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 1.131 2010/12/08 04:02:47 djm Exp $
37 .Dd $Mdocdate: December 8 2010 $
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
172 .Dq .ssh/authorized_keys .
173 .It Cm AuthorizedPrincipalsFile
174 Specifies a file that lists principal names that are accepted for
175 certificate authentication.
176 When using certificates signed by a key listed in
177 .Cm TrustedUserCAKeys ,
178 this file lists names, one of which must appear in the certificate for it
179 to be accepted for authentication.
180 Names are listed one per line preceded by key options (as described
182 .Sx AUTHORIZED_KEYS FILE FORMAT
185 Empty lines and comments starting with
189 .Cm AuthorizedPrincipalsFile
190 may contain tokens of the form %T which are substituted during connection
192 The following tokens are defined: %% is replaced by a literal '%',
193 %h is replaced by the home directory of the user being authenticated, and
194 %u is replaced by the username of that user.
196 .Cm AuthorizedPrincipalsFile
197 is taken to be an absolute path or one relative to the user's home
200 The default is not to use a principals file \(en in this case, the username
201 of the user must appear in a certificate's principals list for it to be
204 .Cm AuthorizedPrincipalsFile
205 is only used when authentication proceeds using a CA listed in
206 .Cm TrustedUserCAKeys
207 and is not consulted for certification authorities trusted via
208 .Pa ~/.ssh/authorized_keys ,
211 key option offers a similar facility (see
215 The contents of the specified file are sent to the remote user before
216 authentication is allowed.
219 then no banner is displayed.
220 This option is only available for protocol version 2.
221 By default, no banner is displayed.
222 .It Cm ChallengeResponseAuthentication
223 Specifies whether challenge-response authentication is allowed (e.g. via
224 PAM or though authentication styles supported in
228 .It Cm ChrootDirectory
229 Specifies the pathname of a directory to
231 to after authentication.
232 All components of the pathname must be root-owned directories that are
233 not writable by any other user or group.
236 changes the working directory to the user's home directory.
238 The pathname may contain the following tokens that are expanded at runtime once
239 the connecting user has been authenticated: %% is replaced by a literal '%',
240 %h is replaced by the home directory of the user being authenticated, and
241 %u is replaced by the username of that user.
245 must contain the necessary files and directories to support the
247 For an interactive session this requires at least a shell, typically
261 For file transfer sessions using
263 no additional configuration of the environment is necessary if the
264 in-process sftp server is used,
265 though sessions which use logging do require
267 inside the chroot directory (see
271 The default is not to
274 Specifies the ciphers allowed for protocol version 2.
275 Multiple ciphers must be comma-separated.
276 The supported ciphers are
291 .Bd -literal -offset 3n
292 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
293 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
296 .It Cm ClientAliveCountMax
297 Sets the number of client alive messages (see below) which may be
300 receiving any messages back from the client.
301 If this threshold is reached while client alive messages are being sent,
302 sshd will disconnect the client, terminating the session.
303 It is important to note that the use of client alive messages is very
307 The client alive messages are sent through the encrypted channel
308 and therefore will not be spoofable.
309 The TCP keepalive option enabled by
312 The client alive mechanism is valuable when the client or
313 server depend on knowing when a connection has become inactive.
315 The default value is 3.
317 .Cm ClientAliveInterval
318 (see below) is set to 15, and
319 .Cm ClientAliveCountMax
320 is left at the default, unresponsive SSH clients
321 will be disconnected after approximately 45 seconds.
322 This option applies to protocol version 2 only.
323 .It Cm ClientAliveInterval
324 Sets a timeout interval in seconds after which if no data has been received
327 will send a message through the encrypted
328 channel to request a response from the client.
330 is 0, indicating that these messages will not be sent to the client.
331 This option applies to protocol version 2 only.
333 Specifies whether compression is allowed, or delayed until
334 the user has authenticated successfully.
343 This keyword can be followed by a list of group name patterns, separated
345 Login is disallowed for users whose primary group or supplementary
346 group list matches one of the patterns.
347 Only group names are valid; a numerical group ID is not recognized.
348 By default, login is allowed for all groups.
349 The allow/deny directives are processed in the following order:
360 for more information on patterns.
362 This keyword can be followed by a list of user name patterns, separated
364 Login is disallowed for user names that match one of the patterns.
365 Only user names are valid; a numerical user ID is not recognized.
366 By default, login is allowed for all users.
367 If the pattern takes the form USER@HOST then USER and HOST
368 are separately checked, restricting logins to particular
369 users from particular hosts.
