Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.85
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
25: .\"
26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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1.85 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.84 2008/03/25 11:58:02 djm Exp $
! 38: .Dd $Mdocdate: March 25 2008 $
1.1 stevesk 39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
1.71 jmc 45: .Nm /etc/ssh/sshd_config
1.1 stevesk 46: .Sh DESCRIPTION
1.53 jmc 47: .Xr sshd 8
1.1 stevesk 48: reads configuration data from
49: .Pa /etc/ssh/sshd_config
50: (or the file specified with
51: .Fl f
52: on the command line).
53: The file contains keyword-argument pairs, one per line.
54: Lines starting with
55: .Ql #
56: and empty lines are interpreted as comments.
1.56 dtucker 57: Arguments may optionally be enclosed in double quotes
58: .Pq \&"
59: in order to represent arguments containing spaces.
1.1 stevesk 60: .Pp
61: The possible
62: keywords and their meanings are as follows (note that
63: keywords are case-insensitive and arguments are case-sensitive):
64: .Bl -tag -width Ds
1.30 djm 65: .It Cm AcceptEnv
66: Specifies what environment variables sent by the client will be copied into
67: the session's
68: .Xr environ 7 .
69: See
70: .Cm SendEnv
71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.31 djm 74: Note that environment passing is only supported for protocol 2.
1.30 djm 75: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 76: .Ql *
1.30 djm 77: and
78: .Ql \&? .
1.31 djm 79: Multiple environment variables may be separated by whitespace or spread
1.30 djm 80: across multiple
81: .Cm AcceptEnv
82: directives.
1.31 djm 83: Be warned that some environment variables could be used to bypass restricted
1.30 djm 84: user environments.
85: For this reason, care should be taken in the use of this directive.
86: The default is not to accept any environment variables.
1.37 djm 87: .It Cm AddressFamily
88: Specifies which address family should be used by
1.53 jmc 89: .Xr sshd 8 .
1.37 djm 90: Valid arguments are
91: .Dq any ,
92: .Dq inet
1.52 jmc 93: (use IPv4 only), or
1.37 djm 94: .Dq inet6
95: (use IPv6 only).
96: The default is
97: .Dq any .
1.1 stevesk 98: .It Cm AllowGroups
99: This keyword can be followed by a list of group name patterns, separated
100: by spaces.
101: If specified, login is allowed only for users whose primary
102: group or supplementary group list matches one of the patterns.
103: Only group names are valid; a numerical group ID is not recognized.
104: By default, login is allowed for all groups.
1.54 jmc 105: The allow/deny directives are processed in the following order:
106: .Cm DenyUsers ,
107: .Cm AllowUsers ,
108: .Cm DenyGroups ,
109: and finally
110: .Cm AllowGroups .
1.49 jmc 111: .Pp
112: See
113: .Sx PATTERNS
114: in
115: .Xr ssh_config 5
116: for more information on patterns.
1.1 stevesk 117: .It Cm AllowTcpForwarding
118: Specifies whether TCP forwarding is permitted.
119: The default is
120: .Dq yes .
121: Note that disabling TCP forwarding does not improve security unless
122: users are also denied shell access, as they can always install their
123: own forwarders.
124: .It Cm AllowUsers
125: This keyword can be followed by a list of user name patterns, separated
126: by spaces.
1.14 jmc 127: If specified, login is allowed only for user names that
1.1 stevesk 128: match one of the patterns.
129: Only user names are valid; a numerical user ID is not recognized.
130: By default, login is allowed for all users.
131: If the pattern takes the form USER@HOST then USER and HOST
132: are separately checked, restricting logins to particular
133: users from particular hosts.
1.54 jmc 134: The allow/deny directives are processed in the following order:
135: .Cm DenyUsers ,
136: .Cm AllowUsers ,
137: .Cm DenyGroups ,
138: and finally
139: .Cm AllowGroups .
1.49 jmc 140: .Pp
141: See
142: .Sx PATTERNS
143: in
144: .Xr ssh_config 5
145: for more information on patterns.
1.1 stevesk 146: .It Cm AuthorizedKeysFile
147: Specifies the file that contains the public keys that can be used
148: for user authentication.
149: .Cm AuthorizedKeysFile
150: may contain tokens of the form %T which are substituted during connection
1.52 jmc 151: setup.
1.17 jmc 152: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 153: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 154: %u is replaced by the username of that user.
155: After expansion,
156: .Cm AuthorizedKeysFile
157: is taken to be an absolute path or one relative to the user's home
158: directory.
159: The default is
160: .Dq .ssh/authorized_keys .
161: .It Cm Banner
162: The contents of the specified file are sent to the remote user before
163: authentication is allowed.
