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Damien Miller32aa1441999-10-29 09:15:49 +10001.\"
Damien Miller32aa1441999-10-29 09:15:49 +10002.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller32aa1441999-10-29 09:15:49 +10003.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4.\" All rights reserved
5.\"
Damien Millere4340be2000-09-16 13:29:08 +11006.\" As far as I am concerned, the code I have written for this software
7.\" can be used freely for any purpose. Any derived versions of this
8.\" software must be clearly marked as such, and if the derived work is
9.\" incompatible with the protocol description in the RFC file, it must be
10.\" called by a name other than "ssh" or "Secure Shell".
Damien Miller32aa1441999-10-29 09:15:49 +100011.\"
Ben Lindstrom92a2e382001-03-05 06:59:27 +000012.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
Damien Millere4340be2000-09-16 13:29:08 +110015.\"
16.\" Redistribution and use in source and binary forms, with or without
17.\" modification, are permitted provided that the following conditions
18.\" are met:
19.\" 1. Redistributions of source code must retain the above copyright
20.\" notice, this list of conditions and the following disclaimer.
21.\" 2. Redistributions in binary form must reproduce the above copyright
22.\" notice, this list of conditions and the following disclaimer in the
23.\" documentation and/or other materials provided with the distribution.
24.\"
25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Damien Miller32aa1441999-10-29 09:15:49 +100035.\"
jcs@openbsd.orgf361df42015-11-15 22:26:49 +000036.\" $OpenBSD: ssh.1,v 1.366 2015/11/15 22:26:49 jcs Exp $
37.Dd $Mdocdate: November 15 2015 $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dt SSH 1
39.Os
40.Sh NAME
41.Nm ssh
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000042.Nd OpenSSH SSH client (remote login program)
Damien Miller32aa1441999-10-29 09:15:49 +100043.Sh SYNOPSIS
44.Nm ssh
Damien Millerbcfbc482010-07-16 13:59:11 +100045.Bk -words
jmc@openbsd.orgb1ba15f2014-10-09 06:21:31 +000046.Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
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
Damien Millerbcfbc482010-07-16 13:59:11 +100049.Op Fl D Oo Ar bind_address : Oc Ns Ar port
Damien Miller03d4d7e2013-04-23 15:21:06 +100050.Op Fl E Ar log_file
Damien Miller32aa1441999-10-29 09:15:49 +100051.Op Fl e Ar escape_char
Ben Lindstrom14f31ab2001-09-12 17:48:04 +000052.Op Fl F Ar configfile
Damien Miller048dc932010-02-12 09:22:04 +110053.Op Fl I Ar pkcs11
Darren Tucker61776952003-10-02 16:19:47 +100054.Op Fl i Ar identity_file
millert@openbsd.org79ec2142015-07-20 15:39:52 +000055.Op Fl L Ar address
Darren Tucker61776952003-10-02 16:19:47 +100056.Op Fl l Ar login_name
57.Op Fl m Ar mac_spec
Darren Tucker7ebfc102004-11-07 20:06:19 +110058.Op Fl O Ar ctl_cmd
Darren Tucker61776952003-10-02 16:19:47 +100059.Op Fl o Ar option
Darren Tucker61776952003-10-02 16:19:47 +100060.Op Fl p Ar port
djm@openbsd.orgc0f55db2015-09-11 03:42:32 +000061.Op Fl Q Ar query_option
millert@openbsd.org79ec2142015-07-20 15:39:52 +000062.Op Fl R Ar address
Darren Tucker7ebfc102004-11-07 20:06:19 +110063.Op Fl S Ar ctl_path
Darren Tucker7ad8dd22010-01-12 19:40:27 +110064.Op Fl W Ar host : Ns Ar port
Damien Millerbcfbc482010-07-16 13:59:11 +100065.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
Darren Tucker61776952003-10-02 16:19:47 +100066.Oo Ar user Ns @ Oc Ns Ar hostname
Damien Miller32aa1441999-10-29 09:15:49 +100067.Op Ar command
Damien Millerbcfbc482010-07-16 13:59:11 +100068.Ek
Damien Miller22c77262000-04-13 12:26:34 +100069.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100070.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000071(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110072executing commands on a remote machine.
mmcc@openbsd.org5e288922015-11-06 00:31:41 +000073It is intended to provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110074two untrusted hosts over an insecure network.
Damien Millera8a0f652014-07-30 12:32:28 +100075X11 connections, arbitrary TCP ports and
76.Ux Ns -domain
77sockets can also be forwarded over the secure channel.
Damien Miller32aa1441999-10-29 09:15:49 +100078.Pp
79.Nm
Damien Miller22c77262000-04-13 12:26:34 +100080connects and logs into the specified
Darren Tucker61776952003-10-02 16:19:47 +100081.Ar hostname
82(with optional
83.Ar user
84name).
Damien Miller32aa1441999-10-29 09:15:49 +100085The user must prove
Damien Millere247cc42000-05-07 12:03:14 +100086his/her identity to the remote machine using one of several methods
mmcc@openbsd.org5e288922015-11-06 00:31:41 +000087(see below).
Damien Millere247cc42000-05-07 12:03:14 +100088.Pp
Darren Tucker61776952003-10-02 16:19:47 +100089If
90.Ar command
91is specified,
Darren Tuckerb18c8672005-12-20 16:10:09 +110092it is executed on the remote host instead of a login shell.
Darren Tucker0b42e6d2004-08-13 21:22:40 +100093.Pp
Damien Miller0bc1bd82000-11-13 22:57:25 +110094The options are as follows:
millert@openbsd.org79ec2142015-07-20 15:39:52 +000095.Pp
millert@openbsd.orgebe27eb2015-07-20 18:44:12 +000096.Bl -tag -width Ds -compact
Darren Tucker61776952003-10-02 16:19:47 +100097.It Fl 1
98Forces
99.Nm
100to try protocol version 1 only.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000101.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000102.It Fl 2
103Forces
104.Nm
105to try protocol version 2 only.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000106.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000107.It Fl 4
108Forces
109.Nm
110to use IPv4 addresses only.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000111.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000112.It Fl 6
113Forces
114.Nm
115to use IPv6 addresses only.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000116.Pp
Damien Millerb1715dc2000-05-30 13:44:51 +1000117.It Fl A
118Enables forwarding of the authentication agent connection.
119This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000120.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000121Agent forwarding should be enabled with caution.
122Users with the ability to bypass file permissions on the remote host
Darren Tuckerae69e1d2009-10-24 11:41:34 +1100123(for the agent's
Darren Tucker98c9aec2009-10-24 11:42:44 +1100124.Ux Ns -domain
125socket) can access the local agent through the forwarded connection.
Damien Miller495dca32003-04-01 21:42:14 +1000126An attacker cannot obtain key material from the agent,
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000127however they can perform operations on the keys that enable them to
128authenticate using the identities loaded into the agent.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000129.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000130.It Fl a
131Disables forwarding of the authentication agent connection.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000132.Pp
Ben Lindstrome0f88042001-04-30 13:06:24 +0000133.It Fl b Ar bind_address
Darren Tuckera5cf8552005-07-14 17:04:18 +1000134Use
135.Ar bind_address
136on the local machine as the source address
137of the connection.
138Only useful on systems with more than one address.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000139.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000140.It Fl C
141Requests compression of all data (including stdin, stdout, stderr, and
Damien Millera8a0f652014-07-30 12:32:28 +1000142data for forwarded X11, TCP and
143.Ux Ns -domain
144connections).
Darren Tucker61776952003-10-02 16:19:47 +1000145The compression algorithm is the same used by
146.Xr gzip 1 ,
147and the
148.Dq level
149can be controlled by the
150.Cm CompressionLevel
151option for protocol version 1.
152Compression is desirable on modem lines and other
153slow connections, but will only slow down things on fast networks.
154The default value can be set on a host-by-host basis in the
155configuration files; see the
156.Cm Compression
157option.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000158.Pp
Damien Miller05202ff2004-06-15 10:30:39 +1000159.It Fl c Ar cipher_spec
160Selects the cipher specification for encrypting the session.
161.Pp
162Protocol version 1 allows specification of a single cipher.
Darren Tucker1e4308e2005-10-03 18:18:40 +1000163The supported values are
Damien Miller05202ff2004-06-15 10:30:39 +1000164.Dq 3des ,
Darren Tuckerff9f4842005-12-20 16:10:45 +1100165.Dq blowfish ,
Damien Miller05202ff2004-06-15 10:30:39 +1000166and
167.Dq des .
Damien Miller35978212005-12-24 14:56:47 +1100168For protocol version 2,
Damien Miller05202ff2004-06-15 10:30:39 +1000169.Ar cipher_spec
170is a comma-separated list of ciphers
171listed in order of preference.
Darren Tucker3a6a51f2009-06-21 17:48:52 +1000172See the
173.Cm Ciphers
Damien Miller081f3c72010-08-03 16:05:25 +1000174keyword in
175.Xr ssh_config 5
176for more information.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000177.Pp
Darren Tuckerc8d64212005-10-03 18:13:42 +1000178.It Fl D Xo
179.Sm off
180.Oo Ar bind_address : Oc
181.Ar port
182.Sm on
183.Xc
Darren Tucker61776952003-10-02 16:19:47 +1000184Specifies a local
185.Dq dynamic
186application-level port forwarding.
187This works by allocating a socket to listen to
188.Ar port
Darren Tuckerc8d64212005-10-03 18:13:42 +1000189on the local side, optionally bound to the specified
190.Ar bind_address .
191Whenever a connection is made to this port, the
Darren Tucker61776952003-10-02 16:19:47 +1000192connection is forwarded over the secure channel, and the application
193protocol is then used to determine where to connect to from the
194remote machine.
195Currently the SOCKS4 and SOCKS5 protocols are supported, and
196.Nm
197will act as a SOCKS server.
198Only root can forward privileged ports.
199Dynamic port forwardings can also be specified in the configuration file.
