Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.82
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
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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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1.82 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.81 2008/02/09 08:04:31 jmc Exp $
! 38: .Dd $Mdocdate: February 9 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
213: The default is not to
214: .Xr chroot 2 .
1.1 stevesk 215: .It Cm Ciphers
216: Specifies the ciphers allowed for protocol version 2.
217: Multiple ciphers must be comma-separated.
1.34 dtucker 218: The supported ciphers are
219: .Dq 3des-cbc ,
220: .Dq aes128-cbc ,
221: .Dq aes192-cbc ,
222: .Dq aes256-cbc ,
223: .Dq aes128-ctr ,
224: .Dq aes192-ctr ,
225: .Dq aes256-ctr ,
1.43 djm 226: .Dq arcfour128 ,
227: .Dq arcfour256 ,
1.34 dtucker 228: .Dq arcfour ,
229: .Dq blowfish-cbc ,
230: and
231: .Dq cast128-cbc .
1.52 jmc 232: The default is:
233: .Bd -literal -offset 3n
234: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
235: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
236: aes192-ctr,aes256-ctr
1.1 stevesk 237: .Ed
238: .It Cm ClientAliveCountMax
1.48 jmc 239: Sets the number of client alive messages (see below) which may be
1.1 stevesk 240: sent without
1.52 jmc 241: .Xr sshd 8
1.17 jmc 242: receiving any messages back from the client.
243: If this threshold is reached while client alive messages are being sent,
1.52 jmc 244: sshd will disconnect the client, terminating the session.
1.17 jmc 245: It is important to note that the use of client alive messages is very
246: different from
1.27 markus 247: .Cm TCPKeepAlive
1.17 jmc 248: (below).
249: The client alive messages are sent through the encrypted channel
250: and therefore will not be spoofable.
251: The TCP keepalive option enabled by
1.27 markus 252: .Cm TCPKeepAlive
1.17 jmc 253: is spoofable.
254: The client alive mechanism is valuable when the client or
1.1 stevesk 255: server depend on knowing when a connection has become inactive.
256: .Pp
1.17 jmc 257: The default value is 3.
258: If
1.1 stevesk 259: .Cm ClientAliveInterval
1.48 jmc 260: (see below) is set to 15, and
1.1 stevesk 261: .Cm ClientAliveCountMax
1.52 jmc 262: is left at the default, unresponsive SSH clients
1.1 stevesk 263: will be disconnected after approximately 45 seconds.
1.57 markus 264: This option applies to protocol version 2 only.
1.42 djm 265: .It Cm ClientAliveInterval
266: Sets a timeout interval in seconds after which if no data has been received
267: from the client,
1.52 jmc 268: .Xr sshd 8
1.42 djm 269: will send a message through the encrypted
270: channel to request a response from the client.
271: The default
272: is 0, indicating that these messages will not be sent to the client.
273: This option applies to protocol version 2 only.
1.3 markus 274: .It Cm Compression
1.44 markus 275: Specifies whether compression is allowed, or delayed until
276: the user has authenticated successfully.
1.3 markus 277: The argument must be
1.44 markus 278: .Dq yes ,
279: .Dq delayed ,
1.3 markus 280: or
281: .Dq no .
282: The default is
1.44 markus 283: .Dq delayed .
1.1 stevesk 284: .It Cm DenyGroups
285: This keyword can be followed by a list of group name patterns, separated
286: by spaces.
287: Login is disallowed for users whose primary group or supplementary
288: group list matches one of the patterns.
289: Only group names are valid; a numerical group ID is not recognized.
290: By default, login is allowed for all groups.
1.54 jmc 291: The allow/deny directives are processed in the following order:
292: .Cm DenyUsers ,
293: .Cm AllowUsers ,
294: .Cm DenyGroups ,
295: and finally
296: .Cm AllowGroups .
1.49 jmc 297: .Pp
298: See
299: .Sx PATTERNS
300: in
301: .Xr ssh_config 5
302: for more information on patterns.
1.1 stevesk 303: .It Cm DenyUsers
304: This keyword can be followed by a list of user name patterns, separated
305: by spaces.
306: Login is disallowed for user names that match one of the patterns.
307: Only user names are valid; a numerical user ID is not recognized.
