Annotation of src/crypto/external/bsd/openssh/dist/sshd_config.5, Revision 1.11
1.11 ! christos 1: .\" $NetBSD: sshd_config.5,v 1.10 2011/07/25 03:03:11 christos Exp $
1.1 christos 2: .\" -*- nroff -*-
3: .\"
4: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
5: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6: .\" All rights reserved
7: .\"
8: .\" As far as I am concerned, the code I have written for this software
9: .\" can be used freely for any purpose. Any derived versions of this
10: .\" software must be clearly marked as such, and if the derived work is
11: .\" incompatible with the protocol description in the RFC file, it must be
12: .\" called by a name other than "ssh" or "Secure Shell".
13: .\"
14: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
15: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
16: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17: .\"
18: .\" Redistribution and use in source and binary forms, with or without
19: .\" modification, are permitted provided that the following conditions
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23: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.11 ! christos 38: .\" $OpenBSD: sshd_config.5,v 1.135 2011/08/02 01:22:11 djm Exp $
! 39: .Dd August 2 2011
1.1 christos 40: .Dt SSHD_CONFIG 5
41: .Os
42: .Sh NAME
43: .Nm sshd_config
44: .Nd OpenSSH SSH daemon configuration file
45: .Sh SYNOPSIS
46: .Nm /etc/ssh/sshd_config
47: .Sh DESCRIPTION
48: .Xr sshd 8
49: reads configuration data from
50: .Pa /etc/ssh/sshd_config
51: (or the file specified with
52: .Fl f
53: on the command line).
54: The file contains keyword-argument pairs, one per line.
55: Lines starting with
56: .Ql #
57: and empty lines are interpreted as comments.
58: Arguments may optionally be enclosed in double quotes
59: .Pq \&"
60: in order to represent arguments containing spaces.
61: .Pp
62: The possible
63: keywords and their meanings are as follows (note that
64: keywords are case-insensitive and arguments are case-sensitive):
65: .Bl -tag -width Ds
66: .It Cm AcceptEnv
67: Specifies what environment variables sent by the client will be copied into
68: the session's
69: .Xr environ 7 .
70: See
71: .Cm SendEnv
72: in
73: .Xr ssh_config 5
74: for how to configure the client.
75: Note that environment passing is only supported for protocol 2.
76: Variables are specified by name, which may contain the wildcard characters
77: .Ql *
78: and
79: .Ql \&? .
80: Multiple environment variables may be separated by whitespace or spread
81: across multiple
82: .Cm AcceptEnv
83: directives.
84: Be warned that some environment variables could be used to bypass restricted
85: user environments.
86: For this reason, care should be taken in the use of this directive.
87: The default is not to accept any environment variables.
88: .It Cm AddressFamily
89: Specifies which address family should be used by
90: .Xr sshd 8 .
91: Valid arguments are
92: .Dq any ,
93: .Dq inet
94: (use IPv4 only), or
95: .Dq inet6
96: (use IPv6 only).
97: The default is
98: .Dq any .
99: .It Cm AllowAgentForwarding
100: Specifies whether
101: .Xr ssh-agent 1
102: forwarding is permitted.
103: The default is
104: .Dq yes .
105: Note that disabling agent forwarding does not improve security
106: unless users are also denied shell access, as they can always install
107: their own forwarders.
108: .It Cm AllowGroups
109: This keyword can be followed by a list of group name patterns, separated
110: by spaces.
111: If specified, login is allowed only for users whose primary
112: group or supplementary group list matches one of the patterns.
113: Only group names are valid; a numerical group ID is not recognized.
114: By default, login is allowed for all groups.
115: The allow/deny directives are processed in the following order:
116: .Cm DenyUsers ,
117: .Cm AllowUsers ,
118: .Cm DenyGroups ,
119: and finally
120: .Cm AllowGroups .
121: .Pp
122: See
123: .Sx PATTERNS
124: in
125: .Xr ssh_config 5
126: for more information on patterns.
127: .It Cm AllowTcpForwarding
128: Specifies whether TCP forwarding is permitted.
129: The default is
130: .Dq yes .
131: Note that disabling TCP forwarding does not improve security unless
132: users are also denied shell access, as they can always install their
133: own forwarders.
134: .It Cm AllowUsers
135: This keyword can be followed by a list of user name patterns, separated
136: by spaces.
137: If specified, login is allowed only for user names that
138: match one of the patterns.
139: Only user names are valid; a numerical user ID is not recognized.
140: By default, login is allowed for all users.
141: If the pattern takes the form USER@HOST then USER and HOST
142: are separately checked, restricting logins to particular
143: users from particular hosts.
144: The allow/deny directives are processed in the following order:
145: .Cm DenyUsers ,
146: .Cm AllowUsers ,
147: .Cm DenyGroups ,
148: and finally
149: .Cm AllowGroups .
150: .Pp
151: See
152: .Sx PATTERNS
153: in
154: .Xr ssh_config 5
155: for more information on patterns.
156: .It Cm AuthorizedKeysFile
157: Specifies the file that contains the public keys that can be used
158: for user authentication.
1.6 adam 159: The format is described in the
160: .Sx AUTHORIZED_KEYS FILE FORMAT
161: section of
162: .Xr sshd 8 .
1.1 christos 163: .Cm AuthorizedKeysFile
164: may contain tokens of the form %T which are substituted during connection
165: setup.
166: The following tokens are defined: %% is replaced by a literal '%',
167: %h is replaced by the home directory of the user being authenticated, and
168: %u is replaced by the username of that user.
