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 1.106 2009/04/21 15:13:17 stevesk Exp $
38 .Dd $Mdocdate: April 21 2009 $
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 (e.g. via
180 PAM or though authentication styles supported in
184 .It Cm ChrootDirectory
187 to after authentication.
188 This path, and all its components, must be root-owned directories that are
189 not writable by any other user or group.
192 changes the working directory to the user's home directory.
194 The path may contain the following tokens that are expanded at runtime once
195 the connecting user has been authenticated: %% is replaced by a literal '%',
196 %h is replaced by the home directory of the user being authenticated, and
197 %u is replaced by the username of that user.
201 must contain the necessary files and directories to support the
203 For an interactive session this requires at least a shell, typically
217 For file transfer sessions using
219 no additional configuration of the environment is necessary if the
220 in-process sftp server is used,
221 though sessions which use logging do require
223 inside the chroot directory (see
227 The default is not to
230 Specifies the ciphers allowed for protocol version 2.
231 Multiple ciphers must be comma-separated.
232 The supported ciphers are
247 .Bd -literal -offset 3n
248 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
249 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
252 .It Cm ClientAliveCountMax
253 Sets the number of client alive messages (see below) which may be
256 receiving any messages back from the client.
257 If this threshold is reached while client alive messages are being sent,
258 sshd will disconnect the client, terminating the session.
259 It is important to note that the use of client alive messages is very
263 The client alive messages are sent through the encrypted channel
264 and therefore will not be spoofable.
265 The TCP keepalive option enabled by
268 The client alive mechanism is valuable when the client or
269 server depend on knowing when a connection has become inactive.
271 The default value is 3.
273 .Cm ClientAliveInterval
274 (see below) is set to 15, and
275 .Cm ClientAliveCountMax
276 is left at the default, unresponsive SSH clients
277 will be disconnected after approximately 45 seconds.
278 This option applies to protocol version 2 only.
279 .It Cm ClientAliveInterval
280 Sets a timeout interval in seconds after which if no data has been received
283 will send a message through the encrypted
284 channel to request a response from the client.
286 is 0, indicating that these messages will not be sent to the client.
287 This option applies to protocol version 2 only.
289 Specifies whether compression is allowed, or delayed until
290 the user has authenticated successfully.
299 This keyword can be followed by a list of group name patterns, separated
301 Login is disallowed for users whose primary group or supplementary
302 group list matches one of the patterns.
303 Only group names are valid; a numerical group ID is not recognized.
304 By default, login is allowed for all groups.
305 The allow/deny directives are processed in the following order:
316 for more information on patterns.
318 This keyword can be followed by a list of user name patterns, separated
320 Login is disallowed for user names that match one of the patterns.
321 Only user names are valid; a numerical user ID is not recognized.
322 By default, login is allowed for all users.
323 If the pattern takes the form USER@HOST then USER and HOST
324 are separately checked, restricting logins to particular
325 users from particular hosts.
326 The allow/deny directives are processed in the following order:
337 for more information on patterns.
339 Forces the execution of the command specified by
341 ignoring any command supplied by the client and
344 The command is invoked by using the user's login shell with the -c option.
345 This applies to shell, command, or subsystem execution.
346 It is most useful inside a
349 The command originally supplied by the client is available in the
350 .Ev SSH_ORIGINAL_COMMAND
351 environment variable.
352 Specifying a command of
354 will force the use of an in-process sftp server that requires no support
356 .Cm ChrootDirectory .
358 Specifies whether remote hosts are allowed to connect to ports
359 forwarded for the client.
362 binds remote port forwardings to the loopback address.
363 This prevents other remote hosts from connecting to forwarded ports.
365 can be used to specify that sshd
366 should allow remote port forwardings to bind to non-loopback addresses, thus
367 allowing other hosts to connect.
370 to force remote port forwardings to be available to the local host only,
372 to force remote port forwardings to bind to the wildcard address, or
374 to allow the client to select the address to which the forwarding is bound.
377 .It Cm GSSAPIAuthentication
378 Specifies whether user authentication based on GSSAPI is allowed.
381 Note that this option applies to protocol version 2 only.
382 .It Cm GSSAPICleanupCredentials
383 Specifies whether to automatically destroy the user's credentials cache
387 Note that this option applies to protocol version 2 only.
388 .It Cm HostbasedAuthentication
389 Specifies whether rhosts or /etc/hosts.equiv authentication together
390 with successful public key client host authentication is allowed
391 (host-based authentication).
