Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.43
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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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.43 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.42 2005/05/19 02:39:55 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 ,
1.43 ! djm 171: .Dq arcfour128 ,
! 172: .Dq arcfour256 ,
1.34 dtucker 173: .Dq arcfour ,
174: .Dq blowfish-cbc ,
175: and
176: .Dq cast128-cbc .
1.1 stevesk 177: The default is
178: .Bd -literal
1.43 ! djm 179: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
! 180: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
! 181: aes192-ctr,aes256-ctr''
1.1 stevesk 182: .Ed
183: .It Cm ClientAliveCountMax
184: Sets the number of client alive messages (see above) which may be
185: sent without
186: .Nm sshd
1.17 jmc 187: receiving any messages back from the client.
188: If this threshold is reached while client alive messages are being sent,
1.1 stevesk 189: .Nm sshd
1.17 jmc 190: will disconnect the client, terminating the session.
191: It is important to note that the use of client alive messages is very
192: different from
1.27 markus 193: .Cm TCPKeepAlive
1.17 jmc 194: (below).
195: The client alive messages are sent through the encrypted channel
196: and therefore will not be spoofable.
197: The TCP keepalive option enabled by
1.27 markus 198: .Cm TCPKeepAlive
1.17 jmc 199: is spoofable.
200: The client alive mechanism is valuable when the client or
1.1 stevesk 201: server depend on knowing when a connection has become inactive.
202: .Pp
1.17 jmc 203: The default value is 3.
204: If
1.1 stevesk 205: .Cm ClientAliveInterval
206: (above) is set to 15, and
207: .Cm ClientAliveCountMax
208: is left at the default, unresponsive ssh clients
209: will be disconnected after approximately 45 seconds.
1.42 djm 210: .It Cm ClientAliveInterval
211: Sets a timeout interval in seconds after which if no data has been received
212: from the client,
213: .Nm sshd
214: will send a message through the encrypted
215: channel to request a response from the client.
216: The default
217: is 0, indicating that these messages will not be sent to the client.
218: This option applies to protocol version 2 only.
1.3 markus 219: .It Cm Compression
220: Specifies whether compression is allowed.
221: The argument must be
222: .Dq yes
223: or
224: .Dq no .
225: The default is
226: .Dq yes .
1.1 stevesk 227: .It Cm DenyGroups
228: This keyword can be followed by a list of group name patterns, separated
229: by spaces.
230: Login is disallowed for users whose primary group or supplementary
231: group list matches one of the patterns.
232: .Ql \&*
233: and
1.16 mouring 234: .Ql \&?
1.1 stevesk 235: can be used as
236: wildcards in the patterns.
237: Only group names are valid; a numerical group ID is not recognized.
238: By default, login is allowed for all groups.
239: .It Cm DenyUsers
240: This keyword can be followed by a list of user name patterns, separated
241: by spaces.
242: Login is disallowed for user names that match one of the patterns.
243: .Ql \&*
244: and
1.16 mouring 245: .Ql \&?
1.1 stevesk 246: can be used as wildcards in the patterns.
247: Only user names are valid; a numerical user ID is not recognized.
248: By default, login is allowed for all users.
249: If the pattern takes the form USER@HOST then USER and HOST
250: are separately checked, restricting logins to particular
251: users from particular hosts.
252: .It Cm GatewayPorts
253: Specifies whether remote hosts are allowed to connect to ports
254: forwarded for the client.
255: By default,
256: .Nm sshd
1.15 jmc 257: binds remote port forwardings to the loopback address.
258: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 259: .Cm GatewayPorts
260: can be used to specify that
261: .Nm sshd
1.39 djm 262: should allow remote port forwardings to bind to non-loopback addresses, thus
263: allowing other hosts to connect.
264: The argument may be
265: .Dq no
266: to force remote port forwardings to be available to the local host only,
1.1 stevesk 267: .Dq yes
1.39 djm 268: to force remote port forwardings to bind to the wildcard address, or
269: .Dq clientspecified
270: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 271: The default is
272: .Dq no .
1.23 markus 273: .It Cm GSSAPIAuthentication
1.25 markus 274: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 275: The default is
1.23 markus 276: .Dq no .
277: Note that this option applies to protocol version 2 only.
278: .It Cm GSSAPICleanupCredentials
279: Specifies whether to automatically destroy the user's credentials cache
280: on logout.
281: The default is
282: .Dq yes .
