3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
38 .\" $OpenBSD: sshd_config.5,v 1.28 2004/02/17 19:35:21 jmc Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH daemon configuration file
46 .Bl -tag -width Ds -compact
47 .It Pa /etc/ssh/sshd_config
51 reads configuration data from
52 .Pa /etc/ssh/sshd_config
53 (or the file specified with
56 The file contains keyword-argument pairs, one per line.
59 and empty lines are interpreted as comments.
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
66 This keyword can be followed by a list of group name patterns, separated
68 If specified, login is allowed only for users whose primary
69 group or supplementary group list matches one of the patterns.
74 wildcards in the patterns.
75 Only group names are valid; a numerical group ID is not recognized.
76 By default, login is allowed for all groups.
78 .It Cm AllowTcpForwarding
79 Specifies whether TCP forwarding is permitted.
82 Note that disabling TCP forwarding does not improve security unless
83 users are also denied shell access, as they can always install their
87 This keyword can be followed by a list of user name patterns, separated
89 If specified, login is allowed only for user names that
90 match one of the patterns.
95 wildcards in the patterns.
96 Only user names are valid; a numerical user ID is not recognized.
97 By default, login is allowed for all users.
98 If the pattern takes the form USER@HOST then USER and HOST
99 are separately checked, restricting logins to particular
100 users from particular hosts.
102 .It Cm AuthorizedKeysFile
103 Specifies the file that contains the public keys that can be used
104 for user authentication.
105 .Cm AuthorizedKeysFile
106 may contain tokens of the form %T which are substituted during connection
108 The following tokens are defined: %% is replaced by a literal '%',
109 %h is replaced by the home directory of the user being authenticated and
110 %u is replaced by the username of that user.
112 .Cm AuthorizedKeysFile
113 is taken to be an absolute path or one relative to the user's home
116 .Dq .ssh/authorized_keys .
118 In some jurisdictions, sending a warning message before authentication
119 may be relevant for getting legal protection.
120 The contents of the specified file are sent to the remote user before
121 authentication is allowed.
122 This option is only available for protocol version 2.
123 By default, no banner is displayed.
125 .It Cm ChallengeResponseAuthentication
126 Specifies whether challenge-response authentication is allowed.
129 this controls the use of PAM (see
132 Note that this affects the effectiveness of the
133 .Cm PasswordAuthentication
140 Specifies the ciphers allowed for protocol version 2.
141 Multiple ciphers must be comma-separated.
145 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
146 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
148 .It Cm ClientAliveInterval
149 Sets a timeout interval in seconds after which if no data has been received
152 will send a message through the encrypted
153 channel to request a response from the client.
155 is 0, indicating that these messages will not be sent to the client.
156 This option applies to protocol version 2 only.
157 .It Cm ClientAliveCountMax
158 Sets the number of client alive messages (see above) which may be
161 receiving any messages back from the client.
162 If this threshold is reached while client alive messages are being sent,
164 will disconnect the client, terminating the session.
165 It is important to note that the use of client alive messages is very
169 The client alive messages are sent through the encrypted channel
170 and therefore will not be spoofable.
171 The TCP keepalive option enabled by
174 The client alive mechanism is valuable when the client or
175 server depend on knowing when a connection has become inactive.
177 The default value is 3.
179 .Cm ClientAliveInterval
180 (above) is set to 15, and
181 .Cm ClientAliveCountMax
182 is left at the default, unresponsive ssh clients
183 will be disconnected after approximately 45 seconds.
185 Specifies whether compression is allowed.
193 This keyword can be followed by a list of group name patterns, separated
195 Login is disallowed for users whose primary group or supplementary
196 group list matches one of the patterns.
201 wildcards in the patterns.
202 Only group names are valid; a numerical group ID is not recognized.
203 By default, login is allowed for all groups.
206 This keyword can be followed by a list of user name patterns, separated
208 Login is disallowed for user names that match one of the patterns.
212 can be used as wildcards in the patterns.
213 Only user names are valid; a numerical user ID is not recognized.
214 By default, login is allowed for all users.
215 If the pattern takes the form USER@HOST then USER and HOST
216 are separately checked, restricting logins to particular
217 users from particular hosts.
219 Specifies whether remote hosts are allowed to connect to ports
220 forwarded for the client.
223 binds remote port forwardings to the loopback address.
224 This prevents other remote hosts from connecting to forwarded ports.
226 can be used to specify that
228 should bind remote port forwardings to the wildcard address,
229 thus allowing remote hosts to connect to forwarded ports.
236 .It Cm GSSAPIAuthentication
237 Specifies whether user authentication based on GSSAPI is allowed.
240 Note that this option applies to protocol version 2 only.
241 .It Cm GSSAPICleanupCredentials
242 Specifies whether to automatically destroy the user's credentials cache
246 Note that this option applies to protocol version 2 only.
247 .It Cm HostbasedAuthentication
248 Specifies whether rhosts or /etc/hosts.equiv authentication together
249 with successful public key client host authentication is allowed
250 (hostbased authentication).
251 This option is similar to
252 .Cm RhostsRSAAuthentication
253 and applies to protocol version 2 only.
