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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 Miller5d43d492006-08-30 11:07:00 +100037.\" $OpenBSD: sshd.8,v 1.234 2006/08/21 08:15:57 dtucker 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
Damien Millerffadc582003-02-24 11:52:26 +110046.Bk -words
Darren Tucker1f203942003-10-15 15:50:42 +100047.Op Fl 46Ddeiqt
Damien Miller32aa1441999-10-29 09:15:49 +100048.Op Fl b Ar bits
49.Op Fl f Ar config_file
50.Op Fl g Ar login_grace_time
51.Op Fl h Ar host_key_file
52.Op Fl k Ar key_gen_time
Ben Lindstromade03f62001-12-06 18:22:17 +000053.Op Fl o Ar option
Damien Miller32aa1441999-10-29 09:15:49 +100054.Op Fl p Ar port
Damien Miller942da032000-08-18 13:59:06 +100055.Op Fl u Ar len
Damien Millerffadc582003-02-24 11:52:26 +110056.Ek
Damien Miller22c77262000-04-13 12:26:34 +100057.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100058.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110059(OpenSSH Daemon) is the daemon program for
Damien Miller32aa1441999-10-29 09:15:49 +100060.Xr ssh 1 .
Damien Miller35dabd02000-05-01 21:10:33 +100061Together these programs replace rlogin and rsh, and
Damien Miller32aa1441999-10-29 09:15:49 +100062provide secure encrypted communications between two untrusted hosts
Damien Miller450a7a12000-03-26 13:04:51 +100063over an insecure network.
Damien Miller32aa1441999-10-29 09:15:49 +100064.Pp
65.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110066listens 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 Miller32aa1441999-10-29 09:15:49 +100074.Pp
75.Nm
Darren Tucker1f203942003-10-15 15:50:42 +100076can be configured using command-line options or a configuration file
77(by default
Damien Miller99cc4a82006-01-31 21:45:53 +110078.Xr sshd_config 5 ) ;
79command-line options override values specified in the
Damien Miller32aa1441999-10-29 09:15:49 +100080configuration file.
Damien Miller6162d121999-11-21 13:23:52 +110081.Nm
82rereads its configuration file when it receives a hangup signal,
Ben Lindstrom49a098d2001-03-05 06:55:18 +000083.Dv SIGHUP ,
Damien Miller208f1ed2006-03-15 11:56:03 +110084by executing itself with the name and options it was started with, e.g.\&
Ben Lindstrom49a098d2001-03-05 06:55:18 +000085.Pa /usr/sbin/sshd .
Damien Miller6162d121999-11-21 13:23:52 +110086.Pp
Damien Miller32aa1441999-10-29 09:15:49 +100087The options are as follows:
88.Bl -tag -width Ds
Darren Tucker1f203942003-10-15 15:50:42 +100089.It Fl 4
90Forces
91.Nm
92to use IPv4 addresses only.
93.It Fl 6
94Forces
95.Nm
96to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +100097.It Fl b Ar bits
Ben Lindstromc65e6a02001-04-23 13:02:16 +000098Specifies the number of bits in the ephemeral protocol version 1
99server key (default 768).
Darren Tucker1f203942003-10-15 15:50:42 +1000100.It Fl D
101When this option is specified,
102.Nm
103will not detach and does not become a daemon.
104This allows easy monitoring of
105.Nm sshd .
Damien Miller32aa1441999-10-29 09:15:49 +1000106.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000107Debug mode.
108The server sends verbose debug output to the system
109log, and does not put itself in the background.
110The server also will not fork and will only process one connection.
111This option is only intended for debugging for the server.
Damien Millerffadc582003-02-24 11:52:26 +1100112Multiple
113.Fl d
114options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100115Maximum is 3.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000116.It Fl e
117When this option is specified,
118.Nm
119will send the output to the standard error instead of the system log.
Damien Miller32aa1441999-10-29 09:15:49 +1000120.It Fl f Ar configuration_file
Damien Miller450a7a12000-03-26 13:04:51 +1000121Specifies the name of the configuration file.
122The default is
Damien Miller05eda432002-02-10 18:32:28 +1100123.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000124.Nm
125refuses to start if there is no configuration file.
126.It Fl g Ar login_grace_time
127Gives the grace time for clients to authenticate themselves (default
Damien Millerc1348632002-09-05 14:35:14 +1000128120 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000129If the client fails to authenticate the user within
130this many seconds, the server disconnects and exits.
