Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.138
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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1.138 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.137 2012/04/12 02:42:32 djm Exp $
! 37: .Dd $Mdocdate: April 12 2012 $
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
1.138 ! djm 201: The default is
! 202: .Dq none ,
! 203: i.e. not to use a principals file \(en in this case, the username
1.121 djm 204: of the user must appear in a certificate's principals list for it to be
205: accepted.
206: Note that
207: .Cm AuthorizedPrincipalsFile
208: is only used when authentication proceeds using a CA listed in
209: .Cm TrustedUserCAKeys
210: and is not consulted for certification authorities trusted via
211: .Pa ~/.ssh/authorized_keys ,
212: though the
213: .Cm principals=
214: key option offers a similar facility (see
215: .Xr sshd 8
216: for details).
1.1 stevesk 217: .It Cm Banner
218: The contents of the specified file are sent to the remote user before
219: authentication is allowed.
1.78 djm 220: If the argument is
221: .Dq none
222: then no banner is displayed.
1.1 stevesk 223: This option is only available for protocol version 2.
224: By default, no banner is displayed.
225: .It Cm ChallengeResponseAuthentication
1.50 jmc 226: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 227: All authentication styles from
228: .Xr login.conf 5
229: are supported.
230: The default is
231: .Dq yes .
1.80 djm 232: .It Cm ChrootDirectory
1.113 stevesk 233: Specifies the pathname of a directory to
1.80 djm 234: .Xr chroot 2
235: to after authentication.
1.113 stevesk 236: All components of the pathname must be root-owned directories that are
1.80 djm 237: not writable by any other user or group.
1.106 stevesk 238: After the chroot,
239: .Xr sshd 8
240: changes the working directory to the user's home directory.
1.80 djm 241: .Pp
1.113 stevesk 242: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 243: the connecting user has been authenticated: %% is replaced by a literal '%',
244: %h is replaced by the home directory of the user being authenticated, and
245: %u is replaced by the username of that user.
246: .Pp
247: The
248: .Cm ChrootDirectory
249: must contain the necessary files and directories to support the
1.103 stevesk 250: user's session.
1.80 djm 251: For an interactive session this requires at least a shell, typically
252: .Xr sh 1 ,
253: and basic
254: .Pa /dev
255: nodes such as
256: .Xr null 4 ,
257: .Xr zero 4 ,
258: .Xr stdin 4 ,
259: .Xr stdout 4 ,
260: .Xr stderr 4 ,
261: .Xr arandom 4
262: and
263: .Xr tty 4
264: devices.
265: For file transfer sessions using
1.105 jmc 266: .Dq sftp ,
1.80 djm 267: no additional configuration of the environment is necessary if the
1.105 jmc 268: in-process sftp server is used,
269: though sessions which use logging do require
1.104 stevesk 270: .Pa /dev/log
271: inside the chroot directory (see
272: .Xr sftp-server 8
1.81 jmc 273: for details).
1.80 djm 274: .Pp
275: The default is not to
276: .Xr chroot 2 .
1.1 stevesk 277: .It Cm Ciphers
278: Specifies the ciphers allowed for protocol version 2.
279: Multiple ciphers must be comma-separated.
1.34 dtucker 280: The supported ciphers are
281: .Dq 3des-cbc ,
282: .Dq aes128-cbc ,
283: .Dq aes192-cbc ,
284: .Dq aes256-cbc ,
285: .Dq aes128-ctr ,
286: .Dq aes192-ctr ,
287: .Dq aes256-ctr ,
1.43 djm 288: .Dq arcfour128 ,
289: .Dq arcfour256 ,
1.34 dtucker 290: .Dq arcfour ,
291: .Dq blowfish-cbc ,
292: and
293: .Dq cast128-cbc .
1.52 jmc 294: The default is:
295: .Bd -literal -offset 3n
1.100 naddy 296: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
297: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
298: aes256-cbc,arcfour
1.1 stevesk 299: .Ed
300: .It Cm ClientAliveCountMax
1.48 jmc 301: Sets the number of client alive messages (see below) which may be
1.1 stevesk 302: sent without
1.52 jmc 303: .Xr sshd 8
1.17 jmc 304: receiving any messages back from the client.
305: If this threshold is reached while client alive messages are being sent,
1.52 jmc 306: sshd will disconnect the client, terminating the session.
1.17 jmc 307: It is important to note that the use of client alive messages is very
308: different from
1.27 markus 309: .Cm TCPKeepAlive
1.17 jmc 310: (below).
311: The client alive messages are sent through the encrypted channel
312: and therefore will not be spoofable.
313: The TCP keepalive option enabled by
1.27 markus 314: .Cm TCPKeepAlive
1.17 jmc 315: is spoofable.
316: The client alive mechanism is valuable when the client or
1.1 stevesk 317: server depend on knowing when a connection has become inactive.
