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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.\"
Damien Millerb9132fc2011-05-29 21:41:40 +100036.\" $OpenBSD: sshd.8,v 1.262 2011/05/23 07:10:21 jmc Exp $
Damien Millerd8478b62011-05-29 21:39:36 +100037.Dd $Mdocdate: May 23 2011 $
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
Damien Miller32aa1441999-10-29 09:15:49 +100047.Op Fl b Ar bits
Darren Tuckere7140f22008-06-10 23:01:51 +100048.Op Fl C Ar connection_spec
Damien Miller0a80ca12010-02-27 07:55:05 +110049.Op Fl c Ar host_certificate_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
53.Op Fl k Ar key_gen_time
Ben Lindstromade03f62001-12-06 18:22:17 +000054.Op Fl o Ar option
Damien Miller32aa1441999-10-29 09:15:49 +100055.Op Fl p Ar port
Damien Miller942da032000-08-18 13:59:06 +100056.Op Fl u Ar len
Damien Millerffadc582003-02-24 11:52:26 +110057.Ek
Damien Miller22c77262000-04-13 12:26:34 +100058.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100059.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110060(OpenSSH Daemon) is the daemon program for
Damien Miller32aa1441999-10-29 09:15:49 +100061.Xr ssh 1 .
Darren Tucker04354b92007-03-21 20:46:54 +110062Together these programs replace
63.Xr rlogin 1
64and
65.Xr rsh 1 ,
66and provide secure encrypted communications between two untrusted hosts
Damien Miller450a7a12000-03-26 13:04:51 +100067over an insecure network.
Damien Miller32aa1441999-10-29 09:15:49 +100068.Pp
69.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110070listens for connections from clients.
Damien Miller22c77262000-04-13 12:26:34 +100071It is normally started at boot from
Damien Miller32aa1441999-10-29 09:15:49 +100072.Pa /etc/rc .
73It forks a new
Damien Miller450a7a12000-03-26 13:04:51 +100074daemon for each incoming connection.
75The forked daemons handle
Damien Miller32aa1441999-10-29 09:15:49 +100076key exchange, encryption, authentication, command execution,
77and data exchange.
Damien Miller32aa1441999-10-29 09:15:49 +100078.Pp
79.Nm
Darren Tucker1f203942003-10-15 15:50:42 +100080can be configured using command-line options or a configuration file
81(by default
Damien Miller99cc4a82006-01-31 21:45:53 +110082.Xr sshd_config 5 ) ;
83command-line options override values specified in the
Damien Miller32aa1441999-10-29 09:15:49 +100084configuration file.
Damien Miller6162d121999-11-21 13:23:52 +110085.Nm
86rereads its configuration file when it receives a hangup signal,
Ben Lindstrom49a098d2001-03-05 06:55:18 +000087.Dv SIGHUP ,
Damien Miller208f1ed2006-03-15 11:56:03 +110088by executing itself with the name and options it was started with, e.g.\&
Ben Lindstrom49a098d2001-03-05 06:55:18 +000089.Pa /usr/sbin/sshd .
Damien Miller6162d121999-11-21 13:23:52 +110090.Pp
Damien Miller32aa1441999-10-29 09:15:49 +100091The options are as follows:
92.Bl -tag -width Ds
Darren Tucker1f203942003-10-15 15:50:42 +100093.It Fl 4
94Forces
95.Nm
96to use IPv4 addresses only.
97.It Fl 6
98Forces
99.Nm
100to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000101.It Fl b Ar bits
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000102Specifies the number of bits in the ephemeral protocol version 1
Darren Tucker7499b0c2008-07-02 22:35:43 +1000103server key (default 1024).
Darren Tuckere7f3f752008-06-10 23:06:01 +1000104.It Fl C Ar connection_spec
105Specify the connection parameters to use for the
106.Fl T
107extended test mode.
108If provided, any
109.Cm Match
110directives in the configuration file
111that would apply to the specified user, host, and address will be set before
112the configuration is written to standard output.
113The connection parameters are supplied as keyword=value pairs.
114The keywords are
115.Dq user ,
116.Dq host ,
117and
118.Dq addr .
119All are required and may be supplied in any order, either with multiple
120.Fl C
121options or as a comma-separated list.
Damien Miller15f5b562010-03-03 10:25:21 +1100122.It Fl c Ar host_certificate_file
123Specifies a path to a certificate file to identify
124.Nm
125during key exchange.
126The certificate file must match a host key file specified using the
127.Fl h
128option or the
129.Cm HostKey
130configuration directive.
Darren Tucker1f203942003-10-15 15:50:42 +1000131.It Fl D
132When this option is specified,
133.Nm
134will not detach and does not become a daemon.
135This allows easy monitoring of
136.Nm sshd .
Damien Miller32aa1441999-10-29 09:15:49 +1000137.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000138Debug mode.
Damien Miller74d98252010-02-02 17:01:46 +1100139The server sends verbose debug output to standard error,
140and does not put itself in the background.
Damien Miller450a7a12000-03-26 13:04:51 +1000141The server also will not fork and will only process one connection.
142This option is only intended for debugging for the server.
Damien Millerffadc582003-02-24 11:52:26 +1100143Multiple
144.Fl d
145options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100146Maximum is 3.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000147.It Fl e
148When this option is specified,
149.Nm
150will send the output to the standard error instead of the system log.
