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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 Miller13ba9c22008-04-03 20:52:51 +110037.\" $OpenBSD: sshd.8,v 1.241 2008/03/27 22:37:57 jmc Exp $
38.Dd $Mdocdate: March 27 2008 $
Damien Miller32aa1441999-10-29 09:15:49 +100039.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 .
Darren Tucker04354b92007-03-21 20:46:54 +110061Together these programs replace
62.Xr rlogin 1
63and
64.Xr rsh 1 ,
65and provide secure encrypted communications between two untrusted hosts
Damien Miller450a7a12000-03-26 13:04:51 +100066over an insecure network.
Damien Miller32aa1441999-10-29 09:15:49 +100067.Pp
68.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110069listens for connections from clients.
Damien Miller22c77262000-04-13 12:26:34 +100070It is normally started at boot from
Damien Miller32aa1441999-10-29 09:15:49 +100071.Pa /etc/rc .
72It forks a new
Damien Miller450a7a12000-03-26 13:04:51 +100073daemon for each incoming connection.
74The forked daemons handle
Damien Miller32aa1441999-10-29 09:15:49 +100075key exchange, encryption, authentication, command execution,
76and data exchange.
Damien Miller32aa1441999-10-29 09:15:49 +100077.Pp
78.Nm
Darren Tucker1f203942003-10-15 15:50:42 +100079can be configured using command-line options or a configuration file
80(by default
Damien Miller99cc4a82006-01-31 21:45:53 +110081.Xr sshd_config 5 ) ;
82command-line options override values specified in the
Damien Miller32aa1441999-10-29 09:15:49 +100083configuration file.
Damien Miller6162d121999-11-21 13:23:52 +110084.Nm
85rereads its configuration file when it receives a hangup signal,
Ben Lindstrom49a098d2001-03-05 06:55:18 +000086.Dv SIGHUP ,
Damien Miller208f1ed2006-03-15 11:56:03 +110087by executing itself with the name and options it was started with, e.g.\&
Ben Lindstrom49a098d2001-03-05 06:55:18 +000088.Pa /usr/sbin/sshd .
Damien Miller6162d121999-11-21 13:23:52 +110089.Pp
Damien Miller32aa1441999-10-29 09:15:49 +100090The options are as follows:
91.Bl -tag -width Ds
Darren Tucker1f203942003-10-15 15:50:42 +100092.It Fl 4
93Forces
94.Nm
95to use IPv4 addresses only.
96.It Fl 6
97Forces
98.Nm
99to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000100.It Fl b Ar bits
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000101Specifies the number of bits in the ephemeral protocol version 1
102server key (default 768).
Darren Tucker1f203942003-10-15 15:50:42 +1000103.It Fl D
104When this option is specified,
105.Nm
106will not detach and does not become a daemon.
107This allows easy monitoring of
108.Nm sshd .
Damien Miller32aa1441999-10-29 09:15:49 +1000109.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000110Debug mode.
111The server sends verbose debug output to the system
112log, and does not put itself in the background.
113The server also will not fork and will only process one connection.
114This option is only intended for debugging for the server.
Damien Millerffadc582003-02-24 11:52:26 +1100115Multiple
116.Fl d
117options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100118Maximum is 3.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000119.It Fl e
120When this option is specified,
121.Nm
122will send the output to the standard error instead of the system log.
Darren Tucker04354b92007-03-21 20:46:54 +1100123.It Fl f Ar config_file
Damien Miller450a7a12000-03-26 13:04:51 +1000124Specifies the name of the configuration file.
125The default is
Damien Miller05eda432002-02-10 18:32:28 +1100126.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000127.Nm
128refuses to start if there is no configuration file.
129.It Fl g Ar login_grace_time
130Gives the grace time for clients to authenticate themselves (default
Damien Millerc1348632002-09-05 14:35:14 +1000131120 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000132If the client fails to authenticate the user within
133this many seconds, the server disconnects and exits.
134A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000135.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100136Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000137This option must be given if
138.Nm
139is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100140host key files are normally not readable by anyone but root).
141The default is
Damien Miller05eda432002-02-10 18:32:28 +1100142.Pa /etc/ssh/ssh_host_key
Damien Miller7fc23732002-01-22 23:19:11 +1100143for protocol version 1, and
Damien Miller05eda432002-02-10 18:32:28 +1100144.Pa /etc/ssh/ssh_host_rsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100145and
Damien Miller05eda432002-02-10 18:32:28 +1100146.Pa /etc/ssh/ssh_host_dsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100147for protocol version 2.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000148It is possible to have multiple host key files for
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000149the different protocol versions and host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000150.It Fl i
151Specifies that
152.Nm
Damien Millerffadc582003-02-24 11:52:26 +1100153is being run from
154.Xr inetd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +1000155.Nm
156is normally not run
157from inetd because it needs to generate the server key before it can
Damien Miller450a7a12000-03-26 13:04:51 +1000158respond to the client, and this may take tens of seconds.
