Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.105
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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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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1.105 ! jmc 37: .\" $OpenBSD: sshd_config.5,v 1.104 2009/04/17 19:40:17 stevesk Exp $
! 38: .Dd $Mdocdate: April 17 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.
191: .Pp
192: The path may contain the following tokens that are expanded at runtime once
193: the connecting user has been authenticated: %% is replaced by a literal '%',
194: %h is replaced by the home directory of the user being authenticated, and
195: %u is replaced by the username of that user.
196: .Pp
197: The
198: .Cm ChrootDirectory
199: must contain the necessary files and directories to support the
1.103 stevesk 200: user's session.
1.80 djm 201: For an interactive session this requires at least a shell, typically
202: .Xr sh 1 ,
203: and basic
204: .Pa /dev
205: nodes such as
206: .Xr null 4 ,
207: .Xr zero 4 ,
208: .Xr stdin 4 ,
209: .Xr stdout 4 ,
210: .Xr stderr 4 ,
211: .Xr arandom 4
212: and
213: .Xr tty 4
214: devices.
215: For file transfer sessions using
1.105 ! jmc 216: .Dq sftp ,
1.80 djm 217: no additional configuration of the environment is necessary if the
1.105 ! jmc 218: in-process sftp server is used,
! 219: though sessions which use logging do require
1.104 stevesk 220: .Pa /dev/log
221: inside the chroot directory (see
222: .Xr sftp-server 8
1.81 jmc 223: for details).
1.80 djm 224: .Pp
225: The default is not to
226: .Xr chroot 2 .
1.1 stevesk 227: .It Cm Ciphers
228: Specifies the ciphers allowed for protocol version 2.
229: Multiple ciphers must be comma-separated.
1.34 dtucker 230: The supported ciphers are
231: .Dq 3des-cbc ,
232: .Dq aes128-cbc ,
233: .Dq aes192-cbc ,
234: .Dq aes256-cbc ,
235: .Dq aes128-ctr ,
236: .Dq aes192-ctr ,
237: .Dq aes256-ctr ,
1.43 djm 238: .Dq arcfour128 ,
239: .Dq arcfour256 ,
1.34 dtucker 240: .Dq arcfour ,
241: .Dq blowfish-cbc ,
242: and
243: .Dq cast128-cbc .
1.52 jmc 244: The default is:
245: .Bd -literal -offset 3n
1.100 naddy 246: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
247: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
248: aes256-cbc,arcfour
1.1 stevesk 249: .Ed
250: .It Cm ClientAliveCountMax
1.48 jmc 251: Sets the number of client alive messages (see below) which may be
1.1 stevesk 252: sent without
1.52 jmc 253: .Xr sshd 8
1.17 jmc 254: receiving any messages back from the client.
255: If this threshold is reached while client alive messages are being sent,
1.52 jmc 256: sshd will disconnect the client, terminating the session.
1.17 jmc 257: It is important to note that the use of client alive messages is very
258: different from
1.27 markus 259: .Cm TCPKeepAlive
1.17 jmc 260: (below).
261: The client alive messages are sent through the encrypted channel
262: and therefore will not be spoofable.
263: The TCP keepalive option enabled by
1.27 markus 264: .Cm TCPKeepAlive
1.17 jmc 265: is spoofable.
266: The client alive mechanism is valuable when the client or
1.1 stevesk 267: server depend on knowing when a connection has become inactive.
268: .Pp
1.17 jmc 269: The default value is 3.
270: If
1.1 stevesk 271: .Cm ClientAliveInterval
1.48 jmc 272: (see below) is set to 15, and
1.1 stevesk 273: .Cm ClientAliveCountMax
1.52 jmc 274: is left at the default, unresponsive SSH clients
1.1 stevesk 275: will be disconnected after approximately 45 seconds.
1.57 markus 276: This option applies to protocol version 2 only.
1.42 djm 277: .It Cm ClientAliveInterval
278: Sets a timeout interval in seconds after which if no data has been received
279: from the client,
1.52 jmc 280: .Xr sshd 8
1.42 djm 281: will send a message through the encrypted
282: channel to request a response from the client.