392 This option is similar to
393 .Cm RhostsRSAAuthentication
394 and applies to protocol version 2 only.
397 .It Cm HostbasedUsesNameFromPacketOnly
398 Specifies whether or not the server will attempt to perform a reverse
399 name lookup when matching the name in the
405 .Cm HostbasedAuthentication .
410 uses the name supplied by the client rather than
411 attempting to resolve the name from the TCP connection itself.
415 Specifies a file containing a private host key
418 .Pa /etc/ssh/ssh_host_key
419 for protocol version 1, and
420 .Pa /etc/ssh/ssh_host_rsa_key
422 .Pa /etc/ssh/ssh_host_dsa_key
423 for protocol version 2.
426 will refuse to use a file if it is group/world-accessible.
427 It is possible to have multiple host key files.
429 keys are used for version 1 and
433 are used for version 2 of the SSH protocol.
439 files will not be used in
440 .Cm RhostsRSAAuthentication
442 .Cm HostbasedAuthentication .
446 .Pa /etc/shosts.equiv
450 .It Cm IgnoreUserKnownHosts
453 should ignore the user's
454 .Pa ~/.ssh/known_hosts
456 .Cm RhostsRSAAuthentication
458 .Cm HostbasedAuthentication .
461 .It Cm KerberosAuthentication
462 Specifies whether the password provided by the user for
463 .Cm PasswordAuthentication
464 will be validated through the Kerberos KDC.
465 To use this option, the server needs a
466 Kerberos servtab which allows the verification of the KDC's identity.
469 .It Cm KerberosGetAFSToken
470 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
471 an AFS token before accessing the user's home directory.
474 .It Cm KerberosOrLocalPasswd
475 If password authentication through Kerberos fails then
476 the password will be validated via any additional local mechanism
481 .It Cm KerberosTicketCleanup
482 Specifies whether to automatically destroy the user's ticket cache
486 .It Cm KeyRegenerationInterval
487 In protocol version 1, the ephemeral server key is automatically regenerated
488 after this many seconds (if it has been used).
489 The purpose of regeneration is to prevent
490 decrypting captured sessions by later breaking into the machine and
492 The key is never stored anywhere.
493 If the value is 0, the key is never regenerated.
494 The default is 3600 (seconds).
496 Specifies the local addresses
499 The following forms may be used:
501 .Bl -item -offset indent -compact
505 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
510 .Ar host No | Ar IPv4_addr No : Ar port
516 .Ar host No | Ar IPv6_addr Oc : Ar port
523 sshd will listen on the address and all prior
526 The default is to listen on all local addresses.
529 options are permitted.
532 options must precede this option for non-port qualified addresses.
533 .It Cm LoginGraceTime
534 The server disconnects after this time if the user has not
535 successfully logged in.
536 If the value is 0, there is no time limit.
537 The default is 120 seconds.
539 Gives the verbosity level that is used when logging messages from
541 The possible values are:
542 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
544 DEBUG and DEBUG1 are equivalent.
545 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
546 Logging with a DEBUG level violates the privacy of users and is not recommended.
548 Specifies the available MAC (message authentication code) algorithms.
549 The MAC algorithm is used in protocol version 2
550 for data integrity protection.
551 Multiple algorithms must be comma-separated.
553 .Bd -literal -offset indent
554 hmac-md5,hmac-sha1,umac-64@openssh.com,
555 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
558 Introduces a conditional block.
559 If all of the criteria on the
561 line are satisfied, the keywords on the following lines override those
562 set in the global section of the config file, until either another
564 line or the end of the file.
568 are one or more criteria-pattern pairs.
569 The available criteria are
575 The match patterns may consist of single entries or comma-separated
576 lists and may use the wildcard and negation operators described in the
583 criteria may additionally contain addresses to match in CIDR
584 address/masklen format, e.g.\&
588 Note that the mask length provided must be consistent with the address -
589 it is an error to specify a mask length that is too long for the address
590 or one with bits set in this host portion of the address.
