Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.178
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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1.178 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.177 2014/08/30 16:32:25 sobrado Exp $
! 37: .Dd $Mdocdate: August 30 2014 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.1 stevesk 123: .It Cm AllowTcpForwarding
124: Specifies whether TCP forwarding is permitted.
1.150 djm 125: The available options are
126: .Dq yes
127: or
128: .Dq all
129: to allow TCP forwarding,
130: .Dq no
131: to prevent all TCP forwarding,
132: .Dq local
133: to allow local (from the perspective of
1.151 jmc 134: .Xr ssh 1 )
135: forwarding only or
1.150 djm 136: .Dq remote
137: to allow remote forwarding only.
1.1 stevesk 138: The default is
139: .Dq yes .
140: Note that disabling TCP forwarding does not improve security unless
141: users are also denied shell access, as they can always install their
142: own forwarders.
1.175 millert 143: .It Cm AllowStreamLocalForwarding
144: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
145: The available options are
146: .Dq yes
147: or
148: .Dq all
149: to allow StreamLocal forwarding,
150: .Dq no
151: to prevent all StreamLocal forwarding,
152: .Dq local
153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
156: .Dq remote
157: to allow remote forwarding only.
158: The default is
159: .Dq yes .
160: Note that disabling StreamLocal forwarding does not improve security unless
161: users are also denied shell access, as they can always install their
162: own forwarders.
1.1 stevesk 163: .It Cm AllowUsers
164: This keyword can be followed by a list of user name patterns, separated
165: by spaces.
1.14 jmc 166: If specified, login is allowed only for user names that
1.1 stevesk 167: match one of the patterns.
168: Only user names are valid; a numerical user ID is not recognized.
169: By default, login is allowed for all users.
170: If the pattern takes the form USER@HOST then USER and HOST
171: are separately checked, restricting logins to particular
172: users from particular hosts.
1.54 jmc 173: The allow/deny directives are processed in the following order:
174: .Cm DenyUsers ,
175: .Cm AllowUsers ,
176: .Cm DenyGroups ,
177: and finally
178: .Cm AllowGroups .
1.49 jmc 179: .Pp
1.161 jmc 180: See PATTERNS in
1.49 jmc 181: .Xr ssh_config 5
182: for more information on patterns.
1.149 djm 183: .It Cm AuthenticationMethods
184: Specifies the authentication methods that must be successfully completed
185: for a user to be granted access.
186: This option must be followed by one or more comma-separated lists of
187: authentication method names.
188: Successful authentication requires completion of every method in at least
189: one of these lists.
190: .Pp
191: For example, an argument of
192: .Dq publickey,password publickey,keyboard-interactive
193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
196: so for this example, it would not be possible to attempt password or
197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
202: .Dq bsdauth ,
203: .Dq pam ,
204: or
205: .Dq skey ,
206: depending on the server configuration.
207: For example,
208: .Dq keyboard-interactive:bsdauth
209: would restrict keyboard interactive authentication to the
210: .Dq bsdauth
211: device.
1.149 djm 212: .Pp
213: This option is only available for SSH protocol 2 and will yield a fatal
214: error if enabled if protocol 1 is also enabled.
215: Note that each authentication method listed should also be explicitly enabled
216: in the configuration.
217: The default is not to require multiple authentication; successful completion
218: of a single authentication method is sufficient.
1.146 djm 219: .It Cm AuthorizedKeysCommand
1.147 jmc 220: Specifies a program to be used to look up the user's public keys.
1.158 djm 221: The program must be owned by root and not writable by group or others.
222: It will be invoked with a single argument of the username
1.146 djm 223: being authenticated, and should produce on standard output zero or
1.161 jmc 224: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 225: .Xr sshd 8 ) .
1.146 djm 226: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
227: and authorize the user then public key authentication continues using the usual
228: .Cm AuthorizedKeysFile
229: files.
230: By default, no AuthorizedKeysCommand is run.
231: .It Cm AuthorizedKeysCommandUser
232: Specifies the user under whose account the AuthorizedKeysCommand is run.
233: It is recommended to use a dedicated user that has no other role on the host
234: than running authorized keys commands.
1.1 stevesk 235: .It Cm AuthorizedKeysFile
236: Specifies the file that contains the public keys that can be used
237: for user authentication.
1.124 djm 238: The format is described in the
1.161 jmc 239: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 240: section of
241: .Xr sshd 8 .
1.1 stevesk 242: .Cm AuthorizedKeysFile
243: may contain tokens of the form %T which are substituted during connection
1.52 jmc 244: setup.
1.17 jmc 245: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 246: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 247: %u is replaced by the username of that user.
248: After expansion,
249: .Cm AuthorizedKeysFile
250: is taken to be an absolute path or one relative to the user's home
251: directory.
1.133 jmc 252: Multiple files may be listed, separated by whitespace.
253: The default is
254: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 255: .It Cm AuthorizedPrincipalsFile
256: Specifies a file that lists principal names that are accepted for
257: certificate authentication.
258: When using certificates signed by a key listed in
259: .Cm TrustedUserCAKeys ,
260: this file lists names, one of which must appear in the certificate for it
261: to be accepted for authentication.
1.125 jmc 262: Names are listed one per line preceded by key options (as described
1.161 jmc 263: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 264: .Xr sshd 8 ) .
1.124 djm 265: Empty lines and comments starting with
1.121 djm 266: .Ql #
267: are ignored.
268: .Pp
269: .Cm AuthorizedPrincipalsFile
270: may contain tokens of the form %T which are substituted during connection
271: setup.
272: The following tokens are defined: %% is replaced by a literal '%',
273: %h is replaced by the home directory of the user being authenticated, and
274: %u is replaced by the username of that user.
275: After expansion,
276: .Cm AuthorizedPrincipalsFile
277: is taken to be an absolute path or one relative to the user's home
278: directory.
279: .Pp
1.138 djm 280: The default is
281: .Dq none ,
282: i.e. not to use a principals file \(en in this case, the username
1.121 djm 283: of the user must appear in a certificate's principals list for it to be
284: accepted.
285: Note that
286: .Cm AuthorizedPrincipalsFile
287: is only used when authentication proceeds using a CA listed in
288: .Cm TrustedUserCAKeys
289: and is not consulted for certification authorities trusted via
290: .Pa ~/.ssh/authorized_keys ,
291: though the
292: .Cm principals=
293: key option offers a similar facility (see
294: .Xr sshd 8
295: for details).
