Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.135
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.135 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.134 2011/06/22 21:57:01 djm Exp $
! 37: .Dd $Mdocdate: June 22 2011 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
120: See
121: .Sx PATTERNS
122: in
123: .Xr ssh_config 5
124: for more information on patterns.
1.1 stevesk 125: .It Cm AllowTcpForwarding
126: Specifies whether TCP forwarding is permitted.
127: The default is
128: .Dq yes .
129: Note that disabling TCP forwarding does not improve security unless
130: users are also denied shell access, as they can always install their
131: own forwarders.
132: .It Cm AllowUsers
133: This keyword can be followed by a list of user name patterns, separated
134: by spaces.
1.14 jmc 135: If specified, login is allowed only for user names that
1.1 stevesk 136: match one of the patterns.
137: Only user names are valid; a numerical user ID is not recognized.
138: By default, login is allowed for all users.
139: If the pattern takes the form USER@HOST then USER and HOST
140: are separately checked, restricting logins to particular
141: users from particular hosts.
1.54 jmc 142: The allow/deny directives are processed in the following order:
143: .Cm DenyUsers ,
144: .Cm AllowUsers ,
145: .Cm DenyGroups ,
146: and finally
147: .Cm AllowGroups .
1.49 jmc 148: .Pp
149: See
150: .Sx PATTERNS
151: in
152: .Xr ssh_config 5
153: for more information on patterns.
1.1 stevesk 154: .It Cm AuthorizedKeysFile
155: Specifies the file that contains the public keys that can be used
156: for user authentication.
1.124 djm 157: The format is described in the
158: .Sx AUTHORIZED_KEYS FILE FORMAT
159: section of
160: .Xr sshd 8 .
1.1 stevesk 161: .Cm AuthorizedKeysFile
162: may contain tokens of the form %T which are substituted during connection
1.52 jmc 163: setup.
1.17 jmc 164: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 165: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 166: %u is replaced by the username of that user.
167: After expansion,
168: .Cm AuthorizedKeysFile
169: is taken to be an absolute path or one relative to the user's home
170: directory.
1.133 jmc 171: Multiple files may be listed, separated by whitespace.
172: The default is
173: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 174: .It Cm AuthorizedPrincipalsFile
175: Specifies a file that lists principal names that are accepted for
176: certificate authentication.
177: When using certificates signed by a key listed in
178: .Cm TrustedUserCAKeys ,
179: this file lists names, one of which must appear in the certificate for it
180: to be accepted for authentication.
1.125 jmc 181: Names are listed one per line preceded by key options (as described
1.124 djm 182: in
183: .Sx AUTHORIZED_KEYS FILE FORMAT
184: in
1.125 jmc 185: .Xr sshd 8 ) .
1.124 djm 186: Empty lines and comments starting with
1.121 djm 187: .Ql #
188: are ignored.
189: .Pp
190: .Cm AuthorizedPrincipalsFile
191: may contain tokens of the form %T which are substituted during connection
192: setup.
193: The following tokens are defined: %% is replaced by a literal '%',
194: %h is replaced by the home directory of the user being authenticated, and
195: %u is replaced by the username of that user.
196: After expansion,
197: .Cm AuthorizedPrincipalsFile
198: is taken to be an absolute path or one relative to the user's home
199: directory.
200: .Pp
1.122 jmc 201: The default is not to use a principals file \(en in this case, the username
1.121 djm 202: of the user must appear in a certificate's principals list for it to be
203: accepted.
204: Note that
205: .Cm AuthorizedPrincipalsFile
206: is only used when authentication proceeds using a CA listed in
207: .Cm TrustedUserCAKeys
208: and is not consulted for certification authorities trusted via
209: .Pa ~/.ssh/authorized_keys ,
210: though the
211: .Cm principals=
212: key option offers a similar facility (see
213: .Xr sshd 8
214: for details).
1.1 stevesk 215: .It Cm Banner
216: The contents of the specified file are sent to the remote user before
217: authentication is allowed.
1.78 djm 218: If the argument is
219: .Dq none
220: then no banner is displayed.
1.1 stevesk 221: This option is only available for protocol version 2.
222: By default, no banner is displayed.
223: .It Cm ChallengeResponseAuthentication
1.50 jmc 224: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 225: All authentication styles from
226: .Xr login.conf 5
227: are supported.
228: The default is
229: .Dq yes .
1.80 djm 230: .It Cm ChrootDirectory
1.113 stevesk 231: Specifies the pathname of a directory to
1.80 djm 232: .Xr chroot 2
233: to after authentication.
1.113 stevesk 234: All components of the pathname must be root-owned directories that are
1.80 djm 235: not writable by any other user or group.
1.106 stevesk 236: After the chroot,
237: .Xr sshd 8
238: changes the working directory to the user's home directory.
1.80 djm 239: .Pp
1.113 stevesk 240: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 241: the connecting user has been authenticated: %% is replaced by a literal '%',
242: %h is replaced by the home directory of the user being authenticated, and
243: %u is replaced by the username of that user.
244: .Pp
245: The
246: .Cm ChrootDirectory
247: must contain the necessary files and directories to support the
1.103 stevesk 248: user's session.
