given host and port on the remote side.
340This works by allocating a socket to listen to
341.Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100342on the local side, optionally bound to the specified
343.Ar bind_address .
344Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000345connection is forwarded over the secure channel, and a connection is
346made to
347.Ar host
348port
349.Ar hostport
350from the remote machine.
351Port forwardings can also be specified in the configuration file.
Darren Tucker61776952003-10-02 16:19:47 +1000352IPv6 addresses can be specified with an alternative syntax:
353.Sm off
354.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100355.Op Ar bind_address No /
Darren Tucker61776952003-10-02 16:19:47 +1000356.Ar port No / Ar host No /
Damien Millerf91ee4c2005-03-01 21:24:33 +1100357.Ar hostport
Darren Tucker61776952003-10-02 16:19:47 +1000358.Xc
359.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100360or by enclosing the address in square brackets.
361Only the superuser can forward privileged ports.
362By default, the local port is bound in accordance with the
363.Cm GatewayPorts
364setting.
365However, an explicit
366.Ar bind_address
367may be used to bind the connection to a specific address.
368The
369.Ar bind_address
370of
371.Dq localhost
Damien Miller36bf7dd2005-03-02 12:02:47 +1100372indicates that the listening port be bound for local use only, while an
373empty address or
374.Sq *
Damien Millerf91ee4c2005-03-01 21:24:33 +1100375indicates that the port should be available from all interfaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000376.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100377Specifies the user to log in as on the remote machine.
378This also may be specified on a per-host basis in the configuration file.
Damien Miller0e220db2004-06-15 10:34:08 +1000379.It Fl M
380Places the
381.Nm
382client into
383.Dq master
384mode for connection sharing.
Damien Miller88b25522005-12-31 16:23:15 +1100385Multiple
386.Fl M
387options places
388.Nm
389into
390.Dq master
391mode with confirmation required before slave connections are accepted.
Damien Miller0e220db2004-06-15 10:34:08 +1000392Refer to the description of
393.Cm ControlMaster
394in
395.Xr ssh_config 5
396for details.
Damien Miller5d1eceb2004-06-30 22:37:57 +1000397.It Fl m Ar mac_spec
398Additionally, for protocol version 2 a comma-separated list of MAC
399(message authentication code) algorithms can
400be specified in order of preference.
401See the
402.Cm MACs
403keyword for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000404.It Fl N
405Do not execute a remote command.
406This is useful for just forwarding ports
407(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000408.It Fl n
409Redirects stdin from
410.Pa /dev/null
411(actually, prevents reading from stdin).
412This must be used when
413.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100414is run in the background.
415A common trick is to use this to run X11 programs on a remote machine.
416For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000417.Ic ssh -n shadows.cs.hut.fi emacs &
418will start an emacs on shadows.cs.hut.fi, and the X11
419connection will be automatically forwarded over an encrypted channel.
420The
421.Nm
422program will be put in the background.
423(This does not work if
424.Nm
425needs to ask for a password or passphrase; see also the
426.Fl f
427option.)
Darren Tuckere04644c2004-12-03 14:08:45 +1100428.It Fl O Ar ctl_cmd
429Control an active connection multiplexing master process.
430When the
431.Fl O
432option is specified, the
433.Ar ctl_cmd
434argument is interpreted and passed to the master process.
435Valid commands are:
436.Dq check
437(check that the master process is running) and
438.Dq exit
439(request the master to exit).
Damien Miller32aa1441999-10-29 09:15:49 +1000440.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000441Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000442This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100443command-line flag.
Darren Tucker61776952003-10-02 16:19:47 +1000444For full details of the options listed below, and their possible values, see
445.Xr ssh_config 5 .
446.Pp
447.Bl -tag -width Ds -offset indent -compact
448.It AddressFamily
449.It BatchMode
450.It BindAddress
451.It ChallengeResponseAuthentication
452.It CheckHostIP
453.It Cipher
454.It Ciphers
455.It ClearAllForwardings
456.It Compression
457.It CompressionLevel
458.It ConnectionAttempts
Darren Tuckeredae0ec2004-05-02 22:15:52 +1000459.It ConnectTimeout
Damien Miller0e220db2004-06-15 10:34:08 +1000460.It ControlMaster
461.It ControlPath
Darren Tucker61776952003-10-02 16:19:47 +1000462.It DynamicForward
Darren Tucker61776952003-10-02 16:19:47 +1000463.It EscapeChar
Darren Tuckere7d4b192006-07-12 22:17:10 +1000464.It ExitOnForwardFailure
Darren Tucker61776952003-10-02 16:19:47 +1000465.It ForwardAgent
466.It ForwardX11
Darren Tucker0a118da2003-10-15 15:54:32 +1000467.It ForwardX11Trusted
Darren Tucker61776952003-10-02 16:19:47 +1000468.It GatewayPorts
469.It GlobalKnownHostsFile
470.It GSSAPIAuthentication
471.It GSSAPIDelegateCredentials
Damien Millere1776152005-03-01 21:47:37 +1100472.It HashKnownHosts
Darren Tucker61776952003-10-02 16:19:47 +1000473.It Host
474.It HostbasedAuthentication
475.It HostKeyAlgorithms
476.It HostKeyAlias
477.It HostName
478.It IdentityFile
Damien Millerbd394c32004-03-08 23:12:36 +1100479.It IdentitiesOnly
Darren Tucker636ca902004-11-05 20:22:00 +1100480.It KbdInteractiveDevices
Damien Millerd27b9472005-12-13 19:29:02 +1100481.It LocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000482.It LocalForward
483.It LogLevel
484.It MACs
485.It NoHostAuthenticationForLocalhost
486.It NumberOfPasswordPrompts
487.It PasswordAuthentication
Damien Millerd27b9472005-12-13 19:29:02 +1100488.It PermitLocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000489.It Port
490.It PreferredAuthentications
491.It Protocol
492.It ProxyCommand
493.It PubkeyAuthentication
Darren Tucker62388b22006-01-20 11:31:47 +1100494.It RekeyLimit
Darren Tucker61776952003-10-02 16:19:47 +1000495.It RemoteForward
496.It RhostsRSAAuthentication
497.It RSAAuthentication
Darren Tucker7a6c0662004-05-02 22:14:03 +1000498.It SendEnv
Damien Miller509b0102003-12-17 16:33:10 +1100499.It ServerAliveInterval
500.It ServerAliveCountMax
Darren Tucker61776952003-10-02 16:19:47 +1000501.It SmartcardDevice
502.It StrictHostKeyChecking
Damien Miller12c150e2003-12-17 16:31:10 +1100503.It TCPKeepAlive
Damien Millerd27b9472005-12-13 19:29:02 +1100504.It Tunnel
505.It TunnelDevice
Darren Tucker61776952003-10-02 16:19:47 +1000506.It UsePrivilegedPort
507.It User
508.It UserKnownHostsFile
509.It VerifyHostKeyDNS
Damien Miller86399202008-06-30 00:04:31 +1000510.It VisualHostKey
Darren Tucker61776952003-10-02 16:19:47 +1000511.It XAuthLocation
512.El
Damien Miller32aa1441999-10-29 09:15:49 +1000513.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100514Port to connect to on the remote host.