1.1 stevesk 296: .It Cm Banner
297: The contents of the specified file are sent to the remote user before
298: authentication is allowed.
1.78 djm 299: If the argument is
300: .Dq none
301: then no banner is displayed.
1.1 stevesk 302: This option is only available for protocol version 2.
303: By default, no banner is displayed.
304: .It Cm ChallengeResponseAuthentication
1.50 jmc 305: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 306: All authentication styles from
307: .Xr login.conf 5
308: are supported.
309: The default is
310: .Dq yes .
1.80 djm 311: .It Cm ChrootDirectory
1.113 stevesk 312: Specifies the pathname of a directory to
1.80 djm 313: .Xr chroot 2
314: to after authentication.
1.113 stevesk 315: All components of the pathname must be root-owned directories that are
1.80 djm 316: not writable by any other user or group.
1.106 stevesk 317: After the chroot,
318: .Xr sshd 8
319: changes the working directory to the user's home directory.
1.80 djm 320: .Pp
1.113 stevesk 321: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 322: the connecting user has been authenticated: %% is replaced by a literal '%',
323: %h is replaced by the home directory of the user being authenticated, and
324: %u is replaced by the username of that user.
325: .Pp
326: The
327: .Cm ChrootDirectory
328: must contain the necessary files and directories to support the
1.103 stevesk 329: user's session.
1.80 djm 330: For an interactive session this requires at least a shell, typically
331: .Xr sh 1 ,
332: and basic
333: .Pa /dev
334: nodes such as
335: .Xr null 4 ,
336: .Xr zero 4 ,
337: .Xr stdin 4 ,
338: .Xr stdout 4 ,
339: .Xr stderr 4 ,
340: .Xr arandom 4
341: and
342: .Xr tty 4
343: devices.
344: For file transfer sessions using
1.105 jmc 345: .Dq sftp ,
1.80 djm 346: no additional configuration of the environment is necessary if the
1.105 jmc 347: in-process sftp server is used,
1.176 schwarze 348: though sessions which use logging may require
1.104 stevesk 349: .Pa /dev/log
1.176 schwarze 350: inside the chroot directory on some operating systems (see
1.104 stevesk 351: .Xr sftp-server 8
1.81 jmc 352: for details).
1.80 djm 353: .Pp
354: The default is not to
355: .Xr chroot 2 .
1.1 stevesk 356: .It Cm Ciphers
357: Specifies the ciphers allowed for protocol version 2.
358: Multiple ciphers must be comma-separated.
1.167 djm 359: The supported ciphers are:
360: .Pp
1.173 naddy 361: .Bl -item -compact -offset indent
362: .It
363: 3des-cbc
364: .It
365: aes128-cbc
366: .It
367: aes192-cbc
368: .It
369: aes256-cbc
370: .It
371: aes128-ctr
372: .It
373: aes192-ctr
374: .It
375: aes256-ctr
376: .It
377: aes128-gcm@openssh.com
378: .It
379: aes256-gcm@openssh.com
380: .It
381: arcfour
382: .It
383: arcfour128
384: .It
385: arcfour256
386: .It
387: blowfish-cbc
388: .It
389: cast128-cbc
390: .It
391: chacha20-poly1305@openssh.com
392: .El
1.167 djm 393: .Pp
1.52 jmc 394: The default is:
1.173 naddy 395: .Bd -literal -offset indent
396: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 397: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 398: chacha20-poly1305@openssh.com
1.1 stevesk 399: .Ed
1.167 djm 400: .Pp
401: The list of available ciphers may also be obtained using the
402: .Fl Q
403: option of
404: .Xr ssh 1 .
1.1 stevesk 405: .It Cm ClientAliveCountMax
1.48 jmc 406: Sets the number of client alive messages (see below) which may be
1.1 stevesk 407: sent without
1.52 jmc 408: .Xr sshd 8
1.17 jmc 409: receiving any messages back from the client.
410: If this threshold is reached while client alive messages are being sent,
1.52 jmc 411: sshd will disconnect the client, terminating the session.
1.17 jmc 412: It is important to note that the use of client alive messages is very
413: different from
1.27 markus 414: .Cm TCPKeepAlive
1.17 jmc 415: (below).
416: The client alive messages are sent through the encrypted channel
417: and therefore will not be spoofable.
418: The TCP keepalive option enabled by
1.27 markus 419: .Cm TCPKeepAlive
1.17 jmc 420: is spoofable.
421: The client alive mechanism is valuable when the client or
1.1 stevesk 422: server depend on knowing when a connection has become inactive.
423: .Pp
1.17 jmc 424: The default value is 3.
425: If
1.1 stevesk 426: .Cm ClientAliveInterval
1.48 jmc 427: (see below) is set to 15, and
1.1 stevesk 428: .Cm ClientAliveCountMax
1.52 jmc 429: is left at the default, unresponsive SSH clients
1.1 stevesk 430: will be disconnected after approximately 45 seconds.
1.57 markus 431: This option applies to protocol version 2 only.
1.42 djm 432: .It Cm ClientAliveInterval
433: Sets a timeout interval in seconds after which if no data has been received
434: from the client,
1.52 jmc 435: .Xr sshd 8
1.42 djm 436: will send a message through the encrypted
437: channel to request a response from the client.
438: The default
439: is 0, indicating that these messages will not be sent to the client.
440: This option applies to protocol version 2 only.
1.3 markus 441: .It Cm Compression
1.44 markus 442: Specifies whether compression is allowed, or delayed until
443: the user has authenticated successfully.
1.3 markus 444: The argument must be
1.44 markus 445: .Dq yes ,
446: .Dq delayed ,
1.3 markus 447: or
448: .Dq no .
449: The default is
1.44 markus 450: .Dq delayed .
1.1 stevesk 451: .It Cm DenyGroups
452: This keyword can be followed by a list of group name patterns, separated
453: by spaces.
454: Login is disallowed for users whose primary group or supplementary
455: group list matches one of the patterns.
456: Only group names are valid; a numerical group ID is not recognized.
457: By default, login is allowed for all groups.
1.54 jmc 458: The allow/deny directives are processed in the following order:
459: .Cm DenyUsers ,
460: .Cm AllowUsers ,
461: .Cm DenyGroups ,
462: and finally
463: .Cm AllowGroups .
