Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.215
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.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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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1.215 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.214 2015/11/13 02:57:46 djm Exp $
! 37: .Dd $Mdocdate: November 13 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
1.215 ! djm 436: The default is
! 437: .Dq none ,
! 438: indicating not to
1.80 djm 439: .Xr chroot 2 .
1.1 stevesk 440: .It Cm Ciphers
441: Specifies the ciphers allowed for protocol version 2.
442: Multiple ciphers must be comma-separated.
1.208 djm 443: If the specified value begins with a
444: .Sq +
445: character, then the specified ciphers will be appended to the default set
446: instead of replacing them.
447: .Pp
1.167 djm 448: The supported ciphers are:
449: .Pp
1.173 naddy 450: .Bl -item -compact -offset indent
451: .It
452: 3des-cbc
453: .It
454: aes128-cbc
455: .It
456: aes192-cbc
457: .It
458: aes256-cbc
459: .It
460: aes128-ctr
461: .It
462: aes192-ctr
463: .It
464: aes256-ctr
465: .It
466: aes128-gcm@openssh.com
467: .It
468: aes256-gcm@openssh.com
469: .It
470: arcfour
471: .It
472: arcfour128
473: .It
474: arcfour256
475: .It
476: blowfish-cbc
477: .It
478: cast128-cbc
479: .It
480: chacha20-poly1305@openssh.com
481: .El
1.167 djm 482: .Pp
1.52 jmc 483: The default is:
1.173 naddy 484: .Bd -literal -offset indent
1.211 jmc 485: chacha20-poly1305@openssh.com,
1.173 naddy 486: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 487: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 488: .Ed
1.167 djm 489: .Pp
490: The list of available ciphers may also be obtained using the
491: .Fl Q
492: option of
1.188 djm 493: .Xr ssh 1
494: with an argument of
495: .Dq cipher .
1.1 stevesk 496: .It Cm ClientAliveCountMax
1.48 jmc 497: Sets the number of client alive messages (see below) which may be
1.1 stevesk 498: sent without
1.52 jmc 499: .Xr sshd 8
1.17 jmc 500: receiving any messages back from the client.
501: If this threshold is reached while client alive messages are being sent,
1.52 jmc 502: sshd will disconnect the client, terminating the session.
1.17 jmc 503: It is important to note that the use of client alive messages is very
504: different from
1.27 markus 505: .Cm TCPKeepAlive
1.17 jmc 506: (below).
507: The client alive messages are sent through the encrypted channel
508: and therefore will not be spoofable.
509: The TCP keepalive option enabled by
1.27 markus 510: .Cm TCPKeepAlive
1.17 jmc 511: is spoofable.
512: The client alive mechanism is valuable when the client or
1.1 stevesk 513: server depend on knowing when a connection has become inactive.
514: .Pp
1.17 jmc 515: The default value is 3.
516: If
1.1 stevesk 517: .Cm ClientAliveInterval
1.48 jmc 518: (see below) is set to 15, and
1.1 stevesk 519: .Cm ClientAliveCountMax
1.52 jmc 520: is left at the default, unresponsive SSH clients
1.1 stevesk 521: will be disconnected after approximately 45 seconds.
1.57 markus 522: This option applies to protocol version 2 only.
1.42 djm 523: .It Cm ClientAliveInterval
524: Sets a timeout interval in seconds after which if no data has been received
525: from the client,
1.52 jmc 526: .Xr sshd 8
1.42 djm 527: will send a message through the encrypted
528: channel to request a response from the client.
529: The default
530: is 0, indicating that these messages will not be sent to the client.
531: This option applies to protocol version 2 only.
1.3 markus 532: .It Cm Compression
1.44 markus 533: Specifies whether compression is allowed, or delayed until
534: the user has authenticated successfully.
1.3 markus 535: The argument must be
1.44 markus 536: .Dq yes ,
537: .Dq delayed ,
1.3 markus 538: or
539: .Dq no .
540: The default is
1.44 markus 541: .Dq delayed .
1.1 stevesk 542: .It Cm DenyGroups
543: This keyword can be followed by a list of group name patterns, separated
544: by spaces.
545: Login is disallowed for users whose primary group or supplementary
546: group list matches one of the patterns.
547: Only group names are valid; a numerical group ID is not recognized.
548: By default, login is allowed for all groups.
1.54 jmc 549: The allow/deny directives are processed in the following order:
550: .Cm DenyUsers ,
551: .Cm AllowUsers ,
552: .Cm DenyGroups ,
553: and finally
554: .Cm AllowGroups .
1.49 jmc 555: .Pp
1.161 jmc 556: See PATTERNS in
1.49 jmc 557: .Xr ssh_config 5
558: for more information on patterns.
1.1 stevesk 559: .It Cm DenyUsers
560: This keyword can be followed by a list of user name patterns, separated
561: by spaces.
562: Login is disallowed for user names that match one of the patterns.
563: Only user names are valid; a numerical user ID is not recognized.
564: By default, login is allowed for all users.
565: If the pattern takes the form USER@HOST then USER and HOST
566: are separately checked, restricting logins to particular
567: users from particular hosts.
1.54 jmc 568: The allow/deny directives are processed in the following order:
569: .Cm DenyUsers ,
570: .Cm AllowUsers ,
571: .Cm DenyGroups ,
572: and finally
573: .Cm AllowGroups .
1.49 jmc 574: .Pp
1.161 jmc 575: See PATTERNS in
1.49 jmc 576: .Xr ssh_config 5
577: for more information on patterns.
1.183 djm 578: .It Cm FingerprintHash
579: Specifies the hash algorithm used when logging key fingerprints.
580: Valid options are:
581: .Dq md5
582: and
583: .Dq sha256 .
584: The default is
585: .Dq sha256 .
1.67 dtucker 586: .It Cm ForceCommand
587: Forces the execution of the command specified by
588: .Cm ForceCommand ,
1.84 djm 589: ignoring any command supplied by the client and
590: .Pa ~/.ssh/rc
591: if present.
1.67 dtucker 592: The command is invoked by using the user's login shell with the -c option.
