Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.189
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.189 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.188 2014/12/22 09:05:17 djm Exp $
1.186 djm 37: .Dd $Mdocdate: December 22 2014 $
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.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.1 stevesk 123: .It Cm AllowTcpForwarding
124: Specifies whether TCP forwarding is permitted.
1.150 djm 125: The available options are
126: .Dq yes
127: or
128: .Dq all
129: to allow TCP forwarding,
130: .Dq no
131: to prevent all TCP forwarding,
132: .Dq local
133: to allow local (from the perspective of
1.151 jmc 134: .Xr ssh 1 )
135: forwarding only or
1.150 djm 136: .Dq remote
137: to allow remote forwarding only.
1.1 stevesk 138: The default is
139: .Dq yes .
140: Note that disabling TCP forwarding does not improve security unless
141: users are also denied shell access, as they can always install their
142: own forwarders.
1.175 millert 143: .It Cm AllowStreamLocalForwarding
144: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
145: The available options are
146: .Dq yes
147: or
148: .Dq all
149: to allow StreamLocal forwarding,
150: .Dq no
151: to prevent all StreamLocal forwarding,
152: .Dq local
153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
156: .Dq remote
157: to allow remote forwarding only.
158: The default is
159: .Dq yes .
160: Note that disabling StreamLocal forwarding does not improve security unless
161: users are also denied shell access, as they can always install their
162: own forwarders.
1.1 stevesk 163: .It Cm AllowUsers
164: This keyword can be followed by a list of user name patterns, separated
165: by spaces.
1.14 jmc 166: If specified, login is allowed only for user names that
1.1 stevesk 167: match one of the patterns.
168: Only user names are valid; a numerical user ID is not recognized.
169: By default, login is allowed for all users.
170: If the pattern takes the form USER@HOST then USER and HOST
171: are separately checked, restricting logins to particular
172: users from particular hosts.
1.54 jmc 173: The allow/deny directives are processed in the following order:
174: .Cm DenyUsers ,
175: .Cm AllowUsers ,
176: .Cm DenyGroups ,
177: and finally
178: .Cm AllowGroups .
1.49 jmc 179: .Pp
1.161 jmc 180: See PATTERNS in
1.49 jmc 181: .Xr ssh_config 5
182: for more information on patterns.
1.149 djm 183: .It Cm AuthenticationMethods
184: Specifies the authentication methods that must be successfully completed
185: for a user to be granted access.
186: This option must be followed by one or more comma-separated lists of
187: authentication method names.
188: Successful authentication requires completion of every method in at least
189: one of these lists.
190: .Pp
191: For example, an argument of
192: .Dq publickey,password publickey,keyboard-interactive
193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
196: so for this example, it would not be possible to attempt password or
197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
202: .Dq bsdauth ,
203: .Dq pam ,
204: or
205: .Dq skey ,
206: depending on the server configuration.
207: For example,
208: .Dq keyboard-interactive:bsdauth
209: would restrict keyboard interactive authentication to the
210: .Dq bsdauth
211: device.
1.185 djm 212: .Pp
213: If the
214: .Dq publickey
1.187 jmc 215: method is listed more than once,
1.185 djm 216: .Xr sshd 8
217: verifies that keys that have been used successfully are not reused for
218: subsequent authentications.
219: For example, an
220: .Cm AuthenticationMethods
221: of
222: .Dq publickey,publickey
223: will require successful authentication using two different public keys.
1.181 djm 224: .Pp
1.149 djm 225: This option is only available for SSH protocol 2 and will yield a fatal
226: error if enabled if protocol 1 is also enabled.
227: Note that each authentication method listed should also be explicitly enabled
228: in the configuration.
229: The default is not to require multiple authentication; successful completion
230: of a single authentication method is sufficient.
1.146 djm 231: .It Cm AuthorizedKeysCommand
1.147 jmc 232: Specifies a program to be used to look up the user's public keys.
1.158 djm 233: The program must be owned by root and not writable by group or others.
234: It will be invoked with a single argument of the username
1.146 djm 235: being authenticated, and should produce on standard output zero or
1.161 jmc 236: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 237: .Xr sshd 8 ) .
1.146 djm 238: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
239: and authorize the user then public key authentication continues using the usual
240: .Cm AuthorizedKeysFile
241: files.
242: By default, no AuthorizedKeysCommand is run.
243: .It Cm AuthorizedKeysCommandUser
244: Specifies the user under whose account the AuthorizedKeysCommand is run.
245: It is recommended to use a dedicated user that has no other role on the host
246: than running authorized keys commands.
1.186 djm 247: If
1.181 djm 248: .Cm AuthorizedKeysCommand
1.186 djm 249: is specified but
250: .Cm AuthorizedKeysCommandUser
251: is not, then
252: .Xr sshd 8
253: will refuse to start.
1.1 stevesk 254: .It Cm AuthorizedKeysFile
255: Specifies the file that contains the public keys that can be used
256: for user authentication.
1.124 djm 257: The format is described in the
1.161 jmc 258: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 259: section of
260: .Xr sshd 8 .
1.1 stevesk 261: .Cm AuthorizedKeysFile
262: may contain tokens of the form %T which are substituted during connection
1.52 jmc 263: setup.
1.17 jmc 264: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 265: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 266: %u is replaced by the username of that user.
267: After expansion,
268: .Cm AuthorizedKeysFile
269: is taken to be an absolute path or one relative to the user's home
270: directory.
1.133 jmc 271: Multiple files may be listed, separated by whitespace.
272: The default is
273: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 274: .It Cm AuthorizedPrincipalsFile
275: Specifies a file that lists principal names that are accepted for
276: certificate authentication.
277: When using certificates signed by a key listed in
278: .Cm TrustedUserCAKeys ,
279: this file lists names, one of which must appear in the certificate for it
280: to be accepted for authentication.
1.125 jmc 281: Names are listed one per line preceded by key options (as described
1.161 jmc 282: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 283: .Xr sshd 8 ) .
1.124 djm 284: Empty lines and comments starting with
1.121 djm 285: .Ql #
286: are ignored.
287: .Pp
288: .Cm AuthorizedPrincipalsFile
289: may contain tokens of the form %T which are substituted during connection
290: setup.
291: The following tokens are defined: %% is replaced by a literal '%',
292: %h is replaced by the home directory of the user being authenticated, and
293: %u is replaced by the username of that user.
294: After expansion,
295: .Cm AuthorizedPrincipalsFile
296: is taken to be an absolute path or one relative to the user's home
297: directory.
298: .Pp
1.138 djm 299: The default is
300: .Dq none ,
301: i.e. not to use a principals file \(en in this case, the username
1.121 djm 302: of the user must appear in a certificate's principals list for it to be
303: accepted.
