Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.127
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
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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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1.127 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.126 2010/08/31 11:54:45 djm Exp $
! 38: .Dd $Mdocdate: August 31 2010 $
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.
1.124 djm 158: The format is described in the
159: .Sx AUTHORIZED_KEYS FILE FORMAT
160: section of
161: .Xr sshd 8 .
1.1 stevesk 162: .Cm AuthorizedKeysFile
163: may contain tokens of the form %T which are substituted during connection
1.52 jmc 164: setup.
1.17 jmc 165: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 166: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 167: %u is replaced by the username of that user.
168: After expansion,
169: .Cm AuthorizedKeysFile
170: is taken to be an absolute path or one relative to the user's home
171: directory.
172: The default is
173: .Dq .ssh/authorized_keys .
1.121 djm 174: .It Cm AuthorizedPrincipalsFile
175: Specifies a file that lists principal names that are accepted for
176: certificate authentication.
177: When using certificates signed by a key listed in
178: .Cm TrustedUserCAKeys ,
179: this file lists names, one of which must appear in the certificate for it
180: to be accepted for authentication.
1.125 jmc 181: Names are listed one per line preceded by key options (as described
1.124 djm 182: in
183: .Sx AUTHORIZED_KEYS FILE FORMAT
184: in
1.125 jmc 185: .Xr sshd 8 ) .
1.124 djm 186: Empty lines and comments starting with
1.121 djm 187: .Ql #
188: are ignored.
189: .Pp
190: .Cm AuthorizedPrincipalsFile
191: may contain tokens of the form %T which are substituted during connection
192: setup.
193: The following tokens are defined: %% 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: After expansion,
197: .Cm AuthorizedPrincipalsFile
198: is taken to be an absolute path or one relative to the user's home
199: directory.
200: .Pp
1.122 jmc 201: The default is not to use a principals file \(en in this case, the username
1.121 djm 202: of the user must appear in a certificate's principals list for it to be
203: accepted.
204: Note that
205: .Cm AuthorizedPrincipalsFile
206: is only used when authentication proceeds using a CA listed in
207: .Cm TrustedUserCAKeys
208: and is not consulted for certification authorities trusted via
209: .Pa ~/.ssh/authorized_keys ,
210: though the
211: .Cm principals=
212: key option offers a similar facility (see
213: .Xr sshd 8
214: for details).
1.1 stevesk 215: .It Cm Banner
216: The contents of the specified file are sent to the remote user before
217: authentication is allowed.
1.78 djm 218: If the argument is
219: .Dq none
220: then no banner is displayed.
1.1 stevesk 221: This option is only available for protocol version 2.
222: By default, no banner is displayed.
223: .It Cm ChallengeResponseAuthentication
1.50 jmc 224: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 225: All authentication styles from
226: .Xr login.conf 5
227: are supported.
228: The default is
229: .Dq yes .
1.80 djm 230: .It Cm ChrootDirectory
1.113 stevesk 231: Specifies the pathname of a directory to
1.80 djm 232: .Xr chroot 2
233: to after authentication.
1.113 stevesk 234: All components of the pathname must be root-owned directories that are
1.80 djm 235: not writable by any other user or group.
1.106 stevesk 236: After the chroot,
237: .Xr sshd 8
238: changes the working directory to the user's home directory.
1.80 djm 239: .Pp
1.113 stevesk 240: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 241: the connecting user has been authenticated: %% is replaced by a literal '%',
242: %h is replaced by the home directory of the user being authenticated, and
243: %u is replaced by the username of that user.
244: .Pp
245: The
246: .Cm ChrootDirectory
247: must contain the necessary files and directories to support the
1.103 stevesk 248: user's session.
1.80 djm 249: For an interactive session this requires at least a shell, typically
250: .Xr sh 1 ,
251: and basic
252: .Pa /dev
253: nodes such as
254: .Xr null 4 ,
255: .Xr zero 4 ,
256: .Xr stdin 4 ,
257: .Xr stdout 4 ,
258: .Xr stderr 4 ,
259: .Xr arandom 4
260: and
261: .Xr tty 4
262: devices.
263: For file transfer sessions using
1.105 jmc 264: .Dq sftp ,
1.80 djm 265: no additional configuration of the environment is necessary if the
1.105 jmc 266: in-process sftp server is used,
267: though sessions which use logging do require
1.104 stevesk 268: .Pa /dev/log
269: inside the chroot directory (see
270: .Xr sftp-server 8
1.81 jmc 271: for details).
1.80 djm 272: .Pp
273: The default is not to
274: .Xr chroot 2 .
1.1 stevesk 275: .It Cm Ciphers
276: Specifies the ciphers allowed for protocol version 2.
277: Multiple ciphers must be comma-separated.
