Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.100
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: .\"
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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
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1.100 ! naddy 37: .\" $OpenBSD: sshd_config.5,v 1.99 2008/12/30 00:46:56 okan Exp $
! 38: .Dd $Mdocdate: December 30 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
1.100 ! naddy 243: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
! 244: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
! 245: aes256-cbc,arcfour
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
1.99 okan 596: .Cm AllowAgentForwarding ,
1.62 dtucker 597: .Cm AllowTcpForwarding ,
1.72 dtucker 598: .Cm Banner ,
1.85 djm 599: .Cm ChrootDirectory ,
1.67 dtucker 600: .Cm ForceCommand ,
1.62 dtucker 601: .Cm GatewayPorts ,
1.87 djm 602: .Cm GSSAPIAuthentication ,
603: .Cm HostbasedAuthentication ,
1.74 jmc 604: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 605: .Cm KerberosAuthentication ,
1.95 dtucker 606: .Cm MaxAuthTries ,
1.94 dtucker 607: .Cm MaxSessions ,
1.72 dtucker 608: .Cm PasswordAuthentication ,
1.97 djm 609: .Cm PermitEmptyPasswords ,
1.66 dtucker 610: .Cm PermitOpen ,
1.79 dtucker 611: .Cm PermitRootLogin ,
1.72 dtucker 612: .Cm RhostsRSAAuthentication ,
613: .Cm RSAAuthentication ,
1.66 dtucker 614: .Cm X11DisplayOffset ,
615: .Cm X11Forwarding ,
1.98 djm 616: .Cm X11UseLocalHost ,
1.60 dtucker 617: and
1.98 djm 618: .Cm ZeroKnowledgePasswordAuthentication .
1.33 dtucker 619: .It Cm MaxAuthTries
620: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 621: connection.
622: Once the number of failures reaches half this value,
623: additional failures are logged.
624: The default is 6.
1.90 djm 625: .It Cm MaxSessions
626: Specifies the maximum number of open sessions permitted per network connection.
627: The default is 10.
1.1 stevesk 628: .It Cm MaxStartups
629: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 630: SSH daemon.
1.1 stevesk 631: Additional connections will be dropped until authentication succeeds or the
632: .Cm LoginGraceTime
633: expires for a connection.
634: The default is 10.
635: .Pp
636: Alternatively, random early drop can be enabled by specifying
637: the three colon separated values
638: .Dq start:rate:full
1.51 jmc 639: (e.g. "10:30:60").
1.53 jmc 640: .Xr sshd 8
1.1 stevesk 641: will refuse connection attempts with a probability of
642: .Dq rate/100
643: (30%)
644: if there are currently
645: .Dq start
646: (10)
647: unauthenticated connections.
648: The probability increases linearly and all connection attempts
649: are refused if the number of unauthenticated connections reaches
650: .Dq full
651: (60).
652: .It Cm PasswordAuthentication
653: Specifies whether password authentication is allowed.
654: The default is
655: .Dq yes .
656: .It Cm PermitEmptyPasswords
657: When password authentication is allowed, it specifies whether the
658: server allows login to accounts with empty password strings.
659: The default is
660: .Dq no .
1.62 dtucker 661: .It Cm PermitOpen
662: Specifies the destinations to which TCP port forwarding is permitted.
663: The forwarding specification must be one of the following forms:
664: .Pp
665: .Bl -item -offset indent -compact
666: .It
667: .Cm PermitOpen
668: .Sm off
669: .Ar host : port
670: .Sm on
671: .It
672: .Cm PermitOpen
673: .Sm off
674: .Ar IPv4_addr : port
675: .Sm on
676: .It
677: .Cm PermitOpen
678: .Sm off
679: .Ar \&[ IPv6_addr \&] : port
680: .Sm on
681: .El
682: .Pp
1.68 dtucker 683: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 684: An argument of
685: .Dq any
686: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 687: By default all port forwarding requests are permitted.
1.1 stevesk 688: .It Cm PermitRootLogin
1.38 jmc 689: Specifies whether root can log in using
1.1 stevesk 690: .Xr ssh 1 .
691: The argument must be
692: .Dq yes ,
693: .Dq without-password ,
1.52 jmc 694: .Dq forced-commands-only ,
1.1 stevesk 695: or
696: .Dq no .
697: The default is
698: .Dq yes .
