Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.172
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
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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
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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
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1.172 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.171 2014/01/29 14:04:51 jmc Exp $
! 37: .Dd $Mdocdate: January 29 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
1.34 dtucker 341: .Dq 3des-cbc ,
342: .Dq aes128-cbc ,
343: .Dq aes192-cbc ,
344: .Dq aes256-cbc ,
345: .Dq aes128-ctr ,
346: .Dq aes192-ctr ,
347: .Dq aes256-ctr ,
1.153 markus 348: .Dq aes128-gcm@openssh.com ,
349: .Dq aes256-gcm@openssh.com ,
1.43 djm 350: .Dq arcfour128 ,
351: .Dq arcfour256 ,
1.34 dtucker 352: .Dq arcfour ,
353: .Dq blowfish-cbc ,
1.167 djm 354: .Dq cast128-cbc ,
1.34 dtucker 355: and
1.167 djm 356: .Dq chacha20-poly1305@openssh.com .
357: .Pp
1.52 jmc 358: The default is:
359: .Bd -literal -offset 3n
1.100 naddy 360: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
1.153 markus 361: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.167 djm 362: chacha20-poly1305@openssh.com,
1.100 naddy 363: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
364: aes256-cbc,arcfour
1.1 stevesk 365: .Ed
1.167 djm 366: .Pp
367: The list of available ciphers may also be obtained using the
368: .Fl Q
369: option of
370: .Xr ssh 1 .
1.1 stevesk 371: .It Cm ClientAliveCountMax
1.48 jmc 372: Sets the number of client alive messages (see below) which may be
1.1 stevesk 373: sent without
1.52 jmc 374: .Xr sshd 8
1.17 jmc 375: receiving any messages back from the client.
376: If this threshold is reached while client alive messages are being sent,
1.52 jmc 377: sshd will disconnect the client, terminating the session.
1.17 jmc 378: It is important to note that the use of client alive messages is very
379: different from
1.27 markus 380: .Cm TCPKeepAlive
1.17 jmc 381: (below).
382: The client alive messages are sent through the encrypted channel
383: and therefore will not be spoofable.
384: The TCP keepalive option enabled by
1.27 markus 385: .Cm TCPKeepAlive
1.17 jmc 386: is spoofable.
387: The client alive mechanism is valuable when the client or
1.1 stevesk 388: server depend on knowing when a connection has become inactive.
389: .Pp
1.17 jmc 390: The default value is 3.
391: If
1.1 stevesk 392: .Cm ClientAliveInterval
1.48 jmc 393: (see below) is set to 15, and
1.1 stevesk 394: .Cm ClientAliveCountMax
1.52 jmc 395: is left at the default, unresponsive SSH clients
1.1 stevesk 396: will be disconnected after approximately 45 seconds.
1.57 markus 397: This option applies to protocol version 2 only.
1.42 djm 398: .It Cm ClientAliveInterval
399: Sets a timeout interval in seconds after which if no data has been received
400: from the client,
1.52 jmc 401: .Xr sshd 8
1.42 djm 402: will send a message through the encrypted
403: channel to request a response from the client.
404: The default
405: is 0, indicating that these messages will not be sent to the client.
406: This option applies to protocol version 2 only.
1.3 markus 407: .It Cm Compression
1.44 markus 408: Specifies whether compression is allowed, or delayed until
409: the user has authenticated successfully.
1.3 markus 410: The argument must be
1.44 markus 411: .Dq yes ,
412: .Dq delayed ,
1.3 markus 413: or
414: .Dq no .
415: The default is
1.44 markus 416: .Dq delayed .
1.1 stevesk 417: .It Cm DenyGroups
418: This keyword can be followed by a list of group name patterns, separated
419: by spaces.
420: Login is disallowed for users whose primary group or supplementary
421: group list matches one of the patterns.
422: Only group names are valid; a numerical group ID is not recognized.
423: By default, login is allowed for all groups.
1.54 jmc 424: The allow/deny directives are processed in the following order:
425: .Cm DenyUsers ,
426: .Cm AllowUsers ,
427: .Cm DenyGroups ,
428: and finally
429: .Cm AllowGroups .
1.49 jmc 430: .Pp
1.161 jmc 431: See PATTERNS in
1.49 jmc 432: .Xr ssh_config 5
433: for more information on patterns.
1.1 stevesk 434: .It Cm DenyUsers
435: This keyword can be followed by a list of user name patterns, separated
436: by spaces.
437: Login is disallowed for user names that match one of the patterns.
438: Only user names are valid; a numerical user ID is not recognized.
439: By default, login is allowed for all users.
440: If the pattern takes the form USER@HOST then USER and HOST
441: are separately checked, restricting logins to particular
442: users from particular hosts.
1.54 jmc 443: The allow/deny directives are processed in the following order:
444: .Cm DenyUsers ,
445: .Cm AllowUsers ,
446: .Cm DenyGroups ,
447: and finally
448: .Cm AllowGroups .
