Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.40.2.1
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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1.40.2.1! brad 37: .\" $OpenBSD: sshd_config.5,v 1.44 2005/07/25 11:59:40 markus 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 ,
1.40.2.1! brad 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
1.40.2.1! brad 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.
1.40.2.1! brad 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
1.40.2.1! brad 220: Specifies whether compression is allowed, or delayed until
! 221: the user has authenticated successfully.
1.3 markus 222: The argument must be
1.40.2.1! brad 223: .Dq yes ,
! 224: .Dq delayed ,
1.3 markus 225: or
226: .Dq no .
227: The default is
1.40.2.1! brad 228: .Dq delayed .
1.1 stevesk 229: .It Cm DenyGroups
230: This keyword can be followed by a list of group name patterns, separated
231: by spaces.
232: Login is disallowed for users whose primary group or supplementary
233: group list matches one of the patterns.
234: .Ql \&*
235: and
1.16 mouring 236: .Ql \&?
1.1 stevesk 237: can be used as
238: wildcards in the patterns.
239: Only group names are valid; a numerical group ID is not recognized.
240: By default, login is allowed for all groups.
241: .It Cm DenyUsers
242: This keyword can be followed by a list of user name patterns, separated
243: by spaces.
244: Login is disallowed for user names that match one of the patterns.
245: .Ql \&*
246: and
1.16 mouring 247: .Ql \&?
1.1 stevesk 248: can be used as wildcards in the patterns.
249: Only user names are valid; a numerical user ID is not recognized.
250: By default, login is allowed for all users.
251: If the pattern takes the form USER@HOST then USER and HOST
252: are separately checked, restricting logins to particular
253: users from particular hosts.
254: .It Cm GatewayPorts
255: Specifies whether remote hosts are allowed to connect to ports
256: forwarded for the client.
257: By default,
258: .Nm sshd
1.15 jmc 259: binds remote port forwardings to the loopback address.
260: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 261: .Cm GatewayPorts
262: can be used to specify that
263: .Nm sshd
1.39 djm 264: should allow remote port forwardings to bind to non-loopback addresses, thus
265: allowing other hosts to connect.
266: The argument may be
267: .Dq no
268: to force remote port forwardings to be available to the local host only,
1.1 stevesk 269: .Dq yes
1.39 djm 270: to force remote port forwardings to bind to the wildcard address, or
271: .Dq clientspecified
272: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 273: The default is
274: .Dq no .
1.23 markus 275: .It Cm GSSAPIAuthentication
1.25 markus 276: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 277: The default is
1.23 markus 278: .Dq no .
279: Note that this option applies to protocol version 2 only.
280: .It Cm GSSAPICleanupCredentials
281: Specifies whether to automatically destroy the user's credentials cache
282: on logout.
283: The default is
284: .Dq yes .
285: Note that this option applies to protocol version 2 only.
1.1 stevesk 286: .It Cm HostbasedAuthentication
287: Specifies whether rhosts or /etc/hosts.equiv authentication together
288: with successful public key client host authentication is allowed
289: (hostbased authentication).
290: This option is similar to
291: .Cm RhostsRSAAuthentication
292: and applies to protocol version 2 only.
293: The default is
294: .Dq no .
295: .It Cm HostKey
296: Specifies a file containing a private host key
297: used by SSH.
298: The default is
299: .Pa /etc/ssh/ssh_host_key
300: for protocol version 1, and
301: .Pa /etc/ssh/ssh_host_rsa_key
302: and
303: .Pa /etc/ssh/ssh_host_dsa_key
304: for protocol version 2.
305: Note that
306: .Nm sshd
307: will refuse to use a file if it is group/world-accessible.
308: It is possible to have multiple host key files.
309: .Dq rsa1
310: keys are used for version 1 and
311: .Dq dsa
312: or
313: .Dq rsa
314: are used for version 2 of the SSH protocol.
315: .It Cm IgnoreRhosts
316: Specifies that
317: .Pa .rhosts
318: and
319: .Pa .shosts
320: files will not be used in
321: .Cm RhostsRSAAuthentication
322: or
323: .Cm HostbasedAuthentication .