1.78 djm 164: If the argument is
165: .Dq none
166: then no banner is displayed.
1.1 stevesk 167: This option is only available for protocol version 2.
168: By default, no banner is displayed.
169: .It Cm ChallengeResponseAuthentication
1.50 jmc 170: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 171: All authentication styles from
172: .Xr login.conf 5
173: are supported.
174: The default is
175: .Dq yes .
1.80 djm 176: .It Cm ChrootDirectory
177: Specifies a path to
178: .Xr chroot 2
179: to after authentication.
180: This path, and all its components, must be root-owned directories that are
181: not writable by any other user or group.
182: .Pp
183: The path may contain the following tokens that are expanded at runtime once
184: the connecting user has been authenticated: %% is replaced by a literal '%',
185: %h is replaced by the home directory of the user being authenticated, and
186: %u is replaced by the username of that user.
187: .Pp
188: The
189: .Cm ChrootDirectory
190: must contain the necessary files and directories to support the
191: users' session.
192: For an interactive session this requires at least a shell, typically
193: .Xr sh 1 ,
194: and basic
195: .Pa /dev
196: nodes such as
197: .Xr null 4 ,
198: .Xr zero 4 ,
199: .Xr stdin 4 ,
200: .Xr stdout 4 ,
201: .Xr stderr 4 ,
202: .Xr arandom 4
203: and
204: .Xr tty 4
205: devices.
206: For file transfer sessions using
207: .Dq sftp ,
208: no additional configuration of the environment is necessary if the
209: in-process sftp server is used (see
210: .Cm Subsystem
1.81 jmc 211: for details).
1.80 djm 212: .Pp
1.85 ! djm 213: Please note that there are many ways to misconfigure a chroot environment
! 214: in ways that compromise security.
! 215: These include:
! 216: .Pp
! 217: .Bl -dash -offset indent -compact
! 218: .It
! 219: Making unsafe setuid binaries available;
! 220: .It
! 221: Having missing or incorrect configuration files in the chroot's
! 222: .Pa /etc
! 223: directory;
! 224: .It
! 225: Hard-linking files between the chroot and outside;
! 226: .It
! 227: Leaving unnecessary
! 228: .Pa /dev
! 229: nodes accessible inside the chroot (especially those for physical drives);
! 230: .It
! 231: Executing scripts or binaries inside the chroot from outside, either
! 232: directly or through facilities such as
! 233: .Xr cron 8 .
! 234: .El
! 235: .Pp
1.80 djm 236: The default is not to
237: .Xr chroot 2 .
1.1 stevesk 238: .It Cm Ciphers
239: Specifies the ciphers allowed for protocol version 2.
240: Multiple ciphers must be comma-separated.
1.34 dtucker 241: The supported ciphers are
242: .Dq 3des-cbc ,
243: .Dq aes128-cbc ,
244: .Dq aes192-cbc ,
245: .Dq aes256-cbc ,
246: .Dq aes128-ctr ,
247: .Dq aes192-ctr ,
248: .Dq aes256-ctr ,
1.43 djm 249: .Dq arcfour128 ,
250: .Dq arcfour256 ,
1.34 dtucker 251: .Dq arcfour ,
252: .Dq blowfish-cbc ,
253: and
254: .Dq cast128-cbc .
1.52 jmc 255: The default is:
256: .Bd -literal -offset 3n
257: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
258: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
259: aes192-ctr,aes256-ctr
1.1 stevesk 260: .Ed
261: .It Cm ClientAliveCountMax
1.48 jmc 262: Sets the number of client alive messages (see below) which may be
1.1 stevesk 263: sent without
1.52 jmc 264: .Xr sshd 8
1.17 jmc 265: receiving any messages back from the client.
266: If this threshold is reached while client alive messages are being sent,
1.52 jmc 267: sshd will disconnect the client, terminating the session.
1.17 jmc 268: It is important to note that the use of client alive messages is very
269: different from
1.27 markus 270: .Cm TCPKeepAlive
1.17 jmc 271: (below).
272: The client alive messages are sent through the encrypted channel
273: and therefore will not be spoofable.
274: The TCP keepalive option enabled by
1.27 markus 275: .Cm TCPKeepAlive
1.17 jmc 276: is spoofable.
277: The client alive mechanism is valuable when the client or
1.1 stevesk 278: server depend on knowing when a connection has become inactive.
279: .Pp
1.17 jmc 280: The default value is 3.
281: If
1.1 stevesk 282: .Cm ClientAliveInterval
1.48 jmc 283: (see below) is set to 15, and
1.1 stevesk 284: .Cm ClientAliveCountMax
1.52 jmc 285: is left at the default, unresponsive SSH clients
1.1 stevesk 286: will be disconnected after approximately 45 seconds.