1.49 jmc 464: .Pp
1.161 jmc 465: See PATTERNS in
1.49 jmc 466: .Xr ssh_config 5
467: for more information on patterns.
1.1 stevesk 468: .It Cm DenyUsers
469: This keyword can be followed by a list of user name patterns, separated
470: by spaces.
471: Login is disallowed for user names that match one of the patterns.
472: Only user names are valid; a numerical user ID is not recognized.
473: By default, login is allowed for all users.
474: If the pattern takes the form USER@HOST then USER and HOST
475: are separately checked, restricting logins to particular
476: users from particular hosts.
1.54 jmc 477: The allow/deny directives are processed in the following order:
478: .Cm DenyUsers ,
479: .Cm AllowUsers ,
480: .Cm DenyGroups ,
481: and finally
482: .Cm AllowGroups .
1.49 jmc 483: .Pp
1.161 jmc 484: See PATTERNS in
1.49 jmc 485: .Xr ssh_config 5
486: for more information on patterns.
1.67 dtucker 487: .It Cm ForceCommand
488: Forces the execution of the command specified by
489: .Cm ForceCommand ,
1.84 djm 490: ignoring any command supplied by the client and
491: .Pa ~/.ssh/rc
492: if present.
1.67 dtucker 493: The command is invoked by using the user's login shell with the -c option.
494: This applies to shell, command, or subsystem execution.
495: It is most useful inside a
496: .Cm Match
497: block.
498: The command originally supplied by the client is available in the
499: .Ev SSH_ORIGINAL_COMMAND
500: environment variable.
1.82 djm 501: Specifying a command of
502: .Dq internal-sftp
503: will force the use of an in-process sftp server that requires no support
504: files when used with
505: .Cm ChrootDirectory .
1.1 stevesk 506: .It Cm GatewayPorts
507: Specifies whether remote hosts are allowed to connect to ports
508: forwarded for the client.
509: By default,
1.52 jmc 510: .Xr sshd 8
1.15 jmc 511: binds remote port forwardings to the loopback address.
512: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 513: .Cm GatewayPorts
1.52 jmc 514: can be used to specify that sshd
1.39 djm 515: should allow remote port forwardings to bind to non-loopback addresses, thus
516: allowing other hosts to connect.
517: The argument may be
518: .Dq no
519: to force remote port forwardings to be available to the local host only,
1.1 stevesk 520: .Dq yes
1.39 djm 521: to force remote port forwardings to bind to the wildcard address, or
522: .Dq clientspecified
523: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 524: The default is
525: .Dq no .
1.23 markus 526: .It Cm GSSAPIAuthentication
1.25 markus 527: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 528: The default is
1.23 markus 529: .Dq no .
530: Note that this option applies to protocol version 2 only.
531: .It Cm GSSAPICleanupCredentials
532: Specifies whether to automatically destroy the user's credentials cache
533: on logout.
534: The default is
535: .Dq yes .
536: Note that this option applies to protocol version 2 only.
1.1 stevesk 537: .It Cm HostbasedAuthentication
538: Specifies whether rhosts or /etc/hosts.equiv authentication together
539: with successful public key client host authentication is allowed
1.50 jmc 540: (host-based authentication).
1.1 stevesk 541: This option is similar to
542: .Cm RhostsRSAAuthentication
543: and applies to protocol version 2 only.
1.70 dtucker 544: The default is
545: .Dq no .
546: .It Cm HostbasedUsesNameFromPacketOnly
547: Specifies whether or not the server will attempt to perform a reverse
548: name lookup when matching the name in the
549: .Pa ~/.shosts ,
550: .Pa ~/.rhosts ,
551: and
552: .Pa /etc/hosts.equiv
553: files during
554: .Cm HostbasedAuthentication .
555: A setting of
556: .Dq yes
557: means that
558: .Xr sshd 8
559: uses the name supplied by the client rather than
560: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 561: The default is
562: .Dq no .
1.117 djm 563: .It Cm HostCertificate
564: Specifies a file containing a public host certificate.
565: The certificate's public key must match a private host key already specified
566: by
567: .Cm HostKey .
568: The default behaviour of
569: .Xr sshd 8
570: is not to load any certificates.
1.1 stevesk 571: .It Cm HostKey
572: Specifies a file containing a private host key
573: used by SSH.
574: The default is
575: .Pa /etc/ssh/ssh_host_key
576: for protocol version 1, and
1.126 djm 577: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 578: .Pa /etc/ssh/ssh_host_ecdsa_key ,
579: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 580: and
1.1 stevesk 581: .Pa /etc/ssh/ssh_host_rsa_key
582: for protocol version 2.
583: Note that
1.52 jmc 584: .Xr sshd 8
1.1 stevesk 585: will refuse to use a file if it is group/world-accessible.
586: It is possible to have multiple host key files.
587: .Dq rsa1
588: keys are used for version 1 and
1.126 djm 589: .Dq dsa ,
1.169 naddy 590: .Dq ecdsa ,
591: .Dq ed25519
1.1 stevesk 592: or
593: .Dq rsa
594: are used for version 2 of the SSH protocol.
1.162 markus 595: It is also possible to specify public host key files instead.
596: In this case operations on the private key will be delegated
597: to an
598: .Xr ssh-agent 1 .
599: .It Cm HostKeyAgent
600: Identifies the UNIX-domain socket used to communicate
601: with an agent that has access to the private host keys.
602: If
603: .Dq SSH_AUTH_SOCK
604: is specified, the location of the socket will be read from the
605: .Ev SSH_AUTH_SOCK
606: environment variable.
1.1 stevesk 607: .It Cm IgnoreRhosts
608: Specifies that
609: .Pa .rhosts
610: and
611: .Pa .shosts
612: files will not be used in
613: .Cm RhostsRSAAuthentication
614: or
615: .Cm HostbasedAuthentication .
616: .Pp
617: .Pa /etc/hosts.equiv
618: and
619: .Pa /etc/shosts.equiv
620: are still used.
621: The default is
622: .Dq yes .
623: .It Cm IgnoreUserKnownHosts
624: Specifies whether
1.52 jmc 625: .Xr sshd 8
1.1 stevesk 626: should ignore the user's
1.41 djm 627: .Pa ~/.ssh/known_hosts
1.1 stevesk 628: during
629: .Cm RhostsRSAAuthentication
630: or
631: .Cm HostbasedAuthentication .
632: The default is
633: .Dq no .
