Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.69
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
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18: .\" modification, are permitted provided that the following conditions
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.69 ! dtucker 37: .\" $OpenBSD: sshd_config.5,v 1.68 2006/07/21 12:43:36 dtucker 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
1.53 jmc 49: .Xr sshd 8
1.1 stevesk 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.
1.56 dtucker 59: Arguments may optionally be enclosed in double quotes
60: .Pq \&"
61: in order to represent arguments containing spaces.
1.1 stevesk 62: .Pp
63: The possible
64: keywords and their meanings are as follows (note that
65: keywords are case-insensitive and arguments are case-sensitive):
66: .Bl -tag -width Ds
1.30 djm 67: .It Cm AcceptEnv
68: Specifies what environment variables sent by the client will be copied into
69: the session's
70: .Xr environ 7 .
71: See
72: .Cm SendEnv
73: in
74: .Xr ssh_config 5
75: for how to configure the client.
1.31 djm 76: Note that environment passing is only supported for protocol 2.
1.30 djm 77: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 78: .Ql *
1.30 djm 79: and
80: .Ql \&? .
1.31 djm 81: Multiple environment variables may be separated by whitespace or spread
1.30 djm 82: across multiple
83: .Cm AcceptEnv
84: directives.
1.31 djm 85: Be warned that some environment variables could be used to bypass restricted
1.30 djm 86: user environments.
87: For this reason, care should be taken in the use of this directive.
88: The default is not to accept any environment variables.
1.37 djm 89: .It Cm AddressFamily
90: Specifies which address family should be used by
1.53 jmc 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
93: .Dq any ,
94: .Dq inet
1.52 jmc 95: (use IPv4 only), or
1.37 djm 96: .Dq inet6
97: (use IPv6 only).
98: The default is
99: .Dq any .
1.1 stevesk 100: .It Cm AllowGroups
101: This keyword can be followed by a list of group name patterns, separated
102: by spaces.
103: If specified, login is allowed only for users whose primary
104: group or supplementary group list matches one of the patterns.
105: Only group names are valid; a numerical group ID is not recognized.
106: By default, login is allowed for all groups.
1.54 jmc 107: The allow/deny directives are processed in the following order:
108: .Cm DenyUsers ,
109: .Cm AllowUsers ,
110: .Cm DenyGroups ,
111: and finally
112: .Cm AllowGroups .
1.49 jmc 113: .Pp
114: See
115: .Sx PATTERNS
116: in
117: .Xr ssh_config 5
118: for more information on patterns.
1.1 stevesk 119: .It Cm AllowTcpForwarding
120: Specifies whether TCP forwarding is permitted.
121: The default is
122: .Dq yes .
123: Note that disabling TCP forwarding does not improve security unless
124: users are also denied shell access, as they can always install their
125: own forwarders.
126: .It Cm AllowUsers
127: This keyword can be followed by a list of user name patterns, separated
128: by spaces.
1.14 jmc 129: If specified, login is allowed only for user names that
1.1 stevesk 130: match one of the patterns.
131: Only user names are valid; a numerical user ID is not recognized.
132: By default, login is allowed for all users.
133: If the pattern takes the form USER@HOST then USER and HOST
134: are separately checked, restricting logins to particular
135: users from particular hosts.
1.54 jmc 136: The allow/deny directives are processed in the following order:
137: .Cm DenyUsers ,
138: .Cm AllowUsers ,
139: .Cm DenyGroups ,
140: and finally
141: .Cm AllowGroups .
1.49 jmc 142: .Pp
143: See
144: .Sx PATTERNS
145: in
146: .Xr ssh_config 5
147: for more information on patterns.
1.1 stevesk 148: .It Cm AuthorizedKeysFile
149: Specifies the file that contains the public keys that can be used
150: for user authentication.
151: .Cm AuthorizedKeysFile
152: may contain tokens of the form %T which are substituted during connection
1.52 jmc 153: setup.
1.17 jmc 154: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 155: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 156: %u is replaced by the username of that user.