318: .Pp
1.17 jmc 319: The default value is 3.
320: If
1.1 stevesk 321: .Cm ClientAliveInterval
1.48 jmc 322: (see below) is set to 15, and
1.1 stevesk 323: .Cm ClientAliveCountMax
1.52 jmc 324: is left at the default, unresponsive SSH clients
1.1 stevesk 325: will be disconnected after approximately 45 seconds.
1.57 markus 326: This option applies to protocol version 2 only.
1.42 djm 327: .It Cm ClientAliveInterval
328: Sets a timeout interval in seconds after which if no data has been received
329: from the client,
1.52 jmc 330: .Xr sshd 8
1.42 djm 331: will send a message through the encrypted
332: channel to request a response from the client.
333: The default
334: is 0, indicating that these messages will not be sent to the client.
335: This option applies to protocol version 2 only.
1.3 markus 336: .It Cm Compression
1.44 markus 337: Specifies whether compression is allowed, or delayed until
338: the user has authenticated successfully.
1.3 markus 339: The argument must be
1.44 markus 340: .Dq yes ,
341: .Dq delayed ,
1.3 markus 342: or
343: .Dq no .
344: The default is
1.44 markus 345: .Dq delayed .
1.1 stevesk 346: .It Cm DenyGroups
347: This keyword can be followed by a list of group name patterns, separated
348: by spaces.
349: Login is disallowed for users whose primary group or supplementary
350: group list matches one of the patterns.
351: Only group names are valid; a numerical group ID is not recognized.
352: By default, login is allowed for all groups.
1.54 jmc 353: The allow/deny directives are processed in the following order:
354: .Cm DenyUsers ,
355: .Cm AllowUsers ,
356: .Cm DenyGroups ,
357: and finally
358: .Cm AllowGroups .
1.49 jmc 359: .Pp
360: See
361: .Sx PATTERNS
362: in
363: .Xr ssh_config 5
364: for more information on patterns.
1.1 stevesk 365: .It Cm DenyUsers
366: This keyword can be followed by a list of user name patterns, separated
367: by spaces.
368: Login is disallowed for user names that match one of the patterns.
369: Only user names are valid; a numerical user ID is not recognized.
370: By default, login is allowed for all users.
371: If the pattern takes the form USER@HOST then USER and HOST
372: are separately checked, restricting logins to particular
373: users from particular hosts.
1.54 jmc 374: The allow/deny directives are processed in the following order:
375: .Cm DenyUsers ,
376: .Cm AllowUsers ,
377: .Cm DenyGroups ,
378: and finally
379: .Cm AllowGroups .
1.49 jmc 380: .Pp
381: See
382: .Sx PATTERNS
383: in
384: .Xr ssh_config 5
385: for more information on patterns.
1.67 dtucker 386: .It Cm ForceCommand
387: Forces the execution of the command specified by
388: .Cm ForceCommand ,
1.84 djm 389: ignoring any command supplied by the client and
390: .Pa ~/.ssh/rc
391: if present.
1.67 dtucker 392: The command is invoked by using the user's login shell with the -c option.
393: This applies to shell, command, or subsystem execution.
394: It is most useful inside a
395: .Cm Match
396: block.
397: The command originally supplied by the client is available in the
398: .Ev SSH_ORIGINAL_COMMAND
399: environment variable.
1.82 djm 400: Specifying a command of
401: .Dq internal-sftp
402: will force the use of an in-process sftp server that requires no support
403: files when used with
404: .Cm ChrootDirectory .
1.1 stevesk 405: .It Cm GatewayPorts
406: Specifies whether remote hosts are allowed to connect to ports
407: forwarded for the client.
408: By default,
1.52 jmc 409: .Xr sshd 8
1.15 jmc 410: binds remote port forwardings to the loopback address.
411: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 412: .Cm GatewayPorts
1.52 jmc 413: can be used to specify that sshd
1.39 djm 414: should allow remote port forwardings to bind to non-loopback addresses, thus
415: allowing other hosts to connect.
416: The argument may be
417: .Dq no
418: to force remote port forwardings to be available to the local host only,
1.1 stevesk 419: .Dq yes
1.39 djm 420: to force remote port forwardings to bind to the wildcard address, or
421: .Dq clientspecified
422: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 423: The default is
424: .Dq no .
1.23 markus 425: .It Cm GSSAPIAuthentication
1.25 markus 426: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 427: The default is
1.23 markus 428: .Dq no .
429: Note that this option applies to protocol version 2 only.
430: .It Cm GSSAPICleanupCredentials
431: Specifies whether to automatically destroy the user's credentials cache
432: on logout.
433: The default is
434: .Dq yes .
435: Note that this option applies to protocol version 2 only.
