Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.40
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
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".
12: .\"
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.
16: .\"
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18: .\" modification, are permitted provided that the following conditions
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.40 ! jmc 37: .\" $OpenBSD: sshd_config.5,v 1.39 2005/03/01 10:09:52 djm Exp $
1.1 stevesk 38: .Dd September 25, 1999
39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
45: .Bl -tag -width Ds -compact
46: .It Pa /etc/ssh/sshd_config
47: .El
48: .Sh DESCRIPTION
49: .Nm sshd
50: reads configuration data from
51: .Pa /etc/ssh/sshd_config
52: (or the file specified with
53: .Fl f
54: on the command line).
55: The file contains keyword-argument pairs, one per line.
56: Lines starting with
57: .Ql #
58: and empty lines are interpreted as comments.
59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
75: .Ql \&*
76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
88: .Nm sshd .
89: Valid arguments are
90: .Dq any ,
91: .Dq inet
92: (use IPv4 only) or
93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.1 stevesk 97: .It Cm AllowGroups
98: This keyword can be followed by a list of group name patterns, separated
99: by spaces.
100: If specified, login is allowed only for users whose primary
101: group or supplementary group list matches one of the patterns.
102: .Ql \&*
103: and
1.16 mouring 104: .Ql \&?
1.1 stevesk 105: can be used as
106: wildcards in the patterns.
107: Only group names are valid; a numerical group ID is not recognized.
108: By default, login is allowed for all groups.
109: .It Cm AllowTcpForwarding
110: Specifies whether TCP forwarding is permitted.
111: The default is
112: .Dq yes .
113: Note that disabling TCP forwarding does not improve security unless
114: users are also denied shell access, as they can always install their
115: own forwarders.
116: .It Cm AllowUsers
117: This keyword can be followed by a list of user name patterns, separated
118: by spaces.
1.14 jmc 119: If specified, login is allowed only for user names that
1.1 stevesk 120: match one of the patterns.
121: .Ql \&*
122: and
1.16 mouring 123: .Ql \&?
1.1 stevesk 124: can be used as
125: wildcards in the patterns.
126: Only user names are valid; a numerical user ID is not recognized.
127: By default, login is allowed for all users.
128: If the pattern takes the form USER@HOST then USER and HOST
129: are separately checked, restricting logins to particular
130: users from particular hosts.
131: .It Cm AuthorizedKeysFile
132: Specifies the file that contains the public keys that can be used
133: for user authentication.
134: .Cm AuthorizedKeysFile
135: may contain tokens of the form %T which are substituted during connection
1.17 jmc 136: set-up.
137: The following tokens are defined: %% is replaced by a literal '%',
1.1 stevesk 138: %h is replaced by the home directory of the user being authenticated and
139: %u is replaced by the username of that user.
140: After expansion,
141: .Cm AuthorizedKeysFile
142: is taken to be an absolute path or one relative to the user's home
143: directory.
144: The default is
145: .Dq .ssh/authorized_keys .
146: .It Cm Banner
147: In some jurisdictions, sending a warning message before authentication
148: may be relevant for getting legal protection.
149: The contents of the specified file are sent to the remote user before
150: authentication is allowed.
151: This option is only available for protocol version 2.
152: By default, no banner is displayed.
153: .It Cm ChallengeResponseAuthentication
154: Specifies whether challenge response authentication is allowed.
155: All authentication styles from
156: .Xr login.conf 5
157: are supported.
158: The default is
159: .Dq yes .
160: .It Cm Ciphers
161: Specifies the ciphers allowed for protocol version 2.
162: Multiple ciphers must be comma-separated.
1.34 dtucker 163: The supported ciphers are
164: .Dq 3des-cbc ,
165: .Dq aes128-cbc ,
166: .Dq aes192-cbc ,
167: .Dq aes256-cbc ,
168: .Dq aes128-ctr ,
169: .Dq aes192-ctr ,
170: .Dq aes256-ctr ,
171: .Dq arcfour ,
172: .Dq blowfish-cbc ,
173: and
174: .Dq cast128-cbc .
1.1 stevesk 175: The default is
176: .Bd -literal
177: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.20 djm 178: aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
1.1 stevesk 179: .Ed
180: .It Cm ClientAliveInterval
181: Sets a timeout interval in seconds after which if no data has been received
182: from the client,
183: .Nm sshd
184: will send a message through the encrypted
185: channel to request a response from the client.