1.34 dtucker 278: The supported ciphers are
279: .Dq 3des-cbc ,
280: .Dq aes128-cbc ,
281: .Dq aes192-cbc ,
282: .Dq aes256-cbc ,
283: .Dq aes128-ctr ,
284: .Dq aes192-ctr ,
285: .Dq aes256-ctr ,
1.43 djm 286: .Dq arcfour128 ,
287: .Dq arcfour256 ,
1.34 dtucker 288: .Dq arcfour ,
289: .Dq blowfish-cbc ,
290: and
291: .Dq cast128-cbc .
1.52 jmc 292: The default is:
293: .Bd -literal -offset 3n
1.100 naddy 294: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
295: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
296: aes256-cbc,arcfour
1.1 stevesk 297: .Ed
298: .It Cm ClientAliveCountMax
1.48 jmc 299: Sets the number of client alive messages (see below) which may be
1.1 stevesk 300: sent without
1.52 jmc 301: .Xr sshd 8
1.17 jmc 302: receiving any messages back from the client.
303: If this threshold is reached while client alive messages are being sent,
1.52 jmc 304: sshd will disconnect the client, terminating the session.
1.17 jmc 305: It is important to note that the use of client alive messages is very
306: different from
1.27 markus 307: .Cm TCPKeepAlive
1.17 jmc 308: (below).
309: The client alive messages are sent through the encrypted channel
310: and therefore will not be spoofable.
311: The TCP keepalive option enabled by
1.27 markus 312: .Cm TCPKeepAlive
1.17 jmc 313: is spoofable.
314: The client alive mechanism is valuable when the client or
1.1 stevesk 315: server depend on knowing when a connection has become inactive.
316: .Pp
1.17 jmc 317: The default value is 3.
318: If
1.1 stevesk 319: .Cm ClientAliveInterval
1.48 jmc 320: (see below) is set to 15, and
1.1 stevesk 321: .Cm ClientAliveCountMax
1.52 jmc 322: is left at the default, unresponsive SSH clients
1.1 stevesk 323: will be disconnected after approximately 45 seconds.
1.57 markus 324: This option applies to protocol version 2 only.
1.42 djm 325: .It Cm ClientAliveInterval
326: Sets a timeout interval in seconds after which if no data has been received
327: from the client,
1.52 jmc 328: .Xr sshd 8
1.42 djm 329: will send a message through the encrypted
330: channel to request a response from the client.
331: The default
332: is 0, indicating that these messages will not be sent to the client.
333: This option applies to protocol version 2 only.
1.3 markus 334: .It Cm Compression
1.44 markus 335: Specifies whether compression is allowed, or delayed until
336: the user has authenticated successfully.
1.3 markus 337: The argument must be
1.44 markus 338: .Dq yes ,
339: .Dq delayed ,
1.3 markus 340: or
341: .Dq no .
342: The default is
1.44 markus 343: .Dq delayed .
1.1 stevesk 344: .It Cm DenyGroups
345: This keyword can be followed by a list of group name patterns, separated
346: by spaces.
347: Login is disallowed for users whose primary group or supplementary
348: group list matches one of the patterns.
349: Only group names are valid; a numerical group ID is not recognized.
350: By default, login is allowed for all groups.
1.54 jmc 351: The allow/deny directives are processed in the following order:
352: .Cm DenyUsers ,
353: .Cm AllowUsers ,
354: .Cm DenyGroups ,
355: and finally
356: .Cm AllowGroups .
1.49 jmc 357: .Pp
358: See
359: .Sx PATTERNS
360: in
361: .Xr ssh_config 5
362: for more information on patterns.
1.1 stevesk 363: .It Cm DenyUsers
364: This keyword can be followed by a list of user name patterns, separated
365: by spaces.
366: Login is disallowed for user names that match one of the patterns.
367: Only user names are valid; a numerical user ID is not recognized.
368: By default, login is allowed for all users.
369: If the pattern takes the form USER@HOST then USER and HOST
370: are separately checked, restricting logins to particular
371: users from particular hosts.
1.54 jmc 372: The allow/deny directives are processed in the following order:
373: .Cm DenyUsers ,
374: .Cm AllowUsers ,
375: .Cm DenyGroups ,
376: and finally
377: .Cm AllowGroups .
1.49 jmc 378: .Pp
379: See
380: .Sx PATTERNS
381: in
382: .Xr ssh_config 5
383: for more information on patterns.
1.67 dtucker 384: .It Cm ForceCommand
385: Forces the execution of the command specified by
386: .Cm ForceCommand ,
1.84 djm 387: ignoring any command supplied by the client and
388: .Pa ~/.ssh/rc
389: if present.
1.67 dtucker 390: The command is invoked by using the user's login shell with the -c option.
391: This applies to shell, command, or subsystem execution.
392: It is most useful inside a
393: .Cm Match
394: block.