Darren Tucker04354b92007-03-21 20:46:54 +1100151.It Fl f Ar config_file
Damien Miller450a7a12000-03-26 13:04:51 +1000152Specifies the name of the configuration file.
153The default is
Damien Miller05eda432002-02-10 18:32:28 +1100154.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000155.Nm
156refuses to start if there is no configuration file.
157.It Fl g Ar login_grace_time
158Gives the grace time for clients to authenticate themselves (default
Damien Millerc1348632002-09-05 14:35:14 +1000159120 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000160If the client fails to authenticate the user within
161this many seconds, the server disconnects and exits.
162A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000163.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100164Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000165This option must be given if
166.Nm
167is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100168host key files are normally not readable by anyone but root).
169The default is
Damien Miller05eda432002-02-10 18:32:28 +1100170.Pa /etc/ssh/ssh_host_key
Damien Miller7fc23732002-01-22 23:19:11 +1100171for protocol version 1, and
Damien Millereb8b60e2010-08-31 22:41:14 +1000172.Pa /etc/ssh/ssh_host_dsa_key ,
173.Pa /etc/ssh/ssh_host_ecdsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100174and
Damien Millereb8b60e2010-08-31 22:41:14 +1000175.Pa /etc/ssh/ssh_host_rsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100176for protocol version 2.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000177It is possible to have multiple host key files for
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000178the different protocol versions and host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000179.It Fl i
180Specifies that
181.Nm
Damien Millerffadc582003-02-24 11:52:26 +1100182is being run from
183.Xr inetd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +1000184.Nm
185is normally not run
186from inetd because it needs to generate the server key before it can
Damien Miller450a7a12000-03-26 13:04:51 +1000187respond to the client, and this may take tens of seconds.
188Clients would have to wait too long if the key was regenerated every time.
Damien Miller208f1ed2006-03-15 11:56:03 +1100189However, with small key sizes (e.g. 512) using
Damien Miller32aa1441999-10-29 09:15:49 +1000190.Nm
191from inetd may
192be feasible.
193.It Fl k Ar key_gen_time
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000194Specifies how often the ephemeral protocol version 1 server key is
195regenerated (default 3600 seconds, or one hour).
Damien Miller450a7a12000-03-26 13:04:51 +1000196The motivation for regenerating the key fairly
Darren Tucker1f203942003-10-15 15:50:42 +1000197often is that the key is not stored anywhere, and after about an hour
Damien Miller32aa1441999-10-29 09:15:49 +1000198it becomes impossible to recover the key for decrypting intercepted
199communications even if the machine is cracked into or physically
Damien Miller450a7a12000-03-26 13:04:51 +1000200seized.
201A value of zero indicates that the key will never be regenerated.
Ben Lindstromade03f62001-12-06 18:22:17 +0000202.It Fl o Ar option
203Can be used to give options in the format used in the configuration file.
204This is useful for specifying options for which there is no separate
205command-line flag.
Darren Tucker1f203942003-10-15 15:50:42 +1000206For full details of the options, and their values, see
207.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000208.It Fl p Ar port
209Specifies the port on which the server listens for connections
210(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100211Multiple port options are permitted.
Damien Millerd7f308f2005-12-24 14:55:16 +1100212Ports specified in the configuration file with the
213.Cm Port
214option are ignored when a command-line port is specified.
215Ports specified using the
216.Cm ListenAddress
217option override command-line ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000218.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000219Quiet mode.
220Nothing is sent to the system log.
221Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000222authentication, and termination of each connection is logged.
Darren Tuckere7140f22008-06-10 23:01:51 +1000223.It Fl T
224Extended test mode.
225Check the validity of the configuration file, output the effective configuration
226to stdout and then exit.
227Optionally,
228.Cm Match
229rules may be applied by specifying the connection parameters using one or more
230.Fl C
231options.
Darren Tuckere7f3f752008-06-10 23:06:01 +1000232.It Fl t
233Test mode.
234Only check the validity of the configuration file and sanity of the keys.
235This is useful for updating
236.Nm
237reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000238.It Fl u Ar len
239This option is used to specify the size of the field
240in the
241.Li utmp
242structure that holds the remote host name.
243If the resolved host name is longer than
244.Ar len ,
245the dotted decimal value will be used instead.
246This allows hosts with very long host names that
247overflow this field to still be uniquely identified.
248Specifying
249.Fl u0
250indicates that only dotted decimal addresses
251should be put into the
252.Pa utmp
253file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000254.Fl u0
Damien Millerffadc582003-02-24 11:52:26 +1100255may also be used to prevent
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000256.Nm
257from making DNS requests unless the authentication
258mechanism or configuration requires it.
259Authentication mechanisms that may require DNS include
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000260.Cm RhostsRSAAuthentication ,
Damien Miller2ac05772006-02-01 22:05:42 +1100261.Cm HostbasedAuthentication ,
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000262and using a
263.Cm from="pattern-list"
264option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000265Configuration options that require DNS include using a
266USER@HOST pattern in
267.Cm AllowUsers
268or
269.Cm DenyUsers .
Damien Miller32aa1441999-10-29 09:15:49 +1000270.El
Damien Miller8bbdf902006-02-01 22:05:25 +1100271.Sh AUTHENTICATION
272The OpenSSH SSH daemon supports SSH protocols 1 and 2.
Darren Tucker7a4a7652009-10-11 21:51:40 +1100273The default is to use protocol 2 only,
Damien Miller8bbdf902006-02-01 22:05:25 +1100274though this can be changed via the
275.Cm Protocol
276option in
277.Xr sshd_config 5 .
