Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.126
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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25: .\"
26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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1.126 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.125 2010/06/30 07:28:34 jmc Exp $
! 38: .Dd $Mdocdate: June 30 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.
1.124 djm 158: The format is described in the
159: .Sx AUTHORIZED_KEYS FILE FORMAT
160: section of
161: .Xr sshd 8 .
1.1 stevesk 162: .Cm AuthorizedKeysFile
163: may contain tokens of the form %T which are substituted during connection
1.52 jmc 164: setup.
1.17 jmc 165: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 166: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 167: %u is replaced by the username of that user.
168: After expansion,
169: .Cm AuthorizedKeysFile
170: is taken to be an absolute path or one relative to the user's home
171: directory.
172: The default is
173: .Dq .ssh/authorized_keys .
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 .
517: .It Cm KerberosAuthentication
1.24 markus 518: Specifies whether the password provided by the user for
1.1 stevesk 519: .Cm PasswordAuthentication
1.24 markus 520: will be validated through the Kerberos KDC.
1.1 stevesk 521: To use this option, the server needs a
522: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 523: The default is
1.29 dtucker 524: .Dq no .
525: .It Cm KerberosGetAFSToken
1.45 djm 526: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 527: an AFS token before accessing the user's home directory.
1.52 jmc 528: The default is
1.1 stevesk 529: .Dq no .
530: .It Cm KerberosOrLocalPasswd
1.52 jmc 531: If password authentication through Kerberos fails then
1.1 stevesk 532: the password will be validated via any additional local mechanism
533: such as
534: .Pa /etc/passwd .
1.52 jmc 535: The default is
1.1 stevesk 536: .Dq yes .
537: .It Cm KerberosTicketCleanup
538: Specifies whether to automatically destroy the user's ticket cache
539: file on logout.
1.52 jmc 540: The default is
1.1 stevesk 541: .Dq yes .
542: .It Cm KeyRegenerationInterval
543: In protocol version 1, the ephemeral server key is automatically regenerated
544: after this many seconds (if it has been used).
545: The purpose of regeneration is to prevent
546: decrypting captured sessions by later breaking into the machine and
547: stealing the keys.
548: The key is never stored anywhere.
549: If the value is 0, the key is never regenerated.
550: The default is 3600 (seconds).
551: .It Cm ListenAddress
552: Specifies the local addresses
1.52 jmc 553: .Xr sshd 8
1.1 stevesk 554: should listen on.
555: The following forms may be used:
556: .Pp
557: .Bl -item -offset indent -compact
558: .It
559: .Cm ListenAddress
560: .Sm off
561: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
562: .Sm on
563: .It
564: .Cm ListenAddress
565: .Sm off
566: .Ar host No | Ar IPv4_addr No : Ar port
567: .Sm on
568: .It
569: .Cm ListenAddress
570: .Sm off
571: .Oo
572: .Ar host No | Ar IPv6_addr Oc : Ar port
573: .Sm on
574: .El
575: .Pp
576: If
577: .Ar port
578: is not specified,
1.52 jmc 579: sshd will listen on the address and all prior
1.1 stevesk 580: .Cm Port
1.17 jmc 581: options specified.
582: The default is to listen on all local addresses.
1.15 jmc 583: Multiple
1.1 stevesk 584: .Cm ListenAddress
1.17 jmc 585: options are permitted.
586: Additionally, any
1.1 stevesk 587: .Cm Port
1.52 jmc 588: options must precede this option for non-port qualified addresses.
1.1 stevesk 589: .It Cm LoginGraceTime
590: The server disconnects after this time if the user has not
591: successfully logged in.
592: If the value is 0, there is no time limit.
1.12 stevesk 593: The default is 120 seconds.
1.1 stevesk 594: .It Cm LogLevel
595: Gives the verbosity level that is used when logging messages from
1.53 jmc 596: .Xr sshd 8 .
1.1 stevesk 597: The possible values are:
1.52 jmc 598: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 599: The default is INFO.
600: DEBUG and DEBUG1 are equivalent.