283: Note that this option applies to protocol version 2 only.
1.1 stevesk 284: .It Cm HostbasedAuthentication
285: Specifies whether rhosts or /etc/hosts.equiv authentication together
286: with successful public key client host authentication is allowed
287: (hostbased authentication).
288: This option is similar to
289: .Cm RhostsRSAAuthentication
290: and applies to protocol version 2 only.
291: The default is
292: .Dq no .
293: .It Cm HostKey
294: Specifies a file containing a private host key
295: used by SSH.
296: The default is
297: .Pa /etc/ssh/ssh_host_key
298: for protocol version 1, and
299: .Pa /etc/ssh/ssh_host_rsa_key
300: and
301: .Pa /etc/ssh/ssh_host_dsa_key
302: for protocol version 2.
303: Note that
304: .Nm sshd
305: will refuse to use a file if it is group/world-accessible.
306: It is possible to have multiple host key files.
307: .Dq rsa1
308: keys are used for version 1 and
309: .Dq dsa
310: or
311: .Dq rsa
312: are used for version 2 of the SSH protocol.
313: .It Cm IgnoreRhosts
314: Specifies that
315: .Pa .rhosts
316: and
317: .Pa .shosts
318: files will not be used in
319: .Cm RhostsRSAAuthentication
320: or
321: .Cm HostbasedAuthentication .
322: .Pp
323: .Pa /etc/hosts.equiv
324: and
325: .Pa /etc/shosts.equiv
326: are still used.
327: The default is
328: .Dq yes .
329: .It Cm IgnoreUserKnownHosts
330: Specifies whether
331: .Nm sshd
332: should ignore the user's
1.41 djm 333: .Pa ~/.ssh/known_hosts
1.1 stevesk 334: during
335: .Cm RhostsRSAAuthentication
336: or
337: .Cm HostbasedAuthentication .
338: The default is
339: .Dq no .
340: .It Cm KerberosAuthentication
1.24 markus 341: Specifies whether the password provided by the user for
1.1 stevesk 342: .Cm PasswordAuthentication
1.24 markus 343: will be validated through the Kerberos KDC.
1.1 stevesk 344: To use this option, the server needs a
345: Kerberos servtab which allows the verification of the KDC's identity.
1.29 dtucker 346: Default is
347: .Dq no .
348: .It Cm KerberosGetAFSToken
349: If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
350: an AFS token before accessing the user's home directory.
1.1 stevesk 351: Default is
352: .Dq no .
353: .It Cm KerberosOrLocalPasswd
354: If set then if password authentication through Kerberos fails then
355: the password will be validated via any additional local mechanism
356: such as
357: .Pa /etc/passwd .
358: Default is
359: .Dq yes .
360: .It Cm KerberosTicketCleanup
361: Specifies whether to automatically destroy the user's ticket cache
362: file on logout.
363: Default is
364: .Dq yes .
365: .It Cm KeyRegenerationInterval
366: In protocol version 1, the ephemeral server key is automatically regenerated
367: after this many seconds (if it has been used).
368: The purpose of regeneration is to prevent
369: decrypting captured sessions by later breaking into the machine and
370: stealing the keys.
371: The key is never stored anywhere.
372: If the value is 0, the key is never regenerated.
373: The default is 3600 (seconds).
374: .It Cm ListenAddress
375: Specifies the local addresses
376: .Nm sshd
377: should listen on.
378: The following forms may be used:
379: .Pp
380: .Bl -item -offset indent -compact
381: .It
382: .Cm ListenAddress
383: .Sm off
384: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
385: .Sm on
386: .It
387: .Cm ListenAddress
388: .Sm off
389: .Ar host No | Ar IPv4_addr No : Ar port
390: .Sm on
391: .It
392: .Cm ListenAddress
393: .Sm off
394: .Oo
395: .Ar host No | Ar IPv6_addr Oc : Ar port
396: .Sm on
397: .El
398: .Pp
399: If
400: .Ar port
401: is not specified,
402: .Nm sshd
403: will listen on the address and all prior
404: .Cm Port
1.17 jmc 405: options specified.
406: The default is to listen on all local addresses.
1.15 jmc 407: Multiple
1.1 stevesk 408: .Cm ListenAddress
1.17 jmc 409: options are permitted.
410: Additionally, any
1.1 stevesk 411: .Cm Port
412: options must precede this option for non port qualified addresses.
