Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.84
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
19: .\" are met:
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
24: .\" documentation and/or other materials provided with the distribution.
25: .\"
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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31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.84 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.83 2008/02/11 07:58:28 jmc Exp $
! 38: .Dd $Mdocdate: February 11 2008 $
1.1 stevesk 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: The contents of the specified file are sent to the remote user before
163: authentication is allowed.
1.78 djm 164: If the argument is
165: .Dq none
166: then no banner is displayed.
1.1 stevesk 167: This option is only available for protocol version 2.
168: By default, no banner is displayed.
169: .It Cm ChallengeResponseAuthentication
1.50 jmc 170: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 171: All authentication styles from
172: .Xr login.conf 5
173: are supported.
174: The default is
175: .Dq yes .
1.80 djm 176: .It Cm ChrootDirectory
177: Specifies a path to
178: .Xr chroot 2
179: to after authentication.
180: This path, and all its components, must be root-owned directories that are
181: not writable by any other user or group.
182: .Pp
183: The path may contain the following tokens that are expanded at runtime once
184: the connecting user has been authenticated: %% is replaced by a literal '%',
185: %h is replaced by the home directory of the user being authenticated, and
186: %u is replaced by the username of that user.
187: .Pp
188: The
189: .Cm ChrootDirectory
190: must contain the necessary files and directories to support the
191: users' session.
192: For an interactive session this requires at least a shell, typically
193: .Xr sh 1 ,
194: and basic
195: .Pa /dev
196: nodes such as
197: .Xr null 4 ,
198: .Xr zero 4 ,
199: .Xr stdin 4 ,
200: .Xr stdout 4 ,
201: .Xr stderr 4 ,
202: .Xr arandom 4
203: and
204: .Xr tty 4
205: devices.
206: For file transfer sessions using
207: .Dq sftp ,
208: no additional configuration of the environment is necessary if the
209: in-process sftp server is used (see
210: .Cm Subsystem
1.81 jmc 211: for details).
1.80 djm 212: .Pp
213: The default is not to
214: .Xr chroot 2 .
1.1 stevesk 215: .It Cm Ciphers
216: Specifies the ciphers allowed for protocol version 2.
217: Multiple ciphers must be comma-separated.
1.34 dtucker 218: The supported ciphers are
219: .Dq 3des-cbc ,
220: .Dq aes128-cbc ,
221: .Dq aes192-cbc ,
222: .Dq aes256-cbc ,
223: .Dq aes128-ctr ,
224: .Dq aes192-ctr ,
225: .Dq aes256-ctr ,
1.43 djm 226: .Dq arcfour128 ,
227: .Dq arcfour256 ,
1.34 dtucker 228: .Dq arcfour ,
229: .Dq blowfish-cbc ,
230: and
231: .Dq cast128-cbc .
1.52 jmc 232: The default is:
233: .Bd -literal -offset 3n
234: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
235: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
236: aes192-ctr,aes256-ctr
1.1 stevesk 237: .Ed
238: .It Cm ClientAliveCountMax
1.48 jmc 239: Sets the number of client alive messages (see below) which may be
1.1 stevesk 240: sent without
1.52 jmc 241: .Xr sshd 8
1.17 jmc 242: receiving any messages back from the client.
243: If this threshold is reached while client alive messages are being sent,
1.52 jmc 244: sshd will disconnect the client, terminating the session.
1.17 jmc 245: It is important to note that the use of client alive messages is very
246: different from
1.27 markus 247: .Cm TCPKeepAlive
1.17 jmc 248: (below).
249: The client alive messages are sent through the encrypted channel
250: and therefore will not be spoofable.
251: The TCP keepalive option enabled by
1.27 markus 252: .Cm TCPKeepAlive
1.17 jmc 253: is spoofable.
254: The client alive mechanism is valuable when the client or
1.1 stevesk 255: server depend on knowing when a connection has become inactive.
256: .Pp
1.17 jmc 257: The default value is 3.