283: The default
284: is 0, indicating that these messages will not be sent to the client.
285: This option applies to protocol version 2 only.
1.3 markus 286: .It Cm Compression
1.44 markus 287: Specifies whether compression is allowed, or delayed until
288: the user has authenticated successfully.
1.3 markus 289: The argument must be
1.44 markus 290: .Dq yes ,
291: .Dq delayed ,
1.3 markus 292: or
293: .Dq no .
294: The default is
1.44 markus 295: .Dq delayed .
1.1 stevesk 296: .It Cm DenyGroups
297: This keyword can be followed by a list of group name patterns, separated
298: by spaces.
299: Login is disallowed for users whose primary group or supplementary
300: group list matches one of the patterns.
301: Only group names are valid; a numerical group ID is not recognized.
302: By default, login is allowed for all groups.
1.54 jmc 303: The allow/deny directives are processed in the following order:
304: .Cm DenyUsers ,
305: .Cm AllowUsers ,
306: .Cm DenyGroups ,
307: and finally
308: .Cm AllowGroups .
1.49 jmc 309: .Pp
310: See
311: .Sx PATTERNS
312: in
313: .Xr ssh_config 5
314: for more information on patterns.
1.1 stevesk 315: .It Cm DenyUsers
316: This keyword can be followed by a list of user name patterns, separated
317: by spaces.
318: Login is disallowed for user names that match one of the patterns.
319: Only user names are valid; a numerical user ID is not recognized.
320: By default, login is allowed for all users.
321: If the pattern takes the form USER@HOST then USER and HOST
322: are separately checked, restricting logins to particular
323: users from particular hosts.
1.54 jmc 324: The allow/deny directives are processed in the following order:
325: .Cm DenyUsers ,
326: .Cm AllowUsers ,
327: .Cm DenyGroups ,
328: and finally
329: .Cm AllowGroups .
1.49 jmc 330: .Pp
331: See
332: .Sx PATTERNS
333: in
334: .Xr ssh_config 5
335: for more information on patterns.
1.67 dtucker 336: .It Cm ForceCommand
337: Forces the execution of the command specified by
338: .Cm ForceCommand ,
1.84 djm 339: ignoring any command supplied by the client and
340: .Pa ~/.ssh/rc
341: if present.
1.67 dtucker 342: The command is invoked by using the user's login shell with the -c option.
343: This applies to shell, command, or subsystem execution.
344: It is most useful inside a
345: .Cm Match
346: block.
347: The command originally supplied by the client is available in the
348: .Ev SSH_ORIGINAL_COMMAND
349: environment variable.
1.82 djm 350: Specifying a command of
351: .Dq internal-sftp
352: will force the use of an in-process sftp server that requires no support
353: files when used with
354: .Cm ChrootDirectory .
1.1 stevesk 355: .It Cm GatewayPorts
356: Specifies whether remote hosts are allowed to connect to ports
357: forwarded for the client.
358: By default,
1.52 jmc 359: .Xr sshd 8
1.15 jmc 360: binds remote port forwardings to the loopback address.
361: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 362: .Cm GatewayPorts
1.52 jmc 363: can be used to specify that sshd
1.39 djm 364: should allow remote port forwardings to bind to non-loopback addresses, thus
365: allowing other hosts to connect.
366: The argument may be
367: .Dq no
368: to force remote port forwardings to be available to the local host only,
1.1 stevesk 369: .Dq yes
1.39 djm 370: to force remote port forwardings to bind to the wildcard address, or
371: .Dq clientspecified
372: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 373: The default is
374: .Dq no .
1.23 markus 375: .It Cm GSSAPIAuthentication
1.25 markus 376: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 377: The default is
1.23 markus 378: .Dq no .
379: Note that this option applies to protocol version 2 only.
380: .It Cm GSSAPICleanupCredentials
381: Specifies whether to automatically destroy the user's credentials cache
382: on logout.
383: The default is
384: .Dq yes .
385: Note that this option applies to protocol version 2 only.
