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 1.102 2009/02/22 23:59:25 djm Exp $
38 .Dd $Mdocdate: February 22 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.
182 this controls the use of PAM (see
185 Note that this affects the effectiveness of the
186 .Cm PasswordAuthentication
192 .It Cm ChrootDirectory
195 to after authentication.
196 This path, and all its components, must be root-owned directories that are
197 not writable by any other user or group.
199 The path may contain the following tokens that are expanded at runtime once
200 the connecting user has been authenticated: %% is replaced by a literal '%',
201 %h is replaced by the home directory of the user being authenticated, and
202 %u is replaced by the username of that user.
206 must contain the necessary files and directories to support the
208 For an interactive session this requires at least a shell, typically
222 For file transfer sessions using
224 no additional configuration of the environment is necessary if the
225 in-process sftp server is used (see
229 The default is not to
232 Specifies the ciphers allowed for protocol version 2.
233 Multiple ciphers must be comma-separated.
234 The supported ciphers are
249 .Bd -literal -offset 3n
250 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
251 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
254 .It Cm ClientAliveCountMax
255 Sets the number of client alive messages (see below) which may be
258 receiving any messages back from the client.
259 If this threshold is reached while client alive messages are being sent,
260 sshd will disconnect the client, terminating the session.
261 It is important to note that the use of client alive messages is very
265 The client alive messages are sent through the encrypted channel
266 and therefore will not be spoofable.
267 The TCP keepalive option enabled by
270 The client alive mechanism is valuable when the client or
271 server depend on knowing when a connection has become inactive.
273 The default value is 3.
275 .Cm ClientAliveInterval
276 (see below) is set to 15, and
277 .Cm ClientAliveCountMax
278 is left at the default, unresponsive SSH clients
279 will be disconnected after approximately 45 seconds.
280 This option applies to protocol version 2 only.
281 .It Cm ClientAliveInterval
282 Sets a timeout interval in seconds after which if no data has been received
285 will send a message through the encrypted
286 channel to request a response from the client.
288 is 0, indicating that these messages will not be sent to the client.
289 This option applies to protocol version 2 only.
291 Specifies whether compression is allowed, or delayed until
292 the user has authenticated successfully.
301 This keyword can be followed by a list of group name patterns, separated
303 Login is disallowed for users whose primary group or supplementary
304 group list matches one of the patterns.
305 Only group names are valid; a numerical group ID is not recognized.
306 By default, login is allowed for all groups.
307 The allow/deny directives are processed in the following order:
318 for more information on patterns.
320 This keyword can be followed by a list of user name patterns, separated
322 Login is disallowed for user names that match one of the patterns.
323 Only user names are valid; a numerical user ID is not recognized.
324 By default, login is allowed for all users.
325 If the pattern takes the form USER@HOST then USER and HOST
326 are separately checked, restricting logins to particular
327 users from particular hosts.
328 The allow/deny directives are processed in the following order:
339 for more information on patterns.
341 Forces the execution of the command specified by
343 ignoring any command supplied by the client and
346 The command is invoked by using the user's login shell with the -c option.
347 This applies to shell, command, or subsystem execution.
348 It is most useful inside a
351 The command originally supplied by the client is available in the
352 .Ev SSH_ORIGINAL_COMMAND
353 environment variable.
354 Specifying a command of
356 will force the use of an in-process sftp server that requires no support
358 .Cm ChrootDirectory .
360 Specifies whether remote hosts are allowed to connect to ports
361 forwarded for the client.
364 binds remote port forwardings to the loopback address.
365 This prevents other remote hosts from connecting to forwarded ports.
367 can be used to specify that sshd
368 should allow remote port forwardings to bind to non-loopback addresses, thus
369 allowing other hosts to connect.
372 to force remote port forwardings to be available to the local host only,
374 to force remote port forwardings to bind to the wildcard address, or
376 to allow the client to select the address to which the forwarding is bound.
379 .It Cm GSSAPIAuthentication
380 Specifies whether user authentication based on GSSAPI is allowed.
383 Note that this option applies to protocol version 2 only.
384 .It Cm GSSAPICleanupCredentials
385 Specifies whether to automatically destroy the user's credentials cache
389 Note that this option applies to protocol version 2 only.
390 .It Cm HostbasedAuthentication
391 Specifies whether rhosts or /etc/hosts.equiv authentication together
392 with successful public key client host authentication is allowed
393 (host-based authentication).
