Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.222
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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33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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1.222 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.221 2016/03/17 17:19:43 djm Exp $
! 37: .Dd $Mdocdate: March 17 2016 $
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.
1.219 jmc 73: The
1.197 dtucker 74: .Ev TERM
1.199 jmc 75: environment variable is always sent whenever the client
1.204 djm 76: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 77: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 78: .Ql *
1.30 djm 79: and
80: .Ql \&? .
1.31 djm 81: Multiple environment variables may be separated by whitespace or spread
1.30 djm 82: across multiple
83: .Cm AcceptEnv
84: directives.
1.31 djm 85: Be warned that some environment variables could be used to bypass restricted
1.30 djm 86: user environments.
87: For this reason, care should be taken in the use of this directive.
88: The default is not to accept any environment variables.
1.37 djm 89: .It Cm AddressFamily
90: Specifies which address family should be used by
1.53 jmc 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
93: .Dq any ,
94: .Dq inet
1.52 jmc 95: (use IPv4 only), or
1.37 djm 96: .Dq inet6
97: (use IPv6 only).
98: The default is
99: .Dq any .
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
105: .Dq yes .
106: Note that disabling agent forwarding does not improve security
107: unless users are also denied shell access, as they can always install
108: their own forwarders.
1.1 stevesk 109: .It Cm AllowGroups
110: This keyword can be followed by a list of group name patterns, separated
111: by spaces.
112: If specified, login is allowed only for users whose primary
113: group or supplementary group list matches one of the patterns.
114: Only group names are valid; a numerical group ID is not recognized.
115: By default, login is allowed for all groups.
1.54 jmc 116: The allow/deny directives are processed in the following order:
117: .Cm DenyUsers ,
118: .Cm AllowUsers ,
119: .Cm DenyGroups ,
120: and finally
121: .Cm AllowGroups .
1.49 jmc 122: .Pp
1.161 jmc 123: See PATTERNS in
1.49 jmc 124: .Xr ssh_config 5
125: for more information on patterns.
1.1 stevesk 126: .It Cm AllowTcpForwarding
127: Specifies whether TCP forwarding is permitted.
1.150 djm 128: The available options are
129: .Dq yes
130: or
131: .Dq all
132: to allow TCP forwarding,
133: .Dq no
134: to prevent all TCP forwarding,
135: .Dq local
136: to allow local (from the perspective of
1.151 jmc 137: .Xr ssh 1 )
138: forwarding only or
1.150 djm 139: .Dq remote
140: to allow remote forwarding only.
1.1 stevesk 141: The default is
142: .Dq yes .
143: Note that disabling TCP forwarding does not improve security unless
144: users are also denied shell access, as they can always install their
145: own forwarders.
1.175 millert 146: .It Cm AllowStreamLocalForwarding
147: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
148: The available options are
149: .Dq yes
150: or
151: .Dq all
152: to allow StreamLocal forwarding,
153: .Dq no
154: to prevent all StreamLocal forwarding,
155: .Dq local
156: to allow local (from the perspective of
157: .Xr ssh 1 )
158: forwarding only or
159: .Dq remote
160: to allow remote forwarding only.
161: The default is
162: .Dq yes .
163: Note that disabling StreamLocal forwarding does not improve security unless
164: users are also denied shell access, as they can always install their
165: own forwarders.
1.1 stevesk 166: .It Cm AllowUsers
167: This keyword can be followed by a list of user name patterns, separated
168: by spaces.
1.14 jmc 169: If specified, login is allowed only for user names that
1.1 stevesk 170: match one of the patterns.
171: Only user names are valid; a numerical user ID is not recognized.
172: By default, login is allowed for all users.
173: If the pattern takes the form USER@HOST then USER and HOST
174: are separately checked, restricting logins to particular
175: users from particular hosts.
1.222 ! jmc 176: HOST criteria may additionally contain addresses to match in CIDR
! 177: address/masklen format.
1.54 jmc 178: The allow/deny directives are processed in the following order:
179: .Cm DenyUsers ,
180: .Cm AllowUsers ,
181: .Cm DenyGroups ,
182: and finally
183: .Cm AllowGroups .
1.49 jmc 184: .Pp
1.161 jmc 185: See PATTERNS in
1.49 jmc 186: .Xr ssh_config 5
187: for more information on patterns.
1.149 djm 188: .It Cm AuthenticationMethods
189: Specifies the authentication methods that must be successfully completed
190: for a user to be granted access.
191: This option must be followed by one or more comma-separated lists of
192: authentication method names.
193: Successful authentication requires completion of every method in at least
194: one of these lists.
195: .Pp
196: For example, an argument of
197: .Dq publickey,password publickey,keyboard-interactive
198: would require the user to complete public key authentication, followed by
199: either password or keyboard interactive authentication.
200: Only methods that are next in one or more lists are offered at each stage,
201: so for this example, it would not be possible to attempt password or
202: keyboard-interactive authentication before public key.
1.157 markus 203: .Pp
204: For keyboard interactive authentication it is also possible to
205: restrict authentication to a specific device by appending a
206: colon followed by the device identifier
207: .Dq bsdauth ,
208: .Dq pam ,
209: or
210: .Dq skey ,
211: depending on the server configuration.
212: For example,
213: .Dq keyboard-interactive:bsdauth
214: would restrict keyboard interactive authentication to the
215: .Dq bsdauth
216: device.
1.185 djm 217: .Pp
218: If the
219: .Dq publickey
1.187 jmc 220: method is listed more than once,
1.185 djm 221: .Xr sshd 8
222: verifies that keys that have been used successfully are not reused for
223: subsequent authentications.
224: For example, an
225: .Cm AuthenticationMethods
226: of
227: .Dq publickey,publickey
228: will require successful authentication using two different public keys.
1.181 djm 229: .Pp
1.219 jmc 230: This option will yield a fatal
1.149 djm 231: error if enabled if protocol 1 is also enabled.
232: Note that each authentication method listed should also be explicitly enabled
233: in the configuration.
234: The default is not to require multiple authentication; successful completion
235: of a single authentication method is sufficient.
1.146 djm 236: .It Cm AuthorizedKeysCommand
1.147 jmc 237: Specifies a program to be used to look up the user's public keys.
1.201 djm 238: The program must be owned by root, not writable by group or others and
239: specified by an absolute path.
240: .Pp
241: Arguments to
242: .Cm AuthorizedKeysCommand
243: may be provided using the following tokens, which will be expanded
244: at runtime: %% is replaced by a literal '%', %u is replaced by the
245: username being authenticated, %h is replaced by the home directory
246: of the user being authenticated, %t is replaced with the key type
247: offered for authentication, %f is replaced with the fingerprint of
248: the key, and %k is replaced with the key being offered for authentication.