1.1 stevesk 386: .It Cm HostbasedAuthentication
387: Specifies whether rhosts or /etc/hosts.equiv authentication together
388: with successful public key client host authentication is allowed
1.50 jmc 389: (host-based authentication).
1.1 stevesk 390: This option is similar to
391: .Cm RhostsRSAAuthentication
392: and applies to protocol version 2 only.
1.70 dtucker 393: The default is
394: .Dq no .
395: .It Cm HostbasedUsesNameFromPacketOnly
396: Specifies whether or not the server will attempt to perform a reverse
397: name lookup when matching the name in the
398: .Pa ~/.shosts ,
399: .Pa ~/.rhosts ,
400: and
401: .Pa /etc/hosts.equiv
402: files during
403: .Cm HostbasedAuthentication .
404: A setting of
405: .Dq yes
406: means that
407: .Xr sshd 8
408: uses the name supplied by the client rather than
409: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 410: The default is
411: .Dq no .
412: .It Cm HostKey
413: Specifies a file containing a private host key
414: used by SSH.
415: The default is
416: .Pa /etc/ssh/ssh_host_key
417: for protocol version 1, and
418: .Pa /etc/ssh/ssh_host_rsa_key
419: and
420: .Pa /etc/ssh/ssh_host_dsa_key
421: for protocol version 2.
422: Note that
1.52 jmc 423: .Xr sshd 8
1.1 stevesk 424: will refuse to use a file if it is group/world-accessible.
425: It is possible to have multiple host key files.
426: .Dq rsa1
427: keys are used for version 1 and
428: .Dq dsa
429: or
430: .Dq rsa
431: are used for version 2 of the SSH protocol.
432: .It Cm IgnoreRhosts
433: Specifies that
434: .Pa .rhosts
435: and
436: .Pa .shosts
437: files will not be used in
438: .Cm RhostsRSAAuthentication
439: or
440: .Cm HostbasedAuthentication .
441: .Pp
442: .Pa /etc/hosts.equiv
443: and
444: .Pa /etc/shosts.equiv
445: are still used.
446: The default is
447: .Dq yes .
448: .It Cm IgnoreUserKnownHosts
449: Specifies whether
1.52 jmc 450: .Xr sshd 8
1.1 stevesk 451: should ignore the user's
1.41 djm 452: .Pa ~/.ssh/known_hosts
1.1 stevesk 453: during
454: .Cm RhostsRSAAuthentication
455: or
456: .Cm HostbasedAuthentication .
457: The default is
458: .Dq no .
459: .It Cm KerberosAuthentication
1.24 markus 460: Specifies whether the password provided by the user for
1.1 stevesk 461: .Cm PasswordAuthentication
1.24 markus 462: will be validated through the Kerberos KDC.
1.1 stevesk 463: To use this option, the server needs a
464: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 465: The default is
1.29 dtucker 466: .Dq no .
467: .It Cm KerberosGetAFSToken
1.45 djm 468: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 469: an AFS token before accessing the user's home directory.
1.52 jmc 470: The default is
1.1 stevesk 471: .Dq no .
472: .It Cm KerberosOrLocalPasswd
1.52 jmc 473: If password authentication through Kerberos fails then
1.1 stevesk 474: the password will be validated via any additional local mechanism
475: such as
476: .Pa /etc/passwd .
1.52 jmc 477: The default is
1.1 stevesk 478: .Dq yes .
479: .It Cm KerberosTicketCleanup
480: Specifies whether to automatically destroy the user's ticket cache
481: file on logout.
1.52 jmc 482: The default is
1.1 stevesk 483: .Dq yes .
484: .It Cm KeyRegenerationInterval
485: In protocol version 1, the ephemeral server key is automatically regenerated
486: after this many seconds (if it has been used).
487: The purpose of regeneration is to prevent
488: decrypting captured sessions by later breaking into the machine and
489: stealing the keys.
490: The key is never stored anywhere.
491: If the value is 0, the key is never regenerated.
492: The default is 3600 (seconds).
493: .It Cm ListenAddress
494: Specifies the local addresses
1.52 jmc 495: .Xr sshd 8
1.1 stevesk 496: should listen on.