1.57 markus 287: This option applies to protocol version 2 only.
1.42 djm 288: .It Cm ClientAliveInterval
289: Sets a timeout interval in seconds after which if no data has been received
290: from the client,
1.52 jmc 291: .Xr sshd 8
1.42 djm 292: will send a message through the encrypted
293: channel to request a response from the client.
294: The default
295: is 0, indicating that these messages will not be sent to the client.
296: This option applies to protocol version 2 only.
1.3 markus 297: .It Cm Compression
1.44 markus 298: Specifies whether compression is allowed, or delayed until
299: the user has authenticated successfully.
1.3 markus 300: The argument must be
1.44 markus 301: .Dq yes ,
302: .Dq delayed ,
1.3 markus 303: or
304: .Dq no .
305: The default is
1.44 markus 306: .Dq delayed .
1.1 stevesk 307: .It Cm DenyGroups
308: This keyword can be followed by a list of group name patterns, separated
309: by spaces.
310: Login is disallowed for users whose primary group or supplementary
311: group list matches one of the patterns.
312: Only group names are valid; a numerical group ID is not recognized.
313: By default, login is allowed for all groups.
1.54 jmc 314: The allow/deny directives are processed in the following order:
315: .Cm DenyUsers ,
316: .Cm AllowUsers ,
317: .Cm DenyGroups ,
318: and finally
319: .Cm AllowGroups .
1.49 jmc 320: .Pp
321: See
322: .Sx PATTERNS
323: in
324: .Xr ssh_config 5
325: for more information on patterns.
1.1 stevesk 326: .It Cm DenyUsers
327: This keyword can be followed by a list of user name patterns, separated
328: by spaces.
329: Login is disallowed for user names that match one of the patterns.
330: Only user names are valid; a numerical user ID is not recognized.
331: By default, login is allowed for all users.
332: If the pattern takes the form USER@HOST then USER and HOST
333: are separately checked, restricting logins to particular
334: users from particular hosts.
1.54 jmc 335: The allow/deny directives are processed in the following order:
336: .Cm DenyUsers ,
337: .Cm AllowUsers ,
338: .Cm DenyGroups ,
339: and finally
340: .Cm AllowGroups .
1.49 jmc 341: .Pp
342: See
343: .Sx PATTERNS
344: in
345: .Xr ssh_config 5
346: for more information on patterns.
1.67 dtucker 347: .It Cm ForceCommand
348: Forces the execution of the command specified by
349: .Cm ForceCommand ,
1.84 djm 350: ignoring any command supplied by the client and
351: .Pa ~/.ssh/rc
352: if present.
1.67 dtucker 353: The command is invoked by using the user's login shell with the -c option.
354: This applies to shell, command, or subsystem execution.
355: It is most useful inside a
356: .Cm Match
357: block.
358: The command originally supplied by the client is available in the
359: .Ev SSH_ORIGINAL_COMMAND
360: environment variable.
1.82 djm 361: Specifying a command of
362: .Dq internal-sftp
363: will force the use of an in-process sftp server that requires no support
364: files when used with
365: .Cm ChrootDirectory .
1.85 ! djm 366: Note that
! 367: .Dq internal-sftp
! 368: is only supported when
! 369: .Cm UsePrivilegeSeparation
! 370: is enabled.
1.1 stevesk 371: .It Cm GatewayPorts
372: Specifies whether remote hosts are allowed to connect to ports
373: forwarded for the client.
374: By default,
1.52 jmc 375: .Xr sshd 8
1.15 jmc 376: binds remote port forwardings to the loopback address.
377: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 378: .Cm GatewayPorts
1.52 jmc 379: can be used to specify that sshd
1.39 djm 380: should allow remote port forwardings to bind to non-loopback addresses, thus
381: allowing other hosts to connect.
382: The argument may be
383: .Dq no
384: to force remote port forwardings to be available to the local host only,
1.1 stevesk 385: .Dq yes
1.39 djm 386: to force remote port forwardings to bind to the wildcard address, or
387: .Dq clientspecified
388: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 389: The default is
390: .Dq no .
1.23 markus 391: .It Cm GSSAPIAuthentication
1.25 markus 392: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 393: The default is
1.23 markus 394: .Dq no .
395: Note that this option applies to protocol version 2 only.
396: .It Cm GSSAPICleanupCredentials
397: Specifies whether to automatically destroy the user's credentials cache
398: on logout.
399: The default is
400: .Dq yes .
401: Note that this option applies to protocol version 2 only.