257 Specifies a file containing a private host key
260 .Pa /etc/ssh/ssh_host_key
261 for protocol version 1, and
262 .Pa /etc/ssh/ssh_host_dsa_key
263 for protocol version 2.
266 will refuse to use a file if it is group/world-accessible.
267 It is possible to have multiple host key files.
269 keys are used for version 1 and
273 are used for version 2 of the SSH protocol.
279 files will not be used in
280 .Cm RhostsRSAAuthentication
282 .Cm HostbasedAuthentication .
286 .Pa /etc/ssh/shosts.equiv
290 .It Cm IgnoreUserKnownHosts
293 should ignore the user's
294 .Pa $HOME/.ssh/known_hosts
296 .Cm RhostsRSAAuthentication
298 .Cm HostbasedAuthentication .
301 .It Cm KerberosAuthentication
302 Specifies whether the password provided by the user for
303 .Cm PasswordAuthentication
304 will be validated through the Kerberos KDC.
305 To use this option, the server needs a
306 Kerberos servtab which allows the verification of the KDC's identity.
309 .It Cm KerberosGetAFSToken
310 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
311 an AFS token before accessing the user's home directory.
314 .It Cm KerberosOrLocalPasswd
315 If set then if password authentication through Kerberos fails then
316 the password will be validated via any additional local mechanism
321 .It Cm KerberosTicketCleanup
322 Specifies whether to automatically destroy the user's ticket cache
326 .It Cm KeyRegenerationInterval
327 In protocol version 1, the ephemeral server key is automatically regenerated
328 after this many seconds (if it has been used).
329 The purpose of regeneration is to prevent
330 decrypting captured sessions by later breaking into the machine and
332 The key is never stored anywhere.
333 If the value is 0, the key is never regenerated.
334 The default is 3600 (seconds).
336 Specifies the local addresses
339 The following forms may be used:
341 .Bl -item -offset indent -compact
345 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
350 .Ar host No | Ar IPv4_addr No : Ar port
356 .Ar host No | Ar IPv6_addr Oc : Ar port
364 will listen on the address and all prior
367 The default is to listen on all local addresses.
370 options are permitted.
373 options must precede this option for non port qualified addresses.
374 .It Cm LoginGraceTime
375 The server disconnects after this time if the user has not
376 successfully logged in.
377 If the value is 0, there is no time limit.
378 The default is 120 seconds.
380 Gives the verbosity level that is used when logging messages from
382 The possible values are:
383 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
385 DEBUG and DEBUG1 are equivalent.
386 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
387 Logging with a DEBUG level violates the privacy of users and is not recommended.
389 Specifies the available MAC (message authentication code) algorithms.
390 The MAC algorithm is used in protocol version 2
391 for data integrity protection.
392 Multiple algorithms must be comma-separated.
394 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
396 Specifies the maximum number of concurrent unauthenticated connections to the
399 Additional connections will be dropped until authentication succeeds or the
401 expires for a connection.
404 Alternatively, random early drop can be enabled by specifying
405 the three colon separated values
409 will refuse connection attempts with a probability of
412 if there are currently
415 unauthenticated connections.
416 The probability increases linearly and all connection attempts
417 are refused if the number of unauthenticated connections reaches
420 .It Cm PasswordAuthentication
421 Specifies whether password authentication is allowed.
426 was built without PAM support, in which case the default is
429 .Cm ChallengeResponseAuthentication
432 and the PAM authentication policy for
436 password authentication will be allowed through the challenge-response
437 mechanism regardless of the value of
438 .Cm PasswordAuthentication .
439 .It Cm PermitEmptyPasswords
440 When password authentication is allowed, it specifies whether the
441 server allows login to accounts with empty password strings.
444 .It Cm PermitRootLogin
445 Specifies whether root can login using
449 .Dq without-password ,
450 .Dq forced-commands-only
456 .Cm ChallengeResponseAuthentication
459 the root user may be allowed in with its password even if
460 .Cm PermitRootLogin is set to
461 .Dq without-password .
463 If this option is set to
465 password authentication is disabled for root. Note that other authentication
466 methods (e.g., keyboard-interactive/PAM) may still allow root to login using
469 If this option is set to
470 .Dq forced-commands-only
471 root login with public key authentication will be allowed,
474 option has been specified
475 (which may be useful for taking remote backups even if root login is
476 normally not allowed).
477 All other authentication methods are disabled for root.
479 If this option is set to
481 root is not allowed to login.
482 .It Cm PermitUserEnvironment
484 .Pa ~/.ssh/environment
488 .Pa ~/.ssh/authorized_keys
493 Enabling environment processing may enable users to bypass access
494 restrictions in some configurations using mechanisms such as
497 Specifies the file that contains the process ID of the
501 .Pa /var/run/sshd.pid .
503 Specifies the port number that
507 Multiple options of this type are permitted.
513 should print the date and time when the user last logged in.
521 when a user logs in interactively.