Darren Tuckerc8d64212005-10-03 18:13:42 +1000200.Pp
Damien Miller7fa96602010-08-05 13:03:13 +1000201IPv6 addresses can be specified by enclosing the address in square brackets.
Darren Tuckerc8d64212005-10-03 18:13:42 +1000202Only the superuser can forward privileged ports.
203By default, the local port is bound in accordance with the
204.Cm GatewayPorts
205setting.
206However, an explicit
207.Ar bind_address
208may be used to bind the connection to a specific address.
209The
210.Ar bind_address
211of
212.Dq localhost
213indicates that the listening port be bound for local use only, while an
214empty address or
215.Sq *
216indicates that the port should be available from all interfaces.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000217.Pp
Damien Miller03d4d7e2013-04-23 15:21:06 +1000218.It Fl E Ar log_file
219Append debug logs to
220.Ar log_file
221instead of standard error.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000222.Pp
Damien Miller2142ba02005-12-24 14:56:29 +1100223.It Fl e Ar escape_char
Damien Miller32aa1441999-10-29 09:15:49 +1000224Sets the escape character for sessions with a pty (default:
225.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100226The escape character is only recognized at the beginning of a line.
227The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000228.Pq Ql \&.
Darren Tucker61776952003-10-02 16:19:47 +1000229closes the connection;
230followed by control-Z suspends the connection;
231and followed by itself sends the escape character once.
Damien Miller7684ee12000-03-17 23:40:15 +1100232Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000233.Dq none
234disables any escapes and makes the session fully transparent.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000235.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000236.It Fl F Ar configfile
237Specifies an alternative per-user configuration file.
238If a configuration file is given on the command line,
239the system-wide configuration file
240.Pq Pa /etc/ssh/ssh_config
241will be ignored.
242The default for the per-user configuration file is
Damien Miller167ea5d2005-05-26 12:04:02 +1000243.Pa ~/.ssh/config .
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000244.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000245.It Fl f
246Requests
247.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100248to go to background just before command execution.
249This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000250.Nm
251is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100252wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000253This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000254.Fl n .
255The recommended way to start X11 programs at a remote site is with
256something like
257.Ic ssh -f host xterm .
Darren Tucker9a2a6092008-07-04 12:53:50 +1000258.Pp
259If the
260.Cm ExitOnForwardFailure
261configuration option is set to
262.Dq yes ,
263then a client started with
264.Fl f
265will wait for all remote port forwards to be successfully established
266before placing itself in the background.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000267.Pp
djm@openbsd.org957fbce2014-10-08 22:20:25 +0000268.It Fl G
269Causes
270.Nm
271to print its configuration after evaluating
272.Cm Host
273and
274.Cm Match
275blocks and exit.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000276.Pp
Damien Miller396691a2000-01-20 22:44:08 +1100277.It Fl g
278Allows remote hosts to connect to local forwarded ports.
Damien Miller49d9bfe2014-07-03 21:26:42 +1000279If used on a multiplexed connection, then this option must be specified
280on the master process.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000281.Pp
Damien Miller7ea845e2010-02-12 09:21:02 +1100282.It Fl I Ar pkcs11
Damien Millerd400da52010-02-12 09:26:23 +1100283Specify the PKCS#11 shared library
Darren Tucker61776952003-10-02 16:19:47 +1000284.Nm
Damien Millera7618442010-02-12 09:26:02 +1100285should use to communicate with a PKCS#11 token providing the user's
Darren Tucker61776952003-10-02 16:19:47 +1000286private RSA key.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000287.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000288.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100289Selects a file from which the identity (private key) for
Damien Millereb8b60e2010-08-31 22:41:14 +1000290public key authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100291The default is
Damien Miller167ea5d2005-05-26 12:04:02 +1000292.Pa ~/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100293for protocol version 1, and
Damien Millereb8b60e2010-08-31 22:41:14 +1000294.Pa ~/.ssh/id_dsa ,
Damien Miller8ba0ead2013-12-18 17:46:27 +1100295.Pa ~/.ssh/id_ecdsa ,
296.Pa ~/.ssh/id_ed25519
Damien Millerfae23862002-01-22 23:32:26 +1100297and
Damien Millereb8b60e2010-08-31 22:41:14 +1000298.Pa ~/.ssh/id_rsa
Damien Millerfae23862002-01-22 23:32:26 +1100299for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100300Identity files may also be specified on
301a per-host basis in the configuration file.
302It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000303.Fl i
304options (and multiple identities specified in
305configuration files).
jmc@openbsd.orgc5f7c082015-09-25 18:19:54 +0000306If no certificates have been explicitly specified by the
djm@openbsd.org4e44a792015-09-24 06:15:11 +0000307.Cm CertificateFile
308directive,
Damien Miller5059d8d2010-03-05 21:31:11 +1100309.Nm
310will also try to load certificate information from the filename obtained
311by appending
312.Pa -cert.pub
313to identity filenames.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000314.Pp
Darren Tucker415bddc2007-06-12 23:43:16 +1000315.It Fl K
316Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
317credentials to the server.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000318.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000319.It Fl k
Damien Millere0113cc2003-11-24 13:10:09 +1100320Disables forwarding (delegation) of GSSAPI credentials to the server.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000321.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000322.It Fl L Xo
323.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100324.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000325.Ar port : host : hostport
326.Sm on
327.Xc
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000328.It Fl L Xo
329.Sm off
330.Oo Ar bind_address : Oc
331.Ar port : remote_socket
332.Sm on
333.Xc
334.It Fl L Xo
335.Sm off
336.Ar local_socket : host : hostport
337.Sm on
338.Xc
339.It Fl L Xo
340.Sm off
341.Ar local_socket : remote_socket
342.Sm on
343.Xc
344Specifies that connections to the given TCP port or Unix socket on the local
345(client) host are to be forwarded to the given host and port, or Unix socket,
346on the remote side.
347This works by allocating a socket to listen to either a TCP
Darren Tucker61776952003-10-02 16:19:47 +1000348.Ar port
Damien Millerf91ee4c2005-03-01 21:24:33 +1100349on the local side, optionally bound to the specified
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000350.Ar bind_address ,
351or to a Unix socket.
352Whenever a connection is made to the local port or socket, the
Darren Tucker61776952003-10-02 16:19:47 +1000353connection is forwarded over the secure channel, and a connection is
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000354made to either
Darren Tucker61776952003-10-02 16:19:47 +1000355.Ar host
356port
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000357.Ar hostport ,
358or the Unix socket
359.Ar remote_socket ,
Darren Tucker61776952003-10-02 16:19:47 +1000360from the remote machine.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000361.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000362Port forwardings can also be specified in the configuration file.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100363Only the superuser can forward privileged ports.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000364IPv6 addresses can be specified by enclosing the address in square brackets.
365.Pp
Damien Millerf91ee4c2005-03-01 21:24:33 +1100366By default, the local port is bound in accordance with the
367.Cm GatewayPorts
368setting.
369However, an explicit
370.Ar bind_address
371may be used to bind the connection to a specific address.
372The
373.Ar bind_address
374of
375.Dq localhost
Damien Miller36bf7dd2005-03-02 12:02:47 +1100376indicates that the listening port be bound for local use only, while an
377empty address or
378.Sq *
Damien Millerf91ee4c2005-03-01 21:24:33 +1100379indicates that the port should be available from all interfaces.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000380.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000381.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100382Specifies the user to log in as on the remote machine.
383This also may be specified on a per-host basis in the configuration file.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000384.Pp
Damien Miller0e220db2004-06-15 10:34:08 +1000385.It Fl M
386Places the
387.Nm
388client into
389.Dq master
390mode for connection sharing.
Damien Miller88b25522005-12-31 16:23:15 +1100391Multiple
392.Fl M
393options places
394.Nm
395into
396.Dq master
397mode with confirmation required before slave connections are accepted.
Damien Miller0e220db2004-06-15 10:34:08 +1000398Refer to the description of
399.Cm ControlMaster
400in
401.Xr ssh_config 5
402for details.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000403.Pp
Damien Miller5d1eceb2004-06-30 22:37:57 +1000404.It Fl m Ar mac_spec
405Additionally, for protocol version 2 a comma-separated list of MAC
406(message authentication code) algorithms can
407be specified in order of preference.
408See the
409.Cm MACs
410keyword for more information.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000411.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000412.It Fl N
413Do not execute a remote command.
414This is useful for just forwarding ports
415(protocol version 2 only).
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000416.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000417.It Fl n
418Redirects stdin from
419.Pa /dev/null
420(actually, prevents reading from stdin).
421This must be used when
422.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100423is run in the background.
424A common trick is to use this to run X11 programs on a remote machine.
425For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000426.Ic ssh -n shadows.cs.hut.fi emacs &
427will start an emacs on shadows.cs.hut.fi, and the X11
428connection will be automatically forwarded over an encrypted channel.
429The
430.Nm
431program will be put in the background.
432(This does not work if
433.Nm
434needs to ask for a password or passphrase; see also the
435.Fl f
436option.)
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000437.Pp
Darren Tuckere04644c2004-12-03 14:08:45 +1100438.It Fl O Ar ctl_cmd
439Control an active connection multiplexing master process.
440When the
441.Fl O
442option is specified, the
443.Ar ctl_cmd
444argument is interpreted and passed to the master process.
445Valid commands are:
446.Dq check
Damien Miller388f6fc2010-05-21 14:57:35 +1000447(check that the master process is running),
448.Dq forward
Damien Miller8cb1cda2011-05-05 14:16:56 +1000449(request forwardings without command execution),
Damien Miller1bcbd0a2011-09-22 21:40:45 +1000450.Dq cancel
451(cancel forwardings),
Darren Tuckere04644c2004-12-03 14:08:45 +1100452.Dq exit
Damien Miller8cb1cda2011-05-05 14:16:56 +1000453(request the master to exit), and
Damien Miller6c3eec72011-05-05 14:16:22 +1000454.Dq stop
455(request the master to stop accepting further multiplexing requests).
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000456.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000457.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000458Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000459This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100460command-line flag.