169: After expansion,
170: .Cm AuthorizedKeysFile
171: is taken to be an absolute path or one relative to the user's home
172: directory.
1.11 ! christos 173: Multiple files may be listed, separated by whitespace.
1.1 christos 174: The default is
1.11 ! christos 175: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.6 adam 176: .It Cm AuthorizedPrincipalsFile
177: Specifies a file that lists principal names that are accepted for
178: certificate authentication.
179: When using certificates signed by a key listed in
180: .Cm TrustedUserCAKeys ,
181: this file lists names, one of which must appear in the certificate for it
182: to be accepted for authentication.
183: Names are listed one per line preceded by key options (as described
184: in
185: .Sx AUTHORIZED_KEYS FILE FORMAT
186: in
187: .Xr sshd 8 ) .
188: Empty lines and comments starting with
189: .Ql #
190: are ignored.
191: .Pp
192: .Cm AuthorizedPrincipalsFile
193: may contain tokens of the form %T which are substituted during connection
194: setup.
195: The following tokens are defined: %% is replaced by a literal '%',
196: %h is replaced by the home directory of the user being authenticated, and
197: %u is replaced by the username of that user.
198: After expansion,
199: .Cm AuthorizedPrincipalsFile
200: is taken to be an absolute path or one relative to the user's home
201: directory.
202: .Pp
203: The default is not to use a principals file \(en in this case, the username
204: of the user must appear in a certificate's principals list for it to be
205: accepted.
206: Note that
207: .Cm AuthorizedPrincipalsFile
208: is only used when authentication proceeds using a CA listed in
209: .Cm TrustedUserCAKeys
210: and is not consulted for certification authorities trusted via
211: .Pa ~/.ssh/authorized_keys ,
212: though the
213: .Cm principals=
214: key option offers a similar facility (see
215: .Xr sshd 8
216: for details).
1.1 christos 217: .It Cm Banner
218: The contents of the specified file are sent to the remote user before
219: authentication is allowed.
220: If the argument is
221: .Dq none
222: then no banner is displayed.
223: This option is only available for protocol version 2.
224: By default, no banner is displayed.
225: .It Cm ChallengeResponseAuthentication
226: Specifies whether challenge-response authentication is allowed.
227: All authentication styles from
228: .Xr login.conf 5
229: are supported.
230: The default is
231: .Dq yes .
232: .It Cm ChrootDirectory
1.6 adam 233: Specifies the pathname of a directory to
1.1 christos 234: .Xr chroot 2
235: to after authentication.
1.6 adam 236: All components of the pathname must be root-owned directories that are
1.1 christos 237: not writable by any other user or group.
1.3 christos 238: After the chroot,
239: .Xr sshd 8
240: changes the working directory to the user's home directory.
1.1 christos 241: .Pp
1.6 adam 242: The pathname may contain the following tokens that are expanded at runtime once
1.1 christos 243: the connecting user has been authenticated: %% is replaced by a literal '%',
244: %h is replaced by the home directory of the user being authenticated, and
245: %u is replaced by the username of that user.
246: .Pp
247: The
248: .Cm ChrootDirectory
249: must contain the necessary files and directories to support the
1.3 christos 250: user's session.
1.1 christos 251: For an interactive session this requires at least a shell, typically
252: .Xr sh 1 ,
253: and basic
254: .Pa /dev
255: nodes such as
256: .Xr null 4 ,
257: .Xr zero 4 ,
258: .Xr stdin 4 ,
259: .Xr stdout 4 ,
260: .Xr stderr 4 ,
261: .Xr arandom 4
262: and
263: .Xr tty 4
264: devices.
265: For file transfer sessions using
266: .Dq sftp ,
267: no additional configuration of the environment is necessary if the
1.3 christos 268: in-process sftp server is used,
269: though sessions which use logging do require
270: .Pa /dev/log
271: inside the chroot directory (see
272: .Xr sftp-server 8
1.1 christos 273: for details).
274: .Pp
275: The default is not to
276: .Xr chroot 2 .
277: .It Cm Ciphers
278: Specifies the ciphers allowed for protocol version 2.
279: Multiple ciphers must be comma-separated.
280: The supported ciphers are
281: .Dq 3des-cbc ,
282: .Dq aes128-cbc ,
283: .Dq aes192-cbc ,
284: .Dq aes256-cbc ,
285: .Dq aes128-ctr ,
286: .Dq aes192-ctr ,
287: .Dq aes256-ctr ,
288: .Dq arcfour128 ,
289: .Dq arcfour256 ,
290: .Dq arcfour ,
291: .Dq blowfish-cbc ,
292: and
293: .Dq cast128-cbc .
294: The default is:
295: .Bd -literal -offset 3n
296: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
297: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
298: aes256-cbc,arcfour
299: .Ed
300: .It Cm ClientAliveCountMax
301: Sets the number of client alive messages (see below) which may be
302: sent without
303: .Xr sshd 8
304: receiving any messages back from the client.
305: If this threshold is reached while client alive messages are being sent,
306: sshd will disconnect the client, terminating the session.
307: It is important to note that the use of client alive messages is very
308: different from
309: .Cm TCPKeepAlive
310: (below).
311: The client alive messages are sent through the encrypted channel
312: and therefore will not be spoofable.
313: The TCP keepalive option enabled by
314: .Cm TCPKeepAlive
315: is spoofable.
