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Damien Miller32aa1441999-10-29 09:15:49 +10001.\"
Damien Miller32aa1441999-10-29 09:15:49 +10002.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller32aa1441999-10-29 09:15:49 +10003.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4.\" All rights reserved
5.\"
Damien Millere4340be2000-09-16 13:29:08 +11006.\" As far as I am concerned, the code I have written for this software
7.\" can be used freely for any purpose. Any derived versions of this
8.\" software must be clearly marked as such, and if the derived work is
9.\" incompatible with the protocol description in the RFC file, it must be
10.\" called by a name other than "ssh" or "Secure Shell".
Damien Miller32aa1441999-10-29 09:15:49 +100011.\"
Ben Lindstrom92a2e382001-03-05 06:59:27 +000012.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
Damien Millere4340be2000-09-16 13:29:08 +110015.\"
16.\" Redistribution and use in source and binary forms, with or without
17.\" modification, are permitted provided that the following conditions
18.\" are met:
19.\" 1. Redistributions of source code must retain the above copyright
20.\" notice, this list of conditions and the following disclaimer.
21.\" 2. Redistributions in binary form must reproduce the above copyright
22.\" notice, this list of conditions and the following disclaimer in the
23.\" documentation and/or other materials provided with the distribution.
24.\"
25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Damien Miller32aa1441999-10-29 09:15:49 +100035.\"
dtucker@openbsd.org01c98d92018-07-22 12:16:59 +000036.\" $OpenBSD: sshd.8,v 1.304 2018/07/22 12:16:59 dtucker Exp $
37.Dd $Mdocdate: July 22 2018 $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dt SSHD 8
39.Os
40.Sh NAME
41.Nm sshd
Ben Lindstromc65e6a02001-04-23 13:02:16 +000042.Nd OpenSSH SSH daemon
Damien Miller32aa1441999-10-29 09:15:49 +100043.Sh SYNOPSIS
44.Nm sshd
Damien Millerffadc582003-02-24 11:52:26 +110045.Bk -words
Darren Tuckere7f3f752008-06-10 23:06:01 +100046.Op Fl 46DdeiqTt
Darren Tuckere7140f22008-06-10 23:01:51 +100047.Op Fl C Ar connection_spec
Damien Miller0a80ca12010-02-27 07:55:05 +110048.Op Fl c Ar host_certificate_file
Damien Miller03d4d7e2013-04-23 15:21:06 +100049.Op Fl E Ar log_file
Damien Miller32aa1441999-10-29 09:15:49 +100050.Op Fl f Ar config_file
51.Op Fl g Ar login_grace_time
52.Op Fl h Ar host_key_file
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 Miller69cb24b2014-04-20 13:29:06 +100061Together these programs replace rlogin and rsh,
Darren Tucker04354b92007-03-21 20:46:54 +110062and provide 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.
Darren Tuckere7f3f752008-06-10 23:06:01 +100097.It Fl C Ar connection_spec
98Specify the connection parameters to use for the
99.Fl T
100extended test mode.
101If provided, any
102.Cm Match
dtucker@openbsd.org@openbsd.org0208a482017-11-03 03:18:53 +0000103directives in the configuration file that would apply are applied before the
104configuration is written to standard output.
105The connection parameters are supplied as keyword=value pairs and may be
106supplied in any order, either with multiple
107.Fl C
108options or as a comma-separated list.
Darren Tuckere7f3f752008-06-10 23:06:01 +1000109The keywords are
dtucker@openbsd.org@openbsd.org0208a482017-11-03 03:18:53 +0000110.Dq addr,
Darren Tuckere7f3f752008-06-10 23:06:01 +1000111.Dq user ,
112.Dq host ,
Darren Tuckerfbcf8272012-05-19 19:37:01 +1000113.Dq laddr ,
114.Dq lport ,
Darren Tuckere7f3f752008-06-10 23:06:01 +1000115and
dtucker@openbsd.org@openbsd.org0208a482017-11-03 03:18:53 +0000116.Dq rdomain
117and correspond to source address, user, resolved source host name,
118local address, local port number and routing domain respectively.
Damien Miller15f5b562010-03-03 10:25:21 +1100119.It Fl c Ar host_certificate_file
120Specifies a path to a certificate file to identify
121.Nm
122during key exchange.
123The certificate file must match a host key file specified using the
124.Fl h
125option or the
126.Cm HostKey
127configuration directive.
Darren Tucker1f203942003-10-15 15:50:42 +1000128.It Fl D
129When this option is specified,
130.Nm
131will not detach and does not become a daemon.
132This allows easy monitoring of
133.Nm sshd .
Damien Miller32aa1441999-10-29 09:15:49 +1000134.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000135Debug mode.
Damien Miller74d98252010-02-02 17:01:46 +1100136The server sends verbose debug output to standard error,
137and does not put itself in the background.
Damien Miller450a7a12000-03-26 13:04:51 +1000138The server also will not fork and will only process one connection.
139This option is only intended for debugging for the server.
Damien Millerffadc582003-02-24 11:52:26 +1100140Multiple
141.Fl d
142options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100143Maximum is 3.
Damien Miller03d4d7e2013-04-23 15:21:06 +1000144.It Fl E Ar log_file
145Append debug logs to
146.Ar log_file
147instead of the system log.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000148.It Fl e
Damien Miller69010322013-04-23 15:21:24 +1000149Write debug logs to standard error instead of the system log.
Darren Tucker04354b92007-03-21 20:46:54 +1100150.It Fl f Ar config_file
Damien Miller450a7a12000-03-26 13:04:51 +1000151Specifies the name of the configuration file.
152The default is
Damien Miller05eda432002-02-10 18:32:28 +1100153.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000154.Nm
155refuses to start if there is no configuration file.