249: If no arguments are specified then the username of the target user
250: will be supplied.
251: .Pp
252: The program should produce on standard output zero or
1.161 jmc 253: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 254: .Xr sshd 8 ) .
1.146 djm 255: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
256: and authorize the user then public key authentication continues using the usual
257: .Cm AuthorizedKeysFile
258: files.
259: By default, no AuthorizedKeysCommand is run.
260: .It Cm AuthorizedKeysCommandUser
261: Specifies the user under whose account the AuthorizedKeysCommand is run.
262: It is recommended to use a dedicated user that has no other role on the host
263: than running authorized keys commands.
1.186 djm 264: If
1.181 djm 265: .Cm AuthorizedKeysCommand
1.186 djm 266: is specified but
267: .Cm AuthorizedKeysCommandUser
268: is not, then
269: .Xr sshd 8
270: will refuse to start.
1.1 stevesk 271: .It Cm AuthorizedKeysFile
272: Specifies the file that contains the public keys that can be used
273: for user authentication.
1.124 djm 274: The format is described in the
1.161 jmc 275: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 276: section of
277: .Xr sshd 8 .
1.1 stevesk 278: .Cm AuthorizedKeysFile
279: may contain tokens of the form %T which are substituted during connection
1.52 jmc 280: setup.
1.17 jmc 281: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 282: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 283: %u is replaced by the username of that user.
284: After expansion,
285: .Cm AuthorizedKeysFile
286: is taken to be an absolute path or one relative to the user's home
287: directory.
1.133 jmc 288: Multiple files may be listed, separated by whitespace.
1.212 djm 289: Alternately this option may be set to
290: .Dq none
291: to skip checking for user keys in files.
1.133 jmc 292: The default is
293: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 294: .It Cm AuthorizedPrincipalsCommand
295: Specifies a program to be used to generate the list of allowed
296: certificate principals as per
297: .Cm AuthorizedPrincipalsFile .
298: The program must be owned by root, not writable by group or others and
299: specified by an absolute path.
300: .Pp
301: Arguments to
302: .Cm AuthorizedPrincipalsCommand
303: may be provided using the following tokens, which will be expanded
304: at runtime: %% is replaced by a literal '%', %u is replaced by the
305: username being authenticated and %h is replaced by the home directory
306: of the user being authenticated.
307: .Pp
308: The program should produce on standard output zero or
309: more lines of
310: .Cm AuthorizedPrincipalsFile
311: output.
312: If either
313: .Cm AuthorizedPrincipalsCommand
314: or
315: .Cm AuthorizedPrincipalsFile
316: is specified, then certificates offered by the client for authentication
317: must contain a principal that is listed.
318: By default, no AuthorizedPrincipalsCommand is run.
319: .It Cm AuthorizedPrincipalsCommandUser
320: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
321: It is recommended to use a dedicated user that has no other role on the host
322: than running authorized principals commands.
323: If
324: .Cm AuthorizedPrincipalsCommand
325: is specified but
326: .Cm AuthorizedPrincipalsCommandUser
327: is not, then
328: .Xr sshd 8
329: will refuse to start.
1.121 djm 330: .It Cm AuthorizedPrincipalsFile
331: Specifies a file that lists principal names that are accepted for
332: certificate authentication.
333: When using certificates signed by a key listed in
334: .Cm TrustedUserCAKeys ,
335: this file lists names, one of which must appear in the certificate for it
336: to be accepted for authentication.
1.125 jmc 337: Names are listed one per line preceded by key options (as described
1.161 jmc 338: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 339: .Xr sshd 8 ) .
1.124 djm 340: Empty lines and comments starting with
1.121 djm 341: .Ql #
342: are ignored.
343: .Pp
344: .Cm AuthorizedPrincipalsFile
345: may contain tokens of the form %T which are substituted during connection
346: setup.
347: The following tokens are defined: %% is replaced by a literal '%',
348: %h is replaced by the home directory of the user being authenticated, and
349: %u is replaced by the username of that user.
350: After expansion,
351: .Cm AuthorizedPrincipalsFile
352: is taken to be an absolute path or one relative to the user's home
353: directory.
354: .Pp
1.138 djm 355: The default is
356: .Dq none ,
357: i.e. not to use a principals file \(en in this case, the username
1.121 djm 358: of the user must appear in a certificate's principals list for it to be
359: accepted.
360: Note that
361: .Cm AuthorizedPrincipalsFile
362: is only used when authentication proceeds using a CA listed in
363: .Cm TrustedUserCAKeys
364: and is not consulted for certification authorities trusted via
365: .Pa ~/.ssh/authorized_keys ,
366: though the
367: .Cm principals=
368: key option offers a similar facility (see
369: .Xr sshd 8
370: for details).
1.1 stevesk 371: .It Cm Banner
372: The contents of the specified file are sent to the remote user before
373: authentication is allowed.
1.78 djm 374: If the argument is
375: .Dq none
376: then no banner is displayed.
1.1 stevesk 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
1.219 jmc 441: Specifies the ciphers allowed.
1.1 stevesk 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.
1.42 djm 522: .It Cm ClientAliveInterval
523: Sets a timeout interval in seconds after which if no data has been received
524: from the client,
1.52 jmc 525: .Xr sshd 8
1.42 djm 526: will send a message through the encrypted
527: channel to request a response from the client.
528: The default
529: is 0, indicating that these messages will not be sent to the client.
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.
1.222 ! jmc 566: HOST criteria may additionally contain addresses to match in CIDR
! 567: address/masklen format.
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: .It Cm GSSAPICleanupCredentials
632: Specifies whether to automatically destroy the user's credentials cache
633: on logout.
634: The default is
635: .Dq yes .
1.203 djm 636: .It Cm GSSAPIStrictAcceptorCheck
637: Determines whether to be strict about the identity of the GSSAPI acceptor
638: a client authenticates against.
639: If set to
640: .Dq yes
641: then the client must authenticate against the
642: .Pa host
643: service on the current hostname.
644: If set to
645: .Dq no
646: then the client may authenticate against any service key stored in the
647: machine's default store.
648: This facility is provided to assist with operation on multi homed machines.
649: The default is
650: .Dq yes .
1.189 djm 651: .It Cm HostbasedAcceptedKeyTypes
652: Specifies the key types that will be accepted for hostbased authentication
653: as a comma-separated pattern list.
1.208 djm 654: Alternately if the specified value begins with a
655: .Sq +
656: character, then the specified key types will be appended to the default set
657: instead of replacing them.
