Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.106
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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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.106 ! stevesk 37: .\" $OpenBSD: sshd_config.5,v 1.105 2009/04/18 18:39:10 jmc Exp $
! 38: .Dd $Mdocdate: April 18 2009 $
1.1 stevesk 39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
1.71 jmc 45: .Nm /etc/ssh/sshd_config
1.1 stevesk 46: .Sh DESCRIPTION
1.53 jmc 47: .Xr sshd 8
1.1 stevesk 48: reads configuration data from
49: .Pa /etc/ssh/sshd_config
50: (or the file specified with
51: .Fl f
52: on the command line).
53: The file contains keyword-argument pairs, one per line.
54: Lines starting with
55: .Ql #
56: and empty lines are interpreted as comments.
1.56 dtucker 57: Arguments may optionally be enclosed in double quotes
58: .Pq \&"
59: in order to represent arguments containing spaces.
1.1 stevesk 60: .Pp
61: The possible
62: keywords and their meanings are as follows (note that
63: keywords are case-insensitive and arguments are case-sensitive):
64: .Bl -tag -width Ds
1.30 djm 65: .It Cm AcceptEnv
66: Specifies what environment variables sent by the client will be copied into
67: the session's
68: .Xr environ 7 .
69: See
70: .Cm SendEnv
71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.31 djm 74: Note that environment passing is only supported for protocol 2.
1.30 djm 75: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 76: .Ql *
1.30 djm 77: and
78: .Ql \&? .
1.31 djm 79: Multiple environment variables may be separated by whitespace or spread
1.30 djm 80: across multiple
81: .Cm AcceptEnv
82: directives.
1.31 djm 83: Be warned that some environment variables could be used to bypass restricted
1.30 djm 84: user environments.
85: For this reason, care should be taken in the use of this directive.
86: The default is not to accept any environment variables.
1.37 djm 87: .It Cm AddressFamily
88: Specifies which address family should be used by
1.53 jmc 89: .Xr sshd 8 .
1.37 djm 90: Valid arguments are
91: .Dq any ,
92: .Dq inet
1.52 jmc 93: (use IPv4 only), or
1.37 djm 94: .Dq inet6
95: (use IPv6 only).
96: The default is
97: .Dq any .
1.89 jmc 98: .It Cm AllowAgentForwarding
99: Specifies whether
100: .Xr ssh-agent 1
101: forwarding is permitted.
102: The default is
103: .Dq yes .
104: Note that disabling agent forwarding does not improve security
105: unless users are also denied shell access, as they can always install
106: their own forwarders.
1.1 stevesk 107: .It Cm AllowGroups
108: This keyword can be followed by a list of group name patterns, separated
109: by spaces.
110: If specified, login is allowed only for users whose primary
111: group or supplementary group list matches one of the patterns.
112: Only group names are valid; a numerical group ID is not recognized.
113: By default, login is allowed for all groups.
1.54 jmc 114: The allow/deny directives are processed in the following order:
115: .Cm DenyUsers ,
116: .Cm AllowUsers ,
117: .Cm DenyGroups ,
118: and finally
119: .Cm AllowGroups .
1.49 jmc 120: .Pp
121: See
122: .Sx PATTERNS
123: in
124: .Xr ssh_config 5
125: for more information on patterns.
1.1 stevesk 126: .It Cm AllowTcpForwarding
127: Specifies whether TCP forwarding is permitted.
128: The default is
129: .Dq yes .
130: Note that disabling TCP forwarding does not improve security unless
131: users are also denied shell access, as they can always install their
132: own forwarders.
133: .It Cm AllowUsers
134: This keyword can be followed by a list of user name patterns, separated
135: by spaces.
1.14 jmc 136: If specified, login is allowed only for user names that
1.1 stevesk 137: match one of the patterns.
138: Only user names are valid; a numerical user ID is not recognized.
139: By default, login is allowed for all users.
140: If the pattern takes the form USER@HOST then USER and HOST
141: are separately checked, restricting logins to particular
142: users from particular hosts.
1.54 jmc 143: The allow/deny directives are processed in the following order:
144: .Cm DenyUsers ,
145: .Cm AllowUsers ,
146: .Cm DenyGroups ,
147: and finally
148: .Cm AllowGroups .
1.49 jmc 149: .Pp
150: See
151: .Sx PATTERNS
152: in
153: .Xr ssh_config 5
154: for more information on patterns.
1.1 stevesk 155: .It Cm AuthorizedKeysFile
156: Specifies the file that contains the public keys that can be used
157: for user authentication.
158: .Cm AuthorizedKeysFile
159: may contain tokens of the form %T which are substituted during connection
1.52 jmc 160: setup.
1.17 jmc 161: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 162: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 163: %u is replaced by the username of that user.