1.1 stevesk 402: .It Cm HostbasedAuthentication
403: Specifies whether rhosts or /etc/hosts.equiv authentication together
404: with successful public key client host authentication is allowed
1.50 jmc 405: (host-based authentication).
1.1 stevesk 406: This option is similar to
407: .Cm RhostsRSAAuthentication
408: and applies to protocol version 2 only.
1.70 dtucker 409: The default is
410: .Dq no .
411: .It Cm HostbasedUsesNameFromPacketOnly
412: Specifies whether or not the server will attempt to perform a reverse
413: name lookup when matching the name in the
414: .Pa ~/.shosts ,
415: .Pa ~/.rhosts ,
416: and
417: .Pa /etc/hosts.equiv
418: files during
419: .Cm HostbasedAuthentication .
420: A setting of
421: .Dq yes
422: means that
423: .Xr sshd 8
424: uses the name supplied by the client rather than
425: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 426: The default is
427: .Dq no .
428: .It Cm HostKey
429: Specifies a file containing a private host key
430: used by SSH.
431: The default is
432: .Pa /etc/ssh/ssh_host_key
433: for protocol version 1, and
434: .Pa /etc/ssh/ssh_host_rsa_key
435: and
436: .Pa /etc/ssh/ssh_host_dsa_key
437: for protocol version 2.
438: Note that
1.52 jmc 439: .Xr sshd 8
1.1 stevesk 440: will refuse to use a file if it is group/world-accessible.
441: It is possible to have multiple host key files.
442: .Dq rsa1
443: keys are used for version 1 and
444: .Dq dsa
445: or
446: .Dq rsa
447: are used for version 2 of the SSH protocol.
448: .It Cm IgnoreRhosts
449: Specifies that
450: .Pa .rhosts
451: and
452: .Pa .shosts
453: files will not be used in
454: .Cm RhostsRSAAuthentication
455: or
456: .Cm HostbasedAuthentication .
457: .Pp
458: .Pa /etc/hosts.equiv
459: and
460: .Pa /etc/shosts.equiv
461: are still used.
462: The default is
463: .Dq yes .
464: .It Cm IgnoreUserKnownHosts
465: Specifies whether
1.52 jmc 466: .Xr sshd 8
1.1 stevesk 467: should ignore the user's
1.41 djm 468: .Pa ~/.ssh/known_hosts
1.1 stevesk 469: during
470: .Cm RhostsRSAAuthentication
471: or
472: .Cm HostbasedAuthentication .
473: The default is
474: .Dq no .
475: .It Cm KerberosAuthentication
1.24 markus 476: Specifies whether the password provided by the user for
1.1 stevesk 477: .Cm PasswordAuthentication
1.24 markus 478: will be validated through the Kerberos KDC.
1.1 stevesk 479: To use this option, the server needs a
480: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 481: The default is
1.29 dtucker 482: .Dq no .
483: .It Cm KerberosGetAFSToken
1.45 djm 484: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 485: an AFS token before accessing the user's home directory.
1.52 jmc 486: The default is
1.1 stevesk 487: .Dq no .
488: .It Cm KerberosOrLocalPasswd
1.52 jmc 489: If password authentication through Kerberos fails then
1.1 stevesk 490: the password will be validated via any additional local mechanism
491: such as
492: .Pa /etc/passwd .
1.52 jmc 493: The default is
1.1 stevesk 494: .Dq yes .
495: .It Cm KerberosTicketCleanup
496: Specifies whether to automatically destroy the user's ticket cache
497: file on logout.
1.52 jmc 498: The default is
1.1 stevesk 499: .Dq yes .
500: .It Cm KeyRegenerationInterval
501: In protocol version 1, the ephemeral server key is automatically regenerated
502: after this many seconds (if it has been used).
503: The purpose of regeneration is to prevent
504: decrypting captured sessions by later breaking into the machine and
505: stealing the keys.
506: The key is never stored anywhere.
507: If the value is 0, the key is never regenerated.
508: The default is 3600 (seconds).
509: .It Cm ListenAddress
510: Specifies the local addresses
1.52 jmc 511: .Xr sshd 8
1.1 stevesk 512: should listen on.
513: The following forms may be used:
514: .Pp
515: .Bl -item -offset indent -compact
516: .It
517: .Cm ListenAddress
518: .Sm off
519: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
520: .Sm on
521: .It
522: .Cm ListenAddress
523: .Sm off
524: .Ar host No | Ar IPv4_addr No : Ar port
525: .Sm on
526: .It
527: .Cm ListenAddress
528: .Sm off
529: .Oo
530: .Ar host No | Ar IPv6_addr Oc : Ar port
531: .Sm on
532: .El
533: .Pp
534: If
535: .Ar port
536: is not specified,
1.52 jmc 537: sshd will listen on the address and all prior
1.1 stevesk 538: .Cm Port
1.17 jmc 539: options specified.