1.1 stevesk 436: .It Cm HostbasedAuthentication
437: Specifies whether rhosts or /etc/hosts.equiv authentication together
438: with successful public key client host authentication is allowed
1.50 jmc 439: (host-based authentication).
1.1 stevesk 440: This option is similar to
441: .Cm RhostsRSAAuthentication
442: and applies to protocol version 2 only.
1.70 dtucker 443: The default is
444: .Dq no .
445: .It Cm HostbasedUsesNameFromPacketOnly
446: Specifies whether or not the server will attempt to perform a reverse
447: name lookup when matching the name in the
448: .Pa ~/.shosts ,
449: .Pa ~/.rhosts ,
450: and
451: .Pa /etc/hosts.equiv
452: files during
453: .Cm HostbasedAuthentication .
454: A setting of
455: .Dq yes
456: means that
457: .Xr sshd 8
458: uses the name supplied by the client rather than
459: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 460: The default is
461: .Dq no .
1.117 djm 462: .It Cm HostCertificate
463: Specifies a file containing a public host certificate.
464: The certificate's public key must match a private host key already specified
465: by
466: .Cm HostKey .
467: The default behaviour of
468: .Xr sshd 8
469: is not to load any certificates.
1.1 stevesk 470: .It Cm HostKey
471: Specifies a file containing a private host key
472: used by SSH.
473: The default is
474: .Pa /etc/ssh/ssh_host_key
475: for protocol version 1, and
1.126 djm 476: .Pa /etc/ssh/ssh_host_dsa_key ,
477: .Pa /etc/ssh/ssh_host_ecdsa_key
478: and
1.1 stevesk 479: .Pa /etc/ssh/ssh_host_rsa_key
480: for protocol version 2.
481: Note that
1.52 jmc 482: .Xr sshd 8
1.1 stevesk 483: will refuse to use a file if it is group/world-accessible.
484: It is possible to have multiple host key files.
485: .Dq rsa1
486: keys are used for version 1 and
1.126 djm 487: .Dq dsa ,
488: .Dq ecdsa
1.1 stevesk 489: or
490: .Dq rsa
491: are used for version 2 of the SSH protocol.
492: .It Cm IgnoreRhosts
493: Specifies that
494: .Pa .rhosts
495: and
496: .Pa .shosts
497: files will not be used in
498: .Cm RhostsRSAAuthentication
499: or
500: .Cm HostbasedAuthentication .
501: .Pp
502: .Pa /etc/hosts.equiv
503: and
504: .Pa /etc/shosts.equiv
505: are still used.
506: The default is
507: .Dq yes .
508: .It Cm IgnoreUserKnownHosts
509: Specifies whether
1.52 jmc 510: .Xr sshd 8
1.1 stevesk 511: should ignore the user's
1.41 djm 512: .Pa ~/.ssh/known_hosts
1.1 stevesk 513: during
514: .Cm RhostsRSAAuthentication
515: or
516: .Cm HostbasedAuthentication .
517: The default is
518: .Dq no .
1.129 djm 519: .It Cm IPQoS
520: Specifies the IPv4 type-of-service or DSCP class for the connection.
521: Accepted values are
522: .Dq af11 ,
523: .Dq af12 ,
524: .Dq af13 ,
1.136 djm 525: .Dq af21 ,
1.129 djm 526: .Dq af22 ,
527: .Dq af23 ,
528: .Dq af31 ,
529: .Dq af32 ,
530: .Dq af33 ,
531: .Dq af41 ,
532: .Dq af42 ,
533: .Dq af43 ,
534: .Dq cs0 ,
535: .Dq cs1 ,
536: .Dq cs2 ,
537: .Dq cs3 ,
538: .Dq cs4 ,
539: .Dq cs5 ,
540: .Dq cs6 ,
541: .Dq cs7 ,
542: .Dq ef ,
543: .Dq lowdelay ,
544: .Dq throughput ,
545: .Dq reliability ,
546: or a numeric value.
1.131 djm 547: This option may take one or two arguments, separated by whitespace.
1.129 djm 548: If one argument is specified, it is used as the packet class unconditionally.
549: If two values are specified, the first is automatically selected for
550: interactive sessions and the second for non-interactive sessions.
551: The default is
552: .Dq lowdelay
553: for interactive sessions and
554: .Dq throughput
555: for non-interactive sessions.
1.1 stevesk 556: .It Cm KerberosAuthentication
1.24 markus 557: Specifies whether the password provided by the user for
1.1 stevesk 558: .Cm PasswordAuthentication
1.24 markus 559: will be validated through the Kerberos KDC.
1.1 stevesk 560: To use this option, the server needs a
561: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 562: The default is
1.29 dtucker 563: .Dq no .
564: .It Cm KerberosGetAFSToken
1.45 djm 565: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 566: an AFS token before accessing the user's home directory.
1.52 jmc 567: The default is
1.1 stevesk 568: .Dq no .
