Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.208
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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1.208 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.207 2015/07/20 00:30:01 djm Exp $
! 37: .Dd $Mdocdate: July 20 2015 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.197 dtucker 73: Note that environment passing is only supported for protocol 2, and
74: that the
75: .Ev TERM
1.199 jmc 76: environment variable is always sent whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
94: .Dq any ,
95: .Dq inet
1.52 jmc 96: (use IPv4 only), or
1.37 djm 97: .Dq inet6
98: (use IPv6 only).
99: The default is
100: .Dq any .
1.89 jmc 101: .It Cm AllowAgentForwarding
102: Specifies whether
103: .Xr ssh-agent 1
104: forwarding is permitted.
105: The default is
106: .Dq yes .
107: Note that disabling agent forwarding does not improve security
108: unless users are also denied shell access, as they can always install
109: their own forwarders.
1.1 stevesk 110: .It Cm AllowGroups
111: This keyword can be followed by a list of group name patterns, separated
112: by spaces.
113: If specified, login is allowed only for users whose primary
114: group or supplementary group list matches one of the patterns.
115: Only group names are valid; a numerical group ID is not recognized.
116: By default, login is allowed for all groups.
1.54 jmc 117: The allow/deny directives are processed in the following order:
118: .Cm DenyUsers ,
119: .Cm AllowUsers ,
120: .Cm DenyGroups ,
121: and finally
122: .Cm AllowGroups .
1.49 jmc 123: .Pp
1.161 jmc 124: See PATTERNS in
1.49 jmc 125: .Xr ssh_config 5
126: for more information on patterns.
1.1 stevesk 127: .It Cm AllowTcpForwarding
128: Specifies whether TCP forwarding is permitted.
1.150 djm 129: The available options are
130: .Dq yes
131: or
132: .Dq all
133: to allow TCP forwarding,
134: .Dq no
135: to prevent all TCP forwarding,
136: .Dq local
137: to allow local (from the perspective of
1.151 jmc 138: .Xr ssh 1 )
139: forwarding only or
1.150 djm 140: .Dq remote
141: to allow remote forwarding only.
1.1 stevesk 142: The default is
143: .Dq yes .
144: Note that disabling TCP forwarding does not improve security unless
145: users are also denied shell access, as they can always install their
146: own forwarders.
1.175 millert 147: .It Cm AllowStreamLocalForwarding
148: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
149: The available options are
150: .Dq yes
151: or
152: .Dq all
153: to allow StreamLocal forwarding,
154: .Dq no
155: to prevent all StreamLocal forwarding,
156: .Dq local
157: to allow local (from the perspective of
158: .Xr ssh 1 )
159: forwarding only or
160: .Dq remote
161: to allow remote forwarding only.
162: The default is
163: .Dq yes .
164: Note that disabling StreamLocal forwarding does not improve security unless
165: users are also denied shell access, as they can always install their
166: own forwarders.
1.1 stevesk 167: .It Cm AllowUsers
168: This keyword can be followed by a list of user name patterns, separated
169: by spaces.
1.14 jmc 170: If specified, login is allowed only for user names that
1.1 stevesk 171: match one of the patterns.
172: Only user names are valid; a numerical user ID is not recognized.
173: By default, login is allowed for all users.
174: If the pattern takes the form USER@HOST then USER and HOST
175: are separately checked, restricting logins to particular
176: users from particular hosts.
1.54 jmc 177: The allow/deny directives are processed in the following order:
178: .Cm DenyUsers ,
179: .Cm AllowUsers ,
180: .Cm DenyGroups ,
181: and finally
182: .Cm AllowGroups .
1.49 jmc 183: .Pp
1.161 jmc 184: See PATTERNS in
1.49 jmc 185: .Xr ssh_config 5
186: for more information on patterns.
1.149 djm 187: .It Cm AuthenticationMethods
188: Specifies the authentication methods that must be successfully completed
189: for a user to be granted access.
190: This option must be followed by one or more comma-separated lists of
191: authentication method names.
192: Successful authentication requires completion of every method in at least
193: one of these lists.
194: .Pp
195: For example, an argument of
196: .Dq publickey,password publickey,keyboard-interactive
197: would require the user to complete public key authentication, followed by
198: either password or keyboard interactive authentication.
199: Only methods that are next in one or more lists are offered at each stage,
200: so for this example, it would not be possible to attempt password or
201: keyboard-interactive authentication before public key.
1.157 markus 202: .Pp
203: For keyboard interactive authentication it is also possible to
204: restrict authentication to a specific device by appending a
205: colon followed by the device identifier
206: .Dq bsdauth ,
207: .Dq pam ,
208: or
209: .Dq skey ,
210: depending on the server configuration.
211: For example,
212: .Dq keyboard-interactive:bsdauth
213: would restrict keyboard interactive authentication to the
214: .Dq bsdauth
215: device.
1.185 djm 216: .Pp
217: If the
218: .Dq publickey
1.187 jmc 219: method is listed more than once,
1.185 djm 220: .Xr sshd 8
221: verifies that keys that have been used successfully are not reused for
222: subsequent authentications.
223: For example, an
224: .Cm AuthenticationMethods
225: of
226: .Dq publickey,publickey
227: will require successful authentication using two different public keys.
1.181 djm 228: .Pp
1.149 djm 229: This option is only available for SSH protocol 2 and will yield a fatal
230: error if enabled if protocol 1 is also enabled.
231: Note that each authentication method listed should also be explicitly enabled
232: in the configuration.
233: The default is not to require multiple authentication; successful completion
234: of a single authentication method is sufficient.
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: .Pp
240: Arguments to
241: .Cm AuthorizedKeysCommand
242: may be provided using the following tokens, which will be expanded
243: at runtime: %% is replaced by a literal '%', %u is replaced by the
244: username being authenticated, %h is replaced by the home directory
245: of the user being authenticated, %t is replaced with the key type
246: offered for authentication, %f is replaced with the fingerprint of
247: the key, and %k is replaced with the key being offered for authentication.
248: If no arguments are specified then the username of the target user
249: will be supplied.
250: .Pp
251: The program should produce on standard output zero or
1.161 jmc 252: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 253: .Xr sshd 8 ) .
1.146 djm 254: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
255: and authorize the user then public key authentication continues using the usual
256: .Cm AuthorizedKeysFile
257: files.
258: By default, no AuthorizedKeysCommand is run.
259: .It Cm AuthorizedKeysCommandUser
260: Specifies the user under whose account the AuthorizedKeysCommand is run.
261: It is recommended to use a dedicated user that has no other role on the host
262: than running authorized keys commands.
1.186 djm 263: If
1.181 djm 264: .Cm AuthorizedKeysCommand
1.186 djm 265: is specified but
266: .Cm AuthorizedKeysCommandUser
267: is not, then
268: .Xr sshd 8
269: will refuse to start.
1.1 stevesk 270: .It Cm AuthorizedKeysFile
271: Specifies the file that contains the public keys that can be used
272: for user authentication.
1.124 djm 273: The format is described in the
1.161 jmc 274: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 275: section of
276: .Xr sshd 8 .
1.1 stevesk 277: .Cm AuthorizedKeysFile
278: may contain tokens of the form %T which are substituted during connection
1.52 jmc 279: setup.
1.17 jmc 280: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 281: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 282: %u is replaced by the username of that user.
283: After expansion,
284: .Cm AuthorizedKeysFile
285: is taken to be an absolute path or one relative to the user's home
286: directory.
1.133 jmc 287: Multiple files may be listed, separated by whitespace.
