Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.219
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
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1.219 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.218 2016/02/16 05:11:04 djm Exp $
! 37: .Dd $Mdocdate: February 16 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.
1.54 jmc 176: The allow/deny directives are processed in the following order:
177: .Cm DenyUsers ,
178: .Cm AllowUsers ,
179: .Cm DenyGroups ,
180: and finally
181: .Cm AllowGroups .
1.49 jmc 182: .Pp
1.161 jmc 183: See PATTERNS in
1.49 jmc 184: .Xr ssh_config 5
185: for more information on patterns.
1.149 djm 186: .It Cm AuthenticationMethods
187: Specifies the authentication methods that must be successfully completed
188: for a user to be granted access.
189: This option must be followed by one or more comma-separated lists of
190: authentication method names.
191: Successful authentication requires completion of every method in at least
192: one of these lists.
193: .Pp
194: For example, an argument of
195: .Dq publickey,password publickey,keyboard-interactive
196: would require the user to complete public key authentication, followed by
197: either password or keyboard interactive authentication.
198: Only methods that are next in one or more lists are offered at each stage,
199: so for this example, it would not be possible to attempt password or
200: keyboard-interactive authentication before public key.
1.157 markus 201: .Pp
202: For keyboard interactive authentication it is also possible to
203: restrict authentication to a specific device by appending a
204: colon followed by the device identifier
205: .Dq bsdauth ,
206: .Dq pam ,
207: or
208: .Dq skey ,
209: depending on the server configuration.
210: For example,
211: .Dq keyboard-interactive:bsdauth
212: would restrict keyboard interactive authentication to the
213: .Dq bsdauth
214: device.
1.185 djm 215: .Pp
216: If the
217: .Dq publickey
1.187 jmc 218: method is listed more than once,
1.185 djm 219: .Xr sshd 8
220: verifies that keys that have been used successfully are not reused for
221: subsequent authentications.
222: For example, an
223: .Cm AuthenticationMethods
224: of
225: .Dq publickey,publickey
226: will require successful authentication using two different public keys.
1.181 djm 227: .Pp
1.219 ! jmc 228: This option will yield a fatal
1.149 djm 229: error if enabled if protocol 1 is also enabled.
230: Note that each authentication method listed should also be explicitly enabled
231: in the configuration.
232: The default is not to require multiple authentication; successful completion
233: of a single authentication method is sufficient.
1.146 djm 234: .It Cm AuthorizedKeysCommand
1.147 jmc 235: Specifies a program to be used to look up the user's public keys.
1.201 djm 236: The program must be owned by root, not writable by group or others and
237: specified by an absolute path.
238: .Pp
239: Arguments to
240: .Cm AuthorizedKeysCommand
241: may be provided using the following tokens, which will be expanded
242: at runtime: %% is replaced by a literal '%', %u is replaced by the
243: username being authenticated, %h is replaced by the home directory
244: of the user being authenticated, %t is replaced with the key type
245: offered for authentication, %f is replaced with the fingerprint of
246: the key, and %k is replaced with the key being offered for authentication.
247: If no arguments are specified then the username of the target user
248: will be supplied.
249: .Pp
250: The program should produce on standard output zero or
1.161 jmc 251: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 252: .Xr sshd 8 ) .
1.146 djm 253: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
254: and authorize the user then public key authentication continues using the usual
255: .Cm AuthorizedKeysFile
256: files.
257: By default, no AuthorizedKeysCommand is run.
258: .It Cm AuthorizedKeysCommandUser
259: Specifies the user under whose account the AuthorizedKeysCommand is run.
260: It is recommended to use a dedicated user that has no other role on the host
261: than running authorized keys commands.
1.186 djm 262: If
1.181 djm 263: .Cm AuthorizedKeysCommand
1.186 djm 264: is specified but
265: .Cm AuthorizedKeysCommandUser
266: is not, then
267: .Xr sshd 8
268: will refuse to start.
1.1 stevesk 269: .It Cm AuthorizedKeysFile
270: Specifies the file that contains the public keys that can be used
271: for user authentication.
1.124 djm 272: The format is described in the
1.161 jmc 273: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 274: section of
275: .Xr sshd 8 .
1.1 stevesk 276: .Cm AuthorizedKeysFile
277: may contain tokens of the form %T which are substituted during connection
1.52 jmc 278: setup.
1.17 jmc 279: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 280: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 281: %u is replaced by the username of that user.
282: After expansion,
283: .Cm AuthorizedKeysFile
284: is taken to be an absolute path or one relative to the user's home
285: directory.
1.133 jmc 286: Multiple files may be listed, separated by whitespace.
1.212 djm 287: Alternately this option may be set to
288: .Dq none
289: to skip checking for user keys in files.
1.133 jmc 290: The default is
291: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 292: .It Cm AuthorizedPrincipalsCommand
293: Specifies a program to be used to generate the list of allowed
294: certificate principals as per
295: .Cm AuthorizedPrincipalsFile .
296: The program must be owned by root, not writable by group or others and
297: specified by an absolute path.
298: .Pp
299: Arguments to
300: .Cm AuthorizedPrincipalsCommand
301: may be provided using the following tokens, which will be expanded
302: at runtime: %% is replaced by a literal '%', %u is replaced by the
303: username being authenticated and %h is replaced by the home directory
304: of the user being authenticated.
305: .Pp
306: The program should produce on standard output zero or
307: more lines of
308: .Cm AuthorizedPrincipalsFile
309: output.
310: If either
311: .Cm AuthorizedPrincipalsCommand
312: or
313: .Cm AuthorizedPrincipalsFile
314: is specified, then certificates offered by the client for authentication
315: must contain a principal that is listed.
316: By default, no AuthorizedPrincipalsCommand is run.
317: .It Cm AuthorizedPrincipalsCommandUser
318: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
319: It is recommended to use a dedicated user that has no other role on the host
320: than running authorized principals commands.
321: If
322: .Cm AuthorizedPrincipalsCommand
323: is specified but
324: .Cm AuthorizedPrincipalsCommandUser
325: is not, then
326: .Xr sshd 8
327: will refuse to start.
1.121 djm 328: .It Cm AuthorizedPrincipalsFile
329: Specifies a file that lists principal names that are accepted for
330: certificate authentication.
331: When using certificates signed by a key listed in
332: .Cm TrustedUserCAKeys ,
333: this file lists names, one of which must appear in the certificate for it
334: to be accepted for authentication.
1.125 jmc 335: Names are listed one per line preceded by key options (as described
1.161 jmc 336: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 337: .Xr sshd 8 ) .
1.124 djm 338: Empty lines and comments starting with
1.121 djm 339: .Ql #
340: are ignored.
341: .Pp
342: .Cm AuthorizedPrincipalsFile
343: may contain tokens of the form %T which are substituted during connection
344: setup.
345: The following tokens are defined: %% is replaced by a literal '%',
346: %h is replaced by the home directory of the user being authenticated, and
347: %u is replaced by the username of that user.
348: After expansion,
349: .Cm AuthorizedPrincipalsFile
350: is taken to be an absolute path or one relative to the user's home
351: directory.