156.It Fl g Ar login_grace_time
157Gives the grace time for clients to authenticate themselves (default
Damien Millerc1348632002-09-05 14:35:14 +1000158120 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000159If the client fails to authenticate the user within
160this many seconds, the server disconnects and exits.
161A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000162.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100163Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000164This option must be given if
165.Nm
166is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100167host key files are normally not readable by anyone but root).
168The default is
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000169.Pa /etc/ssh/ssh_host_ecdsa_key ,
Damien Miller8ba0ead2013-12-18 17:46:27 +1100170.Pa /etc/ssh/ssh_host_ed25519_key
Damien Miller7fc23732002-01-22 23:19:11 +1100171and
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000172.Pa /etc/ssh/ssh_host_rsa_key .
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000173It is possible to have multiple host key files for
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000174the different host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000175.It Fl i
176Specifies that
177.Nm
Damien Millerffadc582003-02-24 11:52:26 +1100178is being run from
179.Xr inetd 8 .
Ben Lindstromade03f62001-12-06 18:22:17 +0000180.It Fl o Ar option
181Can be used to give options in the format used in the configuration file.
182This is useful for specifying options for which there is no separate
183command-line flag.
Darren Tucker1f203942003-10-15 15:50:42 +1000184For full details of the options, and their values, see
185.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000186.It Fl p Ar port
187Specifies the port on which the server listens for connections
188(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100189Multiple port options are permitted.
Damien Millerd7f308f2005-12-24 14:55:16 +1100190Ports specified in the configuration file with the
191.Cm Port
192option are ignored when a command-line port is specified.
193Ports specified using the
194.Cm ListenAddress
195option override command-line ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000196.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000197Quiet mode.
198Nothing is sent to the system log.
199Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000200authentication, and termination of each connection is logged.
Darren Tuckere7140f22008-06-10 23:01:51 +1000201.It Fl T
202Extended test mode.
203Check the validity of the configuration file, output the effective configuration
204to stdout and then exit.
205Optionally,
206.Cm Match
207rules may be applied by specifying the connection parameters using one or more
208.Fl C
209options.
Darren Tuckere7f3f752008-06-10 23:06:01 +1000210.It Fl t
211Test mode.
212Only check the validity of the configuration file and sanity of the keys.
213This is useful for updating
214.Nm
215reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000216.It Fl u Ar len
217This option is used to specify the size of the field
218in the
219.Li utmp
220structure that holds the remote host name.
221If the resolved host name is longer than
222.Ar len ,
223the dotted decimal value will be used instead.
224This allows hosts with very long host names that
225overflow this field to still be uniquely identified.
226Specifying
227.Fl u0
228indicates that only dotted decimal addresses
229should be put into the
230.Pa utmp
231file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000232.Fl u0
Damien Millerffadc582003-02-24 11:52:26 +1100233may also be used to prevent
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000234.Nm
235from making DNS requests unless the authentication
236mechanism or configuration requires it.
237Authentication mechanisms that may require DNS include
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000238.Cm HostbasedAuthentication
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000239and using a
240.Cm from="pattern-list"
241option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000242Configuration options that require DNS include using a
243USER@HOST pattern in
244.Cm AllowUsers
245or
246.Cm DenyUsers .
Damien Miller32aa1441999-10-29 09:15:49 +1000247.El
Damien Miller8bbdf902006-02-01 22:05:25 +1100248.Sh AUTHENTICATION
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000249The OpenSSH SSH daemon supports SSH protocol 2 only.
jmc@openbsd.orga685ae82016-02-17 07:38:19 +0000250Each host has a host-specific key,
251used to identify the host.
Damien Miller99cc4a82006-01-31 21:45:53 +1100252Whenever a client connects, the daemon responds with its public
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000253host key.
Damien Miller99cc4a82006-01-31 21:45:53 +1100254The client compares the
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000255host key against its own database to verify that it has not changed.
256Forward security is provided through a Diffie-Hellman key agreement.
Damien Miller8bbdf902006-02-01 22:05:25 +1100257This key agreement results in a shared session key.
258The rest of the session is encrypted using a symmetric cipher, currently
259128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
260The client selects the encryption algorithm
261to use from those offered by the server.
262Additionally, session integrity is provided
263through a cryptographic message authentication code
djm@openbsd.orgacaf34f2017-05-07 23:12:57 +0000264(hmac-md5, hmac-sha1, umac-64, umac-128,
Damien Miller20bd4532011-08-06 06:17:30 +1000265hmac-sha2-256 or hmac-sha2-512).
Damien Miller8bbdf902006-02-01 22:05:25 +1100266.Pp
267Finally, the server and the client enter an authentication dialog.
Damien Miller99cc4a82006-01-31 21:45:53 +1100268The client tries to authenticate itself using
Damien Miller8bbdf902006-02-01 22:05:25 +1100269host-based authentication,
270public key authentication,
271challenge-response authentication,
272or password authentication.
Damien Miller99cc4a82006-01-31 21:45:53 +1100273.Pp
274Regardless of the authentication type, the account is checked to
275ensure that it is accessible. An account is not accessible if it is
276locked, listed in
277.Cm DenyUsers
278or its group is listed in
279.Cm DenyGroups
Damien Miller10479cc2018-04-10 10:19:02 +1000280\&. The definition of a locked account is system dependent. Some platforms
Damien Miller99cc4a82006-01-31 21:45:53 +1100281have their own account database (eg AIX) and some modify the passwd field (
282.Ql \&*LK\&*
283on Solaris and UnixWare,
284.Ql \&*
285on HP-UX, containing
286.Ql Nologin
287on Tru64,
288a leading
289.Ql \&*LOCKED\&*
290on FreeBSD and a leading
Darren Tucker1a9176b2007-08-17 09:42:32 +1000291.Ql \&!