1.206 markus 658: The default for this option is:
659: .Bd -literal -offset 3n
660: ecdsa-sha2-nistp256-cert-v01@openssh.com,
661: ecdsa-sha2-nistp384-cert-v01@openssh.com,
662: ecdsa-sha2-nistp521-cert-v01@openssh.com,
663: ssh-ed25519-cert-v01@openssh.com,
664: ssh-rsa-cert-v01@openssh.com,
665: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 666: ssh-ed25519,ssh-rsa
1.206 markus 667: .Ed
668: .Pp
1.189 djm 669: The
670: .Fl Q
671: option of
672: .Xr ssh 1
673: may be used to list supported key types.
1.1 stevesk 674: .It Cm HostbasedAuthentication
675: Specifies whether rhosts or /etc/hosts.equiv authentication together
676: with successful public key client host authentication is allowed
1.50 jmc 677: (host-based authentication).
1.70 dtucker 678: The default is
679: .Dq no .
680: .It Cm HostbasedUsesNameFromPacketOnly
681: Specifies whether or not the server will attempt to perform a reverse
682: name lookup when matching the name in the
683: .Pa ~/.shosts ,
684: .Pa ~/.rhosts ,
685: and
686: .Pa /etc/hosts.equiv
687: files during
688: .Cm HostbasedAuthentication .
689: A setting of
690: .Dq yes
691: means that
692: .Xr sshd 8
693: uses the name supplied by the client rather than
694: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 695: The default is
696: .Dq no .
1.117 djm 697: .It Cm HostCertificate
698: Specifies a file containing a public host certificate.
699: The certificate's public key must match a private host key already specified
700: by
701: .Cm HostKey .
702: The default behaviour of
703: .Xr sshd 8
704: is not to load any certificates.
1.1 stevesk 705: .It Cm HostKey
706: Specifies a file containing a private host key
707: used by SSH.
708: The default is
709: .Pa /etc/ssh/ssh_host_key
710: for protocol version 1, and
1.126 djm 711: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 712: .Pa /etc/ssh/ssh_host_ecdsa_key ,
713: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 714: and
1.1 stevesk 715: .Pa /etc/ssh/ssh_host_rsa_key
716: for protocol version 2.
1.206 markus 717: .Pp
1.1 stevesk 718: Note that
1.52 jmc 719: .Xr sshd 8
1.206 markus 720: will refuse to use a file if it is group/world-accessible
721: and that the
722: .Cm HostKeyAlgorithms
723: option restricts which of the keys are actually used by
724: .Xr sshd 8 .
725: .Pp
1.1 stevesk 726: It is possible to have multiple host key files.
727: .Dq rsa1
728: keys are used for version 1 and
1.126 djm 729: .Dq dsa ,
1.169 naddy 730: .Dq ecdsa ,
731: .Dq ed25519
1.1 stevesk 732: or
733: .Dq rsa
734: are used for version 2 of the SSH protocol.
1.162 markus 735: It is also possible to specify public host key files instead.
736: In this case operations on the private key will be delegated
737: to an
738: .Xr ssh-agent 1 .
739: .It Cm HostKeyAgent
740: Identifies the UNIX-domain socket used to communicate
741: with an agent that has access to the private host keys.
742: If
743: .Dq SSH_AUTH_SOCK
744: is specified, the location of the socket will be read from the
745: .Ev SSH_AUTH_SOCK
746: environment variable.
1.206 markus 747: .It Cm HostKeyAlgorithms
1.219 jmc 748: Specifies the host key algorithms
1.206 markus 749: that the server offers.
750: The default for this option is:
751: .Bd -literal -offset 3n
752: ecdsa-sha2-nistp256-cert-v01@openssh.com,
753: ecdsa-sha2-nistp384-cert-v01@openssh.com,
754: ecdsa-sha2-nistp521-cert-v01@openssh.com,
755: ssh-ed25519-cert-v01@openssh.com,
756: ssh-rsa-cert-v01@openssh.com,
757: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 758: ssh-ed25519,ssh-rsa
1.206 markus 759: .Ed
760: .Pp
761: The list of available key types may also be obtained using the
762: .Fl Q
763: option of
764: .Xr ssh 1
765: with an argument of
766: .Dq key .
1.1 stevesk 767: .It Cm IgnoreRhosts
768: Specifies that
769: .Pa .rhosts
770: and
771: .Pa .shosts
772: files will not be used in
773: .Cm RhostsRSAAuthentication
774: or
775: .Cm HostbasedAuthentication .
776: .Pp
777: .Pa /etc/hosts.equiv
778: and
779: .Pa /etc/shosts.equiv
780: are still used.
781: The default is
782: .Dq yes .
783: .It Cm IgnoreUserKnownHosts
784: Specifies whether
1.52 jmc 785: .Xr sshd 8
1.1 stevesk 786: should ignore the user's
1.41 djm 787: .Pa ~/.ssh/known_hosts
1.1 stevesk 788: during
789: .Cm RhostsRSAAuthentication
790: or
791: .Cm HostbasedAuthentication .
792: The default is
793: .Dq no .
1.129 djm 794: .It Cm IPQoS
795: Specifies the IPv4 type-of-service or DSCP class for the connection.
796: Accepted values are
797: .Dq af11 ,
798: .Dq af12 ,
799: .Dq af13 ,
1.136 djm 800: .Dq af21 ,
1.129 djm 801: .Dq af22 ,
802: .Dq af23 ,
803: .Dq af31 ,
804: .Dq af32 ,
805: .Dq af33 ,
806: .Dq af41 ,
807: .Dq af42 ,
808: .Dq af43 ,
809: .Dq cs0 ,
810: .Dq cs1 ,
811: .Dq cs2 ,
812: .Dq cs3 ,
813: .Dq cs4 ,
814: .Dq cs5 ,
815: .Dq cs6 ,
816: .Dq cs7 ,
817: .Dq ef ,
818: .Dq lowdelay ,
819: .Dq throughput ,
820: .Dq reliability ,
821: or a numeric value.
1.131 djm 822: This option may take one or two arguments, separated by whitespace.
1.129 djm 823: If one argument is specified, it is used as the packet class unconditionally.
824: If two values are specified, the first is automatically selected for
825: interactive sessions and the second for non-interactive sessions.
826: The default is
827: .Dq lowdelay
828: for interactive sessions and
829: .Dq throughput
830: for non-interactive sessions.
1.171 jmc 831: .It Cm KbdInteractiveAuthentication
832: Specifies whether to allow keyboard-interactive authentication.
833: The argument to this keyword must be
834: .Dq yes
835: or
836: .Dq no .
837: The default is to use whatever value
838: .Cm ChallengeResponseAuthentication
839: is set to
840: (by default
841: .Dq yes ) .
