Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.165
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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1.165 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.164 2013/10/29 09:48:02 djm Exp $
! 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
655: .Dq ecdh-sha2-nistp256 ,
656: .Dq ecdh-sha2-nistp384 ,
657: .Dq ecdh-sha2-nistp521 ,
1.130 jmc 658: .Dq diffie-hellman-group-exchange-sha256 ,
1.127 djm 659: .Dq diffie-hellman-group-exchange-sha1 ,
660: .Dq diffie-hellman-group14-sha1 ,
661: .Dq diffie-hellman-group1-sha1 .
1.1 stevesk 662: .It Cm KeyRegenerationInterval
663: In protocol version 1, the ephemeral server key is automatically regenerated
664: after this many seconds (if it has been used).
665: The purpose of regeneration is to prevent
666: decrypting captured sessions by later breaking into the machine and
667: stealing the keys.
668: The key is never stored anywhere.
669: If the value is 0, the key is never regenerated.
670: The default is 3600 (seconds).
671: .It Cm ListenAddress
672: Specifies the local addresses
1.52 jmc 673: .Xr sshd 8
1.1 stevesk 674: should listen on.
675: The following forms may be used:
676: .Pp
677: .Bl -item -offset indent -compact
678: .It
679: .Cm ListenAddress
680: .Sm off
681: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
682: .Sm on
683: .It
684: .Cm ListenAddress
685: .Sm off
686: .Ar host No | Ar IPv4_addr No : Ar port
687: .Sm on
688: .It
689: .Cm ListenAddress
690: .Sm off
691: .Oo
692: .Ar host No | Ar IPv6_addr Oc : Ar port
693: .Sm on
694: .El
695: .Pp
696: If
697: .Ar port
698: is not specified,
1.52 jmc 699: sshd will listen on the address and all prior
1.1 stevesk 700: .Cm Port
1.17 jmc 701: options specified.
702: The default is to listen on all local addresses.
1.15 jmc 703: Multiple
1.1 stevesk 704: .Cm ListenAddress
1.17 jmc 705: options are permitted.
706: Additionally, any
1.1 stevesk 707: .Cm Port
1.52 jmc 708: options must precede this option for non-port qualified addresses.
1.1 stevesk 709: .It Cm LoginGraceTime
710: The server disconnects after this time if the user has not
711: successfully logged in.
712: If the value is 0, there is no time limit.
1.12 stevesk 713: The default is 120 seconds.
1.1 stevesk 714: .It Cm LogLevel
715: Gives the verbosity level that is used when logging messages from
1.53 jmc 716: .Xr sshd 8 .
1.1 stevesk 717: The possible values are:
1.52 jmc 718: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 719: The default is INFO.
720: DEBUG and DEBUG1 are equivalent.
721: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
722: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 723: .It Cm MACs
724: Specifies the available MAC (message authentication code) algorithms.
725: The MAC algorithm is used in protocol version 2
726: for data integrity protection.
727: Multiple algorithms must be comma-separated.
1.152 markus 728: The algorithms that contain
729: .Dq -etm
730: calculate the MAC after encryption (encrypt-then-mac).
731: These are considered safer and their use recommended.
1.52 jmc 732: The default is:
1.77 jmc 733: .Bd -literal -offset indent
1.152 markus 734: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
735: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
736: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
737: hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
738: hmac-md5-96-etm@openssh.com,
1.145 markus 739: hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
1.144 naddy 740: hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
741: hmac-sha1-96,hmac-md5-96
1.77 jmc 742: .Ed
1.60 dtucker 743: .It Cm Match
1.61 jmc 744: Introduces a conditional block.
1.65 dtucker 745: If all of the criteria on the
1.60 dtucker 746: .Cm Match
1.65 dtucker 747: line are satisfied, the keywords on the following lines override those
748: set in the global section of the config file, until either another
1.60 dtucker 749: .Cm Match
1.65 dtucker 750: line or the end of the file.
1.91 djm 751: .Pp
1.61 jmc 752: The arguments to
1.60 dtucker 753: .Cm Match
1.163 dtucker 754: are one or more criteria-pattern pairs or the single token
755: .Cm All
756: which matches all criteria.
1.60 dtucker 757: The available criteria are
758: .Cm User ,
1.69 dtucker 759: .Cm Group ,
1.60 dtucker 760: .Cm Host ,
1.139 dtucker 761: .Cm LocalAddress ,
762: .Cm LocalPort ,
1.60 dtucker 763: and
764: .Cm Address .
