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 1.120 2010/03/04 23:17:25 djm Exp $
44 .Nd OpenSSH SSH daemon configuration file
46 .Nm /etc/ssh/sshd_config
49 reads configuration data from
50 .Pa /etc/ssh/sshd_config
51 (or the file specified with
54 The file contains keyword-argument pairs, one per line.
57 and empty lines are interpreted as comments.
58 Arguments may optionally be enclosed in double quotes
60 in order to represent arguments containing spaces.
63 keywords and their meanings are as follows (note that
64 keywords are case-insensitive and arguments are case-sensitive):
67 Specifies what environment variables sent by the client will be copied into
74 for how to configure the client.
75 Note that environment passing is only supported for protocol 2.
76 Variables are specified by name, which may contain the wildcard characters
80 Multiple environment variables may be separated by whitespace or spread
84 Be warned that some environment variables could be used to bypass restricted
86 For this reason, care should be taken in the use of this directive.
87 The default is not to accept any environment variables.
89 Specifies which address family should be used by
99 .It Cm AllowAgentForwarding
102 forwarding is permitted.
105 Note that disabling agent forwarding does not improve security
106 unless users are also denied shell access, as they can always install
107 their own forwarders.
109 This keyword can be followed by a list of group name patterns, separated
111 If specified, login is allowed only for users whose primary
112 group or supplementary group list matches one of the patterns.
113 Only group names are valid; a numerical group ID is not recognized.
114 By default, login is allowed for all groups.
115 The allow/deny directives are processed in the following order:
126 for more information on patterns.
127 .It Cm AllowTcpForwarding
128 Specifies whether TCP forwarding is permitted.
131 Note that disabling TCP forwarding does not improve security unless
132 users are also denied shell access, as they can always install their
135 This keyword can be followed by a list of user name patterns, separated
137 If specified, login is allowed only for user names that
138 match one of the patterns.
139 Only user names are valid; a numerical user ID is not recognized.
140 By default, login is allowed for all users.
141 If the pattern takes the form USER@HOST then USER and HOST
142 are separately checked, restricting logins to particular
143 users from particular hosts.
144 The allow/deny directives are processed in the following order:
155 for more information on patterns.
156 .It Cm AuthorizedKeysFile
157 Specifies the file that contains the public keys that can be used
158 for user authentication.
159 .Cm AuthorizedKeysFile
160 may contain tokens of the form %T which are substituted during connection
162 The following tokens are defined: %% is replaced by a literal '%',
163 %h is replaced by the home directory of the user being authenticated, and
164 %u is replaced by the username of that user.
166 .Cm AuthorizedKeysFile
167 is taken to be an absolute path or one relative to the user's home
170 .Dq .ssh/authorized_keys .
172 The contents of the specified file are sent to the remote user before
173 authentication is allowed.
176 then no banner is displayed.
177 This option is only available for protocol version 2.
178 By default, no banner is displayed.
179 .It Cm ChallengeResponseAuthentication
180 Specifies whether challenge-response authentication is allowed (e.g. via
181 PAM or though authentication styles supported in
185 .It Cm ChrootDirectory
186 Specifies the pathname of a directory to
188 to after authentication.
189 All components of the pathname must be root-owned directories that are
190 not writable by any other user or group.
193 changes the working directory to the user's home directory.
195 The pathname may contain the following tokens that are expanded at runtime once
196 the connecting user has been authenticated: %% is replaced by a literal '%',
197 %h is replaced by the home directory of the user being authenticated, and
198 %u is replaced by the username of that user.
202 must contain the necessary files and directories to support the
204 For an interactive session this requires at least a shell, typically
218 For file transfer sessions using
220 no additional configuration of the environment is necessary if the
221 in-process sftp server is used,
222 though sessions which use logging do 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.
415 .It Cm HostCertificate
416 Specifies a file containing a public host certificate.
417 The certificate's public key must match a private host key already specified
420 The default behaviour of
422 is not to load any certificates.
424 Specifies a file containing a private host key
427 .Pa /etc/ssh/ssh_host_key
428 for protocol version 1, and
429 .Pa /etc/ssh/ssh_host_rsa_key
431 .Pa /etc/ssh/ssh_host_dsa_key
432 for protocol version 2.
435 will refuse to use a file if it is group/world-accessible.
436 It is possible to have multiple host key files.
438 keys are used for version 1 and
442 are used for version 2 of the SSH protocol.