601: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
602: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 603: .It Cm MACs
604: Specifies the available MAC (message authentication code) algorithms.
605: The MAC algorithm is used in protocol version 2
606: for data integrity protection.
607: Multiple algorithms must be comma-separated.
1.52 jmc 608: The default is:
1.77 jmc 609: .Bd -literal -offset indent
610: hmac-md5,hmac-sha1,umac-64@openssh.com,
611: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
612: .Ed
1.60 dtucker 613: .It Cm Match
1.61 jmc 614: Introduces a conditional block.
1.65 dtucker 615: If all of the criteria on the
1.60 dtucker 616: .Cm Match
1.65 dtucker 617: line are satisfied, the keywords on the following lines override those
618: set in the global section of the config file, until either another
1.60 dtucker 619: .Cm Match
1.65 dtucker 620: line or the end of the file.
1.91 djm 621: .Pp
1.61 jmc 622: The arguments to
1.60 dtucker 623: .Cm Match
1.65 dtucker 624: are one or more criteria-pattern pairs.
1.60 dtucker 625: The available criteria are
626: .Cm User ,
1.69 dtucker 627: .Cm Group ,
1.60 dtucker 628: .Cm Host ,
629: and
630: .Cm Address .
1.91 djm 631: The match patterns may consist of single entries or comma-separated
632: lists and may use the wildcard and negation operators described in the
1.92 djm 633: .Sx PATTERNS
1.91 djm 634: section of
1.92 djm 635: .Xr ssh_config 5 .
1.91 djm 636: .Pp
637: The patterns in an
638: .Cm Address
639: criteria may additionally contain addresses to match in CIDR
1.93 jmc 640: address/masklen format, e.g.\&
1.91 djm 641: .Dq 192.0.2.0/24
642: or
643: .Dq 3ffe:ffff::/32 .
644: Note that the mask length provided must be consistent with the address -
645: it is an error to specify a mask length that is too long for the address
1.93 jmc 646: or one with bits set in this host portion of the address.
647: For example,
1.91 djm 648: .Dq 192.0.2.0/33
649: and
1.93 jmc 650: .Dq 192.0.2.0/8
1.91 djm 651: respectively.
652: .Pp
1.60 dtucker 653: Only a subset of keywords may be used on the lines following a
654: .Cm Match
655: keyword.
656: Available keywords are
1.99 okan 657: .Cm AllowAgentForwarding ,
1.62 dtucker 658: .Cm AllowTcpForwarding ,
1.123 djm 659: .Cm AuthorizedKeysFile ,
660: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 661: .Cm Banner ,
1.85 djm 662: .Cm ChrootDirectory ,
1.67 dtucker 663: .Cm ForceCommand ,
1.62 dtucker 664: .Cm GatewayPorts ,
1.87 djm 665: .Cm GSSAPIAuthentication ,
666: .Cm HostbasedAuthentication ,
1.123 djm 667: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 668: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 669: .Cm KerberosAuthentication ,
1.95 dtucker 670: .Cm MaxAuthTries ,
1.94 dtucker 671: .Cm MaxSessions ,
1.72 dtucker 672: .Cm PasswordAuthentication ,
1.97 djm 673: .Cm PermitEmptyPasswords ,
1.66 dtucker 674: .Cm PermitOpen ,
1.79 dtucker 675: .Cm PermitRootLogin ,
1.123 djm 676: .Cm PermitTunnel ,
1.107 dtucker 677: .Cm PubkeyAuthentication ,
1.72 dtucker 678: .Cm RhostsRSAAuthentication ,
679: .Cm RSAAuthentication ,
1.66 dtucker 680: .Cm X11DisplayOffset ,
1.101 djm 681: .Cm X11Forwarding
1.60 dtucker 682: and
1.102 djm 683: .Cm X11UseLocalHost .
1.33 dtucker 684: .It Cm MaxAuthTries
685: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 686: connection.
687: Once the number of failures reaches half this value,
688: additional failures are logged.
689: The default is 6.
1.90 djm 690: .It Cm MaxSessions
691: Specifies the maximum number of open sessions permitted per network connection.
