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Damien Miller32aa1441999-10-29 09:15:49 +10001.\" -*- nroff -*-
2.\"
Damien Miller32aa1441999-10-29 09:15:49 +10003.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller32aa1441999-10-29 09:15:49 +10004.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5.\" All rights reserved
6.\"
Damien Millere4340be2000-09-16 13:29:08 +11007.\" As far as I am concerned, the code I have written for this software
8.\" can be used freely for any purpose. Any derived versions of this
9.\" software must be clearly marked as such, and if the derived work is
10.\" incompatible with the protocol description in the RFC file, it must be
11.\" called by a name other than "ssh" or "Secure Shell".
Damien Miller32aa1441999-10-29 09:15:49 +100012.\"
Ben Lindstrom92a2e382001-03-05 06:59:27 +000013.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
Damien Millere4340be2000-09-16 13:29:08 +110016.\"
17.\" Redistribution and use in source and binary forms, with or without
18.\" modification, are permitted provided that the following conditions
19.\" are met:
20.\" 1. Redistributions of source code must retain the above copyright
21.\" notice, this list of conditions and the following disclaimer.
22.\" 2. Redistributions in binary form must reproduce the above copyright
23.\" notice, this list of conditions and the following disclaimer in the
24.\" documentation and/or other materials provided with the distribution.
25.\"
26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Damien Miller32aa1441999-10-29 09:15:49 +100036.\"
Damien Millerd8769622002-09-30 12:00:55 +100037.\" $OpenBSD: ssh.1,v 1.167 2002/09/27 15:46:21 stevesk 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
Damien Miller147bba32002-09-04 16:46:06 +100051.Op Fl afgknqstvxACNTX1246
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
Ben Lindstrom3541f182002-08-20 19:03:20 +0000356If the
357.Cm ForwardAgent
358variable is set to
359.Dq yes
360(or, see the description of the
361.Fl A
362and
363.Fl a
364options described later) and
365the user is using an authentication agent, the connection to the agent
366is automatically forwarded to the remote side.
Damien Miller32aa1441999-10-29 09:15:49 +1000367.Pp
368Forwarding of arbitrary TCP/IP connections over the secure channel can
Ben Lindstrom979c9812001-07-22 20:45:39 +0000369be specified either on the command line or in a configuration file.
Damien Miller7684ee12000-03-17 23:40:15 +1100370One possible application of TCP/IP forwarding is a secure connection to an
Ben Lindstromebd888d2001-03-05 05:49:29 +0000371electronic purse; another is going through firewalls.
Damien Miller32aa1441999-10-29 09:15:49 +1000372.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000373.Ss Server authentication
374.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000375.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000376automatically maintains and checks a database containing
Damien Miller7684ee12000-03-17 23:40:15 +1100377identifications for all hosts it has ever been used with.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000378Host keys are stored in
Damien Millere247cc42000-05-07 12:03:14 +1000379.Pa $HOME/.ssh/known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100380in the user's home directory.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000381Additionally, the file
Damien Miller05eda432002-02-10 18:32:28 +1100382.Pa /etc/ssh/ssh_known_hosts
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000383is automatically checked for known hosts.
Damien Miller7684ee12000-03-17 23:40:15 +1100384Any new hosts are automatically added to the user's file.
385If a host's identification
Damien Miller32aa1441999-10-29 09:15:49 +1000386ever changes,
387.Nm
388warns about this and disables password authentication to prevent a
Damien Miller7684ee12000-03-17 23:40:15 +1100389trojan horse from getting the user's password.
390Another purpose of
Damien Miller32aa1441999-10-29 09:15:49 +1000391this mechanism is to prevent man-in-the-middle attacks which could
Damien Miller7684ee12000-03-17 23:40:15 +1100392otherwise be used to circumvent the encryption.
393The
Damien Miller32aa1441999-10-29 09:15:49 +1000394.Cm StrictHostKeyChecking
Ben Lindstrom9f049032002-06-21 00:59:05 +0000395option can be used to prevent logins to machines whose
Damien Miller32aa1441999-10-29 09:15:49 +1000396host key is not known or has changed.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100397.Pp
398The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000399.Bl -tag -width Ds
400.It Fl a
Damien Miller450a7a12000-03-26 13:04:51 +1000401Disables forwarding of the authentication agent connection.