316: The client alive mechanism is valuable when the client or
317: server depend on knowing when a connection has become inactive.
318: .Pp
319: The default value is 3.
320: If
321: .Cm ClientAliveInterval
322: (see below) is set to 15, and
323: .Cm ClientAliveCountMax
324: is left at the default, unresponsive SSH clients
325: will be disconnected after approximately 45 seconds.
326: This option applies to protocol version 2 only.
327: .It Cm ClientAliveInterval
328: Sets a timeout interval in seconds after which if no data has been received
329: from the client,
330: .Xr sshd 8
331: will send a message through the encrypted
332: channel to request a response from the client.
333: The default
334: is 0, indicating that these messages will not be sent to the client.
335: This option applies to protocol version 2 only.
336: .It Cm Compression
337: Specifies whether compression is allowed, or delayed until
338: the user has authenticated successfully.
339: The argument must be
340: .Dq yes ,
341: .Dq delayed ,
342: or
343: .Dq no .
344: The default is
345: .Dq delayed .
346: .It Cm DenyGroups
347: This keyword can be followed by a list of group name patterns, separated
348: by spaces.
349: Login is disallowed for users whose primary group or supplementary
350: group list matches one of the patterns.
351: Only group names are valid; a numerical group ID is not recognized.
352: By default, login is allowed for all groups.
353: The allow/deny directives are processed in the following order:
354: .Cm DenyUsers ,
355: .Cm AllowUsers ,
356: .Cm DenyGroups ,
357: and finally
358: .Cm AllowGroups .
359: .Pp
360: See
361: .Sx PATTERNS
362: in
363: .Xr ssh_config 5
364: for more information on patterns.
365: .It Cm DenyUsers
366: This keyword can be followed by a list of user name patterns, separated
367: by spaces.
368: Login is disallowed for user names that match one of the patterns.
369: Only user names are valid; a numerical user ID is not recognized.
370: By default, login is allowed for all users.
371: If the pattern takes the form USER@HOST then USER and HOST
372: are separately checked, restricting logins to particular
373: users from particular hosts.
374: The allow/deny directives are processed in the following order:
375: .Cm DenyUsers ,
376: .Cm AllowUsers ,
377: .Cm DenyGroups ,
378: and finally
379: .Cm AllowGroups .
380: .Pp
381: See
382: .Sx PATTERNS
383: in
384: .Xr ssh_config 5
385: for more information on patterns.
386: .It Cm ForceCommand
387: Forces the execution of the command specified by
388: .Cm ForceCommand ,
389: ignoring any command supplied by the client and
390: .Pa ~/.ssh/rc
391: if present.
392: The command is invoked by using the user's login shell with the -c option.
393: This applies to shell, command, or subsystem execution.
394: It is most useful inside a
395: .Cm Match
396: block.
397: The command originally supplied by the client is available in the
398: .Ev SSH_ORIGINAL_COMMAND
399: environment variable.
400: Specifying a command of
401: .Dq internal-sftp
402: will force the use of an in-process sftp server that requires no support
403: files when used with
404: .Cm ChrootDirectory .
405: .It Cm GatewayPorts
406: Specifies whether remote hosts are allowed to connect to ports
407: forwarded for the client.
408: By default,
409: .Xr sshd 8
410: binds remote port forwardings to the loopback address.
411: This prevents other remote hosts from connecting to forwarded ports.
412: .Cm GatewayPorts
413: can be used to specify that sshd
414: should allow remote port forwardings to bind to non-loopback addresses, thus
415: allowing other hosts to connect.
416: The argument may be
417: .Dq no
418: to force remote port forwardings to be available to the local host only,
419: .Dq yes
420: to force remote port forwardings to bind to the wildcard address, or
421: .Dq clientspecified
422: to allow the client to select the address to which the forwarding is bound.
423: The default is
424: .Dq no .
425: .It Cm GSSAPIAuthentication
426: Specifies whether user authentication based on GSSAPI is allowed.
427: The default is
428: .Dq no .
429: Note that this option applies to protocol version 2 only.
430: .It Cm GSSAPICleanupCredentials
431: Specifies whether to automatically destroy the user's credentials cache
432: on logout.
433: The default is
434: .Dq yes .
435: Note that this option applies to protocol version 2 only.
436: .It Cm HostbasedAuthentication
437: Specifies whether rhosts or /etc/hosts.equiv authentication together
438: with successful public key client host authentication is allowed
439: (host-based authentication).
440: This option is similar to
441: .Cm RhostsRSAAuthentication
442: and applies to protocol version 2 only.
443: The default is
444: .Dq no .
445: .It Cm HostbasedUsesNameFromPacketOnly
446: Specifies whether or not the server will attempt to perform a reverse
447: name lookup when matching the name in the
448: .Pa ~/.shosts ,
449: .Pa ~/.rhosts ,
450: and
451: .Pa /etc/hosts.equiv
452: files during
453: .Cm HostbasedAuthentication .
454: A setting of
455: .Dq yes
456: means that
457: .Xr sshd 8
458: uses the name supplied by the client rather than
459: attempting to resolve the name from the TCP connection itself.
460: The default is
461: .Dq no .
1.6 adam 462: .It Cm HostCertificate
463: Specifies a file containing a public host certificate.
464: The certificate's public key must match a private host key already specified
465: by
466: .Cm HostKey .
467: The default behaviour of
468: .Xr sshd 8
469: is not to load any certificates.
1.1 christos 470: .It Cm HostKey
471: Specifies a file containing a private host key
472: used by SSH.