569: .It Cm KerberosOrLocalPasswd
1.52 jmc 570: If password authentication through Kerberos fails then
1.1 stevesk 571: the password will be validated via any additional local mechanism
572: such as
573: .Pa /etc/passwd .
1.52 jmc 574: The default is
1.1 stevesk 575: .Dq yes .
576: .It Cm KerberosTicketCleanup
577: Specifies whether to automatically destroy the user's ticket cache
578: file on logout.
1.52 jmc 579: The default is
1.1 stevesk 580: .Dq yes .
1.127 djm 581: .It Cm KexAlgorithms
582: Specifies the available KEX (Key Exchange) algorithms.
583: Multiple algorithms must be comma-separated.
584: The default is
585: .Dq ecdh-sha2-nistp256 ,
586: .Dq ecdh-sha2-nistp384 ,
587: .Dq ecdh-sha2-nistp521 ,
1.130 jmc 588: .Dq diffie-hellman-group-exchange-sha256 ,
1.127 djm 589: .Dq diffie-hellman-group-exchange-sha1 ,
590: .Dq diffie-hellman-group14-sha1 ,
591: .Dq diffie-hellman-group1-sha1 .
1.1 stevesk 592: .It Cm KeyRegenerationInterval
593: In protocol version 1, the ephemeral server key is automatically regenerated
594: after this many seconds (if it has been used).
595: The purpose of regeneration is to prevent
596: decrypting captured sessions by later breaking into the machine and
597: stealing the keys.
598: The key is never stored anywhere.
599: If the value is 0, the key is never regenerated.
600: The default is 3600 (seconds).
601: .It Cm ListenAddress
602: Specifies the local addresses
1.52 jmc 603: .Xr sshd 8
1.1 stevesk 604: should listen on.
605: The following forms may be used:
606: .Pp
607: .Bl -item -offset indent -compact
608: .It
609: .Cm ListenAddress
610: .Sm off
611: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
612: .Sm on
613: .It
614: .Cm ListenAddress
615: .Sm off
616: .Ar host No | Ar IPv4_addr No : Ar port
617: .Sm on
618: .It
619: .Cm ListenAddress
620: .Sm off
621: .Oo
622: .Ar host No | Ar IPv6_addr Oc : Ar port
623: .Sm on
624: .El
625: .Pp
626: If
627: .Ar port
628: is not specified,
1.52 jmc 629: sshd will listen on the address and all prior
1.1 stevesk 630: .Cm Port
1.17 jmc 631: options specified.
632: The default is to listen on all local addresses.
1.15 jmc 633: Multiple
1.1 stevesk 634: .Cm ListenAddress
1.17 jmc 635: options are permitted.
636: Additionally, any
1.1 stevesk 637: .Cm Port
1.52 jmc 638: options must precede this option for non-port qualified addresses.
1.1 stevesk 639: .It Cm LoginGraceTime
640: The server disconnects after this time if the user has not
641: successfully logged in.
642: If the value is 0, there is no time limit.
1.12 stevesk 643: The default is 120 seconds.
1.1 stevesk 644: .It Cm LogLevel
645: Gives the verbosity level that is used when logging messages from
1.53 jmc 646: .Xr sshd 8 .
1.1 stevesk 647: The possible values are:
1.52 jmc 648: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 649: The default is INFO.
650: DEBUG and DEBUG1 are equivalent.
651: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
652: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 653: .It Cm MACs
654: Specifies the available MAC (message authentication code) algorithms.
655: The MAC algorithm is used in protocol version 2
656: for data integrity protection.
657: Multiple algorithms must be comma-separated.
1.52 jmc 658: The default is:
1.77 jmc 659: .Bd -literal -offset indent
660: hmac-md5,hmac-sha1,umac-64@openssh.com,
1.135 djm 661: hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
662: hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
663: hmac-sha2-512-96
1.77 jmc 664: .Ed
1.60 dtucker 665: .It Cm Match
1.61 jmc 666: Introduces a conditional block.
1.65 dtucker 667: If all of the criteria on the
1.60 dtucker 668: .Cm Match
1.65 dtucker 669: line are satisfied, the keywords on the following lines override those
670: set in the global section of the config file, until either another
1.60 dtucker 671: .Cm Match
1.65 dtucker 672: line or the end of the file.
1.91 djm 673: .Pp
1.61 jmc 674: The arguments to
1.60 dtucker 675: .Cm Match
1.65 dtucker 676: are one or more criteria-pattern pairs.
1.60 dtucker 677: The available criteria are
678: .Cm User ,
1.69 dtucker 679: .Cm Group ,
1.60 dtucker 680: .Cm Host ,
681: and
682: .Cm Address .
1.91 djm 683: The match patterns may consist of single entries or comma-separated
684: lists and may use the wildcard and negation operators described in the
1.92 djm 685: .Sx PATTERNS
1.91 djm 686: section of
1.92 djm 687: .Xr ssh_config 5 .