Damien Millereb8b60e2010-08-31 22:41:14 +1000278Protocol 2 supports DSA, ECDSA and RSA keys;
Damien Miller8bbdf902006-02-01 22:05:25 +1100279protocol 1 only supports RSA keys.
280For both protocols,
281each host has a host-specific key,
282normally 2048 bits,
283used to identify the host.
Damien Miller99cc4a82006-01-31 21:45:53 +1100284.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100285Forward security for protocol 1 is provided through
286an additional server key,
287normally 768 bits,
288generated when the server starts.
Damien Miller99cc4a82006-01-31 21:45:53 +1100289This key is normally regenerated every hour if it has been used, and
290is never stored on disk.
Damien Miller99cc4a82006-01-31 21:45:53 +1100291Whenever a client connects, the daemon responds with its public
292host and server keys.
293The client compares the
294RSA host key against its own database to verify that it has not changed.
295The client then generates a 256-bit random number.
296It encrypts this
297random number using both the host key and the server key, and sends
298the encrypted number to the server.
299Both sides then use this
300random number as a session key which is used to encrypt all further
301communications in the session.
302The rest of the session is encrypted
303using a conventional cipher, currently Blowfish or 3DES, with 3DES
304being used by default.
305The client selects the encryption algorithm
306to use from those offered by the server.
307.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100308For protocol 2,
309forward security is provided through a Diffie-Hellman key agreement.
310This key agreement results in a shared session key.
311The rest of the session is encrypted using a symmetric cipher, currently
312128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
313The client selects the encryption algorithm
314to use from those offered by the server.
315Additionally, session integrity is provided
316through a cryptographic message authentication code
Damien Millere45796f2007-06-11 14:01:42 +1000317(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
Damien Miller8bbdf902006-02-01 22:05:25 +1100318.Pp
319Finally, the server and the client enter an authentication dialog.
Damien Miller99cc4a82006-01-31 21:45:53 +1100320The client tries to authenticate itself using
Damien Miller8bbdf902006-02-01 22:05:25 +1100321host-based authentication,
322public key authentication,
323challenge-response authentication,
324or password authentication.
Damien Miller99cc4a82006-01-31 21:45:53 +1100325.Pp
326Regardless of the authentication type, the account is checked to
327ensure that it is accessible. An account is not accessible if it is
328locked, listed in
329.Cm DenyUsers
330or its group is listed in
331.Cm DenyGroups
332\&. The definition of a locked account is system dependant. Some platforms
333have their own account database (eg AIX) and some modify the passwd field (
334.Ql \&*LK\&*
335on Solaris and UnixWare,
336.Ql \&*
337on HP-UX, containing
338.Ql Nologin
339on Tru64,
340a leading
341.Ql \&*LOCKED\&*
342on FreeBSD and a leading
Darren Tucker1a9176b2007-08-17 09:42:32 +1000343.Ql \&!
344on most Linuxes).
345If there is a requirement to disable password authentication
Damien Miller99cc4a82006-01-31 21:45:53 +1100346for the account while allowing still public-key, then the passwd field
347should be set to something other than these values (eg
348.Ql NP
349or
350.Ql \&*NP\&*
351).
352.Pp
Damien Miller99cc4a82006-01-31 21:45:53 +1100353If the client successfully authenticates itself, a dialog for
354preparing the session is entered.
355At this time the client may request
356things like allocating a pseudo-tty, forwarding X11 connections,
357forwarding TCP connections, or forwarding the authentication agent
358connection over the secure channel.
359.Pp
Damien Millerc47d7e92006-03-15 11:27:20 +1100360After this, the client either requests a shell or execution of a command.
Damien Miller99cc4a82006-01-31 21:45:53 +1100361The sides then enter session mode.
362In this mode, either side may send
363data at any time, and such data is forwarded to/from the shell or
364command on the server side, and the user terminal in the client side.
365.Pp
366When the user program terminates and all forwarded X11 and other
367connections have been closed, the server sends command exit status to
368the client, and both sides exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000369.Sh LOGIN PROCESS
370When a user successfully logs in,
371.Nm
372does the following:
373.Bl -enum -offset indent
374.It
375If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000376prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000377.Pa /etc/motd
378(unless prevented in the configuration file or by
Damien Miller167ea5d2005-05-26 12:04:02 +1000379.Pa ~/.hushlogin ;
Damien Miller32aa1441999-10-29 09:15:49 +1000380see the
Damien Miller22c77262000-04-13 12:26:34 +1000381.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000382section).
383.It
384If the login is on a tty, records login time.
385.It
386Checks
387.Pa /etc/nologin ;
388if it exists, prints contents and quits
389(unless root).
390.It
391Changes to run with normal user privileges.
392.It
393Sets up basic environment.
394.It
Darren Tucker1f203942003-10-15 15:50:42 +1000395Reads the file
Damien Miller167ea5d2005-05-26 12:04:02 +1000396.Pa ~/.ssh/environment ,
Darren Tucker1f203942003-10-15 15:50:42 +1000397if it exists, and users are allowed to change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000398See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000399.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000400option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000401.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000402.It
403Changes to user's home directory.
404.It
405If
Damien Miller167ea5d2005-05-26 12:04:02 +1000406.Pa ~/.ssh/rc
Damien Miller32aa1441999-10-29 09:15:49 +1000407exists, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100408.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000409exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000410it; otherwise runs xauth.