473: The default is
474: .Pa /etc/ssh/ssh_host_key
475: for protocol version 1, and
1.10 christos 476: .Pa /etc/ssh/ssh_host_dsa_key ,
477: .Pa /etc/ssh/ssh_host_ecdsa_key
478: and
1.1 christos 479: .Pa /etc/ssh/ssh_host_rsa_key
480: for protocol version 2.
481: Note that
482: .Xr sshd 8
483: will refuse to use a file if it is group/world-accessible.
484: It is possible to have multiple host key files.
485: .Dq rsa1
486: keys are used for version 1 and
1.10 christos 487: .Dq dsa ,
488: .Dq ecdsa
1.1 christos 489: or
490: .Dq rsa
491: are used for version 2 of the SSH protocol.
492: .It Cm IgnoreRhosts
493: Specifies that
494: .Pa .rhosts
495: and
496: .Pa .shosts
497: files will not be used in
498: .Cm RhostsRSAAuthentication
499: or
500: .Cm HostbasedAuthentication .
501: .Pp
502: .Pa /etc/hosts.equiv
503: and
504: .Pa /etc/shosts.equiv
505: are still used.
506: The default is
507: .Dq yes .
508: .It Cm IgnoreUserKnownHosts
509: Specifies whether
510: .Xr sshd 8
511: should ignore the user's
512: .Pa ~/.ssh/known_hosts
513: during
514: .Cm RhostsRSAAuthentication
515: or
516: .Cm HostbasedAuthentication .
517: The default is
518: .Dq no .
1.10 christos 519: .It Cm IPQoS
520: Specifies the IPv4 type-of-service or DSCP class for the connection.
521: Accepted values are
522: .Dq af11 ,
523: .Dq af12 ,
524: .Dq af13 ,
525: .Dq af14 ,
526: .Dq af22 ,
527: .Dq af23 ,
528: .Dq af31 ,
529: .Dq af32 ,
530: .Dq af33 ,
531: .Dq af41 ,
532: .Dq af42 ,
533: .Dq af43 ,
534: .Dq cs0 ,
535: .Dq cs1 ,
536: .Dq cs2 ,
537: .Dq cs3 ,
538: .Dq cs4 ,
539: .Dq cs5 ,
540: .Dq cs6 ,
541: .Dq cs7 ,
542: .Dq ef ,
543: .Dq lowdelay ,
544: .Dq throughput ,
545: .Dq reliability ,
546: or a numeric value.
547: This option may take one or two arguments, separated by whitespace.
548: If one argument is specified, it is used as the packet class unconditionally.
549: If two values are specified, the first is automatically selected for
550: interactive sessions and the second for non-interactive sessions.
551: The default is
552: .Dq lowdelay
553: for interactive sessions and
554: .Dq throughput
555: for non-interactive sessions.
1.1 christos 556: .It Cm KerberosAuthentication
557: Specifies whether the password provided by the user for
558: .Cm PasswordAuthentication
559: will be validated through the Kerberos KDC.
560: To use this option, the server needs a
561: Kerberos servtab which allows the verification of the KDC's identity.
562: The default is
563: .Dq no .
564: .It Cm KerberosGetAFSToken
565: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
566: an AFS token before accessing the user's home directory.
567: The default is
568: .Dq no .
569: .It Cm KerberosOrLocalPasswd
570: If password authentication through Kerberos fails then
571: the password will be validated via any additional local mechanism
572: such as
573: .Pa /etc/passwd .
574: The default is
575: .Dq yes .
576: .It Cm KerberosTicketCleanup
577: Specifies whether to automatically destroy the user's ticket cache
578: file on logout.
579: The default is
580: .Dq yes .
1.10 christos 581: .It Cm KexAlgorithms
582: Specifies the available KEX (Key Exchange) algorithms.
583: Multiple algorithms must be comma-separated.
584: The default is
585: .Dq ecdh-sha2-nistp256 ,
586: .Dq ecdh-sha2-nistp384 ,
587: .Dq ecdh-sha2-nistp521 ,
588: .Dq diffie-hellman-group-exchange-sha256 ,
589: .Dq diffie-hellman-group-exchange-sha1 ,
590: .Dq diffie-hellman-group14-sha1 ,
591: .Dq diffie-hellman-group1-sha1 .
1.1 christos 592: .It Cm KeyRegenerationInterval
593: In protocol version 1, the ephemeral server key is automatically regenerated
594: after this many seconds (if it has been used).
595: The purpose of regeneration is to prevent
596: decrypting captured sessions by later breaking into the machine and
597: stealing the keys.
598: The key is never stored anywhere.
599: If the value is 0, the key is never regenerated.
600: The default is 3600 (seconds).
601: .It Cm ListenAddress
602: Specifies the local addresses
603: .Xr sshd 8
604: should listen on.
605: The following forms may be used:
606: .Pp
607: .Bl -item -offset indent -compact
608: .It
609: .Cm ListenAddress
610: .Sm off
611: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
612: .Sm on
613: .It
614: .Cm ListenAddress
615: .Sm off
616: .Ar host No | Ar IPv4_addr No : Ar port
617: .Sm on
618: .It
619: .Cm ListenAddress
620: .Sm off
621: .Oo
622: .Ar host No | Ar IPv6_addr Oc : Ar port
623: .Sm on
624: .El
625: .Pp
626: If
627: .Ar port
628: is not specified,
629: sshd will listen on the address and all prior
630: .Cm Port
631: options specified.