159Clients would have to wait too long if the key was regenerated every time.
Damien Miller208f1ed2006-03-15 11:56:03 +1100160However, with small key sizes (e.g. 512) using
Damien Miller32aa1441999-10-29 09:15:49 +1000161.Nm
162from inetd may
163be feasible.
164.It Fl k Ar key_gen_time
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000165Specifies how often the ephemeral protocol version 1 server key is
166regenerated (default 3600 seconds, or one hour).
Damien Miller450a7a12000-03-26 13:04:51 +1000167The motivation for regenerating the key fairly
Darren Tucker1f203942003-10-15 15:50:42 +1000168often is that the key is not stored anywhere, and after about an hour
Damien Miller32aa1441999-10-29 09:15:49 +1000169it becomes impossible to recover the key for decrypting intercepted
170communications even if the machine is cracked into or physically
Damien Miller450a7a12000-03-26 13:04:51 +1000171seized.
172A value of zero indicates that the key will never be regenerated.
Ben Lindstromade03f62001-12-06 18:22:17 +0000173.It Fl o Ar option
174Can be used to give options in the format used in the configuration file.
175This is useful for specifying options for which there is no separate
176command-line flag.
Darren Tucker1f203942003-10-15 15:50:42 +1000177For full details of the options, and their values, see
178.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000179.It Fl p Ar port
180Specifies the port on which the server listens for connections
181(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100182Multiple port options are permitted.
Damien Millerd7f308f2005-12-24 14:55:16 +1100183Ports specified in the configuration file with the
184.Cm Port
185option are ignored when a command-line port is specified.
186Ports specified using the
187.Cm ListenAddress
188option override command-line ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000189.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000190Quiet mode.
191Nothing is sent to the system log.
192Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000193authentication, and termination of each connection is logged.
Ben Lindstrom794325a2001-08-06 21:09:07 +0000194.It Fl t
195Test mode.
196Only check the validity of the configuration file and sanity of the keys.
Damien Miller9f0f5c62001-12-21 14:45:46 +1100197This is useful for updating
Ben Lindstrom794325a2001-08-06 21:09:07 +0000198.Nm
199reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000200.It Fl u Ar len
201This option is used to specify the size of the field
202in the
203.Li utmp
204structure that holds the remote host name.
205If the resolved host name is longer than
206.Ar len ,
207the dotted decimal value will be used instead.
208This allows hosts with very long host names that
209overflow this field to still be uniquely identified.
210Specifying
211.Fl u0
212indicates that only dotted decimal addresses
213should be put into the
214.Pa utmp
215file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000216.Fl u0
Damien Millerffadc582003-02-24 11:52:26 +1100217may also be used to prevent
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000218.Nm
219from making DNS requests unless the authentication
220mechanism or configuration requires it.
221Authentication mechanisms that may require DNS include
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000222.Cm RhostsRSAAuthentication ,
Damien Miller2ac05772006-02-01 22:05:42 +1100223.Cm HostbasedAuthentication ,
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000224and using a
225.Cm from="pattern-list"
226option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000227Configuration options that require DNS include using a
228USER@HOST pattern in
229.Cm AllowUsers
230or
231.Cm DenyUsers .
Damien Miller32aa1441999-10-29 09:15:49 +1000232.El
Damien Miller8bbdf902006-02-01 22:05:25 +1100233.Sh AUTHENTICATION
234The OpenSSH SSH daemon supports SSH protocols 1 and 2.
235Both protocols are supported by default,
236though this can be changed via the
237.Cm Protocol
238option in
239.Xr sshd_config 5 .
240Protocol 2 supports both RSA and DSA keys;
241protocol 1 only supports RSA keys.
242For both protocols,
243each host has a host-specific key,
244normally 2048 bits,
245used to identify the host.
Damien Miller99cc4a82006-01-31 21:45:53 +1100246.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100247Forward security for protocol 1 is provided through
248an additional server key,
249normally 768 bits,
250generated when the server starts.
Damien Miller99cc4a82006-01-31 21:45:53 +1100251This key is normally regenerated every hour if it has been used, and
252is never stored on disk.