258: If
1.1 stevesk 259: .Cm ClientAliveInterval
1.48 jmc 260: (see below) is set to 15, and
1.1 stevesk 261: .Cm ClientAliveCountMax
1.52 jmc 262: is left at the default, unresponsive SSH clients
1.1 stevesk 263: will be disconnected after approximately 45 seconds.
1.57 markus 264: This option applies to protocol version 2 only.
1.42 djm 265: .It Cm ClientAliveInterval
266: Sets a timeout interval in seconds after which if no data has been received
267: from the client,
1.52 jmc 268: .Xr sshd 8
1.42 djm 269: will send a message through the encrypted
270: channel to request a response from the client.
271: The default
272: is 0, indicating that these messages will not be sent to the client.
273: This option applies to protocol version 2 only.
1.3 markus 274: .It Cm Compression
1.44 markus 275: Specifies whether compression is allowed, or delayed until
276: the user has authenticated successfully.
1.3 markus 277: The argument must be
1.44 markus 278: .Dq yes ,
279: .Dq delayed ,
1.3 markus 280: or
281: .Dq no .
282: The default is
1.44 markus 283: .Dq delayed .
1.1 stevesk 284: .It Cm DenyGroups
285: This keyword can be followed by a list of group name patterns, separated
286: by spaces.
287: Login is disallowed for users whose primary group or supplementary
288: group list matches one of the patterns.
289: Only group names are valid; a numerical group ID is not recognized.
290: By default, login is allowed for all groups.
1.54 jmc 291: The allow/deny directives are processed in the following order:
292: .Cm DenyUsers ,
293: .Cm AllowUsers ,
294: .Cm DenyGroups ,
295: and finally
296: .Cm AllowGroups .
1.49 jmc 297: .Pp
298: See
299: .Sx PATTERNS
300: in
301: .Xr ssh_config 5
302: for more information on patterns.
1.1 stevesk 303: .It Cm DenyUsers
304: This keyword can be followed by a list of user name patterns, separated
305: by spaces.
306: Login is disallowed for user names that match one of the patterns.
307: Only user names are valid; a numerical user ID is not recognized.
308: By default, login is allowed for all users.
309: If the pattern takes the form USER@HOST then USER and HOST
310: are separately checked, restricting logins to particular
311: users from particular hosts.
1.54 jmc 312: The allow/deny directives are processed in the following order:
313: .Cm DenyUsers ,
314: .Cm AllowUsers ,
315: .Cm DenyGroups ,
316: and finally
317: .Cm AllowGroups .
1.49 jmc 318: .Pp
319: See
320: .Sx PATTERNS
321: in
322: .Xr ssh_config 5
323: for more information on patterns.
1.67 dtucker 324: .It Cm ForceCommand
325: Forces the execution of the command specified by
326: .Cm ForceCommand ,
1.84 ! djm 327: ignoring any command supplied by the client and
! 328: .Pa ~/.ssh/rc
! 329: if present.
1.67 dtucker 330: The command is invoked by using the user's login shell with the -c option.
331: This applies to shell, command, or subsystem execution.
332: It is most useful inside a
333: .Cm Match
334: block.
335: The command originally supplied by the client is available in the
336: .Ev SSH_ORIGINAL_COMMAND
337: environment variable.
1.82 djm 338: Specifying a command of
339: .Dq internal-sftp
340: will force the use of an in-process sftp server that requires no support
341: files when used with
342: .Cm ChrootDirectory .
1.1 stevesk 343: .It Cm GatewayPorts
344: Specifies whether remote hosts are allowed to connect to ports
345: forwarded for the client.
346: By default,
1.52 jmc 347: .Xr sshd 8
1.15 jmc 348: binds remote port forwardings to the loopback address.
349: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 350: .Cm GatewayPorts
1.52 jmc 351: can be used to specify that sshd
1.39 djm 352: should allow remote port forwardings to bind to non-loopback addresses, thus
353: allowing other hosts to connect.
