Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.66
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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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.66 ! dtucker 37: .\" $OpenBSD: sshd_config.5,v 1.65 2006/07/18 08:22:23 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.
1.1 stevesk 286: .It Cm GatewayPorts
287: Specifies whether remote hosts are allowed to connect to ports
288: forwarded for the client.
289: By default,
1.52 jmc 290: .Xr sshd 8
1.15 jmc 291: binds remote port forwardings to the loopback address.
292: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 293: .Cm GatewayPorts
1.52 jmc 294: can be used to specify that sshd
1.39 djm 295: should allow remote port forwardings to bind to non-loopback addresses, thus
296: allowing other hosts to connect.
297: The argument may be
298: .Dq no
299: to force remote port forwardings to be available to the local host only,
1.1 stevesk 300: .Dq yes
1.39 djm 301: to force remote port forwardings to bind to the wildcard address, or
302: .Dq clientspecified
303: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 304: The default is
305: .Dq no .
1.23 markus 306: .It Cm GSSAPIAuthentication
1.25 markus 307: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 308: The default is
1.23 markus 309: .Dq no .
310: Note that this option applies to protocol version 2 only.
311: .It Cm GSSAPICleanupCredentials
312: Specifies whether to automatically destroy the user's credentials cache
313: on logout.
314: The default is
315: .Dq yes .
316: Note that this option applies to protocol version 2 only.
1.1 stevesk 317: .It Cm HostbasedAuthentication
318: Specifies whether rhosts or /etc/hosts.equiv authentication together
319: with successful public key client host authentication is allowed
1.50 jmc 320: (host-based authentication).
1.1 stevesk 321: This option is similar to
322: .Cm RhostsRSAAuthentication
323: and applies to protocol version 2 only.
324: The default is
325: .Dq no .
326: .It Cm HostKey
327: Specifies a file containing a private host key
328: used by SSH.
329: The default is
330: .Pa /etc/ssh/ssh_host_key
331: for protocol version 1, and
332: .Pa /etc/ssh/ssh_host_rsa_key
333: and
334: .Pa /etc/ssh/ssh_host_dsa_key
335: for protocol version 2.
336: Note that
1.52 jmc 337: .Xr sshd 8
1.1 stevesk 338: will refuse to use a file if it is group/world-accessible.
339: It is possible to have multiple host key files.
340: .Dq rsa1
341: keys are used for version 1 and
342: .Dq dsa
343: or
344: .Dq rsa
345: are used for version 2 of the SSH protocol.
346: .It Cm IgnoreRhosts
347: Specifies that
348: .Pa .rhosts
349: and
350: .Pa .shosts
351: files will not be used in
352: .Cm RhostsRSAAuthentication
353: or
354: .Cm HostbasedAuthentication .
355: .Pp
356: .Pa /etc/hosts.equiv
357: and
358: .Pa /etc/shosts.equiv
359: are still used.
360: The default is
361: .Dq yes .
362: .It Cm IgnoreUserKnownHosts
363: Specifies whether
1.52 jmc 364: .Xr sshd 8
1.1 stevesk 365: should ignore the user's
1.41 djm 366: .Pa ~/.ssh/known_hosts
1.1 stevesk 367: during
368: .Cm RhostsRSAAuthentication
369: or
370: .Cm HostbasedAuthentication .
371: The default is
372: .Dq no .
373: .It Cm KerberosAuthentication
1.24 markus 374: Specifies whether the password provided by the user for
1.1 stevesk 375: .Cm PasswordAuthentication
1.24 markus 376: will be validated through the Kerberos KDC.
1.1 stevesk 377: To use this option, the server needs a
378: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 379: The default is
1.29 dtucker 380: .Dq no .
381: .It Cm KerberosGetAFSToken
1.45 djm 382: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 383: an AFS token before accessing the user's home directory.
1.52 jmc 384: The default is
1.1 stevesk 385: .Dq no .
386: .It Cm KerberosOrLocalPasswd
1.52 jmc 387: If password authentication through Kerberos fails then
1.1 stevesk 388: the password will be validated via any additional local mechanism
389: such as
390: .Pa /etc/passwd .
1.52 jmc 391: The default is
1.1 stevesk 392: .Dq yes .
393: .It Cm KerberosTicketCleanup
394: Specifies whether to automatically destroy the user's ticket cache
395: file on logout.
1.52 jmc 396: The default is
1.1 stevesk 397: .Dq yes .
398: .It Cm KeyRegenerationInterval
399: In protocol version 1, the ephemeral server key is automatically regenerated
400: after this many seconds (if it has been used).
