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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.\"
Ben Lindstromc001cd32002-06-23 00:32:11 +000037.\" $OpenBSD: ssh.1,v 1.160 2002/06/22 11:51:39 naddy Exp $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dd September 25, 1999
39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000043.Nd OpenSSH SSH client (remote login program)
Damien Miller32aa1441999-10-29 09:15:49 +100044.Sh SYNOPSIS
45.Nm ssh
46.Op Fl l Ar login_name
Ben Lindstroma10aed82001-09-12 16:43:26 +000047.Ar hostname | user@hostname
Damien Miller32aa1441999-10-29 09:15:49 +100048.Op Ar command
49.Pp
50.Nm ssh
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +000051.Op Fl afgknqstvxACNPTX1246
Ben Lindstrome0f88042001-04-30 13:06:24 +000052.Op Fl b Ar bind_address
Damien Miller30c3d422000-05-09 11:02:59 +100053.Op Fl c Ar cipher_spec
Damien Miller32aa1441999-10-29 09:15:49 +100054.Op Fl e Ar escape_char
55.Op Fl i Ar identity_file
56.Op Fl l Ar login_name
Ben Lindstrom06b33aa2001-02-15 03:01:59 +000057.Op Fl m Ar mac_spec
Damien Miller32aa1441999-10-29 09:15:49 +100058.Op Fl o Ar option
59.Op Fl p Ar port
Ben Lindstrom14f31ab2001-09-12 17:48:04 +000060.Op Fl F Ar configfile
Damien Miller32aa1441999-10-29 09:15:49 +100061.Oo Fl L Xo
62.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100063.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110064.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100065.Ar hostport
66.Sm on
67.Xc
68.Oc
69.Oo Fl R Xo
70.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100071.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110072.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100073.Ar hostport
74.Sm on
75.Xc
76.Oc
Ben Lindstrom6e69d532001-09-12 17:59:59 +000077.Op Fl D Ar port
Ben Lindstroma10aed82001-09-12 16:43:26 +000078.Ar hostname | user@hostname
Damien Miller32aa1441999-10-29 09:15:49 +100079.Op Ar command
Damien Miller22c77262000-04-13 12:26:34 +100080.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100081.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000082(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110083executing commands on a remote machine.
84It is intended to replace
Damien Miller32aa1441999-10-29 09:15:49 +100085rlogin and rsh, and provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110086two untrusted hosts over an insecure network.
87X11 connections and
Damien Miller32aa1441999-10-29 09:15:49 +100088arbitrary TCP/IP ports can also be forwarded over the secure channel.
89.Pp
90.Nm
Damien Miller22c77262000-04-13 12:26:34 +100091connects and logs into the specified
Damien Miller32aa1441999-10-29 09:15:49 +100092.Ar hostname .
93The user must prove
Damien Millere247cc42000-05-07 12:03:14 +100094his/her identity to the remote machine using one of several methods
95depending on the protocol version used:
96.Pp
97.Ss SSH protocol version 1
Damien Miller32aa1441999-10-29 09:15:49 +100098.Pp
99First, if the machine the user logs in from is listed in
100.Pa /etc/hosts.equiv
101or
Damien Miller886c63a2000-01-20 23:13:36 +1100102.Pa /etc/shosts.equiv
Damien Miller32aa1441999-10-29 09:15:49 +1000103on the remote machine, and the user names are
104the same on both sides, the user is immediately permitted to log in.
Damien Miller22c77262000-04-13 12:26:34 +1000105Second, if
Damien Miller32aa1441999-10-29 09:15:49 +1000106.Pa \&.rhosts
107or
108.Pa \&.shosts
109exists in the user's home directory on the
110remote machine and contains a line containing the name of the client
111machine and the name of the user on that machine, the user is
Damien Miller7684ee12000-03-17 23:40:15 +1100112permitted to log in.
113This form of authentication alone is normally not
Damien Miller32aa1441999-10-29 09:15:49 +1000114allowed by the server because it is not secure.
115.Pp
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000116The second authentication method is the
Damien Miller32aa1441999-10-29 09:15:49 +1000117.Pa rhosts
118or
119.Pa hosts.equiv
Damien Miller7684ee12000-03-17 23:40:15 +1100120method combined with RSA-based host authentication.
121It means that if the login would be permitted by
Damien Millere247cc42000-05-07 12:03:14 +1000122.Pa $HOME/.rhosts ,
123.Pa $HOME/.shosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000124.Pa /etc/hosts.equiv ,
125or
Damien Miller886c63a2000-01-20 23:13:36 +1100126.Pa /etc/shosts.equiv ,
Damien Miller32aa1441999-10-29 09:15:49 +1000127and if additionally the server can verify the client's
Damien Miller22c77262000-04-13 12:26:34 +1000128host key (see
Damien Miller05eda432002-02-10 18:32:28 +1100129.Pa /etc/ssh/ssh_known_hosts
Damien Miller33e511e1999-11-11 11:43:13 +1100130and
131.Pa $HOME/.ssh/known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +1000132in the
133.Sx FILES
Damien Miller7684ee12000-03-17 23:40:15 +1100134section), only then login is permitted.
135This authentication method closes security holes due to IP
136spoofing, DNS spoofing and routing spoofing.
137[Note to the administrator:
Damien Miller32aa1441999-10-29 09:15:49 +1000138.Pa /etc/hosts.equiv ,
Damien Millere247cc42000-05-07 12:03:14 +1000139.Pa $HOME/.rhosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000140and the rlogin/rsh protocol in general, are inherently insecure and should be
141disabled if security is desired.]