1.129 djm 634: .It Cm IPQoS
635: Specifies the IPv4 type-of-service or DSCP class for the connection.
636: Accepted values are
637: .Dq af11 ,
638: .Dq af12 ,
639: .Dq af13 ,
1.136 djm 640: .Dq af21 ,
1.129 djm 641: .Dq af22 ,
642: .Dq af23 ,
643: .Dq af31 ,
644: .Dq af32 ,
645: .Dq af33 ,
646: .Dq af41 ,
647: .Dq af42 ,
648: .Dq af43 ,
649: .Dq cs0 ,
650: .Dq cs1 ,
651: .Dq cs2 ,
652: .Dq cs3 ,
653: .Dq cs4 ,
654: .Dq cs5 ,
655: .Dq cs6 ,
656: .Dq cs7 ,
657: .Dq ef ,
658: .Dq lowdelay ,
659: .Dq throughput ,
660: .Dq reliability ,
661: or a numeric value.
1.131 djm 662: This option may take one or two arguments, separated by whitespace.
1.129 djm 663: If one argument is specified, it is used as the packet class unconditionally.
664: If two values are specified, the first is automatically selected for
665: interactive sessions and the second for non-interactive sessions.
666: The default is
667: .Dq lowdelay
668: for interactive sessions and
669: .Dq throughput
670: for non-interactive sessions.
1.171 jmc 671: .It Cm KbdInteractiveAuthentication
672: Specifies whether to allow keyboard-interactive authentication.
673: The argument to this keyword must be
674: .Dq yes
675: or
676: .Dq no .
677: The default is to use whatever value
678: .Cm ChallengeResponseAuthentication
679: is set to
680: (by default
681: .Dq yes ) .
1.1 stevesk 682: .It Cm KerberosAuthentication
1.24 markus 683: Specifies whether the password provided by the user for
1.1 stevesk 684: .Cm PasswordAuthentication
1.24 markus 685: will be validated through the Kerberos KDC.
1.1 stevesk 686: To use this option, the server needs a
687: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 688: The default is
1.29 dtucker 689: .Dq no .
690: .It Cm KerberosGetAFSToken
1.45 djm 691: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 692: an AFS token before accessing the user's home directory.
1.52 jmc 693: The default is
1.1 stevesk 694: .Dq no .
695: .It Cm KerberosOrLocalPasswd
1.52 jmc 696: If password authentication through Kerberos fails then
1.1 stevesk 697: the password will be validated via any additional local mechanism
698: such as
699: .Pa /etc/passwd .
1.52 jmc 700: The default is
1.1 stevesk 701: .Dq yes .
702: .It Cm KerberosTicketCleanup
703: Specifies whether to automatically destroy the user's ticket cache
704: file on logout.
1.52 jmc 705: The default is
1.1 stevesk 706: .Dq yes .
1.127 djm 707: .It Cm KexAlgorithms
708: Specifies the available KEX (Key Exchange) algorithms.
709: Multiple algorithms must be comma-separated.
1.173 naddy 710: The supported algorithms are:
711: .Pp
712: .Bl -item -compact -offset indent
713: .It
714: curve25519-sha256@libssh.org
715: .It
716: diffie-hellman-group1-sha1
717: .It
718: diffie-hellman-group14-sha1
719: .It
720: diffie-hellman-group-exchange-sha1
721: .It
722: diffie-hellman-group-exchange-sha256
723: .It
724: ecdh-sha2-nistp256
725: .It
726: ecdh-sha2-nistp384
727: .It
728: ecdh-sha2-nistp521
729: .El
730: .Pp
731: The default is:
1.170 dtucker 732: .Bd -literal -offset indent
733: curve25519-sha256@libssh.org,
734: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
735: diffie-hellman-group-exchange-sha256,
1.173 naddy 736: diffie-hellman-group14-sha1
1.170 dtucker 737: .Ed
1.1 stevesk 738: .It Cm KeyRegenerationInterval
739: In protocol version 1, the ephemeral server key is automatically regenerated
740: after this many seconds (if it has been used).
741: The purpose of regeneration is to prevent
742: decrypting captured sessions by later breaking into the machine and
743: stealing the keys.
744: The key is never stored anywhere.
745: If the value is 0, the key is never regenerated.
746: The default is 3600 (seconds).
747: .It Cm ListenAddress
748: Specifies the local addresses
1.52 jmc 749: .Xr sshd 8
1.1 stevesk 750: should listen on.
751: The following forms may be used:
752: .Pp
753: .Bl -item -offset indent -compact
754: .It
755: .Cm ListenAddress
756: .Sm off
757: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
758: .Sm on
759: .It
760: .Cm ListenAddress
761: .Sm off
762: .Ar host No | Ar IPv4_addr No : Ar port
763: .Sm on
764: .It
765: .Cm ListenAddress
766: .Sm off
767: .Oo
768: .Ar host No | Ar IPv6_addr Oc : Ar port
769: .Sm on
770: .El
771: .Pp
772: If
773: .Ar port
774: is not specified,
1.52 jmc 775: sshd will listen on the address and all prior
1.1 stevesk 776: .Cm Port
1.17 jmc 777: options specified.
778: The default is to listen on all local addresses.
1.15 jmc 779: Multiple
1.1 stevesk 780: .Cm ListenAddress
1.17 jmc 781: options are permitted.
782: Additionally, any
1.1 stevesk 783: .Cm Port
1.52 jmc 784: options must precede this option for non-port qualified addresses.
1.1 stevesk 785: .It Cm LoginGraceTime
786: The server disconnects after this time if the user has not
787: successfully logged in.
788: If the value is 0, there is no time limit.
1.12 stevesk 789: The default is 120 seconds.
1.1 stevesk 790: .It Cm LogLevel
791: Gives the verbosity level that is used when logging messages from
1.53 jmc 792: .Xr sshd 8 .
1.1 stevesk 793: The possible values are:
1.52 jmc 794: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 795: The default is INFO.
796: DEBUG and DEBUG1 are equivalent.
797: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
798: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 799: .It Cm MACs
800: Specifies the available MAC (message authentication code) algorithms.
801: The MAC algorithm is used in protocol version 2
802: for data integrity protection.
803: Multiple algorithms must be comma-separated.
1.152 markus 804: The algorithms that contain
805: .Dq -etm
806: calculate the MAC after encryption (encrypt-then-mac).
807: These are considered safer and their use recommended.
