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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 Miller69010322013-04-23 15:21:24 +100036.\" $OpenBSD: sshd.8,v 1.269 2013/04/07 09:40:27 dtucker Exp $
Damien Miller03d4d7e2013-04-23 15:21:06 +100037.Dd $Mdocdate: April 7 2013 $
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 Miller03d4d7e2013-04-23 15:21:06 +100050.Op Fl E Ar log_file
Damien Miller32aa1441999-10-29 09:15:49 +100051.Op Fl f Ar config_file
52.Op Fl g Ar login_grace_time
53.Op Fl h Ar host_key_file
54.Op Fl k Ar key_gen_time
Ben Lindstromade03f62001-12-06 18:22:17 +000055.Op Fl o Ar option
Damien Miller32aa1441999-10-29 09:15:49 +100056.Op Fl p Ar port
Damien Miller942da032000-08-18 13:59:06 +100057.Op Fl u Ar len
Damien Millerffadc582003-02-24 11:52:26 +110058.Ek
Damien Miller22c77262000-04-13 12:26:34 +100059.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100060.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110061(OpenSSH Daemon) is the daemon program for
Damien Miller32aa1441999-10-29 09:15:49 +100062.Xr ssh 1 .
Darren Tucker04354b92007-03-21 20:46:54 +110063Together these programs replace
64.Xr rlogin 1
65and
66.Xr rsh 1 ,
67and provide secure encrypted communications between two untrusted hosts
Damien Miller450a7a12000-03-26 13:04:51 +100068over an insecure network.
Damien Miller32aa1441999-10-29 09:15:49 +100069.Pp
70.Nm
Damien Miller99cc4a82006-01-31 21:45:53 +110071listens for connections from clients.
Damien Miller22c77262000-04-13 12:26:34 +100072It is normally started at boot from
Damien Miller32aa1441999-10-29 09:15:49 +100073.Pa /etc/rc .
74It forks a new
Damien Miller450a7a12000-03-26 13:04:51 +100075daemon for each incoming connection.
76The forked daemons handle
Damien Miller32aa1441999-10-29 09:15:49 +100077key exchange, encryption, authentication, command execution,
78and data exchange.
Damien Miller32aa1441999-10-29 09:15:49 +100079.Pp
80.Nm
Darren Tucker1f203942003-10-15 15:50:42 +100081can be configured using command-line options or a configuration file
82(by default
Damien Miller99cc4a82006-01-31 21:45:53 +110083.Xr sshd_config 5 ) ;
84command-line options override values specified in the
Damien Miller32aa1441999-10-29 09:15:49 +100085configuration file.
Damien Miller6162d121999-11-21 13:23:52 +110086.Nm
87rereads its configuration file when it receives a hangup signal,
Ben Lindstrom49a098d2001-03-05 06:55:18 +000088.Dv SIGHUP ,
Damien Miller208f1ed2006-03-15 11:56:03 +110089by executing itself with the name and options it was started with, e.g.\&
Ben Lindstrom49a098d2001-03-05 06:55:18 +000090.Pa /usr/sbin/sshd .
Damien Miller6162d121999-11-21 13:23:52 +110091.Pp
Damien Miller32aa1441999-10-29 09:15:49 +100092The options are as follows:
93.Bl -tag -width Ds
Darren Tucker1f203942003-10-15 15:50:42 +100094.It Fl 4
95Forces
96.Nm
97to use IPv4 addresses only.
98.It Fl 6
99Forces
100.Nm
101to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000102.It Fl b Ar bits
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000103Specifies the number of bits in the ephemeral protocol version 1
Darren Tucker7499b0c2008-07-02 22:35:43 +1000104server key (default 1024).
Darren Tuckere7f3f752008-06-10 23:06:01 +1000105.It Fl C Ar connection_spec
106Specify the connection parameters to use for the
107.Fl T
108extended test mode.
109If provided, any
110.Cm Match
111directives in the configuration file
112that would apply to the specified user, host, and address will be set before
113the configuration is written to standard output.
114The connection parameters are supplied as keyword=value pairs.
115The keywords are
116.Dq user ,
117.Dq host ,
Darren Tuckerfbcf8272012-05-19 19:37:01 +1000118.Dq laddr ,
119.Dq lport ,
Darren Tuckere7f3f752008-06-10 23:06:01 +1000120and
121.Dq addr .
122All are required and may be supplied in any order, either with multiple
123.Fl C
124options or as a comma-separated list.
Damien Miller15f5b562010-03-03 10:25:21 +1100125.It Fl c Ar host_certificate_file
126Specifies a path to a certificate file to identify
127.Nm
128during key exchange.
129The certificate file must match a host key file specified using the
130.Fl h
131option or the
132.Cm HostKey
133configuration directive.
Darren Tucker1f203942003-10-15 15:50:42 +1000134.It Fl D
135When this option is specified,
136.Nm
137will not detach and does not become a daemon.
138This allows easy monitoring of
139.Nm sshd .
Damien Miller32aa1441999-10-29 09:15:49 +1000140.It Fl d
Damien Miller450a7a12000-03-26 13:04:51 +1000141Debug mode.
Damien Miller74d98252010-02-02 17:01:46 +1100142The server sends verbose debug output to standard error,
143and does not put itself in the background.
Damien Miller450a7a12000-03-26 13:04:51 +1000144The server also will not fork and will only process one connection.
145This option is only intended for debugging for the server.
Damien Millerffadc582003-02-24 11:52:26 +1100146Multiple
147.Fl d
148options increase the debugging level.
Damien Miller874d77b2000-10-14 16:23:11 +1100149Maximum is 3.
Damien Miller03d4d7e2013-04-23 15:21:06 +1000150.It Fl E Ar log_file
151Append debug logs to
152.Ar log_file
153instead of the system log.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000154.It Fl e
Damien Miller69010322013-04-23 15:21:24 +1000155Write debug logs to standard error instead of the system log.
Darren Tucker04354b92007-03-21 20:46:54 +1100156.It Fl f Ar config_file
Damien Miller450a7a12000-03-26 13:04:51 +1000157Specifies the name of the configuration file.