142.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000143As a third authentication method,
Damien Miller32aa1441999-10-29 09:15:49 +1000144.Nm
145supports RSA based authentication.
146The scheme is based on public-key cryptography: there are cryptosystems
147where encryption and decryption are done using separate keys, and it
148is not possible to derive the decryption key from the encryption key.
Damien Miller7684ee12000-03-17 23:40:15 +1100149RSA is one such system.
Damien Miller22c77262000-04-13 12:26:34 +1000150The idea is that each user creates a public/private
Damien Miller7684ee12000-03-17 23:40:15 +1100151key pair for authentication purposes.
152The server knows the public key, and only the user knows the private key.
Damien Miller22c77262000-04-13 12:26:34 +1000153The file
Damien Miller32aa1441999-10-29 09:15:49 +1000154.Pa $HOME/.ssh/authorized_keys
155lists the public keys that are permitted for logging
Damien Miller7684ee12000-03-17 23:40:15 +1100156in.
157When the user logs in, the
Damien Miller32aa1441999-10-29 09:15:49 +1000158.Nm
159program tells the server which key pair it would like to use for
Damien Miller7684ee12000-03-17 23:40:15 +1100160authentication.
161The server checks if this key is permitted, and if
Damien Miller32aa1441999-10-29 09:15:49 +1000162so, sends the user (actually the
163.Nm
164program running on behalf of the user) a challenge, a random number,
Damien Miller7684ee12000-03-17 23:40:15 +1100165encrypted by the user's public key.
166The challenge can only be
167decrypted using the proper private key.
168The user's client then decrypts the
Damien Miller32aa1441999-10-29 09:15:49 +1000169challenge using the private key, proving that he/she knows the private
170key but without disclosing it to the server.
171.Pp
172.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100173implements the RSA authentication protocol automatically.
174The user creates his/her RSA key pair by running
Damien Miller32aa1441999-10-29 09:15:49 +1000175.Xr ssh-keygen 1 .
Damien Miller22c77262000-04-13 12:26:34 +1000176This stores the private key in
Damien Millere247cc42000-05-07 12:03:14 +1000177.Pa $HOME/.ssh/identity
Damien Miller32aa1441999-10-29 09:15:49 +1000178and the public key in
Damien Millere247cc42000-05-07 12:03:14 +1000179.Pa $HOME/.ssh/identity.pub
Damien Miller7684ee12000-03-17 23:40:15 +1100180in the user's home directory.
181The user should then copy the
Damien Miller32aa1441999-10-29 09:15:49 +1000182.Pa identity.pub
Damien Miller22c77262000-04-13 12:26:34 +1000183to
Damien Millere247cc42000-05-07 12:03:14 +1000184.Pa $HOME/.ssh/authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000185in his/her home directory on the remote machine (the
Damien Miller32aa1441999-10-29 09:15:49 +1000186.Pa authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000187file corresponds to the conventional
Damien Millere247cc42000-05-07 12:03:14 +1000188.Pa $HOME/.rhosts
Damien Miller32aa1441999-10-29 09:15:49 +1000189file, and has one key
Damien Miller7684ee12000-03-17 23:40:15 +1100190per line, though the lines can be very long).
191After this, the user can log in without giving the password.
192RSA authentication is much
Damien Miller32aa1441999-10-29 09:15:49 +1000193more secure than rhosts authentication.
194.Pp
195The most convenient way to use RSA authentication may be with an
Damien Miller7684ee12000-03-17 23:40:15 +1100196authentication agent.
197See
Damien Miller32aa1441999-10-29 09:15:49 +1000198.Xr ssh-agent 1
199for more information.
200.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000201If other authentication methods fail,
Damien Miller32aa1441999-10-29 09:15:49 +1000202.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100203prompts the user for a password.
204The password is sent to the remote
Damien Miller32aa1441999-10-29 09:15:49 +1000205host for checking; however, since all communications are encrypted,
206the password cannot be seen by someone listening on the network.
207.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000208.Ss SSH protocol version 2
209.Pp
Damien Miller07a2d422002-02-05 12:16:15 +1100210When a user connects using protocol version 2
211similar authentication methods are available.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000212Using the default values for
213.Cm PreferredAuthentications ,
Ben Lindstroma9086a12001-08-06 20:58:51 +0000214the client will try to authenticate first using the hostbased method;
215if this method fails public key authentication is attempted,
216and finally if this method fails keyboard-interactive and
217password authentication are tried.
Damien Millere247cc42000-05-07 12:03:14 +1000218.Pp
219The public key method is similar to RSA authentication described
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000220in the previous section and allows the RSA or DSA algorithm to be used:
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000221The client uses his private key,
Damien Millere247cc42000-05-07 12:03:14 +1000222.Pa $HOME/.ssh/id_dsa
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000223or
224.Pa $HOME/.ssh/id_rsa ,
Damien Millere247cc42000-05-07 12:03:14 +1000225to sign the session identifier and sends the result to the server.
226The server checks whether the matching public key is listed in
Ben Lindstromf96704d2001-06-25 04:17:12 +0000227.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000228and grants access if both the key is found and the signature is correct.
229The session identifier is derived from a shared Diffie-Hellman value
230and is only known to the client and the server.