Darren Tucker61776952003-10-02 16:19:47 +1000461For full details of the options listed below, and their possible values, see
462.Xr ssh_config 5 .
463.Pp
464.Bl -tag -width Ds -offset indent -compact
jcs@openbsd.orgf361df42015-11-15 22:26:49 +0000465.It AddKeysToAgent
Darren Tucker61776952003-10-02 16:19:47 +1000466.It AddressFamily
467.It BatchMode
468.It BindAddress
Damien Miller0faf7472013-10-17 11:47:23 +1100469.It CanonicalDomains
Damien Miller38505592013-10-17 11:48:13 +1100470.It CanonicalizeFallbackLocal
471.It CanonicalizeHostname
472.It CanonicalizeMaxDots
473.It CanonicalizePermittedCNAMEs
djm@openbsd.org4e44a792015-09-24 06:15:11 +0000474.It CertificateFile
Darren Tucker61776952003-10-02 16:19:47 +1000475.It ChallengeResponseAuthentication
476.It CheckHostIP
477.It Cipher
478.It Ciphers
479.It ClearAllForwardings
480.It Compression
481.It CompressionLevel
482.It ConnectionAttempts
Darren Tuckeredae0ec2004-05-02 22:15:52 +1000483.It ConnectTimeout
Damien Miller0e220db2004-06-15 10:34:08 +1000484.It ControlMaster
485.It ControlPath
Damien Millerefad7272011-09-22 21:33:53 +1000486.It ControlPersist
Darren Tucker61776952003-10-02 16:19:47 +1000487.It DynamicForward
Darren Tucker61776952003-10-02 16:19:47 +1000488.It EscapeChar
Darren Tuckere7d4b192006-07-12 22:17:10 +1000489.It ExitOnForwardFailure
jmc@openbsd.org8abd8032014-12-22 09:26:31 +0000490.It FingerprintHash
Darren Tucker61776952003-10-02 16:19:47 +1000491.It ForwardAgent
492.It ForwardX11
Damien Millerefad7272011-09-22 21:33:53 +1000493.It ForwardX11Timeout
Darren Tucker0a118da2003-10-15 15:54:32 +1000494.It ForwardX11Trusted
Darren Tucker61776952003-10-02 16:19:47 +1000495.It GatewayPorts
496.It GlobalKnownHostsFile
497.It GSSAPIAuthentication
498.It GSSAPIDelegateCredentials
Damien Millere1776152005-03-01 21:47:37 +1100499.It HashKnownHosts
Darren Tucker61776952003-10-02 16:19:47 +1000500.It Host
501.It HostbasedAuthentication
djm@openbsd.org46347ed2015-01-30 11:43:14 +0000502.It HostbasedKeyTypes
Darren Tucker61776952003-10-02 16:19:47 +1000503.It HostKeyAlgorithms
504.It HostKeyAlias
505.It HostName
506.It IdentityFile
Damien Millerbd394c32004-03-08 23:12:36 +1100507.It IdentitiesOnly
Damien Miller0a184732010-11-20 15:21:03 +1100508.It IPQoS
Damien Millerefad7272011-09-22 21:33:53 +1000509.It KbdInteractiveAuthentication
Darren Tucker636ca902004-11-05 20:22:00 +1100510.It KbdInteractiveDevices
Damien Miller7fe2b1f2010-09-24 22:11:53 +1000511.It KexAlgorithms
Damien Millerd27b9472005-12-13 19:29:02 +1100512.It LocalCommand
Darren Tucker61776952003-10-02 16:19:47 +1000513.It LocalForward
514.It LogLevel
515.It MACs
Damien Millerd77b81f2013-10-17 11:39:00 +1100516.It Match
Darren Tucker61776952003-10-02 16:19:47 +1000517.It NoHostAuthenticationForLocalhost
518.It NumberOfPasswordPrompts
519.It PasswordAuthentication
Damien Millerd27b9472005-12-13 19:29:02 +1100520.It PermitLocalCommand
Damien Miller7ea845e2010-02-12 09:21:02 +1100521.It PKCS11Provider
Darren Tucker61776952003-10-02 16:19:47 +1000522.It Port
523.It PreferredAuthentications
524.It Protocol
525.It ProxyCommand
Damien Millerf2f6c312013-08-21 02:44:58 +1000526.It ProxyUseFdpass
markus@openbsd.org3a1638d2015-07-10 06:21:53 +0000527.It PubkeyAcceptedKeyTypes
Darren Tucker61776952003-10-02 16:19:47 +1000528.It PubkeyAuthentication
Darren Tucker62388b22006-01-20 11:31:47 +1100529.It RekeyLimit
Darren Tucker61776952003-10-02 16:19:47 +1000530.It RemoteForward
Damien Millerf4b32aa2011-05-15 08:47:43 +1000531.It RequestTTY
Darren Tucker61776952003-10-02 16:19:47 +1000532.It RhostsRSAAuthentication
533.It RSAAuthentication
Darren Tucker7a6c0662004-05-02 22:14:03 +1000534.It SendEnv
Damien Miller509b0102003-12-17 16:33:10 +1100535.It ServerAliveInterval
536.It ServerAliveCountMax
Damien Miller6d576562014-07-18 15:02:06 +1000537.It StreamLocalBindMask
538.It StreamLocalBindUnlink
Darren Tucker61776952003-10-02 16:19:47 +1000539.It StrictHostKeyChecking
Damien Miller12c150e2003-12-17 16:31:10 +1100540.It TCPKeepAlive
Damien Millerd27b9472005-12-13 19:29:02 +1100541.It Tunnel
542.It TunnelDevice
djm@openbsd.org1d1092b2015-01-26 12:16:36 +0000543.It UpdateHostKeys
Darren Tucker61776952003-10-02 16:19:47 +1000544.It UsePrivilegedPort
545.It User
546.It UserKnownHostsFile
547.It VerifyHostKeyDNS
Damien Miller86399202008-06-30 00:04:31 +1000548.It VisualHostKey
Darren Tucker61776952003-10-02 16:19:47 +1000549.It XAuthLocation
550.El
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000551.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000552.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100553Port to connect to on the remote host.
554This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000555per-host basis in the configuration file.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000556.Pp
djm@openbsd.orgc0f55db2015-09-11 03:42:32 +0000557.It Fl Q Ar query_option
Damien Millerea111192013-04-23 19:24:32 +1000558Queries
559.Nm
Damien Millerd937dc02013-12-05 10:19:54 +1100560for the algorithms supported for the specified version 2.
561The available features are:
562.Ar cipher
Damien Millerea111192013-04-23 19:24:32 +1000563(supported symmetric ciphers),
Damien Millerd937dc02013-12-05 10:19:54 +1100564.Ar cipher-auth
Damien Miller0fde8ac2013-11-21 14:12:23 +1100565(supported symmetric ciphers that support authenticated encryption),
Damien Millerd937dc02013-12-05 10:19:54 +1100566.Ar mac
Damien Millerea111192013-04-23 19:24:32 +1000567(supported message integrity codes),
Damien Millerd937dc02013-12-05 10:19:54 +1100568.Ar kex
Damien Millerea111192013-04-23 19:24:32 +1000569(key exchange algorithms),
Damien Millerd937dc02013-12-05 10:19:54 +1100570.Ar key
djm@openbsd.orgc0f55db2015-09-11 03:42:32 +0000571(key types),
572.Ar key-cert
573(certificate key types),
574.Ar key-plain
575(non-certificate key types), and
djm@openbsd.org68d2dfc2015-03-03 06:48:58 +0000576.Ar protocol-version
577(supported SSH protocol versions).
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000578.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000579.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100580Quiet mode.
Damien Millereb602472008-02-10 22:21:28 +1100581Causes most warning and diagnostic messages to be suppressed.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000582.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000583.It Fl R Xo
584.Sm off
Damien Millerf91ee4c2005-03-01 21:24:33 +1100585.Oo Ar bind_address : Oc
Darren Tucker61776952003-10-02 16:19:47 +1000586.Ar port : host : hostport
587.Sm on
588.Xc
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000589.It Fl R Xo
590.Sm off
591.Oo Ar bind_address : Oc
592.Ar port : local_socket
593.Sm on
594.Xc
595.It Fl R Xo
596.Sm off
597.Ar remote_socket : host : hostport
598.Sm on
599.Xc
600.It Fl R Xo
601.Sm off
602.Ar remote_socket : local_socket
603.Sm on
604.Xc
605Specifies that connections to the given TCP port or Unix socket on the remote
606(server) host are to be forwarded to the given host and port, or Unix socket,
607on the local side.
608This works by allocating a socket to listen to either a TCP
Damien Miller32aa1441999-10-29 09:15:49 +1000609.Ar port
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000610or to a Unix socket on the remote side.
611Whenever a connection is made to this port or Unix socket, the
612connection is forwarded over the secure channel, and a connection
613is made to either
Damien Miller34132e52000-01-14 15:45:46 +1100614.Ar host
615port
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000616.Ar hostport ,
617or
618.Ar local_socket ,
Damien Miller7684ee12000-03-17 23:40:15 +1100619from the local machine.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100620.Pp
Damien Miller7684ee12000-03-17 23:40:15 +1100621Port forwardings can also be specified in the configuration file.
622Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000623logging in as root on the remote machine.
Damien Miller70b2d552012-04-22 11:26:10 +1000624IPv6 addresses can be specified by enclosing the address in square brackets.
Damien Millerf91ee4c2005-03-01 21:24:33 +1100625.Pp
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000626By default, TCP listening sockets on the server will be bound to the loopback
Damien Millerf91ee4c2005-03-01 21:24:33 +1100627interface only.
Darren Tucker63917bd2008-11-11 16:33:48 +1100628This may be overridden by specifying a
Damien Millerf91ee4c2005-03-01 21:24:33 +1100629.Ar bind_address .
Damien Miller36bf7dd2005-03-02 12:02:47 +1100630An empty
631.Ar bind_address ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100632or the address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100633.Ql * ,
Damien Millerf91ee4c2005-03-01 21:24:33 +1100634indicates that the remote socket should listen on all interfaces.