164: After expansion,
165: .Cm AuthorizedKeysFile
166: is taken to be an absolute path or one relative to the user's home
167: directory.
168: The default is
169: .Dq .ssh/authorized_keys .
170: .It Cm Banner
171: The contents of the specified file are sent to the remote user before
172: authentication is allowed.
1.78 djm 173: If the argument is
174: .Dq none
175: then no banner is displayed.
1.1 stevesk 176: This option is only available for protocol version 2.
177: By default, no banner is displayed.
178: .It Cm ChallengeResponseAuthentication
1.50 jmc 179: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 180: All authentication styles from
181: .Xr login.conf 5
182: are supported.
183: The default is
184: .Dq yes .
1.80 djm 185: .It Cm ChrootDirectory
186: Specifies a path to
187: .Xr chroot 2
188: to after authentication.
189: This path, and all its components, must be root-owned directories that are
190: not writable by any other user or group.
1.106 ! stevesk 191: After the chroot,
! 192: .Xr sshd 8
! 193: changes the working directory to the user's home directory.
1.80 djm 194: .Pp
195: The path may contain the following tokens that are expanded at runtime once
196: the connecting user has been authenticated: %% is replaced by a literal '%',
197: %h is replaced by the home directory of the user being authenticated, and
198: %u is replaced by the username of that user.
199: .Pp
200: The
201: .Cm ChrootDirectory
202: must contain the necessary files and directories to support the
1.103 stevesk 203: user's session.
1.80 djm 204: For an interactive session this requires at least a shell, typically
205: .Xr sh 1 ,
206: and basic
207: .Pa /dev
208: nodes such as
209: .Xr null 4 ,
210: .Xr zero 4 ,
211: .Xr stdin 4 ,
212: .Xr stdout 4 ,
213: .Xr stderr 4 ,
214: .Xr arandom 4
215: and
216: .Xr tty 4
217: devices.
218: For file transfer sessions using
1.105 jmc 219: .Dq sftp ,
1.80 djm 220: no additional configuration of the environment is necessary if the
1.105 jmc 221: in-process sftp server is used,
222: though sessions which use logging do require
1.104 stevesk 223: .Pa /dev/log
224: inside the chroot directory (see
225: .Xr sftp-server 8
1.81 jmc 226: for details).
1.80 djm 227: .Pp
228: The default is not to
229: .Xr chroot 2 .
1.1 stevesk 230: .It Cm Ciphers
231: Specifies the ciphers allowed for protocol version 2.
232: Multiple ciphers must be comma-separated.
1.34 dtucker 233: The supported ciphers are
234: .Dq 3des-cbc ,
235: .Dq aes128-cbc ,
236: .Dq aes192-cbc ,
237: .Dq aes256-cbc ,
238: .Dq aes128-ctr ,
239: .Dq aes192-ctr ,
240: .Dq aes256-ctr ,
1.43 djm 241: .Dq arcfour128 ,
242: .Dq arcfour256 ,
1.34 dtucker 243: .Dq arcfour ,
244: .Dq blowfish-cbc ,
245: and
246: .Dq cast128-cbc .
1.52 jmc 247: The default is:
248: .Bd -literal -offset 3n
1.100 naddy 249: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
250: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
251: aes256-cbc,arcfour
1.1 stevesk 252: .Ed
253: .It Cm ClientAliveCountMax
1.48 jmc 254: Sets the number of client alive messages (see below) which may be
1.1 stevesk 255: sent without
1.52 jmc 256: .Xr sshd 8
1.17 jmc 257: receiving any messages back from the client.
258: If this threshold is reached while client alive messages are being sent,
1.52 jmc 259: sshd will disconnect the client, terminating the session.
1.17 jmc 260: It is important to note that the use of client alive messages is very
261: different from
1.27 markus 262: .Cm TCPKeepAlive
1.17 jmc 263: (below).
264: The client alive messages are sent through the encrypted channel
265: and therefore will not be spoofable.
266: The TCP keepalive option enabled by
1.27 markus 267: .Cm TCPKeepAlive
1.17 jmc 268: is spoofable.
269: The client alive mechanism is valuable when the client or
1.1 stevesk 270: server depend on knowing when a connection has become inactive.
271: .Pp
1.17 jmc 272: The default value is 3.
273: If
1.1 stevesk 274: .Cm ClientAliveInterval
1.48 jmc 275: (see below) is set to 15, and
1.1 stevesk 276: .Cm ClientAliveCountMax
1.52 jmc 277: is left at the default, unresponsive SSH clients
1.1 stevesk 278: will be disconnected after approximately 45 seconds.
1.57 markus 279: This option applies to protocol version 2 only.
1.42 djm 280: .It Cm ClientAliveInterval
281: Sets a timeout interval in seconds after which if no data has been received
282: from the client,
1.52 jmc 283: .Xr sshd 8
1.42 djm 284: will send a message through the encrypted
285: channel to request a response from the client.