231.Pp
232If public key authentication fails or is not available a password
233can be sent encrypted to the remote host for proving the user's identity.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000234.Pp
235Additionally,
236.Nm
237supports hostbased or challenge response authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000238.Pp
239Protocol 2 provides additional mechanisms for confidentiality
Damien Miller30c3d422000-05-09 11:02:59 +1000240(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
Ben Lindstromec26fb12001-03-06 01:05:23 +0000241and integrity (hmac-md5, hmac-sha1).
Damien Millere247cc42000-05-07 12:03:14 +1000242Note that protocol 1 lacks a strong mechanism for ensuring the
243integrity of the connection.
244.Pp
245.Ss Login session and remote execution
246.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000247When the user's identity has been accepted by the server, the server
248either executes the given command, or logs into the machine and gives
Damien Miller7684ee12000-03-17 23:40:15 +1100249the user a normal shell on the remote machine.
250All communication with
Damien Miller32aa1441999-10-29 09:15:49 +1000251the remote command or shell will be automatically encrypted.
252.Pp
253If a pseudo-terminal has been allocated (normal login session), the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000254user may use the escape characters noted below.
Damien Miller32aa1441999-10-29 09:15:49 +1000255.Pp
256If no pseudo tty has been allocated, the
257session is transparent and can be used to reliably transfer binary
Damien Miller7684ee12000-03-17 23:40:15 +1100258data.
259On most systems, setting the escape character to
Damien Miller32aa1441999-10-29 09:15:49 +1000260.Dq none
261will also make the session transparent even if a tty is used.
262.Pp
Ben Lindstroma6885612000-12-09 03:45:32 +0000263The session terminates when the command or shell on the remote
Ben Lindstromebd888d2001-03-05 05:49:29 +0000264machine exits and all X11 and TCP/IP connections have been closed.
Damien Miller32aa1441999-10-29 09:15:49 +1000265The exit status of the remote program is returned as the exit status
266of
267.Nm ssh .
268.Pp
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000269.Ss Escape Characters
270.Pp
271When a pseudo terminal has been requested, ssh supports a number of functions
Ben Lindstrom24643222001-06-25 05:08:11 +0000272through the use of an escape character.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000273.Pp
274A single tilde character can be sent as
275.Ic ~~
Ben Lindstrom66007692001-07-22 20:41:59 +0000276or by following the tilde by a character other than those described below.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000277The escape character must always follow a newline to be interpreted as
278special.
279The escape character can be changed in configuration files using the
280.Cm EscapeChar
Ben Lindstrom24643222001-06-25 05:08:11 +0000281configuration directive or on the command line by the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000282.Fl e
283option.
284.Pp
285The supported escapes (assuming the default
286.Ql ~ )
287are:
288.Bl -tag -width Ds
289.It Cm ~.
290Disconnect
291.It Cm ~^Z
292Background ssh
293.It Cm ~#
294List forwarded connections
295.It Cm ~&
296Background ssh at logout when waiting for forwarded connection / X11 sessions
Damien Miller1f8dddc2001-11-12 11:02:25 +1100297to terminate
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000298.It Cm ~?
299Display a list of escape characters
Ben Lindstrom5589f4b2002-03-22 03:24:32 +0000300.It Cm ~C
301Open command line (only useful for adding port forwardings using the
302.Fl L
303and
304.Fl R
305options)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000306.It Cm ~R
Ben Lindstromf8d24572001-04-22 17:11:56 +0000307Request rekeying of the connection (only useful for SSH protocol version 2
308and if the peer supports it)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000309.El
310.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000311.Ss X11 and TCP forwarding
312.Pp
Ben Lindstrom71215072001-05-17 03:16:18 +0000313If the
314.Cm ForwardX11
315variable is set to
316.Dq yes
317(or, see the description of the
318.Fl X
319and
320.Fl x
321options described later)
322and the user is using X11 (the
Damien Miller32aa1441999-10-29 09:15:49 +1000323.Ev DISPLAY
324environment variable is set), the connection to the X11 display is
325automatically forwarded to the remote side in such a way that any X11
326programs started from the shell (or command) will go through the
327encrypted channel, and the connection to the real X server will be made
Damien Miller7684ee12000-03-17 23:40:15 +1100328from the local machine.
329The user should not manually set
Damien Miller32aa1441999-10-29 09:15:49 +1000330.Ev DISPLAY .
331Forwarding of X11 connections can be
332configured on the command line or in configuration files.
333.Pp
334The
Damien Miller22c77262000-04-13 12:26:34 +1000335.Ev DISPLAY
Damien Miller32aa1441999-10-29 09:15:49 +1000336value set by
337.Nm
338will point to the server machine, but with a display number greater
Damien Miller7684ee12000-03-17 23:40:15 +1100339than zero.
340This is normal, and happens because
Damien Miller32aa1441999-10-29 09:15:49 +1000341.Nm
342creates a
343.Dq proxy
344X server on the server machine for forwarding the
345connections over the encrypted channel.
346.Pp
347.Nm
348will also automatically set up Xauthority data on the server machine.
349For this purpose, it will generate a random authorization cookie,
350store it in Xauthority on the server, and verify that any forwarded
351connections carry this cookie and replace it by the real cookie when
Damien Miller7684ee12000-03-17 23:40:15 +1100352the connection is opened.
353The real authentication cookie is never
Damien Miller32aa1441999-10-29 09:15:49 +1000354sent to the server machine (and no cookies are sent in the plain).
