Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.44.2.2
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: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
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26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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1.44.2.2! brad 37: .\" $OpenBSD: sshd_config.5,v 1.70 2006/08/21 08:14:01 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
1.44.2.2! brad 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.
1.44.2.2! brad 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
1.44.2.2! brad 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
1.44.2.2! brad 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
93: .Dq any ,
94: .Dq inet
1.44.2.2! brad 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.
1.44.2.2! brad 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 .
! 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.
1.44.2.2! brad 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 .
! 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
1.44.2.2! brad 153: setup.
1.17 jmc 154: The following tokens are defined: %% is replaced by a literal '%',
1.44.2.2! brad 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
1.44.2.2! brad 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 .
1.44.2.2! brad 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
1.44.2.1 brad 201: Sets the number of client alive messages (see below) which may be
1.1 stevesk 202: sent without
1.44.2.2! brad 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,
1.44.2.2! brad 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
1.44.2.1 brad 222: (see below) is set to 15, and
1.1 stevesk 223: .Cm ClientAliveCountMax
1.44.2.2! brad 224: is left at the default, unresponsive SSH clients
1.1 stevesk 225: will be disconnected after approximately 45 seconds.
1.44.2.2! brad 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,
1.44.2.2! brad 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.
1.44.2.2! brad 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 .
! 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.
1.44.2.2! brad 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 .
! 280: .Pp
! 281: See
! 282: .Sx PATTERNS
! 283: in
! 284: .Xr ssh_config 5
! 285: for more information on patterns.
! 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,
1.44.2.2! brad 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
1.44.2.2! brad 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
1.44.2.2! brad 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 .
1.44.2.2! brad 338: .It Cm HostbasedUsesNameFromPacketOnly
! 339: Specifies whether or not the server will attempt to perform a reverse
! 340: name lookup when matching the name in the
! 341: .Pa ~/.shosts ,
! 342: .Pa ~/.rhosts ,
! 343: and
! 344: .Pa /etc/hosts.equiv
! 345: files during
! 346: .Cm HostbasedAuthentication .
! 347: A setting of
! 348: .Dq yes
! 349: means that
! 350: .Xr sshd 8
! 351: uses the name supplied by the client rather than
! 352: attempting to resolve the name from the TCP connection itself.
! 353: The default is
! 354: .Dq no .
1.1 stevesk 355: .It Cm HostKey
356: Specifies a file containing a private host key
357: used by SSH.
358: The default is
359: .Pa /etc/ssh/ssh_host_key
360: for protocol version 1, and
361: .Pa /etc/ssh/ssh_host_rsa_key
362: and
363: .Pa /etc/ssh/ssh_host_dsa_key
364: for protocol version 2.
365: Note that
1.44.2.2! brad 366: .Xr sshd 8
1.1 stevesk 367: will refuse to use a file if it is group/world-accessible.
368: It is possible to have multiple host key files.
369: .Dq rsa1
370: keys are used for version 1 and
371: .Dq dsa
372: or
373: .Dq rsa
374: are used for version 2 of the SSH protocol.
375: .It Cm IgnoreRhosts
376: Specifies that
377: .Pa .rhosts
378: and
379: .Pa .shosts
380: files will not be used in
381: .Cm RhostsRSAAuthentication
382: or
383: .Cm HostbasedAuthentication .
384: .Pp
385: .Pa /etc/hosts.equiv
386: and
387: .Pa /etc/shosts.equiv
388: are still used.
389: The default is
390: .Dq yes .
391: .It Cm IgnoreUserKnownHosts
392: Specifies whether
1.44.2.2! brad 393: .Xr sshd 8
1.1 stevesk 394: should ignore the user's
1.41 djm 395: .Pa ~/.ssh/known_hosts
1.1 stevesk 396: during
397: .Cm RhostsRSAAuthentication
398: or
399: .Cm HostbasedAuthentication .
400: The default is
401: .Dq no .
402: .It Cm KerberosAuthentication
1.24 markus 403: Specifies whether the password provided by the user for
1.1 stevesk 404: .Cm PasswordAuthentication
1.24 markus 405: will be validated through the Kerberos KDC.