1.49 jmc 449: .Pp
1.161 jmc 450: See PATTERNS in
1.49 jmc 451: .Xr ssh_config 5
452: for more information on patterns.
1.67 dtucker 453: .It Cm ForceCommand
454: Forces the execution of the command specified by
455: .Cm ForceCommand ,
1.84 djm 456: ignoring any command supplied by the client and
457: .Pa ~/.ssh/rc
458: if present.
1.67 dtucker 459: The command is invoked by using the user's login shell with the -c option.
460: This applies to shell, command, or subsystem execution.
461: It is most useful inside a
462: .Cm Match
463: block.
464: The command originally supplied by the client is available in the
465: .Ev SSH_ORIGINAL_COMMAND
466: environment variable.
1.82 djm 467: Specifying a command of
468: .Dq internal-sftp
469: will force the use of an in-process sftp server that requires no support
470: files when used with
471: .Cm ChrootDirectory .
1.1 stevesk 472: .It Cm GatewayPorts
473: Specifies whether remote hosts are allowed to connect to ports
474: forwarded for the client.
475: By default,
1.52 jmc 476: .Xr sshd 8
1.15 jmc 477: binds remote port forwardings to the loopback address.
478: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 479: .Cm GatewayPorts
1.52 jmc 480: can be used to specify that sshd
1.39 djm 481: should allow remote port forwardings to bind to non-loopback addresses, thus
482: allowing other hosts to connect.
483: The argument may be
484: .Dq no
485: to force remote port forwardings to be available to the local host only,
1.1 stevesk 486: .Dq yes
1.39 djm 487: to force remote port forwardings to bind to the wildcard address, or
488: .Dq clientspecified
489: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 490: The default is
491: .Dq no .
1.23 markus 492: .It Cm GSSAPIAuthentication
1.25 markus 493: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 494: The default is
1.23 markus 495: .Dq no .
496: Note that this option applies to protocol version 2 only.
497: .It Cm GSSAPICleanupCredentials
498: Specifies whether to automatically destroy the user's credentials cache
499: on logout.
500: The default is
501: .Dq yes .
502: Note that this option applies to protocol version 2 only.
1.1 stevesk 503: .It Cm HostbasedAuthentication
504: Specifies whether rhosts or /etc/hosts.equiv authentication together
505: with successful public key client host authentication is allowed
1.50 jmc 506: (host-based authentication).
1.1 stevesk 507: This option is similar to
508: .Cm RhostsRSAAuthentication
509: and applies to protocol version 2 only.
1.70 dtucker 510: The default is
511: .Dq no .
512: .It Cm HostbasedUsesNameFromPacketOnly
513: Specifies whether or not the server will attempt to perform a reverse
514: name lookup when matching the name in the
515: .Pa ~/.shosts ,
516: .Pa ~/.rhosts ,
517: and
518: .Pa /etc/hosts.equiv
519: files during
520: .Cm HostbasedAuthentication .
521: A setting of
522: .Dq yes
523: means that
524: .Xr sshd 8
525: uses the name supplied by the client rather than
526: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 527: The default is
528: .Dq no .
1.117 djm 529: .It Cm HostCertificate
530: Specifies a file containing a public host certificate.
531: The certificate's public key must match a private host key already specified
532: by
533: .Cm HostKey .
534: The default behaviour of
535: .Xr sshd 8
536: is not to load any certificates.
1.1 stevesk 537: .It Cm HostKey
538: Specifies a file containing a private host key
539: used by SSH.
540: The default is
541: .Pa /etc/ssh/ssh_host_key
542: for protocol version 1, and
1.126 djm 543: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 544: .Pa /etc/ssh/ssh_host_ecdsa_key ,
545: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 546: and
1.1 stevesk 547: .Pa /etc/ssh/ssh_host_rsa_key
548: for protocol version 2.
549: Note that
1.52 jmc 550: .Xr sshd 8
1.1 stevesk 551: will refuse to use a file if it is group/world-accessible.
552: It is possible to have multiple host key files.
553: .Dq rsa1
554: keys are used for version 1 and
1.126 djm 555: .Dq dsa ,
1.169 naddy 556: .Dq ecdsa ,
557: .Dq ed25519
1.1 stevesk 558: or
559: .Dq rsa
560: are used for version 2 of the SSH protocol.
1.162 markus 561: It is also possible to specify public host key files instead.
562: In this case operations on the private key will be delegated
563: to an
564: .Xr ssh-agent 1 .
565: .It Cm HostKeyAgent
566: Identifies the UNIX-domain socket used to communicate
567: with an agent that has access to the private host keys.
568: If
569: .Dq SSH_AUTH_SOCK
570: is specified, the location of the socket will be read from the
571: .Ev SSH_AUTH_SOCK
572: environment variable.
1.1 stevesk 573: .It Cm IgnoreRhosts
574: Specifies that
575: .Pa .rhosts
576: and
577: .Pa .shosts
578: files will not be used in
579: .Cm RhostsRSAAuthentication
580: or
581: .Cm HostbasedAuthentication .
