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 1.81 2008/02/09 08:04:31 jmc Exp $
43 .Nd OpenSSH SSH daemon configuration file
45 .Nm /etc/ssh/sshd_config
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
53 The file contains keyword-argument pairs, one per line.
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
59 in order to represent arguments containing spaces.
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
66 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
79 Multiple environment variables may be separated by whitespace or spread
83 Be warned that some environment variables could be used to bypass restricted
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
88 Specifies which address family should be used by
99 This keyword can be followed by a list of group name patterns, separated
101 If specified, login is allowed only for users whose primary
102 group or supplementary group list matches one of the patterns.
103 Only group names are valid; a numerical group ID is not recognized.
104 By default, login is allowed for all groups.
105 The allow/deny directives are processed in the following order:
116 for more information on patterns.
117 .It Cm AllowTcpForwarding
118 Specifies whether TCP forwarding is permitted.
121 Note that disabling TCP forwarding does not improve security unless
122 users are also denied shell access, as they can always install their
125 This keyword can be followed by a list of user name patterns, separated
127 If specified, login is allowed only for user names that
128 match one of the patterns.
129 Only user names are valid; a numerical user ID is not recognized.
130 By default, login is allowed for all users.
131 If the pattern takes the form USER@HOST then USER and HOST
132 are separately checked, restricting logins to particular
133 users from particular hosts.
134 The allow/deny directives are processed in the following order:
145 for more information on patterns.
146 .It Cm AuthorizedKeysFile
147 Specifies the file that contains the public keys that can be used
148 for user authentication.
149 .Cm AuthorizedKeysFile
150 may contain tokens of the form %T which are substituted during connection
152 The following tokens are defined: %% is replaced by a literal '%',
153 %h is replaced by the home directory of the user being authenticated, and
154 %u is replaced by the username of that user.
156 .Cm AuthorizedKeysFile
157 is taken to be an absolute path or one relative to the user's home
160 .Dq .ssh/authorized_keys .
162 The contents of the specified file are sent to the remote user before
163 authentication is allowed.
166 then no banner is displayed.
167 This option is only available for protocol version 2.
168 By default, no banner is displayed.
169 .It Cm ChallengeResponseAuthentication
170 Specifies whether challenge-response authentication is allowed.
171 All authentication styles from
176 .It Cm ChrootDirectory
179 to after authentication.
180 This path, and all its components, must be root-owned directories that are
181 not writable by any other user or group.
183 The path may contain the following tokens that are expanded at runtime once
184 the connecting user has been authenticated: %% is replaced by a literal '%',
185 %h is replaced by the home directory of the user being authenticated, and
186 %u is replaced by the username of that user.
190 must contain the necessary files and directories to support the
192 For an interactive session this requires at least a shell, typically
206 For file transfer sessions using
208 no additional configuration of the environment is necessary if the
209 in-process sftp server is used (see
213 The default is not to
216 Specifies the ciphers allowed for protocol version 2.
217 Multiple ciphers must be comma-separated.
218 The supported ciphers are
233 .Bd -literal -offset 3n
234 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
235 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
236 aes192-ctr,aes256-ctr
238 .It Cm ClientAliveCountMax
239 Sets the number of client alive messages (see below) which may be
242 receiving any messages back from the client.
243 If this threshold is reached while client alive messages are being sent,
244 sshd will disconnect the client, terminating the session.
245 It is important to note that the use of client alive messages is very
249 The client alive messages are sent through the encrypted channel
250 and therefore will not be spoofable.
251 The TCP keepalive option enabled by
254 The client alive mechanism is valuable when the client or
255 server depend on knowing when a connection has become inactive.
257 The default value is 3.
259 .Cm ClientAliveInterval
260 (see below) is set to 15, and
261 .Cm ClientAliveCountMax
262 is left at the default, unresponsive SSH clients
263 will be disconnected after approximately 45 seconds.
264 This option applies to protocol version 2 only.
265 .It Cm ClientAliveInterval
266 Sets a timeout interval in seconds after which if no data has been received
269 will send a message through the encrypted
270 channel to request a response from the client.
272 is 0, indicating that these messages will not be sent to the client.
273 This option applies to protocol version 2 only.
275 Specifies whether compression is allowed, or delayed until
276 the user has authenticated successfully.
285 This keyword can be followed by a list of group name patterns, separated
287 Login is disallowed for users whose primary group or supplementary
288 group list matches one of the patterns.
289 Only group names are valid; a numerical group ID is not recognized.
290 By default, login is allowed for all groups.
291 The allow/deny directives are processed in the following order:
302 for more information on patterns.