158The default is
Damien Miller05eda432002-02-10 18:32:28 +1100159.Pa /etc/ssh/sshd_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000160.Nm
161refuses to start if there is no configuration file.
162.It Fl g Ar login_grace_time
163Gives the grace time for clients to authenticate themselves (default
Damien Millerc1348632002-09-05 14:35:14 +1000164120 seconds).
Damien Miller450a7a12000-03-26 13:04:51 +1000165If the client fails to authenticate the user within
166this many seconds, the server disconnects and exits.
167A value of zero indicates no limit.
Damien Miller32aa1441999-10-29 09:15:49 +1000168.It Fl h Ar host_key_file
Damien Miller7fc23732002-01-22 23:19:11 +1100169Specifies a file from which a host key is read.
Damien Miller32aa1441999-10-29 09:15:49 +1000170This option must be given if
171.Nm
172is not run as root (as the normal
Damien Miller7fc23732002-01-22 23:19:11 +1100173host key files are normally not readable by anyone but root).
174The default is
Damien Miller05eda432002-02-10 18:32:28 +1100175.Pa /etc/ssh/ssh_host_key
Damien Miller7fc23732002-01-22 23:19:11 +1100176for protocol version 1, and
Damien Millereb8b60e2010-08-31 22:41:14 +1000177.Pa /etc/ssh/ssh_host_dsa_key ,
178.Pa /etc/ssh/ssh_host_ecdsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100179and
Damien Millereb8b60e2010-08-31 22:41:14 +1000180.Pa /etc/ssh/ssh_host_rsa_key
Damien Miller7fc23732002-01-22 23:19:11 +1100181for protocol version 2.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000182It is possible to have multiple host key files for
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000183the different protocol versions and host key algorithms.
Damien Miller32aa1441999-10-29 09:15:49 +1000184.It Fl i
185Specifies that
186.Nm
Damien Millerffadc582003-02-24 11:52:26 +1100187is being run from
188.Xr inetd 8 .
Damien Miller32aa1441999-10-29 09:15:49 +1000189.Nm
190is normally not run
191from inetd because it needs to generate the server key before it can
Damien Miller450a7a12000-03-26 13:04:51 +1000192respond to the client, and this may take tens of seconds.
193Clients would have to wait too long if the key was regenerated every time.
Damien Miller208f1ed2006-03-15 11:56:03 +1100194However, with small key sizes (e.g. 512) using
Damien Miller32aa1441999-10-29 09:15:49 +1000195.Nm
196from inetd may
197be feasible.
198.It Fl k Ar key_gen_time
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000199Specifies how often the ephemeral protocol version 1 server key is
200regenerated (default 3600 seconds, or one hour).
Damien Miller450a7a12000-03-26 13:04:51 +1000201The motivation for regenerating the key fairly
Darren Tucker1f203942003-10-15 15:50:42 +1000202often is that the key is not stored anywhere, and after about an hour
Damien Miller32aa1441999-10-29 09:15:49 +1000203it becomes impossible to recover the key for decrypting intercepted
204communications even if the machine is cracked into or physically
Damien Miller450a7a12000-03-26 13:04:51 +1000205seized.
206A value of zero indicates that the key will never be regenerated.
Ben Lindstromade03f62001-12-06 18:22:17 +0000207.It Fl o Ar option
208Can be used to give options in the format used in the configuration file.
209This is useful for specifying options for which there is no separate
210command-line flag.
Darren Tucker1f203942003-10-15 15:50:42 +1000211For full details of the options, and their values, see
212.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000213.It Fl p Ar port
214Specifies the port on which the server listens for connections
215(default 22).
Damien Miller7d1ded42002-01-22 23:09:41 +1100216Multiple port options are permitted.
Damien Millerd7f308f2005-12-24 14:55:16 +1100217Ports specified in the configuration file with the
218.Cm Port
219option are ignored when a command-line port is specified.
220Ports specified using the
221.Cm ListenAddress
222option override command-line ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000223.It Fl q
Damien Miller450a7a12000-03-26 13:04:51 +1000224Quiet mode.
225Nothing is sent to the system log.
226Normally the beginning,
Damien Miller32aa1441999-10-29 09:15:49 +1000227authentication, and termination of each connection is logged.
Darren Tuckere7140f22008-06-10 23:01:51 +1000228.It Fl T
229Extended test mode.
230Check the validity of the configuration file, output the effective configuration
231to stdout and then exit.
232Optionally,
233.Cm Match
234rules may be applied by specifying the connection parameters using one or more
235.Fl C
236options.
Darren Tuckere7f3f752008-06-10 23:06:01 +1000237.It Fl t
238Test mode.
239Only check the validity of the configuration file and sanity of the keys.
240This is useful for updating
241.Nm
242reliably as configuration options may change.
Damien Miller942da032000-08-18 13:59:06 +1000243.It Fl u Ar len
244This option is used to specify the size of the field
245in the
246.Li utmp
247structure that holds the remote host name.
248If the resolved host name is longer than
249.Ar len ,
250the dotted decimal value will be used instead.
251This allows hosts with very long host names that
252overflow this field to still be uniquely identified.
253Specifying
254.Fl u0
255indicates that only dotted decimal addresses
256should be put into the
257.Pa utmp
258file.
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000259.Fl u0
Damien Millerffadc582003-02-24 11:52:26 +1100260may also be used to prevent
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000261.Nm
262from making DNS requests unless the authentication
263mechanism or configuration requires it.
264Authentication mechanisms that may require DNS include
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000265.Cm RhostsRSAAuthentication ,
Damien Miller2ac05772006-02-01 22:05:42 +1100266.Cm HostbasedAuthentication ,
Ben Lindstrom6f672c02001-09-12 17:51:55 +0000267and using a
268.Cm from="pattern-list"
269option in a key file.
Ben Lindstromea03db92002-03-05 01:38:57 +0000270Configuration options that require DNS include using a
271USER@HOST pattern in
272.Cm AllowUsers
273or
274.Cm DenyUsers .
Damien Miller32aa1441999-10-29 09:15:49 +1000275.El
Damien Miller8bbdf902006-02-01 22:05:25 +1100276.Sh AUTHENTICATION
277The OpenSSH SSH daemon supports SSH protocols 1 and 2.