394 This option is similar to
395 .Cm RhostsRSAAuthentication
396 and applies to protocol version 2 only.
399 .It Cm HostbasedUsesNameFromPacketOnly
400 Specifies whether or not the server will attempt to perform a reverse
401 name lookup when matching the name in the
407 .Cm HostbasedAuthentication .
412 uses the name supplied by the client rather than
413 attempting to resolve the name from the TCP connection itself.
417 Specifies a file containing a private host key
420 .Pa /etc/ssh/ssh_host_key
421 for protocol version 1, and
422 .Pa /etc/ssh/ssh_host_rsa_key
424 .Pa /etc/ssh/ssh_host_dsa_key
425 for protocol version 2.
428 will refuse to use a file if it is group/world-accessible.
429 It is possible to have multiple host key files.
431 keys are used for version 1 and
435 are used for version 2 of the SSH protocol.
441 files will not be used in
442 .Cm RhostsRSAAuthentication
444 .Cm HostbasedAuthentication .
448 .Pa /etc/ssh/shosts.equiv
452 .It Cm IgnoreUserKnownHosts
455 should ignore the user's
456 .Pa ~/.ssh/known_hosts
458 .Cm RhostsRSAAuthentication
460 .Cm HostbasedAuthentication .
463 .It Cm KerberosAuthentication
464 Specifies whether the password provided by the user for
465 .Cm PasswordAuthentication
466 will be validated through the Kerberos KDC.
467 To use this option, the server needs a
468 Kerberos servtab which allows the verification of the KDC's identity.
471 .It Cm KerberosGetAFSToken
472 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
473 an AFS token before accessing the user's home directory.
476 .It Cm KerberosOrLocalPasswd
477 If password authentication through Kerberos fails then
478 the password will be validated via any additional local mechanism
483 .It Cm KerberosTicketCleanup
484 Specifies whether to automatically destroy the user's ticket cache
488 .It Cm KeyRegenerationInterval
489 In protocol version 1, the ephemeral server key is automatically regenerated
490 after this many seconds (if it has been used).
491 The purpose of regeneration is to prevent
492 decrypting captured sessions by later breaking into the machine and
494 The key is never stored anywhere.
495 If the value is 0, the key is never regenerated.
496 The default is 3600 (seconds).
498 Specifies the local addresses
501 The following forms may be used:
503 .Bl -item -offset indent -compact
507 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
512 .Ar host No | Ar IPv4_addr No : Ar port
518 .Ar host No | Ar IPv6_addr Oc : Ar port
525 sshd will listen on the address and all prior
528 The default is to listen on all local addresses.
531 options are permitted.
534 options must precede this option for non-port qualified addresses.
535 .It Cm LoginGraceTime
536 The server disconnects after this time if the user has not
537 successfully logged in.
538 If the value is 0, there is no time limit.
539 The default is 120 seconds.
541 Gives the verbosity level that is used when logging messages from
543 The possible values are:
544 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
546 DEBUG and DEBUG1 are equivalent.
547 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
548 Logging with a DEBUG level violates the privacy of users and is not recommended.
550 Specifies the available MAC (message authentication code) algorithms.
551 The MAC algorithm is used in protocol version 2
552 for data integrity protection.
553 Multiple algorithms must be comma-separated.
555 .Bd -literal -offset indent
556 hmac-md5,hmac-sha1,umac-64@openssh.com,
557 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
560 Introduces a conditional block.
561 If all of the criteria on the
563 line are satisfied, the keywords on the following lines override those
564 set in the global section of the config file, until either another
566 line or the end of the file.
570 are one or more criteria-pattern pairs.
571 The available criteria are
577 The match patterns may consist of single entries or comma-separated
578 lists and may use the wildcard and negation operators described in the
585 criteria may additionally contain addresses to match in CIDR
586 address/masklen format, e.g.\&
590 Note that the mask length provided must be consistent with the address -
591 it is an error to specify a mask length that is too long for the address
592 or one with bits set in this host portion of the address.
599 Only a subset of keywords may be used on the lines following a
602 Available keywords are
603 .Cm AllowAgentForwarding ,
604 .Cm AllowTcpForwarding ,
606 .Cm ChrootDirectory ,
609 .Cm GSSAPIAuthentication ,
610 .Cm HostbasedAuthentication ,
611 .Cm KbdInteractiveAuthentication ,
612 .Cm KerberosAuthentication ,
615 .Cm PasswordAuthentication ,
616 .Cm PermitEmptyPasswords ,
618 .Cm PermitRootLogin ,
619 .Cm RhostsRSAAuthentication ,
620 .Cm RSAAuthentication ,
621 .Cm X11DisplayOffset ,
624 .Cm X11UseLocalHost .
