Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.213
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
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.213 ! sobrado 36: .\" $OpenBSD: sshd_config.5,v 1.212 2015/09/11 03:13:36 djm Exp $
! 37: .Dd $Mdocdate: September 11 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.
1.212 djm 288: Alternately this option may be set to
289: .Dq none
290: to skip checking for user keys in files.
1.133 jmc 291: The default is
292: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 293: .It Cm AuthorizedPrincipalsCommand
294: Specifies a program to be used to generate the list of allowed
295: certificate principals as per
296: .Cm AuthorizedPrincipalsFile .
297: The program must be owned by root, not writable by group or others and
298: specified by an absolute path.
299: .Pp
300: Arguments to
301: .Cm AuthorizedPrincipalsCommand
302: may be provided using the following tokens, which will be expanded
303: at runtime: %% is replaced by a literal '%', %u is replaced by the
304: username being authenticated and %h is replaced by the home directory
305: of the user being authenticated.
306: .Pp
307: The program should produce on standard output zero or
308: more lines of
309: .Cm AuthorizedPrincipalsFile
310: output.
311: If either
312: .Cm AuthorizedPrincipalsCommand
313: or
314: .Cm AuthorizedPrincipalsFile
315: is specified, then certificates offered by the client for authentication
316: must contain a principal that is listed.
317: By default, no AuthorizedPrincipalsCommand is run.
318: .It Cm AuthorizedPrincipalsCommandUser
319: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
320: It is recommended to use a dedicated user that has no other role on the host
321: than running authorized principals commands.
322: If
323: .Cm AuthorizedPrincipalsCommand
324: is specified but
325: .Cm AuthorizedPrincipalsCommandUser
326: is not, then
327: .Xr sshd 8
328: will refuse to start.
1.121 djm 329: .It Cm AuthorizedPrincipalsFile
330: Specifies a file that lists principal names that are accepted for
331: certificate authentication.
332: When using certificates signed by a key listed in
333: .Cm TrustedUserCAKeys ,
334: this file lists names, one of which must appear in the certificate for it
335: to be accepted for authentication.
1.125 jmc 336: Names are listed one per line preceded by key options (as described
1.161 jmc 337: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 338: .Xr sshd 8 ) .
1.124 djm 339: Empty lines and comments starting with
1.121 djm 340: .Ql #
341: are ignored.
342: .Pp
343: .Cm AuthorizedPrincipalsFile
344: may contain tokens of the form %T which are substituted during connection
345: setup.
346: The following tokens are defined: %% is replaced by a literal '%',
347: %h is replaced by the home directory of the user being authenticated, and
348: %u is replaced by the username of that user.
349: After expansion,
350: .Cm AuthorizedPrincipalsFile
351: is taken to be an absolute path or one relative to the user's home
352: directory.
353: .Pp
1.138 djm 354: The default is
355: .Dq none ,
356: i.e. not to use a principals file \(en in this case, the username
1.121 djm 357: of the user must appear in a certificate's principals list for it to be
358: accepted.
359: Note that
360: .Cm AuthorizedPrincipalsFile
361: is only used when authentication proceeds using a CA listed in
362: .Cm TrustedUserCAKeys
363: and is not consulted for certification authorities trusted via
364: .Pa ~/.ssh/authorized_keys ,
365: though the
366: .Cm principals=
367: key option offers a similar facility (see
368: .Xr sshd 8
369: for details).
1.1 stevesk 370: .It Cm Banner
371: The contents of the specified file are sent to the remote user before
372: authentication is allowed.
1.78 djm 373: If the argument is
374: .Dq none
375: then no banner is displayed.
1.1 stevesk 376: This option is only available for protocol version 2.
377: By default, no banner is displayed.
378: .It Cm ChallengeResponseAuthentication
1.50 jmc 379: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 380: All authentication styles from
381: .Xr login.conf 5
382: are supported.
383: The default is
384: .Dq yes .
1.80 djm 385: .It Cm ChrootDirectory
1.113 stevesk 386: Specifies the pathname of a directory to
1.80 djm 387: .Xr chroot 2
388: to after authentication.
1.190 deraadt 389: At session startup
390: .Xr sshd 8
391: checks that all components of the pathname are root-owned directories
392: which are not writable by any other user or group.
1.106 stevesk 393: After the chroot,
394: .Xr sshd 8
395: changes the working directory to the user's home directory.
1.80 djm 396: .Pp
1.113 stevesk 397: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 398: the connecting user has been authenticated: %% is replaced by a literal '%',
399: %h is replaced by the home directory of the user being authenticated, and
400: %u is replaced by the username of that user.
401: .Pp
402: The
403: .Cm ChrootDirectory
404: must contain the necessary files and directories to support the
1.103 stevesk 405: user's session.
1.80 djm 406: For an interactive session this requires at least a shell, typically
407: .Xr sh 1 ,
408: and basic
409: .Pa /dev
410: nodes such as
411: .Xr null 4 ,
412: .Xr zero 4 ,
413: .Xr stdin 4 ,
414: .Xr stdout 4 ,
415: .Xr stderr 4 ,
1.180 jmc 416: and
1.80 djm 417: .Xr tty 4
418: devices.
419: For file transfer sessions using
1.105 jmc 420: .Dq sftp ,
1.80 djm 421: no additional configuration of the environment is necessary if the
1.105 jmc 422: in-process sftp server is used,
1.176 schwarze 423: though sessions which use logging may require
1.104 stevesk 424: .Pa /dev/log
1.176 schwarze 425: inside the chroot directory on some operating systems (see
1.104 stevesk 426: .Xr sftp-server 8
1.81 jmc 427: for details).
1.190 deraadt 428: .Pp
1.191 jmc 429: For safety, it is very important that the directory hierarchy be
1.190 deraadt 430: prevented from modification by other processes on the system (especially
431: those outside the jail).
432: Misconfiguration can lead to unsafe environments which
433: .Xr sshd 8
434: cannot detect.
1.80 djm 435: .Pp
436: The default is not to
437: .Xr chroot 2 .
1.1 stevesk 438: .It Cm Ciphers
439: Specifies the ciphers allowed for protocol version 2.
440: Multiple ciphers must be comma-separated.
1.208 djm 441: If the specified value begins with a
442: .Sq +
443: character, then the specified ciphers will be appended to the default set
444: instead of replacing them.
