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 GSSAPIRequireMIC
439 Specifies whether to permit authentication using GSS-API mechanisms
440 and/or contexts that do not support per-message integrity protection.
443 then the server will fail an otherwise valid gssapi-with-mic authentication
444 if per-message integrity protection is not supported.
447 Note that this option applies to protocol version 2 only.
448 .It Cm GSSAPIStrictAcceptorCheck
449 Determines whether to be strict about the identity of the GSSAPI acceptor
450 a client authenticates against. If
452 then the client must authenticate against the
454 service on the current hostname. If
456 then the client may authenticate against any service key stored in the
457 machine's default store. This facility is provided to assist with operation
458 on multi homed machines.
461 Note that this option applies only to protocol version 2 GSSAPI connections,
464 may only work with recent Kerberos GSSAPI libraries.
465 .It Cm GSSAPIStoreCredentialsOnRekey
466 Controls whether the user's GSSAPI credentials should be updated following a
467 successful connection rekeying. This option can be used to accepted renewed
468 or updated credentials from a compatible client. The default is
470 .It Cm HostbasedAuthentication
471 Specifies whether rhosts or /etc/hosts.equiv authentication together
472 with successful public key client host authentication is allowed
473 (host-based authentication).
474 This option is similar to
475 .Cm RhostsRSAAuthentication
476 and applies to protocol version 2 only.
479 .It Cm HostbasedUsesNameFromPacketOnly
480 Specifies whether or not the server will attempt to perform a reverse
481 name lookup when matching the name in the
487 .Cm HostbasedAuthentication .
492 uses the name supplied by the client rather than
493 attempting to resolve the name from the TCP connection itself.
496 .It Cm HostCertificate
497 Specifies a file containing a public host certificate.
498 The certificate's public key must match a private host key already specified
501 The default behaviour of
503 is not to load any certificates.
505 Specifies a file containing a private host key
508 .Pa /etc/ssh/ssh_host_key
509 for protocol version 1, and
510 .Pa /etc/ssh/ssh_host_dsa_key ,
511 .Pa /etc/ssh/ssh_host_ecdsa_key
513 .Pa /etc/ssh/ssh_host_rsa_key
514 for protocol version 2.
517 will refuse to use a file if it is group/world-accessible.
518 It is possible to have multiple host key files.
520 keys are used for version 1 and
525 are used for version 2 of the SSH protocol.
531 files will not be used in
532 .Cm RhostsRSAAuthentication
534 .Cm HostbasedAuthentication .
538 .Pa /etc/shosts.equiv
542 .It Cm IgnoreUserKnownHosts
545 should ignore the user's
546 .Pa ~/.ssh/known_hosts
548 .Cm RhostsRSAAuthentication
550 .Cm HostbasedAuthentication .
554 Specifies the IPv4 type-of-service or DSCP class for the connection.
581 This option may take one or two arguments, separated by whitespace.
582 If one argument is specified, it is used as the packet class unconditionally.
583 If two values are specified, the first is automatically selected for
584 interactive sessions and the second for non-interactive sessions.
587 for interactive sessions and
589 for non-interactive sessions.
590 .It Cm KerberosAuthentication
591 Specifies whether the password provided by the user for
592 .Cm PasswordAuthentication
593 will be validated through the Kerberos KDC.
594 To use this option, the server needs a
595 Kerberos servtab which allows the verification of the KDC's identity.
598 .It Cm KerberosGetAFSToken
599 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
600 an AFS token before accessing the user's home directory.
603 .It Cm KerberosOrLocalPasswd
604 If password authentication through Kerberos fails then
605 the password will be validated via any additional local mechanism
610 .It Cm KerberosTicketCleanup
611 Specifies whether to automatically destroy the user's ticket cache
616 Specifies the available KEX (Key Exchange) algorithms.
617 Multiple algorithms must be comma-separated.
619 .Dq ecdh-sha2-nistp256 ,
620 .Dq ecdh-sha2-nistp384 ,
621 .Dq ecdh-sha2-nistp521 ,
622 .Dq diffie-hellman-group-exchange-sha256 ,
623 .Dq diffie-hellman-group-exchange-sha1 ,
624 .Dq diffie-hellman-group14-sha1 ,
625 .Dq diffie-hellman-group1-sha1 .
