Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.122
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
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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
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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
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1.122 ! jmc 37: .\" $OpenBSD: sshd_config.5,v 1.121 2010/05/07 11:30:30 djm Exp $
! 38: .Dd $Mdocdate: May 7 2010 $
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 .
1.89 jmc 98: .It Cm AllowAgentForwarding
99: Specifies whether
100: .Xr ssh-agent 1
101: forwarding is permitted.
102: The default is
103: .Dq yes .
104: Note that disabling agent forwarding does not improve security
105: unless users are also denied shell access, as they can always install
106: their own forwarders.
1.1 stevesk 107: .It Cm AllowGroups
108: This keyword can be followed by a list of group name patterns, separated
109: by spaces.
110: If specified, login is allowed only for users whose primary
111: group or supplementary group list matches one of the patterns.
112: Only group names are valid; a numerical group ID is not recognized.
113: By default, login is allowed for all groups.
1.54 jmc 114: The allow/deny directives are processed in the following order:
115: .Cm DenyUsers ,
116: .Cm AllowUsers ,
117: .Cm DenyGroups ,
118: and finally
119: .Cm AllowGroups .
1.49 jmc 120: .Pp
121: See
122: .Sx PATTERNS
123: in
124: .Xr ssh_config 5
125: for more information on patterns.
1.1 stevesk 126: .It Cm AllowTcpForwarding
127: Specifies whether TCP forwarding is permitted.
128: The default is
129: .Dq yes .
130: Note that disabling TCP forwarding does not improve security unless
131: users are also denied shell access, as they can always install their
132: own forwarders.
133: .It Cm AllowUsers
134: This keyword can be followed by a list of user name patterns, separated
135: by spaces.
1.14 jmc 136: If specified, login is allowed only for user names that
1.1 stevesk 137: match one of the patterns.
138: Only user names are valid; a numerical user ID is not recognized.
139: By default, login is allowed for all users.
140: If the pattern takes the form USER@HOST then USER and HOST
141: are separately checked, restricting logins to particular
142: users from particular hosts.
1.54 jmc 143: The allow/deny directives are processed in the following order:
144: .Cm DenyUsers ,
145: .Cm AllowUsers ,
146: .Cm DenyGroups ,
147: and finally
148: .Cm AllowGroups .
1.49 jmc 149: .Pp
150: See
151: .Sx PATTERNS
152: in
153: .Xr ssh_config 5
154: for more information on patterns.
1.1 stevesk 155: .It Cm AuthorizedKeysFile
156: Specifies the file that contains the public keys that can be used
157: for user authentication.
158: .Cm AuthorizedKeysFile
159: may contain tokens of the form %T which are substituted during connection
1.52 jmc 160: setup.
1.17 jmc 161: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 162: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 163: %u is replaced by the username of that user.
164: After expansion,
165: .Cm AuthorizedKeysFile
166: is taken to be an absolute path or one relative to the user's home
167: directory.
168: The default is
169: .Dq .ssh/authorized_keys .
1.121 djm 170: .It Cm AuthorizedPrincipalsFile
171: Specifies a file that lists principal names that are accepted for
172: certificate authentication.
173: When using certificates signed by a key listed in
174: .Cm TrustedUserCAKeys ,
175: this file lists names, one of which must appear in the certificate for it
176: to be accepted for authentication.
177: Names are listed one per line; empty lines and comments starting with
178: .Ql #
179: are ignored.
180: .Pp
181: .Cm AuthorizedPrincipalsFile
182: may contain tokens of the form %T which are substituted during connection
183: setup.
184: The following tokens are defined: %% 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: After expansion,
188: .Cm AuthorizedPrincipalsFile
189: is taken to be an absolute path or one relative to the user's home
190: directory.
191: .Pp
1.122 ! jmc 192: The default is not to use a principals file \(en in this case, the username
1.121 djm 193: of the user must appear in a certificate's principals list for it to be
194: accepted.
195: Note that
196: .Cm AuthorizedPrincipalsFile
197: is only used when authentication proceeds using a CA listed in
198: .Cm TrustedUserCAKeys
199: and is not consulted for certification authorities trusted via
200: .Pa ~/.ssh/authorized_keys ,
201: though the
202: .Cm principals=
203: key option offers a similar facility (see
204: .Xr sshd 8
205: for details).
1.1 stevesk 206: .It Cm Banner
207: The contents of the specified file are sent to the remote user before
208: authentication is allowed.
1.78 djm 209: If the argument is
210: .Dq none
211: then no banner is displayed.
1.1 stevesk 212: This option is only available for protocol version 2.
213: By default, no banner is displayed.
214: .It Cm ChallengeResponseAuthentication
1.50 jmc 215: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 216: All authentication styles from
217: .Xr login.conf 5
218: are supported.
219: The default is
220: .Dq yes .
1.80 djm 221: .It Cm ChrootDirectory
1.113 stevesk 222: Specifies the pathname of a directory to
1.80 djm 223: .Xr chroot 2
224: to after authentication.