1.80 djm 249: For an interactive session this requires at least a shell, typically
250: .Xr sh 1 ,
251: and basic
252: .Pa /dev
253: nodes such as
254: .Xr null 4 ,
255: .Xr zero 4 ,
256: .Xr stdin 4 ,
257: .Xr stdout 4 ,
258: .Xr stderr 4 ,
259: .Xr arandom 4
260: and
261: .Xr tty 4
262: devices.
263: For file transfer sessions using
1.105 jmc 264: .Dq sftp ,
1.80 djm 265: no additional configuration of the environment is necessary if the
1.105 jmc 266: in-process sftp server is used,
267: though sessions which use logging do require
1.104 stevesk 268: .Pa /dev/log
269: inside the chroot directory (see
270: .Xr sftp-server 8
1.81 jmc 271: for details).
1.80 djm 272: .Pp
273: The default is not to
274: .Xr chroot 2 .
1.1 stevesk 275: .It Cm Ciphers
276: Specifies the ciphers allowed for protocol version 2.
277: Multiple ciphers must be comma-separated.
1.34 dtucker 278: The supported ciphers are
279: .Dq 3des-cbc ,
280: .Dq aes128-cbc ,
281: .Dq aes192-cbc ,
282: .Dq aes256-cbc ,
283: .Dq aes128-ctr ,
284: .Dq aes192-ctr ,
285: .Dq aes256-ctr ,
1.43 djm 286: .Dq arcfour128 ,
287: .Dq arcfour256 ,
1.34 dtucker 288: .Dq arcfour ,
289: .Dq blowfish-cbc ,
290: and
291: .Dq cast128-cbc .
1.52 jmc 292: The default is:
293: .Bd -literal -offset 3n
1.100 naddy 294: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
295: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
296: aes256-cbc,arcfour
1.1 stevesk 297: .Ed
298: .It Cm ClientAliveCountMax
1.48 jmc 299: Sets the number of client alive messages (see below) which may be
1.1 stevesk 300: sent without
1.52 jmc 301: .Xr sshd 8
1.17 jmc 302: receiving any messages back from the client.
303: If this threshold is reached while client alive messages are being sent,
1.52 jmc 304: sshd will disconnect the client, terminating the session.
1.17 jmc 305: It is important to note that the use of client alive messages is very
306: different from
1.27 markus 307: .Cm TCPKeepAlive
1.17 jmc 308: (below).
309: The client alive messages are sent through the encrypted channel
310: and therefore will not be spoofable.
311: The TCP keepalive option enabled by
1.27 markus 312: .Cm TCPKeepAlive
1.17 jmc 313: is spoofable.
314: The client alive mechanism is valuable when the client or
1.1 stevesk 315: server depend on knowing when a connection has become inactive.
316: .Pp
1.17 jmc 317: The default value is 3.
318: If
1.1 stevesk 319: .Cm ClientAliveInterval
1.48 jmc 320: (see below) is set to 15, and
1.1 stevesk 321: .Cm ClientAliveCountMax
1.52 jmc 322: is left at the default, unresponsive SSH clients
1.1 stevesk 323: will be disconnected after approximately 45 seconds.
1.57 markus 324: This option applies to protocol version 2 only.
1.42 djm 325: .It Cm ClientAliveInterval
326: Sets a timeout interval in seconds after which if no data has been received
327: from the client,
1.52 jmc 328: .Xr sshd 8
1.42 djm 329: will send a message through the encrypted
330: channel to request a response from the client.
331: The default
332: is 0, indicating that these messages will not be sent to the client.
333: This option applies to protocol version 2 only.
1.3 markus 334: .It Cm Compression
1.44 markus 335: Specifies whether compression is allowed, or delayed until
336: the user has authenticated successfully.
1.3 markus 337: The argument must be
1.44 markus 338: .Dq yes ,
339: .Dq delayed ,
1.3 markus 340: or
341: .Dq no .
342: The default is
1.44 markus 343: .Dq delayed .
1.1 stevesk 344: .It Cm DenyGroups
345: This keyword can be followed by a list of group name patterns, separated
346: by spaces.
347: Login is disallowed for users whose primary group or supplementary
348: group list matches one of the patterns.
349: Only group names are valid; a numerical group ID is not recognized.
350: By default, login is allowed for all groups.
1.54 jmc 351: The allow/deny directives are processed in the following order:
352: .Cm DenyUsers ,
353: .Cm AllowUsers ,
354: .Cm DenyGroups ,
355: and finally
356: .Cm AllowGroups .
1.49 jmc 357: .Pp
358: See
359: .Sx PATTERNS
360: in
361: .Xr ssh_config 5
362: for more information on patterns.
1.1 stevesk 363: .It Cm DenyUsers
364: This keyword can be followed by a list of user name patterns, separated
365: by spaces.
366: Login is disallowed for user names that match one of the patterns.
367: Only user names are valid; a numerical user ID is not recognized.
368: By default, login is allowed for all users.
369: If the pattern takes the form USER@HOST then USER and HOST
370: are separately checked, restricting logins to particular
371: users from particular hosts.