288: The default is
289: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 290: .It Cm AuthorizedPrincipalsCommand
291: Specifies a program to be used to generate the list of allowed
292: certificate principals as per
293: .Cm AuthorizedPrincipalsFile .
294: The program must be owned by root, not writable by group or others and
295: specified by an absolute path.
296: .Pp
297: Arguments to
298: .Cm AuthorizedPrincipalsCommand
299: may be provided using the following tokens, which will be expanded
300: at runtime: %% is replaced by a literal '%', %u is replaced by the
301: username being authenticated and %h is replaced by the home directory
302: of the user being authenticated.
303: .Pp
304: The program should produce on standard output zero or
305: more lines of
306: .Cm AuthorizedPrincipalsFile
307: output.
308: If either
309: .Cm AuthorizedPrincipalsCommand
310: or
311: .Cm AuthorizedPrincipalsFile
312: is specified, then certificates offered by the client for authentication
313: must contain a principal that is listed.
314: By default, no AuthorizedPrincipalsCommand is run.
315: .It Cm AuthorizedPrincipalsCommandUser
316: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
317: It is recommended to use a dedicated user that has no other role on the host
318: than running authorized principals commands.
319: If
320: .Cm AuthorizedPrincipalsCommand
321: is specified but
322: .Cm AuthorizedPrincipalsCommandUser
323: is not, then
324: .Xr sshd 8
325: will refuse to start.
1.121 djm 326: .It Cm AuthorizedPrincipalsFile
327: Specifies a file that lists principal names that are accepted for
328: certificate authentication.
329: When using certificates signed by a key listed in
330: .Cm TrustedUserCAKeys ,
331: this file lists names, one of which must appear in the certificate for it
332: to be accepted for authentication.
1.125 jmc 333: Names are listed one per line preceded by key options (as described
1.161 jmc 334: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 335: .Xr sshd 8 ) .
1.124 djm 336: Empty lines and comments starting with
1.121 djm 337: .Ql #
338: are ignored.
339: .Pp
340: .Cm AuthorizedPrincipalsFile
341: may contain tokens of the form %T which are substituted during connection
342: setup.
343: The following tokens are defined: %% is replaced by a literal '%',
344: %h is replaced by the home directory of the user being authenticated, and
345: %u is replaced by the username of that user.
346: After expansion,
347: .Cm AuthorizedPrincipalsFile
348: is taken to be an absolute path or one relative to the user's home
349: directory.
350: .Pp
1.138 djm 351: The default is
352: .Dq none ,
353: i.e. not to use a principals file \(en in this case, the username
1.121 djm 354: of the user must appear in a certificate's principals list for it to be
355: accepted.
356: Note that
357: .Cm AuthorizedPrincipalsFile
358: is only used when authentication proceeds using a CA listed in
359: .Cm TrustedUserCAKeys
360: and is not consulted for certification authorities trusted via
361: .Pa ~/.ssh/authorized_keys ,
362: though the
363: .Cm principals=
364: key option offers a similar facility (see
365: .Xr sshd 8
366: for details).
1.1 stevesk 367: .It Cm Banner
368: The contents of the specified file are sent to the remote user before
369: authentication is allowed.
1.78 djm 370: If the argument is
371: .Dq none
372: then no banner is displayed.
1.1 stevesk 373: This option is only available for protocol version 2.
374: By default, no banner is displayed.
375: .It Cm ChallengeResponseAuthentication
1.50 jmc 376: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 377: All authentication styles from
378: .Xr login.conf 5
379: are supported.
380: The default is
381: .Dq yes .
1.80 djm 382: .It Cm ChrootDirectory
1.113 stevesk 383: Specifies the pathname of a directory to
1.80 djm 384: .Xr chroot 2
385: to after authentication.
1.190 deraadt 386: At session startup
387: .Xr sshd 8
388: checks that all components of the pathname are root-owned directories
389: which are not writable by any other user or group.
1.106 stevesk 390: After the chroot,
391: .Xr sshd 8
392: changes the working directory to the user's home directory.
1.80 djm 393: .Pp
1.113 stevesk 394: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 395: the connecting user has been authenticated: %% is replaced by a literal '%',
396: %h is replaced by the home directory of the user being authenticated, and
397: %u is replaced by the username of that user.
398: .Pp
399: The
400: .Cm ChrootDirectory
401: must contain the necessary files and directories to support the
1.103 stevesk 402: user's session.
1.80 djm 403: For an interactive session this requires at least a shell, typically
404: .Xr sh 1 ,
405: and basic
406: .Pa /dev
407: nodes such as
408: .Xr null 4 ,
409: .Xr zero 4 ,
410: .Xr stdin 4 ,
411: .Xr stdout 4 ,
412: .Xr stderr 4 ,
1.180 jmc 413: and
1.80 djm 414: .Xr tty 4
415: devices.
416: For file transfer sessions using
1.105 jmc 417: .Dq sftp ,
1.80 djm 418: no additional configuration of the environment is necessary if the
1.105 jmc 419: in-process sftp server is used,
1.176 schwarze 420: though sessions which use logging may require
1.104 stevesk 421: .Pa /dev/log
1.176 schwarze 422: inside the chroot directory on some operating systems (see
1.104 stevesk 423: .Xr sftp-server 8
1.81 jmc 424: for details).
1.190 deraadt 425: .Pp
1.191 jmc 426: For safety, it is very important that the directory hierarchy be
1.190 deraadt 427: prevented from modification by other processes on the system (especially
428: those outside the jail).
429: Misconfiguration can lead to unsafe environments which
430: .Xr sshd 8
431: cannot detect.
1.80 djm 432: .Pp
433: The default is not to
434: .Xr chroot 2 .
1.1 stevesk 435: .It Cm Ciphers
436: Specifies the ciphers allowed for protocol version 2.
437: Multiple ciphers must be comma-separated.
1.208 ! djm 438: If the specified value begins with a
! 439: .Sq +
! 440: character, then the specified ciphers will be appended to the default set
! 441: instead of replacing them.
! 442: .Pp
1.167 djm 443: The supported ciphers are:
444: .Pp
1.173 naddy 445: .Bl -item -compact -offset indent
446: .It
447: 3des-cbc
448: .It
449: aes128-cbc
450: .It
451: aes192-cbc
452: .It
453: aes256-cbc
454: .It
455: aes128-ctr
456: .It
457: aes192-ctr
458: .It
459: aes256-ctr
460: .It
461: aes128-gcm@openssh.com
462: .It
463: aes256-gcm@openssh.com
464: .It
465: arcfour
466: .It
467: arcfour128
468: .It
469: arcfour256
470: .It
471: blowfish-cbc
472: .It
473: cast128-cbc
474: .It
475: chacha20-poly1305@openssh.com
476: .El
1.167 djm 477: .Pp
1.52 jmc 478: The default is:
1.173 naddy 479: .Bd -literal -offset indent
480: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 481: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 482: chacha20-poly1305@openssh.com
1.1 stevesk 483: .Ed
1.167 djm 484: .Pp
485: The list of available ciphers may also be obtained using the
486: .Fl Q
487: option of
1.188 djm 488: .Xr ssh 1
489: with an argument of
490: .Dq cipher .
1.1 stevesk 491: .It Cm ClientAliveCountMax
1.48 jmc 492: Sets the number of client alive messages (see below) which may be
1.1 stevesk 493: sent without
1.52 jmc 494: .Xr sshd 8
1.17 jmc 495: receiving any messages back from the client.
496: If this threshold is reached while client alive messages are being sent,
1.52 jmc 497: sshd will disconnect the client, terminating the session.
1.17 jmc 498: It is important to note that the use of client alive messages is very
499: different from
1.27 markus 500: .Cm TCPKeepAlive
1.17 jmc 501: (below).