593: This applies to shell, command, or subsystem execution.
594: It is most useful inside a
595: .Cm Match
596: block.
597: The command originally supplied by the client is available in the
598: .Ev SSH_ORIGINAL_COMMAND
599: environment variable.
1.82 djm 600: Specifying a command of
601: .Dq internal-sftp
602: will force the use of an in-process sftp server that requires no support
603: files when used with
604: .Cm ChrootDirectory .
1.215 ! djm 605: The default is
! 606: .Dq none .
1.1 stevesk 607: .It Cm GatewayPorts
608: Specifies whether remote hosts are allowed to connect to ports
609: forwarded for the client.
610: By default,
1.52 jmc 611: .Xr sshd 8
1.15 jmc 612: binds remote port forwardings to the loopback address.
613: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 614: .Cm GatewayPorts
1.52 jmc 615: can be used to specify that sshd
1.39 djm 616: should allow remote port forwardings to bind to non-loopback addresses, thus
617: allowing other hosts to connect.
618: The argument may be
619: .Dq no
620: to force remote port forwardings to be available to the local host only,
1.1 stevesk 621: .Dq yes
1.39 djm 622: to force remote port forwardings to bind to the wildcard address, or
623: .Dq clientspecified
624: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 625: The default is
626: .Dq no .
1.23 markus 627: .It Cm GSSAPIAuthentication
1.25 markus 628: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 629: The default is
1.23 markus 630: .Dq no .
631: Note that this option applies to protocol version 2 only.
632: .It Cm GSSAPICleanupCredentials
633: Specifies whether to automatically destroy the user's credentials cache
634: on logout.
635: The default is
636: .Dq yes .
637: Note that this option applies to protocol version 2 only.
1.203 djm 638: .It Cm GSSAPIStrictAcceptorCheck
639: Determines whether to be strict about the identity of the GSSAPI acceptor
640: a client authenticates against.
641: If set to
642: .Dq yes
643: then the client must authenticate against the
644: .Pa host
645: service on the current hostname.
646: If set to
647: .Dq no
648: then the client may authenticate against any service key stored in the
649: machine's default store.
650: This facility is provided to assist with operation on multi homed machines.
651: The default is
652: .Dq yes .
1.189 djm 653: .It Cm HostbasedAcceptedKeyTypes
654: Specifies the key types that will be accepted for hostbased authentication
655: as a comma-separated pattern list.
1.208 djm 656: Alternately if the specified value begins with a
657: .Sq +
658: character, then the specified key types will be appended to the default set
659: instead of replacing them.
1.206 markus 660: The default for this option is:
661: .Bd -literal -offset 3n
662: ecdsa-sha2-nistp256-cert-v01@openssh.com,
663: ecdsa-sha2-nistp384-cert-v01@openssh.com,
664: ecdsa-sha2-nistp521-cert-v01@openssh.com,
665: ssh-ed25519-cert-v01@openssh.com,
666: ssh-rsa-cert-v01@openssh.com,
667: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
668: ssh-ed25519,ssh-rsa
669: .Ed
670: .Pp
1.189 djm 671: The
672: .Fl Q
673: option of
674: .Xr ssh 1
675: may be used to list supported key types.
1.1 stevesk 676: .It Cm HostbasedAuthentication
677: Specifies whether rhosts or /etc/hosts.equiv authentication together
678: with successful public key client host authentication is allowed
1.50 jmc 679: (host-based authentication).
1.1 stevesk 680: This option is similar to
681: .Cm RhostsRSAAuthentication
682: and applies to protocol version 2 only.
1.70 dtucker 683: The default is
684: .Dq no .
685: .It Cm HostbasedUsesNameFromPacketOnly
686: Specifies whether or not the server will attempt to perform a reverse
687: name lookup when matching the name in the
688: .Pa ~/.shosts ,
689: .Pa ~/.rhosts ,
690: and
691: .Pa /etc/hosts.equiv
692: files during
693: .Cm HostbasedAuthentication .
694: A setting of
695: .Dq yes
696: means that
697: .Xr sshd 8
698: uses the name supplied by the client rather than
699: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 700: The default is
701: .Dq no .
1.117 djm 702: .It Cm HostCertificate
703: Specifies a file containing a public host certificate.
704: The certificate's public key must match a private host key already specified
705: by
706: .Cm HostKey .
707: The default behaviour of
708: .Xr sshd 8
709: is not to load any certificates.
1.1 stevesk 710: .It Cm HostKey
711: Specifies a file containing a private host key
712: used by SSH.
713: The default is
714: .Pa /etc/ssh/ssh_host_key
715: for protocol version 1, and
1.126 djm 716: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 717: .Pa /etc/ssh/ssh_host_ecdsa_key ,
718: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 719: and
1.1 stevesk 720: .Pa /etc/ssh/ssh_host_rsa_key
721: for protocol version 2.
1.206 markus 722: .Pp
1.1 stevesk 723: Note that
1.52 jmc 724: .Xr sshd 8
1.206 markus 725: will refuse to use a file if it is group/world-accessible
726: and that the
727: .Cm HostKeyAlgorithms
728: option restricts which of the keys are actually used by
729: .Xr sshd 8 .
730: .Pp
1.1 stevesk 731: It is possible to have multiple host key files.
732: .Dq rsa1
733: keys are used for version 1 and
1.126 djm 734: .Dq dsa ,
1.169 naddy 735: .Dq ecdsa ,
736: .Dq ed25519
1.1 stevesk 737: or
738: .Dq rsa
739: are used for version 2 of the SSH protocol.
1.162 markus 740: It is also possible to specify public host key files instead.
741: In this case operations on the private key will be delegated
742: to an
743: .Xr ssh-agent 1 .
744: .It Cm HostKeyAgent
745: Identifies the UNIX-domain socket used to communicate
746: with an agent that has access to the private host keys.
747: If
748: .Dq SSH_AUTH_SOCK
749: is specified, the location of the socket will be read from the
750: .Ev SSH_AUTH_SOCK
751: environment variable.
1.206 markus 752: .It Cm HostKeyAlgorithms
753: Specifies the protocol version 2 host key algorithms
754: that the server offers.
