Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.166
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.166 ! markus 36: .\" $OpenBSD: sshd_config.5,v 1.165 2013/10/29 18:49:32 jmc Exp $
1.165 jmc 37: .Dd $Mdocdate: October 29 2013 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.1 stevesk 123: .It Cm AllowTcpForwarding
124: Specifies whether TCP forwarding is permitted.
1.150 djm 125: The available options are
126: .Dq yes
127: or
128: .Dq all
129: to allow TCP forwarding,
130: .Dq no
131: to prevent all TCP forwarding,
132: .Dq local
133: to allow local (from the perspective of
1.151 jmc 134: .Xr ssh 1 )
135: forwarding only or
1.150 djm 136: .Dq remote
137: to allow remote forwarding only.
1.1 stevesk 138: The default is
139: .Dq yes .
140: Note that disabling TCP forwarding does not improve security unless
141: users are also denied shell access, as they can always install their
142: own forwarders.
143: .It Cm AllowUsers
144: This keyword can be followed by a list of user name patterns, separated
145: by spaces.
1.14 jmc 146: If specified, login is allowed only for user names that
1.1 stevesk 147: match one of the patterns.
148: Only user names are valid; a numerical user ID is not recognized.
149: By default, login is allowed for all users.
150: If the pattern takes the form USER@HOST then USER and HOST
151: are separately checked, restricting logins to particular
152: users from particular hosts.
1.54 jmc 153: The allow/deny directives are processed in the following order:
154: .Cm DenyUsers ,
155: .Cm AllowUsers ,
156: .Cm DenyGroups ,
157: and finally
158: .Cm AllowGroups .
1.49 jmc 159: .Pp
1.161 jmc 160: See PATTERNS in
1.49 jmc 161: .Xr ssh_config 5
162: for more information on patterns.
1.149 djm 163: .It Cm AuthenticationMethods
164: Specifies the authentication methods that must be successfully completed
165: for a user to be granted access.
166: This option must be followed by one or more comma-separated lists of
167: authentication method names.
168: Successful authentication requires completion of every method in at least
169: one of these lists.
170: .Pp
171: For example, an argument of
172: .Dq publickey,password publickey,keyboard-interactive
173: would require the user to complete public key authentication, followed by
174: either password or keyboard interactive authentication.
175: Only methods that are next in one or more lists are offered at each stage,
176: so for this example, it would not be possible to attempt password or
177: keyboard-interactive authentication before public key.
1.157 markus 178: .Pp
179: For keyboard interactive authentication it is also possible to
180: restrict authentication to a specific device by appending a
181: colon followed by the device identifier
182: .Dq bsdauth ,
183: .Dq pam ,
184: or
185: .Dq skey ,
186: depending on the server configuration.
187: For example,
188: .Dq keyboard-interactive:bsdauth
189: would restrict keyboard interactive authentication to the
190: .Dq bsdauth
191: device.
1.149 djm 192: .Pp
193: This option is only available for SSH protocol 2 and will yield a fatal
194: error if enabled if protocol 1 is also enabled.
195: Note that each authentication method listed should also be explicitly enabled
196: in the configuration.
197: The default is not to require multiple authentication; successful completion
198: of a single authentication method is sufficient.
1.146 djm 199: .It Cm AuthorizedKeysCommand
1.147 jmc 200: Specifies a program to be used to look up the user's public keys.
1.158 djm 201: The program must be owned by root and not writable by group or others.
202: It will be invoked with a single argument of the username
1.146 djm 203: being authenticated, and should produce on standard output zero or
1.161 jmc 204: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 205: .Xr sshd 8 ) .
1.146 djm 206: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
207: and authorize the user then public key authentication continues using the usual
208: .Cm AuthorizedKeysFile
209: files.
210: By default, no AuthorizedKeysCommand is run.
211: .It Cm AuthorizedKeysCommandUser
212: Specifies the user under whose account the AuthorizedKeysCommand is run.
213: It is recommended to use a dedicated user that has no other role on the host
214: than running authorized keys commands.
1.1 stevesk 215: .It Cm AuthorizedKeysFile
216: Specifies the file that contains the public keys that can be used
217: for user authentication.
1.124 djm 218: The format is described in the
1.161 jmc 219: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 220: section of
221: .Xr sshd 8 .
1.1 stevesk 222: .Cm AuthorizedKeysFile
223: may contain tokens of the form %T which are substituted during connection
1.52 jmc 224: setup.
1.17 jmc 225: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 226: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 227: %u is replaced by the username of that user.
228: After expansion,
229: .Cm AuthorizedKeysFile
230: is taken to be an absolute path or one relative to the user's home
231: directory.
1.133 jmc 232: Multiple files may be listed, separated by whitespace.
233: The default is
234: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 235: .It Cm AuthorizedPrincipalsFile
236: Specifies a file that lists principal names that are accepted for
237: certificate authentication.
238: When using certificates signed by a key listed in
239: .Cm TrustedUserCAKeys ,
240: this file lists names, one of which must appear in the certificate for it
241: to be accepted for authentication.
1.125 jmc 242: Names are listed one per line preceded by key options (as described
1.161 jmc 243: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 244: .Xr sshd 8 ) .
1.124 djm 245: Empty lines and comments starting with
1.121 djm 246: .Ql #
247: are ignored.
248: .Pp
249: .Cm AuthorizedPrincipalsFile
250: may contain tokens of the form %T which are substituted during connection
251: setup.
252: The following tokens are defined: %% is replaced by a literal '%',
253: %h is replaced by the home directory of the user being authenticated, and
254: %u is replaced by the username of that user.
255: After expansion,
256: .Cm AuthorizedPrincipalsFile
257: is taken to be an absolute path or one relative to the user's home
258: directory.
259: .Pp
1.138 djm 260: The default is
261: .Dq none ,
262: i.e. not to use a principals file \(en in this case, the username
1.121 djm 263: of the user must appear in a certificate's principals list for it to be
264: accepted.