445: .Pp
1.167 djm 446: The supported ciphers are:
447: .Pp
1.173 naddy 448: .Bl -item -compact -offset indent
449: .It
450: 3des-cbc
451: .It
452: aes128-cbc
453: .It
454: aes192-cbc
455: .It
456: aes256-cbc
457: .It
458: aes128-ctr
459: .It
460: aes192-ctr
461: .It
462: aes256-ctr
463: .It
464: aes128-gcm@openssh.com
465: .It
466: aes256-gcm@openssh.com
467: .It
468: arcfour
469: .It
470: arcfour128
471: .It
472: arcfour256
473: .It
474: blowfish-cbc
475: .It
476: cast128-cbc
477: .It
478: chacha20-poly1305@openssh.com
479: .El
1.167 djm 480: .Pp
1.52 jmc 481: The default is:
1.173 naddy 482: .Bd -literal -offset indent
1.211 jmc 483: chacha20-poly1305@openssh.com,
1.173 naddy 484: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 485: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 486: .Ed
1.167 djm 487: .Pp
488: The list of available ciphers may also be obtained using the
489: .Fl Q
490: option of
1.188 djm 491: .Xr ssh 1
492: with an argument of
493: .Dq cipher .
1.1 stevesk 494: .It Cm ClientAliveCountMax
1.48 jmc 495: Sets the number of client alive messages (see below) which may be
1.1 stevesk 496: sent without
1.52 jmc 497: .Xr sshd 8
1.17 jmc 498: receiving any messages back from the client.
499: If this threshold is reached while client alive messages are being sent,
1.52 jmc 500: sshd will disconnect the client, terminating the session.
1.17 jmc 501: It is important to note that the use of client alive messages is very
502: different from
1.27 markus 503: .Cm TCPKeepAlive
1.17 jmc 504: (below).
505: The client alive messages are sent through the encrypted channel
506: and therefore will not be spoofable.
507: The TCP keepalive option enabled by
1.27 markus 508: .Cm TCPKeepAlive
1.17 jmc 509: is spoofable.
510: The client alive mechanism is valuable when the client or
1.1 stevesk 511: server depend on knowing when a connection has become inactive.
512: .Pp
1.17 jmc 513: The default value is 3.
514: If
1.1 stevesk 515: .Cm ClientAliveInterval
1.48 jmc 516: (see below) is set to 15, and
1.1 stevesk 517: .Cm ClientAliveCountMax
1.52 jmc 518: is left at the default, unresponsive SSH clients
1.1 stevesk 519: will be disconnected after approximately 45 seconds.
1.57 markus 520: This option applies to protocol version 2 only.
1.42 djm 521: .It Cm ClientAliveInterval
522: Sets a timeout interval in seconds after which if no data has been received
523: from the client,
1.52 jmc 524: .Xr sshd 8
1.42 djm 525: will send a message through the encrypted
526: channel to request a response from the client.
527: The default
528: is 0, indicating that these messages will not be sent to the client.
529: This option applies to protocol version 2 only.
1.3 markus 530: .It Cm Compression
1.44 markus 531: Specifies whether compression is allowed, or delayed until
532: the user has authenticated successfully.
1.3 markus 533: The argument must be
1.44 markus 534: .Dq yes ,
535: .Dq delayed ,
1.3 markus 536: or
537: .Dq no .
538: The default is
1.44 markus 539: .Dq delayed .
1.1 stevesk 540: .It Cm DenyGroups
541: This keyword can be followed by a list of group name patterns, separated
542: by spaces.
543: Login is disallowed for users whose primary group or supplementary
544: group list matches one of the patterns.
545: Only group names are valid; a numerical group ID is not recognized.
546: By default, login is allowed for all groups.
1.54 jmc 547: The allow/deny directives are processed in the following order:
548: .Cm DenyUsers ,
549: .Cm AllowUsers ,
550: .Cm DenyGroups ,
551: and finally
552: .Cm AllowGroups .
1.49 jmc 553: .Pp
1.161 jmc 554: See PATTERNS in
1.49 jmc 555: .Xr ssh_config 5
556: for more information on patterns.
1.1 stevesk 557: .It Cm DenyUsers
558: This keyword can be followed by a list of user name patterns, separated
559: by spaces.
560: Login is disallowed for user names that match one of the patterns.
561: Only user names are valid; a numerical user ID is not recognized.
562: By default, login is allowed for all users.
563: If the pattern takes the form USER@HOST then USER and HOST
564: are separately checked, restricting logins to particular
565: users from particular hosts.
1.54 jmc 566: The allow/deny directives are processed in the following order:
567: .Cm DenyUsers ,
568: .Cm AllowUsers ,
569: .Cm DenyGroups ,
570: and finally
571: .Cm AllowGroups .
1.49 jmc 572: .Pp
1.161 jmc 573: See PATTERNS in
1.49 jmc 574: .Xr ssh_config 5
575: for more information on patterns.
1.183 djm 576: .It Cm FingerprintHash
577: Specifies the hash algorithm used when logging key fingerprints.
578: Valid options are:
579: .Dq md5
580: and
581: .Dq sha256 .
582: The default is
583: .Dq sha256 .
1.67 dtucker 584: .It Cm ForceCommand
585: Forces the execution of the command specified by
586: .Cm ForceCommand ,
1.84 djm 587: ignoring any command supplied by the client and
588: .Pa ~/.ssh/rc
589: if present.
1.67 dtucker 590: The command is invoked by using the user's login shell with the -c option.
591: This applies to shell, command, or subsystem execution.
592: It is most useful inside a
593: .Cm Match
594: block.
595: The command originally supplied by the client is available in the
596: .Ev SSH_ORIGINAL_COMMAND
597: environment variable.
1.82 djm 598: Specifying a command of
599: .Dq internal-sftp
600: will force the use of an in-process sftp server that requires no support
601: files when used with
602: .Cm ChrootDirectory .
1.1 stevesk 603: .It Cm GatewayPorts
604: Specifies whether remote hosts are allowed to connect to ports
605: forwarded for the client.
606: By default,
1.52 jmc 607: .Xr sshd 8
1.15 jmc 608: binds remote port forwardings to the loopback address.
609: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 610: .Cm GatewayPorts
1.52 jmc 611: can be used to specify that sshd
1.39 djm 612: should allow remote port forwardings to bind to non-loopback addresses, thus
613: allowing other hosts to connect.
614: The argument may be
615: .Dq no
616: to force remote port forwardings to be available to the local host only,
1.1 stevesk 617: .Dq yes
1.39 djm 618: to force remote port forwardings to bind to the wildcard address, or
619: .Dq clientspecified
620: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 621: The default is
622: .Dq no .
1.23 markus 623: .It Cm GSSAPIAuthentication
1.25 markus 624: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 625: The default is
1.23 markus 626: .Dq no .
627: Note that this option applies to protocol version 2 only.
628: .It Cm GSSAPICleanupCredentials
629: Specifies whether to automatically destroy the user's credentials cache
630: on logout.
631: The default is
632: .Dq yes .
633: Note that this option applies to protocol version 2 only.
1.203 djm 634: .It Cm GSSAPIStrictAcceptorCheck
635: Determines whether to be strict about the identity of the GSSAPI acceptor
636: a client authenticates against.
