Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.94
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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1.94 ! dtucker 37: .\" $OpenBSD: sshd_config.5,v 1.93 2008/06/10 07:12:00 jmc Exp $
1.92 djm 38: .Dd $Mdocdate: June 10 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 .
1.89 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
200: users' session.
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
216: .Dq sftp ,
217: no additional configuration of the environment is necessary if the
218: in-process sftp server is used (see
219: .Cm Subsystem
1.81 jmc 220: for details).
1.80 djm 221: .Pp
222: The default is not to
223: .Xr chroot 2 .
1.1 stevesk 224: .It Cm Ciphers
225: Specifies the ciphers allowed for protocol version 2.
226: Multiple ciphers must be comma-separated.
1.34 dtucker 227: The supported ciphers are
228: .Dq 3des-cbc ,
229: .Dq aes128-cbc ,
230: .Dq aes192-cbc ,
231: .Dq aes256-cbc ,
232: .Dq aes128-ctr ,
233: .Dq aes192-ctr ,
234: .Dq aes256-ctr ,
1.43 djm 235: .Dq arcfour128 ,
236: .Dq arcfour256 ,
1.34 dtucker 237: .Dq arcfour ,
238: .Dq blowfish-cbc ,
239: and
240: .Dq cast128-cbc .
1.52 jmc 241: The default is:
242: .Bd -literal -offset 3n
243: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
244: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
245: aes192-ctr,aes256-ctr
1.1 stevesk 246: .Ed
247: .It Cm ClientAliveCountMax
1.48 jmc 248: Sets the number of client alive messages (see below) which may be
1.1 stevesk 249: sent without
1.52 jmc 250: .Xr sshd 8
1.17 jmc 251: receiving any messages back from the client.
252: If this threshold is reached while client alive messages are being sent,
1.52 jmc 253: sshd will disconnect the client, terminating the session.
1.17 jmc 254: It is important to note that the use of client alive messages is very
255: different from
1.27 markus 256: .Cm TCPKeepAlive
1.17 jmc 257: (below).
258: The client alive messages are sent through the encrypted channel
259: and therefore will not be spoofable.
260: The TCP keepalive option enabled by
1.27 markus 261: .Cm TCPKeepAlive
1.17 jmc 262: is spoofable.
263: The client alive mechanism is valuable when the client or
1.1 stevesk 264: server depend on knowing when a connection has become inactive.
265: .Pp
1.17 jmc 266: The default value is 3.
267: If
1.1 stevesk 268: .Cm ClientAliveInterval
1.48 jmc 269: (see below) is set to 15, and
1.1 stevesk 270: .Cm ClientAliveCountMax
1.52 jmc 271: is left at the default, unresponsive SSH clients
1.1 stevesk 272: will be disconnected after approximately 45 seconds.
1.57 markus 273: This option applies to protocol version 2 only.
1.42 djm 274: .It Cm ClientAliveInterval
275: Sets a timeout interval in seconds after which if no data has been received
276: from the client,
1.52 jmc 277: .Xr sshd 8
1.42 djm 278: will send a message through the encrypted
279: channel to request a response from the client.
280: The default
281: is 0, indicating that these messages will not be sent to the client.
282: This option applies to protocol version 2 only.
1.3 markus 283: .It Cm Compression
1.44 markus 284: Specifies whether compression is allowed, or delayed until
285: the user has authenticated successfully.
1.3 markus 286: The argument must be
1.44 markus 287: .Dq yes ,
288: .Dq delayed ,
1.3 markus 289: or
290: .Dq no .
291: The default is
1.44 markus 292: .Dq delayed .
1.1 stevesk 293: .It Cm DenyGroups
294: This keyword can be followed by a list of group name patterns, separated
295: by spaces.
296: Login is disallowed for users whose primary group or supplementary
297: group list matches one of the patterns.
298: Only group names are valid; a numerical group ID is not recognized.
299: By default, login is allowed for all groups.