1.54 jmc 372: The allow/deny directives are processed in the following order:
373: .Cm DenyUsers ,
374: .Cm AllowUsers ,
375: .Cm DenyGroups ,
376: and finally
377: .Cm AllowGroups .
1.49 jmc 378: .Pp
379: See
380: .Sx PATTERNS
381: in
382: .Xr ssh_config 5
383: for more information on patterns.
1.67 dtucker 384: .It Cm ForceCommand
385: Forces the execution of the command specified by
386: .Cm ForceCommand ,
1.84 djm 387: ignoring any command supplied by the client and
388: .Pa ~/.ssh/rc
389: if present.
1.67 dtucker 390: The command is invoked by using the user's login shell with the -c option.
391: This applies to shell, command, or subsystem execution.
392: It is most useful inside a
393: .Cm Match
394: block.
395: The command originally supplied by the client is available in the
396: .Ev SSH_ORIGINAL_COMMAND
397: environment variable.
1.82 djm 398: Specifying a command of
399: .Dq internal-sftp
400: will force the use of an in-process sftp server that requires no support
401: files when used with
402: .Cm ChrootDirectory .
1.1 stevesk 403: .It Cm GatewayPorts
404: Specifies whether remote hosts are allowed to connect to ports
405: forwarded for the client.
406: By default,
1.52 jmc 407: .Xr sshd 8
1.15 jmc 408: binds remote port forwardings to the loopback address.
409: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 410: .Cm GatewayPorts
1.52 jmc 411: can be used to specify that sshd
1.39 djm 412: should allow remote port forwardings to bind to non-loopback addresses, thus
413: allowing other hosts to connect.
414: The argument may be
415: .Dq no
416: to force remote port forwardings to be available to the local host only,
1.1 stevesk 417: .Dq yes
1.39 djm 418: to force remote port forwardings to bind to the wildcard address, or
419: .Dq clientspecified
420: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 421: The default is
422: .Dq no .
1.23 markus 423: .It Cm GSSAPIAuthentication
1.25 markus 424: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 425: The default is
1.23 markus 426: .Dq no .
427: Note that this option applies to protocol version 2 only.
428: .It Cm GSSAPICleanupCredentials
429: Specifies whether to automatically destroy the user's credentials cache
430: on logout.
431: The default is
432: .Dq yes .
433: Note that this option applies to protocol version 2 only.
1.1 stevesk 434: .It Cm HostbasedAuthentication
435: Specifies whether rhosts or /etc/hosts.equiv authentication together
436: with successful public key client host authentication is allowed
1.50 jmc 437: (host-based authentication).
1.1 stevesk 438: This option is similar to
439: .Cm RhostsRSAAuthentication
440: and applies to protocol version 2 only.
1.70 dtucker 441: The default is
442: .Dq no .
443: .It Cm HostbasedUsesNameFromPacketOnly
444: Specifies whether or not the server will attempt to perform a reverse
445: name lookup when matching the name in the
446: .Pa ~/.shosts ,
447: .Pa ~/.rhosts ,
448: and
449: .Pa /etc/hosts.equiv
450: files during
451: .Cm HostbasedAuthentication .
452: A setting of
453: .Dq yes
454: means that
455: .Xr sshd 8
456: uses the name supplied by the client rather than
457: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 458: The default is
459: .Dq no .
1.117 djm 460: .It Cm HostCertificate
461: Specifies a file containing a public host certificate.
462: The certificate's public key must match a private host key already specified
463: by
464: .Cm HostKey .
465: The default behaviour of
466: .Xr sshd 8
467: is not to load any certificates.
1.1 stevesk 468: .It Cm HostKey
469: Specifies a file containing a private host key
470: used by SSH.
471: The default is
472: .Pa /etc/ssh/ssh_host_key
473: for protocol version 1, and
1.126 djm 474: .Pa /etc/ssh/ssh_host_dsa_key ,
475: .Pa /etc/ssh/ssh_host_ecdsa_key
476: and
1.1 stevesk 477: .Pa /etc/ssh/ssh_host_rsa_key
478: for protocol version 2.
479: Note that
1.52 jmc 480: .Xr sshd 8
1.1 stevesk 481: will refuse to use a file if it is group/world-accessible.
482: It is possible to have multiple host key files.
483: .Dq rsa1
484: keys are used for version 1 and
1.126 djm 485: .Dq dsa ,
486: .Dq ecdsa
1.1 stevesk 487: or
488: .Dq rsa
489: are used for version 2 of the SSH protocol.
490: .It Cm IgnoreRhosts
491: Specifies that
492: .Pa .rhosts
493: and
494: .Pa .shosts
495: files will not be used in
496: .Cm RhostsRSAAuthentication
497: or
498: .Cm HostbasedAuthentication .
499: .Pp
500: .Pa /etc/hosts.equiv
501: and
502: .Pa /etc/shosts.equiv
503: are still used.
504: The default is
505: .Dq yes .
506: .It Cm IgnoreUserKnownHosts
507: Specifies whether
1.52 jmc 508: .Xr sshd 8
1.1 stevesk 509: should ignore the user's
1.41 djm 510: .Pa ~/.ssh/known_hosts
1.1 stevesk 511: during
512: .Cm RhostsRSAAuthentication
513: or
514: .Cm HostbasedAuthentication .