1.91 djm 765: The match patterns may consist of single entries or comma-separated
766: lists and may use the wildcard and negation operators described in the
1.161 jmc 767: PATTERNS section of
1.92 djm 768: .Xr ssh_config 5 .
1.91 djm 769: .Pp
770: The patterns in an
771: .Cm Address
772: criteria may additionally contain addresses to match in CIDR
1.93 jmc 773: address/masklen format, e.g.\&
1.91 djm 774: .Dq 192.0.2.0/24
775: or
776: .Dq 3ffe:ffff::/32 .
777: Note that the mask length provided must be consistent with the address -
778: it is an error to specify a mask length that is too long for the address
1.93 jmc 779: or one with bits set in this host portion of the address.
780: For example,
1.91 djm 781: .Dq 192.0.2.0/33
782: and
1.93 jmc 783: .Dq 192.0.2.0/8
1.91 djm 784: respectively.
785: .Pp
1.60 dtucker 786: Only a subset of keywords may be used on the lines following a
787: .Cm Match
788: keyword.
789: Available keywords are
1.142 jmc 790: .Cm AcceptEnv ,
1.99 okan 791: .Cm AllowAgentForwarding ,
1.142 jmc 792: .Cm AllowGroups ,
1.62 dtucker 793: .Cm AllowTcpForwarding ,
1.141 markus 794: .Cm AllowUsers ,
1.149 djm 795: .Cm AuthenticationMethods ,
1.146 djm 796: .Cm AuthorizedKeysCommand ,
797: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 798: .Cm AuthorizedKeysFile ,
1.123 djm 799: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 800: .Cm Banner ,
1.85 djm 801: .Cm ChrootDirectory ,
1.141 markus 802: .Cm DenyGroups ,
803: .Cm DenyUsers ,
1.67 dtucker 804: .Cm ForceCommand ,
1.142 jmc 805: .Cm GatewayPorts ,
1.141 markus 806: .Cm GSSAPIAuthentication ,
1.87 djm 807: .Cm HostbasedAuthentication ,
1.123 djm 808: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 809: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 810: .Cm KerberosAuthentication ,
1.95 dtucker 811: .Cm MaxAuthTries ,
1.94 dtucker 812: .Cm MaxSessions ,
1.72 dtucker 813: .Cm PasswordAuthentication ,
1.97 djm 814: .Cm PermitEmptyPasswords ,
1.66 dtucker 815: .Cm PermitOpen ,
1.79 dtucker 816: .Cm PermitRootLogin ,
1.164 djm 817: .Cm PermitTTY ,
1.123 djm 818: .Cm PermitTunnel ,
1.107 dtucker 819: .Cm PubkeyAuthentication ,
1.159 dtucker 820: .Cm RekeyLimit ,
1.142 jmc 821: .Cm RhostsRSAAuthentication ,
1.141 markus 822: .Cm RSAAuthentication ,
1.66 dtucker 823: .Cm X11DisplayOffset ,
1.101 djm 824: .Cm X11Forwarding
1.60 dtucker 825: and
1.102 djm 826: .Cm X11UseLocalHost .
1.33 dtucker 827: .It Cm MaxAuthTries
828: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 829: connection.
830: Once the number of failures reaches half this value,
831: additional failures are logged.
832: The default is 6.
1.90 djm 833: .It Cm MaxSessions
834: Specifies the maximum number of open sessions permitted per network connection.
835: The default is 10.
1.1 stevesk 836: .It Cm MaxStartups
837: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 838: SSH daemon.
1.1 stevesk 839: Additional connections will be dropped until authentication succeeds or the
840: .Cm LoginGraceTime
841: expires for a connection.
1.156 dtucker 842: The default is 10:30:100.
1.1 stevesk 843: .Pp
844: Alternatively, random early drop can be enabled by specifying
845: the three colon separated values
846: .Dq start:rate:full
1.51 jmc 847: (e.g. "10:30:60").
1.53 jmc 848: .Xr sshd 8
1.1 stevesk 849: will refuse connection attempts with a probability of
850: .Dq rate/100
851: (30%)
852: if there are currently
853: .Dq start
854: (10)
855: unauthenticated connections.