582: .Pp
583: .Pa /etc/hosts.equiv
584: and
585: .Pa /etc/shosts.equiv
586: are still used.
587: The default is
588: .Dq yes .
589: .It Cm IgnoreUserKnownHosts
590: Specifies whether
1.52 jmc 591: .Xr sshd 8
1.1 stevesk 592: should ignore the user's
1.41 djm 593: .Pa ~/.ssh/known_hosts
1.1 stevesk 594: during
595: .Cm RhostsRSAAuthentication
596: or
597: .Cm HostbasedAuthentication .
598: The default is
599: .Dq no .
1.129 djm 600: .It Cm IPQoS
601: Specifies the IPv4 type-of-service or DSCP class for the connection.
602: Accepted values are
603: .Dq af11 ,
604: .Dq af12 ,
605: .Dq af13 ,
1.136 djm 606: .Dq af21 ,
1.129 djm 607: .Dq af22 ,
608: .Dq af23 ,
609: .Dq af31 ,
610: .Dq af32 ,
611: .Dq af33 ,
612: .Dq af41 ,
613: .Dq af42 ,
614: .Dq af43 ,
615: .Dq cs0 ,
616: .Dq cs1 ,
617: .Dq cs2 ,
618: .Dq cs3 ,
619: .Dq cs4 ,
620: .Dq cs5 ,
621: .Dq cs6 ,
622: .Dq cs7 ,
623: .Dq ef ,
624: .Dq lowdelay ,
625: .Dq throughput ,
626: .Dq reliability ,
627: or a numeric value.
1.131 djm 628: This option may take one or two arguments, separated by whitespace.
1.129 djm 629: If one argument is specified, it is used as the packet class unconditionally.
630: If two values are specified, the first is automatically selected for
631: interactive sessions and the second for non-interactive sessions.
632: The default is
633: .Dq lowdelay
634: for interactive sessions and
635: .Dq throughput
636: for non-interactive sessions.
1.171 jmc 637: .It Cm KbdInteractiveAuthentication
638: Specifies whether to allow keyboard-interactive authentication.
639: The argument to this keyword must be
640: .Dq yes
641: or
642: .Dq no .
643: The default is to use whatever value
644: .Cm ChallengeResponseAuthentication
645: is set to
646: (by default
647: .Dq yes ) .
1.1 stevesk 648: .It Cm KerberosAuthentication
1.24 markus 649: Specifies whether the password provided by the user for
1.1 stevesk 650: .Cm PasswordAuthentication
1.24 markus 651: will be validated through the Kerberos KDC.
1.1 stevesk 652: To use this option, the server needs a
653: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 654: The default is
1.29 dtucker 655: .Dq no .
656: .It Cm KerberosGetAFSToken
1.45 djm 657: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 658: an AFS token before accessing the user's home directory.
1.52 jmc 659: The default is
1.1 stevesk 660: .Dq no .
661: .It Cm KerberosOrLocalPasswd
1.52 jmc 662: If password authentication through Kerberos fails then
1.1 stevesk 663: the password will be validated via any additional local mechanism
664: such as
665: .Pa /etc/passwd .
1.52 jmc 666: The default is
1.1 stevesk 667: .Dq yes .
668: .It Cm KerberosTicketCleanup
669: Specifies whether to automatically destroy the user's ticket cache
670: file on logout.
1.52 jmc 671: The default is
1.1 stevesk 672: .Dq yes .
1.127 djm 673: .It Cm KexAlgorithms
674: Specifies the available KEX (Key Exchange) algorithms.
675: Multiple algorithms must be comma-separated.
676: The default is
1.170 dtucker 677: .Bd -literal -offset indent
678: curve25519-sha256@libssh.org,
679: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
680: diffie-hellman-group-exchange-sha256,
681: diffie-hellman-group-exchange-sha1,
682: diffie-hellman-group14-sha1,
683: diffie-hellman-group1-sha1
684: .Ed
1.1 stevesk 685: .It Cm KeyRegenerationInterval
686: In protocol version 1, the ephemeral server key is automatically regenerated
687: after this many seconds (if it has been used).
688: The purpose of regeneration is to prevent
689: decrypting captured sessions by later breaking into the machine and
690: stealing the keys.
691: The key is never stored anywhere.
692: If the value is 0, the key is never regenerated.
693: The default is 3600 (seconds).
694: .It Cm ListenAddress
695: Specifies the local addresses
1.52 jmc 696: .Xr sshd 8
1.1 stevesk 697: should listen on.
698: The following forms may be used:
699: .Pp
700: .Bl -item -offset indent -compact
701: .It
702: .Cm ListenAddress
703: .Sm off
704: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
705: .Sm on
706: .It
707: .Cm ListenAddress
708: .Sm off
709: .Ar host No | Ar IPv4_addr No : Ar port
710: .Sm on
711: .It
712: .Cm ListenAddress
713: .Sm off
714: .Oo
715: .Ar host No | Ar IPv6_addr Oc : Ar port
716: .Sm on
717: .El
718: .Pp
719: If
720: .Ar port
721: is not specified,
1.52 jmc 722: sshd will listen on the address and all prior
1.1 stevesk 723: .Cm Port
1.17 jmc 724: options specified.
