Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.174
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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1.174 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.173 2014/03/28 05:17:11 naddy Exp $
! 37: .Dd $Mdocdate: March 28 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.
143: .It Cm AllowUsers
144: This keyword can be followed by a list of user name patterns, separated
145: by spaces.
1.14 jmc 146: If specified, login is allowed only for user names that
1.1 stevesk 147: match one of the patterns.
148: Only user names are valid; a numerical user ID is not recognized.
149: By default, login is allowed for all users.
150: If the pattern takes the form USER@HOST then USER and HOST
151: are separately checked, restricting logins to particular
152: users from particular hosts.
1.54 jmc 153: The allow/deny directives are processed in the following order:
154: .Cm DenyUsers ,
155: .Cm AllowUsers ,
156: .Cm DenyGroups ,
157: and finally
158: .Cm AllowGroups .
1.49 jmc 159: .Pp
1.161 jmc 160: See PATTERNS in
1.49 jmc 161: .Xr ssh_config 5
162: for more information on patterns.
1.149 djm 163: .It Cm AuthenticationMethods
164: Specifies the authentication methods that must be successfully completed
165: for a user to be granted access.
166: This option must be followed by one or more comma-separated lists of
167: authentication method names.
168: Successful authentication requires completion of every method in at least
169: one of these lists.
170: .Pp
171: For example, an argument of
172: .Dq publickey,password publickey,keyboard-interactive
173: would require the user to complete public key authentication, followed by
174: either password or keyboard interactive authentication.
175: Only methods that are next in one or more lists are offered at each stage,
176: so for this example, it would not be possible to attempt password or
177: keyboard-interactive authentication before public key.
1.157 markus 178: .Pp
179: For keyboard interactive authentication it is also possible to
180: restrict authentication to a specific device by appending a
181: colon followed by the device identifier
182: .Dq bsdauth ,
183: .Dq pam ,
184: or
185: .Dq skey ,
186: depending on the server configuration.
187: For example,
188: .Dq keyboard-interactive:bsdauth
189: would restrict keyboard interactive authentication to the
190: .Dq bsdauth
191: device.
1.149 djm 192: .Pp
193: This option is only available for SSH protocol 2 and will yield a fatal
194: error if enabled if protocol 1 is also enabled.
195: Note that each authentication method listed should also be explicitly enabled
196: in the configuration.
197: The default is not to require multiple authentication; successful completion
198: of a single authentication method is sufficient.
1.146 djm 199: .It Cm AuthorizedKeysCommand
1.147 jmc 200: Specifies a program to be used to look up the user's public keys.
1.158 djm 201: The program must be owned by root and not writable by group or others.
202: It will be invoked with a single argument of the username
1.146 djm 203: being authenticated, and should produce on standard output zero or
1.161 jmc 204: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 205: .Xr sshd 8 ) .
1.146 djm 206: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
207: and authorize the user then public key authentication continues using the usual
208: .Cm AuthorizedKeysFile
209: files.
210: By default, no AuthorizedKeysCommand is run.
211: .It Cm AuthorizedKeysCommandUser
212: Specifies the user under whose account the AuthorizedKeysCommand is run.
213: It is recommended to use a dedicated user that has no other role on the host
214: than running authorized keys commands.
1.1 stevesk 215: .It Cm AuthorizedKeysFile
216: Specifies the file that contains the public keys that can be used
217: for user authentication.
1.124 djm 218: The format is described in the
1.161 jmc 219: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 220: section of
221: .Xr sshd 8 .
1.1 stevesk 222: .Cm AuthorizedKeysFile
223: may contain tokens of the form %T which are substituted during connection
1.52 jmc 224: setup.
1.17 jmc 225: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 226: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 227: %u is replaced by the username of that user.
228: After expansion,
229: .Cm AuthorizedKeysFile
230: is taken to be an absolute path or one relative to the user's home
231: directory.
1.133 jmc 232: Multiple files may be listed, separated by whitespace.
233: The default is
234: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 235: .It Cm AuthorizedPrincipalsFile
236: Specifies a file that lists principal names that are accepted for
237: certificate authentication.
238: When using certificates signed by a key listed in
239: .Cm TrustedUserCAKeys ,
240: this file lists names, one of which must appear in the certificate for it
241: to be accepted for authentication.
1.125 jmc 242: Names are listed one per line preceded by key options (as described
1.161 jmc 243: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 244: .Xr sshd 8 ) .
1.124 djm 245: Empty lines and comments starting with
1.121 djm 246: .Ql #
247: are ignored.
248: .Pp
249: .Cm AuthorizedPrincipalsFile
250: may contain tokens of the form %T which are substituted during connection
251: setup.
252: The following tokens are defined: %% is replaced by a literal '%',
253: %h is replaced by the home directory of the user being authenticated, and
254: %u is replaced by the username of that user.
255: After expansion,
256: .Cm AuthorizedPrincipalsFile
257: is taken to be an absolute path or one relative to the user's home
258: directory.
259: .Pp
1.138 djm 260: The default is
261: .Dq none ,
262: i.e. not to use a principals file \(en in this case, the username
1.121 djm 263: of the user must appear in a certificate's principals list for it to be
264: accepted.
265: Note that
266: .Cm AuthorizedPrincipalsFile
267: is only used when authentication proceeds using a CA listed in
268: .Cm TrustedUserCAKeys
269: and is not consulted for certification authorities trusted via
270: .Pa ~/.ssh/authorized_keys ,
271: though the
272: .Cm principals=
273: key option offers a similar facility (see
274: .Xr sshd 8
275: for details).
1.1 stevesk 276: .It Cm Banner
277: The contents of the specified file are sent to the remote user before
278: authentication is allowed.
1.78 djm 279: If the argument is
280: .Dq none
281: then no banner is displayed.
1.1 stevesk 282: This option is only available for protocol version 2.
283: By default, no banner is displayed.
284: .It Cm ChallengeResponseAuthentication
1.50 jmc 285: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 286: All authentication styles from
287: .Xr login.conf 5
288: are supported.
289: The default is
290: .Dq yes .
