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 1.104 2009/04/17 19:40:17 stevesk 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
98 .It Cm AllowAgentForwarding
101 forwarding is permitted.
104 Note that disabling agent forwarding does not improve security
105 unless users are also denied shell access, as they can always install
106 their own forwarders.
108 This keyword can be followed by a list of group name patterns, separated
110 If specified, login is allowed only for users whose primary
111 group or supplementary group list matches one of the patterns.
112 Only group names are valid; a numerical group ID is not recognized.
113 By default, login is allowed for all groups.
114 The allow/deny directives are processed in the following order:
125 for more information on patterns.
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 which do not use logging,
218 no additional configuration of the environment is necessary if the
219 in-process sftp server is used (see
222 sftp sessions which do use logging require
224 inside the chroot directory (see
228 The default is not to
231 Specifies the ciphers allowed for protocol version 2.
232 Multiple ciphers must be comma-separated.
233 The supported ciphers are
248 .Bd -literal -offset 3n
249 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
250 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
253 .It Cm ClientAliveCountMax
254 Sets the number of client alive messages (see below) which may be
257 receiving any messages back from the client.
258 If this threshold is reached while client alive messages are being sent,
259 sshd will disconnect the client, terminating the session.
260 It is important to note that the use of client alive messages is very
264 The client alive messages are sent through the encrypted channel
265 and therefore will not be spoofable.
266 The TCP keepalive option enabled by
269 The client alive mechanism is valuable when the client or
270 server depend on knowing when a connection has become inactive.
272 The default value is 3.
274 .Cm ClientAliveInterval
275 (see below) is set to 15, and
276 .Cm ClientAliveCountMax
277 is left at the default, unresponsive SSH clients
278 will be disconnected after approximately 45 seconds.
279 This option applies to protocol version 2 only.
280 .It Cm ClientAliveInterval
281 Sets a timeout interval in seconds after which if no data has been received
284 will send a message through the encrypted
285 channel to request a response from the client.
287 is 0, indicating that these messages will not be sent to the client.
288 This option applies to protocol version 2 only.
290 Specifies whether compression is allowed, or delayed until
291 the user has authenticated successfully.
300 This keyword can be followed by a list of group name patterns, separated
302 Login is disallowed for users whose primary group or supplementary
303 group list matches one of the patterns.
304 Only group names are valid; a numerical group ID is not recognized.
305 By default, login is allowed for all groups.
306 The allow/deny directives are processed in the following order:
317 for more information on patterns.
319 This keyword can be followed by a list of user name patterns, separated
321 Login is disallowed for user names that match one of the patterns.
322 Only user names are valid; a numerical user ID is not recognized.
323 By default, login is allowed for all users.
324 If the pattern takes the form USER@HOST then USER and HOST
325 are separately checked, restricting logins to particular
326 users from particular hosts.
327 The allow/deny directives are processed in the following order:
338 for more information on patterns.
340 Forces the execution of the command specified by
342 ignoring any command supplied by the client and
345 The command is invoked by using the user's login shell with the -c option.
346 This applies to shell, command, or subsystem execution.
347 It is most useful inside a
350 The command originally supplied by the client is available in the
351 .Ev SSH_ORIGINAL_COMMAND
352 environment variable.
353 Specifying a command of
355 will force the use of an in-process sftp server that requires no support
357 .Cm ChrootDirectory .
359 Specifies whether remote hosts are allowed to connect to ports
360 forwarded for the client.
363 binds remote port forwardings to the loopback address.
364 This prevents other remote hosts from connecting to forwarded ports.
366 can be used to specify that sshd
367 should allow remote port forwardings to bind to non-loopback addresses, thus
368 allowing other hosts to connect.
371 to force remote port forwardings to be available to the local host only,
373 to force remote port forwardings to bind to the wildcard address, or
375 to allow the client to select the address to which the forwarding is bound.
378 .It Cm GSSAPIAuthentication
379 Specifies whether user authentication based on GSSAPI is allowed.
382 Note that this option applies to protocol version 2 only.
383 .It Cm GSSAPICleanupCredentials
384 Specifies whether to automatically destroy the user's credentials cache
388 Note that this option applies to protocol version 2 only.
389 .It Cm HostbasedAuthentication
390 Specifies whether rhosts or /etc/hosts.equiv authentication together
391 with successful public key client host authentication is allowed
392 (host-based authentication).