Damien Miller99cc4a82006-01-31 21:45:53 +1100253Whenever a client connects, the daemon responds with its public
254host and server keys.
255The client compares the
256RSA host key against its own database to verify that it has not changed.
257The client then generates a 256-bit random number.
258It encrypts this
259random number using both the host key and the server key, and sends
260the encrypted number to the server.
261Both sides then use this
262random number as a session key which is used to encrypt all further
263communications in the session.
264The rest of the session is encrypted
265using a conventional cipher, currently Blowfish or 3DES, with 3DES
266being used by default.
267The client selects the encryption algorithm
268to use from those offered by the server.
269.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100270For protocol 2,
271forward security is provided through a Diffie-Hellman key agreement.
272This key agreement results in a shared session key.
273The rest of the session is encrypted using a symmetric cipher, currently
274128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
275The client selects the encryption algorithm
276to use from those offered by the server.
277Additionally, session integrity is provided
278through a cryptographic message authentication code
Damien Millere45796f2007-06-11 14:01:42 +1000279(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
Damien Miller8bbdf902006-02-01 22:05:25 +1100280.Pp
281Finally, the server and the client enter an authentication dialog.
Damien Miller99cc4a82006-01-31 21:45:53 +1100282The client tries to authenticate itself using
Damien Miller8bbdf902006-02-01 22:05:25 +1100283host-based authentication,
284public key authentication,
285challenge-response authentication,
286or password authentication.
Damien Miller99cc4a82006-01-31 21:45:53 +1100287.Pp
288Regardless of the authentication type, the account is checked to
289ensure that it is accessible. An account is not accessible if it is
290locked, listed in
291.Cm DenyUsers
292or its group is listed in
293.Cm DenyGroups
294\&. The definition of a locked account is system dependant. Some platforms
295have their own account database (eg AIX) and some modify the passwd field (
296.Ql \&*LK\&*
297on Solaris and UnixWare,
298.Ql \&*
299on HP-UX, containing
300.Ql Nologin
301on Tru64,
302a leading
303.Ql \&*LOCKED\&*
304on FreeBSD and a leading
Darren Tucker1a9176b2007-08-17 09:42:32 +1000305.Ql \&!
306on most Linuxes).
307If there is a requirement to disable password authentication
Damien Miller99cc4a82006-01-31 21:45:53 +1100308for the account while allowing still public-key, then the passwd field
309should be set to something other than these values (eg
310.Ql NP
311or
312.Ql \&*NP\&*
313).
314.Pp
Damien Miller99cc4a82006-01-31 21:45:53 +1100315If the client successfully authenticates itself, a dialog for
316preparing the session is entered.
317At this time the client may request
318things like allocating a pseudo-tty, forwarding X11 connections,
319forwarding TCP connections, or forwarding the authentication agent
320connection over the secure channel.
321.Pp
Damien Millerc47d7e92006-03-15 11:27:20 +1100322After this, the client either requests a shell or execution of a command.
Damien Miller99cc4a82006-01-31 21:45:53 +1100323The sides then enter session mode.
324In this mode, either side may send
325data at any time, and such data is forwarded to/from the shell or
326command on the server side, and the user terminal in the client side.
327.Pp
328When the user program terminates and all forwarded X11 and other
329connections have been closed, the server sends command exit status to
330the client, and both sides exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000331.Sh LOGIN PROCESS
332When a user successfully logs in,
333.Nm
334does the following:
335.Bl -enum -offset indent
336.It
337If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000338prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000339.Pa /etc/motd
340(unless prevented in the configuration file or by
Damien Miller167ea5d2005-05-26 12:04:02 +1000341.Pa ~/.hushlogin ;
Damien Miller32aa1441999-10-29 09:15:49 +1000342see the
Damien Miller22c77262000-04-13 12:26:34 +1000343.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000344section).
345.It
346If the login is on a tty, records login time.
347.It
348Checks
349.Pa /etc/nologin ;
350if it exists, prints contents and quits
351(unless root).
352.It
353Changes to run with normal user privileges.
354.It
355Sets up basic environment.
356.It
Darren Tucker1f203942003-10-15 15:50:42 +1000357Reads the file
Damien Miller167ea5d2005-05-26 12:04:02 +1000358.Pa ~/.ssh/environment ,
Darren Tucker1f203942003-10-15 15:50:42 +1000359if it exists, and users are allowed to change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000360See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000361.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000362option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000363.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000364.It
365Changes to user's home directory.