131A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000132.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100133Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000134This option must be given if
135.Nm
136is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100137host key files are normally not readable by anyone but root).
138The default is
Damien Miller05eda432002-02-10 18:32:28 +1100139.Pa /etc/ssh/ssh_host_key
Damien Miller7fc23732002-01-22 23:19:11 +1100140for protocol version 1, and
Damien Miller05eda432002-02-10 18:32:28 +1100141.Pa /etc/ssh/ssh_host_rsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100142and
Damien Miller05eda432002-02-10 18:32:28 +1100143.Pa /etc/ssh/ssh_host_dsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100144for protocol version 2.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000145It is possible to have multiple host key files for
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000146the different protocol versions and host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000147.It Fl i
148Specifies that
149.Nm
Damien Millerffadc582003-02-24 11:52:26 +1100150is being run from
151.Xr inetd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +1000152.Nm
153is normally not run
154from inetd because it needs to generate the server key before it can
Damien Miller450a7a12000-03-26 13:04:51 +1000155respond to the client, and this may take tens of seconds.
156Clients would have to wait too long if the key was regenerated every time.
Damien Miller208f1ed2006-03-15 11:56:03 +1100157However, with small key sizes (e.g. 512) using
Damien Miller32aa1441999-10-29 09:15:49 +1000158.Nm
159from inetd may
160be feasible.
161.It Fl k Ar key_gen_time
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000162Specifies how often the ephemeral protocol version 1 server key is
163regenerated (default 3600 seconds, or one hour).
Damien Miller450a7a12000-03-26 13:04:51 +1000164The motivation for regenerating the key fairly
Darren Tucker1f203942003-10-15 15:50:42 +1000165often is that the key is not stored anywhere, and after about an hour
Damien Miller32aa1441999-10-29 09:15:49 +1000166it becomes impossible to recover the key for decrypting intercepted
167communications even if the machine is cracked into or physically
Damien Miller450a7a12000-03-26 13:04:51 +1000168seized.
169A value of zero indicates that the key will never be regenerated.
Ben Lindstromade03f62001-12-06 18:22:17 +0000170.It Fl o Ar option
171Can be used to give options in the format used in the configuration file.
172This is useful for specifying options for which there is no separate
173command-line flag.
Darren Tucker1f203942003-10-15 15:50:42 +1000174For full details of the options, and their values, see
175.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000176.It Fl p Ar port
177Specifies the port on which the server listens for connections
178(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100179Multiple port options are permitted.
Damien Millerd7f308f2005-12-24 14:55:16 +1100180Ports specified in the configuration file with the
181.Cm Port
182option are ignored when a command-line port is specified.
183Ports specified using the
184.Cm ListenAddress
185option override command-line ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000186.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000187Quiet mode.
188Nothing is sent to the system log.
189Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000190authentication, and termination of each connection is logged.
Ben Lindstrom794325a2001-08-06 21:09:07 +0000191.It Fl t
192Test mode.
193Only check the validity of the configuration file and sanity of the keys.
Damien Miller9f0f5c62001-12-21 14:45:46 +1100194This is useful for updating
Ben Lindstrom794325a2001-08-06 21:09:07 +0000195.Nm
196reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000197.It Fl u Ar len
198This option is used to specify the size of the field
199in the
200.Li utmp
201structure that holds the remote host name.
202If the resolved host name is longer than
203.Ar len ,
204the dotted decimal value will be used instead.
205This allows hosts with very long host names that
206overflow this field to still be uniquely identified.
207Specifying
208.Fl u0
209indicates that only dotted decimal addresses
210should be put into the
211.Pa utmp
212file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000213.Fl u0
Damien Millerffadc582003-02-24 11:52:26 +1100214may also be used to prevent
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000215.Nm
216from making DNS requests unless the authentication
217mechanism or configuration requires it.
218Authentication mechanisms that may require DNS include
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000219.Cm RhostsRSAAuthentication ,
Damien Miller2ac05772006-02-01 22:05:42 +1100220.Cm HostbasedAuthentication ,
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000221and using a
222.Cm from="pattern-list"
223option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000224Configuration options that require DNS include using a
225USER@HOST pattern in
226.Cm AllowUsers
227or
228.Cm DenyUsers .
Damien Miller32aa1441999-10-29 09:15:49 +1000229.El
Damien Miller8bbdf902006-02-01 22:05:25 +1100230.Sh AUTHENTICATION
231The OpenSSH SSH daemon supports SSH protocols 1 and 2.