635Specifying a remote
636.Ar bind_address
Damien Miller36bf7dd2005-03-02 12:02:47 +1100637will only succeed if the server's
638.Cm GatewayPorts
Damien Millerf91ee4c2005-03-01 21:24:33 +1100639option is enabled (see
Damien Miller36bf7dd2005-03-02 12:02:47 +1100640.Xr sshd_config 5 ) .
Damien Millere2f4cc52009-02-14 16:33:49 +1100641.Pp
642If the
643.Ar port
644argument is
Damien Miller65fa4ca2009-02-14 16:34:05 +1100645.Ql 0 ,
Damien Millere2f4cc52009-02-14 16:33:49 +1100646the listen port will be dynamically allocated on the server and reported
647to the client at run time.
Damien Miller388f6fc2010-05-21 14:57:35 +1000648When used together with
649.Ic -O forward
650the allocated port will be printed to the standard output.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000651.Pp
Darren Tucker7ebfc102004-11-07 20:06:19 +1100652.It Fl S Ar ctl_path
Damien Miller67f30d72010-04-16 15:52:03 +1000653Specifies the location of a control socket for connection sharing,
Damien Miller9c60f242010-03-26 11:28:35 +1100654or the string
655.Dq none
656to disable connection sharing.
Damien Miller0e220db2004-06-15 10:34:08 +1000657Refer to the description of
Darren Tuckerba5c5922004-06-18 16:22:39 +1000658.Cm ControlPath
659and
Damien Miller0e220db2004-06-15 10:34:08 +1000660.Cm ControlMaster
661in
662.Xr ssh_config 5
663for details.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000664.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000665.It Fl s
666May be used to request invocation of a subsystem on the remote system.
667Subsystems are a feature of the SSH2 protocol which facilitate the use
668of SSH as a secure transport for other applications (eg.\&
669.Xr sftp 1 ) .
670The subsystem is specified as the remote command.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000671.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000672.It Fl T
dtucker@openbsd.orgf8484da2015-05-06 05:45:17 +0000673Disable pseudo-terminal allocation.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000674.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000675.It Fl t
dtucker@openbsd.orgf8484da2015-05-06 05:45:17 +0000676Force pseudo-terminal allocation.
Darren Tucker61776952003-10-02 16:19:47 +1000677This can be used to execute arbitrary
678screen-based programs on a remote machine, which can be very useful,
Damien Miller208f1ed2006-03-15 11:56:03 +1100679e.g. when implementing menu services.
Darren Tucker61776952003-10-02 16:19:47 +1000680Multiple
681.Fl t
682options force tty allocation, even if
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000683.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000684has no local tty.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000685.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000686.It Fl V
687Display the version number and exit.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000688.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000689.It Fl v
690Verbose mode.
691Causes
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000692.Nm
Darren Tucker61776952003-10-02 16:19:47 +1000693to print debugging messages about its progress.
694This is helpful in
695debugging connection, authentication, and configuration problems.
696Multiple
697.Fl v
698options increase the verbosity.
699The maximum is 3.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000700.Pp
Darren Tucker7ad8dd22010-01-12 19:40:27 +1100701.It Fl W Ar host : Ns Ar port
702Requests that standard input and output on the client be forwarded to
703.Ar host
704on
705.Ar port
706over the secure channel.
707Implies
708.Fl N ,
709.Fl T ,
710.Cm ExitOnForwardFailure
711and
Damien Miller36378c62012-06-20 21:53:25 +1000712.Cm ClearAllForwardings .
713Works with Protocol version 2 only.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000714.Pp
Damien Miller991dba42006-07-10 20:16:27 +1000715.It Fl w Xo
716.Ar local_tun Ns Op : Ns Ar remote_tun
717.Xc
718Requests
719tunnel
720device forwarding with the specified
Damien Millerd27b9472005-12-13 19:29:02 +1100721.Xr tun 4
Damien Miller991dba42006-07-10 20:16:27 +1000722devices between the client
723.Pq Ar local_tun
724and the server
725.Pq Ar remote_tun .
726.Pp
Damien Millercf1e3422005-12-24 14:56:04 +1100727The devices may be specified by numerical ID or the keyword
728.Dq any ,
729which uses the next available tunnel device.
Damien Miller991dba42006-07-10 20:16:27 +1000730If
731.Ar remote_tun
732is not specified, it defaults to
733.Dq any .
Damien Millercf1e3422005-12-24 14:56:04 +1100734See also the
Damien Millerd27b9472005-12-13 19:29:02 +1100735.Cm Tunnel
Damien Miller991dba42006-07-10 20:16:27 +1000736and
737.Cm TunnelDevice
738directives in
Damien Millerd27b9472005-12-13 19:29:02 +1100739.Xr ssh_config 5 .
Damien Miller991dba42006-07-10 20:16:27 +1000740If the
741.Cm Tunnel
742directive is unset, it is set to the default tunnel mode, which is
743.Dq point-to-point .
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000744.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000745.It Fl X
746Enables X11 forwarding.
747This can also be specified on a per-host basis in a configuration file.
748.Pp
749X11 forwarding should be enabled with caution.
750Users with the ability to bypass file permissions on the remote host
751(for the user's X authorization database)
752can access the local X11 display through the forwarded connection.
753An attacker may then be able to perform activities such as keystroke monitoring.
Damien Miller947219e2005-03-02 13:22:30 +1100754.Pp
Damien Millerb022b232005-03-05 11:22:36 +1100755For this reason, X11 forwarding is subjected to X11 SECURITY extension
Damien Miller947219e2005-03-02 13:22:30 +1100756restrictions by default.
757Please refer to the
758.Nm
759.Fl Y
760option and the
761.Cm ForwardX11Trusted
762directive in
763.Xr ssh_config 5
764for more information.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000765.Pp
Darren Tucker61776952003-10-02 16:19:47 +1000766.It Fl x
767Disables X11 forwarding.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000768.Pp
Darren Tucker0a118da2003-10-15 15:54:32 +1000769.It Fl Y
770Enables trusted X11 forwarding.
Damien Miller947219e2005-03-02 13:22:30 +1100771Trusted X11 forwardings are not subjected to the X11 SECURITY extension
772controls.
millert@openbsd.org79ec2142015-07-20 15:39:52 +0000773.Pp
Damien Millere272a5b2008-11-03 19:22:37 +1100774.It Fl y
775Send log information using the
776.Xr syslog 3
777system module.
778By default this information is sent to stderr.
Damien Miller32aa1441999-10-29 09:15:49 +1000779.El
Damien Millere9b333a2005-12-24 14:53:04 +1100780.Pp
781.Nm
782may additionally obtain configuration data from
783a per-user configuration file and a system-wide configuration file.
784The file format and configuration options are described in
785.Xr ssh_config 5 .
Damien Millerc93a8132005-12-24 14:52:13 +1100786.Sh AUTHENTICATION
Darren Tucker94299ec2006-01-20 11:30:14 +1100787The OpenSSH SSH client supports SSH protocols 1 and 2.
Darren Tucker7a4a7652009-10-11 21:51:40 +1100788The default is to use protocol 2 only,
789though this can be changed via the
Damien Millerc93a8132005-12-24 14:52:13 +1100790.Cm Protocol
791option in
Darren Tucker7a4a7652009-10-11 21:51:40 +1100792.Xr ssh_config 5
793or the
Damien Millerc93a8132005-12-24 14:52:13 +1100794.Fl 1
795and
796.Fl 2
797options (see above).
798Both protocols support similar authentication methods,
Darren Tucker7a4a7652009-10-11 21:51:40 +1100799but protocol 2 is the default since
Damien Millerc93a8132005-12-24 14:52:13 +1100800it provides additional mechanisms for confidentiality
801(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
Damien Miller20bd4532011-08-06 06:17:30 +1000802and integrity (hmac-md5, hmac-sha1,
803hmac-sha2-256, hmac-sha2-512,
Darren Tucker427e4092012-10-05 11:02:39 +1000804umac-64, umac-128, hmac-ripemd160).
Damien Millerc93a8132005-12-24 14:52:13 +1100805Protocol 1 lacks a strong mechanism for ensuring the
806integrity of the connection.
807.Pp
808The methods available for authentication are:
Damien Millerfbc94c82006-06-13 13:03:16 +1000809GSSAPI-based authentication,
Damien Millerc93a8132005-12-24 14:52:13 +1100810host-based authentication,
811public key authentication,
812challenge-response authentication,
813and password authentication.
814Authentication methods are tried in the order specified above,
815though protocol 2 has a configuration option to change the default order:
816.Cm PreferredAuthentications .
817.Pp
818Host-based authentication works as follows:
Darren Tuckerd3877b92005-12-20 16:09:36 +1100819If the machine the user logs in from is listed in
820.Pa /etc/hosts.equiv
821or
822.Pa /etc/shosts.equiv
823on the remote machine, and the user names are
824the same on both sides, or if the files
825.Pa ~/.rhosts
826or
827.Pa ~/.shosts
828exist in the user's home directory on the
829remote machine and contain a line containing the name of the client
830machine and the name of the user on that machine, the user is
Damien Millerc93a8132005-12-24 14:52:13 +1100831considered for login.
832Additionally, the server
833.Em must
834be able to verify the client's
835host key (see the description of
Darren Tuckerd3877b92005-12-20 16:09:36 +1100836.Pa /etc/ssh/ssh_known_hosts
837and
Damien Millerc93a8132005-12-24 14:52:13 +1100838.Pa ~/.ssh/known_hosts ,
839below)
840for login to be permitted.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100841This authentication method closes security holes due to IP
Damien Millerc93a8132005-12-24 14:52:13 +1100842spoofing, DNS spoofing, and routing spoofing.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100843[Note to the administrator:
844.Pa /etc/hosts.equiv ,
845.Pa ~/.rhosts ,
846and the rlogin/rsh protocol in general, are inherently insecure and should be
847disabled if security is desired.]
