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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 Lindstrom959de992002-06-23 00:35:25 +000037.\" $OpenBSD: sshd.8,v 1.186 2002/06/22 16:45:29 stevesk Exp $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dd September 25, 1999
39.Dt SSHD 8
40.Os
41.Sh NAME
42.Nm sshd
Ben Lindstromc65e6a02001-04-23 13:02:16 +000043.Nd OpenSSH SSH daemon
Damien Miller32aa1441999-10-29 09:15:49 +100044.Sh SYNOPSIS
45.Nm sshd
Ben Lindstromdfd18502001-09-20 01:06:08 +000046.Op Fl deiqtD46
Damien Miller32aa1441999-10-29 09:15:49 +100047.Op Fl b Ar bits
48.Op Fl f Ar config_file
49.Op Fl g Ar login_grace_time
50.Op Fl h Ar host_key_file
51.Op Fl k Ar key_gen_time
Ben Lindstromade03f62001-12-06 18:22:17 +000052.Op Fl o Ar option
Damien Miller32aa1441999-10-29 09:15:49 +100053.Op Fl p Ar port
Damien Miller942da032000-08-18 13:59:06 +100054.Op Fl u Ar len
Damien Miller22c77262000-04-13 12:26:34 +100055.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100056.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000057(SSH Daemon) is the daemon program for
Damien Miller32aa1441999-10-29 09:15:49 +100058.Xr ssh 1 .
Damien Miller35dabd02000-05-01 21:10:33 +100059Together these programs replace rlogin and rsh, and
Damien Miller32aa1441999-10-29 09:15:49 +100060provide secure encrypted communications between two untrusted hosts
Damien Miller450a7a12000-03-26 13:04:51 +100061over an insecure network.
62The programs are intended to be as easy to
Damien Miller32aa1441999-10-29 09:15:49 +100063install and use as possible.
64.Pp
65.Nm
Damien Miller450a7a12000-03-26 13:04:51 +100066is the daemon that listens for connections from clients.
Damien Miller22c77262000-04-13 12:26:34 +100067It is normally started at boot from
Damien Miller32aa1441999-10-29 09:15:49 +100068.Pa /etc/rc .
69It forks a new
Damien Miller450a7a12000-03-26 13:04:51 +100070daemon for each incoming connection.
71The forked daemons handle
Damien Miller32aa1441999-10-29 09:15:49 +100072key exchange, encryption, authentication, command execution,
73and data exchange.
Damien Millere247cc42000-05-07 12:03:14 +100074This implementation of
75.Nm
76supports both SSH protocol version 1 and 2 simultaneously.
Damien Miller32aa1441999-10-29 09:15:49 +100077.Nm
Damien Miller450a7a12000-03-26 13:04:51 +100078works as follows.
Damien Millere247cc42000-05-07 12:03:14 +100079.Pp
80.Ss SSH protocol version 1
81.Pp
Damien Miller450a7a12000-03-26 13:04:51 +100082Each host has a host-specific RSA key
83(normally 1024 bits) used to identify the host.
84Additionally, when
Damien Miller32aa1441999-10-29 09:15:49 +100085the daemon starts, it generates a server RSA key (normally 768 bits).
86This key is normally regenerated every hour if it has been used, and
87is never stored on disk.
88.Pp
Damien Miller35dabd02000-05-01 21:10:33 +100089Whenever a client connects the daemon responds with its public
90host and server keys.
Damien Miller450a7a12000-03-26 13:04:51 +100091The client compares the
Damien Millere247cc42000-05-07 12:03:14 +100092RSA host key against its own database to verify that it has not changed.
Damien Miller450a7a12000-03-26 13:04:51 +100093The client then generates a 256 bit random number.
94It encrypts this
Damien Miller32aa1441999-10-29 09:15:49 +100095random number using both the host key and the server key, and sends
Damien Miller450a7a12000-03-26 13:04:51 +100096the encrypted number to the server.
Damien Miller35dabd02000-05-01 21:10:33 +100097Both sides then use this
Damien Miller32aa1441999-10-29 09:15:49 +100098random number as a session key which is used to encrypt all further
Damien Miller450a7a12000-03-26 13:04:51 +100099communications in the session.
100The rest of the session is encrypted
Damien Miller35dabd02000-05-01 21:10:33 +1000101using a conventional cipher, currently Blowfish or 3DES, with 3DES
Damien Millerb38eff82000-04-01 11:09:21 +1000102being used by default.
Damien Miller450a7a12000-03-26 13:04:51 +1000103The client selects the encryption algorithm
Damien Miller32aa1441999-10-29 09:15:49 +1000104to use from those offered by the server.