637: If set to
638: .Dq yes
639: then the client must authenticate against the
640: .Pa host
641: service on the current hostname.
642: If set to
643: .Dq no
644: then the client may authenticate against any service key stored in the
645: machine's default store.
646: This facility is provided to assist with operation on multi homed machines.
647: The default is
648: .Dq yes .
1.189 djm 649: .It Cm HostbasedAcceptedKeyTypes
650: Specifies the key types that will be accepted for hostbased authentication
651: as a comma-separated pattern list.
1.208 djm 652: Alternately if the specified value begins with a
653: .Sq +
654: character, then the specified key types will be appended to the default set
655: instead of replacing them.
1.206 markus 656: The default for this option is:
657: .Bd -literal -offset 3n
658: ecdsa-sha2-nistp256-cert-v01@openssh.com,
659: ecdsa-sha2-nistp384-cert-v01@openssh.com,
660: ecdsa-sha2-nistp521-cert-v01@openssh.com,
661: ssh-ed25519-cert-v01@openssh.com,
662: ssh-rsa-cert-v01@openssh.com,
663: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
664: ssh-ed25519,ssh-rsa
665: .Ed
666: .Pp
1.189 djm 667: The
668: .Fl Q
669: option of
670: .Xr ssh 1
671: may be used to list supported key types.
1.1 stevesk 672: .It Cm HostbasedAuthentication
673: Specifies whether rhosts or /etc/hosts.equiv authentication together
674: with successful public key client host authentication is allowed
1.50 jmc 675: (host-based authentication).
1.1 stevesk 676: This option is similar to
677: .Cm RhostsRSAAuthentication
678: and applies to protocol version 2 only.
1.70 dtucker 679: The default is
680: .Dq no .
681: .It Cm HostbasedUsesNameFromPacketOnly
682: Specifies whether or not the server will attempt to perform a reverse
683: name lookup when matching the name in the
684: .Pa ~/.shosts ,
685: .Pa ~/.rhosts ,
686: and
687: .Pa /etc/hosts.equiv
688: files during
689: .Cm HostbasedAuthentication .
690: A setting of
691: .Dq yes
692: means that
693: .Xr sshd 8
694: uses the name supplied by the client rather than
695: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 696: The default is
697: .Dq no .
1.117 djm 698: .It Cm HostCertificate
699: Specifies a file containing a public host certificate.
700: The certificate's public key must match a private host key already specified
701: by
702: .Cm HostKey .
703: The default behaviour of
704: .Xr sshd 8
705: is not to load any certificates.
1.1 stevesk 706: .It Cm HostKey
707: Specifies a file containing a private host key
708: used by SSH.
709: The default is
710: .Pa /etc/ssh/ssh_host_key
711: for protocol version 1, and
1.126 djm 712: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 713: .Pa /etc/ssh/ssh_host_ecdsa_key ,
714: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 715: and
1.1 stevesk 716: .Pa /etc/ssh/ssh_host_rsa_key
717: for protocol version 2.
1.206 markus 718: .Pp
1.1 stevesk 719: Note that
1.52 jmc 720: .Xr sshd 8
1.206 markus 721: will refuse to use a file if it is group/world-accessible
722: and that the
723: .Cm HostKeyAlgorithms
724: option restricts which of the keys are actually used by
725: .Xr sshd 8 .
726: .Pp
1.1 stevesk 727: It is possible to have multiple host key files.
728: .Dq rsa1
729: keys are used for version 1 and
1.126 djm 730: .Dq dsa ,
1.169 naddy 731: .Dq ecdsa ,
732: .Dq ed25519
1.1 stevesk 733: or
734: .Dq rsa
735: are used for version 2 of the SSH protocol.
1.162 markus 736: It is also possible to specify public host key files instead.
737: In this case operations on the private key will be delegated
738: to an
739: .Xr ssh-agent 1 .
740: .It Cm HostKeyAgent
741: Identifies the UNIX-domain socket used to communicate
742: with an agent that has access to the private host keys.
743: If
744: .Dq SSH_AUTH_SOCK
745: is specified, the location of the socket will be read from the
746: .Ev SSH_AUTH_SOCK
747: environment variable.
1.206 markus 748: .It Cm HostKeyAlgorithms
749: Specifies the protocol version 2 host key algorithms
750: that the server offers.
751: The default for this option is:
752: .Bd -literal -offset 3n
753: ecdsa-sha2-nistp256-cert-v01@openssh.com,
754: ecdsa-sha2-nistp384-cert-v01@openssh.com,
755: ecdsa-sha2-nistp521-cert-v01@openssh.com,
756: ssh-ed25519-cert-v01@openssh.com,
757: ssh-rsa-cert-v01@openssh.com,
758: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
759: ssh-ed25519,ssh-rsa
760: .Ed
761: .Pp
762: The list of available key types may also be obtained using the
763: .Fl Q
764: option of
765: .Xr ssh 1
766: with an argument of
767: .Dq key .
1.1 stevesk 768: .It Cm IgnoreRhosts
769: Specifies that
770: .Pa .rhosts
771: and
772: .Pa .shosts
773: files will not be used in
774: .Cm RhostsRSAAuthentication
775: or
776: .Cm HostbasedAuthentication .
777: .Pp
778: .Pa /etc/hosts.equiv
779: and
780: .Pa /etc/shosts.equiv
781: are still used.
782: The default is
783: .Dq yes .
784: .It Cm IgnoreUserKnownHosts
785: Specifies whether
1.52 jmc 786: .Xr sshd 8
1.1 stevesk 787: should ignore the user's
1.41 djm 788: .Pa ~/.ssh/known_hosts
1.1 stevesk 789: during
790: .Cm RhostsRSAAuthentication
791: or
792: .Cm HostbasedAuthentication .
793: The default is
794: .Dq no .
1.129 djm 795: .It Cm IPQoS
796: Specifies the IPv4 type-of-service or DSCP class for the connection.
797: Accepted values are
798: .Dq af11 ,
799: .Dq af12 ,
800: .Dq af13 ,
1.136 djm 801: .Dq af21 ,
1.129 djm 802: .Dq af22 ,
803: .Dq af23 ,
804: .Dq af31 ,
805: .Dq af32 ,
806: .Dq af33 ,
807: .Dq af41 ,
808: .Dq af42 ,
809: .Dq af43 ,
810: .Dq cs0 ,
811: .Dq cs1 ,
812: .Dq cs2 ,
813: .Dq cs3 ,
814: .Dq cs4 ,
815: .Dq cs5 ,
816: .Dq cs6 ,
817: .Dq cs7 ,
818: .Dq ef ,
819: .Dq lowdelay ,
820: .Dq throughput ,
821: .Dq reliability ,
822: or a numeric value.
1.131 djm 823: This option may take one or two arguments, separated by whitespace.