848.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100849Public key authentication works as follows:
850The scheme is based on public-key cryptography,
851using cryptosystems
852where encryption and decryption are done using separate keys,
853and it is unfeasible to derive the decryption key from the encryption key.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100854The idea is that each user creates a public/private
855key pair for authentication purposes.
856The server knows the public key, and only the user knows the private key.
Damien Millerc93a8132005-12-24 14:52:13 +1100857.Nm
858implements public key authentication protocol automatically,
sobrado@openbsd.orgf70b22b2014-08-30 15:33:50 +0000859using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
Damien Millerc93a8132005-12-24 14:52:13 +1100860Protocol 1 is restricted to using only RSA keys,
Damien Millereb8b60e2010-08-31 22:41:14 +1000861but protocol 2 may use any.
Damien Millerfecfd112013-07-18 16:11:50 +1000862The HISTORY section of
Damien Millerc93a8132005-12-24 14:52:13 +1100863.Xr ssl 8
Damien Millerd4427902010-09-10 11:15:10 +1000864contains a brief discussion of the DSA and RSA algorithms.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100865.Pp
866The file
867.Pa ~/.ssh/authorized_keys
868lists the public keys that are permitted for logging in.
869When the user logs in, the
870.Nm
871program tells the server which key pair it would like to use for
872authentication.
Damien Millerc93a8132005-12-24 14:52:13 +1100873The client proves that it has access to the private key
874and the server checks that the corresponding public key
875is authorized to accept the account.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100876.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100877The user creates his/her key pair by running
Darren Tuckerd3877b92005-12-20 16:09:36 +1100878.Xr ssh-keygen 1 .
879This stores the private key in
880.Pa ~/.ssh/identity
Damien Millerc93a8132005-12-24 14:52:13 +1100881(protocol 1),
882.Pa ~/.ssh/id_dsa
883(protocol 2 DSA),
Damien Millereb8b60e2010-08-31 22:41:14 +1000884.Pa ~/.ssh/id_ecdsa
885(protocol 2 ECDSA),
Damien Miller8ba0ead2013-12-18 17:46:27 +1100886.Pa ~/.ssh/id_ed25519
sobrado@openbsd.orgf70b22b2014-08-30 15:33:50 +0000887(protocol 2 Ed25519),
Damien Millerc93a8132005-12-24 14:52:13 +1100888or
889.Pa ~/.ssh/id_rsa
890(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100891and stores the public key in
892.Pa ~/.ssh/identity.pub
Damien Millerc93a8132005-12-24 14:52:13 +1100893(protocol 1),
894.Pa ~/.ssh/id_dsa.pub
895(protocol 2 DSA),
Damien Millereb8b60e2010-08-31 22:41:14 +1000896.Pa ~/.ssh/id_ecdsa.pub
897(protocol 2 ECDSA),
Damien Miller8ba0ead2013-12-18 17:46:27 +1100898.Pa ~/.ssh/id_ed25519.pub
sobrado@openbsd.orgf70b22b2014-08-30 15:33:50 +0000899(protocol 2 Ed25519),
Damien Millerc93a8132005-12-24 14:52:13 +1100900or
901.Pa ~/.ssh/id_rsa.pub
902(protocol 2 RSA)
Darren Tuckerd3877b92005-12-20 16:09:36 +1100903in the user's home directory.
Damien Millerc93a8132005-12-24 14:52:13 +1100904The user should then copy the public key
Darren Tuckerd3877b92005-12-20 16:09:36 +1100905to
906.Pa ~/.ssh/authorized_keys
Damien Millerc93a8132005-12-24 14:52:13 +1100907in his/her home directory on the remote machine.
908The
Darren Tuckerd3877b92005-12-20 16:09:36 +1100909.Pa authorized_keys
910file corresponds to the conventional
911.Pa ~/.rhosts
912file, and has one key
Damien Millerc93a8132005-12-24 14:52:13 +1100913per line, though the lines can be very long.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100914After this, the user can log in without giving the password.
915.Pp
Damien Miller922b5412010-03-05 21:30:54 +1100916A variation on public key authentication
917is available in the form of certificate authentication:
918instead of a set of public/private keys,
919signed certificates are used.
920This has the advantage that a single trusted certification authority
921can be used in place of many public/private keys.
Damien Millerfecfd112013-07-18 16:11:50 +1000922See the CERTIFICATES section of
Damien Miller922b5412010-03-05 21:30:54 +1100923.Xr ssh-keygen 1
924for more information.
925.Pp
926The most convenient way to use public key or certificate authentication
927may be with an authentication agent.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100928See
929.Xr ssh-agent 1
jcs@openbsd.orgf361df42015-11-15 22:26:49 +0000930and (optionally) the
931.Cm AddKeysToAgent
932directive in
933.Xr ssh_config 5
Darren Tuckerd3877b92005-12-20 16:09:36 +1100934for more information.
935.Pp
Damien Millerc93a8132005-12-24 14:52:13 +1100936Challenge-response authentication works as follows:
937The server sends an arbitrary
938.Qq challenge
939text, and prompts for a response.
940Protocol 2 allows multiple challenges and responses;
941protocol 1 is restricted to just one challenge/response.
942Examples of challenge-response authentication include
Damien Millerb7727df2013-08-21 02:43:49 +1000943.Bx
944Authentication (see
Damien Millerc93a8132005-12-24 14:52:13 +1100945.Xr login.conf 5 )
Damien Millerb7727df2013-08-21 02:43:49 +1000946and PAM (some
947.Pf non- Ox
948systems).
Damien Millerc93a8132005-12-24 14:52:13 +1100949.Pp
950Finally, if other authentication methods fail,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100951.Nm
952prompts the user for a password.
953The password is sent to the remote
954host for checking; however, since all communications are encrypted,
955the password cannot be seen by someone listening on the network.
Damien Miller48c94ab2006-01-02 23:38:00 +1100956.Pp
957.Nm
958automatically maintains and checks a database containing
959identification for all hosts it has ever been used with.
960Host keys are stored in
961.Pa ~/.ssh/known_hosts
962in the user's home directory.
963Additionally, the file
964.Pa /etc/ssh/ssh_known_hosts
965is automatically checked for known hosts.
966Any new hosts are automatically added to the user's file.
967If a host's identification ever changes,
968.Nm
969warns about this and disables password authentication to prevent
970server spoofing or man-in-the-middle attacks,
971which could otherwise be used to circumvent the encryption.
972The
973.Cm StrictHostKeyChecking
974option can be used to control logins to machines whose
975host key is not known or has changed.
976.Pp
Darren Tuckerd3877b92005-12-20 16:09:36 +1100977When the user's identity has been accepted by the server, the server
dtucker@openbsd.orgf8484da2015-05-06 05:45:17 +0000978either executes the given command in a non-interactive session or,
979if no command has been specified, logs into the machine and gives
980the user a normal shell as an interactive session.
Darren Tuckerd3877b92005-12-20 16:09:36 +1100981All communication with
982the remote command or shell will be automatically encrypted.
983.Pp
dtucker@openbsd.orgf8484da2015-05-06 05:45:17 +0000984If an interactive session is requested
985.Nm
986by default will only request a pseudo-terminal (pty) for interactive
987sessions when the client has one.
988The flags
989.Fl T
990and
991.Fl t
992can be used to override this behaviour.
993.Pp
994If a pseudo-terminal has been allocated the
Darren Tuckerd3877b92005-12-20 16:09:36 +1100995user may use the escape characters noted below.
996.Pp
dtucker@openbsd.orgf8484da2015-05-06 05:45:17 +0000997If no pseudo-terminal has been allocated,
Darren Tuckerd3877b92005-12-20 16:09:36 +1100998the session is transparent and can be used to reliably transfer binary data.
999On most systems, setting the escape character to
1000.Dq none
1001will also make the session transparent even if a tty is used.
1002.Pp
1003The session terminates when the command or shell on the remote
Damien Miller8bfaf932006-01-14 10:09:30 +11001004machine exits and all X11 and TCP connections have been closed.
Damien Miller52d20612005-12-24 14:52:36 +11001005.Sh ESCAPE CHARACTERS
Darren Tuckerd3877b92005-12-20 16:09:36 +11001006When a pseudo-terminal has been requested,
1007.Nm
1008supports a number of functions through the use of an escape character.
1009.Pp
1010A single tilde character can be sent as
1011.Ic ~~
1012or by following the tilde by a character other than those described below.
1013The escape character must always follow a newline to be interpreted as
1014special.
1015The escape character can be changed in configuration files using the
1016.Cm EscapeChar
1017configuration directive or on the command line by the
1018.Fl e
1019option.
1020.Pp
1021The supported escapes (assuming the default
1022.Ql ~ )
1023are:
1024.Bl -tag -width Ds
1025.It Cm ~.
1026Disconnect.
1027.It Cm ~^Z
1028Background
Damien Miller1164c292006-01-02 23:38:37 +11001029.Nm .
Darren Tuckerd3877b92005-12-20 16:09:36 +11001030.It Cm ~#
1031List forwarded connections.
1032.It Cm ~&
1033Background
1034.Nm
1035at logout when waiting for forwarded connection / X11 sessions to terminate.
1036.It Cm ~?
1037Display a list of escape characters.
1038.It Cm ~B
1039Send a BREAK to the remote system
1040(only useful for SSH protocol version 2 and if the peer supports it).
1041.It Cm ~C
1042Open command line.
1043Currently this allows the addition of port forwardings using the
Damien Miller0164cb82008-11-05 16:30:31 +11001044.Fl L ,
Darren Tuckerd3877b92005-12-20 16:09:36 +11001045.Fl R
Damien Miller0164cb82008-11-05 16:30:31 +11001046and
1047.Fl D
Damien Miller329cb012005-12-24 14:53:23 +11001048options (see above).