515: The default is
516: .Dq no .
1.129 djm 517: .It Cm IPQoS
518: Specifies the IPv4 type-of-service or DSCP class for the connection.
519: Accepted values are
520: .Dq af11 ,
521: .Dq af12 ,
522: .Dq af13 ,
523: .Dq af14 ,
524: .Dq af22 ,
525: .Dq af23 ,
526: .Dq af31 ,
527: .Dq af32 ,
528: .Dq af33 ,
529: .Dq af41 ,
530: .Dq af42 ,
531: .Dq af43 ,
532: .Dq cs0 ,
533: .Dq cs1 ,
534: .Dq cs2 ,
535: .Dq cs3 ,
536: .Dq cs4 ,
537: .Dq cs5 ,
538: .Dq cs6 ,
539: .Dq cs7 ,
540: .Dq ef ,
541: .Dq lowdelay ,
542: .Dq throughput ,
543: .Dq reliability ,
544: or a numeric value.
1.131 djm 545: This option may take one or two arguments, separated by whitespace.
1.129 djm 546: If one argument is specified, it is used as the packet class unconditionally.
547: If two values are specified, the first is automatically selected for
548: interactive sessions and the second for non-interactive sessions.
549: The default is
550: .Dq lowdelay
551: for interactive sessions and
552: .Dq throughput
553: for non-interactive sessions.
1.1 stevesk 554: .It Cm KerberosAuthentication
1.24 markus 555: Specifies whether the password provided by the user for
1.1 stevesk 556: .Cm PasswordAuthentication
1.24 markus 557: will be validated through the Kerberos KDC.
1.1 stevesk 558: To use this option, the server needs a
559: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 560: The default is
1.29 dtucker 561: .Dq no .
562: .It Cm KerberosGetAFSToken
1.45 djm 563: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 564: an AFS token before accessing the user's home directory.
1.52 jmc 565: The default is
1.1 stevesk 566: .Dq no .
567: .It Cm KerberosOrLocalPasswd
1.52 jmc 568: If password authentication through Kerberos fails then
1.1 stevesk 569: the password will be validated via any additional local mechanism
570: such as
571: .Pa /etc/passwd .
1.52 jmc 572: The default is
1.1 stevesk 573: .Dq yes .
574: .It Cm KerberosTicketCleanup
575: Specifies whether to automatically destroy the user's ticket cache
576: file on logout.
1.52 jmc 577: The default is
1.1 stevesk 578: .Dq yes .
1.127 djm 579: .It Cm KexAlgorithms
580: Specifies the available KEX (Key Exchange) algorithms.
581: Multiple algorithms must be comma-separated.
582: The default is
583: .Dq ecdh-sha2-nistp256 ,
584: .Dq ecdh-sha2-nistp384 ,
585: .Dq ecdh-sha2-nistp521 ,
1.130 jmc 586: .Dq diffie-hellman-group-exchange-sha256 ,
1.127 djm 587: .Dq diffie-hellman-group-exchange-sha1 ,
588: .Dq diffie-hellman-group14-sha1 ,
589: .Dq diffie-hellman-group1-sha1 .
1.1 stevesk 590: .It Cm KeyRegenerationInterval
591: In protocol version 1, the ephemeral server key is automatically regenerated
592: after this many seconds (if it has been used).
593: The purpose of regeneration is to prevent
594: decrypting captured sessions by later breaking into the machine and
595: stealing the keys.
596: The key is never stored anywhere.
597: If the value is 0, the key is never regenerated.
598: The default is 3600 (seconds).
599: .It Cm ListenAddress
600: Specifies the local addresses
1.52 jmc 601: .Xr sshd 8
1.1 stevesk 602: should listen on.
603: The following forms may be used:
604: .Pp
605: .Bl -item -offset indent -compact
606: .It
607: .Cm ListenAddress
608: .Sm off
609: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
610: .Sm on
611: .It
612: .Cm ListenAddress
613: .Sm off
614: .Ar host No | Ar IPv4_addr No : Ar port
615: .Sm on
616: .It
617: .Cm ListenAddress
618: .Sm off
619: .Oo
620: .Ar host No | Ar IPv6_addr Oc : Ar port
621: .Sm on
622: .El
623: .Pp
624: If
625: .Ar port
626: is not specified,
1.52 jmc 627: sshd will listen on the address and all prior
1.1 stevesk 628: .Cm Port
1.17 jmc 629: options specified.
630: The default is to listen on all local addresses.
1.15 jmc 631: Multiple
1.1 stevesk 632: .Cm ListenAddress
1.17 jmc 633: options are permitted.
634: Additionally, any
1.1 stevesk 635: .Cm Port
1.52 jmc 636: options must precede this option for non-port qualified addresses.
1.1 stevesk 637: .It Cm LoginGraceTime
638: The server disconnects after this time if the user has not
639: successfully logged in.
640: If the value is 0, there is no time limit.
1.12 stevesk 641: The default is 120 seconds.
1.1 stevesk 642: .It Cm LogLevel
643: Gives the verbosity level that is used when logging messages from
1.53 jmc 644: .Xr sshd 8 .
