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