355.Pp
356If the user is using an authentication agent, the connection to the agent
357is automatically forwarded to the remote side unless disabled on
Ben Lindstrom979c9812001-07-22 20:45:39 +0000358the command line or in a configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000359.Pp
360Forwarding of arbitrary TCP/IP connections over the secure channel can
Ben Lindstrom979c9812001-07-22 20:45:39 +0000361be specified either on the command line or in a configuration file.
Damien Miller7684ee12000-03-17 23:40:15 +1100362One possible application of TCP/IP forwarding is a secure connection to an
Ben Lindstromebd888d2001-03-05 05:49:29 +0000363electronic purse; another is going through firewalls.
Damien Miller32aa1441999-10-29 09:15:49 +1000364.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000365.Ss Server authentication
366.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000367.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000368automatically maintains and checks a database containing
Damien Miller7684ee12000-03-17 23:40:15 +1100369identifications for all hosts it has ever been used with.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000370Host keys are stored in
Damien Millere247cc42000-05-07 12:03:14 +1000371.Pa $HOME/.ssh/known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100372in the user's home directory.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000373Additionally, the file
Damien Miller05eda432002-02-10 18:32:28 +1100374.Pa /etc/ssh/ssh_known_hosts
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000375is automatically checked for known hosts.
Damien Miller7684ee12000-03-17 23:40:15 +1100376Any new hosts are automatically added to the user's file.
377If a host's identification
Damien Miller32aa1441999-10-29 09:15:49 +1000378ever changes,
379.Nm
380warns about this and disables password authentication to prevent a
Damien Miller7684ee12000-03-17 23:40:15 +1100381trojan horse from getting the user's password.
382Another purpose of
Damien Miller32aa1441999-10-29 09:15:49 +1000383this mechanism is to prevent man-in-the-middle attacks which could
Damien Miller7684ee12000-03-17 23:40:15 +1100384otherwise be used to circumvent the encryption.
385The
Damien Miller32aa1441999-10-29 09:15:49 +1000386.Cm StrictHostKeyChecking
Ben Lindstrom9f049032002-06-21 00:59:05 +0000387option can be used to prevent logins to machines whose
Damien Miller32aa1441999-10-29 09:15:49 +1000388host key is not known or has changed.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100389.Pp
390The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000391.Bl -tag -width Ds
392.It Fl a
Damien Miller450a7a12000-03-26 13:04:51 +1000393Disables forwarding of the authentication agent connection.
Damien Millerb1715dc2000-05-30 13:44:51 +1000394.It Fl A
395Enables forwarding of the authentication agent connection.
396This can also be specified on a per-host basis in a configuration file.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000397.It Fl b Ar bind_address
398Specify the interface to transmit from on machines with multiple
399interfaces or aliased addresses.
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000400.It Fl c Ar blowfish|3des|des
Damien Miller22c77262000-04-13 12:26:34 +1000401Selects the cipher to use for encrypting the session.
Damien Miller32aa1441999-10-29 09:15:49 +1000402.Ar 3des
Damien Miller7684ee12000-03-17 23:40:15 +1100403is used by default.
Damien Miller22c77262000-04-13 12:26:34 +1000404It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000405.Ar 3des
406(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
Damien Miller32aa1441999-10-29 09:15:49 +1000407.Ar blowfish
408is a fast block cipher, it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100409.Ar 3des .
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000410.Ar des
411is only supported in the
412.Nm
413client for interoperability with legacy protocol 1 implementations
414that do not support the
415.Ar 3des
416cipher. Its use is strongly discouraged due to cryptographic
417weaknesses.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000418.It Fl c Ar cipher_spec
Damien Miller30c3d422000-05-09 11:02:59 +1000419Additionally, for protocol version 2 a comma-separated list of ciphers can
Damien Miller874d77b2000-10-14 16:23:11 +1100420be specified in order of preference.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000421See
422.Cm Ciphers
423for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000424.It Fl e Ar ch|^ch|none
425Sets the escape character for sessions with a pty (default:
426.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100427The escape character is only recognized at the beginning of a line.
428The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000429.Pq Ql \&.
430closes the connection, followed
431by control-Z suspends the connection, and followed by itself sends the
Damien Miller7684ee12000-03-17 23:40:15 +1100432escape character once.
433Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000434.Dq none
435disables any escapes and makes the session fully transparent.
436.It Fl f
437Requests
438.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100439to go to background just before command execution.
440This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000441.Nm
442is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100443wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000444This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000445.Fl n .
446The recommended way to start X11 programs at a remote site is with
447something like
448.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100449.It Fl g
450Allows remote hosts to connect to local forwarded ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000451.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100452Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100453RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100454The default is
Damien Millere247cc42000-05-07 12:03:14 +1000455.Pa $HOME/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100456for protocol version 1, and
457.Pa $HOME/.ssh/id_rsa
458and
459.Pa $HOME/.ssh/id_dsa
460for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100461Identity files may also be specified on
462a per-host basis in the configuration file.
463It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000464.Fl i
465options (and multiple identities specified in
466configuration files).
Ben Lindstromae996bf2001-08-06 21:27:53 +0000467.It Fl I Ar smartcard_device
468Specifies which smartcard device to use. The argument is
469the device
470.Nm
471should use to communicate with a smartcard used for storing the user's
472private RSA key.
Damien Miller32aa1441999-10-29 09:15:49 +1000473.It Fl k
Damien Miller450a7a12000-03-26 13:04:51 +1000474Disables forwarding of Kerberos tickets and AFS tokens.