Darren Tucker7a4a7652009-10-11 21:51:40 +1100278The default is to use protocol 2 only,
Damien Miller8bbdf902006-02-01 22:05:25 +1100279though this can be changed via the
280.Cm Protocol
281option in
282.Xr sshd_config 5 .
Damien Millereb8b60e2010-08-31 22:41:14 +1000283Protocol 2 supports DSA, ECDSA and RSA keys;
Damien Miller8bbdf902006-02-01 22:05:25 +1100284protocol 1 only supports RSA keys.
285For both protocols,
286each host has a host-specific key,
287normally 2048 bits,
288used to identify the host.
Damien Miller99cc4a82006-01-31 21:45:53 +1100289.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100290Forward security for protocol 1 is provided through
291an additional server key,
292normally 768 bits,
293generated when the server starts.
Damien Miller99cc4a82006-01-31 21:45:53 +1100294This key is normally regenerated every hour if it has been used, and
295is never stored on disk.
Damien Miller99cc4a82006-01-31 21:45:53 +1100296Whenever a client connects, the daemon responds with its public
297host and server keys.
298The client compares the
299RSA host key against its own database to verify that it has not changed.
300The client then generates a 256-bit random number.
301It encrypts this
302random number using both the host key and the server key, and sends
303the encrypted number to the server.
304Both sides then use this
305random number as a session key which is used to encrypt all further
306communications in the session.
307The rest of the session is encrypted
308using a conventional cipher, currently Blowfish or 3DES, with 3DES
309being used by default.
310The client selects the encryption algorithm
311to use from those offered by the server.
312.Pp
Damien Miller8bbdf902006-02-01 22:05:25 +1100313For protocol 2,
314forward security is provided through a Diffie-Hellman key agreement.
315This key agreement results in a shared session key.
316The rest of the session is encrypted using a symmetric cipher, currently
317128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
318The client selects the encryption algorithm
319to use from those offered by the server.
320Additionally, session integrity is provided
321through a cryptographic message authentication code
Darren Tucker427e4092012-10-05 11:02:39 +1000322(hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
Damien Miller20bd4532011-08-06 06:17:30 +1000323hmac-sha2-256 or hmac-sha2-512).
Damien Miller8bbdf902006-02-01 22:05:25 +1100324.Pp
325Finally, the server and the client enter an authentication dialog.
Damien Miller99cc4a82006-01-31 21:45:53 +1100326The client tries to authenticate itself using
Damien Miller8bbdf902006-02-01 22:05:25 +1100327host-based authentication,
328public key authentication,
329challenge-response authentication,
330or password authentication.
Damien Miller99cc4a82006-01-31 21:45:53 +1100331.Pp
332Regardless of the authentication type, the account is checked to
333ensure that it is accessible. An account is not accessible if it is
334locked, listed in
335.Cm DenyUsers
336or its group is listed in
337.Cm DenyGroups
338\&. The definition of a locked account is system dependant. Some platforms
339have their own account database (eg AIX) and some modify the passwd field (
340.Ql \&*LK\&*
341on Solaris and UnixWare,
342.Ql \&*
343on HP-UX, containing
344.Ql Nologin
345on Tru64,
346a leading
347.Ql \&*LOCKED\&*
348on FreeBSD and a leading
Darren Tucker1a9176b2007-08-17 09:42:32 +1000349.Ql \&!
350on most Linuxes).
351If there is a requirement to disable password authentication
Damien Miller99cc4a82006-01-31 21:45:53 +1100352for the account while allowing still public-key, then the passwd field
353should be set to something other than these values (eg
354.Ql NP
355or
356.Ql \&*NP\&*
357).
358.Pp
Damien Miller99cc4a82006-01-31 21:45:53 +1100359If the client successfully authenticates itself, a dialog for
360preparing the session is entered.
361At this time the client may request
362things like allocating a pseudo-tty, forwarding X11 connections,
363forwarding TCP connections, or forwarding the authentication agent
364connection over the secure channel.
365.Pp
Damien Millerc47d7e92006-03-15 11:27:20 +1100366After this, the client either requests a shell or execution of a command.
Damien Miller99cc4a82006-01-31 21:45:53 +1100367The sides then enter session mode.
368In this mode, either side may send
369data at any time, and such data is forwarded to/from the shell or
370command on the server side, and the user terminal in the client side.
371.Pp
372When the user program terminates and all forwarded X11 and other
373connections have been closed, the server sends command exit status to
374the client, and both sides exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000375.Sh LOGIN PROCESS
376When a user successfully logs in,
377.Nm
378does the following:
379.Bl -enum -offset indent
380.It
381If the login is on a tty, and no command has been specified,
Damien Miller22c77262000-04-13 12:26:34 +1000382prints last login time and
Damien Miller32aa1441999-10-29 09:15:49 +1000383.Pa /etc/motd
384(unless prevented in the configuration file or by
Damien Miller167ea5d2005-05-26 12:04:02 +1000385.Pa ~/.hushlogin ;
Damien Miller32aa1441999-10-29 09:15:49 +1000386see the
Damien Miller22c77262000-04-13 12:26:34 +1000387.Sx FILES
Damien Miller32aa1441999-10-29 09:15:49 +1000388section).
389.It
390If the login is on a tty, records login time.
391.It
392Checks
393.Pa /etc/nologin ;
394if it exists, prints contents and quits
395(unless root).
396.It
397Changes to run with normal user privileges.
398.It
399Sets up basic environment.
400.It
Darren Tucker1f203942003-10-15 15:50:42 +1000401Reads the file
Damien Miller167ea5d2005-05-26 12:04:02 +1000402.Pa ~/.ssh/environment ,
Darren Tucker1f203942003-10-15 15:50:42 +1000403if it exists, and users are allowed to change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000404See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000405.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000406option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000407.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000408.It
409Changes to user's home directory.
410.It
411If
Damien Miller167ea5d2005-05-26 12:04:02 +1000412.Pa ~/.ssh/rc
Damien Miller32aa1441999-10-29 09:15:49 +1000413exists, runs it; else if
Damien Millerafcc2252002-02-10 18:32:55 +1100414.Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000415exists, runs
Damien Miller450a7a12000-03-26 13:04:51 +1000416it; otherwise runs xauth.