304: Note that
305: .Cm AuthorizedPrincipalsFile
306: is only used when authentication proceeds using a CA listed in
307: .Cm TrustedUserCAKeys
308: and is not consulted for certification authorities trusted via
309: .Pa ~/.ssh/authorized_keys ,
310: though the
311: .Cm principals=
312: key option offers a similar facility (see
313: .Xr sshd 8
314: for details).
1.1 stevesk 315: .It Cm Banner
316: The contents of the specified file are sent to the remote user before
317: authentication is allowed.
1.78 djm 318: If the argument is
319: .Dq none
320: then no banner is displayed.
1.1 stevesk 321: This option is only available for protocol version 2.
322: By default, no banner is displayed.
323: .It Cm ChallengeResponseAuthentication
1.50 jmc 324: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 325: All authentication styles from
326: .Xr login.conf 5
327: are supported.
328: The default is
329: .Dq yes .
1.80 djm 330: .It Cm ChrootDirectory
1.113 stevesk 331: Specifies the pathname of a directory to
1.80 djm 332: .Xr chroot 2
333: to after authentication.
1.113 stevesk 334: All components of the pathname must be root-owned directories that are
1.80 djm 335: not writable by any other user or group.
1.106 stevesk 336: After the chroot,
337: .Xr sshd 8
338: changes the working directory to the user's home directory.
1.80 djm 339: .Pp
1.113 stevesk 340: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 341: the connecting user has been authenticated: %% is replaced by a literal '%',
342: %h is replaced by the home directory of the user being authenticated, and
343: %u is replaced by the username of that user.
344: .Pp
345: The
346: .Cm ChrootDirectory
347: must contain the necessary files and directories to support the
1.103 stevesk 348: user's session.
1.80 djm 349: For an interactive session this requires at least a shell, typically
350: .Xr sh 1 ,
351: and basic
352: .Pa /dev
353: nodes such as
354: .Xr null 4 ,
355: .Xr zero 4 ,
356: .Xr stdin 4 ,
357: .Xr stdout 4 ,
358: .Xr stderr 4 ,
1.180 jmc 359: and
1.80 djm 360: .Xr tty 4
361: devices.
362: For file transfer sessions using
1.105 jmc 363: .Dq sftp ,
1.80 djm 364: no additional configuration of the environment is necessary if the
1.105 jmc 365: in-process sftp server is used,
1.176 schwarze 366: though sessions which use logging may require
1.104 stevesk 367: .Pa /dev/log
1.176 schwarze 368: inside the chroot directory on some operating systems (see
1.104 stevesk 369: .Xr sftp-server 8
1.81 jmc 370: for details).
1.80 djm 371: .Pp
372: The default is not to
373: .Xr chroot 2 .
1.1 stevesk 374: .It Cm Ciphers
375: Specifies the ciphers allowed for protocol version 2.
376: Multiple ciphers must be comma-separated.
1.167 djm 377: The supported ciphers are:
378: .Pp
1.173 naddy 379: .Bl -item -compact -offset indent
380: .It
381: 3des-cbc
382: .It
383: aes128-cbc
384: .It
385: aes192-cbc
386: .It
387: aes256-cbc
388: .It
389: aes128-ctr
390: .It
391: aes192-ctr
392: .It
393: aes256-ctr
394: .It
395: aes128-gcm@openssh.com
396: .It
397: aes256-gcm@openssh.com
398: .It
399: arcfour
400: .It
401: arcfour128
402: .It
403: arcfour256
404: .It
405: blowfish-cbc
406: .It
407: cast128-cbc
408: .It
409: chacha20-poly1305@openssh.com
410: .El
1.167 djm 411: .Pp
1.52 jmc 412: The default is:
1.173 naddy 413: .Bd -literal -offset indent
414: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 415: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 416: chacha20-poly1305@openssh.com
1.1 stevesk 417: .Ed
1.167 djm 418: .Pp
419: The list of available ciphers may also be obtained using the
420: .Fl Q
421: option of
1.188 djm 422: .Xr ssh 1
423: with an argument of
424: .Dq cipher .
1.1 stevesk 425: .It Cm ClientAliveCountMax
1.48 jmc 426: Sets the number of client alive messages (see below) which may be
1.1 stevesk 427: sent without
1.52 jmc 428: .Xr sshd 8
1.17 jmc 429: receiving any messages back from the client.
430: If this threshold is reached while client alive messages are being sent,
1.52 jmc 431: sshd will disconnect the client, terminating the session.
1.17 jmc 432: It is important to note that the use of client alive messages is very
433: different from
1.27 markus 434: .Cm TCPKeepAlive
1.17 jmc 435: (below).
436: The client alive messages are sent through the encrypted channel
437: and therefore will not be spoofable.
438: The TCP keepalive option enabled by
1.27 markus 439: .Cm TCPKeepAlive
1.17 jmc 440: is spoofable.
441: The client alive mechanism is valuable when the client or
1.1 stevesk 442: server depend on knowing when a connection has become inactive.
443: .Pp
1.17 jmc 444: The default value is 3.
445: If
1.1 stevesk 446: .Cm ClientAliveInterval
1.48 jmc 447: (see below) is set to 15, and
1.1 stevesk 448: .Cm ClientAliveCountMax
1.52 jmc 449: is left at the default, unresponsive SSH clients
1.1 stevesk 450: will be disconnected after approximately 45 seconds.
1.57 markus 451: This option applies to protocol version 2 only.
1.42 djm 452: .It Cm ClientAliveInterval
453: Sets a timeout interval in seconds after which if no data has been received
454: from the client,
1.52 jmc 455: .Xr sshd 8
1.42 djm 456: will send a message through the encrypted
457: channel to request a response from the client.
458: The default
459: is 0, indicating that these messages will not be sent to the client.
460: This option applies to protocol version 2 only.
1.3 markus 461: .It Cm Compression
1.44 markus 462: Specifies whether compression is allowed, or delayed until
463: the user has authenticated successfully.
1.3 markus 464: The argument must be
1.44 markus 465: .Dq yes ,
466: .Dq delayed ,
1.3 markus 467: or
468: .Dq no .
469: The default is
1.44 markus 470: .Dq delayed .
1.1 stevesk 471: .It Cm DenyGroups
472: This keyword can be followed by a list of group name patterns, separated
473: by spaces.
474: Login is disallowed for users whose primary group or supplementary
475: group list matches one of the patterns.
476: Only group names are valid; a numerical group ID is not recognized.
477: By default, login is allowed for all groups.
1.54 jmc 478: The allow/deny directives are processed in the following order:
479: .Cm DenyUsers ,
480: .Cm AllowUsers ,
481: .Cm DenyGroups ,
482: and finally
483: .Cm AllowGroups .