370 The allow/deny directives are processed in the following order:
381 for more information on patterns.
383 Forces the execution of the command specified by
385 ignoring any command supplied by the client and
388 The command is invoked by using the user's login shell with the -c option.
389 This applies to shell, command, or subsystem execution.
390 It is most useful inside a
393 The command originally supplied by the client is available in the
394 .Ev SSH_ORIGINAL_COMMAND
395 environment variable.
396 Specifying a command of
398 will force the use of an in-process sftp server that requires no support
400 .Cm ChrootDirectory .
402 Specifies whether remote hosts are allowed to connect to ports
403 forwarded for the client.
406 binds remote port forwardings to the loopback address.
407 This prevents other remote hosts from connecting to forwarded ports.
409 can be used to specify that sshd
410 should allow remote port forwardings to bind to non-loopback addresses, thus
411 allowing other hosts to connect.
414 to force remote port forwardings to be available to the local host only,
416 to force remote port forwardings to bind to the wildcard address, or
418 to allow the client to select the address to which the forwarding is bound.
421 .It Cm GSSAPIAuthentication
422 Specifies whether user authentication based on GSSAPI is allowed.
425 Note that this option applies to protocol version 2 only.
426 .It Cm GSSAPIKeyExchange
427 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
428 doesn't rely on ssh keys to verify host identity.
431 Note that this option applies to protocol version 2 only.
432 .It Cm GSSAPICleanupCredentials
433 Specifies whether to automatically destroy the user's credentials cache
437 Note that this option applies to protocol version 2 only.
438 .It Cm GSSAPIStrictAcceptorCheck
439 Determines whether to be strict about the identity of the GSSAPI acceptor
440 a client authenticates against. If
442 then the client must authenticate against the
444 service on the current hostname. If
446 then the client may authenticate against any service key stored in the
447 machine's default store. This facility is provided to assist with operation
448 on multi homed machines.
451 Note that this option applies only to protocol version 2 GSSAPI connections,
454 may only work with recent Kerberos GSSAPI libraries.
455 .It Cm GSSAPIStoreCredentialsOnRekey
456 Controls whether the user's GSSAPI credentials should be updated following a
457 successful connection rekeying. This option can be used to accepted renewed
458 or updated credentials from a compatible client. The default is
460 .It Cm HostbasedAuthentication
461 Specifies whether rhosts or /etc/hosts.equiv authentication together
462 with successful public key client host authentication is allowed
463 (host-based authentication).
464 This option is similar to
465 .Cm RhostsRSAAuthentication
466 and applies to protocol version 2 only.
469 .It Cm HostbasedUsesNameFromPacketOnly
470 Specifies whether or not the server will attempt to perform a reverse
471 name lookup when matching the name in the
477 .Cm HostbasedAuthentication .
482 uses the name supplied by the client rather than
483 attempting to resolve the name from the TCP connection itself.
486 .It Cm HostCertificate
487 Specifies a file containing a public host certificate.
488 The certificate's public key must match a private host key already specified
491 The default behaviour of
493 is not to load any certificates.
495 Specifies a file containing a private host key
498 .Pa /etc/ssh/ssh_host_key
499 for protocol version 1, and
500 .Pa /etc/ssh/ssh_host_dsa_key ,
501 .Pa /etc/ssh/ssh_host_ecdsa_key
503 .Pa /etc/ssh/ssh_host_rsa_key
504 for protocol version 2.
507 will refuse to use a file if it is group/world-accessible.
508 It is possible to have multiple host key files.
510 keys are used for version 1 and
515 are used for version 2 of the SSH protocol.
521 files will not be used in
522 .Cm RhostsRSAAuthentication
524 .Cm HostbasedAuthentication .
528 .Pa /etc/shosts.equiv
532 .It Cm IgnoreUserKnownHosts
535 should ignore the user's
536 .Pa ~/.ssh/known_hosts
538 .Cm RhostsRSAAuthentication
540 .Cm HostbasedAuthentication .
544 Specifies the IPv4 type-of-service or DSCP class for the connection.
571 This option may take one or two arguments, separated by whitespace.
572 If one argument is specified, it is used as the packet class unconditionally.
573 If two values are specified, the first is automatically selected for
574 interactive sessions and the second for non-interactive sessions.
577 for interactive sessions and
579 for non-interactive sessions.