265: Note that
266: .Cm AuthorizedPrincipalsFile
267: is only used when authentication proceeds using a CA listed in
268: .Cm TrustedUserCAKeys
269: and is not consulted for certification authorities trusted via
270: .Pa ~/.ssh/authorized_keys ,
271: though the
272: .Cm principals=
273: key option offers a similar facility (see
274: .Xr sshd 8
275: for details).
1.1 stevesk 276: .It Cm Banner
277: The contents of the specified file are sent to the remote user before
278: authentication is allowed.
1.78 djm 279: If the argument is
280: .Dq none
281: then no banner is displayed.
1.1 stevesk 282: This option is only available for protocol version 2.
283: By default, no banner is displayed.
284: .It Cm ChallengeResponseAuthentication
1.50 jmc 285: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 286: All authentication styles from
287: .Xr login.conf 5
288: are supported.
289: The default is
290: .Dq yes .
1.80 djm 291: .It Cm ChrootDirectory
1.113 stevesk 292: Specifies the pathname of a directory to
1.80 djm 293: .Xr chroot 2
294: to after authentication.
1.113 stevesk 295: All components of the pathname must be root-owned directories that are
1.80 djm 296: not writable by any other user or group.
1.106 stevesk 297: After the chroot,
298: .Xr sshd 8
299: changes the working directory to the user's home directory.
1.80 djm 300: .Pp
1.113 stevesk 301: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 302: the connecting user has been authenticated: %% is replaced by a literal '%',
303: %h is replaced by the home directory of the user being authenticated, and
304: %u is replaced by the username of that user.
305: .Pp
306: The
307: .Cm ChrootDirectory
308: must contain the necessary files and directories to support the
1.103 stevesk 309: user's session.
1.80 djm 310: For an interactive session this requires at least a shell, typically
311: .Xr sh 1 ,
312: and basic
313: .Pa /dev
314: nodes such as
315: .Xr null 4 ,
316: .Xr zero 4 ,
317: .Xr stdin 4 ,
318: .Xr stdout 4 ,
319: .Xr stderr 4 ,
320: .Xr arandom 4
321: and
322: .Xr tty 4
323: devices.
324: For file transfer sessions using
1.105 jmc 325: .Dq sftp ,
1.80 djm 326: no additional configuration of the environment is necessary if the
1.105 jmc 327: in-process sftp server is used,
328: though sessions which use logging do require
1.104 stevesk 329: .Pa /dev/log
330: inside the chroot directory (see
331: .Xr sftp-server 8
1.81 jmc 332: for details).
1.80 djm 333: .Pp
334: The default is not to
335: .Xr chroot 2 .
1.1 stevesk 336: .It Cm Ciphers
337: Specifies the ciphers allowed for protocol version 2.
338: Multiple ciphers must be comma-separated.
1.34 dtucker 339: The supported ciphers are
340: .Dq 3des-cbc ,
341: .Dq aes128-cbc ,
342: .Dq aes192-cbc ,
343: .Dq aes256-cbc ,
344: .Dq aes128-ctr ,
345: .Dq aes192-ctr ,
346: .Dq aes256-ctr ,
1.153 markus 347: .Dq aes128-gcm@openssh.com ,
348: .Dq aes256-gcm@openssh.com ,
1.43 djm 349: .Dq arcfour128 ,
350: .Dq arcfour256 ,
1.34 dtucker 351: .Dq arcfour ,
352: .Dq blowfish-cbc ,
353: and
354: .Dq cast128-cbc .
1.52 jmc 355: The default is:
356: .Bd -literal -offset 3n
1.100 naddy 357: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
1.153 markus 358: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.100 naddy 359: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
360: aes256-cbc,arcfour
1.1 stevesk 361: .Ed
362: .It Cm ClientAliveCountMax
1.48 jmc 363: Sets the number of client alive messages (see below) which may be
1.1 stevesk 364: sent without
1.52 jmc 365: .Xr sshd 8
1.17 jmc 366: receiving any messages back from the client.
367: If this threshold is reached while client alive messages are being sent,
1.52 jmc 368: sshd will disconnect the client, terminating the session.
1.17 jmc 369: It is important to note that the use of client alive messages is very
370: different from
1.27 markus 371: .Cm TCPKeepAlive
1.17 jmc 372: (below).
373: The client alive messages are sent through the encrypted channel
374: and therefore will not be spoofable.
375: The TCP keepalive option enabled by
1.27 markus 376: .Cm TCPKeepAlive
1.17 jmc 377: is spoofable.
378: The client alive mechanism is valuable when the client or
1.1 stevesk 379: server depend on knowing when a connection has become inactive.
380: .Pp
1.17 jmc 381: The default value is 3.
382: If
1.1 stevesk 383: .Cm ClientAliveInterval
1.48 jmc 384: (see below) is set to 15, and
1.1 stevesk 385: .Cm ClientAliveCountMax
1.52 jmc 386: is left at the default, unresponsive SSH clients
1.1 stevesk 387: will be disconnected after approximately 45 seconds.
1.57 markus 388: This option applies to protocol version 2 only.
1.42 djm 389: .It Cm ClientAliveInterval
390: Sets a timeout interval in seconds after which if no data has been received
391: from the client,
1.52 jmc 392: .Xr sshd 8
1.42 djm 393: will send a message through the encrypted
394: channel to request a response from the client.
395: The default
396: is 0, indicating that these messages will not be sent to the client.
397: This option applies to protocol version 2 only.
1.3 markus 398: .It Cm Compression
1.44 markus 399: Specifies whether compression is allowed, or delayed until
400: the user has authenticated successfully.
1.3 markus 401: The argument must be
1.44 markus 402: .Dq yes ,
403: .Dq delayed ,
1.3 markus 404: or
405: .Dq no .
406: The default is
1.44 markus 407: .Dq delayed .
