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.
37 .\" $OpenBSD: sshd_config.5,v 1.13 2002/09/16 20:12:11 stevesk Exp $
38 .\" $FreeBSD: src/crypto/openssh/sshd_config.5,v 1.5.2.5 2003/02/11 12:11:54 des Exp $
39 .\" $DragonFly: src/crypto/openssh/Attic/sshd_config.5,v 1.2 2003/06/17 04:24:36 dillon Exp $
40 .Dd September 25, 1999
45 .Nd OpenSSH SSH daemon configuration file
47 .Bl -tag -width Ds -compact
48 .It Pa /etc/ssh/sshd_config
52 reads configuration data from
53 .Pa /etc/ssh/sshd_config
54 (or the file specified with
57 The file contains keyword-argument pairs, one per line.
60 and empty lines are interpreted as comments.
63 keywords and their meanings are as follows (note that
64 keywords are case-insensitive and arguments are case-sensitive):
66 .It Cm AFSTokenPassing
67 Specifies whether an AFS token may be forwarded to the server.
71 This keyword can be followed by a list of group name patterns, separated
73 If specified, login is allowed only for users whose primary
74 group or supplementary group list matches one of the patterns.
79 wildcards in the patterns.
80 Only group names are valid; a numerical group ID is not recognized.
81 By default, login is allowed for all groups.
83 .It Cm AllowTcpForwarding
84 Specifies whether TCP forwarding is permitted.
87 Note that disabling TCP forwarding does not improve security unless
88 users are also denied shell access, as they can always install their
92 This keyword can be followed by a list of user name patterns, separated
94 If specified, login is allowed only for users names that
95 match one of the patterns.
100 wildcards in the patterns.
101 Only user names are valid; a numerical user ID is not recognized.
102 By default, login is allowed for all users.
103 If the pattern takes the form USER@HOST then USER and HOST
104 are separately checked, restricting logins to particular
105 users from particular hosts.
107 .It Cm AuthorizedKeysFile
108 Specifies the file that contains the public keys that can be used
109 for user authentication.
110 .Cm AuthorizedKeysFile
111 may contain tokens of the form %T which are substituted during connection
112 set-up. The following tokens are defined: %% is replaced by a literal '%',
113 %h is replaced by the home directory of the user being authenticated and
114 %u is replaced by the username of that user.
116 .Cm AuthorizedKeysFile
117 is taken to be an absolute path or one relative to the user's home
120 .Dq .ssh/authorized_keys .
122 In some jurisdictions, sending a warning message before authentication
123 may be relevant for getting legal protection.
124 The contents of the specified file are sent to the remote user before
125 authentication is allowed.
126 This option is only available for protocol version 2.
127 By default, no banner is displayed.
129 .It Cm ChallengeResponseAuthentication
130 Specifies whether challenge-response authentication is allowed.
133 this controls the use of PAM (see
136 Note that this affects the effectiveness of the
137 .Cm PasswordAuthentication
144 Specifies the ciphers allowed for protocol version 2.
145 Multiple ciphers must be comma-separated.
149 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
150 aes192-cbc,aes256-cbc''
152 .It Cm ClientAliveInterval
153 Sets a timeout interval in seconds after which if no data has been received
156 will send a message through the encrypted
157 channel to request a response from the client.
159 is 0, indicating that these messages will not be sent to the client.
160 This option applies to protocol version 2 only.
161 .It Cm ClientAliveCountMax
162 Sets the number of client alive messages (see above) which may be
165 receiving any messages back from the client. If this threshold is
166 reached while client alive messages are being sent,
168 will disconnect the client, terminating the session. It is important
169 to note that the use of client alive messages is very different from
171 (below). The client alive messages are sent through the
172 encrypted channel and therefore will not be spoofable. The TCP keepalive
175 is spoofable. The client alive mechanism is valuable when the client or
176 server depend on knowing when a connection has become inactive.
178 The default value is 3. If
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. This
224 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 HostbasedAuthentication
237 Specifies whether rhosts or /etc/hosts.equiv authentication together
238 with successful public key client host authentication is allowed
239 (hostbased authentication).
240 This option is similar to
241 .Cm RhostsRSAAuthentication
242 and applies to protocol version 2 only.
246 Specifies a file containing a private host key
249 .Pa /etc/ssh/ssh_host_key
250 for protocol version 1, and
251 .Pa /etc/ssh/ssh_host_dsa_key
252 for protocol version 2.
255 will refuse to use a file if it is group/world-accessible.
256 It is possible to have multiple host key files.
258 keys are used for version 1 and
262 are used for version 2 of the SSH protocol.
268 files will not be used in
269 .Cm RhostsAuthentication ,
270 .Cm RhostsRSAAuthentication
272 .Cm HostbasedAuthentication .