401: The purpose of regeneration is to prevent
402: decrypting captured sessions by later breaking into the machine and
403: stealing the keys.
404: The key is never stored anywhere.
405: If the value is 0, the key is never regenerated.
406: The default is 3600 (seconds).
407: .It Cm ListenAddress
408: Specifies the local addresses
1.52 jmc 409: .Xr sshd 8
1.1 stevesk 410: should listen on.
411: The following forms may be used:
412: .Pp
413: .Bl -item -offset indent -compact
414: .It
415: .Cm ListenAddress
416: .Sm off
417: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
418: .Sm on
419: .It
420: .Cm ListenAddress
421: .Sm off
422: .Ar host No | Ar IPv4_addr No : Ar port
423: .Sm on
424: .It
425: .Cm ListenAddress
426: .Sm off
427: .Oo
428: .Ar host No | Ar IPv6_addr Oc : Ar port
429: .Sm on
430: .El
431: .Pp
432: If
433: .Ar port
434: is not specified,
1.52 jmc 435: sshd will listen on the address and all prior
1.1 stevesk 436: .Cm Port
1.17 jmc 437: options specified.
438: The default is to listen on all local addresses.
1.15 jmc 439: Multiple
1.1 stevesk 440: .Cm ListenAddress
1.17 jmc 441: options are permitted.
442: Additionally, any
1.1 stevesk 443: .Cm Port
1.52 jmc 444: options must precede this option for non-port qualified addresses.
1.1 stevesk 445: .It Cm LoginGraceTime
446: The server disconnects after this time if the user has not
447: successfully logged in.
448: If the value is 0, there is no time limit.
1.12 stevesk 449: The default is 120 seconds.
1.1 stevesk 450: .It Cm LogLevel
451: Gives the verbosity level that is used when logging messages from
1.53 jmc 452: .Xr sshd 8 .
1.1 stevesk 453: The possible values are:
1.52 jmc 454: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 455: The default is INFO.
456: DEBUG and DEBUG1 are equivalent.
457: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
458: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 459: .It Cm MACs
460: Specifies the available MAC (message authentication code) algorithms.
461: The MAC algorithm is used in protocol version 2
462: for data integrity protection.
463: Multiple algorithms must be comma-separated.
1.52 jmc 464: The default is:
1.1 stevesk 465: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.60 dtucker 466: .It Cm Match
1.61 jmc 467: Introduces a conditional block.
1.65 dtucker 468: If all of the criteria on the
1.60 dtucker 469: .Cm Match
1.65 dtucker 470: line are satisfied, the keywords on the following lines override those
471: set in the global section of the config file, until either another
1.60 dtucker 472: .Cm Match
1.65 dtucker 473: line or the end of the file.
1.61 jmc 474: The arguments to
1.60 dtucker 475: .Cm Match
1.65 dtucker 476: are one or more criteria-pattern pairs.
1.60 dtucker 477: The available criteria are
478: .Cm User ,
479: .Cm Host ,
480: and
481: .Cm Address .
482: Only a subset of keywords may be used on the lines following a
483: .Cm Match
484: keyword.
485: Available keywords are
1.62 dtucker 486: .Cm AllowTcpForwarding ,
487: .Cm GatewayPorts ,
1.66 ! dtucker 488: .Cm PermitOpen ,
! 489: .Cm X11DisplayOffset ,
! 490: .Cm X11Forwarding ,
1.60 dtucker 491: and
1.66 ! dtucker 492: .Cm X11UseLocalHost .
1.33 dtucker 493: .It Cm MaxAuthTries
494: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 495: connection.
496: Once the number of failures reaches half this value,
497: additional failures are logged.
498: The default is 6.
1.1 stevesk 499: .It Cm MaxStartups
500: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 501: SSH daemon.
1.1 stevesk 502: Additional connections will be dropped until authentication succeeds or the
503: .Cm LoginGraceTime
504: expires for a connection.
505: The default is 10.
506: .Pp
507: Alternatively, random early drop can be enabled by specifying
508: the three colon separated values
509: .Dq start:rate:full
1.51 jmc 510: (e.g. "10:30:60").
1.53 jmc 511: .Xr sshd 8
1.1 stevesk 512: will refuse connection attempts with a probability of
513: .Dq rate/100
514: (30%)
515: if there are currently
516: .Dq start
517: (10)
518: unauthenticated connections.
519: The probability increases linearly and all connection attempts
520: are refused if the number of unauthenticated connections reaches
521: .Dq full
522: (60).