626 .It Cm KeyRegenerationInterval
627 In protocol version 1, the ephemeral server key is automatically regenerated
628 after this many seconds (if it has been used).
629 The purpose of regeneration is to prevent
630 decrypting captured sessions by later breaking into the machine and
632 The key is never stored anywhere.
633 If the value is 0, the key is never regenerated.
634 The default is 3600 (seconds).
636 Specifies the local addresses
639 The following forms may be used:
641 .Bl -item -offset indent -compact
645 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
650 .Ar host No | Ar IPv4_addr No : Ar port
656 .Ar host No | Ar IPv6_addr Oc : Ar port
663 sshd will listen on the address and all prior
666 The default is to listen on all local addresses.
669 options are permitted.
672 options must precede this option for non-port qualified addresses.
673 .It Cm LoginGraceTime
674 The server disconnects after this time if the user has not
675 successfully logged in.
676 If the value is 0, there is no time limit.
677 The default is 120 seconds.
679 Gives the verbosity level that is used when logging messages from
681 The possible values are:
682 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
684 DEBUG and DEBUG1 are equivalent.
685 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
686 Logging with a DEBUG level violates the privacy of users and is not recommended.
688 Specifies the available MAC (message authentication code) algorithms.
689 The MAC algorithm is used in protocol version 2
690 for data integrity protection.
691 Multiple algorithms must be comma-separated.
693 .Bd -literal -offset indent
694 hmac-md5,hmac-sha1,umac-64@openssh.com,
695 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
698 Introduces a conditional block.
699 If all of the criteria on the
701 line are satisfied, the keywords on the following lines override those
702 set in the global section of the config file, until either another
704 line or the end of the file.
708 are one or more criteria-pattern pairs.
709 The available criteria are
715 The match patterns may consist of single entries or comma-separated
716 lists and may use the wildcard and negation operators described in the
723 criteria may additionally contain addresses to match in CIDR
724 address/masklen format, e.g.\&
728 Note that the mask length provided must be consistent with the address -
729 it is an error to specify a mask length that is too long for the address
730 or one with bits set in this host portion of the address.
737 Only a subset of keywords may be used on the lines following a
740 Available keywords are
741 .Cm AllowAgentForwarding ,
742 .Cm AllowTcpForwarding ,
743 .Cm AuthorizedKeysFile ,
744 .Cm AuthorizedPrincipalsFile ,
746 .Cm ChrootDirectory ,
749 .Cm GSSAPIAuthentication ,
750 .Cm HostbasedAuthentication ,
751 .Cm HostbasedUsesNameFromPacketOnly ,
752 .Cm KbdInteractiveAuthentication ,
753 .Cm KerberosAuthentication ,
756 .Cm PasswordAuthentication ,
757 .Cm PermitEmptyPasswords ,
759 .Cm PermitRootLogin ,
761 .Cm PubkeyAuthentication ,
762 .Cm RhostsRSAAuthentication ,
763 .Cm RSAAuthentication ,
764 .Cm X11DisplayOffset ,
767 .Cm X11UseLocalHost .
769 Specifies the maximum number of authentication attempts permitted per
771 Once the number of failures reaches half this value,
772 additional failures are logged.
775 Specifies the maximum number of open sessions permitted per network connection.
778 Specifies the maximum number of concurrent unauthenticated connections to the
780 Additional connections will be dropped until authentication succeeds or the
782 expires for a connection.
785 Alternatively, random early drop can be enabled by specifying
786 the three colon separated values
790 will refuse connection attempts with a probability of
793 if there are currently
796 unauthenticated connections.
797 The probability increases linearly and all connection attempts
798 are refused if the number of unauthenticated connections reaches
801 .It Cm PasswordAuthentication
802 Specifies whether password authentication is allowed.
805 .It Cm PermitEmptyPasswords
806 When password authentication is allowed, it specifies whether the
807 server allows login to accounts with empty password strings.