1.49 jmc 484: .Pp
1.161 jmc 485: See PATTERNS in
1.49 jmc 486: .Xr ssh_config 5
487: for more information on patterns.
1.1 stevesk 488: .It Cm DenyUsers
489: This keyword can be followed by a list of user name patterns, separated
490: by spaces.
491: Login is disallowed for user names that match one of the patterns.
492: Only user names are valid; a numerical user ID is not recognized.
493: By default, login is allowed for all users.
494: If the pattern takes the form USER@HOST then USER and HOST
495: are separately checked, restricting logins to particular
496: users from particular hosts.
1.54 jmc 497: The allow/deny directives are processed in the following order:
498: .Cm DenyUsers ,
499: .Cm AllowUsers ,
500: .Cm DenyGroups ,
501: and finally
502: .Cm AllowGroups .
1.49 jmc 503: .Pp
1.161 jmc 504: See PATTERNS in
1.49 jmc 505: .Xr ssh_config 5
506: for more information on patterns.
1.183 djm 507: .It Cm FingerprintHash
508: Specifies the hash algorithm used when logging key fingerprints.
509: Valid options are:
510: .Dq md5
511: and
512: .Dq sha256 .
513: The default is
514: .Dq sha256 .
1.67 dtucker 515: .It Cm ForceCommand
516: Forces the execution of the command specified by
517: .Cm ForceCommand ,
1.84 djm 518: ignoring any command supplied by the client and
519: .Pa ~/.ssh/rc
520: if present.
1.67 dtucker 521: The command is invoked by using the user's login shell with the -c option.
522: This applies to shell, command, or subsystem execution.
523: It is most useful inside a
524: .Cm Match
525: block.
526: The command originally supplied by the client is available in the
527: .Ev SSH_ORIGINAL_COMMAND
528: environment variable.
1.82 djm 529: Specifying a command of
530: .Dq internal-sftp
531: will force the use of an in-process sftp server that requires no support
532: files when used with
533: .Cm ChrootDirectory .
1.1 stevesk 534: .It Cm GatewayPorts
535: Specifies whether remote hosts are allowed to connect to ports
536: forwarded for the client.
537: By default,
1.52 jmc 538: .Xr sshd 8
1.15 jmc 539: binds remote port forwardings to the loopback address.
540: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 541: .Cm GatewayPorts
1.52 jmc 542: can be used to specify that sshd
1.39 djm 543: should allow remote port forwardings to bind to non-loopback addresses, thus
544: allowing other hosts to connect.
545: The argument may be
546: .Dq no
547: to force remote port forwardings to be available to the local host only,
1.1 stevesk 548: .Dq yes
1.39 djm 549: to force remote port forwardings to bind to the wildcard address, or
550: .Dq clientspecified
551: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 552: The default is
553: .Dq no .
1.23 markus 554: .It Cm GSSAPIAuthentication
1.25 markus 555: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 556: The default is
1.23 markus 557: .Dq no .
558: Note that this option applies to protocol version 2 only.
559: .It Cm GSSAPICleanupCredentials
560: Specifies whether to automatically destroy the user's credentials cache
561: on logout.
562: The default is
563: .Dq yes .
564: Note that this option applies to protocol version 2 only.
1.189 ! djm 565: .It Cm HostbasedAcceptedKeyTypes
! 566: Specifies the key types that will be accepted for hostbased authentication
! 567: as a comma-separated pattern list.
! 568: The default
! 569: .Dq *
! 570: will allow all key types.
! 571: The
! 572: .Fl Q
! 573: option of
! 574: .Xr ssh 1
! 575: may be used to list supported key types.
1.1 stevesk 576: .It Cm HostbasedAuthentication
577: Specifies whether rhosts or /etc/hosts.equiv authentication together
578: with successful public key client host authentication is allowed
1.50 jmc 579: (host-based authentication).
1.1 stevesk 580: This option is similar to
581: .Cm RhostsRSAAuthentication
582: and applies to protocol version 2 only.
1.70 dtucker 583: The default is
584: .Dq no .
585: .It Cm HostbasedUsesNameFromPacketOnly
586: Specifies whether or not the server will attempt to perform a reverse
587: name lookup when matching the name in the
588: .Pa ~/.shosts ,
589: .Pa ~/.rhosts ,
590: and
591: .Pa /etc/hosts.equiv
592: files during
593: .Cm HostbasedAuthentication .
594: A setting of
595: .Dq yes
596: means that
597: .Xr sshd 8
598: uses the name supplied by the client rather than
599: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 600: The default is
601: .Dq no .
1.117 djm 602: .It Cm HostCertificate
603: Specifies a file containing a public host certificate.
604: The certificate's public key must match a private host key already specified
605: by
606: .Cm HostKey .
607: The default behaviour of
608: .Xr sshd 8
609: is not to load any certificates.
1.1 stevesk 610: .It Cm HostKey
611: Specifies a file containing a private host key
612: used by SSH.
613: The default is
614: .Pa /etc/ssh/ssh_host_key
615: for protocol version 1, and
1.126 djm 616: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 617: .Pa /etc/ssh/ssh_host_ecdsa_key ,
618: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 619: and
1.1 stevesk 620: .Pa /etc/ssh/ssh_host_rsa_key
621: for protocol version 2.
622: Note that
1.52 jmc 623: .Xr sshd 8
1.1 stevesk 624: will refuse to use a file if it is group/world-accessible.
625: It is possible to have multiple host key files.
626: .Dq rsa1
627: keys are used for version 1 and
1.126 djm 628: .Dq dsa ,
1.169 naddy 629: .Dq ecdsa ,
630: .Dq ed25519
1.1 stevesk 631: or
632: .Dq rsa
633: are used for version 2 of the SSH protocol.
1.162 markus 634: It is also possible to specify public host key files instead.
635: In this case operations on the private key will be delegated
636: to an
637: .Xr ssh-agent 1 .
638: .It Cm HostKeyAgent
639: Identifies the UNIX-domain socket used to communicate
640: with an agent that has access to the private host keys.
641: If
642: .Dq SSH_AUTH_SOCK
643: is specified, the location of the socket will be read from the
644: .Ev SSH_AUTH_SOCK
645: environment variable.
1.1 stevesk 646: .It Cm IgnoreRhosts
647: Specifies that
648: .Pa .rhosts
649: and
650: .Pa .shosts
651: files will not be used in
652: .Cm RhostsRSAAuthentication
653: or
654: .Cm HostbasedAuthentication .
655: .Pp
656: .Pa /etc/hosts.equiv
657: and
658: .Pa /etc/shosts.equiv
659: are still used.