411The
Damien Miller32aa1441999-10-29 09:15:49 +1000412.Dq rc
413files are given the X11
414authentication protocol and cookie in standard input.
Damien Millerfd725cf2006-03-15 11:35:54 +1100415See
416.Sx SSHRC ,
417below.
Damien Miller32aa1441999-10-29 09:15:49 +1000418.It
419Runs user's shell or command.
420.El
Damien Millerfd725cf2006-03-15 11:35:54 +1100421.Sh SSHRC
422If the file
423.Pa ~/.ssh/rc
424exists,
425.Xr sh 1
426runs it after reading the
427environment files but before starting the user's shell or command.
428It must not produce any output on stdout; stderr must be used
429instead.
430If X11 forwarding is in use, it will receive the "proto cookie" pair in
431its standard input (and
432.Ev DISPLAY
433in its environment).
434The script must call
435.Xr xauth 1
436because
437.Nm
438will not run xauth automatically to add X11 cookies.
439.Pp
440The primary purpose of this file is to run any initialization routines
441which may be needed before the user's home directory becomes
442accessible; AFS is a particular example of such an environment.
443.Pp
444This file will probably contain some initialization code followed by
445something similar to:
446.Bd -literal -offset 3n
447if read proto cookie && [ -n "$DISPLAY" ]; then
448 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
449 # X11UseLocalhost=yes
450 echo add unix:`echo $DISPLAY |
451 cut -c11-` $proto $cookie
452 else
453 # X11UseLocalhost=no
454 echo add $DISPLAY $proto $cookie
455 fi | xauth -q -
456fi
457.Ed
458.Pp
459If this file does not exist,
460.Pa /etc/ssh/sshrc
461is run, and if that
462does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000463.Sh AUTHORIZED_KEYS FILE FORMAT
Ben Lindstromf96704d2001-06-25 04:17:12 +0000464.Cm AuthorizedKeysFile
Damien Millerb9132fc2011-05-29 21:41:40 +1000465specifies the files containing public keys for
Damien Millerdcfea272006-03-15 11:31:22 +1100466public key authentication;
Damien Millerb9132fc2011-05-29 21:41:40 +1000467if none is specified, the default is
Damien Millerd8478b62011-05-29 21:39:36 +1000468.Pa ~/.ssh/authorized_keys
469and
470.Pa ~/.ssh/authorized_keys2 .
Damien Miller450a7a12000-03-26 13:04:51 +1000471Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000472key (empty lines and lines starting with a
473.Ql #
474are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000475comments).
Damien Millerdcfea272006-03-15 11:31:22 +1100476Protocol 1 public keys consist of the following space-separated fields:
477options, bits, exponent, modulus, comment.
478Protocol 2 public key consist of:
479options, keytype, base64-encoded key, comment.
480The options field is optional;
481its presence is determined by whether the line starts
Damien Millerb83df8d2002-09-04 16:24:55 +1000482with a number or not (the options field never starts with a number).
Damien Millerdcfea272006-03-15 11:31:22 +1100483The bits, exponent, modulus, and comment fields give the RSA key for
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000484protocol version 1; the
Damien Miller32aa1441999-10-29 09:15:49 +1000485comment field is not used for anything (but may be convenient for the
486user to identify the key).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000487For protocol version 2 the keytype is
Damien Millereb8b60e2010-08-31 22:41:14 +1000488.Dq ecdsa-sha2-nistp256 ,
489.Dq ecdsa-sha2-nistp384 ,
490.Dq ecdsa-sha2-nistp521 ,
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000491.Dq ssh-dss
492or
493.Dq ssh-rsa .
Damien Miller32aa1441999-10-29 09:15:49 +1000494.Pp
495Note that lines in this file are usually several hundred bytes long
Darren Tucker22cc7412004-12-06 22:47:41 +1100496(because of the size of the public key encoding) up to a limit of
4978 kilobytes, which permits DSA keys up to 8 kilobits and RSA
498keys up to 16 kilobits.
Damien Miller450a7a12000-03-26 13:04:51 +1000499You don't want to type them in; instead, copy the
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000500.Pa identity.pub ,
Damien Millerdcfea272006-03-15 11:31:22 +1100501.Pa id_dsa.pub ,
Damien Millereb8b60e2010-08-31 22:41:14 +1000502.Pa id_ecdsa.pub ,
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000503or the
504.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000505file and edit it.
506.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000507.Nm
508enforces a minimum RSA key modulus size for protocol 1
509and protocol 2 keys of 768 bits.
510.Pp
Damien Miller942da032000-08-18 13:59:06 +1000511The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000512specifications.
513No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000514The following option specifications are supported (note
515that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000516.Bl -tag -width Ds
Damien Millercd38c9c2010-03-04 21:51:37 +1100517.It Cm cert-authority
Damien Miller0a80ca12010-02-27 07:55:05 +1100518Specifies that the listed key is a certification authority (CA) that is
519trusted to validate signed certificates for user authentication.
520.Pp
521Certificates may encode access restrictions similar to these key options.
522If both certificate restrictions and key options are present, the most
523restrictive union of the two is applied.
Damien Miller32aa1441999-10-29 09:15:49 +1000524.It Cm command="command"
525Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000526authentication.
527The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000528The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000529otherwise it is run without a tty.