232Both protocols are supported by default,
233though this can be changed via the
234.Cm Protocol
235option in
236.Xr sshd_config 5 .
237Protocol 2 supports both RSA and DSA keys;
238protocol 1 only supports RSA keys.
239For both protocols,
240each host has a host-specific key,
241normally 2048 bits,
242used to identify the host.
Damien Miller99cc4a82006-01-31 21:45:53 +1100243.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100244Forward security for protocol 1 is provided through
245an additional server key,
246normally 768 bits,
247generated when the server starts.
Damien Miller99cc4a82006-01-31 21:45:53 +1100248This key is normally regenerated every hour if it has been used, and
249is never stored on disk.
Damien Miller99cc4a82006-01-31 21:45:53 +1100250Whenever a client connects, the daemon responds with its public
251host and server keys.
252The client compares the
253RSA host key against its own database to verify that it has not changed.
254The client then generates a 256-bit random number.
255It encrypts this
256random number using both the host key and the server key, and sends
257the encrypted number to the server.
258Both sides then use this
259random number as a session key which is used to encrypt all further
260communications in the session.
261The rest of the session is encrypted
262using a conventional cipher, currently Blowfish or 3DES, with 3DES
263being used by default.
264The client selects the encryption algorithm
265to use from those offered by the server.
266.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100267For protocol 2,
268forward security is provided through a Diffie-Hellman key agreement.
269This key agreement results in a shared session key.
270The rest of the session is encrypted using a symmetric cipher, currently
271128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
272The client selects the encryption algorithm
273to use from those offered by the server.
274Additionally, session integrity is provided
275through a cryptographic message authentication code
276(hmac-sha1 or hmac-md5).
277.Pp
278Finally, the server and the client enter an authentication dialog.
Damien Miller99cc4a82006-01-31 21:45:53 +1100279The client tries to authenticate itself using
Damien Miller8bbdf902006-02-01 22:05:25 +1100280host-based authentication,
281public key authentication,
282challenge-response authentication,
283or password authentication.
Damien Miller99cc4a82006-01-31 21:45:53 +1100284.Pp
285Regardless of the authentication type, the account is checked to
286ensure that it is accessible. An account is not accessible if it is
287locked, listed in
288.Cm DenyUsers
289or its group is listed in
290.Cm DenyGroups
291\&. The definition of a locked account is system dependant. Some platforms
292have their own account database (eg AIX) and some modify the passwd field (
293.Ql \&*LK\&*
294on Solaris and UnixWare,
295.Ql \&*
296on HP-UX, containing
297.Ql Nologin
298on Tru64,
299a leading
300.Ql \&*LOCKED\&*
301on FreeBSD and a leading
302.Ql \&!!
303on Linux). If there is a requirement to disable password authentication
304for the account while allowing still public-key, then the passwd field
305should be set to something other than these values (eg
306.Ql NP
307or
308.Ql \&*NP\&*
309).
310.Pp
Damien Miller99cc4a82006-01-31 21:45:53 +1100311If the client successfully authenticates itself, a dialog for
312preparing the session is entered.
313At this time the client may request
314things like allocating a pseudo-tty, forwarding X11 connections,
315forwarding TCP connections, or forwarding the authentication agent
316connection over the secure channel.
317.Pp
Damien Millerc47d7e92006-03-15 11:27:20 +1100318After this, the client either requests a shell or execution of a command.
Damien Miller99cc4a82006-01-31 21:45:53 +1100319The sides then enter session mode.
320In this mode, either side may send
321data at any time, and such data is forwarded to/from the shell or
322command on the server side, and the user terminal in the client side.
323.Pp
324When the user program terminates and all forwarded X11 and other
325connections have been closed, the server sends command exit status to
326the client, and both sides exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000327.Sh LOGIN PROCESS
328When a user successfully logs in,
329.Nm
330does the following:
331.Bl -enum -offset indent
332.It
333If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000334prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000335.Pa /etc/motd
336(unless prevented in the configuration file or by
Damien Miller167ea5d2005-05-26 12:04:02 +1000337.Pa ~/.hushlogin ;
Damien Miller32aa1441999-10-29 09:15:49 +1000338see the
Damien Miller22c77262000-04-13 12:26:34 +1000339.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000340section).
341.It
342If the login is on a tty, records login time.
343.It
344Checks
345.Pa /etc/nologin ;
346if it exists, prints contents and quits
347(unless root).
348.It
349Changes to run with normal user privileges.