580 .It Cm KerberosAuthentication
581 Specifies whether the password provided by the user for
582 .Cm PasswordAuthentication
583 will be validated through the Kerberos KDC.
584 To use this option, the server needs a
585 Kerberos servtab which allows the verification of the KDC's identity.
588 .It Cm KerberosGetAFSToken
589 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
590 an AFS token before accessing the user's home directory.
593 .It Cm KerberosOrLocalPasswd
594 If password authentication through Kerberos fails then
595 the password will be validated via any additional local mechanism
600 .It Cm KerberosTicketCleanup
601 Specifies whether to automatically destroy the user's ticket cache
606 Specifies the available KEX (Key Exchange) algorithms.
607 Multiple algorithms must be comma-separated.
609 .Dq ecdh-sha2-nistp256 ,
610 .Dq ecdh-sha2-nistp384 ,
611 .Dq ecdh-sha2-nistp521 ,
612 .Dq diffie-hellman-group-exchange-sha256 ,
613 .Dq diffie-hellman-group-exchange-sha1 ,
614 .Dq diffie-hellman-group14-sha1 ,
615 .Dq diffie-hellman-group1-sha1 .
616 .It Cm KeyRegenerationInterval
617 In protocol version 1, the ephemeral server key is automatically regenerated
618 after this many seconds (if it has been used).
619 The purpose of regeneration is to prevent
620 decrypting captured sessions by later breaking into the machine and
622 The key is never stored anywhere.
623 If the value is 0, the key is never regenerated.
624 The default is 3600 (seconds).
626 Specifies the local addresses
629 The following forms may be used:
631 .Bl -item -offset indent -compact
635 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
640 .Ar host No | Ar IPv4_addr No : Ar port
646 .Ar host No | Ar IPv6_addr Oc : Ar port
653 sshd will listen on the address and all prior
656 The default is to listen on all local addresses.
659 options are permitted.
662 options must precede this option for non-port qualified addresses.
663 .It Cm LoginGraceTime
664 The server disconnects after this time if the user has not
665 successfully logged in.
666 If the value is 0, there is no time limit.
667 The default is 120 seconds.
669 Gives the verbosity level that is used when logging messages from
671 The possible values are:
672 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
674 DEBUG and DEBUG1 are equivalent.
675 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
676 Logging with a DEBUG level violates the privacy of users and is not recommended.
678 Specifies the available MAC (message authentication code) algorithms.
679 The MAC algorithm is used in protocol version 2
680 for data integrity protection.
681 Multiple algorithms must be comma-separated.
683 .Bd -literal -offset indent
684 hmac-md5,hmac-sha1,umac-64@openssh.com,
685 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
688 Introduces a conditional block.
689 If all of the criteria on the
691 line are satisfied, the keywords on the following lines override those
692 set in the global section of the config file, until either another
694 line or the end of the file.
698 are one or more criteria-pattern pairs.
699 The available criteria are
705 The match patterns may consist of single entries or comma-separated
706 lists and may use the wildcard and negation operators described in the
713 criteria may additionally contain addresses to match in CIDR
714 address/masklen format, e.g.\&
718 Note that the mask length provided must be consistent with the address -
719 it is an error to specify a mask length that is too long for the address
720 or one with bits set in this host portion of the address.
727 Only a subset of keywords may be used on the lines following a
730 Available keywords are
731 .Cm AllowAgentForwarding ,
732 .Cm AllowTcpForwarding ,
733 .Cm AuthorizedKeysFile ,
734 .Cm AuthorizedPrincipalsFile ,
736 .Cm ChrootDirectory ,
739 .Cm GSSAPIAuthentication ,
740 .Cm HostbasedAuthentication ,
741 .Cm HostbasedUsesNameFromPacketOnly ,
742 .Cm KbdInteractiveAuthentication ,
743 .Cm KerberosAuthentication ,
746 .Cm PasswordAuthentication ,
747 .Cm PermitEmptyPasswords ,
749 .Cm PermitRootLogin ,
751 .Cm PubkeyAuthentication ,
752 .Cm RhostsRSAAuthentication ,
753 .Cm RSAAuthentication ,
754 .Cm X11DisplayOffset ,
757 .Cm X11UseLocalHost .
759 Specifies the maximum number of authentication attempts permitted per
761 Once the number of failures reaches half this value,
762 additional failures are logged.
765 Specifies the maximum number of open sessions permitted per network connection.
768 Specifies the maximum number of concurrent unauthenticated connections to the
770 Additional connections will be dropped until authentication succeeds or the
772 expires for a connection.