497: The following forms may be used:
498: .Pp
499: .Bl -item -offset indent -compact
500: .It
501: .Cm ListenAddress
502: .Sm off
503: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
504: .Sm on
505: .It
506: .Cm ListenAddress
507: .Sm off
508: .Ar host No | Ar IPv4_addr No : Ar port
509: .Sm on
510: .It
511: .Cm ListenAddress
512: .Sm off
513: .Oo
514: .Ar host No | Ar IPv6_addr Oc : Ar port
515: .Sm on
516: .El
517: .Pp
518: If
519: .Ar port
520: is not specified,
1.52 jmc 521: sshd will listen on the address and all prior
1.1 stevesk 522: .Cm Port
1.17 jmc 523: options specified.
524: The default is to listen on all local addresses.
1.15 jmc 525: Multiple
1.1 stevesk 526: .Cm ListenAddress
1.17 jmc 527: options are permitted.
528: Additionally, any
1.1 stevesk 529: .Cm Port
1.52 jmc 530: options must precede this option for non-port qualified addresses.
1.1 stevesk 531: .It Cm LoginGraceTime
532: The server disconnects after this time if the user has not
533: successfully logged in.
534: If the value is 0, there is no time limit.
1.12 stevesk 535: The default is 120 seconds.
1.1 stevesk 536: .It Cm LogLevel
537: Gives the verbosity level that is used when logging messages from
1.53 jmc 538: .Xr sshd 8 .
1.1 stevesk 539: The possible values are:
1.52 jmc 540: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 541: The default is INFO.
542: DEBUG and DEBUG1 are equivalent.
543: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
544: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 545: .It Cm MACs
546: Specifies the available MAC (message authentication code) algorithms.
547: The MAC algorithm is used in protocol version 2
548: for data integrity protection.
549: Multiple algorithms must be comma-separated.
1.52 jmc 550: The default is:
1.77 jmc 551: .Bd -literal -offset indent
552: hmac-md5,hmac-sha1,umac-64@openssh.com,
553: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
554: .Ed
1.60 dtucker 555: .It Cm Match
1.61 jmc 556: Introduces a conditional block.
1.65 dtucker 557: If all of the criteria on the
1.60 dtucker 558: .Cm Match
1.65 dtucker 559: line are satisfied, the keywords on the following lines override those
560: set in the global section of the config file, until either another
1.60 dtucker 561: .Cm Match
1.65 dtucker 562: line or the end of the file.
1.91 djm 563: .Pp
1.61 jmc 564: The arguments to
1.60 dtucker 565: .Cm Match
1.65 dtucker 566: are one or more criteria-pattern pairs.
1.60 dtucker 567: The available criteria are
568: .Cm User ,
1.69 dtucker 569: .Cm Group ,
1.60 dtucker 570: .Cm Host ,
571: and
572: .Cm Address .
1.91 djm 573: The match patterns may consist of single entries or comma-separated
574: lists and may use the wildcard and negation operators described in the
1.92 djm 575: .Sx PATTERNS
1.91 djm 576: section of
1.92 djm 577: .Xr ssh_config 5 .
1.91 djm 578: .Pp
579: The patterns in an
580: .Cm Address
581: criteria may additionally contain addresses to match in CIDR
1.93 jmc 582: address/masklen format, e.g.\&
1.91 djm 583: .Dq 192.0.2.0/24
584: or
585: .Dq 3ffe:ffff::/32 .
586: Note that the mask length provided must be consistent with the address -
587: it is an error to specify a mask length that is too long for the address
1.93 jmc 588: or one with bits set in this host portion of the address.
589: For example,
1.91 djm 590: .Dq 192.0.2.0/33
591: and
1.93 jmc 592: .Dq 192.0.2.0/8
1.91 djm 593: respectively.
594: .Pp
1.60 dtucker 595: Only a subset of keywords may be used on the lines following a
596: .Cm Match
597: keyword.