286: The default
287: is 0, indicating that these messages will not be sent to the client.
288: This option applies to protocol version 2 only.
1.3 markus 289: .It Cm Compression
1.44 markus 290: Specifies whether compression is allowed, or delayed until
291: the user has authenticated successfully.
1.3 markus 292: The argument must be
1.44 markus 293: .Dq yes ,
294: .Dq delayed ,
1.3 markus 295: or
296: .Dq no .
297: The default is
1.44 markus 298: .Dq delayed .
1.1 stevesk 299: .It Cm DenyGroups
300: This keyword can be followed by a list of group name patterns, separated
301: by spaces.
302: Login is disallowed for users whose primary group or supplementary
303: group list matches one of the patterns.
304: Only group names are valid; a numerical group ID is not recognized.
305: By default, login is allowed for all groups.
1.54 jmc 306: The allow/deny directives are processed in the following order:
307: .Cm DenyUsers ,
308: .Cm AllowUsers ,
309: .Cm DenyGroups ,
310: and finally
311: .Cm AllowGroups .
1.49 jmc 312: .Pp
313: See
314: .Sx PATTERNS
315: in
316: .Xr ssh_config 5
317: for more information on patterns.
1.1 stevesk 318: .It Cm DenyUsers
319: This keyword can be followed by a list of user name patterns, separated
320: by spaces.
321: Login is disallowed for user names that match one of the patterns.
322: Only user names are valid; a numerical user ID is not recognized.
323: By default, login is allowed for all users.
324: If the pattern takes the form USER@HOST then USER and HOST
325: are separately checked, restricting logins to particular
326: users from particular hosts.
1.54 jmc 327: The allow/deny directives are processed in the following order:
328: .Cm DenyUsers ,
329: .Cm AllowUsers ,
330: .Cm DenyGroups ,
331: and finally
332: .Cm AllowGroups .
1.49 jmc 333: .Pp
334: See
335: .Sx PATTERNS
336: in
337: .Xr ssh_config 5
338: for more information on patterns.
1.67 dtucker 339: .It Cm ForceCommand
340: Forces the execution of the command specified by
341: .Cm ForceCommand ,
1.84 djm 342: ignoring any command supplied by the client and
343: .Pa ~/.ssh/rc
344: if present.
1.67 dtucker 345: The command is invoked by using the user's login shell with the -c option.
346: This applies to shell, command, or subsystem execution.
347: It is most useful inside a
348: .Cm Match
349: block.
350: The command originally supplied by the client is available in the
351: .Ev SSH_ORIGINAL_COMMAND
352: environment variable.
1.82 djm 353: Specifying a command of
354: .Dq internal-sftp
355: will force the use of an in-process sftp server that requires no support
356: files when used with
357: .Cm ChrootDirectory .
1.1 stevesk 358: .It Cm GatewayPorts
359: Specifies whether remote hosts are allowed to connect to ports
360: forwarded for the client.
361: By default,
1.52 jmc 362: .Xr sshd 8
1.15 jmc 363: binds remote port forwardings to the loopback address.
364: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 365: .Cm GatewayPorts
1.52 jmc 366: can be used to specify that sshd
1.39 djm 367: should allow remote port forwardings to bind to non-loopback addresses, thus
368: allowing other hosts to connect.
369: The argument may be
370: .Dq no
371: to force remote port forwardings to be available to the local host only,
1.1 stevesk 372: .Dq yes
1.39 djm 373: to force remote port forwardings to bind to the wildcard address, or
374: .Dq clientspecified
375: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 376: The default is
377: .Dq no .
1.23 markus 378: .It Cm GSSAPIAuthentication
1.25 markus 379: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 380: The default is
1.23 markus 381: .Dq no .
382: Note that this option applies to protocol version 2 only.
383: .It Cm GSSAPICleanupCredentials
384: Specifies whether to automatically destroy the user's credentials cache
385: on logout.
386: The default is
387: .Dq yes .
388: Note that this option applies to protocol version 2 only.
1.1 stevesk 389: .It Cm HostbasedAuthentication
390: Specifies whether rhosts or /etc/hosts.equiv authentication together
391: with successful public key client host authentication is allowed
1.50 jmc 392: (host-based authentication).
1.1 stevesk 393: This option is similar to
394: .Cm RhostsRSAAuthentication
395: and applies to protocol version 2 only.
1.70 dtucker 396: The default is
397: .Dq no .
398: .It Cm HostbasedUsesNameFromPacketOnly
399: Specifies whether or not the server will attempt to perform a reverse
400: name lookup when matching the name in the
401: .Pa ~/.shosts ,
402: .Pa ~/.rhosts ,
403: and
404: .Pa /etc/hosts.equiv
405: files during
406: .Cm HostbasedAuthentication .