699: .Pp
700: If this option is set to
1.52 jmc 701: .Dq without-password ,
1.1 stevesk 702: password authentication is disabled for root.
703: .Pp
704: If this option is set to
1.52 jmc 705: .Dq forced-commands-only ,
1.1 stevesk 706: root login with public key authentication will be allowed,
707: but only if the
708: .Ar command
709: option has been specified
710: (which may be useful for taking remote backups even if root login is
1.17 jmc 711: normally not allowed).
712: All other authentication methods are disabled for root.
1.1 stevesk 713: .Pp
714: If this option is set to
1.52 jmc 715: .Dq no ,
1.38 jmc 716: root is not allowed to log in.
1.46 reyk 717: .It Cm PermitTunnel
718: Specifies whether
719: .Xr tun 4
720: device forwarding is allowed.
1.47 reyk 721: The argument must be
722: .Dq yes ,
1.58 stevesk 723: .Dq point-to-point
724: (layer 3),
725: .Dq ethernet
726: (layer 2), or
1.47 reyk 727: .Dq no .
1.58 stevesk 728: Specifying
729: .Dq yes
730: permits both
731: .Dq point-to-point
732: and
733: .Dq ethernet .
1.46 reyk 734: The default is
735: .Dq no .
1.6 markus 736: .It Cm PermitUserEnvironment
737: Specifies whether
738: .Pa ~/.ssh/environment
1.9 stevesk 739: and
1.6 markus 740: .Cm environment=
741: options in
742: .Pa ~/.ssh/authorized_keys
1.9 stevesk 743: are processed by
1.52 jmc 744: .Xr sshd 8 .
1.6 markus 745: The default is
746: .Dq no .
1.9 stevesk 747: Enabling environment processing may enable users to bypass access
748: restrictions in some configurations using mechanisms such as
749: .Ev LD_PRELOAD .
1.1 stevesk 750: .It Cm PidFile
1.4 stevesk 751: Specifies the file that contains the process ID of the
1.53 jmc 752: SSH daemon.
1.1 stevesk 753: The default is
754: .Pa /var/run/sshd.pid .
755: .It Cm Port
756: Specifies the port number that
1.52 jmc 757: .Xr sshd 8
1.1 stevesk 758: listens on.
759: The default is 22.
760: Multiple options of this type are permitted.
761: See also
762: .Cm ListenAddress .
763: .It Cm PrintLastLog
764: Specifies whether
1.52 jmc 765: .Xr sshd 8
1.36 jaredy 766: should print the date and time of the last user login when a user logs
767: in interactively.
1.1 stevesk 768: The default is
769: .Dq yes .
770: .It Cm PrintMotd
771: Specifies whether
1.52 jmc 772: .Xr sshd 8
1.1 stevesk 773: should print
774: .Pa /etc/motd
775: when a user logs in interactively.
776: (On some systems it is also printed by the shell,
777: .Pa /etc/profile ,
778: or equivalent.)
779: The default is
780: .Dq yes .
781: .It Cm Protocol
782: Specifies the protocol versions
1.52 jmc 783: .Xr sshd 8
1.5 stevesk 784: supports.
1.1 stevesk 785: The possible values are
1.52 jmc 786: .Sq 1
1.1 stevesk 787: and
1.52 jmc 788: .Sq 2 .
1.1 stevesk 789: Multiple versions must be comma-separated.
790: The default is
791: .Dq 2,1 .
1.5 stevesk 792: Note that the order of the protocol list does not indicate preference,
793: because the client selects among multiple protocol versions offered
794: by the server.
795: Specifying
796: .Dq 2,1
797: is identical to
798: .Dq 1,2 .
1.1 stevesk 799: .It Cm PubkeyAuthentication
800: Specifies whether public key authentication is allowed.
801: The default is
802: .Dq yes .
803: Note that this option applies to protocol version 2 only.
804: .It Cm RhostsRSAAuthentication
805: Specifies whether rhosts or /etc/hosts.equiv authentication together
806: with successful RSA host authentication is allowed.
807: The default is
808: .Dq no .
809: This option applies to protocol version 1 only.
810: .It Cm RSAAuthentication
811: Specifies whether pure RSA authentication is allowed.
812: The default is
813: .Dq yes .
814: This option applies to protocol version 1 only.
