2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: sshd_config.5,v 1.135 2011/08/02 01:22:11 djm Exp $
37 .Dd $Mdocdate: August 2 2011 $
42 .Nd OpenSSH SSH daemon configuration file
44 .Nm /etc/ssh/sshd_config
47 reads configuration data from
48 .Pa /etc/ssh/sshd_config
49 (or the file specified with
52 The file contains keyword-argument pairs, one per line.
55 and empty lines are interpreted as comments.
56 Arguments may optionally be enclosed in double quotes
58 in order to represent arguments containing spaces.
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
65 Specifies what environment variables sent by the client will be copied into
72 for how to configure the client.
73 Note that environment passing is only supported for protocol 2.
74 Variables are specified by name, which may contain the wildcard characters
78 Multiple environment variables may be separated by whitespace or spread
82 Be warned that some environment variables could be used to bypass restricted
84 For this reason, care should be taken in the use of this directive.
85 The default is not to accept any environment variables.
87 Specifies which address family should be used by
97 .It Cm AllowAgentForwarding
100 forwarding is permitted.
103 Note that disabling agent forwarding does not improve security
104 unless users are also denied shell access, as they can always install
105 their own forwarders.
107 This keyword can be followed by a list of group name patterns, separated
109 If specified, login is allowed only for users whose primary
110 group or supplementary group list matches one of the patterns.
111 Only group names are valid; a numerical group ID is not recognized.
112 By default, login is allowed for all groups.
113 The allow/deny directives are processed in the following order:
124 for more information on patterns.
125 .It Cm AllowTcpForwarding
126 Specifies whether TCP forwarding is permitted.
129 Note that disabling TCP forwarding does not improve security unless
130 users are also denied shell access, as they can always install their
133 This keyword can be followed by a list of user name patterns, separated
135 If specified, login is allowed only for user names that
136 match one of the patterns.
137 Only user names are valid; a numerical user ID is not recognized.
138 By default, login is allowed for all users.
139 If the pattern takes the form USER@HOST then USER and HOST
140 are separately checked, restricting logins to particular
141 users from particular hosts.
142 The allow/deny directives are processed in the following order:
153 for more information on patterns.
154 .It Cm AuthorizedKeysFile
155 Specifies the file that contains the public keys that can be used
156 for user authentication.
157 The format is described in the
158 .Sx AUTHORIZED_KEYS FILE FORMAT
161 .Cm AuthorizedKeysFile
162 may contain tokens of the form %T which are substituted during connection
164 The following tokens are defined: %% is replaced by a literal '%',
165 %h is replaced by the home directory of the user being authenticated, and
166 %u is replaced by the username of that user.
168 .Cm AuthorizedKeysFile
169 is taken to be an absolute path or one relative to the user's home
171 Multiple files may be listed, separated by whitespace.
173 .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
174 .It Cm AuthorizedPrincipalsFile
175 Specifies a file that lists principal names that are accepted for
176 certificate authentication.
177 When using certificates signed by a key listed in
178 .Cm TrustedUserCAKeys ,
179 this file lists names, one of which must appear in the certificate for it
180 to be accepted for authentication.
181 Names are listed one per line preceded by key options (as described
183 .Sx AUTHORIZED_KEYS FILE FORMAT
186 Empty lines and comments starting with
190 .Cm AuthorizedPrincipalsFile
191 may contain tokens of the form %T which are substituted during connection
193 The following tokens are defined: %% 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.
197 .Cm AuthorizedPrincipalsFile
198 is taken to be an absolute path or one relative to the user's home
201 The default is not to use a principals file \(en in this case, the username
202 of the user must appear in a certificate's principals list for it to be
205 .Cm AuthorizedPrincipalsFile
206 is only used when authentication proceeds using a CA listed in
207 .Cm TrustedUserCAKeys
208 and is not consulted for certification authorities trusted via
209 .Pa ~/.ssh/authorized_keys ,
212 key option offers a similar facility (see
216 The contents of the specified file are sent to the remote user before
217 authentication is allowed.