395: The command originally supplied by the client is available in the
396: .Ev SSH_ORIGINAL_COMMAND
397: environment variable.
1.82 djm 398: Specifying a command of
399: .Dq internal-sftp
400: will force the use of an in-process sftp server that requires no support
401: files when used with
402: .Cm ChrootDirectory .
1.1 stevesk 403: .It Cm GatewayPorts
404: Specifies whether remote hosts are allowed to connect to ports
405: forwarded for the client.
406: By default,
1.52 jmc 407: .Xr sshd 8
1.15 jmc 408: binds remote port forwardings to the loopback address.
409: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 410: .Cm GatewayPorts
1.52 jmc 411: can be used to specify that sshd
1.39 djm 412: should allow remote port forwardings to bind to non-loopback addresses, thus
413: allowing other hosts to connect.
414: The argument may be
415: .Dq no
416: to force remote port forwardings to be available to the local host only,
1.1 stevesk 417: .Dq yes
1.39 djm 418: to force remote port forwardings to bind to the wildcard address, or
419: .Dq clientspecified
420: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 421: The default is
422: .Dq no .
1.23 markus 423: .It Cm GSSAPIAuthentication
1.25 markus 424: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 425: The default is
1.23 markus 426: .Dq no .
427: Note that this option applies to protocol version 2 only.
428: .It Cm GSSAPICleanupCredentials
429: Specifies whether to automatically destroy the user's credentials cache
430: on logout.
431: The default is
432: .Dq yes .
433: Note that this option applies to protocol version 2 only.
1.1 stevesk 434: .It Cm HostbasedAuthentication
435: Specifies whether rhosts or /etc/hosts.equiv authentication together
436: with successful public key client host authentication is allowed
1.50 jmc 437: (host-based authentication).
1.1 stevesk 438: This option is similar to
439: .Cm RhostsRSAAuthentication
440: and applies to protocol version 2 only.
1.70 dtucker 441: The default is
442: .Dq no .
443: .It Cm HostbasedUsesNameFromPacketOnly
444: Specifies whether or not the server will attempt to perform a reverse
445: name lookup when matching the name in the
446: .Pa ~/.shosts ,
447: .Pa ~/.rhosts ,
448: and
449: .Pa /etc/hosts.equiv
450: files during
451: .Cm HostbasedAuthentication .
452: A setting of
453: .Dq yes
454: means that
455: .Xr sshd 8
456: uses the name supplied by the client rather than
457: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 458: The default is
459: .Dq no .
1.117 djm 460: .It Cm HostCertificate
461: Specifies a file containing a public host certificate.
462: The certificate's public key must match a private host key already specified
463: by
464: .Cm HostKey .
465: The default behaviour of
466: .Xr sshd 8
467: is not to load any certificates.
1.1 stevesk 468: .It Cm HostKey
469: Specifies a file containing a private host key
470: used by SSH.
471: The default is
472: .Pa /etc/ssh/ssh_host_key
473: for protocol version 1, and
1.126 djm 474: .Pa /etc/ssh/ssh_host_dsa_key ,
475: .Pa /etc/ssh/ssh_host_ecdsa_key
476: and
1.1 stevesk 477: .Pa /etc/ssh/ssh_host_rsa_key
478: for protocol version 2.
479: Note that
1.52 jmc 480: .Xr sshd 8
1.1 stevesk 481: will refuse to use a file if it is group/world-accessible.
482: It is possible to have multiple host key files.
483: .Dq rsa1
484: keys are used for version 1 and
1.126 djm 485: .Dq dsa ,
486: .Dq ecdsa
1.1 stevesk 487: or
488: .Dq rsa
489: are used for version 2 of the SSH protocol.
490: .It Cm IgnoreRhosts
491: Specifies that
492: .Pa .rhosts
493: and
494: .Pa .shosts
495: files will not be used in
496: .Cm RhostsRSAAuthentication
497: or
498: .Cm HostbasedAuthentication .
499: .Pp
500: .Pa /etc/hosts.equiv
501: and
502: .Pa /etc/shosts.equiv
503: are still used.
504: The default is
505: .Dq yes .
506: .It Cm IgnoreUserKnownHosts
507: Specifies whether
1.52 jmc 508: .Xr sshd 8
1.1 stevesk 509: should ignore the user's
1.41 djm 510: .Pa ~/.ssh/known_hosts
1.1 stevesk 511: during
512: .Cm RhostsRSAAuthentication
513: or
514: .Cm HostbasedAuthentication .
515: The default is
516: .Dq no .
517: .It Cm KerberosAuthentication
1.24 markus 518: Specifies whether the password provided by the user for
1.1 stevesk 519: .Cm PasswordAuthentication
1.24 markus 520: will be validated through the Kerberos KDC.