692: The default is 10.
1.1 stevesk 693: .It Cm MaxStartups
694: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 695: SSH daemon.
1.1 stevesk 696: Additional connections will be dropped until authentication succeeds or the
697: .Cm LoginGraceTime
698: expires for a connection.
699: The default is 10.
700: .Pp
701: Alternatively, random early drop can be enabled by specifying
702: the three colon separated values
703: .Dq start:rate:full
1.51 jmc 704: (e.g. "10:30:60").
1.53 jmc 705: .Xr sshd 8
1.1 stevesk 706: will refuse connection attempts with a probability of
707: .Dq rate/100
708: (30%)
709: if there are currently
710: .Dq start
711: (10)
712: unauthenticated connections.
713: The probability increases linearly and all connection attempts
714: are refused if the number of unauthenticated connections reaches
715: .Dq full
716: (60).
717: .It Cm PasswordAuthentication
718: Specifies whether password authentication is allowed.
719: The default is
720: .Dq yes .
721: .It Cm PermitEmptyPasswords
722: When password authentication is allowed, it specifies whether the
723: server allows login to accounts with empty password strings.
724: The default is
725: .Dq no .
1.62 dtucker 726: .It Cm PermitOpen
727: Specifies the destinations to which TCP port forwarding is permitted.
728: The forwarding specification must be one of the following forms:
729: .Pp
730: .Bl -item -offset indent -compact
731: .It
732: .Cm PermitOpen
733: .Sm off
734: .Ar host : port
735: .Sm on
736: .It
737: .Cm PermitOpen
738: .Sm off
739: .Ar IPv4_addr : port
740: .Sm on
741: .It
742: .Cm PermitOpen
743: .Sm off
744: .Ar \&[ IPv6_addr \&] : port
745: .Sm on
746: .El
747: .Pp
1.68 dtucker 748: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 749: An argument of
750: .Dq any
751: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 752: By default all port forwarding requests are permitted.
1.1 stevesk 753: .It Cm PermitRootLogin
1.38 jmc 754: Specifies whether root can log in using
1.1 stevesk 755: .Xr ssh 1 .
756: The argument must be
757: .Dq yes ,
758: .Dq without-password ,
1.52 jmc 759: .Dq forced-commands-only ,
1.1 stevesk 760: or
761: .Dq no .
762: The default is
763: .Dq yes .
764: .Pp
765: If this option is set to
1.52 jmc 766: .Dq without-password ,
1.1 stevesk 767: password authentication is disabled for root.
768: .Pp
769: If this option is set to
1.52 jmc 770: .Dq forced-commands-only ,
1.1 stevesk 771: root login with public key authentication will be allowed,
772: but only if the
773: .Ar command
774: option has been specified
775: (which may be useful for taking remote backups even if root login is
1.17 jmc 776: normally not allowed).
777: All other authentication methods are disabled for root.
1.1 stevesk 778: .Pp
779: If this option is set to
1.52 jmc 780: .Dq no ,
1.38 jmc 781: root is not allowed to log in.
1.46 reyk 782: .It Cm PermitTunnel
783: Specifies whether
784: .Xr tun 4
785: device forwarding is allowed.
1.47 reyk 786: The argument must be
787: .Dq yes ,
1.58 stevesk 788: .Dq point-to-point
789: (layer 3),
790: .Dq ethernet
791: (layer 2), or
1.47 reyk 792: .Dq no .
1.58 stevesk 793: Specifying
794: .Dq yes
795: permits both
796: .Dq point-to-point
797: and
798: .Dq ethernet .
1.46 reyk 799: The default is
800: .Dq no .
1.6 markus 801: .It Cm PermitUserEnvironment
802: Specifies whether
803: .Pa ~/.ssh/environment
1.9 stevesk 804: and
1.6 markus 805: .Cm environment=
806: options in
807: .Pa ~/.ssh/authorized_keys
1.9 stevesk 808: are processed by
1.52 jmc 809: .Xr sshd 8 .
1.6 markus 810: The default is
811: .Dq no .