352: .Pp
1.138 djm 353: The default is
354: .Dq none ,
355: i.e. not to use a principals file \(en in this case, the username
1.121 djm 356: of the user must appear in a certificate's principals list for it to be
357: accepted.
358: Note that
359: .Cm AuthorizedPrincipalsFile
360: is only used when authentication proceeds using a CA listed in
361: .Cm TrustedUserCAKeys
362: and is not consulted for certification authorities trusted via
363: .Pa ~/.ssh/authorized_keys ,
364: though the
365: .Cm principals=
366: key option offers a similar facility (see
367: .Xr sshd 8
368: for details).
1.1 stevesk 369: .It Cm Banner
370: The contents of the specified file are sent to the remote user before
371: authentication is allowed.
1.78 djm 372: If the argument is
373: .Dq none
374: then no banner is displayed.
1.1 stevesk 375: By default, no banner is displayed.
376: .It Cm ChallengeResponseAuthentication
1.50 jmc 377: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 378: All authentication styles from
379: .Xr login.conf 5
380: are supported.
381: The default is
382: .Dq yes .
1.80 djm 383: .It Cm ChrootDirectory
1.113 stevesk 384: Specifies the pathname of a directory to
1.80 djm 385: .Xr chroot 2
386: to after authentication.
1.190 deraadt 387: At session startup
388: .Xr sshd 8
389: checks that all components of the pathname are root-owned directories
390: which are not writable by any other user or group.
1.106 stevesk 391: After the chroot,
392: .Xr sshd 8
393: changes the working directory to the user's home directory.
1.80 djm 394: .Pp
1.113 stevesk 395: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 396: the connecting user has been authenticated: %% is replaced by a literal '%',
397: %h is replaced by the home directory of the user being authenticated, and
398: %u is replaced by the username of that user.
399: .Pp
400: The
401: .Cm ChrootDirectory
402: must contain the necessary files and directories to support the
1.103 stevesk 403: user's session.
1.80 djm 404: For an interactive session this requires at least a shell, typically
405: .Xr sh 1 ,
406: and basic
407: .Pa /dev
408: nodes such as
409: .Xr null 4 ,
410: .Xr zero 4 ,
411: .Xr stdin 4 ,
412: .Xr stdout 4 ,
413: .Xr stderr 4 ,
1.180 jmc 414: and
1.80 djm 415: .Xr tty 4
416: devices.
417: For file transfer sessions using
1.105 jmc 418: .Dq sftp ,
1.80 djm 419: no additional configuration of the environment is necessary if the
1.105 jmc 420: in-process sftp server is used,
1.176 schwarze 421: though sessions which use logging may require
1.104 stevesk 422: .Pa /dev/log
1.176 schwarze 423: inside the chroot directory on some operating systems (see
1.104 stevesk 424: .Xr sftp-server 8
1.81 jmc 425: for details).
1.190 deraadt 426: .Pp
1.191 jmc 427: For safety, it is very important that the directory hierarchy be
1.190 deraadt 428: prevented from modification by other processes on the system (especially
429: those outside the jail).
430: Misconfiguration can lead to unsafe environments which
431: .Xr sshd 8
432: cannot detect.
1.80 djm 433: .Pp
1.215 djm 434: The default is
435: .Dq none ,
436: indicating not to
1.80 djm 437: .Xr chroot 2 .
1.1 stevesk 438: .It Cm Ciphers
1.219 ! jmc 439: Specifies the ciphers allowed.
1.1 stevesk 440: Multiple ciphers must be comma-separated.
1.208 djm 441: If the specified value begins with a
442: .Sq +
443: character, then the specified ciphers will be appended to the default set
444: instead of replacing them.
445: .Pp
1.167 djm 446: The supported ciphers are:
447: .Pp
1.173 naddy 448: .Bl -item -compact -offset indent
449: .It
450: 3des-cbc
451: .It
452: aes128-cbc
453: .It
454: aes192-cbc
455: .It
456: aes256-cbc
457: .It
458: aes128-ctr
459: .It
460: aes192-ctr
461: .It
462: aes256-ctr
463: .It
464: aes128-gcm@openssh.com
465: .It
466: aes256-gcm@openssh.com
467: .It
468: arcfour
469: .It
470: arcfour128
471: .It
472: arcfour256
473: .It
474: blowfish-cbc
475: .It
476: cast128-cbc
477: .It
478: chacha20-poly1305@openssh.com
479: .El
1.167 djm 480: .Pp
1.52 jmc 481: The default is:
1.173 naddy 482: .Bd -literal -offset indent
1.211 jmc 483: chacha20-poly1305@openssh.com,
1.173 naddy 484: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 485: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 486: .Ed
1.167 djm 487: .Pp
488: The list of available ciphers may also be obtained using the
489: .Fl Q
490: option of
1.188 djm 491: .Xr ssh 1
492: with an argument of
493: .Dq cipher .
1.1 stevesk 494: .It Cm ClientAliveCountMax
1.48 jmc 495: Sets the number of client alive messages (see below) which may be
1.1 stevesk 496: sent without
1.52 jmc 497: .Xr sshd 8
1.17 jmc 498: receiving any messages back from the client.
499: If this threshold is reached while client alive messages are being sent,
1.52 jmc 500: sshd will disconnect the client, terminating the session.
1.17 jmc 501: It is important to note that the use of client alive messages is very
502: different from
1.27 markus 503: .Cm TCPKeepAlive
1.17 jmc 504: (below).
505: The client alive messages are sent through the encrypted channel
506: and therefore will not be spoofable.
507: The TCP keepalive option enabled by
1.27 markus 508: .Cm TCPKeepAlive
1.17 jmc 509: is spoofable.
510: The client alive mechanism is valuable when the client or
1.1 stevesk 511: server depend on knowing when a connection has become inactive.
512: .Pp
1.17 jmc 513: The default value is 3.
514: If
1.1 stevesk 515: .Cm ClientAliveInterval
1.48 jmc 516: (see below) is set to 15, and
1.1 stevesk 517: .Cm ClientAliveCountMax
1.52 jmc 518: is left at the default, unresponsive SSH clients
1.1 stevesk 519: will be disconnected after approximately 45 seconds.
1.42 djm 520: .It Cm ClientAliveInterval
521: Sets a timeout interval in seconds after which if no data has been received
522: from the client,
1.52 jmc 523: .Xr sshd 8
1.42 djm 524: will send a message through the encrypted
525: channel to request a response from the client.
526: The default
527: is 0, indicating that these messages will not be sent to the client.
1.3 markus 528: .It Cm Compression
1.44 markus 529: Specifies whether compression is allowed, or delayed until
530: the user has authenticated successfully.
1.3 markus 531: The argument must be
1.44 markus 532: .Dq yes ,
533: .Dq delayed ,
1.3 markus 534: or
535: .Dq no .
536: The default is
1.44 markus 537: .Dq delayed .
1.1 stevesk 538: .It Cm DenyGroups
539: This keyword can be followed by a list of group name patterns, separated
540: by spaces.