502: The client alive messages are sent through the encrypted channel
503: and therefore will not be spoofable.
504: The TCP keepalive option enabled by
1.27 markus 505: .Cm TCPKeepAlive
1.17 jmc 506: is spoofable.
507: The client alive mechanism is valuable when the client or
1.1 stevesk 508: server depend on knowing when a connection has become inactive.
509: .Pp
1.17 jmc 510: The default value is 3.
511: If
1.1 stevesk 512: .Cm ClientAliveInterval
1.48 jmc 513: (see below) is set to 15, and
1.1 stevesk 514: .Cm ClientAliveCountMax
1.52 jmc 515: is left at the default, unresponsive SSH clients
1.1 stevesk 516: will be disconnected after approximately 45 seconds.
1.57 markus 517: This option applies to protocol version 2 only.
1.42 djm 518: .It Cm ClientAliveInterval
519: Sets a timeout interval in seconds after which if no data has been received
520: from the client,
1.52 jmc 521: .Xr sshd 8
1.42 djm 522: will send a message through the encrypted
523: channel to request a response from the client.
524: The default
525: is 0, indicating that these messages will not be sent to the client.
526: This option applies to protocol version 2 only.
1.3 markus 527: .It Cm Compression
1.44 markus 528: Specifies whether compression is allowed, or delayed until
529: the user has authenticated successfully.
1.3 markus 530: The argument must be
1.44 markus 531: .Dq yes ,
532: .Dq delayed ,
1.3 markus 533: or
534: .Dq no .
535: The default is
1.44 markus 536: .Dq delayed .
1.1 stevesk 537: .It Cm DenyGroups
538: This keyword can be followed by a list of group name patterns, separated
539: by spaces.
540: Login is disallowed for users whose primary group or supplementary
541: group list matches one of the patterns.
542: Only group names are valid; a numerical group ID is not recognized.
543: By default, login is allowed for all groups.
1.54 jmc 544: The allow/deny directives are processed in the following order:
545: .Cm DenyUsers ,
546: .Cm AllowUsers ,
547: .Cm DenyGroups ,
548: and finally
549: .Cm AllowGroups .
1.49 jmc 550: .Pp
1.161 jmc 551: See PATTERNS in
1.49 jmc 552: .Xr ssh_config 5
553: for more information on patterns.
1.1 stevesk 554: .It Cm DenyUsers
555: This keyword can be followed by a list of user name patterns, separated
556: by spaces.
557: Login is disallowed for user names that match one of the patterns.
558: Only user names are valid; a numerical user ID is not recognized.
559: By default, login is allowed for all users.
560: If the pattern takes the form USER@HOST then USER and HOST
561: are separately checked, restricting logins to particular
562: users from particular hosts.
1.54 jmc 563: The allow/deny directives are processed in the following order:
564: .Cm DenyUsers ,
565: .Cm AllowUsers ,
566: .Cm DenyGroups ,
567: and finally
568: .Cm AllowGroups .
1.49 jmc 569: .Pp
1.161 jmc 570: See PATTERNS in
1.49 jmc 571: .Xr ssh_config 5
572: for more information on patterns.
1.183 djm 573: .It Cm FingerprintHash
574: Specifies the hash algorithm used when logging key fingerprints.
575: Valid options are:
576: .Dq md5
577: and
578: .Dq sha256 .
579: The default is
580: .Dq sha256 .
1.67 dtucker 581: .It Cm ForceCommand
582: Forces the execution of the command specified by
583: .Cm ForceCommand ,
1.84 djm 584: ignoring any command supplied by the client and
585: .Pa ~/.ssh/rc
586: if present.
1.67 dtucker 587: The command is invoked by using the user's login shell with the -c option.
588: This applies to shell, command, or subsystem execution.
589: It is most useful inside a
590: .Cm Match
591: block.
592: The command originally supplied by the client is available in the
593: .Ev SSH_ORIGINAL_COMMAND
594: environment variable.
1.82 djm 595: Specifying a command of
596: .Dq internal-sftp
597: will force the use of an in-process sftp server that requires no support
598: files when used with
599: .Cm ChrootDirectory .
1.1 stevesk 600: .It Cm GatewayPorts
601: Specifies whether remote hosts are allowed to connect to ports
602: forwarded for the client.
603: By default,
1.52 jmc 604: .Xr sshd 8
1.15 jmc 605: binds remote port forwardings to the loopback address.
606: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 607: .Cm GatewayPorts
1.52 jmc 608: can be used to specify that sshd
1.39 djm 609: should allow remote port forwardings to bind to non-loopback addresses, thus
610: allowing other hosts to connect.
611: The argument may be
612: .Dq no
613: to force remote port forwardings to be available to the local host only,
1.1 stevesk 614: .Dq yes
1.39 djm 615: to force remote port forwardings to bind to the wildcard address, or
616: .Dq clientspecified
617: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 618: The default is
619: .Dq no .
1.23 markus 620: .It Cm GSSAPIAuthentication
1.25 markus 621: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 622: The default is
1.23 markus 623: .Dq no .
624: Note that this option applies to protocol version 2 only.
625: .It Cm GSSAPICleanupCredentials
626: Specifies whether to automatically destroy the user's credentials cache
627: on logout.
628: The default is
629: .Dq yes .
630: Note that this option applies to protocol version 2 only.
1.203 djm 631: .It Cm GSSAPIStrictAcceptorCheck
632: Determines whether to be strict about the identity of the GSSAPI acceptor
633: a client authenticates against.
634: If set to
635: .Dq yes
636: then the client must authenticate against the
637: .Pa host
638: service on the current hostname.
639: If set to
640: .Dq no
641: then the client may authenticate against any service key stored in the
642: machine's default store.
643: This facility is provided to assist with operation on multi homed machines.
644: The default is
645: .Dq yes .
1.189 djm 646: .It Cm HostbasedAcceptedKeyTypes
647: Specifies the key types that will be accepted for hostbased authentication
648: as a comma-separated pattern list.
1.208 ! djm 649: Alternately if the specified value begins with a
! 650: .Sq +
! 651: character, then the specified key types will be appended to the default set
! 652: instead of replacing them.
1.206 markus 653: The default for this option is:
654: .Bd -literal -offset 3n
655: ecdsa-sha2-nistp256-cert-v01@openssh.com,
656: ecdsa-sha2-nistp384-cert-v01@openssh.com,
657: ecdsa-sha2-nistp521-cert-v01@openssh.com,
658: ssh-ed25519-cert-v01@openssh.com,
659: ssh-rsa-cert-v01@openssh.com,
660: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
661: ssh-ed25519,ssh-rsa
662: .Ed
663: .Pp
1.189 djm 664: The
665: .Fl Q
666: option of
667: .Xr ssh 1
668: may be used to list supported key types.
1.1 stevesk 669: .It Cm HostbasedAuthentication
670: Specifies whether rhosts or /etc/hosts.equiv authentication together
671: with successful public key client host authentication is allowed
1.50 jmc 672: (host-based authentication).
1.1 stevesk 673: This option is similar to
674: .Cm RhostsRSAAuthentication
675: and applies to protocol version 2 only.
1.70 dtucker 676: The default is
677: .Dq no .
678: .It Cm HostbasedUsesNameFromPacketOnly
679: Specifies whether or not the server will attempt to perform a reverse
680: name lookup when matching the name in the
681: .Pa ~/.shosts ,
682: .Pa ~/.rhosts ,
683: and
684: .Pa /etc/hosts.equiv
685: files during
686: .Cm HostbasedAuthentication .