366.It
367If
Damien Miller167ea5d2005-05-26 12:04:02 +1000368.Pa ~/.ssh/rc
Damien Miller32aa1441999-10-29 09:15:49 +1000369exists, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100370.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000371exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000372it; otherwise runs xauth.
373The
Damien Miller32aa1441999-10-29 09:15:49 +1000374.Dq rc
375files are given the X11
376authentication protocol and cookie in standard input.
Damien Millerfd725cf2006-03-15 11:35:54 +1100377See
378.Sx SSHRC ,
379below.
Damien Miller32aa1441999-10-29 09:15:49 +1000380.It
381Runs user's shell or command.
382.El
Damien Millerfd725cf2006-03-15 11:35:54 +1100383.Sh SSHRC
384If the file
385.Pa ~/.ssh/rc
386exists,
387.Xr sh 1
388runs it after reading the
389environment files but before starting the user's shell or command.
390It must not produce any output on stdout; stderr must be used
391instead.
392If X11 forwarding is in use, it will receive the "proto cookie" pair in
393its standard input (and
394.Ev DISPLAY
395in its environment).
396The script must call
397.Xr xauth 1
398because
399.Nm
400will not run xauth automatically to add X11 cookies.
401.Pp
402The primary purpose of this file is to run any initialization routines
403which may be needed before the user's home directory becomes
404accessible; AFS is a particular example of such an environment.
405.Pp
406This file will probably contain some initialization code followed by
407something similar to:
408.Bd -literal -offset 3n
409if read proto cookie && [ -n "$DISPLAY" ]; then
410 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
411 # X11UseLocalhost=yes
412 echo add unix:`echo $DISPLAY |
413 cut -c11-` $proto $cookie
414 else
415 # X11UseLocalhost=no
416 echo add $DISPLAY $proto $cookie
417 fi | xauth -q -
418fi
419.Ed
420.Pp
421If this file does not exist,
422.Pa /etc/ssh/sshrc
423is run, and if that
424does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000425.Sh AUTHORIZED_KEYS FILE FORMAT
Ben Lindstromf96704d2001-06-25 04:17:12 +0000426.Cm AuthorizedKeysFile
Damien Millerdcfea272006-03-15 11:31:22 +1100427specifies the file containing public keys for
428public key authentication;
429if none is specified, the default is
430.Pa ~/.ssh/authorized_keys .
Damien Miller450a7a12000-03-26 13:04:51 +1000431Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000432key (empty lines and lines starting with a
433.Ql #
434are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000435comments).
Damien Millerdcfea272006-03-15 11:31:22 +1100436Protocol 1 public keys consist of the following space-separated fields:
437options, bits, exponent, modulus, comment.
438Protocol 2 public key consist of:
439options, keytype, base64-encoded key, comment.
440The options field is optional;
441its presence is determined by whether the line starts
Damien Millerb83df8d2002-09-04 16:24:55 +1000442with a number or not (the options field never starts with a number).
Damien Millerdcfea272006-03-15 11:31:22 +1100443The bits, exponent, modulus, and comment fields give the RSA key for
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000444protocol version 1; the
Damien Miller32aa1441999-10-29 09:15:49 +1000445comment field is not used for anything (but may be convenient for the
446user to identify the key).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000447For protocol version 2 the keytype is
448.Dq ssh-dss
449or
450.Dq ssh-rsa .
Damien Miller32aa1441999-10-29 09:15:49 +1000451.Pp
452Note that lines in this file are usually several hundred bytes long
Darren Tucker22cc7412004-12-06 22:47:41 +1100453(because of the size of the public key encoding) up to a limit of
4548 kilobytes, which permits DSA keys up to 8 kilobits and RSA
455keys up to 16 kilobits.
Damien Miller450a7a12000-03-26 13:04:51 +1000456You don't want to type them in; instead, copy the
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000457.Pa identity.pub ,
Damien Millerdcfea272006-03-15 11:31:22 +1100458.Pa id_dsa.pub ,
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000459or the
460.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000461file and edit it.
462.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000463.Nm
464enforces a minimum RSA key modulus size for protocol 1
465and protocol 2 keys of 768 bits.
466.Pp
Damien Miller942da032000-08-18 13:59:06 +1000467The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000468specifications.
469No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000470The following option specifications are supported (note
471that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000472.Bl -tag -width Ds
Damien Miller32aa1441999-10-29 09:15:49 +1000473.It Cm command="command"
474Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000475authentication.
476The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000477The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000478otherwise it is run without a tty.