1.1 stevesk 408: .It Cm DenyGroups
409: This keyword can be followed by a list of group name patterns, separated
410: by spaces.
411: Login is disallowed for users whose primary group or supplementary
412: group list matches one of the patterns.
413: Only group names are valid; a numerical group ID is not recognized.
414: By default, login is allowed for all groups.
1.54 jmc 415: The allow/deny directives are processed in the following order:
416: .Cm DenyUsers ,
417: .Cm AllowUsers ,
418: .Cm DenyGroups ,
419: and finally
420: .Cm AllowGroups .
1.49 jmc 421: .Pp
1.161 jmc 422: See PATTERNS in
1.49 jmc 423: .Xr ssh_config 5
424: for more information on patterns.
1.1 stevesk 425: .It Cm DenyUsers
426: This keyword can be followed by a list of user name patterns, separated
427: by spaces.
428: Login is disallowed for user names that match one of the patterns.
429: Only user names are valid; a numerical user ID is not recognized.
430: By default, login is allowed for all users.
431: If the pattern takes the form USER@HOST then USER and HOST
432: are separately checked, restricting logins to particular
433: users from particular hosts.
1.54 jmc 434: The allow/deny directives are processed in the following order:
435: .Cm DenyUsers ,
436: .Cm AllowUsers ,
437: .Cm DenyGroups ,
438: and finally
439: .Cm AllowGroups .
1.49 jmc 440: .Pp
1.161 jmc 441: See PATTERNS in
1.49 jmc 442: .Xr ssh_config 5
443: for more information on patterns.
1.67 dtucker 444: .It Cm ForceCommand
445: Forces the execution of the command specified by
446: .Cm ForceCommand ,
1.84 djm 447: ignoring any command supplied by the client and
448: .Pa ~/.ssh/rc
449: if present.
1.67 dtucker 450: The command is invoked by using the user's login shell with the -c option.
451: This applies to shell, command, or subsystem execution.
452: It is most useful inside a
453: .Cm Match
454: block.
455: The command originally supplied by the client is available in the
456: .Ev SSH_ORIGINAL_COMMAND
457: environment variable.
1.82 djm 458: Specifying a command of
459: .Dq internal-sftp
460: will force the use of an in-process sftp server that requires no support
461: files when used with
462: .Cm ChrootDirectory .
1.1 stevesk 463: .It Cm GatewayPorts
464: Specifies whether remote hosts are allowed to connect to ports
465: forwarded for the client.
466: By default,
1.52 jmc 467: .Xr sshd 8
1.15 jmc 468: binds remote port forwardings to the loopback address.
469: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 470: .Cm GatewayPorts
1.52 jmc 471: can be used to specify that sshd
1.39 djm 472: should allow remote port forwardings to bind to non-loopback addresses, thus
473: allowing other hosts to connect.
474: The argument may be
475: .Dq no
476: to force remote port forwardings to be available to the local host only,
1.1 stevesk 477: .Dq yes
1.39 djm 478: to force remote port forwardings to bind to the wildcard address, or
479: .Dq clientspecified
480: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 481: The default is
482: .Dq no .
1.23 markus 483: .It Cm GSSAPIAuthentication
1.25 markus 484: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 485: The default is
1.23 markus 486: .Dq no .
487: Note that this option applies to protocol version 2 only.
488: .It Cm GSSAPICleanupCredentials
489: Specifies whether to automatically destroy the user's credentials cache
490: on logout.
491: The default is
492: .Dq yes .
493: Note that this option applies to protocol version 2 only.
1.1 stevesk 494: .It Cm HostbasedAuthentication
495: Specifies whether rhosts or /etc/hosts.equiv authentication together
496: with successful public key client host authentication is allowed
1.50 jmc 497: (host-based authentication).
1.1 stevesk 498: This option is similar to
499: .Cm RhostsRSAAuthentication
500: and applies to protocol version 2 only.
1.70 dtucker 501: The default is
502: .Dq no .
503: .It Cm HostbasedUsesNameFromPacketOnly
504: Specifies whether or not the server will attempt to perform a reverse
505: name lookup when matching the name in the
506: .Pa ~/.shosts ,
507: .Pa ~/.rhosts ,
508: and
509: .Pa /etc/hosts.equiv
510: files during
511: .Cm HostbasedAuthentication .
512: A setting of
513: .Dq yes
514: means that
515: .Xr sshd 8
516: uses the name supplied by the client rather than
517: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 518: The default is
519: .Dq no .
1.117 djm 520: .It Cm HostCertificate
521: Specifies a file containing a public host certificate.
522: The certificate's public key must match a private host key already specified
523: by
524: .Cm HostKey .
525: The default behaviour of
526: .Xr sshd 8
527: is not to load any certificates.
1.1 stevesk 528: .It Cm HostKey
529: Specifies a file containing a private host key
530: used by SSH.
531: The default is
532: .Pa /etc/ssh/ssh_host_key
533: for protocol version 1, and
1.126 djm 534: .Pa /etc/ssh/ssh_host_dsa_key ,
535: .Pa /etc/ssh/ssh_host_ecdsa_key
536: and
1.1 stevesk 537: .Pa /etc/ssh/ssh_host_rsa_key
538: for protocol version 2.
539: Note that
1.52 jmc 540: .Xr sshd 8
1.1 stevesk 541: will refuse to use a file if it is group/world-accessible.
542: It is possible to have multiple host key files.
543: .Dq rsa1
544: keys are used for version 1 and
1.126 djm 545: .Dq dsa ,
546: .Dq ecdsa
1.1 stevesk 547: or
548: .Dq rsa
549: are used for version 2 of the SSH protocol.
1.162 markus 550: It is also possible to specify public host key files instead.
551: In this case operations on the private key will be delegated
552: to an
553: .Xr ssh-agent 1 .
554: .It Cm HostKeyAgent
555: Identifies the UNIX-domain socket used to communicate
556: with an agent that has access to the private host keys.