541: Login is disallowed for users whose primary group or supplementary
542: group list matches one of the patterns.
543: Only group names are valid; a numerical group ID is not recognized.
544: By default, login is allowed for all groups.
1.54 jmc 545: The allow/deny directives are processed in the following order:
546: .Cm DenyUsers ,
547: .Cm AllowUsers ,
548: .Cm DenyGroups ,
549: and finally
550: .Cm AllowGroups .
1.49 jmc 551: .Pp
1.161 jmc 552: See PATTERNS in
1.49 jmc 553: .Xr ssh_config 5
554: for more information on patterns.
1.1 stevesk 555: .It Cm DenyUsers
556: This keyword can be followed by a list of user name patterns, separated
557: by spaces.
558: Login is disallowed for user names that match one of the patterns.
559: Only user names are valid; a numerical user ID is not recognized.
560: By default, login is allowed for all users.
561: If the pattern takes the form USER@HOST then USER and HOST
562: are separately checked, restricting logins to particular
563: users from particular hosts.
1.54 jmc 564: The allow/deny directives are processed in the following order:
565: .Cm DenyUsers ,
566: .Cm AllowUsers ,
567: .Cm DenyGroups ,
568: and finally
569: .Cm AllowGroups .
1.49 jmc 570: .Pp
1.161 jmc 571: See PATTERNS in
1.49 jmc 572: .Xr ssh_config 5
573: for more information on patterns.
1.183 djm 574: .It Cm FingerprintHash
575: Specifies the hash algorithm used when logging key fingerprints.
576: Valid options are:
577: .Dq md5
578: and
579: .Dq sha256 .
580: The default is
581: .Dq sha256 .
1.67 dtucker 582: .It Cm ForceCommand
583: Forces the execution of the command specified by
584: .Cm ForceCommand ,
1.84 djm 585: ignoring any command supplied by the client and
586: .Pa ~/.ssh/rc
587: if present.
1.67 dtucker 588: The command is invoked by using the user's login shell with the -c option.
589: This applies to shell, command, or subsystem execution.
590: It is most useful inside a
591: .Cm Match
592: block.
593: The command originally supplied by the client is available in the
594: .Ev SSH_ORIGINAL_COMMAND
595: environment variable.
1.82 djm 596: Specifying a command of
597: .Dq internal-sftp
598: will force the use of an in-process sftp server that requires no support
599: files when used with
600: .Cm ChrootDirectory .
1.215 djm 601: The default is
602: .Dq none .
1.1 stevesk 603: .It Cm GatewayPorts
604: Specifies whether remote hosts are allowed to connect to ports
605: forwarded for the client.
606: By default,
1.52 jmc 607: .Xr sshd 8
1.15 jmc 608: binds remote port forwardings to the loopback address.
609: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 610: .Cm GatewayPorts
1.52 jmc 611: can be used to specify that sshd
1.39 djm 612: should allow remote port forwardings to bind to non-loopback addresses, thus
613: allowing other hosts to connect.
614: The argument may be
615: .Dq no
616: to force remote port forwardings to be available to the local host only,
1.1 stevesk 617: .Dq yes
1.39 djm 618: to force remote port forwardings to bind to the wildcard address, or
619: .Dq clientspecified
620: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 621: The default is
622: .Dq no .
1.23 markus 623: .It Cm GSSAPIAuthentication
1.25 markus 624: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 625: The default is
1.23 markus 626: .Dq no .
627: .It Cm GSSAPICleanupCredentials
628: Specifies whether to automatically destroy the user's credentials cache
629: on logout.
630: The default is
631: .Dq yes .
1.203 djm 632: .It Cm GSSAPIStrictAcceptorCheck
633: Determines whether to be strict about the identity of the GSSAPI acceptor
634: a client authenticates against.
635: If set to
636: .Dq yes
637: then the client must authenticate against the
638: .Pa host
639: service on the current hostname.
640: If set to
641: .Dq no
642: then the client may authenticate against any service key stored in the
643: machine's default store.
644: This facility is provided to assist with operation on multi homed machines.
645: The default is
646: .Dq yes .
1.189 djm 647: .It Cm HostbasedAcceptedKeyTypes
648: Specifies the key types that will be accepted for hostbased authentication
649: as a comma-separated pattern list.
1.208 djm 650: Alternately if the specified value begins with a
651: .Sq +
652: character, then the specified key types will be appended to the default set
653: instead of replacing them.
1.206 markus 654: The default for this option is:
655: .Bd -literal -offset 3n
656: ecdsa-sha2-nistp256-cert-v01@openssh.com,
657: ecdsa-sha2-nistp384-cert-v01@openssh.com,
658: ecdsa-sha2-nistp521-cert-v01@openssh.com,
659: ssh-ed25519-cert-v01@openssh.com,
660: ssh-rsa-cert-v01@openssh.com,
661: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.217 djm 662: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 663: .Ed
664: .Pp
1.189 djm 665: The
666: .Fl Q
667: option of
668: .Xr ssh 1
669: may be used to list supported key types.
1.1 stevesk 670: .It Cm HostbasedAuthentication
671: Specifies whether rhosts or /etc/hosts.equiv authentication together
672: with successful public key client host authentication is allowed
1.50 jmc 673: (host-based authentication).
1.70 dtucker 674: The default is
675: .Dq no .
676: .It Cm HostbasedUsesNameFromPacketOnly
677: Specifies whether or not the server will attempt to perform a reverse
678: name lookup when matching the name in the
679: .Pa ~/.shosts ,
680: .Pa ~/.rhosts ,
681: and
682: .Pa /etc/hosts.equiv
683: files during
684: .Cm HostbasedAuthentication .
685: A setting of
686: .Dq yes
687: means that
688: .Xr sshd 8
689: uses the name supplied by the client rather than
690: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 691: The default is
692: .Dq no .
1.117 djm 693: .It Cm HostCertificate
694: Specifies a file containing a public host certificate.
695: The certificate's public key must match a private host key already specified
696: by
697: .Cm HostKey .
698: The default behaviour of
699: .Xr sshd 8
700: is not to load any certificates.
1.1 stevesk 701: .It Cm HostKey
702: Specifies a file containing a private host key
703: used by SSH.
704: The default is
705: .Pa /etc/ssh/ssh_host_key
706: for protocol version 1, and
1.126 djm 707: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 708: .Pa /etc/ssh/ssh_host_ecdsa_key ,
709: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 710: and
1.1 stevesk 711: .Pa /etc/ssh/ssh_host_rsa_key
712: for protocol version 2.
1.206 markus 713: .Pp
1.1 stevesk 714: Note that
1.52 jmc 715: .Xr sshd 8
1.206 markus 716: will refuse to use a file if it is group/world-accessible
717: and that the
718: .Cm HostKeyAlgorithms
719: option restricts which of the keys are actually used by
720: .Xr sshd 8 .
721: .Pp
1.1 stevesk 722: It is possible to have multiple host key files.