1.113 stevesk 225: All components of the pathname must be root-owned directories that are
1.80 djm 226: not writable by any other user or group.
1.106 stevesk 227: After the chroot,
228: .Xr sshd 8
229: changes the working directory to the user's home directory.
1.80 djm 230: .Pp
1.113 stevesk 231: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 232: the connecting user has been authenticated: %% is replaced by a literal '%',
233: %h is replaced by the home directory of the user being authenticated, and
234: %u is replaced by the username of that user.
235: .Pp
236: The
237: .Cm ChrootDirectory
238: must contain the necessary files and directories to support the
1.103 stevesk 239: user's session.
1.80 djm 240: For an interactive session this requires at least a shell, typically
241: .Xr sh 1 ,
242: and basic
243: .Pa /dev
244: nodes such as
245: .Xr null 4 ,
246: .Xr zero 4 ,
247: .Xr stdin 4 ,
248: .Xr stdout 4 ,
249: .Xr stderr 4 ,
250: .Xr arandom 4
251: and
252: .Xr tty 4
253: devices.
254: For file transfer sessions using
1.105 jmc 255: .Dq sftp ,
1.80 djm 256: no additional configuration of the environment is necessary if the
1.105 jmc 257: in-process sftp server is used,
258: though sessions which use logging do require
1.104 stevesk 259: .Pa /dev/log
260: inside the chroot directory (see
261: .Xr sftp-server 8
1.81 jmc 262: for details).
1.80 djm 263: .Pp
264: The default is not to
265: .Xr chroot 2 .
1.1 stevesk 266: .It Cm Ciphers
267: Specifies the ciphers allowed for protocol version 2.
268: Multiple ciphers must be comma-separated.
1.34 dtucker 269: The supported ciphers are
270: .Dq 3des-cbc ,
271: .Dq aes128-cbc ,
272: .Dq aes192-cbc ,
273: .Dq aes256-cbc ,
274: .Dq aes128-ctr ,
275: .Dq aes192-ctr ,
276: .Dq aes256-ctr ,
1.43 djm 277: .Dq arcfour128 ,
278: .Dq arcfour256 ,
1.34 dtucker 279: .Dq arcfour ,
280: .Dq blowfish-cbc ,
281: and
282: .Dq cast128-cbc .
1.52 jmc 283: The default is:
284: .Bd -literal -offset 3n
1.100 naddy 285: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
286: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
287: aes256-cbc,arcfour
1.1 stevesk 288: .Ed
289: .It Cm ClientAliveCountMax
1.48 jmc 290: Sets the number of client alive messages (see below) which may be
1.1 stevesk 291: sent without
1.52 jmc 292: .Xr sshd 8
1.17 jmc 293: receiving any messages back from the client.
294: If this threshold is reached while client alive messages are being sent,
1.52 jmc 295: sshd will disconnect the client, terminating the session.
1.17 jmc 296: It is important to note that the use of client alive messages is very
297: different from
1.27 markus 298: .Cm TCPKeepAlive
1.17 jmc 299: (below).
300: The client alive messages are sent through the encrypted channel
301: and therefore will not be spoofable.
302: The TCP keepalive option enabled by
1.27 markus 303: .Cm TCPKeepAlive
1.17 jmc 304: is spoofable.
305: The client alive mechanism is valuable when the client or
1.1 stevesk 306: server depend on knowing when a connection has become inactive.
307: .Pp
1.17 jmc 308: The default value is 3.
309: If
1.1 stevesk 310: .Cm ClientAliveInterval
1.48 jmc 311: (see below) is set to 15, and
1.1 stevesk 312: .Cm ClientAliveCountMax
1.52 jmc 313: is left at the default, unresponsive SSH clients
1.1 stevesk 314: will be disconnected after approximately 45 seconds.
1.57 markus 315: This option applies to protocol version 2 only.
1.42 djm 316: .It Cm ClientAliveInterval
317: Sets a timeout interval in seconds after which if no data has been received
318: from the client,
1.52 jmc 319: .Xr sshd 8
1.42 djm 320: will send a message through the encrypted
321: channel to request a response from the client.
322: The default
323: is 0, indicating that these messages will not be sent to the client.
324: This option applies to protocol version 2 only.
1.3 markus 325: .It Cm Compression
1.44 markus 326: Specifies whether compression is allowed, or delayed until
327: the user has authenticated successfully.
1.3 markus 328: The argument must be
1.44 markus 329: .Dq yes ,
330: .Dq delayed ,
1.3 markus 331: or
332: .Dq no .
333: The default is
1.44 markus 334: .Dq delayed .
1.1 stevesk 335: .It Cm DenyGroups
336: This keyword can be followed by a list of group name patterns, separated
337: by spaces.
338: Login is disallowed for users whose primary group or supplementary
339: group list matches one of the patterns.