276 .Pa /etc/ssh/shosts.equiv
280 .It Cm IgnoreUserKnownHosts
283 should ignore the user's
284 .Pa $HOME/.ssh/known_hosts
286 .Cm RhostsRSAAuthentication
288 .Cm HostbasedAuthentication .
292 Specifies whether the system should send TCP keepalive messages to the
294 If they are sent, death of the connection or crash of one
295 of the machines will be properly noticed.
296 However, this means that
297 connections will die if the route is down temporarily, and some people
299 On the other hand, if keepalives are not sent,
300 sessions may hang indefinitely on the server, leaving
302 users and consuming server resources.
306 (to send keepalives), and the server will notice
307 if the network goes down or the client host crashes.
308 This avoids infinitely hanging sessions.
310 To disable keepalives, the value should be set to
312 .It Cm KerberosAuthentication
313 Specifies whether Kerberos authentication is allowed.
314 This can be in the form of a Kerberos ticket, or if
315 .Cm PasswordAuthentication
316 is yes, the password provided by the user will be validated through
318 To use this option, the server needs a
319 Kerberos servtab which allows the verification of the KDC's identity.
322 .It Cm KerberosOrLocalPasswd
323 If set then if password authentication through Kerberos fails then
324 the password will be validated via any additional local mechanism
329 .It Cm KerberosTgtPassing
330 Specifies whether a Kerberos TGT may be forwarded to the server.
333 as this only works when the Kerberos KDC is actually an AFS kaserver.
334 .It Cm KerberosTicketCleanup
335 Specifies whether to automatically destroy the user's ticket cache
339 .It Cm KeyRegenerationInterval
340 In protocol version 1, the ephemeral server key is automatically regenerated
341 after this many seconds (if it has been used).
342 The purpose of regeneration is to prevent
343 decrypting captured sessions by later breaking into the machine and
345 The key is never stored anywhere.
346 If the value is 0, the key is never regenerated.
347 The default is 3600 (seconds).
349 Specifies the local addresses
352 The following forms may be used:
354 .Bl -item -offset indent -compact
358 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
363 .Ar host No | Ar IPv4_addr No : Ar port
369 .Ar host No | Ar IPv6_addr Oc : Ar port
377 will listen on the address and all prior
379 options specified. The default is to listen on all local
382 options are permitted. Additionally, any
384 options must precede this option for non port qualified addresses.
385 .It Cm LoginGraceTime
386 The server disconnects after this time if the user has not
387 successfully logged in.
388 If the value is 0, there is no time limit.
389 The default is 120 seconds.
391 Gives the verbosity level that is used when logging messages from
393 The possible values are:
394 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
395 The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
396 and DEBUG3 each specify higher levels of debugging output.
397 Logging with a DEBUG level violates the privacy of users
398 and is not recommended.
400 Specifies the available MAC (message authentication code) algorithms.
401 The MAC algorithm is used in protocol version 2
402 for data integrity protection.
403 Multiple algorithms must be comma-separated.
405 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
407 Specifies the maximum number of concurrent unauthenticated connections to the
410 Additional connections will be dropped until authentication succeeds or the
412 expires for a connection.
415 Alternatively, random early drop can be enabled by specifying
416 the three colon separated values
420 will refuse connection attempts with a probability of
423 if there are currently
426 unauthenticated connections.
427 The probability increases linearly and all connection attempts
428 are refused if the number of unauthenticated connections reaches
431 .It Cm PasswordAuthentication
432 Specifies whether password authentication is allowed.
436 .Cm ChallengeResponseAuthentication
439 and the PAM authentication policy for
443 password authentication will be allowed through the challenge-response
444 mechanism regardless of the value of
445 .Cm PasswordAuthentication .
446 .It Cm PermitEmptyPasswords
447 When password authentication is allowed, it specifies whether the
448 server allows login to accounts with empty password strings.
451 .It Cm PermitRootLogin
452 Specifies whether root can login using
456 .Dq without-password ,
457 .Dq forced-commands-only
463 .Cm ChallengeResponseAuthentication
466 the root user may be allowed in with its password even if
467 .Cm PermitRootLogin is set to
468 .Dq without-password .
470 If this option is set to
472 password authentication is disabled for root.
474 If this option is set to
475 .Dq forced-commands-only
476 root login with public key authentication will be allowed,
479 option has been specified
480 (which may be useful for taking remote backups even if root login is
481 normally not allowed). All other authentication methods are disabled
484 If this option is set to
486 root is not allowed to login.
487 .It Cm PermitUserEnvironment
489 .Pa ~/.ssh/environment
493 .Pa ~/.ssh/authorized_keys
498 Enabling environment processing may enable users to bypass access
499 restrictions in some configurations using mechanisms such as
502 Specifies the file that contains the process ID of the
506 .Pa /var/run/sshd.pid .
508 Specifies the port number that
512 Multiple options of this type are permitted.
518 should print the date and time when the user last logged in.
526 when a user logs in interactively.