417The
Damien Miller32aa1441999-10-29 09:15:49 +1000418.Dq rc
419files are given the X11
420authentication protocol and cookie in standard input.
Damien Millerfd725cf2006-03-15 11:35:54 +1100421See
422.Sx SSHRC ,
423below.
Damien Miller32aa1441999-10-29 09:15:49 +1000424.It
425Runs user's shell or command.
426.El
Damien Millerfd725cf2006-03-15 11:35:54 +1100427.Sh SSHRC
428If the file
429.Pa ~/.ssh/rc
430exists,
431.Xr sh 1
432runs it after reading the
433environment files but before starting the user's shell or command.
434It must not produce any output on stdout; stderr must be used
435instead.
436If X11 forwarding is in use, it will receive the "proto cookie" pair in
437its standard input (and
438.Ev DISPLAY
439in its environment).
440The script must call
441.Xr xauth 1
442because
443.Nm
444will not run xauth automatically to add X11 cookies.
445.Pp
446The primary purpose of this file is to run any initialization routines
447which may be needed before the user's home directory becomes
448accessible; AFS is a particular example of such an environment.
449.Pp
450This file will probably contain some initialization code followed by
451something similar to:
452.Bd -literal -offset 3n
453if read proto cookie && [ -n "$DISPLAY" ]; then
454 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
455 # X11UseLocalhost=yes
456 echo add unix:`echo $DISPLAY |
457 cut -c11-` $proto $cookie
458 else
459 # X11UseLocalhost=no
460 echo add $DISPLAY $proto $cookie
461 fi | xauth -q -
462fi
463.Ed
464.Pp
465If this file does not exist,
466.Pa /etc/ssh/sshrc
467is run, and if that
468does not exist either, xauth is used to add the cookie.
Damien Miller32aa1441999-10-29 09:15:49 +1000469.Sh AUTHORIZED_KEYS FILE FORMAT
Ben Lindstromf96704d2001-06-25 04:17:12 +0000470.Cm AuthorizedKeysFile
Damien Millerb9132fc2011-05-29 21:41:40 +1000471specifies the files containing public keys for
Damien Millerdcfea272006-03-15 11:31:22 +1100472public key authentication;
Damien Millerb9132fc2011-05-29 21:41:40 +1000473if none is specified, the default is
Damien Millerd8478b62011-05-29 21:39:36 +1000474.Pa ~/.ssh/authorized_keys
475and
476.Pa ~/.ssh/authorized_keys2 .
Damien Miller450a7a12000-03-26 13:04:51 +1000477Each line of the file contains one
Damien Miller32aa1441999-10-29 09:15:49 +1000478key (empty lines and lines starting with a
479.Ql #
480are ignored as
Damien Miller450a7a12000-03-26 13:04:51 +1000481comments).
Damien Millerdcfea272006-03-15 11:31:22 +1100482Protocol 1 public keys consist of the following space-separated fields:
483options, bits, exponent, modulus, comment.
484Protocol 2 public key consist of:
485options, keytype, base64-encoded key, comment.
486The options field is optional;
487its presence is determined by whether the line starts
Damien Millerb83df8d2002-09-04 16:24:55 +1000488with a number or not (the options field never starts with a number).
Damien Millerdcfea272006-03-15 11:31:22 +1100489The bits, exponent, modulus, and comment fields give the RSA key for
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000490protocol version 1; the
Damien Miller32aa1441999-10-29 09:15:49 +1000491comment field is not used for anything (but may be convenient for the
492user to identify the key).
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000493For protocol version 2 the keytype is
Damien Millereb8b60e2010-08-31 22:41:14 +1000494.Dq ecdsa-sha2-nistp256 ,
495.Dq ecdsa-sha2-nistp384 ,
496.Dq ecdsa-sha2-nistp521 ,
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000497.Dq ssh-dss
498or
499.Dq ssh-rsa .
Damien Miller32aa1441999-10-29 09:15:49 +1000500.Pp
501Note that lines in this file are usually several hundred bytes long
Darren Tucker22cc7412004-12-06 22:47:41 +1100502(because of the size of the public key encoding) up to a limit of
5038 kilobytes, which permits DSA keys up to 8 kilobits and RSA
504keys up to 16 kilobits.
Damien Miller450a7a12000-03-26 13:04:51 +1000505You don't want to type them in; instead, copy the
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000506.Pa identity.pub ,
Damien Millerdcfea272006-03-15 11:31:22 +1100507.Pa id_dsa.pub ,
Damien Millereb8b60e2010-08-31 22:41:14 +1000508.Pa id_ecdsa.pub ,
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000509or the
510.Pa id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000511file and edit it.
512.Pp
Ben Lindstrom0d0be022002-04-02 20:39:29 +0000513.Nm
514enforces a minimum RSA key modulus size for protocol 1
515and protocol 2 keys of 768 bits.
516.Pp
Damien Miller942da032000-08-18 13:59:06 +1000517The options (if present) consist of comma-separated option
Damien Miller450a7a12000-03-26 13:04:51 +1000518specifications.
519No spaces are permitted, except within double quotes.
Ben Lindstrom1c0fd092001-09-12 16:36:17 +0000520The following option specifications are supported (note
521that option keywords are case-insensitive):
Damien Miller32aa1441999-10-29 09:15:49 +1000522.Bl -tag -width Ds
Damien Millercd38c9c2010-03-04 21:51:37 +1100523.It Cm cert-authority
Damien Miller0a80ca12010-02-27 07:55:05 +1100524Specifies that the listed key is a certification authority (CA) that is
525trusted to validate signed certificates for user authentication.
526.Pp
527Certificates may encode access restrictions similar to these key options.
528If both certificate restrictions and key options are present, the most
529restrictive union of the two is applied.
Damien Miller32aa1441999-10-29 09:15:49 +1000530.It Cm command="command"
531Specifies that the command is executed whenever this key is used for
Damien Miller450a7a12000-03-26 13:04:51 +1000532authentication.
