Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.206
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
18: .\" are met:
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.
24: .\"
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
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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1.206 ! markus 36: .\" $OpenBSD: sshd_config.5,v 1.205 2015/07/03 03:49:45 djm Exp $
! 37: .Dd $Mdocdate: July 3 2015 $
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.
1.197 dtucker 73: Note that environment passing is only supported for protocol 2, and
74: that the
75: .Ev TERM
1.199 jmc 76: environment variable is always sent whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
94: .Dq any ,
95: .Dq inet
1.52 jmc 96: (use IPv4 only), or
1.37 djm 97: .Dq inet6
98: (use IPv6 only).
99: The default is
100: .Dq any .
1.89 jmc 101: .It Cm AllowAgentForwarding
102: Specifies whether
103: .Xr ssh-agent 1
104: forwarding is permitted.
105: The default is
106: .Dq yes .
107: Note that disabling agent forwarding does not improve security
108: unless users are also denied shell access, as they can always install
109: their own forwarders.
1.1 stevesk 110: .It Cm AllowGroups
111: This keyword can be followed by a list of group name patterns, separated
112: by spaces.
113: If specified, login is allowed only for users whose primary
114: group or supplementary group list matches one of the patterns.
115: Only group names are valid; a numerical group ID is not recognized.
116: By default, login is allowed for all groups.
1.54 jmc 117: The allow/deny directives are processed in the following order:
118: .Cm DenyUsers ,
119: .Cm AllowUsers ,
120: .Cm DenyGroups ,
121: and finally
122: .Cm AllowGroups .
1.49 jmc 123: .Pp
1.161 jmc 124: See PATTERNS in
1.49 jmc 125: .Xr ssh_config 5
126: for more information on patterns.
1.1 stevesk 127: .It Cm AllowTcpForwarding
128: Specifies whether TCP forwarding is permitted.
1.150 djm 129: The available options are
130: .Dq yes
131: or
132: .Dq all
133: to allow TCP forwarding,
134: .Dq no
135: to prevent all TCP forwarding,
136: .Dq local
137: to allow local (from the perspective of
1.151 jmc 138: .Xr ssh 1 )
139: forwarding only or
1.150 djm 140: .Dq remote
141: to allow remote forwarding only.
1.1 stevesk 142: The default is
143: .Dq yes .
144: Note that disabling TCP forwarding does not improve security unless
145: users are also denied shell access, as they can always install their
146: own forwarders.
1.175 millert 147: .It Cm AllowStreamLocalForwarding
148: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
149: The available options are
150: .Dq yes
151: or
152: .Dq all
153: to allow StreamLocal forwarding,
154: .Dq no
155: to prevent all StreamLocal forwarding,
156: .Dq local
157: to allow local (from the perspective of
158: .Xr ssh 1 )
159: forwarding only or
160: .Dq remote
161: to allow remote forwarding only.
162: The default is
163: .Dq yes .
164: Note that disabling StreamLocal forwarding does not improve security unless
165: users are also denied shell access, as they can always install their
166: own forwarders.
1.1 stevesk 167: .It Cm AllowUsers
168: This keyword can be followed by a list of user name patterns, separated
169: by spaces.
1.14 jmc 170: If specified, login is allowed only for user names that
1.1 stevesk 171: match one of the patterns.
172: Only user names are valid; a numerical user ID is not recognized.
173: By default, login is allowed for all users.
174: If the pattern takes the form USER@HOST then USER and HOST
175: are separately checked, restricting logins to particular
176: users from particular hosts.
1.54 jmc 177: The allow/deny directives are processed in the following order:
178: .Cm DenyUsers ,
179: .Cm AllowUsers ,
180: .Cm DenyGroups ,
181: and finally
182: .Cm AllowGroups .
1.49 jmc 183: .Pp
1.161 jmc 184: See PATTERNS in
1.49 jmc 185: .Xr ssh_config 5
186: for more information on patterns.
1.149 djm 187: .It Cm AuthenticationMethods
188: Specifies the authentication methods that must be successfully completed
189: for a user to be granted access.
190: This option must be followed by one or more comma-separated lists of
191: authentication method names.
192: Successful authentication requires completion of every method in at least
193: one of these lists.
194: .Pp
195: For example, an argument of
196: .Dq publickey,password publickey,keyboard-interactive
197: would require the user to complete public key authentication, followed by
198: either password or keyboard interactive authentication.
199: Only methods that are next in one or more lists are offered at each stage,
200: so for this example, it would not be possible to attempt password or
201: keyboard-interactive authentication before public key.
1.157 markus 202: .Pp
203: For keyboard interactive authentication it is also possible to
204: restrict authentication to a specific device by appending a
205: colon followed by the device identifier
206: .Dq bsdauth ,
207: .Dq pam ,
208: or
209: .Dq skey ,
210: depending on the server configuration.
211: For example,
212: .Dq keyboard-interactive:bsdauth
213: would restrict keyboard interactive authentication to the
214: .Dq bsdauth
215: device.
1.185 djm 216: .Pp
217: If the
218: .Dq publickey
1.187 jmc 219: method is listed more than once,
1.185 djm 220: .Xr sshd 8
221: verifies that keys that have been used successfully are not reused for
222: subsequent authentications.
223: For example, an
224: .Cm AuthenticationMethods
225: of
226: .Dq publickey,publickey
227: will require successful authentication using two different public keys.
1.181 djm 228: .Pp
1.149 djm 229: This option is only available for SSH protocol 2 and will yield a fatal
230: error if enabled if protocol 1 is also enabled.
231: Note that each authentication method listed should also be explicitly enabled
232: in the configuration.
233: The default is not to require multiple authentication; successful completion
234: of a single authentication method is sufficient.
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: .Pp
240: Arguments to
241: .Cm AuthorizedKeysCommand
242: may be provided using the following tokens, which will be expanded
243: at runtime: %% is replaced by a literal '%', %u is replaced by the
244: username being authenticated, %h is replaced by the home directory
245: of the user being authenticated, %t is replaced with the key type
246: offered for authentication, %f is replaced with the fingerprint of
247: the key, and %k is replaced with the key being offered for authentication.
248: If no arguments are specified then the username of the target user
249: will be supplied.
250: .Pp
251: The program should produce on standard output zero or
1.161 jmc 252: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 253: .Xr sshd 8 ) .
1.146 djm 254: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
255: and authorize the user then public key authentication continues using the usual
256: .Cm AuthorizedKeysFile
257: files.
258: By default, no AuthorizedKeysCommand is run.
259: .It Cm AuthorizedKeysCommandUser
260: Specifies the user under whose account the AuthorizedKeysCommand is run.
261: It is recommended to use a dedicated user that has no other role on the host
262: than running authorized keys commands.
1.186 djm 263: If
1.181 djm 264: .Cm AuthorizedKeysCommand
1.186 djm 265: is specified but
266: .Cm AuthorizedKeysCommandUser
267: is not, then
268: .Xr sshd 8
269: will refuse to start.
1.1 stevesk 270: .It Cm AuthorizedKeysFile
271: Specifies the file that contains the public keys that can be used
272: for user authentication.
1.124 djm 273: The format is described in the
1.161 jmc 274: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 275: section of
276: .Xr sshd 8 .
1.1 stevesk 277: .Cm AuthorizedKeysFile
278: may contain tokens of the form %T which are substituted during connection
1.52 jmc 279: setup.
1.17 jmc 280: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 281: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 282: %u is replaced by the username of that user.
283: After expansion,
284: .Cm AuthorizedKeysFile
285: is taken to be an absolute path or one relative to the user's home
286: directory.