Damien Millerffadc582003-02-24 11:52:26 +1100479If an 8-bit clean channel is required,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000480one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100481.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000482A quote may be included in the command by quoting it with a backslash.
483This option might be useful
Damien Millerb83df8d2002-09-04 16:24:55 +1000484to restrict certain public keys to perform just a specific operation.
Damien Miller450a7a12000-03-26 13:04:51 +1000485An example might be a key that permits remote backups but nothing else.
Damien Miller7c24b812006-01-14 10:09:56 +1100486Note that the client may specify TCP and/or X11
Damien Miller30c3d422000-05-09 11:02:59 +1000487forwarding unless they are explicitly prohibited.
Damien Millere2754432006-07-24 14:06:47 +1000488The command originally supplied by the client is available in the
489.Ev SSH_ORIGINAL_COMMAND
490environment variable.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000491Note that this option applies to shell, command or subsystem execution.
Damien Miller32aa1441999-10-29 09:15:49 +1000492.It Cm environment="NAME=value"
493Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000494logging in using this key.
495Environment variables set this way
496override other default environment values.
497Multiple options of this type are permitted.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000498Environment processing is disabled by default and is
499controlled via the
500.Cm PermitUserEnvironment
501option.
Ben Lindstrom38b951c2001-12-06 17:47:47 +0000502This option is automatically disabled if
503.Cm UseLogin
504is enabled.
Damien Miller31bdc522006-03-15 11:31:44 +1100505.It Cm from="pattern-list"
506Specifies that in addition to public key authentication, the canonical name
507of the remote host must be present in the comma-separated list of
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100508patterns.
Damien Miller31bdc522006-03-15 11:31:44 +1100509The purpose
510of this option is to optionally increase security: public key authentication
511by itself does not trust the network or name servers or anything (but
512the key); however, if somebody somehow steals the key, the key
513permits an intruder to log in from anywhere in the world.
514This additional option makes using a stolen key more difficult (name
515servers and/or routers would have to be compromised in addition to
516just the key).
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100517.Pp
518See
519.Sx PATTERNS
520in
521.Xr ssh_config 5
522for more information on patterns.
Damien Miller31bdc522006-03-15 11:31:44 +1100523.It Cm no-agent-forwarding
524Forbids authentication agent forwarding when this key is used for
525authentication.
Damien Miller32aa1441999-10-29 09:15:49 +1000526.It Cm no-port-forwarding
Damien Miller7c24b812006-01-14 10:09:56 +1100527Forbids TCP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000528Any port forward requests by the client will return an error.
Damien Miller208f1ed2006-03-15 11:56:03 +1100529This might be used, e.g. in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000530.Cm command
531option.
Damien Miller31bdc522006-03-15 11:31:44 +1100532.It Cm no-pty
533Prevents tty allocation (a request to allocate a pty will fail).
Damien Miller95e80952008-03-27 11:03:05 +1100534.It Cm no-user-rc
Damien Miller13ba9c22008-04-03 20:52:51 +1100535Disables execution of
Damien Miller95e80952008-03-27 11:03:05 +1100536.Pa ~/.ssh/rc .
Damien Miller32aa1441999-10-29 09:15:49 +1000537.It Cm no-X11-forwarding
538Forbids X11 forwarding when this key is used for authentication.
539Any X11 forward requests by the client will return an error.
Damien Millera243fde2001-03-19 23:16:08 +1100540.It Cm permitopen="host:port"
Ben Lindstrom24643222001-06-25 05:08:11 +0000541Limit local
Damien Millera243fde2001-03-19 23:16:08 +1100542.Li ``ssh -L''
Ben Lindstrom4b3564e2001-04-10 02:41:56 +0000543port forwarding such that it may only connect to the specified host and
Ben Lindstromd71ba572001-09-12 18:03:31 +0000544port.
545IPv6 addresses can be specified with an alternative syntax:
Darren Tucker1f203942003-10-15 15:50:42 +1000546.Ar host Ns / Ns Ar port .
Ben Lindstromd71ba572001-09-12 18:03:31 +0000547Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100548.Cm permitopen
Damien Millerfbf486b2003-05-23 18:44:23 +1000549options may be applied separated by commas.
550No pattern matching is performed on the specified hostnames,
551they must be literal domains or addresses.
Damien Millerd27b9472005-12-13 19:29:02 +1100552.It Cm tunnel="n"
553Force a
554.Xr tun 4
555device on the server.
556Without this option, the next available device will be used if
557the client requests a tunnel.