324: .Pp
325: .Pa /etc/hosts.equiv
326: and
327: .Pa /etc/shosts.equiv
328: are still used.
329: The default is
330: .Dq yes .
331: .It Cm IgnoreUserKnownHosts
332: Specifies whether
333: .Nm sshd
334: should ignore the user's
1.40.2.1! brad 335: .Pa ~/.ssh/known_hosts
1.1 stevesk 336: during
337: .Cm RhostsRSAAuthentication
338: or
339: .Cm HostbasedAuthentication .
340: The default is
341: .Dq no .
342: .It Cm KerberosAuthentication
1.24 markus 343: Specifies whether the password provided by the user for
1.1 stevesk 344: .Cm PasswordAuthentication
1.24 markus 345: will be validated through the Kerberos KDC.
1.1 stevesk 346: To use this option, the server needs a
347: Kerberos servtab which allows the verification of the KDC's identity.
1.29 dtucker 348: Default is
349: .Dq no .
350: .It Cm KerberosGetAFSToken
351: If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
352: an AFS token before accessing the user's home directory.
1.1 stevesk 353: Default is
354: .Dq no .
355: .It Cm KerberosOrLocalPasswd
356: If set then if password authentication through Kerberos fails then
357: the password will be validated via any additional local mechanism
358: such as
359: .Pa /etc/passwd .
360: Default is
361: .Dq yes .
362: .It Cm KerberosTicketCleanup
363: Specifies whether to automatically destroy the user's ticket cache
364: file on logout.
365: Default is
366: .Dq yes .
367: .It Cm KeyRegenerationInterval
368: In protocol version 1, the ephemeral server key is automatically regenerated
369: after this many seconds (if it has been used).
370: The purpose of regeneration is to prevent
371: decrypting captured sessions by later breaking into the machine and
372: stealing the keys.
373: The key is never stored anywhere.
374: If the value is 0, the key is never regenerated.
375: The default is 3600 (seconds).
376: .It Cm ListenAddress
377: Specifies the local addresses
378: .Nm sshd
379: should listen on.
380: The following forms may be used:
381: .Pp
382: .Bl -item -offset indent -compact
383: .It
384: .Cm ListenAddress
385: .Sm off
386: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
387: .Sm on
388: .It
389: .Cm ListenAddress
390: .Sm off
391: .Ar host No | Ar IPv4_addr No : Ar port
392: .Sm on
393: .It
394: .Cm ListenAddress
395: .Sm off
396: .Oo
397: .Ar host No | Ar IPv6_addr Oc : Ar port
398: .Sm on
399: .El
400: .Pp
401: If
402: .Ar port
403: is not specified,
404: .Nm sshd
405: will listen on the address and all prior
406: .Cm Port
1.17 jmc 407: options specified.
408: The default is to listen on all local addresses.
1.15 jmc 409: Multiple
1.1 stevesk 410: .Cm ListenAddress
1.17 jmc 411: options are permitted.
412: Additionally, any
1.1 stevesk 413: .Cm Port
414: options must precede this option for non port qualified addresses.
415: .It Cm LoginGraceTime
416: The server disconnects after this time if the user has not
417: successfully logged in.
418: If the value is 0, there is no time limit.
1.12 stevesk 419: The default is 120 seconds.
1.1 stevesk 420: .It Cm LogLevel
421: Gives the verbosity level that is used when logging messages from
422: .Nm sshd .
423: The possible values are:
424: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
1.15 jmc 425: The default is INFO.
426: DEBUG and DEBUG1 are equivalent.
427: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
428: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 429: .It Cm MACs
430: Specifies the available MAC (message authentication code) algorithms.
431: The MAC algorithm is used in protocol version 2
432: for data integrity protection.
433: Multiple algorithms must be comma-separated.
434: The default is
435: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 436: .It Cm MaxAuthTries
437: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 438: connection.
439: Once the number of failures reaches half this value,
440: additional failures are logged.
441: The default is 6.
1.1 stevesk 442: .It Cm MaxStartups
443: Specifies the maximum number of concurrent unauthenticated connections to the
444: .Nm sshd
445: daemon.