448 files will not be used in
449 .Cm RhostsRSAAuthentication
451 .Cm HostbasedAuthentication .
455 .Pa /etc/ssh/shosts.equiv
459 .It Cm IgnoreUserKnownHosts
462 should ignore the user's
463 .Pa ~/.ssh/known_hosts
465 .Cm RhostsRSAAuthentication
467 .Cm HostbasedAuthentication .
470 .It Cm KerberosAuthentication
471 Specifies whether the password provided by the user for
472 .Cm PasswordAuthentication
473 will be validated through the Kerberos KDC.
474 To use this option, the server needs a
475 Kerberos servtab which allows the verification of the KDC's identity.
478 .It Cm KerberosGetAFSToken
479 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
480 an AFS token before accessing the user's home directory.
483 .It Cm KerberosOrLocalPasswd
484 If password authentication through Kerberos fails then
485 the password will be validated via any additional local mechanism
490 .It Cm KerberosTicketCleanup
491 Specifies whether to automatically destroy the user's ticket cache
495 .It Cm KeyRegenerationInterval
496 In protocol version 1, the ephemeral server key is automatically regenerated
497 after this many seconds (if it has been used).
498 The purpose of regeneration is to prevent
499 decrypting captured sessions by later breaking into the machine and
501 The key is never stored anywhere.
502 If the value is 0, the key is never regenerated.
503 The default is 3600 (seconds).
505 Specifies the local addresses
508 The following forms may be used:
510 .Bl -item -offset indent -compact
514 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
519 .Ar host No | Ar IPv4_addr No : Ar port
525 .Ar host No | Ar IPv6_addr Oc : Ar port
532 sshd will listen on the address and all prior
535 The default is to listen on all local addresses.
538 options are permitted.
541 options must precede this option for non-port qualified addresses.
542 .It Cm LoginGraceTime
543 The server disconnects after this time if the user has not
544 successfully logged in.
545 If the value is 0, there is no time limit.
546 The default is 120 seconds.
548 Gives the verbosity level that is used when logging messages from
550 The possible values are:
551 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
553 DEBUG and DEBUG1 are equivalent.
554 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
555 Logging with a DEBUG level violates the privacy of users and is not recommended.
557 Specifies the available MAC (message authentication code) algorithms.
558 The MAC algorithm is used in protocol version 2
559 for data integrity protection.
560 Multiple algorithms must be comma-separated.
562 .Bd -literal -offset indent
563 hmac-md5,hmac-sha1,umac-64@openssh.com,
564 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
567 Introduces a conditional block.
568 If all of the criteria on the
570 line are satisfied, the keywords on the following lines override those
571 set in the global section of the config file, until either another
573 line or the end of the file.
577 are one or more criteria-pattern pairs.
578 The available criteria are
584 The match patterns may consist of single entries or comma-separated
585 lists and may use the wildcard and negation operators described in the
592 criteria may additionally contain addresses to match in CIDR
593 address/masklen format, e.g.\&
597 Note that the mask length provided must be consistent with the address -
598 it is an error to specify a mask length that is too long for the address
599 or one with bits set in this host portion of the address.
606 Only a subset of keywords may be used on the lines following a
609 Available keywords are
610 .Cm AllowAgentForwarding ,
611 .Cm AllowTcpForwarding ,
613 .Cm ChrootDirectory ,
616 .Cm GSSAPIAuthentication ,
617 .Cm HostbasedAuthentication ,
618 .Cm KbdInteractiveAuthentication ,
619 .Cm KerberosAuthentication ,
622 .Cm PasswordAuthentication ,
623 .Cm PermitEmptyPasswords ,
625 .Cm PermitRootLogin ,
626 .Cm PubkeyAuthentication ,
627 .Cm RhostsRSAAuthentication ,
628 .Cm RSAAuthentication ,
629 .Cm X11DisplayOffset ,
632 .Cm X11UseLocalHost .
634 Specifies the maximum number of authentication attempts permitted per
636 Once the number of failures reaches half this value,
637 additional failures are logged.
640 Specifies the maximum number of open sessions permitted per network connection.
643 Specifies the maximum number of concurrent unauthenticated connections to the
645 Additional connections will be dropped until authentication succeeds or the
647 expires for a connection.
650 Alternatively, random early drop can be enabled by specifying
651 the three colon separated values
655 will refuse connection attempts with a probability of
658 if there are currently
661 unauthenticated connections.