755: The default for this option is:
756: .Bd -literal -offset 3n
757: ecdsa-sha2-nistp256-cert-v01@openssh.com,
758: ecdsa-sha2-nistp384-cert-v01@openssh.com,
759: ecdsa-sha2-nistp521-cert-v01@openssh.com,
760: ssh-ed25519-cert-v01@openssh.com,
761: ssh-rsa-cert-v01@openssh.com,
762: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
763: ssh-ed25519,ssh-rsa
764: .Ed
765: .Pp
766: The list of available key types may also be obtained using the
767: .Fl Q
768: option of
769: .Xr ssh 1
770: with an argument of
771: .Dq key .
1.1 stevesk 772: .It Cm IgnoreRhosts
773: Specifies that
774: .Pa .rhosts
775: and
776: .Pa .shosts
777: files will not be used in
778: .Cm RhostsRSAAuthentication
779: or
780: .Cm HostbasedAuthentication .
781: .Pp
782: .Pa /etc/hosts.equiv
783: and
784: .Pa /etc/shosts.equiv
785: are still used.
786: The default is
787: .Dq yes .
788: .It Cm IgnoreUserKnownHosts
789: Specifies whether
1.52 jmc 790: .Xr sshd 8
1.1 stevesk 791: should ignore the user's
1.41 djm 792: .Pa ~/.ssh/known_hosts
1.1 stevesk 793: during
794: .Cm RhostsRSAAuthentication
795: or
796: .Cm HostbasedAuthentication .
797: The default is
798: .Dq no .
1.129 djm 799: .It Cm IPQoS
800: Specifies the IPv4 type-of-service or DSCP class for the connection.
801: Accepted values are
802: .Dq af11 ,
803: .Dq af12 ,
804: .Dq af13 ,
1.136 djm 805: .Dq af21 ,
1.129 djm 806: .Dq af22 ,
807: .Dq af23 ,
808: .Dq af31 ,
809: .Dq af32 ,
810: .Dq af33 ,
811: .Dq af41 ,
812: .Dq af42 ,
813: .Dq af43 ,
814: .Dq cs0 ,
815: .Dq cs1 ,
816: .Dq cs2 ,
817: .Dq cs3 ,
818: .Dq cs4 ,
819: .Dq cs5 ,
820: .Dq cs6 ,
821: .Dq cs7 ,
822: .Dq ef ,
823: .Dq lowdelay ,
824: .Dq throughput ,
825: .Dq reliability ,
826: or a numeric value.
1.131 djm 827: This option may take one or two arguments, separated by whitespace.
1.129 djm 828: If one argument is specified, it is used as the packet class unconditionally.
829: If two values are specified, the first is automatically selected for
830: interactive sessions and the second for non-interactive sessions.
831: The default is
832: .Dq lowdelay
833: for interactive sessions and
834: .Dq throughput
835: for non-interactive sessions.
1.171 jmc 836: .It Cm KbdInteractiveAuthentication
837: Specifies whether to allow keyboard-interactive authentication.
838: The argument to this keyword must be
839: .Dq yes
840: or
841: .Dq no .
842: The default is to use whatever value
843: .Cm ChallengeResponseAuthentication
844: is set to
845: (by default
846: .Dq yes ) .
1.1 stevesk 847: .It Cm KerberosAuthentication
1.24 markus 848: Specifies whether the password provided by the user for
1.1 stevesk 849: .Cm PasswordAuthentication
1.24 markus 850: will be validated through the Kerberos KDC.
1.1 stevesk 851: To use this option, the server needs a
852: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 853: The default is
1.29 dtucker 854: .Dq no .
855: .It Cm KerberosGetAFSToken
1.45 djm 856: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 857: an AFS token before accessing the user's home directory.
1.52 jmc 858: The default is
1.1 stevesk 859: .Dq no .
860: .It Cm KerberosOrLocalPasswd
1.52 jmc 861: If password authentication through Kerberos fails then
1.1 stevesk 862: the password will be validated via any additional local mechanism
863: such as
864: .Pa /etc/passwd .
1.52 jmc 865: The default is
1.1 stevesk 866: .Dq yes .
867: .It Cm KerberosTicketCleanup
868: Specifies whether to automatically destroy the user's ticket cache
869: file on logout.
1.52 jmc 870: The default is
1.1 stevesk 871: .Dq yes .
1.127 djm 872: .It Cm KexAlgorithms
873: Specifies the available KEX (Key Exchange) algorithms.
874: Multiple algorithms must be comma-separated.
1.208 djm 875: Alternately if the specified value begins with a
876: .Sq +
877: character, then the specified methods will be appended to the default set
878: instead of replacing them.
1.173 naddy 879: The supported algorithms are:
880: .Pp
881: .Bl -item -compact -offset indent
882: .It
883: curve25519-sha256@libssh.org
884: .It
885: diffie-hellman-group1-sha1
886: .It
887: diffie-hellman-group14-sha1
888: .It
889: diffie-hellman-group-exchange-sha1
890: .It
891: diffie-hellman-group-exchange-sha256
892: .It
893: ecdh-sha2-nistp256
894: .It
895: ecdh-sha2-nistp384
896: .It
897: ecdh-sha2-nistp521
898: .El
899: .Pp
900: The default is:
1.170 dtucker 901: .Bd -literal -offset indent
902: curve25519-sha256@libssh.org,
903: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
904: diffie-hellman-group-exchange-sha256,
1.173 naddy 905: diffie-hellman-group14-sha1
1.170 dtucker 906: .Ed
1.188 djm 907: .Pp
908: The list of available key exchange algorithms may also be obtained using the
909: .Fl Q
910: option of
911: .Xr ssh 1
912: with an argument of
913: .Dq kex .
1.1 stevesk 914: .It Cm KeyRegenerationInterval
915: In protocol version 1, the ephemeral server key is automatically regenerated
916: after this many seconds (if it has been used).
917: The purpose of regeneration is to prevent
918: decrypting captured sessions by later breaking into the machine and
919: stealing the keys.
920: The key is never stored anywhere.