220 then no banner is displayed.
221 This option is only available for protocol version 2.
222 By default, no banner is displayed.
223 .It Cm ChallengeResponseAuthentication
224 Specifies whether challenge-response authentication is allowed (e.g. via
225 PAM or though authentication styles supported in
229 .It Cm ChrootDirectory
230 Specifies the pathname of a directory to
232 to after authentication.
233 All components of the pathname must be root-owned directories that are
234 not writable by any other user or group.
237 changes the working directory to the user's home directory.
239 The pathname may contain the following tokens that are expanded at runtime once
240 the connecting user has been authenticated: %% is replaced by a literal '%',
241 %h is replaced by the home directory of the user being authenticated, and
242 %u is replaced by the username of that user.
246 must contain the necessary files and directories to support the
248 For an interactive session this requires at least a shell, typically
262 For file transfer sessions using
264 no additional configuration of the environment is necessary if the
265 in-process sftp server is used,
266 though sessions which use logging do require
268 inside the chroot directory (see
272 The default is not to
275 Specifies the ciphers allowed for protocol version 2.
276 Multiple ciphers must be comma-separated.
277 The supported ciphers are
292 .Bd -literal -offset 3n
293 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
294 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
297 .It Cm ClientAliveCountMax
298 Sets the number of client alive messages (see below) which may be
301 receiving any messages back from the client.
302 If this threshold is reached while client alive messages are being sent,
303 sshd will disconnect the client, terminating the session.
304 It is important to note that the use of client alive messages is very
308 The client alive messages are sent through the encrypted channel
309 and therefore will not be spoofable.
310 The TCP keepalive option enabled by
313 The client alive mechanism is valuable when the client or
314 server depend on knowing when a connection has become inactive.
316 The default value is 3.
318 .Cm ClientAliveInterval
319 (see below) is set to 15, and
320 .Cm ClientAliveCountMax
321 is left at the default, unresponsive SSH clients
322 will be disconnected after approximately 45 seconds.
323 This option applies to protocol version 2 only.
324 .It Cm ClientAliveInterval
325 Sets a timeout interval in seconds after which if no data has been received
328 will send a message through the encrypted
329 channel to request a response from the client.
331 is 0, indicating that these messages will not be sent to the client.
332 This option applies to protocol version 2 only.
334 Specifies whether compression is allowed, or delayed until
335 the user has authenticated successfully.
344 This keyword can be followed by a list of group name patterns, separated
346 Login is disallowed for users whose primary group or supplementary
347 group list matches one of the patterns.
348 Only group names are valid; a numerical group ID is not recognized.
349 By default, login is allowed for all groups.
350 The allow/deny directives are processed in the following order:
361 for more information on patterns.
363 This keyword can be followed by a list of user name patterns, separated
365 Login is disallowed for user names that match one of the patterns.
366 Only user names are valid; a numerical user ID is not recognized.
367 By default, login is allowed for all users.
368 If the pattern takes the form USER@HOST then USER and HOST
369 are separately checked, restricting logins to particular
370 users from particular hosts.
371 The allow/deny directives are processed in the following order:
382 for more information on patterns.
384 Forces the execution of the command specified by
386 ignoring any command supplied by the client and
389 The command is invoked by using the user's login shell with the -c option.
390 This applies to shell, command, or subsystem execution.
391 It is most useful inside a
394 The command originally supplied by the client is available in the
395 .Ev SSH_ORIGINAL_COMMAND
396 environment variable.
397 Specifying a command of
399 will force the use of an in-process sftp server that requires no support
401 .Cm ChrootDirectory .
403 Specifies whether remote hosts are allowed to connect to ports
404 forwarded for the client.
407 binds remote port forwardings to the loopback address.
408 This prevents other remote hosts from connecting to forwarded ports.
410 can be used to specify that sshd
411 should allow remote port forwardings to bind to non-loopback addresses, thus
412 allowing other hosts to connect.