687: A setting of
688: .Dq yes
689: means that
690: .Xr sshd 8
691: uses the name supplied by the client rather than
692: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 693: The default is
694: .Dq no .
1.117 djm 695: .It Cm HostCertificate
696: Specifies a file containing a public host certificate.
697: The certificate's public key must match a private host key already specified
698: by
699: .Cm HostKey .
700: The default behaviour of
701: .Xr sshd 8
702: is not to load any certificates.
1.1 stevesk 703: .It Cm HostKey
704: Specifies a file containing a private host key
705: used by SSH.
706: The default is
707: .Pa /etc/ssh/ssh_host_key
708: for protocol version 1, and
1.126 djm 709: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 710: .Pa /etc/ssh/ssh_host_ecdsa_key ,
711: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 712: and
1.1 stevesk 713: .Pa /etc/ssh/ssh_host_rsa_key
714: for protocol version 2.
1.206 markus 715: .Pp
1.1 stevesk 716: Note that
1.52 jmc 717: .Xr sshd 8
1.206 markus 718: will refuse to use a file if it is group/world-accessible
719: and that the
720: .Cm HostKeyAlgorithms
721: option restricts which of the keys are actually used by
722: .Xr sshd 8 .
723: .Pp
1.1 stevesk 724: It is possible to have multiple host key files.
725: .Dq rsa1
726: keys are used for version 1 and
1.126 djm 727: .Dq dsa ,
1.169 naddy 728: .Dq ecdsa ,
729: .Dq ed25519
1.1 stevesk 730: or
731: .Dq rsa
732: are used for version 2 of the SSH protocol.
1.162 markus 733: It is also possible to specify public host key files instead.
734: In this case operations on the private key will be delegated
735: to an
736: .Xr ssh-agent 1 .
737: .It Cm HostKeyAgent
738: Identifies the UNIX-domain socket used to communicate
739: with an agent that has access to the private host keys.
740: If
741: .Dq SSH_AUTH_SOCK
742: is specified, the location of the socket will be read from the
743: .Ev SSH_AUTH_SOCK
744: environment variable.
1.206 markus 745: .It Cm HostKeyAlgorithms
746: Specifies the protocol version 2 host key algorithms
747: that the server offers.
748: The default for this option is:
749: .Bd -literal -offset 3n
750: ecdsa-sha2-nistp256-cert-v01@openssh.com,
751: ecdsa-sha2-nistp384-cert-v01@openssh.com,
752: ecdsa-sha2-nistp521-cert-v01@openssh.com,
753: ssh-ed25519-cert-v01@openssh.com,
754: ssh-rsa-cert-v01@openssh.com,
755: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
756: ssh-ed25519,ssh-rsa
757: .Ed
758: .Pp
759: The list of available key types may also be obtained using the
760: .Fl Q
761: option of
762: .Xr ssh 1
763: with an argument of
764: .Dq key .
1.1 stevesk 765: .It Cm IgnoreRhosts
766: Specifies that
767: .Pa .rhosts
768: and
769: .Pa .shosts
770: files will not be used in
771: .Cm RhostsRSAAuthentication
772: or
773: .Cm HostbasedAuthentication .
774: .Pp
775: .Pa /etc/hosts.equiv
776: and
777: .Pa /etc/shosts.equiv
778: are still used.
779: The default is
780: .Dq yes .
781: .It Cm IgnoreUserKnownHosts
782: Specifies whether
1.52 jmc 783: .Xr sshd 8
1.1 stevesk 784: should ignore the user's
1.41 djm 785: .Pa ~/.ssh/known_hosts
1.1 stevesk 786: during
787: .Cm RhostsRSAAuthentication
788: or
789: .Cm HostbasedAuthentication .
790: The default is
791: .Dq no .
1.129 djm 792: .It Cm IPQoS
793: Specifies the IPv4 type-of-service or DSCP class for the connection.
794: Accepted values are
795: .Dq af11 ,
796: .Dq af12 ,
797: .Dq af13 ,
1.136 djm 798: .Dq af21 ,
1.129 djm 799: .Dq af22 ,
800: .Dq af23 ,
801: .Dq af31 ,
802: .Dq af32 ,
803: .Dq af33 ,
804: .Dq af41 ,
805: .Dq af42 ,
806: .Dq af43 ,
807: .Dq cs0 ,
808: .Dq cs1 ,
809: .Dq cs2 ,
810: .Dq cs3 ,
811: .Dq cs4 ,
812: .Dq cs5 ,
813: .Dq cs6 ,
814: .Dq cs7 ,
815: .Dq ef ,
816: .Dq lowdelay ,
817: .Dq throughput ,
818: .Dq reliability ,
819: or a numeric value.
1.131 djm 820: This option may take one or two arguments, separated by whitespace.
1.129 djm 821: If one argument is specified, it is used as the packet class unconditionally.
822: If two values are specified, the first is automatically selected for
823: interactive sessions and the second for non-interactive sessions.
824: The default is
825: .Dq lowdelay
826: for interactive sessions and
827: .Dq throughput
828: for non-interactive sessions.
1.171 jmc 829: .It Cm KbdInteractiveAuthentication
830: Specifies whether to allow keyboard-interactive authentication.
831: The argument to this keyword must be
832: .Dq yes
833: or
834: .Dq no .
835: The default is to use whatever value
836: .Cm ChallengeResponseAuthentication
837: is set to
838: (by default
839: .Dq yes ) .
1.1 stevesk 840: .It Cm KerberosAuthentication
1.24 markus 841: Specifies whether the password provided by the user for
1.1 stevesk 842: .Cm PasswordAuthentication
1.24 markus 843: will be validated through the Kerberos KDC.
1.1 stevesk 844: To use this option, the server needs a
845: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 846: The default is
1.29 dtucker 847: .Dq no .
848: .It Cm KerberosGetAFSToken
1.45 djm 849: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 850: an AFS token before accessing the user's home directory.
1.52 jmc 851: The default is
1.1 stevesk 852: .Dq no .
853: .It Cm KerberosOrLocalPasswd
1.52 jmc 854: If password authentication through Kerberos fails then
1.1 stevesk 855: the password will be validated via any additional local mechanism
856: such as
857: .Pa /etc/passwd .
1.52 jmc 858: The default is
1.1 stevesk 859: .Dq yes .
860: .It Cm KerberosTicketCleanup
861: Specifies whether to automatically destroy the user's ticket cache
862: file on logout.
1.52 jmc 863: The default is
1.1 stevesk 864: .Dq yes .
1.127 djm 865: .It Cm KexAlgorithms
866: Specifies the available KEX (Key Exchange) algorithms.
867: Multiple algorithms must be comma-separated.
1.208 ! djm 868: Alternately if the specified value begins with a
! 869: .Sq +
! 870: character, then the specified methods will be appended to the default set
! 871: instead of replacing them.
1.173 naddy 872: The supported algorithms are:
873: .Pp
874: .Bl -item -compact -offset indent
875: .It
876: curve25519-sha256@libssh.org
877: .It
878: diffie-hellman-group1-sha1
879: .It
880: diffie-hellman-group14-sha1
881: .It
882: diffie-hellman-group-exchange-sha1
883: .It
884: diffie-hellman-group-exchange-sha256
885: .It
886: ecdh-sha2-nistp256
887: .It
888: ecdh-sha2-nistp384
889: .It
890: ecdh-sha2-nistp521
891: .El
892: .Pp
893: The default is:
1.170 dtucker 894: .Bd -literal -offset indent
895: curve25519-sha256@libssh.org,
896: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
897: diffie-hellman-group-exchange-sha256,
1.173 naddy 898: diffie-hellman-group14-sha1
1.170 dtucker 899: .Ed
1.188 djm 900: .Pp
901: The list of available key exchange algorithms may also be obtained using the
902: .Fl Q
903: option of
904: .Xr ssh 1
905: with an argument of
906: .Dq kex .