597 Only a subset of keywords may be used on the lines following a
600 Available keywords are
601 .Cm AllowAgentForwarding ,
602 .Cm AllowTcpForwarding ,
604 .Cm ChrootDirectory ,
607 .Cm GSSAPIAuthentication ,
608 .Cm HostbasedAuthentication ,
609 .Cm KbdInteractiveAuthentication ,
610 .Cm KerberosAuthentication ,
613 .Cm PasswordAuthentication ,
614 .Cm PermitEmptyPasswords ,
616 .Cm PermitRootLogin ,
617 .Cm RhostsRSAAuthentication ,
618 .Cm RSAAuthentication ,
619 .Cm X11DisplayOffset ,
622 .Cm X11UseLocalHost .
624 Specifies the maximum number of authentication attempts permitted per
626 Once the number of failures reaches half this value,
627 additional failures are logged.
630 Specifies the maximum number of open sessions permitted per network connection.
633 Specifies the maximum number of concurrent unauthenticated connections to the
635 Additional connections will be dropped until authentication succeeds or the
637 expires for a connection.
640 Alternatively, random early drop can be enabled by specifying
641 the three colon separated values
645 will refuse connection attempts with a probability of
648 if there are currently
651 unauthenticated connections.
652 The probability increases linearly and all connection attempts
653 are refused if the number of unauthenticated connections reaches
656 .It Cm PasswordAuthentication
657 Specifies whether password authentication is allowed.
660 .It Cm PermitEmptyPasswords
661 When password authentication is allowed, it specifies whether the
662 server allows login to accounts with empty password strings.
666 Specifies the destinations to which TCP port forwarding is permitted.
667 The forwarding specification must be one of the following forms:
669 .Bl -item -offset indent -compact
683 .Ar \&[ IPv6_addr \&] : port
687 Multiple forwards may be specified by separating them with whitespace.
690 can be used to remove all restrictions and permit any forwarding requests.
691 By default all port forwarding requests are permitted.
692 .It Cm PermitRootLogin
693 Specifies whether root can log in using
697 .Dq without-password ,
698 .Dq forced-commands-only ,
704 If this option is set to
705 .Dq without-password ,
706 password authentication is disabled for root.
708 If this option is set to
709 .Dq forced-commands-only ,
710 root login with public key authentication will be allowed,
713 option has been specified
714 (which may be useful for taking remote backups even if root login is
715 normally not allowed).
716 All other authentication methods are disabled for root.
718 If this option is set to
720 root is not allowed to log in.
724 device forwarding is allowed.
740 .It Cm PermitUserEnvironment
742 .Pa ~/.ssh/environment
746 .Pa ~/.ssh/authorized_keys
751 Enabling environment processing may enable users to bypass access
752 restrictions in some configurations using mechanisms such as
755 Specifies the file that contains the process ID of the
758 .Pa /var/run/sshd.pid .
760 Specifies the port number that
764 Multiple options of this type are permitted.
770 should print the date and time of the last user login when a user logs
779 when a user logs in interactively.
780 (On some systems it is also printed by the shell,
786 Specifies the protocol versions
789 The possible values are
793 Multiple versions must be comma-separated.
796 Note that the order of the protocol list does not indicate preference,
797 because the client selects among multiple protocol versions offered
803 .It Cm PubkeyAuthentication
804 Specifies whether public key authentication is allowed.
807 Note that this option applies to protocol version 2 only.
808 .It Cm RhostsRSAAuthentication
809 Specifies whether rhosts or /etc/hosts.equiv authentication together
810 with successful RSA host authentication is allowed.
813 This option applies to protocol version 1 only.
814 .It Cm RSAAuthentication
815 Specifies whether pure RSA authentication is allowed.
818 This option applies to protocol version 1 only.
820 Defines the number of bits in the ephemeral protocol version 1 server key.
821 The minimum value is 512, and the default is 1024.
825 should check file modes and ownership of the
826 user's files and home directory before accepting login.
827 This is normally desirable because novices sometimes accidentally leave their
828 directory or files world-writable.
832 Configures an external subsystem (e.g. file transfer daemon).
833 Arguments should be a subsystem name and a command (with optional arguments)
834 to execute upon subsystem request.
840 file transfer subsystem.
844 implements an in-process
847 This may simplify configurations using
849 to force a different filesystem root on clients.