340: Only group names are valid; a numerical group ID is not recognized.
341: By default, login is allowed for all groups.
1.54 jmc 342: The allow/deny directives are processed in the following order:
343: .Cm DenyUsers ,
344: .Cm AllowUsers ,
345: .Cm DenyGroups ,
346: and finally
347: .Cm AllowGroups .
1.49 jmc 348: .Pp
349: See
350: .Sx PATTERNS
351: in
352: .Xr ssh_config 5
353: for more information on patterns.
1.1 stevesk 354: .It Cm DenyUsers
355: This keyword can be followed by a list of user name patterns, separated
356: by spaces.
357: Login is disallowed for user names that match one of the patterns.
358: Only user names are valid; a numerical user ID is not recognized.
359: By default, login is allowed for all users.
360: If the pattern takes the form USER@HOST then USER and HOST
361: are separately checked, restricting logins to particular
362: users from particular hosts.
1.54 jmc 363: The allow/deny directives are processed in the following order:
364: .Cm DenyUsers ,
365: .Cm AllowUsers ,
366: .Cm DenyGroups ,
367: and finally
368: .Cm AllowGroups .
1.49 jmc 369: .Pp
370: See
371: .Sx PATTERNS
372: in
373: .Xr ssh_config 5
374: for more information on patterns.
1.67 dtucker 375: .It Cm ForceCommand
376: Forces the execution of the command specified by
377: .Cm ForceCommand ,
1.84 djm 378: ignoring any command supplied by the client and
379: .Pa ~/.ssh/rc
380: if present.
1.67 dtucker 381: The command is invoked by using the user's login shell with the -c option.
382: This applies to shell, command, or subsystem execution.
383: It is most useful inside a
384: .Cm Match
385: block.
386: The command originally supplied by the client is available in the
387: .Ev SSH_ORIGINAL_COMMAND
388: environment variable.
1.82 djm 389: Specifying a command of
390: .Dq internal-sftp
391: will force the use of an in-process sftp server that requires no support
392: files when used with
393: .Cm ChrootDirectory .
1.1 stevesk 394: .It Cm GatewayPorts
395: Specifies whether remote hosts are allowed to connect to ports
396: forwarded for the client.
397: By default,
1.52 jmc 398: .Xr sshd 8
1.15 jmc 399: binds remote port forwardings to the loopback address.
400: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 401: .Cm GatewayPorts
1.52 jmc 402: can be used to specify that sshd
1.39 djm 403: should allow remote port forwardings to bind to non-loopback addresses, thus
404: allowing other hosts to connect.
405: The argument may be
406: .Dq no
407: to force remote port forwardings to be available to the local host only,
1.1 stevesk 408: .Dq yes
1.39 djm 409: to force remote port forwardings to bind to the wildcard address, or
410: .Dq clientspecified
411: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 412: The default is
413: .Dq no .
1.23 markus 414: .It Cm GSSAPIAuthentication
1.25 markus 415: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 416: The default is
1.23 markus 417: .Dq no .
418: Note that this option applies to protocol version 2 only.
419: .It Cm GSSAPICleanupCredentials
420: Specifies whether to automatically destroy the user's credentials cache
421: on logout.
422: The default is
423: .Dq yes .
424: Note that this option applies to protocol version 2 only.
1.1 stevesk 425: .It Cm HostbasedAuthentication
426: Specifies whether rhosts or /etc/hosts.equiv authentication together
427: with successful public key client host authentication is allowed
1.50 jmc 428: (host-based authentication).
1.1 stevesk 429: This option is similar to
430: .Cm RhostsRSAAuthentication
431: and applies to protocol version 2 only.
1.70 dtucker 432: The default is
433: .Dq no .
434: .It Cm HostbasedUsesNameFromPacketOnly
435: Specifies whether or not the server will attempt to perform a reverse
436: name lookup when matching the name in the
437: .Pa ~/.shosts ,
438: .Pa ~/.rhosts ,
439: and
440: .Pa /etc/hosts.equiv
441: files during
442: .Cm HostbasedAuthentication .
443: A setting of
444: .Dq yes
445: means that
446: .Xr sshd 8
447: uses the name supplied by the client rather than
448: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 449: The default is
450: .Dq no .
1.117 djm 451: .It Cm HostCertificate
452: Specifies a file containing a public host certificate.
453: The certificate's public key must match a private host key already specified
454: by
455: .Cm HostKey .
456: The default behaviour of
457: .Xr sshd 8
458: is not to load any certificates.
1.1 stevesk 459: .It Cm HostKey
460: Specifies a file containing a private host key
461: used by SSH.
462: The default is
463: .Pa /etc/ssh/ssh_host_key
464: for protocol version 1, and
465: .Pa /etc/ssh/ssh_host_rsa_key
466: and
467: .Pa /etc/ssh/ssh_host_dsa_key
468: for protocol version 2.