632: The default is to listen on all local addresses.
633: Multiple
634: .Cm ListenAddress
635: options are permitted.
636: Additionally, any
637: .Cm Port
638: options must precede this option for non-port qualified addresses.
639: .It Cm LoginGraceTime
640: The server disconnects after this time if the user has not
641: successfully logged in.
642: If the value is 0, there is no time limit.
1.8 enami 643: The default is 120 seconds but the default
644: .Pa /etc/ssh/sshd_config
645: overwrites it to 600 seconds.
1.1 christos 646: .It Cm LogLevel
647: Gives the verbosity level that is used when logging messages from
648: .Xr sshd 8 .
649: The possible values are:
650: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
651: The default is INFO.
652: DEBUG and DEBUG1 are equivalent.
653: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
654: Logging with a DEBUG level violates the privacy of users and is not recommended.
655: .It Cm MACs
656: Specifies the available MAC (message authentication code) algorithms.
657: The MAC algorithm is used in protocol version 2
658: for data integrity protection.
659: Multiple algorithms must be comma-separated.
660: The default is:
661: .Bd -literal -offset indent
1.11 ! christos 662: hmac-md5,hmac-sha1,umac-64@openssh.com,
1.1 christos 663: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
1.11 ! christos 664: .\" hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
! 665: .\" hmac-sha2-512-96
1.1 christos 666: .Ed
667: .It Cm Match
668: Introduces a conditional block.
669: If all of the criteria on the
670: .Cm Match
671: line are satisfied, the keywords on the following lines override those
672: set in the global section of the config file, until either another
673: .Cm Match
674: line or the end of the file.
675: .Pp
676: The arguments to
677: .Cm Match
678: are one or more criteria-pattern pairs.
679: The available criteria are
680: .Cm User ,
681: .Cm Group ,
682: .Cm Host ,
683: and
684: .Cm Address .
685: The match patterns may consist of single entries or comma-separated
686: lists and may use the wildcard and negation operators described in the
687: .Sx PATTERNS
688: section of
689: .Xr ssh_config 5 .
690: .Pp
691: The patterns in an
692: .Cm Address
693: criteria may additionally contain addresses to match in CIDR
694: address/masklen format, e.g.\&
695: .Dq 192.0.2.0/24
696: or
697: .Dq 3ffe:ffff::/32 .
698: Note that the mask length provided must be consistent with the address -
699: it is an error to specify a mask length that is too long for the address
700: or one with bits set in this host portion of the address.
701: For example,
702: .Dq 192.0.2.0/33
703: and
704: .Dq 192.0.2.0/8
705: respectively.
706: .Pp
707: Only a subset of keywords may be used on the lines following a
708: .Cm Match
709: keyword.
710: Available keywords are
711: .Cm AllowAgentForwarding ,
712: .Cm AllowTcpForwarding ,
1.6 adam 713: .Cm AuthorizedKeysFile ,
714: .Cm AuthorizedPrincipalsFile ,
1.1 christos 715: .Cm Banner ,
716: .Cm ChrootDirectory ,
717: .Cm ForceCommand ,
718: .Cm GatewayPorts ,
719: .Cm GSSAPIAuthentication ,
720: .Cm HostbasedAuthentication ,
1.6 adam 721: .Cm HostbasedUsesNameFromPacketOnly ,
1.1 christos 722: .Cm KbdInteractiveAuthentication ,
723: .Cm KerberosAuthentication ,
724: .Cm MaxAuthTries ,
725: .Cm MaxSessions ,
726: .Cm PasswordAuthentication ,
727: .Cm PermitEmptyPasswords ,
728: .Cm PermitOpen ,
729: .Cm PermitRootLogin ,
1.6 adam 730: .Cm PermitTunnel ,
731: .Cm PubkeyAuthentication ,
1.1 christos 732: .Cm RhostsRSAAuthentication ,
733: .Cm RSAAuthentication ,
734: .Cm X11DisplayOffset ,
735: .Cm X11Forwarding
736: and
737: .Cm X11UseLocalHost .
738: .It Cm MaxAuthTries
739: Specifies the maximum number of authentication attempts permitted per
740: connection.
741: Once the number of failures reaches half this value,
742: additional failures are logged.
743: The default is 6.
744: .It Cm MaxSessions
745: Specifies the maximum number of open sessions permitted per network connection.
746: The default is 10.
747: .It Cm MaxStartups
748: Specifies the maximum number of concurrent unauthenticated connections to the
749: SSH daemon.
750: Additional connections will be dropped until authentication succeeds or the
751: .Cm LoginGraceTime
752: expires for a connection.
753: The default is 10.
754: .Pp
755: Alternatively, random early drop can be enabled by specifying
756: the three colon separated values
757: .Dq start:rate:full
758: (e.g. "10:30:60").
759: .Xr sshd 8
760: will refuse connection attempts with a probability of
761: .Dq rate/100
762: (30%)
763: if there are currently
764: .Dq start
765: (10)
766: unauthenticated connections.
767: The probability increases linearly and all connection attempts
768: are refused if the number of unauthenticated connections reaches
769: .Dq full
770: (60).
771: .It Cm PasswordAuthentication
772: Specifies whether password authentication is allowed.
773: The default is
774: .Dq yes .
775: .It Cm PermitEmptyPasswords
776: When password authentication is allowed, it specifies whether the
777: server allows login to accounts with empty password strings.
778: The default is
779: .Dq no .