723: .Dq rsa1
724: keys are used for version 1 and
1.126 djm 725: .Dq dsa ,
1.169 naddy 726: .Dq ecdsa ,
727: .Dq ed25519
1.1 stevesk 728: or
729: .Dq rsa
730: are used for version 2 of the SSH protocol.
1.162 markus 731: It is also possible to specify public host key files instead.
732: In this case operations on the private key will be delegated
733: to an
734: .Xr ssh-agent 1 .
735: .It Cm HostKeyAgent
736: Identifies the UNIX-domain socket used to communicate
737: with an agent that has access to the private host keys.
738: If
739: .Dq SSH_AUTH_SOCK
740: is specified, the location of the socket will be read from the
741: .Ev SSH_AUTH_SOCK
742: environment variable.
1.206 markus 743: .It Cm HostKeyAlgorithms
1.219 ! jmc 744: Specifies the host key algorithms
1.206 markus 745: that the server offers.
746: The default for this option is:
747: .Bd -literal -offset 3n
748: ecdsa-sha2-nistp256-cert-v01@openssh.com,
749: ecdsa-sha2-nistp384-cert-v01@openssh.com,
750: ecdsa-sha2-nistp521-cert-v01@openssh.com,
751: ssh-ed25519-cert-v01@openssh.com,
752: ssh-rsa-cert-v01@openssh.com,
753: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.217 djm 754: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 755: .Ed
756: .Pp
757: The list of available key types may also be obtained using the
758: .Fl Q
759: option of
760: .Xr ssh 1
761: with an argument of
762: .Dq key .
1.1 stevesk 763: .It Cm IgnoreRhosts
764: Specifies that
765: .Pa .rhosts
766: and
767: .Pa .shosts
768: files will not be used in
769: .Cm RhostsRSAAuthentication
770: or
771: .Cm HostbasedAuthentication .
772: .Pp
773: .Pa /etc/hosts.equiv
774: and
775: .Pa /etc/shosts.equiv
776: are still used.
777: The default is
778: .Dq yes .
779: .It Cm IgnoreUserKnownHosts
780: Specifies whether
1.52 jmc 781: .Xr sshd 8
1.1 stevesk 782: should ignore the user's
1.41 djm 783: .Pa ~/.ssh/known_hosts
1.1 stevesk 784: during
785: .Cm RhostsRSAAuthentication
786: or
787: .Cm HostbasedAuthentication .
788: The default is
789: .Dq no .
1.129 djm 790: .It Cm IPQoS
791: Specifies the IPv4 type-of-service or DSCP class for the connection.
792: Accepted values are
793: .Dq af11 ,
794: .Dq af12 ,
795: .Dq af13 ,
1.136 djm 796: .Dq af21 ,
1.129 djm 797: .Dq af22 ,
798: .Dq af23 ,
799: .Dq af31 ,
800: .Dq af32 ,
801: .Dq af33 ,
802: .Dq af41 ,
803: .Dq af42 ,
804: .Dq af43 ,
805: .Dq cs0 ,
806: .Dq cs1 ,
807: .Dq cs2 ,
808: .Dq cs3 ,
809: .Dq cs4 ,
810: .Dq cs5 ,
811: .Dq cs6 ,
812: .Dq cs7 ,
813: .Dq ef ,
814: .Dq lowdelay ,
815: .Dq throughput ,
816: .Dq reliability ,
817: or a numeric value.
1.131 djm 818: This option may take one or two arguments, separated by whitespace.
1.129 djm 819: If one argument is specified, it is used as the packet class unconditionally.
820: If two values are specified, the first is automatically selected for
821: interactive sessions and the second for non-interactive sessions.
822: The default is
823: .Dq lowdelay
824: for interactive sessions and
825: .Dq throughput
826: for non-interactive sessions.
1.171 jmc 827: .It Cm KbdInteractiveAuthentication
828: Specifies whether to allow keyboard-interactive authentication.
829: The argument to this keyword must be
830: .Dq yes
831: or
832: .Dq no .
833: The default is to use whatever value
834: .Cm ChallengeResponseAuthentication
835: is set to
836: (by default
837: .Dq yes ) .
1.1 stevesk 838: .It Cm KerberosAuthentication
1.24 markus 839: Specifies whether the password provided by the user for
1.1 stevesk 840: .Cm PasswordAuthentication
1.24 markus 841: will be validated through the Kerberos KDC.
1.1 stevesk 842: To use this option, the server needs a
843: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 844: The default is
1.29 dtucker 845: .Dq no .
846: .It Cm KerberosGetAFSToken
1.45 djm 847: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 848: an AFS token before accessing the user's home directory.
1.52 jmc 849: The default is
1.1 stevesk 850: .Dq no .
851: .It Cm KerberosOrLocalPasswd
1.52 jmc 852: If password authentication through Kerberos fails then
1.1 stevesk 853: the password will be validated via any additional local mechanism
854: such as
855: .Pa /etc/passwd .
1.52 jmc 856: The default is
1.1 stevesk 857: .Dq yes .
858: .It Cm KerberosTicketCleanup
859: Specifies whether to automatically destroy the user's ticket cache
860: file on logout.
1.52 jmc 861: The default is
1.1 stevesk 862: .Dq yes .
1.127 djm 863: .It Cm KexAlgorithms
864: Specifies the available KEX (Key Exchange) algorithms.
865: Multiple algorithms must be comma-separated.
1.208 djm 866: Alternately if the specified value begins with a
867: .Sq +
868: character, then the specified methods will be appended to the default set
869: instead of replacing them.
1.173 naddy 870: The supported algorithms are:
871: .Pp
872: .Bl -item -compact -offset indent
873: .It
874: curve25519-sha256@libssh.org
875: .It
876: diffie-hellman-group1-sha1
877: .It
878: diffie-hellman-group14-sha1
879: .It
880: diffie-hellman-group-exchange-sha1
881: .It
882: diffie-hellman-group-exchange-sha256
883: .It
884: ecdh-sha2-nistp256
885: .It
886: ecdh-sha2-nistp384
887: .It
888: ecdh-sha2-nistp521
889: .El
890: .Pp
891: The default is:
1.170 dtucker 892: .Bd -literal -offset indent
893: curve25519-sha256@libssh.org,
894: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
895: diffie-hellman-group-exchange-sha256,
1.173 naddy 896: diffie-hellman-group14-sha1
1.170 dtucker 897: .Ed
1.188 djm 898: .Pp
899: The list of available key exchange algorithms may also be obtained using the
900: .Fl Q
901: option of
902: .Xr ssh 1
903: with an argument of
904: .Dq kex .
1.1 stevesk 905: .It Cm KeyRegenerationInterval
906: In protocol version 1, the ephemeral server key is automatically regenerated
907: after this many seconds (if it has been used).
908: The purpose of regeneration is to prevent
909: decrypting captured sessions by later breaking into the machine and
910: stealing the keys.
911: The key is never stored anywhere.
912: If the value is 0, the key is never regenerated.
913: The default is 3600 (seconds).
914: .It Cm ListenAddress
915: Specifies the local addresses
1.52 jmc 916: .Xr sshd 8
1.1 stevesk 917: should listen on.