775 Alternatively, random early drop can be enabled by specifying
776 the three colon separated values
780 will refuse connection attempts with a probability of
783 if there are currently
786 unauthenticated connections.
787 The probability increases linearly and all connection attempts
788 are refused if the number of unauthenticated connections reaches
791 .It Cm PasswordAuthentication
792 Specifies whether password authentication is allowed.
795 .It Cm PermitEmptyPasswords
796 When password authentication is allowed, it specifies whether the
797 server allows login to accounts with empty password strings.
801 Specifies the destinations to which TCP port forwarding is permitted.
802 The forwarding specification must be one of the following forms:
804 .Bl -item -offset indent -compact
818 .Ar \&[ IPv6_addr \&] : port
822 Multiple forwards may be specified by separating them with whitespace.
825 can be used to remove all restrictions and permit any forwarding requests.
826 By default all port forwarding requests are permitted.
827 .It Cm PermitRootLogin
828 Specifies whether root can log in using
832 .Dq without-password ,
833 .Dq forced-commands-only ,
839 If this option is set to
840 .Dq without-password ,
841 password authentication is disabled for root.
843 If this option is set to
844 .Dq forced-commands-only ,
845 root login with public key authentication will be allowed,
848 option has been specified
849 (which may be useful for taking remote backups even if root login is
850 normally not allowed).
851 All other authentication methods are disabled for root.
853 If this option is set to
855 root is not allowed to log in.
859 device forwarding is allowed.
875 .It Cm PermitUserEnvironment
877 .Pa ~/.ssh/environment
881 .Pa ~/.ssh/authorized_keys
886 Enabling environment processing may enable users to bypass access
887 restrictions in some configurations using mechanisms such as
890 Specifies the file that contains the process ID of the
893 .Pa /var/run/sshd.pid .
895 Specifies the port number that
899 Multiple options of this type are permitted.
905 should print the date and time of the last user login when a user logs
914 when a user logs in interactively.
915 (On some systems it is also printed by the shell,
921 Specifies the protocol versions
924 The possible values are
928 Multiple versions must be comma-separated.
931 Note that the order of the protocol list does not indicate preference,
932 because the client selects among multiple protocol versions offered
938 .It Cm PubkeyAuthentication
939 Specifies whether public key authentication is allowed.
942 Note that this option applies to protocol version 2 only.
944 Specifies a list of revoked public keys.
945 Keys listed in this file will be refused for public key authentication.
946 Note that if this file is not readable, then public key authentication will
947 be refused for all users.
948 .It Cm RhostsRSAAuthentication
949 Specifies whether rhosts or /etc/hosts.equiv authentication together
950 with successful RSA host authentication is allowed.
953 This option applies to protocol version 1 only.
954 .It Cm RSAAuthentication
955 Specifies whether pure RSA authentication is allowed.
958 This option applies to protocol version 1 only.
960 Defines the number of bits in the ephemeral protocol version 1 server key.
961 The minimum value is 512, and the default is 1024.
965 should check file modes and ownership of the
966 user's files and home directory before accepting login.
967 This is normally desirable because novices sometimes accidentally leave their
968 directory or files world-writable.
971 Note that this does not apply to
972 .Cm ChrootDirectory ,
973 whose permissions and ownership are checked unconditionally.
975 Configures an external subsystem (e.g. file transfer daemon).
976 Arguments should be a subsystem name and a command (with optional arguments)
977 to execute upon subsystem request.
983 file transfer subsystem.
987 implements an in-process
990 This may simplify configurations using
992 to force a different filesystem root on clients.
994 By default no subsystems are defined.
995 Note that this option applies to protocol version 2 only.
996 .It Cm SyslogFacility
997 Gives the facility code that is used when logging messages from
999 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1000 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1001 The default is AUTH.
1003 Specifies whether the system should send TCP keepalive messages to the
1005 If they are sent, death of the connection or crash of one
1006 of the machines will be properly noticed.
1007 However, this means that
1008 connections will die if the route is down temporarily, and some people
1010 On the other hand, if TCP keepalives are not sent,
1011 sessions may hang indefinitely on the server, leaving
1013 users and consuming server resources.
1017 (to send TCP keepalive messages), and the server will notice
1018 if the network goes down or the client host crashes.
1019 This avoids infinitely hanging sessions.
1021 To disable TCP keepalive messages, the value should be set to
1023 .It Cm TrustedUserCAKeys
1024 Specifies a file containing public keys of certificate authorities that are
1025 trusted to sign user certificates for authentication.