921: If the value is 0, the key is never regenerated.
922: The default is 3600 (seconds).
923: .It Cm ListenAddress
924: Specifies the local addresses
1.52 jmc 925: .Xr sshd 8
1.1 stevesk 926: should listen on.
927: The following forms may be used:
928: .Pp
929: .Bl -item -offset indent -compact
930: .It
931: .Cm ListenAddress
932: .Sm off
1.180 jmc 933: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 934: .Sm on
935: .It
936: .Cm ListenAddress
937: .Sm off
1.180 jmc 938: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 939: .Sm on
940: .It
941: .Cm ListenAddress
942: .Sm off
943: .Oo
1.180 jmc 944: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 945: .Sm on
946: .El
947: .Pp
948: If
949: .Ar port
950: is not specified,
1.200 dtucker 951: sshd will listen on the address and all
1.1 stevesk 952: .Cm Port
1.17 jmc 953: options specified.
954: The default is to listen on all local addresses.
1.15 jmc 955: Multiple
1.1 stevesk 956: .Cm ListenAddress
1.17 jmc 957: options are permitted.
1.1 stevesk 958: .It Cm LoginGraceTime
959: The server disconnects after this time if the user has not
960: successfully logged in.
961: If the value is 0, there is no time limit.
1.12 stevesk 962: The default is 120 seconds.
1.1 stevesk 963: .It Cm LogLevel
964: Gives the verbosity level that is used when logging messages from
1.53 jmc 965: .Xr sshd 8 .
1.1 stevesk 966: The possible values are:
1.52 jmc 967: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 968: The default is INFO.
969: DEBUG and DEBUG1 are equivalent.
970: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
971: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 972: .It Cm MACs
973: Specifies the available MAC (message authentication code) algorithms.
974: The MAC algorithm is used in protocol version 2
975: for data integrity protection.
976: Multiple algorithms must be comma-separated.
1.208 djm 977: If the specified value begins with a
978: .Sq +
979: character, then the specified algorithms will be appended to the default set
980: instead of replacing them.
981: .Pp
1.152 markus 982: The algorithms that contain
983: .Dq -etm
984: calculate the MAC after encryption (encrypt-then-mac).
985: These are considered safer and their use recommended.
1.173 naddy 986: The supported MACs are:
987: .Pp
988: .Bl -item -compact -offset indent
989: .It
990: hmac-md5
991: .It
992: hmac-md5-96
993: .It
994: hmac-ripemd160
995: .It
996: hmac-sha1
997: .It
998: hmac-sha1-96
999: .It
1000: hmac-sha2-256
1001: .It
1002: hmac-sha2-512
1003: .It
1004: umac-64@openssh.com
1005: .It
1006: umac-128@openssh.com
1007: .It
1008: hmac-md5-etm@openssh.com
1009: .It
1010: hmac-md5-96-etm@openssh.com
1011: .It
1012: hmac-ripemd160-etm@openssh.com
1013: .It
1014: hmac-sha1-etm@openssh.com
1015: .It
1016: hmac-sha1-96-etm@openssh.com
1017: .It
1018: hmac-sha2-256-etm@openssh.com
1019: .It
1020: hmac-sha2-512-etm@openssh.com
1021: .It
1022: umac-64-etm@openssh.com
1023: .It
1024: umac-128-etm@openssh.com
1025: .El
1026: .Pp
1.52 jmc 1027: The default is:
1.77 jmc 1028: .Bd -literal -offset indent
1.152 markus 1029: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1030: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 1031: umac-64@openssh.com,umac-128@openssh.com,
1032: hmac-sha2-256,hmac-sha2-512
1.77 jmc 1033: .Ed
1.188 djm 1034: .Pp
1035: The list of available MAC algorithms may also be obtained using the
1036: .Fl Q
1037: option of
1038: .Xr ssh 1
1039: with an argument of
1040: .Dq mac .
1.60 dtucker 1041: .It Cm Match
1.61 jmc 1042: Introduces a conditional block.
1.65 dtucker 1043: If all of the criteria on the
1.60 dtucker 1044: .Cm Match
1.65 dtucker 1045: line are satisfied, the keywords on the following lines override those
1046: set in the global section of the config file, until either another
1.60 dtucker 1047: .Cm Match
1.65 dtucker 1048: line or the end of the file.
1.172 djm 1049: If a keyword appears in multiple
1050: .Cm Match
1.177 sobrado 1051: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1052: applied.
1.91 djm 1053: .Pp
1.61 jmc 1054: The arguments to
1.60 dtucker 1055: .Cm Match
1.163 dtucker 1056: are one or more criteria-pattern pairs or the single token
1057: .Cm All
1058: which matches all criteria.
1.60 dtucker 1059: The available criteria are
1060: .Cm User ,
1.69 dtucker 1061: .Cm Group ,
1.60 dtucker 1062: .Cm Host ,
1.139 dtucker 1063: .Cm LocalAddress ,
1064: .Cm LocalPort ,
1.60 dtucker 1065: and
1066: .Cm Address .
1.91 djm 1067: The match patterns may consist of single entries or comma-separated
1068: lists and may use the wildcard and negation operators described in the
1.161 jmc 1069: PATTERNS section of
1.92 djm 1070: .Xr ssh_config 5 .
1.91 djm 1071: .Pp
1072: The patterns in an
1073: .Cm Address
1074: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1075: address/masklen format, e.g.\&
1.91 djm 1076: .Dq 192.0.2.0/24
1077: or
1078: .Dq 3ffe:ffff::/32 .
1079: Note that the mask length provided must be consistent with the address -
1080: it is an error to specify a mask length that is too long for the address
1.93 jmc 1081: or one with bits set in this host portion of the address.
1082: For example,
1.91 djm 1083: .Dq 192.0.2.0/33
1084: and
1.93 jmc 1085: .Dq 192.0.2.0/8
1.91 djm 1086: respectively.
1087: .Pp
1.60 dtucker 1088: Only a subset of keywords may be used on the lines following a
1089: .Cm Match
1090: keyword.