725: The default is to listen on all local addresses.
1.15 jmc 726: Multiple
1.1 stevesk 727: .Cm ListenAddress
1.17 jmc 728: options are permitted.
729: Additionally, any
1.1 stevesk 730: .Cm Port
1.52 jmc 731: options must precede this option for non-port qualified addresses.
1.1 stevesk 732: .It Cm LoginGraceTime
733: The server disconnects after this time if the user has not
734: successfully logged in.
735: If the value is 0, there is no time limit.
1.12 stevesk 736: The default is 120 seconds.
1.1 stevesk 737: .It Cm LogLevel
738: Gives the verbosity level that is used when logging messages from
1.53 jmc 739: .Xr sshd 8 .
1.1 stevesk 740: The possible values are:
1.52 jmc 741: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 742: The default is INFO.
743: DEBUG and DEBUG1 are equivalent.
744: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
745: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 746: .It Cm MACs
747: Specifies the available MAC (message authentication code) algorithms.
748: The MAC algorithm is used in protocol version 2
749: for data integrity protection.
750: Multiple algorithms must be comma-separated.
1.152 markus 751: The algorithms that contain
752: .Dq -etm
753: calculate the MAC after encryption (encrypt-then-mac).
754: These are considered safer and their use recommended.
1.52 jmc 755: The default is:
1.77 jmc 756: .Bd -literal -offset indent
1.152 markus 757: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
758: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
759: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
760: hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
761: hmac-md5-96-etm@openssh.com,
1.145 markus 762: hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
1.144 naddy 763: hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
764: hmac-sha1-96,hmac-md5-96
1.77 jmc 765: .Ed
1.60 dtucker 766: .It Cm Match
1.61 jmc 767: Introduces a conditional block.
1.65 dtucker 768: If all of the criteria on the
1.60 dtucker 769: .Cm Match
1.65 dtucker 770: line are satisfied, the keywords on the following lines override those
771: set in the global section of the config file, until either another
1.60 dtucker 772: .Cm Match
1.65 dtucker 773: line or the end of the file.
1.172 ! djm 774: If a keyword appears in multiple
! 775: .Cm Match
! 776: blocks that are satisified, only the first instance of the keyword is
! 777: applied.
1.91 djm 778: .Pp
1.61 jmc 779: The arguments to
1.60 dtucker 780: .Cm Match
1.163 dtucker 781: are one or more criteria-pattern pairs or the single token
782: .Cm All
783: which matches all criteria.
1.60 dtucker 784: The available criteria are
785: .Cm User ,
1.69 dtucker 786: .Cm Group ,
1.60 dtucker 787: .Cm Host ,
1.139 dtucker 788: .Cm LocalAddress ,
789: .Cm LocalPort ,
1.60 dtucker 790: and
791: .Cm Address .
1.91 djm 792: The match patterns may consist of single entries or comma-separated
793: lists and may use the wildcard and negation operators described in the
1.161 jmc 794: PATTERNS section of
1.92 djm 795: .Xr ssh_config 5 .
1.91 djm 796: .Pp
797: The patterns in an
798: .Cm Address
799: criteria may additionally contain addresses to match in CIDR
1.93 jmc 800: address/masklen format, e.g.\&
1.91 djm 801: .Dq 192.0.2.0/24
802: or
803: .Dq 3ffe:ffff::/32 .
804: Note that the mask length provided must be consistent with the address -
805: it is an error to specify a mask length that is too long for the address
1.93 jmc 806: or one with bits set in this host portion of the address.
807: For example,
1.91 djm 808: .Dq 192.0.2.0/33
809: and
1.93 jmc 810: .Dq 192.0.2.0/8
1.91 djm 811: respectively.
812: .Pp
1.60 dtucker 813: Only a subset of keywords may be used on the lines following a
814: .Cm Match
815: keyword.
