Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.37
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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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.37 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.36 2004/09/15 03:25:41 jaredy 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
259: should bind remote port forwardings to the wildcard address,
260: thus allowing remote hosts to connect to forwarded ports.
261: The argument must be
262: .Dq yes
263: or
264: .Dq no .
265: The default is
266: .Dq no .
1.23 markus 267: .It Cm GSSAPIAuthentication
1.25 markus 268: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 269: The default is
1.23 markus 270: .Dq no .
271: Note that this option applies to protocol version 2 only.
272: .It Cm GSSAPICleanupCredentials
273: Specifies whether to automatically destroy the user's credentials cache
274: on logout.
275: The default is
276: .Dq yes .
277: Note that this option applies to protocol version 2 only.
1.1 stevesk 278: .It Cm HostbasedAuthentication
279: Specifies whether rhosts or /etc/hosts.equiv authentication together
280: with successful public key client host authentication is allowed
281: (hostbased authentication).
282: This option is similar to
283: .Cm RhostsRSAAuthentication
284: and applies to protocol version 2 only.
285: The default is
286: .Dq no .
287: .It Cm HostKey
288: Specifies a file containing a private host key
289: used by SSH.
290: The default is
291: .Pa /etc/ssh/ssh_host_key
292: for protocol version 1, and
293: .Pa /etc/ssh/ssh_host_rsa_key
294: and
295: .Pa /etc/ssh/ssh_host_dsa_key
296: for protocol version 2.
297: Note that
298: .Nm sshd
299: will refuse to use a file if it is group/world-accessible.
300: It is possible to have multiple host key files.
301: .Dq rsa1
302: keys are used for version 1 and
303: .Dq dsa
304: or
305: .Dq rsa
306: are used for version 2 of the SSH protocol.
307: .It Cm IgnoreRhosts
308: Specifies that
309: .Pa .rhosts
310: and
311: .Pa .shosts
312: files will not be used in
313: .Cm RhostsRSAAuthentication
314: or
315: .Cm HostbasedAuthentication .
316: .Pp
317: .Pa /etc/hosts.equiv
318: and
319: .Pa /etc/shosts.equiv
320: are still used.
321: The default is
322: .Dq yes .
323: .It Cm IgnoreUserKnownHosts
324: Specifies whether
325: .Nm sshd
326: should ignore the user's
327: .Pa $HOME/.ssh/known_hosts
328: during
329: .Cm RhostsRSAAuthentication
330: or
331: .Cm HostbasedAuthentication .
332: The default is
333: .Dq no .
334: .It Cm KerberosAuthentication
1.24 markus 335: Specifies whether the password provided by the user for
1.1 stevesk 336: .Cm PasswordAuthentication
1.24 markus 337: will be validated through the Kerberos KDC.
1.1 stevesk 338: To use this option, the server needs a
339: Kerberos servtab which allows the verification of the KDC's identity.
1.29 dtucker 340: Default is
341: .Dq no .
342: .It Cm KerberosGetAFSToken
343: If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
344: an AFS token before accessing the user's home directory.
1.1 stevesk 345: Default is
346: .Dq no .
347: .It Cm KerberosOrLocalPasswd
348: If set then if password authentication through Kerberos fails then
349: the password will be validated via any additional local mechanism
350: such as
351: .Pa /etc/passwd .
352: Default is
353: .Dq yes .
354: .It Cm KerberosTicketCleanup
355: Specifies whether to automatically destroy the user's ticket cache
356: file on logout.
357: Default is
358: .Dq yes .
359: .It Cm KeyRegenerationInterval
360: In protocol version 1, the ephemeral server key is automatically regenerated
361: after this many seconds (if it has been used).
362: The purpose of regeneration is to prevent
363: decrypting captured sessions by later breaking into the machine and
364: stealing the keys.
365: The key is never stored anywhere.
366: If the value is 0, the key is never regenerated.
367: The default is 3600 (seconds).
368: .It Cm ListenAddress
369: Specifies the local addresses
370: .Nm sshd
371: should listen on.
372: The following forms may be used:
373: .Pp
374: .Bl -item -offset indent -compact
375: .It
376: .Cm ListenAddress
377: .Sm off
378: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
379: .Sm on
380: .It
381: .Cm ListenAddress
382: .Sm off
383: .Ar host No | Ar IPv4_addr No : Ar port
384: .Sm on
385: .It
386: .Cm ListenAddress
387: .Sm off
388: .Oo
389: .Ar host No | Ar IPv6_addr Oc : Ar port
390: .Sm on
391: .El
392: .Pp
393: If
394: .Ar port
395: is not specified,
396: .Nm sshd
397: will listen on the address and all prior
398: .Cm Port
1.17 jmc 399: options specified.