Damien Millerffadc582003-02-24 11:52:26 +1100530If an 8-bit clean channel is required,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000531one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100532.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000533A quote may be included in the command by quoting it with a backslash.
534This option might be useful
Damien Millerb83df8d2002-09-04 16:24:55 +1000535to restrict certain public keys to perform just a specific operation.
Damien Miller450a7a12000-03-26 13:04:51 +1000536An example might be a key that permits remote backups but nothing else.
Damien Miller7c24b812006-01-14 10:09:56 +1100537Note that the client may specify TCP and/or X11
Damien Miller30c3d422000-05-09 11:02:59 +1000538forwarding unless they are explicitly prohibited.
Damien Millere2754432006-07-24 14:06:47 +1000539The command originally supplied by the client is available in the
540.Ev SSH_ORIGINAL_COMMAND
541environment variable.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000542Note that this option applies to shell, command or subsystem execution.
Damien Miller0a80ca12010-02-27 07:55:05 +1100543Also note that this command may be superseded by either a
544.Xr sshd_config 5
545.Cm ForceCommand
546directive or a command embedded in a certificate.
Damien Miller32aa1441999-10-29 09:15:49 +1000547.It Cm environment="NAME=value"
548Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000549logging in using this key.
550Environment variables set this way
551override other default environment values.
552Multiple options of this type are permitted.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000553Environment processing is disabled by default and is
554controlled via the
555.Cm PermitUserEnvironment
556option.
Ben Lindstrom38b951c2001-12-06 17:47:47 +0000557This option is automatically disabled if
558.Cm UseLogin
559is enabled.
Damien Miller31bdc522006-03-15 11:31:44 +1100560.It Cm from="pattern-list"
Darren Tucker896ad5a2008-06-11 09:34:46 +1000561Specifies that in addition to public key authentication, either the canonical
Darren Tucker11996732008-06-13 04:32:00 +1000562name of the remote host or its IP address must be present in the
Darren Tucker896ad5a2008-06-11 09:34:46 +1000563comma-separated list of patterns.
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100564See
565.Sx PATTERNS
566in
567.Xr ssh_config 5
568for more information on patterns.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000569.Pp
570In addition to the wildcard matching that may be applied to hostnames or
571addresses, a
572.Cm from
Darren Tucker5837b512009-06-21 17:52:27 +1000573stanza may match IP addresses using CIDR address/masklen notation.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000574.Pp
575The purpose of this option is to optionally increase security: public key
576authentication by itself does not trust the network or name servers or
577anything (but the key); however, if somebody somehow steals the key, the key
578permits an intruder to log in from anywhere in the world.
579This additional option makes using a stolen key more difficult (name
580servers and/or routers would have to be compromised in addition to
581just the key).
Damien Miller31bdc522006-03-15 11:31:44 +1100582.It Cm no-agent-forwarding
583Forbids authentication agent forwarding when this key is used for
584authentication.
Damien Miller32aa1441999-10-29 09:15:49 +1000585.It Cm no-port-forwarding
Damien Miller7c24b812006-01-14 10:09:56 +1100586Forbids TCP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000587Any port forward requests by the client will return an error.
Damien Miller208f1ed2006-03-15 11:56:03 +1100588This might be used, e.g. in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000589.Cm command
590option.
Damien Miller31bdc522006-03-15 11:31:44 +1100591.It Cm no-pty
592Prevents tty allocation (a request to allocate a pty will fail).
Damien Miller95e80952008-03-27 11:03:05 +1100593.It Cm no-user-rc
Damien Miller13ba9c22008-04-03 20:52:51 +1100594Disables execution of
Damien Miller95e80952008-03-27 11:03:05 +1100595.Pa ~/.ssh/rc .
Damien Miller32aa1441999-10-29 09:15:49 +1000596.It Cm no-X11-forwarding
597Forbids X11 forwarding when this key is used for authentication.
598Any X11 forward requests by the client will return an error.
Damien Millera243fde2001-03-19 23:16:08 +1100599.It Cm permitopen="host:port"
Ben Lindstrom24643222001-06-25 05:08:11 +0000600Limit local
Damien Millera243fde2001-03-19 23:16:08 +1100601.Li ``ssh -L''
Ben Lindstrom4b3564e2001-04-10 02:41:56 +0000602port forwarding such that it may only connect to the specified host and
Ben Lindstromd71ba572001-09-12 18:03:31 +0000603port.
Damien Miller7fa96602010-08-05 13:03:13 +1000604IPv6 addresses can be specified by enclosing the address in square brackets.
Ben Lindstromd71ba572001-09-12 18:03:31 +0000605Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100606.Cm permitopen
Damien Millerfbf486b2003-05-23 18:44:23 +1000607options may be applied separated by commas.
608No pattern matching is performed on the specified hostnames,
609they must be literal domains or addresses.
Damien Miller30da3442010-05-10 11:58:03 +1000610.It Cm principals="principals"
611On a
612.Cm cert-authority
613line, specifies allowed principals for certificate authentication as a
614comma-separated list.
615At least one name from the list must appear in the certificate's
616list of principals for the certificate to be accepted.
617This option is ignored for keys that are not marked as trusted certificate
618signers using the
619.Cm cert-authority
620option.
Damien Millerd27b9472005-12-13 19:29:02 +1100621.It Cm tunnel="n"
622Force a
623.Xr tun 4
624device on the server.
625Without this option, the next available device will be used if
626the client requests a tunnel.