1.54 jmc 300: The allow/deny directives are processed in the following order:
301: .Cm DenyUsers ,
302: .Cm AllowUsers ,
303: .Cm DenyGroups ,
304: and finally
305: .Cm AllowGroups .
1.49 jmc 306: .Pp
307: See
308: .Sx PATTERNS
309: in
310: .Xr ssh_config 5
311: for more information on patterns.
1.1 stevesk 312: .It Cm DenyUsers
313: This keyword can be followed by a list of user name patterns, separated
314: by spaces.
315: Login is disallowed for user names that match one of the patterns.
316: Only user names are valid; a numerical user ID is not recognized.
317: By default, login is allowed for all users.
318: If the pattern takes the form USER@HOST then USER and HOST
319: are separately checked, restricting logins to particular
320: users from particular hosts.
1.54 jmc 321: The allow/deny directives are processed in the following order:
322: .Cm DenyUsers ,
323: .Cm AllowUsers ,
324: .Cm DenyGroups ,
325: and finally
326: .Cm AllowGroups .
1.49 jmc 327: .Pp
328: See
329: .Sx PATTERNS
330: in
331: .Xr ssh_config 5
332: for more information on patterns.
1.67 dtucker 333: .It Cm ForceCommand
334: Forces the execution of the command specified by
335: .Cm ForceCommand ,
1.84 djm 336: ignoring any command supplied by the client and
337: .Pa ~/.ssh/rc
338: if present.
1.67 dtucker 339: The command is invoked by using the user's login shell with the -c option.
340: This applies to shell, command, or subsystem execution.
341: It is most useful inside a
342: .Cm Match
343: block.
344: The command originally supplied by the client is available in the
345: .Ev SSH_ORIGINAL_COMMAND
346: environment variable.
1.82 djm 347: Specifying a command of
348: .Dq internal-sftp
349: will force the use of an in-process sftp server that requires no support
350: files when used with
351: .Cm ChrootDirectory .
1.1 stevesk 352: .It Cm GatewayPorts
353: Specifies whether remote hosts are allowed to connect to ports
354: forwarded for the client.
355: By default,
1.52 jmc 356: .Xr sshd 8
1.15 jmc 357: binds remote port forwardings to the loopback address.
358: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 359: .Cm GatewayPorts
1.52 jmc 360: can be used to specify that sshd
1.39 djm 361: should allow remote port forwardings to bind to non-loopback addresses, thus
362: allowing other hosts to connect.
363: The argument may be
364: .Dq no
365: to force remote port forwardings to be available to the local host only,
1.1 stevesk 366: .Dq yes
1.39 djm 367: to force remote port forwardings to bind to the wildcard address, or
368: .Dq clientspecified
369: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 370: The default is
371: .Dq no .
1.23 markus 372: .It Cm GSSAPIAuthentication
1.25 markus 373: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 374: The default is
1.23 markus 375: .Dq no .
376: Note that this option applies to protocol version 2 only.
377: .It Cm GSSAPICleanupCredentials
378: Specifies whether to automatically destroy the user's credentials cache
379: on logout.
380: The default is
381: .Dq yes .
382: Note that this option applies to protocol version 2 only.
1.1 stevesk 383: .It Cm HostbasedAuthentication
384: Specifies whether rhosts or /etc/hosts.equiv authentication together
385: with successful public key client host authentication is allowed
1.50 jmc 386: (host-based authentication).
1.1 stevesk 387: This option is similar to
388: .Cm RhostsRSAAuthentication
389: and applies to protocol version 2 only.
1.70 dtucker 390: The default is
391: .Dq no .
392: .It Cm HostbasedUsesNameFromPacketOnly
393: Specifies whether or not the server will attempt to perform a reverse
394: name lookup when matching the name in the
395: .Pa ~/.shosts ,
396: .Pa ~/.rhosts ,
397: and
398: .Pa /etc/hosts.equiv
399: files during
400: .Cm HostbasedAuthentication .