105.Pp
Damien Miller450a7a12000-03-26 13:04:51 +1000106Next, the server and the client enter an authentication dialog.
107The client tries to authenticate itself using
Damien Miller32aa1441999-10-29 09:15:49 +1000108.Pa .rhosts
109authentication,
110.Pa .rhosts
111authentication combined with RSA host
112authentication, RSA challenge-response authentication, or password
113based authentication.
114.Pp
115Rhosts authentication is normally disabled
116because it is fundamentally insecure, but can be enabled in the server
Damien Miller450a7a12000-03-26 13:04:51 +1000117configuration file if desired.
118System security is not improved unless
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000119.Nm rshd ,
120.Nm rlogind ,
Damien Miller32aa1441999-10-29 09:15:49 +1000121and
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000122.Xr rexecd
Damien Miller32aa1441999-10-29 09:15:49 +1000123are disabled (thus completely disabling
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000124.Xr rlogin
Damien Miller32aa1441999-10-29 09:15:49 +1000125and
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000126.Xr rsh
Damien Miller35dabd02000-05-01 21:10:33 +1000127into the machine).
Damien Miller32aa1441999-10-29 09:15:49 +1000128.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000129.Ss SSH protocol version 2
130.Pp
Damien Miller942da032000-08-18 13:59:06 +1000131Version 2 works similarly:
Ben Lindstrom07d24dc2001-08-06 21:18:57 +0000132Each host has a host-specific key (RSA or DSA) used to identify the host.
Damien Millere247cc42000-05-07 12:03:14 +1000133However, when the daemon starts, it does not generate a server key.
134Forward security is provided through a Diffie-Hellman key agreement.
135This key agreement results in a shared session key.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000136.Pp
Ben Lindstromfd2e05b2001-03-05 07:48:45 +0000137The rest of the session is encrypted using a symmetric cipher, currently
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000138128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
Damien Millere247cc42000-05-07 12:03:14 +1000139The client selects the encryption algorithm
140to use from those offered by the server.
141Additionally, session integrity is provided
Damien Miller30c3d422000-05-09 11:02:59 +1000142through a cryptographic message authentication code
Damien Millere247cc42000-05-07 12:03:14 +1000143(hmac-sha1 or hmac-md5).
144.Pp
145Protocol version 2 provides a public key based
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000146user (PubkeyAuthentication) or
147client host (HostbasedAuthentication) authentication method,
148conventional password authentication and challenge response based methods.
Damien Millere247cc42000-05-07 12:03:14 +1000149.Pp
150.Ss Command execution and data forwarding
151.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000152If the client successfully authenticates itself, a dialog for
Damien Miller450a7a12000-03-26 13:04:51 +1000153preparing the session is entered.
154At this time the client may request
Damien Miller32aa1441999-10-29 09:15:49 +1000155things like allocating a pseudo-tty, forwarding X11 connections,
156forwarding TCP/IP connections, or forwarding the authentication agent
157connection over the secure channel.
158.Pp
159Finally, the client either requests a shell or execution of a command.
Damien Miller450a7a12000-03-26 13:04:51 +1000160The sides then enter session mode.
161In this mode, either side may send
Damien Miller32aa1441999-10-29 09:15:49 +1000162data at any time, and such data is forwarded to/from the shell or
163command on the server side, and the user terminal in the client side.
164.Pp
165When the user program terminates and all forwarded X11 and other
166connections have been closed, the server sends command exit status to
167the client, and both sides exit.
168.Pp
169.Nm
170can be configured using command-line options or a configuration
Damien Miller450a7a12000-03-26 13:04:51 +1000171file.
172Command-line options override values specified in the
Damien Miller32aa1441999-10-29 09:15:49 +1000173configuration file.
174.Pp
Damien Miller6162d121999-11-21 13:23:52 +1100175.Nm
176rereads its configuration file when it receives a hangup signal,
Ben Lindstrom49a098d2001-03-05 06:55:18 +0000177.Dv SIGHUP ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000178by executing itself with the name it was started as, i.e.,
Ben Lindstrom49a098d2001-03-05 06:55:18 +0000179.Pa /usr/sbin/sshd .
Damien Miller6162d121999-11-21 13:23:52 +1100180.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000181The options are as follows:
182.Bl -tag -width Ds
183.It Fl b Ar bits
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000184Specifies the number of bits in the ephemeral protocol version 1
185server key (default 768).
Damien Miller32aa1441999-10-29 09:15:49 +1000186.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000187Debug mode.
188The server sends verbose debug output to the system
189log, and does not put itself in the background.
190The server also will not fork and will only process one connection.
