Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.71
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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1.71 ! jmc 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
1.71 ! jmc 45: .Nm /etc/ssh/sshd_config
1.1 stevesk 46: .Sh DESCRIPTION
1.53 jmc 47: .Xr sshd 8
1.1 stevesk 48: reads configuration data from
49: .Pa /etc/ssh/sshd_config
50: (or the file specified with
51: .Fl f
52: on the command line).
53: The file contains keyword-argument pairs, one per line.
54: Lines starting with
55: .Ql #
56: and empty lines are interpreted as comments.
1.56 dtucker 57: Arguments may optionally be enclosed in double quotes
58: .Pq \&"
59: in order to represent arguments containing spaces.
1.1 stevesk 60: .Pp
61: The possible
62: keywords and their meanings are as follows (note that
63: keywords are case-insensitive and arguments are case-sensitive):
64: .Bl -tag -width Ds
1.30 djm 65: .It Cm AcceptEnv
66: Specifies what environment variables sent by the client will be copied into
67: the session's
68: .Xr environ 7 .
69: See
70: .Cm SendEnv
71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.31 djm 74: Note that environment passing is only supported for protocol 2.
1.30 djm 75: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 76: .Ql *
1.30 djm 77: and
78: .Ql \&? .
1.31 djm 79: Multiple environment variables may be separated by whitespace or spread
1.30 djm 80: across multiple
81: .Cm AcceptEnv
82: directives.
1.31 djm 83: Be warned that some environment variables could be used to bypass restricted
1.30 djm 84: user environments.
85: For this reason, care should be taken in the use of this directive.
86: The default is not to accept any environment variables.
1.37 djm 87: .It Cm AddressFamily
88: Specifies which address family should be used by
1.53 jmc 89: .Xr sshd 8 .
1.37 djm 90: Valid arguments are
91: .Dq any ,
92: .Dq inet
1.52 jmc 93: (use IPv4 only), or
1.37 djm 94: .Dq inet6
95: (use IPv6 only).
96: The default is
97: .Dq any .
1.1 stevesk 98: .It Cm AllowGroups
99: This keyword can be followed by a list of group name patterns, separated
100: by spaces.
101: If specified, login is allowed only for users whose primary
102: group or supplementary group list matches one of the patterns.
103: Only group names are valid; a numerical group ID is not recognized.
104: By default, login is allowed for all groups.
1.54 jmc 105: The allow/deny directives are processed in the following order:
106: .Cm DenyUsers ,
107: .Cm AllowUsers ,
108: .Cm DenyGroups ,
109: and finally
110: .Cm AllowGroups .
1.49 jmc 111: .Pp
112: See
113: .Sx PATTERNS
114: in
115: .Xr ssh_config 5
116: for more information on patterns.
1.1 stevesk 117: .It Cm AllowTcpForwarding
118: Specifies whether TCP forwarding is permitted.
119: The default is
120: .Dq yes .
121: Note that disabling TCP forwarding does not improve security unless
122: users are also denied shell access, as they can always install their
123: own forwarders.
124: .It Cm AllowUsers
125: This keyword can be followed by a list of user name patterns, separated
126: by spaces.
1.14 jmc 127: If specified, login is allowed only for user names that
1.1 stevesk 128: match one of the patterns.
129: Only user names are valid; a numerical user ID is not recognized.
130: By default, login is allowed for all users.
131: If the pattern takes the form USER@HOST then USER and HOST
132: are separately checked, restricting logins to particular
133: users from particular hosts.
1.54 jmc 134: The allow/deny directives are processed in the following order:
135: .Cm DenyUsers ,
136: .Cm AllowUsers ,
137: .Cm DenyGroups ,
138: and finally
139: .Cm AllowGroups .
1.49 jmc 140: .Pp
141: See
142: .Sx PATTERNS
143: in
144: .Xr ssh_config 5
145: for more information on patterns.
1.1 stevesk 146: .It Cm AuthorizedKeysFile
147: Specifies the file that contains the public keys that can be used
148: for user authentication.
149: .Cm AuthorizedKeysFile
150: may contain tokens of the form %T which are substituted during connection
1.52 jmc 151: setup.