515This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000516per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000517.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100518Quiet mode.
Damien Millereb602472008-02-10 22:21:28 +1100519Causes most warning and diagnostic messages to be suppressed.
Darren Tucker61776952003-10-02 16:19:47 +1000520.It Fl R Xo
521.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100522.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000523.Ar port : host : hostport
524.Sm on
525.Xc
Damien Miller32aa1441999-10-29 09:15:49 +1000526Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100527forwarded to the given host and port on the local side.
528This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000529.Ar port
530on the remote side, and whenever a connection is made to this port, the
531connection is forwarded over the secure channel, and a connection is
532made to
Damien Miller34132e52000-01-14 15:45:46 +1100533.Ar host
534port
535.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100536from the local machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100537.Pp
Damien Miller7684ee12000-03-17 23:40:15 +1100538Port forwardings can also be specified in the configuration file.
539Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000540logging in as root on the remote machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100541IPv6 addresses can be specified by enclosing the address in square braces or
542using an alternative syntax:
Damien Millerf91ee4c2005-03-01 21:24:33 +1100543.Sm off
Damien Miller36bf7dd2005-03-02 12:02:47 +1100544.Xo
Damien Millerb096ac42005-03-09 11:00:05 +1100545.Op Ar bind_address No /
Damien Miller36bf7dd2005-03-02 12:02:47 +1100546.Ar host No / Ar port No /
547.Ar hostport
Damien Millerf91ee4c2005-03-01 21:24:33 +1100548.Xc .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100549.Sm on
Damien Millerf91ee4c2005-03-01 21:24:33 +1100550.Pp
551By default, the listening socket on the server will be bound to the loopback
552interface only.
553This may be overriden by specifying a
554.Ar bind_address .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100555An empty
556.Ar bind_address ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100557or the address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100558.Ql * ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100559indicates that the remote socket should listen on all interfaces.
560Specifying a remote
561.Ar bind_address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100562will only succeed if the server's
563.Cm GatewayPorts
Damien Millerf91ee4c2005-03-01 21:24:33 +1100564option is enabled (see
Damien Miller36bf7dd2005-03-02 12:02:47 +1100565.Xr sshd_config 5 ) .
Darren Tucker7ebfc102004-11-07 20:06:19 +1100566.It Fl S Ar ctl_path
Damien Miller5d1eceb2004-06-30 22:37:57 +1000567Specifies the location of a control socket for connection sharing.
Damien Miller0e220db2004-06-15 10:34:08 +1000568Refer to the description of
Darren Tuckerba5c5922004-06-18 16:22:39 +1000569.Cm ControlPath
570and
Damien Miller0e220db2004-06-15 10:34:08 +1000571.Cm ControlMaster
572in
573.Xr ssh_config 5
574for details.
Darren Tucker61776952003-10-02 16:19:47 +1000575.It Fl s
576May be used to request invocation of a subsystem on the remote system.
577Subsystems are a feature of the SSH2 protocol which facilitate the use
578of SSH as a secure transport for other applications (eg.\&
579.Xr sftp 1 ) .
580The subsystem is specified as the remote command.
581.It Fl T
582Disable pseudo-tty allocation.
583.It Fl t
584Force pseudo-tty allocation.
585This can be used to execute arbitrary
586screen-based programs on a remote machine, which can be very useful,
Damien Miller208f1ed2006-03-15 11:56:03 +1100587e.g. when implementing menu services.
Darren Tucker61776952003-10-02 16:19:47 +1000588Multiple
589.Fl t
590options force tty allocation, even if
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000591.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000592has no local tty.
593.It Fl V
594Display the version number and exit.
595.It Fl v
596Verbose mode.
597Causes
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000598.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000599to print debugging messages about its progress.
600This is helpful in
601debugging connection, authentication, and configuration problems.
602Multiple
603.Fl v
604options increase the verbosity.
605The maximum is 3.
Damien Miller991dba42006-07-10 20:16:27 +1000606.It Fl w Xo
607.Ar local_tun Ns Op : Ns Ar remote_tun
608.Xc
609Requests
610tunnel
611device forwarding with the specified
Damien Millerd27b9472005-12-13 19:29:02 +1100612.Xr tun 4
Damien Miller991dba42006-07-10 20:16:27 +1000613devices between the client
614.Pq Ar local_tun
615and the server
616.Pq Ar remote_tun .