557: If
558: .Dq SSH_AUTH_SOCK
559: is specified, the location of the socket will be read from the
560: .Ev SSH_AUTH_SOCK
561: environment variable.
1.1 stevesk 562: .It Cm IgnoreRhosts
563: Specifies that
564: .Pa .rhosts
565: and
566: .Pa .shosts
567: files will not be used in
568: .Cm RhostsRSAAuthentication
569: or
570: .Cm HostbasedAuthentication .
571: .Pp
572: .Pa /etc/hosts.equiv
573: and
574: .Pa /etc/shosts.equiv
575: are still used.
576: The default is
577: .Dq yes .
578: .It Cm IgnoreUserKnownHosts
579: Specifies whether
1.52 jmc 580: .Xr sshd 8
1.1 stevesk 581: should ignore the user's
1.41 djm 582: .Pa ~/.ssh/known_hosts
1.1 stevesk 583: during
584: .Cm RhostsRSAAuthentication
585: or
586: .Cm HostbasedAuthentication .
587: The default is
588: .Dq no .
1.129 djm 589: .It Cm IPQoS
590: Specifies the IPv4 type-of-service or DSCP class for the connection.
591: Accepted values are
592: .Dq af11 ,
593: .Dq af12 ,
594: .Dq af13 ,
1.136 djm 595: .Dq af21 ,
1.129 djm 596: .Dq af22 ,
597: .Dq af23 ,
598: .Dq af31 ,
599: .Dq af32 ,
600: .Dq af33 ,
601: .Dq af41 ,
602: .Dq af42 ,
603: .Dq af43 ,
604: .Dq cs0 ,
605: .Dq cs1 ,
606: .Dq cs2 ,
607: .Dq cs3 ,
608: .Dq cs4 ,
609: .Dq cs5 ,
610: .Dq cs6 ,
611: .Dq cs7 ,
612: .Dq ef ,
613: .Dq lowdelay ,
614: .Dq throughput ,
615: .Dq reliability ,
616: or a numeric value.
1.131 djm 617: This option may take one or two arguments, separated by whitespace.
1.129 djm 618: If one argument is specified, it is used as the packet class unconditionally.
619: If two values are specified, the first is automatically selected for
620: interactive sessions and the second for non-interactive sessions.
621: The default is
622: .Dq lowdelay
623: for interactive sessions and
624: .Dq throughput
625: for non-interactive sessions.
1.1 stevesk 626: .It Cm KerberosAuthentication
1.24 markus 627: Specifies whether the password provided by the user for
1.1 stevesk 628: .Cm PasswordAuthentication
1.24 markus 629: will be validated through the Kerberos KDC.
1.1 stevesk 630: To use this option, the server needs a
631: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 632: The default is
1.29 dtucker 633: .Dq no .
634: .It Cm KerberosGetAFSToken
1.45 djm 635: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 636: an AFS token before accessing the user's home directory.
1.52 jmc 637: The default is
1.1 stevesk 638: .Dq no .
639: .It Cm KerberosOrLocalPasswd
1.52 jmc 640: If password authentication through Kerberos fails then
1.1 stevesk 641: the password will be validated via any additional local mechanism
642: such as
643: .Pa /etc/passwd .
1.52 jmc 644: The default is
1.1 stevesk 645: .Dq yes .
646: .It Cm KerberosTicketCleanup
647: Specifies whether to automatically destroy the user's ticket cache
648: file on logout.
1.52 jmc 649: The default is
1.1 stevesk 650: .Dq yes .
1.127 djm 651: .It Cm KexAlgorithms
652: Specifies the available KEX (Key Exchange) algorithms.
653: Multiple algorithms must be comma-separated.
654: The default is
1.166 ! markus 655: .Dq curve25519-sha256@libssh.org ,
1.127 djm 656: .Dq ecdh-sha2-nistp256 ,
657: .Dq ecdh-sha2-nistp384 ,
658: .Dq ecdh-sha2-nistp521 ,
1.130 jmc 659: .Dq diffie-hellman-group-exchange-sha256 ,
1.127 djm 660: .Dq diffie-hellman-group-exchange-sha1 ,
661: .Dq diffie-hellman-group14-sha1 ,
662: .Dq diffie-hellman-group1-sha1 .
1.1 stevesk 663: .It Cm KeyRegenerationInterval
664: In protocol version 1, the ephemeral server key is automatically regenerated
665: after this many seconds (if it has been used).
666: The purpose of regeneration is to prevent
667: decrypting captured sessions by later breaking into the machine and
668: stealing the keys.
669: The key is never stored anywhere.
670: If the value is 0, the key is never regenerated.
671: The default is 3600 (seconds).
672: .It Cm ListenAddress
673: Specifies the local addresses
1.52 jmc 674: .Xr sshd 8
1.1 stevesk 675: should listen on.
676: The following forms may be used:
677: .Pp
678: .Bl -item -offset indent -compact
679: .It
680: .Cm ListenAddress
681: .Sm off
682: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
683: .Sm on
684: .It
685: .Cm ListenAddress
686: .Sm off
687: .Ar host No | Ar IPv4_addr No : Ar port
688: .Sm on
689: .It
690: .Cm ListenAddress
691: .Sm off
692: .Oo
693: .Ar host No | Ar IPv6_addr Oc : Ar port
694: .Sm on
695: .El
696: .Pp
697: If
698: .Ar port
699: is not specified,
1.52 jmc 700: sshd will listen on the address and all prior
1.1 stevesk 701: .Cm Port
1.17 jmc 702: options specified.
703: The default is to listen on all local addresses.
1.15 jmc 704: Multiple
1.1 stevesk 705: .Cm ListenAddress
1.17 jmc 706: options are permitted.
707: Additionally, any
1.1 stevesk 708: .Cm Port
1.52 jmc 709: options must precede this option for non-port qualified addresses.
1.1 stevesk 710: .It Cm LoginGraceTime
711: The server disconnects after this time if the user has not
712: successfully logged in.