816: Available keywords are
1.142 jmc 817: .Cm AcceptEnv ,
1.99 okan 818: .Cm AllowAgentForwarding ,
1.142 jmc 819: .Cm AllowGroups ,
1.62 dtucker 820: .Cm AllowTcpForwarding ,
1.141 markus 821: .Cm AllowUsers ,
1.149 djm 822: .Cm AuthenticationMethods ,
1.146 djm 823: .Cm AuthorizedKeysCommand ,
824: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 825: .Cm AuthorizedKeysFile ,
1.123 djm 826: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 827: .Cm Banner ,
1.85 djm 828: .Cm ChrootDirectory ,
1.141 markus 829: .Cm DenyGroups ,
830: .Cm DenyUsers ,
1.67 dtucker 831: .Cm ForceCommand ,
1.142 jmc 832: .Cm GatewayPorts ,
1.141 markus 833: .Cm GSSAPIAuthentication ,
1.87 djm 834: .Cm HostbasedAuthentication ,
1.123 djm 835: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 836: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 837: .Cm KerberosAuthentication ,
1.95 dtucker 838: .Cm MaxAuthTries ,
1.94 dtucker 839: .Cm MaxSessions ,
1.72 dtucker 840: .Cm PasswordAuthentication ,
1.97 djm 841: .Cm PermitEmptyPasswords ,
1.66 dtucker 842: .Cm PermitOpen ,
1.79 dtucker 843: .Cm PermitRootLogin ,
1.164 djm 844: .Cm PermitTTY ,
1.123 djm 845: .Cm PermitTunnel ,
1.107 dtucker 846: .Cm PubkeyAuthentication ,
1.159 dtucker 847: .Cm RekeyLimit ,
1.142 jmc 848: .Cm RhostsRSAAuthentication ,
1.141 markus 849: .Cm RSAAuthentication ,
1.66 dtucker 850: .Cm X11DisplayOffset ,
1.101 djm 851: .Cm X11Forwarding
1.60 dtucker 852: and
1.102 djm 853: .Cm X11UseLocalHost .
1.33 dtucker 854: .It Cm MaxAuthTries
855: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 856: connection.
857: Once the number of failures reaches half this value,
858: additional failures are logged.
859: The default is 6.
1.90 djm 860: .It Cm MaxSessions
861: Specifies the maximum number of open sessions permitted per network connection.
862: The default is 10.
1.1 stevesk 863: .It Cm MaxStartups
864: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 865: SSH daemon.
1.1 stevesk 866: Additional connections will be dropped until authentication succeeds or the
867: .Cm LoginGraceTime
868: expires for a connection.
1.156 dtucker 869: The default is 10:30:100.
1.1 stevesk 870: .Pp
871: Alternatively, random early drop can be enabled by specifying
872: the three colon separated values
873: .Dq start:rate:full
1.51 jmc 874: (e.g. "10:30:60").
1.53 jmc 875: .Xr sshd 8
1.1 stevesk 876: will refuse connection attempts with a probability of
877: .Dq rate/100
878: (30%)
879: if there are currently
880: .Dq start
881: (10)
882: unauthenticated connections.
883: The probability increases linearly and all connection attempts
884: are refused if the number of unauthenticated connections reaches
885: .Dq full
886: (60).
887: .It Cm PasswordAuthentication
888: Specifies whether password authentication is allowed.
889: The default is
890: .Dq yes .
891: .It Cm PermitEmptyPasswords
892: When password authentication is allowed, it specifies whether the
893: server allows login to accounts with empty password strings.
894: The default is
895: .Dq no .
1.62 dtucker 896: .It Cm PermitOpen
897: Specifies the destinations to which TCP port forwarding is permitted.
898: The forwarding specification must be one of the following forms:
899: .Pp
900: .Bl -item -offset indent -compact
901: .It
902: .Cm PermitOpen
903: .Sm off
904: .Ar host : port
905: .Sm on
906: .It
907: .Cm PermitOpen
908: .Sm off
909: .Ar IPv4_addr : port
910: .Sm on
911: .It
912: .Cm PermitOpen
913: .Sm off
914: .Ar \&[ IPv6_addr \&] : port
915: .Sm on
916: .El
917: .Pp
1.68 dtucker 918: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 919: An argument of
920: .Dq any
921: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 922: An argument of
923: .Dq none
924: can be used to prohibit all forwarding requests.
1.63 jmc 925: By default all port forwarding requests are permitted.
1.1 stevesk 926: .It Cm PermitRootLogin
1.38 jmc 927: Specifies whether root can log in using
1.1 stevesk 928: .Xr ssh 1 .
929: The argument must be
930: .Dq yes ,
931: .Dq without-password ,
1.52 jmc 932: .Dq forced-commands-only ,
1.1 stevesk 933: or
934: .Dq no .
935: The default is
936: .Dq yes .
937: .Pp
938: If this option is set to
1.52 jmc 939: .Dq without-password ,
1.1 stevesk 940: password authentication is disabled for root.
941: .Pp
942: If this option is set to
1.52 jmc 943: .Dq forced-commands-only ,
1.1 stevesk 944: root login with public key authentication will be allowed,
945: but only if the
946: .Ar command
947: option has been specified
948: (which may be useful for taking remote backups even if root login is
1.17 jmc 949: normally not allowed).
950: All other authentication methods are disabled for root.
1.1 stevesk 951: .Pp
952: If this option is set to
1.52 jmc 953: .Dq no ,
1.38 jmc 954: root is not allowed to log in.
1.46 reyk 955: .It Cm PermitTunnel
956: Specifies whether
957: .Xr tun 4
958: device forwarding is allowed.