469: Note that
1.52 jmc 470: .Xr sshd 8
1.1 stevesk 471: will refuse to use a file if it is group/world-accessible.
472: It is possible to have multiple host key files.
473: .Dq rsa1
474: keys are used for version 1 and
475: .Dq dsa
476: or
477: .Dq rsa
478: are used for version 2 of the SSH protocol.
479: .It Cm IgnoreRhosts
480: Specifies that
481: .Pa .rhosts
482: and
483: .Pa .shosts
484: files will not be used in
485: .Cm RhostsRSAAuthentication
486: or
487: .Cm HostbasedAuthentication .
488: .Pp
489: .Pa /etc/hosts.equiv
490: and
491: .Pa /etc/shosts.equiv
492: are still used.
493: The default is
494: .Dq yes .
495: .It Cm IgnoreUserKnownHosts
496: Specifies whether
1.52 jmc 497: .Xr sshd 8
1.1 stevesk 498: should ignore the user's
1.41 djm 499: .Pa ~/.ssh/known_hosts
1.1 stevesk 500: during
501: .Cm RhostsRSAAuthentication
502: or
503: .Cm HostbasedAuthentication .
504: The default is
505: .Dq no .
506: .It Cm KerberosAuthentication
1.24 markus 507: Specifies whether the password provided by the user for
1.1 stevesk 508: .Cm PasswordAuthentication
1.24 markus 509: will be validated through the Kerberos KDC.
1.1 stevesk 510: To use this option, the server needs a
511: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 512: The default is
1.29 dtucker 513: .Dq no .
514: .It Cm KerberosGetAFSToken
1.45 djm 515: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 516: an AFS token before accessing the user's home directory.
1.52 jmc 517: The default is
1.1 stevesk 518: .Dq no .
519: .It Cm KerberosOrLocalPasswd
1.52 jmc 520: If password authentication through Kerberos fails then
1.1 stevesk 521: the password will be validated via any additional local mechanism
522: such as
523: .Pa /etc/passwd .
1.52 jmc 524: The default is
1.1 stevesk 525: .Dq yes .
526: .It Cm KerberosTicketCleanup
527: Specifies whether to automatically destroy the user's ticket cache
528: file on logout.
1.52 jmc 529: The default is
1.1 stevesk 530: .Dq yes .
531: .It Cm KeyRegenerationInterval
532: In protocol version 1, the ephemeral server key is automatically regenerated
533: after this many seconds (if it has been used).
534: The purpose of regeneration is to prevent
535: decrypting captured sessions by later breaking into the machine and
536: stealing the keys.
537: The key is never stored anywhere.
538: If the value is 0, the key is never regenerated.
539: The default is 3600 (seconds).
540: .It Cm ListenAddress
541: Specifies the local addresses
1.52 jmc 542: .Xr sshd 8
1.1 stevesk 543: should listen on.
544: The following forms may be used:
545: .Pp
546: .Bl -item -offset indent -compact
547: .It
548: .Cm ListenAddress
549: .Sm off
550: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
551: .Sm on
552: .It
553: .Cm ListenAddress
554: .Sm off
555: .Ar host No | Ar IPv4_addr No : Ar port
556: .Sm on
557: .It
558: .Cm ListenAddress
559: .Sm off
560: .Oo
561: .Ar host No | Ar IPv6_addr Oc : Ar port
562: .Sm on
563: .El
564: .Pp
565: If
566: .Ar port
567: is not specified,
1.52 jmc 568: sshd will listen on the address and all prior
1.1 stevesk 569: .Cm Port
1.17 jmc 570: options specified.
571: The default is to listen on all local addresses.
1.15 jmc 572: Multiple
1.1 stevesk 573: .Cm ListenAddress
1.17 jmc 574: options are permitted.
575: Additionally, any
1.1 stevesk 576: .Cm Port
1.52 jmc 577: options must precede this option for non-port qualified addresses.
1.1 stevesk 578: .It Cm LoginGraceTime
579: The server disconnects after this time if the user has not
580: successfully logged in.
581: If the value is 0, there is no time limit.
1.12 stevesk 582: The default is 120 seconds.
1.1 stevesk 583: .It Cm LogLevel
584: Gives the verbosity level that is used when logging messages from
1.53 jmc 585: .Xr sshd 8 .
1.1 stevesk 586: The possible values are:
1.52 jmc 587: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 588: The default is INFO.
589: DEBUG and DEBUG1 are equivalent.
590: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
591: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 592: .It Cm MACs
593: Specifies the available MAC (message authentication code) algorithms.
594: The MAC algorithm is used in protocol version 2
595: for data integrity protection.
596: Multiple algorithms must be comma-separated.
1.52 jmc 597: The default is:
1.77 jmc 598: .Bd -literal -offset indent
599: hmac-md5,hmac-sha1,umac-64@openssh.com,
600: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
601: .Ed
1.60 dtucker 602: .It Cm Match
1.61 jmc 603: Introduces a conditional block.