308: By default, login is allowed for all users.
309: If the pattern takes the form USER@HOST then USER and HOST
310: are separately checked, restricting logins to particular
311: users from particular hosts.
1.54 jmc 312: The allow/deny directives are processed in the following order:
313: .Cm DenyUsers ,
314: .Cm AllowUsers ,
315: .Cm DenyGroups ,
316: and finally
317: .Cm AllowGroups .
1.49 jmc 318: .Pp
319: See
320: .Sx PATTERNS
321: in
322: .Xr ssh_config 5
323: for more information on patterns.
1.67 dtucker 324: .It Cm ForceCommand
325: Forces the execution of the command specified by
326: .Cm ForceCommand ,
327: ignoring any command supplied by the client.
328: The command is invoked by using the user's login shell with the -c option.
329: This applies to shell, command, or subsystem execution.
330: It is most useful inside a
331: .Cm Match
332: block.
333: The command originally supplied by the client is available in the
334: .Ev SSH_ORIGINAL_COMMAND
335: environment variable.
1.82 ! djm 336: Specifying a command of
! 337: .Dq internal-sftp
! 338: will force the use of an in-process sftp server that requires no support
! 339: files when used with
! 340: .Cm ChrootDirectory .
1.1 stevesk 341: .It Cm GatewayPorts
342: Specifies whether remote hosts are allowed to connect to ports
343: forwarded for the client.
344: By default,
1.52 jmc 345: .Xr sshd 8
1.15 jmc 346: binds remote port forwardings to the loopback address.
347: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 348: .Cm GatewayPorts
1.52 jmc 349: can be used to specify that sshd
1.39 djm 350: should allow remote port forwardings to bind to non-loopback addresses, thus
351: allowing other hosts to connect.
352: The argument may be
353: .Dq no
354: to force remote port forwardings to be available to the local host only,
1.1 stevesk 355: .Dq yes
1.39 djm 356: to force remote port forwardings to bind to the wildcard address, or
357: .Dq clientspecified
358: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 359: The default is
360: .Dq no .
1.23 markus 361: .It Cm GSSAPIAuthentication
1.25 markus 362: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 363: The default is
1.23 markus 364: .Dq no .
365: Note that this option applies to protocol version 2 only.
366: .It Cm GSSAPICleanupCredentials
367: Specifies whether to automatically destroy the user's credentials cache
368: on logout.
369: The default is
370: .Dq yes .
371: Note that this option applies to protocol version 2 only.
1.1 stevesk 372: .It Cm HostbasedAuthentication
373: Specifies whether rhosts or /etc/hosts.equiv authentication together
374: with successful public key client host authentication is allowed
1.50 jmc 375: (host-based authentication).
1.1 stevesk 376: This option is similar to
377: .Cm RhostsRSAAuthentication
378: and applies to protocol version 2 only.
1.70 dtucker 379: The default is
380: .Dq no .
381: .It Cm HostbasedUsesNameFromPacketOnly
382: Specifies whether or not the server will attempt to perform a reverse
383: name lookup when matching the name in the
384: .Pa ~/.shosts ,
385: .Pa ~/.rhosts ,
386: and
387: .Pa /etc/hosts.equiv
388: files during
389: .Cm HostbasedAuthentication .
390: A setting of
391: .Dq yes
392: means that
393: .Xr sshd 8
394: uses the name supplied by the client rather than
395: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 396: The default is
397: .Dq no .
398: .It Cm HostKey
399: Specifies a file containing a private host key
400: used by SSH.
401: The default is
402: .Pa /etc/ssh/ssh_host_key
403: for protocol version 1, and
404: .Pa /etc/ssh/ssh_host_rsa_key
405: and
406: .Pa /etc/ssh/ssh_host_dsa_key
407: for protocol version 2.
408: Note that
1.52 jmc 409: .Xr sshd 8
1.1 stevesk 410: will refuse to use a file if it is group/world-accessible.
411: It is possible to have multiple host key files.
412: .Dq rsa1
413: keys are used for version 1 and
414: .Dq dsa
415: or
416: .Dq rsa
417: are used for version 2 of the SSH protocol.