191This option is only intended for debugging for the server.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000192Multiple -d options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100193Maximum is 3.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000194.It Fl e
195When this option is specified,
196.Nm
197will send the output to the standard error instead of the system log.
Damien Miller32aa1441999-10-29 09:15:49 +1000198.It Fl f Ar configuration_file
Damien Miller450a7a12000-03-26 13:04:51 +1000199Specifies the name of the configuration file.
200The default is
Damien Miller05eda432002-02-10 18:32:28 +1100201.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000202.Nm
203refuses to start if there is no configuration file.
204.It Fl g Ar login_grace_time
205Gives the grace time for clients to authenticate themselves (default
Kevin Steves9ce907c2001-01-07 11:53:40 +0000206600 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000207If the client fails to authenticate the user within
208this many seconds, the server disconnects and exits.
209A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000210.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100211Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000212This option must be given if
213.Nm
214is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100215host key files are normally not readable by anyone but root).
216The default is
Damien Miller05eda432002-02-10 18:32:28 +1100217.Pa /etc/ssh/ssh_host_key
Damien Miller7fc23732002-01-22 23:19:11 +1100218for protocol version 1, and
Damien Miller05eda432002-02-10 18:32:28 +1100219.Pa /etc/ssh/ssh_host_rsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100220and
Damien Miller05eda432002-02-10 18:32:28 +1100221.Pa /etc/ssh/ssh_host_dsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100222for protocol version 2.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000223It is possible to have multiple host key files for
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000224the different protocol versions and host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000225.It Fl i
226Specifies that
227.Nm
Damien Miller22c77262000-04-13 12:26:34 +1000228is being run from inetd.
Damien Miller32aa1441999-10-29 09:15:49 +1000229.Nm
230is normally not run
231from inetd because it needs to generate the server key before it can
Damien Miller450a7a12000-03-26 13:04:51 +1000232respond to the client, and this may take tens of seconds.
233Clients would have to wait too long if the key was regenerated every time.
Damien Miller7684ee12000-03-17 23:40:15 +1100234However, with small key sizes (e.g., 512) using
Damien Miller32aa1441999-10-29 09:15:49 +1000235.Nm
236from inetd may
237be feasible.
238.It Fl k Ar key_gen_time
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000239Specifies how often the ephemeral protocol version 1 server key is
240regenerated (default 3600 seconds, or one hour).
Damien Miller450a7a12000-03-26 13:04:51 +1000241The motivation for regenerating the key fairly
Damien Miller32aa1441999-10-29 09:15:49 +1000242often is that the key is not stored anywhere, and after about an hour,
243it becomes impossible to recover the key for decrypting intercepted
244communications even if the machine is cracked into or physically
Damien Miller450a7a12000-03-26 13:04:51 +1000245seized.
246A value of zero indicates that the key will never be regenerated.
Ben Lindstromade03f62001-12-06 18:22:17 +0000247.It Fl o Ar option
248Can be used to give options in the format used in the configuration file.
249This is useful for specifying options for which there is no separate
250command-line flag.
Damien Miller32aa1441999-10-29 09:15:49 +1000251.It Fl p Ar port
252Specifies the port on which the server listens for connections
253(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100254Multiple port options are permitted.
255Ports specified in the configuration file are ignored when a
256command-line port is specified.
Damien Miller32aa1441999-10-29 09:15:49 +1000257.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000258Quiet mode.
259Nothing is sent to the system log.
260Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000261authentication, and termination of each connection is logged.
Ben Lindstrom794325a2001-08-06 21:09:07 +0000262.It Fl t
263Test mode.
264Only check the validity of the configuration file and sanity of the keys.
Damien Miller9f0f5c62001-12-21 14:45:46 +1100265This is useful for updating
Ben Lindstrom794325a2001-08-06 21:09:07 +0000266.Nm
267reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000268.It Fl u Ar len
269This option is used to specify the size of the field
270in the
271.Li utmp
272structure that holds the remote host name.
273If the resolved host name is longer than
274.Ar len ,
275the dotted decimal value will be used instead.
276This allows hosts with very long host names that
277overflow this field to still be uniquely identified.
278Specifying
279.Fl u0
280indicates that only dotted decimal addresses
281should be put into the
282.Pa utmp
283file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000284.Fl u0
285is also be used to prevent
286.Nm
287from making DNS requests unless the authentication
288mechanism or configuration requires it.
289Authentication mechanisms that may require DNS include
290.Cm RhostsAuthentication ,
291.Cm RhostsRSAAuthentication ,
292.Cm HostbasedAuthentication
293and using a
294.Cm from="pattern-list"
295option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000296Configuration options that require DNS include using a
297USER@HOST pattern in
298.Cm AllowUsers
299or
300.Cm DenyUsers .