1.80 djm 291: .It Cm ChrootDirectory
1.113 stevesk 292: Specifies the pathname of a directory to
1.80 djm 293: .Xr chroot 2
294: to after authentication.
1.113 stevesk 295: All components of the pathname must be root-owned directories that are
1.80 djm 296: not writable by any other user or group.
1.106 stevesk 297: After the chroot,
298: .Xr sshd 8
299: changes the working directory to the user's home directory.
1.80 djm 300: .Pp
1.113 stevesk 301: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 302: the connecting user has been authenticated: %% is replaced by a literal '%',
303: %h is replaced by the home directory of the user being authenticated, and
304: %u is replaced by the username of that user.
305: .Pp
306: The
307: .Cm ChrootDirectory
308: must contain the necessary files and directories to support the
1.103 stevesk 309: user's session.
1.80 djm 310: For an interactive session this requires at least a shell, typically
311: .Xr sh 1 ,
312: and basic
313: .Pa /dev
314: nodes such as
315: .Xr null 4 ,
316: .Xr zero 4 ,
317: .Xr stdin 4 ,
318: .Xr stdout 4 ,
319: .Xr stderr 4 ,
320: .Xr arandom 4
321: and
322: .Xr tty 4
323: devices.
324: For file transfer sessions using
1.105 jmc 325: .Dq sftp ,
1.80 djm 326: no additional configuration of the environment is necessary if the
1.105 jmc 327: in-process sftp server is used,
328: though sessions which use logging do require
1.104 stevesk 329: .Pa /dev/log
330: inside the chroot directory (see
331: .Xr sftp-server 8
1.81 jmc 332: for details).
1.80 djm 333: .Pp
334: The default is not to
335: .Xr chroot 2 .
1.1 stevesk 336: .It Cm Ciphers
337: Specifies the ciphers allowed for protocol version 2.
338: Multiple ciphers must be comma-separated.
1.167 djm 339: The supported ciphers are:
340: .Pp
1.173 naddy 341: .Bl -item -compact -offset indent
342: .It
343: 3des-cbc
344: .It
345: aes128-cbc
346: .It
347: aes192-cbc
348: .It
349: aes256-cbc
350: .It
351: aes128-ctr
352: .It
353: aes192-ctr
354: .It
355: aes256-ctr
356: .It
357: aes128-gcm@openssh.com
358: .It
359: aes256-gcm@openssh.com
360: .It
361: arcfour
362: .It
363: arcfour128
364: .It
365: arcfour256
366: .It
367: blowfish-cbc
368: .It
369: cast128-cbc
370: .It
371: chacha20-poly1305@openssh.com
372: .El
1.167 djm 373: .Pp
1.52 jmc 374: The default is:
1.173 naddy 375: .Bd -literal -offset indent
376: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 377: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 378: chacha20-poly1305@openssh.com
1.1 stevesk 379: .Ed
1.167 djm 380: .Pp
381: The list of available ciphers may also be obtained using the
382: .Fl Q
383: option of
384: .Xr ssh 1 .
1.1 stevesk 385: .It Cm ClientAliveCountMax
1.48 jmc 386: Sets the number of client alive messages (see below) which may be
1.1 stevesk 387: sent without
1.52 jmc 388: .Xr sshd 8
1.17 jmc 389: receiving any messages back from the client.
390: If this threshold is reached while client alive messages are being sent,
1.52 jmc 391: sshd will disconnect the client, terminating the session.
1.17 jmc 392: It is important to note that the use of client alive messages is very
393: different from
1.27 markus 394: .Cm TCPKeepAlive
1.17 jmc 395: (below).
396: The client alive messages are sent through the encrypted channel
397: and therefore will not be spoofable.
398: The TCP keepalive option enabled by
1.27 markus 399: .Cm TCPKeepAlive
1.17 jmc 400: is spoofable.
401: The client alive mechanism is valuable when the client or
1.1 stevesk 402: server depend on knowing when a connection has become inactive.
403: .Pp
1.17 jmc 404: The default value is 3.
405: If
1.1 stevesk 406: .Cm ClientAliveInterval
1.48 jmc 407: (see below) is set to 15, and
1.1 stevesk 408: .Cm ClientAliveCountMax
1.52 jmc 409: is left at the default, unresponsive SSH clients
1.1 stevesk 410: will be disconnected after approximately 45 seconds.
1.57 markus 411: This option applies to protocol version 2 only.
1.42 djm 412: .It Cm ClientAliveInterval
413: Sets a timeout interval in seconds after which if no data has been received
414: from the client,
1.52 jmc 415: .Xr sshd 8
1.42 djm 416: will send a message through the encrypted
417: channel to request a response from the client.
418: The default
419: is 0, indicating that these messages will not be sent to the client.
420: This option applies to protocol version 2 only.
1.3 markus 421: .It Cm Compression
1.44 markus 422: Specifies whether compression is allowed, or delayed until
423: the user has authenticated successfully.
1.3 markus 424: The argument must be
1.44 markus 425: .Dq yes ,
426: .Dq delayed ,
1.3 markus 427: or
428: .Dq no .
429: The default is
1.44 markus 430: .Dq delayed .
1.1 stevesk 431: .It Cm DenyGroups
432: This keyword can be followed by a list of group name patterns, separated
433: by spaces.
434: Login is disallowed for users whose primary group or supplementary
435: group list matches one of the patterns.
436: Only group names are valid; a numerical group ID is not recognized.
437: By default, login is allowed for all groups.
1.54 jmc 438: The allow/deny directives are processed in the following order:
439: .Cm DenyUsers ,
440: .Cm AllowUsers ,
441: .Cm DenyGroups ,
442: and finally
443: .Cm AllowGroups .
1.49 jmc 444: .Pp
1.161 jmc 445: See PATTERNS in
1.49 jmc 446: .Xr ssh_config 5
447: for more information on patterns.
1.1 stevesk 448: .It Cm DenyUsers
449: This keyword can be followed by a list of user name patterns, separated
450: by spaces.
451: Login is disallowed for user names that match one of the patterns.
452: Only user names are valid; a numerical user ID is not recognized.
453: By default, login is allowed for all users.