418: .It Cm IgnoreRhosts
419: Specifies that
420: .Pa .rhosts
421: and
422: .Pa .shosts
423: files will not be used in
424: .Cm RhostsRSAAuthentication
425: or
426: .Cm HostbasedAuthentication .
427: .Pp
428: .Pa /etc/hosts.equiv
429: and
430: .Pa /etc/shosts.equiv
431: are still used.
432: The default is
433: .Dq yes .
434: .It Cm IgnoreUserKnownHosts
435: Specifies whether
1.52 jmc 436: .Xr sshd 8
1.1 stevesk 437: should ignore the user's
1.41 djm 438: .Pa ~/.ssh/known_hosts
1.1 stevesk 439: during
440: .Cm RhostsRSAAuthentication
441: or
442: .Cm HostbasedAuthentication .
443: The default is
444: .Dq no .
445: .It Cm KerberosAuthentication
1.24 markus 446: Specifies whether the password provided by the user for
1.1 stevesk 447: .Cm PasswordAuthentication
1.24 markus 448: will be validated through the Kerberos KDC.
1.1 stevesk 449: To use this option, the server needs a
450: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 451: The default is
1.29 dtucker 452: .Dq no .
453: .It Cm KerberosGetAFSToken
1.45 djm 454: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 455: an AFS token before accessing the user's home directory.
1.52 jmc 456: The default is
1.1 stevesk 457: .Dq no .
458: .It Cm KerberosOrLocalPasswd
1.52 jmc 459: If password authentication through Kerberos fails then
1.1 stevesk 460: the password will be validated via any additional local mechanism
461: such as
462: .Pa /etc/passwd .
1.52 jmc 463: The default is
1.1 stevesk 464: .Dq yes .
465: .It Cm KerberosTicketCleanup
466: Specifies whether to automatically destroy the user's ticket cache
467: file on logout.
1.52 jmc 468: The default is
1.1 stevesk 469: .Dq yes .
470: .It Cm KeyRegenerationInterval
471: In protocol version 1, the ephemeral server key is automatically regenerated
472: after this many seconds (if it has been used).
473: The purpose of regeneration is to prevent
474: decrypting captured sessions by later breaking into the machine and
475: stealing the keys.
476: The key is never stored anywhere.
477: If the value is 0, the key is never regenerated.
478: The default is 3600 (seconds).
479: .It Cm ListenAddress
480: Specifies the local addresses
1.52 jmc 481: .Xr sshd 8
1.1 stevesk 482: should listen on.
483: The following forms may be used:
484: .Pp
485: .Bl -item -offset indent -compact
486: .It
487: .Cm ListenAddress
488: .Sm off
489: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
490: .Sm on
491: .It
492: .Cm ListenAddress
493: .Sm off
494: .Ar host No | Ar IPv4_addr No : Ar port
495: .Sm on
496: .It
497: .Cm ListenAddress
498: .Sm off
499: .Oo
500: .Ar host No | Ar IPv6_addr Oc : Ar port
501: .Sm on
502: .El
503: .Pp
504: If
505: .Ar port
506: is not specified,
1.52 jmc 507: sshd will listen on the address and all prior
1.1 stevesk 508: .Cm Port
1.17 jmc 509: options specified.
510: The default is to listen on all local addresses.
1.15 jmc 511: Multiple
1.1 stevesk 512: .Cm ListenAddress
1.17 jmc 513: options are permitted.
514: Additionally, any
1.1 stevesk 515: .Cm Port
1.52 jmc 516: options must precede this option for non-port qualified addresses.
1.1 stevesk 517: .It Cm LoginGraceTime
518: The server disconnects after this time if the user has not
519: successfully logged in.
520: If the value is 0, there is no time limit.
1.12 stevesk 521: The default is 120 seconds.
1.1 stevesk 522: .It Cm LogLevel
523: Gives the verbosity level that is used when logging messages from
1.53 jmc 524: .Xr sshd 8 .
1.1 stevesk 525: The possible values are:
1.52 jmc 526: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 527: The default is INFO.
528: DEBUG and DEBUG1 are equivalent.
529: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
530: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 531: .It Cm MACs
532: Specifies the available MAC (message authentication code) algorithms.
533: The MAC algorithm is used in protocol version 2
534: for data integrity protection.
535: Multiple algorithms must be comma-separated.