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000301.It Fl D
302When this option is specified
303.Nm
304will not detach and does not become a daemon.
305This allows easy monitoring of
306.Nm sshd .
Damien Miller34132e52000-01-14 15:45:46 +1100307.It Fl 4
308Forces
309.Nm
310to use IPv4 addresses only.
311.It Fl 6
312Forces
313.Nm
314to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000315.El
316.Sh CONFIGURATION FILE
317.Nm
Damien Miller22c77262000-04-13 12:26:34 +1000318reads configuration data from
Damien Miller05eda432002-02-10 18:32:28 +1100319.Pa /etc/ssh/sshd_config
Damien Miller32aa1441999-10-29 09:15:49 +1000320(or the file specified with
321.Fl f
Damien Miller450a7a12000-03-26 13:04:51 +1000322on the command line).
Ben Lindstrom9f049032002-06-21 00:59:05 +0000323The file format and configuration options are described in
324.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000325.Sh LOGIN PROCESS
326When a user successfully logs in,
327.Nm
328does the following:
329.Bl -enum -offset indent
330.It
331If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000332prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000333.Pa /etc/motd
334(unless prevented in the configuration file or by
335.Pa $HOME/.hushlogin ;
336see the
Damien Miller22c77262000-04-13 12:26:34 +1000337.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000338section).
339.It
340If the login is on a tty, records login time.
341.It
342Checks
343.Pa /etc/nologin ;
344if it exists, prints contents and quits
345(unless root).
346.It
347Changes to run with normal user privileges.
348.It
349Sets up basic environment.
350.It
351Reads
352.Pa $HOME/.ssh/environment
353if it exists.
354.It
355Changes to user's home directory.
356.It
357If
358.Pa $HOME/.ssh/rc
359exists, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100360.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000361exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000362it; otherwise runs xauth.
363The
Damien Miller32aa1441999-10-29 09:15:49 +1000364.Dq rc
365files are given the X11
366authentication protocol and cookie in standard input.
367.It
368Runs user's shell or command.
369.El
370.Sh AUTHORIZED_KEYS FILE FORMAT
Damien Miller32aa1441999-10-29 09:15:49 +1000371.Pa $HOME/.ssh/authorized_keys
Ben Lindstromf96704d2001-06-25 04:17:12 +0000372is the default file that lists the public keys that are
373permitted for RSA authentication in protocol version 1
374and for public key authentication (PubkeyAuthentication)
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000375in protocol version 2.
Ben Lindstromf96704d2001-06-25 04:17:12 +0000376.Cm AuthorizedKeysFile
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000377may be used to specify an alternative file.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000378.Pp
Damien Miller450a7a12000-03-26 13:04:51 +1000379Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000380key (empty lines and lines starting with a
381.Ql #
382are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000383comments).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000384Each RSA public key consists of the following fields, separated by
Damien Miller450a7a12000-03-26 13:04:51 +1000385spaces: options, bits, exponent, modulus, comment.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000386Each protocol version 2 public key consists of:
387options, keytype, base64 encoded key, comment.
388The options fields
389are optional; its presence is determined by whether the line starts
Damien Miller32aa1441999-10-29 09:15:49 +1000390with a number or not (the option field never starts with a number).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000391The bits, exponent, modulus and comment fields give the RSA key for
392protocol version 1; the
Damien Miller32aa1441999-10-29 09:15:49 +1000393comment field is not used for anything (but may be convenient for the
394user to identify the key).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000395For protocol version 2 the keytype is
396.Dq ssh-dss
397or
398.Dq ssh-rsa .
Damien Miller32aa1441999-10-29 09:15:49 +1000399.Pp
400Note that lines in this file are usually several hundred bytes long
Damien Miller450a7a12000-03-26 13:04:51 +1000401(because of the size of the RSA key modulus).
402You don't want to type them in; instead, copy the
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000403.Pa identity.pub ,
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000404.Pa id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000405or the
406.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000407file and edit it.
408.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000409.Nm
410enforces a minimum RSA key modulus size for protocol 1
411and protocol 2 keys of 768 bits.
412.Pp
Damien Miller942da032000-08-18 13:59:06 +1000413The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000414specifications.
415No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000416The following option specifications are supported (note
417that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000418.Bl -tag -width Ds
419.It Cm from="pattern-list"
420Specifies that in addition to RSA authentication, the canonical name
421of the remote host must be present in the comma-separated list of
Damien Miller450a7a12000-03-26 13:04:51 +1000422patterns
423.Pf ( Ql *
424and
425.Ql ?
426serve as wildcards).
427The list may also contain
428patterns negated by prefixing them with
429.Ql ! ;
430if the canonical host name matches a negated pattern, the key is not accepted.