1.17 jmc 152: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 153: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 154: %u is replaced by the username of that user.
155: After expansion,
156: .Cm AuthorizedKeysFile
157: is taken to be an absolute path or one relative to the user's home
158: directory.
159: The default is
160: .Dq .ssh/authorized_keys .
161: .It Cm Banner
162: In some jurisdictions, sending a warning message before authentication
163: may be relevant for getting legal protection.
164: The contents of the specified file are sent to the remote user before
165: authentication is allowed.
166: This option is only available for protocol version 2.
167: By default, no banner is displayed.
168: .It Cm ChallengeResponseAuthentication
1.50 jmc 169: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 170: All authentication styles from
171: .Xr login.conf 5
172: are supported.
173: The default is
174: .Dq yes .
175: .It Cm Ciphers
176: Specifies the ciphers allowed for protocol version 2.
177: Multiple ciphers must be comma-separated.
1.34 dtucker 178: The supported ciphers are
179: .Dq 3des-cbc ,
180: .Dq aes128-cbc ,
181: .Dq aes192-cbc ,
182: .Dq aes256-cbc ,
183: .Dq aes128-ctr ,
184: .Dq aes192-ctr ,
185: .Dq aes256-ctr ,
1.43 djm 186: .Dq arcfour128 ,
187: .Dq arcfour256 ,
1.34 dtucker 188: .Dq arcfour ,
189: .Dq blowfish-cbc ,
190: and
191: .Dq cast128-cbc .
1.52 jmc 192: The default is:
193: .Bd -literal -offset 3n
194: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
195: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
196: aes192-ctr,aes256-ctr
1.1 stevesk 197: .Ed
198: .It Cm ClientAliveCountMax
1.48 jmc 199: Sets the number of client alive messages (see below) which may be
1.1 stevesk 200: sent without
1.52 jmc 201: .Xr sshd 8
1.17 jmc 202: receiving any messages back from the client.
203: If this threshold is reached while client alive messages are being sent,
1.52 jmc 204: sshd will disconnect the client, terminating the session.
1.17 jmc 205: It is important to note that the use of client alive messages is very
206: different from
1.27 markus 207: .Cm TCPKeepAlive
1.17 jmc 208: (below).
209: The client alive messages are sent through the encrypted channel
210: and therefore will not be spoofable.
211: The TCP keepalive option enabled by
1.27 markus 212: .Cm TCPKeepAlive
1.17 jmc 213: is spoofable.
214: The client alive mechanism is valuable when the client or
1.1 stevesk 215: server depend on knowing when a connection has become inactive.
216: .Pp
1.17 jmc 217: The default value is 3.
218: If
1.1 stevesk 219: .Cm ClientAliveInterval
1.48 jmc 220: (see below) is set to 15, and
1.1 stevesk 221: .Cm ClientAliveCountMax
1.52 jmc 222: is left at the default, unresponsive SSH clients
1.1 stevesk 223: will be disconnected after approximately 45 seconds.
1.57 markus 224: This option applies to protocol version 2 only.
1.42 djm 225: .It Cm ClientAliveInterval
226: Sets a timeout interval in seconds after which if no data has been received
227: from the client,
1.52 jmc 228: .Xr sshd 8
1.42 djm 229: will send a message through the encrypted
230: channel to request a response from the client.
231: The default
232: is 0, indicating that these messages will not be sent to the client.
233: This option applies to protocol version 2 only.
1.3 markus 234: .It Cm Compression
1.44 markus 235: Specifies whether compression is allowed, or delayed until
236: the user has authenticated successfully.
1.3 markus 237: The argument must be
1.44 markus 238: .Dq yes ,
239: .Dq delayed ,
1.3 markus 240: or
241: .Dq no .
242: The default is
1.44 markus 243: .Dq delayed .
1.1 stevesk 244: .It Cm DenyGroups
245: This keyword can be followed by a list of group name patterns, separated
246: by spaces.
247: Login is disallowed for users whose primary group or supplementary
248: group list matches one of the patterns.
249: Only group names are valid; a numerical group ID is not recognized.
250: By default, login is allowed for all groups.