1.1 stevesk 521: To use this option, the server needs a
522: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 523: The default is
1.29 dtucker 524: .Dq no .
525: .It Cm KerberosGetAFSToken
1.45 djm 526: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 527: an AFS token before accessing the user's home directory.
1.52 jmc 528: The default is
1.1 stevesk 529: .Dq no .
530: .It Cm KerberosOrLocalPasswd
1.52 jmc 531: If password authentication through Kerberos fails then
1.1 stevesk 532: the password will be validated via any additional local mechanism
533: such as
534: .Pa /etc/passwd .
1.52 jmc 535: The default is
1.1 stevesk 536: .Dq yes .
537: .It Cm KerberosTicketCleanup
538: Specifies whether to automatically destroy the user's ticket cache
539: file on logout.
1.52 jmc 540: The default is
1.1 stevesk 541: .Dq yes .
1.127 ! djm 542: .It Cm KexAlgorithms
! 543: Specifies the available KEX (Key Exchange) algorithms.
! 544: Multiple algorithms must be comma-separated.
! 545: The default is
! 546: .Dq ecdh-sha2-nistp256 ,
! 547: .Dq ecdh-sha2-nistp384 ,
! 548: .Dq ecdh-sha2-nistp521 ,
! 549: .Dq diffie-hellman-group-exchange-sha256 ,
! 550: .Dq diffie-hellman-group-exchange-sha1 ,
! 551: .Dq diffie-hellman-group14-sha1 ,
! 552: .Dq diffie-hellman-group1-sha1 .
1.1 stevesk 553: .It Cm KeyRegenerationInterval
554: In protocol version 1, the ephemeral server key is automatically regenerated
555: after this many seconds (if it has been used).
556: The purpose of regeneration is to prevent
557: decrypting captured sessions by later breaking into the machine and
558: stealing the keys.
559: The key is never stored anywhere.
560: If the value is 0, the key is never regenerated.
561: The default is 3600 (seconds).
562: .It Cm ListenAddress
563: Specifies the local addresses
1.52 jmc 564: .Xr sshd 8
1.1 stevesk 565: should listen on.
566: The following forms may be used:
567: .Pp
568: .Bl -item -offset indent -compact
569: .It
570: .Cm ListenAddress
571: .Sm off
572: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
573: .Sm on
574: .It
575: .Cm ListenAddress
576: .Sm off
577: .Ar host No | Ar IPv4_addr No : Ar port
578: .Sm on
579: .It
580: .Cm ListenAddress
581: .Sm off
582: .Oo
583: .Ar host No | Ar IPv6_addr Oc : Ar port
584: .Sm on
585: .El
586: .Pp
587: If
588: .Ar port
589: is not specified,
1.52 jmc 590: sshd will listen on the address and all prior
1.1 stevesk 591: .Cm Port
1.17 jmc 592: options specified.
593: The default is to listen on all local addresses.
1.15 jmc 594: Multiple
1.1 stevesk 595: .Cm ListenAddress
1.17 jmc 596: options are permitted.
597: Additionally, any
1.1 stevesk 598: .Cm Port
1.52 jmc 599: options must precede this option for non-port qualified addresses.
1.1 stevesk 600: .It Cm LoginGraceTime
601: The server disconnects after this time if the user has not
602: successfully logged in.
603: If the value is 0, there is no time limit.
1.12 stevesk 604: The default is 120 seconds.
1.1 stevesk 605: .It Cm LogLevel
606: Gives the verbosity level that is used when logging messages from
1.53 jmc 607: .Xr sshd 8 .
1.1 stevesk 608: The possible values are:
1.52 jmc 609: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 610: The default is INFO.
611: DEBUG and DEBUG1 are equivalent.
612: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
613: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 614: .It Cm MACs
615: Specifies the available MAC (message authentication code) algorithms.
616: The MAC algorithm is used in protocol version 2
617: for data integrity protection.
618: Multiple algorithms must be comma-separated.
1.52 jmc 619: The default is:
1.77 jmc 620: .Bd -literal -offset indent
621: hmac-md5,hmac-sha1,umac-64@openssh.com,
622: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
623: .Ed
1.60 dtucker 624: .It Cm Match
1.61 jmc 625: Introduces a conditional block.
1.65 dtucker 626: If all of the criteria on the
1.60 dtucker 627: .Cm Match
1.65 dtucker 628: line are satisfied, the keywords on the following lines override those
629: set in the global section of the config file, until either another
1.60 dtucker 630: .Cm Match
1.65 dtucker 631: line or the end of the file.
1.91 djm 632: .Pp
1.61 jmc 633: The arguments to
1.60 dtucker 634: .Cm Match
1.65 dtucker 635: are one or more criteria-pattern pairs.