415 to force remote port forwardings to be available to the local host only,
417 to force remote port forwardings to bind to the wildcard address, or
419 to allow the client to select the address to which the forwarding is bound.
422 .It Cm GSSAPIAuthentication
423 Specifies whether user authentication based on GSSAPI is allowed.
426 Note that this option applies to protocol version 2 only.
427 .It Cm GSSAPICleanupCredentials
428 Specifies whether to automatically destroy the user's credentials cache
432 Note that this option applies to protocol version 2 only.
433 .It Cm HostbasedAuthentication
434 Specifies whether rhosts or /etc/hosts.equiv authentication together
435 with successful public key client host authentication is allowed
436 (host-based authentication).
437 This option is similar to
438 .Cm RhostsRSAAuthentication
439 and applies to protocol version 2 only.
442 .It Cm HostbasedUsesNameFromPacketOnly
443 Specifies whether or not the server will attempt to perform a reverse
444 name lookup when matching the name in the
450 .Cm HostbasedAuthentication .
455 uses the name supplied by the client rather than
456 attempting to resolve the name from the TCP connection itself.
459 .It Cm HostCertificate
460 Specifies a file containing a public host certificate.
461 The certificate's public key must match a private host key already specified
464 The default behaviour of
466 is not to load any certificates.
468 Specifies a file containing a private host key
471 .Pa /etc/ssh/ssh_host_key
472 for protocol version 1, and
473 .Pa /etc/ssh/ssh_host_dsa_key ,
474 .Pa /etc/ssh/ssh_host_ecdsa_key
476 .Pa /etc/ssh/ssh_host_rsa_key
477 for protocol version 2.
480 will refuse to use a file if it is group/world-accessible.
481 It is possible to have multiple host key files.
483 keys are used for version 1 and
488 are used for version 2 of the SSH protocol.
494 files will not be used in
495 .Cm RhostsRSAAuthentication
497 .Cm HostbasedAuthentication .
501 .Pa /etc/shosts.equiv
505 .It Cm IgnoreUserKnownHosts
508 should ignore the user's
509 .Pa ~/.ssh/known_hosts
511 .Cm RhostsRSAAuthentication
513 .Cm HostbasedAuthentication .
517 Specifies the IPv4 type-of-service or DSCP class for the connection.
544 This option may take one or two arguments, separated by whitespace.
545 If one argument is specified, it is used as the packet class unconditionally.
546 If two values are specified, the first is automatically selected for
547 interactive sessions and the second for non-interactive sessions.
550 for interactive sessions and
552 for non-interactive sessions.
553 .It Cm KerberosAuthentication
554 Specifies whether the password provided by the user for
555 .Cm PasswordAuthentication
556 will be validated through the Kerberos KDC.
557 To use this option, the server needs a
558 Kerberos servtab which allows the verification of the KDC's identity.
561 .It Cm KerberosGetAFSToken
562 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
563 an AFS token before accessing the user's home directory.
566 .It Cm KerberosOrLocalPasswd
567 If password authentication through Kerberos fails then
568 the password will be validated via any additional local mechanism
573 .It Cm KerberosTicketCleanup
574 Specifies whether to automatically destroy the user's ticket cache
579 Specifies the available KEX (Key Exchange) algorithms.
580 Multiple algorithms must be comma-separated.
582 .Dq ecdh-sha2-nistp256 ,
583 .Dq ecdh-sha2-nistp384 ,
584 .Dq ecdh-sha2-nistp521 ,
585 .Dq diffie-hellman-group-exchange-sha256 ,
586 .Dq diffie-hellman-group-exchange-sha1 ,
587 .Dq diffie-hellman-group14-sha1 ,
588 .Dq diffie-hellman-group1-sha1 .
589 .It Cm KeyRegenerationInterval
590 In protocol version 1, the ephemeral server key is automatically regenerated
591 after this many seconds (if it has been used).