1.1 stevesk 842: .It Cm KerberosAuthentication
1.24 markus 843: Specifies whether the password provided by the user for
1.1 stevesk 844: .Cm PasswordAuthentication
1.24 markus 845: will be validated through the Kerberos KDC.
1.1 stevesk 846: To use this option, the server needs a
847: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 848: The default is
1.29 dtucker 849: .Dq no .
850: .It Cm KerberosGetAFSToken
1.45 djm 851: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 852: an AFS token before accessing the user's home directory.
1.52 jmc 853: The default is
1.1 stevesk 854: .Dq no .
855: .It Cm KerberosOrLocalPasswd
1.52 jmc 856: If password authentication through Kerberos fails then
1.1 stevesk 857: the password will be validated via any additional local mechanism
858: such as
859: .Pa /etc/passwd .
1.52 jmc 860: The default is
1.1 stevesk 861: .Dq yes .
862: .It Cm KerberosTicketCleanup
863: Specifies whether to automatically destroy the user's ticket cache
864: file on logout.
1.52 jmc 865: The default is
1.1 stevesk 866: .Dq yes .
1.127 djm 867: .It Cm KexAlgorithms
868: Specifies the available KEX (Key Exchange) algorithms.
869: Multiple algorithms must be comma-separated.
1.208 djm 870: Alternately if the specified value begins with a
871: .Sq +
872: character, then the specified methods will be appended to the default set
873: instead of replacing them.
1.173 naddy 874: The supported algorithms are:
875: .Pp
876: .Bl -item -compact -offset indent
877: .It
878: curve25519-sha256@libssh.org
879: .It
880: diffie-hellman-group1-sha1
881: .It
882: diffie-hellman-group14-sha1
883: .It
884: diffie-hellman-group-exchange-sha1
885: .It
886: diffie-hellman-group-exchange-sha256
887: .It
888: ecdh-sha2-nistp256
889: .It
890: ecdh-sha2-nistp384
891: .It
892: ecdh-sha2-nistp521
893: .El
894: .Pp
895: The default is:
1.170 dtucker 896: .Bd -literal -offset indent
897: curve25519-sha256@libssh.org,
898: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
899: diffie-hellman-group-exchange-sha256,
1.173 naddy 900: diffie-hellman-group14-sha1
1.170 dtucker 901: .Ed
1.188 djm 902: .Pp
903: The list of available key exchange algorithms may also be obtained using the
904: .Fl Q
905: option of
906: .Xr ssh 1
907: with an argument of
908: .Dq kex .
1.1 stevesk 909: .It Cm KeyRegenerationInterval
910: In protocol version 1, the ephemeral server key is automatically regenerated
911: after this many seconds (if it has been used).
912: The purpose of regeneration is to prevent
913: decrypting captured sessions by later breaking into the machine and
914: stealing the keys.
915: The key is never stored anywhere.
916: If the value is 0, the key is never regenerated.
917: The default is 3600 (seconds).
918: .It Cm ListenAddress
919: Specifies the local addresses
1.52 jmc 920: .Xr sshd 8
1.1 stevesk 921: should listen on.
922: The following forms may be used:
923: .Pp
924: .Bl -item -offset indent -compact
925: .It
926: .Cm ListenAddress
927: .Sm off
1.180 jmc 928: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 929: .Sm on
930: .It
931: .Cm ListenAddress
932: .Sm off
1.180 jmc 933: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 934: .Sm on
935: .It
936: .Cm ListenAddress
937: .Sm off
938: .Oo
1.180 jmc 939: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 940: .Sm on
941: .El
942: .Pp
943: If
944: .Ar port
945: is not specified,
1.200 dtucker 946: sshd will listen on the address and all
1.1 stevesk 947: .Cm Port
1.17 jmc 948: options specified.
949: The default is to listen on all local addresses.
1.15 jmc 950: Multiple
1.1 stevesk 951: .Cm ListenAddress
1.17 jmc 952: options are permitted.
1.1 stevesk 953: .It Cm LoginGraceTime
954: The server disconnects after this time if the user has not
955: successfully logged in.
956: If the value is 0, there is no time limit.
1.12 stevesk 957: The default is 120 seconds.
1.1 stevesk 958: .It Cm LogLevel
959: Gives the verbosity level that is used when logging messages from
1.53 jmc 960: .Xr sshd 8 .
1.1 stevesk 961: The possible values are:
1.52 jmc 962: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 963: The default is INFO.
964: DEBUG and DEBUG1 are equivalent.
965: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
966: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 967: .It Cm MACs
968: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 969: The MAC algorithm is used for data integrity protection.
1.1 stevesk 970: Multiple algorithms must be comma-separated.
1.208 djm 971: If the specified value begins with a
972: .Sq +
973: character, then the specified algorithms will be appended to the default set
974: instead of replacing them.
975: .Pp
1.152 markus 976: The algorithms that contain
977: .Dq -etm
978: calculate the MAC after encryption (encrypt-then-mac).
979: These are considered safer and their use recommended.
1.173 naddy 980: The supported MACs are:
981: .Pp
982: .Bl -item -compact -offset indent
983: .It
984: hmac-md5
985: .It
986: hmac-md5-96
987: .It
988: hmac-ripemd160
989: .It
990: hmac-sha1
991: .It
992: hmac-sha1-96
993: .It
994: hmac-sha2-256
995: .It
996: hmac-sha2-512
997: .It
998: umac-64@openssh.com
999: .It
1000: umac-128@openssh.com
1001: .It
1002: hmac-md5-etm@openssh.com
1003: .It
1004: hmac-md5-96-etm@openssh.com
1005: .It
1006: hmac-ripemd160-etm@openssh.com
1007: .It
1008: hmac-sha1-etm@openssh.com
1009: .It
1010: hmac-sha1-96-etm@openssh.com
1011: .It
1012: hmac-sha2-256-etm@openssh.com
1013: .It
1014: hmac-sha2-512-etm@openssh.com
1015: .It
1016: umac-64-etm@openssh.com
1017: .It
1018: umac-128-etm@openssh.com
1019: .El
1020: .Pp
1.52 jmc 1021: The default is:
1.77 jmc 1022: .Bd -literal -offset indent
1.152 markus 1023: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1024: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1025: hmac-sha1-etm@openssh.com,
1.173 naddy 1026: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1027: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1028: .Ed
1.188 djm 1029: .Pp
1030: The list of available MAC algorithms may also be obtained using the
1031: .Fl Q
1032: option of
1033: .Xr ssh 1
1034: with an argument of
1035: .Dq mac .
1.60 dtucker 1036: .It Cm Match
1.61 jmc 1037: Introduces a conditional block.