292on most Linuxes).
293If there is a requirement to disable password authentication
Damien Miller99cc4a82006-01-31 21:45:53 +1100294for the account while allowing still public-key, then the passwd field
295should be set to something other than these values (eg
296.Ql NP
297or
298.Ql \&*NP\&*
299).
300.Pp
Damien Miller99cc4a82006-01-31 21:45:53 +1100301If the client successfully authenticates itself, a dialog for
302preparing the session is entered.
303At this time the client may request
304things like allocating a pseudo-tty, forwarding X11 connections,
305forwarding TCP connections, or forwarding the authentication agent
306connection over the secure channel.
307.Pp
Damien Millerc47d7e92006-03-15 11:27:20 +1100308After this, the client either requests a shell or execution of a command.
Damien Miller99cc4a82006-01-31 21:45:53 +1100309The sides then enter session mode.
310In this mode, either side may send
311data at any time, and such data is forwarded to/from the shell or
312command on the server side, and the user terminal in the client side.
313.Pp
314When the user program terminates and all forwarded X11 and other
315connections have been closed, the server sends command exit status to
316the client, and both sides exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000317.Sh LOGIN PROCESS
318When a user successfully logs in,
319.Nm
320does the following:
321.Bl -enum -offset indent
322.It
323If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000324prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000325.Pa /etc/motd
326(unless prevented in the configuration file or by
Damien Miller167ea5d2005-05-26 12:04:02 +1000327.Pa ~/.hushlogin ;
Damien Miller32aa1441999-10-29 09:15:49 +1000328see the
Damien Miller22c77262000-04-13 12:26:34 +1000329.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000330section).
331.It
332If the login is on a tty, records login time.
333.It
334Checks
335.Pa /etc/nologin ;
336if it exists, prints contents and quits
337(unless root).
338.It
339Changes to run with normal user privileges.
340.It
341Sets up basic environment.
342.It
Darren Tucker1f203942003-10-15 15:50:42 +1000343Reads the file
Damien Miller167ea5d2005-05-26 12:04:02 +1000344.Pa ~/.ssh/environment ,
Darren Tucker1f203942003-10-15 15:50:42 +1000345if it exists, and users are allowed to change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000346See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000347.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000348option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000349.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000350.It
351Changes to user's home directory.
352.It
353If
Damien Miller167ea5d2005-05-26 12:04:02 +1000354.Pa ~/.ssh/rc
Damien Miller72e6b5c2014-07-04 09:00:04 +1000355exists and the
356.Xr sshd_config 5
357.Cm PermitUserRC
358option is set, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100359.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000360exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000361it; otherwise runs xauth.
362The
Damien Miller32aa1441999-10-29 09:15:49 +1000363.Dq rc
364files are given the X11
365authentication protocol and cookie in standard input.
Damien Millerfd725cf2006-03-15 11:35:54 +1100366See
367.Sx SSHRC ,
368below.
Damien Miller32aa1441999-10-29 09:15:49 +1000369.It
370Runs user's shell or command.
djm@openbsd.org8d4d1bf2015-05-01 07:11:47 +0000371All commands are run under the user's login shell as specified in the
372system password database.
Damien Miller32aa1441999-10-29 09:15:49 +1000373.El
Damien Millerfd725cf2006-03-15 11:35:54 +1100374.Sh SSHRC
375If the file
376.Pa ~/.ssh/rc
377exists,
378.Xr sh 1
379runs it after reading the
380environment files but before starting the user's shell or command.
381It must not produce any output on stdout; stderr must be used
382instead.
383If X11 forwarding is in use, it will receive the "proto cookie" pair in
384its standard input (and
385.Ev DISPLAY
386in its environment).
387The script must call
388.Xr xauth 1
389because
390.Nm
391will not run xauth automatically to add X11 cookies.
392.Pp
393The primary purpose of this file is to run any initialization routines
394which may be needed before the user's home directory becomes
395accessible; AFS is a particular example of such an environment.
396.Pp
397This file will probably contain some initialization code followed by
398something similar to:
399.Bd -literal -offset 3n
400if read proto cookie && [ -n "$DISPLAY" ]; then
401 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
402 # X11UseLocalhost=yes
403 echo add unix:`echo $DISPLAY |
404 cut -c11-` $proto $cookie
405 else
406 # X11UseLocalhost=no
407 echo add $DISPLAY $proto $cookie
408 fi | xauth -q -
409fi
410.Ed
411.Pp
412If this file does not exist,
413.Pa /etc/ssh/sshrc
414is run, and if that
415does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000416.Sh AUTHORIZED_KEYS FILE FORMAT
Ben Lindstromf96704d2001-06-25 04:17:12 +0000417.Cm AuthorizedKeysFile
Damien Millerb9132fc2011-05-29 21:41:40 +1000418specifies the files containing public keys for
Damien Millerdcfea272006-03-15 11:31:22 +1100419public key authentication;
djm@openbsd.org2bca8a42015-09-11 03:13:36 +0000420if this option is not specified, the default is
Damien Millerd8478b62011-05-29 21:39:36 +1000421.Pa ~/.ssh/authorized_keys
422and
423.Pa ~/.ssh/authorized_keys2 .
Damien Miller450a7a12000-03-26 13:04:51 +1000424Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000425key (empty lines and lines starting with a
426.Ql #
427are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000428comments).
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000429Public keys consist of the following space-separated fields:
Damien Millerdcfea272006-03-15 11:31:22 +1100430options, keytype, base64-encoded key, comment.
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000431The options field is optional.