Damien Miller32aa1441999-10-29 09:15:49 +1000627.El
Damien Miller32aa1441999-10-29 09:15:49 +1000628.Pp
Damien Miller7d2ef022006-03-15 11:32:06 +1100629An example authorized_keys file:
Damien Millerc8f61cf2006-03-15 11:33:25 +1100630.Bd -literal -offset 3n
Damien Miller7d2ef022006-03-15 11:32:06 +1100631# Comments allowed at start of line
632ssh-rsa AAAAB3Nza...LiPk== user@example.net
Damien Millerc8f61cf2006-03-15 11:33:25 +1100633from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
634AAAAB2...19Q== john@example.net
635command="dump /home",no-pty,no-port-forwarding ssh-dss
636AAAAC3...51R== example.net
637permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
638AAAAB5...21S==
639tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
640jane@example.net
Damien Miller7d2ef022006-03-15 11:32:06 +1100641.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000642.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000643The
Damien Millerffadc582003-02-24 11:52:26 +1100644.Pa /etc/ssh/ssh_known_hosts
Damien Miller22c77262000-04-13 12:26:34 +1000645and
Damien Miller167ea5d2005-05-26 12:04:02 +1000646.Pa ~/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000647files contain host public keys for all known hosts.
648The global file should
649be prepared by the administrator (optional), and the per-user file is
Damien Miller9a7f2012006-03-15 11:32:42 +1100650maintained automatically: whenever the user connects from an unknown host,
Damien Miller450a7a12000-03-26 13:04:51 +1000651its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000652.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100653Each line in these files contains the following fields: markers (optional),
654hostnames, bits, exponent, modulus, comment.
Damien Miller450a7a12000-03-26 13:04:51 +1000655The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000656.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100657The marker is optional, but if it is present then it must be one of
658.Dq @cert-authority ,
659to indicate that the line contains a certification authority (CA) key,
660or
661.Dq @revoked ,
662to indicate that the key contained on the line is revoked and must not ever
663be accepted.
664Only one marker should be used on a key line.
665.Pp
Damien Miller31554322003-05-14 13:44:58 +1000666Hostnames is a comma-separated list of patterns
Damien Miller9a7f2012006-03-15 11:32:42 +1100667.Pf ( Ql *
Damien Millerfbf486b2003-05-23 18:44:23 +1000668and
669.Ql \&?
Damien Miller049245d2003-05-14 13:44:42 +1000670act as
Damien Miller32aa1441999-10-29 09:15:49 +1000671wildcards); each pattern in turn is matched against the canonical host
672name (when authenticating a client) or against the user-supplied
Damien Miller450a7a12000-03-26 13:04:51 +1000673name (when authenticating a server).
674A pattern may also be preceded by
Damien Miller049245d2003-05-14 13:44:42 +1000675.Ql \&!
Damien Miller32aa1441999-10-29 09:15:49 +1000676to indicate negation: if the host name matches a negated
677pattern, it is not accepted (by that line) even if it matched another
678pattern on the line.
Darren Tuckerda345532006-07-10 23:04:19 +1000679A hostname or address may optionally be enclosed within
680.Ql \&[
681and
682.Ql \&]
683brackets then followed by
684.Ql \&:
Darren Tucker11318472006-07-12 22:07:59 +1000685and a non-standard port number.
Damien Miller32aa1441999-10-29 09:15:49 +1000686.Pp
Damien Millere1776152005-03-01 21:47:37 +1100687Alternately, hostnames may be stored in a hashed form which hides host names
Damien Miller718fd4b2005-03-02 12:03:23 +1100688and addresses should the file's contents be disclosed.
689Hashed hostnames start with a
690.Ql |
Damien Millere1776152005-03-01 21:47:37 +1100691character.
692Only one hashed hostname may appear on a single line and none of the above
693negation or wildcard operators may be applied.
694.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000695Bits, exponent, and modulus are taken directly from the RSA host key; they
Damien Miller9a7f2012006-03-15 11:32:42 +1100696can be obtained, for example, from
Damien Miller05eda432002-02-10 18:32:28 +1100697.Pa /etc/ssh/ssh_host_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000698The optional comment field continues to the end of the line, and is not used.
699.Pp
700Lines starting with
701.Ql #
702and empty lines are ignored as comments.
703.Pp
704When performing host authentication, authentication is accepted if any
Damien Millera7dab8b2010-03-05 10:42:05 +1100705matching line has the proper key; either one that matches exactly or,
706if the server has presented a certificate for authentication, the key
707of the certification authority that signed the certificate.
708For a key to be trusted as a certification authority, it must use the
709.Dq @cert-authority
710marker described above.
711.Pp
712The known hosts file also provides a facility to mark keys as revoked,
713for example when it is known that the associated private key has been
714stolen.
715Revoked keys are specified by including the
716.Dq @revoked
717marker at the beginning of the key line, and are never accepted for
718authentication or as certification authorities, but instead will
719produce a warning from
720.Xr ssh 1
721when they are encountered.
722.Pp
723It is permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000724recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000725names.
726This will inevitably happen when short forms of host names
727from different domains are put in the file.
728It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000729that the files contain conflicting information; authentication is
730accepted if valid information can be found from either file.
731.Pp
732Note that the lines in these files are typically hundreds of characters
733long, and you definitely don't want to type in the host keys by hand.