454: If the pattern takes the form USER@HOST then USER and HOST
455: are separately checked, restricting logins to particular
456: users from particular hosts.
1.54 jmc 457: The allow/deny directives are processed in the following order:
458: .Cm DenyUsers ,
459: .Cm AllowUsers ,
460: .Cm DenyGroups ,
461: and finally
462: .Cm AllowGroups .
1.49 jmc 463: .Pp
1.161 jmc 464: See PATTERNS in
1.49 jmc 465: .Xr ssh_config 5
466: for more information on patterns.
1.67 dtucker 467: .It Cm ForceCommand
468: Forces the execution of the command specified by
469: .Cm ForceCommand ,
1.84 djm 470: ignoring any command supplied by the client and
471: .Pa ~/.ssh/rc
472: if present.
1.67 dtucker 473: The command is invoked by using the user's login shell with the -c option.
474: This applies to shell, command, or subsystem execution.
475: It is most useful inside a
476: .Cm Match
477: block.
478: The command originally supplied by the client is available in the
479: .Ev SSH_ORIGINAL_COMMAND
480: environment variable.
1.82 djm 481: Specifying a command of
482: .Dq internal-sftp
483: will force the use of an in-process sftp server that requires no support
484: files when used with
485: .Cm ChrootDirectory .
1.1 stevesk 486: .It Cm GatewayPorts
487: Specifies whether remote hosts are allowed to connect to ports
488: forwarded for the client.
489: By default,
1.52 jmc 490: .Xr sshd 8
1.15 jmc 491: binds remote port forwardings to the loopback address.
492: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 493: .Cm GatewayPorts
1.52 jmc 494: can be used to specify that sshd
1.39 djm 495: should allow remote port forwardings to bind to non-loopback addresses, thus
496: allowing other hosts to connect.
497: The argument may be
498: .Dq no
499: to force remote port forwardings to be available to the local host only,
1.1 stevesk 500: .Dq yes
1.39 djm 501: to force remote port forwardings to bind to the wildcard address, or
502: .Dq clientspecified
503: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 504: The default is
505: .Dq no .
1.23 markus 506: .It Cm GSSAPIAuthentication
1.25 markus 507: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 508: The default is
1.23 markus 509: .Dq no .
510: Note that this option applies to protocol version 2 only.
511: .It Cm GSSAPICleanupCredentials
512: Specifies whether to automatically destroy the user's credentials cache
513: on logout.
514: The default is
515: .Dq yes .
516: Note that this option applies to protocol version 2 only.
1.1 stevesk 517: .It Cm HostbasedAuthentication
518: Specifies whether rhosts or /etc/hosts.equiv authentication together
519: with successful public key client host authentication is allowed
1.50 jmc 520: (host-based authentication).
1.1 stevesk 521: This option is similar to
522: .Cm RhostsRSAAuthentication
523: and applies to protocol version 2 only.
1.70 dtucker 524: The default is
525: .Dq no .
526: .It Cm HostbasedUsesNameFromPacketOnly
527: Specifies whether or not the server will attempt to perform a reverse
528: name lookup when matching the name in the
529: .Pa ~/.shosts ,
530: .Pa ~/.rhosts ,
531: and
532: .Pa /etc/hosts.equiv
533: files during
534: .Cm HostbasedAuthentication .
535: A setting of
536: .Dq yes
537: means that
538: .Xr sshd 8
539: uses the name supplied by the client rather than
540: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 541: The default is
542: .Dq no .
1.117 djm 543: .It Cm HostCertificate
544: Specifies a file containing a public host certificate.
545: The certificate's public key must match a private host key already specified
546: by
547: .Cm HostKey .
548: The default behaviour of
549: .Xr sshd 8
550: is not to load any certificates.
1.1 stevesk 551: .It Cm HostKey
552: Specifies a file containing a private host key
553: used by SSH.
554: The default is
555: .Pa /etc/ssh/ssh_host_key
556: for protocol version 1, and
1.126 djm 557: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 558: .Pa /etc/ssh/ssh_host_ecdsa_key ,
559: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 560: and
1.1 stevesk 561: .Pa /etc/ssh/ssh_host_rsa_key
562: for protocol version 2.
563: Note that
1.52 jmc 564: .Xr sshd 8
1.1 stevesk 565: will refuse to use a file if it is group/world-accessible.
566: It is possible to have multiple host key files.
567: .Dq rsa1
568: keys are used for version 1 and
1.126 djm 569: .Dq dsa ,
1.169 naddy 570: .Dq ecdsa ,
571: .Dq ed25519
1.1 stevesk 572: or
573: .Dq rsa
574: are used for version 2 of the SSH protocol.
1.162 markus 575: It is also possible to specify public host key files instead.
576: In this case operations on the private key will be delegated
577: to an
578: .Xr ssh-agent 1 .
579: .It Cm HostKeyAgent
580: Identifies the UNIX-domain socket used to communicate
581: with an agent that has access to the private host keys.
582: If
583: .Dq SSH_AUTH_SOCK
584: is specified, the location of the socket will be read from the
585: .Ev SSH_AUTH_SOCK
586: environment variable.
1.1 stevesk 587: .It Cm IgnoreRhosts
588: Specifies that
589: .Pa .rhosts
590: and
591: .Pa .shosts
592: files will not be used in
593: .Cm RhostsRSAAuthentication
594: or
595: .Cm HostbasedAuthentication .
596: .Pp
597: .Pa /etc/hosts.equiv
598: and
599: .Pa /etc/shosts.equiv
600: are still used.
601: The default is
602: .Dq yes .
603: .It Cm IgnoreUserKnownHosts
604: Specifies whether
1.52 jmc 605: .Xr sshd 8
1.1 stevesk 606: should ignore the user's
1.41 djm 607: .Pa ~/.ssh/known_hosts
1.1 stevesk 608: during
609: .Cm RhostsRSAAuthentication
610: or
611: .Cm HostbasedAuthentication .
612: The default is
613: .Dq no .
1.129 djm 614: .It Cm IPQoS
615: Specifies the IPv4 type-of-service or DSCP class for the connection.