1.133 jmc 287: Multiple files may be listed, separated by whitespace.
288: The default is
289: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 290: .It Cm AuthorizedPrincipalsCommand
291: Specifies a program to be used to generate the list of allowed
292: certificate principals as per
293: .Cm AuthorizedPrincipalsFile .
294: The program must be owned by root, not writable by group or others and
295: specified by an absolute path.
296: .Pp
297: Arguments to
298: .Cm AuthorizedPrincipalsCommand
299: may be provided using the following tokens, which will be expanded
300: at runtime: %% is replaced by a literal '%', %u is replaced by the
301: username being authenticated and %h is replaced by the home directory
302: of the user being authenticated.
303: .Pp
304: The program should produce on standard output zero or
305: more lines of
306: .Cm AuthorizedPrincipalsFile
307: output.
308: If either
309: .Cm AuthorizedPrincipalsCommand
310: or
311: .Cm AuthorizedPrincipalsFile
312: is specified, then certificates offered by the client for authentication
313: must contain a principal that is listed.
314: By default, no AuthorizedPrincipalsCommand is run.
315: .It Cm AuthorizedPrincipalsCommandUser
316: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
317: It is recommended to use a dedicated user that has no other role on the host
318: than running authorized principals commands.
319: If
320: .Cm AuthorizedPrincipalsCommand
321: is specified but
322: .Cm AuthorizedPrincipalsCommandUser
323: is not, then
324: .Xr sshd 8
325: will refuse to start.
1.121 djm 326: .It Cm AuthorizedPrincipalsFile
327: Specifies a file that lists principal names that are accepted for
328: certificate authentication.
329: When using certificates signed by a key listed in
330: .Cm TrustedUserCAKeys ,
331: this file lists names, one of which must appear in the certificate for it
332: to be accepted for authentication.
1.125 jmc 333: Names are listed one per line preceded by key options (as described
1.161 jmc 334: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 335: .Xr sshd 8 ) .
1.124 djm 336: Empty lines and comments starting with
1.121 djm 337: .Ql #
338: are ignored.
339: .Pp
340: .Cm AuthorizedPrincipalsFile
341: may contain tokens of the form %T which are substituted during connection
342: setup.
343: The following tokens are defined: %% is replaced by a literal '%',
344: %h is replaced by the home directory of the user being authenticated, and
345: %u is replaced by the username of that user.
346: After expansion,
347: .Cm AuthorizedPrincipalsFile
348: is taken to be an absolute path or one relative to the user's home
349: directory.
350: .Pp
1.138 djm 351: The default is
352: .Dq none ,
353: i.e. not to use a principals file \(en in this case, the username
1.121 djm 354: of the user must appear in a certificate's principals list for it to be
355: accepted.
356: Note that
357: .Cm AuthorizedPrincipalsFile
358: is only used when authentication proceeds using a CA listed in
359: .Cm TrustedUserCAKeys
360: and is not consulted for certification authorities trusted via
361: .Pa ~/.ssh/authorized_keys ,
362: though the
363: .Cm principals=
364: key option offers a similar facility (see
365: .Xr sshd 8
366: for details).
1.1 stevesk 367: .It Cm Banner
368: The contents of the specified file are sent to the remote user before
369: authentication is allowed.
1.78 djm 370: If the argument is
371: .Dq none
372: then no banner is displayed.
1.1 stevesk 373: This option is only available for protocol version 2.
374: By default, no banner is displayed.
375: .It Cm ChallengeResponseAuthentication
1.50 jmc 376: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 377: All authentication styles from
378: .Xr login.conf 5
379: are supported.
380: The default is
381: .Dq yes .
1.80 djm 382: .It Cm ChrootDirectory
1.113 stevesk 383: Specifies the pathname of a directory to
1.80 djm 384: .Xr chroot 2
385: to after authentication.
1.190 deraadt 386: At session startup
387: .Xr sshd 8
388: checks that all components of the pathname are root-owned directories
389: which are not writable by any other user or group.
1.106 stevesk 390: After the chroot,
391: .Xr sshd 8
392: changes the working directory to the user's home directory.
1.80 djm 393: .Pp
1.113 stevesk 394: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 395: the connecting user has been authenticated: %% is replaced by a literal '%',
396: %h is replaced by the home directory of the user being authenticated, and
397: %u is replaced by the username of that user.
398: .Pp
399: The
400: .Cm ChrootDirectory
401: must contain the necessary files and directories to support the
1.103 stevesk 402: user's session.
1.80 djm 403: For an interactive session this requires at least a shell, typically
404: .Xr sh 1 ,
405: and basic
406: .Pa /dev
407: nodes such as
408: .Xr null 4 ,
409: .Xr zero 4 ,
410: .Xr stdin 4 ,
411: .Xr stdout 4 ,
412: .Xr stderr 4 ,
1.180 jmc 413: and
1.80 djm 414: .Xr tty 4
415: devices.
416: For file transfer sessions using
1.105 jmc 417: .Dq sftp ,
1.80 djm 418: no additional configuration of the environment is necessary if the
1.105 jmc 419: in-process sftp server is used,
1.176 schwarze 420: though sessions which use logging may require
1.104 stevesk 421: .Pa /dev/log
1.176 schwarze 422: inside the chroot directory on some operating systems (see
1.104 stevesk 423: .Xr sftp-server 8
1.81 jmc 424: for details).
1.190 deraadt 425: .Pp
1.191 jmc 426: For safety, it is very important that the directory hierarchy be
1.190 deraadt 427: prevented from modification by other processes on the system (especially
428: those outside the jail).
429: Misconfiguration can lead to unsafe environments which
430: .Xr sshd 8
431: cannot detect.
1.80 djm 432: .Pp
433: The default is not to
434: .Xr chroot 2 .
1.1 stevesk 435: .It Cm Ciphers
436: Specifies the ciphers allowed for protocol version 2.
437: Multiple ciphers must be comma-separated.
1.167 djm 438: The supported ciphers are:
439: .Pp
1.173 naddy 440: .Bl -item -compact -offset indent
441: .It
442: 3des-cbc
443: .It
444: aes128-cbc
445: .It
446: aes192-cbc
447: .It
448: aes256-cbc
449: .It
450: aes128-ctr
451: .It
452: aes192-ctr
453: .It
454: aes256-ctr
455: .It
456: aes128-gcm@openssh.com
457: .It
458: aes256-gcm@openssh.com
459: .It
460: arcfour
461: .It
462: arcfour128
463: .It
464: arcfour256
465: .It
466: blowfish-cbc
467: .It
468: cast128-cbc
469: .It
470: chacha20-poly1305@openssh.com
471: .El
1.167 djm 472: .Pp
1.52 jmc 473: The default is:
1.173 naddy 474: .Bd -literal -offset indent
475: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 476: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 477: chacha20-poly1305@openssh.com
1.1 stevesk 478: .Ed
1.167 djm 479: .Pp
480: The list of available ciphers may also be obtained using the
481: .Fl Q
482: option of
1.188 djm 483: .Xr ssh 1
484: with an argument of
485: .Dq cipher .
1.1 stevesk 486: .It Cm ClientAliveCountMax
1.48 jmc 487: Sets the number of client alive messages (see below) which may be
1.1 stevesk 488: sent without
1.52 jmc 489: .Xr sshd 8
1.17 jmc 490: receiving any messages back from the client.
491: If this threshold is reached while client alive messages are being sent,
1.52 jmc 492: sshd will disconnect the client, terminating the session.