617.Pp
Damien Millercf1e3422005-12-24 14:56:04 +1100618The devices may be specified by numerical ID or the keyword
619.Dq any ,
620which uses the next available tunnel device.
Damien Miller991dba42006-07-10 20:16:27 +1000621If
622.Ar remote_tun
623is not specified, it defaults to
624.Dq any .
Damien Millercf1e3422005-12-24 14:56:04 +1100625See also the
Damien Millerd27b9472005-12-13 19:29:02 +1100626.Cm Tunnel
Damien Miller991dba42006-07-10 20:16:27 +1000627and
628.Cm TunnelDevice
629directives in
Damien Millerd27b9472005-12-13 19:29:02 +1100630.Xr ssh_config 5 .
Damien Miller991dba42006-07-10 20:16:27 +1000631If the
632.Cm Tunnel
633directive is unset, it is set to the default tunnel mode, which is
634.Dq point-to-point .
Darren Tucker61776952003-10-02 16:19:47 +1000635.It Fl X
636Enables X11 forwarding.
637This can also be specified on a per-host basis in a configuration file.
638.Pp
639X11 forwarding should be enabled with caution.
640Users with the ability to bypass file permissions on the remote host
641(for the user's X authorization database)
642can access the local X11 display through the forwarded connection.
643An attacker may then be able to perform activities such as keystroke monitoring.
Damien Miller947219e2005-03-02 13:22:30 +1100644.Pp
Damien Millerb022b232005-03-05 11:22:36 +1100645For this reason, X11 forwarding is subjected to X11 SECURITY extension
Damien Miller947219e2005-03-02 13:22:30 +1100646restrictions by default.
647Please refer to the
648.Nm
649.Fl Y
650option and the
651.Cm ForwardX11Trusted
652directive in
653.Xr ssh_config 5
654for more information.
Darren Tucker61776952003-10-02 16:19:47 +1000655.It Fl x
656Disables X11 forwarding.
Darren Tucker0a118da2003-10-15 15:54:32 +1000657.It Fl Y
658Enables trusted X11 forwarding.
Damien Miller947219e2005-03-02 13:22:30 +1100659Trusted X11 forwardings are not subjected to the X11 SECURITY extension
660controls.
Damien Miller32aa1441999-10-29 09:15:49 +1000661.El
Damien Millere9b333a2005-12-24 14:53:04 +1100662.Pp
663.Nm
664may additionally obtain configuration data from
665a per-user configuration file and a system-wide configuration file.
666The file format and configuration options are described in
667.Xr ssh_config 5 .
668.Pp
669.Nm
670exits with the exit status of the remote command or with 255
671if an error occurred.
Damien Millerc93a8132005-12-24 14:52:13 +1100672.Sh AUTHENTICATION
Darren Tucker94299ec2006-01-20 11:30:14 +1100673The OpenSSH SSH client supports SSH protocols 1 and 2.
Damien Millerc93a8132005-12-24 14:52:13 +1100674Protocol 2 is the default, with
675.Nm
676falling back to protocol 1 if it detects protocol 2 is unsupported.
677These settings may be altered using the
678.Cm Protocol
679option in
680.Xr ssh_config 5 ,
681or enforced using the
682.Fl 1
683and
684.Fl 2
685options (see above).
686Both protocols support similar authentication methods,
687but protocol 2 is preferred since
688it provides additional mechanisms for confidentiality
689(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
Damien Millere45796f2007-06-11 14:01:42 +1000690and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
Damien Millerc93a8132005-12-24 14:52:13 +1100691Protocol 1 lacks a strong mechanism for ensuring the
692integrity of the connection.
693.Pp
694The methods available for authentication are:
Damien Millerfbc94c82006-06-13 13:03:16 +1000695GSSAPI-based authentication,
Damien Millerc93a8132005-12-24 14:52:13 +1100696host-based authentication,
697public key authentication,
698challenge-response authentication,
699and password authentication.
700Authentication methods are tried in the order specified above,
701though protocol 2 has a configuration option to change the default order:
702.Cm PreferredAuthentications .
703.Pp
704Host-based authentication works as follows:
Darren Tuckerd3877b92005-12-20 16:09:36 +1100705If the machine the user logs in from is listed in
706.Pa /etc/hosts.equiv
707or
708.Pa /etc/shosts.equiv
709on the remote machine, and the user names are
710the same on both sides, or if the files
711.Pa ~/.rhosts
712or
713.Pa ~/.shosts
714exist in the user's home directory on the
715remote machine and contain a line containing the name of the client
716machine and the name of the user on that machine, the user is
Damien Millerc93a8132005-12-24 14:52:13 +1100717considered for login.
718Additionally, the server
719.Em must
720be able to verify the client's
721host key (see the description of
Darren Tuckerd3877b92005-12-20 16:09:36 +1100722.Pa /etc/ssh/ssh_known_hosts
723and
Damien Millerc93a8132005-12-24 14:52:13 +1100724.Pa ~/.ssh/known_hosts ,
725below)
726for login to be permitted.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100727This authentication method closes security holes due to IP
Damien Millerc93a8132005-12-24 14:52:13 +1100728spoofing, DNS spoofing, and routing spoofing.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100729[Note to the administrator:
730.Pa /etc/hosts.equiv ,
731.Pa ~/.rhosts ,
732and the rlogin/rsh protocol in general, are inherently insecure and should be
733disabled if security is desired.]
734.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100735Public key authentication works as follows:
736The scheme is based on public-key cryptography,
737using cryptosystems
738where encryption and decryption are done using separate keys,
739and it is unfeasible to derive the decryption key from the encryption key.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100740The idea is that each user creates a public/private
741key pair for authentication purposes.
742The server knows the public key, and only the user knows the private key.
