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 1.105 2009/04/18 18:39:10 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
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 no additional configuration of the environment is necessary if the
218 in-process sftp server is used,
219 though sessions which use logging do require
221 inside the chroot directory (see
225 The default is not to
228 Specifies the ciphers allowed for protocol version 2.
229 Multiple ciphers must be comma-separated.
230 The supported ciphers are
245 .Bd -literal -offset 3n
246 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
247 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
250 .It Cm ClientAliveCountMax
251 Sets the number of client alive messages (see below) which may be
254 receiving any messages back from the client.
255 If this threshold is reached while client alive messages are being sent,
256 sshd will disconnect the client, terminating the session.
257 It is important to note that the use of client alive messages is very
261 The client alive messages are sent through the encrypted channel
262 and therefore will not be spoofable.
263 The TCP keepalive option enabled by
266 The client alive mechanism is valuable when the client or
267 server depend on knowing when a connection has become inactive.
269 The default value is 3.
271 .Cm ClientAliveInterval
272 (see below) is set to 15, and
273 .Cm ClientAliveCountMax
274 is left at the default, unresponsive SSH clients
275 will be disconnected after approximately 45 seconds.
276 This option applies to protocol version 2 only.
277 .It Cm ClientAliveInterval
278 Sets a timeout interval in seconds after which if no data has been received
281 will send a message through the encrypted
282 channel to request a response from the client.
284 is 0, indicating that these messages will not be sent to the client.
285 This option applies to protocol version 2 only.
287 Specifies whether compression is allowed, or delayed until
288 the user has authenticated successfully.
297 This keyword can be followed by a list of group name patterns, separated
299 Login is disallowed for users whose primary group or supplementary
300 group list matches one of the patterns.
301 Only group names are valid; a numerical group ID is not recognized.
302 By default, login is allowed for all groups.
303 The allow/deny directives are processed in the following order:
314 for more information on patterns.
316 This keyword can be followed by a list of user name patterns, separated
318 Login is disallowed for user names that match one of the patterns.
319 Only user names are valid; a numerical user ID is not recognized.
320 By default, login is allowed for all users.
321 If the pattern takes the form USER@HOST then USER and HOST
322 are separately checked, restricting logins to particular
323 users from particular hosts.
324 The allow/deny directives are processed in the following order:
335 for more information on patterns.
337 Forces the execution of the command specified by
339 ignoring any command supplied by the client and
342 The command is invoked by using the user's login shell with the -c option.
343 This applies to shell, command, or subsystem execution.
344 It is most useful inside a
347 The command originally supplied by the client is available in the
348 .Ev SSH_ORIGINAL_COMMAND
349 environment variable.
350 Specifying a command of
352 will force the use of an in-process sftp server that requires no support
354 .Cm ChrootDirectory .
356 Specifies whether remote hosts are allowed to connect to ports
357 forwarded for the client.
360 binds remote port forwardings to the loopback address.
361 This prevents other remote hosts from connecting to forwarded ports.
363 can be used to specify that sshd
364 should allow remote port forwardings to bind to non-loopback addresses, thus
365 allowing other hosts to connect.
368 to force remote port forwardings to be available to the local host only,
370 to force remote port forwardings to bind to the wildcard address, or
372 to allow the client to select the address to which the forwarding is bound.
375 .It Cm GSSAPIAuthentication
376 Specifies whether user authentication based on GSSAPI is allowed.
379 Note that this option applies to protocol version 2 only.
380 .It Cm GSSAPICleanupCredentials
381 Specifies whether to automatically destroy the user's credentials cache
385 Note that this option applies to protocol version 2 only.
386 .It Cm HostbasedAuthentication
387 Specifies whether rhosts or /etc/hosts.equiv authentication together
388 with successful public key client host authentication is allowed
389 (host-based authentication).
390 This option is similar to
391 .Cm RhostsRSAAuthentication
392 and applies to protocol version 2 only.
395 .It Cm HostbasedUsesNameFromPacketOnly
396 Specifies whether or not the server will attempt to perform a reverse
397 name lookup when matching the name in the
403 .Cm HostbasedAuthentication .