1.91 djm 688: .Pp
689: The patterns in an
690: .Cm Address
691: criteria may additionally contain addresses to match in CIDR
1.93 jmc 692: address/masklen format, e.g.\&
1.91 djm 693: .Dq 192.0.2.0/24
694: or
695: .Dq 3ffe:ffff::/32 .
696: Note that the mask length provided must be consistent with the address -
697: it is an error to specify a mask length that is too long for the address
1.93 jmc 698: or one with bits set in this host portion of the address.
699: For example,
1.91 djm 700: .Dq 192.0.2.0/33
701: and
1.93 jmc 702: .Dq 192.0.2.0/8
1.91 djm 703: respectively.
704: .Pp
1.60 dtucker 705: Only a subset of keywords may be used on the lines following a
706: .Cm Match
707: keyword.
708: Available keywords are
1.99 okan 709: .Cm AllowAgentForwarding ,
1.62 dtucker 710: .Cm AllowTcpForwarding ,
1.123 djm 711: .Cm AuthorizedKeysFile ,
712: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 713: .Cm Banner ,
1.85 djm 714: .Cm ChrootDirectory ,
1.67 dtucker 715: .Cm ForceCommand ,
1.62 dtucker 716: .Cm GatewayPorts ,
1.87 djm 717: .Cm GSSAPIAuthentication ,
718: .Cm HostbasedAuthentication ,
1.123 djm 719: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 720: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 721: .Cm KerberosAuthentication ,
1.95 dtucker 722: .Cm MaxAuthTries ,
1.94 dtucker 723: .Cm MaxSessions ,
1.72 dtucker 724: .Cm PasswordAuthentication ,
1.97 djm 725: .Cm PermitEmptyPasswords ,
1.66 dtucker 726: .Cm PermitOpen ,
1.79 dtucker 727: .Cm PermitRootLogin ,
1.123 djm 728: .Cm PermitTunnel ,
1.107 dtucker 729: .Cm PubkeyAuthentication ,
1.72 dtucker 730: .Cm RhostsRSAAuthentication ,
731: .Cm RSAAuthentication ,
1.66 dtucker 732: .Cm X11DisplayOffset ,
1.101 djm 733: .Cm X11Forwarding
1.60 dtucker 734: and
1.102 djm 735: .Cm X11UseLocalHost .
1.33 dtucker 736: .It Cm MaxAuthTries
737: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 738: connection.
739: Once the number of failures reaches half this value,
740: additional failures are logged.
741: The default is 6.
1.90 djm 742: .It Cm MaxSessions
743: Specifies the maximum number of open sessions permitted per network connection.
744: The default is 10.
1.1 stevesk 745: .It Cm MaxStartups
746: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 747: SSH daemon.
1.1 stevesk 748: Additional connections will be dropped until authentication succeeds or the
749: .Cm LoginGraceTime
750: expires for a connection.
751: The default is 10.
752: .Pp
753: Alternatively, random early drop can be enabled by specifying
754: the three colon separated values
755: .Dq start:rate:full
1.51 jmc 756: (e.g. "10:30:60").
1.53 jmc 757: .Xr sshd 8
1.1 stevesk 758: will refuse connection attempts with a probability of
759: .Dq rate/100
760: (30%)
761: if there are currently
762: .Dq start
763: (10)
764: unauthenticated connections.
765: The probability increases linearly and all connection attempts
766: are refused if the number of unauthenticated connections reaches
767: .Dq full
768: (60).
769: .It Cm PasswordAuthentication
770: Specifies whether password authentication is allowed.
771: The default is
772: .Dq yes .
773: .It Cm PermitEmptyPasswords
774: When password authentication is allowed, it specifies whether the
775: server allows login to accounts with empty password strings.
776: The default is
777: .Dq no .
1.62 dtucker 778: .It Cm PermitOpen
779: Specifies the destinations to which TCP port forwarding is permitted.
780: The forwarding specification must be one of the following forms:
781: .Pp
782: .Bl -item -offset indent -compact
783: .It
784: .Cm PermitOpen
785: .Sm off
786: .Ar host : port
787: .Sm on
788: .It
789: .Cm PermitOpen
790: .Sm off
791: .Ar IPv4_addr : port
792: .Sm on
793: .It
794: .Cm PermitOpen
795: .Sm off
796: .Ar \&[ IPv6_addr \&] : port
797: .Sm on
798: .El
799: .Pp
1.68 dtucker 800: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 801: An argument of
802: .Dq any
803: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 804: By default all port forwarding requests are permitted.
1.1 stevesk 805: .It Cm PermitRootLogin
1.38 jmc 806: Specifies whether root can log in using
1.1 stevesk 807: .Xr ssh 1 .