592 The purpose of regeneration is to prevent
593 decrypting captured sessions by later breaking into the machine and
595 The key is never stored anywhere.
596 If the value is 0, the key is never regenerated.
597 The default is 3600 (seconds).
599 Specifies the local addresses
602 The following forms may be used:
604 .Bl -item -offset indent -compact
608 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
613 .Ar host No | Ar IPv4_addr No : Ar port
619 .Ar host No | Ar IPv6_addr Oc : Ar port
626 sshd will listen on the address and all prior
629 The default is to listen on all local addresses.
632 options are permitted.
635 options must precede this option for non-port qualified addresses.
636 .It Cm LoginGraceTime
637 The server disconnects after this time if the user has not
638 successfully logged in.
639 If the value is 0, there is no time limit.
640 The default is 120 seconds.
642 Gives the verbosity level that is used when logging messages from
644 The possible values are:
645 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
647 DEBUG and DEBUG1 are equivalent.
648 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
649 Logging with a DEBUG level violates the privacy of users and is not recommended.
651 Specifies the available MAC (message authentication code) algorithms.
652 The MAC algorithm is used in protocol version 2
653 for data integrity protection.
654 Multiple algorithms must be comma-separated.
656 .Bd -literal -offset indent
657 hmac-md5,hmac-sha1,umac-64@openssh.com,
658 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
659 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
663 Introduces a conditional block.
664 If all of the criteria on the
666 line are satisfied, the keywords on the following lines override those
667 set in the global section of the config file, until either another
669 line or the end of the file.
673 are one or more criteria-pattern pairs.
674 The available criteria are
680 The match patterns may consist of single entries or comma-separated
681 lists and may use the wildcard and negation operators described in the
688 criteria may additionally contain addresses to match in CIDR
689 address/masklen format, e.g.\&
693 Note that the mask length provided must be consistent with the address -
694 it is an error to specify a mask length that is too long for the address
695 or one with bits set in this host portion of the address.
702 Only a subset of keywords may be used on the lines following a
705 Available keywords are
706 .Cm AllowAgentForwarding ,
707 .Cm AllowTcpForwarding ,
708 .Cm AuthorizedKeysFile ,
709 .Cm AuthorizedPrincipalsFile ,
711 .Cm ChrootDirectory ,
714 .Cm GSSAPIAuthentication ,
715 .Cm HostbasedAuthentication ,
716 .Cm HostbasedUsesNameFromPacketOnly ,
717 .Cm KbdInteractiveAuthentication ,
718 .Cm KerberosAuthentication ,
721 .Cm PasswordAuthentication ,
722 .Cm PermitEmptyPasswords ,
724 .Cm PermitRootLogin ,
726 .Cm PubkeyAuthentication ,
727 .Cm RhostsRSAAuthentication ,
728 .Cm RSAAuthentication ,
729 .Cm X11DisplayOffset ,
732 .Cm X11UseLocalHost .
734 Specifies the maximum number of authentication attempts permitted per
736 Once the number of failures reaches half this value,
737 additional failures are logged.
740 Specifies the maximum number of open sessions permitted per network connection.
743 Specifies the maximum number of concurrent unauthenticated connections to the
745 Additional connections will be dropped until authentication succeeds or the
747 expires for a connection.
750 Alternatively, random early drop can be enabled by specifying
751 the three colon separated values
755 will refuse connection attempts with a probability of
758 if there are currently
761 unauthenticated connections.
762 The probability increases linearly and all connection attempts
763 are refused if the number of unauthenticated connections reaches
766 .It Cm PasswordAuthentication
767 Specifies whether password authentication is allowed.
770 .It Cm PermitEmptyPasswords
771 When password authentication is allowed, it specifies whether the
772 server allows login to accounts with empty password strings.
776 Specifies the destinations to which TCP port forwarding is permitted.
777 The forwarding specification must be one of the following forms:
779 .Bl -item -offset indent -compact
793 .Ar \&[ IPv6_addr \&] : port
797 Multiple forwards may be specified by separating them with whitespace.
800 can be used to remove all restrictions and permit any forwarding requests.
