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 1.18 2003/06/02 09:17:34 markus Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon configuration file
45 .Bl -tag -width Ds -compact
46 .It Pa /etc/ssh/sshd_config
50 reads configuration data from
51 .Pa /etc/ssh/sshd_config
52 (or the file specified with
55 The file contains keyword-argument pairs, one per line.
58 and empty lines are interpreted as comments.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
64 .It Cm AFSTokenPassing
65 Specifies whether an AFS token may be forwarded to the server.
69 This keyword can be followed by a list of group name patterns, separated
71 If specified, login is allowed only for users whose primary
72 group or supplementary group list matches one of the patterns.
77 wildcards in the patterns.
78 Only group names are valid; a numerical group ID is not recognized.
79 By default, login is allowed for all groups.
81 .It Cm AllowTcpForwarding
82 Specifies whether TCP forwarding is permitted.
85 Note that disabling TCP forwarding does not improve security unless
86 users are also denied shell access, as they can always install their
90 This keyword can be followed by a list of user name patterns, separated
92 If specified, login is allowed only for user names that
93 match one of the patterns.
98 wildcards in the patterns.
99 Only user names are valid; a numerical user ID is not recognized.
100 By default, login is allowed for all users.
101 If the pattern takes the form USER@HOST then USER and HOST
102 are separately checked, restricting logins to particular
103 users from particular hosts.
105 .It Cm AuthorizedKeysFile
106 Specifies the file that contains the public keys that can be used
107 for user authentication.
108 .Cm AuthorizedKeysFile
109 may contain tokens of the form %T which are substituted during connection
111 The following tokens are defined: %% is replaced by a literal '%',
112 %h is replaced by the home directory of the user being authenticated and
113 %u is replaced by the username of that user.
115 .Cm AuthorizedKeysFile
116 is taken to be an absolute path or one relative to the user's home
119 .Dq .ssh/authorized_keys .
121 In some jurisdictions, sending a warning message before authentication
122 may be relevant for getting legal protection.
123 The contents of the specified file are sent to the remote user before
124 authentication is allowed.
125 This option is only available for protocol version 2.
126 By default, no banner is displayed.
128 .It Cm ChallengeResponseAuthentication
129 Specifies whether challenge response authentication is allowed.
130 All authentication styles from
136 Specifies the ciphers allowed for protocol version 2.
137 Multiple ciphers must be comma-separated.
141 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
142 aes192-cbc,aes256-cbc''
144 .It Cm ClientAliveInterval
145 Sets a timeout interval in seconds after which if no data has been received
148 will send a message through the encrypted
149 channel to request a response from the client.
151 is 0, indicating that these messages will not be sent to the client.
152 This option applies to protocol version 2 only.
153 .It Cm ClientAliveCountMax
154 Sets the number of client alive messages (see above) which may be
157 receiving any messages back from the client.
158 If this threshold is reached while client alive messages are being sent,
160 will disconnect the client, terminating the session.
161 It is important to note that the use of client alive messages is very
165 The client alive messages are sent through the encrypted channel
166 and therefore will not be spoofable.
167 The TCP keepalive option enabled by
170 The client alive mechanism is valuable when the client or
171 server depend on knowing when a connection has become inactive.
173 The default value is 3.
175 .Cm ClientAliveInterval
176 (above) is set to 15, and
177 .Cm ClientAliveCountMax
178 is left at the default, unresponsive ssh clients
179 will be disconnected after approximately 45 seconds.
181 Specifies whether compression is allowed.
189 This keyword can be followed by a list of group name patterns, separated
191 Login is disallowed for users whose primary group or supplementary
192 group list matches one of the patterns.
197 wildcards in the patterns.
198 Only group names are valid; a numerical group ID is not recognized.
199 By default, login is allowed for all groups.
202 This keyword can be followed by a list of user name patterns, separated
204 Login is disallowed for user names that match one of the patterns.
208 can be used as wildcards in the patterns.
209 Only user names are valid; a numerical user ID is not recognized.
210 By default, login is allowed for all users.
211 If the pattern takes the form USER@HOST then USER and HOST
212 are separately checked, restricting logins to particular
213 users from particular hosts.
215 Specifies whether remote hosts are allowed to connect to ports
216 forwarded for the client.
219 binds remote port forwardings to the loopback address.
220 This prevents other remote hosts from connecting to forwarded ports.
222 can be used to specify that
224 should bind remote port forwardings to the wildcard address,
225 thus allowing remote hosts to connect to forwarded ports.
232 .It Cm HostbasedAuthentication
233 Specifies whether rhosts or /etc/hosts.equiv authentication together
234 with successful public key client host authentication is allowed
235 (hostbased authentication).