540: The default is to listen on all local addresses.
1.15 jmc 541: Multiple
1.1 stevesk 542: .Cm ListenAddress
1.17 jmc 543: options are permitted.
544: Additionally, any
1.1 stevesk 545: .Cm Port
1.52 jmc 546: options must precede this option for non-port qualified addresses.
1.1 stevesk 547: .It Cm LoginGraceTime
548: The server disconnects after this time if the user has not
549: successfully logged in.
550: If the value is 0, there is no time limit.
1.12 stevesk 551: The default is 120 seconds.
1.1 stevesk 552: .It Cm LogLevel
553: Gives the verbosity level that is used when logging messages from
1.53 jmc 554: .Xr sshd 8 .
1.1 stevesk 555: The possible values are:
1.52 jmc 556: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 557: The default is INFO.
558: DEBUG and DEBUG1 are equivalent.
559: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
560: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 561: .It Cm MACs
562: Specifies the available MAC (message authentication code) algorithms.
563: The MAC algorithm is used in protocol version 2
564: for data integrity protection.
565: Multiple algorithms must be comma-separated.
1.52 jmc 566: The default is:
1.77 jmc 567: .Bd -literal -offset indent
568: hmac-md5,hmac-sha1,umac-64@openssh.com,
569: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
570: .Ed
1.60 dtucker 571: .It Cm Match
1.61 jmc 572: Introduces a conditional block.
1.65 dtucker 573: If all of the criteria on the
1.60 dtucker 574: .Cm Match
1.65 dtucker 575: line are satisfied, the keywords on the following lines override those
576: set in the global section of the config file, until either another
1.60 dtucker 577: .Cm Match
1.65 dtucker 578: line or the end of the file.
1.61 jmc 579: The arguments to
1.60 dtucker 580: .Cm Match
1.65 dtucker 581: are one or more criteria-pattern pairs.
1.60 dtucker 582: The available criteria are
583: .Cm User ,
1.69 dtucker 584: .Cm Group ,
1.60 dtucker 585: .Cm Host ,
586: and
587: .Cm Address .
588: Only a subset of keywords may be used on the lines following a
589: .Cm Match
590: keyword.
591: Available keywords are
1.62 dtucker 592: .Cm AllowTcpForwarding ,
1.72 dtucker 593: .Cm Banner ,
1.85 ! djm 594: .Cm ChrootDirectory ,
1.67 dtucker 595: .Cm ForceCommand ,
1.62 dtucker 596: .Cm GatewayPorts ,
1.72 dtucker 597: .Cm GSSApiAuthentication ,
1.74 jmc 598: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 599: .Cm KerberosAuthentication ,
600: .Cm PasswordAuthentication ,
1.66 dtucker 601: .Cm PermitOpen ,
1.79 dtucker 602: .Cm PermitRootLogin ,
1.72 dtucker 603: .Cm RhostsRSAAuthentication ,
604: .Cm RSAAuthentication ,
1.66 dtucker 605: .Cm X11DisplayOffset ,
606: .Cm X11Forwarding ,
1.60 dtucker 607: and
1.66 dtucker 608: .Cm X11UseLocalHost .
1.33 dtucker 609: .It Cm MaxAuthTries
610: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 611: connection.
612: Once the number of failures reaches half this value,
613: additional failures are logged.
614: The default is 6.
1.1 stevesk 615: .It Cm MaxStartups
616: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 617: SSH daemon.
1.1 stevesk 618: Additional connections will be dropped until authentication succeeds or the
619: .Cm LoginGraceTime
620: expires for a connection.
621: The default is 10.
622: .Pp
623: Alternatively, random early drop can be enabled by specifying
624: the three colon separated values
625: .Dq start:rate:full
1.51 jmc 626: (e.g. "10:30:60").
1.53 jmc 627: .Xr sshd 8
1.1 stevesk 628: will refuse connection attempts with a probability of
629: .Dq rate/100
630: (30%)
631: if there are currently
632: .Dq start
633: (10)
634: unauthenticated connections.
635: The probability increases linearly and all connection attempts
636: are refused if the number of unauthenticated connections reaches
637: .Dq full
638: (60).
639: .It Cm PasswordAuthentication
640: Specifies whether password authentication is allowed.
641: The default is
642: .Dq yes .
643: .It Cm PermitEmptyPasswords
644: When password authentication is allowed, it specifies whether the
645: server allows login to accounts with empty password strings.
646: The default is
647: .Dq no .