407: A setting of
408: .Dq yes
409: means that
410: .Xr sshd 8
411: uses the name supplied by the client rather than
412: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 413: The default is
414: .Dq no .
415: .It Cm HostKey
416: Specifies a file containing a private host key
417: used by SSH.
418: The default is
419: .Pa /etc/ssh/ssh_host_key
420: for protocol version 1, and
421: .Pa /etc/ssh/ssh_host_rsa_key
422: and
423: .Pa /etc/ssh/ssh_host_dsa_key
424: for protocol version 2.
425: Note that
1.52 jmc 426: .Xr sshd 8
1.1 stevesk 427: will refuse to use a file if it is group/world-accessible.
428: It is possible to have multiple host key files.
429: .Dq rsa1
430: keys are used for version 1 and
431: .Dq dsa
432: or
433: .Dq rsa
434: are used for version 2 of the SSH protocol.
435: .It Cm IgnoreRhosts
436: Specifies that
437: .Pa .rhosts
438: and
439: .Pa .shosts
440: files will not be used in
441: .Cm RhostsRSAAuthentication
442: or
443: .Cm HostbasedAuthentication .
444: .Pp
445: .Pa /etc/hosts.equiv
446: and
447: .Pa /etc/shosts.equiv
448: are still used.
449: The default is
450: .Dq yes .
451: .It Cm IgnoreUserKnownHosts
452: Specifies whether
1.52 jmc 453: .Xr sshd 8
1.1 stevesk 454: should ignore the user's
1.41 djm 455: .Pa ~/.ssh/known_hosts
1.1 stevesk 456: during
457: .Cm RhostsRSAAuthentication
458: or
459: .Cm HostbasedAuthentication .
460: The default is
461: .Dq no .
462: .It Cm KerberosAuthentication
1.24 markus 463: Specifies whether the password provided by the user for
1.1 stevesk 464: .Cm PasswordAuthentication
1.24 markus 465: will be validated through the Kerberos KDC.
1.1 stevesk 466: To use this option, the server needs a
467: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 468: The default is
1.29 dtucker 469: .Dq no .
470: .It Cm KerberosGetAFSToken
1.45 djm 471: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 472: an AFS token before accessing the user's home directory.
1.52 jmc 473: The default is
1.1 stevesk 474: .Dq no .
475: .It Cm KerberosOrLocalPasswd
1.52 jmc 476: If password authentication through Kerberos fails then
1.1 stevesk 477: the password will be validated via any additional local mechanism
478: such as
479: .Pa /etc/passwd .
1.52 jmc 480: The default is
1.1 stevesk 481: .Dq yes .
482: .It Cm KerberosTicketCleanup
483: Specifies whether to automatically destroy the user's ticket cache
484: file on logout.
1.52 jmc 485: The default is
1.1 stevesk 486: .Dq yes .
487: .It Cm KeyRegenerationInterval
488: In protocol version 1, the ephemeral server key is automatically regenerated
489: after this many seconds (if it has been used).
490: The purpose of regeneration is to prevent
491: decrypting captured sessions by later breaking into the machine and
492: stealing the keys.
493: The key is never stored anywhere.
494: If the value is 0, the key is never regenerated.
495: The default is 3600 (seconds).
496: .It Cm ListenAddress
497: Specifies the local addresses
1.52 jmc 498: .Xr sshd 8
1.1 stevesk 499: should listen on.
500: The following forms may be used:
501: .Pp
502: .Bl -item -offset indent -compact
503: .It
504: .Cm ListenAddress
505: .Sm off
506: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
507: .Sm on
508: .It
509: .Cm ListenAddress
510: .Sm off
511: .Ar host No | Ar IPv4_addr No : Ar port
512: .Sm on
513: .It
514: .Cm ListenAddress
515: .Sm off
516: .Oo
517: .Ar host No | Ar IPv6_addr Oc : Ar port
518: .Sm on
519: .El
520: .Pp
521: If
522: .Ar port
523: is not specified,
1.52 jmc 524: sshd will listen on the address and all prior
1.1 stevesk 525: .Cm Port
1.17 jmc 526: options specified.
527: The default is to listen on all local addresses.
1.15 jmc 528: Multiple
1.1 stevesk 529: .Cm ListenAddress
1.17 jmc 530: options are permitted.
531: Additionally, any
1.1 stevesk 532: .Cm Port
1.52 jmc 533: options must precede this option for non-port qualified addresses.
1.1 stevesk 534: .It Cm LoginGraceTime
535: The server disconnects after this time if the user has not
536: successfully logged in.
537: If the value is 0, there is no time limit.
1.12 stevesk 538: The default is 120 seconds.