918: The following forms may be used:
919: .Pp
920: .Bl -item -offset indent -compact
921: .It
922: .Cm ListenAddress
923: .Sm off
1.180 jmc 924: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 925: .Sm on
926: .It
927: .Cm ListenAddress
928: .Sm off
1.180 jmc 929: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 930: .Sm on
931: .It
932: .Cm ListenAddress
933: .Sm off
934: .Oo
1.180 jmc 935: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 936: .Sm on
937: .El
938: .Pp
939: If
940: .Ar port
941: is not specified,
1.200 dtucker 942: sshd will listen on the address and all
1.1 stevesk 943: .Cm Port
1.17 jmc 944: options specified.
945: The default is to listen on all local addresses.
1.15 jmc 946: Multiple
1.1 stevesk 947: .Cm ListenAddress
1.17 jmc 948: options are permitted.
1.1 stevesk 949: .It Cm LoginGraceTime
950: The server disconnects after this time if the user has not
951: successfully logged in.
952: If the value is 0, there is no time limit.
1.12 stevesk 953: The default is 120 seconds.
1.1 stevesk 954: .It Cm LogLevel
955: Gives the verbosity level that is used when logging messages from
1.53 jmc 956: .Xr sshd 8 .
1.1 stevesk 957: The possible values are:
1.52 jmc 958: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 959: The default is INFO.
960: DEBUG and DEBUG1 are equivalent.
961: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
962: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 963: .It Cm MACs
964: Specifies the available MAC (message authentication code) algorithms.
1.219 ! jmc 965: The MAC algorithm is used for data integrity protection.
1.1 stevesk 966: Multiple algorithms must be comma-separated.
1.208 djm 967: If the specified value begins with a
968: .Sq +
969: character, then the specified algorithms will be appended to the default set
970: instead of replacing them.
971: .Pp
1.152 markus 972: The algorithms that contain
973: .Dq -etm
974: calculate the MAC after encryption (encrypt-then-mac).
975: These are considered safer and their use recommended.
1.173 naddy 976: The supported MACs are:
977: .Pp
978: .Bl -item -compact -offset indent
979: .It
980: hmac-md5
981: .It
982: hmac-md5-96
983: .It
984: hmac-ripemd160
985: .It
986: hmac-sha1
987: .It
988: hmac-sha1-96
989: .It
990: hmac-sha2-256
991: .It
992: hmac-sha2-512
993: .It
994: umac-64@openssh.com
995: .It
996: umac-128@openssh.com
997: .It
998: hmac-md5-etm@openssh.com
999: .It
1000: hmac-md5-96-etm@openssh.com
1001: .It
1002: hmac-ripemd160-etm@openssh.com
1003: .It
1004: hmac-sha1-etm@openssh.com
1005: .It
1006: hmac-sha1-96-etm@openssh.com
1007: .It
1008: hmac-sha2-256-etm@openssh.com
1009: .It
1010: hmac-sha2-512-etm@openssh.com
1011: .It
1012: umac-64-etm@openssh.com
1013: .It
1014: umac-128-etm@openssh.com
1015: .El
1016: .Pp
1.52 jmc 1017: The default is:
1.77 jmc 1018: .Bd -literal -offset indent
1.152 markus 1019: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1020: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1021: hmac-sha1-etm@openssh.com,
1.173 naddy 1022: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1023: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1024: .Ed
1.188 djm 1025: .Pp
1026: The list of available MAC algorithms may also be obtained using the
1027: .Fl Q
1028: option of
1029: .Xr ssh 1
1030: with an argument of
1031: .Dq mac .
1.60 dtucker 1032: .It Cm Match
1.61 jmc 1033: Introduces a conditional block.
1.65 dtucker 1034: If all of the criteria on the
1.60 dtucker 1035: .Cm Match
1.65 dtucker 1036: line are satisfied, the keywords on the following lines override those
1037: set in the global section of the config file, until either another
1.60 dtucker 1038: .Cm Match
1.65 dtucker 1039: line or the end of the file.
1.172 djm 1040: If a keyword appears in multiple
1041: .Cm Match
1.177 sobrado 1042: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1043: applied.
1.91 djm 1044: .Pp
1.61 jmc 1045: The arguments to
1.60 dtucker 1046: .Cm Match
1.163 dtucker 1047: are one or more criteria-pattern pairs or the single token
1048: .Cm All
1049: which matches all criteria.
1.60 dtucker 1050: The available criteria are
1051: .Cm User ,
1.69 dtucker 1052: .Cm Group ,
1.60 dtucker 1053: .Cm Host ,
1.139 dtucker 1054: .Cm LocalAddress ,
1055: .Cm LocalPort ,
1.60 dtucker 1056: and
1057: .Cm Address .
1.91 djm 1058: The match patterns may consist of single entries or comma-separated
1059: lists and may use the wildcard and negation operators described in the
1.161 jmc 1060: PATTERNS section of
1.92 djm 1061: .Xr ssh_config 5 .
1.91 djm 1062: .Pp
1063: The patterns in an
1064: .Cm Address
1065: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1066: address/masklen format, e.g.\&
1.91 djm 1067: .Dq 192.0.2.0/24
1068: or
1069: .Dq 3ffe:ffff::/32 .
1070: Note that the mask length provided must be consistent with the address -
1071: it is an error to specify a mask length that is too long for the address
1.93 jmc 1072: or one with bits set in this host portion of the address.
1073: For example,
1.91 djm 1074: .Dq 192.0.2.0/33
1075: and
1.93 jmc 1076: .Dq 192.0.2.0/8
1.91 djm 1077: respectively.
1078: .Pp
1.60 dtucker 1079: Only a subset of keywords may be used on the lines following a
1080: .Cm Match
1081: keyword.
1082: Available keywords are
1.142 jmc 1083: .Cm AcceptEnv ,
1.99 okan 1084: .Cm AllowAgentForwarding ,
1.142 jmc 1085: .Cm AllowGroups ,
1.193 djm 1086: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1087: .Cm AllowTcpForwarding ,
1.141 markus 1088: .Cm AllowUsers ,
1.149 djm 1089: .Cm AuthenticationMethods ,
1.146 djm 1090: .Cm AuthorizedKeysCommand ,
1091: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1092: .Cm AuthorizedKeysFile ,
1.214 djm 1093: .Cm AuthorizedPrincipalsCommand ,
1094: .Cm AuthorizedPrincipalsCommandUser ,
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
1.216 djm 1138: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1139: sessions permitted per network connection.
1140: Multiple sessions may be established by clients that support connection
1141: multiplexing.
1142: Setting
1143: .Cm MaxSessions
1144: to 1 will effectively disable session multiplexing, whereas setting it to 0
1145: will prevent all shell, login and subsystem sessions while still permitting
1146: forwarding.
1.90 djm 1147: The default is 10.
1.1 stevesk 1148: .It Cm MaxStartups
1149: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1150: SSH daemon.
1.1 stevesk 1151: Additional connections will be dropped until authentication succeeds or the
1152: .Cm LoginGraceTime
1153: expires for a connection.