1091: Available keywords are
1.142 jmc 1092: .Cm AcceptEnv ,
1.99 okan 1093: .Cm AllowAgentForwarding ,
1.142 jmc 1094: .Cm AllowGroups ,
1.193 djm 1095: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1096: .Cm AllowTcpForwarding ,
1.141 markus 1097: .Cm AllowUsers ,
1.149 djm 1098: .Cm AuthenticationMethods ,
1.146 djm 1099: .Cm AuthorizedKeysCommand ,
1100: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1101: .Cm AuthorizedKeysFile ,
1.214 djm 1102: .Cm AuthorizedPrincipalsCommand ,
1103: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1104: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1105: .Cm Banner ,
1.85 djm 1106: .Cm ChrootDirectory ,
1.141 markus 1107: .Cm DenyGroups ,
1108: .Cm DenyUsers ,
1.67 dtucker 1109: .Cm ForceCommand ,
1.194 djm 1110: .Cm GatewayPorts ,
1.193 djm 1111: .Cm GSSAPIAuthentication ,
1.189 djm 1112: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1113: .Cm HostbasedAuthentication ,
1.123 djm 1114: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1115: .Cm IPQoS ,
1.74 jmc 1116: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1117: .Cm KerberosAuthentication ,
1.95 dtucker 1118: .Cm MaxAuthTries ,
1.94 dtucker 1119: .Cm MaxSessions ,
1.72 dtucker 1120: .Cm PasswordAuthentication ,
1.97 djm 1121: .Cm PermitEmptyPasswords ,
1.66 dtucker 1122: .Cm PermitOpen ,
1.79 dtucker 1123: .Cm PermitRootLogin ,
1.164 djm 1124: .Cm PermitTTY ,
1.123 djm 1125: .Cm PermitTunnel ,
1.174 djm 1126: .Cm PermitUserRC ,
1.189 djm 1127: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1128: .Cm PubkeyAuthentication ,
1.159 dtucker 1129: .Cm RekeyLimit ,
1.193 djm 1130: .Cm RevokedKeys ,
1.142 jmc 1131: .Cm RhostsRSAAuthentication ,
1.194 djm 1132: .Cm RSAAuthentication ,
1.193 djm 1133: .Cm StreamLocalBindMask ,
1134: .Cm StreamLocalBindUnlink ,
1135: .Cm TrustedUserCAKeys ,
1.66 dtucker 1136: .Cm X11DisplayOffset ,
1.101 djm 1137: .Cm X11Forwarding
1.60 dtucker 1138: and
1.102 djm 1139: .Cm X11UseLocalHost .
1.33 dtucker 1140: .It Cm MaxAuthTries
1141: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1142: connection.
1143: Once the number of failures reaches half this value,
1144: additional failures are logged.
1145: The default is 6.
1.90 djm 1146: .It Cm MaxSessions
1147: Specifies the maximum number of open sessions permitted per network connection.
1148: The default is 10.
1.1 stevesk 1149: .It Cm MaxStartups
1150: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1151: SSH daemon.
1.1 stevesk 1152: Additional connections will be dropped until authentication succeeds or the
1153: .Cm LoginGraceTime
1154: expires for a connection.
1.156 dtucker 1155: The default is 10:30:100.
1.1 stevesk 1156: .Pp
1157: Alternatively, random early drop can be enabled by specifying
1158: the three colon separated values
1159: .Dq start:rate:full
1.51 jmc 1160: (e.g. "10:30:60").
1.53 jmc 1161: .Xr sshd 8
1.1 stevesk 1162: will refuse connection attempts with a probability of
1163: .Dq rate/100
1164: (30%)
1165: if there are currently
1166: .Dq start
1167: (10)
1168: unauthenticated connections.
1169: The probability increases linearly and all connection attempts
1170: are refused if the number of unauthenticated connections reaches
1171: .Dq full
1172: (60).
1173: .It Cm PasswordAuthentication
1174: Specifies whether password authentication is allowed.
1175: The default is
1176: .Dq yes .
1177: .It Cm PermitEmptyPasswords
1178: When password authentication is allowed, it specifies whether the
1179: server allows login to accounts with empty password strings.
1180: The default is
1181: .Dq no .
1.62 dtucker 1182: .It Cm PermitOpen
1183: Specifies the destinations to which TCP port forwarding is permitted.
1184: The forwarding specification must be one of the following forms:
1185: .Pp
1186: .Bl -item -offset indent -compact
1187: .It
1188: .Cm PermitOpen
1189: .Sm off
1190: .Ar host : port
1191: .Sm on
1192: .It
1193: .Cm PermitOpen
1194: .Sm off
1195: .Ar IPv4_addr : port
1196: .Sm on
1197: .It
1198: .Cm PermitOpen
1199: .Sm off
1200: .Ar \&[ IPv6_addr \&] : port
1201: .Sm on
1202: .El
1203: .Pp
1.68 dtucker 1204: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1205: An argument of
1206: .Dq any
1207: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1208: An argument of
1209: .Dq none
1210: can be used to prohibit all forwarding requests.
1.63 jmc 1211: By default all port forwarding requests are permitted.
1.1 stevesk 1212: .It Cm PermitRootLogin
1.38 jmc 1213: Specifies whether root can log in using
1.1 stevesk 1214: .Xr ssh 1 .
1215: The argument must be
1216: .Dq yes ,
1.210 deraadt 1217: .Dq prohibit-password ,
1.1 stevesk 1218: .Dq without-password ,
1.52 jmc 1219: .Dq forced-commands-only ,
1.1 stevesk 1220: or
1221: .Dq no .
1222: The default is
1.210 deraadt 1223: .Dq prohibit-password .
1.1 stevesk 1224: .Pp
1225: If this option is set to
1.210 deraadt 1226: .Dq prohibit-password
1227: or
1.52 jmc 1228: .Dq without-password ,
1.210 deraadt 1229: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1230: .Pp
1231: If this option is set to
1.52 jmc 1232: .Dq forced-commands-only ,
1.1 stevesk 1233: root login with public key authentication will be allowed,
1234: but only if the
1235: .Ar command
1236: option has been specified
1237: (which may be useful for taking remote backups even if root login is
1.17 jmc 1238: normally not allowed).