446: Additional connections will be dropped until authentication succeeds or the
447: .Cm LoginGraceTime
448: expires for a connection.
449: The default is 10.
450: .Pp
451: Alternatively, random early drop can be enabled by specifying
452: the three colon separated values
453: .Dq start:rate:full
454: (e.g., "10:30:60").
455: .Nm sshd
456: will refuse connection attempts with a probability of
457: .Dq rate/100
458: (30%)
459: if there are currently
460: .Dq start
461: (10)
462: unauthenticated connections.
463: The probability increases linearly and all connection attempts
464: are refused if the number of unauthenticated connections reaches
465: .Dq full
466: (60).
467: .It Cm PasswordAuthentication
468: Specifies whether password authentication is allowed.
469: The default is
470: .Dq yes .
471: .It Cm PermitEmptyPasswords
472: When password authentication is allowed, it specifies whether the
473: server allows login to accounts with empty password strings.
474: The default is
475: .Dq no .
476: .It Cm PermitRootLogin
1.38 jmc 477: Specifies whether root can log in using
1.1 stevesk 478: .Xr ssh 1 .
479: The argument must be
480: .Dq yes ,
481: .Dq without-password ,
482: .Dq forced-commands-only
483: or
484: .Dq no .
485: The default is
486: .Dq yes .
487: .Pp
488: If this option is set to
489: .Dq without-password
490: password authentication is disabled for root.
491: .Pp
492: If this option is set to
493: .Dq forced-commands-only
494: root login with public key authentication will be allowed,
495: but only if the
496: .Ar command
497: option has been specified
498: (which may be useful for taking remote backups even if root login is
1.17 jmc 499: normally not allowed).
500: All other authentication methods are disabled for root.
1.1 stevesk 501: .Pp
502: If this option is set to
503: .Dq no
1.38 jmc 504: root is not allowed to log in.
1.6 markus 505: .It Cm PermitUserEnvironment
506: Specifies whether
507: .Pa ~/.ssh/environment
1.9 stevesk 508: and
1.6 markus 509: .Cm environment=
510: options in
511: .Pa ~/.ssh/authorized_keys
1.9 stevesk 512: are processed by
513: .Nm sshd .
1.6 markus 514: The default is
515: .Dq no .
1.9 stevesk 516: Enabling environment processing may enable users to bypass access
517: restrictions in some configurations using mechanisms such as
518: .Ev LD_PRELOAD .
1.1 stevesk 519: .It Cm PidFile
1.4 stevesk 520: Specifies the file that contains the process ID of the
1.1 stevesk 521: .Nm sshd
522: daemon.
523: The default is
524: .Pa /var/run/sshd.pid .
525: .It Cm Port
526: Specifies the port number that
527: .Nm sshd
528: listens on.
529: The default is 22.
530: Multiple options of this type are permitted.
531: See also
532: .Cm ListenAddress .
533: .It Cm PrintLastLog
534: Specifies whether
535: .Nm sshd
1.36 jaredy 536: should print the date and time of the last user login when a user logs
537: in interactively.
1.1 stevesk 538: The default is
539: .Dq yes .
540: .It Cm PrintMotd
541: Specifies whether
542: .Nm sshd
543: should print
544: .Pa /etc/motd
545: when a user logs in interactively.
546: (On some systems it is also printed by the shell,
547: .Pa /etc/profile ,
548: or equivalent.)
549: The default is
550: .Dq yes .
551: .It Cm Protocol
552: Specifies the protocol versions
553: .Nm sshd
1.5 stevesk 554: supports.
1.1 stevesk 555: The possible values are
556: .Dq 1
557: and
558: .Dq 2 .
559: Multiple versions must be comma-separated.
560: The default is
561: .Dq 2,1 .
1.5 stevesk 562: Note that the order of the protocol list does not indicate preference,
563: because the client selects among multiple protocol versions offered
564: by the server.
565: Specifying
566: .Dq 2,1
567: is identical to
568: .Dq 1,2 .