1.65 dtucker 1038: If all of the criteria on the
1.60 dtucker 1039: .Cm Match
1.65 dtucker 1040: line are satisfied, the keywords on the following lines override those
1041: set in the global section of the config file, until either another
1.60 dtucker 1042: .Cm Match
1.65 dtucker 1043: line or the end of the file.
1.172 djm 1044: If a keyword appears in multiple
1045: .Cm Match
1.177 sobrado 1046: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1047: applied.
1.91 djm 1048: .Pp
1.61 jmc 1049: The arguments to
1.60 dtucker 1050: .Cm Match
1.163 dtucker 1051: are one or more criteria-pattern pairs or the single token
1052: .Cm All
1053: which matches all criteria.
1.60 dtucker 1054: The available criteria are
1055: .Cm User ,
1.69 dtucker 1056: .Cm Group ,
1.60 dtucker 1057: .Cm Host ,
1.139 dtucker 1058: .Cm LocalAddress ,
1059: .Cm LocalPort ,
1.60 dtucker 1060: and
1061: .Cm Address .
1.91 djm 1062: The match patterns may consist of single entries or comma-separated
1063: lists and may use the wildcard and negation operators described in the
1.161 jmc 1064: PATTERNS section of
1.92 djm 1065: .Xr ssh_config 5 .
1.91 djm 1066: .Pp
1067: The patterns in an
1068: .Cm Address
1069: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1070: address/masklen format, e.g.\&
1.91 djm 1071: .Dq 192.0.2.0/24
1072: or
1073: .Dq 3ffe:ffff::/32 .
1074: Note that the mask length provided must be consistent with the address -
1075: it is an error to specify a mask length that is too long for the address
1.93 jmc 1076: or one with bits set in this host portion of the address.
1077: For example,
1.91 djm 1078: .Dq 192.0.2.0/33
1079: and
1.93 jmc 1080: .Dq 192.0.2.0/8
1.91 djm 1081: respectively.
1082: .Pp
1.60 dtucker 1083: Only a subset of keywords may be used on the lines following a
1084: .Cm Match
1085: keyword.
1086: Available keywords are
1.142 jmc 1087: .Cm AcceptEnv ,
1.99 okan 1088: .Cm AllowAgentForwarding ,
1.142 jmc 1089: .Cm AllowGroups ,
1.193 djm 1090: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1091: .Cm AllowTcpForwarding ,
1.141 markus 1092: .Cm AllowUsers ,
1.149 djm 1093: .Cm AuthenticationMethods ,
1.146 djm 1094: .Cm AuthorizedKeysCommand ,
1095: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1096: .Cm AuthorizedKeysFile ,
1.214 djm 1097: .Cm AuthorizedPrincipalsCommand ,
1098: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1099: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1100: .Cm Banner ,
1.85 djm 1101: .Cm ChrootDirectory ,
1.141 markus 1102: .Cm DenyGroups ,
1103: .Cm DenyUsers ,
1.67 dtucker 1104: .Cm ForceCommand ,
1.194 djm 1105: .Cm GatewayPorts ,
1.193 djm 1106: .Cm GSSAPIAuthentication ,
1.189 djm 1107: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1108: .Cm HostbasedAuthentication ,
1.123 djm 1109: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1110: .Cm IPQoS ,
1.74 jmc 1111: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1112: .Cm KerberosAuthentication ,
1.95 dtucker 1113: .Cm MaxAuthTries ,
1.94 dtucker 1114: .Cm MaxSessions ,
1.72 dtucker 1115: .Cm PasswordAuthentication ,
1.97 djm 1116: .Cm PermitEmptyPasswords ,
1.66 dtucker 1117: .Cm PermitOpen ,
1.79 dtucker 1118: .Cm PermitRootLogin ,
1.164 djm 1119: .Cm PermitTTY ,
1.123 djm 1120: .Cm PermitTunnel ,
1.174 djm 1121: .Cm PermitUserRC ,
1.189 djm 1122: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1123: .Cm PubkeyAuthentication ,
1.159 dtucker 1124: .Cm RekeyLimit ,
1.193 djm 1125: .Cm RevokedKeys ,
1.142 jmc 1126: .Cm RhostsRSAAuthentication ,
1.194 djm 1127: .Cm RSAAuthentication ,
1.193 djm 1128: .Cm StreamLocalBindMask ,
1129: .Cm StreamLocalBindUnlink ,
1130: .Cm TrustedUserCAKeys ,
1.66 dtucker 1131: .Cm X11DisplayOffset ,
1.101 djm 1132: .Cm X11Forwarding
1.60 dtucker 1133: and
1.102 djm 1134: .Cm X11UseLocalHost .
1.33 dtucker 1135: .It Cm MaxAuthTries
1136: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1137: connection.
1138: Once the number of failures reaches half this value,
1139: additional failures are logged.
1140: The default is 6.
1.90 djm 1141: .It Cm MaxSessions
1.216 djm 1142: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1143: sessions permitted per network connection.
1144: Multiple sessions may be established by clients that support connection
1145: multiplexing.
1146: Setting
1147: .Cm MaxSessions
1148: to 1 will effectively disable session multiplexing, whereas setting it to 0
1149: will prevent all shell, login and subsystem sessions while still permitting
1150: forwarding.
1.90 djm 1151: The default is 10.
1.1 stevesk 1152: .It Cm MaxStartups
1153: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1154: SSH daemon.
1.1 stevesk 1155: Additional connections will be dropped until authentication succeeds or the
1156: .Cm LoginGraceTime
1157: expires for a connection.
1.156 dtucker 1158: The default is 10:30:100.
1.1 stevesk 1159: .Pp
1160: Alternatively, random early drop can be enabled by specifying
1161: the three colon separated values
1162: .Dq start:rate:full
1.51 jmc 1163: (e.g. "10:30:60").
1.53 jmc 1164: .Xr sshd 8
1.1 stevesk 1165: will refuse connection attempts with a probability of
1166: .Dq rate/100
1167: (30%)
1168: if there are currently
1169: .Dq start
1170: (10)
1171: unauthenticated connections.
1172: The probability increases linearly and all connection attempts
1173: are refused if the number of unauthenticated connections reaches
1174: .Dq full
1175: (60).
1176: .It Cm PasswordAuthentication
1177: Specifies whether password authentication is allowed.
1178: The default is
1179: .Dq yes .
1180: .It Cm PermitEmptyPasswords
1181: When password authentication is allowed, it specifies whether the
1182: server allows login to accounts with empty password strings.
1183: The default is
1184: .Dq no .
1.62 dtucker 1185: .It Cm PermitOpen
1186: Specifies the destinations to which TCP port forwarding is permitted.