523: .It Cm PasswordAuthentication
524: Specifies whether password authentication is allowed.
525: The default is
526: .Dq yes .
527: .It Cm PermitEmptyPasswords
528: When password authentication is allowed, it specifies whether the
529: server allows login to accounts with empty password strings.
530: The default is
531: .Dq no .
1.62 dtucker 532: .It Cm PermitOpen
533: Specifies the destinations to which TCP port forwarding is permitted.
534: The forwarding specification must be one of the following forms:
535: .Pp
536: .Bl -item -offset indent -compact
537: .It
538: .Cm PermitOpen
539: .Sm off
540: .Ar host : port
541: .Sm on
542: .It
543: .Cm PermitOpen
544: .Sm off
545: .Ar IPv4_addr : port
546: .Sm on
547: .It
548: .Cm PermitOpen
549: .Sm off
550: .Ar \&[ IPv6_addr \&] : port
551: .Sm on
552: .El
553: .Pp
554: Multiple instances of
555: .Cm PermitOpen
556: are permitted.
557: An argument of
558: .Dq any
559: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 560: By default all port forwarding requests are permitted.
1.1 stevesk 561: .It Cm PermitRootLogin
1.38 jmc 562: Specifies whether root can log in using
1.1 stevesk 563: .Xr ssh 1 .
564: The argument must be
565: .Dq yes ,
566: .Dq without-password ,
1.52 jmc 567: .Dq forced-commands-only ,
1.1 stevesk 568: or
569: .Dq no .
570: The default is
571: .Dq yes .
572: .Pp
573: If this option is set to
1.52 jmc 574: .Dq without-password ,
1.1 stevesk 575: password authentication is disabled for root.
576: .Pp
577: If this option is set to
1.52 jmc 578: .Dq forced-commands-only ,
1.1 stevesk 579: root login with public key authentication will be allowed,
580: but only if the
581: .Ar command
582: option has been specified
583: (which may be useful for taking remote backups even if root login is
1.17 jmc 584: normally not allowed).
585: All other authentication methods are disabled for root.
1.1 stevesk 586: .Pp
587: If this option is set to
1.52 jmc 588: .Dq no ,
1.38 jmc 589: root is not allowed to log in.
1.46 reyk 590: .It Cm PermitTunnel
591: Specifies whether
592: .Xr tun 4
593: device forwarding is allowed.
1.47 reyk 594: The argument must be
595: .Dq yes ,
1.58 stevesk 596: .Dq point-to-point
597: (layer 3),
598: .Dq ethernet
599: (layer 2), or
1.47 reyk 600: .Dq no .
1.58 stevesk 601: Specifying
602: .Dq yes
603: permits both
604: .Dq point-to-point
605: and
606: .Dq ethernet .
1.46 reyk 607: The default is
608: .Dq no .
1.6 markus 609: .It Cm PermitUserEnvironment
610: Specifies whether
611: .Pa ~/.ssh/environment
1.9 stevesk 612: and
1.6 markus 613: .Cm environment=
614: options in
615: .Pa ~/.ssh/authorized_keys
1.9 stevesk 616: are processed by
1.52 jmc 617: .Xr sshd 8 .
1.6 markus 618: The default is
619: .Dq no .
1.9 stevesk 620: Enabling environment processing may enable users to bypass access
621: restrictions in some configurations using mechanisms such as
622: .Ev LD_PRELOAD .
1.1 stevesk 623: .It Cm PidFile
1.4 stevesk 624: Specifies the file that contains the process ID of the
1.53 jmc 625: SSH daemon.
1.1 stevesk 626: The default is
627: .Pa /var/run/sshd.pid .
628: .It Cm Port
629: Specifies the port number that
1.52 jmc 630: .Xr sshd 8
1.1 stevesk 631: listens on.
632: The default is 22.
633: Multiple options of this type are permitted.
634: See also
635: .Cm ListenAddress .
636: .It Cm PrintLastLog
637: Specifies whether
1.52 jmc 638: .Xr sshd 8
1.36 jaredy 639: should print the date and time of the last user login when a user logs
640: in interactively.
1.1 stevesk 641: The default is
642: .Dq yes .
643: .It Cm PrintMotd
644: Specifies whether
1.52 jmc 645: .Xr sshd 8
1.1 stevesk 646: should print
647: .Pa /etc/motd
648: when a user logs in interactively.
649: (On some systems it is also printed by the shell,
650: .Pa /etc/profile ,
651: or equivalent.)
652: The default is
653: .Dq yes .