808: The argument must be
809: .Dq yes ,
810: .Dq without-password ,
1.52 jmc 811: .Dq forced-commands-only ,
1.1 stevesk 812: or
813: .Dq no .
814: The default is
815: .Dq yes .
816: .Pp
817: If this option is set to
1.52 jmc 818: .Dq without-password ,
1.1 stevesk 819: password authentication is disabled for root.
820: .Pp
821: If this option is set to
1.52 jmc 822: .Dq forced-commands-only ,
1.1 stevesk 823: root login with public key authentication will be allowed,
824: but only if the
825: .Ar command
826: option has been specified
827: (which may be useful for taking remote backups even if root login is
1.17 jmc 828: normally not allowed).
829: All other authentication methods are disabled for root.
1.1 stevesk 830: .Pp
831: If this option is set to
1.52 jmc 832: .Dq no ,
1.38 jmc 833: root is not allowed to log in.
1.46 reyk 834: .It Cm PermitTunnel
835: Specifies whether
836: .Xr tun 4
837: device forwarding is allowed.
1.47 reyk 838: The argument must be
839: .Dq yes ,
1.58 stevesk 840: .Dq point-to-point
841: (layer 3),
842: .Dq ethernet
843: (layer 2), or
1.47 reyk 844: .Dq no .
1.58 stevesk 845: Specifying
846: .Dq yes
847: permits both
848: .Dq point-to-point
849: and
850: .Dq ethernet .
1.46 reyk 851: The default is
852: .Dq no .
1.6 markus 853: .It Cm PermitUserEnvironment
854: Specifies whether
855: .Pa ~/.ssh/environment
1.9 stevesk 856: and
1.6 markus 857: .Cm environment=
858: options in
859: .Pa ~/.ssh/authorized_keys
1.9 stevesk 860: are processed by
1.52 jmc 861: .Xr sshd 8 .
1.6 markus 862: The default is
863: .Dq no .
1.9 stevesk 864: Enabling environment processing may enable users to bypass access
865: restrictions in some configurations using mechanisms such as
866: .Ev LD_PRELOAD .
1.1 stevesk 867: .It Cm PidFile
1.4 stevesk 868: Specifies the file that contains the process ID of the
1.53 jmc 869: SSH daemon.
1.1 stevesk 870: The default is
871: .Pa /var/run/sshd.pid .
872: .It Cm Port
873: Specifies the port number that
1.52 jmc 874: .Xr sshd 8
1.1 stevesk 875: listens on.
876: The default is 22.
877: Multiple options of this type are permitted.
878: See also
879: .Cm ListenAddress .
880: .It Cm PrintLastLog
881: Specifies whether
1.52 jmc 882: .Xr sshd 8
1.36 jaredy 883: should print the date and time of the last user login when a user logs
884: in interactively.
1.1 stevesk 885: The default is
886: .Dq yes .
887: .It Cm PrintMotd
888: Specifies whether
1.52 jmc 889: .Xr sshd 8
1.1 stevesk 890: should print
891: .Pa /etc/motd
892: when a user logs in interactively.
893: (On some systems it is also printed by the shell,
894: .Pa /etc/profile ,
895: or equivalent.)
896: The default is
897: .Dq yes .
898: .It Cm Protocol
899: Specifies the protocol versions
1.52 jmc 900: .Xr sshd 8
1.5 stevesk 901: supports.
1.1 stevesk 902: The possible values are
1.52 jmc 903: .Sq 1
1.1 stevesk 904: and
1.52 jmc 905: .Sq 2 .
1.1 stevesk 906: Multiple versions must be comma-separated.
907: The default is
1.109 jmc 908: .Sq 2 .
1.5 stevesk 909: Note that the order of the protocol list does not indicate preference,
910: because the client selects among multiple protocol versions offered
911: by the server.
912: Specifying
913: .Dq 2,1
914: is identical to
915: .Dq 1,2 .
1.1 stevesk 916: .It Cm PubkeyAuthentication
917: Specifies whether public key authentication is allowed.
918: The default is
919: .Dq yes .
920: Note that this option applies to protocol version 2 only.
1.118 djm 921: .It Cm RevokedKeys
922: Specifies a list of revoked public keys.
923: Keys listed in this file will be refused for public key authentication.
924: Note that if this file is not readable, then public key authentication will
925: be refused for all users.
1.1 stevesk 926: .It Cm RhostsRSAAuthentication
927: Specifies whether rhosts or /etc/hosts.equiv authentication together
928: with successful RSA host authentication is allowed.
929: The default is
930: .Dq no .
931: This option applies to protocol version 1 only.
932: .It Cm RSAAuthentication
933: Specifies whether pure RSA authentication is allowed.
934: The default is
935: .Dq yes .
936: This option applies to protocol version 1 only.
937: .It Cm ServerKeyBits
938: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 939: The minimum value is 512, and the default is 1024.