157: After expansion,
158: .Cm AuthorizedKeysFile
159: is taken to be an absolute path or one relative to the user's home
160: directory.
161: The default is
162: .Dq .ssh/authorized_keys .
163: .It Cm Banner
164: In some jurisdictions, sending a warning message before authentication
165: may be relevant for getting legal protection.
166: The contents of the specified file are sent to the remote user before
167: authentication is allowed.
168: This option is only available for protocol version 2.
169: By default, no banner is displayed.
170: .It Cm ChallengeResponseAuthentication
1.50 jmc 171: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 172: All authentication styles from
173: .Xr login.conf 5
174: are supported.
175: The default is
176: .Dq yes .
177: .It Cm Ciphers
178: Specifies the ciphers allowed for protocol version 2.
179: Multiple ciphers must be comma-separated.
1.34 dtucker 180: The supported ciphers are
181: .Dq 3des-cbc ,
182: .Dq aes128-cbc ,
183: .Dq aes192-cbc ,
184: .Dq aes256-cbc ,
185: .Dq aes128-ctr ,
186: .Dq aes192-ctr ,
187: .Dq aes256-ctr ,
1.43 djm 188: .Dq arcfour128 ,
189: .Dq arcfour256 ,
1.34 dtucker 190: .Dq arcfour ,
191: .Dq blowfish-cbc ,
192: and
193: .Dq cast128-cbc .
1.52 jmc 194: The default is:
195: .Bd -literal -offset 3n
196: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
197: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
198: aes192-ctr,aes256-ctr
1.1 stevesk 199: .Ed
200: .It Cm ClientAliveCountMax
1.48 jmc 201: Sets the number of client alive messages (see below) which may be
1.1 stevesk 202: sent without
1.52 jmc 203: .Xr sshd 8
1.17 jmc 204: receiving any messages back from the client.
205: If this threshold is reached while client alive messages are being sent,
1.52 jmc 206: sshd will disconnect the client, terminating the session.
1.17 jmc 207: It is important to note that the use of client alive messages is very
208: different from
1.27 markus 209: .Cm TCPKeepAlive
1.17 jmc 210: (below).
211: The client alive messages are sent through the encrypted channel
212: and therefore will not be spoofable.
213: The TCP keepalive option enabled by
1.27 markus 214: .Cm TCPKeepAlive
1.17 jmc 215: is spoofable.
216: The client alive mechanism is valuable when the client or
1.1 stevesk 217: server depend on knowing when a connection has become inactive.
218: .Pp
1.17 jmc 219: The default value is 3.
220: If
1.1 stevesk 221: .Cm ClientAliveInterval
1.48 jmc 222: (see below) is set to 15, and
1.1 stevesk 223: .Cm ClientAliveCountMax
1.52 jmc 224: is left at the default, unresponsive SSH clients
1.1 stevesk 225: will be disconnected after approximately 45 seconds.
1.57 markus 226: This option applies to protocol version 2 only.
1.42 djm 227: .It Cm ClientAliveInterval
228: Sets a timeout interval in seconds after which if no data has been received
229: from the client,
1.52 jmc 230: .Xr sshd 8
1.42 djm 231: will send a message through the encrypted
232: channel to request a response from the client.
233: The default
234: is 0, indicating that these messages will not be sent to the client.
235: This option applies to protocol version 2 only.
1.3 markus 236: .It Cm Compression
1.44 markus 237: Specifies whether compression is allowed, or delayed until
238: the user has authenticated successfully.
1.3 markus 239: The argument must be
1.44 markus 240: .Dq yes ,
241: .Dq delayed ,
1.3 markus 242: or
243: .Dq no .
244: The default is
1.44 markus 245: .Dq delayed .
1.1 stevesk 246: .It Cm DenyGroups
247: This keyword can be followed by a list of group name patterns, separated
248: by spaces.
249: Login is disallowed for users whose primary group or supplementary
250: group list matches one of the patterns.
251: Only group names are valid; a numerical group ID is not recognized.
252: By default, login is allowed for all groups.