1.54 jmc 251: The allow/deny directives are processed in the following order:
252: .Cm DenyUsers ,
253: .Cm AllowUsers ,
254: .Cm DenyGroups ,
255: and finally
256: .Cm AllowGroups .
1.49 jmc 257: .Pp
258: See
259: .Sx PATTERNS
260: in
261: .Xr ssh_config 5
262: for more information on patterns.
1.1 stevesk 263: .It Cm DenyUsers
264: This keyword can be followed by a list of user name patterns, separated
265: by spaces.
266: Login is disallowed for user names that match one of the patterns.
267: Only user names are valid; a numerical user ID is not recognized.
268: By default, login is allowed for all users.
269: If the pattern takes the form USER@HOST then USER and HOST
270: are separately checked, restricting logins to particular
271: users from particular hosts.
1.54 jmc 272: The allow/deny directives are processed in the following order:
273: .Cm DenyUsers ,
274: .Cm AllowUsers ,
275: .Cm DenyGroups ,
276: and finally
277: .Cm AllowGroups .
1.49 jmc 278: .Pp
279: See
280: .Sx PATTERNS
281: in
282: .Xr ssh_config 5
283: for more information on patterns.
1.67 dtucker 284: .It Cm ForceCommand
285: Forces the execution of the command specified by
286: .Cm ForceCommand ,
287: ignoring any command supplied by the client.
288: The command is invoked by using the user's login shell with the -c option.
289: This applies to shell, command, or subsystem execution.
290: It is most useful inside a
291: .Cm Match
292: block.
293: The command originally supplied by the client is available in the
294: .Ev SSH_ORIGINAL_COMMAND
295: environment variable.
1.1 stevesk 296: .It Cm GatewayPorts
297: Specifies whether remote hosts are allowed to connect to ports
298: forwarded for the client.
299: By default,
1.52 jmc 300: .Xr sshd 8
1.15 jmc 301: binds remote port forwardings to the loopback address.
302: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 303: .Cm GatewayPorts
1.52 jmc 304: can be used to specify that sshd
1.39 djm 305: should allow remote port forwardings to bind to non-loopback addresses, thus
306: allowing other hosts to connect.
307: The argument may be
308: .Dq no
309: to force remote port forwardings to be available to the local host only,
1.1 stevesk 310: .Dq yes
1.39 djm 311: to force remote port forwardings to bind to the wildcard address, or
312: .Dq clientspecified
313: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 314: The default is
315: .Dq no .
1.23 markus 316: .It Cm GSSAPIAuthentication
1.25 markus 317: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 318: The default is
1.23 markus 319: .Dq no .
320: Note that this option applies to protocol version 2 only.
321: .It Cm GSSAPICleanupCredentials
322: Specifies whether to automatically destroy the user's credentials cache
323: on logout.
324: The default is
325: .Dq yes .
326: Note that this option applies to protocol version 2 only.
1.1 stevesk 327: .It Cm HostbasedAuthentication
328: Specifies whether rhosts or /etc/hosts.equiv authentication together
329: with successful public key client host authentication is allowed
1.50 jmc 330: (host-based authentication).
1.1 stevesk 331: This option is similar to
332: .Cm RhostsRSAAuthentication
333: and applies to protocol version 2 only.
1.70 dtucker 334: The default is
335: .Dq no .
336: .It Cm HostbasedUsesNameFromPacketOnly
337: Specifies whether or not the server will attempt to perform a reverse
338: name lookup when matching the name in the
339: .Pa ~/.shosts ,
340: .Pa ~/.rhosts ,
341: and
342: .Pa /etc/hosts.equiv
343: files during
344: .Cm HostbasedAuthentication .
345: A setting of
346: .Dq yes
347: means that
348: .Xr sshd 8
349: uses the name supplied by the client rather than
350: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 351: The default is
352: .Dq no .
353: .It Cm HostKey
354: Specifies a file containing a private host key
355: used by SSH.
356: The default is
357: .Pa /etc/ssh/ssh_host_key
358: for protocol version 1, and
359: .Pa /etc/ssh/ssh_host_rsa_key
360: and
361: .Pa /etc/ssh/ssh_host_dsa_key
362: for protocol version 2.