354: The argument may be
355: .Dq no
356: to force remote port forwardings to be available to the local host only,
1.1 stevesk 357: .Dq yes
1.39 djm 358: to force remote port forwardings to bind to the wildcard address, or
359: .Dq clientspecified
360: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 361: The default is
362: .Dq no .
1.23 markus 363: .It Cm GSSAPIAuthentication
1.25 markus 364: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 365: The default is
1.23 markus 366: .Dq no .
367: Note that this option applies to protocol version 2 only.
368: .It Cm GSSAPICleanupCredentials
369: Specifies whether to automatically destroy the user's credentials cache
370: on logout.
371: The default is
372: .Dq yes .
373: Note that this option applies to protocol version 2 only.
1.1 stevesk 374: .It Cm HostbasedAuthentication
375: Specifies whether rhosts or /etc/hosts.equiv authentication together
376: with successful public key client host authentication is allowed
1.50 jmc 377: (host-based authentication).
1.1 stevesk 378: This option is similar to
379: .Cm RhostsRSAAuthentication
380: and applies to protocol version 2 only.
1.70 dtucker 381: The default is
382: .Dq no .
383: .It Cm HostbasedUsesNameFromPacketOnly
384: Specifies whether or not the server will attempt to perform a reverse
385: name lookup when matching the name in the
386: .Pa ~/.shosts ,
387: .Pa ~/.rhosts ,
388: and
389: .Pa /etc/hosts.equiv
390: files during
391: .Cm HostbasedAuthentication .
392: A setting of
393: .Dq yes
394: means that
395: .Xr sshd 8
396: uses the name supplied by the client rather than
397: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 398: The default is
399: .Dq no .
400: .It Cm HostKey
401: Specifies a file containing a private host key
402: used by SSH.
403: The default is
404: .Pa /etc/ssh/ssh_host_key
405: for protocol version 1, and
406: .Pa /etc/ssh/ssh_host_rsa_key
407: and
408: .Pa /etc/ssh/ssh_host_dsa_key
409: for protocol version 2.
410: Note that
1.52 jmc 411: .Xr sshd 8
1.1 stevesk 412: will refuse to use a file if it is group/world-accessible.
413: It is possible to have multiple host key files.
414: .Dq rsa1
415: keys are used for version 1 and
416: .Dq dsa
417: or
418: .Dq rsa
419: are used for version 2 of the SSH protocol.
420: .It Cm IgnoreRhosts
421: Specifies that
422: .Pa .rhosts
423: and
424: .Pa .shosts
425: files will not be used in
426: .Cm RhostsRSAAuthentication
427: or
428: .Cm HostbasedAuthentication .
429: .Pp
430: .Pa /etc/hosts.equiv
431: and
432: .Pa /etc/shosts.equiv
433: are still used.
434: The default is
435: .Dq yes .
436: .It Cm IgnoreUserKnownHosts
437: Specifies whether
1.52 jmc 438: .Xr sshd 8
1.1 stevesk 439: should ignore the user's
1.41 djm 440: .Pa ~/.ssh/known_hosts
1.1 stevesk 441: during
442: .Cm RhostsRSAAuthentication
443: or
444: .Cm HostbasedAuthentication .
445: The default is
446: .Dq no .
447: .It Cm KerberosAuthentication
1.24 markus 448: Specifies whether the password provided by the user for
1.1 stevesk 449: .Cm PasswordAuthentication
1.24 markus 450: will be validated through the Kerberos KDC.
1.1 stevesk 451: To use this option, the server needs a
452: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 453: The default is
1.29 dtucker 454: .Dq no .
455: .It Cm KerberosGetAFSToken
1.45 djm 456: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 457: an AFS token before accessing the user's home directory.
1.52 jmc 458: The default is
1.1 stevesk 459: .Dq no .
460: .It Cm KerberosOrLocalPasswd
1.52 jmc 461: If password authentication through Kerberos fails then
1.1 stevesk 462: the password will be validated via any additional local mechanism
463: such as
464: .Pa /etc/passwd .
