Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.46
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.
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1.46 ! reyk 37: .\" $OpenBSD: sshd_config.5,v 1.45 2005/09/21 23:36:54 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
1.44 markus 220: Specifies whether compression is allowed, or delayed until
221: the user has authenticated successfully.
1.3 markus 222: The argument must be
1.44 markus 223: .Dq yes ,
224: .Dq delayed ,
1.3 markus 225: or
226: .Dq no .
227: The default is
1.44 markus 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
1.41 djm 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
1.45 djm 351: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 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.
1.46 ! reyk 505: .It Cm PermitTunnel
! 506: Specifies whether
! 507: .Xr tun 4
! 508: device forwarding is allowed.
! 509: The default is
! 510: .Dq no .
1.6 markus 511: .It Cm PermitUserEnvironment
512: Specifies whether
513: .Pa ~/.ssh/environment
1.9 stevesk 514: and
1.6 markus 515: .Cm environment=
516: options in
517: .Pa ~/.ssh/authorized_keys
1.9 stevesk 518: are processed by
519: .Nm sshd .
1.6 markus 520: The default is
521: .Dq no .
1.9 stevesk 522: Enabling environment processing may enable users to bypass access
523: restrictions in some configurations using mechanisms such as
524: .Ev LD_PRELOAD .
1.1 stevesk 525: .It Cm PidFile
1.4 stevesk 526: Specifies the file that contains the process ID of the
1.1 stevesk 527: .Nm sshd
528: daemon.
529: The default is
530: .Pa /var/run/sshd.pid .
531: .It Cm Port
532: Specifies the port number that
533: .Nm sshd
534: listens on.
535: The default is 22.
536: Multiple options of this type are permitted.
537: See also
538: .Cm ListenAddress .
539: .It Cm PrintLastLog
540: Specifies whether
541: .Nm sshd
1.36 jaredy 542: should print the date and time of the last user login when a user logs
543: in interactively.
1.1 stevesk 544: The default is
545: .Dq yes .
546: .It Cm PrintMotd
547: Specifies whether
548: .Nm sshd
549: should print
550: .Pa /etc/motd
551: when a user logs in interactively.
552: (On some systems it is also printed by the shell,
553: .Pa /etc/profile ,
554: or equivalent.)
555: The default is
556: .Dq yes .
557: .It Cm Protocol
558: Specifies the protocol versions
559: .Nm sshd
1.5 stevesk 560: supports.
1.1 stevesk 561: The possible values are
562: .Dq 1
563: and
564: .Dq 2 .
565: Multiple versions must be comma-separated.
566: The default is
567: .Dq 2,1 .
1.5 stevesk 568: Note that the order of the protocol list does not indicate preference,
569: because the client selects among multiple protocol versions offered
570: by the server.
571: Specifying
572: .Dq 2,1
573: is identical to
574: .Dq 1,2 .
1.1 stevesk 575: .It Cm PubkeyAuthentication
576: Specifies whether public key authentication is allowed.
577: The default is
578: .Dq yes .
579: Note that this option applies to protocol version 2 only.
580: .It Cm RhostsRSAAuthentication
581: Specifies whether rhosts or /etc/hosts.equiv authentication together
582: with successful RSA host authentication is allowed.
583: The default is
584: .Dq no .
585: This option applies to protocol version 1 only.
586: .It Cm RSAAuthentication
587: Specifies whether pure RSA authentication is allowed.
588: The default is
589: .Dq yes .
590: This option applies to protocol version 1 only.
591: .It Cm ServerKeyBits
592: Defines the number of bits in the ephemeral protocol version 1 server key.
593: The minimum value is 512, and the default is 768.
594: .It Cm StrictModes
595: Specifies whether
596: .Nm sshd
597: should check file modes and ownership of the
598: user's files and home directory before accepting login.
599: This is normally desirable because novices sometimes accidentally leave their
600: directory or files world-writable.
601: The default is
602: .Dq yes .
603: .It Cm Subsystem
604: Configures an external subsystem (e.g., file transfer daemon).
605: Arguments should be a subsystem name and a command to execute upon subsystem
606: request.
607: The command
608: .Xr sftp-server 8
609: implements the
610: .Dq sftp
611: file transfer subsystem.
612: By default no subsystems are defined.
613: Note that this option applies to protocol version 2 only.
614: .It Cm SyslogFacility
615: Gives the facility code that is used when logging messages from
616: .Nm sshd .
617: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
618: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
619: The default is AUTH.
1.27 markus 620: .It Cm TCPKeepAlive
621: Specifies whether the system should send TCP keepalive messages to the
622: other side.
623: If they are sent, death of the connection or crash of one
624: of the machines will be properly noticed.
625: However, this means that
626: connections will die if the route is down temporarily, and some people
627: find it annoying.
628: On the other hand, if TCP keepalives are not sent,
629: sessions may hang indefinitely on the server, leaving
630: .Dq ghost
631: users and consuming server resources.
632: .Pp
633: The default is
634: .Dq yes
635: (to send TCP keepalive messages), and the server will notice
636: if the network goes down or the client host crashes.
637: This avoids infinitely hanging sessions.
638: .Pp
639: To disable TCP keepalive messages, the value should be set to
640: .Dq no .
1.18 markus 641: .It Cm UseDNS
642: Specifies whether
643: .Nm sshd
1.40 jmc 644: should look up the remote host name and check that
1.18 markus 645: the resolved host name for the remote IP address maps back to the
646: very same IP address.
647: The default is
648: .Dq yes .