350.It
351Sets up basic environment.
352.It
Darren Tucker1f203942003-10-15 15:50:42 +1000353Reads the file
Damien Miller167ea5d2005-05-26 12:04:02 +1000354.Pa ~/.ssh/environment ,
Darren Tucker1f203942003-10-15 15:50:42 +1000355if it exists, and users are allowed to change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000356See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000357.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000358option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000359.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000360.It
361Changes to user's home directory.
362.It
363If
Damien Miller167ea5d2005-05-26 12:04:02 +1000364.Pa ~/.ssh/rc
Damien Miller32aa1441999-10-29 09:15:49 +1000365exists, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100366.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000367exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000368it; otherwise runs xauth.
369The
Damien Miller32aa1441999-10-29 09:15:49 +1000370.Dq rc
371files are given the X11
372authentication protocol and cookie in standard input.
Damien Millerfd725cf2006-03-15 11:35:54 +1100373See
374.Sx SSHRC ,
375below.
Damien Miller32aa1441999-10-29 09:15:49 +1000376.It
377Runs user's shell or command.
378.El
Damien Millerfd725cf2006-03-15 11:35:54 +1100379.Sh SSHRC
380If the file
381.Pa ~/.ssh/rc
382exists,
383.Xr sh 1
384runs it after reading the
385environment files but before starting the user's shell or command.
386It must not produce any output on stdout; stderr must be used
387instead.
388If X11 forwarding is in use, it will receive the "proto cookie" pair in
389its standard input (and
390.Ev DISPLAY
391in its environment).
392The script must call
393.Xr xauth 1
394because
395.Nm
396will not run xauth automatically to add X11 cookies.
397.Pp
398The primary purpose of this file is to run any initialization routines
399which may be needed before the user's home directory becomes
400accessible; AFS is a particular example of such an environment.
401.Pp
402This file will probably contain some initialization code followed by
403something similar to:
404.Bd -literal -offset 3n
405if read proto cookie && [ -n "$DISPLAY" ]; then
406 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
407 # X11UseLocalhost=yes
408 echo add unix:`echo $DISPLAY |
409 cut -c11-` $proto $cookie
410 else
411 # X11UseLocalhost=no
412 echo add $DISPLAY $proto $cookie
413 fi | xauth -q -
414fi
415.Ed
416.Pp
417If this file does not exist,
418.Pa /etc/ssh/sshrc
419is run, and if that
420does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000421.Sh AUTHORIZED_KEYS FILE FORMAT
Ben Lindstromf96704d2001-06-25 04:17:12 +0000422.Cm AuthorizedKeysFile
Damien Millerdcfea272006-03-15 11:31:22 +1100423specifies the file containing public keys for
424public key authentication;
425if none is specified, the default is
426.Pa ~/.ssh/authorized_keys .
Damien Miller450a7a12000-03-26 13:04:51 +1000427Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000428key (empty lines and lines starting with a
429.Ql #
430are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000431comments).
Damien Millerdcfea272006-03-15 11:31:22 +1100432Protocol 1 public keys consist of the following space-separated fields:
433options, bits, exponent, modulus, comment.
434Protocol 2 public key consist of:
435options, keytype, base64-encoded key, comment.
436The options field is optional;
437its presence is determined by whether the line starts
Damien Millerb83df8d2002-09-04 16:24:55 +1000438with a number or not (the options field never starts with a number).
Damien Millerdcfea272006-03-15 11:31:22 +1100439The bits, exponent, modulus, and comment fields give the RSA key for
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000440protocol version 1; the
Damien Miller32aa1441999-10-29 09:15:49 +1000441comment field is not used for anything (but may be convenient for the
442user to identify the key).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000443For protocol version 2 the keytype is
444.Dq ssh-dss
445or
446.Dq ssh-rsa .
Damien Miller32aa1441999-10-29 09:15:49 +1000447.Pp
448Note that lines in this file are usually several hundred bytes long
Darren Tucker22cc7412004-12-06 22:47:41 +1100449(because of the size of the public key encoding) up to a limit of
4508 kilobytes, which permits DSA keys up to 8 kilobits and RSA
451keys up to 16 kilobits.
Damien Miller450a7a12000-03-26 13:04:51 +1000452You don't want to type them in; instead, copy the
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000453.Pa identity.pub ,
Damien Millerdcfea272006-03-15 11:31:22 +1100454.Pa id_dsa.pub ,
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000455or the
456.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000457file and edit it.