856: The probability increases linearly and all connection attempts
857: are refused if the number of unauthenticated connections reaches
858: .Dq full
859: (60).
860: .It Cm PasswordAuthentication
861: Specifies whether password authentication is allowed.
862: The default is
863: .Dq yes .
864: .It Cm PermitEmptyPasswords
865: When password authentication is allowed, it specifies whether the
866: server allows login to accounts with empty password strings.
867: The default is
868: .Dq no .
1.62 dtucker 869: .It Cm PermitOpen
870: Specifies the destinations to which TCP port forwarding is permitted.
871: The forwarding specification must be one of the following forms:
872: .Pp
873: .Bl -item -offset indent -compact
874: .It
875: .Cm PermitOpen
876: .Sm off
877: .Ar host : port
878: .Sm on
879: .It
880: .Cm PermitOpen
881: .Sm off
882: .Ar IPv4_addr : port
883: .Sm on
884: .It
885: .Cm PermitOpen
886: .Sm off
887: .Ar \&[ IPv6_addr \&] : port
888: .Sm on
889: .El
890: .Pp
1.68 dtucker 891: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 892: An argument of
893: .Dq any
894: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 895: An argument of
896: .Dq none
897: can be used to prohibit all forwarding requests.
1.63 jmc 898: By default all port forwarding requests are permitted.
1.1 stevesk 899: .It Cm PermitRootLogin
1.38 jmc 900: Specifies whether root can log in using
1.1 stevesk 901: .Xr ssh 1 .
902: The argument must be
903: .Dq yes ,
904: .Dq without-password ,
1.52 jmc 905: .Dq forced-commands-only ,
1.1 stevesk 906: or
907: .Dq no .
908: The default is
909: .Dq yes .
910: .Pp
911: If this option is set to
1.52 jmc 912: .Dq without-password ,
1.1 stevesk 913: password authentication is disabled for root.
914: .Pp
915: If this option is set to
1.52 jmc 916: .Dq forced-commands-only ,
1.1 stevesk 917: root login with public key authentication will be allowed,
918: but only if the
919: .Ar command
920: option has been specified
921: (which may be useful for taking remote backups even if root login is
1.17 jmc 922: normally not allowed).
923: All other authentication methods are disabled for root.
1.1 stevesk 924: .Pp
925: If this option is set to
1.52 jmc 926: .Dq no ,
1.38 jmc 927: root is not allowed to log in.
1.46 reyk 928: .It Cm PermitTunnel
929: Specifies whether
930: .Xr tun 4
931: device forwarding is allowed.
1.47 reyk 932: The argument must be
933: .Dq yes ,
1.58 stevesk 934: .Dq point-to-point
935: (layer 3),
936: .Dq ethernet
937: (layer 2), or
1.47 reyk 938: .Dq no .
1.58 stevesk 939: Specifying
940: .Dq yes
941: permits both
942: .Dq point-to-point
943: and
944: .Dq ethernet .
1.46 reyk 945: The default is
946: .Dq no .
1.164 djm 947: .It Cm PermitTTY
948: Specifies whether
1.165 ! jmc 949: .Xr pty 4
1.164 djm 950: allocation is permitted.
951: The default is
952: .Dq yes .
1.6 markus 953: .It Cm PermitUserEnvironment
954: Specifies whether
955: .Pa ~/.ssh/environment
1.9 stevesk 956: and
1.6 markus 957: .Cm environment=
958: options in
959: .Pa ~/.ssh/authorized_keys
1.9 stevesk 960: are processed by
1.52 jmc 961: .Xr sshd 8 .
1.6 markus 962: The default is
963: .Dq no .
1.9 stevesk 964: Enabling environment processing may enable users to bypass access
965: restrictions in some configurations using mechanisms such as
966: .Ev LD_PRELOAD .
1.1 stevesk 967: .It Cm PidFile
1.4 stevesk 968: Specifies the file that contains the process ID of the
1.53 jmc 969: SSH daemon.
1.1 stevesk 970: The default is
971: .Pa /var/run/sshd.pid .
972: .It Cm Port
973: Specifies the port number that
1.52 jmc 974: .Xr sshd 8
1.1 stevesk 975: listens on.
976: The default is 22.
977: Multiple options of this type are permitted.
978: See also
979: .Cm ListenAddress .
980: .It Cm PrintLastLog
981: Specifies whether
1.52 jmc 982: .Xr sshd 8
1.36 jaredy 983: should print the date and time of the last user login when a user logs
984: in interactively.