1.129 djm 824: If one argument is specified, it is used as the packet class unconditionally.
825: If two values are specified, the first is automatically selected for
826: interactive sessions and the second for non-interactive sessions.
827: The default is
828: .Dq lowdelay
829: for interactive sessions and
830: .Dq throughput
831: for non-interactive sessions.
1.171 jmc 832: .It Cm KbdInteractiveAuthentication
833: Specifies whether to allow keyboard-interactive authentication.
834: The argument to this keyword must be
835: .Dq yes
836: or
837: .Dq no .
838: The default is to use whatever value
839: .Cm ChallengeResponseAuthentication
840: is set to
841: (by default
842: .Dq yes ) .
1.1 stevesk 843: .It Cm KerberosAuthentication
1.24 markus 844: Specifies whether the password provided by the user for
1.1 stevesk 845: .Cm PasswordAuthentication
1.24 markus 846: will be validated through the Kerberos KDC.
1.1 stevesk 847: To use this option, the server needs a
848: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 849: The default is
1.29 dtucker 850: .Dq no .
851: .It Cm KerberosGetAFSToken
1.45 djm 852: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 853: an AFS token before accessing the user's home directory.
1.52 jmc 854: The default is
1.1 stevesk 855: .Dq no .
856: .It Cm KerberosOrLocalPasswd
1.52 jmc 857: If password authentication through Kerberos fails then
1.1 stevesk 858: the password will be validated via any additional local mechanism
859: such as
860: .Pa /etc/passwd .
1.52 jmc 861: The default is
1.1 stevesk 862: .Dq yes .
863: .It Cm KerberosTicketCleanup
864: Specifies whether to automatically destroy the user's ticket cache
865: file on logout.
1.52 jmc 866: The default is
1.1 stevesk 867: .Dq yes .
1.127 djm 868: .It Cm KexAlgorithms
869: Specifies the available KEX (Key Exchange) algorithms.
870: Multiple algorithms must be comma-separated.
1.208 djm 871: Alternately if the specified value begins with a
872: .Sq +
873: character, then the specified methods will be appended to the default set
874: instead of replacing them.
1.173 naddy 875: The supported algorithms are:
876: .Pp
877: .Bl -item -compact -offset indent
878: .It
879: curve25519-sha256@libssh.org
880: .It
881: diffie-hellman-group1-sha1
882: .It
883: diffie-hellman-group14-sha1
884: .It
885: diffie-hellman-group-exchange-sha1
886: .It
887: diffie-hellman-group-exchange-sha256
888: .It
889: ecdh-sha2-nistp256
890: .It
891: ecdh-sha2-nistp384
892: .It
893: ecdh-sha2-nistp521
894: .El
895: .Pp
896: The default is:
1.170 dtucker 897: .Bd -literal -offset indent
898: curve25519-sha256@libssh.org,
899: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
900: diffie-hellman-group-exchange-sha256,
1.173 naddy 901: diffie-hellman-group14-sha1
1.170 dtucker 902: .Ed
1.188 djm 903: .Pp
904: The list of available key exchange algorithms may also be obtained using the
905: .Fl Q
906: option of
907: .Xr ssh 1
908: with an argument of
909: .Dq kex .
1.1 stevesk 910: .It Cm KeyRegenerationInterval
911: In protocol version 1, the ephemeral server key is automatically regenerated
912: after this many seconds (if it has been used).
913: The purpose of regeneration is to prevent
914: decrypting captured sessions by later breaking into the machine and
915: stealing the keys.
916: The key is never stored anywhere.
917: If the value is 0, the key is never regenerated.
918: The default is 3600 (seconds).
919: .It Cm ListenAddress
920: Specifies the local addresses
1.52 jmc 921: .Xr sshd 8
1.1 stevesk 922: should listen on.
923: The following forms may be used:
924: .Pp
925: .Bl -item -offset indent -compact
926: .It
927: .Cm ListenAddress
928: .Sm off
1.180 jmc 929: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 930: .Sm on
931: .It
932: .Cm ListenAddress
933: .Sm off
1.180 jmc 934: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 935: .Sm on
936: .It
937: .Cm ListenAddress
938: .Sm off
939: .Oo
1.180 jmc 940: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 941: .Sm on
942: .El
943: .Pp
944: If
945: .Ar port
946: is not specified,
1.200 dtucker 947: sshd will listen on the address and all
1.1 stevesk 948: .Cm Port
1.17 jmc 949: options specified.
950: The default is to listen on all local addresses.
1.15 jmc 951: Multiple
1.1 stevesk 952: .Cm ListenAddress
1.17 jmc 953: options are permitted.
1.1 stevesk 954: .It Cm LoginGraceTime
955: The server disconnects after this time if the user has not
956: successfully logged in.
957: If the value is 0, there is no time limit.
1.12 stevesk 958: The default is 120 seconds.
1.1 stevesk 959: .It Cm LogLevel
960: Gives the verbosity level that is used when logging messages from
1.53 jmc 961: .Xr sshd 8 .
1.1 stevesk 962: The possible values are:
1.52 jmc 963: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 964: The default is INFO.
965: DEBUG and DEBUG1 are equivalent.
966: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
967: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 968: .It Cm MACs
969: Specifies the available MAC (message authentication code) algorithms.
970: The MAC algorithm is used in protocol version 2
971: for data integrity protection.
972: Multiple algorithms must be comma-separated.
1.208 djm 973: If the specified value begins with a
974: .Sq +
975: character, then the specified algorithms will be appended to the default set
976: instead of replacing them.
977: .Pp
1.152 markus 978: The algorithms that contain
979: .Dq -etm
980: calculate the MAC after encryption (encrypt-then-mac).
981: These are considered safer and their use recommended.
1.173 naddy 982: The supported MACs are:
983: .Pp
984: .Bl -item -compact -offset indent
985: .It
986: hmac-md5
987: .It
988: hmac-md5-96
989: .It
990: hmac-ripemd160
991: .It
992: hmac-sha1
993: .It
994: hmac-sha1-96
995: .It
996: hmac-sha2-256
997: .It
998: hmac-sha2-512
999: .It
1000: umac-64@openssh.com
1001: .It
1002: umac-128@openssh.com
1003: .It
1004: hmac-md5-etm@openssh.com
1005: .It
1006: hmac-md5-96-etm@openssh.com
1007: .It
1008: hmac-ripemd160-etm@openssh.com
1009: .It
1010: hmac-sha1-etm@openssh.com
1011: .It
1012: hmac-sha1-96-etm@openssh.com
1013: .It
1014: hmac-sha2-256-etm@openssh.com
1015: .It
1016: hmac-sha2-512-etm@openssh.com
1017: .It
1018: umac-64-etm@openssh.com
1019: .It
1020: umac-128-etm@openssh.com
1021: .El
1022: .Pp
1.52 jmc 1023: The default is:
1.77 jmc 1024: .Bd -literal -offset indent
1.152 markus 1025: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1026: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 1027: umac-64@openssh.com,umac-128@openssh.com,
1028: hmac-sha2-256,hmac-sha2-512
1.77 jmc 1029: .Ed
1.188 djm 1030: .Pp
1031: The list of available MAC algorithms may also be obtained using the
1032: .Fl Q
1033: option of
1034: .Xr ssh 1
1035: with an argument of
1036: .Dq mac .