616: Accepted values are
617: .Dq af11 ,
618: .Dq af12 ,
619: .Dq af13 ,
1.136 djm 620: .Dq af21 ,
1.129 djm 621: .Dq af22 ,
622: .Dq af23 ,
623: .Dq af31 ,
624: .Dq af32 ,
625: .Dq af33 ,
626: .Dq af41 ,
627: .Dq af42 ,
628: .Dq af43 ,
629: .Dq cs0 ,
630: .Dq cs1 ,
631: .Dq cs2 ,
632: .Dq cs3 ,
633: .Dq cs4 ,
634: .Dq cs5 ,
635: .Dq cs6 ,
636: .Dq cs7 ,
637: .Dq ef ,
638: .Dq lowdelay ,
639: .Dq throughput ,
640: .Dq reliability ,
641: or a numeric value.
1.131 djm 642: This option may take one or two arguments, separated by whitespace.
1.129 djm 643: If one argument is specified, it is used as the packet class unconditionally.
644: If two values are specified, the first is automatically selected for
645: interactive sessions and the second for non-interactive sessions.
646: The default is
647: .Dq lowdelay
648: for interactive sessions and
649: .Dq throughput
650: for non-interactive sessions.
1.171 jmc 651: .It Cm KbdInteractiveAuthentication
652: Specifies whether to allow keyboard-interactive authentication.
653: The argument to this keyword must be
654: .Dq yes
655: or
656: .Dq no .
657: The default is to use whatever value
658: .Cm ChallengeResponseAuthentication
659: is set to
660: (by default
661: .Dq yes ) .
1.1 stevesk 662: .It Cm KerberosAuthentication
1.24 markus 663: Specifies whether the password provided by the user for
1.1 stevesk 664: .Cm PasswordAuthentication
1.24 markus 665: will be validated through the Kerberos KDC.
1.1 stevesk 666: To use this option, the server needs a
667: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 668: The default is
1.29 dtucker 669: .Dq no .
670: .It Cm KerberosGetAFSToken
1.45 djm 671: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 672: an AFS token before accessing the user's home directory.
1.52 jmc 673: The default is
1.1 stevesk 674: .Dq no .
675: .It Cm KerberosOrLocalPasswd
1.52 jmc 676: If password authentication through Kerberos fails then
1.1 stevesk 677: the password will be validated via any additional local mechanism
678: such as
679: .Pa /etc/passwd .
1.52 jmc 680: The default is
1.1 stevesk 681: .Dq yes .
682: .It Cm KerberosTicketCleanup
683: Specifies whether to automatically destroy the user's ticket cache
684: file on logout.
1.52 jmc 685: The default is
1.1 stevesk 686: .Dq yes .
1.127 djm 687: .It Cm KexAlgorithms
688: Specifies the available KEX (Key Exchange) algorithms.
689: Multiple algorithms must be comma-separated.
1.173 naddy 690: The supported algorithms are:
691: .Pp
692: .Bl -item -compact -offset indent
693: .It
694: curve25519-sha256@libssh.org
695: .It
696: diffie-hellman-group1-sha1
697: .It
698: diffie-hellman-group14-sha1
699: .It
700: diffie-hellman-group-exchange-sha1
701: .It
702: diffie-hellman-group-exchange-sha256
703: .It
704: ecdh-sha2-nistp256
705: .It
706: ecdh-sha2-nistp384
707: .It
708: ecdh-sha2-nistp521
709: .El
710: .Pp
711: The default is:
1.170 dtucker 712: .Bd -literal -offset indent
713: curve25519-sha256@libssh.org,
714: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
715: diffie-hellman-group-exchange-sha256,
1.173 naddy 716: diffie-hellman-group14-sha1
1.170 dtucker 717: .Ed
1.1 stevesk 718: .It Cm KeyRegenerationInterval
719: In protocol version 1, the ephemeral server key is automatically regenerated
720: after this many seconds (if it has been used).
721: The purpose of regeneration is to prevent
722: decrypting captured sessions by later breaking into the machine and
723: stealing the keys.
724: The key is never stored anywhere.
725: If the value is 0, the key is never regenerated.
726: The default is 3600 (seconds).
727: .It Cm ListenAddress
728: Specifies the local addresses
1.52 jmc 729: .Xr sshd 8
1.1 stevesk 730: should listen on.
731: The following forms may be used:
732: .Pp
733: .Bl -item -offset indent -compact
734: .It
735: .Cm ListenAddress
736: .Sm off
737: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
738: .Sm on
739: .It
740: .Cm ListenAddress
741: .Sm off
742: .Ar host No | Ar IPv4_addr No : Ar port
743: .Sm on
744: .It
745: .Cm ListenAddress
746: .Sm off
747: .Oo
748: .Ar host No | Ar IPv6_addr Oc : Ar port
749: .Sm on
750: .El
751: .Pp
752: If
753: .Ar port
754: is not specified,
1.52 jmc 755: sshd will listen on the address and all prior
1.1 stevesk 756: .Cm Port
1.17 jmc 757: options specified.
758: The default is to listen on all local addresses.
1.15 jmc 759: Multiple
1.1 stevesk 760: .Cm ListenAddress
1.17 jmc 761: options are permitted.
762: Additionally, any
1.1 stevesk 763: .Cm Port
1.52 jmc 764: options must precede this option for non-port qualified addresses.
1.1 stevesk 765: .It Cm LoginGraceTime
766: The server disconnects after this time if the user has not
767: successfully logged in.
768: If the value is 0, there is no time limit.
1.12 stevesk 769: The default is 120 seconds.
1.1 stevesk 770: .It Cm LogLevel
771: Gives the verbosity level that is used when logging messages from
1.53 jmc 772: .Xr sshd 8 .
1.1 stevesk 773: The possible values are:
1.52 jmc 774: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 775: The default is INFO.
776: DEBUG and DEBUG1 are equivalent.
777: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
778: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 779: .It Cm MACs
780: Specifies the available MAC (message authentication code) algorithms.
781: The MAC algorithm is used in protocol version 2
782: for data integrity protection.
783: Multiple algorithms must be comma-separated.
1.152 markus 784: The algorithms that contain
785: .Dq -etm
786: calculate the MAC after encryption (encrypt-then-mac).