458.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000459.Nm
460enforces a minimum RSA key modulus size for protocol 1
461and protocol 2 keys of 768 bits.
462.Pp
Damien Miller942da032000-08-18 13:59:06 +1000463The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000464specifications.
465No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000466The following option specifications are supported (note
467that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000468.Bl -tag -width Ds
Damien Miller32aa1441999-10-29 09:15:49 +1000469.It Cm command="command"
470Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000471authentication.
472The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000473The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000474otherwise it is run without a tty.
Damien Millerffadc582003-02-24 11:52:26 +1100475If an 8-bit clean channel is required,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000476one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100477.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000478A quote may be included in the command by quoting it with a backslash.
479This option might be useful
Damien Millerb83df8d2002-09-04 16:24:55 +1000480to restrict certain public keys to perform just a specific operation.
Damien Miller450a7a12000-03-26 13:04:51 +1000481An example might be a key that permits remote backups but nothing else.
Damien Miller7c24b812006-01-14 10:09:56 +1100482Note that the client may specify TCP and/or X11
Damien Miller30c3d422000-05-09 11:02:59 +1000483forwarding unless they are explicitly prohibited.
Damien Millere2754432006-07-24 14:06:47 +1000484The command originally supplied by the client is available in the
485.Ev SSH_ORIGINAL_COMMAND
486environment variable.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000487Note that this option applies to shell, command or subsystem execution.
Damien Miller32aa1441999-10-29 09:15:49 +1000488.It Cm environment="NAME=value"
489Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000490logging in using this key.
491Environment variables set this way
492override other default environment values.
493Multiple options of this type are permitted.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000494Environment processing is disabled by default and is
495controlled via the
496.Cm PermitUserEnvironment
497option.
Ben Lindstrom38b951c2001-12-06 17:47:47 +0000498This option is automatically disabled if
499.Cm UseLogin
500is enabled.
Damien Miller31bdc522006-03-15 11:31:44 +1100501.It Cm from="pattern-list"
502Specifies that in addition to public key authentication, the canonical name
503of the remote host must be present in the comma-separated list of
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100504patterns.
Damien Miller31bdc522006-03-15 11:31:44 +1100505The purpose
506of this option is to optionally increase security: public key authentication
507by itself does not trust the network or name servers or anything (but
508the key); however, if somebody somehow steals the key, the key
509permits an intruder to log in from anywhere in the world.
510This additional option makes using a stolen key more difficult (name
511servers and/or routers would have to be compromised in addition to
512just the key).
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100513.Pp
514See
515.Sx PATTERNS
516in
517.Xr ssh_config 5
518for more information on patterns.
Damien Miller31bdc522006-03-15 11:31:44 +1100519.It Cm no-agent-forwarding
520Forbids authentication agent forwarding when this key is used for
521authentication.
Damien Miller32aa1441999-10-29 09:15:49 +1000522.It Cm no-port-forwarding
Damien Miller7c24b812006-01-14 10:09:56 +1100523Forbids TCP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000524Any port forward requests by the client will return an error.
Damien Miller208f1ed2006-03-15 11:56:03 +1100525This might be used, e.g. in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000526.Cm command
527option.
Damien Miller31bdc522006-03-15 11:31:44 +1100528.It Cm no-pty
529Prevents tty allocation (a request to allocate a pty will fail).
Damien Miller32aa1441999-10-29 09:15:49 +1000530.It Cm no-X11-forwarding
531Forbids X11 forwarding when this key is used for authentication.
532Any X11 forward requests by the client will return an error.
Damien Millera243fde2001-03-19 23:16:08 +1100533.It Cm permitopen="host:port"
Ben Lindstrom24643222001-06-25 05:08:11 +0000534Limit local
Damien Millera243fde2001-03-19 23:16:08 +1100535.Li ``ssh -L''
Ben Lindstrom4b3564e2001-04-10 02:41:56 +0000536port forwarding such that it may only connect to the specified host and
Ben Lindstromd71ba572001-09-12 18:03:31 +0000537port.
538IPv6 addresses can be specified with an alternative syntax:
Darren Tucker1f203942003-10-15 15:50:42 +1000539.Ar host Ns / Ns Ar port .
Ben Lindstromd71ba572001-09-12 18:03:31 +0000540Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100541.Cm permitopen
Damien Millerfbf486b2003-05-23 18:44:23 +1000542options may be applied separated by commas.
543No pattern matching is performed on the specified hostnames,
544they must be literal domains or addresses.