Damien Millerb1715dc2000-05-30 13:44:51 +1000402.It Fl A
403Enables forwarding of the authentication agent connection.
404This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000405.Pp
406Agent forwarding should be enabled with caution. Users with the
407ability to bypass file permissions on the remote host (for the agent's
408Unix-domain socket) can access the local agent through the forwarded
409connection. An attacker cannot obtain key material from the agent,
410however they can perform operations on the keys that enable them to
411authenticate using the identities loaded into the agent.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000412.It Fl b Ar bind_address
413Specify the interface to transmit from on machines with multiple
414interfaces or aliased addresses.
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000415.It Fl c Ar blowfish|3des|des
Damien Miller22c77262000-04-13 12:26:34 +1000416Selects the cipher to use for encrypting the session.
Damien Miller32aa1441999-10-29 09:15:49 +1000417.Ar 3des
Damien Miller7684ee12000-03-17 23:40:15 +1100418is used by default.
Damien Miller22c77262000-04-13 12:26:34 +1000419It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000420.Ar 3des
421(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
Damien Miller32aa1441999-10-29 09:15:49 +1000422.Ar blowfish
423is a fast block cipher, it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100424.Ar 3des .
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000425.Ar des
426is only supported in the
427.Nm
428client for interoperability with legacy protocol 1 implementations
429that do not support the
430.Ar 3des
431cipher. Its use is strongly discouraged due to cryptographic
432weaknesses.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000433.It Fl c Ar cipher_spec
Damien Miller30c3d422000-05-09 11:02:59 +1000434Additionally, for protocol version 2 a comma-separated list of ciphers can
Damien Miller874d77b2000-10-14 16:23:11 +1100435be specified in order of preference.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000436See
437.Cm Ciphers
438for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000439.It Fl e Ar ch|^ch|none
440Sets the escape character for sessions with a pty (default:
441.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100442The escape character is only recognized at the beginning of a line.
443The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000444.Pq Ql \&.
445closes the connection, followed
446by control-Z suspends the connection, and followed by itself sends the
Damien Miller7684ee12000-03-17 23:40:15 +1100447escape character once.
448Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000449.Dq none
450disables any escapes and makes the session fully transparent.
451.It Fl f
452Requests
453.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100454to go to background just before command execution.
455This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000456.Nm
457is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100458wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000459This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000460.Fl n .
461The recommended way to start X11 programs at a remote site is with
462something like
463.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100464.It Fl g
465Allows remote hosts to connect to local forwarded ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000466.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100467Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100468RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100469The default is
Damien Millere247cc42000-05-07 12:03:14 +1000470.Pa $HOME/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100471for protocol version 1, and
472.Pa $HOME/.ssh/id_rsa
473and
474.Pa $HOME/.ssh/id_dsa
475for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100476Identity files may also be specified on
477a per-host basis in the configuration file.
478It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000479.Fl i
480options (and multiple identities specified in
481configuration files).
Ben Lindstromae996bf2001-08-06 21:27:53 +0000482.It Fl I Ar smartcard_device
483Specifies which smartcard device to use. The argument is
484the device
485.Nm
486should use to communicate with a smartcard used for storing the user's
487private RSA key.
Damien Miller32aa1441999-10-29 09:15:49 +1000488.It Fl k
Damien Miller450a7a12000-03-26 13:04:51 +1000489Disables forwarding of Kerberos tickets and AFS tokens.
490This may also be specified on a per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000491.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100492Specifies the user to log in as on the remote machine.
493This also may be specified on a per-host basis in the configuration file.
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000494.It Fl m Ar mac_spec
495Additionally, for protocol version 2 a comma-separated list of MAC
496(message authentication code) algorithms can
497be specified in order of preference.
498See the
499.Cm MACs
500keyword for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000501.It Fl n
502Redirects stdin from
503.Pa /dev/null
504(actually, prevents reading from stdin).
505This must be used when
506.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100507is run in the background.
508A common trick is to use this to run X11 programs on a remote machine.