1.60 dtucker 1037: .It Cm Match
1.61 jmc 1038: Introduces a conditional block.
1.65 dtucker 1039: If all of the criteria on the
1.60 dtucker 1040: .Cm Match
1.65 dtucker 1041: line are satisfied, the keywords on the following lines override those
1042: set in the global section of the config file, until either another
1.60 dtucker 1043: .Cm Match
1.65 dtucker 1044: line or the end of the file.
1.172 djm 1045: If a keyword appears in multiple
1046: .Cm Match
1.177 sobrado 1047: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1048: applied.
1.91 djm 1049: .Pp
1.61 jmc 1050: The arguments to
1.60 dtucker 1051: .Cm Match
1.163 dtucker 1052: are one or more criteria-pattern pairs or the single token
1053: .Cm All
1054: which matches all criteria.
1.60 dtucker 1055: The available criteria are
1056: .Cm User ,
1.69 dtucker 1057: .Cm Group ,
1.60 dtucker 1058: .Cm Host ,
1.139 dtucker 1059: .Cm LocalAddress ,
1060: .Cm LocalPort ,
1.60 dtucker 1061: and
1062: .Cm Address .
1.91 djm 1063: The match patterns may consist of single entries or comma-separated
1064: lists and may use the wildcard and negation operators described in the
1.161 jmc 1065: PATTERNS section of
1.92 djm 1066: .Xr ssh_config 5 .
1.91 djm 1067: .Pp
1068: The patterns in an
1069: .Cm Address
1070: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1071: address/masklen format, e.g.\&
1.91 djm 1072: .Dq 192.0.2.0/24
1073: or
1074: .Dq 3ffe:ffff::/32 .
1075: Note that the mask length provided must be consistent with the address -
1076: it is an error to specify a mask length that is too long for the address
1.93 jmc 1077: or one with bits set in this host portion of the address.
1078: For example,
1.91 djm 1079: .Dq 192.0.2.0/33
1080: and
1.93 jmc 1081: .Dq 192.0.2.0/8
1.91 djm 1082: respectively.
1083: .Pp
1.60 dtucker 1084: Only a subset of keywords may be used on the lines following a
1085: .Cm Match
1086: keyword.
1087: Available keywords are
1.142 jmc 1088: .Cm AcceptEnv ,
1.99 okan 1089: .Cm AllowAgentForwarding ,
1.142 jmc 1090: .Cm AllowGroups ,
1.193 djm 1091: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1092: .Cm AllowTcpForwarding ,
1.141 markus 1093: .Cm AllowUsers ,
1.149 djm 1094: .Cm AuthenticationMethods ,
1.146 djm 1095: .Cm AuthorizedKeysCommand ,
1096: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1097: .Cm AuthorizedKeysFile ,
1.123 djm 1098: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1099: .Cm Banner ,
1.85 djm 1100: .Cm ChrootDirectory ,
1.141 markus 1101: .Cm DenyGroups ,
1102: .Cm DenyUsers ,
1.67 dtucker 1103: .Cm ForceCommand ,
1.194 djm 1104: .Cm GatewayPorts ,
1.193 djm 1105: .Cm GSSAPIAuthentication ,
1.189 djm 1106: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1107: .Cm HostbasedAuthentication ,
1.123 djm 1108: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1109: .Cm IPQoS ,
1.74 jmc 1110: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1111: .Cm KerberosAuthentication ,
1.95 dtucker 1112: .Cm MaxAuthTries ,
1.94 dtucker 1113: .Cm MaxSessions ,
1.72 dtucker 1114: .Cm PasswordAuthentication ,
1.97 djm 1115: .Cm PermitEmptyPasswords ,
1.66 dtucker 1116: .Cm PermitOpen ,
1.79 dtucker 1117: .Cm PermitRootLogin ,
1.164 djm 1118: .Cm PermitTTY ,
1.123 djm 1119: .Cm PermitTunnel ,
1.174 djm 1120: .Cm PermitUserRC ,
1.189 djm 1121: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1122: .Cm PubkeyAuthentication ,
1.159 dtucker 1123: .Cm RekeyLimit ,
1.193 djm 1124: .Cm RevokedKeys ,
1.142 jmc 1125: .Cm RhostsRSAAuthentication ,
1.194 djm 1126: .Cm RSAAuthentication ,
1.193 djm 1127: .Cm StreamLocalBindMask ,
1128: .Cm StreamLocalBindUnlink ,
1129: .Cm TrustedUserCAKeys ,
1.66 dtucker 1130: .Cm X11DisplayOffset ,
1.101 djm 1131: .Cm X11Forwarding
1.60 dtucker 1132: and
1.102 djm 1133: .Cm X11UseLocalHost .
1.33 dtucker 1134: .It Cm MaxAuthTries
1135: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1136: connection.
1137: Once the number of failures reaches half this value,
1138: additional failures are logged.
1139: The default is 6.
1.90 djm 1140: .It Cm MaxSessions
1141: Specifies the maximum number of open sessions permitted per network connection.
1142: The default is 10.
1.1 stevesk 1143: .It Cm MaxStartups
1144: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1145: SSH daemon.
1.1 stevesk 1146: Additional connections will be dropped until authentication succeeds or the
1147: .Cm LoginGraceTime
1148: expires for a connection.
1.156 dtucker 1149: The default is 10:30:100.
1.1 stevesk 1150: .Pp
1151: Alternatively, random early drop can be enabled by specifying
1152: the three colon separated values
1153: .Dq start:rate:full
1.51 jmc 1154: (e.g. "10:30:60").
1.53 jmc 1155: .Xr sshd 8
1.1 stevesk 1156: will refuse connection attempts with a probability of
1157: .Dq rate/100
1158: (30%)
1159: if there are currently
1160: .Dq start
1161: (10)
1162: unauthenticated connections.
1163: The probability increases linearly and all connection attempts
1164: are refused if the number of unauthenticated connections reaches
1165: .Dq full
1166: (60).
1167: .It Cm PasswordAuthentication
1168: Specifies whether password authentication is allowed.
1169: The default is
1170: .Dq yes .
1171: .It Cm PermitEmptyPasswords
1172: When password authentication is allowed, it specifies whether the
1173: server allows login to accounts with empty password strings.