304 This keyword can be followed by a list of user name patterns, separated
306 Login is disallowed for user names that match one of the patterns.
307 Only user names are valid; a numerical user ID is not recognized.
308 By default, login is allowed for all users.
309 If the pattern takes the form USER@HOST then USER and HOST
310 are separately checked, restricting logins to particular
311 users from particular hosts.
312 The allow/deny directives are processed in the following order:
323 for more information on patterns.
325 Forces the execution of the command specified by
327 ignoring any command supplied by the client.
328 The command is invoked by using the user's login shell with the -c option.
329 This applies to shell, command, or subsystem execution.
330 It is most useful inside a
333 The command originally supplied by the client is available in the
334 .Ev SSH_ORIGINAL_COMMAND
335 environment variable.
337 Specifies whether remote hosts are allowed to connect to ports
338 forwarded for the client.
341 binds remote port forwardings to the loopback address.
342 This prevents other remote hosts from connecting to forwarded ports.
344 can be used to specify that sshd
345 should allow remote port forwardings to bind to non-loopback addresses, thus
346 allowing other hosts to connect.
349 to force remote port forwardings to be available to the local host only,
351 to force remote port forwardings to bind to the wildcard address, or
353 to allow the client to select the address to which the forwarding is bound.
356 .It Cm GSSAPIAuthentication
357 Specifies whether user authentication based on GSSAPI is allowed.
360 Note that this option applies to protocol version 2 only.
361 .It Cm GSSAPICleanupCredentials
362 Specifies whether to automatically destroy the user's credentials cache
366 Note that this option applies to protocol version 2 only.
367 .It Cm HostbasedAuthentication
368 Specifies whether rhosts or /etc/hosts.equiv authentication together
369 with successful public key client host authentication is allowed
370 (host-based authentication).
371 This option is similar to
372 .Cm RhostsRSAAuthentication
373 and applies to protocol version 2 only.
376 .It Cm HostbasedUsesNameFromPacketOnly
377 Specifies whether or not the server will attempt to perform a reverse
378 name lookup when matching the name in the
384 .Cm HostbasedAuthentication .
389 uses the name supplied by the client rather than
390 attempting to resolve the name from the TCP connection itself.
394 Specifies a file containing a private host key
397 .Pa /etc/ssh/ssh_host_key
398 for protocol version 1, and
399 .Pa /etc/ssh/ssh_host_rsa_key
401 .Pa /etc/ssh/ssh_host_dsa_key
402 for protocol version 2.
405 will refuse to use a file if it is group/world-accessible.
406 It is possible to have multiple host key files.
408 keys are used for version 1 and
412 are used for version 2 of the SSH protocol.
418 files will not be used in
419 .Cm RhostsRSAAuthentication
421 .Cm HostbasedAuthentication .
425 .Pa /etc/shosts.equiv
429 .It Cm IgnoreUserKnownHosts
432 should ignore the user's
433 .Pa ~/.ssh/known_hosts
435 .Cm RhostsRSAAuthentication
437 .Cm HostbasedAuthentication .
440 .It Cm KerberosAuthentication
441 Specifies whether the password provided by the user for
442 .Cm PasswordAuthentication
443 will be validated through the Kerberos KDC.
444 To use this option, the server needs a
445 Kerberos servtab which allows the verification of the KDC's identity.
448 .It Cm KerberosGetAFSToken
449 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
450 an AFS token before accessing the user's home directory.
453 .It Cm KerberosOrLocalPasswd
454 If password authentication through Kerberos fails then
455 the password will be validated via any additional local mechanism
460 .It Cm KerberosTicketCleanup
461 Specifies whether to automatically destroy the user's ticket cache
465 .It Cm KeyRegenerationInterval
466 In protocol version 1, the ephemeral server key is automatically regenerated
467 after this many seconds (if it has been used).
468 The purpose of regeneration is to prevent
469 decrypting captured sessions by later breaking into the machine and
471 The key is never stored anywhere.
472 If the value is 0, the key is never regenerated.
473 The default is 3600 (seconds).
475 Specifies the local addresses
478 The following forms may be used:
480 .Bl -item -offset indent -compact
484 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
489 .Ar host No | Ar IPv4_addr No : Ar port
495 .Ar host No | Ar IPv6_addr Oc : Ar port
502 sshd will listen on the address and all prior
505 The default is to listen on all local addresses.
508 options are permitted.
511 options must precede this option for non-port qualified addresses.
512 .It Cm LoginGraceTime
513 The server disconnects after this time if the user has not
514 successfully logged in.