1.54 jmc 253: The allow/deny directives are processed in the following order:
254: .Cm DenyUsers ,
255: .Cm AllowUsers ,
256: .Cm DenyGroups ,
257: and finally
258: .Cm AllowGroups .
1.49 jmc 259: .Pp
260: See
261: .Sx PATTERNS
262: in
263: .Xr ssh_config 5
264: for more information on patterns.
1.1 stevesk 265: .It Cm DenyUsers
266: This keyword can be followed by a list of user name patterns, separated
267: by spaces.
268: Login is disallowed for user names that match one of the patterns.
269: Only user names are valid; a numerical user ID is not recognized.
270: By default, login is allowed for all users.
271: If the pattern takes the form USER@HOST then USER and HOST
272: are separately checked, restricting logins to particular
273: users from particular hosts.
1.54 jmc 274: The allow/deny directives are processed in the following order:
275: .Cm DenyUsers ,
276: .Cm AllowUsers ,
277: .Cm DenyGroups ,
278: and finally
279: .Cm AllowGroups .
1.49 jmc 280: .Pp
281: See
282: .Sx PATTERNS
283: in
284: .Xr ssh_config 5
285: for more information on patterns.
1.67 dtucker 286: .It Cm ForceCommand
287: Forces the execution of the command specified by
288: .Cm ForceCommand ,
289: ignoring any command supplied by the client.
290: The command is invoked by using the user's login shell with the -c option.
291: This applies to shell, command, or subsystem execution.
292: It is most useful inside a
293: .Cm Match
294: block.
295: The command originally supplied by the client is available in the
296: .Ev SSH_ORIGINAL_COMMAND
297: environment variable.
1.1 stevesk 298: .It Cm GatewayPorts
299: Specifies whether remote hosts are allowed to connect to ports
300: forwarded for the client.
301: By default,
1.52 jmc 302: .Xr sshd 8
1.15 jmc 303: binds remote port forwardings to the loopback address.
304: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 305: .Cm GatewayPorts
1.52 jmc 306: can be used to specify that sshd
1.39 djm 307: should allow remote port forwardings to bind to non-loopback addresses, thus
308: allowing other hosts to connect.
309: The argument may be
310: .Dq no
311: to force remote port forwardings to be available to the local host only,
1.1 stevesk 312: .Dq yes
1.39 djm 313: to force remote port forwardings to bind to the wildcard address, or
314: .Dq clientspecified
315: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 316: The default is
317: .Dq no .
1.23 markus 318: .It Cm GSSAPIAuthentication
1.25 markus 319: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 320: The default is
1.23 markus 321: .Dq no .
322: Note that this option applies to protocol version 2 only.
323: .It Cm GSSAPICleanupCredentials
324: Specifies whether to automatically destroy the user's credentials cache
325: on logout.
326: The default is
327: .Dq yes .
328: Note that this option applies to protocol version 2 only.
1.1 stevesk 329: .It Cm HostbasedAuthentication
330: Specifies whether rhosts or /etc/hosts.equiv authentication together
331: with successful public key client host authentication is allowed
1.50 jmc 332: (host-based authentication).
1.1 stevesk 333: This option is similar to
334: .Cm RhostsRSAAuthentication
335: and applies to protocol version 2 only.
336: The default is
337: .Dq no .
338: .It Cm HostKey
339: Specifies a file containing a private host key
340: used by SSH.
341: The default is
342: .Pa /etc/ssh/ssh_host_key
343: for protocol version 1, and
344: .Pa /etc/ssh/ssh_host_rsa_key
345: and
346: .Pa /etc/ssh/ssh_host_dsa_key
347: for protocol version 2.
348: Note that
1.52 jmc 349: .Xr sshd 8
1.1 stevesk 350: will refuse to use a file if it is group/world-accessible.
351: It is possible to have multiple host key files.
352: .Dq rsa1
353: keys are used for version 1 and
354: .Dq dsa
355: or
356: .Dq rsa
357: are used for version 2 of the SSH protocol.
358: .It Cm IgnoreRhosts
359: Specifies that
360: .Pa .rhosts
361: and
362: .Pa .shosts
363: files will not be used in
364: .Cm RhostsRSAAuthentication
365: or
366: .Cm HostbasedAuthentication .