1.52 jmc 465: The default is
1.1 stevesk 466: .Dq yes .
467: .It Cm KerberosTicketCleanup
468: Specifies whether to automatically destroy the user's ticket cache
469: file on logout.
1.52 jmc 470: The default is
1.1 stevesk 471: .Dq yes .
472: .It Cm KeyRegenerationInterval
473: In protocol version 1, the ephemeral server key is automatically regenerated
474: after this many seconds (if it has been used).
475: The purpose of regeneration is to prevent
476: decrypting captured sessions by later breaking into the machine and
477: stealing the keys.
478: The key is never stored anywhere.
479: If the value is 0, the key is never regenerated.
480: The default is 3600 (seconds).
481: .It Cm ListenAddress
482: Specifies the local addresses
1.52 jmc 483: .Xr sshd 8
1.1 stevesk 484: should listen on.
485: The following forms may be used:
486: .Pp
487: .Bl -item -offset indent -compact
488: .It
489: .Cm ListenAddress
490: .Sm off
491: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
492: .Sm on
493: .It
494: .Cm ListenAddress
495: .Sm off
496: .Ar host No | Ar IPv4_addr No : Ar port
497: .Sm on
498: .It
499: .Cm ListenAddress
500: .Sm off
501: .Oo
502: .Ar host No | Ar IPv6_addr Oc : Ar port
503: .Sm on
504: .El
505: .Pp
506: If
507: .Ar port
508: is not specified,
1.52 jmc 509: sshd will listen on the address and all prior
1.1 stevesk 510: .Cm Port
1.17 jmc 511: options specified.
512: The default is to listen on all local addresses.
1.15 jmc 513: Multiple
1.1 stevesk 514: .Cm ListenAddress
1.17 jmc 515: options are permitted.
516: Additionally, any
1.1 stevesk 517: .Cm Port
1.52 jmc 518: options must precede this option for non-port qualified addresses.
1.1 stevesk 519: .It Cm LoginGraceTime
520: The server disconnects after this time if the user has not
521: successfully logged in.
522: If the value is 0, there is no time limit.
1.12 stevesk 523: The default is 120 seconds.
1.1 stevesk 524: .It Cm LogLevel
525: Gives the verbosity level that is used when logging messages from
1.53 jmc 526: .Xr sshd 8 .
1.1 stevesk 527: The possible values are:
1.52 jmc 528: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 529: The default is INFO.
530: DEBUG and DEBUG1 are equivalent.
531: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
532: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 533: .It Cm MACs
534: Specifies the available MAC (message authentication code) algorithms.
535: The MAC algorithm is used in protocol version 2
536: for data integrity protection.
537: Multiple algorithms must be comma-separated.
1.52 jmc 538: The default is:
1.77 jmc 539: .Bd -literal -offset indent
540: hmac-md5,hmac-sha1,umac-64@openssh.com,
541: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
542: .Ed
1.60 dtucker 543: .It Cm Match
1.61 jmc 544: Introduces a conditional block.
1.65 dtucker 545: If all of the criteria on the
1.60 dtucker 546: .Cm Match
1.65 dtucker 547: line are satisfied, the keywords on the following lines override those
548: set in the global section of the config file, until either another
1.60 dtucker 549: .Cm Match
1.65 dtucker 550: line or the end of the file.
1.61 jmc 551: The arguments to
1.60 dtucker 552: .Cm Match
1.65 dtucker 553: are one or more criteria-pattern pairs.
1.60 dtucker 554: The available criteria are
555: .Cm User ,
1.69 dtucker 556: .Cm Group ,
1.60 dtucker 557: .Cm Host ,
558: and
559: .Cm Address .
560: Only a subset of keywords may be used on the lines following a
561: .Cm Match
562: keyword.