1.1 stevesk 645: The possible values are:
1.52 jmc 646: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 647: The default is INFO.
648: DEBUG and DEBUG1 are equivalent.
649: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
650: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 651: .It Cm MACs
652: Specifies the available MAC (message authentication code) algorithms.
653: The MAC algorithm is used in protocol version 2
654: for data integrity protection.
655: Multiple algorithms must be comma-separated.
1.52 jmc 656: The default is:
1.77 jmc 657: .Bd -literal -offset indent
658: hmac-md5,hmac-sha1,umac-64@openssh.com,
1.135 ! djm 659: hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
! 660: hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
! 661: hmac-sha2-512-96
1.77 jmc 662: .Ed
1.60 dtucker 663: .It Cm Match
1.61 jmc 664: Introduces a conditional block.
1.65 dtucker 665: If all of the criteria on the
1.60 dtucker 666: .Cm Match
1.65 dtucker 667: line are satisfied, the keywords on the following lines override those
668: set in the global section of the config file, until either another
1.60 dtucker 669: .Cm Match
1.65 dtucker 670: line or the end of the file.
1.91 djm 671: .Pp
1.61 jmc 672: The arguments to
1.60 dtucker 673: .Cm Match
1.65 dtucker 674: are one or more criteria-pattern pairs.
1.60 dtucker 675: The available criteria are
676: .Cm User ,
1.69 dtucker 677: .Cm Group ,
1.60 dtucker 678: .Cm Host ,
679: and
680: .Cm Address .
1.91 djm 681: The match patterns may consist of single entries or comma-separated
682: lists and may use the wildcard and negation operators described in the
1.92 djm 683: .Sx PATTERNS
1.91 djm 684: section of
1.92 djm 685: .Xr ssh_config 5 .
1.91 djm 686: .Pp
687: The patterns in an
688: .Cm Address
689: criteria may additionally contain addresses to match in CIDR
1.93 jmc 690: address/masklen format, e.g.\&
1.91 djm 691: .Dq 192.0.2.0/24
692: or
693: .Dq 3ffe:ffff::/32 .
694: Note that the mask length provided must be consistent with the address -
695: it is an error to specify a mask length that is too long for the address
1.93 jmc 696: or one with bits set in this host portion of the address.
697: For example,
1.91 djm 698: .Dq 192.0.2.0/33
699: and
1.93 jmc 700: .Dq 192.0.2.0/8
1.91 djm 701: respectively.
702: .Pp
1.60 dtucker 703: Only a subset of keywords may be used on the lines following a
704: .Cm Match
705: keyword.
706: Available keywords are
1.99 okan 707: .Cm AllowAgentForwarding ,
1.62 dtucker 708: .Cm AllowTcpForwarding ,
1.123 djm 709: .Cm AuthorizedKeysFile ,
710: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 711: .Cm Banner ,
1.85 djm 712: .Cm ChrootDirectory ,
1.67 dtucker 713: .Cm ForceCommand ,
1.62 dtucker 714: .Cm GatewayPorts ,
1.87 djm 715: .Cm GSSAPIAuthentication ,
716: .Cm HostbasedAuthentication ,
1.123 djm 717: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 718: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 719: .Cm KerberosAuthentication ,
1.95 dtucker 720: .Cm MaxAuthTries ,
1.94 dtucker 721: .Cm MaxSessions ,
1.72 dtucker 722: .Cm PasswordAuthentication ,
1.97 djm 723: .Cm PermitEmptyPasswords ,
1.66 dtucker 724: .Cm PermitOpen ,
1.79 dtucker 725: .Cm PermitRootLogin ,
1.123 djm 726: .Cm PermitTunnel ,
1.107 dtucker 727: .Cm PubkeyAuthentication ,
1.72 dtucker 728: .Cm RhostsRSAAuthentication ,
729: .Cm RSAAuthentication ,
1.66 dtucker 730: .Cm X11DisplayOffset ,
1.101 djm 731: .Cm X11Forwarding
1.60 dtucker 732: and
1.102 djm 733: .Cm X11UseLocalHost .
1.33 dtucker 734: .It Cm MaxAuthTries
735: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 736: connection.
737: Once the number of failures reaches half this value,
738: additional failures are logged.
739: The default is 6.
1.90 djm 740: .It Cm MaxSessions
741: Specifies the maximum number of open sessions permitted per network connection.
742: The default is 10.
1.1 stevesk 743: .It Cm MaxStartups
744: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 745: SSH daemon.
1.1 stevesk 746: Additional connections will be dropped until authentication succeeds or the
747: .Cm LoginGraceTime
748: expires for a connection.
749: The default is 10.
750: .Pp
751: Alternatively, random early drop can be enabled by specifying
752: the three colon separated values
753: .Dq start:rate:full
1.51 jmc 754: (e.g. "10:30:60").
1.53 jmc 755: .Xr sshd 8
1.1 stevesk 756: will refuse connection attempts with a probability of
757: .Dq rate/100
758: (30%)
759: if there are currently
760: .Dq start
761: (10)
762: unauthenticated connections.
763: The probability increases linearly and all connection attempts
764: are refused if the number of unauthenticated connections reaches
765: .Dq full
766: (60).