780: .It Cm PermitOpen
781: Specifies the destinations to which TCP port forwarding is permitted.
782: The forwarding specification must be one of the following forms:
783: .Pp
784: .Bl -item -offset indent -compact
785: .It
786: .Cm PermitOpen
787: .Sm off
788: .Ar host : port
789: .Sm on
790: .It
791: .Cm PermitOpen
792: .Sm off
793: .Ar IPv4_addr : port
794: .Sm on
795: .It
796: .Cm PermitOpen
797: .Sm off
798: .Ar \&[ IPv6_addr \&] : port
799: .Sm on
800: .El
801: .Pp
802: Multiple forwards may be specified by separating them with whitespace.
803: An argument of
804: .Dq any
805: can be used to remove all restrictions and permit any forwarding requests.
806: By default all port forwarding requests are permitted.
807: .It Cm PermitRootLogin
808: Specifies whether root can log in using
809: .Xr ssh 1 .
810: The argument must be
811: .Dq yes ,
812: .Dq without-password ,
813: .Dq forced-commands-only ,
814: or
815: .Dq no .
816: The default is
1.2 christos 817: .Dq no .
1.1 christos 818: .Pp
819: If this option is set to
820: .Dq without-password ,
821: password authentication is disabled for root.
822: .Pp
823: If this option is set to
824: .Dq forced-commands-only ,
825: root login with public key authentication will be allowed,
826: but only if the
827: .Ar command
828: option has been specified
829: (which may be useful for taking remote backups even if root login is
830: normally not allowed).
831: All other authentication methods are disabled for root.
832: .Pp
833: If this option is set to
834: .Dq no ,
835: root is not allowed to log in.
836: .It Cm PermitTunnel
837: Specifies whether
838: .Xr tun 4
839: device forwarding is allowed.
840: The argument must be
841: .Dq yes ,
842: .Dq point-to-point
843: (layer 3),
844: .Dq ethernet
845: (layer 2), or
846: .Dq no .
847: Specifying
848: .Dq yes
849: permits both
850: .Dq point-to-point
851: and
852: .Dq ethernet .
853: The default is
854: .Dq no .
855: .It Cm PermitUserEnvironment
856: Specifies whether
857: .Pa ~/.ssh/environment
858: and
859: .Cm environment=
860: options in
861: .Pa ~/.ssh/authorized_keys
862: are processed by
863: .Xr sshd 8 .
864: The default is
865: .Dq no .
866: Enabling environment processing may enable users to bypass access
867: restrictions in some configurations using mechanisms such as
868: .Ev LD_PRELOAD .
869: .It Cm PidFile
870: Specifies the file that contains the process ID of the
871: SSH daemon.
872: The default is
873: .Pa /var/run/sshd.pid .
874: .It Cm Port
875: Specifies the port number that
876: .Xr sshd 8
877: listens on.
878: The default is 22.
879: Multiple options of this type are permitted.
880: See also
881: .Cm ListenAddress .
882: .It Cm PrintLastLog
883: Specifies whether
884: .Xr sshd 8
885: should print the date and time of the last user login when a user logs
886: in interactively.
887: The default is
888: .Dq yes .
889: .It Cm PrintMotd
890: Specifies whether
891: .Xr sshd 8
892: should print
893: .Pa /etc/motd
894: when a user logs in interactively.
895: (On some systems it is also printed by the shell,
896: .Pa /etc/profile ,
897: or equivalent.)
898: The default is
899: .Dq yes .
900: .It Cm Protocol
901: Specifies the protocol versions
902: .Xr sshd 8
903: supports.
904: The possible values are
905: .Sq 1
906: and
907: .Sq 2 .
908: Multiple versions must be comma-separated.
909: The default is
1.6 adam 910: .Sq 2 .
1.1 christos 911: Note that the order of the protocol list does not indicate preference,
912: because the client selects among multiple protocol versions offered
913: by the server.
914: Specifying
915: .Dq 2,1
916: is identical to
917: .Dq 1,2 .
918: .It Cm PubkeyAuthentication
919: Specifies whether public key authentication is allowed.
920: The default is
921: .Dq yes .
922: Note that this option applies to protocol version 2 only.
1.6 adam 923: .It Cm RevokedKeys
924: Specifies a list of revoked public keys.
925: Keys listed in this file will be refused for public key authentication.
926: Note that if this file is not readable, then public key authentication will
927: be refused for all users.
1.1 christos 928: .It Cm RhostsRSAAuthentication
929: Specifies whether rhosts or /etc/hosts.equiv authentication together
930: with successful RSA host authentication is allowed.
931: The default is
932: .Dq no .
933: This option applies to protocol version 1 only.
934: .It Cm RSAAuthentication
935: Specifies whether pure RSA authentication is allowed.
936: The default is
937: .Dq yes .
938: This option applies to protocol version 1 only.
939: .It Cm ServerKeyBits
940: Defines the number of bits in the ephemeral protocol version 1 server key.
941: The minimum value is 512, and the default is 1024.
942: .It Cm StrictModes
943: Specifies whether
944: .Xr sshd 8
945: should check file modes and ownership of the
946: user's files and home directory before accepting login.
947: This is normally desirable because novices sometimes accidentally leave their
948: directory or files world-writable.
949: The default is
950: .Dq yes .
1.6 adam 951: Note that this does not apply to
952: .Cm ChrootDirectory ,
953: whose permissions and ownership are checked unconditionally.
1.1 christos 954: .It Cm Subsystem
955: Configures an external subsystem (e.g. file transfer daemon).