1239: All other authentication methods are disabled for root.
1.1 stevesk 1240: .Pp
1241: If this option is set to
1.52 jmc 1242: .Dq no ,
1.38 jmc 1243: root is not allowed to log in.
1.46 reyk 1244: .It Cm PermitTunnel
1245: Specifies whether
1246: .Xr tun 4
1247: device forwarding is allowed.
1.47 reyk 1248: The argument must be
1249: .Dq yes ,
1.58 stevesk 1250: .Dq point-to-point
1251: (layer 3),
1252: .Dq ethernet
1253: (layer 2), or
1.47 reyk 1254: .Dq no .
1.58 stevesk 1255: Specifying
1256: .Dq yes
1257: permits both
1258: .Dq point-to-point
1259: and
1260: .Dq ethernet .
1.46 reyk 1261: The default is
1262: .Dq no .
1.178 djm 1263: .Pp
1264: Independent of this setting, the permissions of the selected
1265: .Xr tun 4
1266: device must allow access to the user.
1.164 djm 1267: .It Cm PermitTTY
1268: Specifies whether
1.165 jmc 1269: .Xr pty 4
1.164 djm 1270: allocation is permitted.
1271: The default is
1272: .Dq yes .
1.6 markus 1273: .It Cm PermitUserEnvironment
1274: Specifies whether
1275: .Pa ~/.ssh/environment
1.9 stevesk 1276: and
1.6 markus 1277: .Cm environment=
1278: options in
1279: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1280: are processed by
1.52 jmc 1281: .Xr sshd 8 .
1.6 markus 1282: The default is
1283: .Dq no .
1.9 stevesk 1284: Enabling environment processing may enable users to bypass access
1285: restrictions in some configurations using mechanisms such as
1286: .Ev LD_PRELOAD .
1.174 djm 1287: .It Cm PermitUserRC
1288: Specifies whether any
1289: .Pa ~/.ssh/rc
1290: file is executed.
1291: The default is
1292: .Dq yes .
1.1 stevesk 1293: .It Cm PidFile
1.4 stevesk 1294: Specifies the file that contains the process ID of the
1.195 dtucker 1295: SSH daemon, or
1296: .Dq none
1297: to not write one.
1.1 stevesk 1298: The default is
1299: .Pa /var/run/sshd.pid .
1300: .It Cm Port
1301: Specifies the port number that
1.52 jmc 1302: .Xr sshd 8
1.1 stevesk 1303: listens on.
1304: The default is 22.
1305: Multiple options of this type are permitted.
1306: See also
1307: .Cm ListenAddress .
1308: .It Cm PrintLastLog
1309: Specifies whether
1.52 jmc 1310: .Xr sshd 8
1.36 jaredy 1311: should print the date and time of the last user login when a user logs
1312: in interactively.
1.1 stevesk 1313: The default is
1314: .Dq yes .
1315: .It Cm PrintMotd
1316: Specifies whether
1.52 jmc 1317: .Xr sshd 8
1.1 stevesk 1318: should print
1319: .Pa /etc/motd
1320: when a user logs in interactively.
1321: (On some systems it is also printed by the shell,
1322: .Pa /etc/profile ,
1323: or equivalent.)
1324: The default is
1325: .Dq yes .
1326: .It Cm Protocol
1327: Specifies the protocol versions
1.52 jmc 1328: .Xr sshd 8
1.5 stevesk 1329: supports.
1.1 stevesk 1330: The possible values are
1.52 jmc 1331: .Sq 1
1.1 stevesk 1332: and
1.52 jmc 1333: .Sq 2 .
1.1 stevesk 1334: Multiple versions must be comma-separated.
1335: The default is
1.109 jmc 1336: .Sq 2 .
1.5 stevesk 1337: Note that the order of the protocol list does not indicate preference,
1338: because the client selects among multiple protocol versions offered
1339: by the server.
1340: Specifying
1341: .Dq 2,1
1342: is identical to
1343: .Dq 1,2 .
1.189 djm 1344: .It Cm PubkeyAcceptedKeyTypes
1345: Specifies the key types that will be accepted for public key authentication
1346: as a comma-separated pattern list.
1.208 djm 1347: Alternately if the specified value begins with a
1348: .Sq +
1349: character, then the specified key types will be appended to the default set
1350: instead of replacing them.
1.206 markus 1351: The default for this option is:
1352: .Bd -literal -offset 3n
1353: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1354: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1355: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1356: ssh-ed25519-cert-v01@openssh.com,
1357: ssh-rsa-cert-v01@openssh.com,
1358: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1359: ssh-ed25519,ssh-rsa
1360: .Ed
1361: .Pp
1.189 djm 1362: The
1363: .Fl Q
1364: option of
1365: .Xr ssh 1
1366: may be used to list supported key types.
1.1 stevesk 1367: .It Cm PubkeyAuthentication
1368: Specifies whether public key authentication is allowed.
1369: The default is
1370: .Dq yes .
1371: Note that this option applies to protocol version 2 only.
1.159 dtucker 1372: .It Cm RekeyLimit
1373: Specifies the maximum amount of data that may be transmitted before the
1374: session key is renegotiated, optionally followed a maximum amount of
1375: time that may pass before the session key is renegotiated.
1376: The first argument is specified in bytes and may have a suffix of
1377: .Sq K ,
1378: .Sq M ,
1379: or
1380: .Sq G
1381: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1382: The default is between
1383: .Sq 1G
1384: and
1385: .Sq 4G ,
1386: depending on the cipher.
1387: The optional second value is specified in seconds and may use any of the
1388: units documented in the
1389: .Sx TIME FORMATS
1.160 jmc 1390: section.
1.159 dtucker 1391: The default value for
1392: .Cm RekeyLimit
1393: is
1394: .Dq default none ,
1395: which means that rekeying is performed after the cipher's default amount
1396: of data has been sent or received and no time based rekeying is done.
1397: This option applies to protocol version 2 only.