400: The default is to listen on all local addresses.
1.15 jmc 401: Multiple
1.1 stevesk 402: .Cm ListenAddress
1.17 jmc 403: options are permitted.
404: Additionally, any
1.1 stevesk 405: .Cm Port
406: options must precede this option for non port qualified addresses.
407: .It Cm LoginGraceTime
408: The server disconnects after this time if the user has not
409: successfully logged in.
410: If the value is 0, there is no time limit.
1.12 stevesk 411: The default is 120 seconds.
1.1 stevesk 412: .It Cm LogLevel
413: Gives the verbosity level that is used when logging messages from
414: .Nm sshd .
415: The possible values are:
416: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.15 jmc 417: The default is INFO.
418: DEBUG and DEBUG1 are equivalent.
419: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
420: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 421: .It Cm MACs
422: Specifies the available MAC (message authentication code) algorithms.
423: The MAC algorithm is used in protocol version 2
424: for data integrity protection.
425: Multiple algorithms must be comma-separated.
426: The default is
427: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 428: .It Cm MaxAuthTries
429: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 430: connection.
431: Once the number of failures reaches half this value,
432: additional failures are logged.
433: The default is 6.
1.1 stevesk 434: .It Cm MaxStartups
435: Specifies the maximum number of concurrent unauthenticated connections to the
436: .Nm sshd
437: daemon.
438: Additional connections will be dropped until authentication succeeds or the
439: .Cm LoginGraceTime
440: expires for a connection.
441: The default is 10.
442: .Pp
443: Alternatively, random early drop can be enabled by specifying
444: the three colon separated values
445: .Dq start:rate:full
446: (e.g., "10:30:60").
447: .Nm sshd
448: will refuse connection attempts with a probability of
449: .Dq rate/100
450: (30%)
451: if there are currently
452: .Dq start
453: (10)
454: unauthenticated connections.
455: The probability increases linearly and all connection attempts
456: are refused if the number of unauthenticated connections reaches
457: .Dq full
458: (60).
459: .It Cm PasswordAuthentication
460: Specifies whether password authentication is allowed.
461: The default is
462: .Dq yes .
463: .It Cm PermitEmptyPasswords
464: When password authentication is allowed, it specifies whether the
465: server allows login to accounts with empty password strings.
466: The default is
467: .Dq no .
468: .It Cm PermitRootLogin
469: Specifies whether root can login using
470: .Xr ssh 1 .
471: The argument must be
472: .Dq yes ,
473: .Dq without-password ,
474: .Dq forced-commands-only
475: or
476: .Dq no .
477: The default is
478: .Dq yes .
479: .Pp
480: If this option is set to
481: .Dq without-password
482: password authentication is disabled for root.
483: .Pp
484: If this option is set to
485: .Dq forced-commands-only
486: root login with public key authentication will be allowed,
487: but only if the
488: .Ar command
489: option has been specified
490: (which may be useful for taking remote backups even if root login is
1.17 jmc 491: normally not allowed).
492: All other authentication methods are disabled for root.
1.1 stevesk 493: .Pp
494: If this option is set to
495: .Dq no
496: root is not allowed to login.
1.6 markus 497: .It Cm PermitUserEnvironment
498: Specifies whether
499: .Pa ~/.ssh/environment
1.9 stevesk 500: and
1.6 markus 501: .Cm environment=
502: options in
503: .Pa ~/.ssh/authorized_keys
1.9 stevesk 504: are processed by
505: .Nm sshd .
1.6 markus 506: The default is
507: .Dq no .
1.9 stevesk 508: Enabling environment processing may enable users to bypass access
509: restrictions in some configurations using mechanisms such as
510: .Ev LD_PRELOAD .
1.1 stevesk 511: .It Cm PidFile
1.4 stevesk 512: Specifies the file that contains the process ID of the
1.1 stevesk 513: .Nm sshd
514: daemon.
515: The default is
516: .Pa /var/run/sshd.pid .
517: .It Cm Port
518: Specifies the port number that
519: .Nm sshd
520: listens on.
521: The default is 22.
522: Multiple options of this type are permitted.
523: See also
524: .Cm ListenAddress .
525: .It Cm PrintLastLog
526: Specifies whether
527: .Nm sshd
1.36 jaredy 528: should print the date and time of the last user login when a user logs
529: in interactively.