1.65 dtucker 604: If all of the criteria on the
1.60 dtucker 605: .Cm Match
1.65 dtucker 606: line are satisfied, the keywords on the following lines override those
607: set in the global section of the config file, until either another
1.60 dtucker 608: .Cm Match
1.65 dtucker 609: line or the end of the file.
1.91 djm 610: .Pp
1.61 jmc 611: The arguments to
1.60 dtucker 612: .Cm Match
1.65 dtucker 613: are one or more criteria-pattern pairs.
1.60 dtucker 614: The available criteria are
615: .Cm User ,
1.69 dtucker 616: .Cm Group ,
1.60 dtucker 617: .Cm Host ,
618: and
619: .Cm Address .
1.91 djm 620: The match patterns may consist of single entries or comma-separated
621: lists and may use the wildcard and negation operators described in the
1.92 djm 622: .Sx PATTERNS
1.91 djm 623: section of
1.92 djm 624: .Xr ssh_config 5 .
1.91 djm 625: .Pp
626: The patterns in an
627: .Cm Address
628: criteria may additionally contain addresses to match in CIDR
1.93 jmc 629: address/masklen format, e.g.\&
1.91 djm 630: .Dq 192.0.2.0/24
631: or
632: .Dq 3ffe:ffff::/32 .
633: Note that the mask length provided must be consistent with the address -
634: it is an error to specify a mask length that is too long for the address
1.93 jmc 635: or one with bits set in this host portion of the address.
636: For example,
1.91 djm 637: .Dq 192.0.2.0/33
638: and
1.93 jmc 639: .Dq 192.0.2.0/8
1.91 djm 640: respectively.
641: .Pp
1.60 dtucker 642: Only a subset of keywords may be used on the lines following a
643: .Cm Match
644: keyword.
645: Available keywords are
1.99 okan 646: .Cm AllowAgentForwarding ,
1.62 dtucker 647: .Cm AllowTcpForwarding ,
1.72 dtucker 648: .Cm Banner ,
1.85 djm 649: .Cm ChrootDirectory ,
1.67 dtucker 650: .Cm ForceCommand ,
1.62 dtucker 651: .Cm GatewayPorts ,
1.87 djm 652: .Cm GSSAPIAuthentication ,
653: .Cm HostbasedAuthentication ,
1.74 jmc 654: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 655: .Cm KerberosAuthentication ,
1.95 dtucker 656: .Cm MaxAuthTries ,
1.94 dtucker 657: .Cm MaxSessions ,
1.72 dtucker 658: .Cm PasswordAuthentication ,
1.97 djm 659: .Cm PermitEmptyPasswords ,
1.66 dtucker 660: .Cm PermitOpen ,
1.79 dtucker 661: .Cm PermitRootLogin ,
1.107 dtucker 662: .Cm PubkeyAuthentication ,
1.72 dtucker 663: .Cm RhostsRSAAuthentication ,
664: .Cm RSAAuthentication ,
1.66 dtucker 665: .Cm X11DisplayOffset ,
1.101 djm 666: .Cm X11Forwarding
1.60 dtucker 667: and
1.102 djm 668: .Cm X11UseLocalHost .
1.33 dtucker 669: .It Cm MaxAuthTries
670: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 671: connection.
672: Once the number of failures reaches half this value,
673: additional failures are logged.
674: The default is 6.
1.90 djm 675: .It Cm MaxSessions
676: Specifies the maximum number of open sessions permitted per network connection.
677: The default is 10.
1.1 stevesk 678: .It Cm MaxStartups
679: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 680: SSH daemon.
1.1 stevesk 681: Additional connections will be dropped until authentication succeeds or the
682: .Cm LoginGraceTime
683: expires for a connection.
684: The default is 10.
685: .Pp
686: Alternatively, random early drop can be enabled by specifying
687: the three colon separated values
688: .Dq start:rate:full
1.51 jmc 689: (e.g. "10:30:60").
1.53 jmc 690: .Xr sshd 8
1.1 stevesk 691: will refuse connection attempts with a probability of
692: .Dq rate/100
693: (30%)
694: if there are currently
695: .Dq start
696: (10)
697: unauthenticated connections.
698: The probability increases linearly and all connection attempts
699: are refused if the number of unauthenticated connections reaches
700: .Dq full
701: (60).
702: .It Cm PasswordAuthentication
703: Specifies whether password authentication is allowed.
704: The default is
705: .Dq yes .
706: .It Cm PermitEmptyPasswords
707: When password authentication is allowed, it specifies whether the
708: server allows login to accounts with empty password strings.
709: The default is
710: .Dq no .
1.62 dtucker 711: .It Cm PermitOpen
712: Specifies the destinations to which TCP port forwarding is permitted.