Damien Millerd27b9472005-12-13 19:29:02 +1100545.It Cm tunnel="n"
546Force a
547.Xr tun 4
548device on the server.
549Without this option, the next available device will be used if
550the client requests a tunnel.
Damien Miller32aa1441999-10-29 09:15:49 +1000551.El
Damien Miller32aa1441999-10-29 09:15:49 +1000552.Pp
Damien Miller7d2ef022006-03-15 11:32:06 +1100553An example authorized_keys file:
Damien Millerc8f61cf2006-03-15 11:33:25 +1100554.Bd -literal -offset 3n
Damien Miller7d2ef022006-03-15 11:32:06 +1100555# Comments allowed at start of line
556ssh-rsa AAAAB3Nza...LiPk== user@example.net
Damien Millerc8f61cf2006-03-15 11:33:25 +1100557from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
558AAAAB2...19Q== john@example.net
559command="dump /home",no-pty,no-port-forwarding ssh-dss
560AAAAC3...51R== example.net
561permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
562AAAAB5...21S==
563tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
564jane@example.net
Damien Miller7d2ef022006-03-15 11:32:06 +1100565.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000566.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000567The
Damien Millerffadc582003-02-24 11:52:26 +1100568.Pa /etc/ssh/ssh_known_hosts
Damien Miller22c77262000-04-13 12:26:34 +1000569and
Damien Miller167ea5d2005-05-26 12:04:02 +1000570.Pa ~/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000571files contain host public keys for all known hosts.
572The global file should
573be prepared by the administrator (optional), and the per-user file is
Damien Miller9a7f2012006-03-15 11:32:42 +1100574maintained automatically: whenever the user connects from an unknown host,
Damien Miller450a7a12000-03-26 13:04:51 +1000575its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000576.Pp
577Each line in these files contains the following fields: hostnames,
Damien Miller450a7a12000-03-26 13:04:51 +1000578bits, exponent, modulus, comment.
579The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000580.Pp
Damien Miller31554322003-05-14 13:44:58 +1000581Hostnames is a comma-separated list of patterns
Damien Miller9a7f2012006-03-15 11:32:42 +1100582.Pf ( Ql *
Damien Millerfbf486b2003-05-23 18:44:23 +1000583and
584.Ql \&?
Damien Miller049245d2003-05-14 13:44:42 +1000585act as
Damien Miller32aa1441999-10-29 09:15:49 +1000586wildcards); each pattern in turn is matched against the canonical host
587name (when authenticating a client) or against the user-supplied
Damien Miller450a7a12000-03-26 13:04:51 +1000588name (when authenticating a server).
589A pattern may also be preceded by
Damien Miller049245d2003-05-14 13:44:42 +1000590.Ql \&!
Damien Miller32aa1441999-10-29 09:15:49 +1000591to indicate negation: if the host name matches a negated
592pattern, it is not accepted (by that line) even if it matched another
593pattern on the line.
Darren Tuckerda345532006-07-10 23:04:19 +1000594A hostname or address may optionally be enclosed within
595.Ql \&[
596and
597.Ql \&]
598brackets then followed by
599.Ql \&:
Darren Tucker11318472006-07-12 22:07:59 +1000600and a non-standard port number.
Damien Miller32aa1441999-10-29 09:15:49 +1000601.Pp
Damien Millere1776152005-03-01 21:47:37 +1100602Alternately, hostnames may be stored in a hashed form which hides host names
Damien Miller718fd4b2005-03-02 12:03:23 +1100603and addresses should the file's contents be disclosed.
604Hashed hostnames start with a
605.Ql |
Damien Millere1776152005-03-01 21:47:37 +1100606character.
607Only one hashed hostname may appear on a single line and none of the above
608negation or wildcard operators may be applied.
609.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000610Bits, exponent, and modulus are taken directly from the RSA host key; they
Damien Miller9a7f2012006-03-15 11:32:42 +1100611can be obtained, for example, from
Damien Miller05eda432002-02-10 18:32:28 +1100612.Pa /etc/ssh/ssh_host_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000613The optional comment field continues to the end of the line, and is not used.
614.Pp
615Lines starting with
616.Ql #
617and empty lines are ignored as comments.
618.Pp
619When performing host authentication, authentication is accepted if any
Damien Miller450a7a12000-03-26 13:04:51 +1000620matching line has the proper key.
621It is thus permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000622recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000623names.
624This will inevitably happen when short forms of host names
625from different domains are put in the file.