1.1 stevesk 649: .It Cm UseLogin
650: Specifies whether
651: .Xr login 1
652: is used for interactive login sessions.
653: The default is
654: .Dq no .
655: Note that
656: .Xr login 1
657: is never used for remote command execution.
658: Note also, that if this is enabled,
659: .Cm X11Forwarding
660: will be disabled because
661: .Xr login 1
662: does not know how to handle
663: .Xr xauth 1
1.15 jmc 664: cookies.
665: If
1.1 stevesk 666: .Cm UsePrivilegeSeparation
667: is specified, it will be disabled after authentication.
668: .It Cm UsePrivilegeSeparation
669: Specifies whether
670: .Nm sshd
1.2 stevesk 671: separates privileges by creating an unprivileged child process
1.15 jmc 672: to deal with incoming network traffic.
673: After successful authentication, another process will be created that has
674: the privilege of the authenticated user.
675: The goal of privilege separation is to prevent privilege
1.1 stevesk 676: escalation by containing any corruption within the unprivileged processes.
677: The default is
678: .Dq yes .
679: .It Cm X11DisplayOffset
680: Specifies the first display number available for
681: .Nm sshd Ns 's
682: X11 forwarding.
683: This prevents
684: .Nm sshd
685: from interfering with real X11 servers.
686: The default is 10.
687: .It Cm X11Forwarding
688: Specifies whether X11 forwarding is permitted.
1.13 stevesk 689: The argument must be
690: .Dq yes
691: or
692: .Dq no .
1.1 stevesk 693: The default is
694: .Dq no .
1.13 stevesk 695: .Pp
696: When X11 forwarding is enabled, there may be additional exposure to
697: the server and to client displays if the
698: .Nm sshd
699: proxy display is configured to listen on the wildcard address (see
700: .Cm X11UseLocalhost
701: below), however this is not the default.
702: Additionally, the authentication spoofing and authentication data
703: verification and substitution occur on the client side.
704: The security risk of using X11 forwarding is that the client's X11
705: display server may be exposed to attack when the ssh client requests
706: forwarding (see the warnings for
707: .Cm ForwardX11
708: in
1.19 jmc 709: .Xr ssh_config 5 ) .
1.13 stevesk 710: A system administrator may have a stance in which they want to
711: protect clients that may expose themselves to attack by unwittingly
712: requesting X11 forwarding, which can warrant a
713: .Dq no
714: setting.
715: .Pp
716: Note that disabling X11 forwarding does not prevent users from
717: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 718: X11 forwarding is automatically disabled if
719: .Cm UseLogin
720: is enabled.
721: .It Cm X11UseLocalhost
722: Specifies whether
723: .Nm sshd
724: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 725: the wildcard address.
726: By default,
1.1 stevesk 727: .Nm sshd
728: binds the forwarding server to the loopback address and sets the
729: hostname part of the
730: .Ev DISPLAY
731: environment variable to
732: .Dq localhost .
1.8 stevesk 733: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 734: However, some older X11 clients may not function with this
735: configuration.
736: .Cm X11UseLocalhost
737: may be set to
738: .Dq no
739: to specify that the forwarding server should be bound to the wildcard
740: address.
741: The argument must be
742: .Dq yes
743: or
744: .Dq no .
745: The default is
746: .Dq yes .
747: .It Cm XAuthLocation
1.11 stevesk 748: Specifies the full pathname of the
1.1 stevesk 749: .Xr xauth 1
750: program.
751: The default is
752: .Pa /usr/X11R6/bin/xauth .
753: .El
754: .Ss Time Formats
755: .Nm sshd
756: command-line arguments and configuration file options that specify time
757: may be expressed using a sequence of the form:
758: .Sm off
1.7 stevesk 759: .Ar time Op Ar qualifier ,
1.1 stevesk 760: .Sm on
761: where
762: .Ar time
763: is a positive integer value and
764: .Ar qualifier
765: is one of the following:
766: .Pp
767: .Bl -tag -width Ds -compact -offset indent
768: .It Cm <none>
769: seconds
770: .It Cm s | Cm S
771: seconds
772: .It Cm m | Cm M
773: minutes
774: .It Cm h | Cm H
775: hours
776: .It Cm d | Cm D
777: days
778: .It Cm w | Cm W
779: weeks
780: .El
781: .Pp
782: Each member of the sequence is added together to calculate
783: the total time value.
784: .Pp
785: Time format examples:
786: .Pp
787: .Bl -tag -width Ds -compact -offset indent
788: .It 600
789: 600 seconds (10 minutes)
790: .It 10m
791: 10 minutes
792: .It 1h30m
793: 1 hour 30 minutes (90 minutes)
794: .El
795: .Sh FILES
796: .Bl -tag -width Ds
797: .It Pa /etc/ssh/sshd_config
798: Contains configuration data for
799: .Nm sshd .
800: This file should be writable by root only, but it is recommended
801: (though not necessary) that it be world-readable.
802: .El
1.19 jmc 803: .Sh SEE ALSO
804: .Xr sshd 8
1.1 stevesk 805: .Sh AUTHORS
806: OpenSSH is a derivative of the original and free
807: ssh 1.2.12 release by Tatu Ylonen.
808: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
809: Theo de Raadt and Dug Song
810: removed many bugs, re-added newer features and
811: created OpenSSH.
812: Markus Friedl contributed the support for SSH
813: protocol versions 1.5 and 2.0.
814: Niels Provos and Markus Friedl contributed support
815: for privilege separation.
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