1.1 stevesk 406: To use this option, the server needs a
407: Kerberos servtab which allows the verification of the KDC's identity.
1.44.2.2! brad 408: The default is
1.29 dtucker 409: .Dq no .
410: .It Cm KerberosGetAFSToken
1.44.2.1 brad 411: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 412: an AFS token before accessing the user's home directory.
1.44.2.2! brad 413: The default is
1.1 stevesk 414: .Dq no .
415: .It Cm KerberosOrLocalPasswd
1.44.2.2! brad 416: If password authentication through Kerberos fails then
1.1 stevesk 417: the password will be validated via any additional local mechanism
418: such as
419: .Pa /etc/passwd .
1.44.2.2! brad 420: The default is
1.1 stevesk 421: .Dq yes .
422: .It Cm KerberosTicketCleanup
423: Specifies whether to automatically destroy the user's ticket cache
424: file on logout.
1.44.2.2! brad 425: The default is
1.1 stevesk 426: .Dq yes .
427: .It Cm KeyRegenerationInterval
428: In protocol version 1, the ephemeral server key is automatically regenerated
429: after this many seconds (if it has been used).
430: The purpose of regeneration is to prevent
431: decrypting captured sessions by later breaking into the machine and
432: stealing the keys.
433: The key is never stored anywhere.
434: If the value is 0, the key is never regenerated.
435: The default is 3600 (seconds).
436: .It Cm ListenAddress
437: Specifies the local addresses
1.44.2.2! brad 438: .Xr sshd 8
1.1 stevesk 439: should listen on.
440: The following forms may be used:
441: .Pp
442: .Bl -item -offset indent -compact
443: .It
444: .Cm ListenAddress
445: .Sm off
446: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
447: .Sm on
448: .It
449: .Cm ListenAddress
450: .Sm off
451: .Ar host No | Ar IPv4_addr No : Ar port
452: .Sm on
453: .It
454: .Cm ListenAddress
455: .Sm off
456: .Oo
457: .Ar host No | Ar IPv6_addr Oc : Ar port
458: .Sm on
459: .El
460: .Pp
461: If
462: .Ar port
463: is not specified,
1.44.2.2! brad 464: sshd will listen on the address and all prior
1.1 stevesk 465: .Cm Port
1.17 jmc 466: options specified.
467: The default is to listen on all local addresses.
1.15 jmc 468: Multiple
1.1 stevesk 469: .Cm ListenAddress
1.17 jmc 470: options are permitted.
471: Additionally, any
1.1 stevesk 472: .Cm Port
1.44.2.2! brad 473: options must precede this option for non-port qualified addresses.
1.1 stevesk 474: .It Cm LoginGraceTime
475: The server disconnects after this time if the user has not
476: successfully logged in.
477: If the value is 0, there is no time limit.
1.12 stevesk 478: The default is 120 seconds.
1.1 stevesk 479: .It Cm LogLevel
480: Gives the verbosity level that is used when logging messages from
1.44.2.2! brad 481: .Xr sshd 8 .
1.1 stevesk 482: The possible values are:
1.44.2.2! brad 483: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 484: The default is INFO.
485: DEBUG and DEBUG1 are equivalent.
486: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
487: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 488: .It Cm MACs
489: Specifies the available MAC (message authentication code) algorithms.
490: The MAC algorithm is used in protocol version 2
491: for data integrity protection.
492: Multiple algorithms must be comma-separated.
1.44.2.2! brad 493: The default is:
1.1 stevesk 494: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.44.2.2! brad 495: .It Cm Match
! 496: Introduces a conditional block.
! 497: If all of the criteria on the
! 498: .Cm Match
! 499: line are satisfied, the keywords on the following lines override those
! 500: set in the global section of the config file, until either another
! 501: .Cm Match
! 502: line or the end of the file.
! 503: The arguments to
! 504: .Cm Match
! 505: are one or more criteria-pattern pairs.