1174: The default is
1175: .Dq no .
1.62 dtucker 1176: .It Cm PermitOpen
1177: Specifies the destinations to which TCP port forwarding is permitted.
1178: The forwarding specification must be one of the following forms:
1179: .Pp
1180: .Bl -item -offset indent -compact
1181: .It
1182: .Cm PermitOpen
1183: .Sm off
1184: .Ar host : port
1185: .Sm on
1186: .It
1187: .Cm PermitOpen
1188: .Sm off
1189: .Ar IPv4_addr : port
1190: .Sm on
1191: .It
1192: .Cm PermitOpen
1193: .Sm off
1194: .Ar \&[ IPv6_addr \&] : port
1195: .Sm on
1196: .El
1197: .Pp
1.68 dtucker 1198: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1199: An argument of
1200: .Dq any
1201: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1202: An argument of
1203: .Dq none
1204: can be used to prohibit all forwarding requests.
1.63 jmc 1205: By default all port forwarding requests are permitted.
1.1 stevesk 1206: .It Cm PermitRootLogin
1.38 jmc 1207: Specifies whether root can log in using
1.1 stevesk 1208: .Xr ssh 1 .
1209: The argument must be
1210: .Dq yes ,
1.210 deraadt 1211: .Dq prohibit-password ,
1.1 stevesk 1212: .Dq without-password ,
1.52 jmc 1213: .Dq forced-commands-only ,
1.1 stevesk 1214: or
1215: .Dq no .
1216: The default is
1.210 deraadt 1217: .Dq prohibit-password .
1.1 stevesk 1218: .Pp
1219: If this option is set to
1.210 deraadt 1220: .Dq prohibit-password
1221: or
1.52 jmc 1222: .Dq without-password ,
1.210 deraadt 1223: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1224: .Pp
1225: If this option is set to
1.52 jmc 1226: .Dq forced-commands-only ,
1.1 stevesk 1227: root login with public key authentication will be allowed,
1228: but only if the
1229: .Ar command
1230: option has been specified
1231: (which may be useful for taking remote backups even if root login is
1.17 jmc 1232: normally not allowed).
1233: All other authentication methods are disabled for root.
1.1 stevesk 1234: .Pp
1235: If this option is set to
1.52 jmc 1236: .Dq no ,
1.38 jmc 1237: root is not allowed to log in.
1.46 reyk 1238: .It Cm PermitTunnel
1239: Specifies whether
1240: .Xr tun 4
1241: device forwarding is allowed.
1.47 reyk 1242: The argument must be
1243: .Dq yes ,
1.58 stevesk 1244: .Dq point-to-point
1245: (layer 3),
1246: .Dq ethernet
1247: (layer 2), or
1.47 reyk 1248: .Dq no .
1.58 stevesk 1249: Specifying
1250: .Dq yes
1251: permits both
1252: .Dq point-to-point
1253: and
1254: .Dq ethernet .
1.46 reyk 1255: The default is
1256: .Dq no .
1.178 djm 1257: .Pp
1258: Independent of this setting, the permissions of the selected
1259: .Xr tun 4
1260: device must allow access to the user.
1.164 djm 1261: .It Cm PermitTTY
1262: Specifies whether
1.165 jmc 1263: .Xr pty 4
1.164 djm 1264: allocation is permitted.
1265: The default is
1266: .Dq yes .
1.6 markus 1267: .It Cm PermitUserEnvironment
1268: Specifies whether
1269: .Pa ~/.ssh/environment
1.9 stevesk 1270: and
1.6 markus 1271: .Cm environment=
1272: options in
1273: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1274: are processed by
1.52 jmc 1275: .Xr sshd 8 .
1.6 markus 1276: The default is
1277: .Dq no .
1.9 stevesk 1278: Enabling environment processing may enable users to bypass access
1279: restrictions in some configurations using mechanisms such as
1280: .Ev LD_PRELOAD .
1.174 djm 1281: .It Cm PermitUserRC
1282: Specifies whether any
1283: .Pa ~/.ssh/rc
1284: file is executed.
1285: The default is
1286: .Dq yes .
1.1 stevesk 1287: .It Cm PidFile
1.4 stevesk 1288: Specifies the file that contains the process ID of the
1.195 dtucker 1289: SSH daemon, or
1290: .Dq none
1291: to not write one.
1.1 stevesk 1292: The default is
1293: .Pa /var/run/sshd.pid .
1294: .It Cm Port
1295: Specifies the port number that
1.52 jmc 1296: .Xr sshd 8
1.1 stevesk 1297: listens on.
1298: The default is 22.
1299: Multiple options of this type are permitted.
1300: See also
1301: .Cm ListenAddress .
1302: .It Cm PrintLastLog
1303: Specifies whether
1.52 jmc 1304: .Xr sshd 8
1.36 jaredy 1305: should print the date and time of the last user login when a user logs
1306: in interactively.
1.1 stevesk 1307: The default is
1308: .Dq yes .
1309: .It Cm PrintMotd
1310: Specifies whether
1.52 jmc 1311: .Xr sshd 8
1.1 stevesk 1312: should print
1313: .Pa /etc/motd
1314: when a user logs in interactively.
1315: (On some systems it is also printed by the shell,
1316: .Pa /etc/profile ,
1317: or equivalent.)
1318: The default is
1319: .Dq yes .
1320: .It Cm Protocol
1321: Specifies the protocol versions
1.52 jmc 1322: .Xr sshd 8
1.5 stevesk 1323: supports.
1.1 stevesk 1324: The possible values are
1.52 jmc 1325: .Sq 1
1.1 stevesk 1326: and
1.52 jmc 1327: .Sq 2 .
1.1 stevesk 1328: Multiple versions must be comma-separated.
1329: The default is
1.109 jmc 1330: .Sq 2 .
1.5 stevesk 1331: Note that the order of the protocol list does not indicate preference,
1332: because the client selects among multiple protocol versions offered
1333: by the server.
1334: Specifying
1335: .Dq 2,1
1336: is identical to
1337: .Dq 1,2 .
1.189 djm 1338: .It Cm PubkeyAcceptedKeyTypes
1339: Specifies the key types that will be accepted for public key authentication
1340: as a comma-separated pattern list.
1.208 djm 1341: Alternately if the specified value begins with a
1342: .Sq +
1343: character, then the specified key types will be appended to the default set
1344: instead of replacing them.
1.206 markus 1345: The default for this option is:
1346: .Bd -literal -offset 3n
1347: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1348: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1349: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1350: ssh-ed25519-cert-v01@openssh.com,
1351: ssh-rsa-cert-v01@openssh.com,
1352: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1353: ssh-ed25519,ssh-rsa
1354: .Ed
1355: .Pp
1.189 djm 1356: The
1357: .Fl Q
1358: option of
1359: .Xr ssh 1
1360: may be used to list supported key types.