401: A setting of
402: .Dq yes
403: means that
404: .Xr sshd 8
405: uses the name supplied by the client rather than
406: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 407: The default is
408: .Dq no .
409: .It Cm HostKey
410: Specifies a file containing a private host key
411: used by SSH.
412: The default is
413: .Pa /etc/ssh/ssh_host_key
414: for protocol version 1, and
415: .Pa /etc/ssh/ssh_host_rsa_key
416: and
417: .Pa /etc/ssh/ssh_host_dsa_key
418: for protocol version 2.
419: Note that
1.52 jmc 420: .Xr sshd 8
1.1 stevesk 421: will refuse to use a file if it is group/world-accessible.
422: It is possible to have multiple host key files.
423: .Dq rsa1
424: keys are used for version 1 and
425: .Dq dsa
426: or
427: .Dq rsa
428: are used for version 2 of the SSH protocol.
429: .It Cm IgnoreRhosts
430: Specifies that
431: .Pa .rhosts
432: and
433: .Pa .shosts
434: files will not be used in
435: .Cm RhostsRSAAuthentication
436: or
437: .Cm HostbasedAuthentication .
438: .Pp
439: .Pa /etc/hosts.equiv
440: and
441: .Pa /etc/shosts.equiv
442: are still used.
443: The default is
444: .Dq yes .
445: .It Cm IgnoreUserKnownHosts
446: Specifies whether
1.52 jmc 447: .Xr sshd 8
1.1 stevesk 448: should ignore the user's
1.41 djm 449: .Pa ~/.ssh/known_hosts
1.1 stevesk 450: during
451: .Cm RhostsRSAAuthentication
452: or
453: .Cm HostbasedAuthentication .
454: The default is
455: .Dq no .
456: .It Cm KerberosAuthentication
1.24 markus 457: Specifies whether the password provided by the user for
1.1 stevesk 458: .Cm PasswordAuthentication
1.24 markus 459: will be validated through the Kerberos KDC.
1.1 stevesk 460: To use this option, the server needs a
461: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 462: The default is
1.29 dtucker 463: .Dq no .
464: .It Cm KerberosGetAFSToken
1.45 djm 465: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 466: an AFS token before accessing the user's home directory.
1.52 jmc 467: The default is
1.1 stevesk 468: .Dq no .
469: .It Cm KerberosOrLocalPasswd
1.52 jmc 470: If password authentication through Kerberos fails then
1.1 stevesk 471: the password will be validated via any additional local mechanism
472: such as
473: .Pa /etc/passwd .
1.52 jmc 474: The default is
1.1 stevesk 475: .Dq yes .
476: .It Cm KerberosTicketCleanup
477: Specifies whether to automatically destroy the user's ticket cache
478: file on logout.
1.52 jmc 479: The default is
1.1 stevesk 480: .Dq yes .
481: .It Cm KeyRegenerationInterval
482: In protocol version 1, the ephemeral server key is automatically regenerated
483: after this many seconds (if it has been used).
484: The purpose of regeneration is to prevent
485: decrypting captured sessions by later breaking into the machine and
486: stealing the keys.
487: The key is never stored anywhere.
488: If the value is 0, the key is never regenerated.
489: The default is 3600 (seconds).
490: .It Cm ListenAddress
491: Specifies the local addresses
1.52 jmc 492: .Xr sshd 8
1.1 stevesk 493: should listen on.
494: The following forms may be used:
495: .Pp
496: .Bl -item -offset indent -compact
497: .It
498: .Cm ListenAddress
499: .Sm off
500: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
501: .Sm on
502: .It
503: .Cm ListenAddress
504: .Sm off
505: .Ar host No | Ar IPv4_addr No : Ar port
506: .Sm on
507: .It
508: .Cm ListenAddress
509: .Sm off
510: .Oo
511: .Ar host No | Ar IPv6_addr Oc : Ar port
512: .Sm on
513: .El
514: .Pp
515: If
516: .Ar port
517: is not specified,
1.52 jmc 518: sshd will listen on the address and all prior
1.1 stevesk 519: .Cm Port
1.17 jmc 520: options specified.