598: Available keywords are
1.99 okan 599: .Cm AllowAgentForwarding ,
1.62 dtucker 600: .Cm AllowTcpForwarding ,
1.72 dtucker 601: .Cm Banner ,
1.85 djm 602: .Cm ChrootDirectory ,
1.67 dtucker 603: .Cm ForceCommand ,
1.62 dtucker 604: .Cm GatewayPorts ,
1.87 djm 605: .Cm GSSAPIAuthentication ,
606: .Cm HostbasedAuthentication ,
1.74 jmc 607: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 608: .Cm KerberosAuthentication ,
1.95 dtucker 609: .Cm MaxAuthTries ,
1.94 dtucker 610: .Cm MaxSessions ,
1.72 dtucker 611: .Cm PasswordAuthentication ,
1.97 djm 612: .Cm PermitEmptyPasswords ,
1.66 dtucker 613: .Cm PermitOpen ,
1.79 dtucker 614: .Cm PermitRootLogin ,
1.72 dtucker 615: .Cm RhostsRSAAuthentication ,
616: .Cm RSAAuthentication ,
1.66 dtucker 617: .Cm X11DisplayOffset ,
1.101 djm 618: .Cm X11Forwarding
1.60 dtucker 619: and
1.102 djm 620: .Cm X11UseLocalHost .
1.33 dtucker 621: .It Cm MaxAuthTries
622: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 623: connection.
624: Once the number of failures reaches half this value,
625: additional failures are logged.
626: The default is 6.
1.90 djm 627: .It Cm MaxSessions
628: Specifies the maximum number of open sessions permitted per network connection.
629: The default is 10.
1.1 stevesk 630: .It Cm MaxStartups
631: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 632: SSH daemon.
1.1 stevesk 633: Additional connections will be dropped until authentication succeeds or the
634: .Cm LoginGraceTime
635: expires for a connection.
636: The default is 10.
637: .Pp
638: Alternatively, random early drop can be enabled by specifying
639: the three colon separated values
640: .Dq start:rate:full
1.51 jmc 641: (e.g. "10:30:60").
1.53 jmc 642: .Xr sshd 8
1.1 stevesk 643: will refuse connection attempts with a probability of
644: .Dq rate/100
645: (30%)
646: if there are currently
647: .Dq start
648: (10)
649: unauthenticated connections.
650: The probability increases linearly and all connection attempts
651: are refused if the number of unauthenticated connections reaches
652: .Dq full
653: (60).
654: .It Cm PasswordAuthentication
655: Specifies whether password authentication is allowed.
656: The default is
657: .Dq yes .
658: .It Cm PermitEmptyPasswords
659: When password authentication is allowed, it specifies whether the
660: server allows login to accounts with empty password strings.
661: The default is
662: .Dq no .
1.62 dtucker 663: .It Cm PermitOpen
664: Specifies the destinations to which TCP port forwarding is permitted.
665: The forwarding specification must be one of the following forms:
666: .Pp
667: .Bl -item -offset indent -compact
668: .It
669: .Cm PermitOpen
670: .Sm off
671: .Ar host : port
672: .Sm on
673: .It
674: .Cm PermitOpen
675: .Sm off
676: .Ar IPv4_addr : port
677: .Sm on
678: .It
679: .Cm PermitOpen
680: .Sm off
681: .Ar \&[ IPv6_addr \&] : port
682: .Sm on
683: .El
684: .Pp
1.68 dtucker 685: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 686: An argument of
687: .Dq any
688: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 689: By default all port forwarding requests are permitted.
1.1 stevesk 690: .It Cm PermitRootLogin
1.38 jmc 691: Specifies whether root can log in using
1.1 stevesk 692: .Xr ssh 1 .
693: The argument must be
694: .Dq yes ,
695: .Dq without-password ,
1.52 jmc 696: .Dq forced-commands-only ,
1.1 stevesk 697: or
698: .Dq no .
699: The default is
700: .Dq yes .
701: .Pp
702: If this option is set to
1.52 jmc 703: .Dq without-password ,
1.1 stevesk 704: password authentication is disabled for root.
705: .Pp
706: If this option is set to
1.52 jmc 707: .Dq forced-commands-only ,
1.1 stevesk 708: root login with public key authentication will be allowed,
709: but only if the
710: .Ar command
711: option has been specified
712: (which may be useful for taking remote backups even if root login is
1.17 jmc 713: normally not allowed).