533The command supplied by the user (if any) is ignored.
Ben Lindstrom23124ea2001-09-14 23:14:25 +0000534The command is run on a pty if the client requests a pty;
Damien Miller450a7a12000-03-26 13:04:51 +1000535otherwise it is run without a tty.
Damien Millerffadc582003-02-24 11:52:26 +1100536If an 8-bit clean channel is required,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000537one must not request a pty or should specify
Damien Miller33804262001-02-04 23:20:18 +1100538.Cm no-pty .
Damien Miller450a7a12000-03-26 13:04:51 +1000539A quote may be included in the command by quoting it with a backslash.
540This option might be useful
Damien Millerb83df8d2002-09-04 16:24:55 +1000541to restrict certain public keys to perform just a specific operation.
Damien Miller450a7a12000-03-26 13:04:51 +1000542An example might be a key that permits remote backups but nothing else.
Damien Miller7c24b812006-01-14 10:09:56 +1100543Note that the client may specify TCP and/or X11
Damien Miller30c3d422000-05-09 11:02:59 +1000544forwarding unless they are explicitly prohibited.
Damien Millere2754432006-07-24 14:06:47 +1000545The command originally supplied by the client is available in the
546.Ev SSH_ORIGINAL_COMMAND
547environment variable.
Ben Lindstrom20daef72001-09-20 00:54:01 +0000548Note that this option applies to shell, command or subsystem execution.
Damien Miller0a80ca12010-02-27 07:55:05 +1100549Also note that this command may be superseded by either a
550.Xr sshd_config 5
551.Cm ForceCommand
552directive or a command embedded in a certificate.
Damien Miller32aa1441999-10-29 09:15:49 +1000553.It Cm environment="NAME=value"
554Specifies that the string is to be added to the environment when
Damien Miller450a7a12000-03-26 13:04:51 +1000555logging in using this key.
556Environment variables set this way
557override other default environment values.
558Multiple options of this type are permitted.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000559Environment processing is disabled by default and is
560controlled via the
561.Cm PermitUserEnvironment
562option.
Ben Lindstrom38b951c2001-12-06 17:47:47 +0000563This option is automatically disabled if
564.Cm UseLogin
565is enabled.
Damien Miller31bdc522006-03-15 11:31:44 +1100566.It Cm from="pattern-list"
Darren Tucker896ad5a2008-06-11 09:34:46 +1000567Specifies that in addition to public key authentication, either the canonical
Darren Tucker11996732008-06-13 04:32:00 +1000568name of the remote host or its IP address must be present in the
Darren Tucker896ad5a2008-06-11 09:34:46 +1000569comma-separated list of patterns.
Damien Millerc7d5b5e2006-03-15 11:55:08 +1100570See
571.Sx PATTERNS
572in
573.Xr ssh_config 5
574for more information on patterns.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000575.Pp
576In addition to the wildcard matching that may be applied to hostnames or
577addresses, a
578.Cm from
Darren Tucker5837b512009-06-21 17:52:27 +1000579stanza may match IP addresses using CIDR address/masklen notation.
Darren Tucker896ad5a2008-06-11 09:34:46 +1000580.Pp
581The purpose of this option is to optionally increase security: public key
582authentication by itself does not trust the network or name servers or
583anything (but the key); however, if somebody somehow steals the key, the key
584permits an intruder to log in from anywhere in the world.
585This additional option makes using a stolen key more difficult (name
586servers and/or routers would have to be compromised in addition to
587just the key).
Damien Miller31bdc522006-03-15 11:31:44 +1100588.It Cm no-agent-forwarding
589Forbids authentication agent forwarding when this key is used for
590authentication.
Damien Miller32aa1441999-10-29 09:15:49 +1000591.It Cm no-port-forwarding
Damien Miller7c24b812006-01-14 10:09:56 +1100592Forbids TCP forwarding when this key is used for authentication.
Damien Miller450a7a12000-03-26 13:04:51 +1000593Any port forward requests by the client will return an error.
Damien Miller208f1ed2006-03-15 11:56:03 +1100594This might be used, e.g. in connection with the
Damien Miller32aa1441999-10-29 09:15:49 +1000595.Cm command
596option.
Damien Miller31bdc522006-03-15 11:31:44 +1100597.It Cm no-pty
598Prevents tty allocation (a request to allocate a pty will fail).
Damien Miller95e80952008-03-27 11:03:05 +1100599.It Cm no-user-rc
Damien Miller13ba9c22008-04-03 20:52:51 +1100600Disables execution of
Damien Miller95e80952008-03-27 11:03:05 +1100601.Pa ~/.ssh/rc .
Damien Miller32aa1441999-10-29 09:15:49 +1000602.It Cm no-X11-forwarding
603Forbids X11 forwarding when this key is used for authentication.
604Any X11 forward requests by the client will return an error.
Damien Millera243fde2001-03-19 23:16:08 +1100605.It Cm permitopen="host:port"
Ben Lindstrom24643222001-06-25 05:08:11 +0000606Limit local
Damien Millera243fde2001-03-19 23:16:08 +1100607.Li ``ssh -L''
Ben Lindstrom4b3564e2001-04-10 02:41:56 +0000608port forwarding such that it may only connect to the specified host and
Ben Lindstromd71ba572001-09-12 18:03:31 +0000609port.
Damien Miller7fa96602010-08-05 13:03:13 +1000610IPv6 addresses can be specified by enclosing the address in square brackets.
Ben Lindstromd71ba572001-09-12 18:03:31 +0000611Multiple
Damien Millera243fde2001-03-19 23:16:08 +1100612.Cm permitopen
Damien Millerfbf486b2003-05-23 18:44:23 +1000613options may be applied separated by commas.
614No pattern matching is performed on the specified hostnames,
615they must be literal domains or addresses.
Darren Tucker1338b9e2011-10-02 18:57:35 +1100616A port specification of
617.Cm *
618matches any port.
Damien Miller30da3442010-05-10 11:58:03 +1000619.It Cm principals="principals"
620On a
621.Cm cert-authority
622line, specifies allowed principals for certificate authentication as a
623comma-separated list.
624At least one name from the list must appear in the certificate's
625list of principals for the certificate to be accepted.