1.1 stevesk 1361: .It Cm PubkeyAuthentication
1362: Specifies whether public key authentication is allowed.
1363: The default is
1364: .Dq yes .
1365: Note that this option applies to protocol version 2 only.
1.159 dtucker 1366: .It Cm RekeyLimit
1367: Specifies the maximum amount of data that may be transmitted before the
1368: session key is renegotiated, optionally followed a maximum amount of
1369: time that may pass before the session key is renegotiated.
1370: The first argument is specified in bytes and may have a suffix of
1371: .Sq K ,
1372: .Sq M ,
1373: or
1374: .Sq G
1375: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1376: The default is between
1377: .Sq 1G
1378: and
1379: .Sq 4G ,
1380: depending on the cipher.
1381: The optional second value is specified in seconds and may use any of the
1382: units documented in the
1383: .Sx TIME FORMATS
1.160 jmc 1384: section.
1.159 dtucker 1385: The default value for
1386: .Cm RekeyLimit
1387: is
1388: .Dq default none ,
1389: which means that rekeying is performed after the cipher's default amount
1390: of data has been sent or received and no time based rekeying is done.
1391: This option applies to protocol version 2 only.
1.118 djm 1392: .It Cm RevokedKeys
1.195 dtucker 1393: Specifies revoked public keys file, or
1394: .Dq none
1395: to not use one.
1.118 djm 1396: Keys listed in this file will be refused for public key authentication.
1397: Note that if this file is not readable, then public key authentication will
1398: be refused for all users.
1.154 djm 1399: Keys may be specified as a text file, listing one public key per line, or as
1400: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1401: .Xr ssh-keygen 1 .
1.161 jmc 1402: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1403: .Xr ssh-keygen 1 .
1.1 stevesk 1404: .It Cm RhostsRSAAuthentication
1405: Specifies whether rhosts or /etc/hosts.equiv authentication together
1406: with successful RSA host authentication is allowed.
1407: The default is
1408: .Dq no .
1409: This option applies to protocol version 1 only.
1410: .It Cm RSAAuthentication
1411: Specifies whether pure RSA authentication is allowed.
1412: The default is
1413: .Dq yes .
1414: This option applies to protocol version 1 only.
1415: .It Cm ServerKeyBits
1416: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1417: The default and minimum value is 1024.
1.175 millert 1418: .It Cm StreamLocalBindMask
1419: Sets the octal file creation mode mask
1420: .Pq umask
1421: used when creating a Unix-domain socket file for local or remote
1422: port forwarding.
1423: This option is only used for port forwarding to a Unix-domain socket file.
1424: .Pp
1425: The default value is 0177, which creates a Unix-domain socket file that is
1426: readable and writable only by the owner.
1427: Note that not all operating systems honor the file mode on Unix-domain
1428: socket files.
1429: .It Cm StreamLocalBindUnlink
1430: Specifies whether to remove an existing Unix-domain socket file for local
1431: or remote port forwarding before creating a new one.
1432: If the socket file already exists and
1433: .Cm StreamLocalBindUnlink
1434: is not enabled,
1435: .Nm sshd
1436: will be unable to forward the port to the Unix-domain socket file.
1437: This option is only used for port forwarding to a Unix-domain socket file.
1438: .Pp
1439: The argument must be
1440: .Dq yes
1441: or
1442: .Dq no .
1443: The default is
1444: .Dq no .
1.1 stevesk 1445: .It Cm StrictModes
1446: Specifies whether
1.52 jmc 1447: .Xr sshd 8
1.1 stevesk 1448: should check file modes and ownership of the
1449: user's files and home directory before accepting login.
1450: This is normally desirable because novices sometimes accidentally leave their
1451: directory or files world-writable.
1452: The default is
1453: .Dq yes .
1.112 djm 1454: Note that this does not apply to
1455: .Cm ChrootDirectory ,
1456: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1457: .It Cm Subsystem
1.51 jmc 1458: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1459: Arguments should be a subsystem name and a command (with optional arguments)
1460: to execute upon subsystem request.
1.80 djm 1461: .Pp
1.1 stevesk 1462: The command
1463: .Xr sftp-server 8
1464: implements the
1465: .Dq sftp
1466: file transfer subsystem.
1.80 djm 1467: .Pp
1468: Alternately the name
1469: .Dq internal-sftp
1470: implements an in-process
1471: .Dq sftp
1472: server.
1473: This may simplify configurations using
1474: .Cm ChrootDirectory
1475: to force a different filesystem root on clients.
1476: .Pp
1.1 stevesk 1477: By default no subsystems are defined.
1478: Note that this option applies to protocol version 2 only.
1479: .It Cm SyslogFacility
1480: Gives the facility code that is used when logging messages from
1.53 jmc 1481: .Xr sshd 8 .
1.1 stevesk 1482: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1483: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1484: The default is AUTH.
1.27 markus 1485: .It Cm TCPKeepAlive
1486: Specifies whether the system should send TCP keepalive messages to the
1487: other side.
1488: If they are sent, death of the connection or crash of one
1489: of the machines will be properly noticed.
1490: However, this means that
1491: connections will die if the route is down temporarily, and some people
1492: find it annoying.
1493: On the other hand, if TCP keepalives are not sent,
1494: sessions may hang indefinitely on the server, leaving
1495: .Dq ghost
1496: users and consuming server resources.
1497: .Pp
1498: The default is
1499: .Dq yes
1500: (to send TCP keepalive messages), and the server will notice
1501: if the network goes down or the client host crashes.
1502: This avoids infinitely hanging sessions.
1503: .Pp
1504: To disable TCP keepalive messages, the value should be set to
1505: .Dq no .
1.118 djm 1506: .It Cm TrustedUserCAKeys
1507: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1508: trusted to sign user certificates for authentication, or
1509: .Dq none
1510: to not use one.
1.119 jmc 1511: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1512: .Ql #
1513: are allowed.
1514: If a certificate is presented for authentication and has its signing CA key
1515: listed in this file, then it may be used for authentication for any user
1516: listed in the certificate's principals list.
1517: Note that certificates that lack a list of principals will not be permitted
1518: for authentication using
1519: .Cm TrustedUserCAKeys .
1.161 jmc 1520: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1521: .Xr ssh-keygen 1 .
1.18 markus 1522: .It Cm UseDNS
1523: Specifies whether
1.52 jmc 1524: .Xr sshd 8
1.207 djm 1525: should look up the remote host name, and to check that
1.18 markus 1526: the resolved host name for the remote IP address maps back to the
1527: very same IP address.
1.207 djm 1528: .Pp
1529: If this option is set to
1530: .Dq no
1531: (the default) then only addresses and not host names may be used in
1532: .Pa ~/.ssh/known_hosts
1533: .Cm from
1534: and
1.211 jmc 1535: .Nm
1.207 djm 1536: .Cm Match
1537: .Cm Host
1538: directives.