521: The default is to listen on all local addresses.
1.15 jmc 522: Multiple
1.1 stevesk 523: .Cm ListenAddress
1.17 jmc 524: options are permitted.
525: Additionally, any
1.1 stevesk 526: .Cm Port
1.52 jmc 527: options must precede this option for non-port qualified addresses.
1.1 stevesk 528: .It Cm LoginGraceTime
529: The server disconnects after this time if the user has not
530: successfully logged in.
531: If the value is 0, there is no time limit.
1.12 stevesk 532: The default is 120 seconds.
1.1 stevesk 533: .It Cm LogLevel
534: Gives the verbosity level that is used when logging messages from
1.53 jmc 535: .Xr sshd 8 .
1.1 stevesk 536: The possible values are:
1.52 jmc 537: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 538: The default is INFO.
539: DEBUG and DEBUG1 are equivalent.
540: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
541: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 542: .It Cm MACs
543: Specifies the available MAC (message authentication code) algorithms.
544: The MAC algorithm is used in protocol version 2
545: for data integrity protection.
546: Multiple algorithms must be comma-separated.
1.52 jmc 547: The default is:
1.77 jmc 548: .Bd -literal -offset indent
549: hmac-md5,hmac-sha1,umac-64@openssh.com,
550: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
551: .Ed
1.60 dtucker 552: .It Cm Match
1.61 jmc 553: Introduces a conditional block.
1.65 dtucker 554: If all of the criteria on the
1.60 dtucker 555: .Cm Match
1.65 dtucker 556: line are satisfied, the keywords on the following lines override those
557: set in the global section of the config file, until either another
1.60 dtucker 558: .Cm Match
1.65 dtucker 559: line or the end of the file.
1.91 djm 560: .Pp
1.61 jmc 561: The arguments to
1.60 dtucker 562: .Cm Match
1.65 dtucker 563: are one or more criteria-pattern pairs.
1.60 dtucker 564: The available criteria are
565: .Cm User ,
1.69 dtucker 566: .Cm Group ,
1.60 dtucker 567: .Cm Host ,
568: and
569: .Cm Address .
1.91 djm 570: The match patterns may consist of single entries or comma-separated
571: lists and may use the wildcard and negation operators described in the
1.92 djm 572: .Sx PATTERNS
1.91 djm 573: section of
1.92 djm 574: .Xr ssh_config 5 .
1.91 djm 575: .Pp
576: The patterns in an
577: .Cm Address
578: criteria may additionally contain addresses to match in CIDR
1.93 jmc 579: address/masklen format, e.g.\&
1.91 djm 580: .Dq 192.0.2.0/24
581: or
582: .Dq 3ffe:ffff::/32 .
583: Note that the mask length provided must be consistent with the address -
584: it is an error to specify a mask length that is too long for the address
1.93 jmc 585: or one with bits set in this host portion of the address.
586: For example,
1.91 djm 587: .Dq 192.0.2.0/33
588: and
1.93 jmc 589: .Dq 192.0.2.0/8
1.91 djm 590: respectively.
591: .Pp
1.60 dtucker 592: Only a subset of keywords may be used on the lines following a
593: .Cm Match
594: keyword.
595: Available keywords are
1.62 dtucker 596: .Cm AllowTcpForwarding ,
1.72 dtucker 597: .Cm Banner ,
1.85 djm 598: .Cm ChrootDirectory ,
1.67 dtucker 599: .Cm ForceCommand ,
1.62 dtucker 600: .Cm GatewayPorts ,
1.87 djm 601: .Cm GSSAPIAuthentication ,
602: .Cm HostbasedAuthentication ,
1.74 jmc 603: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 604: .Cm KerberosAuthentication ,
1.94 ! dtucker 605: .Cm MaxSessions ,
1.72 dtucker 606: .Cm PasswordAuthentication ,
1.66 dtucker 607: .Cm PermitOpen ,
1.79 dtucker 608: .Cm PermitRootLogin ,
1.72 dtucker 609: .Cm RhostsRSAAuthentication ,
610: .Cm RSAAuthentication ,
1.66 dtucker 611: .Cm X11DisplayOffset ,
612: .Cm X11Forwarding ,
1.60 dtucker 613: and
1.66 dtucker 614: .Cm X11UseLocalHost .