1.60 dtucker 636: The available criteria are
637: .Cm User ,
1.69 dtucker 638: .Cm Group ,
1.60 dtucker 639: .Cm Host ,
640: and
641: .Cm Address .
1.91 djm 642: The match patterns may consist of single entries or comma-separated
643: lists and may use the wildcard and negation operators described in the
1.92 djm 644: .Sx PATTERNS
1.91 djm 645: section of
1.92 djm 646: .Xr ssh_config 5 .
1.91 djm 647: .Pp
648: The patterns in an
649: .Cm Address
650: criteria may additionally contain addresses to match in CIDR
1.93 jmc 651: address/masklen format, e.g.\&
1.91 djm 652: .Dq 192.0.2.0/24
653: or
654: .Dq 3ffe:ffff::/32 .
655: Note that the mask length provided must be consistent with the address -
656: it is an error to specify a mask length that is too long for the address
1.93 jmc 657: or one with bits set in this host portion of the address.
658: For example,
1.91 djm 659: .Dq 192.0.2.0/33
660: and
1.93 jmc 661: .Dq 192.0.2.0/8
1.91 djm 662: respectively.
663: .Pp
1.60 dtucker 664: Only a subset of keywords may be used on the lines following a
665: .Cm Match
666: keyword.
667: Available keywords are
1.99 okan 668: .Cm AllowAgentForwarding ,
1.62 dtucker 669: .Cm AllowTcpForwarding ,
1.123 djm 670: .Cm AuthorizedKeysFile ,
671: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 672: .Cm Banner ,
1.85 djm 673: .Cm ChrootDirectory ,
1.67 dtucker 674: .Cm ForceCommand ,
1.62 dtucker 675: .Cm GatewayPorts ,
1.87 djm 676: .Cm GSSAPIAuthentication ,
677: .Cm HostbasedAuthentication ,
1.123 djm 678: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 679: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 680: .Cm KerberosAuthentication ,
1.95 dtucker 681: .Cm MaxAuthTries ,
1.94 dtucker 682: .Cm MaxSessions ,
1.72 dtucker 683: .Cm PasswordAuthentication ,
1.97 djm 684: .Cm PermitEmptyPasswords ,
1.66 dtucker 685: .Cm PermitOpen ,
1.79 dtucker 686: .Cm PermitRootLogin ,
1.123 djm 687: .Cm PermitTunnel ,
1.107 dtucker 688: .Cm PubkeyAuthentication ,
1.72 dtucker 689: .Cm RhostsRSAAuthentication ,
690: .Cm RSAAuthentication ,
1.66 dtucker 691: .Cm X11DisplayOffset ,
1.101 djm 692: .Cm X11Forwarding
1.60 dtucker 693: and
1.102 djm 694: .Cm X11UseLocalHost .
1.33 dtucker 695: .It Cm MaxAuthTries
696: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 697: connection.
698: Once the number of failures reaches half this value,
699: additional failures are logged.
700: The default is 6.
1.90 djm 701: .It Cm MaxSessions
702: Specifies the maximum number of open sessions permitted per network connection.
703: The default is 10.
1.1 stevesk 704: .It Cm MaxStartups
705: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 706: SSH daemon.
1.1 stevesk 707: Additional connections will be dropped until authentication succeeds or the
708: .Cm LoginGraceTime
709: expires for a connection.
710: The default is 10.
711: .Pp
712: Alternatively, random early drop can be enabled by specifying
713: the three colon separated values
714: .Dq start:rate:full
1.51 jmc 715: (e.g. "10:30:60").
1.53 jmc 716: .Xr sshd 8
1.1 stevesk 717: will refuse connection attempts with a probability of
718: .Dq rate/100
719: (30%)
720: if there are currently
721: .Dq start
722: (10)
723: unauthenticated connections.
724: The probability increases linearly and all connection attempts
725: are refused if the number of unauthenticated connections reaches
726: .Dq full
727: (60).
728: .It Cm PasswordAuthentication
729: Specifies whether password authentication is allowed.
730: The default is
731: .Dq yes .
732: .It Cm PermitEmptyPasswords
733: When password authentication is allowed, it specifies whether the
734: server allows login to accounts with empty password strings.
735: The default is
736: .Dq no .
1.62 dtucker 737: .It Cm PermitOpen
738: Specifies the destinations to which TCP port forwarding is permitted.
739: The forwarding specification must be one of the following forms:
740: .Pp
741: .Bl -item -offset indent -compact
742: .It
743: .Cm PermitOpen
744: .Sm off
745: .Ar host : port
746: .Sm on
747: .It
748: .Cm PermitOpen
749: .Sm off
750: .Ar IPv4_addr : port
751: .Sm on
752: .It
753: .Cm PermitOpen
754: .Sm off
755: .Ar \&[ IPv6_addr \&] : port
756: .Sm on
757: .El
758: .Pp
1.68 dtucker 759: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 760: An argument of
761: .Dq any
762: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 763: By default all port forwarding requests are permitted.