1.1 stevesk 569: .It Cm PubkeyAuthentication
570: Specifies whether public key authentication is allowed.
571: The default is
572: .Dq yes .
573: Note that this option applies to protocol version 2 only.
574: .It Cm RhostsRSAAuthentication
575: Specifies whether rhosts or /etc/hosts.equiv authentication together
576: with successful RSA host authentication is allowed.
577: The default is
578: .Dq no .
579: This option applies to protocol version 1 only.
580: .It Cm RSAAuthentication
581: Specifies whether pure RSA authentication is allowed.
582: The default is
583: .Dq yes .
584: This option applies to protocol version 1 only.
585: .It Cm ServerKeyBits
586: Defines the number of bits in the ephemeral protocol version 1 server key.
587: The minimum value is 512, and the default is 768.
588: .It Cm StrictModes
589: Specifies whether
590: .Nm sshd
591: should check file modes and ownership of the
592: user's files and home directory before accepting login.
593: This is normally desirable because novices sometimes accidentally leave their
594: directory or files world-writable.
595: The default is
596: .Dq yes .
597: .It Cm Subsystem
598: Configures an external subsystem (e.g., file transfer daemon).
599: Arguments should be a subsystem name and a command to execute upon subsystem
600: request.
601: The command
602: .Xr sftp-server 8
603: implements the
604: .Dq sftp
605: file transfer subsystem.
606: By default no subsystems are defined.
607: Note that this option applies to protocol version 2 only.
608: .It Cm SyslogFacility
609: Gives the facility code that is used when logging messages from
610: .Nm sshd .
611: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
612: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
613: The default is AUTH.
1.27 markus 614: .It Cm TCPKeepAlive
615: Specifies whether the system should send TCP keepalive messages to the
616: other side.
617: If they are sent, death of the connection or crash of one
618: of the machines will be properly noticed.
619: However, this means that
620: connections will die if the route is down temporarily, and some people
621: find it annoying.
622: On the other hand, if TCP keepalives are not sent,
623: sessions may hang indefinitely on the server, leaving
624: .Dq ghost
625: users and consuming server resources.
626: .Pp
627: The default is
628: .Dq yes
629: (to send TCP keepalive messages), and the server will notice
630: if the network goes down or the client host crashes.
631: This avoids infinitely hanging sessions.
632: .Pp
633: To disable TCP keepalive messages, the value should be set to
634: .Dq no .
1.18 markus 635: .It Cm UseDNS
636: Specifies whether
637: .Nm sshd
1.40 jmc 638: should look up the remote host name and check that
1.18 markus 639: the resolved host name for the remote IP address maps back to the
640: very same IP address.
641: The default is
642: .Dq yes .
1.1 stevesk 643: .It Cm UseLogin
644: Specifies whether
645: .Xr login 1
646: is used for interactive login sessions.
647: The default is
648: .Dq no .
649: Note that
650: .Xr login 1
651: is never used for remote command execution.
652: Note also, that if this is enabled,
653: .Cm X11Forwarding
654: will be disabled because
655: .Xr login 1
656: does not know how to handle
657: .Xr xauth 1
1.15 jmc 658: cookies.
659: If
1.1 stevesk 660: .Cm UsePrivilegeSeparation
661: is specified, it will be disabled after authentication.
662: .It Cm UsePrivilegeSeparation
663: Specifies whether
664: .Nm sshd
1.2 stevesk 665: separates privileges by creating an unprivileged child process
1.15 jmc 666: to deal with incoming network traffic.
667: After successful authentication, another process will be created that has
668: the privilege of the authenticated user.
669: The goal of privilege separation is to prevent privilege
1.1 stevesk 670: escalation by containing any corruption within the unprivileged processes.
671: The default is
672: .Dq yes .
673: .It Cm X11DisplayOffset
674: Specifies the first display number available for
675: .Nm sshd Ns 's
676: X11 forwarding.
677: This prevents
678: .Nm sshd
679: from interfering with real X11 servers.
680: The default is 10.
681: .It Cm X11Forwarding
682: Specifies whether X11 forwarding is permitted.