1.118 djm 1398: .It Cm RevokedKeys
1.195 dtucker 1399: Specifies revoked public keys file, or
1400: .Dq none
1401: to not use one.
1.118 djm 1402: Keys listed in this file will be refused for public key authentication.
1403: Note that if this file is not readable, then public key authentication will
1404: be refused for all users.
1.154 djm 1405: Keys may be specified as a text file, listing one public key per line, or as
1406: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1407: .Xr ssh-keygen 1 .
1.161 jmc 1408: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1409: .Xr ssh-keygen 1 .
1.1 stevesk 1410: .It Cm RhostsRSAAuthentication
1411: Specifies whether rhosts or /etc/hosts.equiv authentication together
1412: with successful RSA host authentication is allowed.
1413: The default is
1414: .Dq no .
1415: This option applies to protocol version 1 only.
1416: .It Cm RSAAuthentication
1417: Specifies whether pure RSA authentication is allowed.
1418: The default is
1419: .Dq yes .
1420: This option applies to protocol version 1 only.
1421: .It Cm ServerKeyBits
1422: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1423: The default and minimum value is 1024.
1.175 millert 1424: .It Cm StreamLocalBindMask
1425: Sets the octal file creation mode mask
1426: .Pq umask
1427: used when creating a Unix-domain socket file for local or remote
1428: port forwarding.
1429: This option is only used for port forwarding to a Unix-domain socket file.
1430: .Pp
1431: The default value is 0177, which creates a Unix-domain socket file that is
1432: readable and writable only by the owner.
1433: Note that not all operating systems honor the file mode on Unix-domain
1434: socket files.
1435: .It Cm StreamLocalBindUnlink
1436: Specifies whether to remove an existing Unix-domain socket file for local
1437: or remote port forwarding before creating a new one.
1438: If the socket file already exists and
1439: .Cm StreamLocalBindUnlink
1440: is not enabled,
1441: .Nm sshd
1442: will be unable to forward the port to the Unix-domain socket file.
1443: This option is only used for port forwarding to a Unix-domain socket file.
1444: .Pp
1445: The argument must be
1446: .Dq yes
1447: or
1448: .Dq no .
1449: The default is
1450: .Dq no .
1.1 stevesk 1451: .It Cm StrictModes
1452: Specifies whether
1.52 jmc 1453: .Xr sshd 8
1.1 stevesk 1454: should check file modes and ownership of the
1455: user's files and home directory before accepting login.
1456: This is normally desirable because novices sometimes accidentally leave their
1457: directory or files world-writable.
1458: The default is
1459: .Dq yes .
1.112 djm 1460: Note that this does not apply to
1461: .Cm ChrootDirectory ,
1462: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1463: .It Cm Subsystem
1.51 jmc 1464: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1465: Arguments should be a subsystem name and a command (with optional arguments)
1466: to execute upon subsystem request.
1.80 djm 1467: .Pp
1.1 stevesk 1468: The command
1469: .Xr sftp-server 8
1470: implements the
1471: .Dq sftp
1472: file transfer subsystem.
1.80 djm 1473: .Pp
1474: Alternately the name
1475: .Dq internal-sftp
1476: implements an in-process
1477: .Dq sftp
1478: server.
1479: This may simplify configurations using
1480: .Cm ChrootDirectory
1481: to force a different filesystem root on clients.
1482: .Pp
1.1 stevesk 1483: By default no subsystems are defined.
1484: Note that this option applies to protocol version 2 only.
1485: .It Cm SyslogFacility
1486: Gives the facility code that is used when logging messages from
1.53 jmc 1487: .Xr sshd 8 .
1.1 stevesk 1488: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1489: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1490: The default is AUTH.
1.27 markus 1491: .It Cm TCPKeepAlive
1492: Specifies whether the system should send TCP keepalive messages to the
1493: other side.
1494: If they are sent, death of the connection or crash of one
1495: of the machines will be properly noticed.
1496: However, this means that
1497: connections will die if the route is down temporarily, and some people
1498: find it annoying.
1499: On the other hand, if TCP keepalives are not sent,
1500: sessions may hang indefinitely on the server, leaving
1501: .Dq ghost
1502: users and consuming server resources.
1503: .Pp
1504: The default is
1505: .Dq yes
1506: (to send TCP keepalive messages), and the server will notice
1507: if the network goes down or the client host crashes.
1508: This avoids infinitely hanging sessions.
1509: .Pp
1510: To disable TCP keepalive messages, the value should be set to
1511: .Dq no .
1.118 djm 1512: .It Cm TrustedUserCAKeys
1513: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1514: trusted to sign user certificates for authentication, or
1515: .Dq none
1516: to not use one.
1.119 jmc 1517: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1518: .Ql #
1519: are allowed.
1520: If a certificate is presented for authentication and has its signing CA key
1521: listed in this file, then it may be used for authentication for any user
1522: listed in the certificate's principals list.
1523: Note that certificates that lack a list of principals will not be permitted
1524: for authentication using
1525: .Cm TrustedUserCAKeys .
1.161 jmc 1526: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1527: .Xr ssh-keygen 1 .
1.18 markus 1528: .It Cm UseDNS
1529: Specifies whether
1.52 jmc 1530: .Xr sshd 8
1.207 djm 1531: should look up the remote host name, and to check that
1.18 markus 1532: the resolved host name for the remote IP address maps back to the
1533: very same IP address.
1.207 djm 1534: .Pp
1535: If this option is set to
1536: .Dq no
1537: (the default) then only addresses and not host names may be used in
1538: .Pa ~/.ssh/known_hosts
1539: .Cm from
1540: and
1.211 jmc 1541: .Nm
1.207 djm 1542: .Cm Match
1543: .Cm Host
1544: directives.
1.1 stevesk 1545: .It Cm UseLogin
1546: Specifies whether
1547: .Xr login 1
1548: is used for interactive login sessions.
1549: The default is
1550: .Dq no .
1551: Note that
1552: .Xr login 1
1553: is never used for remote command execution.
1554: Note also, that if this is enabled,
1555: .Cm X11Forwarding
1556: will be disabled because
1557: .Xr login 1
1558: does not know how to handle
1559: .Xr xauth 1
1.15 jmc 1560: cookies.