1.9 stevesk 812: Enabling environment processing may enable users to bypass access
813: restrictions in some configurations using mechanisms such as
814: .Ev LD_PRELOAD .
1.1 stevesk 815: .It Cm PidFile
1.4 stevesk 816: Specifies the file that contains the process ID of the
1.53 jmc 817: SSH daemon.
1.1 stevesk 818: The default is
819: .Pa /var/run/sshd.pid .
820: .It Cm Port
821: Specifies the port number that
1.52 jmc 822: .Xr sshd 8
1.1 stevesk 823: listens on.
824: The default is 22.
825: Multiple options of this type are permitted.
826: See also
827: .Cm ListenAddress .
828: .It Cm PrintLastLog
829: Specifies whether
1.52 jmc 830: .Xr sshd 8
1.36 jaredy 831: should print the date and time of the last user login when a user logs
832: in interactively.
1.1 stevesk 833: The default is
834: .Dq yes .
835: .It Cm PrintMotd
836: Specifies whether
1.52 jmc 837: .Xr sshd 8
1.1 stevesk 838: should print
839: .Pa /etc/motd
840: when a user logs in interactively.
841: (On some systems it is also printed by the shell,
842: .Pa /etc/profile ,
843: or equivalent.)
844: The default is
845: .Dq yes .
846: .It Cm Protocol
847: Specifies the protocol versions
1.52 jmc 848: .Xr sshd 8
1.5 stevesk 849: supports.
1.1 stevesk 850: The possible values are
1.52 jmc 851: .Sq 1
1.1 stevesk 852: and
1.52 jmc 853: .Sq 2 .
1.1 stevesk 854: Multiple versions must be comma-separated.
855: The default is
1.109 jmc 856: .Sq 2 .
1.5 stevesk 857: Note that the order of the protocol list does not indicate preference,
858: because the client selects among multiple protocol versions offered
859: by the server.
860: Specifying
861: .Dq 2,1
862: is identical to
863: .Dq 1,2 .
1.1 stevesk 864: .It Cm PubkeyAuthentication
865: Specifies whether public key authentication is allowed.
866: The default is
867: .Dq yes .
868: Note that this option applies to protocol version 2 only.
1.118 djm 869: .It Cm RevokedKeys
870: Specifies a list of revoked public keys.
871: Keys listed in this file will be refused for public key authentication.
872: Note that if this file is not readable, then public key authentication will
873: be refused for all users.
1.1 stevesk 874: .It Cm RhostsRSAAuthentication
875: Specifies whether rhosts or /etc/hosts.equiv authentication together
876: with successful RSA host authentication is allowed.
877: The default is
878: .Dq no .
879: This option applies to protocol version 1 only.
880: .It Cm RSAAuthentication
881: Specifies whether pure RSA authentication is allowed.
882: The default is
883: .Dq yes .
884: This option applies to protocol version 1 only.
885: .It Cm ServerKeyBits
886: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 887: The minimum value is 512, and the default is 1024.
1.1 stevesk 888: .It Cm StrictModes
889: Specifies whether
1.52 jmc 890: .Xr sshd 8
1.1 stevesk 891: should check file modes and ownership of the
892: user's files and home directory before accepting login.
893: This is normally desirable because novices sometimes accidentally leave their
894: directory or files world-writable.
895: The default is
896: .Dq yes .
1.112 djm 897: Note that this does not apply to
898: .Cm ChrootDirectory ,
899: whose permissions and ownership are checked unconditionally.
1.1 stevesk 900: .It Cm Subsystem
1.51 jmc 901: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 902: Arguments should be a subsystem name and a command (with optional arguments)
903: to execute upon subsystem request.
1.80 djm 904: .Pp
1.1 stevesk 905: The command
906: .Xr sftp-server 8
907: implements the
908: .Dq sftp
909: file transfer subsystem.
1.80 djm 910: .Pp
911: Alternately the name
912: .Dq internal-sftp
913: implements an in-process
914: .Dq sftp
915: server.
916: This may simplify configurations using
917: .Cm ChrootDirectory
918: to force a different filesystem root on clients.