1.1 stevesk 907: .It Cm KeyRegenerationInterval
908: In protocol version 1, the ephemeral server key is automatically regenerated
909: after this many seconds (if it has been used).
910: The purpose of regeneration is to prevent
911: decrypting captured sessions by later breaking into the machine and
912: stealing the keys.
913: The key is never stored anywhere.
914: If the value is 0, the key is never regenerated.
915: The default is 3600 (seconds).
916: .It Cm ListenAddress
917: Specifies the local addresses
1.52 jmc 918: .Xr sshd 8
1.1 stevesk 919: should listen on.
920: The following forms may be used:
921: .Pp
922: .Bl -item -offset indent -compact
923: .It
924: .Cm ListenAddress
925: .Sm off
1.180 jmc 926: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 927: .Sm on
928: .It
929: .Cm ListenAddress
930: .Sm off
1.180 jmc 931: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 932: .Sm on
933: .It
934: .Cm ListenAddress
935: .Sm off
936: .Oo
1.180 jmc 937: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 938: .Sm on
939: .El
940: .Pp
941: If
942: .Ar port
943: is not specified,
1.200 dtucker 944: sshd will listen on the address and all
1.1 stevesk 945: .Cm Port
1.17 jmc 946: options specified.
947: The default is to listen on all local addresses.
1.15 jmc 948: Multiple
1.1 stevesk 949: .Cm ListenAddress
1.17 jmc 950: options are permitted.
1.1 stevesk 951: .It Cm LoginGraceTime
952: The server disconnects after this time if the user has not
953: successfully logged in.
954: If the value is 0, there is no time limit.
1.12 stevesk 955: The default is 120 seconds.
1.1 stevesk 956: .It Cm LogLevel
957: Gives the verbosity level that is used when logging messages from
1.53 jmc 958: .Xr sshd 8 .
1.1 stevesk 959: The possible values are:
1.52 jmc 960: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 961: The default is INFO.
962: DEBUG and DEBUG1 are equivalent.
963: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
964: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 965: .It Cm MACs
966: Specifies the available MAC (message authentication code) algorithms.
967: The MAC algorithm is used in protocol version 2
968: for data integrity protection.
969: Multiple algorithms must be comma-separated.
1.208 ! djm 970: If the specified value begins with a
! 971: .Sq +
! 972: character, then the specified algorithms will be appended to the default set
! 973: instead of replacing them.
! 974: .Pp
1.152 markus 975: The algorithms that contain
976: .Dq -etm
977: calculate the MAC after encryption (encrypt-then-mac).
978: These are considered safer and their use recommended.
1.173 naddy 979: The supported MACs are:
980: .Pp
981: .Bl -item -compact -offset indent
982: .It
983: hmac-md5
984: .It
985: hmac-md5-96
986: .It
987: hmac-ripemd160
988: .It
989: hmac-sha1
990: .It
991: hmac-sha1-96
992: .It
993: hmac-sha2-256
994: .It
995: hmac-sha2-512
996: .It
997: umac-64@openssh.com
998: .It
999: umac-128@openssh.com
1000: .It
1001: hmac-md5-etm@openssh.com
1002: .It
1003: hmac-md5-96-etm@openssh.com
1004: .It
1005: hmac-ripemd160-etm@openssh.com
1006: .It
1007: hmac-sha1-etm@openssh.com
1008: .It
1009: hmac-sha1-96-etm@openssh.com
1010: .It
1011: hmac-sha2-256-etm@openssh.com
1012: .It
1013: hmac-sha2-512-etm@openssh.com
1014: .It
1015: umac-64-etm@openssh.com
1016: .It
1017: umac-128-etm@openssh.com
1018: .El
1019: .Pp
1.52 jmc 1020: The default is:
1.77 jmc 1021: .Bd -literal -offset indent
1.152 markus 1022: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1023: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 1024: umac-64@openssh.com,umac-128@openssh.com,
1025: hmac-sha2-256,hmac-sha2-512
1.77 jmc 1026: .Ed
1.188 djm 1027: .Pp
1028: The list of available MAC algorithms may also be obtained using the
1029: .Fl Q
1030: option of
1031: .Xr ssh 1
1032: with an argument of
1033: .Dq mac .
1.60 dtucker 1034: .It Cm Match
1.61 jmc 1035: Introduces a conditional block.
1.65 dtucker 1036: If all of the criteria on the
1.60 dtucker 1037: .Cm Match
1.65 dtucker 1038: line are satisfied, the keywords on the following lines override those
1039: set in the global section of the config file, until either another
1.60 dtucker 1040: .Cm Match
1.65 dtucker 1041: line or the end of the file.
1.172 djm 1042: If a keyword appears in multiple
1043: .Cm Match
1.177 sobrado 1044: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1045: applied.
1.91 djm 1046: .Pp
1.61 jmc 1047: The arguments to
1.60 dtucker 1048: .Cm Match
1.163 dtucker 1049: are one or more criteria-pattern pairs or the single token
1050: .Cm All
1051: which matches all criteria.
1.60 dtucker 1052: The available criteria are
1053: .Cm User ,
1.69 dtucker 1054: .Cm Group ,
1.60 dtucker 1055: .Cm Host ,
1.139 dtucker 1056: .Cm LocalAddress ,
1057: .Cm LocalPort ,
1.60 dtucker 1058: and
1059: .Cm Address .
1.91 djm 1060: The match patterns may consist of single entries or comma-separated
1061: lists and may use the wildcard and negation operators described in the
1.161 jmc 1062: PATTERNS section of
1.92 djm 1063: .Xr ssh_config 5 .
1.91 djm 1064: .Pp
1065: The patterns in an
1066: .Cm Address
1067: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1068: address/masklen format, e.g.\&
1.91 djm 1069: .Dq 192.0.2.0/24
1070: or
1071: .Dq 3ffe:ffff::/32 .
1072: Note that the mask length provided must be consistent with the address -
1073: it is an error to specify a mask length that is too long for the address
1.93 jmc 1074: or one with bits set in this host portion of the address.
1075: For example,
1.91 djm 1076: .Dq 192.0.2.0/33
1077: and
1.93 jmc 1078: .Dq 192.0.2.0/8
1.91 djm 1079: respectively.
1080: .Pp
1.60 dtucker 1081: Only a subset of keywords may be used on the lines following a
1082: .Cm Match
1083: keyword.