Damien Millera7dab8b2010-03-05 10:42:05 +1100734Rather, generate them by a script,
735.Xr ssh-keyscan 1
Damien Miller22c77262000-04-13 12:26:34 +1000736or by taking
Damien Miller05eda432002-02-10 18:32:28 +1100737.Pa /etc/ssh/ssh_host_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000738and adding the host names at the front.
Damien Millera7dab8b2010-03-05 10:42:05 +1100739.Xr ssh-keygen 1
Damien Miller98339052010-03-05 21:30:35 +1100740also offers some basic automated editing for
Damien Millera7dab8b2010-03-05 10:42:05 +1100741.Pa ~/.ssh/known_hosts
742including removing hosts matching a host name and converting all host
743names to their hashed representations.
Damien Millercc00f5e2006-03-15 11:33:00 +1100744.Pp
745An example ssh_known_hosts file:
746.Bd -literal -offset 3n
747# Comments allowed at start of line
748closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
749cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
Damien Millere1776152005-03-01 21:47:37 +1100750# A hashed hostname
751|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
752AAAA1234.....=
Damien Millera7dab8b2010-03-05 10:42:05 +1100753# A revoked key
754@revoked * ssh-rsa AAAAB5W...
755# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
756@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
Damien Millere1776152005-03-01 21:47:37 +1100757.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000758.Sh FILES
Damien Millerd8702e82006-03-15 11:33:56 +1100759.Bl -tag -width Ds -compact
Damien Millerafdae612010-08-31 22:31:14 +1000760.It Pa ~/.hushlogin
Damien Miller70a908e2005-03-01 21:17:09 +1100761This file is used to suppress printing the last login time and
762.Pa /etc/motd ,
763if
764.Cm PrintLastLog
765and
766.Cm PrintMotd ,
767respectively,
768are enabled.
769It does not suppress printing of the banner specified by
770.Cm Banner .
Damien Millerd8702e82006-03-15 11:33:56 +1100771.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000772.It Pa ~/.rhosts
Damien Milleradc35b92006-03-15 11:35:27 +1100773This file is used for host-based authentication (see
774.Xr ssh 1
775for more information).
776On some machines this file may need to be
777world-readable if the user's home directory is on an NFS partition,
778because
779.Nm
780reads it as root.
781Additionally, this file must be owned by the user,
782and must not have write permissions for anyone else.
783The recommended
784permission for most machines is read/write for the user, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000785accessible by others.
786.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000787.It Pa ~/.shosts
Damien Milleradc35b92006-03-15 11:35:27 +1100788This file is used in exactly the same way as
789.Pa .rhosts ,
790but allows host-based authentication without permitting login with
791rlogin/rsh.
Damien Miller32aa1441999-10-29 09:15:49 +1000792.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000793.It Pa ~/.ssh/
Damien Miller520e6152008-02-10 22:46:22 +1100794This directory is the default location for all user-specific configuration
795and authentication information.
796There is no general requirement to keep the entire contents of this directory
797secret, but the recommended permissions are read/write/execute for the user,
798and not accessible by others.
799.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000800.It Pa ~/.ssh/authorized_keys
Damien Millereb8b60e2010-08-31 22:41:14 +1000801Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in
802as this user.
Damien Millerd8702e82006-03-15 11:33:56 +1100803The format of this file is described above.
Damien Miller5d43d492006-08-30 11:07:00 +1000804The content of the file is not highly sensitive, but the recommended
Damien Milleradc35b92006-03-15 11:35:27 +1100805permissions are read/write for the user, and not accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000806.Pp
Damien Miller5d43d492006-08-30 11:07:00 +1000807If this file, the
808.Pa ~/.ssh
809directory, or the user's home directory are writable
810by other users, then the file could be modified or replaced by unauthorized
811users.
812In this case,
813.Nm
814will not allow it to be used unless the
815.Cm StrictModes
816option has been set to
817.Dq no .
Damien Miller5d43d492006-08-30 11:07:00 +1000818.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000819.It Pa ~/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000820This file is read into the environment at login (if it exists).
821It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000822.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000823and assignment lines of the form name=value.
824The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000825only by the user; it need not be readable by anyone else.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000826Environment processing is disabled by default and is
827controlled via the
828.Cm PermitUserEnvironment
829option.
Damien Millerd8702e82006-03-15 11:33:56 +1100830.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000831.It Pa ~/.ssh/known_hosts
Damien Milleradc35b92006-03-15 11:35:27 +1100832Contains a list of host keys for all hosts the user has logged into
833that are not already in the systemwide list of known host keys.
834The format of this file is described above.
835This file should be writable only by root/the owner and
Damien Millerd8702e82006-03-15 11:33:56 +1100836can, but need not be, world-readable.
837.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000838.It Pa ~/.ssh/rc
Damien Millerfd725cf2006-03-15 11:35:54 +1100839Contains initialization routines to be run before
840the user's home directory becomes accessible.
Damien Miller32aa1441999-10-29 09:15:49 +1000841This file should be writable only by the user, and need not be
842readable by anyone else.
Damien Millerd8702e82006-03-15 11:33:56 +1100843.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000844.It Pa /etc/hosts.allow
845.It Pa /etc/hosts.deny
Damien Millerd8702e82006-03-15 11:33:56 +1100846Access controls that should be enforced by tcp-wrappers are defined here.
847Further details are described in
848.Xr hosts_access 5 .
849.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000850.It Pa /etc/hosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100851This file is for host-based authentication (see
852.Xr ssh 1 ) .
853It should only be writable by root.