1.17 jmc 493: It is important to note that the use of client alive messages is very
494: different from
1.27 markus 495: .Cm TCPKeepAlive
1.17 jmc 496: (below).
497: The client alive messages are sent through the encrypted channel
498: and therefore will not be spoofable.
499: The TCP keepalive option enabled by
1.27 markus 500: .Cm TCPKeepAlive
1.17 jmc 501: is spoofable.
502: The client alive mechanism is valuable when the client or
1.1 stevesk 503: server depend on knowing when a connection has become inactive.
504: .Pp
1.17 jmc 505: The default value is 3.
506: If
1.1 stevesk 507: .Cm ClientAliveInterval
1.48 jmc 508: (see below) is set to 15, and
1.1 stevesk 509: .Cm ClientAliveCountMax
1.52 jmc 510: is left at the default, unresponsive SSH clients
1.1 stevesk 511: will be disconnected after approximately 45 seconds.
1.57 markus 512: This option applies to protocol version 2 only.
1.42 djm 513: .It Cm ClientAliveInterval
514: Sets a timeout interval in seconds after which if no data has been received
515: from the client,
1.52 jmc 516: .Xr sshd 8
1.42 djm 517: will send a message through the encrypted
518: channel to request a response from the client.
519: The default
520: is 0, indicating that these messages will not be sent to the client.
521: This option applies to protocol version 2 only.
1.3 markus 522: .It Cm Compression
1.44 markus 523: Specifies whether compression is allowed, or delayed until
524: the user has authenticated successfully.
1.3 markus 525: The argument must be
1.44 markus 526: .Dq yes ,
527: .Dq delayed ,
1.3 markus 528: or
529: .Dq no .
530: The default is
1.44 markus 531: .Dq delayed .
1.1 stevesk 532: .It Cm DenyGroups
533: This keyword can be followed by a list of group name patterns, separated
534: by spaces.
535: Login is disallowed for users whose primary group or supplementary
536: group list matches one of the patterns.
537: Only group names are valid; a numerical group ID is not recognized.
538: By default, login is allowed for all groups.
1.54 jmc 539: The allow/deny directives are processed in the following order:
540: .Cm DenyUsers ,
541: .Cm AllowUsers ,
542: .Cm DenyGroups ,
543: and finally
544: .Cm AllowGroups .
1.49 jmc 545: .Pp
1.161 jmc 546: See PATTERNS in
1.49 jmc 547: .Xr ssh_config 5
548: for more information on patterns.
1.1 stevesk 549: .It Cm DenyUsers
550: This keyword can be followed by a list of user name patterns, separated
551: by spaces.
552: Login is disallowed for user names that match one of the patterns.
553: Only user names are valid; a numerical user ID is not recognized.
554: By default, login is allowed for all users.
555: If the pattern takes the form USER@HOST then USER and HOST
556: are separately checked, restricting logins to particular
557: users from particular hosts.
1.54 jmc 558: The allow/deny directives are processed in the following order:
559: .Cm DenyUsers ,
560: .Cm AllowUsers ,
561: .Cm DenyGroups ,
562: and finally
563: .Cm AllowGroups .
1.49 jmc 564: .Pp
1.161 jmc 565: See PATTERNS in
1.49 jmc 566: .Xr ssh_config 5
567: for more information on patterns.
1.183 djm 568: .It Cm FingerprintHash
569: Specifies the hash algorithm used when logging key fingerprints.
570: Valid options are:
571: .Dq md5
572: and
573: .Dq sha256 .
574: The default is
575: .Dq sha256 .
1.67 dtucker 576: .It Cm ForceCommand
577: Forces the execution of the command specified by
578: .Cm ForceCommand ,
1.84 djm 579: ignoring any command supplied by the client and
580: .Pa ~/.ssh/rc
581: if present.
1.67 dtucker 582: The command is invoked by using the user's login shell with the -c option.
583: This applies to shell, command, or subsystem execution.
584: It is most useful inside a
585: .Cm Match
586: block.
587: The command originally supplied by the client is available in the
588: .Ev SSH_ORIGINAL_COMMAND
589: environment variable.
1.82 djm 590: Specifying a command of
591: .Dq internal-sftp
592: will force the use of an in-process sftp server that requires no support
593: files when used with
594: .Cm ChrootDirectory .
1.1 stevesk 595: .It Cm GatewayPorts
596: Specifies whether remote hosts are allowed to connect to ports
597: forwarded for the client.
598: By default,
1.52 jmc 599: .Xr sshd 8
1.15 jmc 600: binds remote port forwardings to the loopback address.
601: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 602: .Cm GatewayPorts
1.52 jmc 603: can be used to specify that sshd
1.39 djm 604: should allow remote port forwardings to bind to non-loopback addresses, thus
605: allowing other hosts to connect.
606: The argument may be
607: .Dq no
608: to force remote port forwardings to be available to the local host only,
1.1 stevesk 609: .Dq yes
1.39 djm 610: to force remote port forwardings to bind to the wildcard address, or
611: .Dq clientspecified
612: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 613: The default is
614: .Dq no .
1.23 markus 615: .It Cm GSSAPIAuthentication
1.25 markus 616: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 617: The default is
1.23 markus 618: .Dq no .
619: Note that this option applies to protocol version 2 only.
620: .It Cm GSSAPICleanupCredentials
621: Specifies whether to automatically destroy the user's credentials cache
622: on logout.
623: The default is
624: .Dq yes .
625: Note that this option applies to protocol version 2 only.
1.203 djm 626: .It Cm GSSAPIStrictAcceptorCheck
627: Determines whether to be strict about the identity of the GSSAPI acceptor
628: a client authenticates against.
629: If set to
630: .Dq yes
631: then the client must authenticate against the
632: .Pa host
633: service on the current hostname.
634: If set to
635: .Dq no
636: then the client may authenticate against any service key stored in the
637: machine's default store.
638: This facility is provided to assist with operation on multi homed machines.
639: The default is
640: .Dq yes .
1.189 djm 641: .It Cm HostbasedAcceptedKeyTypes
642: Specifies the key types that will be accepted for hostbased authentication
643: as a comma-separated pattern list.
1.206 ! markus 644: The default for this option is:
! 645: .Bd -literal -offset 3n
! 646: ecdsa-sha2-nistp256-cert-v01@openssh.com,
! 647: ecdsa-sha2-nistp384-cert-v01@openssh.com,
! 648: ecdsa-sha2-nistp521-cert-v01@openssh.com,
! 649: ssh-ed25519-cert-v01@openssh.com,
! 650: ssh-rsa-cert-v01@openssh.com,
! 651: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
! 652: ssh-ed25519,ssh-rsa
! 653: .Ed
! 654: .Pp
1.189 djm 655: The
656: .Fl Q
657: option of
658: .Xr ssh 1
659: may be used to list supported key types.
1.1 stevesk 660: .It Cm HostbasedAuthentication
661: Specifies whether rhosts or /etc/hosts.equiv authentication together
662: with successful public key client host authentication is allowed
1.50 jmc 663: (host-based authentication).
1.1 stevesk 664: This option is similar to
665: .Cm RhostsRSAAuthentication
666: and applies to protocol version 2 only.
1.70 dtucker 667: The default is
668: .Dq no .
669: .It Cm HostbasedUsesNameFromPacketOnly
670: Specifies whether or not the server will attempt to perform a reverse
671: name lookup when matching the name in the
672: .Pa ~/.shosts ,
673: .Pa ~/.rhosts ,
674: and
675: .Pa /etc/hosts.equiv
676: files during
677: .Cm HostbasedAuthentication .