919: .Pp
1.1 stevesk 920: By default no subsystems are defined.
921: Note that this option applies to protocol version 2 only.
922: .It Cm SyslogFacility
923: Gives the facility code that is used when logging messages from
1.53 jmc 924: .Xr sshd 8 .
1.1 stevesk 925: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
926: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
927: The default is AUTH.
1.27 markus 928: .It Cm TCPKeepAlive
929: Specifies whether the system should send TCP keepalive messages to the
930: other side.
931: If they are sent, death of the connection or crash of one
932: of the machines will be properly noticed.
933: However, this means that
934: connections will die if the route is down temporarily, and some people
935: find it annoying.
936: On the other hand, if TCP keepalives are not sent,
937: sessions may hang indefinitely on the server, leaving
938: .Dq ghost
939: users and consuming server resources.
940: .Pp
941: The default is
942: .Dq yes
943: (to send TCP keepalive messages), and the server will notice
944: if the network goes down or the client host crashes.
945: This avoids infinitely hanging sessions.
946: .Pp
947: To disable TCP keepalive messages, the value should be set to
948: .Dq no .
1.118 djm 949: .It Cm TrustedUserCAKeys
950: Specifies a file containing public keys of certificate authorities that are
1.120 djm 951: trusted to sign user certificates for authentication.
1.119 jmc 952: Keys are listed one per line; empty lines and comments starting with
1.118 djm 953: .Ql #
954: are allowed.
955: If a certificate is presented for authentication and has its signing CA key
956: listed in this file, then it may be used for authentication for any user
957: listed in the certificate's principals list.
958: Note that certificates that lack a list of principals will not be permitted
959: for authentication using
960: .Cm TrustedUserCAKeys .
1.119 jmc 961: For more details on certificates, see the
1.118 djm 962: .Sx CERTIFICATES
963: section in
964: .Xr ssh-keygen 1 .
1.18 markus 965: .It Cm UseDNS
966: Specifies whether
1.52 jmc 967: .Xr sshd 8
1.40 jmc 968: should look up the remote host name and check that
1.18 markus 969: the resolved host name for the remote IP address maps back to the
970: very same IP address.
971: The default is
972: .Dq yes .
1.1 stevesk 973: .It Cm UseLogin
974: Specifies whether
975: .Xr login 1
976: is used for interactive login sessions.
977: The default is
978: .Dq no .
979: Note that
980: .Xr login 1
981: is never used for remote command execution.
982: Note also, that if this is enabled,
983: .Cm X11Forwarding
984: will be disabled because
985: .Xr login 1
986: does not know how to handle
987: .Xr xauth 1
1.15 jmc 988: cookies.
989: If
1.1 stevesk 990: .Cm UsePrivilegeSeparation
991: is specified, it will be disabled after authentication.
992: .It Cm UsePrivilegeSeparation
993: Specifies whether
1.52 jmc 994: .Xr sshd 8
1.2 stevesk 995: separates privileges by creating an unprivileged child process
1.15 jmc 996: to deal with incoming network traffic.
997: After successful authentication, another process will be created that has
998: the privilege of the authenticated user.
999: The goal of privilege separation is to prevent privilege
1.1 stevesk 1000: escalation by containing any corruption within the unprivileged processes.
1001: The default is
1002: .Dq yes .
1003: .It Cm X11DisplayOffset
1004: Specifies the first display number available for
1.52 jmc 1005: .Xr sshd 8 Ns 's
1.1 stevesk 1006: X11 forwarding.
1.52 jmc 1007: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1008: The default is 10.
1009: .It Cm X11Forwarding
1010: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1011: The argument must be
1012: .Dq yes
1013: or
1014: .Dq no .
1.1 stevesk 1015: The default is
1016: .Dq no .
1.13 stevesk 1017: .Pp
1018: When X11 forwarding is enabled, there may be additional exposure to
1019: the server and to client displays if the
1.52 jmc 1020: .Xr sshd 8
1.13 stevesk 1021: proxy display is configured to listen on the wildcard address (see
1022: .Cm X11UseLocalhost
1.52 jmc 1023: below), though this is not the default.