1.33 dtucker 615: .It Cm MaxAuthTries
616: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 617: connection.
618: Once the number of failures reaches half this value,
619: additional failures are logged.
620: The default is 6.
1.90 djm 621: .It Cm MaxSessions
622: Specifies the maximum number of open sessions permitted per network connection.
623: The default is 10.
1.1 stevesk 624: .It Cm MaxStartups
625: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 626: SSH daemon.
1.1 stevesk 627: Additional connections will be dropped until authentication succeeds or the
628: .Cm LoginGraceTime
629: expires for a connection.
630: The default is 10.
631: .Pp
632: Alternatively, random early drop can be enabled by specifying
633: the three colon separated values
634: .Dq start:rate:full
1.51 jmc 635: (e.g. "10:30:60").
1.53 jmc 636: .Xr sshd 8
1.1 stevesk 637: will refuse connection attempts with a probability of
638: .Dq rate/100
639: (30%)
640: if there are currently
641: .Dq start
642: (10)
643: unauthenticated connections.
644: The probability increases linearly and all connection attempts
645: are refused if the number of unauthenticated connections reaches
646: .Dq full
647: (60).
648: .It Cm PasswordAuthentication
649: Specifies whether password authentication is allowed.
650: The default is
651: .Dq yes .
652: .It Cm PermitEmptyPasswords
653: When password authentication is allowed, it specifies whether the
654: server allows login to accounts with empty password strings.
655: The default is
656: .Dq no .
1.62 dtucker 657: .It Cm PermitOpen
658: Specifies the destinations to which TCP port forwarding is permitted.
659: The forwarding specification must be one of the following forms:
660: .Pp
661: .Bl -item -offset indent -compact
662: .It
663: .Cm PermitOpen
664: .Sm off
665: .Ar host : port
666: .Sm on
667: .It
668: .Cm PermitOpen
669: .Sm off
670: .Ar IPv4_addr : port
671: .Sm on
672: .It
673: .Cm PermitOpen
674: .Sm off
675: .Ar \&[ IPv6_addr \&] : port
676: .Sm on
677: .El
678: .Pp
1.68 dtucker 679: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 680: An argument of
681: .Dq any
682: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 683: By default all port forwarding requests are permitted.
1.1 stevesk 684: .It Cm PermitRootLogin
1.38 jmc 685: Specifies whether root can log in using
1.1 stevesk 686: .Xr ssh 1 .
687: The argument must be
688: .Dq yes ,
689: .Dq without-password ,
1.52 jmc 690: .Dq forced-commands-only ,
1.1 stevesk 691: or
692: .Dq no .
693: The default is
694: .Dq yes .
695: .Pp
696: If this option is set to
1.52 jmc 697: .Dq without-password ,
1.1 stevesk 698: password authentication is disabled for root.
699: .Pp
700: If this option is set to
1.52 jmc 701: .Dq forced-commands-only ,
1.1 stevesk 702: root login with public key authentication will be allowed,
703: but only if the
704: .Ar command
705: option has been specified
706: (which may be useful for taking remote backups even if root login is
1.17 jmc 707: normally not allowed).
708: All other authentication methods are disabled for root.
1.1 stevesk 709: .Pp
710: If this option is set to
1.52 jmc 711: .Dq no ,
1.38 jmc 712: root is not allowed to log in.
1.46 reyk 713: .It Cm PermitTunnel
714: Specifies whether
715: .Xr tun 4
716: device forwarding is allowed.