1.156 dtucker 1154: The default is 10:30:100.
1.1 stevesk 1155: .Pp
1156: Alternatively, random early drop can be enabled by specifying
1157: the three colon separated values
1158: .Dq start:rate:full
1.51 jmc 1159: (e.g. "10:30:60").
1.53 jmc 1160: .Xr sshd 8
1.1 stevesk 1161: will refuse connection attempts with a probability of
1162: .Dq rate/100
1163: (30%)
1164: if there are currently
1165: .Dq start
1166: (10)
1167: unauthenticated connections.
1168: The probability increases linearly and all connection attempts
1169: are refused if the number of unauthenticated connections reaches
1170: .Dq full
1171: (60).
1172: .It Cm PasswordAuthentication
1173: Specifies whether password authentication is allowed.
1174: The default is
1175: .Dq yes .
1176: .It Cm PermitEmptyPasswords
1177: When password authentication is allowed, it specifies whether the
1178: server allows login to accounts with empty password strings.
1179: The default is
1180: .Dq no .
1.62 dtucker 1181: .It Cm PermitOpen
1182: Specifies the destinations to which TCP port forwarding is permitted.
1183: The forwarding specification must be one of the following forms:
1184: .Pp
1185: .Bl -item -offset indent -compact
1186: .It
1187: .Cm PermitOpen
1188: .Sm off
1189: .Ar host : port
1190: .Sm on
1191: .It
1192: .Cm PermitOpen
1193: .Sm off
1194: .Ar IPv4_addr : port
1195: .Sm on
1196: .It
1197: .Cm PermitOpen
1198: .Sm off
1199: .Ar \&[ IPv6_addr \&] : port
1200: .Sm on
1201: .El
1202: .Pp
1.68 dtucker 1203: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1204: An argument of
1205: .Dq any
1206: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1207: An argument of
1208: .Dq none
1209: can be used to prohibit all forwarding requests.
1.63 jmc 1210: By default all port forwarding requests are permitted.
1.1 stevesk 1211: .It Cm PermitRootLogin
1.38 jmc 1212: Specifies whether root can log in using
1.1 stevesk 1213: .Xr ssh 1 .
1214: The argument must be
1215: .Dq yes ,
1.210 deraadt 1216: .Dq prohibit-password ,
1.1 stevesk 1217: .Dq without-password ,
1.52 jmc 1218: .Dq forced-commands-only ,
1.1 stevesk 1219: or
1220: .Dq no .
1221: The default is
1.210 deraadt 1222: .Dq prohibit-password .
1.1 stevesk 1223: .Pp
1224: If this option is set to
1.210 deraadt 1225: .Dq prohibit-password
1226: or
1.52 jmc 1227: .Dq without-password ,
1.210 deraadt 1228: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1229: .Pp
1230: If this option is set to
1.52 jmc 1231: .Dq forced-commands-only ,
1.1 stevesk 1232: root login with public key authentication will be allowed,
1233: but only if the
1234: .Ar command
1235: option has been specified
1236: (which may be useful for taking remote backups even if root login is
1.17 jmc 1237: normally not allowed).
1238: All other authentication methods are disabled for root.
1.1 stevesk 1239: .Pp
1240: If this option is set to
1.52 jmc 1241: .Dq no ,
1.38 jmc 1242: root is not allowed to log in.
1.46 reyk 1243: .It Cm PermitTunnel
1244: Specifies whether
1245: .Xr tun 4
1246: device forwarding is allowed.
1.47 reyk 1247: The argument must be
1248: .Dq yes ,
1.58 stevesk 1249: .Dq point-to-point
1250: (layer 3),
1251: .Dq ethernet
1252: (layer 2), or
1.47 reyk 1253: .Dq no .
1.58 stevesk 1254: Specifying
1255: .Dq yes
1256: permits both
1257: .Dq point-to-point
1258: and
1259: .Dq ethernet .
1.46 reyk 1260: The default is
1261: .Dq no .
1.178 djm 1262: .Pp
1263: Independent of this setting, the permissions of the selected
1264: .Xr tun 4
1265: device must allow access to the user.
1.164 djm 1266: .It Cm PermitTTY
1267: Specifies whether
1.165 jmc 1268: .Xr pty 4
1.164 djm 1269: allocation is permitted.
1270: The default is
1271: .Dq yes .
1.6 markus 1272: .It Cm PermitUserEnvironment
1273: Specifies whether
1274: .Pa ~/.ssh/environment
1.9 stevesk 1275: and
1.6 markus 1276: .Cm environment=
1277: options in
1278: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1279: are processed by
1.52 jmc 1280: .Xr sshd 8 .
1.6 markus 1281: The default is
1282: .Dq no .
1.9 stevesk 1283: Enabling environment processing may enable users to bypass access
1284: restrictions in some configurations using mechanisms such as
1285: .Ev LD_PRELOAD .
1.174 djm 1286: .It Cm PermitUserRC
1287: Specifies whether any
1288: .Pa ~/.ssh/rc
1289: file is executed.
1290: The default is
1291: .Dq yes .
1.1 stevesk 1292: .It Cm PidFile
1.4 stevesk 1293: Specifies the file that contains the process ID of the
1.195 dtucker 1294: SSH daemon, or
1295: .Dq none
1296: to not write one.
1.1 stevesk 1297: The default is
1298: .Pa /var/run/sshd.pid .
1299: .It Cm Port
1300: Specifies the port number that
1.52 jmc 1301: .Xr sshd 8
1.1 stevesk 1302: listens on.
1303: The default is 22.
1304: Multiple options of this type are permitted.
1305: See also
1306: .Cm ListenAddress .
1307: .It Cm PrintLastLog
1308: Specifies whether
1.52 jmc 1309: .Xr sshd 8
1.36 jaredy 1310: should print the date and time of the last user login when a user logs
1311: in interactively.
1.1 stevesk 1312: The default is
1313: .Dq yes .
1314: .It Cm PrintMotd
1315: Specifies whether
1.52 jmc 1316: .Xr sshd 8
1.1 stevesk 1317: should print
1318: .Pa /etc/motd
1319: when a user logs in interactively.
1320: (On some systems it is also printed by the shell,
1321: .Pa /etc/profile ,
1322: or equivalent.)
1323: The default is
1324: .Dq yes .
1325: .It Cm Protocol
1326: Specifies the protocol versions
1.52 jmc 1327: .Xr sshd 8
1.5 stevesk 1328: supports.
1.1 stevesk 1329: The possible values are
1.52 jmc 1330: .Sq 1
1.1 stevesk 1331: and
1.52 jmc 1332: .Sq 2 .
1.1 stevesk 1333: Multiple versions must be comma-separated.
1334: The default is
1.109 jmc 1335: .Sq 2 .
1.218 djm 1336: Protocol 1 suffers from a number of cryptographic weaknesses and should
1337: not be used.
1338: It is only offered to support legacy devices.
1339: .Pp
1.5 stevesk 1340: Note that the order of the protocol list does not indicate preference,
1341: because the client selects among multiple protocol versions offered
1342: by the server.