Damien Millerff773642011-09-22 21:39:48 +10001049It also allows the cancellation of existing port-forwardings
1050with
Damien Miller57e8ad32006-07-10 20:20:52 +10001051.Sm off
Damien Millerff773642011-09-22 21:39:48 +10001052.Fl KL Oo Ar bind_address : Oc Ar port
Damien Miller57e8ad32006-07-10 20:20:52 +10001053.Sm on
Damien Millerff773642011-09-22 21:39:48 +10001054for local,
1055.Sm off
1056.Fl KR Oo Ar bind_address : Oc Ar port
1057.Sm on
1058for remote and
1059.Sm off
1060.Fl KD Oo Ar bind_address : Oc Ar port
1061.Sm on
1062for dynamic port-forwardings.
Darren Tuckerd3877b92005-12-20 16:09:36 +11001063.Ic !\& Ns Ar command
1064allows the user to execute a local command if the
1065.Ic PermitLocalCommand
1066option is enabled in
1067.Xr ssh_config 5 .
1068Basic help is available, using the
1069.Fl h
1070option.
1071.It Cm ~R
1072Request rekeying of the connection
1073(only useful for SSH protocol version 2 and if the peer supports it).
Darren Tucker50a48d02012-09-06 21:25:37 +10001074.It Cm ~V
1075Decrease the verbosity
1076.Pq Ic LogLevel
1077when errors are being written to stderr.
1078.It Cm ~v
Darren Tucker83d0af62012-09-07 11:21:03 +10001079Increase the verbosity
Darren Tucker50a48d02012-09-06 21:25:37 +10001080.Pq Ic LogLevel
1081when errors are being written to stderr.
Darren Tuckerd3877b92005-12-20 16:09:36 +11001082.El
Damien Millerf3177182006-01-14 10:09:13 +11001083.Sh TCP FORWARDING
1084Forwarding of arbitrary TCP connections over the secure channel can
1085be specified either on the command line or in a configuration file.
1086One possible application of TCP forwarding is a secure connection to a
1087mail server; another is going through firewalls.
1088.Pp
1089In the example below, we look at encrypting communication between
1090an IRC client and server, even though the IRC server does not directly
1091support encrypted communications.
1092This works as follows:
1093the user connects to the remote host using
1094.Nm ,
1095specifying a port to be used to forward connections
1096to the remote server.
1097After that it is possible to start the service which is to be encrypted
1098on the client machine,
1099connecting to the same local port,
1100and
1101.Nm
1102will encrypt and forward the connection.
1103.Pp
1104The following example tunnels an IRC session from client machine
1105.Dq 127.0.0.1
1106(localhost)
1107to remote server
1108.Dq server.example.com :
1109.Bd -literal -offset 4n
1110$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1111$ irc -c '#users' -p 1234 pinky 127.0.0.1
1112.Ed
1113.Pp
1114This tunnels a connection to IRC server
1115.Dq server.example.com ,
1116joining channel
1117.Dq #users ,
1118nickname
1119.Dq pinky ,
1120using port 1234.
1121It doesn't matter which port is used,
1122as long as it's greater than 1023
1123(remember, only root can open sockets on privileged ports)
1124and doesn't conflict with any ports already in use.
1125The connection is forwarded to port 6667 on the remote server,
1126since that's the standard port for IRC services.
1127.Pp
1128The
1129.Fl f
1130option backgrounds
1131.Nm
1132and the remote command
1133.Dq sleep 10
1134is specified to allow an amount of time
1135(10 seconds, in the example)
1136to start the service which is to be tunnelled.
1137If no connections are made within the time specified,
1138.Nm
1139will exit.
Damien Millerf3177182006-01-14 10:09:13 +11001140.Sh X11 FORWARDING
Darren Tuckerd3877b92005-12-20 16:09:36 +11001141If the
1142.Cm ForwardX11
1143variable is set to
1144.Dq yes
1145(or see the description of the
Damien Millerd7f308f2005-12-24 14:55:16 +11001146.Fl X ,
1147.Fl x ,
Darren Tuckerd3877b92005-12-20 16:09:36 +11001148and
Damien Millerd7f308f2005-12-24 14:55:16 +11001149.Fl Y
Damien Miller9a765b22005-12-24 14:53:44 +11001150options above)
Darren Tuckerd3877b92005-12-20 16:09:36 +11001151and the user is using X11 (the
1152.Ev DISPLAY
1153environment variable is set), the connection to the X11 display is
1154automatically forwarded to the remote side in such a way that any X11
1155programs started from the shell (or command) will go through the
1156encrypted channel, and the connection to the real X server will be made
1157from the local machine.
1158The user should not manually set
1159.Ev DISPLAY .
1160Forwarding of X11 connections can be
1161configured on the command line or in configuration files.
1162.Pp
1163The
1164.Ev DISPLAY
1165value set by
1166.Nm
1167will point to the server machine, but with a display number greater than zero.
1168This is normal, and happens because
1169.Nm
1170creates a
1171.Dq proxy
1172X server on the server machine for forwarding the
1173connections over the encrypted channel.
1174.Pp
1175.Nm
1176will also automatically set up Xauthority data on the server machine.
1177For this purpose, it will generate a random authorization cookie,
1178store it in Xauthority on the server, and verify that any forwarded
1179connections carry this cookie and replace it by the real cookie when
1180the connection is opened.
1181The real authentication cookie is never
1182sent to the server machine (and no cookies are sent in the plain).
1183.Pp
1184If the
1185.Cm ForwardAgent
1186variable is set to
1187.Dq yes
1188(or see the description of the
1189.Fl A
1190and
1191.Fl a
Damien Miller9a765b22005-12-24 14:53:44 +11001192options above) and
Darren Tuckerd3877b92005-12-20 16:09:36 +11001193the user is using an authentication agent, the connection to the agent
1194is automatically forwarded to the remote side.
Damien Millerbbc59092006-01-31 21:46:51 +11001195.Sh VERIFYING HOST KEYS
1196When connecting to a server for the first time,
1197a fingerprint of the server's public key is presented to the user
1198(unless the option
1199.Cm StrictHostKeyChecking
1200has been disabled).
1201Fingerprints can be determined using
1202.Xr ssh-keygen 1 :
1203.Pp
1204.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1205.Pp
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001206If the fingerprint is already known, it can be matched
1207and the key can be accepted or rejected.
djm@openbsd.orgf9487372015-05-22 05:28:45 +00001208If only legacy (MD5) fingerprints for the server are available, the
1209.Xr ssh-keygen 1
1210.Fl E
1211option may be used to downgrade the fingerprint algorithm to match.
1212.Pp
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001213Because of the difficulty of comparing host keys
djm@openbsd.org56d1c832014-12-21 22:27:55 +00001214just by looking at fingerprint strings,
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001215there is also support to compare host keys visually,
1216using
1217.Em random art .
1218By setting the
Damien Miller10288242008-06-30 00:04:03 +10001219.Cm VisualHostKey
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001220option to
Damien Miller10288242008-06-30 00:04:03 +10001221.Dq yes ,
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001222a small ASCII graphic gets displayed on every login to a server, no matter
1223if the session itself is interactive or not.
1224By learning the pattern a known server produces, a user can easily
1225find out that the host key has changed when a completely different pattern
1226is displayed.
1227Because these patterns are not unambiguous however, a pattern that looks
1228similar to the pattern remembered only gives a good probability that the
1229host key is the same, not guaranteed proof.
1230.Pp
1231To get a listing of the fingerprints along with their random art for
1232all known hosts, the following command line can be used:
1233.Pp
1234.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1235.Pp
Damien Millerbbc59092006-01-31 21:46:51 +11001236If the fingerprint is unknown,
1237an alternative method of verification is available:
1238SSH fingerprints verified by DNS.
1239An additional resource record (RR),
1240SSHFP,
1241is added to a zonefile
1242and the connecting client is able to match the fingerprint
1243with that of the key presented.
1244.Pp
1245In this example, we are connecting a client to a server,
1246.Dq host.example.com .
1247The SSHFP resource records should first be added to the zonefile for
1248host.example.com:
1249.Bd -literal -offset indent
Damien Millera1b3d632006-03-26 00:07:02 +11001250$ ssh-keygen -r host.example.com.
Damien Millerbbc59092006-01-31 21:46:51 +11001251.Ed
1252.Pp
1253The output lines will have to be added to the zonefile.
1254To check that the zone is answering fingerprint queries:
1255.Pp
1256.Dl $ dig -t SSHFP host.example.com
1257.Pp
1258Finally the client connects:
1259.Bd -literal -offset indent
1260$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1261[...]
1262Matching host key fingerprint found in DNS.
1263Are you sure you want to continue connecting (yes/no)?
1264.Ed
1265.Pp
1266See the
1267.Cm VerifyHostKeyDNS
1268option in
1269.Xr ssh_config 5
1270for more information.
Darren Tucker248dd132006-01-20 11:30:58 +11001271.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1272.Nm
1273contains support for Virtual Private Network (VPN) tunnelling
1274using the
1275.Xr tun 4
1276network pseudo-device,
1277allowing two networks to be joined securely.
1278The
1279.Xr sshd_config 5
1280configuration option
1281.Cm PermitTunnel
1282controls whether the server supports this,
1283and at what level (layer 2 or 3 traffic).
1284.Pp
1285The following example would connect client network 10.0.50.0/24
Damien Miller3975ee22006-11-05 05:31:33 +11001286with remote network 10.0.99.0/24 using a point-to-point connection
1287from 10.1.1.1 to 10.1.1.2,
1288provided that the SSH server running on the gateway to the remote network,
1289at 192.168.1.15, allows it.
1290.Pp
1291On the client:
Darren Tucker248dd132006-01-20 11:30:58 +11001292.Bd -literal -offset indent
1293# ssh -f -w 0:1 192.168.1.15 true
Damien Miller3975ee22006-11-05 05:31:33 +11001294# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1295# route add 10.0.99.0/24 10.1.1.2
1296.Ed
1297.Pp
1298On the server:
1299.Bd -literal -offset indent
1300# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1301# route add 10.0.50.0/24 10.1.1.1
Darren Tucker248dd132006-01-20 11:30:58 +11001302.Ed
1303.Pp
1304Client access may be more finely tuned via the
1305.Pa /root/.ssh/authorized_keys
1306file (see below) and the
1307.Cm PermitRootLogin
1308server option.