1084: Available keywords are
1.142 jmc 1085: .Cm AcceptEnv ,
1.99 okan 1086: .Cm AllowAgentForwarding ,
1.142 jmc 1087: .Cm AllowGroups ,
1.193 djm 1088: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1089: .Cm AllowTcpForwarding ,
1.141 markus 1090: .Cm AllowUsers ,
1.149 djm 1091: .Cm AuthenticationMethods ,
1.146 djm 1092: .Cm AuthorizedKeysCommand ,
1093: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1094: .Cm AuthorizedKeysFile ,
1.123 djm 1095: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1096: .Cm Banner ,
1.85 djm 1097: .Cm ChrootDirectory ,
1.141 markus 1098: .Cm DenyGroups ,
1099: .Cm DenyUsers ,
1.67 dtucker 1100: .Cm ForceCommand ,
1.194 djm 1101: .Cm GatewayPorts ,
1.193 djm 1102: .Cm GSSAPIAuthentication ,
1.189 djm 1103: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1104: .Cm HostbasedAuthentication ,
1.123 djm 1105: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1106: .Cm IPQoS ,
1.74 jmc 1107: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1108: .Cm KerberosAuthentication ,
1.95 dtucker 1109: .Cm MaxAuthTries ,
1.94 dtucker 1110: .Cm MaxSessions ,
1.72 dtucker 1111: .Cm PasswordAuthentication ,
1.97 djm 1112: .Cm PermitEmptyPasswords ,
1.66 dtucker 1113: .Cm PermitOpen ,
1.79 dtucker 1114: .Cm PermitRootLogin ,
1.164 djm 1115: .Cm PermitTTY ,
1.123 djm 1116: .Cm PermitTunnel ,
1.174 djm 1117: .Cm PermitUserRC ,
1.189 djm 1118: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1119: .Cm PubkeyAuthentication ,
1.159 dtucker 1120: .Cm RekeyLimit ,
1.193 djm 1121: .Cm RevokedKeys ,
1.142 jmc 1122: .Cm RhostsRSAAuthentication ,
1.194 djm 1123: .Cm RSAAuthentication ,
1.193 djm 1124: .Cm StreamLocalBindMask ,
1125: .Cm StreamLocalBindUnlink ,
1126: .Cm TrustedUserCAKeys ,
1.66 dtucker 1127: .Cm X11DisplayOffset ,
1.101 djm 1128: .Cm X11Forwarding
1.60 dtucker 1129: and
1.102 djm 1130: .Cm X11UseLocalHost .
1.33 dtucker 1131: .It Cm MaxAuthTries
1132: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1133: connection.
1134: Once the number of failures reaches half this value,
1135: additional failures are logged.
1136: The default is 6.
1.90 djm 1137: .It Cm MaxSessions
1138: Specifies the maximum number of open sessions permitted per network connection.
1139: The default is 10.
1.1 stevesk 1140: .It Cm MaxStartups
1141: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1142: SSH daemon.
1.1 stevesk 1143: Additional connections will be dropped until authentication succeeds or the
1144: .Cm LoginGraceTime
1145: expires for a connection.
1.156 dtucker 1146: The default is 10:30:100.
1.1 stevesk 1147: .Pp
1148: Alternatively, random early drop can be enabled by specifying
1149: the three colon separated values
1150: .Dq start:rate:full
1.51 jmc 1151: (e.g. "10:30:60").
1.53 jmc 1152: .Xr sshd 8
1.1 stevesk 1153: will refuse connection attempts with a probability of
1154: .Dq rate/100
1155: (30%)
1156: if there are currently
1157: .Dq start
1158: (10)
1159: unauthenticated connections.
1160: The probability increases linearly and all connection attempts
1161: are refused if the number of unauthenticated connections reaches
1162: .Dq full
1163: (60).
1164: .It Cm PasswordAuthentication
1165: Specifies whether password authentication is allowed.
1166: The default is
1167: .Dq yes .
1168: .It Cm PermitEmptyPasswords
1169: When password authentication is allowed, it specifies whether the
1170: server allows login to accounts with empty password strings.
1171: The default is
1172: .Dq no .
1.62 dtucker 1173: .It Cm PermitOpen
1174: Specifies the destinations to which TCP port forwarding is permitted.
1175: The forwarding specification must be one of the following forms:
1176: .Pp
1177: .Bl -item -offset indent -compact
1178: .It
1179: .Cm PermitOpen
1180: .Sm off
1181: .Ar host : port
1182: .Sm on
1183: .It
1184: .Cm PermitOpen
1185: .Sm off
1186: .Ar IPv4_addr : port
1187: .Sm on
1188: .It
1189: .Cm PermitOpen
1190: .Sm off
1191: .Ar \&[ IPv6_addr \&] : port
1192: .Sm on
1193: .El
1194: .Pp
1.68 dtucker 1195: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1196: An argument of
1197: .Dq any
1198: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1199: An argument of
1200: .Dq none
1201: can be used to prohibit all forwarding requests.
1.63 jmc 1202: By default all port forwarding requests are permitted.
1.1 stevesk 1203: .It Cm PermitRootLogin
1.38 jmc 1204: Specifies whether root can log in using
1.1 stevesk 1205: .Xr ssh 1 .
1206: The argument must be
1207: .Dq yes ,
1208: .Dq without-password ,
1.52 jmc 1209: .Dq forced-commands-only ,
1.1 stevesk 1210: or
1211: .Dq no .
1212: The default is
1.196 djm 1213: .Dq no .
1.1 stevesk 1214: .Pp
1215: If this option is set to
1.52 jmc 1216: .Dq without-password ,
1.1 stevesk 1217: password authentication is disabled for root.
1218: .Pp
1219: If this option is set to
1.52 jmc 1220: .Dq forced-commands-only ,
1.1 stevesk 1221: root login with public key authentication will be allowed,
1222: but only if the
1223: .Ar command
1224: option has been specified
1225: (which may be useful for taking remote backups even if root login is
1.17 jmc 1226: normally not allowed).
1227: All other authentication methods are disabled for root.
1.1 stevesk 1228: .Pp
1229: If this option is set to
1.52 jmc 1230: .Dq no ,
1.38 jmc 1231: root is not allowed to log in.
1.46 reyk 1232: .It Cm PermitTunnel
1233: Specifies whether
1234: .Xr tun 4
1235: device forwarding is allowed.
1.47 reyk 1236: The argument must be
1237: .Dq yes ,
1.58 stevesk 1238: .Dq point-to-point
1239: (layer 3),
1240: .Dq ethernet
1241: (layer 2), or
1.47 reyk 1242: .Dq no .
1.58 stevesk 1243: Specifying
1244: .Dq yes
1245: permits both
1246: .Dq point-to-point
1247: and
1248: .Dq ethernet .
1.46 reyk 1249: The default is
1250: .Dq no .
1.178 djm 1251: .Pp
1252: Independent of this setting, the permissions of the selected
1253: .Xr tun 4
1254: device must allow access to the user.
1.164 djm 1255: .It Cm PermitTTY
1256: Specifies whether
1.165 jmc 1257: .Xr pty 4
1.164 djm 1258: allocation is permitted.
1259: The default is
1260: .Dq yes .
1.6 markus 1261: .It Cm PermitUserEnvironment
1262: Specifies whether
1263: .Pa ~/.ssh/environment
1.9 stevesk 1264: and
1.6 markus 1265: .Cm environment=
1266: options in
1267: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1268: are processed by
1.52 jmc 1269: .Xr sshd 8 .
1.6 markus 1270: The default is
1271: .Dq no .
1.9 stevesk 1272: Enabling environment processing may enable users to bypass access
1273: restrictions in some configurations using mechanisms such as
1274: .Ev LD_PRELOAD .
1.174 djm 1275: .It Cm PermitUserRC
1276: Specifies whether any
1277: .Pa ~/.ssh/rc
1278: file is executed.
1279: The default is
1280: .Dq yes .
1.1 stevesk 1281: .It Cm PidFile
1.4 stevesk 1282: Specifies the file that contains the process ID of the
1.195 dtucker 1283: SSH daemon, or
1284: .Dq none
1285: to not write one.
1.1 stevesk 1286: The default is
1287: .Pa /var/run/sshd.pid .
1288: .It Cm Port
1289: Specifies the port number that
1.52 jmc 1290: .Xr sshd 8
1.1 stevesk 1291: listens on.
1292: The default is 22.
1293: Multiple options of this type are permitted.
1294: See also
1295: .Cm ListenAddress .
1296: .It Cm PrintLastLog
1297: Specifies whether
1.52 jmc 1298: .Xr sshd 8
1.36 jaredy 1299: should print the date and time of the last user login when a user logs
1300: in interactively.