Damien Miller32aa1441999-10-29 09:15:49 +1000558.El
Damien Miller32aa1441999-10-29 09:15:49 +1000559.Pp
Damien Miller7d2ef022006-03-15 11:32:06 +1100560An example authorized_keys file:
Damien Millerc8f61cf2006-03-15 11:33:25 +1100561.Bd -literal -offset 3n
Damien Miller7d2ef022006-03-15 11:32:06 +1100562# Comments allowed at start of line
563ssh-rsa AAAAB3Nza...LiPk== user@example.net
Damien Millerc8f61cf2006-03-15 11:33:25 +1100564from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
565AAAAB2...19Q== john@example.net
566command="dump /home",no-pty,no-port-forwarding ssh-dss
567AAAAC3...51R== example.net
568permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
569AAAAB5...21S==
570tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
571jane@example.net
Damien Miller7d2ef022006-03-15 11:32:06 +1100572.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000573.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000574The
Damien Millerffadc582003-02-24 11:52:26 +1100575.Pa /etc/ssh/ssh_known_hosts
Damien Miller22c77262000-04-13 12:26:34 +1000576and
Damien Miller167ea5d2005-05-26 12:04:02 +1000577.Pa ~/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000578files contain host public keys for all known hosts.
579The global file should
580be prepared by the administrator (optional), and the per-user file is
Damien Miller9a7f2012006-03-15 11:32:42 +1100581maintained automatically: whenever the user connects from an unknown host,
Damien Miller450a7a12000-03-26 13:04:51 +1000582its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000583.Pp
584Each line in these files contains the following fields: hostnames,
Damien Miller450a7a12000-03-26 13:04:51 +1000585bits, exponent, modulus, comment.
586The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000587.Pp
Damien Miller31554322003-05-14 13:44:58 +1000588Hostnames is a comma-separated list of patterns
Damien Miller9a7f2012006-03-15 11:32:42 +1100589.Pf ( Ql *
Damien Millerfbf486b2003-05-23 18:44:23 +1000590and
591.Ql \&?
Damien Miller049245d2003-05-14 13:44:42 +1000592act as
Damien Miller32aa1441999-10-29 09:15:49 +1000593wildcards); each pattern in turn is matched against the canonical host
594name (when authenticating a client) or against the user-supplied
Damien Miller450a7a12000-03-26 13:04:51 +1000595name (when authenticating a server).
596A pattern may also be preceded by
Damien Miller049245d2003-05-14 13:44:42 +1000597.Ql \&!
Damien Miller32aa1441999-10-29 09:15:49 +1000598to indicate negation: if the host name matches a negated
599pattern, it is not accepted (by that line) even if it matched another
600pattern on the line.
Darren Tuckerda345532006-07-10 23:04:19 +1000601A hostname or address may optionally be enclosed within
602.Ql \&[
603and
604.Ql \&]
605brackets then followed by
606.Ql \&:
Darren Tucker11318472006-07-12 22:07:59 +1000607and a non-standard port number.
Damien Miller32aa1441999-10-29 09:15:49 +1000608.Pp
Damien Millere1776152005-03-01 21:47:37 +1100609Alternately, hostnames may be stored in a hashed form which hides host names
Damien Miller718fd4b2005-03-02 12:03:23 +1100610and addresses should the file's contents be disclosed.
611Hashed hostnames start with a
612.Ql |
Damien Millere1776152005-03-01 21:47:37 +1100613character.
614Only one hashed hostname may appear on a single line and none of the above
615negation or wildcard operators may be applied.
616.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000617Bits, exponent, and modulus are taken directly from the RSA host key; they
Damien Miller9a7f2012006-03-15 11:32:42 +1100618can be obtained, for example, from
Damien Miller05eda432002-02-10 18:32:28 +1100619.Pa /etc/ssh/ssh_host_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000620The optional comment field continues to the end of the line, and is not used.
621.Pp
622Lines starting with
623.Ql #
624and empty lines are ignored as comments.
625.Pp
626When performing host authentication, authentication is accepted if any
Damien Miller450a7a12000-03-26 13:04:51 +1000627matching line has the proper key.
628It is thus permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000629recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000630names.
631This will inevitably happen when short forms of host names
632from different domains are put in the file.
633It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000634that the files contain conflicting information; authentication is
635accepted if valid information can be found from either file.
636.Pp
637Note that the lines in these files are typically hundreds of characters
638long, and you definitely don't want to type in the host keys by hand.
