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 1.88 2008/05/07 05:49:37 pyr 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 AllowAgentForwarding
120 forwarding is permitted.
123 Note that disabling Agent forwarding does not improve security
124 unless users are also denied shell access, as they can always install
125 their own forwarders.
126 .It Cm AllowTcpForwarding
127 Specifies whether TCP forwarding is permitted.
130 Note that disabling TCP forwarding does not improve security unless
131 users are also denied shell access, as they can always install their
134 This keyword can be followed by a list of user name patterns, separated
136 If specified, login is allowed only for user names that
137 match one of the patterns.
138 Only user names are valid; a numerical user ID is not recognized.
139 By default, login is allowed for all users.
140 If the pattern takes the form USER@HOST then USER and HOST
141 are separately checked, restricting logins to particular
142 users from particular hosts.
143 The allow/deny directives are processed in the following order:
154 for more information on patterns.
155 .It Cm AuthorizedKeysFile
156 Specifies the file that contains the public keys that can be used
157 for user authentication.
158 .Cm AuthorizedKeysFile
159 may contain tokens of the form %T which are substituted during connection
161 The following tokens are defined: %% is replaced by a literal '%',
162 %h is replaced by the home directory of the user being authenticated, and
163 %u is replaced by the username of that user.
165 .Cm AuthorizedKeysFile
166 is taken to be an absolute path or one relative to the user's home
169 .Dq .ssh/authorized_keys .
171 The contents of the specified file are sent to the remote user before
172 authentication is allowed.
175 then no banner is displayed.
176 This option is only available for protocol version 2.
177 By default, no banner is displayed.
178 .It Cm ChallengeResponseAuthentication
179 Specifies whether challenge-response authentication is allowed.
180 All authentication styles from
185 .It Cm ChrootDirectory
188 to after authentication.
189 This path, and all its components, must be root-owned directories that are
190 not writable by any other user or group.
192 The path may contain the following tokens that are expanded at runtime once
193 the connecting user has been authenticated: %% is replaced by a literal '%',
194 %h is replaced by the home directory of the user being authenticated, and
195 %u is replaced by the username of that user.
199 must contain the necessary files and directories to support the
201 For an interactive session this requires at least a shell, typically
215 For file transfer sessions using
217 no additional configuration of the environment is necessary if the
218 in-process sftp server is used (see
222 The default is not to
225 Specifies the ciphers allowed for protocol version 2.
226 Multiple ciphers must be comma-separated.
227 The supported ciphers are
242 .Bd -literal -offset 3n
243 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
244 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
245 aes192-ctr,aes256-ctr
247 .It Cm ClientAliveCountMax
248 Sets the number of client alive messages (see below) which may be
251 receiving any messages back from the client.
252 If this threshold is reached while client alive messages are being sent,
253 sshd will disconnect the client, terminating the session.
254 It is important to note that the use of client alive messages is very
258 The client alive messages are sent through the encrypted channel
259 and therefore will not be spoofable.
260 The TCP keepalive option enabled by
263 The client alive mechanism is valuable when the client or
264 server depend on knowing when a connection has become inactive.
266 The default value is 3.
268 .Cm ClientAliveInterval
269 (see below) is set to 15, and
270 .Cm ClientAliveCountMax
271 is left at the default, unresponsive SSH clients
272 will be disconnected after approximately 45 seconds.
273 This option applies to protocol version 2 only.
274 .It Cm ClientAliveInterval
275 Sets a timeout interval in seconds after which if no data has been received
278 will send a message through the encrypted
279 channel to request a response from the client.
281 is 0, indicating that these messages will not be sent to the client.
282 This option applies to protocol version 2 only.
284 Specifies whether compression is allowed, or delayed until
285 the user has authenticated successfully.
294 This keyword can be followed by a list of group name patterns, separated
296 Login is disallowed for users whose primary group or supplementary
297 group list matches one of the patterns.
298 Only group names are valid; a numerical group ID is not recognized.
299 By default, login is allowed for all groups.
300 The allow/deny directives are processed in the following order:
311 for more information on patterns.
313 This keyword can be followed by a list of user name patterns, separated
315 Login is disallowed for user names that match one of the patterns.
316 Only user names are valid; a numerical user ID is not recognized.
317 By default, login is allowed for all users.
318 If the pattern takes the form USER@HOST then USER and HOST
319 are separately checked, restricting logins to particular
320 users from particular hosts.
321 The allow/deny directives are processed in the following order:
332 for more information on patterns.