408 uses the name supplied by the client rather than
409 attempting to resolve the name from the TCP connection itself.
413 Specifies a file containing a private host key
416 .Pa /etc/ssh/ssh_host_key
417 for protocol version 1, and
418 .Pa /etc/ssh/ssh_host_rsa_key
420 .Pa /etc/ssh/ssh_host_dsa_key
421 for protocol version 2.
424 will refuse to use a file if it is group/world-accessible.
425 It is possible to have multiple host key files.
427 keys are used for version 1 and
431 are used for version 2 of the SSH protocol.
437 files will not be used in
438 .Cm RhostsRSAAuthentication
440 .Cm HostbasedAuthentication .
444 .Pa /etc/shosts.equiv
448 .It Cm IgnoreUserKnownHosts
451 should ignore the user's
452 .Pa ~/.ssh/known_hosts
454 .Cm RhostsRSAAuthentication
456 .Cm HostbasedAuthentication .
459 .It Cm KerberosAuthentication
460 Specifies whether the password provided by the user for
461 .Cm PasswordAuthentication
462 will be validated through the Kerberos KDC.
463 To use this option, the server needs a
464 Kerberos servtab which allows the verification of the KDC's identity.
467 .It Cm KerberosGetAFSToken
468 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
469 an AFS token before accessing the user's home directory.
472 .It Cm KerberosOrLocalPasswd
473 If password authentication through Kerberos fails then
474 the password will be validated via any additional local mechanism
479 .It Cm KerberosTicketCleanup
480 Specifies whether to automatically destroy the user's ticket cache
484 .It Cm KeyRegenerationInterval
485 In protocol version 1, the ephemeral server key is automatically regenerated
486 after this many seconds (if it has been used).
487 The purpose of regeneration is to prevent
488 decrypting captured sessions by later breaking into the machine and
490 The key is never stored anywhere.
491 If the value is 0, the key is never regenerated.
492 The default is 3600 (seconds).
494 Specifies the local addresses
497 The following forms may be used:
499 .Bl -item -offset indent -compact
503 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
508 .Ar host No | Ar IPv4_addr No : Ar port
514 .Ar host No | Ar IPv6_addr Oc : Ar port
521 sshd will listen on the address and all prior
524 The default is to listen on all local addresses.
527 options are permitted.
530 options must precede this option for non-port qualified addresses.
531 .It Cm LoginGraceTime
532 The server disconnects after this time if the user has not
533 successfully logged in.
534 If the value is 0, there is no time limit.
535 The default is 120 seconds.
537 Gives the verbosity level that is used when logging messages from
539 The possible values are:
540 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
542 DEBUG and DEBUG1 are equivalent.
543 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
544 Logging with a DEBUG level violates the privacy of users and is not recommended.
546 Specifies the available MAC (message authentication code) algorithms.
547 The MAC algorithm is used in protocol version 2
548 for data integrity protection.
549 Multiple algorithms must be comma-separated.
551 .Bd -literal -offset indent
552 hmac-md5,hmac-sha1,umac-64@openssh.com,
553 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
556 Introduces a conditional block.
557 If all of the criteria on the
559 line are satisfied, the keywords on the following lines override those
560 set in the global section of the config file, until either another
562 line or the end of the file.
566 are one or more criteria-pattern pairs.
567 The available criteria are
573 The match patterns may consist of single entries or comma-separated
574 lists and may use the wildcard and negation operators described in the
581 criteria may additionally contain addresses to match in CIDR
582 address/masklen format, e.g.\&
586 Note that the mask length provided must be consistent with the address -
587 it is an error to specify a mask length that is too long for the address
588 or one with bits set in this host portion of the address.
595 Only a subset of keywords may be used on the lines following a
598 Available keywords are
599 .Cm AllowAgentForwarding ,
600 .Cm AllowTcpForwarding ,
602 .Cm ChrootDirectory ,
605 .Cm GSSAPIAuthentication ,
606 .Cm HostbasedAuthentication ,
607 .Cm KbdInteractiveAuthentication ,
608 .Cm KerberosAuthentication ,
611 .Cm PasswordAuthentication ,
612 .Cm PermitEmptyPasswords ,
614 .Cm PermitRootLogin ,
615 .Cm RhostsRSAAuthentication ,
616 .Cm RSAAuthentication ,
617 .Cm X11DisplayOffset ,
620 .Cm X11UseLocalHost .