475This may also be specified on a per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000476.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100477Specifies the user to log in as on the remote machine.
478This also may be specified on a per-host basis in the configuration file.
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000479.It Fl m Ar mac_spec
480Additionally, for protocol version 2 a comma-separated list of MAC
481(message authentication code) algorithms can
482be specified in order of preference.
483See the
484.Cm MACs
485keyword for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000486.It Fl n
487Redirects stdin from
488.Pa /dev/null
489(actually, prevents reading from stdin).
490This must be used when
491.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100492is run in the background.
493A common trick is to use this to run X11 programs on a remote machine.
494For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000495.Ic ssh -n shadows.cs.hut.fi emacs &
496will start an emacs on shadows.cs.hut.fi, and the X11
497connection will be automatically forwarded over an encrypted channel.
498The
499.Nm
500program will be put in the background.
501(This does not work if
502.Nm
503needs to ask for a password or passphrase; see also the
504.Fl f
505option.)
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000506.It Fl N
507Do not execute a remote command.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000508This is useful for just forwarding ports
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000509(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000510.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000511Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000512This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100513command-line flag.
Damien Miller32aa1441999-10-29 09:15:49 +1000514.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100515Port to connect to on the remote host.
516This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000517per-host basis in the configuration file.
518.It Fl P
519Use a non-privileged port for outgoing connections.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000520This can be used if a firewall does
Damien Miller32aa1441999-10-29 09:15:49 +1000521not permit connections from privileged ports.
Damien Millera34a28b1999-12-14 10:47:15 +1100522Note that this option turns off
Damien Miller32aa1441999-10-29 09:15:49 +1000523.Cm RhostsAuthentication
524and
Kevin Stevesfcec7f82000-12-15 19:55:48 +0000525.Cm RhostsRSAAuthentication
526for older servers.
Damien Miller32aa1441999-10-29 09:15:49 +1000527.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100528Quiet mode.
529Causes all warning and diagnostic messages to be suppressed.
Damien Miller832562e2001-01-30 09:30:01 +1100530.It Fl s
Ben Lindstrom24643222001-06-25 05:08:11 +0000531May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
532of SSH as a secure transport for other applications (eg. sftp). The
Damien Miller832562e2001-01-30 09:30:01 +1100533subsystem is specified as the remote command.
Damien Miller32aa1441999-10-29 09:15:49 +1000534.It Fl t
Damien Miller7684ee12000-03-17 23:40:15 +1100535Force pseudo-tty allocation.
Damien Miller450a7a12000-03-26 13:04:51 +1000536This can be used to execute arbitrary
Damien Miller7684ee12000-03-17 23:40:15 +1100537screen-based programs on a remote machine, which can be very useful,
538e.g., when implementing menu services.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000539Multiple
540.Fl t
541options force tty allocation, even if
542.Nm
543has no local tty.
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000544.It Fl T
Ben Lindstromc72745a2000-12-02 19:03:54 +0000545Disable pseudo-tty allocation.
Damien Miller32aa1441999-10-29 09:15:49 +1000546.It Fl v
Damien Miller7684ee12000-03-17 23:40:15 +1100547Verbose mode.
548Causes
Damien Miller32aa1441999-10-29 09:15:49 +1000549.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100550to print debugging messages about its progress.
551This is helpful in
Damien Miller32aa1441999-10-29 09:15:49 +1000552debugging connection, authentication, and configuration problems.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000553Multiple
554.Fl v
555options increases the verbosity.
Damien Miller874d77b2000-10-14 16:23:11 +1100556Maximum is 3.
Damien Miller32aa1441999-10-29 09:15:49 +1000557.It Fl x
Damien Miller7684ee12000-03-17 23:40:15 +1100558Disables X11 forwarding.
Damien Miller32aa1441999-10-29 09:15:49 +1000559.It Fl X
560Enables X11 forwarding.
Damien Millerb1715dc2000-05-30 13:44:51 +1000561This can also be specified on a per-host basis in a configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000562.It Fl C
563Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller7684ee12000-03-17 23:40:15 +1100564data for forwarded X11 and TCP/IP connections).
565The compression algorithm is the same used by
Damien Miller396691a2000-01-20 22:44:08 +1100566.Xr gzip 1 ,
567and the
Damien Miller32aa1441999-10-29 09:15:49 +1000568.Dq level
569can be controlled by the
570.Cm CompressionLevel
Ben Lindstrom9f049032002-06-21 00:59:05 +0000571option.
Damien Miller7684ee12000-03-17 23:40:15 +1100572Compression is desirable on modem lines and other
Damien Miller32aa1441999-10-29 09:15:49 +1000573slow connections, but will only slow down things on fast networks.
574The default value can be set on a host-by-host basis in the
575configuration files; see the
Ben Lindstrome2b9b062001-08-06 20:50:55 +0000576.Cm Compression
Ben Lindstrom9f049032002-06-21 00:59:05 +0000577option.
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000578.It Fl F Ar configfile
579Specifies an alternative per-user configuration file.
580If a configuration file is given on the command line,
581the system-wide configuration file
Damien Miller05eda432002-02-10 18:32:28 +1100582.Pq Pa /etc/ssh/ssh_config
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000583will be ignored.
584The default for the per-user configuration file is
585.Pa $HOME/.ssh/config .
Damien Miller32aa1441999-10-29 09:15:49 +1000586.It Fl L Ar port:host:hostport
587Specifies that the given port on the local (client) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100588forwarded to the given host and port on the remote side.