1.52 jmc 536: The default is:
1.77 jmc 537: .Bd -literal -offset indent
538: hmac-md5,hmac-sha1,umac-64@openssh.com,
539: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
540: .Ed
1.60 dtucker 541: .It Cm Match
1.61 jmc 542: Introduces a conditional block.
1.65 dtucker 543: If all of the criteria on the
1.60 dtucker 544: .Cm Match
1.65 dtucker 545: line are satisfied, the keywords on the following lines override those
546: set in the global section of the config file, until either another
1.60 dtucker 547: .Cm Match
1.65 dtucker 548: line or the end of the file.
1.61 jmc 549: The arguments to
1.60 dtucker 550: .Cm Match
1.65 dtucker 551: are one or more criteria-pattern pairs.
1.60 dtucker 552: The available criteria are
553: .Cm User ,
1.69 dtucker 554: .Cm Group ,
1.60 dtucker 555: .Cm Host ,
556: and
557: .Cm Address .
558: Only a subset of keywords may be used on the lines following a
559: .Cm Match
560: keyword.
561: Available keywords are
1.62 dtucker 562: .Cm AllowTcpForwarding ,
1.72 dtucker 563: .Cm Banner ,
1.67 dtucker 564: .Cm ForceCommand ,
1.62 dtucker 565: .Cm GatewayPorts ,
1.72 dtucker 566: .Cm GSSApiAuthentication ,
1.74 jmc 567: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 568: .Cm KerberosAuthentication ,
569: .Cm PasswordAuthentication ,
1.66 dtucker 570: .Cm PermitOpen ,
1.79 dtucker 571: .Cm PermitRootLogin ,
1.72 dtucker 572: .Cm RhostsRSAAuthentication ,
573: .Cm RSAAuthentication ,
1.66 dtucker 574: .Cm X11DisplayOffset ,
575: .Cm X11Forwarding ,
1.60 dtucker 576: and
1.66 dtucker 577: .Cm X11UseLocalHost .
1.33 dtucker 578: .It Cm MaxAuthTries
579: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 580: connection.
581: Once the number of failures reaches half this value,
582: additional failures are logged.
583: The default is 6.
1.1 stevesk 584: .It Cm MaxStartups
585: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 586: SSH daemon.
1.1 stevesk 587: Additional connections will be dropped until authentication succeeds or the
588: .Cm LoginGraceTime
589: expires for a connection.
590: The default is 10.
591: .Pp
592: Alternatively, random early drop can be enabled by specifying
593: the three colon separated values
594: .Dq start:rate:full
1.51 jmc 595: (e.g. "10:30:60").
1.53 jmc 596: .Xr sshd 8
1.1 stevesk 597: will refuse connection attempts with a probability of
598: .Dq rate/100
599: (30%)
600: if there are currently
601: .Dq start
602: (10)
603: unauthenticated connections.
604: The probability increases linearly and all connection attempts
605: are refused if the number of unauthenticated connections reaches
606: .Dq full
607: (60).
608: .It Cm PasswordAuthentication
609: Specifies whether password authentication is allowed.
610: The default is
611: .Dq yes .
612: .It Cm PermitEmptyPasswords
613: When password authentication is allowed, it specifies whether the
614: server allows login to accounts with empty password strings.
615: The default is
616: .Dq no .
1.62 dtucker 617: .It Cm PermitOpen
618: Specifies the destinations to which TCP port forwarding is permitted.
619: The forwarding specification must be one of the following forms:
620: .Pp
621: .Bl -item -offset indent -compact
622: .It
623: .Cm PermitOpen
624: .Sm off
625: .Ar host : port
626: .Sm on
627: .It
628: .Cm PermitOpen
629: .Sm off
630: .Ar IPv4_addr : port
631: .Sm on
632: .It
633: .Cm PermitOpen
634: .Sm off
635: .Ar \&[ IPv6_addr \&] : port
636: .Sm on
637: .El
638: .Pp
1.68 dtucker 639: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 640: An argument of
641: .Dq any
642: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 643: By default all port forwarding requests are permitted.
1.1 stevesk 644: .It Cm PermitRootLogin
1.38 jmc 645: Specifies whether root can log in using
1.1 stevesk 646: .Xr ssh 1 .