432The keytype is
Damien Millereb8b60e2010-08-31 22:41:14 +1000433.Dq ecdsa-sha2-nistp256 ,
434.Dq ecdsa-sha2-nistp384 ,
435.Dq ecdsa-sha2-nistp521 ,
Damien Millera7827c12013-12-07 11:24:30 +1100436.Dq ssh-ed25519 ,
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000437.Dq ssh-dss
438or
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000439.Dq ssh-rsa ;
440the comment field is not used for anything (but may be convenient for the
441user to identify the key).
Damien Miller32aa1441999-10-29 09:15:49 +1000442.Pp
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000443Note that lines in this file can be several hundred bytes long
Darren Tucker22cc7412004-12-06 22:47:41 +1100444(because of the size of the public key encoding) up to a limit of
4458 kilobytes, which permits DSA keys up to 8 kilobits and RSA
446keys up to 16 kilobits.
Damien Miller450a7a12000-03-26 13:04:51 +1000447You don't want to type them in; instead, copy the
Damien Millerdcfea272006-03-15 11:31:22 +1100448.Pa id_dsa.pub ,
Damien Millereb8b60e2010-08-31 22:41:14 +1000449.Pa id_ecdsa.pub ,
Damien Miller8ba0ead2013-12-18 17:46:27 +1100450.Pa id_ed25519.pub ,
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000451or the
452.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000453file and edit it.
454.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000455.Nm
dtucker@openbsd.org055e09e2018-03-03 06:37:53 +0000456enforces a minimum RSA key modulus size of 1024 bits.
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000457.Pp
Damien Miller942da032000-08-18 13:59:06 +1000458The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000459specifications.
460No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000461The following option specifications are supported (note
462that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000463.Bl -tag -width Ds
djm@openbsd.org383f10f2015-11-16 00:30:02 +0000464.It Cm agent-forwarding
465Enable authentication agent forwarding previously disabled by the
466.Cm restrict
467option.
Damien Millercd38c9c2010-03-04 21:51:37 +1100468.It Cm cert-authority
Damien Miller0a80ca12010-02-27 07:55:05 +1100469Specifies that the listed key is a certification authority (CA) that is
470trusted to validate signed certificates for user authentication.
471.Pp
472Certificates may encode access restrictions similar to these key options.
473If both certificate restrictions and key options are present, the most
474restrictive union of the two is applied.
Damien Miller32aa1441999-10-29 09:15:49 +1000475.It Cm command="command"
476Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000477authentication.
478The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000479The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000480otherwise it is run without a tty.
Damien Millerffadc582003-02-24 11:52:26 +1100481If an 8-bit clean channel is required,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000482one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100483.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000484A quote may be included in the command by quoting it with a backslash.
djm@openbsd.orgfd6dcef2016-11-30 02:57:40 +0000485.Pp
Damien Miller450a7a12000-03-26 13:04:51 +1000486This option might be useful
Damien Millerb83df8d2002-09-04 16:24:55 +1000487to restrict certain public keys to perform just a specific operation.
Damien Miller450a7a12000-03-26 13:04:51 +1000488An example might be a key that permits remote backups but nothing else.
Damien Miller7c24b812006-01-14 10:09:56 +1100489Note that the client may specify TCP and/or X11
djm@openbsd.orgfd6dcef2016-11-30 02:57:40 +0000490forwarding unless they are explicitly prohibited, e.g. using the
491.Cm restrict
492key option.
493.Pp
Damien Millere2754432006-07-24 14:06:47 +1000494The command originally supplied by the client is available in the
495.Ev SSH_ORIGINAL_COMMAND
496environment variable.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000497Note that this option applies to shell, command or subsystem execution.
djm@openbsd.orgfd6dcef2016-11-30 02:57:40 +0000498Also note that this command may be superseded by a
Damien Miller0a80ca12010-02-27 07:55:05 +1100499.Xr sshd_config 5
500.Cm ForceCommand
djm@openbsd.orgfd6dcef2016-11-30 02:57:40 +0000501directive.
502.Pp
503If a command is specified and a forced-command is embedded in a certificate
504used for authentication, then the certificate will be accepted only if the
505two commands are identical.
Damien Miller32aa1441999-10-29 09:15:49 +1000506.It Cm environment="NAME=value"
507Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000508logging in using this key.
509Environment variables set this way
510override other default environment values.
511Multiple options of this type are permitted.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000512Environment processing is disabled by default and is
513controlled via the
514.Cm PermitUserEnvironment
515option.
jmc@openbsd.org037fdc12018-03-14 06:56:20 +0000516.It Cm expiry-time="timespec"
517Specifies a time after which the key will not be accepted.
518The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time
519in the system time-zone.
Damien Miller31bdc522006-03-15 11:31:44 +1100520.It Cm from="pattern-list"
Darren Tucker896ad5a2008-06-11 09:34:46 +1000521Specifies that in addition to public key authentication, either the canonical
Darren Tucker11996732008-06-13 04:32:00 +1000522name of the remote host or its IP address must be present in the
Darren Tucker896ad5a2008-06-11 09:34:46 +1000523comma-separated list of patterns.
Damien Millerfecfd112013-07-18 16:11:50 +1000524See PATTERNS in
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100525.Xr ssh_config 5
526for more information on patterns.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000527.Pp
528In addition to the wildcard matching that may be applied to hostnames or
529addresses, a
530.Cm from
Darren Tucker5837b512009-06-21 17:52:27 +1000531stanza may match IP addresses using CIDR address/masklen notation.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000532.Pp
533The purpose of this option is to optionally increase security: public key
534authentication by itself does not trust the network or name servers or
535anything (but the key); however, if somebody somehow steals the key, the key
536permits an intruder to log in from anywhere in the world.