678: A setting of
679: .Dq yes
680: means that
681: .Xr sshd 8
682: uses the name supplied by the client rather than
683: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 684: The default is
685: .Dq no .
1.117 djm 686: .It Cm HostCertificate
687: Specifies a file containing a public host certificate.
688: The certificate's public key must match a private host key already specified
689: by
690: .Cm HostKey .
691: The default behaviour of
692: .Xr sshd 8
693: is not to load any certificates.
1.1 stevesk 694: .It Cm HostKey
695: Specifies a file containing a private host key
696: used by SSH.
697: The default is
698: .Pa /etc/ssh/ssh_host_key
699: for protocol version 1, and
1.126 djm 700: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 701: .Pa /etc/ssh/ssh_host_ecdsa_key ,
702: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 703: and
1.1 stevesk 704: .Pa /etc/ssh/ssh_host_rsa_key
705: for protocol version 2.
1.206 ! markus 706: .Pp
1.1 stevesk 707: Note that
1.52 jmc 708: .Xr sshd 8
1.206 ! markus 709: will refuse to use a file if it is group/world-accessible
! 710: and that the
! 711: .Cm HostKeyAlgorithms
! 712: option restricts which of the keys are actually used by
! 713: .Xr sshd 8 .
! 714: .Pp
1.1 stevesk 715: It is possible to have multiple host key files.
716: .Dq rsa1
717: keys are used for version 1 and
1.126 djm 718: .Dq dsa ,
1.169 naddy 719: .Dq ecdsa ,
720: .Dq ed25519
1.1 stevesk 721: or
722: .Dq rsa
723: are used for version 2 of the SSH protocol.
1.162 markus 724: It is also possible to specify public host key files instead.
725: In this case operations on the private key will be delegated
726: to an
727: .Xr ssh-agent 1 .
728: .It Cm HostKeyAgent
729: Identifies the UNIX-domain socket used to communicate
730: with an agent that has access to the private host keys.
731: If
732: .Dq SSH_AUTH_SOCK
733: is specified, the location of the socket will be read from the
734: .Ev SSH_AUTH_SOCK
735: environment variable.
1.206 ! markus 736: .It Cm HostKeyAlgorithms
! 737: Specifies the protocol version 2 host key algorithms
! 738: that the server offers.
! 739: The default for this option is:
! 740: .Bd -literal -offset 3n
! 741: ecdsa-sha2-nistp256-cert-v01@openssh.com,
! 742: ecdsa-sha2-nistp384-cert-v01@openssh.com,
! 743: ecdsa-sha2-nistp521-cert-v01@openssh.com,
! 744: ssh-ed25519-cert-v01@openssh.com,
! 745: ssh-rsa-cert-v01@openssh.com,
! 746: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
! 747: ssh-ed25519,ssh-rsa
! 748: .Ed
! 749: .Pp
! 750: The list of available key types may also be obtained using the
! 751: .Fl Q
! 752: option of
! 753: .Xr ssh 1
! 754: with an argument of
! 755: .Dq key .
1.1 stevesk 756: .It Cm IgnoreRhosts
757: Specifies that
758: .Pa .rhosts
759: and
760: .Pa .shosts
761: files will not be used in
762: .Cm RhostsRSAAuthentication
763: or
764: .Cm HostbasedAuthentication .
765: .Pp
766: .Pa /etc/hosts.equiv
767: and
768: .Pa /etc/shosts.equiv
769: are still used.
770: The default is
771: .Dq yes .
772: .It Cm IgnoreUserKnownHosts
773: Specifies whether
1.52 jmc 774: .Xr sshd 8
1.1 stevesk 775: should ignore the user's
1.41 djm 776: .Pa ~/.ssh/known_hosts
1.1 stevesk 777: during
778: .Cm RhostsRSAAuthentication
779: or
780: .Cm HostbasedAuthentication .
781: The default is
782: .Dq no .
1.129 djm 783: .It Cm IPQoS
784: Specifies the IPv4 type-of-service or DSCP class for the connection.
785: Accepted values are
786: .Dq af11 ,
787: .Dq af12 ,
788: .Dq af13 ,
1.136 djm 789: .Dq af21 ,
1.129 djm 790: .Dq af22 ,
791: .Dq af23 ,
792: .Dq af31 ,
793: .Dq af32 ,
794: .Dq af33 ,
795: .Dq af41 ,
796: .Dq af42 ,
797: .Dq af43 ,
798: .Dq cs0 ,
799: .Dq cs1 ,
800: .Dq cs2 ,
801: .Dq cs3 ,
802: .Dq cs4 ,
803: .Dq cs5 ,
804: .Dq cs6 ,
805: .Dq cs7 ,
806: .Dq ef ,
807: .Dq lowdelay ,
808: .Dq throughput ,
809: .Dq reliability ,
810: or a numeric value.
1.131 djm 811: This option may take one or two arguments, separated by whitespace.
1.129 djm 812: If one argument is specified, it is used as the packet class unconditionally.
813: If two values are specified, the first is automatically selected for
814: interactive sessions and the second for non-interactive sessions.
815: The default is
816: .Dq lowdelay
817: for interactive sessions and
818: .Dq throughput
819: for non-interactive sessions.
1.171 jmc 820: .It Cm KbdInteractiveAuthentication
821: Specifies whether to allow keyboard-interactive authentication.
822: The argument to this keyword must be
823: .Dq yes
824: or
825: .Dq no .
826: The default is to use whatever value
827: .Cm ChallengeResponseAuthentication
828: is set to
829: (by default
830: .Dq yes ) .
1.1 stevesk 831: .It Cm KerberosAuthentication
1.24 markus 832: Specifies whether the password provided by the user for
1.1 stevesk 833: .Cm PasswordAuthentication
1.24 markus 834: will be validated through the Kerberos KDC.
1.1 stevesk 835: To use this option, the server needs a
836: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 837: The default is
1.29 dtucker 838: .Dq no .
839: .It Cm KerberosGetAFSToken
1.45 djm 840: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 841: an AFS token before accessing the user's home directory.
1.52 jmc 842: The default is
1.1 stevesk 843: .Dq no .
844: .It Cm KerberosOrLocalPasswd
1.52 jmc 845: If password authentication through Kerberos fails then
1.1 stevesk 846: the password will be validated via any additional local mechanism
847: such as
848: .Pa /etc/passwd .
1.52 jmc 849: The default is
1.1 stevesk 850: .Dq yes .
851: .It Cm KerberosTicketCleanup
852: Specifies whether to automatically destroy the user's ticket cache
853: file on logout.
1.52 jmc 854: The default is
1.1 stevesk 855: .Dq yes .
1.127 djm 856: .It Cm KexAlgorithms
857: Specifies the available KEX (Key Exchange) algorithms.
858: Multiple algorithms must be comma-separated.
1.173 naddy 859: The supported algorithms are:
860: .Pp
861: .Bl -item -compact -offset indent
862: .It
863: curve25519-sha256@libssh.org
864: .It
865: diffie-hellman-group1-sha1
866: .It
867: diffie-hellman-group14-sha1
868: .It
869: diffie-hellman-group-exchange-sha1
870: .It
871: diffie-hellman-group-exchange-sha256
872: .It
873: ecdh-sha2-nistp256
874: .It
875: ecdh-sha2-nistp384
876: .It
877: ecdh-sha2-nistp521
878: .El
879: .Pp
880: The default is:
1.170 dtucker 881: .Bd -literal -offset indent
882: curve25519-sha256@libssh.org,
883: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
884: diffie-hellman-group-exchange-sha256,
1.173 naddy 885: diffie-hellman-group14-sha1
1.170 dtucker 886: .Ed
1.188 djm 887: .Pp
888: The list of available key exchange algorithms may also be obtained using the
889: .Fl Q
890: option of
891: .Xr ssh 1
892: with an argument of
893: .Dq kex .