563: Available keywords are
1.62 dtucker 564: .Cm AllowTcpForwarding ,
1.72 dtucker 565: .Cm Banner ,
1.67 dtucker 566: .Cm ForceCommand ,
1.62 dtucker 567: .Cm GatewayPorts ,
1.72 dtucker 568: .Cm GSSApiAuthentication ,
1.74 jmc 569: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 570: .Cm KerberosAuthentication ,
571: .Cm PasswordAuthentication ,
1.66 dtucker 572: .Cm PermitOpen ,
1.79 dtucker 573: .Cm PermitRootLogin ,
1.72 dtucker 574: .Cm RhostsRSAAuthentication ,
575: .Cm RSAAuthentication ,
1.66 dtucker 576: .Cm X11DisplayOffset ,
577: .Cm X11Forwarding ,
1.60 dtucker 578: and
1.66 dtucker 579: .Cm X11UseLocalHost .
1.33 dtucker 580: .It Cm MaxAuthTries
581: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 582: connection.
583: Once the number of failures reaches half this value,
584: additional failures are logged.
585: The default is 6.
1.1 stevesk 586: .It Cm MaxStartups
587: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 588: SSH daemon.
1.1 stevesk 589: Additional connections will be dropped until authentication succeeds or the
590: .Cm LoginGraceTime
591: expires for a connection.
592: The default is 10.
593: .Pp
594: Alternatively, random early drop can be enabled by specifying
595: the three colon separated values
596: .Dq start:rate:full
1.51 jmc 597: (e.g. "10:30:60").
1.53 jmc 598: .Xr sshd 8
1.1 stevesk 599: will refuse connection attempts with a probability of
600: .Dq rate/100
601: (30%)
602: if there are currently
603: .Dq start
604: (10)
605: unauthenticated connections.
606: The probability increases linearly and all connection attempts
607: are refused if the number of unauthenticated connections reaches
608: .Dq full
609: (60).
610: .It Cm PasswordAuthentication
611: Specifies whether password authentication is allowed.
612: The default is
613: .Dq yes .
614: .It Cm PermitEmptyPasswords
615: When password authentication is allowed, it specifies whether the
616: server allows login to accounts with empty password strings.
617: The default is
618: .Dq no .
1.62 dtucker 619: .It Cm PermitOpen
620: Specifies the destinations to which TCP port forwarding is permitted.
621: The forwarding specification must be one of the following forms:
622: .Pp
623: .Bl -item -offset indent -compact
624: .It
625: .Cm PermitOpen
626: .Sm off
627: .Ar host : port
628: .Sm on
629: .It
630: .Cm PermitOpen
631: .Sm off
632: .Ar IPv4_addr : port
633: .Sm on
634: .It
635: .Cm PermitOpen
636: .Sm off
637: .Ar \&[ IPv6_addr \&] : port
638: .Sm on
639: .El
640: .Pp
1.68 dtucker 641: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 642: An argument of
643: .Dq any
644: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 645: By default all port forwarding requests are permitted.
1.1 stevesk 646: .It Cm PermitRootLogin
1.38 jmc 647: Specifies whether root can log in using
1.1 stevesk 648: .Xr ssh 1 .
649: The argument must be
650: .Dq yes ,
651: .Dq without-password ,
1.52 jmc 652: .Dq forced-commands-only ,
1.1 stevesk 653: or
654: .Dq no .
655: The default is
656: .Dq yes .
657: .Pp
658: If this option is set to
1.52 jmc 659: .Dq without-password ,
1.1 stevesk 660: password authentication is disabled for root.
661: .Pp
662: If this option is set to
1.52 jmc 663: .Dq forced-commands-only ,
1.1 stevesk 664: root login with public key authentication will be allowed,
665: but only if the
666: .Ar command
667: option has been specified
668: (which may be useful for taking remote backups even if root login is
1.17 jmc 669: normally not allowed).
670: All other authentication methods are disabled for root.
1.1 stevesk 671: .Pp
672: If this option is set to
1.52 jmc 673: .Dq no ,
1.38 jmc 674: root is not allowed to log in.