! 506: The available criteria are
! 507: .Cm User ,
! 508: .Cm Group ,
! 509: .Cm Host ,
! 510: and
! 511: .Cm Address .
! 512: Only a subset of keywords may be used on the lines following a
! 513: .Cm Match
! 514: keyword.
! 515: Available keywords are
! 516: .Cm AllowTcpForwarding ,
! 517: .Cm ForceCommand ,
! 518: .Cm GatewayPorts ,
! 519: .Cm PermitOpen ,
! 520: .Cm X11DisplayOffset ,
! 521: .Cm X11Forwarding ,
! 522: and
! 523: .Cm X11UseLocalHost .
1.33 dtucker 524: .It Cm MaxAuthTries
525: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 526: connection.
527: Once the number of failures reaches half this value,
528: additional failures are logged.
529: The default is 6.
1.1 stevesk 530: .It Cm MaxStartups
531: Specifies the maximum number of concurrent unauthenticated connections to the
1.44.2.2! brad 532: SSH daemon.
1.1 stevesk 533: Additional connections will be dropped until authentication succeeds or the
534: .Cm LoginGraceTime
535: expires for a connection.
536: The default is 10.
537: .Pp
538: Alternatively, random early drop can be enabled by specifying
539: the three colon separated values
540: .Dq start:rate:full
1.44.2.2! brad 541: (e.g. "10:30:60").
! 542: .Xr sshd 8
1.1 stevesk 543: will refuse connection attempts with a probability of
544: .Dq rate/100
545: (30%)
546: if there are currently
547: .Dq start
548: (10)
549: unauthenticated connections.
550: The probability increases linearly and all connection attempts
551: are refused if the number of unauthenticated connections reaches
552: .Dq full
553: (60).
554: .It Cm PasswordAuthentication
555: Specifies whether password authentication is allowed.
556: The default is
557: .Dq yes .
558: .It Cm PermitEmptyPasswords
559: When password authentication is allowed, it specifies whether the
560: server allows login to accounts with empty password strings.
561: The default is
562: .Dq no .
1.44.2.2! brad 563: .It Cm PermitOpen
! 564: Specifies the destinations to which TCP port forwarding is permitted.
! 565: The forwarding specification must be one of the following forms:
! 566: .Pp
! 567: .Bl -item -offset indent -compact
! 568: .It
! 569: .Cm PermitOpen
! 570: .Sm off
! 571: .Ar host : port
! 572: .Sm on
! 573: .It
! 574: .Cm PermitOpen
! 575: .Sm off
! 576: .Ar IPv4_addr : port
! 577: .Sm on
! 578: .It
! 579: .Cm PermitOpen
! 580: .Sm off
! 581: .Ar \&[ IPv6_addr \&] : port
! 582: .Sm on
! 583: .El
! 584: .Pp
! 585: Multiple forwards may be specified by separating them with whitespace.
! 586: An argument of
! 587: .Dq any
! 588: can be used to remove all restrictions and permit any forwarding requests.
! 589: By default all port forwarding requests are permitted.
1.1 stevesk 590: .It Cm PermitRootLogin
1.38 jmc 591: Specifies whether root can log in using
1.1 stevesk 592: .Xr ssh 1 .
593: The argument must be
594: .Dq yes ,
595: .Dq without-password ,
1.44.2.2! brad 596: .Dq forced-commands-only ,
1.1 stevesk 597: or
598: .Dq no .
599: The default is
600: .Dq yes .
601: .Pp
602: If this option is set to
1.44.2.2! brad 603: .Dq without-password ,
1.1 stevesk 604: password authentication is disabled for root.
605: .Pp
606: If this option is set to
1.44.2.2! brad 607: .Dq forced-commands-only ,
1.1 stevesk 608: root login with public key authentication will be allowed,
609: but only if the
610: .Ar command
611: option has been specified
612: (which may be useful for taking remote backups even if root login is
1.17 jmc 613: normally not allowed).
614: All other authentication methods are disabled for root.