1.13 stevesk 683: The argument must be
684: .Dq yes
685: or
686: .Dq no .
1.1 stevesk 687: The default is
688: .Dq no .
1.13 stevesk 689: .Pp
690: When X11 forwarding is enabled, there may be additional exposure to
691: the server and to client displays if the
692: .Nm sshd
693: proxy display is configured to listen on the wildcard address (see
694: .Cm X11UseLocalhost
695: below), however this is not the default.
696: Additionally, the authentication spoofing and authentication data
697: verification and substitution occur on the client side.
698: The security risk of using X11 forwarding is that the client's X11
699: display server may be exposed to attack when the ssh client requests
700: forwarding (see the warnings for
701: .Cm ForwardX11
702: in
1.19 jmc 703: .Xr ssh_config 5 ) .
1.13 stevesk 704: A system administrator may have a stance in which they want to
705: protect clients that may expose themselves to attack by unwittingly
706: requesting X11 forwarding, which can warrant a
707: .Dq no
708: setting.
709: .Pp
710: Note that disabling X11 forwarding does not prevent users from
711: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 712: X11 forwarding is automatically disabled if
713: .Cm UseLogin
714: is enabled.
715: .It Cm X11UseLocalhost
716: Specifies whether
717: .Nm sshd
718: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 719: the wildcard address.
720: By default,
1.1 stevesk 721: .Nm sshd
722: binds the forwarding server to the loopback address and sets the
723: hostname part of the
724: .Ev DISPLAY
725: environment variable to
726: .Dq localhost .
1.8 stevesk 727: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 728: However, some older X11 clients may not function with this
729: configuration.
730: .Cm X11UseLocalhost
731: may be set to
732: .Dq no
733: to specify that the forwarding server should be bound to the wildcard
734: address.
735: The argument must be
736: .Dq yes
737: or
738: .Dq no .
739: The default is
740: .Dq yes .
741: .It Cm XAuthLocation
1.11 stevesk 742: Specifies the full pathname of the
1.1 stevesk 743: .Xr xauth 1
744: program.
745: The default is
746: .Pa /usr/X11R6/bin/xauth .
747: .El
748: .Ss Time Formats
749: .Nm sshd
750: command-line arguments and configuration file options that specify time
751: may be expressed using a sequence of the form:
752: .Sm off
1.7 stevesk 753: .Ar time Op Ar qualifier ,
1.1 stevesk 754: .Sm on
755: where
756: .Ar time
757: is a positive integer value and
758: .Ar qualifier
759: is one of the following:
760: .Pp
761: .Bl -tag -width Ds -compact -offset indent
762: .It Cm <none>
763: seconds
764: .It Cm s | Cm S
765: seconds
766: .It Cm m | Cm M
767: minutes
768: .It Cm h | Cm H
769: hours
770: .It Cm d | Cm D
771: days
772: .It Cm w | Cm W
773: weeks
774: .El
775: .Pp
776: Each member of the sequence is added together to calculate
777: the total time value.
778: .Pp
779: Time format examples:
780: .Pp
781: .Bl -tag -width Ds -compact -offset indent
782: .It 600
783: 600 seconds (10 minutes)
784: .It 10m
785: 10 minutes
786: .It 1h30m
787: 1 hour 30 minutes (90 minutes)
788: .El
789: .Sh FILES
790: .Bl -tag -width Ds
791: .It Pa /etc/ssh/sshd_config
792: Contains configuration data for
793: .Nm sshd .
794: This file should be writable by root only, but it is recommended
795: (though not necessary) that it be world-readable.
796: .El
1.19 jmc 797: .Sh SEE ALSO
798: .Xr sshd 8
1.1 stevesk 799: .Sh AUTHORS
800: OpenSSH is a derivative of the original and free
801: ssh 1.2.12 release by Tatu Ylonen.
802: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
803: Theo de Raadt and Dug Song
804: removed many bugs, re-added newer features and
805: created OpenSSH.
806: Markus Friedl contributed the support for SSH
807: protocol versions 1.5 and 2.0.
808: Niels Provos and Markus Friedl contributed support
809: for privilege separation.
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