509For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000510.Ic ssh -n shadows.cs.hut.fi emacs &
511will start an emacs on shadows.cs.hut.fi, and the X11
512connection will be automatically forwarded over an encrypted channel.
513The
514.Nm
515program will be put in the background.
516(This does not work if
517.Nm
518needs to ask for a password or passphrase; see also the
519.Fl f
520option.)
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000521.It Fl N
522Do not execute a remote command.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000523This is useful for just forwarding ports
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000524(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000525.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000526Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000527This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100528command-line flag.
Damien Miller32aa1441999-10-29 09:15:49 +1000529.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100530Port to connect to on the remote host.
531This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000532per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000533.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100534Quiet mode.
535Causes all warning and diagnostic messages to be suppressed.
Damien Miller832562e2001-01-30 09:30:01 +1100536.It Fl s
Ben Lindstrom24643222001-06-25 05:08:11 +0000537May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
538of SSH as a secure transport for other applications (eg. sftp). The
Damien Miller832562e2001-01-30 09:30:01 +1100539subsystem is specified as the remote command.
Damien Miller32aa1441999-10-29 09:15:49 +1000540.It Fl t
Damien Miller7684ee12000-03-17 23:40:15 +1100541Force pseudo-tty allocation.
Damien Miller450a7a12000-03-26 13:04:51 +1000542This can be used to execute arbitrary
Damien Miller7684ee12000-03-17 23:40:15 +1100543screen-based programs on a remote machine, which can be very useful,
544e.g., when implementing menu services.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000545Multiple
546.Fl t
547options force tty allocation, even if
548.Nm
549has no local tty.
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000550.It Fl T
Ben Lindstromc72745a2000-12-02 19:03:54 +0000551Disable pseudo-tty allocation.
Damien Miller32aa1441999-10-29 09:15:49 +1000552.It Fl v
Damien Miller7684ee12000-03-17 23:40:15 +1100553Verbose mode.
554Causes
Damien Miller32aa1441999-10-29 09:15:49 +1000555.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100556to print debugging messages about its progress.
557This is helpful in
Damien Miller32aa1441999-10-29 09:15:49 +1000558debugging connection, authentication, and configuration problems.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000559Multiple
560.Fl v
561options increases the verbosity.
Damien Miller874d77b2000-10-14 16:23:11 +1100562Maximum is 3.
Damien Miller32aa1441999-10-29 09:15:49 +1000563.It Fl x
Damien Miller7684ee12000-03-17 23:40:15 +1100564Disables X11 forwarding.
Damien Miller32aa1441999-10-29 09:15:49 +1000565.It Fl X
566Enables X11 forwarding.
Damien Millerb1715dc2000-05-30 13:44:51 +1000567This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000568.Pp
569X11 forwarding should be enabled with caution. Users with the ability
570to bypass file permissions on the remote host (for the user's X
571authorization database) can access the local X11 display through the
572forwarded connection. An attacker may then be able to perform
573activities such as keystroke monitoring.
Damien Miller32aa1441999-10-29 09:15:49 +1000574.It Fl C
575Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller7684ee12000-03-17 23:40:15 +1100576data for forwarded X11 and TCP/IP connections).
577The compression algorithm is the same used by
Damien Miller396691a2000-01-20 22:44:08 +1100578.Xr gzip 1 ,
579and the
Damien Miller32aa1441999-10-29 09:15:49 +1000580.Dq level
581can be controlled by the
582.Cm CompressionLevel
Damien Millerd8769622002-09-30 12:00:55 +1000583option for protocol version 1.
Damien Miller7684ee12000-03-17 23:40:15 +1100584Compression is desirable on modem lines and other
Damien Miller32aa1441999-10-29 09:15:49 +1000585slow connections, but will only slow down things on fast networks.
586The default value can be set on a host-by-host basis in the
587configuration files; see the
Ben Lindstrome2b9b062001-08-06 20:50:55 +0000588.Cm Compression
Ben Lindstrom9f049032002-06-21 00:59:05 +0000589option.
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000590.It Fl F Ar configfile
591Specifies an alternative per-user configuration file.
592If a configuration file is given on the command line,
593the system-wide configuration file
Damien Miller05eda432002-02-10 18:32:28 +1100594.Pq Pa /etc/ssh/ssh_config
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000595will be ignored.