1.1 stevesk 764: .It Cm PermitRootLogin
1.38 jmc 765: Specifies whether root can log in using
1.1 stevesk 766: .Xr ssh 1 .
767: The argument must be
768: .Dq yes ,
769: .Dq without-password ,
1.52 jmc 770: .Dq forced-commands-only ,
1.1 stevesk 771: or
772: .Dq no .
773: The default is
774: .Dq yes .
775: .Pp
776: If this option is set to
1.52 jmc 777: .Dq without-password ,
1.1 stevesk 778: password authentication is disabled for root.
779: .Pp
780: If this option is set to
1.52 jmc 781: .Dq forced-commands-only ,
1.1 stevesk 782: root login with public key authentication will be allowed,
783: but only if the
784: .Ar command
785: option has been specified
786: (which may be useful for taking remote backups even if root login is
1.17 jmc 787: normally not allowed).
788: All other authentication methods are disabled for root.
1.1 stevesk 789: .Pp
790: If this option is set to
1.52 jmc 791: .Dq no ,
1.38 jmc 792: root is not allowed to log in.
1.46 reyk 793: .It Cm PermitTunnel
794: Specifies whether
795: .Xr tun 4
796: device forwarding is allowed.
1.47 reyk 797: The argument must be
798: .Dq yes ,
1.58 stevesk 799: .Dq point-to-point
800: (layer 3),
801: .Dq ethernet
802: (layer 2), or
1.47 reyk 803: .Dq no .
1.58 stevesk 804: Specifying
805: .Dq yes
806: permits both
807: .Dq point-to-point
808: and
809: .Dq ethernet .
1.46 reyk 810: The default is
811: .Dq no .
1.6 markus 812: .It Cm PermitUserEnvironment
813: Specifies whether
814: .Pa ~/.ssh/environment
1.9 stevesk 815: and
1.6 markus 816: .Cm environment=
817: options in
818: .Pa ~/.ssh/authorized_keys
1.9 stevesk 819: are processed by
1.52 jmc 820: .Xr sshd 8 .
1.6 markus 821: The default is
822: .Dq no .
1.9 stevesk 823: Enabling environment processing may enable users to bypass access
824: restrictions in some configurations using mechanisms such as
825: .Ev LD_PRELOAD .
1.1 stevesk 826: .It Cm PidFile
1.4 stevesk 827: Specifies the file that contains the process ID of the
1.53 jmc 828: SSH daemon.
1.1 stevesk 829: The default is
830: .Pa /var/run/sshd.pid .
831: .It Cm Port
832: Specifies the port number that
1.52 jmc 833: .Xr sshd 8
1.1 stevesk 834: listens on.
835: The default is 22.
836: Multiple options of this type are permitted.
837: See also
838: .Cm ListenAddress .
839: .It Cm PrintLastLog
840: Specifies whether
1.52 jmc 841: .Xr sshd 8
1.36 jaredy 842: should print the date and time of the last user login when a user logs
843: in interactively.
1.1 stevesk 844: The default is
845: .Dq yes .
846: .It Cm PrintMotd
847: Specifies whether
1.52 jmc 848: .Xr sshd 8
1.1 stevesk 849: should print
850: .Pa /etc/motd
851: when a user logs in interactively.
852: (On some systems it is also printed by the shell,
853: .Pa /etc/profile ,
854: or equivalent.)
855: The default is
856: .Dq yes .
857: .It Cm Protocol
858: Specifies the protocol versions
1.52 jmc 859: .Xr sshd 8
1.5 stevesk 860: supports.
1.1 stevesk 861: The possible values are
1.52 jmc 862: .Sq 1
1.1 stevesk 863: and
1.52 jmc 864: .Sq 2 .
1.1 stevesk 865: Multiple versions must be comma-separated.
866: The default is
1.109 jmc 867: .Sq 2 .
1.5 stevesk 868: Note that the order of the protocol list does not indicate preference,
869: because the client selects among multiple protocol versions offered
870: by the server.
871: Specifying
872: .Dq 2,1
873: is identical to
874: .Dq 1,2 .
1.1 stevesk 875: .It Cm PubkeyAuthentication
876: Specifies whether public key authentication is allowed.
877: The default is
878: .Dq yes .
879: Note that this option applies to protocol version 2 only.
1.118 djm 880: .It Cm RevokedKeys
881: Specifies a list of revoked public keys.
882: Keys listed in this file will be refused for public key authentication.
883: Note that if this file is not readable, then public key authentication will
884: be refused for all users.
1.1 stevesk 885: .It Cm RhostsRSAAuthentication
886: Specifies whether rhosts or /etc/hosts.equiv authentication together
887: with successful RSA host authentication is allowed.