1.1 stevesk 530: The default is
531: .Dq yes .
532: .It Cm PrintMotd
533: Specifies whether
534: .Nm sshd
535: should print
536: .Pa /etc/motd
537: when a user logs in interactively.
538: (On some systems it is also printed by the shell,
539: .Pa /etc/profile ,
540: or equivalent.)
541: The default is
542: .Dq yes .
543: .It Cm Protocol
544: Specifies the protocol versions
545: .Nm sshd
1.5 stevesk 546: supports.
1.1 stevesk 547: The possible values are
548: .Dq 1
549: and
550: .Dq 2 .
551: Multiple versions must be comma-separated.
552: The default is
553: .Dq 2,1 .
1.5 stevesk 554: Note that the order of the protocol list does not indicate preference,
555: because the client selects among multiple protocol versions offered
556: by the server.
557: Specifying
558: .Dq 2,1
559: is identical to
560: .Dq 1,2 .
1.1 stevesk 561: .It Cm PubkeyAuthentication
562: Specifies whether public key authentication is allowed.
563: The default is
564: .Dq yes .
565: Note that this option applies to protocol version 2 only.
566: .It Cm RhostsRSAAuthentication
567: Specifies whether rhosts or /etc/hosts.equiv authentication together
568: with successful RSA host authentication is allowed.
569: The default is
570: .Dq no .
571: This option applies to protocol version 1 only.
572: .It Cm RSAAuthentication
573: Specifies whether pure RSA authentication is allowed.
574: The default is
575: .Dq yes .
576: This option applies to protocol version 1 only.
577: .It Cm ServerKeyBits
578: Defines the number of bits in the ephemeral protocol version 1 server key.
579: The minimum value is 512, and the default is 768.
580: .It Cm StrictModes
581: Specifies whether
582: .Nm sshd
583: should check file modes and ownership of the
584: user's files and home directory before accepting login.
585: This is normally desirable because novices sometimes accidentally leave their
586: directory or files world-writable.
587: The default is
588: .Dq yes .
589: .It Cm Subsystem
590: Configures an external subsystem (e.g., file transfer daemon).
591: Arguments should be a subsystem name and a command to execute upon subsystem
592: request.
593: The command
594: .Xr sftp-server 8
595: implements the
596: .Dq sftp
597: file transfer subsystem.
598: By default no subsystems are defined.
599: Note that this option applies to protocol version 2 only.
600: .It Cm SyslogFacility
601: Gives the facility code that is used when logging messages from
602: .Nm sshd .
603: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
604: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
605: The default is AUTH.
1.27 markus 606: .It Cm TCPKeepAlive
607: Specifies whether the system should send TCP keepalive messages to the
608: other side.
609: If they are sent, death of the connection or crash of one
610: of the machines will be properly noticed.
611: However, this means that
612: connections will die if the route is down temporarily, and some people
613: find it annoying.
614: On the other hand, if TCP keepalives are not sent,
615: sessions may hang indefinitely on the server, leaving
616: .Dq ghost
617: users and consuming server resources.
618: .Pp
619: The default is
620: .Dq yes
621: (to send TCP keepalive messages), and the server will notice
622: if the network goes down or the client host crashes.
623: This avoids infinitely hanging sessions.
624: .Pp
625: To disable TCP keepalive messages, the value should be set to
626: .Dq no .
1.18 markus 627: .It Cm UseDNS
628: Specifies whether
629: .Nm sshd
630: should lookup the remote host name and check that
631: the resolved host name for the remote IP address maps back to the
632: very same IP address.
633: The default is
634: .Dq yes .
1.1 stevesk 635: .It Cm UseLogin
636: Specifies whether
637: .Xr login 1
638: is used for interactive login sessions.
639: The default is
640: .Dq no .
641: Note that
642: .Xr login 1
643: is never used for remote command execution.
644: Note also, that if this is enabled,
645: .Cm X11Forwarding
646: will be disabled because
647: .Xr login 1
648: does not know how to handle
649: .Xr xauth 1
1.15 jmc 650: cookies.
651: If
1.1 stevesk 652: .Cm UsePrivilegeSeparation
653: is specified, it will be disabled after authentication.
654: .It Cm UsePrivilegeSeparation
655: Specifies whether
656: .Nm sshd
1.2 stevesk 657: separates privileges by creating an unprivileged child process
1.15 jmc 658: to deal with incoming network traffic.
659: After successful authentication, another process will be created that has
660: the privilege of the authenticated user.
661: The goal of privilege separation is to prevent privilege
1.1 stevesk 662: escalation by containing any corruption within the unprivileged processes.