660: The default is
661: .Dq yes .
662: .It Cm IgnoreUserKnownHosts
663: Specifies whether
1.52 jmc 664: .Xr sshd 8
1.1 stevesk 665: should ignore the user's
1.41 djm 666: .Pa ~/.ssh/known_hosts
1.1 stevesk 667: during
668: .Cm RhostsRSAAuthentication
669: or
670: .Cm HostbasedAuthentication .
671: The default is
672: .Dq no .
1.129 djm 673: .It Cm IPQoS
674: Specifies the IPv4 type-of-service or DSCP class for the connection.
675: Accepted values are
676: .Dq af11 ,
677: .Dq af12 ,
678: .Dq af13 ,
1.136 djm 679: .Dq af21 ,
1.129 djm 680: .Dq af22 ,
681: .Dq af23 ,
682: .Dq af31 ,
683: .Dq af32 ,
684: .Dq af33 ,
685: .Dq af41 ,
686: .Dq af42 ,
687: .Dq af43 ,
688: .Dq cs0 ,
689: .Dq cs1 ,
690: .Dq cs2 ,
691: .Dq cs3 ,
692: .Dq cs4 ,
693: .Dq cs5 ,
694: .Dq cs6 ,
695: .Dq cs7 ,
696: .Dq ef ,
697: .Dq lowdelay ,
698: .Dq throughput ,
699: .Dq reliability ,
700: or a numeric value.
1.131 djm 701: This option may take one or two arguments, separated by whitespace.
1.129 djm 702: If one argument is specified, it is used as the packet class unconditionally.
703: If two values are specified, the first is automatically selected for
704: interactive sessions and the second for non-interactive sessions.
705: The default is
706: .Dq lowdelay
707: for interactive sessions and
708: .Dq throughput
709: for non-interactive sessions.
1.171 jmc 710: .It Cm KbdInteractiveAuthentication
711: Specifies whether to allow keyboard-interactive authentication.
712: The argument to this keyword must be
713: .Dq yes
714: or
715: .Dq no .
716: The default is to use whatever value
717: .Cm ChallengeResponseAuthentication
718: is set to
719: (by default
720: .Dq yes ) .
1.1 stevesk 721: .It Cm KerberosAuthentication
1.24 markus 722: Specifies whether the password provided by the user for
1.1 stevesk 723: .Cm PasswordAuthentication
1.24 markus 724: will be validated through the Kerberos KDC.
1.1 stevesk 725: To use this option, the server needs a
726: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 727: The default is
1.29 dtucker 728: .Dq no .
729: .It Cm KerberosGetAFSToken
1.45 djm 730: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 731: an AFS token before accessing the user's home directory.
1.52 jmc 732: The default is
1.1 stevesk 733: .Dq no .
734: .It Cm KerberosOrLocalPasswd
1.52 jmc 735: If password authentication through Kerberos fails then
1.1 stevesk 736: the password will be validated via any additional local mechanism
737: such as
738: .Pa /etc/passwd .
1.52 jmc 739: The default is
1.1 stevesk 740: .Dq yes .
741: .It Cm KerberosTicketCleanup
742: Specifies whether to automatically destroy the user's ticket cache
743: file on logout.
1.52 jmc 744: The default is
1.1 stevesk 745: .Dq yes .
1.127 djm 746: .It Cm KexAlgorithms
747: Specifies the available KEX (Key Exchange) algorithms.
748: Multiple algorithms must be comma-separated.
1.173 naddy 749: The supported algorithms are:
750: .Pp
751: .Bl -item -compact -offset indent
752: .It
753: curve25519-sha256@libssh.org
754: .It
755: diffie-hellman-group1-sha1
756: .It
757: diffie-hellman-group14-sha1
758: .It
759: diffie-hellman-group-exchange-sha1
760: .It
761: diffie-hellman-group-exchange-sha256
762: .It
763: ecdh-sha2-nistp256
764: .It
765: ecdh-sha2-nistp384
766: .It
767: ecdh-sha2-nistp521
768: .El
769: .Pp
770: The default is:
1.170 dtucker 771: .Bd -literal -offset indent
772: curve25519-sha256@libssh.org,
773: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
774: diffie-hellman-group-exchange-sha256,
1.173 naddy 775: diffie-hellman-group14-sha1
1.170 dtucker 776: .Ed
1.188 djm 777: .Pp
778: The list of available key exchange algorithms may also be obtained using the
779: .Fl Q
780: option of
781: .Xr ssh 1
782: with an argument of
783: .Dq kex .
1.1 stevesk 784: .It Cm KeyRegenerationInterval
785: In protocol version 1, the ephemeral server key is automatically regenerated
786: after this many seconds (if it has been used).
787: The purpose of regeneration is to prevent
788: decrypting captured sessions by later breaking into the machine and
789: stealing the keys.
790: The key is never stored anywhere.
791: If the value is 0, the key is never regenerated.
792: The default is 3600 (seconds).
793: .It Cm ListenAddress
794: Specifies the local addresses
1.52 jmc 795: .Xr sshd 8
1.1 stevesk 796: should listen on.
797: The following forms may be used:
798: .Pp
799: .Bl -item -offset indent -compact
800: .It
801: .Cm ListenAddress
802: .Sm off
1.180 jmc 803: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 804: .Sm on
805: .It
806: .Cm ListenAddress
807: .Sm off
1.180 jmc 808: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 809: .Sm on
810: .It
811: .Cm ListenAddress
812: .Sm off
813: .Oo
1.180 jmc 814: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 815: .Sm on
816: .El
817: .Pp
818: If
819: .Ar port
820: is not specified,
1.52 jmc 821: sshd will listen on the address and all prior
1.1 stevesk 822: .Cm Port
1.17 jmc 823: options specified.
824: The default is to listen on all local addresses.
1.15 jmc 825: Multiple
1.1 stevesk 826: .Cm ListenAddress
1.17 jmc 827: options are permitted.
828: Additionally, any
1.1 stevesk 829: .Cm Port
1.52 jmc 830: options must precede this option for non-port qualified addresses.
1.1 stevesk 831: .It Cm LoginGraceTime
832: The server disconnects after this time if the user has not
833: successfully logged in.
834: If the value is 0, there is no time limit.
1.12 stevesk 835: The default is 120 seconds.
1.1 stevesk 836: .It Cm LogLevel
837: Gives the verbosity level that is used when logging messages from
1.53 jmc 838: .Xr sshd 8 .
1.1 stevesk 839: The possible values are:
1.52 jmc 840: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 841: The default is INFO.
842: DEBUG and DEBUG1 are equivalent.
843: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
844: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 845: .It Cm MACs
846: Specifies the available MAC (message authentication code) algorithms.