1561: If
1.1 stevesk 1562: .Cm UsePrivilegeSeparation
1563: is specified, it will be disabled after authentication.
1564: .It Cm UsePrivilegeSeparation
1565: Specifies whether
1.52 jmc 1566: .Xr sshd 8
1.2 stevesk 1567: separates privileges by creating an unprivileged child process
1.15 jmc 1568: to deal with incoming network traffic.
1569: After successful authentication, another process will be created that has
1570: the privilege of the authenticated user.
1571: The goal of privilege separation is to prevent privilege
1.1 stevesk 1572: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1573: The argument must be
1574: .Dq yes ,
1575: .Dq no ,
1576: or
1577: .Dq sandbox .
1.134 djm 1578: If
1579: .Cm UsePrivilegeSeparation
1580: is set to
1581: .Dq sandbox
1582: then the pre-authentication unprivileged process is subject to additional
1583: restrictions.
1.213 sobrado 1584: The default is
1585: .Dq sandbox .
1.137 djm 1586: .It Cm VersionAddendum
1587: Optionally specifies additional text to append to the SSH protocol banner
1588: sent by the server upon connection.
1589: The default is
1590: .Dq none .
1.1 stevesk 1591: .It Cm X11DisplayOffset
1592: Specifies the first display number available for
1.52 jmc 1593: .Xr sshd 8 Ns 's
1.1 stevesk 1594: X11 forwarding.
1.52 jmc 1595: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1596: The default is 10.
1597: .It Cm X11Forwarding
1598: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1599: The argument must be
1600: .Dq yes
1601: or
1602: .Dq no .
1.1 stevesk 1603: The default is
1604: .Dq no .
1.13 stevesk 1605: .Pp
1606: When X11 forwarding is enabled, there may be additional exposure to
1607: the server and to client displays if the
1.52 jmc 1608: .Xr sshd 8
1.13 stevesk 1609: proxy display is configured to listen on the wildcard address (see
1610: .Cm X11UseLocalhost
1.52 jmc 1611: below), though this is not the default.
1.13 stevesk 1612: Additionally, the authentication spoofing and authentication data
1613: verification and substitution occur on the client side.
1614: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1615: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1616: forwarding (see the warnings for
1617: .Cm ForwardX11
1618: in
1.19 jmc 1619: .Xr ssh_config 5 ) .
1.13 stevesk 1620: A system administrator may have a stance in which they want to
1621: protect clients that may expose themselves to attack by unwittingly
1622: requesting X11 forwarding, which can warrant a
1623: .Dq no
1624: setting.
1625: .Pp
1626: Note that disabling X11 forwarding does not prevent users from
1627: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1628: X11 forwarding is automatically disabled if
1629: .Cm UseLogin
1630: is enabled.
1631: .It Cm X11UseLocalhost
1632: Specifies whether
1.52 jmc 1633: .Xr sshd 8
1.1 stevesk 1634: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1635: the wildcard address.
1636: By default,
1.52 jmc 1637: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1638: hostname part of the
1639: .Ev DISPLAY
1640: environment variable to
1641: .Dq localhost .
1.8 stevesk 1642: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1643: However, some older X11 clients may not function with this
1644: configuration.
1645: .Cm X11UseLocalhost
1646: may be set to
1647: .Dq no
1648: to specify that the forwarding server should be bound to the wildcard
1649: address.
1650: The argument must be
1651: .Dq yes
1652: or
1653: .Dq no .
1654: The default is
1655: .Dq yes .
1656: .It Cm XAuthLocation
1.11 stevesk 1657: Specifies the full pathname of the
1.1 stevesk 1658: .Xr xauth 1
1.195 dtucker 1659: program, or
1660: .Dq none
1661: to not use one.
1.1 stevesk 1662: The default is
1663: .Pa /usr/X11R6/bin/xauth .
1664: .El
1.55 jmc 1665: .Sh TIME FORMATS
1.53 jmc 1666: .Xr sshd 8
1.1 stevesk 1667: command-line arguments and configuration file options that specify time
1668: may be expressed using a sequence of the form:
1669: .Sm off
1.7 stevesk 1670: .Ar time Op Ar qualifier ,
1.1 stevesk 1671: .Sm on
1672: where
1673: .Ar time
1674: is a positive integer value and
1675: .Ar qualifier
1676: is one of the following:
1677: .Pp
1678: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1679: .It Aq Cm none
1.1 stevesk 1680: seconds
1681: .It Cm s | Cm S
1682: seconds
1683: .It Cm m | Cm M
1684: minutes
1685: .It Cm h | Cm H
1686: hours
1687: .It Cm d | Cm D
1688: days
1689: .It Cm w | Cm W
1690: weeks
1691: .El
1692: .Pp
1693: Each member of the sequence is added together to calculate
1694: the total time value.
1695: .Pp
1696: Time format examples:
1697: .Pp
1698: .Bl -tag -width Ds -compact -offset indent
1699: .It 600
1700: 600 seconds (10 minutes)
1701: .It 10m
1702: 10 minutes
1703: .It 1h30m
1704: 1 hour 30 minutes (90 minutes)
1705: .El
1706: .Sh FILES
1707: .Bl -tag -width Ds
1708: .It Pa /etc/ssh/sshd_config
1709: Contains configuration data for
1.53 jmc 1710: .Xr sshd 8 .
1.1 stevesk 1711: This file should be writable by root only, but it is recommended
1712: (though not necessary) that it be world-readable.
1713: .El
1.19 jmc 1714: .Sh SEE ALSO
1715: .Xr sshd 8
1.1 stevesk 1716: .Sh AUTHORS
1717: OpenSSH is a derivative of the original and free
1718: ssh 1.2.12 release by Tatu Ylonen.
1719: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1720: Theo de Raadt and Dug Song
1721: removed many bugs, re-added newer features and
1722: created OpenSSH.
1723: Markus Friedl contributed the support for SSH
1724: protocol versions 1.5 and 2.0.
1725: Niels Provos and Markus Friedl contributed support
1726: for privilege separation.
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