596The default for the per-user configuration file is
597.Pa $HOME/.ssh/config .
Damien Miller32aa1441999-10-29 09:15:49 +1000598.It Fl L Ar port:host:hostport
599Specifies that the given port on the local (client) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100600forwarded to the given host and port on the remote side.
601This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000602.Ar port
603on the local side, and whenever a connection is made to this port, the
604connection is forwarded over the secure channel, and a connection is
605made to
Damien Miller34132e52000-01-14 15:45:46 +1100606.Ar host
607port
608.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100609from the remote machine.
610Port forwardings can also be specified in the configuration file.
611Only root can forward privileged ports.
Damien Miller34132e52000-01-14 15:45:46 +1100612IPv6 addresses can be specified with an alternative syntax:
613.Ar port/host/hostport
Damien Miller32aa1441999-10-29 09:15:49 +1000614.It Fl R Ar port:host:hostport
615Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100616forwarded to the given host and port on the local side.
617This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000618.Ar port
619on the remote side, and whenever a connection is made to this port, the
620connection is forwarded over the secure channel, and a connection is
621made to
Damien Miller34132e52000-01-14 15:45:46 +1100622.Ar host
623port
624.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100625from the local machine.
626Port forwardings can also be specified in the configuration file.
627Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000628logging in as root on the remote machine.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000629IPv6 addresses can be specified with an alternative syntax:
630.Ar port/host/hostport
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000631.It Fl D Ar port
632Specifies a local
633.Dq dynamic
634application-level port forwarding.
635This works by allocating a socket to listen to
636.Ar port
637on the local side, and whenever a connection is made to this port, the
638connection is forwarded over the secure channel, and the application
639protocol is then used to determine where to connect to from the
640remote machine. Currently the SOCKS4 protocol is supported, and
641.Nm
642will act as a SOCKS4 server.
643Only root can forward privileged ports.
644Dynamic port forwardings can also be specified in the configuration file.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000645.It Fl 1
646Forces
647.Nm
648to try protocol version 1 only.
Damien Miller4af51302000-04-16 11:18:38 +1000649.It Fl 2
650Forces
651.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000652to try protocol version 2 only.
Damien Miller34132e52000-01-14 15:45:46 +1100653.It Fl 4
654Forces
655.Nm
656to use IPv4 addresses only.
657.It Fl 6
658Forces
659.Nm
660to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000661.El
662.Sh CONFIGURATION FILES
663.Nm
Ben Lindstrom9f049032002-06-21 00:59:05 +0000664may additionally obtain configuration data from
665a per-user configuration file and a system-wide configuration file.
666The file format and configuration options are described in
667.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000668.Sh ENVIRONMENT
669.Nm
670will normally set the following environment variables:
671.Bl -tag -width Ds
672.It Ev DISPLAY
673The
674.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +1100675variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +1000676It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +1000677.Nm
678to point to a value of the form
679.Dq hostname:n
680where hostname indicates
Damien Miller7684ee12000-03-17 23:40:15 +1100681the host where the shell runs, and n is an integer >= 1.
682.Nm
683uses this special value to forward X11 connections over the secure
684channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000685The user should normally not set
686.Ev DISPLAY
687explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +1000688will render the X11 connection insecure (and will require the user to
689manually copy any required authorization cookies).
690.It Ev HOME
691Set to the path of the user's home directory.
692.It Ev LOGNAME
693Synonym for
694.Ev USER ;
695set for compatibility with systems that use this variable.
696.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +0000697Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +1100698.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +1000699Set to the default
700.Ev PATH ,
701as specified when compiling
702.Nm ssh .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +0000703.It Ev SSH_ASKPASS
704If
705.Nm
706needs a passphrase, it will read the passphrase from the current
707terminal if it was run from a terminal.
708If
709.Nm
710does not have a terminal associated with it but
711.Ev DISPLAY
712and
713.Ev SSH_ASKPASS
714are set, it will execute the program specified by
715.Ev SSH_ASKPASS
716and open an X11 window to read the passphrase.
717This is particularly useful when calling
718.Nm
719from a
720.Pa .Xsession
721or related script.