1.62 dtucker 648: .It Cm PermitOpen
649: Specifies the destinations to which TCP port forwarding is permitted.
650: The forwarding specification must be one of the following forms:
651: .Pp
652: .Bl -item -offset indent -compact
653: .It
654: .Cm PermitOpen
655: .Sm off
656: .Ar host : port
657: .Sm on
658: .It
659: .Cm PermitOpen
660: .Sm off
661: .Ar IPv4_addr : port
662: .Sm on
663: .It
664: .Cm PermitOpen
665: .Sm off
666: .Ar \&[ IPv6_addr \&] : port
667: .Sm on
668: .El
669: .Pp
1.68 dtucker 670: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 671: An argument of
672: .Dq any
673: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 674: By default all port forwarding requests are permitted.
1.1 stevesk 675: .It Cm PermitRootLogin
1.38 jmc 676: Specifies whether root can log in using
1.1 stevesk 677: .Xr ssh 1 .
678: The argument must be
679: .Dq yes ,
680: .Dq without-password ,
1.52 jmc 681: .Dq forced-commands-only ,
1.1 stevesk 682: or
683: .Dq no .
684: The default is
685: .Dq yes .
686: .Pp
687: If this option is set to
1.52 jmc 688: .Dq without-password ,
1.1 stevesk 689: password authentication is disabled for root.
690: .Pp
691: If this option is set to
1.52 jmc 692: .Dq forced-commands-only ,
1.1 stevesk 693: root login with public key authentication will be allowed,
694: but only if the
695: .Ar command
696: option has been specified
697: (which may be useful for taking remote backups even if root login is
1.17 jmc 698: normally not allowed).
699: All other authentication methods are disabled for root.
1.1 stevesk 700: .Pp
701: If this option is set to
1.52 jmc 702: .Dq no ,
1.38 jmc 703: root is not allowed to log in.
1.46 reyk 704: .It Cm PermitTunnel
705: Specifies whether
706: .Xr tun 4
707: device forwarding is allowed.
1.47 reyk 708: The argument must be
709: .Dq yes ,
1.58 stevesk 710: .Dq point-to-point
711: (layer 3),
712: .Dq ethernet
713: (layer 2), or
1.47 reyk 714: .Dq no .
1.58 stevesk 715: Specifying
716: .Dq yes
717: permits both
718: .Dq point-to-point
719: and
720: .Dq ethernet .
1.46 reyk 721: The default is
722: .Dq no .
1.6 markus 723: .It Cm PermitUserEnvironment
724: Specifies whether
725: .Pa ~/.ssh/environment
1.9 stevesk 726: and
1.6 markus 727: .Cm environment=
728: options in
729: .Pa ~/.ssh/authorized_keys
1.9 stevesk 730: are processed by
1.52 jmc 731: .Xr sshd 8 .
1.6 markus 732: The default is
733: .Dq no .
1.9 stevesk 734: Enabling environment processing may enable users to bypass access
735: restrictions in some configurations using mechanisms such as
736: .Ev LD_PRELOAD .
1.1 stevesk 737: .It Cm PidFile
1.4 stevesk 738: Specifies the file that contains the process ID of the
1.53 jmc 739: SSH daemon.
1.1 stevesk 740: The default is
741: .Pa /var/run/sshd.pid .
742: .It Cm Port
743: Specifies the port number that
1.52 jmc 744: .Xr sshd 8
1.1 stevesk 745: listens on.
746: The default is 22.
747: Multiple options of this type are permitted.
748: See also
749: .Cm ListenAddress .
750: .It Cm PrintLastLog
751: Specifies whether
1.52 jmc 752: .Xr sshd 8
1.36 jaredy 753: should print the date and time of the last user login when a user logs
754: in interactively.
1.1 stevesk 755: The default is
756: .Dq yes .
757: .It Cm PrintMotd
758: Specifies whether
1.52 jmc 759: .Xr sshd 8
1.1 stevesk 760: should print
761: .Pa /etc/motd
762: when a user logs in interactively.
763: (On some systems it is also printed by the shell,
764: .Pa /etc/profile ,
765: or equivalent.)
766: The default is
767: .Dq yes .
768: .It Cm Protocol
769: Specifies the protocol versions
1.52 jmc 770: .Xr sshd 8
1.5 stevesk 771: supports.
1.1 stevesk 772: The possible values are
1.52 jmc 773: .Sq 1
1.1 stevesk 774: and
1.52 jmc 775: .Sq 2 .
1.1 stevesk 776: Multiple versions must be comma-separated.
777: The default is
778: .Dq 2,1 .