662 The probability increases linearly and all connection attempts
663 are refused if the number of unauthenticated connections reaches
666 .It Cm PasswordAuthentication
667 Specifies whether password authentication is allowed.
672 .It Cm PermitEmptyPasswords
673 When password authentication is allowed, it specifies whether the
674 server allows login to accounts with empty password strings.
678 Specifies the destinations to which TCP port forwarding is permitted.
679 The forwarding specification must be one of the following forms:
681 .Bl -item -offset indent -compact
695 .Ar \&[ IPv6_addr \&] : port
699 Multiple forwards may be specified by separating them with whitespace.
702 can be used to remove all restrictions and permit any forwarding requests.
703 By default all port forwarding requests are permitted.
704 .It Cm PermitRootLogin
705 Specifies whether root can log in using
709 .Dq without-password ,
710 .Dq forced-commands-only ,
716 .Cm ChallengeResponseAuthentication
719 the root user may be allowed in with its password even if
720 .Cm PermitRootLogin is set to
721 .Dq without-password .
723 If this option is set to
724 .Dq without-password ,
725 password authentication is disabled for root.
727 If this option is set to
728 .Dq forced-commands-only ,
729 root login with public key authentication will be allowed,
732 option has been specified
733 (which may be useful for taking remote backups even if root login is
734 normally not allowed).
735 All other authentication methods are disabled for root.
737 If this option is set to
739 root is not allowed to log in.
743 device forwarding is allowed.
759 .It Cm PermitUserEnvironment
761 .Pa ~/.ssh/environment
765 .Pa ~/.ssh/authorized_keys
770 Enabling environment processing may enable users to bypass access
771 restrictions in some configurations using mechanisms such as
774 Specifies the file that contains the process ID of the
777 .Pa /var/run/sshd.pid .
779 Specifies the port number that
783 Multiple options of this type are permitted.
789 should print the date and time of the last user login when a user logs
798 when a user logs in interactively.
799 (On some systems it is also printed by the shell,
805 Specifies the protocol versions
808 The possible values are
812 Multiple versions must be comma-separated.
815 Note that the order of the protocol list does not indicate preference,
816 because the client selects among multiple protocol versions offered
822 .It Cm PubkeyAuthentication
823 Specifies whether public key authentication is allowed.
826 Note that this option applies to protocol version 2 only.
828 Specifies a list of revoked public keys.
829 Keys listed in this file will be refused for public key authentication.
830 Note that if this file is not readable, then public key authentication will
831 be refused for all users.
832 .It Cm RhostsRSAAuthentication
833 Specifies whether rhosts or
835 authentication together
836 with successful RSA host authentication is allowed.
839 This option applies to protocol version 1 only.
840 .It Cm RSAAuthentication
841 Specifies whether pure RSA authentication is allowed.
844 This option applies to protocol version 1 only.
846 Defines the number of bits in the ephemeral protocol version 1 server key.
847 The minimum value is 512, and the default is 1024.
851 should check file modes and ownership of the
852 user's files and home directory before accepting login.
853 This is normally desirable because novices sometimes accidentally leave their
854 directory or files world-writable.
857 Note that this does not apply to
858 .Cm ChrootDirectory ,
859 whose permissions and ownership are checked unconditionally.
861 Configures an external subsystem (e.g. file transfer daemon).
862 Arguments should be a subsystem name and a command (with optional arguments)
863 to execute upon subsystem request.
869 file transfer subsystem.
873 implements an in-process
876 This may simplify configurations using
878 to force a different filesystem root on clients.
880 By default no subsystems are defined.
881 Note that this option applies to protocol version 2 only.
882 .It Cm SyslogFacility
883 Gives the facility code that is used when logging messages from
885 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
886 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
889 Specifies whether the system should send TCP keepalive messages to the
891 If they are sent, death of the connection or crash of one
892 of the machines will be properly noticed.
893 However, this means that
894 connections will die if the route is down temporarily, and some people
896 On the other hand, if TCP keepalives are not sent,
897 sessions may hang indefinitely on the server, leaving
899 users and consuming server resources.
903 (to send TCP keepalive messages), and the server will notice
904 if the network goes down or the client host crashes.
905 This avoids infinitely hanging sessions.
907 To disable TCP keepalive messages, the value should be set to
909 .It Cm TrustedUserCAKeys
910 Specifies a file containing public keys of certificate authorities that are
911 trusted to sign user certificates for authentication.