413: .It Cm LoginGraceTime
414: The server disconnects after this time if the user has not
415: successfully logged in.
416: If the value is 0, there is no time limit.
1.12 stevesk 417: The default is 120 seconds.
1.1 stevesk 418: .It Cm LogLevel
419: Gives the verbosity level that is used when logging messages from
420: .Nm sshd .
421: The possible values are:
422: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.15 jmc 423: The default is INFO.
424: DEBUG and DEBUG1 are equivalent.
425: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
426: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 427: .It Cm MACs
428: Specifies the available MAC (message authentication code) algorithms.
429: The MAC algorithm is used in protocol version 2
430: for data integrity protection.
431: Multiple algorithms must be comma-separated.
432: The default is
433: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 434: .It Cm MaxAuthTries
435: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 436: connection.
437: Once the number of failures reaches half this value,
438: additional failures are logged.
439: The default is 6.
1.1 stevesk 440: .It Cm MaxStartups
441: Specifies the maximum number of concurrent unauthenticated connections to the
442: .Nm sshd
443: daemon.
444: Additional connections will be dropped until authentication succeeds or the
445: .Cm LoginGraceTime
446: expires for a connection.
447: The default is 10.
448: .Pp
449: Alternatively, random early drop can be enabled by specifying
450: the three colon separated values
451: .Dq start:rate:full
452: (e.g., "10:30:60").
453: .Nm sshd
454: will refuse connection attempts with a probability of
455: .Dq rate/100
456: (30%)
457: if there are currently
458: .Dq start
459: (10)
460: unauthenticated connections.
461: The probability increases linearly and all connection attempts
462: are refused if the number of unauthenticated connections reaches
463: .Dq full
464: (60).
465: .It Cm PasswordAuthentication
466: Specifies whether password authentication is allowed.
467: The default is
468: .Dq yes .
469: .It Cm PermitEmptyPasswords
470: When password authentication is allowed, it specifies whether the
471: server allows login to accounts with empty password strings.
472: The default is
473: .Dq no .
474: .It Cm PermitRootLogin
1.38 jmc 475: Specifies whether root can log in using
1.1 stevesk 476: .Xr ssh 1 .
477: The argument must be
478: .Dq yes ,
479: .Dq without-password ,
480: .Dq forced-commands-only
481: or
482: .Dq no .
483: The default is
484: .Dq yes .
485: .Pp
486: If this option is set to
487: .Dq without-password
488: password authentication is disabled for root.
489: .Pp
490: If this option is set to
491: .Dq forced-commands-only
492: root login with public key authentication will be allowed,
493: but only if the
494: .Ar command
495: option has been specified
496: (which may be useful for taking remote backups even if root login is
1.17 jmc 497: normally not allowed).
498: All other authentication methods are disabled for root.
1.1 stevesk 499: .Pp
500: If this option is set to
501: .Dq no
1.38 jmc 502: root is not allowed to log in.
1.6 markus 503: .It Cm PermitUserEnvironment
504: Specifies whether
505: .Pa ~/.ssh/environment
1.9 stevesk 506: and
1.6 markus 507: .Cm environment=
508: options in
509: .Pa ~/.ssh/authorized_keys
1.9 stevesk 510: are processed by
511: .Nm sshd .
1.6 markus 512: The default is
513: .Dq no .
1.9 stevesk 514: Enabling environment processing may enable users to bypass access
515: restrictions in some configurations using mechanisms such as
516: .Ev LD_PRELOAD .
1.1 stevesk 517: .It Cm PidFile
1.4 stevesk 518: Specifies the file that contains the process ID of the
1.1 stevesk 519: .Nm sshd
520: daemon.
521: The default is
522: .Pa /var/run/sshd.pid .
523: .It Cm Port
524: Specifies the port number that
525: .Nm sshd
526: listens on.
527: The default is 22.
528: Multiple options of this type are permitted.
529: See also
530: .Cm ListenAddress .
531: .It Cm PrintLastLog
532: Specifies whether
533: .Nm sshd
1.36 jaredy 534: should print the date and time of the last user login when a user logs
535: in interactively.
1.1 stevesk 536: The default is
537: .Dq yes .
538: .It Cm PrintMotd
539: Specifies whether
540: .Nm sshd
541: should print
542: .Pa /etc/motd
543: when a user logs in interactively.
544: (On some systems it is also printed by the shell,
545: .Pa /etc/profile ,
546: or equivalent.)