393 This option is similar to
394 .Cm RhostsRSAAuthentication
395 and applies to protocol version 2 only.
398 .It Cm HostbasedUsesNameFromPacketOnly
399 Specifies whether or not the server will attempt to perform a reverse
400 name lookup when matching the name in the
406 .Cm HostbasedAuthentication .
411 uses the name supplied by the client rather than
412 attempting to resolve the name from the TCP connection itself.
416 Specifies a file containing a private host key
419 .Pa /etc/ssh/ssh_host_key
420 for protocol version 1, and
421 .Pa /etc/ssh/ssh_host_rsa_key
423 .Pa /etc/ssh/ssh_host_dsa_key
424 for protocol version 2.
427 will refuse to use a file if it is group/world-accessible.
428 It is possible to have multiple host key files.
430 keys are used for version 1 and
434 are used for version 2 of the SSH protocol.
440 files will not be used in
441 .Cm RhostsRSAAuthentication
443 .Cm HostbasedAuthentication .
447 .Pa /etc/shosts.equiv
451 .It Cm IgnoreUserKnownHosts
454 should ignore the user's
455 .Pa ~/.ssh/known_hosts
457 .Cm RhostsRSAAuthentication
459 .Cm HostbasedAuthentication .
462 .It Cm KerberosAuthentication
463 Specifies whether the password provided by the user for
464 .Cm PasswordAuthentication
465 will be validated through the Kerberos KDC.
466 To use this option, the server needs a
467 Kerberos servtab which allows the verification of the KDC's identity.
470 .It Cm KerberosGetAFSToken
471 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
472 an AFS token before accessing the user's home directory.
475 .It Cm KerberosOrLocalPasswd
476 If password authentication through Kerberos fails then
477 the password will be validated via any additional local mechanism
482 .It Cm KerberosTicketCleanup
483 Specifies whether to automatically destroy the user's ticket cache
487 .It Cm KeyRegenerationInterval
488 In protocol version 1, the ephemeral server key is automatically regenerated
489 after this many seconds (if it has been used).
490 The purpose of regeneration is to prevent
491 decrypting captured sessions by later breaking into the machine and
493 The key is never stored anywhere.
494 If the value is 0, the key is never regenerated.
495 The default is 3600 (seconds).
497 Specifies the local addresses
500 The following forms may be used:
502 .Bl -item -offset indent -compact
506 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
511 .Ar host No | Ar IPv4_addr No : Ar port
517 .Ar host No | Ar IPv6_addr Oc : Ar port
524 sshd will listen on the address and all prior
527 The default is to listen on all local addresses.
530 options are permitted.
533 options must precede this option for non-port qualified addresses.
534 .It Cm LoginGraceTime
535 The server disconnects after this time if the user has not
536 successfully logged in.
537 If the value is 0, there is no time limit.
538 The default is 120 seconds.
540 Gives the verbosity level that is used when logging messages from
542 The possible values are:
543 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
545 DEBUG and DEBUG1 are equivalent.
546 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
547 Logging with a DEBUG level violates the privacy of users and is not recommended.
549 Specifies the available MAC (message authentication code) algorithms.
550 The MAC algorithm is used in protocol version 2
551 for data integrity protection.
552 Multiple algorithms must be comma-separated.
554 .Bd -literal -offset indent
555 hmac-md5,hmac-sha1,umac-64@openssh.com,
556 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
559 Introduces a conditional block.
560 If all of the criteria on the
562 line are satisfied, the keywords on the following lines override those
563 set in the global section of the config file, until either another
565 line or the end of the file.
569 are one or more criteria-pattern pairs.
570 The available criteria are
576 The match patterns may consist of single entries or comma-separated
577 lists and may use the wildcard and negation operators described in the
584 criteria may additionally contain addresses to match in CIDR
585 address/masklen format, e.g.\&
589 Note that the mask length provided must be consistent with the address -
590 it is an error to specify a mask length that is too long for the address
591 or one with bits set in this host portion of the address.