713: The forwarding specification must be one of the following forms:
714: .Pp
715: .Bl -item -offset indent -compact
716: .It
717: .Cm PermitOpen
718: .Sm off
719: .Ar host : port
720: .Sm on
721: .It
722: .Cm PermitOpen
723: .Sm off
724: .Ar IPv4_addr : port
725: .Sm on
726: .It
727: .Cm PermitOpen
728: .Sm off
729: .Ar \&[ IPv6_addr \&] : port
730: .Sm on
731: .El
732: .Pp
1.68 dtucker 733: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 734: An argument of
735: .Dq any
736: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 737: By default all port forwarding requests are permitted.
1.1 stevesk 738: .It Cm PermitRootLogin
1.38 jmc 739: Specifies whether root can log in using
1.1 stevesk 740: .Xr ssh 1 .
741: The argument must be
742: .Dq yes ,
743: .Dq without-password ,
1.52 jmc 744: .Dq forced-commands-only ,
1.1 stevesk 745: or
746: .Dq no .
747: The default is
748: .Dq yes .
749: .Pp
750: If this option is set to
1.52 jmc 751: .Dq without-password ,
1.1 stevesk 752: password authentication is disabled for root.
753: .Pp
754: If this option is set to
1.52 jmc 755: .Dq forced-commands-only ,
1.1 stevesk 756: root login with public key authentication will be allowed,
757: but only if the
758: .Ar command
759: option has been specified
760: (which may be useful for taking remote backups even if root login is
1.17 jmc 761: normally not allowed).
762: All other authentication methods are disabled for root.
1.1 stevesk 763: .Pp
764: If this option is set to
1.52 jmc 765: .Dq no ,
1.38 jmc 766: root is not allowed to log in.
1.46 reyk 767: .It Cm PermitTunnel
768: Specifies whether
769: .Xr tun 4
770: device forwarding is allowed.
1.47 reyk 771: The argument must be
772: .Dq yes ,
1.58 stevesk 773: .Dq point-to-point
774: (layer 3),
775: .Dq ethernet
776: (layer 2), or
1.47 reyk 777: .Dq no .
1.58 stevesk 778: Specifying
779: .Dq yes
780: permits both
781: .Dq point-to-point
782: and
783: .Dq ethernet .
1.46 reyk 784: The default is
785: .Dq no .
1.6 markus 786: .It Cm PermitUserEnvironment
787: Specifies whether
788: .Pa ~/.ssh/environment
1.9 stevesk 789: and
1.6 markus 790: .Cm environment=
791: options in
792: .Pa ~/.ssh/authorized_keys
1.9 stevesk 793: are processed by
1.52 jmc 794: .Xr sshd 8 .
1.6 markus 795: The default is
796: .Dq no .
1.9 stevesk 797: Enabling environment processing may enable users to bypass access
798: restrictions in some configurations using mechanisms such as
799: .Ev LD_PRELOAD .
1.1 stevesk 800: .It Cm PidFile
1.4 stevesk 801: Specifies the file that contains the process ID of the
1.53 jmc 802: SSH daemon.
1.1 stevesk 803: The default is
804: .Pa /var/run/sshd.pid .
805: .It Cm Port
806: Specifies the port number that
1.52 jmc 807: .Xr sshd 8
1.1 stevesk 808: listens on.
809: The default is 22.
810: Multiple options of this type are permitted.
811: See also
812: .Cm ListenAddress .
813: .It Cm PrintLastLog
814: Specifies whether
1.52 jmc 815: .Xr sshd 8
1.36 jaredy 816: should print the date and time of the last user login when a user logs
817: in interactively.
1.1 stevesk 818: The default is
819: .Dq yes .
820: .It Cm PrintMotd
821: Specifies whether
1.52 jmc 822: .Xr sshd 8
1.1 stevesk 823: should print
824: .Pa /etc/motd
825: when a user logs in interactively.
826: (On some systems it is also printed by the shell,
827: .Pa /etc/profile ,
828: or equivalent.)
829: The default is
830: .Dq yes .
831: .It Cm Protocol
832: Specifies the protocol versions
1.52 jmc 833: .Xr sshd 8
1.5 stevesk 834: supports.
1.1 stevesk 835: The possible values are
1.52 jmc 836: .Sq 1
1.1 stevesk 837: and
1.52 jmc 838: .Sq 2 .
1.1 stevesk 839: Multiple versions must be comma-separated.
840: The default is
1.109 jmc 841: .Sq 2 .
1.5 stevesk 842: Note that the order of the protocol list does not indicate preference,
843: because the client selects among multiple protocol versions offered
844: by the server.
845: Specifying
846: .Dq 2,1
847: is identical to
848: .Dq 1,2 .
1.1 stevesk 849: .It Cm PubkeyAuthentication
850: Specifies whether public key authentication is allowed.
851: The default is
852: .Dq yes .
853: Note that this option applies to protocol version 2 only.
1.118 djm 854: .It Cm RevokedKeys
855: Specifies a list of revoked public keys.
856: Keys listed in this file will be refused for public key authentication.