912 Keys are listed one per line; empty lines and comments starting with
915 If a certificate is presented for authentication and has its signing CA key
916 listed in this file, then it may be used for authentication for any user
917 listed in the certificate's principals list.
918 Note that certificates that lack a list of principals will not be permitted
919 for authentication using
920 .Cm TrustedUserCAKeys .
921 For more details on certificates, see the
928 should look up the remote host name and check that
929 the resolved host name for the remote IP address maps back to the
930 very same IP address.
936 is used for interactive login sessions.
941 is never used for remote command execution.
942 Note also, that if this is enabled,
944 will be disabled because
946 does not know how to handle
950 .Cm UsePrivilegeSeparation
951 is specified, it will be disabled after authentication.
953 Enables the Pluggable Authentication Module interface.
956 this will enable PAM authentication using
957 .Cm ChallengeResponseAuthentication
959 .Cm PasswordAuthentication
960 in addition to PAM account and session module processing for all
961 authentication types.
963 Because PAM challenge-response authentication usually serves an equivalent
964 role to password authentication, you should disable either
965 .Cm PasswordAuthentication
967 .Cm ChallengeResponseAuthentication.
971 is enabled, you will not be able to run
976 .It Cm UsePrivilegeSeparation
979 separates privileges by creating an unprivileged child process
980 to deal with incoming network traffic.
981 After successful authentication, another process will be created that has
982 the privilege of the authenticated user.
983 The goal of privilege separation is to prevent privilege
984 escalation by containing any corruption within the unprivileged processes.
987 .It Cm VersionAddendum
988 Specifies a string to append to the regular version string to identify
989 OS- or site-specific modifications.
991 .Dq FreeBSD-20100308 .
992 .It Cm X11DisplayOffset
993 Specifies the first display number available for
996 This prevents sshd from interfering with real X11 servers.
999 Specifies whether X11 forwarding is permitted.
1000 The argument must be
1007 When X11 forwarding is enabled, there may be additional exposure to
1008 the server and to client displays if the
1010 proxy display is configured to listen on the wildcard address (see
1012 below), though this is not the default.
1013 Additionally, the authentication spoofing and authentication data
1014 verification and substitution occur on the client side.
1015 The security risk of using X11 forwarding is that the client's X11
1016 display server may be exposed to attack when the SSH client requests
1017 forwarding (see the warnings for
1020 .Xr ssh_config 5 ) .
1021 A system administrator may have a stance in which they want to
1022 protect clients that may expose themselves to attack by unwittingly
1023 requesting X11 forwarding, which can warrant a
1027 Note that disabling X11 forwarding does not prevent users from
1028 forwarding X11 traffic, as users can always install their own forwarders.
1029 X11 forwarding is automatically disabled if
1032 .It Cm X11UseLocalhost
1035 should bind the X11 forwarding server to the loopback address or to
1036 the wildcard address.
1038 sshd binds the forwarding server to the loopback address and sets the
1039 hostname part of the
1041 environment variable to
1043 This prevents remote hosts from connecting to the proxy display.
1044 However, some older X11 clients may not function with this
1049 to specify that the forwarding server should be bound to the wildcard
1051 The argument must be
1057 .It Cm XAuthLocation
1058 Specifies the full pathname of the
1062 .Pa /usr/local/bin/xauth .
1066 command-line arguments and configuration file options that specify time
1067 may be expressed using a sequence of the form:
1069 .Ar time Op Ar qualifier ,
1073 is a positive integer value and
1075 is one of the following:
1077 .Bl -tag -width Ds -compact -offset indent
1092 Each member of the sequence is added together to calculate
1093 the total time value.
1095 Time format examples:
1097 .Bl -tag -width Ds -compact -offset indent
1099 600 seconds (10 minutes)
1103 1 hour 30 minutes (90 minutes)
1107 .It Pa /etc/ssh/sshd_config
1108 Contains configuration data for
1110 This file should be writable by root only, but it is recommended
1111 (though not necessary) that it be world-readable.
1116 OpenSSH is a derivative of the original and free
1117 ssh 1.2.12 release by Tatu Ylonen.
1118 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1119 Theo de Raadt and Dug Song
1120 removed many bugs, re-added newer features and
1122 Markus Friedl contributed the support for SSH
1123 protocol versions 1.5 and 2.0.
1124 Niels Provos and Markus Friedl contributed support
1125 for privilege separation.
projects / FreeBSD / releng / 8.1.git / blob