1.47 reyk 717: The argument must be
718: .Dq yes ,
1.58 stevesk 719: .Dq point-to-point
720: (layer 3),
721: .Dq ethernet
722: (layer 2), or
1.47 reyk 723: .Dq no .
1.58 stevesk 724: Specifying
725: .Dq yes
726: permits both
727: .Dq point-to-point
728: and
729: .Dq ethernet .
1.46 reyk 730: The default is
731: .Dq no .
1.6 markus 732: .It Cm PermitUserEnvironment
733: Specifies whether
734: .Pa ~/.ssh/environment
1.9 stevesk 735: and
1.6 markus 736: .Cm environment=
737: options in
738: .Pa ~/.ssh/authorized_keys
1.9 stevesk 739: are processed by
1.52 jmc 740: .Xr sshd 8 .
1.6 markus 741: The default is
742: .Dq no .
1.9 stevesk 743: Enabling environment processing may enable users to bypass access
744: restrictions in some configurations using mechanisms such as
745: .Ev LD_PRELOAD .
1.1 stevesk 746: .It Cm PidFile
1.4 stevesk 747: Specifies the file that contains the process ID of the
1.53 jmc 748: SSH daemon.
1.1 stevesk 749: The default is
750: .Pa /var/run/sshd.pid .
751: .It Cm Port
752: Specifies the port number that
1.52 jmc 753: .Xr sshd 8
1.1 stevesk 754: listens on.
755: The default is 22.
756: Multiple options of this type are permitted.
757: See also
758: .Cm ListenAddress .
759: .It Cm PrintLastLog
760: Specifies whether
1.52 jmc 761: .Xr sshd 8
1.36 jaredy 762: should print the date and time of the last user login when a user logs
763: in interactively.
1.1 stevesk 764: The default is
765: .Dq yes .
766: .It Cm PrintMotd
767: Specifies whether
1.52 jmc 768: .Xr sshd 8
1.1 stevesk 769: should print
770: .Pa /etc/motd
771: when a user logs in interactively.
772: (On some systems it is also printed by the shell,
773: .Pa /etc/profile ,
774: or equivalent.)
775: The default is
776: .Dq yes .
777: .It Cm Protocol
778: Specifies the protocol versions
1.52 jmc 779: .Xr sshd 8
1.5 stevesk 780: supports.
1.1 stevesk 781: The possible values are
1.52 jmc 782: .Sq 1
1.1 stevesk 783: and
1.52 jmc 784: .Sq 2 .
1.1 stevesk 785: Multiple versions must be comma-separated.
786: The default is
787: .Dq 2,1 .
1.5 stevesk 788: Note that the order of the protocol list does not indicate preference,
789: because the client selects among multiple protocol versions offered
790: by the server.
791: Specifying
792: .Dq 2,1
793: is identical to
794: .Dq 1,2 .
1.1 stevesk 795: .It Cm PubkeyAuthentication
796: Specifies whether public key authentication is allowed.
797: The default is
798: .Dq yes .
799: Note that this option applies to protocol version 2 only.
800: .It Cm RhostsRSAAuthentication
801: Specifies whether rhosts or /etc/hosts.equiv authentication together
802: with successful RSA host authentication is allowed.
803: The default is
804: .Dq no .
805: This option applies to protocol version 1 only.
806: .It Cm RSAAuthentication
807: Specifies whether pure RSA authentication is allowed.
808: The default is
809: .Dq yes .
810: This option applies to protocol version 1 only.
811: .It Cm ServerKeyBits
812: Defines the number of bits in the ephemeral protocol version 1 server key.
813: The minimum value is 512, and the default is 768.
814: .It Cm StrictModes
815: Specifies whether
1.52 jmc 816: .Xr sshd 8
1.1 stevesk 817: should check file modes and ownership of the
818: user's files and home directory before accepting login.
819: This is normally desirable because novices sometimes accidentally leave their
820: directory or files world-writable.