431The purpose
Damien Miller32aa1441999-10-29 09:15:49 +1000432of this option is to optionally increase security: RSA authentication
433by itself does not trust the network or name servers or anything (but
434the key); however, if somebody somehow steals the key, the key
Damien Miller450a7a12000-03-26 13:04:51 +1000435permits an intruder to log in from anywhere in the world.
436This additional option makes using a stolen key more difficult (name
Damien Miller32aa1441999-10-29 09:15:49 +1000437servers and/or routers would have to be compromised in addition to
438just the key).
439.It Cm command="command"
440Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000441authentication.
442The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000443The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000444otherwise it is run without a tty.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000445If a 8-bit clean channel is required,
446one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100447.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000448A quote may be included in the command by quoting it with a backslash.
449This option might be useful
450to restrict certain RSA keys to perform just a specific operation.
451An example might be a key that permits remote backups but nothing else.
Damien Miller30c3d422000-05-09 11:02:59 +1000452Note that the client may specify TCP/IP and/or X11
453forwarding unless they are explicitly prohibited.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000454Note that this option applies to shell, command or subsystem execution.
Damien Miller32aa1441999-10-29 09:15:49 +1000455.It Cm environment="NAME=value"
456Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000457logging in using this key.
458Environment variables set this way
459override other default environment values.
460Multiple options of this type are permitted.
Ben Lindstrom38b951c2001-12-06 17:47:47 +0000461This option is automatically disabled if
462.Cm UseLogin
463is enabled.
Damien Miller32aa1441999-10-29 09:15:49 +1000464.It Cm no-port-forwarding
465Forbids TCP/IP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000466Any port forward requests by the client will return an error.
467This might be used, e.g., in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000468.Cm command
469option.
470.It Cm no-X11-forwarding
471Forbids X11 forwarding when this key is used for authentication.
472Any X11 forward requests by the client will return an error.
473.It Cm no-agent-forwarding
474Forbids authentication agent forwarding when this key is used for
475authentication.
476.It Cm no-pty
477Prevents tty allocation (a request to allocate a pty will fail).
Damien Millera243fde2001-03-19 23:16:08 +1100478.It Cm permitopen="host:port"
Ben Lindstrom24643222001-06-25 05:08:11 +0000479Limit local
Damien Millera243fde2001-03-19 23:16:08 +1100480.Li ``ssh -L''
Ben Lindstrom4b3564e2001-04-10 02:41:56 +0000481port forwarding such that it may only connect to the specified host and
Ben Lindstromd71ba572001-09-12 18:03:31 +0000482port.
483IPv6 addresses can be specified with an alternative syntax:
484.Ar host/port .
485Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100486.Cm permitopen
Ben Lindstrom24643222001-06-25 05:08:11 +0000487options may be applied separated by commas. No pattern matching is
488performed on the specified hostnames, they must be literal domains or
Damien Millera243fde2001-03-19 23:16:08 +1100489addresses.
Damien Miller32aa1441999-10-29 09:15:49 +1000490.El
491.Ss Examples
4921024 33 12121.\|.\|.\|312314325 ylo@foo.bar
493.Pp
494from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
495.Pp
496command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
Damien Millera243fde2001-03-19 23:16:08 +1100497.Pp
498permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
Damien Miller32aa1441999-10-29 09:15:49 +1000499.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000500The
Damien Miller05eda432002-02-10 18:32:28 +1100501.Pa /etc/ssh/ssh_known_hosts ,
Damien Miller22c77262000-04-13 12:26:34 +1000502and
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000503.Pa $HOME/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000504files contain host public keys for all known hosts.
505The global file should
506be prepared by the administrator (optional), and the per-user file is
Damien Miller942da032000-08-18 13:59:06 +1000507maintained automatically: whenever the user connects from an unknown host
Damien Miller450a7a12000-03-26 13:04:51 +1000508its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000509.Pp
510Each line in these files contains the following fields: hostnames,
Damien Miller450a7a12000-03-26 13:04:51 +1000511bits, exponent, modulus, comment.
512The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000513.Pp
514Hostnames is a comma-separated list of patterns ('*' and '?' act as
515wildcards); each pattern in turn is matched against the canonical host
516name (when authenticating a client) or against the user-supplied
Damien Miller450a7a12000-03-26 13:04:51 +1000517name (when authenticating a server).
518A pattern may also be preceded by
Damien Miller32aa1441999-10-29 09:15:49 +1000519.Ql !
520to indicate negation: if the host name matches a negated
521pattern, it is not accepted (by that line) even if it matched another
522pattern on the line.