654: .It Cm Protocol
655: Specifies the protocol versions
1.52 jmc 656: .Xr sshd 8
1.5 stevesk 657: supports.
1.1 stevesk 658: The possible values are
1.52 jmc 659: .Sq 1
1.1 stevesk 660: and
1.52 jmc 661: .Sq 2 .
1.1 stevesk 662: Multiple versions must be comma-separated.
663: The default is
664: .Dq 2,1 .
1.5 stevesk 665: Note that the order of the protocol list does not indicate preference,
666: because the client selects among multiple protocol versions offered
667: by the server.
668: Specifying
669: .Dq 2,1
670: is identical to
671: .Dq 1,2 .
1.1 stevesk 672: .It Cm PubkeyAuthentication
673: Specifies whether public key authentication is allowed.
674: The default is
675: .Dq yes .
676: Note that this option applies to protocol version 2 only.
677: .It Cm RhostsRSAAuthentication
678: Specifies whether rhosts or /etc/hosts.equiv authentication together
679: with successful RSA host authentication is allowed.
680: The default is
681: .Dq no .
682: This option applies to protocol version 1 only.
683: .It Cm RSAAuthentication
684: Specifies whether pure RSA authentication is allowed.
685: The default is
686: .Dq yes .
687: This option applies to protocol version 1 only.
688: .It Cm ServerKeyBits
689: Defines the number of bits in the ephemeral protocol version 1 server key.
690: The minimum value is 512, and the default is 768.
691: .It Cm StrictModes
692: Specifies whether
1.52 jmc 693: .Xr sshd 8
1.1 stevesk 694: should check file modes and ownership of the
695: user's files and home directory before accepting login.
696: This is normally desirable because novices sometimes accidentally leave their
697: directory or files world-writable.
698: The default is
699: .Dq yes .
700: .It Cm Subsystem
1.51 jmc 701: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 702: Arguments should be a subsystem name and a command (with optional arguments)
703: to execute upon subsystem request.
1.1 stevesk 704: The command
705: .Xr sftp-server 8
706: implements the
707: .Dq sftp
708: file transfer subsystem.
709: By default no subsystems are defined.
710: Note that this option applies to protocol version 2 only.
711: .It Cm SyslogFacility
712: Gives the facility code that is used when logging messages from
1.53 jmc 713: .Xr sshd 8 .
1.1 stevesk 714: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
715: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
716: The default is AUTH.
1.27 markus 717: .It Cm TCPKeepAlive
718: Specifies whether the system should send TCP keepalive messages to the
719: other side.
720: If they are sent, death of the connection or crash of one
721: of the machines will be properly noticed.
722: However, this means that
723: connections will die if the route is down temporarily, and some people
724: find it annoying.
725: On the other hand, if TCP keepalives are not sent,
726: sessions may hang indefinitely on the server, leaving
727: .Dq ghost
728: users and consuming server resources.
729: .Pp
730: The default is
731: .Dq yes
732: (to send TCP keepalive messages), and the server will notice
733: if the network goes down or the client host crashes.
734: This avoids infinitely hanging sessions.
735: .Pp
736: To disable TCP keepalive messages, the value should be set to
737: .Dq no .
1.18 markus 738: .It Cm UseDNS
739: Specifies whether
1.52 jmc 740: .Xr sshd 8
1.40 jmc 741: should look up the remote host name and check that
1.18 markus 742: the resolved host name for the remote IP address maps back to the
743: very same IP address.
744: The default is
745: .Dq yes .
1.1 stevesk 746: .It Cm UseLogin
747: Specifies whether
748: .Xr login 1
749: is used for interactive login sessions.
750: The default is
751: .Dq no .
752: Note that
753: .Xr login 1
754: is never used for remote command execution.
755: Note also, that if this is enabled,
756: .Cm X11Forwarding
757: will be disabled because
758: .Xr login 1
759: does not know how to handle
760: .Xr xauth 1
1.15 jmc 761: cookies.
762: If
1.1 stevesk 763: .Cm UsePrivilegeSeparation
764: is specified, it will be disabled after authentication.
765: .It Cm UsePrivilegeSeparation
766: Specifies whether
1.52 jmc 767: .Xr sshd 8
1.2 stevesk 768: separates privileges by creating an unprivileged child process
1.15 jmc 769: to deal with incoming network traffic.
770: After successful authentication, another process will be created that has
771: the privilege of the authenticated user.
772: The goal of privilege separation is to prevent privilege
1.1 stevesk 773: escalation by containing any corruption within the unprivileged processes.
774: The default is
775: .Dq yes .