186: The default
187: is 0, indicating that these messages will not be sent to the client.
188: This option applies to protocol version 2 only.
189: .It Cm ClientAliveCountMax
190: Sets the number of client alive messages (see above) which may be
191: sent without
192: .Nm sshd
1.17 jmc 193: receiving any messages back from the client.
194: If this threshold is reached while client alive messages are being sent,
1.1 stevesk 195: .Nm sshd
1.17 jmc 196: will disconnect the client, terminating the session.
197: It is important to note that the use of client alive messages is very
198: different from
1.27 markus 199: .Cm TCPKeepAlive
1.17 jmc 200: (below).
201: The client alive messages are sent through the encrypted channel
202: and therefore will not be spoofable.
203: The TCP keepalive option enabled by
1.27 markus 204: .Cm TCPKeepAlive
1.17 jmc 205: is spoofable.
206: The client alive mechanism is valuable when the client or
1.1 stevesk 207: server depend on knowing when a connection has become inactive.
208: .Pp
1.17 jmc 209: The default value is 3.
210: If
1.1 stevesk 211: .Cm ClientAliveInterval
212: (above) is set to 15, and
213: .Cm ClientAliveCountMax
214: is left at the default, unresponsive ssh clients
215: will be disconnected after approximately 45 seconds.
1.3 markus 216: .It Cm Compression
217: Specifies whether compression is allowed.
218: The argument must be
219: .Dq yes
220: or
221: .Dq no .
222: The default is
223: .Dq yes .
1.1 stevesk 224: .It Cm DenyGroups
225: This keyword can be followed by a list of group name patterns, separated
226: by spaces.
227: Login is disallowed for users whose primary group or supplementary
228: group list matches one of the patterns.
229: .Ql \&*
230: and
1.16 mouring 231: .Ql \&?
1.1 stevesk 232: can be used as
233: wildcards in the patterns.
234: Only group names are valid; a numerical group ID is not recognized.
235: By default, login is allowed for all groups.
236: .It Cm DenyUsers
237: This keyword can be followed by a list of user name patterns, separated
238: by spaces.
239: Login is disallowed for user names that match one of the patterns.
240: .Ql \&*
241: and
1.16 mouring 242: .Ql \&?
1.1 stevesk 243: can be used as wildcards in the patterns.
244: Only user names are valid; a numerical user ID is not recognized.
245: By default, login is allowed for all users.
246: If the pattern takes the form USER@HOST then USER and HOST
247: are separately checked, restricting logins to particular
248: users from particular hosts.
249: .It Cm GatewayPorts
250: Specifies whether remote hosts are allowed to connect to ports
251: forwarded for the client.
252: By default,
253: .Nm sshd
1.15 jmc 254: binds remote port forwardings to the loopback address.
255: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 256: .Cm GatewayPorts
257: can be used to specify that
258: .Nm sshd
1.39 djm 259: should allow remote port forwardings to bind to non-loopback addresses, thus
260: allowing other hosts to connect.
261: The argument may be
262: .Dq no
263: to force remote port forwardings to be available to the local host only,
1.1 stevesk 264: .Dq yes
1.39 djm 265: to force remote port forwardings to bind to the wildcard address, or
266: .Dq clientspecified
267: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 268: The default is
269: .Dq no .
1.23 markus 270: .It Cm GSSAPIAuthentication
1.25 markus 271: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 272: The default is
1.23 markus 273: .Dq no .
274: Note that this option applies to protocol version 2 only.
275: .It Cm GSSAPICleanupCredentials
276: Specifies whether to automatically destroy the user's credentials cache
277: on logout.
278: The default is
279: .Dq yes .
280: Note that this option applies to protocol version 2 only.
1.1 stevesk 281: .It Cm HostbasedAuthentication
282: Specifies whether rhosts or /etc/hosts.equiv authentication together
283: with successful public key client host authentication is allowed
284: (hostbased authentication).
285: This option is similar to
286: .Cm RhostsRSAAuthentication
287: and applies to protocol version 2 only.
288: The default is
289: .Dq no .
290: .It Cm HostKey
291: Specifies a file containing a private host key
292: used by SSH.