363: Note that
1.52 jmc 364: .Xr sshd 8
1.1 stevesk 365: will refuse to use a file if it is group/world-accessible.
366: It is possible to have multiple host key files.
367: .Dq rsa1
368: keys are used for version 1 and
369: .Dq dsa
370: or
371: .Dq rsa
372: are used for version 2 of the SSH protocol.
373: .It Cm IgnoreRhosts
374: Specifies that
375: .Pa .rhosts
376: and
377: .Pa .shosts
378: files will not be used in
379: .Cm RhostsRSAAuthentication
380: or
381: .Cm HostbasedAuthentication .
382: .Pp
383: .Pa /etc/hosts.equiv
384: and
385: .Pa /etc/shosts.equiv
386: are still used.
387: The default is
388: .Dq yes .
389: .It Cm IgnoreUserKnownHosts
390: Specifies whether
1.52 jmc 391: .Xr sshd 8
1.1 stevesk 392: should ignore the user's
1.41 djm 393: .Pa ~/.ssh/known_hosts
1.1 stevesk 394: during
395: .Cm RhostsRSAAuthentication
396: or
397: .Cm HostbasedAuthentication .
398: The default is
399: .Dq no .
400: .It Cm KerberosAuthentication
1.24 markus 401: Specifies whether the password provided by the user for
1.1 stevesk 402: .Cm PasswordAuthentication
1.24 markus 403: will be validated through the Kerberos KDC.
1.1 stevesk 404: To use this option, the server needs a
405: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 406: The default is
1.29 dtucker 407: .Dq no .
408: .It Cm KerberosGetAFSToken
1.45 djm 409: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 410: an AFS token before accessing the user's home directory.
1.52 jmc 411: The default is
1.1 stevesk 412: .Dq no .
413: .It Cm KerberosOrLocalPasswd
1.52 jmc 414: If password authentication through Kerberos fails then
1.1 stevesk 415: the password will be validated via any additional local mechanism
416: such as
417: .Pa /etc/passwd .
1.52 jmc 418: The default is
1.1 stevesk 419: .Dq yes .
420: .It Cm KerberosTicketCleanup
421: Specifies whether to automatically destroy the user's ticket cache
422: file on logout.
1.52 jmc 423: The default is
1.1 stevesk 424: .Dq yes .
425: .It Cm KeyRegenerationInterval
426: In protocol version 1, the ephemeral server key is automatically regenerated
427: after this many seconds (if it has been used).
428: The purpose of regeneration is to prevent
429: decrypting captured sessions by later breaking into the machine and
430: stealing the keys.
431: The key is never stored anywhere.
432: If the value is 0, the key is never regenerated.
433: The default is 3600 (seconds).
434: .It Cm ListenAddress
435: Specifies the local addresses
1.52 jmc 436: .Xr sshd 8
1.1 stevesk 437: should listen on.
438: The following forms may be used:
439: .Pp
440: .Bl -item -offset indent -compact
441: .It
442: .Cm ListenAddress
443: .Sm off
444: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
445: .Sm on
446: .It
447: .Cm ListenAddress
448: .Sm off
449: .Ar host No | Ar IPv4_addr No : Ar port
450: .Sm on
451: .It
452: .Cm ListenAddress
453: .Sm off
454: .Oo
455: .Ar host No | Ar IPv6_addr Oc : Ar port
456: .Sm on
457: .El
458: .Pp
459: If
460: .Ar port
461: is not specified,
1.52 jmc 462: sshd will listen on the address and all prior
1.1 stevesk 463: .Cm Port
1.17 jmc 464: options specified.
465: The default is to listen on all local addresses.
1.15 jmc 466: Multiple
1.1 stevesk 467: .Cm ListenAddress
1.17 jmc 468: options are permitted.
469: Additionally, any
1.1 stevesk 470: .Cm Port
1.52 jmc 471: options must precede this option for non-port qualified addresses.
1.1 stevesk 472: .It Cm LoginGraceTime
473: The server disconnects after this time if the user has not
474: successfully logged in.
475: If the value is 0, there is no time limit.
1.12 stevesk 476: The default is 120 seconds.