647: The argument must be
648: .Dq yes ,
649: .Dq without-password ,
1.52 jmc 650: .Dq forced-commands-only ,
1.1 stevesk 651: or
652: .Dq no .
653: The default is
654: .Dq yes .
655: .Pp
656: If this option is set to
1.52 jmc 657: .Dq without-password ,
1.1 stevesk 658: password authentication is disabled for root.
659: .Pp
660: If this option is set to
1.52 jmc 661: .Dq forced-commands-only ,
1.1 stevesk 662: root login with public key authentication will be allowed,
663: but only if the
664: .Ar command
665: option has been specified
666: (which may be useful for taking remote backups even if root login is
1.17 jmc 667: normally not allowed).
668: All other authentication methods are disabled for root.
1.1 stevesk 669: .Pp
670: If this option is set to
1.52 jmc 671: .Dq no ,
1.38 jmc 672: root is not allowed to log in.
1.46 reyk 673: .It Cm PermitTunnel
674: Specifies whether
675: .Xr tun 4
676: device forwarding is allowed.
1.47 reyk 677: The argument must be
678: .Dq yes ,
1.58 stevesk 679: .Dq point-to-point
680: (layer 3),
681: .Dq ethernet
682: (layer 2), or
1.47 reyk 683: .Dq no .
1.58 stevesk 684: Specifying
685: .Dq yes
686: permits both
687: .Dq point-to-point
688: and
689: .Dq ethernet .
1.46 reyk 690: The default is
691: .Dq no .
1.6 markus 692: .It Cm PermitUserEnvironment
693: Specifies whether
694: .Pa ~/.ssh/environment
1.9 stevesk 695: and
1.6 markus 696: .Cm environment=
697: options in
698: .Pa ~/.ssh/authorized_keys
1.9 stevesk 699: are processed by
1.52 jmc 700: .Xr sshd 8 .
1.6 markus 701: The default is
702: .Dq no .
1.9 stevesk 703: Enabling environment processing may enable users to bypass access
704: restrictions in some configurations using mechanisms such as
705: .Ev LD_PRELOAD .
1.1 stevesk 706: .It Cm PidFile
1.4 stevesk 707: Specifies the file that contains the process ID of the
1.53 jmc 708: SSH daemon.
1.1 stevesk 709: The default is
710: .Pa /var/run/sshd.pid .
711: .It Cm Port
712: Specifies the port number that
1.52 jmc 713: .Xr sshd 8
1.1 stevesk 714: listens on.
715: The default is 22.
716: Multiple options of this type are permitted.
717: See also
718: .Cm ListenAddress .
719: .It Cm PrintLastLog
720: Specifies whether
1.52 jmc 721: .Xr sshd 8
1.36 jaredy 722: should print the date and time of the last user login when a user logs
723: in interactively.
1.1 stevesk 724: The default is
725: .Dq yes .
726: .It Cm PrintMotd
727: Specifies whether
1.52 jmc 728: .Xr sshd 8
1.1 stevesk 729: should print
730: .Pa /etc/motd
731: when a user logs in interactively.
732: (On some systems it is also printed by the shell,
733: .Pa /etc/profile ,
734: or equivalent.)
735: The default is
736: .Dq yes .
737: .It Cm Protocol
738: Specifies the protocol versions
1.52 jmc 739: .Xr sshd 8
1.5 stevesk 740: supports.
1.1 stevesk 741: The possible values are
1.52 jmc 742: .Sq 1
1.1 stevesk 743: and
1.52 jmc 744: .Sq 2 .
1.1 stevesk 745: Multiple versions must be comma-separated.
746: The default is
747: .Dq 2,1 .
1.5 stevesk 748: Note that the order of the protocol list does not indicate preference,
749: because the client selects among multiple protocol versions offered
750: by the server.
751: Specifying
752: .Dq 2,1
753: is identical to
754: .Dq 1,2 .
1.1 stevesk 755: .It Cm PubkeyAuthentication
756: Specifies whether public key authentication is allowed.
757: The default is
758: .Dq yes .
759: Note that this option applies to protocol version 2 only.
760: .It Cm RhostsRSAAuthentication
761: Specifies whether rhosts or /etc/hosts.equiv authentication together
762: with successful RSA host authentication is allowed.