Damien Millere93eaaa2006-03-15 11:05:59 +11001309The following entry would permit connections on
Darren Tucker248dd132006-01-20 11:30:58 +11001310.Xr tun 4
Damien Millere93eaaa2006-03-15 11:05:59 +11001311device 1 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001312.Dq jane
Damien Millere93eaaa2006-03-15 11:05:59 +11001313and on tun device 2 from user
Darren Tucker248dd132006-01-20 11:30:58 +11001314.Dq john ,
1315if
1316.Cm PermitRootLogin
1317is set to
1318.Dq forced-commands-only :
1319.Bd -literal -offset 2n
1320tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
Damien Miller9f67a212006-03-15 11:05:35 +11001321tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Darren Tucker248dd132006-01-20 11:30:58 +11001322.Ed
1323.Pp
Darren Tuckerffe88e12006-10-18 07:53:06 +10001324Since an SSH-based setup entails a fair amount of overhead,
Darren Tucker248dd132006-01-20 11:30:58 +11001325it may be more suited to temporary setups,
1326such as for wireless VPNs.
1327More permanent VPNs are better provided by tools such as
1328.Xr ipsecctl 8
1329and
1330.Xr isakmpd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +10001331.Sh ENVIRONMENT
1332.Nm
1333will normally set the following environment variables:
Damien Miller6aa22902006-01-06 14:48:34 +11001334.Bl -tag -width "SSH_ORIGINAL_COMMAND"
Damien Miller32aa1441999-10-29 09:15:49 +10001335.It Ev DISPLAY
1336The
1337.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +11001338variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +10001339It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +10001340.Nm
1341to point to a value of the form
Damien Miller14af93e2006-01-02 23:38:21 +11001342.Dq hostname:n ,
1343where
1344.Dq hostname
1345indicates the host where the shell runs, and
1346.Sq n
1347is an integer \*(Ge 1.
Damien Miller7684ee12000-03-17 23:40:15 +11001348.Nm
1349uses this special value to forward X11 connections over the secure
1350channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001351The user should normally not set
1352.Ev DISPLAY
1353explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +10001354will render the X11 connection insecure (and will require the user to
1355manually copy any required authorization cookies).
1356.It Ev HOME
1357Set to the path of the user's home directory.
1358.It Ev LOGNAME
1359Synonym for
1360.Ev USER ;
1361set for compatibility with systems that use this variable.
1362.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +00001363Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +11001364.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +10001365Set to the default
1366.Ev PATH ,
1367as specified when compiling
Damien Miller1164c292006-01-02 23:38:37 +11001368.Nm .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001369.It Ev SSH_ASKPASS
1370If
1371.Nm
1372needs a passphrase, it will read the passphrase from the current
1373terminal if it was run from a terminal.
1374If
1375.Nm
1376does not have a terminal associated with it but
1377.Ev DISPLAY
1378and
1379.Ev SSH_ASKPASS
1380are set, it will execute the program specified by
1381.Ev SSH_ASKPASS
1382and open an X11 window to read the passphrase.
1383This is particularly useful when calling
1384.Nm
1385from a
Darren Tucker4e4fe002004-11-05 20:01:03 +11001386.Pa .xsession
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001387or related script.
1388(Note that on some machines it
1389may be necessary to redirect the input from
1390.Pa /dev/null
1391to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +10001392.It Ev SSH_AUTH_SOCK
Damien Miller14af93e2006-01-02 23:38:21 +11001393Identifies the path of a
1394.Ux Ns -domain
1395socket used to communicate with the agent.
Damien Millerf37e2462002-09-19 11:47:55 +10001396.It Ev SSH_CONNECTION
1397Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +11001398The variable contains
Damien Miller14af93e2006-01-02 23:38:21 +11001399four space-separated values: client IP address, client port number,
1400server IP address, and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001401.It Ev SSH_ORIGINAL_COMMAND
Damien Miller14af93e2006-01-02 23:38:21 +11001402This variable contains the original command line if a forced command
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001403is executed.
1404It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +10001405.It Ev SSH_TTY
1406This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +11001407with the current shell or command.
1408If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +10001409this variable is not set.
1410.It Ev TZ
Damien Miller6dbdb6a2005-12-13 19:25:43 +11001411This variable is set to indicate the present time zone if it
Damien Miller208f1ed2006-03-15 11:56:03 +11001412was set when the daemon was started (i.e. the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +10001413on to new connections).
1414.It Ev USER
1415Set to the name of the user logging in.
1416.El
1417.Pp
Damien Miller22c77262000-04-13 12:26:34 +10001418Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +10001419.Nm
Damien Miller22c77262000-04-13 12:26:34 +10001420reads
Damien Miller167ea5d2005-05-26 12:04:02 +10001421.Pa ~/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +10001422and adds lines of the format
1423.Dq VARNAME=value
Damien Miller14af93e2006-01-02 23:38:21 +11001424to the environment if the file exists and users are allowed to
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001425change their environment.
Darren Tucker61776952003-10-02 16:19:47 +10001426For more information, see the
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001427.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +00001428option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +00001429.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +10001430.Sh FILES
Damien Miller7655f5c2006-01-06 14:48:18 +11001431.Bl -tag -width Ds -compact
Damien Millerafdae612010-08-31 22:31:14 +10001432.It Pa ~/.rhosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001433This file is used for host-based authentication (see above).
Ben Lindstromebd888d2001-03-05 05:49:29 +00001434On some machines this file may need to be
Damien Miller1bcdb502006-01-06 14:49:38 +11001435world-readable if the user's home directory is on an NFS partition,
Damien Miller32aa1441999-10-29 09:15:49 +10001436because
1437.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001438reads it as root.
1439Additionally, this file must be owned by the user,
1440and must not have write permissions for anyone else.
1441The recommended
Damien Miller32aa1441999-10-29 09:15:49 +10001442permission for most machines is read/write for the user, and not
1443accessible by others.
1444.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001445.It Pa ~/.shosts
Damien Miller1bcdb502006-01-06 14:49:38 +11001446This file is used in exactly the same way as
1447.Pa .rhosts ,
1448but allows host-based authentication without permitting login with
1449rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001450.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001451.It Pa ~/.ssh/
Damien Miller520e6152008-02-10 22:46:22 +11001452This directory is the default location for all user-specific configuration
1453and authentication information.
1454There is no general requirement to keep the entire contents of this directory
1455secret, but the recommended permissions are read/write/execute for the user,
1456and not accessible by others.
1457.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001458.It Pa ~/.ssh/authorized_keys
sobrado@openbsd.orgf70b22b2014-08-30 15:33:50 +00001459Lists the public keys (DSA, ECDSA, Ed25519, RSA)
Damien Miller8ba0ead2013-12-18 17:46:27 +11001460that can be used for logging in as this user.
Damien Millerfb8ea742006-01-06 14:48:52 +11001461The format of this file is described in the
1462.Xr sshd 8
1463manual page.
Damien Millerfb8ea742006-01-06 14:48:52 +11001464This file is not highly sensitive, but the recommended
1465permissions are read/write for the user, and not accessible by others.
1466.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001467.It Pa ~/.ssh/config
Damien Millerfb8ea742006-01-06 14:48:52 +11001468This is the per-user configuration file.
1469The file format and configuration options are described in
1470.Xr ssh_config 5 .
1471Because of the potential for abuse, this file must have strict permissions:
Damien Millerd93340c2013-07-18 16:14:34 +10001472read/write for the user, and not writable by others.
Damien Millerfb8ea742006-01-06 14:48:52 +11001473.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001474.It Pa ~/.ssh/environment
Damien Miller4c102ee2006-01-06 14:49:17 +11001475Contains additional definitions for environment variables; see
1476.Sx ENVIRONMENT ,
Damien Millerfb8ea742006-01-06 14:48:52 +11001477above.
1478.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001479.It Pa ~/.ssh/identity
1480.It Pa ~/.ssh/id_dsa
Damien Millereb8b60e2010-08-31 22:41:14 +10001481.It Pa ~/.ssh/id_ecdsa
Damien Miller8ba0ead2013-12-18 17:46:27 +11001482.It Pa ~/.ssh/id_ed25519
Damien Millerafdae612010-08-31 22:31:14 +10001483.It Pa ~/.ssh/id_rsa
Damien Millerfb8ea742006-01-06 14:48:52 +11001484Contains the private key for authentication.
1485These files
1486contain sensitive data and should be readable by the user but not
1487accessible by others (read/write/execute).
1488.Nm
1489will simply ignore a private key file if it is accessible by others.
1490It is possible to specify a passphrase when
1491generating the key which will be used to encrypt the
1492sensitive part of this file using 3DES.
1493.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001494.It Pa ~/.ssh/identity.pub
1495.It Pa ~/.ssh/id_dsa.pub
Damien Millereb8b60e2010-08-31 22:41:14 +10001496.It Pa ~/.ssh/id_ecdsa.pub
Damien Miller8ba0ead2013-12-18 17:46:27 +11001497.It Pa ~/.ssh/id_ed25519.pub
Damien Millerafdae612010-08-31 22:31:14 +10001498.It Pa ~/.ssh/id_rsa.pub
Damien Millerfb8ea742006-01-06 14:48:52 +11001499Contains the public key for authentication.
1500These files are not
1501sensitive and can (but need not) be readable by anyone.
Damien Millerfb8ea742006-01-06 14:48:52 +11001502.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001503.It Pa ~/.ssh/known_hosts
Damien Millere87eb4c2006-01-14 10:08:36 +11001504Contains a list of host keys for all hosts the user has logged into
1505that are not already in the systemwide list of known host keys.