1.173 naddy 808: The supported MACs are:
809: .Pp
810: .Bl -item -compact -offset indent
811: .It
812: hmac-md5
813: .It
814: hmac-md5-96
815: .It
816: hmac-ripemd160
817: .It
818: hmac-sha1
819: .It
820: hmac-sha1-96
821: .It
822: hmac-sha2-256
823: .It
824: hmac-sha2-512
825: .It
826: umac-64@openssh.com
827: .It
828: umac-128@openssh.com
829: .It
830: hmac-md5-etm@openssh.com
831: .It
832: hmac-md5-96-etm@openssh.com
833: .It
834: hmac-ripemd160-etm@openssh.com
835: .It
836: hmac-sha1-etm@openssh.com
837: .It
838: hmac-sha1-96-etm@openssh.com
839: .It
840: hmac-sha2-256-etm@openssh.com
841: .It
842: hmac-sha2-512-etm@openssh.com
843: .It
844: umac-64-etm@openssh.com
845: .It
846: umac-128-etm@openssh.com
847: .El
848: .Pp
1.52 jmc 849: The default is:
1.77 jmc 850: .Bd -literal -offset indent
1.152 markus 851: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
852: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 853: umac-64@openssh.com,umac-128@openssh.com,
854: hmac-sha2-256,hmac-sha2-512
1.77 jmc 855: .Ed
1.60 dtucker 856: .It Cm Match
1.61 jmc 857: Introduces a conditional block.
1.65 dtucker 858: If all of the criteria on the
1.60 dtucker 859: .Cm Match
1.65 dtucker 860: line are satisfied, the keywords on the following lines override those
861: set in the global section of the config file, until either another
1.60 dtucker 862: .Cm Match
1.65 dtucker 863: line or the end of the file.
1.172 djm 864: If a keyword appears in multiple
865: .Cm Match
1.177 sobrado 866: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 867: applied.
1.91 djm 868: .Pp
1.61 jmc 869: The arguments to
1.60 dtucker 870: .Cm Match
1.163 dtucker 871: are one or more criteria-pattern pairs or the single token
872: .Cm All
873: which matches all criteria.
1.60 dtucker 874: The available criteria are
875: .Cm User ,
1.69 dtucker 876: .Cm Group ,
1.60 dtucker 877: .Cm Host ,
1.139 dtucker 878: .Cm LocalAddress ,
879: .Cm LocalPort ,
1.60 dtucker 880: and
881: .Cm Address .
1.91 djm 882: The match patterns may consist of single entries or comma-separated
883: lists and may use the wildcard and negation operators described in the
1.161 jmc 884: PATTERNS section of
1.92 djm 885: .Xr ssh_config 5 .
1.91 djm 886: .Pp
887: The patterns in an
888: .Cm Address
889: criteria may additionally contain addresses to match in CIDR
1.93 jmc 890: address/masklen format, e.g.\&
1.91 djm 891: .Dq 192.0.2.0/24
892: or
893: .Dq 3ffe:ffff::/32 .
894: Note that the mask length provided must be consistent with the address -
895: it is an error to specify a mask length that is too long for the address
1.93 jmc 896: or one with bits set in this host portion of the address.
897: For example,
1.91 djm 898: .Dq 192.0.2.0/33
899: and
1.93 jmc 900: .Dq 192.0.2.0/8
1.91 djm 901: respectively.
902: .Pp
1.60 dtucker 903: Only a subset of keywords may be used on the lines following a
904: .Cm Match
905: keyword.
906: Available keywords are
1.142 jmc 907: .Cm AcceptEnv ,
1.99 okan 908: .Cm AllowAgentForwarding ,
1.142 jmc 909: .Cm AllowGroups ,
1.62 dtucker 910: .Cm AllowTcpForwarding ,
1.141 markus 911: .Cm AllowUsers ,
1.149 djm 912: .Cm AuthenticationMethods ,
1.146 djm 913: .Cm AuthorizedKeysCommand ,
914: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 915: .Cm AuthorizedKeysFile ,
1.123 djm 916: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 917: .Cm Banner ,
1.85 djm 918: .Cm ChrootDirectory ,
1.141 markus 919: .Cm DenyGroups ,
920: .Cm DenyUsers ,
1.67 dtucker 921: .Cm ForceCommand ,
1.142 jmc 922: .Cm GatewayPorts ,
1.141 markus 923: .Cm GSSAPIAuthentication ,
1.87 djm 924: .Cm HostbasedAuthentication ,
1.123 djm 925: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 926: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 927: .Cm KerberosAuthentication ,
1.95 dtucker 928: .Cm MaxAuthTries ,
1.94 dtucker 929: .Cm MaxSessions ,
1.72 dtucker 930: .Cm PasswordAuthentication ,
1.97 djm 931: .Cm PermitEmptyPasswords ,
1.66 dtucker 932: .Cm PermitOpen ,
1.79 dtucker 933: .Cm PermitRootLogin ,
1.164 djm 934: .Cm PermitTTY ,
1.123 djm 935: .Cm PermitTunnel ,
1.174 djm 936: .Cm PermitUserRC ,
1.107 dtucker 937: .Cm PubkeyAuthentication ,
1.159 dtucker 938: .Cm RekeyLimit ,
1.142 jmc 939: .Cm RhostsRSAAuthentication ,
1.141 markus 940: .Cm RSAAuthentication ,
1.66 dtucker 941: .Cm X11DisplayOffset ,
1.101 djm 942: .Cm X11Forwarding
1.60 dtucker 943: and
1.102 djm 944: .Cm X11UseLocalHost .
1.33 dtucker 945: .It Cm MaxAuthTries
946: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 947: connection.
948: Once the number of failures reaches half this value,
949: additional failures are logged.
950: The default is 6.
1.90 djm 951: .It Cm MaxSessions
952: Specifies the maximum number of open sessions permitted per network connection.
953: The default is 10.
1.1 stevesk 954: .It Cm MaxStartups
955: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 956: SSH daemon.
1.1 stevesk 957: Additional connections will be dropped until authentication succeeds or the
958: .Cm LoginGraceTime
959: expires for a connection.
1.156 dtucker 960: The default is 10:30:100.
1.1 stevesk 961: .Pp
962: Alternatively, random early drop can be enabled by specifying
963: the three colon separated values
964: .Dq start:rate:full
1.51 jmc 965: (e.g. "10:30:60").