722(Note that on some machines it
723may be necessary to redirect the input from
724.Pa /dev/null
725to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +1000726.It Ev SSH_AUTH_SOCK
Ben Lindstrome59433d2001-09-12 16:41:37 +0000727Identifies the path of a unix-domain socket used to communicate with the
Damien Miller32aa1441999-10-29 09:15:49 +1000728agent.
Damien Millerf37e2462002-09-19 11:47:55 +1000729.It Ev SSH_CONNECTION
730Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +1100731The variable contains
Damien Millerf37e2462002-09-19 11:47:55 +1000732four space-separated values: client ip-address, client port number,
733server ip-address and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000734.It Ev SSH_ORIGINAL_COMMAND
735The variable contains the original command line if a forced command
736is executed.
737It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +1000738.It Ev SSH_TTY
739This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +1100740with the current shell or command.
741If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +1000742this variable is not set.
743.It Ev TZ
744The timezone variable is set to indicate the present timezone if it
Damien Miller942da032000-08-18 13:59:06 +1000745was set when the daemon was started (i.e., the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +1000746on to new connections).
747.It Ev USER
748Set to the name of the user logging in.
749.El
750.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000751Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +1000752.Nm
Damien Miller22c77262000-04-13 12:26:34 +1000753reads
754.Pa $HOME/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +1000755and adds lines of the format
756.Dq VARNAME=value
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000757to the environment if the file exists and if users are allowed to
758change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000759See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000760.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000761option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000762.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000763.Sh FILES
Damien Miller98c7ad62000-03-09 21:27:49 +1100764.Bl -tag -width Ds
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000765.It Pa $HOME/.ssh/known_hosts
Ben Lindstrome59433d2001-09-12 16:41:37 +0000766Records host keys for all hosts the user has logged into that are not
Damien Miller32aa1441999-10-29 09:15:49 +1000767in
Damien Miller05eda432002-02-10 18:32:28 +1100768.Pa /etc/ssh/ssh_known_hosts .
Damien Miller32aa1441999-10-29 09:15:49 +1000769See
770.Xr sshd 8 .
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000771.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
772Contains the authentication identity of the user.
773They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
Damien Millere247cc42000-05-07 12:03:14 +1000774These files
775contain sensitive data and should be readable by the user but not
Damien Miller32aa1441999-10-29 09:15:49 +1000776accessible by others (read/write/execute).
777Note that
778.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000779ignores a private key file if it is accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000780It is possible to specify a passphrase when
781generating the key; the passphrase will be used to encrypt the
782sensitive part of this file using 3DES.
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000783.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000784Contains the public key for authentication (public part of the
Damien Miller7684ee12000-03-17 23:40:15 +1100785identity file in human-readable form).
Damien Millere247cc42000-05-07 12:03:14 +1000786The contents of the
787.Pa $HOME/.ssh/identity.pub
788file should be added to
Damien Miller32aa1441999-10-29 09:15:49 +1000789.Pa $HOME/.ssh/authorized_keys
790on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000791where the user wishes to log in using protocol version 1 RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000792The contents of the
793.Pa $HOME/.ssh/id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000794and
795.Pa $HOME/.ssh/id_rsa.pub
Damien Millere247cc42000-05-07 12:03:14 +1000796file should be added to
Ben Lindstromf96704d2001-06-25 04:17:12 +0000797.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000798on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000799where the user wishes to log in using protocol version 2 DSA/RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000800These files are not
Damien Miller7684ee12000-03-17 23:40:15 +1100801sensitive and can (but need not) be readable by anyone.
Damien Millere247cc42000-05-07 12:03:14 +1000802These files are
Kevin Steves3c034ae2001-02-05 13:47:11 +0000803never used automatically and are not necessary; they are only provided for
Damien Miller32aa1441999-10-29 09:15:49 +1000804the convenience of the user.
805.It Pa $HOME/.ssh/config
Damien Miller7684ee12000-03-17 23:40:15 +1100806This is the per-user configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000807The file format and configuration options are described in
808.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000809.It Pa $HOME/.ssh/authorized_keys
Ben Lindstromf96704d2001-06-25 04:17:12 +0000810Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Miller7684ee12000-03-17 23:40:15 +1100811The format of this file is described in the
Damien Miller32aa1441999-10-29 09:15:49 +1000812.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100813manual page.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000814In the simplest form the format is the same as the .pub
815identity files.