1.1 stevesk 615: .Pp
616: If this option is set to
1.44.2.2! brad 617: .Dq no ,
1.38 jmc 618: root is not allowed to log in.
1.44.2.1 brad 619: .It Cm PermitTunnel
620: Specifies whether
621: .Xr tun 4
622: device forwarding is allowed.
623: The argument must be
624: .Dq yes ,
1.44.2.2! brad 625: .Dq point-to-point
! 626: (layer 3),
1.44.2.1 brad 627: .Dq ethernet
1.44.2.2! brad 628: (layer 2), or
1.44.2.1 brad 629: .Dq no .
1.44.2.2! brad 630: Specifying
! 631: .Dq yes
! 632: permits both
! 633: .Dq point-to-point
! 634: and
! 635: .Dq ethernet .
1.44.2.1 brad 636: The default is
637: .Dq no .
1.6 markus 638: .It Cm PermitUserEnvironment
639: Specifies whether
640: .Pa ~/.ssh/environment
1.9 stevesk 641: and
1.6 markus 642: .Cm environment=
643: options in
644: .Pa ~/.ssh/authorized_keys
1.9 stevesk 645: are processed by
1.44.2.2! brad 646: .Xr sshd 8 .
1.6 markus 647: The default is
648: .Dq no .
1.9 stevesk 649: Enabling environment processing may enable users to bypass access
650: restrictions in some configurations using mechanisms such as
651: .Ev LD_PRELOAD .
1.1 stevesk 652: .It Cm PidFile
1.4 stevesk 653: Specifies the file that contains the process ID of the
1.44.2.2! brad 654: SSH daemon.
1.1 stevesk 655: The default is
656: .Pa /var/run/sshd.pid .
657: .It Cm Port
658: Specifies the port number that
1.44.2.2! brad 659: .Xr sshd 8
1.1 stevesk 660: listens on.
661: The default is 22.
662: Multiple options of this type are permitted.
663: See also
664: .Cm ListenAddress .
665: .It Cm PrintLastLog
666: Specifies whether
1.44.2.2! brad 667: .Xr sshd 8
1.36 jaredy 668: should print the date and time of the last user login when a user logs
669: in interactively.
1.1 stevesk 670: The default is
671: .Dq yes .
672: .It Cm PrintMotd
673: Specifies whether
1.44.2.2! brad 674: .Xr sshd 8
1.1 stevesk 675: should print
676: .Pa /etc/motd
677: when a user logs in interactively.
678: (On some systems it is also printed by the shell,
679: .Pa /etc/profile ,
680: or equivalent.)
681: The default is
682: .Dq yes .
683: .It Cm Protocol
684: Specifies the protocol versions
1.44.2.2! brad 685: .Xr sshd 8
1.5 stevesk 686: supports.
1.1 stevesk 687: The possible values are
1.44.2.2! brad 688: .Sq 1
1.1 stevesk 689: and
1.44.2.2! brad 690: .Sq 2 .
1.1 stevesk 691: Multiple versions must be comma-separated.
692: The default is
693: .Dq 2,1 .
1.5 stevesk 694: Note that the order of the protocol list does not indicate preference,
695: because the client selects among multiple protocol versions offered
696: by the server.
697: Specifying
698: .Dq 2,1
699: is identical to
700: .Dq 1,2 .
1.1 stevesk 701: .It Cm PubkeyAuthentication
702: Specifies whether public key authentication is allowed.
703: The default is
704: .Dq yes .
705: Note that this option applies to protocol version 2 only.
706: .It Cm RhostsRSAAuthentication
707: Specifies whether rhosts or /etc/hosts.equiv authentication together
708: with successful RSA host authentication is allowed.
709: The default is
710: .Dq no .
711: This option applies to protocol version 1 only.
712: .It Cm RSAAuthentication
713: Specifies whether pure RSA authentication is allowed.
714: The default is
715: .Dq yes .
716: This option applies to protocol version 1 only.
717: .It Cm ServerKeyBits
718: Defines the number of bits in the ephemeral protocol version 1 server key.