847: The MAC algorithm is used in protocol version 2
848: for data integrity protection.
849: Multiple algorithms must be comma-separated.
1.152 markus 850: The algorithms that contain
851: .Dq -etm
852: calculate the MAC after encryption (encrypt-then-mac).
853: These are considered safer and their use recommended.
1.173 naddy 854: The supported MACs are:
855: .Pp
856: .Bl -item -compact -offset indent
857: .It
858: hmac-md5
859: .It
860: hmac-md5-96
861: .It
862: hmac-ripemd160
863: .It
864: hmac-sha1
865: .It
866: hmac-sha1-96
867: .It
868: hmac-sha2-256
869: .It
870: hmac-sha2-512
871: .It
872: umac-64@openssh.com
873: .It
874: umac-128@openssh.com
875: .It
876: hmac-md5-etm@openssh.com
877: .It
878: hmac-md5-96-etm@openssh.com
879: .It
880: hmac-ripemd160-etm@openssh.com
881: .It
882: hmac-sha1-etm@openssh.com
883: .It
884: hmac-sha1-96-etm@openssh.com
885: .It
886: hmac-sha2-256-etm@openssh.com
887: .It
888: hmac-sha2-512-etm@openssh.com
889: .It
890: umac-64-etm@openssh.com
891: .It
892: umac-128-etm@openssh.com
893: .El
894: .Pp
1.52 jmc 895: The default is:
1.77 jmc 896: .Bd -literal -offset indent
1.152 markus 897: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
898: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 899: umac-64@openssh.com,umac-128@openssh.com,
900: hmac-sha2-256,hmac-sha2-512
1.77 jmc 901: .Ed
1.188 djm 902: .Pp
903: The list of available MAC algorithms may also be obtained using the
904: .Fl Q
905: option of
906: .Xr ssh 1
907: with an argument of
908: .Dq mac .
1.60 dtucker 909: .It Cm Match
1.61 jmc 910: Introduces a conditional block.
1.65 dtucker 911: If all of the criteria on the
1.60 dtucker 912: .Cm Match
1.65 dtucker 913: line are satisfied, the keywords on the following lines override those
914: set in the global section of the config file, until either another
1.60 dtucker 915: .Cm Match
1.65 dtucker 916: line or the end of the file.
1.172 djm 917: If a keyword appears in multiple
918: .Cm Match
1.177 sobrado 919: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 920: applied.
1.91 djm 921: .Pp
1.61 jmc 922: The arguments to
1.60 dtucker 923: .Cm Match
1.163 dtucker 924: are one or more criteria-pattern pairs or the single token
925: .Cm All
926: which matches all criteria.
1.60 dtucker 927: The available criteria are
928: .Cm User ,
1.69 dtucker 929: .Cm Group ,
1.60 dtucker 930: .Cm Host ,
1.139 dtucker 931: .Cm LocalAddress ,
932: .Cm LocalPort ,
1.60 dtucker 933: and
934: .Cm Address .
1.91 djm 935: The match patterns may consist of single entries or comma-separated
936: lists and may use the wildcard and negation operators described in the
1.161 jmc 937: PATTERNS section of
1.92 djm 938: .Xr ssh_config 5 .
1.91 djm 939: .Pp
940: The patterns in an
941: .Cm Address
942: criteria may additionally contain addresses to match in CIDR
1.93 jmc 943: address/masklen format, e.g.\&
1.91 djm 944: .Dq 192.0.2.0/24
945: or
946: .Dq 3ffe:ffff::/32 .
947: Note that the mask length provided must be consistent with the address -
948: it is an error to specify a mask length that is too long for the address
1.93 jmc 949: or one with bits set in this host portion of the address.
950: For example,
1.91 djm 951: .Dq 192.0.2.0/33
952: and
1.93 jmc 953: .Dq 192.0.2.0/8
1.91 djm 954: respectively.
955: .Pp
1.60 dtucker 956: Only a subset of keywords may be used on the lines following a
957: .Cm Match
958: keyword.
959: Available keywords are
1.142 jmc 960: .Cm AcceptEnv ,
1.99 okan 961: .Cm AllowAgentForwarding ,
1.142 jmc 962: .Cm AllowGroups ,
1.62 dtucker 963: .Cm AllowTcpForwarding ,
1.141 markus 964: .Cm AllowUsers ,
1.149 djm 965: .Cm AuthenticationMethods ,
1.146 djm 966: .Cm AuthorizedKeysCommand ,
967: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 968: .Cm AuthorizedKeysFile ,
1.123 djm 969: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 970: .Cm Banner ,
1.85 djm 971: .Cm ChrootDirectory ,
1.141 markus 972: .Cm DenyGroups ,
973: .Cm DenyUsers ,
1.67 dtucker 974: .Cm ForceCommand ,
1.142 jmc 975: .Cm GatewayPorts ,
1.141 markus 976: .Cm GSSAPIAuthentication ,
1.189 ! djm 977: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 978: .Cm HostbasedAuthentication ,
1.123 djm 979: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 980: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 981: .Cm KerberosAuthentication ,
1.95 dtucker 982: .Cm MaxAuthTries ,
1.94 dtucker 983: .Cm MaxSessions ,
1.72 dtucker 984: .Cm PasswordAuthentication ,
1.97 djm 985: .Cm PermitEmptyPasswords ,
1.66 dtucker 986: .Cm PermitOpen ,
1.79 dtucker 987: .Cm PermitRootLogin ,
1.164 djm 988: .Cm PermitTTY ,
1.123 djm 989: .Cm PermitTunnel ,
1.174 djm 990: .Cm PermitUserRC ,
1.189 ! djm 991: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 992: .Cm PubkeyAuthentication ,
1.159 dtucker 993: .Cm RekeyLimit ,
1.142 jmc 994: .Cm RhostsRSAAuthentication ,
1.141 markus 995: .Cm RSAAuthentication ,
1.66 dtucker 996: .Cm X11DisplayOffset ,
1.101 djm 997: .Cm X11Forwarding
1.60 dtucker 998: and
1.102 djm 999: .Cm X11UseLocalHost .
1.33 dtucker 1000: .It Cm MaxAuthTries
1001: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1002: connection.
1003: Once the number of failures reaches half this value,
1004: additional failures are logged.
1005: The default is 6.
1.90 djm 1006: .It Cm MaxSessions
1007: Specifies the maximum number of open sessions permitted per network connection.
1008: The default is 10.
1.1 stevesk 1009: .It Cm MaxStartups
1010: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1011: SSH daemon.
1.1 stevesk 1012: Additional connections will be dropped until authentication succeeds or the
1013: .Cm LoginGraceTime
1014: expires for a connection.