1.1 stevesk 539: .It Cm LogLevel
540: Gives the verbosity level that is used when logging messages from
1.53 jmc 541: .Xr sshd 8 .
1.1 stevesk 542: The possible values are:
1.52 jmc 543: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 544: The default is INFO.
545: DEBUG and DEBUG1 are equivalent.
546: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
547: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 548: .It Cm MACs
549: Specifies the available MAC (message authentication code) algorithms.
550: The MAC algorithm is used in protocol version 2
551: for data integrity protection.
552: Multiple algorithms must be comma-separated.
1.52 jmc 553: The default is:
1.77 jmc 554: .Bd -literal -offset indent
555: hmac-md5,hmac-sha1,umac-64@openssh.com,
556: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
557: .Ed
1.60 dtucker 558: .It Cm Match
1.61 jmc 559: Introduces a conditional block.
1.65 dtucker 560: If all of the criteria on the
1.60 dtucker 561: .Cm Match
1.65 dtucker 562: line are satisfied, the keywords on the following lines override those
563: set in the global section of the config file, until either another
1.60 dtucker 564: .Cm Match
1.65 dtucker 565: line or the end of the file.
1.91 djm 566: .Pp
1.61 jmc 567: The arguments to
1.60 dtucker 568: .Cm Match
1.65 dtucker 569: are one or more criteria-pattern pairs.
1.60 dtucker 570: The available criteria are
571: .Cm User ,
1.69 dtucker 572: .Cm Group ,
1.60 dtucker 573: .Cm Host ,
574: and
575: .Cm Address .
1.91 djm 576: The match patterns may consist of single entries or comma-separated
577: lists and may use the wildcard and negation operators described in the
1.92 djm 578: .Sx PATTERNS
1.91 djm 579: section of
1.92 djm 580: .Xr ssh_config 5 .
1.91 djm 581: .Pp
582: The patterns in an
583: .Cm Address
584: criteria may additionally contain addresses to match in CIDR
1.93 jmc 585: address/masklen format, e.g.\&
1.91 djm 586: .Dq 192.0.2.0/24
587: or
588: .Dq 3ffe:ffff::/32 .
589: Note that the mask length provided must be consistent with the address -
590: it is an error to specify a mask length that is too long for the address
1.93 jmc 591: or one with bits set in this host portion of the address.
592: For example,
1.91 djm 593: .Dq 192.0.2.0/33
594: and
1.93 jmc 595: .Dq 192.0.2.0/8
1.91 djm 596: respectively.
597: .Pp
1.60 dtucker 598: Only a subset of keywords may be used on the lines following a
599: .Cm Match
600: keyword.
601: Available keywords are
1.99 okan 602: .Cm AllowAgentForwarding ,
1.62 dtucker 603: .Cm AllowTcpForwarding ,
1.72 dtucker 604: .Cm Banner ,
1.85 djm 605: .Cm ChrootDirectory ,
1.67 dtucker 606: .Cm ForceCommand ,
1.62 dtucker 607: .Cm GatewayPorts ,
1.87 djm 608: .Cm GSSAPIAuthentication ,
609: .Cm HostbasedAuthentication ,
1.74 jmc 610: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 611: .Cm KerberosAuthentication ,
1.95 dtucker 612: .Cm MaxAuthTries ,
1.94 dtucker 613: .Cm MaxSessions ,
1.72 dtucker 614: .Cm PasswordAuthentication ,
1.97 djm 615: .Cm PermitEmptyPasswords ,
1.66 dtucker 616: .Cm PermitOpen ,
1.79 dtucker 617: .Cm PermitRootLogin ,
1.72 dtucker 618: .Cm RhostsRSAAuthentication ,
619: .Cm RSAAuthentication ,
1.66 dtucker 620: .Cm X11DisplayOffset ,
1.101 djm 621: .Cm X11Forwarding
1.60 dtucker 622: and
1.102 djm 623: .Cm X11UseLocalHost .
1.33 dtucker 624: .It Cm MaxAuthTries
625: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 626: connection.
627: Once the number of failures reaches half this value,
628: additional failures are logged.
629: The default is 6.
1.90 djm 630: .It Cm MaxSessions
631: Specifies the maximum number of open sessions permitted per network connection.
632: The default is 10.
1.1 stevesk 633: .It Cm MaxStartups
634: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 635: SSH daemon.
1.1 stevesk 636: Additional connections will be dropped until authentication succeeds or the
637: .Cm LoginGraceTime
638: expires for a connection.
639: The default is 10.
640: .Pp
641: Alternatively, random early drop can be enabled by specifying
642: the three colon separated values
643: .Dq start:rate:full
1.51 jmc 644: (e.g. "10:30:60").