589This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000590.Ar port
591on the local side, and whenever a connection is made to this port, the
592connection is forwarded over the secure channel, and a connection is
593made to
Damien Miller34132e52000-01-14 15:45:46 +1100594.Ar host
595port
596.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100597from the remote machine.
598Port forwardings can also be specified in the configuration file.
599Only root can forward privileged ports.
Damien Miller34132e52000-01-14 15:45:46 +1100600IPv6 addresses can be specified with an alternative syntax:
601.Ar port/host/hostport
Damien Miller32aa1441999-10-29 09:15:49 +1000602.It Fl R Ar port:host:hostport
603Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100604forwarded to the given host and port on the local side.
605This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000606.Ar port
607on the remote side, and whenever a connection is made to this port, the
608connection is forwarded over the secure channel, and a connection is
609made to
Damien Miller34132e52000-01-14 15:45:46 +1100610.Ar host
611port
612.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100613from the local machine.
614Port forwardings can also be specified in the configuration file.
615Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000616logging in as root on the remote machine.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000617IPv6 addresses can be specified with an alternative syntax:
618.Ar port/host/hostport
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000619.It Fl D Ar port
620Specifies a local
621.Dq dynamic
622application-level port forwarding.
623This works by allocating a socket to listen to
624.Ar port
625on the local side, and whenever a connection is made to this port, the
626connection is forwarded over the secure channel, and the application
627protocol is then used to determine where to connect to from the
628remote machine. Currently the SOCKS4 protocol is supported, and
629.Nm
630will act as a SOCKS4 server.
631Only root can forward privileged ports.
632Dynamic port forwardings can also be specified in the configuration file.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000633.It Fl 1
634Forces
635.Nm
636to try protocol version 1 only.
Damien Miller4af51302000-04-16 11:18:38 +1000637.It Fl 2
638Forces
639.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000640to try protocol version 2 only.
Damien Miller34132e52000-01-14 15:45:46 +1100641.It Fl 4
642Forces
643.Nm
644to use IPv4 addresses only.
645.It Fl 6
646Forces
647.Nm
648to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000649.El
650.Sh CONFIGURATION FILES
651.Nm
Ben Lindstrom9f049032002-06-21 00:59:05 +0000652may additionally obtain configuration data from
653a per-user configuration file and a system-wide configuration file.
654The file format and configuration options are described in
655.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000656.Sh ENVIRONMENT
657.Nm
658will normally set the following environment variables:
659.Bl -tag -width Ds
660.It Ev DISPLAY
661The
662.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +1100663variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +1000664It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +1000665.Nm
666to point to a value of the form
667.Dq hostname:n
668where hostname indicates
Damien Miller7684ee12000-03-17 23:40:15 +1100669the host where the shell runs, and n is an integer >= 1.
670.Nm
671uses this special value to forward X11 connections over the secure
672channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000673The user should normally not set
674.Ev DISPLAY
675explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +1000676will render the X11 connection insecure (and will require the user to
677manually copy any required authorization cookies).
678.It Ev HOME
679Set to the path of the user's home directory.
680.It Ev LOGNAME
681Synonym for
682.Ev USER ;
683set for compatibility with systems that use this variable.
684.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +0000685Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +1100686.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +1000687Set to the default
688.Ev PATH ,
689as specified when compiling
690.Nm ssh .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +0000691.It Ev SSH_ASKPASS
692If
693.Nm
694needs a passphrase, it will read the passphrase from the current
695terminal if it was run from a terminal.
696If
697.Nm
698does not have a terminal associated with it but
699.Ev DISPLAY
700and
701.Ev SSH_ASKPASS
702are set, it will execute the program specified by
703.Ev SSH_ASKPASS
704and open an X11 window to read the passphrase.
705This is particularly useful when calling
706.Nm
707from a
708.Pa .Xsession
709or related script.
710(Note that on some machines it
711may be necessary to redirect the input from
712.Pa /dev/null
713to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +1000714.It Ev SSH_AUTH_SOCK
Ben Lindstrome59433d2001-09-12 16:41:37 +0000715Identifies the path of a unix-domain socket used to communicate with the
Damien Miller32aa1441999-10-29 09:15:49 +1000716agent.
717.It Ev SSH_CLIENT
Damien Miller7684ee12000-03-17 23:40:15 +1100718Identifies the client end of the connection.
719The variable contains
Damien Miller32aa1441999-10-29 09:15:49 +1000720three space-separated values: client ip-address, client port number,
721and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000722.It Ev SSH_ORIGINAL_COMMAND
723The variable contains the original command line if a forced command
724is executed.
725It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +1000726.It Ev SSH_TTY
727This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +1100728with the current shell or command.
729If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +1000730this variable is not set.
731.It Ev TZ
732The timezone variable is set to indicate the present timezone if it
Damien Miller942da032000-08-18 13:59:06 +1000733was set when the daemon was started (i.e., the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +1000734on to new connections).
735.It Ev USER
736Set to the name of the user logging in.
737.El
738.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000739Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +1000740.Nm
Damien Miller22c77262000-04-13 12:26:34 +1000741reads
742.Pa $HOME/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +1000743and adds lines of the format
744.Dq VARNAME=value
745to the environment.