Damien Millerc93a8132005-12-24 14:52:13 +1100743.Nm
744implements public key authentication protocol automatically,
745using either the RSA or DSA algorithms.
746Protocol 1 is restricted to using only RSA keys,
747but protocol 2 may use either.
748The
749.Sx HISTORY
750section of
751.Xr ssl 8
752contains a brief discussion of the two algorithms.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100753.Pp
754The file
755.Pa ~/.ssh/authorized_keys
756lists the public keys that are permitted for logging in.
757When the user logs in, the
758.Nm
759program tells the server which key pair it would like to use for
760authentication.
Damien Millerc93a8132005-12-24 14:52:13 +1100761The client proves that it has access to the private key
762and the server checks that the corresponding public key
763is authorized to accept the account.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100764.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100765The user creates his/her key pair by running
Darren Tuckerd3877b92005-12-20 16:09:36 +1100766.Xr ssh-keygen 1 .
767This stores the private key in
768.Pa ~/.ssh/identity
Damien Millerc93a8132005-12-24 14:52:13 +1100769(protocol 1),
770.Pa ~/.ssh/id_dsa
771(protocol 2 DSA),
772or
773.Pa ~/.ssh/id_rsa
774(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100775and stores the public key in
776.Pa ~/.ssh/identity.pub
Damien Millerc93a8132005-12-24 14:52:13 +1100777(protocol 1),
778.Pa ~/.ssh/id_dsa.pub
779(protocol 2 DSA),
780or
781.Pa ~/.ssh/id_rsa.pub
782(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100783in the user's home directory.
Damien Millerc93a8132005-12-24 14:52:13 +1100784The user should then copy the public key
Darren Tuckerd3877b92005-12-20 16:09:36 +1100785to
786.Pa ~/.ssh/authorized_keys
Damien Millerc93a8132005-12-24 14:52:13 +1100787in his/her home directory on the remote machine.
788The
Darren Tuckerd3877b92005-12-20 16:09:36 +1100789.Pa authorized_keys
790file corresponds to the conventional
791.Pa ~/.rhosts
792file, and has one key
Damien Millerc93a8132005-12-24 14:52:13 +1100793per line, though the lines can be very long.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100794After this, the user can log in without giving the password.
795.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100796The most convenient way to use public key authentication may be with an
Darren Tuckerd3877b92005-12-20 16:09:36 +1100797authentication agent.
798See
799.Xr ssh-agent 1
800for more information.
801.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100802Challenge-response authentication works as follows:
803The server sends an arbitrary
804.Qq challenge
805text, and prompts for a response.
806Protocol 2 allows multiple challenges and responses;
807protocol 1 is restricted to just one challenge/response.
808Examples of challenge-response authentication include
809BSD Authentication (see
810.Xr login.conf 5 )
811and PAM (some non-OpenBSD systems).
812.Pp
813Finally, if other authentication methods fail,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100814.Nm
815prompts the user for a password.
816The password is sent to the remote
817host for checking; however, since all communications are encrypted,
818the password cannot be seen by someone listening on the network.
Damien Miller48c94ab2006-01-02 23:38:00 +1100819.Pp
820.Nm
821automatically maintains and checks a database containing
822identification for all hosts it has ever been used with.
823Host keys are stored in
824.Pa ~/.ssh/known_hosts
825in the user's home directory.
826Additionally, the file
827.Pa /etc/ssh/ssh_known_hosts
828is automatically checked for known hosts.
829Any new hosts are automatically added to the user's file.
830If a host's identification ever changes,
831.Nm
832warns about this and disables password authentication to prevent
833server spoofing or man-in-the-middle attacks,
834which could otherwise be used to circumvent the encryption.
835The
836.Cm StrictHostKeyChecking
837option can be used to control logins to machines whose
838host key is not known or has changed.
839.Pp
Darren Tuckerd3877b92005-12-20 16:09:36 +1100840When the user's identity has been accepted by the server, the server
841either executes the given command, or logs into the machine and gives
842the user a normal shell on the remote machine.
843All communication with
844the remote command or shell will be automatically encrypted.
845.Pp
846If a pseudo-terminal has been allocated (normal login session), the
847user may use the escape characters noted below.
848.Pp
849If no pseudo-tty has been allocated,
850the session is transparent and can be used to reliably transfer binary data.
851On most systems, setting the escape character to
852.Dq none
853will also make the session transparent even if a tty is used.
854.Pp
855The session terminates when the command or shell on the remote
Damien Miller8bfaf932006-01-14 10:09:30 +1100856machine exits and all X11 and TCP connections have been closed.
Damien Miller52d20612005-12-24 14:52:36 +1100857.Sh ESCAPE CHARACTERS
Darren Tuckerd3877b92005-12-20 16:09:36 +1100858When a pseudo-terminal has been requested,
859.Nm
860supports a number of functions through the use of an escape character.
861.Pp
862A single tilde character can be sent as
863.Ic ~~
864or by following the tilde by a character other than those described below.
865The escape character must always follow a newline to be interpreted as
866special.
867The escape character can be changed in configuration files using the
868.Cm EscapeChar
869configuration directive or on the command line by the
870.Fl e
871option.
872.Pp
873The supported escapes (assuming the default
874.Ql ~ )
875are:
876.Bl -tag -width Ds
877.It Cm ~.
878Disconnect.
879.It Cm ~^Z
880Background
Damien Miller1164c292006-01-02 23:38:37 +1100881.Nm .
Darren Tuckerd3877b92005-12-20 16:09:36 +1100882.It Cm ~#
883List forwarded connections.
884.It Cm ~&
885Background
886.Nm
887at logout when waiting for forwarded connection / X11 sessions to terminate.
888.It Cm ~?
889Display a list of escape characters.
890.It Cm ~B
891Send a BREAK to the remote system
892(only useful for SSH protocol version 2 and if the peer supports it).