713: If the value is 0, there is no time limit.
1.12 stevesk 714: The default is 120 seconds.
1.1 stevesk 715: .It Cm LogLevel
716: Gives the verbosity level that is used when logging messages from
1.53 jmc 717: .Xr sshd 8 .
1.1 stevesk 718: The possible values are:
1.52 jmc 719: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 720: The default is INFO.
721: DEBUG and DEBUG1 are equivalent.
722: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
723: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 724: .It Cm MACs
725: Specifies the available MAC (message authentication code) algorithms.
726: The MAC algorithm is used in protocol version 2
727: for data integrity protection.
728: Multiple algorithms must be comma-separated.
1.152 markus 729: The algorithms that contain
730: .Dq -etm
731: calculate the MAC after encryption (encrypt-then-mac).
732: These are considered safer and their use recommended.
1.52 jmc 733: The default is:
1.77 jmc 734: .Bd -literal -offset indent
1.152 markus 735: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
736: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
737: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
738: hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
739: hmac-md5-96-etm@openssh.com,
1.145 markus 740: hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
1.144 naddy 741: hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
742: hmac-sha1-96,hmac-md5-96
1.77 jmc 743: .Ed
1.60 dtucker 744: .It Cm Match
1.61 jmc 745: Introduces a conditional block.
1.65 dtucker 746: If all of the criteria on the
1.60 dtucker 747: .Cm Match
1.65 dtucker 748: line are satisfied, the keywords on the following lines override those
749: set in the global section of the config file, until either another
1.60 dtucker 750: .Cm Match
1.65 dtucker 751: line or the end of the file.
1.91 djm 752: .Pp
1.61 jmc 753: The arguments to
1.60 dtucker 754: .Cm Match
1.163 dtucker 755: are one or more criteria-pattern pairs or the single token
756: .Cm All
757: which matches all criteria.
1.60 dtucker 758: The available criteria are
759: .Cm User ,
1.69 dtucker 760: .Cm Group ,
1.60 dtucker 761: .Cm Host ,
1.139 dtucker 762: .Cm LocalAddress ,
763: .Cm LocalPort ,
1.60 dtucker 764: and
765: .Cm Address .
1.91 djm 766: The match patterns may consist of single entries or comma-separated
767: lists and may use the wildcard and negation operators described in the
1.161 jmc 768: PATTERNS section of
1.92 djm 769: .Xr ssh_config 5 .
1.91 djm 770: .Pp
771: The patterns in an
772: .Cm Address
773: criteria may additionally contain addresses to match in CIDR
1.93 jmc 774: address/masklen format, e.g.\&
1.91 djm 775: .Dq 192.0.2.0/24
776: or
777: .Dq 3ffe:ffff::/32 .
778: Note that the mask length provided must be consistent with the address -
779: it is an error to specify a mask length that is too long for the address
1.93 jmc 780: or one with bits set in this host portion of the address.
781: For example,
1.91 djm 782: .Dq 192.0.2.0/33
783: and
1.93 jmc 784: .Dq 192.0.2.0/8
1.91 djm 785: respectively.
786: .Pp
1.60 dtucker 787: Only a subset of keywords may be used on the lines following a
788: .Cm Match
789: keyword.
790: Available keywords are
1.142 jmc 791: .Cm AcceptEnv ,
1.99 okan 792: .Cm AllowAgentForwarding ,
1.142 jmc 793: .Cm AllowGroups ,
1.62 dtucker 794: .Cm AllowTcpForwarding ,
1.141 markus 795: .Cm AllowUsers ,
1.149 djm 796: .Cm AuthenticationMethods ,
1.146 djm 797: .Cm AuthorizedKeysCommand ,
798: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 799: .Cm AuthorizedKeysFile ,
1.123 djm 800: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 801: .Cm Banner ,
1.85 djm 802: .Cm ChrootDirectory ,
1.141 markus 803: .Cm DenyGroups ,
804: .Cm DenyUsers ,
1.67 dtucker 805: .Cm ForceCommand ,
1.142 jmc 806: .Cm GatewayPorts ,
1.141 markus 807: .Cm GSSAPIAuthentication ,
1.87 djm 808: .Cm HostbasedAuthentication ,
1.123 djm 809: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 810: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 811: .Cm KerberosAuthentication ,
1.95 dtucker 812: .Cm MaxAuthTries ,
1.94 dtucker 813: .Cm MaxSessions ,
1.72 dtucker 814: .Cm PasswordAuthentication ,
1.97 djm 815: .Cm PermitEmptyPasswords ,
1.66 dtucker 816: .Cm PermitOpen ,
1.79 dtucker 817: .Cm PermitRootLogin ,
1.164 djm 818: .Cm PermitTTY ,
1.123 djm 819: .Cm PermitTunnel ,
1.107 dtucker 820: .Cm PubkeyAuthentication ,
1.159 dtucker 821: .Cm RekeyLimit ,
1.142 jmc 822: .Cm RhostsRSAAuthentication ,
1.141 markus 823: .Cm RSAAuthentication ,
1.66 dtucker 824: .Cm X11DisplayOffset ,
1.101 djm 825: .Cm X11Forwarding
1.60 dtucker 826: and
1.102 djm 827: .Cm X11UseLocalHost .
1.33 dtucker 828: .It Cm MaxAuthTries
829: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 830: connection.
831: Once the number of failures reaches half this value,
832: additional failures are logged.
833: The default is 6.
1.90 djm 834: .It Cm MaxSessions
835: Specifies the maximum number of open sessions permitted per network connection.
836: The default is 10.
1.1 stevesk 837: .It Cm MaxStartups
838: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 839: SSH daemon.
1.1 stevesk 840: Additional connections will be dropped until authentication succeeds or the
841: .Cm LoginGraceTime
842: expires for a connection.
1.156 dtucker 843: The default is 10:30:100.