1.1 stevesk 477: .It Cm LogLevel
478: Gives the verbosity level that is used when logging messages from
1.53 jmc 479: .Xr sshd 8 .
1.1 stevesk 480: The possible values are:
1.52 jmc 481: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 482: The default is INFO.
483: DEBUG and DEBUG1 are equivalent.
484: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
485: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 486: .It Cm MACs
487: Specifies the available MAC (message authentication code) algorithms.
488: The MAC algorithm is used in protocol version 2
489: for data integrity protection.
490: Multiple algorithms must be comma-separated.
1.52 jmc 491: The default is:
1.1 stevesk 492: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.60 dtucker 493: .It Cm Match
1.61 jmc 494: Introduces a conditional block.
1.65 dtucker 495: If all of the criteria on the
1.60 dtucker 496: .Cm Match
1.65 dtucker 497: line are satisfied, the keywords on the following lines override those
498: set in the global section of the config file, until either another
1.60 dtucker 499: .Cm Match
1.65 dtucker 500: line or the end of the file.
1.61 jmc 501: The arguments to
1.60 dtucker 502: .Cm Match
1.65 dtucker 503: are one or more criteria-pattern pairs.
1.60 dtucker 504: The available criteria are
505: .Cm User ,
1.69 dtucker 506: .Cm Group ,
1.60 dtucker 507: .Cm Host ,
508: and
509: .Cm Address .
510: Only a subset of keywords may be used on the lines following a
511: .Cm Match
512: keyword.
513: Available keywords are
1.62 dtucker 514: .Cm AllowTcpForwarding ,
1.67 dtucker 515: .Cm ForceCommand ,
1.62 dtucker 516: .Cm GatewayPorts ,
1.66 dtucker 517: .Cm PermitOpen ,
518: .Cm X11DisplayOffset ,
519: .Cm X11Forwarding ,
1.60 dtucker 520: and
1.66 dtucker 521: .Cm X11UseLocalHost .
1.33 dtucker 522: .It Cm MaxAuthTries
523: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 524: connection.
525: Once the number of failures reaches half this value,
526: additional failures are logged.
527: The default is 6.
1.1 stevesk 528: .It Cm MaxStartups
529: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 530: SSH daemon.
1.1 stevesk 531: Additional connections will be dropped until authentication succeeds or the
532: .Cm LoginGraceTime
533: expires for a connection.
534: The default is 10.
535: .Pp
536: Alternatively, random early drop can be enabled by specifying
537: the three colon separated values
538: .Dq start:rate:full
1.51 jmc 539: (e.g. "10:30:60").
1.53 jmc 540: .Xr sshd 8
1.1 stevesk 541: will refuse connection attempts with a probability of
542: .Dq rate/100
543: (30%)
544: if there are currently
545: .Dq start
546: (10)
547: unauthenticated connections.
548: The probability increases linearly and all connection attempts
549: are refused if the number of unauthenticated connections reaches
550: .Dq full
551: (60).
552: .It Cm PasswordAuthentication
553: Specifies whether password authentication is allowed.
554: The default is
555: .Dq yes .
556: .It Cm PermitEmptyPasswords
557: When password authentication is allowed, it specifies whether the
558: server allows login to accounts with empty password strings.
559: The default is
560: .Dq no .
1.62 dtucker 561: .It Cm PermitOpen
562: Specifies the destinations to which TCP port forwarding is permitted.
563: The forwarding specification must be one of the following forms:
564: .Pp
565: .Bl -item -offset indent -compact
566: .It
567: .Cm PermitOpen
568: .Sm off
569: .Ar host : port
570: .Sm on
571: .It
572: .Cm PermitOpen
573: .Sm off
574: .Ar IPv4_addr : port
575: .Sm on
576: .It
577: .Cm PermitOpen
578: .Sm off
579: .Ar \&[ IPv6_addr \&] : port
580: .Sm on
581: .El
582: .Pp
1.68 dtucker 583: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 584: An argument of
585: .Dq any
586: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 587: By default all port forwarding requests are permitted.
1.1 stevesk 588: .It Cm PermitRootLogin
1.38 jmc 589: Specifies whether root can log in using
1.1 stevesk 590: .Xr ssh 1 .