1.46 reyk 675: .It Cm PermitTunnel
676: Specifies whether
677: .Xr tun 4
678: device forwarding is allowed.
1.47 reyk 679: The argument must be
680: .Dq yes ,
1.58 stevesk 681: .Dq point-to-point
682: (layer 3),
683: .Dq ethernet
684: (layer 2), or
1.47 reyk 685: .Dq no .
1.58 stevesk 686: Specifying
687: .Dq yes
688: permits both
689: .Dq point-to-point
690: and
691: .Dq ethernet .
1.46 reyk 692: The default is
693: .Dq no .
1.6 markus 694: .It Cm PermitUserEnvironment
695: Specifies whether
696: .Pa ~/.ssh/environment
1.9 stevesk 697: and
1.6 markus 698: .Cm environment=
699: options in
700: .Pa ~/.ssh/authorized_keys
1.9 stevesk 701: are processed by
1.52 jmc 702: .Xr sshd 8 .
1.6 markus 703: The default is
704: .Dq no .
1.9 stevesk 705: Enabling environment processing may enable users to bypass access
706: restrictions in some configurations using mechanisms such as
707: .Ev LD_PRELOAD .
1.1 stevesk 708: .It Cm PidFile
1.4 stevesk 709: Specifies the file that contains the process ID of the
1.53 jmc 710: SSH daemon.
1.1 stevesk 711: The default is
712: .Pa /var/run/sshd.pid .
713: .It Cm Port
714: Specifies the port number that
1.52 jmc 715: .Xr sshd 8
1.1 stevesk 716: listens on.
717: The default is 22.
718: Multiple options of this type are permitted.
719: See also
720: .Cm ListenAddress .
721: .It Cm PrintLastLog
722: Specifies whether
1.52 jmc 723: .Xr sshd 8
1.36 jaredy 724: should print the date and time of the last user login when a user logs
725: in interactively.
1.1 stevesk 726: The default is
727: .Dq yes .
728: .It Cm PrintMotd
729: Specifies whether
1.52 jmc 730: .Xr sshd 8
1.1 stevesk 731: should print
732: .Pa /etc/motd
733: when a user logs in interactively.
734: (On some systems it is also printed by the shell,
735: .Pa /etc/profile ,
736: or equivalent.)
737: The default is
738: .Dq yes .
739: .It Cm Protocol
740: Specifies the protocol versions
1.52 jmc 741: .Xr sshd 8
1.5 stevesk 742: supports.
1.1 stevesk 743: The possible values are
1.52 jmc 744: .Sq 1
1.1 stevesk 745: and
1.52 jmc 746: .Sq 2 .
1.1 stevesk 747: Multiple versions must be comma-separated.
748: The default is
749: .Dq 2,1 .
1.5 stevesk 750: Note that the order of the protocol list does not indicate preference,
751: because the client selects among multiple protocol versions offered
752: by the server.
753: Specifying
754: .Dq 2,1
755: is identical to
756: .Dq 1,2 .
1.1 stevesk 757: .It Cm PubkeyAuthentication
758: Specifies whether public key authentication is allowed.
759: The default is
760: .Dq yes .
761: Note that this option applies to protocol version 2 only.
762: .It Cm RhostsRSAAuthentication
763: Specifies whether rhosts or /etc/hosts.equiv authentication together
764: with successful RSA host authentication is allowed.
765: The default is
766: .Dq no .
767: This option applies to protocol version 1 only.
768: .It Cm RSAAuthentication
769: Specifies whether pure RSA authentication is allowed.
770: The default is
771: .Dq yes .
772: This option applies to protocol version 1 only.
773: .It Cm ServerKeyBits
774: Defines the number of bits in the ephemeral protocol version 1 server key.
775: The minimum value is 512, and the default is 768.
776: .It Cm StrictModes
777: Specifies whether
1.52 jmc 778: .Xr sshd 8
1.1 stevesk 779: should check file modes and ownership of the
780: user's files and home directory before accepting login.