622 Specifies the maximum number of authentication attempts permitted per
624 Once the number of failures reaches half this value,
625 additional failures are logged.
628 Specifies the maximum number of open sessions permitted per network connection.
631 Specifies the maximum number of concurrent unauthenticated connections to the
633 Additional connections will be dropped until authentication succeeds or the
635 expires for a connection.
638 Alternatively, random early drop can be enabled by specifying
639 the three colon separated values
643 will refuse connection attempts with a probability of
646 if there are currently
649 unauthenticated connections.
650 The probability increases linearly and all connection attempts
651 are refused if the number of unauthenticated connections reaches
654 .It Cm PasswordAuthentication
655 Specifies whether password authentication is allowed.
658 .It Cm PermitEmptyPasswords
659 When password authentication is allowed, it specifies whether the
660 server allows login to accounts with empty password strings.
664 Specifies the destinations to which TCP port forwarding is permitted.
665 The forwarding specification must be one of the following forms:
667 .Bl -item -offset indent -compact
681 .Ar \&[ IPv6_addr \&] : port
685 Multiple forwards may be specified by separating them with whitespace.
688 can be used to remove all restrictions and permit any forwarding requests.
689 By default all port forwarding requests are permitted.
690 .It Cm PermitRootLogin
691 Specifies whether root can log in using
695 .Dq without-password ,
696 .Dq forced-commands-only ,
702 If this option is set to
703 .Dq without-password ,
704 password authentication is disabled for root.
706 If this option is set to
707 .Dq forced-commands-only ,
708 root login with public key authentication will be allowed,
711 option has been specified
712 (which may be useful for taking remote backups even if root login is
713 normally not allowed).
714 All other authentication methods are disabled for root.
716 If this option is set to
718 root is not allowed to log in.
722 device forwarding is allowed.
738 .It Cm PermitUserEnvironment
740 .Pa ~/.ssh/environment
744 .Pa ~/.ssh/authorized_keys
749 Enabling environment processing may enable users to bypass access
750 restrictions in some configurations using mechanisms such as
753 Specifies the file that contains the process ID of the
756 .Pa /var/run/sshd.pid .
758 Specifies the port number that
762 Multiple options of this type are permitted.
768 should print the date and time of the last user login when a user logs
777 when a user logs in interactively.
778 (On some systems it is also printed by the shell,
784 Specifies the protocol versions
787 The possible values are
791 Multiple versions must be comma-separated.
794 Note that the order of the protocol list does not indicate preference,
795 because the client selects among multiple protocol versions offered
801 .It Cm PubkeyAuthentication
802 Specifies whether public key authentication is allowed.
805 Note that this option applies to protocol version 2 only.
806 .It Cm RhostsRSAAuthentication
807 Specifies whether rhosts or /etc/hosts.equiv authentication together
808 with successful RSA host authentication is allowed.
811 This option applies to protocol version 1 only.
812 .It Cm RSAAuthentication
813 Specifies whether pure RSA authentication is allowed.
816 This option applies to protocol version 1 only.
818 Defines the number of bits in the ephemeral protocol version 1 server key.
819 The minimum value is 512, and the default is 1024.
823 should check file modes and ownership of the
824 user's files and home directory before accepting login.
825 This is normally desirable because novices sometimes accidentally leave their
826 directory or files world-writable.
830 Configures an external subsystem (e.g. file transfer daemon).
831 Arguments should be a subsystem name and a command (with optional arguments)
832 to execute upon subsystem request.
838 file transfer subsystem.
842 implements an in-process
845 This may simplify configurations using
847 to force a different filesystem root on clients.
849 By default no subsystems are defined.
850 Note that this option applies to protocol version 2 only.
851 .It Cm SyslogFacility
852 Gives the facility code that is used when logging messages from
854 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
855 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
858 Specifies whether the system should send TCP keepalive messages to the
860 If they are sent, death of the connection or crash of one
861 of the machines will be properly noticed.