1.156 dtucker 1015: The default is 10:30:100.
1.1 stevesk 1016: .Pp
1017: Alternatively, random early drop can be enabled by specifying
1018: the three colon separated values
1019: .Dq start:rate:full
1.51 jmc 1020: (e.g. "10:30:60").
1.53 jmc 1021: .Xr sshd 8
1.1 stevesk 1022: will refuse connection attempts with a probability of
1023: .Dq rate/100
1024: (30%)
1025: if there are currently
1026: .Dq start
1027: (10)
1028: unauthenticated connections.
1029: The probability increases linearly and all connection attempts
1030: are refused if the number of unauthenticated connections reaches
1031: .Dq full
1032: (60).
1033: .It Cm PasswordAuthentication
1034: Specifies whether password authentication is allowed.
1035: The default is
1036: .Dq yes .
1037: .It Cm PermitEmptyPasswords
1038: When password authentication is allowed, it specifies whether the
1039: server allows login to accounts with empty password strings.
1040: The default is
1041: .Dq no .
1.62 dtucker 1042: .It Cm PermitOpen
1043: Specifies the destinations to which TCP port forwarding is permitted.
1044: The forwarding specification must be one of the following forms:
1045: .Pp
1046: .Bl -item -offset indent -compact
1047: .It
1048: .Cm PermitOpen
1049: .Sm off
1050: .Ar host : port
1051: .Sm on
1052: .It
1053: .Cm PermitOpen
1054: .Sm off
1055: .Ar IPv4_addr : port
1056: .Sm on
1057: .It
1058: .Cm PermitOpen
1059: .Sm off
1060: .Ar \&[ IPv6_addr \&] : port
1061: .Sm on
1062: .El
1063: .Pp
1.68 dtucker 1064: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1065: An argument of
1066: .Dq any
1067: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1068: An argument of
1069: .Dq none
1070: can be used to prohibit all forwarding requests.
1.63 jmc 1071: By default all port forwarding requests are permitted.
1.1 stevesk 1072: .It Cm PermitRootLogin
1.38 jmc 1073: Specifies whether root can log in using
1.1 stevesk 1074: .Xr ssh 1 .
1075: The argument must be
1076: .Dq yes ,
1077: .Dq without-password ,
1.52 jmc 1078: .Dq forced-commands-only ,
1.1 stevesk 1079: or
1080: .Dq no .
1081: The default is
1082: .Dq yes .
1083: .Pp
1084: If this option is set to
1.52 jmc 1085: .Dq without-password ,
1.1 stevesk 1086: password authentication is disabled for root.
1087: .Pp
1088: If this option is set to
1.52 jmc 1089: .Dq forced-commands-only ,
1.1 stevesk 1090: root login with public key authentication will be allowed,
1091: but only if the
1092: .Ar command
1093: option has been specified
1094: (which may be useful for taking remote backups even if root login is
1.17 jmc 1095: normally not allowed).
1096: All other authentication methods are disabled for root.
1.1 stevesk 1097: .Pp
1098: If this option is set to
1.52 jmc 1099: .Dq no ,
1.38 jmc 1100: root is not allowed to log in.
1.46 reyk 1101: .It Cm PermitTunnel
1102: Specifies whether
1103: .Xr tun 4
1104: device forwarding is allowed.
1.47 reyk 1105: The argument must be
1106: .Dq yes ,
1.58 stevesk 1107: .Dq point-to-point
1108: (layer 3),
1109: .Dq ethernet
1110: (layer 2), or
1.47 reyk 1111: .Dq no .
1.58 stevesk 1112: Specifying
1113: .Dq yes
1114: permits both
1115: .Dq point-to-point
1116: and
1117: .Dq ethernet .
1.46 reyk 1118: The default is
1119: .Dq no .
1.178 djm 1120: .Pp
1121: Independent of this setting, the permissions of the selected
1122: .Xr tun 4
1123: device must allow access to the user.
1.164 djm 1124: .It Cm PermitTTY
1125: Specifies whether
1.165 jmc 1126: .Xr pty 4
1.164 djm 1127: allocation is permitted.
1128: The default is
1129: .Dq yes .
1.6 markus 1130: .It Cm PermitUserEnvironment
1131: Specifies whether
1132: .Pa ~/.ssh/environment
1.9 stevesk 1133: and
1.6 markus 1134: .Cm environment=
1135: options in
1136: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1137: are processed by
1.52 jmc 1138: .Xr sshd 8 .
1.6 markus 1139: The default is
1140: .Dq no .
1.9 stevesk 1141: Enabling environment processing may enable users to bypass access
1142: restrictions in some configurations using mechanisms such as
1143: .Ev LD_PRELOAD .
1.174 djm 1144: .It Cm PermitUserRC
1145: Specifies whether any
1146: .Pa ~/.ssh/rc
1147: file is executed.
1148: The default is
1149: .Dq yes .
1.1 stevesk 1150: .It Cm PidFile
1.4 stevesk 1151: Specifies the file that contains the process ID of the
1.53 jmc 1152: SSH daemon.
1.1 stevesk 1153: The default is
1154: .Pa /var/run/sshd.pid .
1155: .It Cm Port
1156: Specifies the port number that
1.52 jmc 1157: .Xr sshd 8
1.1 stevesk 1158: listens on.
1159: The default is 22.
1160: Multiple options of this type are permitted.
1161: See also
1162: .Cm ListenAddress .
1163: .It Cm PrintLastLog
1164: Specifies whether
1.52 jmc 1165: .Xr sshd 8
1.36 jaredy 1166: should print the date and time of the last user login when a user logs
1167: in interactively.
1.1 stevesk 1168: The default is
1169: .Dq yes .
1170: .It Cm PrintMotd
1171: Specifies whether
1.52 jmc 1172: .Xr sshd 8
1.1 stevesk 1173: should print
1174: .Pa /etc/motd
1175: when a user logs in interactively.
1176: (On some systems it is also printed by the shell,
1177: .Pa /etc/profile ,
1178: or equivalent.)
1179: The default is
1180: .Dq yes .
1181: .It Cm Protocol
1182: Specifies the protocol versions
1.52 jmc 1183: .Xr sshd 8
1.5 stevesk 1184: supports.
1.1 stevesk 1185: The possible values are
1.52 jmc 1186: .Sq 1
1.1 stevesk 1187: and
1.52 jmc 1188: .Sq 2 .
1.1 stevesk 1189: Multiple versions must be comma-separated.
1190: The default is
1.109 jmc 1191: .Sq 2 .
1.5 stevesk 1192: Note that the order of the protocol list does not indicate preference,
1193: because the client selects among multiple protocol versions offered
1194: by the server.