522 (On some systems it is also printed by the shell,
528 Specifies the protocol versions
531 The possible values are
535 Multiple versions must be comma-separated.
538 Note that the order of the protocol list does not indicate preference,
539 because the client selects among multiple protocol versions offered
545 .It Cm PubkeyAuthentication
546 Specifies whether public key authentication is allowed.
549 Note that this option applies to protocol version 2 only.
550 .It Cm RhostsRSAAuthentication
551 Specifies whether rhosts or
553 authentication together
554 with successful RSA host authentication is allowed.
557 This option applies to protocol version 1 only.
558 .It Cm RSAAuthentication
559 Specifies whether pure RSA authentication is allowed.
562 This option applies to protocol version 1 only.
564 Defines the number of bits in the ephemeral protocol version 1 server key.
565 The minimum value is 512, and the default is 768.
569 should check file modes and ownership of the
570 user's files and home directory before accepting login.
571 This is normally desirable because novices sometimes accidentally leave their
572 directory or files world-writable.
576 Configures an external subsystem (e.g., file transfer daemon).
577 Arguments should be a subsystem name and a command to execute upon subsystem
583 file transfer subsystem.
584 By default no subsystems are defined.
585 Note that this option applies to protocol version 2 only.
586 .It Cm SyslogFacility
587 Gives the facility code that is used when logging messages from
589 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
590 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
593 Specifies whether the system should send TCP keepalive messages to the
595 If they are sent, death of the connection or crash of one
596 of the machines will be properly noticed.
597 However, this means that
598 connections will die if the route is down temporarily, and some people
600 On the other hand, if TCP keepalives are not sent,
601 sessions may hang indefinitely on the server, leaving
603 users and consuming server resources.
607 (to send TCP keepalive messages), and the server will notice
608 if the network goes down or the client host crashes.
609 This avoids infinitely hanging sessions.
611 To disable TCP keepalive messages, the value should be set to
616 should lookup the remote host name and check that
617 the resolved host name for the remote IP address maps back to the
618 very same IP address.
624 is used for interactive login sessions.
629 is never used for remote command execution.
630 Note also, that if this is enabled,
632 will be disabled because
634 does not know how to handle
638 .Cm UsePrivilegeSeparation
639 is specified, it will be disabled after authentication.
641 Enables PAM authentication (via challenge-response) and session set up.
642 If you enable this, you should probably disable
643 .Cm PasswordAuthentication .
646 then you will not be able to run sshd as a non-root user. The default is
648 .It Cm UsePrivilegeSeparation
651 separates privileges by creating an unprivileged child process
652 to deal with incoming network traffic.
653 After successful authentication, another process will be created that has
654 the privilege of the authenticated user.
655 The goal of privilege separation is to prevent privilege
656 escalation by containing any corruption within the unprivileged processes.
659 .It Cm VersionAddendum
660 Specifies a string to append to the regular version string to identify
661 OS- or site-specific modifications.
663 .Dq FreeBSD-20040419 .
664 .It Cm X11DisplayOffset
665 Specifies the first display number available for
670 from interfering with real X11 servers.
673 Specifies whether X11 forwarding is permitted.
681 When X11 forwarding is enabled, there may be additional exposure to
682 the server and to client displays if the
684 proxy display is configured to listen on the wildcard address (see
686 below), however this is not the default.
687 Additionally, the authentication spoofing and authentication data
688 verification and substitution occur on the client side.
689 The security risk of using X11 forwarding is that the client's X11
690 display server may be exposed to attack when the ssh client requests
691 forwarding (see the warnings for
695 A system administrator may have a stance in which they want to
696 protect clients that may expose themselves to attack by unwittingly
697 requesting X11 forwarding, which can warrant a
701 Note that disabling X11 forwarding does not prevent users from
702 forwarding X11 traffic, as users can always install their own forwarders.
703 X11 forwarding is automatically disabled if
706 .It Cm X11UseLocalhost
709 should bind the X11 forwarding server to the loopback address or to
710 the wildcard address.
713 binds the forwarding server to the loopback address and sets the
716 environment variable to
718 This prevents remote hosts from connecting to the proxy display.
719 However, some older X11 clients may not function with this
724 to specify that the forwarding server should be bound to the wildcard
733 Specifies the full pathname of the
737 .Pa /usr/X11R6/bin/xauth .
741 command-line arguments and configuration file options that specify time
742 may be expressed using a sequence of the form:
744 .Ar time Op Ar qualifier ,
748 is a positive integer value and
750 is one of the following:
752 .Bl -tag -width Ds -compact -offset indent
767 Each member of the sequence is added together to calculate
768 the total time value.
770 Time format examples:
772 .Bl -tag -width Ds -compact -offset indent
774 600 seconds (10 minutes)
778 1 hour 30 minutes (90 minutes)
782 .It Pa /etc/ssh/sshd_config
783 Contains configuration data for
785 This file should be writable by root only, but it is recommended
786 (though not necessary) that it be world-readable.
791 OpenSSH is a derivative of the original and free
792 ssh 1.2.12 release by Tatu Ylonen.
793 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
794 Theo de Raadt and Dug Song
795 removed many bugs, re-added newer features and
797 Markus Friedl contributed the support for SSH
798 protocol versions 1.5 and 2.0.
799 Niels Provos and Markus Friedl contributed support
800 for privilege separation.