1.1 stevesk 894: .It Cm KeyRegenerationInterval
895: In protocol version 1, the ephemeral server key is automatically regenerated
896: after this many seconds (if it has been used).
897: The purpose of regeneration is to prevent
898: decrypting captured sessions by later breaking into the machine and
899: stealing the keys.
900: The key is never stored anywhere.
901: If the value is 0, the key is never regenerated.
902: The default is 3600 (seconds).
903: .It Cm ListenAddress
904: Specifies the local addresses
1.52 jmc 905: .Xr sshd 8
1.1 stevesk 906: should listen on.
907: The following forms may be used:
908: .Pp
909: .Bl -item -offset indent -compact
910: .It
911: .Cm ListenAddress
912: .Sm off
1.180 jmc 913: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 914: .Sm on
915: .It
916: .Cm ListenAddress
917: .Sm off
1.180 jmc 918: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 919: .Sm on
920: .It
921: .Cm ListenAddress
922: .Sm off
923: .Oo
1.180 jmc 924: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 925: .Sm on
926: .El
927: .Pp
928: If
929: .Ar port
930: is not specified,
1.200 dtucker 931: sshd will listen on the address and all
1.1 stevesk 932: .Cm Port
1.17 jmc 933: options specified.
934: The default is to listen on all local addresses.
1.15 jmc 935: Multiple
1.1 stevesk 936: .Cm ListenAddress
1.17 jmc 937: options are permitted.
1.1 stevesk 938: .It Cm LoginGraceTime
939: The server disconnects after this time if the user has not
940: successfully logged in.
941: If the value is 0, there is no time limit.
1.12 stevesk 942: The default is 120 seconds.
1.1 stevesk 943: .It Cm LogLevel
944: Gives the verbosity level that is used when logging messages from
1.53 jmc 945: .Xr sshd 8 .
1.1 stevesk 946: The possible values are:
1.52 jmc 947: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 948: The default is INFO.
949: DEBUG and DEBUG1 are equivalent.
950: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
951: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 952: .It Cm MACs
953: Specifies the available MAC (message authentication code) algorithms.
954: The MAC algorithm is used in protocol version 2
955: for data integrity protection.
956: Multiple algorithms must be comma-separated.
1.152 markus 957: The algorithms that contain
958: .Dq -etm
959: calculate the MAC after encryption (encrypt-then-mac).
960: These are considered safer and their use recommended.
1.173 naddy 961: The supported MACs are:
962: .Pp
963: .Bl -item -compact -offset indent
964: .It
965: hmac-md5
966: .It
967: hmac-md5-96
968: .It
969: hmac-ripemd160
970: .It
971: hmac-sha1
972: .It
973: hmac-sha1-96
974: .It
975: hmac-sha2-256
976: .It
977: hmac-sha2-512
978: .It
979: umac-64@openssh.com
980: .It
981: umac-128@openssh.com
982: .It
983: hmac-md5-etm@openssh.com
984: .It
985: hmac-md5-96-etm@openssh.com
986: .It
987: hmac-ripemd160-etm@openssh.com
988: .It
989: hmac-sha1-etm@openssh.com
990: .It
991: hmac-sha1-96-etm@openssh.com
992: .It
993: hmac-sha2-256-etm@openssh.com
994: .It
995: hmac-sha2-512-etm@openssh.com
996: .It
997: umac-64-etm@openssh.com
998: .It
999: umac-128-etm@openssh.com
1000: .El
1001: .Pp
1.52 jmc 1002: The default is:
1.77 jmc 1003: .Bd -literal -offset indent
1.152 markus 1004: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1005: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 1006: umac-64@openssh.com,umac-128@openssh.com,
1007: hmac-sha2-256,hmac-sha2-512
1.77 jmc 1008: .Ed
1.188 djm 1009: .Pp
1010: The list of available MAC algorithms may also be obtained using the
1011: .Fl Q
1012: option of
1013: .Xr ssh 1
1014: with an argument of
1015: .Dq mac .
1.60 dtucker 1016: .It Cm Match
1.61 jmc 1017: Introduces a conditional block.
1.65 dtucker 1018: If all of the criteria on the
1.60 dtucker 1019: .Cm Match
1.65 dtucker 1020: line are satisfied, the keywords on the following lines override those
1021: set in the global section of the config file, until either another
1.60 dtucker 1022: .Cm Match
1.65 dtucker 1023: line or the end of the file.
1.172 djm 1024: If a keyword appears in multiple
1025: .Cm Match
1.177 sobrado 1026: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1027: applied.
1.91 djm 1028: .Pp
1.61 jmc 1029: The arguments to
1.60 dtucker 1030: .Cm Match
1.163 dtucker 1031: are one or more criteria-pattern pairs or the single token
1032: .Cm All
1033: which matches all criteria.
1.60 dtucker 1034: The available criteria are
1035: .Cm User ,
1.69 dtucker 1036: .Cm Group ,
1.60 dtucker 1037: .Cm Host ,
1.139 dtucker 1038: .Cm LocalAddress ,
1039: .Cm LocalPort ,
1.60 dtucker 1040: and
1041: .Cm Address .
1.91 djm 1042: The match patterns may consist of single entries or comma-separated
1043: lists and may use the wildcard and negation operators described in the
1.161 jmc 1044: PATTERNS section of
1.92 djm 1045: .Xr ssh_config 5 .
1.91 djm 1046: .Pp
1047: The patterns in an
1048: .Cm Address
1049: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1050: address/masklen format, e.g.\&
1.91 djm 1051: .Dq 192.0.2.0/24
1052: or
1053: .Dq 3ffe:ffff::/32 .
1054: Note that the mask length provided must be consistent with the address -
1055: it is an error to specify a mask length that is too long for the address
1.93 jmc 1056: or one with bits set in this host portion of the address.
1057: For example,
1.91 djm 1058: .Dq 192.0.2.0/33
1059: and
1.93 jmc 1060: .Dq 192.0.2.0/8
1.91 djm 1061: respectively.
1062: .Pp
1.60 dtucker 1063: Only a subset of keywords may be used on the lines following a
1064: .Cm Match
1065: keyword.