1.53 jmc 645: .Xr sshd 8
1.1 stevesk 646: will refuse connection attempts with a probability of
647: .Dq rate/100
648: (30%)
649: if there are currently
650: .Dq start
651: (10)
652: unauthenticated connections.
653: The probability increases linearly and all connection attempts
654: are refused if the number of unauthenticated connections reaches
655: .Dq full
656: (60).
657: .It Cm PasswordAuthentication
658: Specifies whether password authentication is allowed.
659: The default is
660: .Dq yes .
661: .It Cm PermitEmptyPasswords
662: When password authentication is allowed, it specifies whether the
663: server allows login to accounts with empty password strings.
664: The default is
665: .Dq no .
1.62 dtucker 666: .It Cm PermitOpen
667: Specifies the destinations to which TCP port forwarding is permitted.
668: The forwarding specification must be one of the following forms:
669: .Pp
670: .Bl -item -offset indent -compact
671: .It
672: .Cm PermitOpen
673: .Sm off
674: .Ar host : port
675: .Sm on
676: .It
677: .Cm PermitOpen
678: .Sm off
679: .Ar IPv4_addr : port
680: .Sm on
681: .It
682: .Cm PermitOpen
683: .Sm off
684: .Ar \&[ IPv6_addr \&] : port
685: .Sm on
686: .El
687: .Pp
1.68 dtucker 688: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 689: An argument of
690: .Dq any
691: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 692: By default all port forwarding requests are permitted.
1.1 stevesk 693: .It Cm PermitRootLogin
1.38 jmc 694: Specifies whether root can log in using
1.1 stevesk 695: .Xr ssh 1 .
696: The argument must be
697: .Dq yes ,
698: .Dq without-password ,
1.52 jmc 699: .Dq forced-commands-only ,
1.1 stevesk 700: or
701: .Dq no .
702: The default is
703: .Dq yes .
704: .Pp
705: If this option is set to
1.52 jmc 706: .Dq without-password ,
1.1 stevesk 707: password authentication is disabled for root.
708: .Pp
709: If this option is set to
1.52 jmc 710: .Dq forced-commands-only ,
1.1 stevesk 711: root login with public key authentication will be allowed,
712: but only if the
713: .Ar command
714: option has been specified
715: (which may be useful for taking remote backups even if root login is
1.17 jmc 716: normally not allowed).
717: All other authentication methods are disabled for root.
1.1 stevesk 718: .Pp
719: If this option is set to
1.52 jmc 720: .Dq no ,
1.38 jmc 721: root is not allowed to log in.
1.46 reyk 722: .It Cm PermitTunnel
723: Specifies whether
724: .Xr tun 4
725: device forwarding is allowed.
1.47 reyk 726: The argument must be
727: .Dq yes ,
1.58 stevesk 728: .Dq point-to-point
729: (layer 3),
730: .Dq ethernet
731: (layer 2), or
1.47 reyk 732: .Dq no .
1.58 stevesk 733: Specifying
734: .Dq yes
735: permits both
736: .Dq point-to-point
737: and
738: .Dq ethernet .
1.46 reyk 739: The default is
740: .Dq no .
1.6 markus 741: .It Cm PermitUserEnvironment
742: Specifies whether
743: .Pa ~/.ssh/environment
1.9 stevesk 744: and
1.6 markus 745: .Cm environment=
746: options in
747: .Pa ~/.ssh/authorized_keys
1.9 stevesk 748: are processed by
1.52 jmc 749: .Xr sshd 8 .
1.6 markus 750: The default is
751: .Dq no .
1.9 stevesk 752: Enabling environment processing may enable users to bypass access
753: restrictions in some configurations using mechanisms such as
754: .Ev LD_PRELOAD .
1.1 stevesk 755: .It Cm PidFile
1.4 stevesk 756: Specifies the file that contains the process ID of the
1.53 jmc 757: SSH daemon.
1.1 stevesk 758: The default is
759: .Pa /var/run/sshd.pid .
760: .It Cm Port
761: Specifies the port number that
1.52 jmc 762: .Xr sshd 8
1.1 stevesk 763: listens on.
764: The default is 22.
765: Multiple options of this type are permitted.
766: See also
767: .Cm ListenAddress .
768: .It Cm PrintLastLog
769: Specifies whether
1.52 jmc 770: .Xr sshd 8
1.36 jaredy 771: should print the date and time of the last user login when a user logs
772: in interactively.
1.1 stevesk 773: The default is
774: .Dq yes .
775: .It Cm PrintMotd
776: Specifies whether
1.52 jmc 777: .Xr sshd 8
1.1 stevesk 778: should print
779: .Pa /etc/motd
780: when a user logs in interactively.
781: (On some systems it is also printed by the shell,
782: .Pa /etc/profile ,
783: or equivalent.)
784: The default is
785: .Dq yes .