719: The minimum value is 512, and the default is 768.
720: .It Cm StrictModes
721: Specifies whether
1.44.2.2! brad 722: .Xr sshd 8
1.1 stevesk 723: should check file modes and ownership of the
724: user's files and home directory before accepting login.
725: This is normally desirable because novices sometimes accidentally leave their
726: directory or files world-writable.
727: The default is
728: .Dq yes .
729: .It Cm Subsystem
1.44.2.2! brad 730: Configures an external subsystem (e.g. file transfer daemon).
! 731: Arguments should be a subsystem name and a command (with optional arguments)
! 732: to execute upon subsystem request.
1.1 stevesk 733: The command
734: .Xr sftp-server 8
735: implements the
736: .Dq sftp
737: file transfer subsystem.
738: By default no subsystems are defined.
739: Note that this option applies to protocol version 2 only.
740: .It Cm SyslogFacility
741: Gives the facility code that is used when logging messages from
1.44.2.2! brad 742: .Xr sshd 8 .
1.1 stevesk 743: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
744: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
745: The default is AUTH.
1.27 markus 746: .It Cm TCPKeepAlive
747: Specifies whether the system should send TCP keepalive messages to the
748: other side.
749: If they are sent, death of the connection or crash of one
750: of the machines will be properly noticed.
751: However, this means that
752: connections will die if the route is down temporarily, and some people
753: find it annoying.
754: On the other hand, if TCP keepalives are not sent,
755: sessions may hang indefinitely on the server, leaving
756: .Dq ghost
757: users and consuming server resources.
758: .Pp
759: The default is
760: .Dq yes
761: (to send TCP keepalive messages), and the server will notice
762: if the network goes down or the client host crashes.
763: This avoids infinitely hanging sessions.
764: .Pp
765: To disable TCP keepalive messages, the value should be set to
766: .Dq no .
1.18 markus 767: .It Cm UseDNS
768: Specifies whether
1.44.2.2! brad 769: .Xr sshd 8
1.40 jmc 770: should look up the remote host name and check that
1.18 markus 771: the resolved host name for the remote IP address maps back to the
772: very same IP address.
773: The default is
774: .Dq yes .
1.1 stevesk 775: .It Cm UseLogin
776: Specifies whether
777: .Xr login 1
778: is used for interactive login sessions.
779: The default is
780: .Dq no .
781: Note that
782: .Xr login 1
783: is never used for remote command execution.
784: Note also, that if this is enabled,
785: .Cm X11Forwarding
786: will be disabled because
787: .Xr login 1
788: does not know how to handle
789: .Xr xauth 1
1.15 jmc 790: cookies.
791: If
1.1 stevesk 792: .Cm UsePrivilegeSeparation
793: is specified, it will be disabled after authentication.
794: .It Cm UsePrivilegeSeparation
795: Specifies whether
1.44.2.2! brad 796: .Xr sshd 8
1.2 stevesk 797: separates privileges by creating an unprivileged child process
1.15 jmc 798: to deal with incoming network traffic.
799: After successful authentication, another process will be created that has
800: the privilege of the authenticated user.
801: The goal of privilege separation is to prevent privilege
1.1 stevesk 802: escalation by containing any corruption within the unprivileged processes.
803: The default is
804: .Dq yes .
805: .It Cm X11DisplayOffset
806: Specifies the first display number available for
1.44.2.2! brad 807: .Xr sshd 8 Ns 's
1.1 stevesk 808: X11 forwarding.
1.44.2.2! brad 809: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 810: The default is 10.
811: .It Cm X11Forwarding
812: Specifies whether X11 forwarding is permitted.
1.13 stevesk 813: The argument must be
814: .Dq yes
815: or
816: .Dq no .
1.1 stevesk 817: The default is
818: .Dq no .
1.13 stevesk 819: .Pp
820: When X11 forwarding is enabled, there may be additional exposure to
821: the server and to client displays if the
1.44.2.2! brad 822: .Xr sshd 8
1.13 stevesk 823: proxy display is configured to listen on the wildcard address (see
824: .Cm X11UseLocalhost
1.44.2.2! brad 825: below), though this is not the default.