821: The default is
822: .Dq yes .
823: .It Cm Subsystem
1.51 jmc 824: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 825: Arguments should be a subsystem name and a command (with optional arguments)
826: to execute upon subsystem request.
1.80 djm 827: .Pp
1.1 stevesk 828: The command
829: .Xr sftp-server 8
830: implements the
831: .Dq sftp
832: file transfer subsystem.
1.80 djm 833: .Pp
834: Alternately the name
835: .Dq internal-sftp
836: implements an in-process
837: .Dq sftp
838: server.
839: This may simplify configurations using
840: .Cm ChrootDirectory
841: to force a different filesystem root on clients.
842: .Pp
1.1 stevesk 843: By default no subsystems are defined.
844: Note that this option applies to protocol version 2 only.
845: .It Cm SyslogFacility
846: Gives the facility code that is used when logging messages from
1.53 jmc 847: .Xr sshd 8 .
1.1 stevesk 848: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
849: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
850: The default is AUTH.
1.27 markus 851: .It Cm TCPKeepAlive
852: Specifies whether the system should send TCP keepalive messages to the
853: other side.
854: If they are sent, death of the connection or crash of one
855: of the machines will be properly noticed.
856: However, this means that
857: connections will die if the route is down temporarily, and some people
858: find it annoying.
859: On the other hand, if TCP keepalives are not sent,
860: sessions may hang indefinitely on the server, leaving
861: .Dq ghost
862: users and consuming server resources.
863: .Pp
864: The default is
865: .Dq yes
866: (to send TCP keepalive messages), and the server will notice
867: if the network goes down or the client host crashes.
868: This avoids infinitely hanging sessions.
869: .Pp
870: To disable TCP keepalive messages, the value should be set to
871: .Dq no .
1.18 markus 872: .It Cm UseDNS
873: Specifies whether
1.52 jmc 874: .Xr sshd 8
1.40 jmc 875: should look up the remote host name and check that
1.18 markus 876: the resolved host name for the remote IP address maps back to the
877: very same IP address.
878: The default is
879: .Dq yes .
1.1 stevesk 880: .It Cm UseLogin
881: Specifies whether
882: .Xr login 1
883: is used for interactive login sessions.
884: The default is
885: .Dq no .
886: Note that
887: .Xr login 1
888: is never used for remote command execution.
889: Note also, that if this is enabled,
890: .Cm X11Forwarding
891: will be disabled because
892: .Xr login 1
893: does not know how to handle
894: .Xr xauth 1
1.15 jmc 895: cookies.
896: If
1.1 stevesk 897: .Cm UsePrivilegeSeparation
898: is specified, it will be disabled after authentication.
899: .It Cm UsePrivilegeSeparation
900: Specifies whether
1.52 jmc 901: .Xr sshd 8
1.2 stevesk 902: separates privileges by creating an unprivileged child process
1.15 jmc 903: to deal with incoming network traffic.
904: After successful authentication, another process will be created that has
905: the privilege of the authenticated user.
906: The goal of privilege separation is to prevent privilege
1.1 stevesk 907: escalation by containing any corruption within the unprivileged processes.
908: The default is
909: .Dq yes .
910: .It Cm X11DisplayOffset
911: Specifies the first display number available for
1.52 jmc 912: .Xr sshd 8 Ns 's
1.1 stevesk 913: X11 forwarding.
1.52 jmc 914: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 915: The default is 10.
916: .It Cm X11Forwarding
917: Specifies whether X11 forwarding is permitted.
1.13 stevesk 918: The argument must be
919: .Dq yes
920: or
921: .Dq no .
1.1 stevesk 922: The default is
923: .Dq no .
1.13 stevesk 924: .Pp
925: When X11 forwarding is enabled, there may be additional exposure to
926: the server and to client displays if the
1.52 jmc 927: .Xr sshd 8
1.13 stevesk 928: proxy display is configured to listen on the wildcard address (see
929: .Cm X11UseLocalhost
1.52 jmc 930: below), though this is not the default.