1.47 reyk 959: The argument must be
960: .Dq yes ,
1.58 stevesk 961: .Dq point-to-point
962: (layer 3),
963: .Dq ethernet
964: (layer 2), or
1.47 reyk 965: .Dq no .
1.58 stevesk 966: Specifying
967: .Dq yes
968: permits both
969: .Dq point-to-point
970: and
971: .Dq ethernet .
1.46 reyk 972: The default is
973: .Dq no .
1.164 djm 974: .It Cm PermitTTY
975: Specifies whether
1.165 jmc 976: .Xr pty 4
1.164 djm 977: allocation is permitted.
978: The default is
979: .Dq yes .
1.6 markus 980: .It Cm PermitUserEnvironment
981: Specifies whether
982: .Pa ~/.ssh/environment
1.9 stevesk 983: and
1.6 markus 984: .Cm environment=
985: options in
986: .Pa ~/.ssh/authorized_keys
1.9 stevesk 987: are processed by
1.52 jmc 988: .Xr sshd 8 .
1.6 markus 989: The default is
990: .Dq no .
1.9 stevesk 991: Enabling environment processing may enable users to bypass access
992: restrictions in some configurations using mechanisms such as
993: .Ev LD_PRELOAD .
1.1 stevesk 994: .It Cm PidFile
1.4 stevesk 995: Specifies the file that contains the process ID of the
1.53 jmc 996: SSH daemon.
1.1 stevesk 997: The default is
998: .Pa /var/run/sshd.pid .
999: .It Cm Port
1000: Specifies the port number that
1.52 jmc 1001: .Xr sshd 8
1.1 stevesk 1002: listens on.
1003: The default is 22.
1004: Multiple options of this type are permitted.
1005: See also
1006: .Cm ListenAddress .
1007: .It Cm PrintLastLog
1008: Specifies whether
1.52 jmc 1009: .Xr sshd 8
1.36 jaredy 1010: should print the date and time of the last user login when a user logs
1011: in interactively.
1.1 stevesk 1012: The default is
1013: .Dq yes .
1014: .It Cm PrintMotd
1015: Specifies whether
1.52 jmc 1016: .Xr sshd 8
1.1 stevesk 1017: should print
1018: .Pa /etc/motd
1019: when a user logs in interactively.
1020: (On some systems it is also printed by the shell,
1021: .Pa /etc/profile ,
1022: or equivalent.)
1023: The default is
1024: .Dq yes .
1025: .It Cm Protocol
1026: Specifies the protocol versions
1.52 jmc 1027: .Xr sshd 8
1.5 stevesk 1028: supports.
1.1 stevesk 1029: The possible values are
1.52 jmc 1030: .Sq 1
1.1 stevesk 1031: and
1.52 jmc 1032: .Sq 2 .
1.1 stevesk 1033: Multiple versions must be comma-separated.
1034: The default is
1.109 jmc 1035: .Sq 2 .
1.5 stevesk 1036: Note that the order of the protocol list does not indicate preference,
1037: because the client selects among multiple protocol versions offered
1038: by the server.
1039: Specifying
1040: .Dq 2,1
1041: is identical to
1042: .Dq 1,2 .
1.1 stevesk 1043: .It Cm PubkeyAuthentication
1044: Specifies whether public key authentication is allowed.
1045: The default is
1046: .Dq yes .
1047: Note that this option applies to protocol version 2 only.
1.159 dtucker 1048: .It Cm RekeyLimit
1049: Specifies the maximum amount of data that may be transmitted before the
1050: session key is renegotiated, optionally followed a maximum amount of
1051: time that may pass before the session key is renegotiated.
1052: The first argument is specified in bytes and may have a suffix of
1053: .Sq K ,
1054: .Sq M ,
1055: or
1056: .Sq G
1057: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1058: The default is between
1059: .Sq 1G
1060: and
1061: .Sq 4G ,
1062: depending on the cipher.
1063: The optional second value is specified in seconds and may use any of the
1064: units documented in the
1065: .Sx TIME FORMATS
1.160 jmc 1066: section.
1.159 dtucker 1067: The default value for
1068: .Cm RekeyLimit
1069: is
1070: .Dq default none ,
1071: which means that rekeying is performed after the cipher's default amount
1072: of data has been sent or received and no time based rekeying is done.
1073: This option applies to protocol version 2 only.
1.118 djm 1074: .It Cm RevokedKeys
1.154 djm 1075: Specifies revoked public keys.
1.118 djm 1076: Keys listed in this file will be refused for public key authentication.
1077: Note that if this file is not readable, then public key authentication will
1078: be refused for all users.
1.154 djm 1079: Keys may be specified as a text file, listing one public key per line, or as
1080: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1081: .Xr ssh-keygen 1 .
1.161 jmc 1082: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1083: .Xr ssh-keygen 1 .
1.1 stevesk 1084: .It Cm RhostsRSAAuthentication
1085: Specifies whether rhosts or /etc/hosts.equiv authentication together
1086: with successful RSA host authentication is allowed.
1087: The default is
1088: .Dq no .