893.It Cm ~C
894Open command line.
895Currently this allows the addition of port forwardings using the
896.Fl L
897and
898.Fl R
Damien Miller329cb012005-12-24 14:53:23 +1100899options (see above).
Darren Tuckerd3877b92005-12-20 16:09:36 +1100900It also allows the cancellation of existing remote port-forwardings
901using
Damien Miller57e8ad32006-07-10 20:20:52 +1000902.Sm off
903.Fl KR Oo Ar bind_address : Oc Ar port .
904.Sm on
Darren Tuckerd3877b92005-12-20 16:09:36 +1100905.Ic !\& Ns Ar command
906allows the user to execute a local command if the
907.Ic PermitLocalCommand
908option is enabled in
909.Xr ssh_config 5 .
910Basic help is available, using the
911.Fl h
912option.
913.It Cm ~R
914Request rekeying of the connection
915(only useful for SSH protocol version 2 and if the peer supports it).
916.El
Damien Millerf3177182006-01-14 10:09:13 +1100917.Sh TCP FORWARDING
918Forwarding of arbitrary TCP connections over the secure channel can
919be specified either on the command line or in a configuration file.
920One possible application of TCP forwarding is a secure connection to a
921mail server; another is going through firewalls.
922.Pp
923In the example below, we look at encrypting communication between
924an IRC client and server, even though the IRC server does not directly
925support encrypted communications.
926This works as follows:
927the user connects to the remote host using
928.Nm ,
929specifying a port to be used to forward connections
930to the remote server.
931After that it is possible to start the service which is to be encrypted
932on the client machine,
933connecting to the same local port,
934and
935.Nm
936will encrypt and forward the connection.
937.Pp
938The following example tunnels an IRC session from client machine
939.Dq 127.0.0.1
940(localhost)
941to remote server
942.Dq server.example.com :
943.Bd -literal -offset 4n
944$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
945$ irc -c '#users' -p 1234 pinky 127.0.0.1
946.Ed
947.Pp
948This tunnels a connection to IRC server
949.Dq server.example.com ,
950joining channel
951.Dq #users ,
952nickname
953.Dq pinky ,
954using port 1234.
955It doesn't matter which port is used,
956as long as it's greater than 1023
957(remember, only root can open sockets on privileged ports)
958and doesn't conflict with any ports already in use.
959The connection is forwarded to port 6667 on the remote server,
960since that's the standard port for IRC services.
961.Pp
962The
963.Fl f
964option backgrounds
965.Nm
966and the remote command
967.Dq sleep 10
968is specified to allow an amount of time
969(10 seconds, in the example)
970to start the service which is to be tunnelled.
971If no connections are made within the time specified,
972.Nm
973will exit.
Damien Millerf3177182006-01-14 10:09:13 +1100974.Sh X11 FORWARDING
Darren Tuckerd3877b92005-12-20 16:09:36 +1100975If the
976.Cm ForwardX11
977variable is set to
978.Dq yes
979(or see the description of the
Damien Millerd7f308f2005-12-24 14:55:16 +1100980.Fl X ,
981.Fl x ,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100982and
Damien Millerd7f308f2005-12-24 14:55:16 +1100983.Fl Y
Damien Miller9a765b22005-12-24 14:53:44 +1100984options above)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100985and the user is using X11 (the
986.Ev DISPLAY
987environment variable is set), the connection to the X11 display is
988automatically forwarded to the remote side in such a way that any X11
989programs started from the shell (or command) will go through the
990encrypted channel, and the connection to the real X server will be made
991from the local machine.
992The user should not manually set
993.Ev DISPLAY .
994Forwarding of X11 connections can be
995configured on the command line or in configuration files.
996.Pp
997The
998.Ev DISPLAY
999value set by
1000.Nm
1001will point to the server machine, but with a display number greater than zero.
1002This is normal, and happens because
1003.Nm
1004creates a
1005.Dq proxy
1006X server on the server machine for forwarding the
1007connections over the encrypted channel.
1008.Pp
1009.Nm
1010will also automatically set up Xauthority data on the server machine.
1011For this purpose, it will generate a random authorization cookie,
1012store it in Xauthority on the server, and verify that any forwarded
1013connections carry this cookie and replace it by the real cookie when
1014the connection is opened.
1015The real authentication cookie is never
1016sent to the server machine (and no cookies are sent in the plain).
1017.Pp
1018If the
1019.Cm ForwardAgent
1020variable is set to
1021.Dq yes
1022(or see the description of the
1023.Fl A
1024and
1025.Fl a
Damien Miller9a765b22005-12-24 14:53:44 +11001026options above) and
Darren Tuckerd3877b92005-12-20 16:09:36 +11001027the user is using an authentication agent, the connection to the agent
1028is automatically forwarded to the remote side.
Damien Millerbbc59092006-01-31 21:46:51 +11001029.Sh VERIFYING HOST KEYS
1030When connecting to a server for the first time,
1031a fingerprint of the server's public key is presented to the user
1032(unless the option
1033.Cm StrictHostKeyChecking
1034has been disabled).
1035Fingerprints can be determined using
1036.Xr ssh-keygen 1 :
1037.Pp
1038.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1039.Pp
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001040If the fingerprint is already known, it can be matched
1041and the key can be accepted or rejected.
1042Because of the difficulty of comparing host keys
1043just by looking at hex strings,
1044there is also support to compare host keys visually,
1045using
1046.Em random art .
1047By setting the
Damien Miller10288242008-06-30 00:04:03 +10001048.Cm VisualHostKey
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001049option to
Damien Miller10288242008-06-30 00:04:03 +10001050.Dq yes ,
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001051a small ASCII graphic gets displayed on every login to a server, no matter
1052if the session itself is interactive or not.