1.1 stevesk 985: The default is
986: .Dq yes .
987: .It Cm PrintMotd
988: Specifies whether
1.52 jmc 989: .Xr sshd 8
1.1 stevesk 990: should print
991: .Pa /etc/motd
992: when a user logs in interactively.
993: (On some systems it is also printed by the shell,
994: .Pa /etc/profile ,
995: or equivalent.)
996: The default is
997: .Dq yes .
998: .It Cm Protocol
999: Specifies the protocol versions
1.52 jmc 1000: .Xr sshd 8
1.5 stevesk 1001: supports.
1.1 stevesk 1002: The possible values are
1.52 jmc 1003: .Sq 1
1.1 stevesk 1004: and
1.52 jmc 1005: .Sq 2 .
1.1 stevesk 1006: Multiple versions must be comma-separated.
1007: The default is
1.109 jmc 1008: .Sq 2 .
1.5 stevesk 1009: Note that the order of the protocol list does not indicate preference,
1010: because the client selects among multiple protocol versions offered
1011: by the server.
1012: Specifying
1013: .Dq 2,1
1014: is identical to
1015: .Dq 1,2 .
1.1 stevesk 1016: .It Cm PubkeyAuthentication
1017: Specifies whether public key authentication is allowed.
1018: The default is
1019: .Dq yes .
1020: Note that this option applies to protocol version 2 only.
1.159 dtucker 1021: .It Cm RekeyLimit
1022: Specifies the maximum amount of data that may be transmitted before the
1023: session key is renegotiated, optionally followed a maximum amount of
1024: time that may pass before the session key is renegotiated.
1025: The first argument is specified in bytes and may have a suffix of
1026: .Sq K ,
1027: .Sq M ,
1028: or
1029: .Sq G
1030: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1031: The default is between
1032: .Sq 1G
1033: and
1034: .Sq 4G ,
1035: depending on the cipher.
1036: The optional second value is specified in seconds and may use any of the
1037: units documented in the
1038: .Sx TIME FORMATS
1.160 jmc 1039: section.
1.159 dtucker 1040: The default value for
1041: .Cm RekeyLimit
1042: is
1043: .Dq default none ,
1044: which means that rekeying is performed after the cipher's default amount
1045: of data has been sent or received and no time based rekeying is done.
1046: This option applies to protocol version 2 only.
1.118 djm 1047: .It Cm RevokedKeys
1.154 djm 1048: Specifies revoked public keys.
1.118 djm 1049: Keys listed in this file will be refused for public key authentication.
1050: Note that if this file is not readable, then public key authentication will
1051: be refused for all users.
1.154 djm 1052: Keys may be specified as a text file, listing one public key per line, or as
1053: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1054: .Xr ssh-keygen 1 .
1.161 jmc 1055: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1056: .Xr ssh-keygen 1 .
1.1 stevesk 1057: .It Cm RhostsRSAAuthentication
1058: Specifies whether rhosts or /etc/hosts.equiv authentication together
1059: with successful RSA host authentication is allowed.
1060: The default is
1061: .Dq no .
1062: This option applies to protocol version 1 only.
1063: .It Cm RSAAuthentication
1064: Specifies whether pure RSA authentication is allowed.
1065: The default is
1066: .Dq yes .
1067: This option applies to protocol version 1 only.
1068: .It Cm ServerKeyBits
1069: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1070: The minimum value is 512, and the default is 1024.
1.1 stevesk 1071: .It Cm StrictModes
1072: Specifies whether
1.52 jmc 1073: .Xr sshd 8
1.1 stevesk 1074: should check file modes and ownership of the
1075: user's files and home directory before accepting login.
1076: This is normally desirable because novices sometimes accidentally leave their
1077: directory or files world-writable.
1078: The default is
1079: .Dq yes .
1.112 djm 1080: Note that this does not apply to
1081: .Cm ChrootDirectory ,
1082: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1083: .It Cm Subsystem
1.51 jmc 1084: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1085: Arguments should be a subsystem name and a command (with optional arguments)
1086: to execute upon subsystem request.
1.80 djm 1087: .Pp
1.1 stevesk 1088: The command
1089: .Xr sftp-server 8
1090: implements the
1091: .Dq sftp
1092: file transfer subsystem.