1089: This option applies to protocol version 1 only.
1090: .It Cm RSAAuthentication
1091: Specifies whether pure RSA authentication is allowed.
1092: The default is
1093: .Dq yes .
1094: This option applies to protocol version 1 only.
1095: .It Cm ServerKeyBits
1096: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1097: The minimum value is 512, and the default is 1024.
1.1 stevesk 1098: .It Cm StrictModes
1099: Specifies whether
1.52 jmc 1100: .Xr sshd 8
1.1 stevesk 1101: should check file modes and ownership of the
1102: user's files and home directory before accepting login.
1103: This is normally desirable because novices sometimes accidentally leave their
1104: directory or files world-writable.
1105: The default is
1106: .Dq yes .
1.112 djm 1107: Note that this does not apply to
1108: .Cm ChrootDirectory ,
1109: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1110: .It Cm Subsystem
1.51 jmc 1111: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1112: Arguments should be a subsystem name and a command (with optional arguments)
1113: to execute upon subsystem request.
1.80 djm 1114: .Pp
1.1 stevesk 1115: The command
1116: .Xr sftp-server 8
1117: implements the
1118: .Dq sftp
1119: file transfer subsystem.
1.80 djm 1120: .Pp
1121: Alternately the name
1122: .Dq internal-sftp
1123: implements an in-process
1124: .Dq sftp
1125: server.
1126: This may simplify configurations using
1127: .Cm ChrootDirectory
1128: to force a different filesystem root on clients.
1129: .Pp
1.1 stevesk 1130: By default no subsystems are defined.
1131: Note that this option applies to protocol version 2 only.
1132: .It Cm SyslogFacility
1133: Gives the facility code that is used when logging messages from
1.53 jmc 1134: .Xr sshd 8 .
1.1 stevesk 1135: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1136: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1137: The default is AUTH.
1.27 markus 1138: .It Cm TCPKeepAlive
1139: Specifies whether the system should send TCP keepalive messages to the
1140: other side.
1141: If they are sent, death of the connection or crash of one
1142: of the machines will be properly noticed.
1143: However, this means that
1144: connections will die if the route is down temporarily, and some people
1145: find it annoying.
1146: On the other hand, if TCP keepalives are not sent,
1147: sessions may hang indefinitely on the server, leaving
1148: .Dq ghost
1149: users and consuming server resources.
1150: .Pp
1151: The default is
1152: .Dq yes
1153: (to send TCP keepalive messages), and the server will notice
1154: if the network goes down or the client host crashes.
1155: This avoids infinitely hanging sessions.
1156: .Pp
1157: To disable TCP keepalive messages, the value should be set to
1158: .Dq no .
1.118 djm 1159: .It Cm TrustedUserCAKeys
1160: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1161: trusted to sign user certificates for authentication.
1.119 jmc 1162: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1163: .Ql #
1164: are allowed.
1165: If a certificate is presented for authentication and has its signing CA key
1166: listed in this file, then it may be used for authentication for any user
1167: listed in the certificate's principals list.
1168: Note that certificates that lack a list of principals will not be permitted
1169: for authentication using
1170: .Cm TrustedUserCAKeys .
1.161 jmc 1171: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1172: .Xr ssh-keygen 1 .
1.18 markus 1173: .It Cm UseDNS
1174: Specifies whether
1.52 jmc 1175: .Xr sshd 8
1.40 jmc 1176: should look up the remote host name and check that
1.18 markus 1177: the resolved host name for the remote IP address maps back to the
1178: very same IP address.
1179: The default is
1180: .Dq yes .
1.1 stevesk 1181: .It Cm UseLogin
1182: Specifies whether
1183: .Xr login 1
1184: is used for interactive login sessions.
1185: The default is
1186: .Dq no .
1187: Note that
1188: .Xr login 1
1189: is never used for remote command execution.
1190: Note also, that if this is enabled,
1191: .Cm X11Forwarding
1192: will be disabled because
1193: .Xr login 1
1194: does not know how to handle
1195: .Xr xauth 1
1.15 jmc 1196: cookies.
1197: If
1.1 stevesk 1198: .Cm UsePrivilegeSeparation
1199: is specified, it will be disabled after authentication.
1200: .It Cm UsePrivilegeSeparation
1201: Specifies whether
1.52 jmc 1202: .Xr sshd 8
1.2 stevesk 1203: separates privileges by creating an unprivileged child process
1.15 jmc 1204: to deal with incoming network traffic.
1205: After successful authentication, another process will be created that has
1206: the privilege of the authenticated user.
1207: The goal of privilege separation is to prevent privilege
1.1 stevesk 1208: escalation by containing any corruption within the unprivileged processes.
1209: The default is
1210: .Dq yes .
1.134 djm 1211: If
1212: .Cm UsePrivilegeSeparation
1213: is set to
1214: .Dq sandbox
1215: then the pre-authentication unprivileged process is subject to additional
1216: restrictions.