787: These are considered safer and their use recommended.
1.173 naddy 788: The supported MACs are:
789: .Pp
790: .Bl -item -compact -offset indent
791: .It
792: hmac-md5
793: .It
794: hmac-md5-96
795: .It
796: hmac-ripemd160
797: .It
798: hmac-sha1
799: .It
800: hmac-sha1-96
801: .It
802: hmac-sha2-256
803: .It
804: hmac-sha2-512
805: .It
806: umac-64@openssh.com
807: .It
808: umac-128@openssh.com
809: .It
810: hmac-md5-etm@openssh.com
811: .It
812: hmac-md5-96-etm@openssh.com
813: .It
814: hmac-ripemd160-etm@openssh.com
815: .It
816: hmac-sha1-etm@openssh.com
817: .It
818: hmac-sha1-96-etm@openssh.com
819: .It
820: hmac-sha2-256-etm@openssh.com
821: .It
822: hmac-sha2-512-etm@openssh.com
823: .It
824: umac-64-etm@openssh.com
825: .It
826: umac-128-etm@openssh.com
827: .El
828: .Pp
1.52 jmc 829: The default is:
1.77 jmc 830: .Bd -literal -offset indent
1.152 markus 831: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
832: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 833: umac-64@openssh.com,umac-128@openssh.com,
834: hmac-sha2-256,hmac-sha2-512
1.77 jmc 835: .Ed
1.60 dtucker 836: .It Cm Match
1.61 jmc 837: Introduces a conditional block.
1.65 dtucker 838: If all of the criteria on the
1.60 dtucker 839: .Cm Match
1.65 dtucker 840: line are satisfied, the keywords on the following lines override those
841: set in the global section of the config file, until either another
1.60 dtucker 842: .Cm Match
1.65 dtucker 843: line or the end of the file.
1.172 djm 844: If a keyword appears in multiple
845: .Cm Match
846: blocks that are satisified, only the first instance of the keyword is
847: applied.
1.91 djm 848: .Pp
1.61 jmc 849: The arguments to
1.60 dtucker 850: .Cm Match
1.163 dtucker 851: are one or more criteria-pattern pairs or the single token
852: .Cm All
853: which matches all criteria.
1.60 dtucker 854: The available criteria are
855: .Cm User ,
1.69 dtucker 856: .Cm Group ,
1.60 dtucker 857: .Cm Host ,
1.139 dtucker 858: .Cm LocalAddress ,
859: .Cm LocalPort ,
1.60 dtucker 860: and
861: .Cm Address .
1.91 djm 862: The match patterns may consist of single entries or comma-separated
863: lists and may use the wildcard and negation operators described in the
1.161 jmc 864: PATTERNS section of
1.92 djm 865: .Xr ssh_config 5 .
1.91 djm 866: .Pp
867: The patterns in an
868: .Cm Address
869: criteria may additionally contain addresses to match in CIDR
1.93 jmc 870: address/masklen format, e.g.\&
1.91 djm 871: .Dq 192.0.2.0/24
872: or
873: .Dq 3ffe:ffff::/32 .
874: Note that the mask length provided must be consistent with the address -
875: it is an error to specify a mask length that is too long for the address
1.93 jmc 876: or one with bits set in this host portion of the address.
877: For example,
1.91 djm 878: .Dq 192.0.2.0/33
879: and
1.93 jmc 880: .Dq 192.0.2.0/8
1.91 djm 881: respectively.
882: .Pp
1.60 dtucker 883: Only a subset of keywords may be used on the lines following a
884: .Cm Match
885: keyword.
886: Available keywords are
1.142 jmc 887: .Cm AcceptEnv ,
1.99 okan 888: .Cm AllowAgentForwarding ,
1.142 jmc 889: .Cm AllowGroups ,
1.62 dtucker 890: .Cm AllowTcpForwarding ,
1.141 markus 891: .Cm AllowUsers ,
1.149 djm 892: .Cm AuthenticationMethods ,
1.146 djm 893: .Cm AuthorizedKeysCommand ,
894: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 895: .Cm AuthorizedKeysFile ,
1.123 djm 896: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 897: .Cm Banner ,
1.85 djm 898: .Cm ChrootDirectory ,
1.141 markus 899: .Cm DenyGroups ,
900: .Cm DenyUsers ,
1.67 dtucker 901: .Cm ForceCommand ,
1.142 jmc 902: .Cm GatewayPorts ,
1.141 markus 903: .Cm GSSAPIAuthentication ,
1.87 djm 904: .Cm HostbasedAuthentication ,
1.123 djm 905: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 906: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 907: .Cm KerberosAuthentication ,
1.95 dtucker 908: .Cm MaxAuthTries ,
1.94 dtucker 909: .Cm MaxSessions ,
1.72 dtucker 910: .Cm PasswordAuthentication ,
1.97 djm 911: .Cm PermitEmptyPasswords ,
1.66 dtucker 912: .Cm PermitOpen ,
1.79 dtucker 913: .Cm PermitRootLogin ,
1.164 djm 914: .Cm PermitTTY ,
1.123 djm 915: .Cm PermitTunnel ,
1.174 ! djm 916: .Cm PermitUserRC ,
1.107 dtucker 917: .Cm PubkeyAuthentication ,
1.159 dtucker 918: .Cm RekeyLimit ,
1.142 jmc 919: .Cm RhostsRSAAuthentication ,
1.141 markus 920: .Cm RSAAuthentication ,
1.66 dtucker 921: .Cm X11DisplayOffset ,
1.101 djm 922: .Cm X11Forwarding
1.60 dtucker 923: and
1.102 djm 924: .Cm X11UseLocalHost .
1.33 dtucker 925: .It Cm MaxAuthTries
926: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 927: connection.
928: Once the number of failures reaches half this value,
929: additional failures are logged.
930: The default is 6.
1.90 djm 931: .It Cm MaxSessions
932: Specifies the maximum number of open sessions permitted per network connection.
933: The default is 10.
1.1 stevesk 934: .It Cm MaxStartups
935: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 936: SSH daemon.