663: The default is
664: .Dq yes .
665: .It Cm X11DisplayOffset
666: Specifies the first display number available for
667: .Nm sshd Ns 's
668: X11 forwarding.
669: This prevents
670: .Nm sshd
671: from interfering with real X11 servers.
672: The default is 10.
673: .It Cm X11Forwarding
674: Specifies whether X11 forwarding is permitted.
1.13 stevesk 675: The argument must be
676: .Dq yes
677: or
678: .Dq no .
1.1 stevesk 679: The default is
680: .Dq no .
1.13 stevesk 681: .Pp
682: When X11 forwarding is enabled, there may be additional exposure to
683: the server and to client displays if the
684: .Nm sshd
685: proxy display is configured to listen on the wildcard address (see
686: .Cm X11UseLocalhost
687: below), however this is not the default.
688: Additionally, the authentication spoofing and authentication data
689: verification and substitution occur on the client side.
690: The security risk of using X11 forwarding is that the client's X11
691: display server may be exposed to attack when the ssh client requests
692: forwarding (see the warnings for
693: .Cm ForwardX11
694: in
1.19 jmc 695: .Xr ssh_config 5 ) .
1.13 stevesk 696: A system administrator may have a stance in which they want to
697: protect clients that may expose themselves to attack by unwittingly
698: requesting X11 forwarding, which can warrant a
699: .Dq no
700: setting.
701: .Pp
702: Note that disabling X11 forwarding does not prevent users from
703: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 704: X11 forwarding is automatically disabled if
705: .Cm UseLogin
706: is enabled.
707: .It Cm X11UseLocalhost
708: Specifies whether
709: .Nm sshd
710: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 711: the wildcard address.
712: By default,
1.1 stevesk 713: .Nm sshd
714: binds the forwarding server to the loopback address and sets the
715: hostname part of the
716: .Ev DISPLAY
717: environment variable to
718: .Dq localhost .
1.8 stevesk 719: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 720: However, some older X11 clients may not function with this
721: configuration.
722: .Cm X11UseLocalhost
723: may be set to
724: .Dq no
725: to specify that the forwarding server should be bound to the wildcard
726: address.
727: The argument must be
728: .Dq yes
729: or
730: .Dq no .
731: The default is
732: .Dq yes .
733: .It Cm XAuthLocation
1.11 stevesk 734: Specifies the full pathname of the
1.1 stevesk 735: .Xr xauth 1
736: program.
737: The default is
738: .Pa /usr/X11R6/bin/xauth .
739: .El
740: .Ss Time Formats
741: .Nm sshd
742: command-line arguments and configuration file options that specify time
743: may be expressed using a sequence of the form:
744: .Sm off
1.7 stevesk 745: .Ar time Op Ar qualifier ,
1.1 stevesk 746: .Sm on
747: where
748: .Ar time
749: is a positive integer value and
750: .Ar qualifier
751: is one of the following:
752: .Pp
753: .Bl -tag -width Ds -compact -offset indent
754: .It Cm <none>
755: seconds
756: .It Cm s | Cm S
757: seconds
758: .It Cm m | Cm M
759: minutes
760: .It Cm h | Cm H
761: hours
762: .It Cm d | Cm D
763: days
764: .It Cm w | Cm W
765: weeks
766: .El
767: .Pp
768: Each member of the sequence is added together to calculate
769: the total time value.
770: .Pp
771: Time format examples:
772: .Pp
773: .Bl -tag -width Ds -compact -offset indent
774: .It 600
775: 600 seconds (10 minutes)
776: .It 10m
777: 10 minutes
778: .It 1h30m
779: 1 hour 30 minutes (90 minutes)
780: .El
781: .Sh FILES
782: .Bl -tag -width Ds
783: .It Pa /etc/ssh/sshd_config
784: Contains configuration data for
785: .Nm sshd .
786: This file should be writable by root only, but it is recommended
787: (though not necessary) that it be world-readable.
788: .El
1.19 jmc 789: .Sh SEE ALSO
790: .Xr sshd 8
1.1 stevesk 791: .Sh AUTHORS
792: OpenSSH is a derivative of the original and free
793: ssh 1.2.12 release by Tatu Ylonen.
794: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
795: Theo de Raadt and Dug Song
796: removed many bugs, re-added newer features and
797: created OpenSSH.
798: Markus Friedl contributed the support for SSH
799: protocol versions 1.5 and 2.0.
800: Niels Provos and Markus Friedl contributed support
801: for privilege separation.
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