1.13 stevesk 826: Additionally, the authentication spoofing and authentication data
827: verification and substitution occur on the client side.
828: The security risk of using X11 forwarding is that the client's X11
1.44.2.2! brad 829: display server may be exposed to attack when the SSH client requests
1.13 stevesk 830: forwarding (see the warnings for
831: .Cm ForwardX11
832: in
1.19 jmc 833: .Xr ssh_config 5 ) .
1.13 stevesk 834: A system administrator may have a stance in which they want to
835: protect clients that may expose themselves to attack by unwittingly
836: requesting X11 forwarding, which can warrant a
837: .Dq no
838: setting.
839: .Pp
840: Note that disabling X11 forwarding does not prevent users from
841: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 842: X11 forwarding is automatically disabled if
843: .Cm UseLogin
844: is enabled.
845: .It Cm X11UseLocalhost
846: Specifies whether
1.44.2.2! brad 847: .Xr sshd 8
1.1 stevesk 848: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 849: the wildcard address.
850: By default,
1.44.2.2! brad 851: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 852: hostname part of the
853: .Ev DISPLAY
854: environment variable to
855: .Dq localhost .
1.8 stevesk 856: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 857: However, some older X11 clients may not function with this
858: configuration.
859: .Cm X11UseLocalhost
860: may be set to
861: .Dq no
862: to specify that the forwarding server should be bound to the wildcard
863: address.
864: The argument must be
865: .Dq yes
866: or
867: .Dq no .
868: The default is
869: .Dq yes .
870: .It Cm XAuthLocation
1.11 stevesk 871: Specifies the full pathname of the
1.1 stevesk 872: .Xr xauth 1
873: program.
874: The default is
875: .Pa /usr/X11R6/bin/xauth .
876: .El
1.44.2.2! brad 877: .Sh TIME FORMATS
! 878: .Xr sshd 8
1.1 stevesk 879: command-line arguments and configuration file options that specify time
880: may be expressed using a sequence of the form:
881: .Sm off
1.7 stevesk 882: .Ar time Op Ar qualifier ,
1.1 stevesk 883: .Sm on
884: where
885: .Ar time
886: is a positive integer value and
887: .Ar qualifier
888: is one of the following:
889: .Pp
890: .Bl -tag -width Ds -compact -offset indent
1.44.2.2! brad 891: .It Aq Cm none
1.1 stevesk 892: seconds
893: .It Cm s | Cm S
894: seconds
895: .It Cm m | Cm M
896: minutes
897: .It Cm h | Cm H
898: hours
899: .It Cm d | Cm D
900: days
901: .It Cm w | Cm W
902: weeks
903: .El
904: .Pp
905: Each member of the sequence is added together to calculate
906: the total time value.
907: .Pp
908: Time format examples:
909: .Pp
910: .Bl -tag -width Ds -compact -offset indent
911: .It 600
912: 600 seconds (10 minutes)
913: .It 10m
914: 10 minutes
915: .It 1h30m
916: 1 hour 30 minutes (90 minutes)
917: .El
918: .Sh FILES
919: .Bl -tag -width Ds
920: .It Pa /etc/ssh/sshd_config
921: Contains configuration data for
1.44.2.2! brad 922: .Xr sshd 8 .
1.1 stevesk 923: This file should be writable by root only, but it is recommended
924: (though not necessary) that it be world-readable.
925: .El
1.19 jmc 926: .Sh SEE ALSO
927: .Xr sshd 8
1.1 stevesk 928: .Sh AUTHORS
929: OpenSSH is a derivative of the original and free
930: ssh 1.2.12 release by Tatu Ylonen.
931: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
932: Theo de Raadt and Dug Song
933: removed many bugs, re-added newer features and
934: created OpenSSH.
935: Markus Friedl contributed the support for SSH
936: protocol versions 1.5 and 2.0.
937: Niels Provos and Markus Friedl contributed support
938: for privilege separation.