334 Forces the execution of the command specified by
336 ignoring any command supplied by the client and
339 The command is invoked by using the user's login shell with the -c option.
340 This applies to shell, command, or subsystem execution.
341 It is most useful inside a
344 The command originally supplied by the client is available in the
345 .Ev SSH_ORIGINAL_COMMAND
346 environment variable.
347 Specifying a command of
349 will force the use of an in-process sftp server that requires no support
351 .Cm ChrootDirectory .
353 Specifies whether remote hosts are allowed to connect to ports
354 forwarded for the client.
357 binds remote port forwardings to the loopback address.
358 This prevents other remote hosts from connecting to forwarded ports.
360 can be used to specify that sshd
361 should allow remote port forwardings to bind to non-loopback addresses, thus
362 allowing other hosts to connect.
365 to force remote port forwardings to be available to the local host only,
367 to force remote port forwardings to bind to the wildcard address, or
369 to allow the client to select the address to which the forwarding is bound.
372 .It Cm GSSAPIAuthentication
373 Specifies whether user authentication based on GSSAPI is allowed.
376 Note that this option applies to protocol version 2 only.
377 .It Cm GSSAPICleanupCredentials
378 Specifies whether to automatically destroy the user's credentials cache
382 Note that this option applies to protocol version 2 only.
383 .It Cm HostbasedAuthentication
384 Specifies whether rhosts or /etc/hosts.equiv authentication together
385 with successful public key client host authentication is allowed
386 (host-based authentication).
387 This option is similar to
388 .Cm RhostsRSAAuthentication
389 and applies to protocol version 2 only.
392 .It Cm HostbasedUsesNameFromPacketOnly
393 Specifies whether or not the server will attempt to perform a reverse
394 name lookup when matching the name in the
400 .Cm HostbasedAuthentication .
405 uses the name supplied by the client rather than
406 attempting to resolve the name from the TCP connection itself.
410 Specifies a file containing a private host key
413 .Pa /etc/ssh/ssh_host_key
414 for protocol version 1, and
415 .Pa /etc/ssh/ssh_host_rsa_key
417 .Pa /etc/ssh/ssh_host_dsa_key
418 for protocol version 2.
421 will refuse to use a file if it is group/world-accessible.
422 It is possible to have multiple host key files.
424 keys are used for version 1 and
428 are used for version 2 of the SSH protocol.
434 files will not be used in
435 .Cm RhostsRSAAuthentication
437 .Cm HostbasedAuthentication .
441 .Pa /etc/shosts.equiv
445 .It Cm IgnoreUserKnownHosts
448 should ignore the user's
449 .Pa ~/.ssh/known_hosts
451 .Cm RhostsRSAAuthentication
453 .Cm HostbasedAuthentication .
456 .It Cm KerberosAuthentication
457 Specifies whether the password provided by the user for
458 .Cm PasswordAuthentication
459 will be validated through the Kerberos KDC.
460 To use this option, the server needs a
461 Kerberos servtab which allows the verification of the KDC's identity.
464 .It Cm KerberosGetAFSToken
465 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
466 an AFS token before accessing the user's home directory.
469 .It Cm KerberosOrLocalPasswd
470 If password authentication through Kerberos fails then
471 the password will be validated via any additional local mechanism
476 .It Cm KerberosTicketCleanup
477 Specifies whether to automatically destroy the user's ticket cache
481 .It Cm KeyRegenerationInterval
482 In protocol version 1, the ephemeral server key is automatically regenerated
483 after this many seconds (if it has been used).
484 The purpose of regeneration is to prevent
485 decrypting captured sessions by later breaking into the machine and
487 The key is never stored anywhere.
488 If the value is 0, the key is never regenerated.
489 The default is 3600 (seconds).
491 Specifies the local addresses
494 The following forms may be used:
496 .Bl -item -offset indent -compact
500 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
505 .Ar host No | Ar IPv4_addr No : Ar port
511 .Ar host No | Ar IPv6_addr Oc : Ar port
518 sshd will listen on the address and all prior
521 The default is to listen on all local addresses.
524 options are permitted.
527 options must precede this option for non-port qualified addresses.
528 .It Cm LoginGraceTime
529 The server disconnects after this time if the user has not
530 successfully logged in.
531 If the value is 0, there is no time limit.
532 The default is 120 seconds.
534 Gives the verbosity level that is used when logging messages from
536 The possible values are:
537 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
539 DEBUG and DEBUG1 are equivalent.
540 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
541 Logging with a DEBUG level violates the privacy of users and is not recommended.
543 Specifies the available MAC (message authentication code) algorithms.
544 The MAC algorithm is used in protocol version 2
545 for data integrity protection.