786: .It Cm Protocol
787: Specifies the protocol versions
1.52 jmc 788: .Xr sshd 8
1.5 stevesk 789: supports.
1.1 stevesk 790: The possible values are
1.52 jmc 791: .Sq 1
1.1 stevesk 792: and
1.52 jmc 793: .Sq 2 .
1.1 stevesk 794: Multiple versions must be comma-separated.
795: The default is
796: .Dq 2,1 .
1.5 stevesk 797: Note that the order of the protocol list does not indicate preference,
798: because the client selects among multiple protocol versions offered
799: by the server.
800: Specifying
801: .Dq 2,1
802: is identical to
803: .Dq 1,2 .
1.1 stevesk 804: .It Cm PubkeyAuthentication
805: Specifies whether public key authentication is allowed.
806: The default is
807: .Dq yes .
808: Note that this option applies to protocol version 2 only.
809: .It Cm RhostsRSAAuthentication
810: Specifies whether rhosts or /etc/hosts.equiv authentication together
811: with successful RSA host authentication is allowed.
812: The default is
813: .Dq no .
814: This option applies to protocol version 1 only.
815: .It Cm RSAAuthentication
816: Specifies whether pure RSA authentication is allowed.
817: The default is
818: .Dq yes .
819: This option applies to protocol version 1 only.
820: .It Cm ServerKeyBits
821: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 822: The minimum value is 512, and the default is 1024.
1.1 stevesk 823: .It Cm StrictModes
824: Specifies whether
1.52 jmc 825: .Xr sshd 8
1.1 stevesk 826: should check file modes and ownership of the
827: user's files and home directory before accepting login.
828: This is normally desirable because novices sometimes accidentally leave their
829: directory or files world-writable.
830: The default is
831: .Dq yes .
832: .It Cm Subsystem
1.51 jmc 833: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 834: Arguments should be a subsystem name and a command (with optional arguments)
835: to execute upon subsystem request.
1.80 djm 836: .Pp
1.1 stevesk 837: The command
838: .Xr sftp-server 8
839: implements the
840: .Dq sftp
841: file transfer subsystem.
1.80 djm 842: .Pp
843: Alternately the name
844: .Dq internal-sftp
845: implements an in-process
846: .Dq sftp
847: server.
848: This may simplify configurations using
849: .Cm ChrootDirectory
850: to force a different filesystem root on clients.
851: .Pp
1.1 stevesk 852: By default no subsystems are defined.
853: Note that this option applies to protocol version 2 only.
854: .It Cm SyslogFacility
855: Gives the facility code that is used when logging messages from
1.53 jmc 856: .Xr sshd 8 .
1.1 stevesk 857: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
858: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
859: The default is AUTH.
1.27 markus 860: .It Cm TCPKeepAlive
861: Specifies whether the system should send TCP keepalive messages to the
862: other side.
863: If they are sent, death of the connection or crash of one
864: of the machines will be properly noticed.
865: However, this means that
866: connections will die if the route is down temporarily, and some people
867: find it annoying.
868: On the other hand, if TCP keepalives are not sent,
869: sessions may hang indefinitely on the server, leaving
870: .Dq ghost
871: users and consuming server resources.
872: .Pp
873: The default is
874: .Dq yes
875: (to send TCP keepalive messages), and the server will notice
876: if the network goes down or the client host crashes.
877: This avoids infinitely hanging sessions.
878: .Pp
879: To disable TCP keepalive messages, the value should be set to
880: .Dq no .
1.18 markus 881: .It Cm UseDNS
882: Specifies whether
1.52 jmc 883: .Xr sshd 8
1.40 jmc 884: should look up the remote host name and check that
1.18 markus 885: the resolved host name for the remote IP address maps back to the
886: very same IP address.
887: The default is
888: .Dq yes .
1.1 stevesk 889: .It Cm UseLogin
890: Specifies whether
891: .Xr login 1
892: is used for interactive login sessions.
893: The default is
894: .Dq no .
895: Note that
896: .Xr login 1
897: is never used for remote command execution.
898: Note also, that if this is enabled,
899: .Cm X11Forwarding
900: will be disabled because
901: .Xr login 1
902: does not know how to handle
903: .Xr xauth 1
1.15 jmc 904: cookies.
905: If
1.1 stevesk 906: .Cm UsePrivilegeSeparation
907: is specified, it will be disabled after authentication.
908: .It Cm UsePrivilegeSeparation
909: Specifies whether
1.52 jmc 910: .Xr sshd 8
1.2 stevesk 911: separates privileges by creating an unprivileged child process
1.15 jmc 912: to deal with incoming network traffic.
913: After successful authentication, another process will be created that has
914: the privilege of the authenticated user.
915: The goal of privilege separation is to prevent privilege
1.1 stevesk 916: escalation by containing any corruption within the unprivileged processes.