591: The argument must be
592: .Dq yes ,
593: .Dq without-password ,
1.52 jmc 594: .Dq forced-commands-only ,
1.1 stevesk 595: or
596: .Dq no .
597: The default is
598: .Dq yes .
599: .Pp
600: If this option is set to
1.52 jmc 601: .Dq without-password ,
1.1 stevesk 602: password authentication is disabled for root.
603: .Pp
604: If this option is set to
1.52 jmc 605: .Dq forced-commands-only ,
1.1 stevesk 606: root login with public key authentication will be allowed,
607: but only if the
608: .Ar command
609: option has been specified
610: (which may be useful for taking remote backups even if root login is
1.17 jmc 611: normally not allowed).
612: All other authentication methods are disabled for root.
1.1 stevesk 613: .Pp
614: If this option is set to
1.52 jmc 615: .Dq no ,
1.38 jmc 616: root is not allowed to log in.
1.46 reyk 617: .It Cm PermitTunnel
618: Specifies whether
619: .Xr tun 4
620: device forwarding is allowed.
1.47 reyk 621: The argument must be
622: .Dq yes ,
1.58 stevesk 623: .Dq point-to-point
624: (layer 3),
625: .Dq ethernet
626: (layer 2), or
1.47 reyk 627: .Dq no .
1.58 stevesk 628: Specifying
629: .Dq yes
630: permits both
631: .Dq point-to-point
632: and
633: .Dq ethernet .
1.46 reyk 634: The default is
635: .Dq no .
1.6 markus 636: .It Cm PermitUserEnvironment
637: Specifies whether
638: .Pa ~/.ssh/environment
1.9 stevesk 639: and
1.6 markus 640: .Cm environment=
641: options in
642: .Pa ~/.ssh/authorized_keys
1.9 stevesk 643: are processed by
1.52 jmc 644: .Xr sshd 8 .
1.6 markus 645: The default is
646: .Dq no .
1.9 stevesk 647: Enabling environment processing may enable users to bypass access
648: restrictions in some configurations using mechanisms such as
649: .Ev LD_PRELOAD .
1.1 stevesk 650: .It Cm PidFile
1.4 stevesk 651: Specifies the file that contains the process ID of the
1.53 jmc 652: SSH daemon.
1.1 stevesk 653: The default is
654: .Pa /var/run/sshd.pid .
655: .It Cm Port
656: Specifies the port number that
1.52 jmc 657: .Xr sshd 8
1.1 stevesk 658: listens on.
659: The default is 22.
660: Multiple options of this type are permitted.
661: See also
662: .Cm ListenAddress .
663: .It Cm PrintLastLog
664: Specifies whether
1.52 jmc 665: .Xr sshd 8
1.36 jaredy 666: should print the date and time of the last user login when a user logs
667: in interactively.
1.1 stevesk 668: The default is
669: .Dq yes .
670: .It Cm PrintMotd
671: Specifies whether
1.52 jmc 672: .Xr sshd 8
1.1 stevesk 673: should print
674: .Pa /etc/motd
675: when a user logs in interactively.
676: (On some systems it is also printed by the shell,
677: .Pa /etc/profile ,
678: or equivalent.)
679: The default is
680: .Dq yes .
681: .It Cm Protocol
682: Specifies the protocol versions
1.52 jmc 683: .Xr sshd 8
1.5 stevesk 684: supports.
1.1 stevesk 685: The possible values are
1.52 jmc 686: .Sq 1
1.1 stevesk 687: and
1.52 jmc 688: .Sq 2 .
1.1 stevesk 689: Multiple versions must be comma-separated.
690: The default is
691: .Dq 2,1 .
1.5 stevesk 692: Note that the order of the protocol list does not indicate preference,
693: because the client selects among multiple protocol versions offered
694: by the server.
695: Specifying
696: .Dq 2,1
697: is identical to
698: .Dq 1,2 .
1.1 stevesk 699: .It Cm PubkeyAuthentication
700: Specifies whether public key authentication is allowed.
701: The default is
702: .Dq yes .
703: Note that this option applies to protocol version 2 only.