626 Specifies the maximum number of authentication attempts permitted per
628 Once the number of failures reaches half this value,
629 additional failures are logged.
632 Specifies the maximum number of open sessions permitted per network connection.
635 Specifies the maximum number of concurrent unauthenticated connections to the
637 Additional connections will be dropped until authentication succeeds or the
639 expires for a connection.
642 Alternatively, random early drop can be enabled by specifying
643 the three colon separated values
647 will refuse connection attempts with a probability of
650 if there are currently
653 unauthenticated connections.
654 The probability increases linearly and all connection attempts
655 are refused if the number of unauthenticated connections reaches
658 .It Cm PasswordAuthentication
659 Specifies whether password authentication is allowed.
663 .Cm ChallengeResponseAuthentication
669 and the PAM authentication policy for
673 password authentication will be allowed through the challenge-response
674 mechanism regardless of the value of
675 .Cm PasswordAuthentication .
676 .It Cm PermitEmptyPasswords
677 When password authentication is allowed, it specifies whether the
678 server allows login to accounts with empty password strings.
682 Specifies the destinations to which TCP port forwarding is permitted.
683 The forwarding specification must be one of the following forms:
685 .Bl -item -offset indent -compact
699 .Ar \&[ IPv6_addr \&] : port
703 Multiple forwards may be specified by separating them with whitespace.
706 can be used to remove all restrictions and permit any forwarding requests.
707 By default all port forwarding requests are permitted.
708 .It Cm PermitRootLogin
709 Specifies whether root can log in using
713 .Dq without-password ,
714 .Dq forced-commands-only ,
720 .Cm ChallengeResponseAuthentication
723 the root user may be allowed in with its password even if
724 .Cm PermitRootLogin is set to
725 .Dq without-password .
727 If this option is set to
728 .Dq without-password ,
729 password authentication is disabled for root.
731 If this option is set to
732 .Dq forced-commands-only ,
733 root login with public key authentication will be allowed,
736 option has been specified
737 (which may be useful for taking remote backups even if root login is
738 normally not allowed).
739 All other authentication methods are disabled for root.
741 If this option is set to
743 root is not allowed to log in.
747 device forwarding is allowed.
763 .It Cm PermitUserEnvironment
765 .Pa ~/.ssh/environment
769 .Pa ~/.ssh/authorized_keys
774 Enabling environment processing may enable users to bypass access
775 restrictions in some configurations using mechanisms such as
778 Specifies the file that contains the process ID of the
781 .Pa /var/run/sshd.pid .
783 Specifies the port number that
787 Multiple options of this type are permitted.
793 should print the date and time of the last user login when a user logs
802 when a user logs in interactively.
803 (On some systems it is also printed by the shell,
809 Specifies the protocol versions
812 The possible values are
816 Multiple versions must be comma-separated.
819 Note that the order of the protocol list does not indicate preference,
820 because the client selects among multiple protocol versions offered
826 .It Cm PubkeyAuthentication
827 Specifies whether public key authentication is allowed.
830 Note that this option applies to protocol version 2 only.
831 .It Cm RhostsRSAAuthentication
832 Specifies whether rhosts or
834 authentication together
835 with successful RSA host authentication is allowed.
838 This option applies to protocol version 1 only.
839 .It Cm RSAAuthentication
840 Specifies whether pure RSA authentication is allowed.
843 This option applies to protocol version 1 only.
845 Defines the number of bits in the ephemeral protocol version 1 server key.
846 The minimum value is 512, and the default is 1024.
850 should check file modes and ownership of the
851 user's files and home directory before accepting login.
852 This is normally desirable because novices sometimes accidentally leave their
853 directory or files world-writable.
857 Configures an external subsystem (e.g. file transfer daemon).
858 Arguments should be a subsystem name and a command (with optional arguments)
859 to execute upon subsystem request.
865 file transfer subsystem.
869 implements an in-process
872 This may simplify configurations using
874 to force a different filesystem root on clients.
876 By default no subsystems are defined.
877 Note that this option applies to protocol version 2 only.
878 .It Cm SyslogFacility
879 Gives the facility code that is used when logging messages from
881 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
882 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
885 Specifies whether the system should send TCP keepalive messages to the
887 If they are sent, death of the connection or crash of one
888 of the machines will be properly noticed.