367: .Pp
368: .Pa /etc/hosts.equiv
369: and
370: .Pa /etc/shosts.equiv
371: are still used.
372: The default is
373: .Dq yes .
374: .It Cm IgnoreUserKnownHosts
375: Specifies whether
1.52 jmc 376: .Xr sshd 8
1.1 stevesk 377: should ignore the user's
1.41 djm 378: .Pa ~/.ssh/known_hosts
1.1 stevesk 379: during
380: .Cm RhostsRSAAuthentication
381: or
382: .Cm HostbasedAuthentication .
383: The default is
384: .Dq no .
385: .It Cm KerberosAuthentication
1.24 markus 386: Specifies whether the password provided by the user for
1.1 stevesk 387: .Cm PasswordAuthentication
1.24 markus 388: will be validated through the Kerberos KDC.
1.1 stevesk 389: To use this option, the server needs a
390: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 391: The default is
1.29 dtucker 392: .Dq no .
393: .It Cm KerberosGetAFSToken
1.45 djm 394: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 395: an AFS token before accessing the user's home directory.
1.52 jmc 396: The default is
1.1 stevesk 397: .Dq no .
398: .It Cm KerberosOrLocalPasswd
1.52 jmc 399: If password authentication through Kerberos fails then
1.1 stevesk 400: the password will be validated via any additional local mechanism
401: such as
402: .Pa /etc/passwd .
1.52 jmc 403: The default is
1.1 stevesk 404: .Dq yes .
405: .It Cm KerberosTicketCleanup
406: Specifies whether to automatically destroy the user's ticket cache
407: file on logout.
1.52 jmc 408: The default is
1.1 stevesk 409: .Dq yes .
410: .It Cm KeyRegenerationInterval
411: In protocol version 1, the ephemeral server key is automatically regenerated
412: after this many seconds (if it has been used).
413: The purpose of regeneration is to prevent
414: decrypting captured sessions by later breaking into the machine and
415: stealing the keys.
416: The key is never stored anywhere.
417: If the value is 0, the key is never regenerated.
418: The default is 3600 (seconds).
419: .It Cm ListenAddress
420: Specifies the local addresses
1.52 jmc 421: .Xr sshd 8
1.1 stevesk 422: should listen on.
423: The following forms may be used:
424: .Pp
425: .Bl -item -offset indent -compact
426: .It
427: .Cm ListenAddress
428: .Sm off
429: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
430: .Sm on
431: .It
432: .Cm ListenAddress
433: .Sm off
434: .Ar host No | Ar IPv4_addr No : Ar port
435: .Sm on
436: .It
437: .Cm ListenAddress
438: .Sm off
439: .Oo
440: .Ar host No | Ar IPv6_addr Oc : Ar port
441: .Sm on
442: .El
443: .Pp
444: If
445: .Ar port
446: is not specified,
1.52 jmc 447: sshd will listen on the address and all prior
1.1 stevesk 448: .Cm Port
1.17 jmc 449: options specified.
450: The default is to listen on all local addresses.
1.15 jmc 451: Multiple
1.1 stevesk 452: .Cm ListenAddress
1.17 jmc 453: options are permitted.
454: Additionally, any
1.1 stevesk 455: .Cm Port
1.52 jmc 456: options must precede this option for non-port qualified addresses.
1.1 stevesk 457: .It Cm LoginGraceTime
458: The server disconnects after this time if the user has not
459: successfully logged in.
460: If the value is 0, there is no time limit.
1.12 stevesk 461: The default is 120 seconds.
1.1 stevesk 462: .It Cm LogLevel
463: Gives the verbosity level that is used when logging messages from
1.53 jmc 464: .Xr sshd 8 .
1.1 stevesk 465: The possible values are:
1.52 jmc 466: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 467: The default is INFO.
468: DEBUG and DEBUG1 are equivalent.
469: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
470: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 471: .It Cm MACs
472: Specifies the available MAC (message authentication code) algorithms.