767: .It Cm PasswordAuthentication
768: Specifies whether password authentication is allowed.
769: The default is
770: .Dq yes .
771: .It Cm PermitEmptyPasswords
772: When password authentication is allowed, it specifies whether the
773: server allows login to accounts with empty password strings.
774: The default is
775: .Dq no .
1.62 dtucker 776: .It Cm PermitOpen
777: Specifies the destinations to which TCP port forwarding is permitted.
778: The forwarding specification must be one of the following forms:
779: .Pp
780: .Bl -item -offset indent -compact
781: .It
782: .Cm PermitOpen
783: .Sm off
784: .Ar host : port
785: .Sm on
786: .It
787: .Cm PermitOpen
788: .Sm off
789: .Ar IPv4_addr : port
790: .Sm on
791: .It
792: .Cm PermitOpen
793: .Sm off
794: .Ar \&[ IPv6_addr \&] : port
795: .Sm on
796: .El
797: .Pp
1.68 dtucker 798: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 799: An argument of
800: .Dq any
801: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 802: By default all port forwarding requests are permitted.
1.1 stevesk 803: .It Cm PermitRootLogin
1.38 jmc 804: Specifies whether root can log in using
1.1 stevesk 805: .Xr ssh 1 .
806: The argument must be
807: .Dq yes ,
808: .Dq without-password ,
1.52 jmc 809: .Dq forced-commands-only ,
1.1 stevesk 810: or
811: .Dq no .
812: The default is
813: .Dq yes .
814: .Pp
815: If this option is set to
1.52 jmc 816: .Dq without-password ,
1.1 stevesk 817: password authentication is disabled for root.
818: .Pp
819: If this option is set to
1.52 jmc 820: .Dq forced-commands-only ,
1.1 stevesk 821: root login with public key authentication will be allowed,
822: but only if the
823: .Ar command
824: option has been specified
825: (which may be useful for taking remote backups even if root login is
1.17 jmc 826: normally not allowed).
827: All other authentication methods are disabled for root.
1.1 stevesk 828: .Pp
829: If this option is set to
1.52 jmc 830: .Dq no ,
1.38 jmc 831: root is not allowed to log in.
1.46 reyk 832: .It Cm PermitTunnel
833: Specifies whether
834: .Xr tun 4
835: device forwarding is allowed.
1.47 reyk 836: The argument must be
837: .Dq yes ,
1.58 stevesk 838: .Dq point-to-point
839: (layer 3),
840: .Dq ethernet
841: (layer 2), or
1.47 reyk 842: .Dq no .
1.58 stevesk 843: Specifying
844: .Dq yes
845: permits both
846: .Dq point-to-point
847: and
848: .Dq ethernet .
1.46 reyk 849: The default is
850: .Dq no .
1.6 markus 851: .It Cm PermitUserEnvironment
852: Specifies whether
853: .Pa ~/.ssh/environment
1.9 stevesk 854: and
1.6 markus 855: .Cm environment=
856: options in
857: .Pa ~/.ssh/authorized_keys
1.9 stevesk 858: are processed by
1.52 jmc 859: .Xr sshd 8 .
1.6 markus 860: The default is
861: .Dq no .
1.9 stevesk 862: Enabling environment processing may enable users to bypass access
863: restrictions in some configurations using mechanisms such as
864: .Ev LD_PRELOAD .
1.1 stevesk 865: .It Cm PidFile
1.4 stevesk 866: Specifies the file that contains the process ID of the
1.53 jmc 867: SSH daemon.
1.1 stevesk 868: The default is
869: .Pa /var/run/sshd.pid .
870: .It Cm Port
871: Specifies the port number that
1.52 jmc 872: .Xr sshd 8
1.1 stevesk 873: listens on.
874: The default is 22.
875: Multiple options of this type are permitted.
876: See also
877: .Cm ListenAddress .
878: .It Cm PrintLastLog
879: Specifies whether
1.52 jmc 880: .Xr sshd 8
1.36 jaredy 881: should print the date and time of the last user login when a user logs
882: in interactively.
1.1 stevesk 883: The default is
884: .Dq yes .
885: .It Cm PrintMotd
886: Specifies whether
1.52 jmc 887: .Xr sshd 8
1.1 stevesk 888: should print
889: .Pa /etc/motd
890: when a user logs in interactively.
891: (On some systems it is also printed by the shell,
892: .Pa /etc/profile ,
893: or equivalent.)
894: The default is
895: .Dq yes .
896: .It Cm Protocol
897: Specifies the protocol versions
1.52 jmc 898: .Xr sshd 8
1.5 stevesk 899: supports.
1.1 stevesk 900: The possible values are
1.52 jmc 901: .Sq 1
1.1 stevesk 902: and
1.52 jmc 903: .Sq 2 .
1.1 stevesk 904: Multiple versions must be comma-separated.
905: The default is
1.109 jmc 906: .Sq 2 .
1.5 stevesk 907: Note that the order of the protocol list does not indicate preference,
908: because the client selects among multiple protocol versions offered
909: by the server.
910: Specifying
911: .Dq 2,1
912: is identical to
913: .Dq 1,2 .