815: .It Cm ServerKeyBits
816: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 817: The minimum value is 512, and the default is 1024.
1.1 stevesk 818: .It Cm StrictModes
819: Specifies whether
1.52 jmc 820: .Xr sshd 8
1.1 stevesk 821: should check file modes and ownership of the
822: user's files and home directory before accepting login.
823: This is normally desirable because novices sometimes accidentally leave their
824: directory or files world-writable.
825: The default is
826: .Dq yes .
827: .It Cm Subsystem
1.51 jmc 828: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 829: Arguments should be a subsystem name and a command (with optional arguments)
830: to execute upon subsystem request.
1.80 djm 831: .Pp
1.1 stevesk 832: The command
833: .Xr sftp-server 8
834: implements the
835: .Dq sftp
836: file transfer subsystem.
1.80 djm 837: .Pp
838: Alternately the name
839: .Dq internal-sftp
840: implements an in-process
841: .Dq sftp
842: server.
843: This may simplify configurations using
844: .Cm ChrootDirectory
845: to force a different filesystem root on clients.
846: .Pp
1.1 stevesk 847: By default no subsystems are defined.
848: Note that this option applies to protocol version 2 only.
849: .It Cm SyslogFacility
850: Gives the facility code that is used when logging messages from
1.53 jmc 851: .Xr sshd 8 .
1.1 stevesk 852: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
853: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
854: The default is AUTH.
1.27 markus 855: .It Cm TCPKeepAlive
856: Specifies whether the system should send TCP keepalive messages to the
857: other side.
858: If they are sent, death of the connection or crash of one
859: of the machines will be properly noticed.
860: However, this means that
861: connections will die if the route is down temporarily, and some people
862: find it annoying.
863: On the other hand, if TCP keepalives are not sent,
864: sessions may hang indefinitely on the server, leaving
865: .Dq ghost
866: users and consuming server resources.
867: .Pp
868: The default is
869: .Dq yes
870: (to send TCP keepalive messages), and the server will notice
871: if the network goes down or the client host crashes.
872: This avoids infinitely hanging sessions.
873: .Pp
874: To disable TCP keepalive messages, the value should be set to
875: .Dq no .
1.18 markus 876: .It Cm UseDNS
877: Specifies whether
1.52 jmc 878: .Xr sshd 8
1.40 jmc 879: should look up the remote host name and check that
1.18 markus 880: the resolved host name for the remote IP address maps back to the
881: very same IP address.
882: The default is
883: .Dq yes .
1.1 stevesk 884: .It Cm UseLogin
885: Specifies whether
886: .Xr login 1
887: is used for interactive login sessions.
888: The default is
889: .Dq no .
890: Note that
891: .Xr login 1
892: is never used for remote command execution.
893: Note also, that if this is enabled,
894: .Cm X11Forwarding
895: will be disabled because
896: .Xr login 1
897: does not know how to handle
898: .Xr xauth 1
1.15 jmc 899: cookies.
900: If
1.1 stevesk 901: .Cm UsePrivilegeSeparation
902: is specified, it will be disabled after authentication.
903: .It Cm UsePrivilegeSeparation
904: Specifies whether
1.52 jmc 905: .Xr sshd 8
1.2 stevesk 906: separates privileges by creating an unprivileged child process
1.15 jmc 907: to deal with incoming network traffic.
908: After successful authentication, another process will be created that has
909: the privilege of the authenticated user.
910: The goal of privilege separation is to prevent privilege
1.1 stevesk 911: escalation by containing any corruption within the unprivileged processes.
912: The default is
913: .Dq yes .
914: .It Cm X11DisplayOffset
915: Specifies the first display number available for
1.52 jmc 916: .Xr sshd 8 Ns 's
1.1 stevesk 917: X11 forwarding.
1.52 jmc 918: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 919: The default is 10.
920: .It Cm X11Forwarding
921: Specifies whether X11 forwarding is permitted.
1.13 stevesk 922: The argument must be
923: .Dq yes
924: or
925: .Dq no .
1.1 stevesk 926: The default is
927: .Dq no .
1.13 stevesk 928: .Pp
929: When X11 forwarding is enabled, there may be additional exposure to
930: the server and to client displays if the
1.52 jmc 931: .Xr sshd 8
1.13 stevesk 932: proxy display is configured to listen on the wildcard address (see
933: .Cm X11UseLocalhost
1.52 jmc 934: below), though this is not the default.