598 Only a subset of keywords may be used on the lines following a
601 Available keywords are
602 .Cm AllowAgentForwarding ,
603 .Cm AllowTcpForwarding ,
605 .Cm ChrootDirectory ,
608 .Cm GSSAPIAuthentication ,
609 .Cm HostbasedAuthentication ,
610 .Cm KbdInteractiveAuthentication ,
611 .Cm KerberosAuthentication ,
614 .Cm PasswordAuthentication ,
615 .Cm PermitEmptyPasswords ,
617 .Cm PermitRootLogin ,
618 .Cm RhostsRSAAuthentication ,
619 .Cm RSAAuthentication ,
620 .Cm X11DisplayOffset ,
623 .Cm X11UseLocalHost .
625 Specifies the maximum number of authentication attempts permitted per
627 Once the number of failures reaches half this value,
628 additional failures are logged.
631 Specifies the maximum number of open sessions permitted per network connection.
634 Specifies the maximum number of concurrent unauthenticated connections to the
636 Additional connections will be dropped until authentication succeeds or the
638 expires for a connection.
641 Alternatively, random early drop can be enabled by specifying
642 the three colon separated values
646 will refuse connection attempts with a probability of
649 if there are currently
652 unauthenticated connections.
653 The probability increases linearly and all connection attempts
654 are refused if the number of unauthenticated connections reaches
657 .It Cm PasswordAuthentication
658 Specifies whether password authentication is allowed.
661 .It Cm PermitEmptyPasswords
662 When password authentication is allowed, it specifies whether the
663 server allows login to accounts with empty password strings.
667 Specifies the destinations to which TCP port forwarding is permitted.
668 The forwarding specification must be one of the following forms:
670 .Bl -item -offset indent -compact
684 .Ar \&[ IPv6_addr \&] : port
688 Multiple forwards may be specified by separating them with whitespace.
691 can be used to remove all restrictions and permit any forwarding requests.
692 By default all port forwarding requests are permitted.
693 .It Cm PermitRootLogin
694 Specifies whether root can log in using
698 .Dq without-password ,
699 .Dq forced-commands-only ,
705 If this option is set to
706 .Dq without-password ,
707 password authentication is disabled for root.
709 If this option is set to
710 .Dq forced-commands-only ,
711 root login with public key authentication will be allowed,
714 option has been specified
715 (which may be useful for taking remote backups even if root login is
716 normally not allowed).
717 All other authentication methods are disabled for root.
719 If this option is set to
721 root is not allowed to log in.
725 device forwarding is allowed.
741 .It Cm PermitUserEnvironment
743 .Pa ~/.ssh/environment
747 .Pa ~/.ssh/authorized_keys
752 Enabling environment processing may enable users to bypass access
753 restrictions in some configurations using mechanisms such as
756 Specifies the file that contains the process ID of the
759 .Pa /var/run/sshd.pid .
761 Specifies the port number that
765 Multiple options of this type are permitted.
771 should print the date and time of the last user login when a user logs
780 when a user logs in interactively.
781 (On some systems it is also printed by the shell,
787 Specifies the protocol versions
790 The possible values are
794 Multiple versions must be comma-separated.
797 Note that the order of the protocol list does not indicate preference,
798 because the client selects among multiple protocol versions offered
804 .It Cm PubkeyAuthentication
805 Specifies whether public key authentication is allowed.
808 Note that this option applies to protocol version 2 only.
809 .It Cm RhostsRSAAuthentication
810 Specifies whether rhosts or /etc/hosts.equiv authentication together
811 with successful RSA host authentication is allowed.
814 This option applies to protocol version 1 only.
815 .It Cm RSAAuthentication
816 Specifies whether pure RSA authentication is allowed.
819 This option applies to protocol version 1 only.
821 Defines the number of bits in the ephemeral protocol version 1 server key.
822 The minimum value is 512, and the default is 1024.
826 should check file modes and ownership of the
827 user's files and home directory before accepting login.
828 This is normally desirable because novices sometimes accidentally leave their
829 directory or files world-writable.
833 Configures an external subsystem (e.g. file transfer daemon).
834 Arguments should be a subsystem name and a command (with optional arguments)
835 to execute upon subsystem request.
841 file transfer subsystem.
845 implements an in-process
848 This may simplify configurations using
850 to force a different filesystem root on clients.
852 By default no subsystems are defined.
853 Note that this option applies to protocol version 2 only.
854 .It Cm SyslogFacility
855 Gives the facility code that is used when logging messages from
857 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
858 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
861 Specifies whether the system should send TCP keepalive messages to the
863 If they are sent, death of the connection or crash of one
864 of the machines will be properly noticed.