857: Note that if this file is not readable, then public key authentication will
858: be refused for all users.
1.1 stevesk 859: .It Cm RhostsRSAAuthentication
860: Specifies whether rhosts or /etc/hosts.equiv authentication together
861: with successful RSA host authentication is allowed.
862: The default is
863: .Dq no .
864: This option applies to protocol version 1 only.
865: .It Cm RSAAuthentication
866: Specifies whether pure RSA authentication is allowed.
867: The default is
868: .Dq yes .
869: This option applies to protocol version 1 only.
870: .It Cm ServerKeyBits
871: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 872: The minimum value is 512, and the default is 1024.
1.1 stevesk 873: .It Cm StrictModes
874: Specifies whether
1.52 jmc 875: .Xr sshd 8
1.1 stevesk 876: should check file modes and ownership of the
877: user's files and home directory before accepting login.
878: This is normally desirable because novices sometimes accidentally leave their
879: directory or files world-writable.
880: The default is
881: .Dq yes .
1.112 djm 882: Note that this does not apply to
883: .Cm ChrootDirectory ,
884: whose permissions and ownership are checked unconditionally.
1.1 stevesk 885: .It Cm Subsystem
1.51 jmc 886: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 887: Arguments should be a subsystem name and a command (with optional arguments)
888: to execute upon subsystem request.
1.80 djm 889: .Pp
1.1 stevesk 890: The command
891: .Xr sftp-server 8
892: implements the
893: .Dq sftp
894: file transfer subsystem.
1.80 djm 895: .Pp
896: Alternately the name
897: .Dq internal-sftp
898: implements an in-process
899: .Dq sftp
900: server.
901: This may simplify configurations using
902: .Cm ChrootDirectory
903: to force a different filesystem root on clients.
904: .Pp
1.1 stevesk 905: By default no subsystems are defined.
906: Note that this option applies to protocol version 2 only.
907: .It Cm SyslogFacility
908: Gives the facility code that is used when logging messages from
1.53 jmc 909: .Xr sshd 8 .
1.1 stevesk 910: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
911: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
912: The default is AUTH.
1.27 markus 913: .It Cm TCPKeepAlive
914: Specifies whether the system should send TCP keepalive messages to the
915: other side.
916: If they are sent, death of the connection or crash of one
917: of the machines will be properly noticed.
918: However, this means that
919: connections will die if the route is down temporarily, and some people
920: find it annoying.
921: On the other hand, if TCP keepalives are not sent,
922: sessions may hang indefinitely on the server, leaving
923: .Dq ghost
924: users and consuming server resources.
925: .Pp
926: The default is
927: .Dq yes
928: (to send TCP keepalive messages), and the server will notice
929: if the network goes down or the client host crashes.
930: This avoids infinitely hanging sessions.
931: .Pp
932: To disable TCP keepalive messages, the value should be set to
933: .Dq no .
1.118 djm 934: .It Cm TrustedUserCAKeys
935: Specifies a file containing public keys of certificate authorities that are
1.120 djm 936: trusted to sign user certificates for authentication.
1.119 jmc 937: Keys are listed one per line; empty lines and comments starting with
1.118 djm 938: .Ql #
939: are allowed.
940: If a certificate is presented for authentication and has its signing CA key
941: listed in this file, then it may be used for authentication for any user
942: listed in the certificate's principals list.
943: Note that certificates that lack a list of principals will not be permitted
944: for authentication using
945: .Cm TrustedUserCAKeys .
1.119 jmc 946: For more details on certificates, see the
1.118 djm 947: .Sx CERTIFICATES
948: section in
949: .Xr ssh-keygen 1 .
1.18 markus 950: .It Cm UseDNS
951: Specifies whether
1.52 jmc 952: .Xr sshd 8
1.40 jmc 953: should look up the remote host name and check that
1.18 markus 954: the resolved host name for the remote IP address maps back to the
955: very same IP address.
956: The default is
957: .Dq yes .
1.1 stevesk 958: .It Cm UseLogin
959: Specifies whether
960: .Xr login 1
961: is used for interactive login sessions.
962: The default is
963: .Dq no .
964: Note that
965: .Xr login 1
966: is never used for remote command execution.
967: Note also, that if this is enabled,
968: .Cm X11Forwarding
969: will be disabled because
970: .Xr login 1
971: does not know how to handle
972: .Xr xauth 1
1.15 jmc 973: cookies.
974: If
1.1 stevesk 975: .Cm UsePrivilegeSeparation
976: is specified, it will be disabled after authentication.
977: .It Cm UsePrivilegeSeparation
978: Specifies whether
1.52 jmc 979: .Xr sshd 8
1.2 stevesk 980: separates privileges by creating an unprivileged child process
1.15 jmc 981: to deal with incoming network traffic.
982: After successful authentication, another process will be created that has
983: the privilege of the authenticated user.
984: The goal of privilege separation is to prevent privilege
1.1 stevesk 985: escalation by containing any corruption within the unprivileged processes.