1053By learning the pattern a known server produces, a user can easily
1054find out that the host key has changed when a completely different pattern
1055is displayed.
1056Because these patterns are not unambiguous however, a pattern that looks
1057similar to the pattern remembered only gives a good probability that the
1058host key is the same, not guaranteed proof.
1059.Pp
1060To get a listing of the fingerprints along with their random art for
1061all known hosts, the following command line can be used:
1062.Pp
1063.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1064.Pp
Damien Millerbbc59092006-01-31 21:46:51 +11001065If the fingerprint is unknown,
1066an alternative method of verification is available:
1067SSH fingerprints verified by DNS.
1068An additional resource record (RR),
1069SSHFP,
1070is added to a zonefile
1071and the connecting client is able to match the fingerprint
1072with that of the key presented.
1073.Pp
1074In this example, we are connecting a client to a server,
1075.Dq host.example.com .
1076The SSHFP resource records should first be added to the zonefile for
1077host.example.com:
1078.Bd -literal -offset indent
Damien Millera1b3d632006-03-26 00:07:02 +11001079$ ssh-keygen -r host.example.com.
Damien Millerbbc59092006-01-31 21:46:51 +11001080.Ed
1081.Pp
1082The output lines will have to be added to the zonefile.
1083To check that the zone is answering fingerprint queries:
1084.Pp
1085.Dl $ dig -t SSHFP host.example.com
1086.Pp
1087Finally the client connects:
1088.Bd -literal -offset indent
1089$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1090[...]
1091Matching host key fingerprint found in DNS.
1092Are you sure you want to continue connecting (yes/no)?
1093.Ed
1094.Pp
1095See the
1096.Cm VerifyHostKeyDNS
1097option in
1098.Xr ssh_config 5
1099for more information.
Darren Tucker248dd132006-01-20 11:30:58 +11001100.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1101.Nm
1102contains support for Virtual Private Network (VPN) tunnelling
1103using the
1104.Xr tun 4
1105network pseudo-device,
1106allowing two networks to be joined securely.
1107The
1108.Xr sshd_config 5
1109configuration option
1110.Cm PermitTunnel
1111controls whether the server supports this,
1112and at what level (layer 2 or 3 traffic).
1113.Pp
1114The following example would connect client network 10.0.50.0/24
Damien Miller3975ee22006-11-05 05:31:33 +11001115with remote network 10.0.99.0/24 using a point-to-point connection
1116from 10.1.1.1 to 10.1.1.2,
1117provided that the SSH server running on the gateway to the remote network,
1118at 192.168.1.15, allows it.
1119.Pp
1120On the client:
Darren Tucker248dd132006-01-20 11:30:58 +11001121.Bd -literal -offset indent
1122# ssh -f -w 0:1 192.168.1.15 true
Damien Miller3975ee22006-11-05 05:31:33 +11001123# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1124# route add 10.0.99.0/24 10.1.1.2
1125.Ed
1126.Pp
1127On the server:
1128.Bd -literal -offset indent
1129# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1130# route add 10.0.50.0/24 10.1.1.1
Darren Tucker248dd132006-01-20 11:30:58 +11001131.Ed
1132.Pp
1133Client access may be more finely tuned via the
1134.Pa /root/.ssh/authorized_keys
1135file (see below) and the
1136.Cm PermitRootLogin
1137server option.
Damien Millere93eaaa2006-03-15 11:05:59 +11001138The following entry would permit connections on
Darren Tucker248dd132006-01-20 11:30:58 +11001139.Xr tun 4
Damien Millere93eaaa2006-03-15 11:05:59 +11001140device 1 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001141.Dq jane
Damien Millere93eaaa2006-03-15 11:05:59 +11001142and on tun device 2 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001143.Dq john ,
1144if
1145.Cm PermitRootLogin
1146is set to
1147.Dq forced-commands-only :
1148.Bd -literal -offset 2n
1149tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
Damien Miller9f67a212006-03-15 11:05:35 +11001150tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Darren Tucker248dd132006-01-20 11:30:58 +11001151.Ed
1152.Pp
Darren Tuckerffe88e12006-10-18 07:53:06 +10001153Since an SSH-based setup entails a fair amount of overhead,
Darren Tucker248dd132006-01-20 11:30:58 +11001154it may be more suited to temporary setups,
1155such as for wireless VPNs.
1156More permanent VPNs are better provided by tools such as
1157.Xr ipsecctl 8
1158and
1159.Xr isakmpd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +10001160.Sh ENVIRONMENT
1161.Nm
1162will normally set the following environment variables:
Damien Miller6aa22902006-01-06 14:48:34 +11001163.Bl -tag -width "SSH_ORIGINAL_COMMAND"
Damien Miller32aa1441999-10-29 09:15:49 +10001164.It Ev DISPLAY
1165The
1166.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +11001167variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +10001168It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +10001169.Nm
1170to point to a value of the form
Damien Miller14af93e2006-01-02 23:38:21 +11001171.Dq hostname:n ,
1172where
1173.Dq hostname
1174indicates the host where the shell runs, and
1175.Sq n
1176is an integer \*(Ge 1.
Damien Miller7684ee12000-03-17 23:40:15 +11001177.Nm
1178uses this special value to forward X11 connections over the secure
1179channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001180The user should normally not set
1181.Ev DISPLAY
1182explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +10001183will render the X11 connection insecure (and will require the user to
1184manually copy any required authorization cookies).
1185.It Ev HOME
1186Set to the path of the user's home directory.
1187.It Ev LOGNAME
1188Synonym for
1189.Ev USER ;
1190set for compatibility with systems that use this variable.
1191.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +00001192Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +11001193.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +10001194Set to the default
1195.Ev PATH ,
1196as specified when compiling
Damien Miller1164c292006-01-02 23:38:37 +11001197.Nm .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001198.It Ev SSH_ASKPASS
1199If
1200.Nm
1201needs a passphrase, it will read the passphrase from the current
1202terminal if it was run from a terminal.