1066: Available keywords are
1.142 jmc 1067: .Cm AcceptEnv ,
1.99 okan 1068: .Cm AllowAgentForwarding ,
1.142 jmc 1069: .Cm AllowGroups ,
1.193 djm 1070: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1071: .Cm AllowTcpForwarding ,
1.141 markus 1072: .Cm AllowUsers ,
1.149 djm 1073: .Cm AuthenticationMethods ,
1.146 djm 1074: .Cm AuthorizedKeysCommand ,
1075: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1076: .Cm AuthorizedKeysFile ,
1.123 djm 1077: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1078: .Cm Banner ,
1.85 djm 1079: .Cm ChrootDirectory ,
1.141 markus 1080: .Cm DenyGroups ,
1081: .Cm DenyUsers ,
1.67 dtucker 1082: .Cm ForceCommand ,
1.194 djm 1083: .Cm GatewayPorts ,
1.193 djm 1084: .Cm GSSAPIAuthentication ,
1.189 djm 1085: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1086: .Cm HostbasedAuthentication ,
1.123 djm 1087: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1088: .Cm IPQoS ,
1.74 jmc 1089: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1090: .Cm KerberosAuthentication ,
1.95 dtucker 1091: .Cm MaxAuthTries ,
1.94 dtucker 1092: .Cm MaxSessions ,
1.72 dtucker 1093: .Cm PasswordAuthentication ,
1.97 djm 1094: .Cm PermitEmptyPasswords ,
1.66 dtucker 1095: .Cm PermitOpen ,
1.79 dtucker 1096: .Cm PermitRootLogin ,
1.164 djm 1097: .Cm PermitTTY ,
1.123 djm 1098: .Cm PermitTunnel ,
1.174 djm 1099: .Cm PermitUserRC ,
1.189 djm 1100: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1101: .Cm PubkeyAuthentication ,
1.159 dtucker 1102: .Cm RekeyLimit ,
1.193 djm 1103: .Cm RevokedKeys ,
1.142 jmc 1104: .Cm RhostsRSAAuthentication ,
1.194 djm 1105: .Cm RSAAuthentication ,
1.193 djm 1106: .Cm StreamLocalBindMask ,
1107: .Cm StreamLocalBindUnlink ,
1108: .Cm TrustedUserCAKeys ,
1.66 dtucker 1109: .Cm X11DisplayOffset ,
1.101 djm 1110: .Cm X11Forwarding
1.60 dtucker 1111: and
1.102 djm 1112: .Cm X11UseLocalHost .
1.33 dtucker 1113: .It Cm MaxAuthTries
1114: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1115: connection.
1116: Once the number of failures reaches half this value,
1117: additional failures are logged.
1118: The default is 6.
1.90 djm 1119: .It Cm MaxSessions
1120: Specifies the maximum number of open sessions permitted per network connection.
1121: The default is 10.
1.1 stevesk 1122: .It Cm MaxStartups
1123: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1124: SSH daemon.
1.1 stevesk 1125: Additional connections will be dropped until authentication succeeds or the
1126: .Cm LoginGraceTime
1127: expires for a connection.
1.156 dtucker 1128: The default is 10:30:100.
1.1 stevesk 1129: .Pp
1130: Alternatively, random early drop can be enabled by specifying
1131: the three colon separated values
1132: .Dq start:rate:full
1.51 jmc 1133: (e.g. "10:30:60").
1.53 jmc 1134: .Xr sshd 8
1.1 stevesk 1135: will refuse connection attempts with a probability of
1136: .Dq rate/100
1137: (30%)
1138: if there are currently
1139: .Dq start
1140: (10)
1141: unauthenticated connections.
1142: The probability increases linearly and all connection attempts
1143: are refused if the number of unauthenticated connections reaches
1144: .Dq full
1145: (60).
1146: .It Cm PasswordAuthentication
1147: Specifies whether password authentication is allowed.
1148: The default is
1149: .Dq yes .
1150: .It Cm PermitEmptyPasswords
1151: When password authentication is allowed, it specifies whether the
1152: server allows login to accounts with empty password strings.
1153: The default is
1154: .Dq no .
1.62 dtucker 1155: .It Cm PermitOpen
1156: Specifies the destinations to which TCP port forwarding is permitted.
1157: The forwarding specification must be one of the following forms:
1158: .Pp
1159: .Bl -item -offset indent -compact
1160: .It
1161: .Cm PermitOpen
1162: .Sm off
1163: .Ar host : port
1164: .Sm on
1165: .It
1166: .Cm PermitOpen
1167: .Sm off
1168: .Ar IPv4_addr : port
1169: .Sm on
1170: .It
1171: .Cm PermitOpen
1172: .Sm off
1173: .Ar \&[ IPv6_addr \&] : port
1174: .Sm on
1175: .El
1176: .Pp
1.68 dtucker 1177: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1178: An argument of
1179: .Dq any
1180: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1181: An argument of
1182: .Dq none
1183: can be used to prohibit all forwarding requests.
1.63 jmc 1184: By default all port forwarding requests are permitted.
1.1 stevesk 1185: .It Cm PermitRootLogin
1.38 jmc 1186: Specifies whether root can log in using
1.1 stevesk 1187: .Xr ssh 1 .
1188: The argument must be
1189: .Dq yes ,
1190: .Dq without-password ,
1.52 jmc 1191: .Dq forced-commands-only ,
1.1 stevesk 1192: or
1193: .Dq no .
1194: The default is
1.196 djm 1195: .Dq no .
1.1 stevesk 1196: .Pp
1197: If this option is set to
1.52 jmc 1198: .Dq without-password ,
1.1 stevesk 1199: password authentication is disabled for root.
1200: .Pp
1201: If this option is set to
1.52 jmc 1202: .Dq forced-commands-only ,
1.1 stevesk 1203: root login with public key authentication will be allowed,
1204: but only if the
1205: .Ar command
1206: option has been specified
1207: (which may be useful for taking remote backups even if root login is
1.17 jmc 1208: normally not allowed).
1209: All other authentication methods are disabled for root.
1.1 stevesk 1210: .Pp
1211: If this option is set to
1.52 jmc 1212: .Dq no ,
1.38 jmc 1213: root is not allowed to log in.
1.46 reyk 1214: .It Cm PermitTunnel
1215: Specifies whether
1216: .Xr tun 4
1217: device forwarding is allowed.
1.47 reyk 1218: The argument must be
1219: .Dq yes ,
1.58 stevesk 1220: .Dq point-to-point
1221: (layer 3),
1222: .Dq ethernet
1223: (layer 2), or
1.47 reyk 1224: .Dq no .
1.58 stevesk 1225: Specifying
1226: .Dq yes
1227: permits both
1228: .Dq point-to-point
1229: and
1230: .Dq ethernet .
1.46 reyk 1231: The default is
1232: .Dq no .
1.178 djm 1233: .Pp
1234: Independent of this setting, the permissions of the selected
1235: .Xr tun 4
1236: device must allow access to the user.
1.164 djm 1237: .It Cm PermitTTY
1238: Specifies whether
1.165 jmc 1239: .Xr pty 4
1.164 djm 1240: allocation is permitted.
1241: The default is
1242: .Dq yes .
1.6 markus 1243: .It Cm PermitUserEnvironment
1244: Specifies whether
1245: .Pa ~/.ssh/environment
1.9 stevesk 1246: and
1.6 markus 1247: .Cm environment=
1248: options in
1249: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1250: are processed by
1.52 jmc 1251: .Xr sshd 8 .
1.6 markus 1252: The default is
1253: .Dq no .
1.9 stevesk 1254: Enabling environment processing may enable users to bypass access
1255: restrictions in some configurations using mechanisms such as
1256: .Ev LD_PRELOAD .
1.174 djm 1257: .It Cm PermitUserRC
1258: Specifies whether any
1259: .Pa ~/.ssh/rc
1260: file is executed.
1261: The default is
1262: .Dq yes .
1.1 stevesk 1263: .It Cm PidFile
1.4 stevesk 1264: Specifies the file that contains the process ID of the
1.195 dtucker 1265: SSH daemon, or
1266: .Dq none
1267: to not write one.
1.1 stevesk 1268: The default is
1269: .Pa /var/run/sshd.pid .
1270: .It Cm Port
1271: Specifies the port number that
1.52 jmc 1272: .Xr sshd 8
1.1 stevesk 1273: listens on.
1274: The default is 22.
1275: Multiple options of this type are permitted.
1276: See also
1277: .Cm ListenAddress .
1278: .It Cm PrintLastLog
1279: Specifies whether
1.52 jmc 1280: .Xr sshd 8
1.36 jaredy 1281: should print the date and time of the last user login when a user logs
1282: in interactively.