1.5 stevesk 779: Note that the order of the protocol list does not indicate preference,
780: because the client selects among multiple protocol versions offered
781: by the server.
782: Specifying
783: .Dq 2,1
784: is identical to
785: .Dq 1,2 .
1.1 stevesk 786: .It Cm PubkeyAuthentication
787: Specifies whether public key authentication is allowed.
788: The default is
789: .Dq yes .
790: Note that this option applies to protocol version 2 only.
791: .It Cm RhostsRSAAuthentication
792: Specifies whether rhosts or /etc/hosts.equiv authentication together
793: with successful RSA host authentication is allowed.
794: The default is
795: .Dq no .
796: This option applies to protocol version 1 only.
797: .It Cm RSAAuthentication
798: Specifies whether pure RSA authentication is allowed.
799: The default is
800: .Dq yes .
801: This option applies to protocol version 1 only.
802: .It Cm ServerKeyBits
803: Defines the number of bits in the ephemeral protocol version 1 server key.
804: The minimum value is 512, and the default is 768.
805: .It Cm StrictModes
806: Specifies whether
1.52 jmc 807: .Xr sshd 8
1.1 stevesk 808: should check file modes and ownership of the
809: user's files and home directory before accepting login.
810: This is normally desirable because novices sometimes accidentally leave their
811: directory or files world-writable.
812: The default is
813: .Dq yes .
814: .It Cm Subsystem
1.51 jmc 815: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 816: Arguments should be a subsystem name and a command (with optional arguments)
817: to execute upon subsystem request.
1.80 djm 818: .Pp
1.1 stevesk 819: The command
820: .Xr sftp-server 8
821: implements the
822: .Dq sftp
823: file transfer subsystem.
1.80 djm 824: .Pp
825: Alternately the name
826: .Dq internal-sftp
827: implements an in-process
828: .Dq sftp
829: server.
830: This may simplify configurations using
831: .Cm ChrootDirectory
832: to force a different filesystem root on clients.
1.85 ! djm 833: Note that
! 834: .Dq internal-sftp
! 835: is only supported when
! 836: .Cm UsePrivilegeSeparation
! 837: is enabled.
1.80 djm 838: .Pp
1.1 stevesk 839: By default no subsystems are defined.
840: Note that this option applies to protocol version 2 only.
841: .It Cm SyslogFacility
842: Gives the facility code that is used when logging messages from
1.53 jmc 843: .Xr sshd 8 .
1.1 stevesk 844: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
845: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
846: The default is AUTH.
1.27 markus 847: .It Cm TCPKeepAlive
848: Specifies whether the system should send TCP keepalive messages to the
849: other side.
850: If they are sent, death of the connection or crash of one
851: of the machines will be properly noticed.
852: However, this means that
853: connections will die if the route is down temporarily, and some people
854: find it annoying.
855: On the other hand, if TCP keepalives are not sent,
856: sessions may hang indefinitely on the server, leaving
857: .Dq ghost
858: users and consuming server resources.
859: .Pp
860: The default is
861: .Dq yes
862: (to send TCP keepalive messages), and the server will notice
863: if the network goes down or the client host crashes.
864: This avoids infinitely hanging sessions.
865: .Pp
866: To disable TCP keepalive messages, the value should be set to
867: .Dq no .
1.18 markus 868: .It Cm UseDNS
869: Specifies whether
1.52 jmc 870: .Xr sshd 8
1.40 jmc 871: should look up the remote host name and check that
1.18 markus 872: the resolved host name for the remote IP address maps back to the
873: very same IP address.
874: The default is
875: .Dq yes .
1.1 stevesk 876: .It Cm UseLogin
877: Specifies whether
878: .Xr login 1
879: is used for interactive login sessions.
880: The default is
881: .Dq no .
882: Note that
883: .Xr login 1
884: is never used for remote command execution.
885: Note also, that if this is enabled,
886: .Cm X11Forwarding
887: will be disabled because
888: .Xr login 1
889: does not know how to handle
890: .Xr xauth 1
1.15 jmc 891: cookies.
892: If
1.1 stevesk 893: .Cm UsePrivilegeSeparation
894: is specified, it will be disabled after authentication.
895: .It Cm UsePrivilegeSeparation
896: Specifies whether
1.52 jmc 897: .Xr sshd 8
1.2 stevesk 898: separates privileges by creating an unprivileged child process
1.15 jmc 899: to deal with incoming network traffic.
900: After successful authentication, another process will be created that has
901: the privilege of the authenticated user.
902: The goal of privilege separation is to prevent privilege
1.1 stevesk 903: escalation by containing any corruption within the unprivileged processes.
904: The default is
905: .Dq yes .