746.Sh FILES
Damien Miller98c7ad62000-03-09 21:27:49 +1100747.Bl -tag -width Ds
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000748.It Pa $HOME/.ssh/known_hosts
Ben Lindstrome59433d2001-09-12 16:41:37 +0000749Records host keys for all hosts the user has logged into that are not
Damien Miller32aa1441999-10-29 09:15:49 +1000750in
Damien Miller05eda432002-02-10 18:32:28 +1100751.Pa /etc/ssh/ssh_known_hosts .
Damien Miller32aa1441999-10-29 09:15:49 +1000752See
753.Xr sshd 8 .
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000754.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
755Contains the authentication identity of the user.
756They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
Damien Millere247cc42000-05-07 12:03:14 +1000757These files
758contain sensitive data and should be readable by the user but not
Damien Miller32aa1441999-10-29 09:15:49 +1000759accessible by others (read/write/execute).
760Note that
761.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000762ignores a private key file if it is accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000763It is possible to specify a passphrase when
764generating the key; the passphrase will be used to encrypt the
765sensitive part of this file using 3DES.
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000766.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000767Contains the public key for authentication (public part of the
Damien Miller7684ee12000-03-17 23:40:15 +1100768identity file in human-readable form).
Damien Millere247cc42000-05-07 12:03:14 +1000769The contents of the
770.Pa $HOME/.ssh/identity.pub
771file should be added to
Damien Miller32aa1441999-10-29 09:15:49 +1000772.Pa $HOME/.ssh/authorized_keys
773on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000774where the user wishes to log in using protocol version 1 RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000775The contents of the
776.Pa $HOME/.ssh/id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000777and
778.Pa $HOME/.ssh/id_rsa.pub
Damien Millere247cc42000-05-07 12:03:14 +1000779file should be added to
Ben Lindstromf96704d2001-06-25 04:17:12 +0000780.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000781on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000782where the user wishes to log in using protocol version 2 DSA/RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000783These files are not
Damien Miller7684ee12000-03-17 23:40:15 +1100784sensitive and can (but need not) be readable by anyone.
Damien Millere247cc42000-05-07 12:03:14 +1000785These files are
Kevin Steves3c034ae2001-02-05 13:47:11 +0000786never used automatically and are not necessary; they are only provided for
Damien Miller32aa1441999-10-29 09:15:49 +1000787the convenience of the user.
788.It Pa $HOME/.ssh/config
Damien Miller7684ee12000-03-17 23:40:15 +1100789This is the per-user configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000790The file format and configuration options are described in
791.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000792.It Pa $HOME/.ssh/authorized_keys
Ben Lindstromf96704d2001-06-25 04:17:12 +0000793Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Miller7684ee12000-03-17 23:40:15 +1100794The format of this file is described in the
Damien Miller32aa1441999-10-29 09:15:49 +1000795.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100796manual page.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000797In the simplest form the format is the same as the .pub
798identity files.
Damien Millere247cc42000-05-07 12:03:14 +1000799This file is not highly sensitive, but the recommended
800permissions are read/write for the user, and not accessible by others.
Damien Miller05eda432002-02-10 18:32:28 +1100801.It Pa /etc/ssh/ssh_known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100802Systemwide list of known host keys.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000803This file should be prepared by the
Damien Miller32aa1441999-10-29 09:15:49 +1000804system administrator to contain the public host keys of all machines in the
Damien Miller7684ee12000-03-17 23:40:15 +1100805organization.
806This file should be world-readable.
807This file contains
Damien Miller32aa1441999-10-29 09:15:49 +1000808public keys, one per line, in the following format (fields separated
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000809by spaces): system name, public key and optional comment field.
Damien Miller7684ee12000-03-17 23:40:15 +1100810When different names are used
Damien Miller32aa1441999-10-29 09:15:49 +1000811for the same machine, all such names should be listed, separated by
Damien Miller7684ee12000-03-17 23:40:15 +1100812commas.
813The format is described on the
Damien Miller32aa1441999-10-29 09:15:49 +1000814.Xr sshd 8
815manual page.
816.Pp
817The canonical system name (as returned by name servers) is used by
818.Xr sshd 8
819to verify the client host when logging in; other names are needed because
820.Nm
821does not convert the user-supplied name to a canonical name before
822checking the key, because someone with access to the name servers
823would then be able to fool host authentication.
Damien Miller05eda432002-02-10 18:32:28 +1100824.It Pa /etc/ssh/ssh_config
Damien Miller7684ee12000-03-17 23:40:15 +1100825Systemwide configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000826The file format and configuration options are described in
827.Xr ssh_config 5 .
Damien Miller05eda432002-02-10 18:32:28 +1100828.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
Damien Miller705499b2001-11-12 11:05:38 +1100829These three files contain the private parts of the host keys
830and are used for
831.Cm RhostsRSAAuthentication
832and
833.Cm HostbasedAuthentication .
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000834If the protocol version 1
835.Cm RhostsRSAAuthentication
Ben Lindstromcb72e4f2002-06-21 00:41:51 +0000836method is used,
Damien Miller705499b2001-11-12 11:05:38 +1100837.Nm
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000838must be setuid root, since the host key is readable only by root.
839For protocol version 2,
840.Nm
841uses
842.Xr ssh-keysign 8
843to access the host keys for
844.Cm HostbasedAuthentication .
845This eliminates the requirement that
846.Nm
847be setuid root when that authentication method is used.
848By default
849.Nm
850is not setuid root.