1195: Specifying
1196: .Dq 2,1
1197: is identical to
1198: .Dq 1,2 .
1.189 ! djm 1199: .It Cm PubkeyAcceptedKeyTypes
! 1200: Specifies the key types that will be accepted for public key authentication
! 1201: as a comma-separated pattern list.
! 1202: The default
! 1203: .Dq *
! 1204: will allow all key types.
! 1205: The
! 1206: .Fl Q
! 1207: option of
! 1208: .Xr ssh 1
! 1209: may be used to list supported key types.
1.1 stevesk 1210: .It Cm PubkeyAuthentication
1211: Specifies whether public key authentication is allowed.
1212: The default is
1213: .Dq yes .
1214: Note that this option applies to protocol version 2 only.
1.159 dtucker 1215: .It Cm RekeyLimit
1216: Specifies the maximum amount of data that may be transmitted before the
1217: session key is renegotiated, optionally followed a maximum amount of
1218: time that may pass before the session key is renegotiated.
1219: The first argument is specified in bytes and may have a suffix of
1220: .Sq K ,
1221: .Sq M ,
1222: or
1223: .Sq G
1224: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1225: The default is between
1226: .Sq 1G
1227: and
1228: .Sq 4G ,
1229: depending on the cipher.
1230: The optional second value is specified in seconds and may use any of the
1231: units documented in the
1232: .Sx TIME FORMATS
1.160 jmc 1233: section.
1.159 dtucker 1234: The default value for
1235: .Cm RekeyLimit
1236: is
1237: .Dq default none ,
1238: which means that rekeying is performed after the cipher's default amount
1239: of data has been sent or received and no time based rekeying is done.
1240: This option applies to protocol version 2 only.
1.118 djm 1241: .It Cm RevokedKeys
1.154 djm 1242: Specifies revoked public keys.
1.118 djm 1243: Keys listed in this file will be refused for public key authentication.
1244: Note that if this file is not readable, then public key authentication will
1245: be refused for all users.
1.154 djm 1246: Keys may be specified as a text file, listing one public key per line, or as
1247: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1248: .Xr ssh-keygen 1 .
1.161 jmc 1249: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1250: .Xr ssh-keygen 1 .
1.1 stevesk 1251: .It Cm RhostsRSAAuthentication
1252: Specifies whether rhosts or /etc/hosts.equiv authentication together
1253: with successful RSA host authentication is allowed.
1254: The default is
1255: .Dq no .
1256: This option applies to protocol version 1 only.
1257: .It Cm RSAAuthentication
1258: Specifies whether pure RSA authentication is allowed.
1259: The default is
1260: .Dq yes .
1261: This option applies to protocol version 1 only.
1262: .It Cm ServerKeyBits
1263: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1264: The minimum value is 512, and the default is 1024.
1.175 millert 1265: .It Cm StreamLocalBindMask
1266: Sets the octal file creation mode mask
1267: .Pq umask
1268: used when creating a Unix-domain socket file for local or remote
1269: port forwarding.
1270: This option is only used for port forwarding to a Unix-domain socket file.
1271: .Pp
1272: The default value is 0177, which creates a Unix-domain socket file that is
1273: readable and writable only by the owner.
1274: Note that not all operating systems honor the file mode on Unix-domain
1275: socket files.
1276: .It Cm StreamLocalBindUnlink
1277: Specifies whether to remove an existing Unix-domain socket file for local
1278: or remote port forwarding before creating a new one.
1279: If the socket file already exists and
1280: .Cm StreamLocalBindUnlink
1281: is not enabled,
1282: .Nm sshd
1283: will be unable to forward the port to the Unix-domain socket file.
1284: This option is only used for port forwarding to a Unix-domain socket file.
1285: .Pp
1286: The argument must be
1287: .Dq yes
1288: or
1289: .Dq no .
1290: The default is
1291: .Dq no .
1.1 stevesk 1292: .It Cm StrictModes
1293: Specifies whether
1.52 jmc 1294: .Xr sshd 8
1.1 stevesk 1295: should check file modes and ownership of the
1296: user's files and home directory before accepting login.
1297: This is normally desirable because novices sometimes accidentally leave their
1298: directory or files world-writable.
1299: The default is
1300: .Dq yes .
1.112 djm 1301: Note that this does not apply to
1302: .Cm ChrootDirectory ,
1303: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1304: .It Cm Subsystem
1.51 jmc 1305: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1306: Arguments should be a subsystem name and a command (with optional arguments)
1307: to execute upon subsystem request.
1.80 djm 1308: .Pp
1.1 stevesk 1309: The command
1310: .Xr sftp-server 8
1311: implements the
1312: .Dq sftp
1313: file transfer subsystem.
1.80 djm 1314: .Pp
1315: Alternately the name
1316: .Dq internal-sftp
1317: implements an in-process
1318: .Dq sftp
1319: server.
1320: This may simplify configurations using
1321: .Cm ChrootDirectory
1322: to force a different filesystem root on clients.
1323: .Pp
1.1 stevesk 1324: By default no subsystems are defined.
1325: Note that this option applies to protocol version 2 only.
1326: .It Cm SyslogFacility
1327: Gives the facility code that is used when logging messages from
1.53 jmc 1328: .Xr sshd 8 .
1.1 stevesk 1329: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1330: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1331: The default is AUTH.
1.27 markus 1332: .It Cm TCPKeepAlive
1333: Specifies whether the system should send TCP keepalive messages to the
1334: other side.
1335: If they are sent, death of the connection or crash of one
1336: of the machines will be properly noticed.
1337: However, this means that
1338: connections will die if the route is down temporarily, and some people
1339: find it annoying.
1340: On the other hand, if TCP keepalives are not sent,
1341: sessions may hang indefinitely on the server, leaving
1342: .Dq ghost
1343: users and consuming server resources.
1344: .Pp
1345: The default is
1346: .Dq yes
1347: (to send TCP keepalive messages), and the server will notice
1348: if the network goes down or the client host crashes.
1349: This avoids infinitely hanging sessions.
1350: .Pp
1351: To disable TCP keepalive messages, the value should be set to
1352: .Dq no .
1.118 djm 1353: .It Cm TrustedUserCAKeys
1354: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1355: trusted to sign user certificates for authentication.
1.119 jmc 1356: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1357: .Ql #
1358: are allowed.
1359: If a certificate is presented for authentication and has its signing CA key
1360: listed in this file, then it may be used for authentication for any user
1361: listed in the certificate's principals list.
1362: Note that certificates that lack a list of principals will not be permitted
1363: for authentication using
1364: .Cm TrustedUserCAKeys .
1.161 jmc 1365: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1366: .Xr ssh-keygen 1 .