862 However, this means that
863 connections will die if the route is down temporarily, and some people
865 On the other hand, if TCP keepalives are not sent,
866 sessions may hang indefinitely on the server, leaving
868 users and consuming server resources.
872 (to send TCP keepalive messages), and the server will notice
873 if the network goes down or the client host crashes.
874 This avoids infinitely hanging sessions.
876 To disable TCP keepalive messages, the value should be set to
881 should look up the remote host name and check that
882 the resolved host name for the remote IP address maps back to the
883 very same IP address.
889 is used for interactive login sessions.
894 is never used for remote command execution.
895 Note also, that if this is enabled,
897 will be disabled because
899 does not know how to handle
903 .Cm UsePrivilegeSeparation
904 is specified, it will be disabled after authentication.
906 Enables the Pluggable Authentication Module interface.
909 this will enable PAM authentication using
910 .Cm ChallengeResponseAuthentication
912 .Cm PasswordAuthentication
913 in addition to PAM account and session module processing for all
914 authentication types.
916 Because PAM challenge-response authentication usually serves an equivalent
917 role to password authentication, you should disable either
918 .Cm PasswordAuthentication
920 .Cm ChallengeResponseAuthentication.
924 is enabled, you will not be able to run
929 .It Cm UsePrivilegeSeparation
932 separates privileges by creating an unprivileged child process
933 to deal with incoming network traffic.
934 After successful authentication, another process will be created that has
935 the privilege of the authenticated user.
936 The goal of privilege separation is to prevent privilege
937 escalation by containing any corruption within the unprivileged processes.
940 .It Cm X11DisplayOffset
941 Specifies the first display number available for
944 This prevents sshd from interfering with real X11 servers.
947 Specifies whether X11 forwarding is permitted.
955 When X11 forwarding is enabled, there may be additional exposure to
956 the server and to client displays if the
958 proxy display is configured to listen on the wildcard address (see
960 below), though this is not the default.
961 Additionally, the authentication spoofing and authentication data
962 verification and substitution occur on the client side.
963 The security risk of using X11 forwarding is that the client's X11
964 display server may be exposed to attack when the SSH client requests
965 forwarding (see the warnings for
969 A system administrator may have a stance in which they want to
970 protect clients that may expose themselves to attack by unwittingly
971 requesting X11 forwarding, which can warrant a
975 Note that disabling X11 forwarding does not prevent users from
976 forwarding X11 traffic, as users can always install their own forwarders.
977 X11 forwarding is automatically disabled if
980 .It Cm X11UseLocalhost
983 should bind the X11 forwarding server to the loopback address or to
984 the wildcard address.
986 sshd binds the forwarding server to the loopback address and sets the
989 environment variable to
991 This prevents remote hosts from connecting to the proxy display.
992 However, some older X11 clients may not function with this
997 to specify that the forwarding server should be bound to the wildcard
1005 .It Cm XAuthLocation
1006 Specifies the full pathname of the
1010 .Pa /usr/X11R6/bin/xauth .
1014 command-line arguments and configuration file options that specify time
1015 may be expressed using a sequence of the form:
1017 .Ar time Op Ar qualifier ,
1021 is a positive integer value and
1023 is one of the following:
1025 .Bl -tag -width Ds -compact -offset indent
1040 Each member of the sequence is added together to calculate
1041 the total time value.
1043 Time format examples:
1045 .Bl -tag -width Ds -compact -offset indent
1047 600 seconds (10 minutes)
1051 1 hour 30 minutes (90 minutes)
1055 .It Pa /etc/ssh/sshd_config
1056 Contains configuration data for
1058 This file should be writable by root only, but it is recommended
1059 (though not necessary) that it be world-readable.
1064 OpenSSH is a derivative of the original and free
1065 ssh 1.2.12 release by Tatu Ylonen.
1066 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1067 Theo de Raadt and Dug Song
1068 removed many bugs, re-added newer features and
1070 Markus Friedl contributed the support for SSH
1071 protocol versions 1.5 and 2.0.
1072 Niels Provos and Markus Friedl contributed support
1073 for privilege separation.
andersk / openssh.git / blob