888: The default is
889: .Dq no .
890: This option applies to protocol version 1 only.
891: .It Cm RSAAuthentication
892: Specifies whether pure RSA authentication is allowed.
893: The default is
894: .Dq yes .
895: This option applies to protocol version 1 only.
896: .It Cm ServerKeyBits
897: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 898: The minimum value is 512, and the default is 1024.
1.1 stevesk 899: .It Cm StrictModes
900: Specifies whether
1.52 jmc 901: .Xr sshd 8
1.1 stevesk 902: should check file modes and ownership of the
903: user's files and home directory before accepting login.
904: This is normally desirable because novices sometimes accidentally leave their
905: directory or files world-writable.
906: The default is
907: .Dq yes .
1.112 djm 908: Note that this does not apply to
909: .Cm ChrootDirectory ,
910: whose permissions and ownership are checked unconditionally.
1.1 stevesk 911: .It Cm Subsystem
1.51 jmc 912: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 913: Arguments should be a subsystem name and a command (with optional arguments)
914: to execute upon subsystem request.
1.80 djm 915: .Pp
1.1 stevesk 916: The command
917: .Xr sftp-server 8
918: implements the
919: .Dq sftp
920: file transfer subsystem.
1.80 djm 921: .Pp
922: Alternately the name
923: .Dq internal-sftp
924: implements an in-process
925: .Dq sftp
926: server.
927: This may simplify configurations using
928: .Cm ChrootDirectory
929: to force a different filesystem root on clients.
930: .Pp
1.1 stevesk 931: By default no subsystems are defined.
932: Note that this option applies to protocol version 2 only.
933: .It Cm SyslogFacility
934: Gives the facility code that is used when logging messages from
1.53 jmc 935: .Xr sshd 8 .
1.1 stevesk 936: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
937: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
938: The default is AUTH.
1.27 markus 939: .It Cm TCPKeepAlive
940: Specifies whether the system should send TCP keepalive messages to the
941: other side.
942: If they are sent, death of the connection or crash of one
943: of the machines will be properly noticed.
944: However, this means that
945: connections will die if the route is down temporarily, and some people
946: find it annoying.
947: On the other hand, if TCP keepalives are not sent,
948: sessions may hang indefinitely on the server, leaving
949: .Dq ghost
950: users and consuming server resources.
951: .Pp
952: The default is
953: .Dq yes
954: (to send TCP keepalive messages), and the server will notice
955: if the network goes down or the client host crashes.
956: This avoids infinitely hanging sessions.
957: .Pp
958: To disable TCP keepalive messages, the value should be set to
959: .Dq no .
1.118 djm 960: .It Cm TrustedUserCAKeys
961: Specifies a file containing public keys of certificate authorities that are
1.120 djm 962: trusted to sign user certificates for authentication.
1.119 jmc 963: Keys are listed one per line; empty lines and comments starting with
1.118 djm 964: .Ql #
965: are allowed.
966: If a certificate is presented for authentication and has its signing CA key
967: listed in this file, then it may be used for authentication for any user
968: listed in the certificate's principals list.
969: Note that certificates that lack a list of principals will not be permitted
970: for authentication using
971: .Cm TrustedUserCAKeys .
1.119 jmc 972: For more details on certificates, see the
1.118 djm 973: .Sx CERTIFICATES
974: section in
975: .Xr ssh-keygen 1 .
1.18 markus 976: .It Cm UseDNS
977: Specifies whether
1.52 jmc 978: .Xr sshd 8
1.40 jmc 979: should look up the remote host name and check that
1.18 markus 980: the resolved host name for the remote IP address maps back to the
981: very same IP address.
982: The default is
983: .Dq yes .
1.1 stevesk 984: .It Cm UseLogin
985: Specifies whether
986: .Xr login 1
987: is used for interactive login sessions.
988: The default is
989: .Dq no .
990: Note that
991: .Xr login 1
992: is never used for remote command execution.
993: Note also, that if this is enabled,
994: .Cm X11Forwarding
995: will be disabled because
996: .Xr login 1
997: does not know how to handle
998: .Xr xauth 1
1.15 jmc 999: cookies.
1000: If
1.1 stevesk 1001: .Cm UsePrivilegeSeparation
1002: is specified, it will be disabled after authentication.
1003: .It Cm UsePrivilegeSeparation
1004: Specifies whether
1.52 jmc 1005: .Xr sshd 8
1.2 stevesk 1006: separates privileges by creating an unprivileged child process
1.15 jmc 1007: to deal with incoming network traffic.
1008: After successful authentication, another process will be created that has
1009: the privilege of the authenticated user.
1010: The goal of privilege separation is to prevent privilege
1.1 stevesk 1011: escalation by containing any corruption within the unprivileged processes.
1012: The default is
1013: .Dq yes .