293: The default is
294: .Pa /etc/ssh/ssh_host_key
295: for protocol version 1, and
296: .Pa /etc/ssh/ssh_host_rsa_key
297: and
298: .Pa /etc/ssh/ssh_host_dsa_key
299: for protocol version 2.
300: Note that
301: .Nm sshd
302: will refuse to use a file if it is group/world-accessible.
303: It is possible to have multiple host key files.
304: .Dq rsa1
305: keys are used for version 1 and
306: .Dq dsa
307: or
308: .Dq rsa
309: are used for version 2 of the SSH protocol.
310: .It Cm IgnoreRhosts
311: Specifies that
312: .Pa .rhosts
313: and
314: .Pa .shosts
315: files will not be used in
316: .Cm RhostsRSAAuthentication
317: or
318: .Cm HostbasedAuthentication .
319: .Pp
320: .Pa /etc/hosts.equiv
321: and
322: .Pa /etc/shosts.equiv
323: are still used.
324: The default is
325: .Dq yes .
326: .It Cm IgnoreUserKnownHosts
327: Specifies whether
328: .Nm sshd
329: should ignore the user's
330: .Pa $HOME/.ssh/known_hosts
331: during
332: .Cm RhostsRSAAuthentication
333: or
334: .Cm HostbasedAuthentication .
335: The default is
336: .Dq no .
337: .It Cm KerberosAuthentication
1.24 markus 338: Specifies whether the password provided by the user for
1.1 stevesk 339: .Cm PasswordAuthentication
1.24 markus 340: will be validated through the Kerberos KDC.
1.1 stevesk 341: To use this option, the server needs a
342: Kerberos servtab which allows the verification of the KDC's identity.
1.29 dtucker 343: Default is
344: .Dq no .
345: .It Cm KerberosGetAFSToken
346: If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
347: an AFS token before accessing the user's home directory.
1.1 stevesk 348: Default is
349: .Dq no .
350: .It Cm KerberosOrLocalPasswd
351: If set then if password authentication through Kerberos fails then
352: the password will be validated via any additional local mechanism
353: such as
354: .Pa /etc/passwd .
355: Default is
356: .Dq yes .
357: .It Cm KerberosTicketCleanup
358: Specifies whether to automatically destroy the user's ticket cache
359: file on logout.
360: Default is
361: .Dq yes .
362: .It Cm KeyRegenerationInterval
363: In protocol version 1, the ephemeral server key is automatically regenerated
364: after this many seconds (if it has been used).
365: The purpose of regeneration is to prevent
366: decrypting captured sessions by later breaking into the machine and
367: stealing the keys.
368: The key is never stored anywhere.
369: If the value is 0, the key is never regenerated.
370: The default is 3600 (seconds).
371: .It Cm ListenAddress
372: Specifies the local addresses
373: .Nm sshd
374: should listen on.
375: The following forms may be used:
376: .Pp
377: .Bl -item -offset indent -compact
378: .It
379: .Cm ListenAddress
380: .Sm off
381: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
382: .Sm on
383: .It
384: .Cm ListenAddress
385: .Sm off
386: .Ar host No | Ar IPv4_addr No : Ar port
387: .Sm on
388: .It
389: .Cm ListenAddress
390: .Sm off
391: .Oo
392: .Ar host No | Ar IPv6_addr Oc : Ar port
393: .Sm on
394: .El
395: .Pp
396: If
397: .Ar port
398: is not specified,
399: .Nm sshd
400: will listen on the address and all prior
401: .Cm Port
1.17 jmc 402: options specified.
403: The default is to listen on all local addresses.
1.15 jmc 404: Multiple
1.1 stevesk 405: .Cm ListenAddress
1.17 jmc 406: options are permitted.
407: Additionally, any
1.1 stevesk 408: .Cm Port
409: options must precede this option for non port qualified addresses.
410: .It Cm LoginGraceTime
411: The server disconnects after this time if the user has not
412: successfully logged in.
413: If the value is 0, there is no time limit.
1.12 stevesk 414: The default is 120 seconds.
1.1 stevesk 415: .It Cm LogLevel
416: Gives the verbosity level that is used when logging messages from
417: .Nm sshd .
418: The possible values are:
419: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.15 jmc 420: The default is INFO.