523.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000524Bits, exponent, and modulus are taken directly from the RSA host key; they
Damien Miller32aa1441999-10-29 09:15:49 +1000525can be obtained, e.g., from
Damien Miller05eda432002-02-10 18:32:28 +1100526.Pa /etc/ssh/ssh_host_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000527The optional comment field continues to the end of the line, and is not used.
528.Pp
529Lines starting with
530.Ql #
531and empty lines are ignored as comments.
532.Pp
533When performing host authentication, authentication is accepted if any
Damien Miller450a7a12000-03-26 13:04:51 +1000534matching line has the proper key.
535It is thus permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000536recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000537names.
538This will inevitably happen when short forms of host names
539from different domains are put in the file.
540It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000541that the files contain conflicting information; authentication is
542accepted if valid information can be found from either file.
543.Pp
544Note that the lines in these files are typically hundreds of characters
545long, and you definitely don't want to type in the host keys by hand.
546Rather, generate them by a script
Damien Miller22c77262000-04-13 12:26:34 +1000547or by taking
Damien Miller05eda432002-02-10 18:32:28 +1100548.Pa /etc/ssh/ssh_host_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000549and adding the host names at the front.
550.Ss Examples
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000551.Bd -literal
552closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
553cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
554.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000555.Sh FILES
556.Bl -tag -width Ds
Damien Miller05eda432002-02-10 18:32:28 +1100557.It Pa /etc/ssh/sshd_config
Damien Miller32aa1441999-10-29 09:15:49 +1000558Contains configuration data for
559.Nm sshd .
Ben Lindstrom9f049032002-06-21 00:59:05 +0000560The file format and configuration options are described in
561.Xr sshd_config 5 .
Damien Miller05eda432002-02-10 18:32:28 +1100562.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000563These three files contain the private parts of the host keys.
Ben Lindstromd7f5b512001-03-05 06:57:23 +0000564These files should only be owned by root, readable only by root, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000565accessible to others.
566Note that
567.Nm
568does not start if this file is group/world-accessible.
Damien Miller05eda432002-02-10 18:32:28 +1100569.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000570These three files contain the public parts of the host keys.
Ben Lindstromd7f5b512001-03-05 06:57:23 +0000571These files should be world-readable but writable only by
Damien Miller450a7a12000-03-26 13:04:51 +1000572root.
Ben Lindstromd7f5b512001-03-05 06:57:23 +0000573Their contents should match the respective private parts.
574These files are not
575really used for anything; they are provided for the convenience of
576the user so their contents can be copied to known hosts files.
577These files are created using
Damien Miller32aa1441999-10-29 09:15:49 +1000578.Xr ssh-keygen 1 .
Ben Lindstromae1c51c2001-06-25 04:14:59 +0000579.It Pa /etc/moduli
Damien Millere39cacc2000-11-29 12:18:44 +1100580Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
Ben Lindstrom624e3f22002-06-23 00:32:57 +0000581.It Pa /var/empty
582.Xr chroot 2
583directory used by
584.Nm
585during privilege separation in the pre-authentication phase.
586The directory should not contain any files and must be owned by root
587and not group or world-writable.
Damien Miller886c63a2000-01-20 23:13:36 +1100588.It Pa /var/run/sshd.pid
Damien Miller32aa1441999-10-29 09:15:49 +1000589Contains the process ID of the
590.Nm
591listening for connections (if there are several daemons running
Ben Lindstrom959de992002-06-23 00:35:25 +0000592concurrently for different ports, this contains the process ID of the one
Damien Miller450a7a12000-03-26 13:04:51 +1000593started last).
Damien Miller942da032000-08-18 13:59:06 +1000594The content of this file is not sensitive; it can be world-readable.
Damien Miller32aa1441999-10-29 09:15:49 +1000595.It Pa $HOME/.ssh/authorized_keys
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000596Lists the public keys (RSA or DSA) that can be used to log into the user's account.
Damien Millere247cc42000-05-07 12:03:14 +1000597This file must be readable by root (which may on some machines imply
598it being world-readable if the user's home directory resides on an NFS
599volume).
600It is recommended that it not be accessible by others.
601The format of this file is described above.
602Users will place the contents of their
Ben Lindstromf96704d2001-06-25 04:17:12 +0000603.Pa identity.pub ,
Damien Millere247cc42000-05-07 12:03:14 +1000604.Pa id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000605and/or
606.Pa id_rsa.pub
Damien Millere247cc42000-05-07 12:03:14 +1000607files into this file, as described in
608.Xr ssh-keygen 1 .
Damien Miller05eda432002-02-10 18:32:28 +1100609.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
Damien Miller5ce662a1999-11-11 17:57:39 +1100610These files are consulted when using rhosts with RSA host
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000611authentication or protocol version 2 hostbased authentication
612to check the public key of the host.