1026 Keys are listed one per line; empty lines and comments starting with
1029 If a certificate is presented for authentication and has its signing CA key
1030 listed in this file, then it may be used for authentication for any user
1031 listed in the certificate's principals list.
1032 Note that certificates that lack a list of principals will not be permitted
1033 for authentication using
1034 .Cm TrustedUserCAKeys .
1035 For more details on certificates, see the
1042 should look up the remote host name and check that
1043 the resolved host name for the remote IP address maps back to the
1044 very same IP address.
1050 is used for interactive login sessions.
1055 is never used for remote command execution.
1056 Note also, that if this is enabled,
1058 will be disabled because
1060 does not know how to handle
1064 .Cm UsePrivilegeSeparation
1065 is specified, it will be disabled after authentication.
1067 Enables the Pluggable Authentication Module interface.
1070 this will enable PAM authentication using
1071 .Cm ChallengeResponseAuthentication
1073 .Cm PasswordAuthentication
1074 in addition to PAM account and session module processing for all
1075 authentication types.
1077 Because PAM challenge-response authentication usually serves an equivalent
1078 role to password authentication, you should disable either
1079 .Cm PasswordAuthentication
1081 .Cm ChallengeResponseAuthentication.
1085 is enabled, you will not be able to run
1090 .It Cm UsePrivilegeSeparation
1093 separates privileges by creating an unprivileged child process
1094 to deal with incoming network traffic.
1095 After successful authentication, another process will be created that has
1096 the privilege of the authenticated user.
1097 The goal of privilege separation is to prevent privilege
1098 escalation by containing any corruption within the unprivileged processes.
1101 .It Cm X11DisplayOffset
1102 Specifies the first display number available for
1105 This prevents sshd from interfering with real X11 servers.
1107 .It Cm X11Forwarding
1108 Specifies whether X11 forwarding is permitted.
1109 The argument must be
1116 When X11 forwarding is enabled, there may be additional exposure to
1117 the server and to client displays if the
1119 proxy display is configured to listen on the wildcard address (see
1121 below), though this is not the default.
1122 Additionally, the authentication spoofing and authentication data
1123 verification and substitution occur on the client side.
1124 The security risk of using X11 forwarding is that the client's X11
1125 display server may be exposed to attack when the SSH client requests
1126 forwarding (see the warnings for
1129 .Xr ssh_config 5 ) .
1130 A system administrator may have a stance in which they want to
1131 protect clients that may expose themselves to attack by unwittingly
1132 requesting X11 forwarding, which can warrant a
1136 Note that disabling X11 forwarding does not prevent users from
1137 forwarding X11 traffic, as users can always install their own forwarders.
1138 X11 forwarding is automatically disabled if
1141 .It Cm X11UseLocalhost
1144 should bind the X11 forwarding server to the loopback address or to
1145 the wildcard address.
1147 sshd binds the forwarding server to the loopback address and sets the
1148 hostname part of the
1150 environment variable to
1152 This prevents remote hosts from connecting to the proxy display.
1153 However, some older X11 clients may not function with this
1158 to specify that the forwarding server should be bound to the wildcard
1160 The argument must be
1166 .It Cm XAuthLocation
1167 Specifies the full pathname of the
1171 .Pa /usr/X11R6/bin/xauth .
1175 command-line arguments and configuration file options that specify time
1176 may be expressed using a sequence of the form:
1178 .Ar time Op Ar qualifier ,
1182 is a positive integer value and
1184 is one of the following:
1186 .Bl -tag -width Ds -compact -offset indent
1201 Each member of the sequence is added together to calculate
1202 the total time value.
1204 Time format examples:
1206 .Bl -tag -width Ds -compact -offset indent
1208 600 seconds (10 minutes)
1212 1 hour 30 minutes (90 minutes)
1216 .It Pa /etc/ssh/sshd_config
1217 Contains configuration data for
1219 This file should be writable by root only, but it is recommended
1220 (though not necessary) that it be world-readable.
1225 OpenSSH is a derivative of the original and free
1226 ssh 1.2.12 release by Tatu Ylonen.
1227 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1228 Theo de Raadt and Dug Song
1229 removed many bugs, re-added newer features and
1231 Markus Friedl contributed the support for SSH
1232 protocol versions 1.5 and 2.0.
1233 Niels Provos and Markus Friedl contributed support
1234 for privilege separation.