1.13 stevesk 1024: Additionally, the authentication spoofing and authentication data
1025: verification and substitution occur on the client side.
1026: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1027: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1028: forwarding (see the warnings for
1029: .Cm ForwardX11
1030: in
1.19 jmc 1031: .Xr ssh_config 5 ) .
1.13 stevesk 1032: A system administrator may have a stance in which they want to
1033: protect clients that may expose themselves to attack by unwittingly
1034: requesting X11 forwarding, which can warrant a
1035: .Dq no
1036: setting.
1037: .Pp
1038: Note that disabling X11 forwarding does not prevent users from
1039: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1040: X11 forwarding is automatically disabled if
1041: .Cm UseLogin
1042: is enabled.
1043: .It Cm X11UseLocalhost
1044: Specifies whether
1.52 jmc 1045: .Xr sshd 8
1.1 stevesk 1046: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1047: the wildcard address.
1048: By default,
1.52 jmc 1049: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1050: hostname part of the
1051: .Ev DISPLAY
1052: environment variable to
1053: .Dq localhost .
1.8 stevesk 1054: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1055: However, some older X11 clients may not function with this
1056: configuration.
1057: .Cm X11UseLocalhost
1058: may be set to
1059: .Dq no
1060: to specify that the forwarding server should be bound to the wildcard
1061: address.
1062: The argument must be
1063: .Dq yes
1064: or
1065: .Dq no .
1066: The default is
1067: .Dq yes .
1068: .It Cm XAuthLocation
1.11 stevesk 1069: Specifies the full pathname of the
1.1 stevesk 1070: .Xr xauth 1
1071: program.
1072: The default is
1073: .Pa /usr/X11R6/bin/xauth .
1074: .El
1.55 jmc 1075: .Sh TIME FORMATS
1.53 jmc 1076: .Xr sshd 8
1.1 stevesk 1077: command-line arguments and configuration file options that specify time
1078: may be expressed using a sequence of the form:
1079: .Sm off
1.7 stevesk 1080: .Ar time Op Ar qualifier ,
1.1 stevesk 1081: .Sm on
1082: where
1083: .Ar time
1084: is a positive integer value and
1085: .Ar qualifier
1086: is one of the following:
1087: .Pp
1088: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1089: .It Aq Cm none
1.1 stevesk 1090: seconds
1091: .It Cm s | Cm S
1092: seconds
1093: .It Cm m | Cm M
1094: minutes
1095: .It Cm h | Cm H
1096: hours
1097: .It Cm d | Cm D
1098: days
1099: .It Cm w | Cm W
1100: weeks
1101: .El
1102: .Pp
1103: Each member of the sequence is added together to calculate
1104: the total time value.
1105: .Pp
1106: Time format examples:
1107: .Pp
1108: .Bl -tag -width Ds -compact -offset indent
1109: .It 600
1110: 600 seconds (10 minutes)
1111: .It 10m
1112: 10 minutes
1113: .It 1h30m
1114: 1 hour 30 minutes (90 minutes)
1115: .El
1116: .Sh FILES
1117: .Bl -tag -width Ds
1118: .It Pa /etc/ssh/sshd_config
1119: Contains configuration data for
1.53 jmc 1120: .Xr sshd 8 .
1.1 stevesk 1121: This file should be writable by root only, but it is recommended
1122: (though not necessary) that it be world-readable.
1123: .El
1.19 jmc 1124: .Sh SEE ALSO
1125: .Xr sshd 8
1.1 stevesk 1126: .Sh AUTHORS
1127: OpenSSH is a derivative of the original and free
1128: ssh 1.2.12 release by Tatu Ylonen.
1129: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1130: Theo de Raadt and Dug Song
1131: removed many bugs, re-added newer features and
1132: created OpenSSH.
1133: Markus Friedl contributed the support for SSH
1134: protocol versions 1.5 and 2.0.
1135: Niels Provos and Markus Friedl contributed support
1136: for privilege separation.