639Rather, generate them by a script
Damien Miller22c77262000-04-13 12:26:34 +1000640or by taking
Damien Miller05eda432002-02-10 18:32:28 +1100641.Pa /etc/ssh/ssh_host_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000642and adding the host names at the front.
Damien Millercc00f5e2006-03-15 11:33:00 +1100643.Pp
644An example ssh_known_hosts file:
645.Bd -literal -offset 3n
646# Comments allowed at start of line
647closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
648cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
Damien Millere1776152005-03-01 21:47:37 +1100649# A hashed hostname
650|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
651AAAA1234.....=
652.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000653.Sh FILES
Damien Millerd8702e82006-03-15 11:33:56 +1100654.Bl -tag -width Ds -compact
655.It ~/.hushlogin
Damien Miller70a908e2005-03-01 21:17:09 +1100656This file is used to suppress printing the last login time and
657.Pa /etc/motd ,
658if
659.Cm PrintLastLog
660and
661.Cm PrintMotd ,
662respectively,
663are enabled.
664It does not suppress printing of the banner specified by
665.Cm Banner .
Damien Millerd8702e82006-03-15 11:33:56 +1100666.Pp
667.It ~/.rhosts
Damien Milleradc35b92006-03-15 11:35:27 +1100668This file is used for host-based authentication (see
669.Xr ssh 1
670for more information).
671On some machines this file may need to be
672world-readable if the user's home directory is on an NFS partition,
673because
674.Nm
675reads it as root.
676Additionally, this file must be owned by the user,
677and must not have write permissions for anyone else.
678The recommended
679permission for most machines is read/write for the user, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000680accessible by others.
681.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100682.It ~/.shosts
Damien Milleradc35b92006-03-15 11:35:27 +1100683This file is used in exactly the same way as
684.Pa .rhosts ,
685but allows host-based authentication without permitting login with
686rlogin/rsh.
Damien Miller32aa1441999-10-29 09:15:49 +1000687.Pp
Damien Miller520e6152008-02-10 22:46:22 +1100688.It ~/.ssh/
689This directory is the default location for all user-specific configuration
690and authentication information.
691There is no general requirement to keep the entire contents of this directory
692secret, but the recommended permissions are read/write/execute for the user,
693and not accessible by others.
694.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100695.It ~/.ssh/authorized_keys
Damien Milleradc35b92006-03-15 11:35:27 +1100696Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Millerd8702e82006-03-15 11:33:56 +1100697The format of this file is described above.
Damien Miller5d43d492006-08-30 11:07:00 +1000698The content of the file is not highly sensitive, but the recommended
Damien Milleradc35b92006-03-15 11:35:27 +1100699permissions are read/write for the user, and not accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000700.Pp
Damien Miller5d43d492006-08-30 11:07:00 +1000701If this file, the
702.Pa ~/.ssh
703directory, or the user's home directory are writable
704by other users, then the file could be modified or replaced by unauthorized
705users.
706In this case,
707.Nm
708will not allow it to be used unless the
709.Cm StrictModes
710option has been set to
711.Dq no .
712The recommended permissions can be set by executing
713.Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
714.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100715.It ~/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000716This file is read into the environment at login (if it exists).
717It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000718.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000719and assignment lines of the form name=value.
720The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000721only by the user; it need not be readable by anyone else.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000722Environment processing is disabled by default and is
723controlled via the
724.Cm PermitUserEnvironment
725option.
Damien Millerd8702e82006-03-15 11:33:56 +1100726.Pp
727.It ~/.ssh/known_hosts
Damien Milleradc35b92006-03-15 11:35:27 +1100728Contains a list of host keys for all hosts the user has logged into
729that are not already in the systemwide list of known host keys.
730The format of this file is described above.
731This file should be writable only by root/the owner and
Damien Millerd8702e82006-03-15 11:33:56 +1100732can, but need not be, world-readable.
733.Pp
734.It ~/.ssh/rc
Damien Millerfd725cf2006-03-15 11:35:54 +1100735Contains initialization routines to be run before
736the user's home directory becomes accessible.
Damien Miller32aa1441999-10-29 09:15:49 +1000737This file should be writable only by the user, and need not be
738readable by anyone else.
Damien Millerd8702e82006-03-15 11:33:56 +1100739.Pp
740.It /etc/hosts.allow
741.It /etc/hosts.deny
742Access controls that should be enforced by tcp-wrappers are defined here.
743Further details are described in
744.Xr hosts_access 5 .
745.Pp
746.It /etc/hosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100747This file is for host-based authentication (see
748.Xr ssh 1 ) .
749It should only be writable by root.