781: This is normally desirable because novices sometimes accidentally leave their
782: directory or files world-writable.
783: The default is
784: .Dq yes .
785: .It Cm Subsystem
1.51 jmc 786: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 787: Arguments should be a subsystem name and a command (with optional arguments)
788: to execute upon subsystem request.
1.80 djm 789: .Pp
1.1 stevesk 790: The command
791: .Xr sftp-server 8
792: implements the
793: .Dq sftp
794: file transfer subsystem.
1.80 djm 795: .Pp
796: Alternately the name
797: .Dq internal-sftp
798: implements an in-process
799: .Dq sftp
800: server.
801: This may simplify configurations using
802: .Cm ChrootDirectory
803: to force a different filesystem root on clients.
804: .Pp
1.1 stevesk 805: By default no subsystems are defined.
806: Note that this option applies to protocol version 2 only.
807: .It Cm SyslogFacility
808: Gives the facility code that is used when logging messages from
1.53 jmc 809: .Xr sshd 8 .
1.1 stevesk 810: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
811: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
812: The default is AUTH.
1.27 markus 813: .It Cm TCPKeepAlive
814: Specifies whether the system should send TCP keepalive messages to the
815: other side.
816: If they are sent, death of the connection or crash of one
817: of the machines will be properly noticed.
818: However, this means that
819: connections will die if the route is down temporarily, and some people
820: find it annoying.
821: On the other hand, if TCP keepalives are not sent,
822: sessions may hang indefinitely on the server, leaving
823: .Dq ghost
824: users and consuming server resources.
825: .Pp
826: The default is
827: .Dq yes
828: (to send TCP keepalive messages), and the server will notice
829: if the network goes down or the client host crashes.
830: This avoids infinitely hanging sessions.
831: .Pp
832: To disable TCP keepalive messages, the value should be set to
833: .Dq no .
1.18 markus 834: .It Cm UseDNS
835: Specifies whether
1.52 jmc 836: .Xr sshd 8
1.40 jmc 837: should look up the remote host name and check that
1.18 markus 838: the resolved host name for the remote IP address maps back to the
839: very same IP address.
840: The default is
841: .Dq yes .
1.1 stevesk 842: .It Cm UseLogin
843: Specifies whether
844: .Xr login 1
845: is used for interactive login sessions.
846: The default is
847: .Dq no .
848: Note that
849: .Xr login 1
850: is never used for remote command execution.
851: Note also, that if this is enabled,
852: .Cm X11Forwarding
853: will be disabled because
854: .Xr login 1
855: does not know how to handle
856: .Xr xauth 1
1.15 jmc 857: cookies.
858: If
1.1 stevesk 859: .Cm UsePrivilegeSeparation
860: is specified, it will be disabled after authentication.
861: .It Cm UsePrivilegeSeparation
862: Specifies whether
1.52 jmc 863: .Xr sshd 8
1.2 stevesk 864: separates privileges by creating an unprivileged child process
1.15 jmc 865: to deal with incoming network traffic.
866: After successful authentication, another process will be created that has
867: the privilege of the authenticated user.
868: The goal of privilege separation is to prevent privilege
1.1 stevesk 869: escalation by containing any corruption within the unprivileged processes.
870: The default is
871: .Dq yes .
872: .It Cm X11DisplayOffset
873: Specifies the first display number available for
1.52 jmc 874: .Xr sshd 8 Ns 's
1.1 stevesk 875: X11 forwarding.
1.52 jmc 876: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 877: The default is 10.
878: .It Cm X11Forwarding
879: Specifies whether X11 forwarding is permitted.
1.13 stevesk 880: The argument must be
881: .Dq yes
882: or
883: .Dq no .
1.1 stevesk 884: The default is
885: .Dq no .