763: The default is
764: .Dq no .
765: This option applies to protocol version 1 only.
766: .It Cm RSAAuthentication
767: Specifies whether pure RSA authentication is allowed.
768: The default is
769: .Dq yes .
770: This option applies to protocol version 1 only.
771: .It Cm ServerKeyBits
772: Defines the number of bits in the ephemeral protocol version 1 server key.
773: The minimum value is 512, and the default is 768.
774: .It Cm StrictModes
775: Specifies whether
1.52 jmc 776: .Xr sshd 8
1.1 stevesk 777: should check file modes and ownership of the
778: user's files and home directory before accepting login.
779: This is normally desirable because novices sometimes accidentally leave their
780: directory or files world-writable.
781: The default is
782: .Dq yes .
783: .It Cm Subsystem
1.51 jmc 784: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 785: Arguments should be a subsystem name and a command (with optional arguments)
786: to execute upon subsystem request.
1.80 djm 787: .Pp
1.1 stevesk 788: The command
789: .Xr sftp-server 8
790: implements the
791: .Dq sftp
792: file transfer subsystem.
1.80 djm 793: .Pp
794: Alternately the name
795: .Dq internal-sftp
796: implements an in-process
797: .Dq sftp
798: server.
799: This may simplify configurations using
800: .Cm ChrootDirectory
801: to force a different filesystem root on clients.
802: .Pp
1.1 stevesk 803: By default no subsystems are defined.
804: Note that this option applies to protocol version 2 only.
805: .It Cm SyslogFacility
806: Gives the facility code that is used when logging messages from
1.53 jmc 807: .Xr sshd 8 .
1.1 stevesk 808: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
809: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
810: The default is AUTH.
1.27 markus 811: .It Cm TCPKeepAlive
812: Specifies whether the system should send TCP keepalive messages to the
813: other side.
814: If they are sent, death of the connection or crash of one
815: of the machines will be properly noticed.
816: However, this means that
817: connections will die if the route is down temporarily, and some people
818: find it annoying.
819: On the other hand, if TCP keepalives are not sent,
820: sessions may hang indefinitely on the server, leaving
821: .Dq ghost
822: users and consuming server resources.
823: .Pp
824: The default is
825: .Dq yes
826: (to send TCP keepalive messages), and the server will notice
827: if the network goes down or the client host crashes.
828: This avoids infinitely hanging sessions.
829: .Pp
830: To disable TCP keepalive messages, the value should be set to
831: .Dq no .
1.18 markus 832: .It Cm UseDNS
833: Specifies whether
1.52 jmc 834: .Xr sshd 8
1.40 jmc 835: should look up the remote host name and check that
1.18 markus 836: the resolved host name for the remote IP address maps back to the
837: very same IP address.
838: The default is
839: .Dq yes .
1.1 stevesk 840: .It Cm UseLogin
841: Specifies whether
842: .Xr login 1
843: is used for interactive login sessions.
844: The default is
845: .Dq no .
846: Note that
847: .Xr login 1
848: is never used for remote command execution.
849: Note also, that if this is enabled,
850: .Cm X11Forwarding
851: will be disabled because
852: .Xr login 1
853: does not know how to handle
854: .Xr xauth 1
1.15 jmc 855: cookies.
856: If
1.1 stevesk 857: .Cm UsePrivilegeSeparation
858: is specified, it will be disabled after authentication.
859: .It Cm UsePrivilegeSeparation
860: Specifies whether
1.52 jmc 861: .Xr sshd 8
1.2 stevesk 862: separates privileges by creating an unprivileged child process
1.15 jmc 863: to deal with incoming network traffic.
864: After successful authentication, another process will be created that has
865: the privilege of the authenticated user.
866: The goal of privilege separation is to prevent privilege
1.1 stevesk 867: escalation by containing any corruption within the unprivileged processes.
868: The default is
869: .Dq yes .
870: .It Cm X11DisplayOffset
871: Specifies the first display number available for
1.52 jmc 872: .Xr sshd 8 Ns 's
1.1 stevesk 873: X11 forwarding.
1.52 jmc 874: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 875: The default is 10.
876: .It Cm X11Forwarding
877: Specifies whether X11 forwarding is permitted.