Damien Millerfb8ea742006-01-06 14:48:52 +11001506See
Damien Millere87eb4c2006-01-14 10:08:36 +11001507.Xr sshd 8
1508for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001509.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001510.It Pa ~/.ssh/rc
Damien Millerfb8ea742006-01-06 14:48:52 +11001511Commands in this file are executed by
1512.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001513when the user logs in, just before the user's shell (or command) is
Damien Millerfb8ea742006-01-06 14:48:52 +11001514started.
1515See the
1516.Xr sshd 8
1517manual page for more information.
1518.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001519.It Pa /etc/hosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001520This file is for host-based authentication (see above).
1521It should only be writable by root.
Damien Miller7655f5c2006-01-06 14:48:18 +11001522.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001523.It Pa /etc/shosts.equiv
Damien Miller1bcdb502006-01-06 14:49:38 +11001524This file is used in exactly the same way as
1525.Pa hosts.equiv ,
1526but allows host-based authentication without permitting login with
1527rlogin/rsh.
Damien Miller7655f5c2006-01-06 14:48:18 +11001528.Pp
Damien Millerfb8ea742006-01-06 14:48:52 +11001529.It Pa /etc/ssh/ssh_config
1530Systemwide configuration file.
1531The file format and configuration options are described in
1532.Xr ssh_config 5 .
1533.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001534.It Pa /etc/ssh/ssh_host_key
1535.It Pa /etc/ssh/ssh_host_dsa_key
Damien Millereb8b60e2010-08-31 22:41:14 +10001536.It Pa /etc/ssh/ssh_host_ecdsa_key
Damien Miller8ba0ead2013-12-18 17:46:27 +11001537.It Pa /etc/ssh/ssh_host_ed25519_key
Damien Millerafdae612010-08-31 22:31:14 +10001538.It Pa /etc/ssh/ssh_host_rsa_key
Damien Millerb9902cf2012-06-20 21:52:58 +10001539These files contain the private parts of the host keys
Damien Miller7e76e1f2006-01-14 10:08:57 +11001540and are used for host-based authentication.
1541If protocol version 1 is used,
Damien Millerfb8ea742006-01-06 14:48:52 +11001542.Nm
1543must be setuid root, since the host key is readable only by root.
1544For protocol version 2,
1545.Nm
1546uses
1547.Xr ssh-keysign 8
Damien Miller7e76e1f2006-01-14 10:08:57 +11001548to access the host keys,
1549eliminating the requirement that
Damien Millerfb8ea742006-01-06 14:48:52 +11001550.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001551be setuid root when host-based authentication is used.
Damien Millerfb8ea742006-01-06 14:48:52 +11001552By default
1553.Nm
1554is not setuid root.
1555.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001556.It Pa /etc/ssh/ssh_known_hosts
Damien Millerfb8ea742006-01-06 14:48:52 +11001557Systemwide list of known host keys.
1558This file should be prepared by the
1559system administrator to contain the public host keys of all machines in the
1560organization.
Damien Millere87eb4c2006-01-14 10:08:36 +11001561It should be world-readable.
1562See
Damien Millerfb8ea742006-01-06 14:48:52 +11001563.Xr sshd 8
Damien Millere87eb4c2006-01-14 10:08:36 +11001564for further details of the format of this file.
Damien Millerfb8ea742006-01-06 14:48:52 +11001565.Pp
Damien Millerafdae612010-08-31 22:31:14 +10001566.It Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +10001567Commands in this file are executed by
1568.Nm
Damien Miller7e76e1f2006-01-14 10:08:57 +11001569when the user logs in, just before the user's shell (or command) is started.
Damien Miller32aa1441999-10-29 09:15:49 +10001570See the
1571.Xr sshd 8
1572manual page for more information.
Damien Miller7b28dc52000-09-05 13:34:53 +11001573.El
Damien Millerdaa7b222010-09-10 11:19:33 +10001574.Sh EXIT STATUS
1575.Nm
1576exits with the exit status of the remote command or with 255
1577if an error occurred.
Damien Miller32aa1441999-10-29 09:15:49 +10001578.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +10001579.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +11001580.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +10001581.Xr ssh-add 1 ,
1582.Xr ssh-agent 1 ,
1583.Xr ssh-keygen 1 ,
Damien Miller128a0f12006-01-06 14:50:11 +11001584.Xr ssh-keyscan 1 ,
Darren Tucker248dd132006-01-20 11:30:58 +11001585.Xr tun 4 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +00001586.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +00001587.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +00001588.Xr sshd 8
Darren Tucker628a3fd2012-10-05 10:50:15 +10001589.Sh STANDARDS
Ben Lindstrom160ec622001-04-22 17:17:46 +00001590.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001591.%A S. Lehtinen
1592.%A C. Lonvick
1593.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001594.%R RFC 4250
Darren Tucker628a3fd2012-10-05 10:50:15 +10001595.%T The Secure Shell (SSH) Protocol Assigned Numbers
Damien Miller39a93a32006-03-15 11:34:45 +11001596.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001597.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001598.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001599.%A T. Ylonen
1600.%A C. Lonvick
1601.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001602.%R RFC 4251
Darren Tucker628a3fd2012-10-05 10:50:15 +10001603.%T The Secure Shell (SSH) Protocol Architecture
Damien Miller39a93a32006-03-15 11:34:45 +11001604.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001605.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001606.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001607.%A T. Ylonen
1608.%A C. Lonvick
1609.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001610.%R RFC 4252
Darren Tucker628a3fd2012-10-05 10:50:15 +10001611.%T The Secure Shell (SSH) Authentication Protocol
Damien Miller39a93a32006-03-15 11:34:45 +11001612.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001613.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001614.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001615.%A T. Ylonen
1616.%A C. Lonvick
1617.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001618.%R RFC 4253
Darren Tucker628a3fd2012-10-05 10:50:15 +10001619.%T The Secure Shell (SSH) Transport Layer Protocol
Damien Miller39a93a32006-03-15 11:34:45 +11001620.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001621.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001622.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001623.%A T. Ylonen
1624.%A C. Lonvick
1625.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001626.%R RFC 4254
Darren Tucker628a3fd2012-10-05 10:50:15 +10001627.%T The Secure Shell (SSH) Connection Protocol
Damien Miller39a93a32006-03-15 11:34:45 +11001628.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001629.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001630.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001631.%A J. Schlyter
1632.%A W. Griffin
1633.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001634.%R RFC 4255
Darren Tucker628a3fd2012-10-05 10:50:15 +10001635.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
Damien Miller39a93a32006-03-15 11:34:45 +11001636.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001637.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001638.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001639.%A F. Cusack
1640.%A M. Forssen
1641.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001642.%R RFC 4256
Darren Tucker628a3fd2012-10-05 10:50:15 +10001643.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
Damien Miller39a93a32006-03-15 11:34:45 +11001644.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001645.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001646.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001647.%A J. Galbraith
1648.%A P. Remaker
1649.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001650.%R RFC 4335
Darren Tucker628a3fd2012-10-05 10:50:15 +10001651.%T The Secure Shell (SSH) Session Channel Break Extension
Damien Miller39a93a32006-03-15 11:34:45 +11001652.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001653.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001654.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001655.%A M. Bellare
1656.%A T. Kohno
1657.%A C. Namprempre
1658.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001659.%R RFC 4344
Darren Tucker628a3fd2012-10-05 10:50:15 +10001660.%T The Secure Shell (SSH) Transport Layer Encryption Modes
Damien Miller39a93a32006-03-15 11:34:45 +11001661.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001662.Pp
Damien Miller39a93a32006-03-15 11:34:45 +11001663.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001664.%A B. Harris
1665.%D January 2006
Damien Miller39a93a32006-03-15 11:34:45 +11001666.%R RFC 4345
Darren Tucker628a3fd2012-10-05 10:50:15 +10001667.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
Ben Lindstrom160ec622001-04-22 17:17:46 +00001668.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001669.Pp
Damien Miller3e96d742006-03-25 23:39:29 +11001670.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001671.%A M. Friedl
1672.%A N. Provos
1673.%A W. Simpson
1674.%D March 2006
Damien Miller3e96d742006-03-25 23:39:29 +11001675.%R RFC 4419
Darren Tucker628a3fd2012-10-05 10:50:15 +10001676.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
Damien Miller3e96d742006-03-25 23:39:29 +11001677.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001678.Pp
Damien Millerc0367fb2007-01-05 16:25:46 +11001679.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001680.%A J. Galbraith
1681.%A R. Thayer
1682.%D November 2006
Damien Millerc0367fb2007-01-05 16:25:46 +11001683.%R RFC 4716
Darren Tucker628a3fd2012-10-05 10:50:15 +10001684.%T The Secure Shell (SSH) Public Key File Format
Damien Millerc0367fb2007-01-05 16:25:46 +11001685.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001686.Pp
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001687.Rs
Darren Tucker628a3fd2012-10-05 10:50:15 +10001688.%A D. Stebila
1689.%A J. Green
1690.%D December 2009
Damien Miller1ca94692010-09-24 22:01:22 +10001691.%R RFC 5656
Darren Tucker628a3fd2012-10-05 10:50:15 +10001692.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
Damien Miller1ca94692010-09-24 22:01:22 +10001693.Re
Darren Tucker628a3fd2012-10-05 10:50:15 +10001694.Pp
Damien Miller1ca94692010-09-24 22:01:22 +10001695.Rs
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001696.%A A. Perrig
1697.%A D. Song
1698.%D 1999
Darren Tucker628a3fd2012-10-05 10:50:15 +10001699.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1700.%T Hash Visualization: a New Technique to improve Real-World Security
Darren Tuckerf6bffb12008-06-14 09:04:26 +10001701.Re
Damien Millerf1ce5052003-06-11 22:04:39 +10001702.Sh AUTHORS
1703OpenSSH is a derivative of the original and free
1704ssh 1.2.12 release by Tatu Ylonen.
1705Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1706Theo de Raadt and Dug Song
1707removed many bugs, re-added newer features and
1708created OpenSSH.
1709Markus Friedl contributed the support for SSH
1710protocol versions 1.5 and 2.0.