1343: Specifying
1344: .Dq 2,1
1345: is identical to
1346: .Dq 1,2 .
1.189 djm 1347: .It Cm PubkeyAcceptedKeyTypes
1348: Specifies the key types that will be accepted for public key authentication
1349: as a comma-separated pattern list.
1.208 djm 1350: Alternately if the specified value begins with a
1351: .Sq +
1352: character, then the specified key types will be appended to the default set
1353: instead of replacing them.
1.206 markus 1354: The default for this option is:
1355: .Bd -literal -offset 3n
1356: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1357: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1358: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1359: ssh-ed25519-cert-v01@openssh.com,
1360: ssh-rsa-cert-v01@openssh.com,
1361: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.217 djm 1362: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1363: .Ed
1364: .Pp
1.189 djm 1365: The
1366: .Fl Q
1367: option of
1368: .Xr ssh 1
1369: may be used to list supported key types.
1.1 stevesk 1370: .It Cm PubkeyAuthentication
1371: Specifies whether public key authentication is allowed.
1372: The default is
1373: .Dq yes .
1.159 dtucker 1374: .It Cm RekeyLimit
1375: Specifies the maximum amount of data that may be transmitted before the
1376: session key is renegotiated, optionally followed a maximum amount of
1377: time that may pass before the session key is renegotiated.
1378: The first argument is specified in bytes and may have a suffix of
1379: .Sq K ,
1380: .Sq M ,
1381: or
1382: .Sq G
1383: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1384: The default is between
1385: .Sq 1G
1386: and
1387: .Sq 4G ,
1388: depending on the cipher.
1389: The optional second value is specified in seconds and may use any of the
1390: units documented in the
1391: .Sx TIME FORMATS
1.160 jmc 1392: section.
1.159 dtucker 1393: The default value for
1394: .Cm RekeyLimit
1395: is
1396: .Dq default none ,
1397: which means that rekeying is performed after the cipher's default amount
1398: of data has been sent or received and no time based rekeying is done.
1.118 djm 1399: .It Cm RevokedKeys
1.195 dtucker 1400: Specifies revoked public keys file, or
1401: .Dq none
1402: to not use one.
1.118 djm 1403: Keys listed in this file will be refused for public key authentication.
1404: Note that if this file is not readable, then public key authentication will
1405: be refused for all users.
1.154 djm 1406: Keys may be specified as a text file, listing one public key per line, or as
1407: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1408: .Xr ssh-keygen 1 .
1.161 jmc 1409: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1410: .Xr ssh-keygen 1 .
1.1 stevesk 1411: .It Cm RhostsRSAAuthentication
1412: Specifies whether rhosts or /etc/hosts.equiv authentication together
1413: with successful RSA host authentication is allowed.
1414: The default is
1415: .Dq no .
1416: This option applies to protocol version 1 only.
1417: .It Cm RSAAuthentication
1418: Specifies whether pure RSA authentication is allowed.
1419: The default is
1420: .Dq yes .
1421: This option applies to protocol version 1 only.
1422: .It Cm ServerKeyBits
1423: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 djm 1424: The default and minimum value is 1024.
1.175 millert 1425: .It Cm StreamLocalBindMask
1426: Sets the octal file creation mode mask
1427: .Pq umask
1428: used when creating a Unix-domain socket file for local or remote
1429: port forwarding.
1430: This option is only used for port forwarding to a Unix-domain socket file.
1431: .Pp
1432: The default value is 0177, which creates a Unix-domain socket file that is
1433: readable and writable only by the owner.
1434: Note that not all operating systems honor the file mode on Unix-domain
1435: socket files.
1436: .It Cm StreamLocalBindUnlink
1437: Specifies whether to remove an existing Unix-domain socket file for local
1438: or remote port forwarding before creating a new one.
1439: If the socket file already exists and
1440: .Cm StreamLocalBindUnlink
1441: is not enabled,
1442: .Nm sshd
1443: will be unable to forward the port to the Unix-domain socket file.
1444: This option is only used for port forwarding to a Unix-domain socket file.
1445: .Pp
1446: The argument must be
1447: .Dq yes
1448: or
1449: .Dq no .
1450: The default is
1451: .Dq no .
1.1 stevesk 1452: .It Cm StrictModes
1453: Specifies whether
1.52 jmc 1454: .Xr sshd 8
1.1 stevesk 1455: should check file modes and ownership of the
1456: user's files and home directory before accepting login.
1457: This is normally desirable because novices sometimes accidentally leave their
1458: directory or files world-writable.
1459: The default is
1460: .Dq yes .
1.112 djm 1461: Note that this does not apply to
1462: .Cm ChrootDirectory ,
1463: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1464: .It Cm Subsystem
1.51 jmc 1465: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1466: Arguments should be a subsystem name and a command (with optional arguments)
1467: to execute upon subsystem request.
1.80 djm 1468: .Pp
1.1 stevesk 1469: The command
1470: .Xr sftp-server 8
1471: implements the
1472: .Dq sftp
1473: file transfer subsystem.
1.80 djm 1474: .Pp
1475: Alternately the name
1476: .Dq internal-sftp
1477: implements an in-process
1478: .Dq sftp
1479: server.
1480: This may simplify configurations using
1481: .Cm ChrootDirectory
1482: to force a different filesystem root on clients.
1483: .Pp
1.1 stevesk 1484: By default no subsystems are defined.
1485: .It Cm SyslogFacility
1486: Gives the facility code that is used when logging messages from
1.53 jmc 1487: .Xr sshd 8 .
1.1 stevesk 1488: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1489: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1490: The default is AUTH.
1.27 markus 1491: .It Cm TCPKeepAlive
1492: Specifies whether the system should send TCP keepalive messages to the
1493: other side.
1494: If they are sent, death of the connection or crash of one
1495: of the machines will be properly noticed.
1496: However, this means that
1497: connections will die if the route is down temporarily, and some people
1498: find it annoying.
1499: On the other hand, if TCP keepalives are not sent,
1500: sessions may hang indefinitely on the server, leaving
1501: .Dq ghost
1502: users and consuming server resources.
1503: .Pp
1504: The default is
1505: .Dq yes
1506: (to send TCP keepalive messages), and the server will notice
1507: if the network goes down or the client host crashes.
1508: This avoids infinitely hanging sessions.
1509: .Pp
1510: To disable TCP keepalive messages, the value should be set to
1511: .Dq no .
1.118 djm 1512: .It Cm TrustedUserCAKeys
1513: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1514: trusted to sign user certificates for authentication, or
1515: .Dq none
1516: to not use one.
1.119 jmc 1517: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1518: .Ql #
1519: are allowed.
1520: If a certificate is presented for authentication and has its signing CA key
1521: listed in this file, then it may be used for authentication for any user
1522: listed in the certificate's principals list.
1523: Note that certificates that lack a list of principals will not be permitted
1524: for authentication using
1525: .Cm TrustedUserCAKeys .
1.161 jmc 1526: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1527: .Xr ssh-keygen 1 .