1.1 stevesk 914: .It Cm PubkeyAuthentication
915: Specifies whether public key authentication is allowed.
916: The default is
917: .Dq yes .
918: Note that this option applies to protocol version 2 only.
1.118 djm 919: .It Cm RevokedKeys
920: Specifies a list of revoked public keys.
921: Keys listed in this file will be refused for public key authentication.
922: Note that if this file is not readable, then public key authentication will
923: be refused for all users.
1.1 stevesk 924: .It Cm RhostsRSAAuthentication
925: Specifies whether rhosts or /etc/hosts.equiv authentication together
926: with successful RSA host authentication is allowed.
927: The default is
928: .Dq no .
929: This option applies to protocol version 1 only.
930: .It Cm RSAAuthentication
931: Specifies whether pure RSA authentication is allowed.
932: The default is
933: .Dq yes .
934: This option applies to protocol version 1 only.
935: .It Cm ServerKeyBits
936: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 937: The minimum value is 512, and the default is 1024.
1.1 stevesk 938: .It Cm StrictModes
939: Specifies whether
1.52 jmc 940: .Xr sshd 8
1.1 stevesk 941: should check file modes and ownership of the
942: user's files and home directory before accepting login.
943: This is normally desirable because novices sometimes accidentally leave their
944: directory or files world-writable.
945: The default is
946: .Dq yes .
1.112 djm 947: Note that this does not apply to
948: .Cm ChrootDirectory ,
949: whose permissions and ownership are checked unconditionally.
1.1 stevesk 950: .It Cm Subsystem
1.51 jmc 951: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 952: Arguments should be a subsystem name and a command (with optional arguments)
953: to execute upon subsystem request.
1.80 djm 954: .Pp
1.1 stevesk 955: The command
956: .Xr sftp-server 8
957: implements the
958: .Dq sftp
959: file transfer subsystem.
1.80 djm 960: .Pp
961: Alternately the name
962: .Dq internal-sftp
963: implements an in-process
964: .Dq sftp
965: server.
966: This may simplify configurations using
967: .Cm ChrootDirectory
968: to force a different filesystem root on clients.
969: .Pp
1.1 stevesk 970: By default no subsystems are defined.
971: Note that this option applies to protocol version 2 only.
972: .It Cm SyslogFacility
973: Gives the facility code that is used when logging messages from
1.53 jmc 974: .Xr sshd 8 .
1.1 stevesk 975: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
976: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
977: The default is AUTH.
1.27 markus 978: .It Cm TCPKeepAlive
979: Specifies whether the system should send TCP keepalive messages to the
980: other side.
981: If they are sent, death of the connection or crash of one
982: of the machines will be properly noticed.
983: However, this means that
984: connections will die if the route is down temporarily, and some people
985: find it annoying.
986: On the other hand, if TCP keepalives are not sent,
987: sessions may hang indefinitely on the server, leaving
988: .Dq ghost
989: users and consuming server resources.
990: .Pp
991: The default is
992: .Dq yes
993: (to send TCP keepalive messages), and the server will notice
994: if the network goes down or the client host crashes.
995: This avoids infinitely hanging sessions.
996: .Pp
997: To disable TCP keepalive messages, the value should be set to
998: .Dq no .
1.118 djm 999: .It Cm TrustedUserCAKeys
1000: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1001: trusted to sign user certificates for authentication.
1.119 jmc 1002: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1003: .Ql #
1004: are allowed.
1005: If a certificate is presented for authentication and has its signing CA key
1006: listed in this file, then it may be used for authentication for any user
1007: listed in the certificate's principals list.
1008: Note that certificates that lack a list of principals will not be permitted
1009: for authentication using
1010: .Cm TrustedUserCAKeys .
1.119 jmc 1011: For more details on certificates, see the
1.118 djm 1012: .Sx CERTIFICATES
1013: section in
1014: .Xr ssh-keygen 1 .
1.18 markus 1015: .It Cm UseDNS
1016: Specifies whether
1.52 jmc 1017: .Xr sshd 8
1.40 jmc 1018: should look up the remote host name and check that
1.18 markus 1019: the resolved host name for the remote IP address maps back to the
1020: very same IP address.
1021: The default is
1022: .Dq yes .
1.1 stevesk 1023: .It Cm UseLogin
1024: Specifies whether
1025: .Xr login 1
1026: is used for interactive login sessions.
1027: The default is
1028: .Dq no .
1029: Note that
1030: .Xr login 1
1031: is never used for remote command execution.
1032: Note also, that if this is enabled,
1033: .Cm X11Forwarding
1034: will be disabled because
1035: .Xr login 1
1036: does not know how to handle
1037: .Xr xauth 1
1.15 jmc 1038: cookies.
1039: If
1.1 stevesk 1040: .Cm UsePrivilegeSeparation
1041: is specified, it will be disabled after authentication.
1042: .It Cm UsePrivilegeSeparation
1043: Specifies whether
1.52 jmc 1044: .Xr sshd 8
1.2 stevesk 1045: separates privileges by creating an unprivileged child process
1.15 jmc 1046: to deal with incoming network traffic.
1047: After successful authentication, another process will be created that has
1048: the privilege of the authenticated user.