Damien Millere247cc42000-05-07 12:03:14 +1000816This file is not highly sensitive, but the recommended
817permissions are read/write for the user, and not accessible by others.
Damien Miller05eda432002-02-10 18:32:28 +1100818.It Pa /etc/ssh/ssh_known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100819Systemwide list of known host keys.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000820This file should be prepared by the
Damien Miller32aa1441999-10-29 09:15:49 +1000821system administrator to contain the public host keys of all machines in the
Damien Miller7684ee12000-03-17 23:40:15 +1100822organization.
823This file should be world-readable.
824This file contains
Damien Miller32aa1441999-10-29 09:15:49 +1000825public keys, one per line, in the following format (fields separated
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000826by spaces): system name, public key and optional comment field.
Damien Miller7684ee12000-03-17 23:40:15 +1100827When different names are used
Damien Miller32aa1441999-10-29 09:15:49 +1000828for the same machine, all such names should be listed, separated by
Damien Miller7684ee12000-03-17 23:40:15 +1100829commas.
830The format is described on the
Damien Miller32aa1441999-10-29 09:15:49 +1000831.Xr sshd 8
832manual page.
833.Pp
834The canonical system name (as returned by name servers) is used by
835.Xr sshd 8
836to verify the client host when logging in; other names are needed because
837.Nm
838does not convert the user-supplied name to a canonical name before
839checking the key, because someone with access to the name servers
840would then be able to fool host authentication.
Damien Miller05eda432002-02-10 18:32:28 +1100841.It Pa /etc/ssh/ssh_config
Damien Miller7684ee12000-03-17 23:40:15 +1100842Systemwide configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000843The file format and configuration options are described in
844.Xr ssh_config 5 .
Damien Miller05eda432002-02-10 18:32:28 +1100845.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 +1100846These three files contain the private parts of the host keys
847and are used for
848.Cm RhostsRSAAuthentication
849and
850.Cm HostbasedAuthentication .
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000851If the protocol version 1
852.Cm RhostsRSAAuthentication
Ben Lindstromcb72e4f2002-06-21 00:41:51 +0000853method is used,
Damien Miller705499b2001-11-12 11:05:38 +1100854.Nm
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000855must be setuid root, since the host key is readable only by root.
856For protocol version 2,
857.Nm
858uses
859.Xr ssh-keysign 8
860to access the host keys for
861.Cm HostbasedAuthentication .
862This eliminates the requirement that
863.Nm
864be setuid root when that authentication method is used.
865By default
866.Nm
867is not setuid root.
Damien Miller32aa1441999-10-29 09:15:49 +1000868.It Pa $HOME/.rhosts
869This file is used in
870.Pa \&.rhosts
871authentication to list the
Damien Miller7684ee12000-03-17 23:40:15 +1100872host/user pairs that are permitted to log in.
873(Note that this file is
Damien Miller32aa1441999-10-29 09:15:49 +1000874also used by rlogin and rsh, which makes using this file insecure.)
875Each line of the file contains a host name (in the canonical form
876returned by name servers), and then a user name on that host,
Damien Miller7684ee12000-03-17 23:40:15 +1100877separated by a space.
Ben Lindstromebd888d2001-03-05 05:49:29 +0000878On some machines this file may need to be
Damien Miller32aa1441999-10-29 09:15:49 +1000879world-readable if the user's home directory is on a NFS partition,
880because
881.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100882reads it as root.
883Additionally, this file must be owned by the user,
884and must not have write permissions for anyone else.
885The recommended
Damien Miller32aa1441999-10-29 09:15:49 +1000886permission for most machines is read/write for the user, and not
887accessible by others.
888.Pp
889Note that by default
890.Xr sshd 8
891will be installed so that it requires successful RSA host
Damien Miller7684ee12000-03-17 23:40:15 +1100892authentication before permitting \s+2.\s0rhosts authentication.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000893If the server machine does not have the client's host key in
Damien Miller05eda432002-02-10 18:32:28 +1100894.Pa /etc/ssh/ssh_known_hosts ,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000895it can be stored in
Damien Miller32aa1441999-10-29 09:15:49 +1000896.Pa $HOME/.ssh/known_hosts .