1203If
1204.Nm
1205does not have a terminal associated with it but
1206.Ev DISPLAY
1207and
1208.Ev SSH_ASKPASS
1209are set, it will execute the program specified by
1210.Ev SSH_ASKPASS
1211and open an X11 window to read the passphrase.
1212This is particularly useful when calling
1213.Nm
1214from a
Darren Tucker4e4fe002004-11-05 20:01:03 +11001215.Pa .xsession
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001216or related script.
1217(Note that on some machines it
1218may be necessary to redirect the input from
1219.Pa /dev/null
1220to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +10001221.It Ev SSH_AUTH_SOCK
Damien Miller14af93e2006-01-02 23:38:21 +11001222Identifies the path of a
1223.Ux Ns -domain
1224socket used to communicate with the agent.
Damien Millerf37e2462002-09-19 11:47:55 +10001225.It Ev SSH_CONNECTION
1226Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +11001227The variable contains
Damien Miller14af93e2006-01-02 23:38:21 +11001228four space-separated values: client IP address, client port number,
1229server IP address, and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001230.It Ev SSH_ORIGINAL_COMMAND
Damien Miller14af93e2006-01-02 23:38:21 +11001231This variable contains the original command line if a forced command
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001232is executed.
1233It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +10001234.It Ev SSH_TTY
1235This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +11001236with the current shell or command.
1237If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +10001238this variable is not set.
1239.It Ev TZ
Damien Miller6dbdb6a2005-12-13 19:25:43 +11001240This variable is set to indicate the present time zone if it
Damien Miller208f1ed2006-03-15 11:56:03 +11001241was set when the daemon was started (i.e. the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +10001242on to new connections).
1243.It Ev USER
1244Set to the name of the user logging in.
1245.El
1246.Pp
Damien Miller22c77262000-04-13 12:26:34 +10001247Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +10001248.Nm
Damien Miller22c77262000-04-13 12:26:34 +10001249reads
Damien Miller167ea5d2005-05-26 12:04:02 +10001250.Pa ~/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +10001251and adds lines of the format
1252.Dq VARNAME=value
Damien Miller14af93e2006-01-02 23:38:21 +11001253to the environment if the file exists and users are allowed to
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001254change their environment.
Darren Tucker61776952003-10-02 16:19:47 +10001255For more information, see the
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001256.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +00001257option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001258.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +10001259.Sh FILES
Damien Miller7655f5c2006-01-06 14:48:18 +11001260.Bl -tag -width Ds -compact
Damien Miller7655f5c2006-01-06 14:48:18 +11001261.It ~/.rhosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001262This file is used for host-based authentication (see above).
Ben Lindstromebd888d2001-03-05 05:49:29 +00001263On some machines this file may need to be
Damien Miller1bcdb502006-01-06 14:49:38 +11001264world-readable if the user's home directory is on an NFS partition,
Damien Miller32aa1441999-10-29 09:15:49 +10001265because
1266.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001267reads it as root.
1268Additionally, this file must be owned by the user,
1269and must not have write permissions for anyone else.
1270The recommended
Damien Miller32aa1441999-10-29 09:15:49 +10001271permission for most machines is read/write for the user, and not
1272accessible by others.
1273.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001274.It ~/.shosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001275This file is used in exactly the same way as
1276.Pa .rhosts ,
1277but allows host-based authentication without permitting login with
1278rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001279.Pp
Damien Miller520e6152008-02-10 22:46:22 +11001280.It ~/.ssh/
1281This directory is the default location for all user-specific configuration
1282and authentication information.
1283There is no general requirement to keep the entire contents of this directory
1284secret, but the recommended permissions are read/write/execute for the user,
1285and not accessible by others.
1286.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001287.It ~/.ssh/authorized_keys
1288Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1289The format of this file is described in the
1290.Xr sshd 8
1291manual page.
Damien Millerfb8ea742006-01-06 14:48:52 +11001292This file is not highly sensitive, but the recommended
1293permissions are read/write for the user, and not accessible by others.
1294.Pp
1295.It ~/.ssh/config
1296This is the per-user configuration file.
1297The file format and configuration options are described in
1298.Xr ssh_config 5 .
1299Because of the potential for abuse, this file must have strict permissions:
1300read/write for the user, and not accessible by others.
1301.Pp
1302.It ~/.ssh/environment
Damien Miller4c102ee2006-01-06 14:49:17 +11001303Contains additional definitions for environment variables; see
1304.Sx ENVIRONMENT ,
Damien Millerfb8ea742006-01-06 14:48:52 +11001305above.
1306.Pp
1307.It ~/.ssh/identity
1308.It ~/.ssh/id_dsa
1309.It ~/.ssh/id_rsa
1310Contains the private key for authentication.
1311These files
1312contain sensitive data and should be readable by the user but not
1313accessible by others (read/write/execute).
1314.Nm
1315will simply ignore a private key file if it is accessible by others.
1316It is possible to specify a passphrase when
1317generating the key which will be used to encrypt the
1318sensitive part of this file using 3DES.
1319.Pp
1320.It ~/.ssh/identity.pub
1321.It ~/.ssh/id_dsa.pub
1322.It ~/.ssh/id_rsa.pub
1323Contains the public key for authentication.
1324These files are not
1325sensitive and can (but need not) be readable by anyone.
Damien Millerfb8ea742006-01-06 14:48:52 +11001326.Pp
1327.It ~/.ssh/known_hosts
Damien Millere87eb4c2006-01-14 10:08:36 +11001328Contains a list of host keys for all hosts the user has logged into
1329that are not already in the systemwide list of known host keys.