1.1 stevesk 844: .Pp
845: Alternatively, random early drop can be enabled by specifying
846: the three colon separated values
847: .Dq start:rate:full
1.51 jmc 848: (e.g. "10:30:60").
1.53 jmc 849: .Xr sshd 8
1.1 stevesk 850: will refuse connection attempts with a probability of
851: .Dq rate/100
852: (30%)
853: if there are currently
854: .Dq start
855: (10)
856: unauthenticated connections.
857: The probability increases linearly and all connection attempts
858: are refused if the number of unauthenticated connections reaches
859: .Dq full
860: (60).
861: .It Cm PasswordAuthentication
862: Specifies whether password authentication is allowed.
863: The default is
864: .Dq yes .
865: .It Cm PermitEmptyPasswords
866: When password authentication is allowed, it specifies whether the
867: server allows login to accounts with empty password strings.
868: The default is
869: .Dq no .
1.62 dtucker 870: .It Cm PermitOpen
871: Specifies the destinations to which TCP port forwarding is permitted.
872: The forwarding specification must be one of the following forms:
873: .Pp
874: .Bl -item -offset indent -compact
875: .It
876: .Cm PermitOpen
877: .Sm off
878: .Ar host : port
879: .Sm on
880: .It
881: .Cm PermitOpen
882: .Sm off
883: .Ar IPv4_addr : port
884: .Sm on
885: .It
886: .Cm PermitOpen
887: .Sm off
888: .Ar \&[ IPv6_addr \&] : port
889: .Sm on
890: .El
891: .Pp
1.68 dtucker 892: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 893: An argument of
894: .Dq any
895: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 896: An argument of
897: .Dq none
898: can be used to prohibit all forwarding requests.
1.63 jmc 899: By default all port forwarding requests are permitted.
1.1 stevesk 900: .It Cm PermitRootLogin
1.38 jmc 901: Specifies whether root can log in using
1.1 stevesk 902: .Xr ssh 1 .
903: The argument must be
904: .Dq yes ,
905: .Dq without-password ,
1.52 jmc 906: .Dq forced-commands-only ,
1.1 stevesk 907: or
908: .Dq no .
909: The default is
910: .Dq yes .
911: .Pp
912: If this option is set to
1.52 jmc 913: .Dq without-password ,
1.1 stevesk 914: password authentication is disabled for root.
915: .Pp
916: If this option is set to
1.52 jmc 917: .Dq forced-commands-only ,
1.1 stevesk 918: root login with public key authentication will be allowed,
919: but only if the
920: .Ar command
921: option has been specified
922: (which may be useful for taking remote backups even if root login is
1.17 jmc 923: normally not allowed).
924: All other authentication methods are disabled for root.
1.1 stevesk 925: .Pp
926: If this option is set to
1.52 jmc 927: .Dq no ,
1.38 jmc 928: root is not allowed to log in.
1.46 reyk 929: .It Cm PermitTunnel
930: Specifies whether
931: .Xr tun 4
932: device forwarding is allowed.
1.47 reyk 933: The argument must be
934: .Dq yes ,
1.58 stevesk 935: .Dq point-to-point
936: (layer 3),
937: .Dq ethernet
938: (layer 2), or
1.47 reyk 939: .Dq no .
1.58 stevesk 940: Specifying
941: .Dq yes
942: permits both
943: .Dq point-to-point
944: and
945: .Dq ethernet .
1.46 reyk 946: The default is
947: .Dq no .
1.164 djm 948: .It Cm PermitTTY
949: Specifies whether
1.165 jmc 950: .Xr pty 4
1.164 djm 951: allocation is permitted.
952: The default is
953: .Dq yes .
1.6 markus 954: .It Cm PermitUserEnvironment
955: Specifies whether
956: .Pa ~/.ssh/environment
1.9 stevesk 957: and
1.6 markus 958: .Cm environment=
959: options in
960: .Pa ~/.ssh/authorized_keys
1.9 stevesk 961: are processed by
1.52 jmc 962: .Xr sshd 8 .
1.6 markus 963: The default is
964: .Dq no .
1.9 stevesk 965: Enabling environment processing may enable users to bypass access
966: restrictions in some configurations using mechanisms such as
967: .Ev LD_PRELOAD .
1.1 stevesk 968: .It Cm PidFile
1.4 stevesk 969: Specifies the file that contains the process ID of the
1.53 jmc 970: SSH daemon.
1.1 stevesk 971: The default is
972: .Pa /var/run/sshd.pid .
973: .It Cm Port
974: Specifies the port number that
1.52 jmc 975: .Xr sshd 8
1.1 stevesk 976: listens on.
977: The default is 22.
978: Multiple options of this type are permitted.
979: See also
980: .Cm ListenAddress .
981: .It Cm PrintLastLog
982: Specifies whether
1.52 jmc 983: .Xr sshd 8
1.36 jaredy 984: should print the date and time of the last user login when a user logs
985: in interactively.
1.1 stevesk 986: The default is
987: .Dq yes .
988: .It Cm PrintMotd
989: Specifies whether
1.52 jmc 990: .Xr sshd 8
1.1 stevesk 991: should print
992: .Pa /etc/motd
993: when a user logs in interactively.
994: (On some systems it is also printed by the shell,
995: .Pa /etc/profile ,
996: or equivalent.)
997: The default is
998: .Dq yes .
999: .It Cm Protocol
1000: Specifies the protocol versions
1.52 jmc 1001: .Xr sshd 8
1.5 stevesk 1002: supports.
1.1 stevesk 1003: The possible values are
1.52 jmc 1004: .Sq 1
1.1 stevesk 1005: and
1.52 jmc 1006: .Sq 2 .
1.1 stevesk 1007: Multiple versions must be comma-separated.
1008: The default is
1.109 jmc 1009: .Sq 2 .
1.5 stevesk 1010: Note that the order of the protocol list does not indicate preference,
1011: because the client selects among multiple protocol versions offered
1012: by the server.