547: The default is
548: .Dq yes .
549: .It Cm Protocol
550: Specifies the protocol versions
551: .Nm sshd
1.5 stevesk 552: supports.
1.1 stevesk 553: The possible values are
554: .Dq 1
555: and
556: .Dq 2 .
557: Multiple versions must be comma-separated.
558: The default is
559: .Dq 2,1 .
1.5 stevesk 560: Note that the order of the protocol list does not indicate preference,
561: because the client selects among multiple protocol versions offered
562: by the server.
563: Specifying
564: .Dq 2,1
565: is identical to
566: .Dq 1,2 .
1.1 stevesk 567: .It Cm PubkeyAuthentication
568: Specifies whether public key authentication is allowed.
569: The default is
570: .Dq yes .
571: Note that this option applies to protocol version 2 only.
572: .It Cm RhostsRSAAuthentication
573: Specifies whether rhosts or /etc/hosts.equiv authentication together
574: with successful RSA host authentication is allowed.
575: The default is
576: .Dq no .
577: This option applies to protocol version 1 only.
578: .It Cm RSAAuthentication
579: Specifies whether pure RSA authentication is allowed.
580: The default is
581: .Dq yes .
582: This option applies to protocol version 1 only.
583: .It Cm ServerKeyBits
584: Defines the number of bits in the ephemeral protocol version 1 server key.
585: The minimum value is 512, and the default is 768.
586: .It Cm StrictModes
587: Specifies whether
588: .Nm sshd
589: should check file modes and ownership of the
590: user's files and home directory before accepting login.
591: This is normally desirable because novices sometimes accidentally leave their
592: directory or files world-writable.
593: The default is
594: .Dq yes .
595: .It Cm Subsystem
596: Configures an external subsystem (e.g., file transfer daemon).
597: Arguments should be a subsystem name and a command to execute upon subsystem
598: request.
599: The command
600: .Xr sftp-server 8
601: implements the
602: .Dq sftp
603: file transfer subsystem.
604: By default no subsystems are defined.
605: Note that this option applies to protocol version 2 only.
606: .It Cm SyslogFacility
607: Gives the facility code that is used when logging messages from
608: .Nm sshd .
609: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
610: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
611: The default is AUTH.
1.27 markus 612: .It Cm TCPKeepAlive
613: Specifies whether the system should send TCP keepalive messages to the
614: other side.
615: If they are sent, death of the connection or crash of one
616: of the machines will be properly noticed.
617: However, this means that
618: connections will die if the route is down temporarily, and some people
619: find it annoying.
620: On the other hand, if TCP keepalives are not sent,
621: sessions may hang indefinitely on the server, leaving
622: .Dq ghost
623: users and consuming server resources.
624: .Pp
625: The default is
626: .Dq yes
627: (to send TCP keepalive messages), and the server will notice
628: if the network goes down or the client host crashes.
629: This avoids infinitely hanging sessions.
630: .Pp
631: To disable TCP keepalive messages, the value should be set to
632: .Dq no .
1.18 markus 633: .It Cm UseDNS
634: Specifies whether
635: .Nm sshd
1.40 jmc 636: should look up the remote host name and check that
1.18 markus 637: the resolved host name for the remote IP address maps back to the
638: very same IP address.
639: The default is
640: .Dq yes .
1.1 stevesk 641: .It Cm UseLogin
642: Specifies whether
643: .Xr login 1
644: is used for interactive login sessions.
645: The default is
646: .Dq no .
647: Note that
648: .Xr login 1
649: is never used for remote command execution.
650: Note also, that if this is enabled,
651: .Cm X11Forwarding
652: will be disabled because
653: .Xr login 1
654: does not know how to handle
655: .Xr xauth 1
1.15 jmc 656: cookies.
657: If
1.1 stevesk 658: .Cm UsePrivilegeSeparation
659: is specified, it will be disabled after authentication.
660: .It Cm UsePrivilegeSeparation
661: Specifies whether
662: .Nm sshd
1.2 stevesk 663: separates privileges by creating an unprivileged child process
1.15 jmc 664: to deal with incoming network traffic.
665: After successful authentication, another process will be created that has
666: the privilege of the authenticated user.
667: The goal of privilege separation is to prevent privilege
1.1 stevesk 668: escalation by containing any corruption within the unprivileged processes.
669: The default is
670: .Dq yes .