1.1 stevesk 937: Additional connections will be dropped until authentication succeeds or the
938: .Cm LoginGraceTime
939: expires for a connection.
1.156 dtucker 940: The default is 10:30:100.
1.1 stevesk 941: .Pp
942: Alternatively, random early drop can be enabled by specifying
943: the three colon separated values
944: .Dq start:rate:full
1.51 jmc 945: (e.g. "10:30:60").
1.53 jmc 946: .Xr sshd 8
1.1 stevesk 947: will refuse connection attempts with a probability of
948: .Dq rate/100
949: (30%)
950: if there are currently
951: .Dq start
952: (10)
953: unauthenticated connections.
954: The probability increases linearly and all connection attempts
955: are refused if the number of unauthenticated connections reaches
956: .Dq full
957: (60).
958: .It Cm PasswordAuthentication
959: Specifies whether password authentication is allowed.
960: The default is
961: .Dq yes .
962: .It Cm PermitEmptyPasswords
963: When password authentication is allowed, it specifies whether the
964: server allows login to accounts with empty password strings.
965: The default is
966: .Dq no .
1.62 dtucker 967: .It Cm PermitOpen
968: Specifies the destinations to which TCP port forwarding is permitted.
969: The forwarding specification must be one of the following forms:
970: .Pp
971: .Bl -item -offset indent -compact
972: .It
973: .Cm PermitOpen
974: .Sm off
975: .Ar host : port
976: .Sm on
977: .It
978: .Cm PermitOpen
979: .Sm off
980: .Ar IPv4_addr : port
981: .Sm on
982: .It
983: .Cm PermitOpen
984: .Sm off
985: .Ar \&[ IPv6_addr \&] : port
986: .Sm on
987: .El
988: .Pp
1.68 dtucker 989: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 990: An argument of
991: .Dq any
992: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 993: An argument of
994: .Dq none
995: can be used to prohibit all forwarding requests.
1.63 jmc 996: By default all port forwarding requests are permitted.
1.1 stevesk 997: .It Cm PermitRootLogin
1.38 jmc 998: Specifies whether root can log in using
1.1 stevesk 999: .Xr ssh 1 .
1000: The argument must be
1001: .Dq yes ,
1002: .Dq without-password ,
1.52 jmc 1003: .Dq forced-commands-only ,
1.1 stevesk 1004: or
1005: .Dq no .
1006: The default is
1007: .Dq yes .
1008: .Pp
1009: If this option is set to
1.52 jmc 1010: .Dq without-password ,
1.1 stevesk 1011: password authentication is disabled for root.
1012: .Pp
1013: If this option is set to
1.52 jmc 1014: .Dq forced-commands-only ,
1.1 stevesk 1015: root login with public key authentication will be allowed,
1016: but only if the
1017: .Ar command
1018: option has been specified
1019: (which may be useful for taking remote backups even if root login is
1.17 jmc 1020: normally not allowed).
1021: All other authentication methods are disabled for root.
1.1 stevesk 1022: .Pp
1023: If this option is set to
1.52 jmc 1024: .Dq no ,
1.38 jmc 1025: root is not allowed to log in.
1.46 reyk 1026: .It Cm PermitTunnel
1027: Specifies whether
1028: .Xr tun 4
1029: device forwarding is allowed.
1.47 reyk 1030: The argument must be
1031: .Dq yes ,
1.58 stevesk 1032: .Dq point-to-point
1033: (layer 3),
1034: .Dq ethernet
1035: (layer 2), or
1.47 reyk 1036: .Dq no .
1.58 stevesk 1037: Specifying
1038: .Dq yes
1039: permits both
1040: .Dq point-to-point
1041: and
1042: .Dq ethernet .
1.46 reyk 1043: The default is
1044: .Dq no .
1.164 djm 1045: .It Cm PermitTTY
1046: Specifies whether
1.165 jmc 1047: .Xr pty 4
1.164 djm 1048: allocation is permitted.
1049: The default is
1050: .Dq yes .
1.6 markus 1051: .It Cm PermitUserEnvironment
1052: Specifies whether
1053: .Pa ~/.ssh/environment
1.9 stevesk 1054: and
1.6 markus 1055: .Cm environment=
1056: options in
1057: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1058: are processed by
1.52 jmc 1059: .Xr sshd 8 .
1.6 markus 1060: The default is
1061: .Dq no .
1.9 stevesk 1062: Enabling environment processing may enable users to bypass access
1063: restrictions in some configurations using mechanisms such as
1064: .Ev LD_PRELOAD .
1.174 ! djm 1065: .It Cm PermitUserRC
! 1066: Specifies whether any
! 1067: .Pa ~/.ssh/rc
! 1068: file is executed.
! 1069: The default is
! 1070: .Dq yes .
1.1 stevesk 1071: .It Cm PidFile
1.4 stevesk 1072: Specifies the file that contains the process ID of the
1.53 jmc 1073: SSH daemon.
1.1 stevesk 1074: The default is
1075: .Pa /var/run/sshd.pid .
1076: .It Cm Port
1077: Specifies the port number that
1.52 jmc 1078: .Xr sshd 8
1.1 stevesk 1079: listens on.
1080: The default is 22.
1081: Multiple options of this type are permitted.
1082: See also
1083: .Cm ListenAddress .
1084: .It Cm PrintLastLog
1085: Specifies whether
1.52 jmc 1086: .Xr sshd 8
1.36 jaredy 1087: should print the date and time of the last user login when a user logs
1088: in interactively.
1.1 stevesk 1089: The default is
1090: .Dq yes .
1091: .It Cm PrintMotd
1092: Specifies whether
1.52 jmc 1093: .Xr sshd 8
1.1 stevesk 1094: should print
1095: .Pa /etc/motd
1096: when a user logs in interactively.
1097: (On some systems it is also printed by the shell,
1098: .Pa /etc/profile ,
1099: or equivalent.)
1100: The default is
1101: .Dq yes .
1102: .It Cm Protocol
1103: Specifies the protocol versions
1.52 jmc 1104: .Xr sshd 8
1.5 stevesk 1105: supports.