537This additional option makes using a stolen key more difficult (name
538servers and/or routers would have to be compromised in addition to
539just the key).
Damien Miller31bdc522006-03-15 11:31:44 +1100540.It Cm no-agent-forwarding
541Forbids authentication agent forwarding when this key is used for
542authentication.
Damien Miller32aa1441999-10-29 09:15:49 +1000543.It Cm no-port-forwarding
Damien Miller7c24b812006-01-14 10:09:56 +1100544Forbids TCP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000545Any port forward requests by the client will return an error.
Damien Miller208f1ed2006-03-15 11:56:03 +1100546This might be used, e.g. in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000547.Cm command
548option.
Damien Miller31bdc522006-03-15 11:31:44 +1100549.It Cm no-pty
550Prevents tty allocation (a request to allocate a pty will fail).
Damien Miller95e80952008-03-27 11:03:05 +1100551.It Cm no-user-rc
Damien Miller13ba9c22008-04-03 20:52:51 +1100552Disables execution of
Damien Miller95e80952008-03-27 11:03:05 +1100553.Pa ~/.ssh/rc .
Damien Miller32aa1441999-10-29 09:15:49 +1000554.It Cm no-X11-forwarding
555Forbids X11 forwarding when this key is used for authentication.
556Any X11 forward requests by the client will return an error.
djm@openbsd.org87ddd672018-06-19 02:59:41 +0000557.It Cm permitlisten="[host:]port"
jmc@openbsd.org6ff6fda2018-06-07 11:26:14 +0000558Limit remote port forwarding with the
djm@openbsd.org803d8962018-06-06 18:24:15 +0000559.Xr ssh 1
560.Fl R
djm@openbsd.org87ddd672018-06-19 02:59:41 +0000561option such that it may only listen on the specified host (optional) and port.
djm@openbsd.org803d8962018-06-06 18:24:15 +0000562IPv6 addresses can be specified by enclosing the address in square brackets.
563Multiple
564.Cm permitlisten
565options may be applied separated by commas.
566Hostnames may include wildcards as described in the PATTERNS section in
567.Xr ssh_config 5 .
568A port specification of
569.Cm *
570matches any port.
571Note that the setting of
572.Cm GatewayPorts
573may further restrict listen addresses.
djm@openbsd.org87ddd672018-06-19 02:59:41 +0000574Note that
575.Xr ssh 1
576will send a hostname of
577.Dq localhost
578if a listen host was not specified when the forwarding was requested, and
jmc@openbsd.orgf535ff92018-06-19 05:36:57 +0000579that this name is treated differently to the explicit localhost addresses
djm@openbsd.org87ddd672018-06-19 02:59:41 +0000580.Dq 127.0.0.1
581and
582.Dq ::1 .
Damien Millera243fde2001-03-19 23:16:08 +1100583.It Cm permitopen="host:port"
jmc@openbsd.org6ff6fda2018-06-07 11:26:14 +0000584Limit local port forwarding with the
bentley@openbsd.orgda8af832014-11-15 14:41:03 +0000585.Xr ssh 1
586.Fl L
djm@openbsd.org803d8962018-06-06 18:24:15 +0000587option such that it may only connect to the specified host and port.
Damien Miller7fa96602010-08-05 13:03:13 +1000588IPv6 addresses can be specified by enclosing the address in square brackets.
Ben Lindstromd71ba572001-09-12 18:03:31 +0000589Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100590.Cm permitopen
Damien Millerfbf486b2003-05-23 18:44:23 +1000591options may be applied separated by commas.
592No pattern matching is performed on the specified hostnames,
593they must be literal domains or addresses.
Darren Tucker1338b9e2011-10-02 18:57:35 +1100594A port specification of
595.Cm *
596matches any port.
djm@openbsd.org383f10f2015-11-16 00:30:02 +0000597.It Cm port-forwarding
598Enable port forwarding previously disabled by the
599.Cm restrict
djm@openbsd.org2b428f92018-02-05 04:02:53 +0000600option.
Damien Miller30da3442010-05-10 11:58:03 +1000601.It Cm principals="principals"
602On a
603.Cm cert-authority
604line, specifies allowed principals for certificate authentication as a
605comma-separated list.
606At least one name from the list must appear in the certificate's
607list of principals for the certificate to be accepted.
608This option is ignored for keys that are not marked as trusted certificate
609signers using the
610.Cm cert-authority
611option.
djm@openbsd.org383f10f2015-11-16 00:30:02 +0000612.It Cm pty
613Permits tty allocation previously disabled by the
614.Cm restrict
615option.
616.It Cm restrict
617Enable all restrictions, i.e. disable port, agent and X11 forwarding,
618as well as disabling PTY allocation
619and execution of
620.Pa ~/.ssh/rc .
621If any future restriction capabilities are added to authorized_keys files
622they will be included in this set.
Damien Millerd27b9472005-12-13 19:29:02 +1100623.It Cm tunnel="n"
624Force a
625.Xr tun 4
626device on the server.
627Without this option, the next available device will be used if
628the client requests a tunnel.
djm@openbsd.org383f10f2015-11-16 00:30:02 +0000629.It Cm user-rc
630Enables execution of
631.Pa ~/.ssh/rc
632previously disabled by the
633.Cm restrict
634option.
635.It Cm X11-forwarding
636Permits X11 forwarding previously disabled by the
637.Cm restrict
638option.