1187: The forwarding specification must be one of the following forms:
1188: .Pp
1189: .Bl -item -offset indent -compact
1190: .It
1191: .Cm PermitOpen
1192: .Sm off
1193: .Ar host : port
1194: .Sm on
1195: .It
1196: .Cm PermitOpen
1197: .Sm off
1198: .Ar IPv4_addr : port
1199: .Sm on
1200: .It
1201: .Cm PermitOpen
1202: .Sm off
1203: .Ar \&[ IPv6_addr \&] : port
1204: .Sm on
1205: .El
1206: .Pp
1.68 dtucker 1207: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1208: An argument of
1209: .Dq any
1210: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1211: An argument of
1212: .Dq none
1213: can be used to prohibit all forwarding requests.
1.63 jmc 1214: By default all port forwarding requests are permitted.
1.1 stevesk 1215: .It Cm PermitRootLogin
1.38 jmc 1216: Specifies whether root can log in using
1.1 stevesk 1217: .Xr ssh 1 .
1218: The argument must be
1219: .Dq yes ,
1.210 deraadt 1220: .Dq prohibit-password ,
1.1 stevesk 1221: .Dq without-password ,
1.52 jmc 1222: .Dq forced-commands-only ,
1.1 stevesk 1223: or
1224: .Dq no .
1225: The default is
1.210 deraadt 1226: .Dq prohibit-password .
1.1 stevesk 1227: .Pp
1228: If this option is set to
1.210 deraadt 1229: .Dq prohibit-password
1230: or
1.52 jmc 1231: .Dq without-password ,
1.210 deraadt 1232: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1233: .Pp
1234: If this option is set to
1.52 jmc 1235: .Dq forced-commands-only ,
1.1 stevesk 1236: root login with public key authentication will be allowed,
1237: but only if the
1238: .Ar command
1239: option has been specified
1240: (which may be useful for taking remote backups even if root login is
1.17 jmc 1241: normally not allowed).
1242: All other authentication methods are disabled for root.
1.1 stevesk 1243: .Pp
1244: If this option is set to
1.52 jmc 1245: .Dq no ,
1.38 jmc 1246: root is not allowed to log in.
1.46 reyk 1247: .It Cm PermitTunnel
1248: Specifies whether
1249: .Xr tun 4
1250: device forwarding is allowed.
1.47 reyk 1251: The argument must be
1252: .Dq yes ,
1.58 stevesk 1253: .Dq point-to-point
1254: (layer 3),
1255: .Dq ethernet
1256: (layer 2), or
1.47 reyk 1257: .Dq no .
1.58 stevesk 1258: Specifying
1259: .Dq yes
1260: permits both
1261: .Dq point-to-point
1262: and
1263: .Dq ethernet .
1.46 reyk 1264: The default is
1265: .Dq no .
1.178 djm 1266: .Pp
1267: Independent of this setting, the permissions of the selected
1268: .Xr tun 4
1269: device must allow access to the user.
1.164 djm 1270: .It Cm PermitTTY
1271: Specifies whether
1.165 jmc 1272: .Xr pty 4
1.164 djm 1273: allocation is permitted.
1274: The default is
1275: .Dq yes .
1.6 markus 1276: .It Cm PermitUserEnvironment
1277: Specifies whether
1278: .Pa ~/.ssh/environment
1.9 stevesk 1279: and
1.6 markus 1280: .Cm environment=
1281: options in
1282: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1283: are processed by
1.52 jmc 1284: .Xr sshd 8 .
1.6 markus 1285: The default is
1286: .Dq no .
1.9 stevesk 1287: Enabling environment processing may enable users to bypass access
1288: restrictions in some configurations using mechanisms such as
1289: .Ev LD_PRELOAD .
1.174 djm 1290: .It Cm PermitUserRC
1291: Specifies whether any
1292: .Pa ~/.ssh/rc
1293: file is executed.
1294: The default is
1295: .Dq yes .
1.1 stevesk 1296: .It Cm PidFile
1.4 stevesk 1297: Specifies the file that contains the process ID of the
1.195 dtucker 1298: SSH daemon, or
1299: .Dq none
1300: to not write one.
1.1 stevesk 1301: The default is
1302: .Pa /var/run/sshd.pid .
1303: .It Cm Port
1304: Specifies the port number that
1.52 jmc 1305: .Xr sshd 8
1.1 stevesk 1306: listens on.
1307: The default is 22.
1308: Multiple options of this type are permitted.
1309: See also
1310: .Cm ListenAddress .
1311: .It Cm PrintLastLog
1312: Specifies whether
1.52 jmc 1313: .Xr sshd 8
1.36 jaredy 1314: should print the date and time of the last user login when a user logs
1315: in interactively.
1.1 stevesk 1316: The default is
1317: .Dq yes .
1318: .It Cm PrintMotd
1319: Specifies whether
1.52 jmc 1320: .Xr sshd 8
1.1 stevesk 1321: should print
1322: .Pa /etc/motd
1323: when a user logs in interactively.
1324: (On some systems it is also printed by the shell,
1325: .Pa /etc/profile ,
1326: or equivalent.)
1327: The default is
1328: .Dq yes .
1329: .It Cm Protocol
1330: Specifies the protocol versions
1.52 jmc 1331: .Xr sshd 8
1.5 stevesk 1332: supports.
1.1 stevesk 1333: The possible values are
1.52 jmc 1334: .Sq 1
1.1 stevesk 1335: and
1.52 jmc 1336: .Sq 2 .
1.1 stevesk 1337: Multiple versions must be comma-separated.
1338: The default is
1.109 jmc 1339: .Sq 2 .
1.218 djm 1340: Protocol 1 suffers from a number of cryptographic weaknesses and should
1341: not be used.
1342: It is only offered to support legacy devices.
1343: .Pp
1.5 stevesk 1344: Note that the order of the protocol list does not indicate preference,
1345: because the client selects among multiple protocol versions offered
1346: by the server.
1347: Specifying
1348: .Dq 2,1
1349: is identical to
1350: .Dq 1,2 .
1.189 djm 1351: .It Cm PubkeyAcceptedKeyTypes
1352: Specifies the key types that will be accepted for public key authentication
1353: as a comma-separated pattern list.
1.208 djm 1354: Alternately if the specified value begins with a
1355: .Sq +
1356: character, then the specified key types will be appended to the default set
1357: instead of replacing them.
1.206 markus 1358: The default for this option is:
1359: .Bd -literal -offset 3n
1360: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1361: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1362: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1363: ssh-ed25519-cert-v01@openssh.com,
1364: ssh-rsa-cert-v01@openssh.com,
1365: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1366: ssh-ed25519,ssh-rsa
1.206 markus 1367: .Ed
1368: .Pp
1.189 djm 1369: The
1370: .Fl Q
1371: option of
1372: .Xr ssh 1
1373: may be used to list supported key types.