236 This option is similar to
237 .Cm RhostsRSAAuthentication
238 and applies to protocol version 2 only.
242 Specifies a file containing a private host key
245 .Pa /etc/ssh/ssh_host_key
246 for protocol version 1, and
247 .Pa /etc/ssh/ssh_host_rsa_key
249 .Pa /etc/ssh/ssh_host_dsa_key
250 for protocol version 2.
253 will refuse to use a file if it is group/world-accessible.
254 It is possible to have multiple host key files.
256 keys are used for version 1 and
260 are used for version 2 of the SSH protocol.
266 files will not be used in
267 .Cm RhostsAuthentication ,
268 .Cm RhostsRSAAuthentication
270 .Cm HostbasedAuthentication .
274 .Pa /etc/shosts.equiv
278 .It Cm IgnoreUserKnownHosts
281 should ignore the user's
282 .Pa $HOME/.ssh/known_hosts
284 .Cm RhostsRSAAuthentication
286 .Cm HostbasedAuthentication .
290 Specifies whether the system should send TCP keepalive messages to the
292 If they are sent, death of the connection or crash of one
293 of the machines will be properly noticed.
294 However, this means that
295 connections will die if the route is down temporarily, and some people
297 On the other hand, if keepalives are not sent,
298 sessions may hang indefinitely on the server, leaving
300 users and consuming server resources.
304 (to send keepalives), and the server will notice
305 if the network goes down or the client host crashes.
306 This avoids infinitely hanging sessions.
308 To disable keepalives, the value should be set to
310 .It Cm KerberosAuthentication
311 Specifies whether Kerberos authentication is allowed.
312 This can be in the form of a Kerberos ticket, or if
313 .Cm PasswordAuthentication
314 is yes, the password provided by the user will be validated through
316 To use this option, the server needs a
317 Kerberos servtab which allows the verification of the KDC's identity.
320 .It Cm KerberosOrLocalPasswd
321 If set then if password authentication through Kerberos fails then
322 the password will be validated via any additional local mechanism
327 .It Cm KerberosTgtPassing
328 Specifies whether a Kerberos TGT may be forwarded to the server.
331 as this only works when the Kerberos KDC is actually an AFS kaserver.
332 .It Cm KerberosTicketCleanup
333 Specifies whether to automatically destroy the user's ticket cache
337 .It Cm KeyRegenerationInterval
338 In protocol version 1, the ephemeral server key is automatically regenerated
339 after this many seconds (if it has been used).
340 The purpose of regeneration is to prevent
341 decrypting captured sessions by later breaking into the machine and
343 The key is never stored anywhere.
344 If the value is 0, the key is never regenerated.
345 The default is 3600 (seconds).
347 Specifies the local addresses
350 The following forms may be used:
352 .Bl -item -offset indent -compact
356 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
361 .Ar host No | Ar IPv4_addr No : Ar port
367 .Ar host No | Ar IPv6_addr Oc : Ar port
375 will listen on the address and all prior
378 The default is to listen on all local addresses.
381 options are permitted.
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.
396 DEBUG and DEBUG1 are equivalent.
397 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
398 Logging with a DEBUG level violates the privacy of users 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.
435 .It Cm PermitEmptyPasswords
436 When password authentication is allowed, it specifies whether the
437 server allows login to accounts with empty password strings.
440 .It Cm PermitRootLogin
441 Specifies whether root can login using
445 .Dq without-password ,
446 .Dq forced-commands-only
452 If this option is set to
454 password authentication is disabled for root.
456 If this option is set to
457 .Dq forced-commands-only
458 root login with public key authentication will be allowed,
461 option has been specified
462 (which may be useful for taking remote backups even if root login is
463 normally not allowed).
464 All other authentication methods are disabled for root.
466 If this option is set to
468 root is not allowed to login.
469 .It Cm PermitUserEnvironment
471 .Pa ~/.ssh/environment
475 .Pa ~/.ssh/authorized_keys
480 Enabling environment processing may enable users to bypass access
481 restrictions in some configurations using mechanisms such as
484 Specifies the file that contains the process ID of the
488 .Pa /var/run/sshd.pid .
490 Specifies the port number that
494 Multiple options of this type are permitted.
500 should print the date and time when the user last logged in.
508 when a user logs in interactively.
509 (On some systems it is also printed by the shell,
515 Specifies the protocol versions
518 The possible values are
522 Multiple versions must be comma-separated.
525 Note that the order of the protocol list does not indicate preference,
526 because the client selects among multiple protocol versions offered
532 .It Cm PubkeyAuthentication
533 Specifies whether public key authentication is allowed.
536 Note that this option applies to protocol version 2 only.
537 .It Cm RhostsAuthentication
538 Specifies whether authentication using rhosts or /etc/hosts.equiv
540 Normally, this method should not be permitted because it is insecure.
541 .Cm RhostsRSAAuthentication
543 instead, because it performs RSA-based host authentication in addition
544 to normal rhosts or /etc/hosts.equiv authentication.