515 If the value is 0, there is no time limit.
516 The default is 120 seconds.
518 Gives the verbosity level that is used when logging messages from
520 The possible values are:
521 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
523 DEBUG and DEBUG1 are equivalent.
524 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
525 Logging with a DEBUG level violates the privacy of users and is not recommended.
527 Specifies the available MAC (message authentication code) algorithms.
528 The MAC algorithm is used in protocol version 2
529 for data integrity protection.
530 Multiple algorithms must be comma-separated.
532 .Bd -literal -offset indent
533 hmac-md5,hmac-sha1,umac-64@openssh.com,
534 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
537 Introduces a conditional block.
538 If all of the criteria on the
540 line are satisfied, the keywords on the following lines override those
541 set in the global section of the config file, until either another
543 line or the end of the file.
546 are one or more criteria-pattern pairs.
547 The available criteria are
553 Only a subset of keywords may be used on the lines following a
556 Available keywords are
557 .Cm AllowTcpForwarding ,
561 .Cm GSSApiAuthentication ,
562 .Cm KbdInteractiveAuthentication ,
563 .Cm KerberosAuthentication ,
564 .Cm PasswordAuthentication ,
566 .Cm PermitRootLogin ,
567 .Cm RhostsRSAAuthentication ,
568 .Cm RSAAuthentication ,
569 .Cm X11DisplayOffset ,
572 .Cm X11UseLocalHost .
574 Specifies the maximum number of authentication attempts permitted per
576 Once the number of failures reaches half this value,
577 additional failures are logged.
580 Specifies the maximum number of concurrent unauthenticated connections to the
582 Additional connections will be dropped until authentication succeeds or the
584 expires for a connection.
587 Alternatively, random early drop can be enabled by specifying
588 the three colon separated values
592 will refuse connection attempts with a probability of
595 if there are currently
598 unauthenticated connections.
599 The probability increases linearly and all connection attempts
600 are refused if the number of unauthenticated connections reaches
603 .It Cm PasswordAuthentication
604 Specifies whether password authentication is allowed.
607 .It Cm PermitEmptyPasswords
608 When password authentication is allowed, it specifies whether the
609 server allows login to accounts with empty password strings.
613 Specifies the destinations to which TCP port forwarding is permitted.
614 The forwarding specification must be one of the following forms:
616 .Bl -item -offset indent -compact
630 .Ar \&[ IPv6_addr \&] : port
634 Multiple forwards may be specified by separating them with whitespace.
637 can be used to remove all restrictions and permit any forwarding requests.
638 By default all port forwarding requests are permitted.
639 .It Cm PermitRootLogin
640 Specifies whether root can log in using
644 .Dq without-password ,
645 .Dq forced-commands-only ,
651 If this option is set to
652 .Dq without-password ,
653 password authentication is disabled for root.
655 If this option is set to
656 .Dq forced-commands-only ,
657 root login with public key authentication will be allowed,
660 option has been specified
661 (which may be useful for taking remote backups even if root login is
662 normally not allowed).
663 All other authentication methods are disabled for root.
665 If this option is set to
667 root is not allowed to log in.
671 device forwarding is allowed.
687 .It Cm PermitUserEnvironment
689 .Pa ~/.ssh/environment
693 .Pa ~/.ssh/authorized_keys
698 Enabling environment processing may enable users to bypass access
699 restrictions in some configurations using mechanisms such as
702 Specifies the file that contains the process ID of the
705 .Pa /var/run/sshd.pid .
707 Specifies the port number that
711 Multiple options of this type are permitted.
717 should print the date and time of the last user login when a user logs
726 when a user logs in interactively.
727 (On some systems it is also printed by the shell,
733 Specifies the protocol versions
736 The possible values are
740 Multiple versions must be comma-separated.
743 Note that the order of the protocol list does not indicate preference,
744 because the client selects among multiple protocol versions offered
750 .It Cm PubkeyAuthentication
751 Specifies whether public key authentication is allowed.
754 Note that this option applies to protocol version 2 only.
755 .It Cm RhostsRSAAuthentication
756 Specifies whether rhosts or /etc/hosts.equiv authentication together
757 with successful RSA host authentication is allowed.
760 This option applies to protocol version 1 only.
761 .It Cm RSAAuthentication
762 Specifies whether pure RSA authentication is allowed.
765 This option applies to protocol version 1 only.
767 Defines the number of bits in the ephemeral protocol version 1 server key.
768 The minimum value is 512, and the default is 768.
772 should check file modes and ownership of the
773 user's files and home directory before accepting login.
774 This is normally desirable because novices sometimes accidentally leave their
775 directory or files world-writable.