714: All other authentication methods are disabled for root.
1.1 stevesk 715: .Pp
716: If this option is set to
1.52 jmc 717: .Dq no ,
1.38 jmc 718: root is not allowed to log in.
1.46 reyk 719: .It Cm PermitTunnel
720: Specifies whether
721: .Xr tun 4
722: device forwarding is allowed.
1.47 reyk 723: The argument must be
724: .Dq yes ,
1.58 stevesk 725: .Dq point-to-point
726: (layer 3),
727: .Dq ethernet
728: (layer 2), or
1.47 reyk 729: .Dq no .
1.58 stevesk 730: Specifying
731: .Dq yes
732: permits both
733: .Dq point-to-point
734: and
735: .Dq ethernet .
1.46 reyk 736: The default is
737: .Dq no .
1.6 markus 738: .It Cm PermitUserEnvironment
739: Specifies whether
740: .Pa ~/.ssh/environment
1.9 stevesk 741: and
1.6 markus 742: .Cm environment=
743: options in
744: .Pa ~/.ssh/authorized_keys
1.9 stevesk 745: are processed by
1.52 jmc 746: .Xr sshd 8 .
1.6 markus 747: The default is
748: .Dq no .
1.9 stevesk 749: Enabling environment processing may enable users to bypass access
750: restrictions in some configurations using mechanisms such as
751: .Ev LD_PRELOAD .
1.1 stevesk 752: .It Cm PidFile
1.4 stevesk 753: Specifies the file that contains the process ID of the
1.53 jmc 754: SSH daemon.
1.1 stevesk 755: The default is
756: .Pa /var/run/sshd.pid .
757: .It Cm Port
758: Specifies the port number that
1.52 jmc 759: .Xr sshd 8
1.1 stevesk 760: listens on.
761: The default is 22.
762: Multiple options of this type are permitted.
763: See also
764: .Cm ListenAddress .
765: .It Cm PrintLastLog
766: Specifies whether
1.52 jmc 767: .Xr sshd 8
1.36 jaredy 768: should print the date and time of the last user login when a user logs
769: in interactively.
1.1 stevesk 770: The default is
771: .Dq yes .
772: .It Cm PrintMotd
773: Specifies whether
1.52 jmc 774: .Xr sshd 8
1.1 stevesk 775: should print
776: .Pa /etc/motd
777: when a user logs in interactively.
778: (On some systems it is also printed by the shell,
779: .Pa /etc/profile ,
780: or equivalent.)
781: The default is
782: .Dq yes .
783: .It Cm Protocol
784: Specifies the protocol versions
1.52 jmc 785: .Xr sshd 8
1.5 stevesk 786: supports.
1.1 stevesk 787: The possible values are
1.52 jmc 788: .Sq 1
1.1 stevesk 789: and
1.52 jmc 790: .Sq 2 .
1.1 stevesk 791: Multiple versions must be comma-separated.
792: The default is
793: .Dq 2,1 .
1.5 stevesk 794: Note that the order of the protocol list does not indicate preference,
795: because the client selects among multiple protocol versions offered
796: by the server.
797: Specifying
798: .Dq 2,1
799: is identical to
800: .Dq 1,2 .
1.1 stevesk 801: .It Cm PubkeyAuthentication
802: Specifies whether public key authentication is allowed.
803: The default is
804: .Dq yes .
805: Note that this option applies to protocol version 2 only.
806: .It Cm RhostsRSAAuthentication
807: Specifies whether rhosts or /etc/hosts.equiv authentication together
808: with successful RSA host authentication is allowed.
809: The default is
810: .Dq no .
811: This option applies to protocol version 1 only.
812: .It Cm RSAAuthentication
813: Specifies whether pure RSA authentication is allowed.
814: The default is
815: .Dq yes .
816: This option applies to protocol version 1 only.
817: .It Cm ServerKeyBits
818: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 819: The minimum value is 512, and the default is 1024.