1.137 djm 1217: .It Cm VersionAddendum
1218: Optionally specifies additional text to append to the SSH protocol banner
1219: sent by the server upon connection.
1220: The default is
1221: .Dq none .
1.1 stevesk 1222: .It Cm X11DisplayOffset
1223: Specifies the first display number available for
1.52 jmc 1224: .Xr sshd 8 Ns 's
1.1 stevesk 1225: X11 forwarding.
1.52 jmc 1226: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1227: The default is 10.
1228: .It Cm X11Forwarding
1229: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1230: The argument must be
1231: .Dq yes
1232: or
1233: .Dq no .
1.1 stevesk 1234: The default is
1235: .Dq no .
1.13 stevesk 1236: .Pp
1237: When X11 forwarding is enabled, there may be additional exposure to
1238: the server and to client displays if the
1.52 jmc 1239: .Xr sshd 8
1.13 stevesk 1240: proxy display is configured to listen on the wildcard address (see
1241: .Cm X11UseLocalhost
1.52 jmc 1242: below), though this is not the default.
1.13 stevesk 1243: Additionally, the authentication spoofing and authentication data
1244: verification and substitution occur on the client side.
1245: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1246: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1247: forwarding (see the warnings for
1248: .Cm ForwardX11
1249: in
1.19 jmc 1250: .Xr ssh_config 5 ) .
1.13 stevesk 1251: A system administrator may have a stance in which they want to
1252: protect clients that may expose themselves to attack by unwittingly
1253: requesting X11 forwarding, which can warrant a
1254: .Dq no
1255: setting.
1256: .Pp
1257: Note that disabling X11 forwarding does not prevent users from
1258: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1259: X11 forwarding is automatically disabled if
1260: .Cm UseLogin
1261: is enabled.
1262: .It Cm X11UseLocalhost
1263: Specifies whether
1.52 jmc 1264: .Xr sshd 8
1.1 stevesk 1265: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1266: the wildcard address.
1267: By default,
1.52 jmc 1268: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1269: hostname part of the
1270: .Ev DISPLAY
1271: environment variable to
1272: .Dq localhost .
1.8 stevesk 1273: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1274: However, some older X11 clients may not function with this
1275: configuration.
1276: .Cm X11UseLocalhost
1277: may be set to
1278: .Dq no
1279: to specify that the forwarding server should be bound to the wildcard
1280: address.
1281: The argument must be
1282: .Dq yes
1283: or
1284: .Dq no .
1285: The default is
1286: .Dq yes .
1287: .It Cm XAuthLocation
1.11 stevesk 1288: Specifies the full pathname of the
1.1 stevesk 1289: .Xr xauth 1
1290: program.
1291: The default is
1292: .Pa /usr/X11R6/bin/xauth .
1293: .El
1.55 jmc 1294: .Sh TIME FORMATS
1.53 jmc 1295: .Xr sshd 8
1.1 stevesk 1296: command-line arguments and configuration file options that specify time
1297: may be expressed using a sequence of the form:
1298: .Sm off
1.7 stevesk 1299: .Ar time Op Ar qualifier ,
1.1 stevesk 1300: .Sm on
1301: where
1302: .Ar time
1303: is a positive integer value and
1304: .Ar qualifier
1305: is one of the following:
1306: .Pp
1307: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1308: .It Aq Cm none
1.1 stevesk 1309: seconds
1310: .It Cm s | Cm S
1311: seconds
1312: .It Cm m | Cm M
1313: minutes
1314: .It Cm h | Cm H
1315: hours
1316: .It Cm d | Cm D
1317: days
1318: .It Cm w | Cm W
1319: weeks
1320: .El
1321: .Pp
1322: Each member of the sequence is added together to calculate
1323: the total time value.
1324: .Pp
1325: Time format examples:
1326: .Pp
1327: .Bl -tag -width Ds -compact -offset indent
1328: .It 600
1329: 600 seconds (10 minutes)
1330: .It 10m
1331: 10 minutes
1332: .It 1h30m
1333: 1 hour 30 minutes (90 minutes)
1334: .El
1335: .Sh FILES
1336: .Bl -tag -width Ds
1337: .It Pa /etc/ssh/sshd_config
1338: Contains configuration data for
1.53 jmc 1339: .Xr sshd 8 .
1.1 stevesk 1340: This file should be writable by root only, but it is recommended
1341: (though not necessary) that it be world-readable.
1342: .El
1.19 jmc 1343: .Sh SEE ALSO
1344: .Xr sshd 8
1.1 stevesk 1345: .Sh AUTHORS
1346: OpenSSH is a derivative of the original and free
1347: ssh 1.2.12 release by Tatu Ylonen.
1348: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1349: Theo de Raadt and Dug Song
1350: removed many bugs, re-added newer features and
1351: created OpenSSH.
1352: Markus Friedl contributed the support for SSH
1353: protocol versions 1.5 and 2.0.
1354: Niels Provos and Markus Friedl contributed support
1355: for privilege separation.
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