Damien Millerd8702e82006-03-15 11:33:56 +1100750.Pp
751.It /etc/moduli
752Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
753The file format is described in
754.Xr moduli 5 .
755.Pp
756.It /etc/motd
757See
758.Xr motd 5 .
759.Pp
760.It /etc/nologin
761If this file exists,
762.Nm
763refuses to let anyone except root log in.
764The contents of the file
765are displayed to anyone trying to log in, and non-root connections are
766refused.
767The file should be world-readable.
768.Pp
769.It /etc/shosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100770This file is used in exactly the same way as
771.Pa hosts.equiv ,
772but allows host-based authentication without permitting login with
773rlogin/rsh.
Damien Millerd8702e82006-03-15 11:33:56 +1100774.Pp
775.It /etc/ssh/ssh_host_key
776.It /etc/ssh/ssh_host_dsa_key
777.It /etc/ssh/ssh_host_rsa_key
778These three files contain the private parts of the host keys.
779These files should only be owned by root, readable only by root, and not
780accessible to others.
781Note that
782.Nm
Damien Milleredd03752006-03-15 11:36:45 +1100783does not start if these files are group/world-accessible.
Damien Millerd8702e82006-03-15 11:33:56 +1100784.Pp
785.It /etc/ssh/ssh_host_key.pub
786.It /etc/ssh/ssh_host_dsa_key.pub
787.It /etc/ssh/ssh_host_rsa_key.pub
788These three files contain the public parts of the host keys.
789These files should be world-readable but writable only by
790root.
791Their contents should match the respective private parts.
792These files are not
793really used for anything; they are provided for the convenience of
794the user so their contents can be copied to known hosts files.
795These files are created using
796.Xr ssh-keygen 1 .
797.Pp
Darren Tucker04354b92007-03-21 20:46:54 +1100798.It /etc/ssh/ssh_known_hosts
799Systemwide list of known host keys.
800This file should be prepared by the
801system administrator to contain the public host keys of all machines in the
802organization.
803The format of this file is described above.
804This file should be writable only by root/the owner and
805should be world-readable.
806.Pp
Damien Millerd8702e82006-03-15 11:33:56 +1100807.It /etc/ssh/sshd_config
808Contains configuration data for
809.Nm sshd .
810The file format and configuration options are described in
811.Xr sshd_config 5 .
812.Pp
813.It /etc/ssh/sshrc
Damien Millerfd725cf2006-03-15 11:35:54 +1100814Similar to
815.Pa ~/.ssh/rc ,
816it can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000817machine-specific login-time initializations globally.
818This file should be writable only by root, and should be world-readable.
Damien Millerd8702e82006-03-15 11:33:56 +1100819.Pp
820.It /var/empty
821.Xr chroot 2
822directory used by
823.Nm
824during privilege separation in the pre-authentication phase.
825The directory should not contain any files and must be owned by root
826and not group or world-writable.
827.Pp
828.It /var/run/sshd.pid
829Contains the process ID of the
830.Nm
831listening for connections (if there are several daemons running
832concurrently for different ports, this contains the process ID of the one
833started last).
834The content of this file is not sensitive; it can be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000835.El
Damien Miller32aa1441999-10-29 09:15:49 +1000836.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000837.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100838.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000839.Xr ssh 1 ,
840.Xr ssh-add 1 ,
841.Xr ssh-agent 1 ,
842.Xr ssh-keygen 1 ,
Darren Tucker04354b92007-03-21 20:46:54 +1100843.Xr ssh-keyscan 1 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000844.Xr chroot 2 ,
845.Xr hosts_access 5 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000846.Xr login.conf 5 ,
847.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000848.Xr sshd_config 5 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000849.Xr inetd 8 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000850.Xr sftp-server 8
Damien Millerf1ce5052003-06-11 22:04:39 +1000851.Sh AUTHORS
852OpenSSH is a derivative of the original and free
853ssh 1.2.12 release by Tatu Ylonen.
854Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
855Theo de Raadt and Dug Song
856removed many bugs, re-added newer features and
857created OpenSSH.
858Markus Friedl contributed the support for SSH
859protocol versions 1.5 and 2.0.
860Niels Provos and Markus Friedl contributed support
861for privilege separation.
Damien Millerc47d7e92006-03-15 11:27:20 +1100862.Sh CAVEATS
863System security is not improved unless
864.Nm rshd ,
865.Nm rlogind ,
866and
867.Nm rexecd
868are disabled (thus completely disabling
869.Xr rlogin
870and
871.Xr rsh
872into the machine).