671: .It Cm X11DisplayOffset
672: Specifies the first display number available for
673: .Nm sshd Ns 's
674: X11 forwarding.
675: This prevents
676: .Nm sshd
677: from interfering with real X11 servers.
678: The default is 10.
679: .It Cm X11Forwarding
680: Specifies whether X11 forwarding is permitted.
1.13 stevesk 681: The argument must be
682: .Dq yes
683: or
684: .Dq no .
1.1 stevesk 685: The default is
686: .Dq no .
1.13 stevesk 687: .Pp
688: When X11 forwarding is enabled, there may be additional exposure to
689: the server and to client displays if the
690: .Nm sshd
691: proxy display is configured to listen on the wildcard address (see
692: .Cm X11UseLocalhost
693: below), however this is not the default.
694: Additionally, the authentication spoofing and authentication data
695: verification and substitution occur on the client side.
696: The security risk of using X11 forwarding is that the client's X11
697: display server may be exposed to attack when the ssh client requests
698: forwarding (see the warnings for
699: .Cm ForwardX11
700: in
1.19 jmc 701: .Xr ssh_config 5 ) .
1.13 stevesk 702: A system administrator may have a stance in which they want to
703: protect clients that may expose themselves to attack by unwittingly
704: requesting X11 forwarding, which can warrant a
705: .Dq no
706: setting.
707: .Pp
708: Note that disabling X11 forwarding does not prevent users from
709: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 710: X11 forwarding is automatically disabled if
711: .Cm UseLogin
712: is enabled.
713: .It Cm X11UseLocalhost
714: Specifies whether
715: .Nm sshd
716: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 717: the wildcard address.
718: By default,
1.1 stevesk 719: .Nm sshd
720: binds the forwarding server to the loopback address and sets the
721: hostname part of the
722: .Ev DISPLAY
723: environment variable to
724: .Dq localhost .
1.8 stevesk 725: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 726: However, some older X11 clients may not function with this
727: configuration.
728: .Cm X11UseLocalhost
729: may be set to
730: .Dq no
731: to specify that the forwarding server should be bound to the wildcard
732: address.
733: The argument must be
734: .Dq yes
735: or
736: .Dq no .
737: The default is
738: .Dq yes .
739: .It Cm XAuthLocation
1.11 stevesk 740: Specifies the full pathname of the
1.1 stevesk 741: .Xr xauth 1
742: program.
743: The default is
744: .Pa /usr/X11R6/bin/xauth .
745: .El
746: .Ss Time Formats
747: .Nm sshd
748: command-line arguments and configuration file options that specify time
749: may be expressed using a sequence of the form:
750: .Sm off
1.7 stevesk 751: .Ar time Op Ar qualifier ,
1.1 stevesk 752: .Sm on
753: where
754: .Ar time
755: is a positive integer value and
756: .Ar qualifier
757: is one of the following:
758: .Pp
759: .Bl -tag -width Ds -compact -offset indent
760: .It Cm <none>
761: seconds
762: .It Cm s | Cm S
763: seconds
764: .It Cm m | Cm M
765: minutes
766: .It Cm h | Cm H
767: hours
768: .It Cm d | Cm D
769: days
770: .It Cm w | Cm W
771: weeks
772: .El
773: .Pp
774: Each member of the sequence is added together to calculate
775: the total time value.
776: .Pp
777: Time format examples:
778: .Pp
779: .Bl -tag -width Ds -compact -offset indent
780: .It 600
781: 600 seconds (10 minutes)
782: .It 10m
783: 10 minutes
784: .It 1h30m
785: 1 hour 30 minutes (90 minutes)
786: .El
787: .Sh FILES
788: .Bl -tag -width Ds
789: .It Pa /etc/ssh/sshd_config
790: Contains configuration data for
791: .Nm sshd .
792: This file should be writable by root only, but it is recommended
793: (though not necessary) that it be world-readable.
794: .El
1.19 jmc 795: .Sh SEE ALSO
796: .Xr sshd 8
1.1 stevesk 797: .Sh AUTHORS
798: OpenSSH is a derivative of the original and free
799: ssh 1.2.12 release by Tatu Ylonen.
800: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
801: Theo de Raadt and Dug Song
802: removed many bugs, re-added newer features and
803: created OpenSSH.
804: Markus Friedl contributed the support for SSH
805: protocol versions 1.5 and 2.0.
806: Niels Provos and Markus Friedl contributed support
807: for privilege separation.
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