1.53 jmc 966: .Xr sshd 8
1.1 stevesk 967: will refuse connection attempts with a probability of
968: .Dq rate/100
969: (30%)
970: if there are currently
971: .Dq start
972: (10)
973: unauthenticated connections.
974: The probability increases linearly and all connection attempts
975: are refused if the number of unauthenticated connections reaches
976: .Dq full
977: (60).
978: .It Cm PasswordAuthentication
979: Specifies whether password authentication is allowed.
980: The default is
981: .Dq yes .
982: .It Cm PermitEmptyPasswords
983: When password authentication is allowed, it specifies whether the
984: server allows login to accounts with empty password strings.
985: The default is
986: .Dq no .
1.62 dtucker 987: .It Cm PermitOpen
988: Specifies the destinations to which TCP port forwarding is permitted.
989: The forwarding specification must be one of the following forms:
990: .Pp
991: .Bl -item -offset indent -compact
992: .It
993: .Cm PermitOpen
994: .Sm off
995: .Ar host : port
996: .Sm on
997: .It
998: .Cm PermitOpen
999: .Sm off
1000: .Ar IPv4_addr : port
1001: .Sm on
1002: .It
1003: .Cm PermitOpen
1004: .Sm off
1005: .Ar \&[ IPv6_addr \&] : port
1006: .Sm on
1007: .El
1008: .Pp
1.68 dtucker 1009: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1010: An argument of
1011: .Dq any
1012: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1013: An argument of
1014: .Dq none
1015: can be used to prohibit all forwarding requests.
1.63 jmc 1016: By default all port forwarding requests are permitted.
1.1 stevesk 1017: .It Cm PermitRootLogin
1.38 jmc 1018: Specifies whether root can log in using
1.1 stevesk 1019: .Xr ssh 1 .
1020: The argument must be
1021: .Dq yes ,
1022: .Dq without-password ,
1.52 jmc 1023: .Dq forced-commands-only ,
1.1 stevesk 1024: or
1025: .Dq no .
1026: The default is
1027: .Dq yes .
1028: .Pp
1029: If this option is set to
1.52 jmc 1030: .Dq without-password ,
1.1 stevesk 1031: password authentication is disabled for root.
1032: .Pp
1033: If this option is set to
1.52 jmc 1034: .Dq forced-commands-only ,
1.1 stevesk 1035: root login with public key authentication will be allowed,
1036: but only if the
1037: .Ar command
1038: option has been specified
1039: (which may be useful for taking remote backups even if root login is
1.17 jmc 1040: normally not allowed).
1041: All other authentication methods are disabled for root.
1.1 stevesk 1042: .Pp
1043: If this option is set to
1.52 jmc 1044: .Dq no ,
1.38 jmc 1045: root is not allowed to log in.
1.46 reyk 1046: .It Cm PermitTunnel
1047: Specifies whether
1048: .Xr tun 4
1049: device forwarding is allowed.
1.47 reyk 1050: The argument must be
1051: .Dq yes ,
1.58 stevesk 1052: .Dq point-to-point
1053: (layer 3),
1054: .Dq ethernet
1055: (layer 2), or
1.47 reyk 1056: .Dq no .
1.58 stevesk 1057: Specifying
1058: .Dq yes
1059: permits both
1060: .Dq point-to-point
1061: and
1062: .Dq ethernet .
1.46 reyk 1063: The default is
1064: .Dq no .
1.178 ! djm 1065: .Pp
! 1066: Independent of this setting, the permissions of the selected
! 1067: .Xr tun 4
! 1068: device must allow access to the user.
1.164 djm 1069: .It Cm PermitTTY
1070: Specifies whether
1.165 jmc 1071: .Xr pty 4
1.164 djm 1072: allocation is permitted.
1073: The default is
1074: .Dq yes .
1.6 markus 1075: .It Cm PermitUserEnvironment
1076: Specifies whether
1077: .Pa ~/.ssh/environment
1.9 stevesk 1078: and
1.6 markus 1079: .Cm environment=
1080: options in
1081: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1082: are processed by
1.52 jmc 1083: .Xr sshd 8 .
1.6 markus 1084: The default is
1085: .Dq no .
1.9 stevesk 1086: Enabling environment processing may enable users to bypass access
1087: restrictions in some configurations using mechanisms such as
1088: .Ev LD_PRELOAD .
1.174 djm 1089: .It Cm PermitUserRC
1090: Specifies whether any
1091: .Pa ~/.ssh/rc
1092: file is executed.
1093: The default is
1094: .Dq yes .
1.1 stevesk 1095: .It Cm PidFile
1.4 stevesk 1096: Specifies the file that contains the process ID of the
1.53 jmc 1097: SSH daemon.
1.1 stevesk 1098: The default is
1099: .Pa /var/run/sshd.pid .
1100: .It Cm Port
1101: Specifies the port number that
1.52 jmc 1102: .Xr sshd 8
1.1 stevesk 1103: listens on.
1104: The default is 22.
1105: Multiple options of this type are permitted.
1106: See also
1107: .Cm ListenAddress .
1108: .It Cm PrintLastLog
1109: Specifies whether
1.52 jmc 1110: .Xr sshd 8
1.36 jaredy 1111: should print the date and time of the last user login when a user logs
1112: in interactively.
1.1 stevesk 1113: The default is
1114: .Dq yes .
1115: .It Cm PrintMotd
1116: Specifies whether
1.52 jmc 1117: .Xr sshd 8
1.1 stevesk 1118: should print
1119: .Pa /etc/motd
1120: when a user logs in interactively.
1121: (On some systems it is also printed by the shell,
1122: .Pa /etc/profile ,
1123: or equivalent.)
1124: The default is
1125: .Dq yes .
1126: .It Cm Protocol
1127: Specifies the protocol versions
1.52 jmc 1128: .Xr sshd 8
1.5 stevesk 1129: supports.
1.1 stevesk 1130: The possible values are
1.52 jmc 1131: .Sq 1
1.1 stevesk 1132: and
1.52 jmc 1133: .Sq 2 .
1.1 stevesk 1134: Multiple versions must be comma-separated.
1135: The default is
1.109 jmc 1136: .Sq 2 .
1.5 stevesk 1137: Note that the order of the protocol list does not indicate preference,
1138: because the client selects among multiple protocol versions offered
1139: by the server.
1140: Specifying
1141: .Dq 2,1
1142: is identical to
1143: .Dq 1,2 .