421: DEBUG and DEBUG1 are equivalent.
422: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
423: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 424: .It Cm MACs
425: Specifies the available MAC (message authentication code) algorithms.
426: The MAC algorithm is used in protocol version 2
427: for data integrity protection.
428: Multiple algorithms must be comma-separated.
429: The default is
430: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 431: .It Cm MaxAuthTries
432: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 433: connection.
434: Once the number of failures reaches half this value,
435: additional failures are logged.
436: The default is 6.
1.1 stevesk 437: .It Cm MaxStartups
438: Specifies the maximum number of concurrent unauthenticated connections to the
439: .Nm sshd
440: daemon.
441: Additional connections will be dropped until authentication succeeds or the
442: .Cm LoginGraceTime
443: expires for a connection.
444: The default is 10.
445: .Pp
446: Alternatively, random early drop can be enabled by specifying
447: the three colon separated values
448: .Dq start:rate:full
449: (e.g., "10:30:60").
450: .Nm sshd
451: will refuse connection attempts with a probability of
452: .Dq rate/100
453: (30%)
454: if there are currently
455: .Dq start
456: (10)
457: unauthenticated connections.
458: The probability increases linearly and all connection attempts
459: are refused if the number of unauthenticated connections reaches
460: .Dq full
461: (60).
462: .It Cm PasswordAuthentication
463: Specifies whether password authentication is allowed.
464: The default is
465: .Dq yes .
466: .It Cm PermitEmptyPasswords
467: When password authentication is allowed, it specifies whether the
468: server allows login to accounts with empty password strings.
469: The default is
470: .Dq no .
471: .It Cm PermitRootLogin
1.38 jmc 472: Specifies whether root can log in using
1.1 stevesk 473: .Xr ssh 1 .
474: The argument must be
475: .Dq yes ,
476: .Dq without-password ,
477: .Dq forced-commands-only
478: or
479: .Dq no .
480: The default is
481: .Dq yes .
482: .Pp
483: If this option is set to
484: .Dq without-password
485: password authentication is disabled for root.
486: .Pp
487: If this option is set to
488: .Dq forced-commands-only
489: root login with public key authentication will be allowed,
490: but only if the
491: .Ar command
492: option has been specified
493: (which may be useful for taking remote backups even if root login is
1.17 jmc 494: normally not allowed).
495: All other authentication methods are disabled for root.
1.1 stevesk 496: .Pp
497: If this option is set to
498: .Dq no
1.38 jmc 499: root is not allowed to log in.
1.6 markus 500: .It Cm PermitUserEnvironment
501: Specifies whether
502: .Pa ~/.ssh/environment
1.9 stevesk 503: and
1.6 markus 504: .Cm environment=
505: options in
506: .Pa ~/.ssh/authorized_keys
1.9 stevesk 507: are processed by
508: .Nm sshd .
1.6 markus 509: The default is
510: .Dq no .
1.9 stevesk 511: Enabling environment processing may enable users to bypass access
512: restrictions in some configurations using mechanisms such as
513: .Ev LD_PRELOAD .
1.1 stevesk 514: .It Cm PidFile
1.4 stevesk 515: Specifies the file that contains the process ID of the
1.1 stevesk 516: .Nm sshd
517: daemon.
518: The default is
519: .Pa /var/run/sshd.pid .
520: .It Cm Port
521: Specifies the port number that
522: .Nm sshd
523: listens on.
524: The default is 22.
525: Multiple options of this type are permitted.
526: See also
527: .Cm ListenAddress .
528: .It Cm PrintLastLog
529: Specifies whether
530: .Nm sshd
1.36 jaredy 531: should print the date and time of the last user login when a user logs
532: in interactively.
1.1 stevesk 533: The default is
534: .Dq yes .
535: .It Cm PrintMotd
536: Specifies whether
537: .Nm sshd
538: should print
539: .Pa /etc/motd
540: when a user logs in interactively.
541: (On some systems it is also printed by the shell,
542: .Pa /etc/profile ,
543: or equivalent.)
544: The default is
545: .Dq yes .
546: .It Cm Protocol
547: Specifies the protocol versions
548: .Nm sshd
1.5 stevesk 549: supports.
1.1 stevesk 550: The possible values are
551: .Dq 1
552: and
553: .Dq 2 .