1.1 stevesk 1106: The possible values are
1.52 jmc 1107: .Sq 1
1.1 stevesk 1108: and
1.52 jmc 1109: .Sq 2 .
1.1 stevesk 1110: Multiple versions must be comma-separated.
1111: The default is
1.109 jmc 1112: .Sq 2 .
1.5 stevesk 1113: Note that the order of the protocol list does not indicate preference,
1114: because the client selects among multiple protocol versions offered
1115: by the server.
1116: Specifying
1117: .Dq 2,1
1118: is identical to
1119: .Dq 1,2 .
1.1 stevesk 1120: .It Cm PubkeyAuthentication
1121: Specifies whether public key authentication is allowed.
1122: The default is
1123: .Dq yes .
1124: Note that this option applies to protocol version 2 only.
1.159 dtucker 1125: .It Cm RekeyLimit
1126: Specifies the maximum amount of data that may be transmitted before the
1127: session key is renegotiated, optionally followed a maximum amount of
1128: time that may pass before the session key is renegotiated.
1129: The first argument is specified in bytes and may have a suffix of
1130: .Sq K ,
1131: .Sq M ,
1132: or
1133: .Sq G
1134: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1135: The default is between
1136: .Sq 1G
1137: and
1138: .Sq 4G ,
1139: depending on the cipher.
1140: The optional second value is specified in seconds and may use any of the
1141: units documented in the
1142: .Sx TIME FORMATS
1.160 jmc 1143: section.
1.159 dtucker 1144: The default value for
1145: .Cm RekeyLimit
1146: is
1147: .Dq default none ,
1148: which means that rekeying is performed after the cipher's default amount
1149: of data has been sent or received and no time based rekeying is done.
1150: This option applies to protocol version 2 only.
1.118 djm 1151: .It Cm RevokedKeys
1.154 djm 1152: Specifies revoked public keys.
1.118 djm 1153: Keys listed in this file will be refused for public key authentication.
1154: Note that if this file is not readable, then public key authentication will
1155: be refused for all users.
1.154 djm 1156: Keys may be specified as a text file, listing one public key per line, or as
1157: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1158: .Xr ssh-keygen 1 .
1.161 jmc 1159: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1160: .Xr ssh-keygen 1 .
1.1 stevesk 1161: .It Cm RhostsRSAAuthentication
1162: Specifies whether rhosts or /etc/hosts.equiv authentication together
1163: with successful RSA host authentication is allowed.
1164: The default is
1165: .Dq no .
1166: This option applies to protocol version 1 only.
1167: .It Cm RSAAuthentication
1168: Specifies whether pure RSA authentication is allowed.
1169: The default is
1170: .Dq yes .
1171: This option applies to protocol version 1 only.
1172: .It Cm ServerKeyBits
1173: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1174: The minimum value is 512, and the default is 1024.
1.1 stevesk 1175: .It Cm StrictModes
1176: Specifies whether
1.52 jmc 1177: .Xr sshd 8
1.1 stevesk 1178: should check file modes and ownership of the
1179: user's files and home directory before accepting login.
1180: This is normally desirable because novices sometimes accidentally leave their
1181: directory or files world-writable.
1182: The default is
1183: .Dq yes .
1.112 djm 1184: Note that this does not apply to
1185: .Cm ChrootDirectory ,
1186: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1187: .It Cm Subsystem
1.51 jmc 1188: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1189: Arguments should be a subsystem name and a command (with optional arguments)
1190: to execute upon subsystem request.
1.80 djm 1191: .Pp
1.1 stevesk 1192: The command
1193: .Xr sftp-server 8
1194: implements the
1195: .Dq sftp
1196: file transfer subsystem.
1.80 djm 1197: .Pp
1198: Alternately the name
1199: .Dq internal-sftp
1200: implements an in-process
1201: .Dq sftp
1202: server.
1203: This may simplify configurations using
1204: .Cm ChrootDirectory
1205: to force a different filesystem root on clients.
1206: .Pp
1.1 stevesk 1207: By default no subsystems are defined.
1208: Note that this option applies to protocol version 2 only.
1209: .It Cm SyslogFacility
1210: Gives the facility code that is used when logging messages from
1.53 jmc 1211: .Xr sshd 8 .
1.1 stevesk 1212: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1213: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1214: The default is AUTH.
1.27 markus 1215: .It Cm TCPKeepAlive
1216: Specifies whether the system should send TCP keepalive messages to the
1217: other side.
1218: If they are sent, death of the connection or crash of one
1219: of the machines will be properly noticed.
1220: However, this means that
1221: connections will die if the route is down temporarily, and some people
1222: find it annoying.
1223: On the other hand, if TCP keepalives are not sent,
1224: sessions may hang indefinitely on the server, leaving
1225: .Dq ghost
1226: users and consuming server resources.
1227: .Pp
1228: The default is
1229: .Dq yes
1230: (to send TCP keepalive messages), and the server will notice
1231: if the network goes down or the client host crashes.
1232: This avoids infinitely hanging sessions.
1233: .Pp
1234: To disable TCP keepalive messages, the value should be set to
1235: .Dq no .
1.118 djm 1236: .It Cm TrustedUserCAKeys
1237: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1238: trusted to sign user certificates for authentication.
1.119 jmc 1239: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1240: .Ql #
1241: are allowed.
1242: If a certificate is presented for authentication and has its signing CA key
1243: listed in this file, then it may be used for authentication for any user
1244: listed in the certificate's principals list.
1245: Note that certificates that lack a list of principals will not be permitted
1246: for authentication using
1247: .Cm TrustedUserCAKeys .
1.161 jmc 1248: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1249: .Xr ssh-keygen 1 .
1.18 markus 1250: .It Cm UseDNS
1251: Specifies whether
1.52 jmc 1252: .Xr sshd 8
1.40 jmc 1253: should look up the remote host name and check that
1.18 markus 1254: the resolved host name for the remote IP address maps back to the
1255: very same IP address.
1256: The default is
1257: .Dq yes .
1.1 stevesk 1258: .It Cm UseLogin
1259: Specifies whether
1260: .Xr login 1
1261: is used for interactive login sessions.
1262: The default is
1263: .Dq no .