956: Arguments should be a subsystem name and a command (with optional arguments)
957: to execute upon subsystem request.
958: .Pp
959: The command
960: .Xr sftp-server 8
961: implements the
962: .Dq sftp
963: file transfer subsystem.
964: .Pp
965: Alternately the name
966: .Dq internal-sftp
967: implements an in-process
968: .Dq sftp
969: server.
970: This may simplify configurations using
971: .Cm ChrootDirectory
972: to force a different filesystem root on clients.
973: .Pp
974: By default no subsystems are defined.
975: Note that this option applies to protocol version 2 only.
976: .It Cm SyslogFacility
977: Gives the facility code that is used when logging messages from
978: .Xr sshd 8 .
979: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
980: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
981: The default is AUTH.
982: .It Cm TCPKeepAlive
983: Specifies whether the system should send TCP keepalive messages to the
984: other side.
985: If they are sent, death of the connection or crash of one
986: of the machines will be properly noticed.
987: However, this means that
988: connections will die if the route is down temporarily, and some people
989: find it annoying.
990: On the other hand, if TCP keepalives are not sent,
991: sessions may hang indefinitely on the server, leaving
992: .Dq ghost
993: users and consuming server resources.
994: .Pp
995: The default is
996: .Dq yes
997: (to send TCP keepalive messages), and the server will notice
998: if the network goes down or the client host crashes.
999: This avoids infinitely hanging sessions.
1000: .Pp
1001: To disable TCP keepalive messages, the value should be set to
1002: .Dq no .
1.6 adam 1003: .It Cm TrustedUserCAKeys
1004: Specifies a file containing public keys of certificate authorities that are
1005: trusted to sign user certificates for authentication.
1006: Keys are listed one per line; empty lines and comments starting with
1007: .Ql #
1008: are allowed.
1009: If a certificate is presented for authentication and has its signing CA key
1010: listed in this file, then it may be used for authentication for any user
1011: listed in the certificate's principals list.
1012: Note that certificates that lack a list of principals will not be permitted
1013: for authentication using
1014: .Cm TrustedUserCAKeys .
1015: For more details on certificates, see the
1016: .Sx CERTIFICATES
1017: section in
1018: .Xr ssh-keygen 1 .
1.1 christos 1019: .It Cm UseDNS
1020: Specifies whether
1021: .Xr sshd 8
1022: should look up the remote host name and check that
1023: the resolved host name for the remote IP address maps back to the
1024: very same IP address.
1025: The default is
1026: .Dq yes .
1027: .It Cm UseLogin
1028: Specifies whether
1029: .Xr login 1
1030: is used for interactive login sessions.
1031: The default is
1032: .Dq no .
1033: Note that
1034: .Xr login 1
1035: is never used for remote command execution.
1036: Note also, that if this is enabled,
1037: .Cm X11Forwarding
1038: will be disabled because
1039: .Xr login 1
1040: does not know how to handle
1041: .Xr xauth 1
1042: cookies.
1043: If
1044: .Cm UsePrivilegeSeparation
1045: is specified, it will be disabled after authentication.
1046: .It Cm UsePrivilegeSeparation
1047: Specifies whether
1048: .Xr sshd 8
1049: separates privileges by creating an unprivileged child process
1050: to deal with incoming network traffic.
1051: After successful authentication, another process will be created that has
1052: the privilege of the authenticated user.
1053: The goal of privilege separation is to prevent privilege
1054: escalation by containing any corruption within the unprivileged processes.
1055: The default is
1056: .Dq yes .
1.11 ! christos 1057: If
! 1058: .Cm UsePrivilegeSeparation
! 1059: is set to
! 1060: .Dq sandbox
! 1061: then the pre-authentication unprivileged process is subject to additional
! 1062: restrictions.
1.1 christos 1063: .It Cm X11DisplayOffset
1064: Specifies the first display number available for
1065: .Xr sshd 8 Ns 's
1066: X11 forwarding.
1067: This prevents sshd from interfering with real X11 servers.
1068: The default is 10.
1069: .It Cm X11Forwarding
1070: Specifies whether X11 forwarding is permitted.
1071: The argument must be
1072: .Dq yes
1073: or
1074: .Dq no .
1075: The default is
1076: .Dq no .
1077: .Pp
1078: When X11 forwarding is enabled, there may be additional exposure to
1079: the server and to client displays if the
1080: .Xr sshd 8
1081: proxy display is configured to listen on the wildcard address (see
1082: .Cm X11UseLocalhost
1083: below), though this is not the default.
1084: Additionally, the authentication spoofing and authentication data
1085: verification and substitution occur on the client side.
1086: The security risk of using X11 forwarding is that the client's X11
1087: display server may be exposed to attack when the SSH client requests
1088: forwarding (see the warnings for
1089: .Cm ForwardX11
1090: in
1091: .Xr ssh_config 5 ) .
1092: A system administrator may have a stance in which they want to
1093: protect clients that may expose themselves to attack by unwittingly
1094: requesting X11 forwarding, which can warrant a
1095: .Dq no
1096: setting.
1097: .Pp
1098: Note that disabling X11 forwarding does not prevent users from
1099: forwarding X11 traffic, as users can always install their own forwarders.
1100: X11 forwarding is automatically disabled if
1101: .Cm UseLogin
1102: is enabled.
1103: .It Cm X11UseLocalhost
1104: Specifies whether
1105: .Xr sshd 8
1106: should bind the X11 forwarding server to the loopback address or to
1107: the wildcard address.