1.13 stevesk 935: Additionally, the authentication spoofing and authentication data
936: verification and substitution occur on the client side.
937: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 938: display server may be exposed to attack when the SSH client requests
1.13 stevesk 939: forwarding (see the warnings for
940: .Cm ForwardX11
941: in
1.19 jmc 942: .Xr ssh_config 5 ) .
1.13 stevesk 943: A system administrator may have a stance in which they want to
944: protect clients that may expose themselves to attack by unwittingly
945: requesting X11 forwarding, which can warrant a
946: .Dq no
947: setting.
948: .Pp
949: Note that disabling X11 forwarding does not prevent users from
950: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 951: X11 forwarding is automatically disabled if
952: .Cm UseLogin
953: is enabled.
954: .It Cm X11UseLocalhost
955: Specifies whether
1.52 jmc 956: .Xr sshd 8
1.1 stevesk 957: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 958: the wildcard address.
959: By default,
1.52 jmc 960: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 961: hostname part of the
962: .Ev DISPLAY
963: environment variable to
964: .Dq localhost .
1.8 stevesk 965: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 966: However, some older X11 clients may not function with this
967: configuration.
968: .Cm X11UseLocalhost
969: may be set to
970: .Dq no
971: to specify that the forwarding server should be bound to the wildcard
972: address.
973: The argument must be
974: .Dq yes
975: or
976: .Dq no .
977: The default is
978: .Dq yes .
979: .It Cm XAuthLocation
1.11 stevesk 980: Specifies the full pathname of the
1.1 stevesk 981: .Xr xauth 1
982: program.
983: The default is
984: .Pa /usr/X11R6/bin/xauth .
1.98 djm 985: .It Cm ZeroKnowledgePasswordAuthentication
986: Specifies whether to use zero knowledge password authentication.
987: This authentication method avoids exposure of password to untrusted
988: hosts.
989: The argument to this keyword must be
990: .Dq yes
991: or
992: .Dq no .
993: The default is currently
994: .Dq no
995: as this method is considered experimental.
1.1 stevesk 996: .El
1.55 jmc 997: .Sh TIME FORMATS
1.53 jmc 998: .Xr sshd 8
1.1 stevesk 999: command-line arguments and configuration file options that specify time
1000: may be expressed using a sequence of the form:
1001: .Sm off
1.7 stevesk 1002: .Ar time Op Ar qualifier ,
1.1 stevesk 1003: .Sm on
1004: where
1005: .Ar time
1006: is a positive integer value and
1007: .Ar qualifier
1008: is one of the following:
1009: .Pp
1010: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1011: .It Aq Cm none
1.1 stevesk 1012: seconds
1013: .It Cm s | Cm S
1014: seconds
1015: .It Cm m | Cm M
1016: minutes
1017: .It Cm h | Cm H
1018: hours
1019: .It Cm d | Cm D
1020: days
1021: .It Cm w | Cm W
1022: weeks
1023: .El
1024: .Pp
1025: Each member of the sequence is added together to calculate
1026: the total time value.
1027: .Pp
1028: Time format examples:
1029: .Pp
1030: .Bl -tag -width Ds -compact -offset indent
1031: .It 600
1032: 600 seconds (10 minutes)
1033: .It 10m
1034: 10 minutes
1035: .It 1h30m
1036: 1 hour 30 minutes (90 minutes)
1037: .El
1038: .Sh FILES
1039: .Bl -tag -width Ds
1040: .It Pa /etc/ssh/sshd_config
1041: Contains configuration data for
1.53 jmc 1042: .Xr sshd 8 .
1.1 stevesk 1043: This file should be writable by root only, but it is recommended
1044: (though not necessary) that it be world-readable.
1045: .El
1.19 jmc 1046: .Sh SEE ALSO
1047: .Xr sshd 8
1.1 stevesk 1048: .Sh AUTHORS
1049: OpenSSH is a derivative of the original and free
1050: ssh 1.2.12 release by Tatu Ylonen.
1051: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1052: Theo de Raadt and Dug Song
1053: removed many bugs, re-added newer features and
1054: created OpenSSH.
1055: Markus Friedl contributed the support for SSH
1056: protocol versions 1.5 and 2.0.
1057: Niels Provos and Markus Friedl contributed support
1058: for privilege separation.
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