473: The MAC algorithm is used in protocol version 2
474: for data integrity protection.
475: Multiple algorithms must be comma-separated.
1.52 jmc 476: The default is:
1.1 stevesk 477: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.60 dtucker 478: .It Cm Match
1.61 jmc 479: Introduces a conditional block.
1.65 dtucker 480: If all of the criteria on the
1.60 dtucker 481: .Cm Match
1.65 dtucker 482: line are satisfied, the keywords on the following lines override those
483: set in the global section of the config file, until either another
1.60 dtucker 484: .Cm Match
1.65 dtucker 485: line or the end of the file.
1.61 jmc 486: The arguments to
1.60 dtucker 487: .Cm Match
1.65 dtucker 488: are one or more criteria-pattern pairs.
1.60 dtucker 489: The available criteria are
490: .Cm User ,
1.69 ! dtucker 491: .Cm Group ,
1.60 dtucker 492: .Cm Host ,
493: and
494: .Cm Address .
495: Only a subset of keywords may be used on the lines following a
496: .Cm Match
497: keyword.
498: Available keywords are
1.62 dtucker 499: .Cm AllowTcpForwarding ,
1.67 dtucker 500: .Cm ForceCommand ,
1.62 dtucker 501: .Cm GatewayPorts ,
1.66 dtucker 502: .Cm PermitOpen ,
503: .Cm X11DisplayOffset ,
504: .Cm X11Forwarding ,
1.60 dtucker 505: and
1.66 dtucker 506: .Cm X11UseLocalHost .
1.33 dtucker 507: .It Cm MaxAuthTries
508: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 509: connection.
510: Once the number of failures reaches half this value,
511: additional failures are logged.
512: The default is 6.
1.1 stevesk 513: .It Cm MaxStartups
514: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 515: SSH daemon.
1.1 stevesk 516: Additional connections will be dropped until authentication succeeds or the
517: .Cm LoginGraceTime
518: expires for a connection.
519: The default is 10.
520: .Pp
521: Alternatively, random early drop can be enabled by specifying
522: the three colon separated values
523: .Dq start:rate:full
1.51 jmc 524: (e.g. "10:30:60").
1.53 jmc 525: .Xr sshd 8
1.1 stevesk 526: will refuse connection attempts with a probability of
527: .Dq rate/100
528: (30%)
529: if there are currently
530: .Dq start
531: (10)
532: unauthenticated connections.
533: The probability increases linearly and all connection attempts
534: are refused if the number of unauthenticated connections reaches
535: .Dq full
536: (60).
537: .It Cm PasswordAuthentication
538: Specifies whether password authentication is allowed.
539: The default is
540: .Dq yes .
541: .It Cm PermitEmptyPasswords
542: When password authentication is allowed, it specifies whether the
543: server allows login to accounts with empty password strings.
544: The default is
545: .Dq no .
1.62 dtucker 546: .It Cm PermitOpen
547: Specifies the destinations to which TCP port forwarding is permitted.
548: The forwarding specification must be one of the following forms:
549: .Pp
550: .Bl -item -offset indent -compact
551: .It
552: .Cm PermitOpen
553: .Sm off
554: .Ar host : port
555: .Sm on
556: .It
557: .Cm PermitOpen
558: .Sm off
559: .Ar IPv4_addr : port
560: .Sm on
561: .It
562: .Cm PermitOpen
563: .Sm off
564: .Ar \&[ IPv6_addr \&] : port
565: .Sm on
566: .El
567: .Pp
1.68 dtucker 568: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 569: An argument of
570: .Dq any
571: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 572: By default all port forwarding requests are permitted.
1.1 stevesk 573: .It Cm PermitRootLogin
1.38 jmc 574: Specifies whether root can log in using
1.1 stevesk 575: .Xr ssh 1 .
576: The argument must be
577: .Dq yes ,
578: .Dq without-password ,
1.52 jmc 579: .Dq forced-commands-only ,
1.1 stevesk 580: or
581: .Dq no .
582: The default is
583: .Dq yes .