704: .It Cm RhostsRSAAuthentication
705: Specifies whether rhosts or /etc/hosts.equiv authentication together
706: with successful RSA host authentication is allowed.
707: The default is
708: .Dq no .
709: This option applies to protocol version 1 only.
710: .It Cm RSAAuthentication
711: Specifies whether pure RSA authentication is allowed.
712: The default is
713: .Dq yes .
714: This option applies to protocol version 1 only.
715: .It Cm ServerKeyBits
716: Defines the number of bits in the ephemeral protocol version 1 server key.
717: The minimum value is 512, and the default is 768.
718: .It Cm StrictModes
719: Specifies whether
1.52 jmc 720: .Xr sshd 8
1.1 stevesk 721: should check file modes and ownership of the
722: user's files and home directory before accepting login.
723: This is normally desirable because novices sometimes accidentally leave their
724: directory or files world-writable.
725: The default is
726: .Dq yes .
727: .It Cm Subsystem
1.51 jmc 728: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 729: Arguments should be a subsystem name and a command (with optional arguments)
730: to execute upon subsystem request.
1.1 stevesk 731: The command
732: .Xr sftp-server 8
733: implements the
734: .Dq sftp
735: file transfer subsystem.
736: By default no subsystems are defined.
737: Note that this option applies to protocol version 2 only.
738: .It Cm SyslogFacility
739: Gives the facility code that is used when logging messages from
1.53 jmc 740: .Xr sshd 8 .
1.1 stevesk 741: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
742: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
743: The default is AUTH.
1.27 markus 744: .It Cm TCPKeepAlive
745: Specifies whether the system should send TCP keepalive messages to the
746: other side.
747: If they are sent, death of the connection or crash of one
748: of the machines will be properly noticed.
749: However, this means that
750: connections will die if the route is down temporarily, and some people
751: find it annoying.
752: On the other hand, if TCP keepalives are not sent,
753: sessions may hang indefinitely on the server, leaving
754: .Dq ghost
755: users and consuming server resources.
756: .Pp
757: The default is
758: .Dq yes
759: (to send TCP keepalive messages), and the server will notice
760: if the network goes down or the client host crashes.
761: This avoids infinitely hanging sessions.
762: .Pp
763: To disable TCP keepalive messages, the value should be set to
764: .Dq no .
1.18 markus 765: .It Cm UseDNS
766: Specifies whether
1.52 jmc 767: .Xr sshd 8
1.40 jmc 768: should look up the remote host name and check that
1.18 markus 769: the resolved host name for the remote IP address maps back to the
770: very same IP address.
771: The default is
772: .Dq yes .
1.1 stevesk 773: .It Cm UseLogin
774: Specifies whether
775: .Xr login 1
776: is used for interactive login sessions.
777: The default is
778: .Dq no .
779: Note that
780: .Xr login 1
781: is never used for remote command execution.
782: Note also, that if this is enabled,
783: .Cm X11Forwarding
784: will be disabled because
785: .Xr login 1
786: does not know how to handle
787: .Xr xauth 1
1.15 jmc 788: cookies.
789: If
1.1 stevesk 790: .Cm UsePrivilegeSeparation
791: is specified, it will be disabled after authentication.
792: .It Cm UsePrivilegeSeparation
793: Specifies whether
1.52 jmc 794: .Xr sshd 8
1.2 stevesk 795: separates privileges by creating an unprivileged child process
1.15 jmc 796: to deal with incoming network traffic.
797: After successful authentication, another process will be created that has
798: the privilege of the authenticated user.
799: The goal of privilege separation is to prevent privilege
1.1 stevesk 800: escalation by containing any corruption within the unprivileged processes.
801: The default is
802: .Dq yes .
803: .It Cm X11DisplayOffset
804: Specifies the first display number available for
1.52 jmc 805: .Xr sshd 8 Ns 's
1.1 stevesk 806: X11 forwarding.
1.52 jmc 807: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 808: The default is 10.
809: .It Cm X11Forwarding
810: Specifies whether X11 forwarding is permitted.
1.13 stevesk 811: The argument must be
812: .Dq yes
813: or
814: .Dq no .