801 By default all port forwarding requests are permitted.
802 .It Cm PermitRootLogin
803 Specifies whether root can log in using
807 .Dq without-password ,
808 .Dq forced-commands-only ,
814 If this option is set to
815 .Dq without-password ,
816 password authentication is disabled for root.
818 If this option is set to
819 .Dq forced-commands-only ,
820 root login with public key authentication will be allowed,
823 option has been specified
824 (which may be useful for taking remote backups even if root login is
825 normally not allowed).
826 All other authentication methods are disabled for root.
828 If this option is set to
830 root is not allowed to log in.
834 device forwarding is allowed.
850 .It Cm PermitUserEnvironment
852 .Pa ~/.ssh/environment
856 .Pa ~/.ssh/authorized_keys
861 Enabling environment processing may enable users to bypass access
862 restrictions in some configurations using mechanisms such as
865 Specifies the file that contains the process ID of the
868 .Pa /var/run/sshd.pid .
870 Specifies the port number that
874 Multiple options of this type are permitted.
880 should print the date and time of the last user login when a user logs
889 when a user logs in interactively.
890 (On some systems it is also printed by the shell,
896 Specifies the protocol versions
899 The possible values are
903 Multiple versions must be comma-separated.
906 Note that the order of the protocol list does not indicate preference,
907 because the client selects among multiple protocol versions offered
913 .It Cm PubkeyAuthentication
914 Specifies whether public key authentication is allowed.
917 Note that this option applies to protocol version 2 only.
919 Specifies a list of revoked public keys.
920 Keys listed in this file will be refused for public key authentication.
921 Note that if this file is not readable, then public key authentication will
922 be refused for all users.
923 .It Cm RhostsRSAAuthentication
924 Specifies whether rhosts or /etc/hosts.equiv authentication together
925 with successful RSA host authentication is allowed.
928 This option applies to protocol version 1 only.
929 .It Cm RSAAuthentication
930 Specifies whether pure RSA authentication is allowed.
933 This option applies to protocol version 1 only.
935 Defines the number of bits in the ephemeral protocol version 1 server key.
936 The minimum value is 512, and the default is 1024.
940 should check file modes and ownership of the
941 user's files and home directory before accepting login.
942 This is normally desirable because novices sometimes accidentally leave their
943 directory or files world-writable.
946 Note that this does not apply to
947 .Cm ChrootDirectory ,
948 whose permissions and ownership are checked unconditionally.
950 Configures an external subsystem (e.g. file transfer daemon).
951 Arguments should be a subsystem name and a command (with optional arguments)
952 to execute upon subsystem request.
958 file transfer subsystem.
962 implements an in-process
965 This may simplify configurations using
967 to force a different filesystem root on clients.
969 By default no subsystems are defined.
970 Note that this option applies to protocol version 2 only.
971 .It Cm SyslogFacility
972 Gives the facility code that is used when logging messages from
974 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
975 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
978 Specifies whether the system should send TCP keepalive messages to the
980 If they are sent, death of the connection or crash of one
981 of the machines will be properly noticed.
982 However, this means that
983 connections will die if the route is down temporarily, and some people
985 On the other hand, if TCP keepalives are not sent,
986 sessions may hang indefinitely on the server, leaving
988 users and consuming server resources.
992 (to send TCP keepalive messages), and the server will notice
993 if the network goes down or the client host crashes.
994 This avoids infinitely hanging sessions.
996 To disable TCP keepalive messages, the value should be set to
998 .It Cm TrustedUserCAKeys
999 Specifies a file containing public keys of certificate authorities that are
1000 trusted to sign user certificates for authentication.
1001 Keys are listed one per line; empty lines and comments starting with
1004 If a certificate is presented for authentication and has its signing CA key
1005 listed in this file, then it may be used for authentication for any user
1006 listed in the certificate's principals list.
1007 Note that certificates that lack a list of principals will not be permitted
1008 for authentication using
1009 .Cm TrustedUserCAKeys .
1010 For more details on certificates, see the
1017 should look up the remote host name and check that
1018 the resolved host name for the remote IP address maps back to the
1019 very same IP address.