Damien Miller32aa1441999-10-29 09:15:49 +1000639.El
Damien Miller32aa1441999-10-29 09:15:49 +1000640.Pp
Damien Miller7d2ef022006-03-15 11:32:06 +1100641An example authorized_keys file:
Damien Millerc8f61cf2006-03-15 11:33:25 +1100642.Bd -literal -offset 3n
Damien Miller7d2ef022006-03-15 11:32:06 +1100643# Comments allowed at start of line
644ssh-rsa AAAAB3Nza...LiPk== user@example.net
Damien Millerc8f61cf2006-03-15 11:33:25 +1100645from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
646AAAAB2...19Q== john@example.net
dtucker@openbsd.org01c98d92018-07-22 12:16:59 +0000647command="dump /home",no-pty,no-port-forwarding ssh-rsa
Damien Millerc8f61cf2006-03-15 11:33:25 +1100648AAAAC3...51R== example.net
dtucker@openbsd.org01c98d92018-07-22 12:16:59 +0000649permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa
Damien Millerc8f61cf2006-03-15 11:33:25 +1100650AAAAB5...21S==
dtucker@openbsd.org01c98d92018-07-22 12:16:59 +0000651permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
djm@openbsd.org87ddd672018-06-19 02:59:41 +0000652AAAAB5...21S==
Damien Millerc8f61cf2006-03-15 11:33:25 +1100653tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
654jane@example.net
djm@openbsd.org383f10f2015-11-16 00:30:02 +0000655restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
656user@example.net
657restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
658user@example.net
Damien Miller7d2ef022006-03-15 11:32:06 +1100659.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000660.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000661The
Damien Millerffadc582003-02-24 11:52:26 +1100662.Pa /etc/ssh/ssh_known_hosts
Damien Miller22c77262000-04-13 12:26:34 +1000663and
Damien Miller167ea5d2005-05-26 12:04:02 +1000664.Pa ~/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000665files contain host public keys for all known hosts.
666The global file should
667be prepared by the administrator (optional), and the per-user file is
dtucker@openbsd.org6ba9f892017-01-30 23:27:39 +0000668maintained automatically: whenever the user connects to an unknown host,
Damien Miller450a7a12000-03-26 13:04:51 +1000669its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000670.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100671Each line in these files contains the following fields: markers (optional),
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000672hostnames, keytype, base64-encoded key, comment.
Damien Miller450a7a12000-03-26 13:04:51 +1000673The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000674.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100675The marker is optional, but if it is present then it must be one of
676.Dq @cert-authority ,
677to indicate that the line contains a certification authority (CA) key,
678or
679.Dq @revoked ,
680to indicate that the key contained on the line is revoked and must not ever
681be accepted.
682Only one marker should be used on a key line.
683.Pp
Damien Miller31554322003-05-14 13:44:58 +1000684Hostnames is a comma-separated list of patterns
Damien Miller9a7f2012006-03-15 11:32:42 +1100685.Pf ( Ql *
Damien Millerfbf486b2003-05-23 18:44:23 +1000686and
687.Ql \&?
Damien Miller049245d2003-05-14 13:44:42 +1000688act as
djm@openbsd.org6f8ca3b2017-06-24 05:35:05 +0000689wildcards); each pattern in turn is matched against the host name.
690When
691.Nm sshd
692is authenticating a client, such as when using
693.Cm HostbasedAuthentication ,
694this will be the canonical client host name.
695When
696.Xr ssh 1
jmc@openbsd.orge2004d42017-06-24 06:28:50 +0000697is authenticating a server, this will be the host name
djm@openbsd.org6f8ca3b2017-06-24 05:35:05 +0000698given by the user, the value of the
699.Xr ssh 1
700.Cm HostkeyAlias
701if it was specified, or the canonical server hostname if the
702.Xr ssh 1
703.Cm CanonicalizeHostname
704option was used.
705.Pp
Damien Miller450a7a12000-03-26 13:04:51 +1000706A pattern may also be preceded by
Damien Miller049245d2003-05-14 13:44:42 +1000707.Ql \&!
Damien Miller32aa1441999-10-29 09:15:49 +1000708to indicate negation: if the host name matches a negated
709pattern, it is not accepted (by that line) even if it matched another
710pattern on the line.
Darren Tuckerda345532006-07-10 23:04:19 +1000711A hostname or address may optionally be enclosed within
712.Ql \&[
713and
714.Ql \&]
715brackets then followed by
716.Ql \&:
Darren Tucker11318472006-07-12 22:07:59 +1000717and a non-standard port number.
Damien Miller32aa1441999-10-29 09:15:49 +1000718.Pp
Damien Millere1776152005-03-01 21:47:37 +1100719Alternately, hostnames may be stored in a hashed form which hides host names
Damien Miller718fd4b2005-03-02 12:03:23 +1100720and addresses should the file's contents be disclosed.
721Hashed hostnames start with a
722.Ql |
Damien Millere1776152005-03-01 21:47:37 +1100723character.
724Only one hashed hostname may appear on a single line and none of the above
725negation or wildcard operators may be applied.
726.Pp
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000727The keytype and base64-encoded key are taken directly from the host key; they
Damien Miller9a7f2012006-03-15 11:32:42 +1100728can be obtained, for example, from
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000729.Pa /etc/ssh/ssh_host_rsa_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000730The optional comment field continues to the end of the line, and is not used.
731.Pp
732Lines starting with
733.Ql #
734and empty lines are ignored as comments.
735.Pp
736When performing host authentication, authentication is accepted if any
Damien Millera7dab8b2010-03-05 10:42:05 +1100737matching line has the proper key; either one that matches exactly or,
738if the server has presented a certificate for authentication, the key
739of the certification authority that signed the certificate.
740For a key to be trusted as a certification authority, it must use the
741.Dq @cert-authority
742marker described above.
743.Pp
744The known hosts file also provides a facility to mark keys as revoked,
745for example when it is known that the associated private key has been
746stolen.