1.1 stevesk 1144: .It Cm PubkeyAuthentication
1145: Specifies whether public key authentication is allowed.
1146: The default is
1147: .Dq yes .
1148: Note that this option applies to protocol version 2 only.
1.159 dtucker 1149: .It Cm RekeyLimit
1150: Specifies the maximum amount of data that may be transmitted before the
1151: session key is renegotiated, optionally followed a maximum amount of
1152: time that may pass before the session key is renegotiated.
1153: The first argument is specified in bytes and may have a suffix of
1154: .Sq K ,
1155: .Sq M ,
1156: or
1157: .Sq G
1158: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1159: The default is between
1160: .Sq 1G
1161: and
1162: .Sq 4G ,
1163: depending on the cipher.
1164: The optional second value is specified in seconds and may use any of the
1165: units documented in the
1166: .Sx TIME FORMATS
1.160 jmc 1167: section.
1.159 dtucker 1168: The default value for
1169: .Cm RekeyLimit
1170: is
1171: .Dq default none ,
1172: which means that rekeying is performed after the cipher's default amount
1173: of data has been sent or received and no time based rekeying is done.
1174: This option applies to protocol version 2 only.
1.118 djm 1175: .It Cm RevokedKeys
1.154 djm 1176: Specifies revoked public keys.
1.118 djm 1177: Keys listed in this file will be refused for public key authentication.
1178: Note that if this file is not readable, then public key authentication will
1179: be refused for all users.
1.154 djm 1180: Keys may be specified as a text file, listing one public key per line, or as
1181: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1182: .Xr ssh-keygen 1 .
1.161 jmc 1183: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1184: .Xr ssh-keygen 1 .
1.1 stevesk 1185: .It Cm RhostsRSAAuthentication
1186: Specifies whether rhosts or /etc/hosts.equiv authentication together
1187: with successful RSA host authentication is allowed.
1188: The default is
1189: .Dq no .
1190: This option applies to protocol version 1 only.
1191: .It Cm RSAAuthentication
1192: Specifies whether pure RSA authentication is allowed.
1193: The default is
1194: .Dq yes .
1195: This option applies to protocol version 1 only.
1196: .It Cm ServerKeyBits
1197: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1198: The minimum value is 512, and the default is 1024.
1.175 millert 1199: .It Cm StreamLocalBindMask
1200: Sets the octal file creation mode mask
1201: .Pq umask
1202: used when creating a Unix-domain socket file for local or remote
1203: port forwarding.
1204: This option is only used for port forwarding to a Unix-domain socket file.
1205: .Pp
1206: The default value is 0177, which creates a Unix-domain socket file that is
1207: readable and writable only by the owner.
1208: Note that not all operating systems honor the file mode on Unix-domain
1209: socket files.
1210: .It Cm StreamLocalBindUnlink
1211: Specifies whether to remove an existing Unix-domain socket file for local
1212: or remote port forwarding before creating a new one.
1213: If the socket file already exists and
1214: .Cm StreamLocalBindUnlink
1215: is not enabled,
1216: .Nm sshd
1217: will be unable to forward the port to the Unix-domain socket file.
1218: This option is only used for port forwarding to a Unix-domain socket file.
1219: .Pp
1220: The argument must be
1221: .Dq yes
1222: or
1223: .Dq no .
1224: The default is
1225: .Dq no .
1.1 stevesk 1226: .It Cm StrictModes
1227: Specifies whether
1.52 jmc 1228: .Xr sshd 8
1.1 stevesk 1229: should check file modes and ownership of the
1230: user's files and home directory before accepting login.
1231: This is normally desirable because novices sometimes accidentally leave their
1232: directory or files world-writable.
1233: The default is
1234: .Dq yes .
1.112 djm 1235: Note that this does not apply to
1236: .Cm ChrootDirectory ,
1237: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1238: .It Cm Subsystem
1.51 jmc 1239: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1240: Arguments should be a subsystem name and a command (with optional arguments)
1241: to execute upon subsystem request.
1.80 djm 1242: .Pp
1.1 stevesk 1243: The command
1244: .Xr sftp-server 8
1245: implements the
1246: .Dq sftp
1247: file transfer subsystem.
1.80 djm 1248: .Pp
1249: Alternately the name
1250: .Dq internal-sftp
1251: implements an in-process
1252: .Dq sftp
1253: server.
1254: This may simplify configurations using
1255: .Cm ChrootDirectory
1256: to force a different filesystem root on clients.
1257: .Pp
1.1 stevesk 1258: By default no subsystems are defined.
1259: Note that this option applies to protocol version 2 only.
1260: .It Cm SyslogFacility
1261: Gives the facility code that is used when logging messages from
1.53 jmc 1262: .Xr sshd 8 .
1.1 stevesk 1263: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1264: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1265: The default is AUTH.
1.27 markus 1266: .It Cm TCPKeepAlive
1267: Specifies whether the system should send TCP keepalive messages to the
1268: other side.
1269: If they are sent, death of the connection or crash of one
1270: of the machines will be properly noticed.
1271: However, this means that
1272: connections will die if the route is down temporarily, and some people
1273: find it annoying.
1274: On the other hand, if TCP keepalives are not sent,
1275: sessions may hang indefinitely on the server, leaving
1276: .Dq ghost
1277: users and consuming server resources.
1278: .Pp
1279: The default is
1280: .Dq yes
1281: (to send TCP keepalive messages), and the server will notice
1282: if the network goes down or the client host crashes.
1283: This avoids infinitely hanging sessions.
1284: .Pp
1285: To disable TCP keepalive messages, the value should be set to
1286: .Dq no .
1.118 djm 1287: .It Cm TrustedUserCAKeys
1288: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1289: trusted to sign user certificates for authentication.
1.119 jmc 1290: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1291: .Ql #
1292: are allowed.
1293: If a certificate is presented for authentication and has its signing CA key
1294: listed in this file, then it may be used for authentication for any user
1295: listed in the certificate's principals list.
1296: Note that certificates that lack a list of principals will not be permitted
1297: for authentication using
1298: .Cm TrustedUserCAKeys .
1.161 jmc 1299: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1300: .Xr ssh-keygen 1 .
1.18 markus 1301: .It Cm UseDNS
1302: Specifies whether
1.52 jmc 1303: .Xr sshd 8
1.40 jmc 1304: should look up the remote host name and check that
1.18 markus 1305: the resolved host name for the remote IP address maps back to the
1306: very same IP address.