Damien Miller450a7a12000-03-26 13:04:51 +1000613The key must be listed in one of these files to be accepted.
Damien Miller33e511e1999-11-11 11:43:13 +1100614The client uses the same files
Ben Lindstromebd888d2001-03-05 05:49:29 +0000615to verify that it is connecting to the correct remote host.
Damien Miller450a7a12000-03-26 13:04:51 +1000616These files should be writable only by root/the owner.
Damien Miller05eda432002-02-10 18:32:28 +1100617.Pa /etc/ssh/ssh_known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +1000618should be world-readable, and
619.Pa $HOME/.ssh/known_hosts
620can but need not be world-readable.
621.It Pa /etc/nologin
Damien Miller22c77262000-04-13 12:26:34 +1000622If this file exists,
Damien Miller32aa1441999-10-29 09:15:49 +1000623.Nm
Damien Miller450a7a12000-03-26 13:04:51 +1000624refuses to let anyone except root log in.
625The contents of the file
Damien Miller32aa1441999-10-29 09:15:49 +1000626are displayed to anyone trying to log in, and non-root connections are
Damien Miller450a7a12000-03-26 13:04:51 +1000627refused.
628The file should be world-readable.
Damien Miller32aa1441999-10-29 09:15:49 +1000629.It Pa /etc/hosts.allow, /etc/hosts.deny
Ben Lindstrom6149a6c2001-10-03 17:15:32 +0000630Access controls that should be enforced by tcp-wrappers are defined here.
631Further details are described in
Damien Miller32aa1441999-10-29 09:15:49 +1000632.Xr hosts_access 5 .
633.It Pa $HOME/.rhosts
634This file contains host-username pairs, separated by a space, one per
Damien Miller450a7a12000-03-26 13:04:51 +1000635line.
636The given user on the corresponding host is permitted to log in
637without password.
638The same file is used by rlogind and rshd.
Damien Miller32aa1441999-10-29 09:15:49 +1000639The file must
640be writable only by the user; it is recommended that it not be
641accessible by others.
642.Pp
Damien Miller450a7a12000-03-26 13:04:51 +1000643If is also possible to use netgroups in the file.
644Either host or user
Damien Miller32aa1441999-10-29 09:15:49 +1000645name may be of the form +@groupname to specify all hosts or all users
646in the group.
647.It Pa $HOME/.shosts
648For ssh,
649this file is exactly the same as for
650.Pa .rhosts .
651However, this file is
652not used by rlogin and rshd, so using this permits access using SSH only.
Damien Miller942da032000-08-18 13:59:06 +1000653.It Pa /etc/hosts.equiv
Damien Miller32aa1441999-10-29 09:15:49 +1000654This file is used during
655.Pa .rhosts
Damien Miller450a7a12000-03-26 13:04:51 +1000656authentication.
657In the simplest form, this file contains host names, one per line.
658Users on
Damien Miller32aa1441999-10-29 09:15:49 +1000659those hosts are permitted to log in without a password, provided they
Damien Miller450a7a12000-03-26 13:04:51 +1000660have the same user name on both machines.
661The host name may also be
Damien Miller32aa1441999-10-29 09:15:49 +1000662followed by a user name; such users are permitted to log in as
663.Em any
Damien Miller450a7a12000-03-26 13:04:51 +1000664user on this machine (except root).
665Additionally, the syntax
Damien Miller32aa1441999-10-29 09:15:49 +1000666.Dq +@group
Damien Miller450a7a12000-03-26 13:04:51 +1000667can be used to specify netgroups.
668Negated entries start with
Damien Miller32aa1441999-10-29 09:15:49 +1000669.Ql \&- .
670.Pp
671If the client host/user is successfully matched in this file, login is
672automatically permitted provided the client and server user names are the
Damien Miller450a7a12000-03-26 13:04:51 +1000673same.
674Additionally, successful RSA host authentication is normally required.
675This file must be writable only by root; it is recommended
Damien Miller32aa1441999-10-29 09:15:49 +1000676that it be world-readable.
677.Pp
678.Sy "Warning: It is almost never a good idea to use user names in"
679.Pa hosts.equiv .
680Beware that it really means that the named user(s) can log in as
681.Em anybody ,
682which includes bin, daemon, adm, and other accounts that own critical
Damien Miller450a7a12000-03-26 13:04:51 +1000683binaries and directories.
684Using a user name practically grants the user root access.
685The only valid use for user names that I can think
Damien Miller32aa1441999-10-29 09:15:49 +1000686of is in negative entries.