584: .Pp
585: If this option is set to
1.52 jmc 586: .Dq without-password ,
1.1 stevesk 587: password authentication is disabled for root.
588: .Pp
589: If this option is set to
1.52 jmc 590: .Dq forced-commands-only ,
1.1 stevesk 591: root login with public key authentication will be allowed,
592: but only if the
593: .Ar command
594: option has been specified
595: (which may be useful for taking remote backups even if root login is
1.17 jmc 596: normally not allowed).
597: All other authentication methods are disabled for root.
1.1 stevesk 598: .Pp
599: If this option is set to
1.52 jmc 600: .Dq no ,
1.38 jmc 601: root is not allowed to log in.
1.46 reyk 602: .It Cm PermitTunnel
603: Specifies whether
604: .Xr tun 4
605: device forwarding is allowed.
1.47 reyk 606: The argument must be
607: .Dq yes ,
1.58 stevesk 608: .Dq point-to-point
609: (layer 3),
610: .Dq ethernet
611: (layer 2), or
1.47 reyk 612: .Dq no .
1.58 stevesk 613: Specifying
614: .Dq yes
615: permits both
616: .Dq point-to-point
617: and
618: .Dq ethernet .
1.46 reyk 619: The default is
620: .Dq no .
1.6 markus 621: .It Cm PermitUserEnvironment
622: Specifies whether
623: .Pa ~/.ssh/environment
1.9 stevesk 624: and
1.6 markus 625: .Cm environment=
626: options in
627: .Pa ~/.ssh/authorized_keys
1.9 stevesk 628: are processed by
1.52 jmc 629: .Xr sshd 8 .
1.6 markus 630: The default is
631: .Dq no .
1.9 stevesk 632: Enabling environment processing may enable users to bypass access
633: restrictions in some configurations using mechanisms such as
634: .Ev LD_PRELOAD .
1.1 stevesk 635: .It Cm PidFile
1.4 stevesk 636: Specifies the file that contains the process ID of the
1.53 jmc 637: SSH daemon.
1.1 stevesk 638: The default is
639: .Pa /var/run/sshd.pid .
640: .It Cm Port
641: Specifies the port number that
1.52 jmc 642: .Xr sshd 8
1.1 stevesk 643: listens on.
644: The default is 22.
645: Multiple options of this type are permitted.
646: See also
647: .Cm ListenAddress .
648: .It Cm PrintLastLog
649: Specifies whether
1.52 jmc 650: .Xr sshd 8
1.36 jaredy 651: should print the date and time of the last user login when a user logs
652: in interactively.
1.1 stevesk 653: The default is
654: .Dq yes .
655: .It Cm PrintMotd
656: Specifies whether
1.52 jmc 657: .Xr sshd 8
1.1 stevesk 658: should print
659: .Pa /etc/motd
660: when a user logs in interactively.
661: (On some systems it is also printed by the shell,
662: .Pa /etc/profile ,
663: or equivalent.)
664: The default is
665: .Dq yes .
666: .It Cm Protocol
667: Specifies the protocol versions
1.52 jmc 668: .Xr sshd 8
1.5 stevesk 669: supports.
1.1 stevesk 670: The possible values are
1.52 jmc 671: .Sq 1
1.1 stevesk 672: and
1.52 jmc 673: .Sq 2 .
1.1 stevesk 674: Multiple versions must be comma-separated.
675: The default is
676: .Dq 2,1 .
1.5 stevesk 677: Note that the order of the protocol list does not indicate preference,
678: because the client selects among multiple protocol versions offered
679: by the server.
680: Specifying
681: .Dq 2,1
682: is identical to
683: .Dq 1,2 .
1.1 stevesk 684: .It Cm PubkeyAuthentication
685: Specifies whether public key authentication is allowed.
686: The default is
687: .Dq yes .
688: Note that this option applies to protocol version 2 only.
689: .It Cm RhostsRSAAuthentication
690: Specifies whether rhosts or /etc/hosts.equiv authentication together
691: with successful RSA host authentication is allowed.
692: The default is
693: .Dq no .