1307: The default is
1308: .Dq yes .
1.1 stevesk 1309: .It Cm UseLogin
1310: Specifies whether
1311: .Xr login 1
1312: is used for interactive login sessions.
1313: The default is
1314: .Dq no .
1315: Note that
1316: .Xr login 1
1317: is never used for remote command execution.
1318: Note also, that if this is enabled,
1319: .Cm X11Forwarding
1320: will be disabled because
1321: .Xr login 1
1322: does not know how to handle
1323: .Xr xauth 1
1.15 jmc 1324: cookies.
1325: If
1.1 stevesk 1326: .Cm UsePrivilegeSeparation
1327: is specified, it will be disabled after authentication.
1328: .It Cm UsePrivilegeSeparation
1329: Specifies whether
1.52 jmc 1330: .Xr sshd 8
1.2 stevesk 1331: separates privileges by creating an unprivileged child process
1.15 jmc 1332: to deal with incoming network traffic.
1333: After successful authentication, another process will be created that has
1334: the privilege of the authenticated user.
1335: The goal of privilege separation is to prevent privilege
1.1 stevesk 1336: escalation by containing any corruption within the unprivileged processes.
1337: The default is
1338: .Dq yes .
1.134 djm 1339: If
1340: .Cm UsePrivilegeSeparation
1341: is set to
1342: .Dq sandbox
1343: then the pre-authentication unprivileged process is subject to additional
1344: restrictions.
1.137 djm 1345: .It Cm VersionAddendum
1346: Optionally specifies additional text to append to the SSH protocol banner
1347: sent by the server upon connection.
1348: The default is
1349: .Dq none .
1.1 stevesk 1350: .It Cm X11DisplayOffset
1351: Specifies the first display number available for
1.52 jmc 1352: .Xr sshd 8 Ns 's
1.1 stevesk 1353: X11 forwarding.
1.52 jmc 1354: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1355: The default is 10.
1356: .It Cm X11Forwarding
1357: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1358: The argument must be
1359: .Dq yes
1360: or
1361: .Dq no .
1.1 stevesk 1362: The default is
1363: .Dq no .
1.13 stevesk 1364: .Pp
1365: When X11 forwarding is enabled, there may be additional exposure to
1366: the server and to client displays if the
1.52 jmc 1367: .Xr sshd 8
1.13 stevesk 1368: proxy display is configured to listen on the wildcard address (see
1369: .Cm X11UseLocalhost
1.52 jmc 1370: below), though this is not the default.
1.13 stevesk 1371: Additionally, the authentication spoofing and authentication data
1372: verification and substitution occur on the client side.
1373: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1374: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1375: forwarding (see the warnings for
1376: .Cm ForwardX11
1377: in
1.19 jmc 1378: .Xr ssh_config 5 ) .
1.13 stevesk 1379: A system administrator may have a stance in which they want to
1380: protect clients that may expose themselves to attack by unwittingly
1381: requesting X11 forwarding, which can warrant a
1382: .Dq no
1383: setting.
1384: .Pp
1385: Note that disabling X11 forwarding does not prevent users from
1386: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1387: X11 forwarding is automatically disabled if
1388: .Cm UseLogin
1389: is enabled.
1390: .It Cm X11UseLocalhost
1391: Specifies whether
1.52 jmc 1392: .Xr sshd 8
1.1 stevesk 1393: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1394: the wildcard address.
1395: By default,
1.52 jmc 1396: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1397: hostname part of the
1398: .Ev DISPLAY
1399: environment variable to
1400: .Dq localhost .
1.8 stevesk 1401: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1402: However, some older X11 clients may not function with this
1403: configuration.
1404: .Cm X11UseLocalhost
1405: may be set to
1406: .Dq no
1407: to specify that the forwarding server should be bound to the wildcard
1408: address.
1409: The argument must be
1410: .Dq yes
1411: or
1412: .Dq no .
1413: The default is
1414: .Dq yes .
1415: .It Cm XAuthLocation
1.11 stevesk 1416: Specifies the full pathname of the
1.1 stevesk 1417: .Xr xauth 1
1418: program.
1419: The default is
1420: .Pa /usr/X11R6/bin/xauth .
1421: .El
1.55 jmc 1422: .Sh TIME FORMATS
1.53 jmc 1423: .Xr sshd 8
1.1 stevesk 1424: command-line arguments and configuration file options that specify time
1425: may be expressed using a sequence of the form:
1426: .Sm off
1.7 stevesk 1427: .Ar time Op Ar qualifier ,
1.1 stevesk 1428: .Sm on
1429: where
1430: .Ar time
1431: is a positive integer value and
1432: .Ar qualifier
1433: is one of the following:
1434: .Pp
1435: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1436: .It Aq Cm none
1.1 stevesk 1437: seconds
1438: .It Cm s | Cm S
1439: seconds
1440: .It Cm m | Cm M
1441: minutes
1442: .It Cm h | Cm H
1443: hours
1444: .It Cm d | Cm D
1445: days
1446: .It Cm w | Cm W
1447: weeks
1448: .El
1449: .Pp
1450: Each member of the sequence is added together to calculate
1451: the total time value.
1452: .Pp
1453: Time format examples:
1454: .Pp
1455: .Bl -tag -width Ds -compact -offset indent
1456: .It 600
1457: 600 seconds (10 minutes)
1458: .It 10m
1459: 10 minutes
1460: .It 1h30m
1461: 1 hour 30 minutes (90 minutes)
1462: .El
1463: .Sh FILES
1464: .Bl -tag -width Ds
1465: .It Pa /etc/ssh/sshd_config
1466: Contains configuration data for
1.53 jmc 1467: .Xr sshd 8 .
1.1 stevesk 1468: This file should be writable by root only, but it is recommended
1469: (though not necessary) that it be world-readable.
1470: .El
1.19 jmc 1471: .Sh SEE ALSO
1472: .Xr sshd 8
1.1 stevesk 1473: .Sh AUTHORS
1474: OpenSSH is a derivative of the original and free
1475: ssh 1.2.12 release by Tatu Ylonen.
1476: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1477: Theo de Raadt and Dug Song
1478: removed many bugs, re-added newer features and
1479: created OpenSSH.
1480: Markus Friedl contributed the support for SSH
1481: protocol versions 1.5 and 2.0.
1482: Niels Provos and Markus Friedl contributed support
1483: for privilege separation.
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