626This option is ignored for keys that are not marked as trusted certificate
627signers using the
628.Cm cert-authority
629option.
Damien Millerd27b9472005-12-13 19:29:02 +1100630.It Cm tunnel="n"
631Force a
632.Xr tun 4
633device on the server.
634Without this option, the next available device will be used if
635the client requests a tunnel.
Damien Miller32aa1441999-10-29 09:15:49 +1000636.El
Damien Miller32aa1441999-10-29 09:15:49 +1000637.Pp
Damien Miller7d2ef022006-03-15 11:32:06 +1100638An example authorized_keys file:
Damien Millerc8f61cf2006-03-15 11:33:25 +1100639.Bd -literal -offset 3n
Damien Miller7d2ef022006-03-15 11:32:06 +1100640# Comments allowed at start of line
641ssh-rsa AAAAB3Nza...LiPk== user@example.net
Damien Millerc8f61cf2006-03-15 11:33:25 +1100642from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
643AAAAB2...19Q== john@example.net
644command="dump /home",no-pty,no-port-forwarding ssh-dss
645AAAAC3...51R== example.net
646permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
647AAAAB5...21S==
648tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
649jane@example.net
Damien Miller7d2ef022006-03-15 11:32:06 +1100650.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000651.Sh SSH_KNOWN_HOSTS FILE FORMAT
Damien Miller22c77262000-04-13 12:26:34 +1000652The
Damien Millerffadc582003-02-24 11:52:26 +1100653.Pa /etc/ssh/ssh_known_hosts
Damien Miller22c77262000-04-13 12:26:34 +1000654and
Damien Miller167ea5d2005-05-26 12:04:02 +1000655.Pa ~/.ssh/known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000656files contain host public keys for all known hosts.
657The global file should
658be prepared by the administrator (optional), and the per-user file is
Damien Miller9a7f2012006-03-15 11:32:42 +1100659maintained automatically: whenever the user connects from an unknown host,
Damien Miller450a7a12000-03-26 13:04:51 +1000660its key is added to the per-user file.
Damien Miller32aa1441999-10-29 09:15:49 +1000661.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100662Each line in these files contains the following fields: markers (optional),
663hostnames, bits, exponent, modulus, comment.
Damien Miller450a7a12000-03-26 13:04:51 +1000664The fields are separated by spaces.
Damien Miller32aa1441999-10-29 09:15:49 +1000665.Pp
Damien Millera7dab8b2010-03-05 10:42:05 +1100666The marker is optional, but if it is present then it must be one of
667.Dq @cert-authority ,
668to indicate that the line contains a certification authority (CA) key,
669or
670.Dq @revoked ,
671to indicate that the key contained on the line is revoked and must not ever
672be accepted.
673Only one marker should be used on a key line.
674.Pp
Damien Miller31554322003-05-14 13:44:58 +1000675Hostnames is a comma-separated list of patterns
Damien Miller9a7f2012006-03-15 11:32:42 +1100676.Pf ( Ql *
Damien Millerfbf486b2003-05-23 18:44:23 +1000677and
678.Ql \&?
Damien Miller049245d2003-05-14 13:44:42 +1000679act as
Damien Miller32aa1441999-10-29 09:15:49 +1000680wildcards); each pattern in turn is matched against the canonical host
681name (when authenticating a client) or against the user-supplied
Damien Miller450a7a12000-03-26 13:04:51 +1000682name (when authenticating a server).
683A pattern may also be preceded by
Damien Miller049245d2003-05-14 13:44:42 +1000684.Ql \&!
Damien Miller32aa1441999-10-29 09:15:49 +1000685to indicate negation: if the host name matches a negated
686pattern, it is not accepted (by that line) even if it matched another
687pattern on the line.
Darren Tuckerda345532006-07-10 23:04:19 +1000688A hostname or address may optionally be enclosed within
689.Ql \&[
690and
691.Ql \&]
692brackets then followed by
693.Ql \&:
Darren Tucker11318472006-07-12 22:07:59 +1000694and a non-standard port number.
Damien Miller32aa1441999-10-29 09:15:49 +1000695.Pp
Damien Millere1776152005-03-01 21:47:37 +1100696Alternately, hostnames may be stored in a hashed form which hides host names
Damien Miller718fd4b2005-03-02 12:03:23 +1100697and addresses should the file's contents be disclosed.
698Hashed hostnames start with a
699.Ql |
Damien Millere1776152005-03-01 21:47:37 +1100700character.
701Only one hashed hostname may appear on a single line and none of the above
702negation or wildcard operators may be applied.
703.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000704Bits, exponent, and modulus are taken directly from the RSA host key; they
Damien Miller9a7f2012006-03-15 11:32:42 +1100705can be obtained, for example, from
Damien Miller05eda432002-02-10 18:32:28 +1100706.Pa /etc/ssh/ssh_host_key.pub .
Damien Miller32aa1441999-10-29 09:15:49 +1000707The optional comment field continues to the end of the line, and is not used.
708.Pp
709Lines starting with
710.Ql #
711and empty lines are ignored as comments.
712.Pp
713When performing host authentication, authentication is accepted if any
Damien Millera7dab8b2010-03-05 10:42:05 +1100714matching line has the proper key; either one that matches exactly or,
715if the server has presented a certificate for authentication, the key
716of the certification authority that signed the certificate.
717For a key to be trusted as a certification authority, it must use the
718.Dq @cert-authority
719marker described above.
720.Pp
721The known hosts file also provides a facility to mark keys as revoked,
722for example when it is known that the associated private key has been
723stolen.
724Revoked keys are specified by including the
725.Dq @revoked
726marker at the beginning of the key line, and are never accepted for
727authentication or as certification authorities, but instead will
728produce a warning from
729.Xr ssh 1
730when they are encountered.
731.Pp
732It is permissible (but not
Damien Miller32aa1441999-10-29 09:15:49 +1000733recommended) to have several lines or different host keys for the same
Damien Miller450a7a12000-03-26 13:04:51 +1000734names.
735This will inevitably happen when short forms of host names
736from different domains are put in the file.