694: This option applies to protocol version 1 only.
695: .It Cm RSAAuthentication
696: Specifies whether pure RSA authentication is allowed.
697: The default is
698: .Dq yes .
699: This option applies to protocol version 1 only.
700: .It Cm ServerKeyBits
701: Defines the number of bits in the ephemeral protocol version 1 server key.
702: The minimum value is 512, and the default is 768.
703: .It Cm StrictModes
704: Specifies whether
1.52 jmc 705: .Xr sshd 8
1.1 stevesk 706: should check file modes and ownership of the
707: user's files and home directory before accepting login.
708: This is normally desirable because novices sometimes accidentally leave their
709: directory or files world-writable.
710: The default is
711: .Dq yes .
712: .It Cm Subsystem
1.51 jmc 713: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 714: Arguments should be a subsystem name and a command (with optional arguments)
715: to execute upon subsystem request.
1.1 stevesk 716: The command
717: .Xr sftp-server 8
718: implements the
719: .Dq sftp
720: file transfer subsystem.
721: By default no subsystems are defined.
722: Note that this option applies to protocol version 2 only.
723: .It Cm SyslogFacility
724: Gives the facility code that is used when logging messages from
1.53 jmc 725: .Xr sshd 8 .
1.1 stevesk 726: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
727: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
728: The default is AUTH.
1.27 markus 729: .It Cm TCPKeepAlive
730: Specifies whether the system should send TCP keepalive messages to the
731: other side.
732: If they are sent, death of the connection or crash of one
733: of the machines will be properly noticed.
734: However, this means that
735: connections will die if the route is down temporarily, and some people
736: find it annoying.
737: On the other hand, if TCP keepalives are not sent,
738: sessions may hang indefinitely on the server, leaving
739: .Dq ghost
740: users and consuming server resources.
741: .Pp
742: The default is
743: .Dq yes
744: (to send TCP keepalive messages), and the server will notice
745: if the network goes down or the client host crashes.
746: This avoids infinitely hanging sessions.
747: .Pp
748: To disable TCP keepalive messages, the value should be set to
749: .Dq no .
1.18 markus 750: .It Cm UseDNS
751: Specifies whether
1.52 jmc 752: .Xr sshd 8
1.40 jmc 753: should look up the remote host name and check that
1.18 markus 754: the resolved host name for the remote IP address maps back to the
755: very same IP address.
756: The default is
757: .Dq yes .
1.1 stevesk 758: .It Cm UseLogin
759: Specifies whether
760: .Xr login 1
761: is used for interactive login sessions.
762: The default is
763: .Dq no .
764: Note that
765: .Xr login 1
766: is never used for remote command execution.
767: Note also, that if this is enabled,
768: .Cm X11Forwarding
769: will be disabled because
770: .Xr login 1
771: does not know how to handle
772: .Xr xauth 1
1.15 jmc 773: cookies.
774: If
1.1 stevesk 775: .Cm UsePrivilegeSeparation
776: is specified, it will be disabled after authentication.
777: .It Cm UsePrivilegeSeparation
778: Specifies whether
1.52 jmc 779: .Xr sshd 8
1.2 stevesk 780: separates privileges by creating an unprivileged child process
1.15 jmc 781: to deal with incoming network traffic.
782: After successful authentication, another process will be created that has
783: the privilege of the authenticated user.
784: The goal of privilege separation is to prevent privilege
1.1 stevesk 785: escalation by containing any corruption within the unprivileged processes.
786: The default is
787: .Dq yes .
788: .It Cm X11DisplayOffset
789: Specifies the first display number available for
1.52 jmc 790: .Xr sshd 8 Ns 's
1.1 stevesk 791: X11 forwarding.
1.52 jmc 792: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 793: The default is 10.
794: .It Cm X11Forwarding
795: Specifies whether X11 forwarding is permitted.
1.13 stevesk 796: The argument must be
797: .Dq yes
798: or
799: .Dq no .
1.1 stevesk 800: The default is
801: .Dq no .