889 However, this means that
890 connections will die if the route is down temporarily, and some people
892 On the other hand, if TCP keepalives are not sent,
893 sessions may hang indefinitely on the server, leaving
895 users and consuming server resources.
899 (to send TCP keepalive messages), and the server will notice
900 if the network goes down or the client host crashes.
901 This avoids infinitely hanging sessions.
903 To disable TCP keepalive messages, the value should be set to
908 should look up the remote host name and check that
909 the resolved host name for the remote IP address maps back to the
910 very same IP address.
916 is used for interactive login sessions.
921 is never used for remote command execution.
922 Note also, that if this is enabled,
924 will be disabled because
926 does not know how to handle
930 .Cm UsePrivilegeSeparation
931 is specified, it will be disabled after authentication.
933 Enables the Pluggable Authentication Module interface.
936 this will enable PAM authentication using
937 .Cm ChallengeResponseAuthentication
939 .Cm PasswordAuthentication
940 in addition to PAM account and session module processing for all
941 authentication types.
943 Because PAM challenge-response authentication usually serves an equivalent
944 role to password authentication, you should disable either
945 .Cm PasswordAuthentication
947 .Cm ChallengeResponseAuthentication.
951 is enabled, you will not be able to run
956 .It Cm UsePrivilegeSeparation
959 separates privileges by creating an unprivileged child process
960 to deal with incoming network traffic.
961 After successful authentication, another process will be created that has
962 the privilege of the authenticated user.
963 The goal of privilege separation is to prevent privilege
964 escalation by containing any corruption within the unprivileged processes.
967 .It Cm VersionAddendum
968 Specifies a string to append to the regular version string to identify
969 OS- or site-specific modifications.
971 .Dq DragonFly-20090620 .
972 .It Cm X11DisplayOffset
973 Specifies the first display number available for
976 This prevents sshd from interfering with real X11 servers.
979 Specifies whether X11 forwarding is permitted.
987 When X11 forwarding is enabled, there may be additional exposure to
988 the server and to client displays if the
990 proxy display is configured to listen on the wildcard address (see
992 below), though this is not the default.
993 Additionally, the authentication spoofing and authentication data
994 verification and substitution occur on the client side.
995 The security risk of using X11 forwarding is that the client's X11
996 display server may be exposed to attack when the SSH client requests
997 forwarding (see the warnings for
1000 .Xr ssh_config 5 ) .
1001 A system administrator may have a stance in which they want to
1002 protect clients that may expose themselves to attack by unwittingly
1003 requesting X11 forwarding, which can warrant a
1007 Note that disabling X11 forwarding does not prevent users from
1008 forwarding X11 traffic, as users can always install their own forwarders.
1009 X11 forwarding is automatically disabled if
1012 .It Cm X11UseLocalhost
1015 should bind the X11 forwarding server to the loopback address or to
1016 the wildcard address.
1018 sshd binds the forwarding server to the loopback address and sets the
1019 hostname part of the
1021 environment variable to
1023 This prevents remote hosts from connecting to the proxy display.
1024 However, some older X11 clients may not function with this
1029 to specify that the forwarding server should be bound to the wildcard
1031 The argument must be
1037 .It Cm XAuthLocation
1038 Specifies the full pathname of the
1042 .Pa /usr/X11R6/bin/xauth .
1046 command-line arguments and configuration file options that specify time
1047 may be expressed using a sequence of the form:
1049 .Ar time Op Ar qualifier ,
1053 is a positive integer value and
1055 is one of the following:
1057 .Bl -tag -width Ds -compact -offset indent
1072 Each member of the sequence is added together to calculate
1073 the total time value.
1075 Time format examples:
1077 .Bl -tag -width Ds -compact -offset indent
1079 600 seconds (10 minutes)
1083 1 hour 30 minutes (90 minutes)
1087 .It Pa /etc/ssh/sshd_config
1088 Contains configuration data for
1090 This file should be writable by root only, but it is recommended
1091 (though not necessary) that it be world-readable.
1096 OpenSSH is a derivative of the original and free
1097 ssh 1.2.12 release by Tatu Ylonen.
1098 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1099 Theo de Raadt and Dug Song
1100 removed many bugs, re-added newer features and
1102 Markus Friedl contributed the support for SSH
1103 protocol versions 1.5 and 2.0.
1104 Niels Provos and Markus Friedl contributed support
1105 for privilege separation.
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