906: .It Cm X11DisplayOffset
907: Specifies the first display number available for
1.52 jmc 908: .Xr sshd 8 Ns 's
1.1 stevesk 909: X11 forwarding.
1.52 jmc 910: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 911: The default is 10.
912: .It Cm X11Forwarding
913: Specifies whether X11 forwarding is permitted.
1.13 stevesk 914: The argument must be
915: .Dq yes
916: or
917: .Dq no .
1.1 stevesk 918: The default is
919: .Dq no .
1.13 stevesk 920: .Pp
921: When X11 forwarding is enabled, there may be additional exposure to
922: the server and to client displays if the
1.52 jmc 923: .Xr sshd 8
1.13 stevesk 924: proxy display is configured to listen on the wildcard address (see
925: .Cm X11UseLocalhost
1.52 jmc 926: below), though this is not the default.
1.13 stevesk 927: Additionally, the authentication spoofing and authentication data
928: verification and substitution occur on the client side.
929: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 930: display server may be exposed to attack when the SSH client requests
1.13 stevesk 931: forwarding (see the warnings for
932: .Cm ForwardX11
933: in
1.19 jmc 934: .Xr ssh_config 5 ) .
1.13 stevesk 935: A system administrator may have a stance in which they want to
936: protect clients that may expose themselves to attack by unwittingly
937: requesting X11 forwarding, which can warrant a
938: .Dq no
939: setting.
940: .Pp
941: Note that disabling X11 forwarding does not prevent users from
942: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 943: X11 forwarding is automatically disabled if
944: .Cm UseLogin
945: is enabled.
946: .It Cm X11UseLocalhost
947: Specifies whether
1.52 jmc 948: .Xr sshd 8
1.1 stevesk 949: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 950: the wildcard address.
951: By default,
1.52 jmc 952: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 953: hostname part of the
954: .Ev DISPLAY
955: environment variable to
956: .Dq localhost .
1.8 stevesk 957: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 958: However, some older X11 clients may not function with this
959: configuration.
960: .Cm X11UseLocalhost
961: may be set to
962: .Dq no
963: to specify that the forwarding server should be bound to the wildcard
964: address.
965: The argument must be
966: .Dq yes
967: or
968: .Dq no .
969: The default is
970: .Dq yes .
971: .It Cm XAuthLocation
1.11 stevesk 972: Specifies the full pathname of the
1.1 stevesk 973: .Xr xauth 1
974: program.
975: The default is
976: .Pa /usr/X11R6/bin/xauth .
977: .El
1.55 jmc 978: .Sh TIME FORMATS
1.53 jmc 979: .Xr sshd 8
1.1 stevesk 980: command-line arguments and configuration file options that specify time
981: may be expressed using a sequence of the form:
982: .Sm off
1.7 stevesk 983: .Ar time Op Ar qualifier ,
1.1 stevesk 984: .Sm on
985: where
986: .Ar time
987: is a positive integer value and
988: .Ar qualifier
989: is one of the following:
990: .Pp
991: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 992: .It Aq Cm none
1.1 stevesk 993: seconds
994: .It Cm s | Cm S
995: seconds
996: .It Cm m | Cm M
997: minutes
998: .It Cm h | Cm H
999: hours
1000: .It Cm d | Cm D
1001: days
1002: .It Cm w | Cm W
1003: weeks
1004: .El
1005: .Pp
1006: Each member of the sequence is added together to calculate
1007: the total time value.
1008: .Pp
1009: Time format examples:
1010: .Pp
1011: .Bl -tag -width Ds -compact -offset indent
1012: .It 600
1013: 600 seconds (10 minutes)
1014: .It 10m
1015: 10 minutes
1016: .It 1h30m
1017: 1 hour 30 minutes (90 minutes)
1018: .El
1019: .Sh FILES
1020: .Bl -tag -width Ds
1021: .It Pa /etc/ssh/sshd_config
1022: Contains configuration data for
1.53 jmc 1023: .Xr sshd 8 .
1.1 stevesk 1024: This file should be writable by root only, but it is recommended
1025: (though not necessary) that it be world-readable.
1026: .El
1.19 jmc 1027: .Sh SEE ALSO
1028: .Xr sshd 8
1.1 stevesk 1029: .Sh AUTHORS
1030: OpenSSH is a derivative of the original and free
1031: ssh 1.2.12 release by Tatu Ylonen.
1032: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1033: Theo de Raadt and Dug Song
1034: removed many bugs, re-added newer features and
1035: created OpenSSH.
1036: Markus Friedl contributed the support for SSH
1037: protocol versions 1.5 and 2.0.
1038: Niels Provos and Markus Friedl contributed support
1039: for privilege separation.
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