554: Multiple versions must be comma-separated.
555: The default is
556: .Dq 2,1 .
1.5 stevesk 557: Note that the order of the protocol list does not indicate preference,
558: because the client selects among multiple protocol versions offered
559: by the server.
560: Specifying
561: .Dq 2,1
562: is identical to
563: .Dq 1,2 .
1.1 stevesk 564: .It Cm PubkeyAuthentication
565: Specifies whether public key authentication is allowed.
566: The default is
567: .Dq yes .
568: Note that this option applies to protocol version 2 only.
569: .It Cm RhostsRSAAuthentication
570: Specifies whether rhosts or /etc/hosts.equiv authentication together
571: with successful RSA host authentication is allowed.
572: The default is
573: .Dq no .
574: This option applies to protocol version 1 only.
575: .It Cm RSAAuthentication
576: Specifies whether pure RSA authentication is allowed.
577: The default is
578: .Dq yes .
579: This option applies to protocol version 1 only.
580: .It Cm ServerKeyBits
581: Defines the number of bits in the ephemeral protocol version 1 server key.
582: The minimum value is 512, and the default is 768.
583: .It Cm StrictModes
584: Specifies whether
585: .Nm sshd
586: should check file modes and ownership of the
587: user's files and home directory before accepting login.
588: This is normally desirable because novices sometimes accidentally leave their
589: directory or files world-writable.
590: The default is
591: .Dq yes .
592: .It Cm Subsystem
593: Configures an external subsystem (e.g., file transfer daemon).
594: Arguments should be a subsystem name and a command to execute upon subsystem
595: request.
596: The command
597: .Xr sftp-server 8
598: implements the
599: .Dq sftp
600: file transfer subsystem.
601: By default no subsystems are defined.
602: Note that this option applies to protocol version 2 only.
603: .It Cm SyslogFacility
604: Gives the facility code that is used when logging messages from
605: .Nm sshd .
606: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
607: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
608: The default is AUTH.
1.27 markus 609: .It Cm TCPKeepAlive
610: Specifies whether the system should send TCP keepalive messages to the
611: other side.
612: If they are sent, death of the connection or crash of one
613: of the machines will be properly noticed.
614: However, this means that
615: connections will die if the route is down temporarily, and some people
616: find it annoying.
617: On the other hand, if TCP keepalives are not sent,
618: sessions may hang indefinitely on the server, leaving
619: .Dq ghost
620: users and consuming server resources.
621: .Pp
622: The default is
623: .Dq yes
624: (to send TCP keepalive messages), and the server will notice
625: if the network goes down or the client host crashes.
626: This avoids infinitely hanging sessions.
627: .Pp
628: To disable TCP keepalive messages, the value should be set to
629: .Dq no .
1.18 markus 630: .It Cm UseDNS
631: Specifies whether
632: .Nm sshd
1.40 ! jmc 633: should look up the remote host name and check that
1.18 markus 634: the resolved host name for the remote IP address maps back to the
635: very same IP address.
636: The default is
637: .Dq yes .
1.1 stevesk 638: .It Cm UseLogin
639: Specifies whether
640: .Xr login 1
641: is used for interactive login sessions.
642: The default is
643: .Dq no .
644: Note that
645: .Xr login 1
646: is never used for remote command execution.
647: Note also, that if this is enabled,
648: .Cm X11Forwarding
649: will be disabled because
650: .Xr login 1
651: does not know how to handle
652: .Xr xauth 1
1.15 jmc 653: cookies.
654: If
1.1 stevesk 655: .Cm UsePrivilegeSeparation
656: is specified, it will be disabled after authentication.
657: .It Cm UsePrivilegeSeparation
658: Specifies whether
659: .Nm sshd
1.2 stevesk 660: separates privileges by creating an unprivileged child process
1.15 jmc 661: to deal with incoming network traffic.
662: After successful authentication, another process will be created that has
663: the privilege of the authenticated user.
664: The goal of privilege separation is to prevent privilege
1.1 stevesk 665: escalation by containing any corruption within the unprivileged processes.
666: The default is
667: .Dq yes .
668: .It Cm X11DisplayOffset
669: Specifies the first display number available for
670: .Nm sshd Ns 's
671: X11 forwarding.