1.18 markus 1528: .It Cm UseDNS
1529: Specifies whether
1.52 jmc 1530: .Xr sshd 8
1.207 djm 1531: should look up the remote host name, and to check that
1.18 markus 1532: the resolved host name for the remote IP address maps back to the
1533: very same IP address.
1.207 djm 1534: .Pp
1535: If this option is set to
1536: .Dq no
1537: (the default) then only addresses and not host names may be used in
1538: .Pa ~/.ssh/known_hosts
1539: .Cm from
1540: and
1.211 jmc 1541: .Nm
1.207 djm 1542: .Cm Match
1543: .Cm Host
1544: directives.
1.1 stevesk 1545: .It Cm UseLogin
1546: Specifies whether
1547: .Xr login 1
1548: is used for interactive login sessions.
1549: The default is
1550: .Dq no .
1551: Note that
1552: .Xr login 1
1553: is never used for remote command execution.
1554: Note also, that if this is enabled,
1555: .Cm X11Forwarding
1556: will be disabled because
1557: .Xr login 1
1558: does not know how to handle
1559: .Xr xauth 1
1.15 jmc 1560: cookies.
1561: If
1.1 stevesk 1562: .Cm UsePrivilegeSeparation
1563: is specified, it will be disabled after authentication.
1564: .It Cm UsePrivilegeSeparation
1565: Specifies whether
1.52 jmc 1566: .Xr sshd 8
1.2 stevesk 1567: separates privileges by creating an unprivileged child process
1.15 jmc 1568: to deal with incoming network traffic.
1569: After successful authentication, another process will be created that has
1570: the privilege of the authenticated user.
1571: The goal of privilege separation is to prevent privilege
1.1 stevesk 1572: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1573: The argument must be
1574: .Dq yes ,
1575: .Dq no ,
1576: or
1577: .Dq sandbox .
1.134 djm 1578: If
1579: .Cm UsePrivilegeSeparation
1580: is set to
1581: .Dq sandbox
1582: then the pre-authentication unprivileged process is subject to additional
1583: restrictions.
1.213 sobrado 1584: The default is
1585: .Dq sandbox .
1.137 djm 1586: .It Cm VersionAddendum
1587: Optionally specifies additional text to append to the SSH protocol banner
1588: sent by the server upon connection.
1589: The default is
1590: .Dq none .
1.1 stevesk 1591: .It Cm X11DisplayOffset
1592: Specifies the first display number available for
1.52 jmc 1593: .Xr sshd 8 Ns 's
1.1 stevesk 1594: X11 forwarding.
1.52 jmc 1595: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1596: The default is 10.
1597: .It Cm X11Forwarding
1598: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1599: The argument must be
1600: .Dq yes
1601: or
1602: .Dq no .
1.1 stevesk 1603: The default is
1604: .Dq no .
1.13 stevesk 1605: .Pp
1606: When X11 forwarding is enabled, there may be additional exposure to
1607: the server and to client displays if the
1.52 jmc 1608: .Xr sshd 8
1.13 stevesk 1609: proxy display is configured to listen on the wildcard address (see
1610: .Cm X11UseLocalhost
1.52 jmc 1611: below), though this is not the default.
1.13 stevesk 1612: Additionally, the authentication spoofing and authentication data
1613: verification and substitution occur on the client side.
1614: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1615: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1616: forwarding (see the warnings for
1617: .Cm ForwardX11
1618: in
1.19 jmc 1619: .Xr ssh_config 5 ) .
1.13 stevesk 1620: A system administrator may have a stance in which they want to
1621: protect clients that may expose themselves to attack by unwittingly
1622: requesting X11 forwarding, which can warrant a
1623: .Dq no
1624: setting.
1625: .Pp
1626: Note that disabling X11 forwarding does not prevent users from
1627: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1628: X11 forwarding is automatically disabled if
1629: .Cm UseLogin
1630: is enabled.
1631: .It Cm X11UseLocalhost
1632: Specifies whether
1.52 jmc 1633: .Xr sshd 8
1.1 stevesk 1634: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1635: the wildcard address.
1636: By default,
1.52 jmc 1637: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1638: hostname part of the
1639: .Ev DISPLAY
1640: environment variable to
1641: .Dq localhost .
1.8 stevesk 1642: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1643: However, some older X11 clients may not function with this
1644: configuration.
1645: .Cm X11UseLocalhost
1646: may be set to
1647: .Dq no
1648: to specify that the forwarding server should be bound to the wildcard
1649: address.
1650: The argument must be
1651: .Dq yes
1652: or
1653: .Dq no .
1654: The default is
1655: .Dq yes .
1656: .It Cm XAuthLocation
1.11 stevesk 1657: Specifies the full pathname of the
1.1 stevesk 1658: .Xr xauth 1
1.195 dtucker 1659: program, or
1660: .Dq none
1661: to not use one.
1.1 stevesk 1662: The default is
1663: .Pa /usr/X11R6/bin/xauth .
1664: .El
1.55 jmc 1665: .Sh TIME FORMATS
1.53 jmc 1666: .Xr sshd 8
1.1 stevesk 1667: command-line arguments and configuration file options that specify time
1668: may be expressed using a sequence of the form:
1669: .Sm off
1.7 stevesk 1670: .Ar time Op Ar qualifier ,
1.1 stevesk 1671: .Sm on
1672: where
1673: .Ar time
1674: is a positive integer value and
1675: .Ar qualifier
1676: is one of the following:
1677: .Pp
1678: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1679: .It Aq Cm none
1.1 stevesk 1680: seconds
1681: .It Cm s | Cm S
1682: seconds
1683: .It Cm m | Cm M
1684: minutes
1685: .It Cm h | Cm H
1686: hours
1687: .It Cm d | Cm D
1688: days
1689: .It Cm w | Cm W
1690: weeks
1691: .El
1692: .Pp
1693: Each member of the sequence is added together to calculate
1694: the total time value.
1695: .Pp
1696: Time format examples:
1697: .Pp
1698: .Bl -tag -width Ds -compact -offset indent
1699: .It 600
1700: 600 seconds (10 minutes)
1701: .It 10m
1702: 10 minutes
1703: .It 1h30m
1704: 1 hour 30 minutes (90 minutes)
1705: .El
1706: .Sh FILES
1707: .Bl -tag -width Ds
1708: .It Pa /etc/ssh/sshd_config
1709: Contains configuration data for
1.53 jmc 1710: .Xr sshd 8 .
1.1 stevesk 1711: This file should be writable by root only, but it is recommended
1712: (though not necessary) that it be world-readable.
1713: .El
1.19 jmc 1714: .Sh SEE ALSO
1715: .Xr sshd 8
1.1 stevesk 1716: .Sh AUTHORS
1717: OpenSSH is a derivative of the original and free
1718: ssh 1.2.12 release by Tatu Ylonen.
1719: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1720: Theo de Raadt and Dug Song
1721: removed many bugs, re-added newer features and
1722: created OpenSSH.
1723: Markus Friedl contributed the support for SSH
1724: protocol versions 1.5 and 2.0.
1725: Niels Provos and Markus Friedl contributed support
1726: for privilege separation.
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