917: The default is
918: .Dq yes .
919: .It Cm X11DisplayOffset
920: Specifies the first display number available for
1.52 jmc 921: .Xr sshd 8 Ns 's
1.1 stevesk 922: X11 forwarding.
1.52 jmc 923: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 924: The default is 10.
925: .It Cm X11Forwarding
926: Specifies whether X11 forwarding is permitted.
1.13 stevesk 927: The argument must be
928: .Dq yes
929: or
930: .Dq no .
1.1 stevesk 931: The default is
932: .Dq no .
1.13 stevesk 933: .Pp
934: When X11 forwarding is enabled, there may be additional exposure to
935: the server and to client displays if the
1.52 jmc 936: .Xr sshd 8
1.13 stevesk 937: proxy display is configured to listen on the wildcard address (see
938: .Cm X11UseLocalhost
1.52 jmc 939: below), though this is not the default.
1.13 stevesk 940: Additionally, the authentication spoofing and authentication data
941: verification and substitution occur on the client side.
942: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 943: display server may be exposed to attack when the SSH client requests
1.13 stevesk 944: forwarding (see the warnings for
945: .Cm ForwardX11
946: in
1.19 jmc 947: .Xr ssh_config 5 ) .
1.13 stevesk 948: A system administrator may have a stance in which they want to
949: protect clients that may expose themselves to attack by unwittingly
950: requesting X11 forwarding, which can warrant a
951: .Dq no
952: setting.
953: .Pp
954: Note that disabling X11 forwarding does not prevent users from
955: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 956: X11 forwarding is automatically disabled if
957: .Cm UseLogin
958: is enabled.
959: .It Cm X11UseLocalhost
960: Specifies whether
1.52 jmc 961: .Xr sshd 8
1.1 stevesk 962: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 963: the wildcard address.
964: By default,
1.52 jmc 965: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 966: hostname part of the
967: .Ev DISPLAY
968: environment variable to
969: .Dq localhost .
1.8 stevesk 970: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 971: However, some older X11 clients may not function with this
972: configuration.
973: .Cm X11UseLocalhost
974: may be set to
975: .Dq no
976: to specify that the forwarding server should be bound to the wildcard
977: address.
978: The argument must be
979: .Dq yes
980: or
981: .Dq no .
982: The default is
983: .Dq yes .
984: .It Cm XAuthLocation
1.11 stevesk 985: Specifies the full pathname of the
1.1 stevesk 986: .Xr xauth 1
987: program.
988: The default is
989: .Pa /usr/X11R6/bin/xauth .
990: .El
1.55 jmc 991: .Sh TIME FORMATS
1.53 jmc 992: .Xr sshd 8
1.1 stevesk 993: command-line arguments and configuration file options that specify time
994: may be expressed using a sequence of the form:
995: .Sm off
1.7 stevesk 996: .Ar time Op Ar qualifier ,
1.1 stevesk 997: .Sm on
998: where
999: .Ar time
1000: is a positive integer value and
1001: .Ar qualifier
1002: is one of the following:
1003: .Pp
1004: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1005: .It Aq Cm none
1.1 stevesk 1006: seconds
1007: .It Cm s | Cm S
1008: seconds
1009: .It Cm m | Cm M
1010: minutes
1011: .It Cm h | Cm H
1012: hours
1013: .It Cm d | Cm D
1014: days
1015: .It Cm w | Cm W
1016: weeks
1017: .El
1018: .Pp
1019: Each member of the sequence is added together to calculate
1020: the total time value.
1021: .Pp
1022: Time format examples:
1023: .Pp
1024: .Bl -tag -width Ds -compact -offset indent
1025: .It 600
1026: 600 seconds (10 minutes)
1027: .It 10m
1028: 10 minutes
1029: .It 1h30m
1030: 1 hour 30 minutes (90 minutes)
1031: .El
1032: .Sh FILES
1033: .Bl -tag -width Ds
1034: .It Pa /etc/ssh/sshd_config
1035: Contains configuration data for
1.53 jmc 1036: .Xr sshd 8 .
1.1 stevesk 1037: This file should be writable by root only, but it is recommended
1038: (though not necessary) that it be world-readable.
1039: .El
1.19 jmc 1040: .Sh SEE ALSO
1041: .Xr sshd 8
1.1 stevesk 1042: .Sh AUTHORS
1043: OpenSSH is a derivative of the original and free
1044: ssh 1.2.12 release by Tatu Ylonen.
1045: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1046: Theo de Raadt and Dug Song
1047: removed many bugs, re-added newer features and
1048: created OpenSSH.
1049: Markus Friedl contributed the support for SSH
1050: protocol versions 1.5 and 2.0.
1051: Niels Provos and Markus Friedl contributed support
1052: for privilege separation.
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