Damien Millerd8702e82006-03-15 11:33:56 +1100854.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000855.It Pa /etc/moduli
Damien Millerd8702e82006-03-15 11:33:56 +1100856Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
857The file format is described in
858.Xr moduli 5 .
859.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000860.It Pa /etc/motd
Damien Millerd8702e82006-03-15 11:33:56 +1100861See
862.Xr motd 5 .
863.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000864.It Pa /etc/nologin
Damien Millerd8702e82006-03-15 11:33:56 +1100865If this file exists,
866.Nm
867refuses to let anyone except root log in.
868The contents of the file
869are displayed to anyone trying to log in, and non-root connections are
870refused.
871The file should be world-readable.
872.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000873.It Pa /etc/shosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100874This file is used in exactly the same way as
875.Pa hosts.equiv ,
876but allows host-based authentication without permitting login with
877rlogin/rsh.
Damien Millerd8702e82006-03-15 11:33:56 +1100878.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000879.It Pa /etc/ssh/ssh_host_key
880.It Pa /etc/ssh/ssh_host_dsa_key
Damien Millereb8b60e2010-08-31 22:41:14 +1000881.It Pa /etc/ssh/ssh_host_ecdsa_key
Damien Millerafdae612010-08-31 22:31:14 +1000882.It Pa /etc/ssh/ssh_host_rsa_key
Damien Millerd8702e82006-03-15 11:33:56 +1100883These three files contain the private parts of the host keys.
884These files should only be owned by root, readable only by root, and not
885accessible to others.
886Note that
887.Nm
Damien Milleredd03752006-03-15 11:36:45 +1100888does not start if these files are group/world-accessible.
Damien Millerd8702e82006-03-15 11:33:56 +1100889.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000890.It Pa /etc/ssh/ssh_host_key.pub
891.It Pa /etc/ssh/ssh_host_dsa_key.pub
Damien Millereb8b60e2010-08-31 22:41:14 +1000892.It Pa /etc/ssh/ssh_host_ecdsa_key.pub
Damien Millerafdae612010-08-31 22:31:14 +1000893.It Pa /etc/ssh/ssh_host_rsa_key.pub
Damien Millerd8702e82006-03-15 11:33:56 +1100894These three files contain the public parts of the host keys.
895These files should be world-readable but writable only by
896root.
897Their contents should match the respective private parts.
898These files are not
899really used for anything; they are provided for the convenience of
900the user so their contents can be copied to known hosts files.
901These files are created using
902.Xr ssh-keygen 1 .
903.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000904.It Pa /etc/ssh/ssh_known_hosts
Darren Tucker04354b92007-03-21 20:46:54 +1100905Systemwide list of known host keys.
906This file should be prepared by the
907system administrator to contain the public host keys of all machines in the
908organization.
909The format of this file is described above.
910This file should be writable only by root/the owner and
911should be world-readable.
912.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000913.It Pa /etc/ssh/sshd_config
Damien Millerd8702e82006-03-15 11:33:56 +1100914Contains configuration data for
915.Nm sshd .
916The file format and configuration options are described in
917.Xr sshd_config 5 .
918.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000919.It Pa /etc/ssh/sshrc
Damien Millerfd725cf2006-03-15 11:35:54 +1100920Similar to
921.Pa ~/.ssh/rc ,
922it can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000923machine-specific login-time initializations globally.
924This file should be writable only by root, and should be world-readable.
Damien Millerd8702e82006-03-15 11:33:56 +1100925.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000926.It Pa /var/empty
Damien Millerd8702e82006-03-15 11:33:56 +1100927.Xr chroot 2
928directory used by
929.Nm
930during privilege separation in the pre-authentication phase.
931The directory should not contain any files and must be owned by root
932and not group or world-writable.
933.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000934.It Pa /var/run/sshd.pid
Damien Millerd8702e82006-03-15 11:33:56 +1100935Contains the process ID of the
936.Nm
937listening for connections (if there are several daemons running
938concurrently for different ports, this contains the process ID of the one
939started last).
940The content of this file is not sensitive; it can be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000941.El
Damien Miller32aa1441999-10-29 09:15:49 +1000942.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000943.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100944.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000945.Xr ssh 1 ,
946.Xr ssh-add 1 ,
947.Xr ssh-agent 1 ,
948.Xr ssh-keygen 1 ,
Darren Tucker04354b92007-03-21 20:46:54 +1100949.Xr ssh-keyscan 1 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000950.Xr chroot 2 ,
951.Xr hosts_access 5 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000952.Xr login.conf 5 ,
953.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000954.Xr sshd_config 5 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000955.Xr inetd 8 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000956.Xr sftp-server 8
Damien Millerf1ce5052003-06-11 22:04:39 +1000957.Sh AUTHORS
958OpenSSH is a derivative of the original and free
959ssh 1.2.12 release by Tatu Ylonen.
960Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
961Theo de Raadt and Dug Song
962removed many bugs, re-added newer features and
963created OpenSSH.
964Markus Friedl contributed the support for SSH
965protocol versions 1.5 and 2.0.
966Niels Provos and Markus Friedl contributed support
967for privilege separation.
Damien Millerc47d7e92006-03-15 11:27:20 +1100968.Sh CAVEATS
969System security is not improved unless
970.Nm rshd ,
971.Nm rlogind ,
972and
973.Nm rexecd
974are disabled (thus completely disabling
975.Xr rlogin
976and
977.Xr rsh
978into the machine).