851 By default no subsystems are defined.
852 Note that this option applies to protocol version 2 only.
853 .It Cm SyslogFacility
854 Gives the facility code that is used when logging messages from
856 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
857 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
860 Specifies whether the system should send TCP keepalive messages to the
862 If they are sent, death of the connection or crash of one
863 of the machines will be properly noticed.
864 However, this means that
865 connections will die if the route is down temporarily, and some people
867 On the other hand, if TCP keepalives are not sent,
868 sessions may hang indefinitely on the server, leaving
870 users and consuming server resources.
874 (to send TCP keepalive messages), and the server will notice
875 if the network goes down or the client host crashes.
876 This avoids infinitely hanging sessions.
878 To disable TCP keepalive messages, the value should be set to
883 should look up the remote host name and check that
884 the resolved host name for the remote IP address maps back to the
885 very same IP address.
891 is used for interactive login sessions.
896 is never used for remote command execution.
897 Note also, that if this is enabled,
899 will be disabled because
901 does not know how to handle
905 .Cm UsePrivilegeSeparation
906 is specified, it will be disabled after authentication.
908 Enables the Pluggable Authentication Module interface.
911 this will enable PAM authentication using
912 .Cm ChallengeResponseAuthentication
914 .Cm PasswordAuthentication
915 in addition to PAM account and session module processing for all
916 authentication types.
918 Because PAM challenge-response authentication usually serves an equivalent
919 role to password authentication, you should disable either
920 .Cm PasswordAuthentication
922 .Cm ChallengeResponseAuthentication.
926 is enabled, you will not be able to run
931 .It Cm UsePrivilegeSeparation
934 separates privileges by creating an unprivileged child process
935 to deal with incoming network traffic.
936 After successful authentication, another process will be created that has
937 the privilege of the authenticated user.
938 The goal of privilege separation is to prevent privilege
939 escalation by containing any corruption within the unprivileged processes.
942 .It Cm X11DisplayOffset
943 Specifies the first display number available for
946 This prevents sshd from interfering with real X11 servers.
949 Specifies whether X11 forwarding is permitted.
957 When X11 forwarding is enabled, there may be additional exposure to
958 the server and to client displays if the
960 proxy display is configured to listen on the wildcard address (see
962 below), though this is not the default.
963 Additionally, the authentication spoofing and authentication data
964 verification and substitution occur on the client side.
965 The security risk of using X11 forwarding is that the client's X11
966 display server may be exposed to attack when the SSH client requests
967 forwarding (see the warnings for
971 A system administrator may have a stance in which they want to
972 protect clients that may expose themselves to attack by unwittingly
973 requesting X11 forwarding, which can warrant a
977 Note that disabling X11 forwarding does not prevent users from
978 forwarding X11 traffic, as users can always install their own forwarders.
979 X11 forwarding is automatically disabled if
982 .It Cm X11UseLocalhost
985 should bind the X11 forwarding server to the loopback address or to
986 the wildcard address.
988 sshd binds the forwarding server to the loopback address and sets the
991 environment variable to
993 This prevents remote hosts from connecting to the proxy display.
994 However, some older X11 clients may not function with this
999 to specify that the forwarding server should be bound to the wildcard
1001 The argument must be
1007 .It Cm XAuthLocation
1008 Specifies the full pathname of the
1012 .Pa /usr/X11R6/bin/xauth .
1016 command-line arguments and configuration file options that specify time
1017 may be expressed using a sequence of the form:
1019 .Ar time Op Ar qualifier ,
1023 is a positive integer value and
1025 is one of the following:
1027 .Bl -tag -width Ds -compact -offset indent
1042 Each member of the sequence is added together to calculate
1043 the total time value.
1045 Time format examples:
1047 .Bl -tag -width Ds -compact -offset indent
1049 600 seconds (10 minutes)
1053 1 hour 30 minutes (90 minutes)
1057 .It Pa /etc/ssh/sshd_config
1058 Contains configuration data for
1060 This file should be writable by root only, but it is recommended
1061 (though not necessary) that it be world-readable.
1066 OpenSSH is a derivative of the original and free
1067 ssh 1.2.12 release by Tatu Ylonen.
1068 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1069 Theo de Raadt and Dug Song
1070 removed many bugs, re-added newer features and
1072 Markus Friedl contributed the support for SSH
1073 protocol versions 1.5 and 2.0.
1074 Niels Provos and Markus Friedl contributed support
1075 for privilege separation.
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