1264: Note that
1265: .Xr login 1
1266: is never used for remote command execution.
1267: Note also, that if this is enabled,
1268: .Cm X11Forwarding
1269: will be disabled because
1270: .Xr login 1
1271: does not know how to handle
1272: .Xr xauth 1
1.15 jmc 1273: cookies.
1274: If
1.1 stevesk 1275: .Cm UsePrivilegeSeparation
1276: is specified, it will be disabled after authentication.
1277: .It Cm UsePrivilegeSeparation
1278: Specifies whether
1.52 jmc 1279: .Xr sshd 8
1.2 stevesk 1280: separates privileges by creating an unprivileged child process
1.15 jmc 1281: to deal with incoming network traffic.
1282: After successful authentication, another process will be created that has
1283: the privilege of the authenticated user.
1284: The goal of privilege separation is to prevent privilege
1.1 stevesk 1285: escalation by containing any corruption within the unprivileged processes.
1286: The default is
1287: .Dq yes .
1.134 djm 1288: If
1289: .Cm UsePrivilegeSeparation
1290: is set to
1291: .Dq sandbox
1292: then the pre-authentication unprivileged process is subject to additional
1293: restrictions.
1.137 djm 1294: .It Cm VersionAddendum
1295: Optionally specifies additional text to append to the SSH protocol banner
1296: sent by the server upon connection.
1297: The default is
1298: .Dq none .
1.1 stevesk 1299: .It Cm X11DisplayOffset
1300: Specifies the first display number available for
1.52 jmc 1301: .Xr sshd 8 Ns 's
1.1 stevesk 1302: X11 forwarding.
1.52 jmc 1303: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1304: The default is 10.
1305: .It Cm X11Forwarding
1306: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1307: The argument must be
1308: .Dq yes
1309: or
1310: .Dq no .
1.1 stevesk 1311: The default is
1312: .Dq no .
1.13 stevesk 1313: .Pp
1314: When X11 forwarding is enabled, there may be additional exposure to
1315: the server and to client displays if the
1.52 jmc 1316: .Xr sshd 8
1.13 stevesk 1317: proxy display is configured to listen on the wildcard address (see
1318: .Cm X11UseLocalhost
1.52 jmc 1319: below), though this is not the default.
1.13 stevesk 1320: Additionally, the authentication spoofing and authentication data
1321: verification and substitution occur on the client side.
1322: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1323: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1324: forwarding (see the warnings for
1325: .Cm ForwardX11
1326: in
1.19 jmc 1327: .Xr ssh_config 5 ) .
1.13 stevesk 1328: A system administrator may have a stance in which they want to
1329: protect clients that may expose themselves to attack by unwittingly
1330: requesting X11 forwarding, which can warrant a
1331: .Dq no
1332: setting.
1333: .Pp
1334: Note that disabling X11 forwarding does not prevent users from
1335: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1336: X11 forwarding is automatically disabled if
1337: .Cm UseLogin
1338: is enabled.
1339: .It Cm X11UseLocalhost
1340: Specifies whether
1.52 jmc 1341: .Xr sshd 8
1.1 stevesk 1342: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1343: the wildcard address.
1344: By default,
1.52 jmc 1345: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1346: hostname part of the
1347: .Ev DISPLAY
1348: environment variable to
1349: .Dq localhost .
1.8 stevesk 1350: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1351: However, some older X11 clients may not function with this
1352: configuration.
1353: .Cm X11UseLocalhost
1354: may be set to
1355: .Dq no
1356: to specify that the forwarding server should be bound to the wildcard
1357: address.
1358: The argument must be
1359: .Dq yes
1360: or
1361: .Dq no .
1362: The default is
1363: .Dq yes .
1364: .It Cm XAuthLocation
1.11 stevesk 1365: Specifies the full pathname of the
1.1 stevesk 1366: .Xr xauth 1
1367: program.
1368: The default is
1369: .Pa /usr/X11R6/bin/xauth .
1370: .El
1.55 jmc 1371: .Sh TIME FORMATS
1.53 jmc 1372: .Xr sshd 8
1.1 stevesk 1373: command-line arguments and configuration file options that specify time
1374: may be expressed using a sequence of the form:
1375: .Sm off
1.7 stevesk 1376: .Ar time Op Ar qualifier ,
1.1 stevesk 1377: .Sm on
1378: where
1379: .Ar time
1380: is a positive integer value and
1381: .Ar qualifier
1382: is one of the following:
1383: .Pp
1384: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1385: .It Aq Cm none
1.1 stevesk 1386: seconds
1387: .It Cm s | Cm S
1388: seconds
1389: .It Cm m | Cm M
1390: minutes
1391: .It Cm h | Cm H
1392: hours
1393: .It Cm d | Cm D
1394: days
1395: .It Cm w | Cm W
1396: weeks
1397: .El
1398: .Pp
1399: Each member of the sequence is added together to calculate
1400: the total time value.
1401: .Pp
1402: Time format examples:
1403: .Pp
1404: .Bl -tag -width Ds -compact -offset indent
1405: .It 600
1406: 600 seconds (10 minutes)
1407: .It 10m
1408: 10 minutes
1409: .It 1h30m
1410: 1 hour 30 minutes (90 minutes)
1411: .El
1412: .Sh FILES
1413: .Bl -tag -width Ds
1414: .It Pa /etc/ssh/sshd_config
1415: Contains configuration data for
1.53 jmc 1416: .Xr sshd 8 .
1.1 stevesk 1417: This file should be writable by root only, but it is recommended
1418: (though not necessary) that it be world-readable.
1419: .El
1.19 jmc 1420: .Sh SEE ALSO
1421: .Xr sshd 8
1.1 stevesk 1422: .Sh AUTHORS
1423: OpenSSH is a derivative of the original and free
1424: ssh 1.2.12 release by Tatu Ylonen.
1425: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1426: Theo de Raadt and Dug Song
1427: removed many bugs, re-added newer features and
1428: created OpenSSH.
1429: Markus Friedl contributed the support for SSH
1430: protocol versions 1.5 and 2.0.
1431: Niels Provos and Markus Friedl contributed support
1432: for privilege separation.
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