626It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000627that the files contain conflicting information; authentication is
628accepted if valid information can be found from either file.
629.Pp
630Note that the lines in these files are typically hundreds of characters
631long, and you definitely don't want to type in the host keys by hand.
632Rather, generate them by a script
Damien Miller22c77262000-04-13 12:26:34 +1000633or by taking
Damien Miller05eda432002-02-10 18:32:28 +1100634.Pa /etc/ssh/ssh_host_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000635and adding the host names at the front.
Damien Millercc00f5e2006-03-15 11:33:00 +1100636.Pp
637An example ssh_known_hosts file:
638.Bd -literal -offset 3n
639# Comments allowed at start of line
640closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
641cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
Damien Millere1776152005-03-01 21:47:37 +1100642# A hashed hostname
643|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
644AAAA1234.....=
645.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000646.Sh FILES
Damien Millerd8702e82006-03-15 11:33:56 +1100647.Bl -tag -width Ds -compact
648.It ~/.hushlogin
Damien Miller70a908e2005-03-01 21:17:09 +1100649This file is used to suppress printing the last login time and
650.Pa /etc/motd ,
651if
652.Cm PrintLastLog
653and
654.Cm PrintMotd ,
655respectively,
656are enabled.
657It does not suppress printing of the banner specified by
658.Cm Banner .
Damien Millerd8702e82006-03-15 11:33:56 +1100659.Pp
660.It ~/.rhosts
Damien Milleradc35b92006-03-15 11:35:27 +1100661This file is used for host-based authentication (see
662.Xr ssh 1
663for more information).
664On some machines this file may need to be
665world-readable if the user's home directory is on an NFS partition,
666because
667.Nm
668reads it as root.
669Additionally, this file must be owned by the user,
670and must not have write permissions for anyone else.
671The recommended
672permission for most machines is read/write for the user, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000673accessible by others.
674.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100675.It ~/.shosts
Damien Milleradc35b92006-03-15 11:35:27 +1100676This file is used in exactly the same way as
677.Pa .rhosts ,
678but allows host-based authentication without permitting login with
679rlogin/rsh.
Damien Miller32aa1441999-10-29 09:15:49 +1000680.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100681.It ~/.ssh/authorized_keys
Damien Milleradc35b92006-03-15 11:35:27 +1100682Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Millerd8702e82006-03-15 11:33:56 +1100683The format of this file is described above.
Damien Miller5d43d492006-08-30 11:07:00 +1000684The content of the file is not highly sensitive, but the recommended
Damien Milleradc35b92006-03-15 11:35:27 +1100685permissions are read/write for the user, and not accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000686.Pp
Damien Miller5d43d492006-08-30 11:07:00 +1000687If this file, the
688.Pa ~/.ssh
689directory, or the user's home directory are writable
690by other users, then the file could be modified or replaced by unauthorized
691users.
692In this case,
693.Nm
694will not allow it to be used unless the
695.Cm StrictModes
696option has been set to
697.Dq no .
698The recommended permissions can be set by executing
699.Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
700.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100701.It ~/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000702This file is read into the environment at login (if it exists).
703It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000704.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000705and assignment lines of the form name=value.
706The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000707only by the user; it need not be readable by anyone else.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000708Environment processing is disabled by default and is
709controlled via the
710.Cm PermitUserEnvironment
711option.
Damien Millerd8702e82006-03-15 11:33:56 +1100712.Pp
713.It ~/.ssh/known_hosts
Damien Milleradc35b92006-03-15 11:35:27 +1100714Contains a list of host keys for all hosts the user has logged into
715that are not already in the systemwide list of known host keys.
716The format of this file is described above.
717This file should be writable only by root/the owner and
Damien Millerd8702e82006-03-15 11:33:56 +1100718can, but need not be, world-readable.
719.Pp
720.It ~/.ssh/rc
Damien Millerfd725cf2006-03-15 11:35:54 +1100721Contains initialization routines to be run before
722the user's home directory becomes accessible.
Damien Miller32aa1441999-10-29 09:15:49 +1000723This file should be writable only by the user, and need not be
724readable by anyone else.
Damien Millerd8702e82006-03-15 11:33:56 +1100725.Pp
726.It /etc/hosts.allow
727.It /etc/hosts.deny
728Access controls that should be enforced by tcp-wrappers are defined here.
729Further details are described in
730.Xr hosts_access 5 .