737It is possible
Damien Miller32aa1441999-10-29 09:15:49 +1000738that the files contain conflicting information; authentication is
739accepted if valid information can be found from either file.
740.Pp
741Note that the lines in these files are typically hundreds of characters
742long, and you definitely don't want to type in the host keys by hand.
Damien Millera7dab8b2010-03-05 10:42:05 +1100743Rather, generate them by a script,
744.Xr ssh-keyscan 1
Damien Miller22c77262000-04-13 12:26:34 +1000745or by taking
Damien Miller05eda432002-02-10 18:32:28 +1100746.Pa /etc/ssh/ssh_host_key.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000747and adding the host names at the front.
Damien Millera7dab8b2010-03-05 10:42:05 +1100748.Xr ssh-keygen 1
Damien Miller98339052010-03-05 21:30:35 +1100749also offers some basic automated editing for
Damien Millera7dab8b2010-03-05 10:42:05 +1100750.Pa ~/.ssh/known_hosts
751including removing hosts matching a host name and converting all host
752names to their hashed representations.
Damien Millercc00f5e2006-03-15 11:33:00 +1100753.Pp
754An example ssh_known_hosts file:
755.Bd -literal -offset 3n
756# Comments allowed at start of line
757closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
758cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
Damien Millere1776152005-03-01 21:47:37 +1100759# A hashed hostname
760|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
761AAAA1234.....=
Damien Millera7dab8b2010-03-05 10:42:05 +1100762# A revoked key
763@revoked * ssh-rsa AAAAB5W...
764# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
765@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
Damien Millere1776152005-03-01 21:47:37 +1100766.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000767.Sh FILES
Damien Millerd8702e82006-03-15 11:33:56 +1100768.Bl -tag -width Ds -compact
Damien Millerafdae612010-08-31 22:31:14 +1000769.It Pa ~/.hushlogin
Damien Miller70a908e2005-03-01 21:17:09 +1100770This file is used to suppress printing the last login time and
771.Pa /etc/motd ,
772if
773.Cm PrintLastLog
774and
775.Cm PrintMotd ,
776respectively,
777are enabled.
778It does not suppress printing of the banner specified by
779.Cm Banner .
Damien Millerd8702e82006-03-15 11:33:56 +1100780.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000781.It Pa ~/.rhosts
Damien Milleradc35b92006-03-15 11:35:27 +1100782This file is used for host-based authentication (see
783.Xr ssh 1
784for more information).
785On some machines this file may need to be
786world-readable if the user's home directory is on an NFS partition,
787because
788.Nm
789reads it as root.
790Additionally, this file must be owned by the user,
791and must not have write permissions for anyone else.
792The recommended
793permission for most machines is read/write for the user, and not
Damien Miller32aa1441999-10-29 09:15:49 +1000794accessible by others.
795.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000796.It Pa ~/.shosts
Damien Milleradc35b92006-03-15 11:35:27 +1100797This file is used in exactly the same way as
798.Pa .rhosts ,
799but allows host-based authentication without permitting login with
800rlogin/rsh.
Damien Miller32aa1441999-10-29 09:15:49 +1000801.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000802.It Pa ~/.ssh/
Damien Miller520e6152008-02-10 22:46:22 +1100803This directory is the default location for all user-specific configuration
804and authentication information.
805There is no general requirement to keep the entire contents of this directory
806secret, but the recommended permissions are read/write/execute for the user,
807and not accessible by others.
808.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000809.It Pa ~/.ssh/authorized_keys
Damien Millereb8b60e2010-08-31 22:41:14 +1000810Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in
811as this user.
Damien Millerd8702e82006-03-15 11:33:56 +1100812The format of this file is described above.
Damien Miller5d43d492006-08-30 11:07:00 +1000813The content of the file is not highly sensitive, but the recommended
Damien Milleradc35b92006-03-15 11:35:27 +1100814permissions are read/write for the user, and not accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000815.Pp
Damien Miller5d43d492006-08-30 11:07:00 +1000816If this file, the
817.Pa ~/.ssh
818directory, or the user's home directory are writable
819by other users, then the file could be modified or replaced by unauthorized
820users.
821In this case,
822.Nm
823will not allow it to be used unless the
824.Cm StrictModes
825option has been set to
826.Dq no .
Damien Miller5d43d492006-08-30 11:07:00 +1000827.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000828.It Pa ~/.ssh/environment
Damien Miller450a7a12000-03-26 13:04:51 +1000829This file is read into the environment at login (if it exists).
830It can only contain empty lines, comment lines (that start with
Damien Miller32aa1441999-10-29 09:15:49 +1000831.Ql # ) ,
Damien Miller450a7a12000-03-26 13:04:51 +1000832and assignment lines of the form name=value.
833The file should be writable
Damien Miller32aa1441999-10-29 09:15:49 +1000834only by the user; it need not be readable by anyone else.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000835Environment processing is disabled by default and is
836controlled via the
837.Cm PermitUserEnvironment
838option.
Damien Millerd8702e82006-03-15 11:33:56 +1100839.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000840.It Pa ~/.ssh/known_hosts
Damien Milleradc35b92006-03-15 11:35:27 +1100841Contains a list of host keys for all hosts the user has logged into
842that are not already in the systemwide list of known host keys.
843The format of this file is described above.
844This file should be writable only by root/the owner and
Damien Millerd8702e82006-03-15 11:33:56 +1100845can, but need not be, world-readable.
846.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000847.It Pa ~/.ssh/rc
Damien Millerfd725cf2006-03-15 11:35:54 +1100848Contains initialization routines to be run before
849the user's home directory becomes accessible.
Damien Miller32aa1441999-10-29 09:15:49 +1000850This file should be writable only by the user, and need not be
851readable by anyone else.
Damien Millerd8702e82006-03-15 11:33:56 +1100852.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000853.It Pa /etc/hosts.allow
854.It Pa /etc/hosts.deny
Damien Millerd8702e82006-03-15 11:33:56 +1100855Access controls that should be enforced by tcp-wrappers are defined here.
856Further details are described in
857.Xr hosts_access 5 .
858.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000859.It Pa /etc/hosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100860This file is for host-based authentication (see
861.Xr ssh 1 ) .