1.1 stevesk 1539: .It Cm UseLogin
1540: Specifies whether
1541: .Xr login 1
1542: is used for interactive login sessions.
1543: The default is
1544: .Dq no .
1545: Note that
1546: .Xr login 1
1547: is never used for remote command execution.
1548: Note also, that if this is enabled,
1549: .Cm X11Forwarding
1550: will be disabled because
1551: .Xr login 1
1552: does not know how to handle
1553: .Xr xauth 1
1.15 jmc 1554: cookies.
1555: If
1.1 stevesk 1556: .Cm UsePrivilegeSeparation
1557: is specified, it will be disabled after authentication.
1558: .It Cm UsePrivilegeSeparation
1559: Specifies whether
1.52 jmc 1560: .Xr sshd 8
1.2 stevesk 1561: separates privileges by creating an unprivileged child process
1.15 jmc 1562: to deal with incoming network traffic.
1563: After successful authentication, another process will be created that has
1564: the privilege of the authenticated user.
1565: The goal of privilege separation is to prevent privilege
1.1 stevesk 1566: escalation by containing any corruption within the unprivileged processes.
1.213 ! sobrado 1567: The argument must be
! 1568: .Dq yes ,
! 1569: .Dq no ,
! 1570: or
! 1571: .Dq sandbox .
1.134 djm 1572: If
1573: .Cm UsePrivilegeSeparation
1574: is set to
1575: .Dq sandbox
1576: then the pre-authentication unprivileged process is subject to additional
1577: restrictions.
1.213 ! sobrado 1578: The default is
! 1579: .Dq sandbox .
1.137 djm 1580: .It Cm VersionAddendum
1581: Optionally specifies additional text to append to the SSH protocol banner
1582: sent by the server upon connection.
1583: The default is
1584: .Dq none .
1.1 stevesk 1585: .It Cm X11DisplayOffset
1586: Specifies the first display number available for
1.52 jmc 1587: .Xr sshd 8 Ns 's
1.1 stevesk 1588: X11 forwarding.
1.52 jmc 1589: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1590: The default is 10.
1591: .It Cm X11Forwarding
1592: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1593: The argument must be
1594: .Dq yes
1595: or
1596: .Dq no .
1.1 stevesk 1597: The default is
1598: .Dq no .
1.13 stevesk 1599: .Pp
1600: When X11 forwarding is enabled, there may be additional exposure to
1601: the server and to client displays if the
1.52 jmc 1602: .Xr sshd 8
1.13 stevesk 1603: proxy display is configured to listen on the wildcard address (see
1604: .Cm X11UseLocalhost
1.52 jmc 1605: below), though this is not the default.
1.13 stevesk 1606: Additionally, the authentication spoofing and authentication data
1607: verification and substitution occur on the client side.
1608: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1609: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1610: forwarding (see the warnings for
1611: .Cm ForwardX11
1612: in
1.19 jmc 1613: .Xr ssh_config 5 ) .
1.13 stevesk 1614: A system administrator may have a stance in which they want to
1615: protect clients that may expose themselves to attack by unwittingly
1616: requesting X11 forwarding, which can warrant a
1617: .Dq no
1618: setting.
1619: .Pp
1620: Note that disabling X11 forwarding does not prevent users from
1621: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1622: X11 forwarding is automatically disabled if
1623: .Cm UseLogin
1624: is enabled.
1625: .It Cm X11UseLocalhost
1626: Specifies whether
1.52 jmc 1627: .Xr sshd 8
1.1 stevesk 1628: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1629: the wildcard address.
1630: By default,
1.52 jmc 1631: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1632: hostname part of the
1633: .Ev DISPLAY
1634: environment variable to
1635: .Dq localhost .
1.8 stevesk 1636: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1637: However, some older X11 clients may not function with this
1638: configuration.
1639: .Cm X11UseLocalhost
1640: may be set to
1641: .Dq no
1642: to specify that the forwarding server should be bound to the wildcard
1643: address.
1644: The argument must be
1645: .Dq yes
1646: or
1647: .Dq no .
1648: The default is
1649: .Dq yes .
1650: .It Cm XAuthLocation
1.11 stevesk 1651: Specifies the full pathname of the
1.1 stevesk 1652: .Xr xauth 1
1.195 dtucker 1653: program, or
1654: .Dq none
1655: to not use one.
1.1 stevesk 1656: The default is
1657: .Pa /usr/X11R6/bin/xauth .
1658: .El
1.55 jmc 1659: .Sh TIME FORMATS
1.53 jmc 1660: .Xr sshd 8
1.1 stevesk 1661: command-line arguments and configuration file options that specify time
1662: may be expressed using a sequence of the form:
1663: .Sm off
1.7 stevesk 1664: .Ar time Op Ar qualifier ,
1.1 stevesk 1665: .Sm on
1666: where
1667: .Ar time
1668: is a positive integer value and
1669: .Ar qualifier
1670: is one of the following:
1671: .Pp
1672: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1673: .It Aq Cm none
1.1 stevesk 1674: seconds
1675: .It Cm s | Cm S
1676: seconds
1677: .It Cm m | Cm M
1678: minutes
1679: .It Cm h | Cm H
1680: hours
1681: .It Cm d | Cm D
1682: days
1683: .It Cm w | Cm W
1684: weeks
1685: .El
1686: .Pp
1687: Each member of the sequence is added together to calculate
1688: the total time value.
1689: .Pp
1690: Time format examples:
1691: .Pp
1692: .Bl -tag -width Ds -compact -offset indent
1693: .It 600
1694: 600 seconds (10 minutes)
1695: .It 10m
1696: 10 minutes
1697: .It 1h30m
1698: 1 hour 30 minutes (90 minutes)
1699: .El
1700: .Sh FILES
1701: .Bl -tag -width Ds
1702: .It Pa /etc/ssh/sshd_config
1703: Contains configuration data for
1.53 jmc 1704: .Xr sshd 8 .
1.1 stevesk 1705: This file should be writable by root only, but it is recommended
1706: (though not necessary) that it be world-readable.
1707: .El
1.19 jmc 1708: .Sh SEE ALSO
1709: .Xr sshd 8
1.1 stevesk 1710: .Sh AUTHORS
1711: OpenSSH is a derivative of the original and free
1712: ssh 1.2.12 release by Tatu Ylonen.
1713: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1714: Theo de Raadt and Dug Song
1715: removed many bugs, re-added newer features and
1716: created OpenSSH.
1717: Markus Friedl contributed the support for SSH
1718: protocol versions 1.5 and 2.0.
1719: Niels Provos and Markus Friedl contributed support
1720: for privilege separation.
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