672: This prevents
673: .Nm sshd
674: from interfering with real X11 servers.
675: The default is 10.
676: .It Cm X11Forwarding
677: Specifies whether X11 forwarding is permitted.
1.13 stevesk 678: The argument must be
679: .Dq yes
680: or
681: .Dq no .
1.1 stevesk 682: The default is
683: .Dq no .
1.13 stevesk 684: .Pp
685: When X11 forwarding is enabled, there may be additional exposure to
686: the server and to client displays if the
687: .Nm sshd
688: proxy display is configured to listen on the wildcard address (see
689: .Cm X11UseLocalhost
690: below), however this is not the default.
691: Additionally, the authentication spoofing and authentication data
692: verification and substitution occur on the client side.
693: The security risk of using X11 forwarding is that the client's X11
694: display server may be exposed to attack when the ssh client requests
695: forwarding (see the warnings for
696: .Cm ForwardX11
697: in
1.19 jmc 698: .Xr ssh_config 5 ) .
1.13 stevesk 699: A system administrator may have a stance in which they want to
700: protect clients that may expose themselves to attack by unwittingly
701: requesting X11 forwarding, which can warrant a
702: .Dq no
703: setting.
704: .Pp
705: Note that disabling X11 forwarding does not prevent users from
706: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 707: X11 forwarding is automatically disabled if
708: .Cm UseLogin
709: is enabled.
710: .It Cm X11UseLocalhost
711: Specifies whether
712: .Nm sshd
713: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 714: the wildcard address.
715: By default,
1.1 stevesk 716: .Nm sshd
717: binds the forwarding server to the loopback address and sets the
718: hostname part of the
719: .Ev DISPLAY
720: environment variable to
721: .Dq localhost .
1.8 stevesk 722: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 723: However, some older X11 clients may not function with this
724: configuration.
725: .Cm X11UseLocalhost
726: may be set to
727: .Dq no
728: to specify that the forwarding server should be bound to the wildcard
729: address.
730: The argument must be
731: .Dq yes
732: or
733: .Dq no .
734: The default is
735: .Dq yes .
736: .It Cm XAuthLocation
1.11 stevesk 737: Specifies the full pathname of the
1.1 stevesk 738: .Xr xauth 1
739: program.
740: The default is
741: .Pa /usr/X11R6/bin/xauth .
742: .El
743: .Ss Time Formats
744: .Nm sshd
745: command-line arguments and configuration file options that specify time
746: may be expressed using a sequence of the form:
747: .Sm off
1.7 stevesk 748: .Ar time Op Ar qualifier ,
1.1 stevesk 749: .Sm on
750: where
751: .Ar time
752: is a positive integer value and
753: .Ar qualifier
754: is one of the following:
755: .Pp
756: .Bl -tag -width Ds -compact -offset indent
757: .It Cm <none>
758: seconds
759: .It Cm s | Cm S
760: seconds
761: .It Cm m | Cm M
762: minutes
763: .It Cm h | Cm H
764: hours
765: .It Cm d | Cm D
766: days
767: .It Cm w | Cm W
768: weeks
769: .El
770: .Pp
771: Each member of the sequence is added together to calculate
772: the total time value.
773: .Pp
774: Time format examples:
775: .Pp
776: .Bl -tag -width Ds -compact -offset indent
777: .It 600
778: 600 seconds (10 minutes)
779: .It 10m
780: 10 minutes
781: .It 1h30m
782: 1 hour 30 minutes (90 minutes)
783: .El
784: .Sh FILES
785: .Bl -tag -width Ds
786: .It Pa /etc/ssh/sshd_config
787: Contains configuration data for
788: .Nm sshd .
789: This file should be writable by root only, but it is recommended
790: (though not necessary) that it be world-readable.
791: .El
1.19 jmc 792: .Sh SEE ALSO
793: .Xr sshd 8
1.1 stevesk 794: .Sh AUTHORS
795: OpenSSH is a derivative of the original and free
796: ssh 1.2.12 release by Tatu Ylonen.
797: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
798: Theo de Raadt and Dug Song
799: removed many bugs, re-added newer features and
800: created OpenSSH.
801: Markus Friedl contributed the support for SSH
802: protocol versions 1.5 and 2.0.
803: Niels Provos and Markus Friedl contributed support
804: for privilege separation.
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