811 Specifies the destinations to which TCP port forwarding is permitted.
812 The forwarding specification must be one of the following forms:
814 .Bl -item -offset indent -compact
828 .Ar \&[ IPv6_addr \&] : port
832 Multiple forwards may be specified by separating them with whitespace.
835 can be used to remove all restrictions and permit any forwarding requests.
836 By default all port forwarding requests are permitted.
837 .It Cm PermitRootLogin
838 Specifies whether root can log in using
842 .Dq without-password ,
843 .Dq forced-commands-only ,
849 If this option is set to
850 .Dq without-password ,
851 password authentication is disabled for root.
853 If this option is set to
854 .Dq forced-commands-only ,
855 root login with public key authentication will be allowed,
858 option has been specified
859 (which may be useful for taking remote backups even if root login is
860 normally not allowed).
861 All other authentication methods are disabled for root.
863 If this option is set to
865 root is not allowed to log in.
869 device forwarding is allowed.
885 .It Cm PermitUserEnvironment
887 .Pa ~/.ssh/environment
891 .Pa ~/.ssh/authorized_keys
896 Enabling environment processing may enable users to bypass access
897 restrictions in some configurations using mechanisms such as
900 Specifies the file that contains the process ID of the
903 .Pa /var/run/sshd.pid .
905 Specifies the port number that
909 Multiple options of this type are permitted.
915 should print the date and time of the last user login when a user logs
924 when a user logs in interactively.
925 (On some systems it is also printed by the shell,
931 Specifies the protocol versions
934 The possible values are
938 Multiple versions must be comma-separated.
941 Note that the order of the protocol list does not indicate preference,
942 because the client selects among multiple protocol versions offered
948 .It Cm PubkeyAuthentication
949 Specifies whether public key authentication is allowed.
952 Note that this option applies to protocol version 2 only.
954 Specifies a list of revoked public keys.
955 Keys listed in this file will be refused for public key authentication.
956 Note that if this file is not readable, then public key authentication will
957 be refused for all users.
958 .It Cm RhostsRSAAuthentication
959 Specifies whether rhosts or /etc/hosts.equiv authentication together
960 with successful RSA host authentication is allowed.
963 This option applies to protocol version 1 only.
964 .It Cm RSAAuthentication
965 Specifies whether pure RSA authentication is allowed.
968 This option applies to protocol version 1 only.
970 Defines the number of bits in the ephemeral protocol version 1 server key.
971 The minimum value is 512, and the default is 1024.
975 should check file modes and ownership of the
976 user's files and home directory before accepting login.
977 This is normally desirable because novices sometimes accidentally leave their
978 directory or files world-writable.
981 Note that this does not apply to
982 .Cm ChrootDirectory ,
983 whose permissions and ownership are checked unconditionally.
985 Configures an external subsystem (e.g. file transfer daemon).
986 Arguments should be a subsystem name and a command (with optional arguments)
987 to execute upon subsystem request.
993 file transfer subsystem.
997 implements an in-process
1000 This may simplify configurations using
1002 to force a different filesystem root on clients.
1004 By default no subsystems are defined.
1005 Note that this option applies to protocol version 2 only.
1006 .It Cm SyslogFacility
1007 Gives the facility code that is used when logging messages from
1009 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1010 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1011 The default is AUTH.
1013 Specifies whether the system should send TCP keepalive messages to the
1015 If they are sent, death of the connection or crash of one
1016 of the machines will be properly noticed.
1017 However, this means that
1018 connections will die if the route is down temporarily, and some people
1020 On the other hand, if TCP keepalives are not sent,
1021 sessions may hang indefinitely on the server, leaving
1023 users and consuming server resources.
1027 (to send TCP keepalive messages), and the server will notice
1028 if the network goes down or the client host crashes.
1029 This avoids infinitely hanging sessions.
1031 To disable TCP keepalive messages, the value should be set to
1033 .It Cm TrustedUserCAKeys
1034 Specifies a file containing public keys of certificate authorities that are
1035 trusted to sign user certificates for authentication.
1036 Keys are listed one per line; empty lines and comments starting with
1039 If a certificate is presented for authentication and has its signing CA key
1040 listed in this file, then it may be used for authentication for any user
1041 listed in the certificate's principals list.