1014: .It Cm X11DisplayOffset
1015: Specifies the first display number available for
1.52 jmc 1016: .Xr sshd 8 Ns 's
1.1 stevesk 1017: X11 forwarding.
1.52 jmc 1018: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1019: The default is 10.
1020: .It Cm X11Forwarding
1021: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1022: The argument must be
1023: .Dq yes
1024: or
1025: .Dq no .
1.1 stevesk 1026: The default is
1027: .Dq no .
1.13 stevesk 1028: .Pp
1029: When X11 forwarding is enabled, there may be additional exposure to
1030: the server and to client displays if the
1.52 jmc 1031: .Xr sshd 8
1.13 stevesk 1032: proxy display is configured to listen on the wildcard address (see
1033: .Cm X11UseLocalhost
1.52 jmc 1034: below), though this is not the default.
1.13 stevesk 1035: Additionally, the authentication spoofing and authentication data
1036: verification and substitution occur on the client side.
1037: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1038: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1039: forwarding (see the warnings for
1040: .Cm ForwardX11
1041: in
1.19 jmc 1042: .Xr ssh_config 5 ) .
1.13 stevesk 1043: A system administrator may have a stance in which they want to
1044: protect clients that may expose themselves to attack by unwittingly
1045: requesting X11 forwarding, which can warrant a
1046: .Dq no
1047: setting.
1048: .Pp
1049: Note that disabling X11 forwarding does not prevent users from
1050: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1051: X11 forwarding is automatically disabled if
1052: .Cm UseLogin
1053: is enabled.
1054: .It Cm X11UseLocalhost
1055: Specifies whether
1.52 jmc 1056: .Xr sshd 8
1.1 stevesk 1057: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1058: the wildcard address.
1059: By default,
1.52 jmc 1060: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1061: hostname part of the
1062: .Ev DISPLAY
1063: environment variable to
1064: .Dq localhost .
1.8 stevesk 1065: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1066: However, some older X11 clients may not function with this
1067: configuration.
1068: .Cm X11UseLocalhost
1069: may be set to
1070: .Dq no
1071: to specify that the forwarding server should be bound to the wildcard
1072: address.
1073: The argument must be
1074: .Dq yes
1075: or
1076: .Dq no .
1077: The default is
1078: .Dq yes .
1079: .It Cm XAuthLocation
1.11 stevesk 1080: Specifies the full pathname of the
1.1 stevesk 1081: .Xr xauth 1
1082: program.
1083: The default is
1084: .Pa /usr/X11R6/bin/xauth .
1085: .El
1.55 jmc 1086: .Sh TIME FORMATS
1.53 jmc 1087: .Xr sshd 8
1.1 stevesk 1088: command-line arguments and configuration file options that specify time
1089: may be expressed using a sequence of the form:
1090: .Sm off
1.7 stevesk 1091: .Ar time Op Ar qualifier ,
1.1 stevesk 1092: .Sm on
1093: where
1094: .Ar time
1095: is a positive integer value and
1096: .Ar qualifier
1097: is one of the following:
1098: .Pp
1099: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1100: .It Aq Cm none
1.1 stevesk 1101: seconds
1102: .It Cm s | Cm S
1103: seconds
1104: .It Cm m | Cm M
1105: minutes
1106: .It Cm h | Cm H
1107: hours
1108: .It Cm d | Cm D
1109: days
1110: .It Cm w | Cm W
1111: weeks
1112: .El
1113: .Pp
1114: Each member of the sequence is added together to calculate
1115: the total time value.
1116: .Pp
1117: Time format examples:
1118: .Pp
1119: .Bl -tag -width Ds -compact -offset indent
1120: .It 600
1121: 600 seconds (10 minutes)
1122: .It 10m
1123: 10 minutes
1124: .It 1h30m
1125: 1 hour 30 minutes (90 minutes)
1126: .El
1127: .Sh FILES
1128: .Bl -tag -width Ds
1129: .It Pa /etc/ssh/sshd_config
1130: Contains configuration data for
1.53 jmc 1131: .Xr sshd 8 .
1.1 stevesk 1132: This file should be writable by root only, but it is recommended
1133: (though not necessary) that it be world-readable.
1134: .El
1.19 jmc 1135: .Sh SEE ALSO
1136: .Xr sshd 8
1.1 stevesk 1137: .Sh AUTHORS
1138: OpenSSH is a derivative of the original and free
1139: ssh 1.2.12 release by Tatu Ylonen.
1140: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1141: Theo de Raadt and Dug Song
1142: removed many bugs, re-added newer features and
1143: created OpenSSH.
1144: Markus Friedl contributed the support for SSH
1145: protocol versions 1.5 and 2.0.
1146: Niels Provos and Markus Friedl contributed support
1147: for privilege separation.
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