776: .It Cm X11DisplayOffset
777: Specifies the first display number available for
1.52 jmc 778: .Xr sshd 8 Ns 's
1.1 stevesk 779: X11 forwarding.
1.52 jmc 780: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 781: The default is 10.
782: .It Cm X11Forwarding
783: Specifies whether X11 forwarding is permitted.
1.13 stevesk 784: The argument must be
785: .Dq yes
786: or
787: .Dq no .
1.1 stevesk 788: The default is
789: .Dq no .
1.13 stevesk 790: .Pp
791: When X11 forwarding is enabled, there may be additional exposure to
792: the server and to client displays if the
1.52 jmc 793: .Xr sshd 8
1.13 stevesk 794: proxy display is configured to listen on the wildcard address (see
795: .Cm X11UseLocalhost
1.52 jmc 796: below), though this is not the default.
1.13 stevesk 797: Additionally, the authentication spoofing and authentication data
798: verification and substitution occur on the client side.
799: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 800: display server may be exposed to attack when the SSH client requests
1.13 stevesk 801: forwarding (see the warnings for
802: .Cm ForwardX11
803: in
1.19 jmc 804: .Xr ssh_config 5 ) .
1.13 stevesk 805: A system administrator may have a stance in which they want to
806: protect clients that may expose themselves to attack by unwittingly
807: requesting X11 forwarding, which can warrant a
808: .Dq no
809: setting.
810: .Pp
811: Note that disabling X11 forwarding does not prevent users from
812: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 813: X11 forwarding is automatically disabled if
814: .Cm UseLogin
815: is enabled.
816: .It Cm X11UseLocalhost
817: Specifies whether
1.52 jmc 818: .Xr sshd 8
1.1 stevesk 819: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 820: the wildcard address.
821: By default,
1.52 jmc 822: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 823: hostname part of the
824: .Ev DISPLAY
825: environment variable to
826: .Dq localhost .
1.8 stevesk 827: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 828: However, some older X11 clients may not function with this
829: configuration.
830: .Cm X11UseLocalhost
831: may be set to
832: .Dq no
833: to specify that the forwarding server should be bound to the wildcard
834: address.
835: The argument must be
836: .Dq yes
837: or
838: .Dq no .
839: The default is
840: .Dq yes .
841: .It Cm XAuthLocation
1.11 stevesk 842: Specifies the full pathname of the
1.1 stevesk 843: .Xr xauth 1
844: program.
845: The default is
846: .Pa /usr/X11R6/bin/xauth .
847: .El
1.55 jmc 848: .Sh TIME FORMATS
1.53 jmc 849: .Xr sshd 8
1.1 stevesk 850: command-line arguments and configuration file options that specify time
851: may be expressed using a sequence of the form:
852: .Sm off
1.7 stevesk 853: .Ar time Op Ar qualifier ,
1.1 stevesk 854: .Sm on
855: where
856: .Ar time
857: is a positive integer value and
858: .Ar qualifier
859: is one of the following:
860: .Pp
861: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 862: .It Aq Cm none
1.1 stevesk 863: seconds
864: .It Cm s | Cm S
865: seconds
866: .It Cm m | Cm M
867: minutes
868: .It Cm h | Cm H
869: hours
870: .It Cm d | Cm D
871: days
872: .It Cm w | Cm W
873: weeks
874: .El
875: .Pp
876: Each member of the sequence is added together to calculate
877: the total time value.
878: .Pp
879: Time format examples:
880: .Pp
881: .Bl -tag -width Ds -compact -offset indent
882: .It 600
883: 600 seconds (10 minutes)
884: .It 10m
885: 10 minutes
886: .It 1h30m
887: 1 hour 30 minutes (90 minutes)
888: .El
889: .Sh FILES
890: .Bl -tag -width Ds
891: .It Pa /etc/ssh/sshd_config
892: Contains configuration data for
1.53 jmc 893: .Xr sshd 8 .
1.1 stevesk 894: This file should be writable by root only, but it is recommended
895: (though not necessary) that it be world-readable.
896: .El
1.19 jmc 897: .Sh SEE ALSO
898: .Xr sshd 8
1.1 stevesk 899: .Sh AUTHORS
900: OpenSSH is a derivative of the original and free
901: ssh 1.2.12 release by Tatu Ylonen.
902: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
903: Theo de Raadt and Dug Song
904: removed many bugs, re-added newer features and
905: created OpenSSH.
906: Markus Friedl contributed the support for SSH
907: protocol versions 1.5 and 2.0.
908: Niels Provos and Markus Friedl contributed support
909: for privilege separation.
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