1.18 markus 1367: .It Cm UseDNS
1368: Specifies whether
1.52 jmc 1369: .Xr sshd 8
1.40 jmc 1370: should look up the remote host name and check that
1.18 markus 1371: the resolved host name for the remote IP address maps back to the
1372: very same IP address.
1373: The default is
1374: .Dq yes .
1.1 stevesk 1375: .It Cm UseLogin
1376: Specifies whether
1377: .Xr login 1
1378: is used for interactive login sessions.
1379: The default is
1380: .Dq no .
1381: Note that
1382: .Xr login 1
1383: is never used for remote command execution.
1384: Note also, that if this is enabled,
1385: .Cm X11Forwarding
1386: will be disabled because
1387: .Xr login 1
1388: does not know how to handle
1389: .Xr xauth 1
1.15 jmc 1390: cookies.
1391: If
1.1 stevesk 1392: .Cm UsePrivilegeSeparation
1393: is specified, it will be disabled after authentication.
1394: .It Cm UsePrivilegeSeparation
1395: Specifies whether
1.52 jmc 1396: .Xr sshd 8
1.2 stevesk 1397: separates privileges by creating an unprivileged child process
1.15 jmc 1398: to deal with incoming network traffic.
1399: After successful authentication, another process will be created that has
1400: the privilege of the authenticated user.
1401: The goal of privilege separation is to prevent privilege
1.1 stevesk 1402: escalation by containing any corruption within the unprivileged processes.
1403: The default is
1404: .Dq yes .
1.134 djm 1405: If
1406: .Cm UsePrivilegeSeparation
1407: is set to
1408: .Dq sandbox
1409: then the pre-authentication unprivileged process is subject to additional
1410: restrictions.
1.137 djm 1411: .It Cm VersionAddendum
1412: Optionally specifies additional text to append to the SSH protocol banner
1413: sent by the server upon connection.
1414: The default is
1415: .Dq none .
1.1 stevesk 1416: .It Cm X11DisplayOffset
1417: Specifies the first display number available for
1.52 jmc 1418: .Xr sshd 8 Ns 's
1.1 stevesk 1419: X11 forwarding.
1.52 jmc 1420: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1421: The default is 10.
1422: .It Cm X11Forwarding
1423: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1424: The argument must be
1425: .Dq yes
1426: or
1427: .Dq no .
1.1 stevesk 1428: The default is
1429: .Dq no .
1.13 stevesk 1430: .Pp
1431: When X11 forwarding is enabled, there may be additional exposure to
1432: the server and to client displays if the
1.52 jmc 1433: .Xr sshd 8
1.13 stevesk 1434: proxy display is configured to listen on the wildcard address (see
1435: .Cm X11UseLocalhost
1.52 jmc 1436: below), though this is not the default.
1.13 stevesk 1437: Additionally, the authentication spoofing and authentication data
1438: verification and substitution occur on the client side.
1439: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1440: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1441: forwarding (see the warnings for
1442: .Cm ForwardX11
1443: in
1.19 jmc 1444: .Xr ssh_config 5 ) .
1.13 stevesk 1445: A system administrator may have a stance in which they want to
1446: protect clients that may expose themselves to attack by unwittingly
1447: requesting X11 forwarding, which can warrant a
1448: .Dq no
1449: setting.
1450: .Pp
1451: Note that disabling X11 forwarding does not prevent users from
1452: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1453: X11 forwarding is automatically disabled if
1454: .Cm UseLogin
1455: is enabled.
1456: .It Cm X11UseLocalhost
1457: Specifies whether
1.52 jmc 1458: .Xr sshd 8
1.1 stevesk 1459: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1460: the wildcard address.
1461: By default,
1.52 jmc 1462: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1463: hostname part of the
1464: .Ev DISPLAY
1465: environment variable to
1466: .Dq localhost .
1.8 stevesk 1467: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1468: However, some older X11 clients may not function with this
1469: configuration.
1470: .Cm X11UseLocalhost
1471: may be set to
1472: .Dq no
1473: to specify that the forwarding server should be bound to the wildcard
1474: address.
1475: The argument must be
1476: .Dq yes
1477: or
1478: .Dq no .
1479: The default is
1480: .Dq yes .
1481: .It Cm XAuthLocation
1.11 stevesk 1482: Specifies the full pathname of the
1.1 stevesk 1483: .Xr xauth 1
1484: program.
1485: The default is
1486: .Pa /usr/X11R6/bin/xauth .
1487: .El
1.55 jmc 1488: .Sh TIME FORMATS
1.53 jmc 1489: .Xr sshd 8
1.1 stevesk 1490: command-line arguments and configuration file options that specify time
1491: may be expressed using a sequence of the form:
1492: .Sm off
1.7 stevesk 1493: .Ar time Op Ar qualifier ,
1.1 stevesk 1494: .Sm on
1495: where
1496: .Ar time
1497: is a positive integer value and
1498: .Ar qualifier
1499: is one of the following:
1500: .Pp
1501: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1502: .It Aq Cm none
1.1 stevesk 1503: seconds
1504: .It Cm s | Cm S
1505: seconds
1506: .It Cm m | Cm M
1507: minutes
1508: .It Cm h | Cm H
1509: hours
1510: .It Cm d | Cm D
1511: days
1512: .It Cm w | Cm W
1513: weeks
1514: .El
1515: .Pp
1516: Each member of the sequence is added together to calculate
1517: the total time value.
1518: .Pp
1519: Time format examples:
1520: .Pp
1521: .Bl -tag -width Ds -compact -offset indent
1522: .It 600
1523: 600 seconds (10 minutes)
1524: .It 10m
1525: 10 minutes
1526: .It 1h30m
1527: 1 hour 30 minutes (90 minutes)
1528: .El
1529: .Sh FILES
1530: .Bl -tag -width Ds
1531: .It Pa /etc/ssh/sshd_config
1532: Contains configuration data for
1.53 jmc 1533: .Xr sshd 8 .
1.1 stevesk 1534: This file should be writable by root only, but it is recommended
1535: (though not necessary) that it be world-readable.
1536: .El
1.19 jmc 1537: .Sh SEE ALSO
1538: .Xr sshd 8
1.1 stevesk 1539: .Sh AUTHORS
1540: OpenSSH is a derivative of the original and free
1541: ssh 1.2.12 release by Tatu Ylonen.
1542: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1543: Theo de Raadt and Dug Song
1544: removed many bugs, re-added newer features and
1545: created OpenSSH.
1546: Markus Friedl contributed the support for SSH
1547: protocol versions 1.5 and 2.0.
1548: Niels Provos and Markus Friedl contributed support
1549: for privilege separation.
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