1013: Specifying
1014: .Dq 2,1
1015: is identical to
1016: .Dq 1,2 .
1.1 stevesk 1017: .It Cm PubkeyAuthentication
1018: Specifies whether public key authentication is allowed.
1019: The default is
1020: .Dq yes .
1021: Note that this option applies to protocol version 2 only.
1.159 dtucker 1022: .It Cm RekeyLimit
1023: Specifies the maximum amount of data that may be transmitted before the
1024: session key is renegotiated, optionally followed a maximum amount of
1025: time that may pass before the session key is renegotiated.
1026: The first argument is specified in bytes and may have a suffix of
1027: .Sq K ,
1028: .Sq M ,
1029: or
1030: .Sq G
1031: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1032: The default is between
1033: .Sq 1G
1034: and
1035: .Sq 4G ,
1036: depending on the cipher.
1037: The optional second value is specified in seconds and may use any of the
1038: units documented in the
1039: .Sx TIME FORMATS
1.160 jmc 1040: section.
1.159 dtucker 1041: The default value for
1042: .Cm RekeyLimit
1043: is
1044: .Dq default none ,
1045: which means that rekeying is performed after the cipher's default amount
1046: of data has been sent or received and no time based rekeying is done.
1047: This option applies to protocol version 2 only.
1.118 djm 1048: .It Cm RevokedKeys
1.154 djm 1049: Specifies revoked public keys.
1.118 djm 1050: Keys listed in this file will be refused for public key authentication.
1051: Note that if this file is not readable, then public key authentication will
1052: be refused for all users.
1.154 djm 1053: Keys may be specified as a text file, listing one public key per line, or as
1054: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1055: .Xr ssh-keygen 1 .
1.161 jmc 1056: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1057: .Xr ssh-keygen 1 .
1.1 stevesk 1058: .It Cm RhostsRSAAuthentication
1059: Specifies whether rhosts or /etc/hosts.equiv authentication together
1060: with successful RSA host authentication is allowed.
1061: The default is
1062: .Dq no .
1063: This option applies to protocol version 1 only.
1064: .It Cm RSAAuthentication
1065: Specifies whether pure RSA authentication is allowed.
1066: The default is
1067: .Dq yes .
1068: This option applies to protocol version 1 only.
1069: .It Cm ServerKeyBits
1070: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1071: The minimum value is 512, and the default is 1024.
1.1 stevesk 1072: .It Cm StrictModes
1073: Specifies whether
1.52 jmc 1074: .Xr sshd 8
1.1 stevesk 1075: should check file modes and ownership of the
1076: user's files and home directory before accepting login.
1077: This is normally desirable because novices sometimes accidentally leave their
1078: directory or files world-writable.
1079: The default is
1080: .Dq yes .
1.112 djm 1081: Note that this does not apply to
1082: .Cm ChrootDirectory ,
1083: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1084: .It Cm Subsystem
1.51 jmc 1085: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1086: Arguments should be a subsystem name and a command (with optional arguments)
1087: to execute upon subsystem request.
1.80 djm 1088: .Pp
1.1 stevesk 1089: The command
1090: .Xr sftp-server 8
1091: implements the
1092: .Dq sftp
1093: file transfer subsystem.
1.80 djm 1094: .Pp
1095: Alternately the name
1096: .Dq internal-sftp
1097: implements an in-process
1098: .Dq sftp
1099: server.
1100: This may simplify configurations using
1101: .Cm ChrootDirectory
1102: to force a different filesystem root on clients.
1103: .Pp
1.1 stevesk 1104: By default no subsystems are defined.
1105: Note that this option applies to protocol version 2 only.
1106: .It Cm SyslogFacility
1107: Gives the facility code that is used when logging messages from
1.53 jmc 1108: .Xr sshd 8 .
1.1 stevesk 1109: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1110: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1111: The default is AUTH.
1.27 markus 1112: .It Cm TCPKeepAlive
1113: Specifies whether the system should send TCP keepalive messages to the
1114: other side.
1115: If they are sent, death of the connection or crash of one
1116: of the machines will be properly noticed.
1117: However, this means that
1118: connections will die if the route is down temporarily, and some people
1119: find it annoying.
1120: On the other hand, if TCP keepalives are not sent,
1121: sessions may hang indefinitely on the server, leaving
1122: .Dq ghost
1123: users and consuming server resources.
1124: .Pp
1125: The default is
1126: .Dq yes
1127: (to send TCP keepalive messages), and the server will notice
1128: if the network goes down or the client host crashes.
1129: This avoids infinitely hanging sessions.
1130: .Pp
1131: To disable TCP keepalive messages, the value should be set to
1132: .Dq no .
1.118 djm 1133: .It Cm TrustedUserCAKeys
1134: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1135: trusted to sign user certificates for authentication.
1.119 jmc 1136: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1137: .Ql #
1138: are allowed.
1139: If a certificate is presented for authentication and has its signing CA key
1140: listed in this file, then it may be used for authentication for any user
1141: listed in the certificate's principals list.
1142: Note that certificates that lack a list of principals will not be permitted
1143: for authentication using
1144: .Cm TrustedUserCAKeys .
1.161 jmc 1145: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1146: .Xr ssh-keygen 1 .
1.18 markus 1147: .It Cm UseDNS
1148: Specifies whether
1.52 jmc 1149: .Xr sshd 8
1.40 jmc 1150: should look up the remote host name and check that
1.18 markus 1151: the resolved host name for the remote IP address maps back to the
1152: very same IP address.
1153: The default is
1154: .Dq yes .
1.1 stevesk 1155: .It Cm UseLogin
1156: Specifies whether
1157: .Xr login 1
1158: is used for interactive login sessions.
1159: The default is
1160: .Dq no .
1161: Note that
1162: .Xr login 1
1163: is never used for remote command execution.
1164: Note also, that if this is enabled,
1165: .Cm X11Forwarding
1166: will be disabled because
1167: .Xr login 1
1168: does not know how to handle
1169: .Xr xauth 1
1.15 jmc 1170: cookies.