779 Configures an external subsystem (e.g. file transfer daemon).
780 Arguments should be a subsystem name and a command (with optional arguments)
781 to execute upon subsystem request.
787 file transfer subsystem.
791 implements an in-process
794 This may simplify configurations using
796 to force a different filesystem root on clients.
798 By default no subsystems are defined.
799 Note that this option applies to protocol version 2 only.
800 .It Cm SyslogFacility
801 Gives the facility code that is used when logging messages from
803 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
804 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
807 Specifies whether the system should send TCP keepalive messages to the
809 If they are sent, death of the connection or crash of one
810 of the machines will be properly noticed.
811 However, this means that
812 connections will die if the route is down temporarily, and some people
814 On the other hand, if TCP keepalives are not sent,
815 sessions may hang indefinitely on the server, leaving
817 users and consuming server resources.
821 (to send TCP keepalive messages), and the server will notice
822 if the network goes down or the client host crashes.
823 This avoids infinitely hanging sessions.
825 To disable TCP keepalive messages, the value should be set to
830 should look up the remote host name and check that
831 the resolved host name for the remote IP address maps back to the
832 very same IP address.
838 is used for interactive login sessions.
843 is never used for remote command execution.
844 Note also, that if this is enabled,
846 will be disabled because
848 does not know how to handle
852 .Cm UsePrivilegeSeparation
853 is specified, it will be disabled after authentication.
855 Enables the Pluggable Authentication Module interface.
858 this will enable PAM authentication using
859 .Cm ChallengeResponseAuthentication
861 .Cm PasswordAuthentication
862 in addition to PAM account and session module processing for all
863 authentication types.
865 Because PAM challenge-response authentication usually serves an equivalent
866 role to password authentication, you should disable either
867 .Cm PasswordAuthentication
869 .Cm ChallengeResponseAuthentication.
873 is enabled, you will not be able to run
878 .It Cm UsePrivilegeSeparation
881 separates privileges by creating an unprivileged child process
882 to deal with incoming network traffic.
883 After successful authentication, another process will be created that has
884 the privilege of the authenticated user.
885 The goal of privilege separation is to prevent privilege
886 escalation by containing any corruption within the unprivileged processes.
889 .It Cm X11DisplayOffset
890 Specifies the first display number available for
893 This prevents sshd from interfering with real X11 servers.
896 Specifies whether X11 forwarding is permitted.
904 When X11 forwarding is enabled, there may be additional exposure to
905 the server and to client displays if the
907 proxy display is configured to listen on the wildcard address (see
909 below), though this is not the default.
910 Additionally, the authentication spoofing and authentication data
911 verification and substitution occur on the client side.
912 The security risk of using X11 forwarding is that the client's X11
913 display server may be exposed to attack when the SSH client requests
914 forwarding (see the warnings for
918 A system administrator may have a stance in which they want to
919 protect clients that may expose themselves to attack by unwittingly
920 requesting X11 forwarding, which can warrant a
924 Note that disabling X11 forwarding does not prevent users from
925 forwarding X11 traffic, as users can always install their own forwarders.
926 X11 forwarding is automatically disabled if
929 .It Cm X11UseLocalhost
932 should bind the X11 forwarding server to the loopback address or to
933 the wildcard address.
935 sshd binds the forwarding server to the loopback address and sets the
938 environment variable to
940 This prevents remote hosts from connecting to the proxy display.
941 However, some older X11 clients may not function with this
946 to specify that the forwarding server should be bound to the wildcard
955 Specifies the full pathname of the
959 .Pa /usr/X11R6/bin/xauth .
963 command-line arguments and configuration file options that specify time
964 may be expressed using a sequence of the form:
966 .Ar time Op Ar qualifier ,
970 is a positive integer value and
972 is one of the following:
974 .Bl -tag -width Ds -compact -offset indent
989 Each member of the sequence is added together to calculate
990 the total time value.
992 Time format examples:
994 .Bl -tag -width Ds -compact -offset indent
996 600 seconds (10 minutes)
1000 1 hour 30 minutes (90 minutes)
1004 .It Pa /etc/ssh/sshd_config
1005 Contains configuration data for
1007 This file should be writable by root only, but it is recommended
1008 (though not necessary) that it be world-readable.
1013 OpenSSH is a derivative of the original and free
1014 ssh 1.2.12 release by Tatu Ylonen.
1015 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1016 Theo de Raadt and Dug Song
1017 removed many bugs, re-added newer features and
1019 Markus Friedl contributed the support for SSH
1020 protocol versions 1.5 and 2.0.
1021 Niels Provos and Markus Friedl contributed support
1022 for privilege separation.
andersk / openssh.git / blob