1.13 stevesk 802: .Pp
803: When X11 forwarding is enabled, there may be additional exposure to
804: the server and to client displays if the
1.52 jmc 805: .Xr sshd 8
1.13 stevesk 806: proxy display is configured to listen on the wildcard address (see
807: .Cm X11UseLocalhost
1.52 jmc 808: below), though this is not the default.
1.13 stevesk 809: Additionally, the authentication spoofing and authentication data
810: verification and substitution occur on the client side.
811: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 812: display server may be exposed to attack when the SSH client requests
1.13 stevesk 813: forwarding (see the warnings for
814: .Cm ForwardX11
815: in
1.19 jmc 816: .Xr ssh_config 5 ) .
1.13 stevesk 817: A system administrator may have a stance in which they want to
818: protect clients that may expose themselves to attack by unwittingly
819: requesting X11 forwarding, which can warrant a
820: .Dq no
821: setting.
822: .Pp
823: Note that disabling X11 forwarding does not prevent users from
824: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 825: X11 forwarding is automatically disabled if
826: .Cm UseLogin
827: is enabled.
828: .It Cm X11UseLocalhost
829: Specifies whether
1.52 jmc 830: .Xr sshd 8
1.1 stevesk 831: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 832: the wildcard address.
833: By default,
1.52 jmc 834: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 835: hostname part of the
836: .Ev DISPLAY
837: environment variable to
838: .Dq localhost .
1.8 stevesk 839: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 840: However, some older X11 clients may not function with this
841: configuration.
842: .Cm X11UseLocalhost
843: may be set to
844: .Dq no
845: to specify that the forwarding server should be bound to the wildcard
846: address.
847: The argument must be
848: .Dq yes
849: or
850: .Dq no .
851: The default is
852: .Dq yes .
853: .It Cm XAuthLocation
1.11 stevesk 854: Specifies the full pathname of the
1.1 stevesk 855: .Xr xauth 1
856: program.
857: The default is
858: .Pa /usr/X11R6/bin/xauth .
859: .El
1.55 jmc 860: .Sh TIME FORMATS
1.53 jmc 861: .Xr sshd 8
1.1 stevesk 862: command-line arguments and configuration file options that specify time
863: may be expressed using a sequence of the form:
864: .Sm off
1.7 stevesk 865: .Ar time Op Ar qualifier ,
1.1 stevesk 866: .Sm on
867: where
868: .Ar time
869: is a positive integer value and
870: .Ar qualifier
871: is one of the following:
872: .Pp
873: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 874: .It Aq Cm none
1.1 stevesk 875: seconds
876: .It Cm s | Cm S
877: seconds
878: .It Cm m | Cm M
879: minutes
880: .It Cm h | Cm H
881: hours
882: .It Cm d | Cm D
883: days
884: .It Cm w | Cm W
885: weeks
886: .El
887: .Pp
888: Each member of the sequence is added together to calculate
889: the total time value.
890: .Pp
891: Time format examples:
892: .Pp
893: .Bl -tag -width Ds -compact -offset indent
894: .It 600
895: 600 seconds (10 minutes)
896: .It 10m
897: 10 minutes
898: .It 1h30m
899: 1 hour 30 minutes (90 minutes)
900: .El
901: .Sh FILES
902: .Bl -tag -width Ds
903: .It Pa /etc/ssh/sshd_config
904: Contains configuration data for
1.53 jmc 905: .Xr sshd 8 .
1.1 stevesk 906: This file should be writable by root only, but it is recommended
907: (though not necessary) that it be world-readable.
908: .El
1.19 jmc 909: .Sh SEE ALSO
910: .Xr sshd 8
1.1 stevesk 911: .Sh AUTHORS
912: OpenSSH is a derivative of the original and free
913: ssh 1.2.12 release by Tatu Ylonen.
914: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
915: Theo de Raadt and Dug Song
916: removed many bugs, re-added newer features and
917: created OpenSSH.
918: Markus Friedl contributed the support for SSH
919: protocol versions 1.5 and 2.0.
920: Niels Provos and Markus Friedl contributed support
921: for privilege separation.
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