1.13 stevesk 878: The argument must be
879: .Dq yes
880: or
881: .Dq no .
1.1 stevesk 882: The default is
883: .Dq no .
1.13 stevesk 884: .Pp
885: When X11 forwarding is enabled, there may be additional exposure to
886: the server and to client displays if the
1.52 jmc 887: .Xr sshd 8
1.13 stevesk 888: proxy display is configured to listen on the wildcard address (see
889: .Cm X11UseLocalhost
1.52 jmc 890: below), though this is not the default.
1.13 stevesk 891: Additionally, the authentication spoofing and authentication data
892: verification and substitution occur on the client side.
893: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 894: display server may be exposed to attack when the SSH client requests
1.13 stevesk 895: forwarding (see the warnings for
896: .Cm ForwardX11
897: in
1.19 jmc 898: .Xr ssh_config 5 ) .
1.13 stevesk 899: A system administrator may have a stance in which they want to
900: protect clients that may expose themselves to attack by unwittingly
901: requesting X11 forwarding, which can warrant a
902: .Dq no
903: setting.
904: .Pp
905: Note that disabling X11 forwarding does not prevent users from
906: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 907: X11 forwarding is automatically disabled if
908: .Cm UseLogin
909: is enabled.
910: .It Cm X11UseLocalhost
911: Specifies whether
1.52 jmc 912: .Xr sshd 8
1.1 stevesk 913: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 914: the wildcard address.
915: By default,
1.52 jmc 916: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 917: hostname part of the
918: .Ev DISPLAY
919: environment variable to
920: .Dq localhost .
1.8 stevesk 921: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 922: However, some older X11 clients may not function with this
923: configuration.
924: .Cm X11UseLocalhost
925: may be set to
926: .Dq no
927: to specify that the forwarding server should be bound to the wildcard
928: address.
929: The argument must be
930: .Dq yes
931: or
932: .Dq no .
933: The default is
934: .Dq yes .
935: .It Cm XAuthLocation
1.11 stevesk 936: Specifies the full pathname of the
1.1 stevesk 937: .Xr xauth 1
938: program.
939: The default is
940: .Pa /usr/X11R6/bin/xauth .
941: .El
1.55 jmc 942: .Sh TIME FORMATS
1.53 jmc 943: .Xr sshd 8
1.1 stevesk 944: command-line arguments and configuration file options that specify time
945: may be expressed using a sequence of the form:
946: .Sm off
1.7 stevesk 947: .Ar time Op Ar qualifier ,
1.1 stevesk 948: .Sm on
949: where
950: .Ar time
951: is a positive integer value and
952: .Ar qualifier
953: is one of the following:
954: .Pp
955: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 956: .It Aq Cm none
1.1 stevesk 957: seconds
958: .It Cm s | Cm S
959: seconds
960: .It Cm m | Cm M
961: minutes
962: .It Cm h | Cm H
963: hours
964: .It Cm d | Cm D
965: days
966: .It Cm w | Cm W
967: weeks
968: .El
969: .Pp
970: Each member of the sequence is added together to calculate
971: the total time value.
972: .Pp
973: Time format examples:
974: .Pp
975: .Bl -tag -width Ds -compact -offset indent
976: .It 600
977: 600 seconds (10 minutes)
978: .It 10m
979: 10 minutes
980: .It 1h30m
981: 1 hour 30 minutes (90 minutes)
982: .El
983: .Sh FILES
984: .Bl -tag -width Ds
985: .It Pa /etc/ssh/sshd_config
986: Contains configuration data for
1.53 jmc 987: .Xr sshd 8 .
1.1 stevesk 988: This file should be writable by root only, but it is recommended
989: (though not necessary) that it be world-readable.
990: .El
1.19 jmc 991: .Sh SEE ALSO
992: .Xr sshd 8
1.1 stevesk 993: .Sh AUTHORS
994: OpenSSH is a derivative of the original and free
995: ssh 1.2.12 release by Tatu Ylonen.
996: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
997: Theo de Raadt and Dug Song
998: removed many bugs, re-added newer features and
999: created OpenSSH.
1000: Markus Friedl contributed the support for SSH
1001: protocol versions 1.5 and 2.0.
1002: Niels Provos and Markus Friedl contributed support
1003: for privilege separation.
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