1.80 djm 1093: .Pp
1094: Alternately the name
1095: .Dq internal-sftp
1096: implements an in-process
1097: .Dq sftp
1098: server.
1099: This may simplify configurations using
1100: .Cm ChrootDirectory
1101: to force a different filesystem root on clients.
1102: .Pp
1.1 stevesk 1103: By default no subsystems are defined.
1104: Note that this option applies to protocol version 2 only.
1105: .It Cm SyslogFacility
1106: Gives the facility code that is used when logging messages from
1.53 jmc 1107: .Xr sshd 8 .
1.1 stevesk 1108: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1109: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1110: The default is AUTH.
1.27 markus 1111: .It Cm TCPKeepAlive
1112: Specifies whether the system should send TCP keepalive messages to the
1113: other side.
1114: If they are sent, death of the connection or crash of one
1115: of the machines will be properly noticed.
1116: However, this means that
1117: connections will die if the route is down temporarily, and some people
1118: find it annoying.
1119: On the other hand, if TCP keepalives are not sent,
1120: sessions may hang indefinitely on the server, leaving
1121: .Dq ghost
1122: users and consuming server resources.
1123: .Pp
1124: The default is
1125: .Dq yes
1126: (to send TCP keepalive messages), and the server will notice
1127: if the network goes down or the client host crashes.
1128: This avoids infinitely hanging sessions.
1129: .Pp
1130: To disable TCP keepalive messages, the value should be set to
1131: .Dq no .
1.118 djm 1132: .It Cm TrustedUserCAKeys
1133: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1134: trusted to sign user certificates for authentication.
1.119 jmc 1135: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1136: .Ql #
1137: are allowed.
1138: If a certificate is presented for authentication and has its signing CA key
1139: listed in this file, then it may be used for authentication for any user
1140: listed in the certificate's principals list.
1141: Note that certificates that lack a list of principals will not be permitted
1142: for authentication using
1143: .Cm TrustedUserCAKeys .
1.161 jmc 1144: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1145: .Xr ssh-keygen 1 .
1.18 markus 1146: .It Cm UseDNS
1147: Specifies whether
1.52 jmc 1148: .Xr sshd 8
1.40 jmc 1149: should look up the remote host name and check that
1.18 markus 1150: the resolved host name for the remote IP address maps back to the
1151: very same IP address.
1152: The default is
1153: .Dq yes .
1.1 stevesk 1154: .It Cm UseLogin
1155: Specifies whether
1156: .Xr login 1
1157: is used for interactive login sessions.
1158: The default is
1159: .Dq no .
1160: Note that
1161: .Xr login 1
1162: is never used for remote command execution.
1163: Note also, that if this is enabled,
1164: .Cm X11Forwarding
1165: will be disabled because
1166: .Xr login 1
1167: does not know how to handle
1168: .Xr xauth 1
1.15 jmc 1169: cookies.
1170: If
1.1 stevesk 1171: .Cm UsePrivilegeSeparation
1172: is specified, it will be disabled after authentication.
1173: .It Cm UsePrivilegeSeparation
1174: Specifies whether
1.52 jmc 1175: .Xr sshd 8
1.2 stevesk 1176: separates privileges by creating an unprivileged child process
1.15 jmc 1177: to deal with incoming network traffic.
1178: After successful authentication, another process will be created that has
1179: the privilege of the authenticated user.
1180: The goal of privilege separation is to prevent privilege
1.1 stevesk 1181: escalation by containing any corruption within the unprivileged processes.
1182: The default is
1183: .Dq yes .
1.134 djm 1184: If
1185: .Cm UsePrivilegeSeparation
1186: is set to
1187: .Dq sandbox
1188: then the pre-authentication unprivileged process is subject to additional
1189: restrictions.
1.137 djm 1190: .It Cm VersionAddendum
1191: Optionally specifies additional text to append to the SSH protocol banner
1192: sent by the server upon connection.
1193: The default is
1194: .Dq none .
1.1 stevesk 1195: .It Cm X11DisplayOffset
1196: Specifies the first display number available for
1.52 jmc 1197: .Xr sshd 8 Ns 's
1.1 stevesk 1198: X11 forwarding.
1.52 jmc 1199: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1200: The default is 10.
1201: .It Cm X11Forwarding
1202: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1203: The argument must be
1204: .Dq yes
1205: or
1206: .Dq no .