1171: If
1.1 stevesk 1172: .Cm UsePrivilegeSeparation
1173: is specified, it will be disabled after authentication.
1174: .It Cm UsePrivilegeSeparation
1175: Specifies whether
1.52 jmc 1176: .Xr sshd 8
1.2 stevesk 1177: separates privileges by creating an unprivileged child process
1.15 jmc 1178: to deal with incoming network traffic.
1179: After successful authentication, another process will be created that has
1180: the privilege of the authenticated user.
1181: The goal of privilege separation is to prevent privilege
1.1 stevesk 1182: escalation by containing any corruption within the unprivileged processes.
1183: The default is
1184: .Dq yes .
1.134 djm 1185: If
1186: .Cm UsePrivilegeSeparation
1187: is set to
1188: .Dq sandbox
1189: then the pre-authentication unprivileged process is subject to additional
1190: restrictions.
1.137 djm 1191: .It Cm VersionAddendum
1192: Optionally specifies additional text to append to the SSH protocol banner
1193: sent by the server upon connection.
1194: The default is
1195: .Dq none .
1.1 stevesk 1196: .It Cm X11DisplayOffset
1197: Specifies the first display number available for
1.52 jmc 1198: .Xr sshd 8 Ns 's
1.1 stevesk 1199: X11 forwarding.
1.52 jmc 1200: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1201: The default is 10.
1202: .It Cm X11Forwarding
1203: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1204: The argument must be
1205: .Dq yes
1206: or
1207: .Dq no .
1.1 stevesk 1208: The default is
1209: .Dq no .
1.13 stevesk 1210: .Pp
1211: When X11 forwarding is enabled, there may be additional exposure to
1212: the server and to client displays if the
1.52 jmc 1213: .Xr sshd 8
1.13 stevesk 1214: proxy display is configured to listen on the wildcard address (see
1215: .Cm X11UseLocalhost
1.52 jmc 1216: below), though this is not the default.
1.13 stevesk 1217: Additionally, the authentication spoofing and authentication data
1218: verification and substitution occur on the client side.
1219: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1220: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1221: forwarding (see the warnings for
1222: .Cm ForwardX11
1223: in
1.19 jmc 1224: .Xr ssh_config 5 ) .
1.13 stevesk 1225: A system administrator may have a stance in which they want to
1226: protect clients that may expose themselves to attack by unwittingly
1227: requesting X11 forwarding, which can warrant a
1228: .Dq no
1229: setting.
1230: .Pp
1231: Note that disabling X11 forwarding does not prevent users from
1232: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1233: X11 forwarding is automatically disabled if
1234: .Cm UseLogin
1235: is enabled.
1236: .It Cm X11UseLocalhost
1237: Specifies whether
1.52 jmc 1238: .Xr sshd 8
1.1 stevesk 1239: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1240: the wildcard address.
1241: By default,
1.52 jmc 1242: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1243: hostname part of the
1244: .Ev DISPLAY
1245: environment variable to
1246: .Dq localhost .
1.8 stevesk 1247: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1248: However, some older X11 clients may not function with this
1249: configuration.
1250: .Cm X11UseLocalhost
1251: may be set to
1252: .Dq no
1253: to specify that the forwarding server should be bound to the wildcard
1254: address.
1255: The argument must be
1256: .Dq yes
1257: or
1258: .Dq no .
1259: The default is
1260: .Dq yes .
1261: .It Cm XAuthLocation
1.11 stevesk 1262: Specifies the full pathname of the
1.1 stevesk 1263: .Xr xauth 1
1264: program.
1265: The default is
1266: .Pa /usr/X11R6/bin/xauth .
1267: .El
1.55 jmc 1268: .Sh TIME FORMATS
1.53 jmc 1269: .Xr sshd 8
1.1 stevesk 1270: command-line arguments and configuration file options that specify time
1271: may be expressed using a sequence of the form:
1272: .Sm off
1.7 stevesk 1273: .Ar time Op Ar qualifier ,
1.1 stevesk 1274: .Sm on
1275: where
1276: .Ar time
1277: is a positive integer value and
1278: .Ar qualifier
1279: is one of the following:
1280: .Pp
1281: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1282: .It Aq Cm none
1.1 stevesk 1283: seconds
1284: .It Cm s | Cm S
1285: seconds
1286: .It Cm m | Cm M
1287: minutes
1288: .It Cm h | Cm H
1289: hours
1290: .It Cm d | Cm D
1291: days
1292: .It Cm w | Cm W
1293: weeks
1294: .El
1295: .Pp
1296: Each member of the sequence is added together to calculate
1297: the total time value.
1298: .Pp
1299: Time format examples:
1300: .Pp
1301: .Bl -tag -width Ds -compact -offset indent
1302: .It 600
1303: 600 seconds (10 minutes)
1304: .It 10m
1305: 10 minutes
1306: .It 1h30m
1307: 1 hour 30 minutes (90 minutes)
1308: .El
1309: .Sh FILES
1310: .Bl -tag -width Ds
1311: .It Pa /etc/ssh/sshd_config
1312: Contains configuration data for
1.53 jmc 1313: .Xr sshd 8 .
1.1 stevesk 1314: This file should be writable by root only, but it is recommended
1315: (though not necessary) that it be world-readable.
1316: .El
1.19 jmc 1317: .Sh SEE ALSO
1318: .Xr sshd 8
1.1 stevesk 1319: .Sh AUTHORS
1320: OpenSSH is a derivative of the original and free
1321: ssh 1.2.12 release by Tatu Ylonen.
1322: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1323: Theo de Raadt and Dug Song
1324: removed many bugs, re-added newer features and
1325: created OpenSSH.
1326: Markus Friedl contributed the support for SSH
1327: protocol versions 1.5 and 2.0.
1328: Niels Provos and Markus Friedl contributed support
1329: for privilege separation.