1.1 stevesk 1374: .It Cm PubkeyAuthentication
1375: Specifies whether public key authentication is allowed.
1376: The default is
1377: .Dq yes .
1.159 dtucker 1378: .It Cm RekeyLimit
1379: Specifies the maximum amount of data that may be transmitted before the
1380: session key is renegotiated, optionally followed a maximum amount of
1381: time that may pass before the session key is renegotiated.
1382: The first argument is specified in bytes and may have a suffix of
1383: .Sq K ,
1384: .Sq M ,
1385: or
1386: .Sq G
1387: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1388: The default is between
1389: .Sq 1G
1390: and
1391: .Sq 4G ,
1392: depending on the cipher.
1393: The optional second value is specified in seconds and may use any of the
1394: units documented in the
1395: .Sx TIME FORMATS
1.160 jmc 1396: section.
1.159 dtucker 1397: The default value for
1398: .Cm RekeyLimit
1399: is
1400: .Dq default none ,
1401: which means that rekeying is performed after the cipher's default amount
1402: of data has been sent or received and no time based rekeying is done.
1.118 djm 1403: .It Cm RevokedKeys
1.195 dtucker 1404: Specifies revoked public keys file, or
1405: .Dq none
1406: to not use one.
1.118 djm 1407: Keys listed in this file will be refused for public key authentication.
1408: Note that if this file is not readable, then public key authentication will
1409: be refused for all users.
1.154 djm 1410: Keys may be specified as a text file, listing one public key per line, or as
1411: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1412: .Xr ssh-keygen 1 .
1.161 jmc 1413: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1414: .Xr ssh-keygen 1 .
1.1 stevesk 1415: .It Cm RhostsRSAAuthentication
1416: Specifies whether rhosts or /etc/hosts.equiv authentication together
1417: with successful RSA host authentication is allowed.
1418: The default is
1419: .Dq no .
1420: This option applies to protocol version 1 only.
1421: .It Cm RSAAuthentication
1422: Specifies whether pure RSA authentication is allowed.
1423: The default is
1424: .Dq yes .
1425: This option applies to protocol version 1 only.
1426: .It Cm ServerKeyBits
1427: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1428: The default and minimum value is 1024.
1.175 millert 1429: .It Cm StreamLocalBindMask
1430: Sets the octal file creation mode mask
1431: .Pq umask
1432: used when creating a Unix-domain socket file for local or remote
1433: port forwarding.
1434: This option is only used for port forwarding to a Unix-domain socket file.
1435: .Pp
1436: The default value is 0177, which creates a Unix-domain socket file that is
1437: readable and writable only by the owner.
1438: Note that not all operating systems honor the file mode on Unix-domain
1439: socket files.
1440: .It Cm StreamLocalBindUnlink
1441: Specifies whether to remove an existing Unix-domain socket file for local
1442: or remote port forwarding before creating a new one.
1443: If the socket file already exists and
1444: .Cm StreamLocalBindUnlink
1445: is not enabled,
1446: .Nm sshd
1447: will be unable to forward the port to the Unix-domain socket file.
1448: This option is only used for port forwarding to a Unix-domain socket file.
1449: .Pp
1450: The argument must be
1451: .Dq yes
1452: or
1453: .Dq no .
1454: The default is
1455: .Dq no .
1.1 stevesk 1456: .It Cm StrictModes
1457: Specifies whether
1.52 jmc 1458: .Xr sshd 8
1.1 stevesk 1459: should check file modes and ownership of the
1460: user's files and home directory before accepting login.
1461: This is normally desirable because novices sometimes accidentally leave their
1462: directory or files world-writable.
1463: The default is
1464: .Dq yes .
1.112 djm 1465: Note that this does not apply to
1466: .Cm ChrootDirectory ,
1467: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1468: .It Cm Subsystem
1.51 jmc 1469: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1470: Arguments should be a subsystem name and a command (with optional arguments)
1471: to execute upon subsystem request.
1.80 djm 1472: .Pp
1.1 stevesk 1473: The command
1474: .Xr sftp-server 8
1475: implements the
1476: .Dq sftp
1477: file transfer subsystem.
1.80 djm 1478: .Pp
1479: Alternately the name
1480: .Dq internal-sftp
1481: implements an in-process
1482: .Dq sftp
1483: server.
1484: This may simplify configurations using
1485: .Cm ChrootDirectory
1486: to force a different filesystem root on clients.
1487: .Pp
1.1 stevesk 1488: By default no subsystems are defined.
1489: .It Cm SyslogFacility
1490: Gives the facility code that is used when logging messages from
1.53 jmc 1491: .Xr sshd 8 .
1.1 stevesk 1492: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1493: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1494: The default is AUTH.
1.27 markus 1495: .It Cm TCPKeepAlive
1496: Specifies whether the system should send TCP keepalive messages to the
1497: other side.
1498: If they are sent, death of the connection or crash of one
1499: of the machines will be properly noticed.
1500: However, this means that
1501: connections will die if the route is down temporarily, and some people
1502: find it annoying.
1503: On the other hand, if TCP keepalives are not sent,
1504: sessions may hang indefinitely on the server, leaving
1505: .Dq ghost
1506: users and consuming server resources.
1507: .Pp
1508: The default is
1509: .Dq yes
1510: (to send TCP keepalive messages), and the server will notice
1511: if the network goes down or the client host crashes.
1512: This avoids infinitely hanging sessions.
1513: .Pp
1514: To disable TCP keepalive messages, the value should be set to
1515: .Dq no .
1.118 djm 1516: .It Cm TrustedUserCAKeys
1517: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1518: trusted to sign user certificates for authentication, or
1519: .Dq none
1520: to not use one.
1.119 jmc 1521: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1522: .Ql #
1523: are allowed.
1524: If a certificate is presented for authentication and has its signing CA key
1525: listed in this file, then it may be used for authentication for any user
1526: listed in the certificate's principals list.
1527: Note that certificates that lack a list of principals will not be permitted
1528: for authentication using
1529: .Cm TrustedUserCAKeys .
1.161 jmc 1530: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1531: .Xr ssh-keygen 1 .
1.18 markus 1532: .It Cm UseDNS
1533: Specifies whether
1.52 jmc 1534: .Xr sshd 8
1.207 djm 1535: should look up the remote host name, and to check that
1.18 markus 1536: the resolved host name for the remote IP address maps back to the
1537: very same IP address.
1.207 djm 1538: .Pp
1539: If this option is set to
1540: .Dq no
1541: (the default) then only addresses and not host names may be used in
1.221 djm 1542: .Pa ~/.ssh/authorized_keys
1.207 djm 1543: .Cm from
1544: and
1.211 jmc 1545: .Nm
1.207 djm 1546: .Cm Match
1547: .Cm Host
1548: directives.