1025 is used for interactive login sessions.
1030 is never used for remote command execution.
1031 Note also, that if this is enabled,
1033 will be disabled because
1035 does not know how to handle
1039 .Cm UsePrivilegeSeparation
1040 is specified, it will be disabled after authentication.
1042 Enables the Pluggable Authentication Module interface.
1045 this will enable PAM authentication using
1046 .Cm ChallengeResponseAuthentication
1048 .Cm PasswordAuthentication
1049 in addition to PAM account and session module processing for all
1050 authentication types.
1052 Because PAM challenge-response authentication usually serves an equivalent
1053 role to password authentication, you should disable either
1054 .Cm PasswordAuthentication
1056 .Cm ChallengeResponseAuthentication.
1060 is enabled, you will not be able to run
1065 .It Cm UsePrivilegeSeparation
1068 separates privileges by creating an unprivileged child process
1069 to deal with incoming network traffic.
1070 After successful authentication, another process will be created that has
1071 the privilege of the authenticated user.
1072 The goal of privilege separation is to prevent privilege
1073 escalation by containing any corruption within the unprivileged processes.
1077 .Cm UsePrivilegeSeparation
1080 then the pre-authentication unprivileged process is subject to additional
1082 .It Cm X11DisplayOffset
1083 Specifies the first display number available for
1086 This prevents sshd from interfering with real X11 servers.
1088 .It Cm X11Forwarding
1089 Specifies whether X11 forwarding is permitted.
1090 The argument must be
1097 When X11 forwarding is enabled, there may be additional exposure to
1098 the server and to client displays if the
1100 proxy display is configured to listen on the wildcard address (see
1102 below), though this is not the default.
1103 Additionally, the authentication spoofing and authentication data
1104 verification and substitution occur on the client side.
1105 The security risk of using X11 forwarding is that the client's X11
1106 display server may be exposed to attack when the SSH client requests
1107 forwarding (see the warnings for
1110 .Xr ssh_config 5 ) .
1111 A system administrator may have a stance in which they want to
1112 protect clients that may expose themselves to attack by unwittingly
1113 requesting X11 forwarding, which can warrant a
1117 Note that disabling X11 forwarding does not prevent users from
1118 forwarding X11 traffic, as users can always install their own forwarders.
1119 X11 forwarding is automatically disabled if
1122 .It Cm X11UseLocalhost
1125 should bind the X11 forwarding server to the loopback address or to
1126 the wildcard address.
1128 sshd binds the forwarding server to the loopback address and sets the
1129 hostname part of the
1131 environment variable to
1133 This prevents remote hosts from connecting to the proxy display.
1134 However, some older X11 clients may not function with this
1139 to specify that the forwarding server should be bound to the wildcard
1141 The argument must be
1147 .It Cm XAuthLocation
1148 Specifies the full pathname of the
1152 .Pa /usr/X11R6/bin/xauth .
1156 command-line arguments and configuration file options that specify time
1157 may be expressed using a sequence of the form:
1159 .Ar time Op Ar qualifier ,
1163 is a positive integer value and
1165 is one of the following:
1167 .Bl -tag -width Ds -compact -offset indent
1182 Each member of the sequence is added together to calculate
1183 the total time value.
1185 Time format examples:
1187 .Bl -tag -width Ds -compact -offset indent
1189 600 seconds (10 minutes)
1193 1 hour 30 minutes (90 minutes)
1197 .It Pa /etc/ssh/sshd_config
1198 Contains configuration data for
1200 This file should be writable by root only, but it is recommended
1201 (though not necessary) that it be world-readable.
1206 OpenSSH is a derivative of the original and free
1207 ssh 1.2.12 release by Tatu Ylonen.
1208 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1209 Theo de Raadt and Dug Song
1210 removed many bugs, re-added newer features and
1212 Markus Friedl contributed the support for SSH
1213 protocol versions 1.5 and 2.0.
1214 Niels Provos and Markus Friedl contributed support
1215 for privilege separation.