1.1 stevesk 1283: The default is
1284: .Dq yes .
1285: .It Cm PrintMotd
1286: Specifies whether
1.52 jmc 1287: .Xr sshd 8
1.1 stevesk 1288: should print
1289: .Pa /etc/motd
1290: when a user logs in interactively.
1291: (On some systems it is also printed by the shell,
1292: .Pa /etc/profile ,
1293: or equivalent.)
1294: The default is
1295: .Dq yes .
1296: .It Cm Protocol
1297: Specifies the protocol versions
1.52 jmc 1298: .Xr sshd 8
1.5 stevesk 1299: supports.
1.1 stevesk 1300: The possible values are
1.52 jmc 1301: .Sq 1
1.1 stevesk 1302: and
1.52 jmc 1303: .Sq 2 .
1.1 stevesk 1304: Multiple versions must be comma-separated.
1305: The default is
1.109 jmc 1306: .Sq 2 .
1.5 stevesk 1307: Note that the order of the protocol list does not indicate preference,
1308: because the client selects among multiple protocol versions offered
1309: by the server.
1310: Specifying
1311: .Dq 2,1
1312: is identical to
1313: .Dq 1,2 .
1.189 djm 1314: .It Cm PubkeyAcceptedKeyTypes
1315: Specifies the key types that will be accepted for public key authentication
1316: as a comma-separated pattern list.
1.206 ! markus 1317: The default for this option is:
! 1318: .Bd -literal -offset 3n
! 1319: ecdsa-sha2-nistp256-cert-v01@openssh.com,
! 1320: ecdsa-sha2-nistp384-cert-v01@openssh.com,
! 1321: ecdsa-sha2-nistp521-cert-v01@openssh.com,
! 1322: ssh-ed25519-cert-v01@openssh.com,
! 1323: ssh-rsa-cert-v01@openssh.com,
! 1324: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
! 1325: ssh-ed25519,ssh-rsa
! 1326: .Ed
! 1327: .Pp
1.189 djm 1328: The
1329: .Fl Q
1330: option of
1331: .Xr ssh 1
1332: may be used to list supported key types.
1.1 stevesk 1333: .It Cm PubkeyAuthentication
1334: Specifies whether public key authentication is allowed.
1335: The default is
1336: .Dq yes .
1337: Note that this option applies to protocol version 2 only.
1.159 dtucker 1338: .It Cm RekeyLimit
1339: Specifies the maximum amount of data that may be transmitted before the
1340: session key is renegotiated, optionally followed a maximum amount of
1341: time that may pass before the session key is renegotiated.
1342: The first argument is specified in bytes and may have a suffix of
1343: .Sq K ,
1344: .Sq M ,
1345: or
1346: .Sq G
1347: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1348: The default is between
1349: .Sq 1G
1350: and
1351: .Sq 4G ,
1352: depending on the cipher.
1353: The optional second value is specified in seconds and may use any of the
1354: units documented in the
1355: .Sx TIME FORMATS
1.160 jmc 1356: section.
1.159 dtucker 1357: The default value for
1358: .Cm RekeyLimit
1359: is
1360: .Dq default none ,
1361: which means that rekeying is performed after the cipher's default amount
1362: of data has been sent or received and no time based rekeying is done.
1363: This option applies to protocol version 2 only.
1.118 djm 1364: .It Cm RevokedKeys
1.195 dtucker 1365: Specifies revoked public keys file, or
1366: .Dq none
1367: to not use one.
1.118 djm 1368: Keys listed in this file will be refused for public key authentication.
1369: Note that if this file is not readable, then public key authentication will
1370: be refused for all users.
1.154 djm 1371: Keys may be specified as a text file, listing one public key per line, or as
1372: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1373: .Xr ssh-keygen 1 .
1.161 jmc 1374: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1375: .Xr ssh-keygen 1 .
1.1 stevesk 1376: .It Cm RhostsRSAAuthentication
1377: Specifies whether rhosts or /etc/hosts.equiv authentication together
1378: with successful RSA host authentication is allowed.
1379: The default is
1380: .Dq no .
1381: This option applies to protocol version 1 only.
1382: .It Cm RSAAuthentication
1383: Specifies whether pure RSA authentication is allowed.
1384: The default is
1385: .Dq yes .
1386: This option applies to protocol version 1 only.
1387: .It Cm ServerKeyBits
1388: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1389: The default and minimum value is 1024.
1.175 millert 1390: .It Cm StreamLocalBindMask
1391: Sets the octal file creation mode mask
1392: .Pq umask
1393: used when creating a Unix-domain socket file for local or remote
1394: port forwarding.
1395: This option is only used for port forwarding to a Unix-domain socket file.
1396: .Pp
1397: The default value is 0177, which creates a Unix-domain socket file that is
1398: readable and writable only by the owner.
1399: Note that not all operating systems honor the file mode on Unix-domain
1400: socket files.
1401: .It Cm StreamLocalBindUnlink
1402: Specifies whether to remove an existing Unix-domain socket file for local
1403: or remote port forwarding before creating a new one.
1404: If the socket file already exists and
1405: .Cm StreamLocalBindUnlink
1406: is not enabled,
1407: .Nm sshd
1408: will be unable to forward the port to the Unix-domain socket file.
1409: This option is only used for port forwarding to a Unix-domain socket file.
1410: .Pp
1411: The argument must be
1412: .Dq yes
1413: or
1414: .Dq no .
1415: The default is
1416: .Dq no .
1.1 stevesk 1417: .It Cm StrictModes
1418: Specifies whether
1.52 jmc 1419: .Xr sshd 8
1.1 stevesk 1420: should check file modes and ownership of the
1421: user's files and home directory before accepting login.
1422: This is normally desirable because novices sometimes accidentally leave their
1423: directory or files world-writable.
1424: The default is
1425: .Dq yes .
1.112 djm 1426: Note that this does not apply to
1427: .Cm ChrootDirectory ,
1428: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1429: .It Cm Subsystem
1.51 jmc 1430: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1431: Arguments should be a subsystem name and a command (with optional arguments)
1432: to execute upon subsystem request.
1.80 djm 1433: .Pp
1.1 stevesk 1434: The command
1435: .Xr sftp-server 8
1436: implements the
1437: .Dq sftp
1438: file transfer subsystem.
1.80 djm 1439: .Pp
1440: Alternately the name
1441: .Dq internal-sftp
1442: implements an in-process
1443: .Dq sftp
1444: server.
1445: This may simplify configurations using
1446: .Cm ChrootDirectory
1447: to force a different filesystem root on clients.
1448: .Pp
1.1 stevesk 1449: By default no subsystems are defined.
1450: Note that this option applies to protocol version 2 only.
1451: .It Cm SyslogFacility
1452: Gives the facility code that is used when logging messages from
1.53 jmc 1453: .Xr sshd 8 .
1.1 stevesk 1454: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1455: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1456: The default is AUTH.
1.27 markus 1457: .It Cm TCPKeepAlive
1458: Specifies whether the system should send TCP keepalive messages to the
1459: other side.
1460: If they are sent, death of the connection or crash of one
1461: of the machines will be properly noticed.
1462: However, this means that
1463: connections will die if the route is down temporarily, and some people
1464: find it annoying.
1465: On the other hand, if TCP keepalives are not sent,
1466: sessions may hang indefinitely on the server, leaving
1467: .Dq ghost
1468: users and consuming server resources.
1469: .Pp
1470: The default is
1471: .Dq yes
1472: (to send TCP keepalive messages), and the server will notice
1473: if the network goes down or the client host crashes.