1.1 stevesk 1549: .It Cm UseLogin
1550: Specifies whether
1551: .Xr login 1
1552: is used for interactive login sessions.
1553: The default is
1554: .Dq no .
1555: Note that
1556: .Xr login 1
1557: is never used for remote command execution.
1558: Note also, that if this is enabled,
1559: .Cm X11Forwarding
1560: will be disabled because
1561: .Xr login 1
1562: does not know how to handle
1563: .Xr xauth 1
1.15 jmc 1564: cookies.
1565: If
1.1 stevesk 1566: .Cm UsePrivilegeSeparation
1567: is specified, it will be disabled after authentication.
1568: .It Cm UsePrivilegeSeparation
1569: Specifies whether
1.52 jmc 1570: .Xr sshd 8
1.2 stevesk 1571: separates privileges by creating an unprivileged child process
1.15 jmc 1572: to deal with incoming network traffic.
1573: After successful authentication, another process will be created that has
1574: the privilege of the authenticated user.
1575: The goal of privilege separation is to prevent privilege
1.1 stevesk 1576: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1577: The argument must be
1578: .Dq yes ,
1579: .Dq no ,
1580: or
1581: .Dq sandbox .
1.134 djm 1582: If
1583: .Cm UsePrivilegeSeparation
1584: is set to
1585: .Dq sandbox
1586: then the pre-authentication unprivileged process is subject to additional
1587: restrictions.
1.213 sobrado 1588: The default is
1589: .Dq sandbox .
1.137 djm 1590: .It Cm VersionAddendum
1591: Optionally specifies additional text to append to the SSH protocol banner
1592: sent by the server upon connection.
1593: The default is
1594: .Dq none .
1.1 stevesk 1595: .It Cm X11DisplayOffset
1596: Specifies the first display number available for
1.52 jmc 1597: .Xr sshd 8 Ns 's
1.1 stevesk 1598: X11 forwarding.
1.52 jmc 1599: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1600: The default is 10.
1601: .It Cm X11Forwarding
1602: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1603: The argument must be
1604: .Dq yes
1605: or
1606: .Dq no .
1.1 stevesk 1607: The default is
1608: .Dq no .
1.13 stevesk 1609: .Pp
1610: When X11 forwarding is enabled, there may be additional exposure to
1611: the server and to client displays if the
1.52 jmc 1612: .Xr sshd 8
1.13 stevesk 1613: proxy display is configured to listen on the wildcard address (see
1614: .Cm X11UseLocalhost
1.52 jmc 1615: below), though this is not the default.
1.13 stevesk 1616: Additionally, the authentication spoofing and authentication data
1617: verification and substitution occur on the client side.
1618: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1619: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1620: forwarding (see the warnings for
1621: .Cm ForwardX11
1622: in
1.19 jmc 1623: .Xr ssh_config 5 ) .
1.13 stevesk 1624: A system administrator may have a stance in which they want to
1625: protect clients that may expose themselves to attack by unwittingly
1626: requesting X11 forwarding, which can warrant a
1627: .Dq no
1628: setting.
1629: .Pp
1630: Note that disabling X11 forwarding does not prevent users from
1631: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1632: X11 forwarding is automatically disabled if
1633: .Cm UseLogin
1634: is enabled.
1635: .It Cm X11UseLocalhost
1636: Specifies whether
1.52 jmc 1637: .Xr sshd 8
1.1 stevesk 1638: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1639: the wildcard address.
1640: By default,
1.52 jmc 1641: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1642: hostname part of the
1643: .Ev DISPLAY
1644: environment variable to
1645: .Dq localhost .
1.8 stevesk 1646: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1647: However, some older X11 clients may not function with this
1648: configuration.
1649: .Cm X11UseLocalhost
1650: may be set to
1651: .Dq no
1652: to specify that the forwarding server should be bound to the wildcard
1653: address.
1654: The argument must be
1655: .Dq yes
1656: or
1657: .Dq no .
1658: The default is
1659: .Dq yes .
1660: .It Cm XAuthLocation
1.11 stevesk 1661: Specifies the full pathname of the
1.1 stevesk 1662: .Xr xauth 1
1.195 dtucker 1663: program, or
1664: .Dq none
1665: to not use one.
1.1 stevesk 1666: The default is
1667: .Pa /usr/X11R6/bin/xauth .
1668: .El
1.55 jmc 1669: .Sh TIME FORMATS
1.53 jmc 1670: .Xr sshd 8
1.1 stevesk 1671: command-line arguments and configuration file options that specify time
1672: may be expressed using a sequence of the form:
1673: .Sm off
1.7 stevesk 1674: .Ar time Op Ar qualifier ,
1.1 stevesk 1675: .Sm on
1676: where
1677: .Ar time
1678: is a positive integer value and
1679: .Ar qualifier
1680: is one of the following:
1681: .Pp
1682: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1683: .It Aq Cm none
1.1 stevesk 1684: seconds
1685: .It Cm s | Cm S
1686: seconds
1687: .It Cm m | Cm M
1688: minutes
1689: .It Cm h | Cm H
1690: hours
1691: .It Cm d | Cm D
1692: days
1693: .It Cm w | Cm W
1694: weeks
1695: .El
1696: .Pp
1697: Each member of the sequence is added together to calculate
1698: the total time value.
1699: .Pp
1700: Time format examples:
1701: .Pp
1702: .Bl -tag -width Ds -compact -offset indent
1703: .It 600
1704: 600 seconds (10 minutes)
1705: .It 10m
1706: 10 minutes
1707: .It 1h30m
1708: 1 hour 30 minutes (90 minutes)
1709: .El
1710: .Sh FILES
1711: .Bl -tag -width Ds
1712: .It Pa /etc/ssh/sshd_config
1713: Contains configuration data for
1.53 jmc 1714: .Xr sshd 8 .
1.1 stevesk 1715: This file should be writable by root only, but it is recommended
1716: (though not necessary) that it be world-readable.
1717: .El
1.19 jmc 1718: .Sh SEE ALSO
1719: .Xr sshd 8
1.1 stevesk 1720: .Sh AUTHORS
1721: OpenSSH is a derivative of the original and free
1722: ssh 1.2.12 release by Tatu Ylonen.
1723: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1724: Theo de Raadt and Dug Song
1725: removed many bugs, re-added newer features and
1726: created OpenSSH.
1727: Markus Friedl contributed the support for SSH
1728: protocol versions 1.5 and 2.0.
1729: Niels Provos and Markus Friedl contributed support
1730: for privilege separation.
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