1042 Note that certificates that lack a list of principals will not be permitted
1043 for authentication using
1044 .Cm TrustedUserCAKeys .
1045 For more details on certificates, see the
1052 should look up the remote host name and check that
1053 the resolved host name for the remote IP address maps back to the
1054 very same IP address.
1060 is used for interactive login sessions.
1065 is never used for remote command execution.
1066 Note also, that if this is enabled,
1068 will be disabled because
1070 does not know how to handle
1074 .Cm UsePrivilegeSeparation
1075 is specified, it will be disabled after authentication.
1077 Enables the Pluggable Authentication Module interface.
1080 this will enable PAM authentication using
1081 .Cm ChallengeResponseAuthentication
1083 .Cm PasswordAuthentication
1084 in addition to PAM account and session module processing for all
1085 authentication types.
1087 Because PAM challenge-response authentication usually serves an equivalent
1088 role to password authentication, you should disable either
1089 .Cm PasswordAuthentication
1091 .Cm ChallengeResponseAuthentication.
1095 is enabled, you will not be able to run
1100 .It Cm UsePrivilegeSeparation
1103 separates privileges by creating an unprivileged child process
1104 to deal with incoming network traffic.
1105 After successful authentication, another process will be created that has
1106 the privilege of the authenticated user.
1107 The goal of privilege separation is to prevent privilege
1108 escalation by containing any corruption within the unprivileged processes.
1111 .It Cm X11DisplayOffset
1112 Specifies the first display number available for
1115 This prevents sshd from interfering with real X11 servers.
1117 .It Cm X11Forwarding
1118 Specifies whether X11 forwarding is permitted.
1119 The argument must be
1126 When X11 forwarding is enabled, there may be additional exposure to
1127 the server and to client displays if the
1129 proxy display is configured to listen on the wildcard address (see
1131 below), though this is not the default.
1132 Additionally, the authentication spoofing and authentication data
1133 verification and substitution occur on the client side.
1134 The security risk of using X11 forwarding is that the client's X11
1135 display server may be exposed to attack when the SSH client requests
1136 forwarding (see the warnings for
1139 .Xr ssh_config 5 ) .
1140 A system administrator may have a stance in which they want to
1141 protect clients that may expose themselves to attack by unwittingly
1142 requesting X11 forwarding, which can warrant a
1146 Note that disabling X11 forwarding does not prevent users from
1147 forwarding X11 traffic, as users can always install their own forwarders.
1148 X11 forwarding is automatically disabled if
1151 .It Cm X11UseLocalhost
1154 should bind the X11 forwarding server to the loopback address or to
1155 the wildcard address.
1157 sshd binds the forwarding server to the loopback address and sets the
1158 hostname part of the
1160 environment variable to
1162 This prevents remote hosts from connecting to the proxy display.
1163 However, some older X11 clients may not function with this
1168 to specify that the forwarding server should be bound to the wildcard
1170 The argument must be
1176 .It Cm XAuthLocation
1177 Specifies the full pathname of the
1181 .Pa /usr/X11R6/bin/xauth .
1185 command-line arguments and configuration file options that specify time
1186 may be expressed using a sequence of the form:
1188 .Ar time Op Ar qualifier ,
1192 is a positive integer value and
1194 is one of the following:
1196 .Bl -tag -width Ds -compact -offset indent
1211 Each member of the sequence is added together to calculate
1212 the total time value.
1214 Time format examples:
1216 .Bl -tag -width Ds -compact -offset indent
1218 600 seconds (10 minutes)
1222 1 hour 30 minutes (90 minutes)
1226 .It Pa /etc/ssh/sshd_config
1227 Contains configuration data for
1229 This file should be writable by root only, but it is recommended
1230 (though not necessary) that it be world-readable.
1235 OpenSSH is a derivative of the original and free
1236 ssh 1.2.12 release by Tatu Ylonen.
1237 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1238 Theo de Raadt and Dug Song
1239 removed many bugs, re-added newer features and
1241 Markus Friedl contributed the support for SSH
1242 protocol versions 1.5 and 2.0.
1243 Niels Provos and Markus Friedl contributed support
1244 for privilege separation.