1049: The goal of privilege separation is to prevent privilege
1.1 stevesk 1050: escalation by containing any corruption within the unprivileged processes.
1051: The default is
1052: .Dq yes .
1.134 djm 1053: If
1054: .Cm UsePrivilegeSeparation
1055: is set to
1056: .Dq sandbox
1057: then the pre-authentication unprivileged process is subject to additional
1058: restrictions.
1.1 stevesk 1059: .It Cm X11DisplayOffset
1060: Specifies the first display number available for
1.52 jmc 1061: .Xr sshd 8 Ns 's
1.1 stevesk 1062: X11 forwarding.
1.52 jmc 1063: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1064: The default is 10.
1065: .It Cm X11Forwarding
1066: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1067: The argument must be
1068: .Dq yes
1069: or
1070: .Dq no .
1.1 stevesk 1071: The default is
1072: .Dq no .
1.13 stevesk 1073: .Pp
1074: When X11 forwarding is enabled, there may be additional exposure to
1075: the server and to client displays if the
1.52 jmc 1076: .Xr sshd 8
1.13 stevesk 1077: proxy display is configured to listen on the wildcard address (see
1078: .Cm X11UseLocalhost
1.52 jmc 1079: below), though this is not the default.
1.13 stevesk 1080: Additionally, the authentication spoofing and authentication data
1081: verification and substitution occur on the client side.
1082: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1083: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1084: forwarding (see the warnings for
1085: .Cm ForwardX11
1086: in
1.19 jmc 1087: .Xr ssh_config 5 ) .
1.13 stevesk 1088: A system administrator may have a stance in which they want to
1089: protect clients that may expose themselves to attack by unwittingly
1090: requesting X11 forwarding, which can warrant a
1091: .Dq no
1092: setting.
1093: .Pp
1094: Note that disabling X11 forwarding does not prevent users from
1095: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1096: X11 forwarding is automatically disabled if
1097: .Cm UseLogin
1098: is enabled.
1099: .It Cm X11UseLocalhost
1100: Specifies whether
1.52 jmc 1101: .Xr sshd 8
1.1 stevesk 1102: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1103: the wildcard address.
1104: By default,
1.52 jmc 1105: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1106: hostname part of the
1107: .Ev DISPLAY
1108: environment variable to
1109: .Dq localhost .
1.8 stevesk 1110: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1111: However, some older X11 clients may not function with this
1112: configuration.
1113: .Cm X11UseLocalhost
1114: may be set to
1115: .Dq no
1116: to specify that the forwarding server should be bound to the wildcard
1117: address.
1118: The argument must be
1119: .Dq yes
1120: or
1121: .Dq no .
1122: The default is
1123: .Dq yes .
1124: .It Cm XAuthLocation
1.11 stevesk 1125: Specifies the full pathname of the
1.1 stevesk 1126: .Xr xauth 1
1127: program.
1128: The default is
1129: .Pa /usr/X11R6/bin/xauth .
1130: .El
1.55 jmc 1131: .Sh TIME FORMATS
1.53 jmc 1132: .Xr sshd 8
1.1 stevesk 1133: command-line arguments and configuration file options that specify time
1134: may be expressed using a sequence of the form:
1135: .Sm off
1.7 stevesk 1136: .Ar time Op Ar qualifier ,
1.1 stevesk 1137: .Sm on
1138: where
1139: .Ar time
1140: is a positive integer value and
1141: .Ar qualifier
1142: is one of the following:
1143: .Pp
1144: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1145: .It Aq Cm none
1.1 stevesk 1146: seconds
1147: .It Cm s | Cm S
1148: seconds
1149: .It Cm m | Cm M
1150: minutes
1151: .It Cm h | Cm H
1152: hours
1153: .It Cm d | Cm D
1154: days
1155: .It Cm w | Cm W
1156: weeks
1157: .El
1158: .Pp
1159: Each member of the sequence is added together to calculate
1160: the total time value.
1161: .Pp
1162: Time format examples:
1163: .Pp
1164: .Bl -tag -width Ds -compact -offset indent
1165: .It 600
1166: 600 seconds (10 minutes)
1167: .It 10m
1168: 10 minutes
1169: .It 1h30m
1170: 1 hour 30 minutes (90 minutes)
1171: .El
1172: .Sh FILES
1173: .Bl -tag -width Ds
1174: .It Pa /etc/ssh/sshd_config
1175: Contains configuration data for
1.53 jmc 1176: .Xr sshd 8 .
1.1 stevesk 1177: This file should be writable by root only, but it is recommended
1178: (though not necessary) that it be world-readable.
1179: .El
1.19 jmc 1180: .Sh SEE ALSO
1181: .Xr sshd 8
1.1 stevesk 1182: .Sh AUTHORS
1183: OpenSSH is a derivative of the original and free
1184: ssh 1.2.12 release by Tatu Ylonen.
1185: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1186: Theo de Raadt and Dug Song
1187: removed many bugs, re-added newer features and
1188: created OpenSSH.
1189: Markus Friedl contributed the support for SSH
1190: protocol versions 1.5 and 2.0.
1191: Niels Provos and Markus Friedl contributed support
1192: for privilege separation.
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