1.13 stevesk 931: Additionally, the authentication spoofing and authentication data
932: verification and substitution occur on the client side.
933: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 934: display server may be exposed to attack when the SSH client requests
1.13 stevesk 935: forwarding (see the warnings for
936: .Cm ForwardX11
937: in
1.19 jmc 938: .Xr ssh_config 5 ) .
1.13 stevesk 939: A system administrator may have a stance in which they want to
940: protect clients that may expose themselves to attack by unwittingly
941: requesting X11 forwarding, which can warrant a
942: .Dq no
943: setting.
944: .Pp
945: Note that disabling X11 forwarding does not prevent users from
946: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 947: X11 forwarding is automatically disabled if
948: .Cm UseLogin
949: is enabled.
950: .It Cm X11UseLocalhost
951: Specifies whether
1.52 jmc 952: .Xr sshd 8
1.1 stevesk 953: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 954: the wildcard address.
955: By default,
1.52 jmc 956: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 957: hostname part of the
958: .Ev DISPLAY
959: environment variable to
960: .Dq localhost .
1.8 stevesk 961: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 962: However, some older X11 clients may not function with this
963: configuration.
964: .Cm X11UseLocalhost
965: may be set to
966: .Dq no
967: to specify that the forwarding server should be bound to the wildcard
968: address.
969: The argument must be
970: .Dq yes
971: or
972: .Dq no .
973: The default is
974: .Dq yes .
975: .It Cm XAuthLocation
1.11 stevesk 976: Specifies the full pathname of the
1.1 stevesk 977: .Xr xauth 1
978: program.
979: The default is
980: .Pa /usr/X11R6/bin/xauth .
981: .El
1.55 jmc 982: .Sh TIME FORMATS
1.53 jmc 983: .Xr sshd 8
1.1 stevesk 984: command-line arguments and configuration file options that specify time
985: may be expressed using a sequence of the form:
986: .Sm off
1.7 stevesk 987: .Ar time Op Ar qualifier ,
1.1 stevesk 988: .Sm on
989: where
990: .Ar time
991: is a positive integer value and
992: .Ar qualifier
993: is one of the following:
994: .Pp
995: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 996: .It Aq Cm none
1.1 stevesk 997: seconds
998: .It Cm s | Cm S
999: seconds
1000: .It Cm m | Cm M
1001: minutes
1002: .It Cm h | Cm H
1003: hours
1004: .It Cm d | Cm D
1005: days
1006: .It Cm w | Cm W
1007: weeks
1008: .El
1009: .Pp
1010: Each member of the sequence is added together to calculate
1011: the total time value.
1012: .Pp
1013: Time format examples:
1014: .Pp
1015: .Bl -tag -width Ds -compact -offset indent
1016: .It 600
1017: 600 seconds (10 minutes)
1018: .It 10m
1019: 10 minutes
1020: .It 1h30m
1021: 1 hour 30 minutes (90 minutes)
1022: .El
1023: .Sh FILES
1024: .Bl -tag -width Ds
1025: .It Pa /etc/ssh/sshd_config
1026: Contains configuration data for
1.53 jmc 1027: .Xr sshd 8 .
1.1 stevesk 1028: This file should be writable by root only, but it is recommended
1029: (though not necessary) that it be world-readable.
1030: .El
1.19 jmc 1031: .Sh SEE ALSO
1032: .Xr sshd 8
1.1 stevesk 1033: .Sh AUTHORS
1034: OpenSSH is a derivative of the original and free
1035: ssh 1.2.12 release by Tatu Ylonen.
1036: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1037: Theo de Raadt and Dug Song
1038: removed many bugs, re-added newer features and
1039: created OpenSSH.
1040: Markus Friedl contributed the support for SSH
1041: protocol versions 1.5 and 2.0.
1042: Niels Provos and Markus Friedl contributed support
1043: for privilege separation.