1.1 stevesk 820: .It Cm StrictModes
821: Specifies whether
1.52 jmc 822: .Xr sshd 8
1.1 stevesk 823: should check file modes and ownership of the
824: user's files and home directory before accepting login.
825: This is normally desirable because novices sometimes accidentally leave their
826: directory or files world-writable.
827: The default is
828: .Dq yes .
829: .It Cm Subsystem
1.51 jmc 830: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 831: Arguments should be a subsystem name and a command (with optional arguments)
832: to execute upon subsystem request.
1.80 djm 833: .Pp
1.1 stevesk 834: The command
835: .Xr sftp-server 8
836: implements the
837: .Dq sftp
838: file transfer subsystem.
1.80 djm 839: .Pp
840: Alternately the name
841: .Dq internal-sftp
842: implements an in-process
843: .Dq sftp
844: server.
845: This may simplify configurations using
846: .Cm ChrootDirectory
847: to force a different filesystem root on clients.
848: .Pp
1.1 stevesk 849: By default no subsystems are defined.
850: Note that this option applies to protocol version 2 only.
851: .It Cm SyslogFacility
852: Gives the facility code that is used when logging messages from
1.53 jmc 853: .Xr sshd 8 .
1.1 stevesk 854: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
855: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
856: The default is AUTH.
1.27 markus 857: .It Cm TCPKeepAlive
858: Specifies whether the system should send TCP keepalive messages to the
859: other side.
860: If they are sent, death of the connection or crash of one
861: of the machines will be properly noticed.
862: However, this means that
863: connections will die if the route is down temporarily, and some people
864: find it annoying.
865: On the other hand, if TCP keepalives are not sent,
866: sessions may hang indefinitely on the server, leaving
867: .Dq ghost
868: users and consuming server resources.
869: .Pp
870: The default is
871: .Dq yes
872: (to send TCP keepalive messages), and the server will notice
873: if the network goes down or the client host crashes.
874: This avoids infinitely hanging sessions.
875: .Pp
876: To disable TCP keepalive messages, the value should be set to
877: .Dq no .
1.18 markus 878: .It Cm UseDNS
879: Specifies whether
1.52 jmc 880: .Xr sshd 8
1.40 jmc 881: should look up the remote host name and check that
1.18 markus 882: the resolved host name for the remote IP address maps back to the
883: very same IP address.
884: The default is
885: .Dq yes .
1.1 stevesk 886: .It Cm UseLogin
887: Specifies whether
888: .Xr login 1
889: is used for interactive login sessions.
890: The default is
891: .Dq no .
892: Note that
893: .Xr login 1
894: is never used for remote command execution.
895: Note also, that if this is enabled,
896: .Cm X11Forwarding
897: will be disabled because
898: .Xr login 1
899: does not know how to handle
900: .Xr xauth 1
1.15 jmc 901: cookies.
902: If
1.1 stevesk 903: .Cm UsePrivilegeSeparation
904: is specified, it will be disabled after authentication.
905: .It Cm UsePrivilegeSeparation
906: Specifies whether
1.52 jmc 907: .Xr sshd 8
1.2 stevesk 908: separates privileges by creating an unprivileged child process
1.15 jmc 909: to deal with incoming network traffic.
910: After successful authentication, another process will be created that has
911: the privilege of the authenticated user.
912: The goal of privilege separation is to prevent privilege
1.1 stevesk 913: escalation by containing any corruption within the unprivileged processes.
914: The default is
915: .Dq yes .
916: .It Cm X11DisplayOffset
917: Specifies the first display number available for
1.52 jmc 918: .Xr sshd 8 Ns 's
1.1 stevesk 919: X11 forwarding.
1.52 jmc 920: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 921: The default is 10.
922: .It Cm X11Forwarding
923: Specifies whether X11 forwarding is permitted.
1.13 stevesk 924: The argument must be
925: .Dq yes
926: or
927: .Dq no .
1.1 stevesk 928: The default is
929: .Dq no .