747Revoked keys are specified by including the
748.Dq @revoked
749marker at the beginning of the key line, and are never accepted for
750authentication or as certification authorities, but instead will
751produce a warning from
752.Xr ssh 1
753when they are encountered.
754.Pp
755It is permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000756recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000757names.
758This will inevitably happen when short forms of host names
759from different domains are put in the file.
760It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000761that the files contain conflicting information; authentication is
762accepted if valid information can be found from either file.
763.Pp
764Note that the lines in these files are typically hundreds of characters
765long, and you definitely don't want to type in the host keys by hand.
Damien Millera7dab8b2010-03-05 10:42:05 +1100766Rather, generate them by a script,
767.Xr ssh-keyscan 1
naddy@openbsd.orgffe65492016-08-15 12:32:04 +0000768or by taking, for example,
769.Pa /etc/ssh/ssh_host_rsa_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000770and adding the host names at the front.
Damien Millera7dab8b2010-03-05 10:42:05 +1100771.Xr ssh-keygen 1
Damien Miller98339052010-03-05 21:30:35 +1100772also offers some basic automated editing for
Damien Millera7dab8b2010-03-05 10:42:05 +1100773.Pa ~/.ssh/known_hosts
774including removing hosts matching a host name and converting all host
775names to their hashed representations.
Damien Millercc00f5e2006-03-15 11:33:00 +1100776.Pp
777An example ssh_known_hosts file:
778.Bd -literal -offset 3n
779# Comments allowed at start of line
780closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
781cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
Damien Millere1776152005-03-01 21:47:37 +1100782# A hashed hostname
783|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
784AAAA1234.....=
Damien Millera7dab8b2010-03-05 10:42:05 +1100785# A revoked key
786@revoked * ssh-rsa AAAAB5W...
787# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
788@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
Damien Millere1776152005-03-01 21:47:37 +1100789.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000790.Sh FILES
Damien Millerd8702e82006-03-15 11:33:56 +1100791.Bl -tag -width Ds -compact
Damien Millerafdae612010-08-31 22:31:14 +1000792.It Pa ~/.hushlogin
Damien Miller70a908e2005-03-01 21:17:09 +1100793This file is used to suppress printing the last login time and
794.Pa /etc/motd ,
795if
796.Cm PrintLastLog
797and
798.Cm PrintMotd ,
799respectively,
800are enabled.
801It does not suppress printing of the banner specified by
802.Cm Banner .
Damien Millerd8702e82006-03-15 11:33:56 +1100803.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000804.It Pa ~/.rhosts
Damien Milleradc35b92006-03-15 11:35:27 +1100805This file is used for host-based authentication (see
806.Xr ssh 1
807for more information).
808On some machines this file may need to be
809world-readable if the user's home directory is on an NFS partition,
810because
811.Nm
812reads it as root.
813Additionally, this file must be owned by the user,
814and must not have write permissions for anyone else.
815The recommended
816permission for most machines is read/write for the user, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000817accessible by others.
818.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000819.It Pa ~/.shosts
Damien Milleradc35b92006-03-15 11:35:27 +1100820This file is used in exactly the same way as
821.Pa .rhosts ,
822but allows host-based authentication without permitting login with
823rlogin/rsh.
Damien Miller32aa1441999-10-29 09:15:49 +1000824.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000825.It Pa ~/.ssh/
Damien Miller520e6152008-02-10 22:46:22 +1100826This directory is the default location for all user-specific configuration
827and authentication information.
828There is no general requirement to keep the entire contents of this directory
829secret, but the recommended permissions are read/write/execute for the user,
830and not accessible by others.
831.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000832.It Pa ~/.ssh/authorized_keys
sobrado@openbsd.orgf70b22b2014-08-30 15:33:50 +0000833Lists the public keys (DSA, ECDSA, Ed25519, RSA)
Damien Miller8ba0ead2013-12-18 17:46:27 +1100834that can be used for logging in as this user.
Damien Millerd8702e82006-03-15 11:33:56 +1100835The format of this file is described above.
Damien Miller5d43d492006-08-30 11:07:00 +1000836The content of the file is not highly sensitive, but the recommended
Damien Milleradc35b92006-03-15 11:35:27 +1100837permissions are read/write for the user, and not accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000838.Pp
Damien Miller5d43d492006-08-30 11:07:00 +1000839If this file, the
840.Pa ~/.ssh
841directory, or the user's home directory are writable
842by other users, then the file could be modified or replaced by unauthorized
843users.
844In this case,
845.Nm
846will not allow it to be used unless the
847.Cm StrictModes
848option has been set to
849.Dq no .
Damien Miller5d43d492006-08-30 11:07:00 +1000850.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000851.It Pa ~/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000852This file is read into the environment at login (if it exists).
853It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000854.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000855and assignment lines of the form name=value.
856The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000857only by the user; it need not be readable by anyone else.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000858Environment processing is disabled by default and is
859controlled via the
860.Cm PermitUserEnvironment
861option.
Damien Millerd8702e82006-03-15 11:33:56 +1100862.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000863.It Pa ~/.ssh/known_hosts
Damien Milleradc35b92006-03-15 11:35:27 +1100864Contains a list of host keys for all hosts the user has logged into
865that are not already in the systemwide list of known host keys.
866The format of this file is described above.
867This file should be writable only by root/the owner and
Damien Millerd8702e82006-03-15 11:33:56 +1100868can, but need not be, world-readable.
869.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000870.It Pa ~/.ssh/rc
Damien Millerfd725cf2006-03-15 11:35:54 +1100871Contains initialization routines to be run before
872the user's home directory becomes accessible.
Damien Miller32aa1441999-10-29 09:15:49 +1000873This file should be writable only by the user, and need not be
874readable by anyone else.