527 (On some systems it is also printed by the shell,
533 Specifies the protocol versions
536 The possible values are
540 Multiple versions must be comma-separated.
543 Note that the order of the protocol list does not indicate preference,
544 because the client selects among multiple protocol versions offered
550 .It Cm PubkeyAuthentication
551 Specifies whether public key authentication is allowed.
554 Note that this option applies to protocol version 2 only.
555 .It Cm RhostsAuthentication
556 Specifies whether authentication using rhosts or
559 Normally, this method should not be permitted because it is insecure.
560 .Cm RhostsRSAAuthentication
562 instead, because it performs RSA-based host authentication in addition
568 This option applies to protocol version 1 only.
569 .It Cm RhostsRSAAuthentication
570 Specifies whether rhosts or
572 authentication together
573 with successful RSA host authentication is allowed.
576 This option applies to protocol version 1 only.
577 .It Cm RSAAuthentication
578 Specifies whether pure RSA authentication is allowed.
581 This option applies to protocol version 1 only.
583 Defines the number of bits in the ephemeral protocol version 1 server key.
584 The minimum value is 512, and the default is 768.
588 should check file modes and ownership of the
589 user's files and home directory before accepting login.
590 This is normally desirable because novices sometimes accidentally leave their
591 directory or files world-writable.
595 Configures an external subsystem (e.g., file transfer daemon).
596 Arguments should be a subsystem name and a command to execute upon subsystem
602 file transfer subsystem.
603 By default no subsystems are defined.
604 Note that this option applies to protocol version 2 only.
605 .It Cm SyslogFacility
606 Gives the facility code that is used when logging messages from
608 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
609 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
614 is used for interactive login sessions.
619 is never used for remote command execution.
620 Note also, that if this is enabled,
622 will be disabled because
624 does not know how to handle
627 .Cm UsePrivilegeSeparation
628 is specified, it will be disabled after authentication.
629 .It Cm UsePrivilegeSeparation
632 separates privileges by creating an unprivileged child process
633 to deal with incoming network traffic. After successful authentication,
634 another process will be created that has the privilege of the authenticated
635 user. The goal of privilege separation is to prevent privilege
636 escalation by containing any corruption within the unprivileged processes.
639 .It Cm VerifyReverseMapping
642 should try to verify the remote host name and check that
643 the resolved host name for the remote IP address maps back to the
644 very same IP address.
647 .It Cm VersionAddendum
648 Specifies a string to append to the regular version string to identify
649 OS- or site-specific modifications.
651 .Dq FreeBSD-20030201 .
652 .It Cm X11DisplayOffset
653 Specifies the first display number available for
658 from interfering with real X11 servers.
661 Specifies whether X11 forwarding is permitted.
669 When X11 forwarding is enabled, there may be additional exposure to
670 the server and to client displays if the
672 proxy display is configured to listen on the wildcard address (see
674 below), however this is not the default.
675 Additionally, the authentication spoofing and authentication data
676 verification and substitution occur on the client side.
677 The security risk of using X11 forwarding is that the client's X11
678 display server may be exposed to attack when the ssh client requests
679 forwarding (see the warnings for
683 A system administrator may have a stance in which they want to
684 protect clients that may expose themselves to attack by unwittingly
685 requesting X11 forwarding, which can warrant a
689 Note that disabling X11 forwarding does not prevent users from
690 forwarding X11 traffic, as users can always install their own forwarders.
691 X11 forwarding is automatically disabled if
694 .It Cm X11UseLocalhost
697 should bind the X11 forwarding server to the loopback address or to
698 the wildcard address. By default,
700 binds the forwarding server to the loopback address and sets the
703 environment variable to
705 This prevents remote hosts from connecting to the proxy display.
706 However, some older X11 clients may not function with this
711 to specify that the forwarding server should be bound to the wildcard
720 Specifies the full pathname of the
724 .Pa /usr/X11R6/bin/xauth .
729 command-line arguments and configuration file options that specify time
730 may be expressed using a sequence of the form:
732 .Ar time Op Ar qualifier ,
736 is a positive integer value and
738 is one of the following:
740 .Bl -tag -width Ds -compact -offset indent
755 Each member of the sequence is added together to calculate
756 the total time value.
758 Time format examples:
760 .Bl -tag -width Ds -compact -offset indent
762 600 seconds (10 minutes)
766 1 hour 30 minutes (90 minutes)
770 .It Pa /etc/ssh/sshd_config
771 Contains configuration data for
773 This file should be writable by root only, but it is recommended
774 (though not necessary) that it be world-readable.
777 OpenSSH is a derivative of the original and free
778 ssh 1.2.12 release by Tatu Ylonen.
779 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
780 Theo de Raadt and Dug Song
781 removed many bugs, re-added newer features and
783 Markus Friedl contributed the support for SSH
784 protocol versions 1.5 and 2.0.
785 Niels Provos and Markus Friedl contributed support
786 for privilege separation.