1.1 stevesk 1207: The default is
1208: .Dq no .
1.13 stevesk 1209: .Pp
1210: When X11 forwarding is enabled, there may be additional exposure to
1211: the server and to client displays if the
1.52 jmc 1212: .Xr sshd 8
1.13 stevesk 1213: proxy display is configured to listen on the wildcard address (see
1214: .Cm X11UseLocalhost
1.52 jmc 1215: below), though this is not the default.
1.13 stevesk 1216: Additionally, the authentication spoofing and authentication data
1217: verification and substitution occur on the client side.
1218: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1219: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1220: forwarding (see the warnings for
1221: .Cm ForwardX11
1222: in
1.19 jmc 1223: .Xr ssh_config 5 ) .
1.13 stevesk 1224: A system administrator may have a stance in which they want to
1225: protect clients that may expose themselves to attack by unwittingly
1226: requesting X11 forwarding, which can warrant a
1227: .Dq no
1228: setting.
1229: .Pp
1230: Note that disabling X11 forwarding does not prevent users from
1231: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1232: X11 forwarding is automatically disabled if
1233: .Cm UseLogin
1234: is enabled.
1235: .It Cm X11UseLocalhost
1236: Specifies whether
1.52 jmc 1237: .Xr sshd 8
1.1 stevesk 1238: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1239: the wildcard address.
1240: By default,
1.52 jmc 1241: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1242: hostname part of the
1243: .Ev DISPLAY
1244: environment variable to
1245: .Dq localhost .
1.8 stevesk 1246: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1247: However, some older X11 clients may not function with this
1248: configuration.
1249: .Cm X11UseLocalhost
1250: may be set to
1251: .Dq no
1252: to specify that the forwarding server should be bound to the wildcard
1253: address.
1254: The argument must be
1255: .Dq yes
1256: or
1257: .Dq no .
1258: The default is
1259: .Dq yes .
1260: .It Cm XAuthLocation
1.11 stevesk 1261: Specifies the full pathname of the
1.1 stevesk 1262: .Xr xauth 1
1263: program.
1264: The default is
1265: .Pa /usr/X11R6/bin/xauth .
1266: .El
1.55 jmc 1267: .Sh TIME FORMATS
1.53 jmc 1268: .Xr sshd 8
1.1 stevesk 1269: command-line arguments and configuration file options that specify time
1270: may be expressed using a sequence of the form:
1271: .Sm off
1.7 stevesk 1272: .Ar time Op Ar qualifier ,
1.1 stevesk 1273: .Sm on
1274: where
1275: .Ar time
1276: is a positive integer value and
1277: .Ar qualifier
1278: is one of the following:
1279: .Pp
1280: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1281: .It Aq Cm none
1.1 stevesk 1282: seconds
1283: .It Cm s | Cm S
1284: seconds
1285: .It Cm m | Cm M
1286: minutes
1287: .It Cm h | Cm H
1288: hours
1289: .It Cm d | Cm D
1290: days
1291: .It Cm w | Cm W
1292: weeks
1293: .El
1294: .Pp
1295: Each member of the sequence is added together to calculate
1296: the total time value.
1297: .Pp
1298: Time format examples:
1299: .Pp
1300: .Bl -tag -width Ds -compact -offset indent
1301: .It 600
1302: 600 seconds (10 minutes)
1303: .It 10m
1304: 10 minutes
1305: .It 1h30m
1306: 1 hour 30 minutes (90 minutes)
1307: .El
1308: .Sh FILES
1309: .Bl -tag -width Ds
1310: .It Pa /etc/ssh/sshd_config
1311: Contains configuration data for
1.53 jmc 1312: .Xr sshd 8 .
1.1 stevesk 1313: This file should be writable by root only, but it is recommended
1314: (though not necessary) that it be world-readable.
1315: .El
1.19 jmc 1316: .Sh SEE ALSO
1317: .Xr sshd 8
1.1 stevesk 1318: .Sh AUTHORS
1319: OpenSSH is a derivative of the original and free
1320: ssh 1.2.12 release by Tatu Ylonen.
1321: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1322: Theo de Raadt and Dug Song
1323: removed many bugs, re-added newer features and
1324: created OpenSSH.
1325: Markus Friedl contributed the support for SSH
1326: protocol versions 1.5 and 2.0.
1327: Niels Provos and Markus Friedl contributed support
1328: for privilege separation.
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