897The easiest way to do this is to
898connect back to the client from the server machine using ssh; this
Damien Millere247cc42000-05-07 12:03:14 +1000899will automatically add the host key to
Damien Miller32aa1441999-10-29 09:15:49 +1000900.Pa $HOME/.ssh/known_hosts .
901.It Pa $HOME/.shosts
902This file is used exactly the same way as
903.Pa \&.rhosts .
904The purpose for
905having this file is to be able to use rhosts authentication with
906.Nm
907without permitting login with
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000908.Nm rlogin
Damien Miller32aa1441999-10-29 09:15:49 +1000909or
910.Xr rsh 1 .
911.It Pa /etc/hosts.equiv
912This file is used during
Damien Miller7684ee12000-03-17 23:40:15 +1100913.Pa \&.rhosts authentication.
914It contains
Damien Miller32aa1441999-10-29 09:15:49 +1000915canonical hosts names, one per line (the full format is described on
916the
917.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100918manual page).
919If the client host is found in this file, login is
Damien Miller32aa1441999-10-29 09:15:49 +1000920automatically permitted provided client and server user names are the
Damien Miller7684ee12000-03-17 23:40:15 +1100921same.
922Additionally, successful RSA host authentication is normally
923required.
924This file should only be writable by root.
Damien Miller886c63a2000-01-20 23:13:36 +1100925.It Pa /etc/shosts.equiv
Damien Miller22c77262000-04-13 12:26:34 +1000926This file is processed exactly as
Damien Miller32aa1441999-10-29 09:15:49 +1000927.Pa /etc/hosts.equiv .
928This file may be useful to permit logins using
929.Nm
930but not using rsh/rlogin.
Damien Miller05eda432002-02-10 18:32:28 +1100931.It Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000932Commands in this file are executed by
933.Nm
934when the user logs in just before the user's shell (or command) is started.
935See the
936.Xr sshd 8
937manual page for more information.
938.It Pa $HOME/.ssh/rc
939Commands in this file are executed by
940.Nm
941when the user logs in just before the user's shell (or command) is
942started.
Damien Miller22c77262000-04-13 12:26:34 +1000943See the
Damien Miller32aa1441999-10-29 09:15:49 +1000944.Xr sshd 8
945manual page for more information.
Damien Miller4f0fa561999-12-26 14:24:41 +1100946.It Pa $HOME/.ssh/environment
947Contains additional definitions for environment variables, see section
948.Sx ENVIRONMENT
949above.
Damien Miller7b28dc52000-09-05 13:34:53 +1100950.El
Damien Miller07a2d422002-02-05 12:16:15 +1100951.Sh DIAGNOSTICS
952.Nm
953exits with the exit status of the remote command or with 255
954if an error occurred.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100955.Sh AUTHORS
Ben Lindstrom035782e2001-01-29 08:34:16 +0000956OpenSSH is a derivative of the original and free
957ssh 1.2.12 release by Tatu Ylonen.
958Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
959Theo de Raadt and Dug Song
960removed many bugs, re-added newer features and
961created OpenSSH.
962Markus Friedl contributed the support for SSH
963protocol versions 1.5 and 2.0.
Damien Miller32aa1441999-10-29 09:15:49 +1000964.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000965.Xr rsh 1 ,
966.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100967.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000968.Xr ssh-add 1 ,
969.Xr ssh-agent 1 ,
970.Xr ssh-keygen 1 ,
971.Xr telnet 1 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +0000972.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +0000973.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000974.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +0000975.Rs
976.%A T. Ylonen
977.%A T. Kivinen
978.%A M. Saarinen
979.%A T. Rinne
980.%A S. Lehtinen
981.%T "SSH Protocol Architecture"
Ben Lindstromf1813842002-03-27 17:18:31 +0000982.%N draft-ietf-secsh-architecture-12.txt
983.%D January 2002
Ben Lindstrom160ec622001-04-22 17:17:46 +0000984.%O work in progress material
985.Re