1108: By default,
1109: sshd binds the forwarding server to the loopback address and sets the
1110: hostname part of the
1111: .Ev DISPLAY
1112: environment variable to
1113: .Dq localhost .
1114: This prevents remote hosts from connecting to the proxy display.
1115: However, some older X11 clients may not function with this
1116: configuration.
1117: .Cm X11UseLocalhost
1118: may be set to
1119: .Dq no
1120: to specify that the forwarding server should be bound to the wildcard
1121: address.
1122: The argument must be
1123: .Dq yes
1124: or
1125: .Dq no .
1126: The default is
1127: .Dq yes .
1128: .It Cm XAuthLocation
1129: Specifies the full pathname of the
1130: .Xr xauth 1
1131: program.
1132: The default is
1133: .Pa /usr/X11R6/bin/xauth .
1.7 adam 1134: .It Cm UseLPK
1135: Specifies whether LDAP public key retrieval must be used or not. It allow
1136: an easy centralisation of public keys within an LDAP directory. The argument must be
1137: .Dq yes
1138: or
1139: .Dq no .
1140: .It Cm LpkLdapConf
1141: Specifies whether LDAP Public keys should parse the specified ldap.conf file
1142: instead of sshd_config Tokens. The argument must be a valid path to an ldap.conf
1143: file like
1144: .Pa /etc/ldap.conf
1145: .It Cm LpkServers
1146: Specifies LDAP one or more [:space:] separated server's url the following form may be used:
1147: .Pp
1148: LpkServers ldaps://127.0.0.1 ldap://127.0.0.2 ldap://127.0.0.3
1149: .It Cm LpkUserDN
1150: Specifies the LDAP user DN.
1151: .Pp
1152: LpkUserDN ou=users,dc=phear,dc=org
1153: .It Cm LpkGroupDN
1154: Specifies the LDAP groups DN.
1155: .Pp
1156: LpkGroupDN ou=groups,dc=phear,dc=org
1157: .It Cm LpkBindDN
1158: Specifies the LDAP bind DN to use if necessary.
1159: .Pp
1160: LpkBindDN cn=Manager,dc=phear,dc=org
1161: .It Cm LpkBindPw
1162: Specifies the LDAP bind credential.
1163: .Pp
1164: LpkBindPw secret
1165: .It Cm LpkServerGroup
1166: Specifies one or more [:space:] separated group the server is part of.
1167: .Pp
1168: LpkServerGroup unix mail prod
1169: .It Cm LpkFilter
1170: Specifies an additional LDAP filter to use for finding SSH keys
1171: .Pp
1172: LpkFilter (hostAccess=master.phear.org)
1173: .It Cm LpkForceTLS
1174: Specifies if the LDAP server connection must be tried, forced or not used. The argument must be
1175: .Dq yes
1176: or
1177: .Dq no
1178: or
1179: .Dq try .
1180: .It Cm LpkSearchTimelimit
1.9 agc 1181: Specifies the search time limit before the search is considered over. value is
1.7 adam 1182: in seconds.
1183: .Pp
1184: LpkSearchTimelimit 3
1185: .It Cm LpkBindTimelimit
1.9 agc 1186: Specifies the bind time limit before the connection is considered dead. value is
1.7 adam 1187: in seconds.
1188: .Pp
1189: LpkBindTimelimit 3
1.1 christos 1190: .El
1191: .Sh TIME FORMATS
1192: .Xr sshd 8
1193: command-line arguments and configuration file options that specify time
1194: may be expressed using a sequence of the form:
1195: .Sm off
1196: .Ar time Op Ar qualifier ,
1197: .Sm on
1198: where
1199: .Ar time
1200: is a positive integer value and
1201: .Ar qualifier
1202: is one of the following:
1203: .Pp
1204: .Bl -tag -width Ds -compact -offset indent
1205: .It Aq Cm none
1206: seconds
1207: .It Cm s | Cm S
1208: seconds
1209: .It Cm m | Cm M
1210: minutes
1211: .It Cm h | Cm H
1212: hours
1213: .It Cm d | Cm D
1214: days
1215: .It Cm w | Cm W
1216: weeks
1217: .El
1218: .Pp
1219: Each member of the sequence is added together to calculate
1220: the total time value.
1221: .Pp
1222: Time format examples:
1223: .Pp
1224: .Bl -tag -width Ds -compact -offset indent
1225: .It 600
1226: 600 seconds (10 minutes)
1227: .It 10m
1228: 10 minutes
1229: .It 1h30m
1230: 1 hour 30 minutes (90 minutes)
1231: .El
1232: .Sh FILES
1233: .Bl -tag -width Ds
1234: .It Pa /etc/ssh/sshd_config
1235: Contains configuration data for
1236: .Xr sshd 8 .
1237: This file should be writable by root only, but it is recommended
1238: (though not necessary) that it be world-readable.
1239: .El
1240: .Sh SEE ALSO
1241: .Xr sshd 8
1242: .Sh AUTHORS
1243: OpenSSH is a derivative of the original and free
1244: ssh 1.2.12 release by Tatu Ylonen.
1245: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1246: Theo de Raadt and Dug Song
1247: removed many bugs, re-added newer features and
1248: created OpenSSH.
1249: Markus Friedl contributed the support for SSH
1250: protocol versions 1.5 and 2.0.
1251: Niels Provos and Markus Friedl contributed support
1252: for privilege separation.
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