546 Multiple algorithms must be comma-separated.
548 .Bd -literal -offset indent
549 hmac-md5,hmac-sha1,umac-64@openssh.com,
550 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
553 Introduces a conditional block.
554 If all of the criteria on the
556 line are satisfied, the keywords on the following lines override those
557 set in the global section of the config file, until either another
559 line or the end of the file.
562 are one or more criteria-pattern pairs.
563 The available criteria are
569 Only a subset of keywords may be used on the lines following a
572 Available keywords are
573 .Cm AllowTcpForwarding ,
575 .Cm ChrootDirectory ,
578 .Cm GSSAPIAuthentication ,
579 .Cm HostbasedAuthentication ,
580 .Cm KbdInteractiveAuthentication ,
581 .Cm KerberosAuthentication ,
582 .Cm PasswordAuthentication ,
584 .Cm PermitRootLogin ,
585 .Cm RhostsRSAAuthentication ,
586 .Cm RSAAuthentication ,
587 .Cm X11DisplayOffset ,
590 .Cm X11UseLocalHost .
592 Specifies the maximum number of authentication attempts permitted per
594 Once the number of failures reaches half this value,
595 additional failures are logged.
598 Specifies the maximum number of concurrent unauthenticated connections to the
600 Additional connections will be dropped until authentication succeeds or the
602 expires for a connection.
605 Alternatively, random early drop can be enabled by specifying
606 the three colon separated values
610 will refuse connection attempts with a probability of
613 if there are currently
616 unauthenticated connections.
617 The probability increases linearly and all connection attempts
618 are refused if the number of unauthenticated connections reaches
621 .It Cm PasswordAuthentication
622 Specifies whether password authentication is allowed.
625 .It Cm PermitEmptyPasswords
626 When password authentication is allowed, it specifies whether the
627 server allows login to accounts with empty password strings.
631 Specifies the destinations to which TCP port forwarding is permitted.
632 The forwarding specification must be one of the following forms:
634 .Bl -item -offset indent -compact
648 .Ar \&[ IPv6_addr \&] : port
652 Multiple forwards may be specified by separating them with whitespace.
655 can be used to remove all restrictions and permit any forwarding requests.
656 By default all port forwarding requests are permitted.
657 .It Cm PermitRootLogin
658 Specifies whether root can log in using
662 .Dq without-password ,
663 .Dq forced-commands-only ,
669 If this option is set to
670 .Dq without-password ,
671 password authentication is disabled for root.
673 If this option is set to
674 .Dq forced-commands-only ,
675 root login with public key authentication will be allowed,
678 option has been specified
679 (which may be useful for taking remote backups even if root login is
680 normally not allowed).
681 All other authentication methods are disabled for root.
683 If this option is set to
685 root is not allowed to log in.
689 device forwarding is allowed.
705 .It Cm PermitUserEnvironment
707 .Pa ~/.ssh/environment
711 .Pa ~/.ssh/authorized_keys
716 Enabling environment processing may enable users to bypass access
717 restrictions in some configurations using mechanisms such as
720 Specifies the file that contains the process ID of the
723 .Pa /var/run/sshd.pid .
725 Specifies the port number that
729 Multiple options of this type are permitted.
735 should print the date and time of the last user login when a user logs
744 when a user logs in interactively.
745 (On some systems it is also printed by the shell,
751 Specifies the protocol versions
754 The possible values are
758 Multiple versions must be comma-separated.
761 Note that the order of the protocol list does not indicate preference,
762 because the client selects among multiple protocol versions offered
768 .It Cm PubkeyAuthentication
769 Specifies whether public key authentication is allowed.
772 Note that this option applies to protocol version 2 only.
773 .It Cm RhostsRSAAuthentication
774 Specifies whether rhosts or /etc/hosts.equiv authentication together
775 with successful RSA host authentication is allowed.
778 This option applies to protocol version 1 only.
779 .It Cm RSAAuthentication
780 Specifies whether pure RSA authentication is allowed.
783 This option applies to protocol version 1 only.
785 Defines the number of bits in the ephemeral protocol version 1 server key.
786 The minimum value is 512, and the default is 768.
790 should check file modes and ownership of the
791 user's files and home directory before accepting login.
792 This is normally desirable because novices sometimes accidentally leave their
793 directory or files world-writable.
797 Configures an external subsystem (e.g. file transfer daemon).
798 Arguments should be a subsystem name and a command (with optional arguments)
799 to execute upon subsystem request.