Damien Millerfb8ea742006-01-06 14:48:52 +11001330See
Damien Millere87eb4c2006-01-14 10:08:36 +11001331.Xr sshd 8
1332for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001333.Pp
1334.It ~/.ssh/rc
1335Commands in this file are executed by
1336.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001337when the user logs in, just before the user's shell (or command) is
Damien Millerfb8ea742006-01-06 14:48:52 +11001338started.
1339See the
1340.Xr sshd 8
1341manual page for more information.
1342.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001343.It /etc/hosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001344This file is for host-based authentication (see above).
1345It should only be writable by root.
Damien Miller7655f5c2006-01-06 14:48:18 +11001346.Pp
1347.It /etc/shosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001348This file is used in exactly the same way as
1349.Pa hosts.equiv ,
1350but allows host-based authentication without permitting login with
1351rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001352.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001353.It Pa /etc/ssh/ssh_config
1354Systemwide configuration file.
1355The file format and configuration options are described in
1356.Xr ssh_config 5 .
1357.Pp
1358.It /etc/ssh/ssh_host_key
1359.It /etc/ssh/ssh_host_dsa_key
1360.It /etc/ssh/ssh_host_rsa_key
1361These three files contain the private parts of the host keys
Damien Miller7e76e1f2006-01-14 10:08:57 +11001362and are used for host-based authentication.
1363If protocol version 1 is used,
Damien Millerfb8ea742006-01-06 14:48:52 +11001364.Nm
1365must be setuid root, since the host key is readable only by root.
1366For protocol version 2,
1367.Nm
1368uses
1369.Xr ssh-keysign 8
Damien Miller7e76e1f2006-01-14 10:08:57 +11001370to access the host keys,
1371eliminating the requirement that
Damien Millerfb8ea742006-01-06 14:48:52 +11001372.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001373be setuid root when host-based authentication is used.
Damien Millerfb8ea742006-01-06 14:48:52 +11001374By default
1375.Nm
1376is not setuid root.
1377.Pp
1378.It /etc/ssh/ssh_known_hosts
1379Systemwide list of known host keys.
1380This file should be prepared by the
1381system administrator to contain the public host keys of all machines in the
1382organization.
Damien Millere87eb4c2006-01-14 10:08:36 +11001383It should be world-readable.
1384See
Damien Millerfb8ea742006-01-06 14:48:52 +11001385.Xr sshd 8
Damien Millere87eb4c2006-01-14 10:08:36 +11001386for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001387.Pp
Damien Miller7655f5c2006-01-06 14:48:18 +11001388.It /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +10001389Commands in this file are executed by
1390.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001391when the user logs in, just before the user's shell (or command) is started.
Damien Miller32aa1441999-10-29 09:15:49 +10001392See the
1393.Xr sshd 8
1394manual page for more information.
Damien Miller7b28dc52000-09-05 13:34:53 +11001395.El
Damien Miller32aa1441999-10-29 09:15:49 +10001396.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +10001397.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +11001398.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +10001399.Xr ssh-add 1 ,
1400.Xr ssh-agent 1 ,
1401.Xr ssh-keygen 1 ,
Damien Miller128a0f12006-01-06 14:50:11 +11001402.Xr ssh-keyscan 1 ,
Darren Tucker248dd132006-01-20 11:30:58 +11001403.Xr tun 4 ,
Darren Tucker61776952003-10-02 16:19:47 +10001404.Xr hosts.equiv 5 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +00001405.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +00001406.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +00001407.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +00001408.Rs
Damien Miller39a93a32006-03-15 11:34:45 +11001409.%R RFC 4250
1410.%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1411.%D 2006
1412.Re
1413.Rs
1414.%R RFC 4251
1415.%T "The Secure Shell (SSH) Protocol Architecture"
1416.%D 2006
1417.Re
1418.Rs
1419.%R RFC 4252
1420.%T "The Secure Shell (SSH) Authentication Protocol"
1421.%D 2006
1422.Re
1423.Rs
1424.%R RFC 4253
1425.%T "The Secure Shell (SSH) Transport Layer Protocol"
1426.%D 2006
1427.Re
1428.Rs
1429.%R RFC 4254
1430.%T "The Secure Shell (SSH) Connection Protocol"
1431.%D 2006
1432.Re
1433.Rs
1434.%R RFC 4255
1435.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1436.%D 2006
1437.Re
1438.Rs
1439.%R RFC 4256
1440.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1441.%D 2006
1442.Re
1443.Rs
1444.%R RFC 4335
1445.%T "The Secure Shell (SSH) Session Channel Break Extension"
1446.%D 2006
1447.Re
1448.Rs
1449.%R RFC 4344
1450.%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1451.%D 2006
1452.Re
1453.Rs
1454.%R RFC 4345
1455.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1456.%D 2006
Ben Lindstrom160ec622001-04-22 17:17:46 +00001457.Re
Damien Miller3e96d742006-03-25 23:39:29 +11001458.Rs
1459.%R RFC 4419
1460.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1461.%D 2006
1462.Re
Damien Millerc0367fb2007-01-05 16:25:46 +11001463.Rs
1464.%R RFC 4716
1465.%T "The Secure Shell (SSH) Public Key File Format"
1466.%D 2006
1467.Re
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001468.Rs
1469.%T "Hash Visualization: a New Technique to improve Real-World Security"
1470.%A A. Perrig
1471.%A D. Song
1472.%D 1999
1473.%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1474.Re
Damien Millerf1ce5052003-06-11 22:04:39 +10001475.Sh AUTHORS
1476OpenSSH is a derivative of the original and free
1477ssh 1.2.12 release by Tatu Ylonen.
1478Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1479Theo de Raadt and Dug Song
1480removed many bugs, re-added newer features and
1481created OpenSSH.
1482Markus Friedl contributed the support for SSH
1483protocol versions 1.5 and 2.0.