Damien Millerd8702e82006-03-15 11:33:56 +1100875.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000876.It Pa /etc/hosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100877This file is for host-based authentication (see
878.Xr ssh 1 ) .
879It should only be writable by root.
Damien Millerd8702e82006-03-15 11:33:56 +1100880.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000881.It Pa /etc/moduli
djm@openbsd.orgdeae7d52016-02-05 03:07:06 +0000882Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
883key exchange method.
Damien Millerd8702e82006-03-15 11:33:56 +1100884The file format is described in
885.Xr moduli 5 .
djm@openbsd.orgdeae7d52016-02-05 03:07:06 +0000886If no usable groups are found in this file then fixed internal groups will
887be used.
Damien Millerd8702e82006-03-15 11:33:56 +1100888.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000889.It Pa /etc/motd
Damien Millerd8702e82006-03-15 11:33:56 +1100890See
891.Xr motd 5 .
892.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000893.It Pa /etc/nologin
Damien Millerd8702e82006-03-15 11:33:56 +1100894If this file exists,
895.Nm
896refuses to let anyone except root log in.
897The contents of the file
898are displayed to anyone trying to log in, and non-root connections are
899refused.
900The file should be world-readable.
901.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000902.It Pa /etc/shosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100903This file is used in exactly the same way as
904.Pa hosts.equiv ,
905but allows host-based authentication without permitting login with
906rlogin/rsh.
Damien Millerd8702e82006-03-15 11:33:56 +1100907.Pp
Damien Millereb8b60e2010-08-31 22:41:14 +1000908.It Pa /etc/ssh/ssh_host_ecdsa_key
Damien Miller8ba0ead2013-12-18 17:46:27 +1100909.It Pa /etc/ssh/ssh_host_ed25519_key
Damien Millerafdae612010-08-31 22:31:14 +1000910.It Pa /etc/ssh/ssh_host_rsa_key
Damien Millerb9902cf2012-06-20 21:52:58 +1000911These files contain the private parts of the host keys.
Damien Millerd8702e82006-03-15 11:33:56 +1100912These files should only be owned by root, readable only by root, and not
913accessible to others.
914Note that
915.Nm
Damien Milleredd03752006-03-15 11:36:45 +1100916does not start if these files are group/world-accessible.
Damien Millerd8702e82006-03-15 11:33:56 +1100917.Pp
Damien Millereb8b60e2010-08-31 22:41:14 +1000918.It Pa /etc/ssh/ssh_host_ecdsa_key.pub
Damien Miller8ba0ead2013-12-18 17:46:27 +1100919.It Pa /etc/ssh/ssh_host_ed25519_key.pub
Damien Millerafdae612010-08-31 22:31:14 +1000920.It Pa /etc/ssh/ssh_host_rsa_key.pub
Damien Millerb9902cf2012-06-20 21:52:58 +1000921These files contain the public parts of the host keys.
Damien Millerd8702e82006-03-15 11:33:56 +1100922These files should be world-readable but writable only by
923root.
924Their contents should match the respective private parts.
925These files are not
926really used for anything; they are provided for the convenience of
927the user so their contents can be copied to known hosts files.
928These files are created using
929.Xr ssh-keygen 1 .
930.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000931.It Pa /etc/ssh/ssh_known_hosts
Darren Tucker04354b92007-03-21 20:46:54 +1100932Systemwide list of known host keys.
933This file should be prepared by the
934system administrator to contain the public host keys of all machines in the
935organization.
936The format of this file is described above.
937This file should be writable only by root/the owner and
938should be world-readable.
939.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000940.It Pa /etc/ssh/sshd_config
Damien Millerd8702e82006-03-15 11:33:56 +1100941Contains configuration data for
942.Nm sshd .
943The file format and configuration options are described in
944.Xr sshd_config 5 .
945.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000946.It Pa /etc/ssh/sshrc
Damien Millerfd725cf2006-03-15 11:35:54 +1100947Similar to
948.Pa ~/.ssh/rc ,
949it can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000950machine-specific login-time initializations globally.
951This file should be writable only by root, and should be world-readable.
Damien Millerd8702e82006-03-15 11:33:56 +1100952.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000953.It Pa /var/empty
Damien Millerd8702e82006-03-15 11:33:56 +1100954.Xr chroot 2
955directory used by
956.Nm
957during privilege separation in the pre-authentication phase.
958The directory should not contain any files and must be owned by root
959and not group or world-writable.
960.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000961.It Pa /var/run/sshd.pid
Damien Millerd8702e82006-03-15 11:33:56 +1100962Contains the process ID of the
963.Nm
964listening for connections (if there are several daemons running
965concurrently for different ports, this contains the process ID of the one
966started last).
967The content of this file is not sensitive; it can be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000968.El
Damien Miller32aa1441999-10-29 09:15:49 +1000969.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000970.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100971.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000972.Xr ssh 1 ,
973.Xr ssh-add 1 ,
974.Xr ssh-agent 1 ,
975.Xr ssh-keygen 1 ,
Darren Tucker04354b92007-03-21 20:46:54 +1100976.Xr ssh-keyscan 1 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000977.Xr chroot 2 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000978.Xr login.conf 5 ,
979.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000980.Xr sshd_config 5 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000981.Xr inetd 8 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000982.Xr sftp-server 8
Damien Millerf1ce5052003-06-11 22:04:39 +1000983.Sh AUTHORS
984OpenSSH is a derivative of the original and free
985ssh 1.2.12 release by Tatu Ylonen.
986Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
987Theo de Raadt and Dug Song
988removed many bugs, re-added newer features and
989created OpenSSH.
990Markus Friedl contributed the support for SSH
991protocol versions 1.5 and 2.0.
992Niels Provos and Markus Friedl contributed support
993for privilege separation.