731.Pp
732.It /etc/hosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100733This file is for host-based authentication (see
734.Xr ssh 1 ) .
735It should only be writable by root.
Damien Millerd8702e82006-03-15 11:33:56 +1100736.Pp
737.It /etc/moduli
738Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
739The file format is described in
740.Xr moduli 5 .
741.Pp
742.It /etc/motd
743See
744.Xr motd 5 .
745.Pp
746.It /etc/nologin
747If this file exists,
748.Nm
749refuses to let anyone except root log in.
750The contents of the file
751are displayed to anyone trying to log in, and non-root connections are
752refused.
753The file should be world-readable.
754.Pp
755.It /etc/shosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100756This file is used in exactly the same way as
757.Pa hosts.equiv ,
758but allows host-based authentication without permitting login with
759rlogin/rsh.
Damien Millerd8702e82006-03-15 11:33:56 +1100760.Pp
Damien Milleradc35b92006-03-15 11:35:27 +1100761.It /etc/ssh/ssh_known_hosts
762Systemwide list of known host keys.
763This file should be prepared by the
764system administrator to contain the public host keys of all machines in the
765organization.
766The format of this file is described above.
767This file should be writable only by root/the owner and
768should be world-readable.
769.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100770.It /etc/ssh/ssh_host_key
771.It /etc/ssh/ssh_host_dsa_key
772.It /etc/ssh/ssh_host_rsa_key
773These three files contain the private parts of the host keys.
774These files should only be owned by root, readable only by root, and not
775accessible to others.
776Note that
777.Nm
Damien Milleredd03752006-03-15 11:36:45 +1100778does not start if these files are group/world-accessible.
Damien Millerd8702e82006-03-15 11:33:56 +1100779.Pp
780.It /etc/ssh/ssh_host_key.pub
781.It /etc/ssh/ssh_host_dsa_key.pub
782.It /etc/ssh/ssh_host_rsa_key.pub
783These three files contain the public parts of the host keys.
784These files should be world-readable but writable only by
785root.
786Their contents should match the respective private parts.
787These files are not
788really used for anything; they are provided for the convenience of
789the user so their contents can be copied to known hosts files.
790These files are created using
791.Xr ssh-keygen 1 .
792.Pp
793.It /etc/ssh/sshd_config
794Contains configuration data for
795.Nm sshd .
796The file format and configuration options are described in
797.Xr sshd_config 5 .
798.Pp
799.It /etc/ssh/sshrc
Damien Millerfd725cf2006-03-15 11:35:54 +1100800Similar to
801.Pa ~/.ssh/rc ,
802it can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000803machine-specific login-time initializations globally.
804This file should be writable only by root, and should be world-readable.
Damien Millerd8702e82006-03-15 11:33:56 +1100805.Pp
806.It /var/empty
807.Xr chroot 2
808directory used by
809.Nm
810during privilege separation in the pre-authentication phase.
811The directory should not contain any files and must be owned by root
812and not group or world-writable.
813.Pp
814.It /var/run/sshd.pid
815Contains the process ID of the
816.Nm
817listening for connections (if there are several daemons running
818concurrently for different ports, this contains the process ID of the one
819started last).
820The content of this file is not sensitive; it can be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000821.El
Damien Miller32aa1441999-10-29 09:15:49 +1000822.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000823.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100824.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000825.Xr ssh 1 ,
826.Xr ssh-add 1 ,
827.Xr ssh-agent 1 ,
828.Xr ssh-keygen 1 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000829.Xr chroot 2 ,
830.Xr hosts_access 5 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000831.Xr login.conf 5 ,
832.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000833.Xr sshd_config 5 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000834.Xr inetd 8 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000835.Xr sftp-server 8
Damien Millerf1ce5052003-06-11 22:04:39 +1000836.Sh AUTHORS
837OpenSSH is a derivative of the original and free
838ssh 1.2.12 release by Tatu Ylonen.
839Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
840Theo de Raadt and Dug Song
841removed many bugs, re-added newer features and
842created OpenSSH.
843Markus Friedl contributed the support for SSH
844protocol versions 1.5 and 2.0.
845Niels Provos and Markus Friedl contributed support
846for privilege separation.
Damien Millerc47d7e92006-03-15 11:27:20 +1100847.Sh CAVEATS
848System security is not improved unless
849.Nm rshd ,
850.Nm rlogind ,
851and
852.Nm rexecd
853are disabled (thus completely disabling
854.Xr rlogin
855and
856.Xr rsh
857into the machine).