1.13 stevesk 886: .Pp
887: When X11 forwarding is enabled, there may be additional exposure to
888: the server and to client displays if the
1.52 jmc 889: .Xr sshd 8
1.13 stevesk 890: proxy display is configured to listen on the wildcard address (see
891: .Cm X11UseLocalhost
1.52 jmc 892: below), though this is not the default.
1.13 stevesk 893: Additionally, the authentication spoofing and authentication data
894: verification and substitution occur on the client side.
895: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 896: display server may be exposed to attack when the SSH client requests
1.13 stevesk 897: forwarding (see the warnings for
898: .Cm ForwardX11
899: in
1.19 jmc 900: .Xr ssh_config 5 ) .
1.13 stevesk 901: A system administrator may have a stance in which they want to
902: protect clients that may expose themselves to attack by unwittingly
903: requesting X11 forwarding, which can warrant a
904: .Dq no
905: setting.
906: .Pp
907: Note that disabling X11 forwarding does not prevent users from
908: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 909: X11 forwarding is automatically disabled if
910: .Cm UseLogin
911: is enabled.
912: .It Cm X11UseLocalhost
913: Specifies whether
1.52 jmc 914: .Xr sshd 8
1.1 stevesk 915: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 916: the wildcard address.
917: By default,
1.52 jmc 918: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 919: hostname part of the
920: .Ev DISPLAY
921: environment variable to
922: .Dq localhost .
1.8 stevesk 923: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 924: However, some older X11 clients may not function with this
925: configuration.
926: .Cm X11UseLocalhost
927: may be set to
928: .Dq no
929: to specify that the forwarding server should be bound to the wildcard
930: address.
931: The argument must be
932: .Dq yes
933: or
934: .Dq no .
935: The default is
936: .Dq yes .
937: .It Cm XAuthLocation
1.11 stevesk 938: Specifies the full pathname of the
1.1 stevesk 939: .Xr xauth 1
940: program.
941: The default is
942: .Pa /usr/X11R6/bin/xauth .
943: .El
1.55 jmc 944: .Sh TIME FORMATS
1.53 jmc 945: .Xr sshd 8
1.1 stevesk 946: command-line arguments and configuration file options that specify time
947: may be expressed using a sequence of the form:
948: .Sm off
1.7 stevesk 949: .Ar time Op Ar qualifier ,
1.1 stevesk 950: .Sm on
951: where
952: .Ar time
953: is a positive integer value and
954: .Ar qualifier
955: is one of the following:
956: .Pp
957: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 958: .It Aq Cm none
1.1 stevesk 959: seconds
960: .It Cm s | Cm S
961: seconds
962: .It Cm m | Cm M
963: minutes
964: .It Cm h | Cm H
965: hours
966: .It Cm d | Cm D
967: days
968: .It Cm w | Cm W
969: weeks
970: .El
971: .Pp
972: Each member of the sequence is added together to calculate
973: the total time value.
974: .Pp
975: Time format examples:
976: .Pp
977: .Bl -tag -width Ds -compact -offset indent
978: .It 600
979: 600 seconds (10 minutes)
980: .It 10m
981: 10 minutes
982: .It 1h30m
983: 1 hour 30 minutes (90 minutes)
984: .El
985: .Sh FILES
986: .Bl -tag -width Ds
987: .It Pa /etc/ssh/sshd_config
988: Contains configuration data for
1.53 jmc 989: .Xr sshd 8 .
1.1 stevesk 990: This file should be writable by root only, but it is recommended
991: (though not necessary) that it be world-readable.
992: .El
1.19 jmc 993: .Sh SEE ALSO
994: .Xr sshd 8
1.1 stevesk 995: .Sh AUTHORS
996: OpenSSH is a derivative of the original and free
997: ssh 1.2.12 release by Tatu Ylonen.
998: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
999: Theo de Raadt and Dug Song
1000: removed many bugs, re-added newer features and
1001: created OpenSSH.
1002: Markus Friedl contributed the support for SSH
1003: protocol versions 1.5 and 2.0.
1004: Niels Provos and Markus Friedl contributed support
1005: for privilege separation.
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