1.1 stevesk 1301: The default is
1302: .Dq yes .
1303: .It Cm PrintMotd
1304: Specifies whether
1.52 jmc 1305: .Xr sshd 8
1.1 stevesk 1306: should print
1307: .Pa /etc/motd
1308: when a user logs in interactively.
1309: (On some systems it is also printed by the shell,
1310: .Pa /etc/profile ,
1311: or equivalent.)
1312: The default is
1313: .Dq yes .
1314: .It Cm Protocol
1315: Specifies the protocol versions
1.52 jmc 1316: .Xr sshd 8
1.5 stevesk 1317: supports.
1.1 stevesk 1318: The possible values are
1.52 jmc 1319: .Sq 1
1.1 stevesk 1320: and
1.52 jmc 1321: .Sq 2 .
1.1 stevesk 1322: Multiple versions must be comma-separated.
1323: The default is
1.109 jmc 1324: .Sq 2 .
1.5 stevesk 1325: Note that the order of the protocol list does not indicate preference,
1326: because the client selects among multiple protocol versions offered
1327: by the server.
1328: Specifying
1329: .Dq 2,1
1330: is identical to
1331: .Dq 1,2 .
1.189 djm 1332: .It Cm PubkeyAcceptedKeyTypes
1333: Specifies the key types that will be accepted for public key authentication
1334: as a comma-separated pattern list.
1.208 ! djm 1335: Alternately if the specified value begins with a
! 1336: .Sq +
! 1337: character, then the specified key types will be appended to the default set
! 1338: instead of replacing them.
1.206 markus 1339: The default for this option is:
1340: .Bd -literal -offset 3n
1341: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1342: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1343: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1344: ssh-ed25519-cert-v01@openssh.com,
1345: ssh-rsa-cert-v01@openssh.com,
1346: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1347: ssh-ed25519,ssh-rsa
1348: .Ed
1349: .Pp
1.189 djm 1350: The
1351: .Fl Q
1352: option of
1353: .Xr ssh 1
1354: may be used to list supported key types.
1.1 stevesk 1355: .It Cm PubkeyAuthentication
1356: Specifies whether public key authentication is allowed.
1357: The default is
1358: .Dq yes .
1359: Note that this option applies to protocol version 2 only.
1.159 dtucker 1360: .It Cm RekeyLimit
1361: Specifies the maximum amount of data that may be transmitted before the
1362: session key is renegotiated, optionally followed a maximum amount of
1363: time that may pass before the session key is renegotiated.
1364: The first argument is specified in bytes and may have a suffix of
1365: .Sq K ,
1366: .Sq M ,
1367: or
1368: .Sq G
1369: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1370: The default is between
1371: .Sq 1G
1372: and
1373: .Sq 4G ,
1374: depending on the cipher.
1375: The optional second value is specified in seconds and may use any of the
1376: units documented in the
1377: .Sx TIME FORMATS
1.160 jmc 1378: section.
1.159 dtucker 1379: The default value for
1380: .Cm RekeyLimit
1381: is
1382: .Dq default none ,
1383: which means that rekeying is performed after the cipher's default amount
1384: of data has been sent or received and no time based rekeying is done.
1385: This option applies to protocol version 2 only.
1.118 djm 1386: .It Cm RevokedKeys
1.195 dtucker 1387: Specifies revoked public keys file, or
1388: .Dq none
1389: to not use one.
1.118 djm 1390: Keys listed in this file will be refused for public key authentication.
1391: Note that if this file is not readable, then public key authentication will
1392: be refused for all users.
1.154 djm 1393: Keys may be specified as a text file, listing one public key per line, or as
1394: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1395: .Xr ssh-keygen 1 .
1.161 jmc 1396: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1397: .Xr ssh-keygen 1 .
1.1 stevesk 1398: .It Cm RhostsRSAAuthentication
1399: Specifies whether rhosts or /etc/hosts.equiv authentication together
1400: with successful RSA host authentication is allowed.
1401: The default is
1402: .Dq no .
1403: This option applies to protocol version 1 only.
1404: .It Cm RSAAuthentication
1405: Specifies whether pure RSA authentication is allowed.
1406: The default is
1407: .Dq yes .
1408: This option applies to protocol version 1 only.
1409: .It Cm ServerKeyBits
1410: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1411: The default and minimum value is 1024.
1.175 millert 1412: .It Cm StreamLocalBindMask
1413: Sets the octal file creation mode mask
1414: .Pq umask
1415: used when creating a Unix-domain socket file for local or remote
1416: port forwarding.
1417: This option is only used for port forwarding to a Unix-domain socket file.
1418: .Pp
1419: The default value is 0177, which creates a Unix-domain socket file that is
1420: readable and writable only by the owner.
1421: Note that not all operating systems honor the file mode on Unix-domain
1422: socket files.
1423: .It Cm StreamLocalBindUnlink
1424: Specifies whether to remove an existing Unix-domain socket file for local
1425: or remote port forwarding before creating a new one.
1426: If the socket file already exists and
1427: .Cm StreamLocalBindUnlink
1428: is not enabled,
1429: .Nm sshd
1430: will be unable to forward the port to the Unix-domain socket file.
1431: This option is only used for port forwarding to a Unix-domain socket file.
1432: .Pp
1433: The argument must be
1434: .Dq yes
1435: or
1436: .Dq no .
1437: The default is
1438: .Dq no .
1.1 stevesk 1439: .It Cm StrictModes
1440: Specifies whether
1.52 jmc 1441: .Xr sshd 8
1.1 stevesk 1442: should check file modes and ownership of the
1443: user's files and home directory before accepting login.
1444: This is normally desirable because novices sometimes accidentally leave their
1445: directory or files world-writable.
1446: The default is
1447: .Dq yes .
1.112 djm 1448: Note that this does not apply to
1449: .Cm ChrootDirectory ,
1450: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1451: .It Cm Subsystem
1.51 jmc 1452: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1453: Arguments should be a subsystem name and a command (with optional arguments)
1454: to execute upon subsystem request.
1.80 djm 1455: .Pp
1.1 stevesk 1456: The command
1457: .Xr sftp-server 8
1458: implements the
1459: .Dq sftp
1460: file transfer subsystem.
1.80 djm 1461: .Pp
1462: Alternately the name
1463: .Dq internal-sftp
1464: implements an in-process
1465: .Dq sftp
1466: server.
1467: This may simplify configurations using
1468: .Cm ChrootDirectory
1469: to force a different filesystem root on clients.
1470: .Pp
1.1 stevesk 1471: By default no subsystems are defined.
1472: Note that this option applies to protocol version 2 only.
1473: .It Cm SyslogFacility
1474: Gives the facility code that is used when logging messages from
1.53 jmc 1475: .Xr sshd 8 .
1.1 stevesk 1476: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1477: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1478: The default is AUTH.
1.27 markus 1479: .It Cm TCPKeepAlive
1480: Specifies whether the system should send TCP keepalive messages to the
1481: other side.
1482: If they are sent, death of the connection or crash of one
1483: of the machines will be properly noticed.
1484: However, this means that
1485: connections will die if the route is down temporarily, and some people
1486: find it annoying.
1487: On the other hand, if TCP keepalives are not sent,
1488: sessions may hang indefinitely on the server, leaving
1489: .Dq ghost
1490: users and consuming server resources.
1491: .Pp
1492: The default is
1493: .Dq yes
1494: (to send TCP keepalive messages), and the server will notice
1495: if the network goes down or the client host crashes.
1496: This avoids infinitely hanging sessions.
1497: .Pp
1498: To disable TCP keepalive messages, the value should be set to
1499: .Dq no .