1.1 stevesk 940: .It Cm StrictModes
941: Specifies whether
1.52 jmc 942: .Xr sshd 8
1.1 stevesk 943: should check file modes and ownership of the
944: user's files and home directory before accepting login.
945: This is normally desirable because novices sometimes accidentally leave their
946: directory or files world-writable.
947: The default is
948: .Dq yes .
1.112 djm 949: Note that this does not apply to
950: .Cm ChrootDirectory ,
951: whose permissions and ownership are checked unconditionally.
1.1 stevesk 952: .It Cm Subsystem
1.51 jmc 953: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 954: Arguments should be a subsystem name and a command (with optional arguments)
955: to execute upon subsystem request.
1.80 djm 956: .Pp
1.1 stevesk 957: The command
958: .Xr sftp-server 8
959: implements the
960: .Dq sftp
961: file transfer subsystem.
1.80 djm 962: .Pp
963: Alternately the name
964: .Dq internal-sftp
965: implements an in-process
966: .Dq sftp
967: server.
968: This may simplify configurations using
969: .Cm ChrootDirectory
970: to force a different filesystem root on clients.
971: .Pp
1.1 stevesk 972: By default no subsystems are defined.
973: Note that this option applies to protocol version 2 only.
974: .It Cm SyslogFacility
975: Gives the facility code that is used when logging messages from
1.53 jmc 976: .Xr sshd 8 .
1.1 stevesk 977: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
978: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
979: The default is AUTH.
1.27 markus 980: .It Cm TCPKeepAlive
981: Specifies whether the system should send TCP keepalive messages to the
982: other side.
983: If they are sent, death of the connection or crash of one
984: of the machines will be properly noticed.
985: However, this means that
986: connections will die if the route is down temporarily, and some people
987: find it annoying.
988: On the other hand, if TCP keepalives are not sent,
989: sessions may hang indefinitely on the server, leaving
990: .Dq ghost
991: users and consuming server resources.
992: .Pp
993: The default is
994: .Dq yes
995: (to send TCP keepalive messages), and the server will notice
996: if the network goes down or the client host crashes.
997: This avoids infinitely hanging sessions.
998: .Pp
999: To disable TCP keepalive messages, the value should be set to
1000: .Dq no .
1.118 djm 1001: .It Cm TrustedUserCAKeys
1002: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1003: trusted to sign user certificates for authentication.
1.119 jmc 1004: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1005: .Ql #
1006: are allowed.
1007: If a certificate is presented for authentication and has its signing CA key
1008: listed in this file, then it may be used for authentication for any user
1009: listed in the certificate's principals list.
1010: Note that certificates that lack a list of principals will not be permitted
1011: for authentication using
1012: .Cm TrustedUserCAKeys .
1.119 jmc 1013: For more details on certificates, see the
1.118 djm 1014: .Sx CERTIFICATES
1015: section in
1016: .Xr ssh-keygen 1 .
1.18 markus 1017: .It Cm UseDNS
1018: Specifies whether
1.52 jmc 1019: .Xr sshd 8
1.40 jmc 1020: should look up the remote host name and check that
1.18 markus 1021: the resolved host name for the remote IP address maps back to the
1022: very same IP address.
1023: The default is
1024: .Dq yes .
1.1 stevesk 1025: .It Cm UseLogin
1026: Specifies whether
1027: .Xr login 1
1028: is used for interactive login sessions.
1029: The default is
1030: .Dq no .
1031: Note that
1032: .Xr login 1
1033: is never used for remote command execution.
1034: Note also, that if this is enabled,
1035: .Cm X11Forwarding
1036: will be disabled because
1037: .Xr login 1
1038: does not know how to handle
1039: .Xr xauth 1
1.15 jmc 1040: cookies.
1041: If
1.1 stevesk 1042: .Cm UsePrivilegeSeparation
1043: is specified, it will be disabled after authentication.
1044: .It Cm UsePrivilegeSeparation
1045: Specifies whether
1.52 jmc 1046: .Xr sshd 8
1.2 stevesk 1047: separates privileges by creating an unprivileged child process
1.15 jmc 1048: to deal with incoming network traffic.
1049: After successful authentication, another process will be created that has
1050: the privilege of the authenticated user.
1051: The goal of privilege separation is to prevent privilege
1.1 stevesk 1052: escalation by containing any corruption within the unprivileged processes.
1053: The default is
1054: .Dq yes .
1.134 djm 1055: If
1056: .Cm UsePrivilegeSeparation
1057: is set to
1058: .Dq sandbox
1059: then the pre-authentication unprivileged process is subject to additional
1060: restrictions.
1.137 djm 1061: .It Cm VersionAddendum
1062: Optionally specifies additional text to append to the SSH protocol banner
1063: sent by the server upon connection.