862It should only be writable by root.
Damien Millerd8702e82006-03-15 11:33:56 +1100863.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000864.It Pa /etc/moduli
Damien Millerd8702e82006-03-15 11:33:56 +1100865Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
866The file format is described in
867.Xr moduli 5 .
868.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000869.It Pa /etc/motd
Damien Millerd8702e82006-03-15 11:33:56 +1100870See
871.Xr motd 5 .
872.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000873.It Pa /etc/nologin
Damien Millerd8702e82006-03-15 11:33:56 +1100874If this file exists,
875.Nm
876refuses to let anyone except root log in.
877The contents of the file
878are displayed to anyone trying to log in, and non-root connections are
879refused.
880The file should be world-readable.
881.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000882.It Pa /etc/shosts.equiv
Damien Miller445121f2006-03-15 11:36:18 +1100883This file is used in exactly the same way as
884.Pa hosts.equiv ,
885but allows host-based authentication without permitting login with
886rlogin/rsh.
Damien Millerd8702e82006-03-15 11:33:56 +1100887.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000888.It Pa /etc/ssh/ssh_host_key
889.It Pa /etc/ssh/ssh_host_dsa_key
Damien Millereb8b60e2010-08-31 22:41:14 +1000890.It Pa /etc/ssh/ssh_host_ecdsa_key
Damien Millerafdae612010-08-31 22:31:14 +1000891.It Pa /etc/ssh/ssh_host_rsa_key
Damien Millerb9902cf2012-06-20 21:52:58 +1000892These files contain the private parts of the host keys.
Damien Millerd8702e82006-03-15 11:33:56 +1100893These files should only be owned by root, readable only by root, and not
894accessible to others.
895Note that
896.Nm
Damien Milleredd03752006-03-15 11:36:45 +1100897does not start if these files are group/world-accessible.
Damien Millerd8702e82006-03-15 11:33:56 +1100898.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000899.It Pa /etc/ssh/ssh_host_key.pub
900.It Pa /etc/ssh/ssh_host_dsa_key.pub
Damien Millereb8b60e2010-08-31 22:41:14 +1000901.It Pa /etc/ssh/ssh_host_ecdsa_key.pub
Damien Millerafdae612010-08-31 22:31:14 +1000902.It Pa /etc/ssh/ssh_host_rsa_key.pub
Damien Millerb9902cf2012-06-20 21:52:58 +1000903These files contain the public parts of the host keys.
Damien Millerd8702e82006-03-15 11:33:56 +1100904These files should be world-readable but writable only by
905root.
906Their contents should match the respective private parts.
907These files are not
908really used for anything; they are provided for the convenience of
909the user so their contents can be copied to known hosts files.
910These files are created using
911.Xr ssh-keygen 1 .
912.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000913.It Pa /etc/ssh/ssh_known_hosts
Darren Tucker04354b92007-03-21 20:46:54 +1100914Systemwide list of known host keys.
915This file should be prepared by the
916system administrator to contain the public host keys of all machines in the
917organization.
918The format of this file is described above.
919This file should be writable only by root/the owner and
920should be world-readable.
921.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000922.It Pa /etc/ssh/sshd_config
Damien Millerd8702e82006-03-15 11:33:56 +1100923Contains configuration data for
924.Nm sshd .
925The file format and configuration options are described in
926.Xr sshd_config 5 .
927.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000928.It Pa /etc/ssh/sshrc
Damien Millerfd725cf2006-03-15 11:35:54 +1100929Similar to
930.Pa ~/.ssh/rc ,
931it can be used to specify
Damien Miller450a7a12000-03-26 13:04:51 +1000932machine-specific login-time initializations globally.
933This file should be writable only by root, and should be world-readable.
Damien Millerd8702e82006-03-15 11:33:56 +1100934.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000935.It Pa /var/empty
Damien Millerd8702e82006-03-15 11:33:56 +1100936.Xr chroot 2
937directory used by
938.Nm
939during privilege separation in the pre-authentication phase.
940The directory should not contain any files and must be owned by root
941and not group or world-writable.
942.Pp
Damien Millerafdae612010-08-31 22:31:14 +1000943.It Pa /var/run/sshd.pid
Damien Millerd8702e82006-03-15 11:33:56 +1100944Contains the process ID of the
945.Nm
946listening for connections (if there are several daemons running
947concurrently for different ports, this contains the process ID of the one
948started last).
949The content of this file is not sensitive; it can be world-readable.
Damien Miller37023962000-07-11 17:31:38 +1000950.El
Damien Miller32aa1441999-10-29 09:15:49 +1000951.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000952.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100953.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000954.Xr ssh 1 ,
955.Xr ssh-add 1 ,
956.Xr ssh-agent 1 ,
957.Xr ssh-keygen 1 ,
Darren Tucker04354b92007-03-21 20:46:54 +1100958.Xr ssh-keyscan 1 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000959.Xr chroot 2 ,
960.Xr hosts_access 5 ,
Ben Lindstromc8e29ce2001-08-06 20:55:28 +0000961.Xr login.conf 5 ,
962.Xr moduli 5 ,
Ben Lindstrom9f049032002-06-21 00:59:05 +0000963.Xr sshd_config 5 ,
Darren Tucker1f203942003-10-15 15:50:42 +1000964.Xr inetd 8 ,
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000965.Xr sftp-server 8
Damien Millerf1ce5052003-06-11 22:04:39 +1000966.Sh AUTHORS
967OpenSSH is a derivative of the original and free
968ssh 1.2.12 release by Tatu Ylonen.
969Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
970Theo de Raadt and Dug Song
971removed many bugs, re-added newer features and
972created OpenSSH.
973Markus Friedl contributed the support for SSH
974protocol versions 1.5 and 2.0.
975Niels Provos and Markus Friedl contributed support
976for privilege separation.
Damien Millerc47d7e92006-03-15 11:27:20 +1100977.Sh CAVEATS
978System security is not improved unless
979.Nm rshd ,
980.Nm rlogind ,
981and
982.Nm rexecd
983are disabled (thus completely disabling
984.Xr rlogin
985and
986.Xr rsh
987into the machine).