1.118 djm 1500: .It Cm TrustedUserCAKeys
1501: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1502: trusted to sign user certificates for authentication, or
1503: .Dq none
1504: to not use one.
1.119 jmc 1505: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1506: .Ql #
1507: are allowed.
1508: If a certificate is presented for authentication and has its signing CA key
1509: listed in this file, then it may be used for authentication for any user
1510: listed in the certificate's principals list.
1511: Note that certificates that lack a list of principals will not be permitted
1512: for authentication using
1513: .Cm TrustedUserCAKeys .
1.161 jmc 1514: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1515: .Xr ssh-keygen 1 .
1.18 markus 1516: .It Cm UseDNS
1517: Specifies whether
1.52 jmc 1518: .Xr sshd 8
1.207 djm 1519: should look up the remote host name, and to check that
1.18 markus 1520: the resolved host name for the remote IP address maps back to the
1521: very same IP address.
1.207 djm 1522: .Pp
1523: If this option is set to
1524: .Dq no
1525: (the default) then only addresses and not host names may be used in
1526: .Pa ~/.ssh/known_hosts
1527: .Cm from
1528: and
1529: .Xr sshd_config 5
1530: .Cm Match
1531: .Cm Host
1532: directives.
1.1 stevesk 1533: .It Cm UseLogin
1534: Specifies whether
1535: .Xr login 1
1536: is used for interactive login sessions.
1537: The default is
1538: .Dq no .
1539: Note that
1540: .Xr login 1
1541: is never used for remote command execution.
1542: Note also, that if this is enabled,
1543: .Cm X11Forwarding
1544: will be disabled because
1545: .Xr login 1
1546: does not know how to handle
1547: .Xr xauth 1
1.15 jmc 1548: cookies.
1549: If
1.1 stevesk 1550: .Cm UsePrivilegeSeparation
1551: is specified, it will be disabled after authentication.
1552: .It Cm UsePrivilegeSeparation
1553: Specifies whether
1.52 jmc 1554: .Xr sshd 8
1.2 stevesk 1555: separates privileges by creating an unprivileged child process
1.15 jmc 1556: to deal with incoming network traffic.
1557: After successful authentication, another process will be created that has
1558: the privilege of the authenticated user.
1559: The goal of privilege separation is to prevent privilege
1.1 stevesk 1560: escalation by containing any corruption within the unprivileged processes.
1561: The default is
1562: .Dq yes .
1.134 djm 1563: If
1564: .Cm UsePrivilegeSeparation
1565: is set to
1566: .Dq sandbox
1567: then the pre-authentication unprivileged process is subject to additional
1568: restrictions.
1.137 djm 1569: .It Cm VersionAddendum
1570: Optionally specifies additional text to append to the SSH protocol banner
1571: sent by the server upon connection.
1572: The default is
1573: .Dq none .
1.1 stevesk 1574: .It Cm X11DisplayOffset
1575: Specifies the first display number available for
1.52 jmc 1576: .Xr sshd 8 Ns 's
1.1 stevesk 1577: X11 forwarding.
1.52 jmc 1578: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1579: The default is 10.
1580: .It Cm X11Forwarding
1581: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1582: The argument must be
1583: .Dq yes
1584: or
1585: .Dq no .
1.1 stevesk 1586: The default is
1587: .Dq no .
1.13 stevesk 1588: .Pp
1589: When X11 forwarding is enabled, there may be additional exposure to
1590: the server and to client displays if the
1.52 jmc 1591: .Xr sshd 8
1.13 stevesk 1592: proxy display is configured to listen on the wildcard address (see
1593: .Cm X11UseLocalhost
1.52 jmc 1594: below), though this is not the default.
1.13 stevesk 1595: Additionally, the authentication spoofing and authentication data
1596: verification and substitution occur on the client side.
1597: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1598: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1599: forwarding (see the warnings for
1600: .Cm ForwardX11
1601: in
1.19 jmc 1602: .Xr ssh_config 5 ) .
1.13 stevesk 1603: A system administrator may have a stance in which they want to
1604: protect clients that may expose themselves to attack by unwittingly
1605: requesting X11 forwarding, which can warrant a
1606: .Dq no
1607: setting.
1608: .Pp
1609: Note that disabling X11 forwarding does not prevent users from
1610: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1611: X11 forwarding is automatically disabled if
1612: .Cm UseLogin
1613: is enabled.
1614: .It Cm X11UseLocalhost
1615: Specifies whether
1.52 jmc 1616: .Xr sshd 8
1.1 stevesk 1617: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1618: the wildcard address.
1619: By default,
1.52 jmc 1620: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1621: hostname part of the
1622: .Ev DISPLAY
1623: environment variable to
1624: .Dq localhost .
1.8 stevesk 1625: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1626: However, some older X11 clients may not function with this
1627: configuration.
1628: .Cm X11UseLocalhost
1629: may be set to
1630: .Dq no
1631: to specify that the forwarding server should be bound to the wildcard
1632: address.
1633: The argument must be
1634: .Dq yes
1635: or
1636: .Dq no .
1637: The default is
1638: .Dq yes .
1639: .It Cm XAuthLocation
1.11 stevesk 1640: Specifies the full pathname of the
1.1 stevesk 1641: .Xr xauth 1
1.195 dtucker 1642: program, or
1643: .Dq none
1644: to not use one.
1.1 stevesk 1645: The default is
1646: .Pa /usr/X11R6/bin/xauth .
1647: .El
1.55 jmc 1648: .Sh TIME FORMATS
1.53 jmc 1649: .Xr sshd 8
1.1 stevesk 1650: command-line arguments and configuration file options that specify time
1651: may be expressed using a sequence of the form:
1652: .Sm off
1.7 stevesk 1653: .Ar time Op Ar qualifier ,
1.1 stevesk 1654: .Sm on
1655: where
1656: .Ar time
1657: is a positive integer value and
1658: .Ar qualifier
1659: is one of the following:
1660: .Pp
1661: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1662: .It Aq Cm none
1.1 stevesk 1663: seconds
1664: .It Cm s | Cm S
1665: seconds
1666: .It Cm m | Cm M
1667: minutes
1668: .It Cm h | Cm H
1669: hours
1670: .It Cm d | Cm D
1671: days
1672: .It Cm w | Cm W
1673: weeks
1674: .El
1675: .Pp
1676: Each member of the sequence is added together to calculate
1677: the total time value.
1678: .Pp
1679: Time format examples:
1680: .Pp
1681: .Bl -tag -width Ds -compact -offset indent
1682: .It 600
1683: 600 seconds (10 minutes)
1684: .It 10m
1685: 10 minutes
1686: .It 1h30m
1687: 1 hour 30 minutes (90 minutes)
1688: .El
1689: .Sh FILES
1690: .Bl -tag -width Ds
1691: .It Pa /etc/ssh/sshd_config
1692: Contains configuration data for
1.53 jmc 1693: .Xr sshd 8 .
1.1 stevesk 1694: This file should be writable by root only, but it is recommended
1695: (though not necessary) that it be world-readable.
1696: .El
1.19 jmc 1697: .Sh SEE ALSO
1698: .Xr sshd 8
1.1 stevesk 1699: .Sh AUTHORS
1700: OpenSSH is a derivative of the original and free
1701: ssh 1.2.12 release by Tatu Ylonen.
1702: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1703: Theo de Raadt and Dug Song
1704: removed many bugs, re-added newer features and
1705: created OpenSSH.
1706: Markus Friedl contributed the support for SSH
1707: protocol versions 1.5 and 2.0.
1708: Niels Provos and Markus Friedl contributed support
1709: for privilege separation.
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