865 However, this means that
866 connections will die if the route is down temporarily, and some people
868 On the other hand, if TCP keepalives are not sent,
869 sessions may hang indefinitely on the server, leaving
871 users and consuming server resources.
875 (to send TCP keepalive messages), and the server will notice
876 if the network goes down or the client host crashes.
877 This avoids infinitely hanging sessions.
879 To disable TCP keepalive messages, the value should be set to
884 should look up the remote host name and check that
885 the resolved host name for the remote IP address maps back to the
886 very same IP address.
892 is used for interactive login sessions.
897 is never used for remote command execution.
898 Note also, that if this is enabled,
900 will be disabled because
902 does not know how to handle
906 .Cm UsePrivilegeSeparation
907 is specified, it will be disabled after authentication.
909 Enables the Pluggable Authentication Module interface.
912 this will enable PAM authentication using
913 .Cm ChallengeResponseAuthentication
915 .Cm PasswordAuthentication
916 in addition to PAM account and session module processing for all
917 authentication types.
919 Because PAM challenge-response authentication usually serves an equivalent
920 role to password authentication, you should disable either
921 .Cm PasswordAuthentication
923 .Cm ChallengeResponseAuthentication.
927 is enabled, you will not be able to run
932 .It Cm UsePrivilegeSeparation
935 separates privileges by creating an unprivileged child process
936 to deal with incoming network traffic.
937 After successful authentication, another process will be created that has
938 the privilege of the authenticated user.
939 The goal of privilege separation is to prevent privilege
940 escalation by containing any corruption within the unprivileged processes.
943 .It Cm X11DisplayOffset
944 Specifies the first display number available for
947 This prevents sshd from interfering with real X11 servers.
950 Specifies whether X11 forwarding is permitted.
958 When X11 forwarding is enabled, there may be additional exposure to
959 the server and to client displays if the
961 proxy display is configured to listen on the wildcard address (see
963 below), though this is not the default.
964 Additionally, the authentication spoofing and authentication data
965 verification and substitution occur on the client side.
966 The security risk of using X11 forwarding is that the client's X11
967 display server may be exposed to attack when the SSH client requests
968 forwarding (see the warnings for
972 A system administrator may have a stance in which they want to
973 protect clients that may expose themselves to attack by unwittingly
974 requesting X11 forwarding, which can warrant a
978 Note that disabling X11 forwarding does not prevent users from
979 forwarding X11 traffic, as users can always install their own forwarders.
980 X11 forwarding is automatically disabled if
983 .It Cm X11UseLocalhost
986 should bind the X11 forwarding server to the loopback address or to
987 the wildcard address.
989 sshd binds the forwarding server to the loopback address and sets the
992 environment variable to
994 This prevents remote hosts from connecting to the proxy display.
995 However, some older X11 clients may not function with this
1000 to specify that the forwarding server should be bound to the wildcard
1002 The argument must be
1008 .It Cm XAuthLocation
1009 Specifies the full pathname of the
1013 .Pa /usr/X11R6/bin/xauth .
1017 command-line arguments and configuration file options that specify time
1018 may be expressed using a sequence of the form:
1020 .Ar time Op Ar qualifier ,
1024 is a positive integer value and
1026 is one of the following:
1028 .Bl -tag -width Ds -compact -offset indent
1043 Each member of the sequence is added together to calculate
1044 the total time value.
1046 Time format examples:
1048 .Bl -tag -width Ds -compact -offset indent
1050 600 seconds (10 minutes)
1054 1 hour 30 minutes (90 minutes)
1058 .It Pa /etc/ssh/sshd_config
1059 Contains configuration data for
1061 This file should be writable by root only, but it is recommended
1062 (though not necessary) that it be world-readable.
1067 OpenSSH is a derivative of the original and free
1068 ssh 1.2.12 release by Tatu Ylonen.
1069 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1070 Theo de Raadt and Dug Song
1071 removed many bugs, re-added newer features and
1073 Markus Friedl contributed the support for SSH
1074 protocol versions 1.5 and 2.0.
1075 Niels Provos and Markus Friedl contributed support
1076 for privilege separation.
andersk / openssh.git / blob