1.13 stevesk 930: .Pp
931: When X11 forwarding is enabled, there may be additional exposure to
932: the server and to client displays if the
1.52 jmc 933: .Xr sshd 8
1.13 stevesk 934: proxy display is configured to listen on the wildcard address (see
935: .Cm X11UseLocalhost
1.52 jmc 936: below), though this is not the default.
1.13 stevesk 937: Additionally, the authentication spoofing and authentication data
938: verification and substitution occur on the client side.
939: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 940: display server may be exposed to attack when the SSH client requests
1.13 stevesk 941: forwarding (see the warnings for
942: .Cm ForwardX11
943: in
1.19 jmc 944: .Xr ssh_config 5 ) .
1.13 stevesk 945: A system administrator may have a stance in which they want to
946: protect clients that may expose themselves to attack by unwittingly
947: requesting X11 forwarding, which can warrant a
948: .Dq no
949: setting.
950: .Pp
951: Note that disabling X11 forwarding does not prevent users from
952: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 953: X11 forwarding is automatically disabled if
954: .Cm UseLogin
955: is enabled.
956: .It Cm X11UseLocalhost
957: Specifies whether
1.52 jmc 958: .Xr sshd 8
1.1 stevesk 959: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 960: the wildcard address.
961: By default,
1.52 jmc 962: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 963: hostname part of the
964: .Ev DISPLAY
965: environment variable to
966: .Dq localhost .
1.8 stevesk 967: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 968: However, some older X11 clients may not function with this
969: configuration.
970: .Cm X11UseLocalhost
971: may be set to
972: .Dq no
973: to specify that the forwarding server should be bound to the wildcard
974: address.
975: The argument must be
976: .Dq yes
977: or
978: .Dq no .
979: The default is
980: .Dq yes .
981: .It Cm XAuthLocation
1.11 stevesk 982: Specifies the full pathname of the
1.1 stevesk 983: .Xr xauth 1
984: program.
985: The default is
986: .Pa /usr/X11R6/bin/xauth .
987: .El
1.55 jmc 988: .Sh TIME FORMATS
1.53 jmc 989: .Xr sshd 8
1.1 stevesk 990: command-line arguments and configuration file options that specify time
991: may be expressed using a sequence of the form:
992: .Sm off
1.7 stevesk 993: .Ar time Op Ar qualifier ,
1.1 stevesk 994: .Sm on
995: where
996: .Ar time
997: is a positive integer value and
998: .Ar qualifier
999: is one of the following:
1000: .Pp
1001: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1002: .It Aq Cm none
1.1 stevesk 1003: seconds
1004: .It Cm s | Cm S
1005: seconds
1006: .It Cm m | Cm M
1007: minutes
1008: .It Cm h | Cm H
1009: hours
1010: .It Cm d | Cm D
1011: days
1012: .It Cm w | Cm W
1013: weeks
1014: .El
1015: .Pp
1016: Each member of the sequence is added together to calculate
1017: the total time value.
1018: .Pp
1019: Time format examples:
1020: .Pp
1021: .Bl -tag -width Ds -compact -offset indent
1022: .It 600
1023: 600 seconds (10 minutes)
1024: .It 10m
1025: 10 minutes
1026: .It 1h30m
1027: 1 hour 30 minutes (90 minutes)
1028: .El
1029: .Sh FILES
1030: .Bl -tag -width Ds
1031: .It Pa /etc/ssh/sshd_config
1032: Contains configuration data for
1.53 jmc 1033: .Xr sshd 8 .
1.1 stevesk 1034: This file should be writable by root only, but it is recommended
1035: (though not necessary) that it be world-readable.
1036: .El
1.19 jmc 1037: .Sh SEE ALSO
1038: .Xr sshd 8
1.1 stevesk 1039: .Sh AUTHORS
1040: OpenSSH is a derivative of the original and free
1041: ssh 1.2.12 release by Tatu Ylonen.
1042: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1043: Theo de Raadt and Dug Song
1044: removed many bugs, re-added newer features and
1045: created OpenSSH.
1046: Markus Friedl contributed the support for SSH
1047: protocol versions 1.5 and 2.0.
1048: Niels Provos and Markus Friedl contributed support
1049: for privilege separation.
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