1064: The default is
1065: .Dq none .
1.1 stevesk 1066: .It Cm X11DisplayOffset
1067: Specifies the first display number available for
1.52 jmc 1068: .Xr sshd 8 Ns 's
1.1 stevesk 1069: X11 forwarding.
1.52 jmc 1070: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1071: The default is 10.
1072: .It Cm X11Forwarding
1073: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1074: The argument must be
1075: .Dq yes
1076: or
1077: .Dq no .
1.1 stevesk 1078: The default is
1079: .Dq no .
1.13 stevesk 1080: .Pp
1081: When X11 forwarding is enabled, there may be additional exposure to
1082: the server and to client displays if the
1.52 jmc 1083: .Xr sshd 8
1.13 stevesk 1084: proxy display is configured to listen on the wildcard address (see
1085: .Cm X11UseLocalhost
1.52 jmc 1086: below), though this is not the default.
1.13 stevesk 1087: Additionally, the authentication spoofing and authentication data
1088: verification and substitution occur on the client side.
1089: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1090: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1091: forwarding (see the warnings for
1092: .Cm ForwardX11
1093: in
1.19 jmc 1094: .Xr ssh_config 5 ) .
1.13 stevesk 1095: A system administrator may have a stance in which they want to
1096: protect clients that may expose themselves to attack by unwittingly
1097: requesting X11 forwarding, which can warrant a
1098: .Dq no
1099: setting.
1100: .Pp
1101: Note that disabling X11 forwarding does not prevent users from
1102: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1103: X11 forwarding is automatically disabled if
1104: .Cm UseLogin
1105: is enabled.
1106: .It Cm X11UseLocalhost
1107: Specifies whether
1.52 jmc 1108: .Xr sshd 8
1.1 stevesk 1109: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1110: the wildcard address.
1111: By default,
1.52 jmc 1112: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1113: hostname part of the
1114: .Ev DISPLAY
1115: environment variable to
1116: .Dq localhost .
1.8 stevesk 1117: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1118: However, some older X11 clients may not function with this
1119: configuration.
1120: .Cm X11UseLocalhost
1121: may be set to
1122: .Dq no
1123: to specify that the forwarding server should be bound to the wildcard
1124: address.
1125: The argument must be
1126: .Dq yes
1127: or
1128: .Dq no .
1129: The default is
1130: .Dq yes .
1131: .It Cm XAuthLocation
1.11 stevesk 1132: Specifies the full pathname of the
1.1 stevesk 1133: .Xr xauth 1
1134: program.
1135: The default is
1136: .Pa /usr/X11R6/bin/xauth .
1137: .El
1.55 jmc 1138: .Sh TIME FORMATS
1.53 jmc 1139: .Xr sshd 8
1.1 stevesk 1140: command-line arguments and configuration file options that specify time
1141: may be expressed using a sequence of the form:
1142: .Sm off
1.7 stevesk 1143: .Ar time Op Ar qualifier ,
1.1 stevesk 1144: .Sm on
1145: where
1146: .Ar time
1147: is a positive integer value and
1148: .Ar qualifier
1149: is one of the following:
1150: .Pp
1151: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1152: .It Aq Cm none
1.1 stevesk 1153: seconds
1154: .It Cm s | Cm S
1155: seconds
1156: .It Cm m | Cm M
1157: minutes
1158: .It Cm h | Cm H
1159: hours
1160: .It Cm d | Cm D
1161: days
1162: .It Cm w | Cm W
1163: weeks
1164: .El
1165: .Pp
1166: Each member of the sequence is added together to calculate
1167: the total time value.
1168: .Pp
1169: Time format examples:
1170: .Pp
1171: .Bl -tag -width Ds -compact -offset indent
1172: .It 600
1173: 600 seconds (10 minutes)
1174: .It 10m
1175: 10 minutes
1176: .It 1h30m
1177: 1 hour 30 minutes (90 minutes)
1178: .El
1179: .Sh FILES
1180: .Bl -tag -width Ds
1181: .It Pa /etc/ssh/sshd_config
1182: Contains configuration data for
1.53 jmc 1183: .Xr sshd 8 .
1.1 stevesk 1184: This file should be writable by root only, but it is recommended
1185: (though not necessary) that it be world-readable.
1186: .El
1.19 jmc 1187: .Sh SEE ALSO
1188: .Xr sshd 8
1.1 stevesk 1189: .Sh AUTHORS
1190: OpenSSH is a derivative of the original and free
1191: ssh 1.2.12 release by Tatu Ylonen.
1192: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1193: Theo de Raadt and Dug Song
1194: removed many bugs, re-added newer features and
1195: created OpenSSH.
1196: Markus Friedl contributed the support for SSH
1197: protocol versions 1.5 and 2.0.
1198: Niels Provos and Markus Friedl contributed support
1199: for privilege separation.
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