805 file transfer subsystem.
809 implements an in-process
812 This may simplify configurations using
814 to force a different filesystem root on clients.
816 By default no subsystems are defined.
817 Note that this option applies to protocol version 2 only.
818 .It Cm SyslogFacility
819 Gives the facility code that is used when logging messages from
821 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
822 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
825 Specifies whether the system should send TCP keepalive messages to the
827 If they are sent, death of the connection or crash of one
828 of the machines will be properly noticed.
829 However, this means that
830 connections will die if the route is down temporarily, and some people
832 On the other hand, if TCP keepalives are not sent,
833 sessions may hang indefinitely on the server, leaving
835 users and consuming server resources.
839 (to send TCP keepalive messages), and the server will notice
840 if the network goes down or the client host crashes.
841 This avoids infinitely hanging sessions.
843 To disable TCP keepalive messages, the value should be set to
848 should look up the remote host name and check that
849 the resolved host name for the remote IP address maps back to the
850 very same IP address.
856 is used for interactive login sessions.
861 is never used for remote command execution.
862 Note also, that if this is enabled,
864 will be disabled because
866 does not know how to handle
870 .Cm UsePrivilegeSeparation
871 is specified, it will be disabled after authentication.
873 Enables the Pluggable Authentication Module interface.
876 this will enable PAM authentication using
877 .Cm ChallengeResponseAuthentication
879 .Cm PasswordAuthentication
880 in addition to PAM account and session module processing for all
881 authentication types.
883 Because PAM challenge-response authentication usually serves an equivalent
884 role to password authentication, you should disable either
885 .Cm PasswordAuthentication
887 .Cm ChallengeResponseAuthentication.
891 is enabled, you will not be able to run
896 .It Cm UsePrivilegeSeparation
899 separates privileges by creating an unprivileged child process
900 to deal with incoming network traffic.
901 After successful authentication, another process will be created that has
902 the privilege of the authenticated user.
903 The goal of privilege separation is to prevent privilege
904 escalation by containing any corruption within the unprivileged processes.
907 .It Cm X11DisplayOffset
908 Specifies the first display number available for
911 This prevents sshd from interfering with real X11 servers.
914 Specifies whether X11 forwarding is permitted.
922 When X11 forwarding is enabled, there may be additional exposure to
923 the server and to client displays if the
925 proxy display is configured to listen on the wildcard address (see
927 below), though this is not the default.
928 Additionally, the authentication spoofing and authentication data
929 verification and substitution occur on the client side.
930 The security risk of using X11 forwarding is that the client's X11
931 display server may be exposed to attack when the SSH client requests
932 forwarding (see the warnings for
936 A system administrator may have a stance in which they want to
937 protect clients that may expose themselves to attack by unwittingly
938 requesting X11 forwarding, which can warrant a
942 Note that disabling X11 forwarding does not prevent users from
943 forwarding X11 traffic, as users can always install their own forwarders.
944 X11 forwarding is automatically disabled if
947 .It Cm X11UseLocalhost
950 should bind the X11 forwarding server to the loopback address or to
951 the wildcard address.
953 sshd binds the forwarding server to the loopback address and sets the
956 environment variable to
958 This prevents remote hosts from connecting to the proxy display.
959 However, some older X11 clients may not function with this
964 to specify that the forwarding server should be bound to the wildcard
973 Specifies the full pathname of the
977 .Pa /usr/X11R6/bin/xauth .
981 command-line arguments and configuration file options that specify time
982 may be expressed using a sequence of the form:
984 .Ar time Op Ar qualifier ,
988 is a positive integer value and
990 is one of the following:
992 .Bl -tag -width Ds -compact -offset indent
1007 Each member of the sequence is added together to calculate
1008 the total time value.
1010 Time format examples:
1012 .Bl -tag -width Ds -compact -offset indent
1014 600 seconds (10 minutes)
1018 1 hour 30 minutes (90 minutes)
1022 .It Pa /etc/ssh/sshd_config
1023 Contains configuration data for
1025 This file should be writable by root only, but it is recommended
1026 (though not necessary) that it be world-readable.
1031 OpenSSH is a derivative of the original and free
1032 ssh 1.2.12 release by Tatu Ylonen.
1033 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1034 Theo de Raadt and Dug Song
1035 removed many bugs, re-added newer features and
1037 Markus Friedl contributed the support for SSH
1038 protocol versions 1.5 and 2.0.
1039 Niels Provos and Markus Friedl contributed support
1040 for privilege separation.
andersk / openssh.git / blob