1.1 stevesk 815: The default is
816: .Dq no .
1.13 stevesk 817: .Pp
818: When X11 forwarding is enabled, there may be additional exposure to
819: the server and to client displays if the
1.52 jmc 820: .Xr sshd 8
1.13 stevesk 821: proxy display is configured to listen on the wildcard address (see
822: .Cm X11UseLocalhost
1.52 jmc 823: below), though this is not the default.
1.13 stevesk 824: Additionally, the authentication spoofing and authentication data
825: verification and substitution occur on the client side.
826: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 827: display server may be exposed to attack when the SSH client requests
1.13 stevesk 828: forwarding (see the warnings for
829: .Cm ForwardX11
830: in
1.19 jmc 831: .Xr ssh_config 5 ) .
1.13 stevesk 832: A system administrator may have a stance in which they want to
833: protect clients that may expose themselves to attack by unwittingly
834: requesting X11 forwarding, which can warrant a
835: .Dq no
836: setting.
837: .Pp
838: Note that disabling X11 forwarding does not prevent users from
839: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 840: X11 forwarding is automatically disabled if
841: .Cm UseLogin
842: is enabled.
843: .It Cm X11UseLocalhost
844: Specifies whether
1.52 jmc 845: .Xr sshd 8
1.1 stevesk 846: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 847: the wildcard address.
848: By default,
1.52 jmc 849: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 850: hostname part of the
851: .Ev DISPLAY
852: environment variable to
853: .Dq localhost .
1.8 stevesk 854: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 855: However, some older X11 clients may not function with this
856: configuration.
857: .Cm X11UseLocalhost
858: may be set to
859: .Dq no
860: to specify that the forwarding server should be bound to the wildcard
861: address.
862: The argument must be
863: .Dq yes
864: or
865: .Dq no .
866: The default is
867: .Dq yes .
868: .It Cm XAuthLocation
1.11 stevesk 869: Specifies the full pathname of the
1.1 stevesk 870: .Xr xauth 1
871: program.
872: The default is
873: .Pa /usr/X11R6/bin/xauth .
874: .El
1.55 jmc 875: .Sh TIME FORMATS
1.53 jmc 876: .Xr sshd 8
1.1 stevesk 877: command-line arguments and configuration file options that specify time
878: may be expressed using a sequence of the form:
879: .Sm off
1.7 stevesk 880: .Ar time Op Ar qualifier ,
1.1 stevesk 881: .Sm on
882: where
883: .Ar time
884: is a positive integer value and
885: .Ar qualifier
886: is one of the following:
887: .Pp
888: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 889: .It Aq Cm none
1.1 stevesk 890: seconds
891: .It Cm s | Cm S
892: seconds
893: .It Cm m | Cm M
894: minutes
895: .It Cm h | Cm H
896: hours
897: .It Cm d | Cm D
898: days
899: .It Cm w | Cm W
900: weeks
901: .El
902: .Pp
903: Each member of the sequence is added together to calculate
904: the total time value.
905: .Pp
906: Time format examples:
907: .Pp
908: .Bl -tag -width Ds -compact -offset indent
909: .It 600
910: 600 seconds (10 minutes)
911: .It 10m
912: 10 minutes
913: .It 1h30m
914: 1 hour 30 minutes (90 minutes)
915: .El
916: .Sh FILES
917: .Bl -tag -width Ds
918: .It Pa /etc/ssh/sshd_config
919: Contains configuration data for
1.53 jmc 920: .Xr sshd 8 .
1.1 stevesk 921: This file should be writable by root only, but it is recommended
922: (though not necessary) that it be world-readable.
923: .El
1.19 jmc 924: .Sh SEE ALSO
925: .Xr sshd 8
1.1 stevesk 926: .Sh AUTHORS
927: OpenSSH is a derivative of the original and free
928: ssh 1.2.12 release by Tatu Ylonen.
929: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
930: Theo de Raadt and Dug Song
931: removed many bugs, re-added newer features and
932: created OpenSSH.
933: Markus Friedl contributed the support for SSH
934: protocol versions 1.5 and 2.0.
935: Niels Provos and Markus Friedl contributed support
936: for privilege separation.
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