547 This option applies to protocol version 1 only.
548 .It Cm RhostsRSAAuthentication
549 Specifies whether rhosts or /etc/hosts.equiv authentication together
550 with successful RSA host authentication is allowed.
553 This option applies to protocol version 1 only.
554 .It Cm RSAAuthentication
555 Specifies whether pure RSA authentication is allowed.
558 This option applies to protocol version 1 only.
560 Defines the number of bits in the ephemeral protocol version 1 server key.
561 The minimum value is 512, and the default is 768.
565 should check file modes and ownership of the
566 user's files and home directory before accepting login.
567 This is normally desirable because novices sometimes accidentally leave their
568 directory or files world-writable.
572 Configures an external subsystem (e.g., file transfer daemon).
573 Arguments should be a subsystem name and a command to execute upon subsystem
579 file transfer subsystem.
580 By default no subsystems are defined.
581 Note that this option applies to protocol version 2 only.
582 .It Cm SyslogFacility
583 Gives the facility code that is used when logging messages from
585 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
586 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
591 should lookup the remote host name and check that
592 the resolved host name for the remote IP address maps back to the
593 very same IP address.
599 is used for interactive login sessions.
604 is never used for remote command execution.
605 Note also, that if this is enabled,
607 will be disabled because
609 does not know how to handle
613 .Cm UsePrivilegeSeparation
614 is specified, it will be disabled after authentication.
616 Enables PAM authentication (via challenge-response) and session set up.
617 If you enable this, you should probably disable
618 .Cm PasswordAuthentication .
621 then you will not be able to run sshd as a non-root user.
622 .It Cm UsePrivilegeSeparation
625 separates privileges by creating an unprivileged child process
626 to deal with incoming network traffic.
627 After successful authentication, another process will be created that has
628 the privilege of the authenticated user.
629 The goal of privilege separation is to prevent privilege
630 escalation by containing any corruption within the unprivileged processes.
633 .It Cm X11DisplayOffset
634 Specifies the first display number available for
639 from interfering with real X11 servers.
642 Specifies whether X11 forwarding is permitted.
650 When X11 forwarding is enabled, there may be additional exposure to
651 the server and to client displays if the
653 proxy display is configured to listen on the wildcard address (see
655 below), however this is not the default.
656 Additionally, the authentication spoofing and authentication data
657 verification and substitution occur on the client side.
658 The security risk of using X11 forwarding is that the client's X11
659 display server may be exposed to attack when the ssh client requests
660 forwarding (see the warnings for
664 A system administrator may have a stance in which they want to
665 protect clients that may expose themselves to attack by unwittingly
666 requesting X11 forwarding, which can warrant a
670 Note that disabling X11 forwarding does not prevent users from
671 forwarding X11 traffic, as users can always install their own forwarders.
672 X11 forwarding is automatically disabled if
675 .It Cm X11UseLocalhost
678 should bind the X11 forwarding server to the loopback address or to
679 the wildcard address.
682 binds the forwarding server to the loopback address and sets the
685 environment variable to
687 This prevents remote hosts from connecting to the proxy display.
688 However, some older X11 clients may not function with this
693 to specify that the forwarding server should be bound to the wildcard
702 Specifies the full pathname of the
706 .Pa /usr/X11R6/bin/xauth .
711 command-line arguments and configuration file options that specify time
712 may be expressed using a sequence of the form:
714 .Ar time Op Ar qualifier ,
718 is a positive integer value and
720 is one of the following:
722 .Bl -tag -width Ds -compact -offset indent
737 Each member of the sequence is added together to calculate
738 the total time value.
740 Time format examples:
742 .Bl -tag -width Ds -compact -offset indent
744 600 seconds (10 minutes)
748 1 hour 30 minutes (90 minutes)
752 .It Pa /etc/ssh/sshd_config
753 Contains configuration data for
755 This file should be writable by root only, but it is recommended
756 (though not necessary) that it be world-readable.
759 OpenSSH is a derivative of the original and free
760 ssh 1.2.12 release by Tatu Ylonen.
761 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
762 Theo de Raadt and Dug Song
763 removed many bugs, re-added newer features and
765 Markus Friedl contributed the support for SSH
766 protocol versions 1.5 and 2.0.
767 Niels Provos and Markus Friedl contributed support
768 for privilege separation.