1474: This avoids infinitely hanging sessions.
1475: .Pp
1476: To disable TCP keepalive messages, the value should be set to
1477: .Dq no .
1.118 djm 1478: .It Cm TrustedUserCAKeys
1479: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1480: trusted to sign user certificates for authentication, or
1481: .Dq none
1482: to not use one.
1.119 jmc 1483: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1484: .Ql #
1485: are allowed.
1486: If a certificate is presented for authentication and has its signing CA key
1487: listed in this file, then it may be used for authentication for any user
1488: listed in the certificate's principals list.
1489: Note that certificates that lack a list of principals will not be permitted
1490: for authentication using
1491: .Cm TrustedUserCAKeys .
1.161 jmc 1492: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1493: .Xr ssh-keygen 1 .
1.18 markus 1494: .It Cm UseDNS
1495: Specifies whether
1.52 jmc 1496: .Xr sshd 8
1.40 jmc 1497: should look up the remote host name and check that
1.18 markus 1498: the resolved host name for the remote IP address maps back to the
1499: very same IP address.
1500: The default is
1.192 deraadt 1501: .Dq no .
1.1 stevesk 1502: .It Cm UseLogin
1503: Specifies whether
1504: .Xr login 1
1505: is used for interactive login sessions.
1506: The default is
1507: .Dq no .
1508: Note that
1509: .Xr login 1
1510: is never used for remote command execution.
1511: Note also, that if this is enabled,
1512: .Cm X11Forwarding
1513: will be disabled because
1514: .Xr login 1
1515: does not know how to handle
1516: .Xr xauth 1
1.15 jmc 1517: cookies.
1518: If
1.1 stevesk 1519: .Cm UsePrivilegeSeparation
1520: is specified, it will be disabled after authentication.
1521: .It Cm UsePrivilegeSeparation
1522: Specifies whether
1.52 jmc 1523: .Xr sshd 8
1.2 stevesk 1524: separates privileges by creating an unprivileged child process
1.15 jmc 1525: to deal with incoming network traffic.
1526: After successful authentication, another process will be created that has
1527: the privilege of the authenticated user.
1528: The goal of privilege separation is to prevent privilege
1.1 stevesk 1529: escalation by containing any corruption within the unprivileged processes.
1530: The default is
1531: .Dq yes .
1.134 djm 1532: If
1533: .Cm UsePrivilegeSeparation
1534: is set to
1535: .Dq sandbox
1536: then the pre-authentication unprivileged process is subject to additional
1537: restrictions.
1.137 djm 1538: .It Cm VersionAddendum
1539: Optionally specifies additional text to append to the SSH protocol banner
1540: sent by the server upon connection.
1541: The default is
1542: .Dq none .
1.1 stevesk 1543: .It Cm X11DisplayOffset
1544: Specifies the first display number available for
1.52 jmc 1545: .Xr sshd 8 Ns 's
1.1 stevesk 1546: X11 forwarding.
1.52 jmc 1547: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1548: The default is 10.
1549: .It Cm X11Forwarding
1550: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1551: The argument must be
1552: .Dq yes
1553: or
1554: .Dq no .
1.1 stevesk 1555: The default is
1556: .Dq no .
1.13 stevesk 1557: .Pp
1558: When X11 forwarding is enabled, there may be additional exposure to
1559: the server and to client displays if the
1.52 jmc 1560: .Xr sshd 8
1.13 stevesk 1561: proxy display is configured to listen on the wildcard address (see
1562: .Cm X11UseLocalhost
1.52 jmc 1563: below), though this is not the default.
1.13 stevesk 1564: Additionally, the authentication spoofing and authentication data
1565: verification and substitution occur on the client side.
1566: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1567: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1568: forwarding (see the warnings for
1569: .Cm ForwardX11
1570: in
1.19 jmc 1571: .Xr ssh_config 5 ) .
1.13 stevesk 1572: A system administrator may have a stance in which they want to
1573: protect clients that may expose themselves to attack by unwittingly
1574: requesting X11 forwarding, which can warrant a
1575: .Dq no
1576: setting.
1577: .Pp
1578: Note that disabling X11 forwarding does not prevent users from
1579: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1580: X11 forwarding is automatically disabled if
1581: .Cm UseLogin
1582: is enabled.
1583: .It Cm X11UseLocalhost
1584: Specifies whether
1.52 jmc 1585: .Xr sshd 8
1.1 stevesk 1586: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1587: the wildcard address.
1588: By default,
1.52 jmc 1589: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1590: hostname part of the
1591: .Ev DISPLAY
1592: environment variable to
1593: .Dq localhost .
1.8 stevesk 1594: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1595: However, some older X11 clients may not function with this
1596: configuration.
1597: .Cm X11UseLocalhost
1598: may be set to
1599: .Dq no
1600: to specify that the forwarding server should be bound to the wildcard
1601: address.
1602: The argument must be
1603: .Dq yes
1604: or
1605: .Dq no .
1606: The default is
1607: .Dq yes .
1608: .It Cm XAuthLocation
1.11 stevesk 1609: Specifies the full pathname of the
1.1 stevesk 1610: .Xr xauth 1
1.195 dtucker 1611: program, or
1612: .Dq none
1613: to not use one.
1.1 stevesk 1614: The default is
1615: .Pa /usr/X11R6/bin/xauth .
1616: .El
1.55 jmc 1617: .Sh TIME FORMATS
1.53 jmc 1618: .Xr sshd 8
1.1 stevesk 1619: command-line arguments and configuration file options that specify time
1620: may be expressed using a sequence of the form:
1621: .Sm off
1.7 stevesk 1622: .Ar time Op Ar qualifier ,
1.1 stevesk 1623: .Sm on
1624: where
1625: .Ar time
1626: is a positive integer value and
1627: .Ar qualifier
1628: is one of the following:
1629: .Pp
1630: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1631: .It Aq Cm none
1.1 stevesk 1632: seconds
1633: .It Cm s | Cm S
1634: seconds
1635: .It Cm m | Cm M
1636: minutes
1637: .It Cm h | Cm H
1638: hours
1639: .It Cm d | Cm D
1640: days
1641: .It Cm w | Cm W
1642: weeks
1643: .El
1644: .Pp
1645: Each member of the sequence is added together to calculate
1646: the total time value.
1647: .Pp
1648: Time format examples:
1649: .Pp
1650: .Bl -tag -width Ds -compact -offset indent
1651: .It 600
1652: 600 seconds (10 minutes)
1653: .It 10m
1654: 10 minutes
1655: .It 1h30m
1656: 1 hour 30 minutes (90 minutes)
1657: .El
1658: .Sh FILES
1659: .Bl -tag -width Ds
1660: .It Pa /etc/ssh/sshd_config
1661: Contains configuration data for
1.53 jmc 1662: .Xr sshd 8 .
1.1 stevesk 1663: This file should be writable by root only, but it is recommended
1664: (though not necessary) that it be world-readable.
1665: .El
1.19 jmc 1666: .Sh SEE ALSO
1667: .Xr sshd 8
1.1 stevesk 1668: .Sh AUTHORS
1669: OpenSSH is a derivative of the original and free
1670: ssh 1.2.12 release by Tatu Ylonen.
1671: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1672: Theo de Raadt and Dug Song
1673: removed many bugs, re-added newer features and
1674: created OpenSSH.
1675: Markus Friedl contributed the support for SSH
1676: protocol versions 1.5 and 2.0.
1677: Niels Provos and Markus Friedl contributed support
1678: for privilege separation.