986: The default is
987: .Dq yes .
988: .It Cm X11DisplayOffset
989: Specifies the first display number available for
1.52 jmc 990: .Xr sshd 8 Ns 's
1.1 stevesk 991: X11 forwarding.
1.52 jmc 992: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 993: The default is 10.
994: .It Cm X11Forwarding
995: Specifies whether X11 forwarding is permitted.
1.13 stevesk 996: The argument must be
997: .Dq yes
998: or
999: .Dq no .
1.1 stevesk 1000: The default is
1001: .Dq no .
1.13 stevesk 1002: .Pp
1003: When X11 forwarding is enabled, there may be additional exposure to
1004: the server and to client displays if the
1.52 jmc 1005: .Xr sshd 8
1.13 stevesk 1006: proxy display is configured to listen on the wildcard address (see
1007: .Cm X11UseLocalhost
1.52 jmc 1008: below), though this is not the default.
1.13 stevesk 1009: Additionally, the authentication spoofing and authentication data
1010: verification and substitution occur on the client side.
1011: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1012: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1013: forwarding (see the warnings for
1014: .Cm ForwardX11
1015: in
1.19 jmc 1016: .Xr ssh_config 5 ) .
1.13 stevesk 1017: A system administrator may have a stance in which they want to
1018: protect clients that may expose themselves to attack by unwittingly
1019: requesting X11 forwarding, which can warrant a
1020: .Dq no
1021: setting.
1022: .Pp
1023: Note that disabling X11 forwarding does not prevent users from
1024: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1025: X11 forwarding is automatically disabled if
1026: .Cm UseLogin
1027: is enabled.
1028: .It Cm X11UseLocalhost
1029: Specifies whether
1.52 jmc 1030: .Xr sshd 8
1.1 stevesk 1031: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1032: the wildcard address.
1033: By default,
1.52 jmc 1034: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1035: hostname part of the
1036: .Ev DISPLAY
1037: environment variable to
1038: .Dq localhost .
1.8 stevesk 1039: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1040: However, some older X11 clients may not function with this
1041: configuration.
1042: .Cm X11UseLocalhost
1043: may be set to
1044: .Dq no
1045: to specify that the forwarding server should be bound to the wildcard
1046: address.
1047: The argument must be
1048: .Dq yes
1049: or
1050: .Dq no .
1051: The default is
1052: .Dq yes .
1053: .It Cm XAuthLocation
1.11 stevesk 1054: Specifies the full pathname of the
1.1 stevesk 1055: .Xr xauth 1
1056: program.
1057: The default is
1058: .Pa /usr/X11R6/bin/xauth .
1059: .El
1.55 jmc 1060: .Sh TIME FORMATS
1.53 jmc 1061: .Xr sshd 8
1.1 stevesk 1062: command-line arguments and configuration file options that specify time
1063: may be expressed using a sequence of the form:
1064: .Sm off
1.7 stevesk 1065: .Ar time Op Ar qualifier ,
1.1 stevesk 1066: .Sm on
1067: where
1068: .Ar time
1069: is a positive integer value and
1070: .Ar qualifier
1071: is one of the following:
1072: .Pp
1073: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1074: .It Aq Cm none
1.1 stevesk 1075: seconds
1076: .It Cm s | Cm S
1077: seconds
1078: .It Cm m | Cm M
1079: minutes
1080: .It Cm h | Cm H
1081: hours
1082: .It Cm d | Cm D
1083: days
1084: .It Cm w | Cm W
1085: weeks
1086: .El
1087: .Pp
1088: Each member of the sequence is added together to calculate
1089: the total time value.
1090: .Pp
1091: Time format examples:
1092: .Pp
1093: .Bl -tag -width Ds -compact -offset indent
1094: .It 600
1095: 600 seconds (10 minutes)
1096: .It 10m
1097: 10 minutes
1098: .It 1h30m
1099: 1 hour 30 minutes (90 minutes)
1100: .El
1101: .Sh FILES
1102: .Bl -tag -width Ds
1103: .It Pa /etc/ssh/sshd_config
1104: Contains configuration data for
1.53 jmc 1105: .Xr sshd 8 .
1.1 stevesk 1106: This file should be writable by root only, but it is recommended
1107: (though not necessary) that it be world-readable.
1108: .El
1.19 jmc 1109: .Sh SEE ALSO
1110: .Xr sshd 8
1.1 stevesk 1111: .Sh AUTHORS
1112: OpenSSH is a derivative of the original and free
1113: ssh 1.2.12 release by Tatu Ylonen.
1114: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1115: Theo de Raadt and Dug Song
1116: removed many bugs, re-added newer features and
1117: created OpenSSH.
1118: Markus Friedl contributed the support for SSH
1119: protocol versions 1.5 and 2.0.
1120: Niels Provos and Markus Friedl contributed support
1121: for privilege separation.