687.Pp
688Note that this warning also applies to rsh/rlogin.
Damien Miller886c63a2000-01-20 23:13:36 +1100689.It Pa /etc/shosts.equiv
Damien Miller32aa1441999-10-29 09:15:49 +1000690This is processed exactly as
691.Pa /etc/hosts.equiv .
692However, this file may be useful in environments that want to run both
693rsh/rlogin and ssh.
694.It Pa $HOME/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000695This file is read into the environment at login (if it exists).
696It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000697.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000698and assignment lines of the form name=value.
699The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000700only by the user; it need not be readable by anyone else.
701.It Pa $HOME/.ssh/rc
702If this file exists, it is run with /bin/sh after reading the
Damien Miller450a7a12000-03-26 13:04:51 +1000703environment files but before starting the user's shell or command.
Ben Lindstroma11e2702002-04-05 22:18:48 +0000704It must not produce any output on stdout; stderr must be used
705instead.
706If X11 forwarding is in use, it will receive the "proto cookie" pair in
707its standard input (and
Damien Miller32aa1441999-10-29 09:15:49 +1000708.Ev DISPLAY
Ben Lindstroma11e2702002-04-05 22:18:48 +0000709in its environment).
710The script must call
Damien Miller32aa1441999-10-29 09:15:49 +1000711.Xr xauth 1
Ben Lindstroma11e2702002-04-05 22:18:48 +0000712because
713.Nm
714will not run xauth automatically to add X11 cookies.
Damien Miller32aa1441999-10-29 09:15:49 +1000715.Pp
716The primary purpose of this file is to run any initialization routines
717which may be needed before the user's home directory becomes
718accessible; AFS is a particular example of such an environment.
719.Pp
720This file will probably contain some initialization code followed by
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000721something similar to:
722.Bd -literal
Ben Lindstroma11e2702002-04-05 22:18:48 +0000723if read proto cookie && [ -n "$DISPLAY" ]; then
724 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
725 # X11UseLocalhost=yes
726 xauth add unix:`echo $DISPLAY |
727 cut -c11-` $proto $cookie
728 else
729 # X11UseLocalhost=no
730 xauth add $DISPLAY $proto $cookie
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000731 fi
Ben Lindstroma11e2702002-04-05 22:18:48 +0000732fi
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000733.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000734.Pp
735If this file does not exist,
Damien Miller05eda432002-02-10 18:32:28 +1100736.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000737is run, and if that
Ben Lindstroma11e2702002-04-05 22:18:48 +0000738does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000739.Pp
740This file should be writable only by the user, and need not be
741readable by anyone else.
Damien Miller05eda432002-02-10 18:32:28 +1100742.It Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000743Like
744.Pa $HOME/.ssh/rc .
745This can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000746machine-specific login-time initializations globally.
747This file should be writable only by root, and should be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000748.El
Damien Miller0bc1bd82000-11-13 22:57:25 +1100749.Sh AUTHORS
Ben Lindstrom8eec2c82001-01-29 08:39:16 +0000750OpenSSH is a derivative of the original and free
751ssh 1.2.12 release by Tatu Ylonen.
752Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
753Theo de Raadt and Dug Song
754removed many bugs, re-added newer features and
755created OpenSSH.
756Markus Friedl contributed the support for SSH
757protocol versions 1.5 and 2.0.
Ben Lindstrom000dda52002-03-22 02:33:12 +0000758Niels Provos and Markus Friedl contributed support
759for privilege separation.
Damien Miller32aa1441999-10-29 09:15:49 +1000760.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000761.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100762.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000763.Xr ssh 1 ,
764.Xr ssh-add 1 ,
765.Xr ssh-agent 1 ,
766.Xr ssh-keygen 1 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000767.Xr login.conf 5 ,
768.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000769.Xr sshd_config 5 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000770.Xr sftp-server 8
Ben Lindstrom160ec622001-04-22 17:17:46 +0000771.Rs
772.%A T. Ylonen
773.%A T. Kivinen
774.%A M. Saarinen
775.%A T. Rinne
776.%A S. Lehtinen
777.%T "SSH Protocol Architecture"
Ben Lindstromf1813842002-03-27 17:18:31 +0000778.%N draft-ietf-secsh-architecture-12.txt
779.%D January 2002
Ben Lindstrom160ec622001-04-22 17:17:46 +0000780.%O work in progress material
781.Re
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000782.Rs
783.%A M. Friedl
784.%A N. Provos
785.%A W. A. Simpson
786.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
Ben Lindstromf1813842002-03-27 17:18:31 +0000787.%N draft-ietf-secsh-dh-group-exchange-02.txt
788.%D January 2002
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000789.%O work in progress material
790.Re