Damien Miller32aa1441999-10-29 09:15:49 +1000851.It Pa $HOME/.rhosts
852This file is used in
853.Pa \&.rhosts
854authentication to list the
Damien Miller7684ee12000-03-17 23:40:15 +1100855host/user pairs that are permitted to log in.
856(Note that this file is
Damien Miller32aa1441999-10-29 09:15:49 +1000857also used by rlogin and rsh, which makes using this file insecure.)
858Each line of the file contains a host name (in the canonical form
859returned by name servers), and then a user name on that host,
Damien Miller7684ee12000-03-17 23:40:15 +1100860separated by a space.
Ben Lindstromebd888d2001-03-05 05:49:29 +0000861On some machines this file may need to be
Damien Miller32aa1441999-10-29 09:15:49 +1000862world-readable if the user's home directory is on a NFS partition,
863because
864.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100865reads it as root.
866Additionally, this file must be owned by the user,
867and must not have write permissions for anyone else.
868The recommended
Damien Miller32aa1441999-10-29 09:15:49 +1000869permission for most machines is read/write for the user, and not
870accessible by others.
871.Pp
872Note that by default
873.Xr sshd 8
874will be installed so that it requires successful RSA host
Damien Miller7684ee12000-03-17 23:40:15 +1100875authentication before permitting \s+2.\s0rhosts authentication.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000876If the server machine does not have the client's host key in
Damien Miller05eda432002-02-10 18:32:28 +1100877.Pa /etc/ssh/ssh_known_hosts ,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000878it can be stored in
Damien Miller32aa1441999-10-29 09:15:49 +1000879.Pa $HOME/.ssh/known_hosts .
880The easiest way to do this is to
881connect back to the client from the server machine using ssh; this
Damien Millere247cc42000-05-07 12:03:14 +1000882will automatically add the host key to
Damien Miller32aa1441999-10-29 09:15:49 +1000883.Pa $HOME/.ssh/known_hosts .
884.It Pa $HOME/.shosts
885This file is used exactly the same way as
886.Pa \&.rhosts .
887The purpose for
888having this file is to be able to use rhosts authentication with
889.Nm
890without permitting login with
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000891.Nm rlogin
Damien Miller32aa1441999-10-29 09:15:49 +1000892or
893.Xr rsh 1 .
894.It Pa /etc/hosts.equiv
895This file is used during
Damien Miller7684ee12000-03-17 23:40:15 +1100896.Pa \&.rhosts authentication.
897It contains
Damien Miller32aa1441999-10-29 09:15:49 +1000898canonical hosts names, one per line (the full format is described on
899the
900.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100901manual page).
902If the client host is found in this file, login is
Damien Miller32aa1441999-10-29 09:15:49 +1000903automatically permitted provided client and server user names are the
Damien Miller7684ee12000-03-17 23:40:15 +1100904same.
905Additionally, successful RSA host authentication is normally
906required.
907This file should only be writable by root.
Damien Miller886c63a2000-01-20 23:13:36 +1100908.It Pa /etc/shosts.equiv
Damien Miller22c77262000-04-13 12:26:34 +1000909This file is processed exactly as
Damien Miller32aa1441999-10-29 09:15:49 +1000910.Pa /etc/hosts.equiv .
911This file may be useful to permit logins using
912.Nm
913but not using rsh/rlogin.
Damien Miller05eda432002-02-10 18:32:28 +1100914.It Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000915Commands in this file are executed by
916.Nm
917when the user logs in just before the user's shell (or command) is started.
918See the
919.Xr sshd 8
920manual page for more information.
921.It Pa $HOME/.ssh/rc
922Commands in this file are executed by
923.Nm
924when the user logs in just before the user's shell (or command) is
925started.
Damien Miller22c77262000-04-13 12:26:34 +1000926See the
Damien Miller32aa1441999-10-29 09:15:49 +1000927.Xr sshd 8
928manual page for more information.
Damien Miller4f0fa561999-12-26 14:24:41 +1100929.It Pa $HOME/.ssh/environment
930Contains additional definitions for environment variables, see section
931.Sx ENVIRONMENT
932above.
Damien Miller7b28dc52000-09-05 13:34:53 +1100933.El
Damien Miller07a2d422002-02-05 12:16:15 +1100934.Sh DIAGNOSTICS
935.Nm
936exits with the exit status of the remote command or with 255
937if an error occurred.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100938.Sh AUTHORS
Ben Lindstrom035782e2001-01-29 08:34:16 +0000939OpenSSH is a derivative of the original and free
940ssh 1.2.12 release by Tatu Ylonen.
941Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
942Theo de Raadt and Dug Song
943removed many bugs, re-added newer features and
944created OpenSSH.
945Markus Friedl contributed the support for SSH
946protocol versions 1.5 and 2.0.
Damien Miller32aa1441999-10-29 09:15:49 +1000947.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000948.Xr rsh 1 ,
949.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100950.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000951.Xr ssh-add 1 ,
952.Xr ssh-agent 1 ,
953.Xr ssh-keygen 1 ,
954.Xr telnet 1 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +0000955.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +0000956.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000957.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +0000958.Rs
959.%A T. Ylonen
960.%A T. Kivinen
961.%A M. Saarinen
962.%A T. Rinne
963.%A S. Lehtinen
964.%T "SSH Protocol Architecture"
Ben Lindstromf1813842002-03-27 17:18:31 +0000965.%N draft-ietf-secsh-architecture-12.txt
966.%D January 2002
Ben Lindstrom160ec622001-04-22 17:17:46 +0000967.%O work in progress material
968.Re