Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.202
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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1.202 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.201 2015/05/21 06:38:35 djm Exp $
! 37: .Dd $Mdocdate: May 21 2015 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.197 dtucker 73: Note that environment passing is only supported for protocol 2, and
74: that the
75: .Ev TERM
1.199 jmc 76: environment variable is always sent whenever the client
1.198 dtucker 77: requests a pseudo-terminal is requested as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
94: .Dq any ,
95: .Dq inet
1.52 jmc 96: (use IPv4 only), or
1.37 djm 97: .Dq inet6
98: (use IPv6 only).
99: The default is
100: .Dq any .
1.89 jmc 101: .It Cm AllowAgentForwarding
102: Specifies whether
103: .Xr ssh-agent 1
104: forwarding is permitted.
105: The default is
106: .Dq yes .
107: Note that disabling agent forwarding does not improve security
108: unless users are also denied shell access, as they can always install
109: their own forwarders.
1.1 stevesk 110: .It Cm AllowGroups
111: This keyword can be followed by a list of group name patterns, separated
112: by spaces.
113: If specified, login is allowed only for users whose primary
114: group or supplementary group list matches one of the patterns.
115: Only group names are valid; a numerical group ID is not recognized.
116: By default, login is allowed for all groups.
1.54 jmc 117: The allow/deny directives are processed in the following order:
118: .Cm DenyUsers ,
119: .Cm AllowUsers ,
120: .Cm DenyGroups ,
121: and finally
122: .Cm AllowGroups .
1.49 jmc 123: .Pp
1.161 jmc 124: See PATTERNS in
1.49 jmc 125: .Xr ssh_config 5
126: for more information on patterns.
1.1 stevesk 127: .It Cm AllowTcpForwarding
128: Specifies whether TCP forwarding is permitted.
1.150 djm 129: The available options are
130: .Dq yes
131: or
132: .Dq all
133: to allow TCP forwarding,
134: .Dq no
135: to prevent all TCP forwarding,
136: .Dq local
137: to allow local (from the perspective of
1.151 jmc 138: .Xr ssh 1 )
139: forwarding only or
1.150 djm 140: .Dq remote
141: to allow remote forwarding only.
1.1 stevesk 142: The default is
143: .Dq yes .
144: Note that disabling TCP forwarding does not improve security unless
145: users are also denied shell access, as they can always install their
146: own forwarders.
1.175 millert 147: .It Cm AllowStreamLocalForwarding
148: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
149: The available options are
150: .Dq yes
151: or
152: .Dq all
153: to allow StreamLocal forwarding,
154: .Dq no
155: to prevent all StreamLocal forwarding,
156: .Dq local
157: to allow local (from the perspective of
158: .Xr ssh 1 )
159: forwarding only or
160: .Dq remote
161: to allow remote forwarding only.
162: The default is
163: .Dq yes .
164: Note that disabling StreamLocal forwarding does not improve security unless
165: users are also denied shell access, as they can always install their
166: own forwarders.
1.1 stevesk 167: .It Cm AllowUsers
168: This keyword can be followed by a list of user name patterns, separated
169: by spaces.
1.14 jmc 170: If specified, login is allowed only for user names that
1.1 stevesk 171: match one of the patterns.
172: Only user names are valid; a numerical user ID is not recognized.
173: By default, login is allowed for all users.
174: If the pattern takes the form USER@HOST then USER and HOST
175: are separately checked, restricting logins to particular
176: users from particular hosts.
1.54 jmc 177: The allow/deny directives are processed in the following order:
178: .Cm DenyUsers ,
179: .Cm AllowUsers ,
180: .Cm DenyGroups ,
181: and finally
182: .Cm AllowGroups .
1.49 jmc 183: .Pp
1.161 jmc 184: See PATTERNS in
1.49 jmc 185: .Xr ssh_config 5
186: for more information on patterns.
1.149 djm 187: .It Cm AuthenticationMethods
188: Specifies the authentication methods that must be successfully completed
189: for a user to be granted access.
190: This option must be followed by one or more comma-separated lists of
191: authentication method names.
192: Successful authentication requires completion of every method in at least
193: one of these lists.
194: .Pp
195: For example, an argument of
196: .Dq publickey,password publickey,keyboard-interactive
197: would require the user to complete public key authentication, followed by
198: either password or keyboard interactive authentication.
199: Only methods that are next in one or more lists are offered at each stage,
200: so for this example, it would not be possible to attempt password or
201: keyboard-interactive authentication before public key.
1.157 markus 202: .Pp
203: For keyboard interactive authentication it is also possible to
204: restrict authentication to a specific device by appending a
205: colon followed by the device identifier
206: .Dq bsdauth ,
207: .Dq pam ,
208: or
209: .Dq skey ,
210: depending on the server configuration.
211: For example,
212: .Dq keyboard-interactive:bsdauth
213: would restrict keyboard interactive authentication to the
214: .Dq bsdauth
215: device.
1.185 djm 216: .Pp
217: If the
218: .Dq publickey
1.187 jmc 219: method is listed more than once,
1.185 djm 220: .Xr sshd 8
221: verifies that keys that have been used successfully are not reused for
222: subsequent authentications.
223: For example, an
224: .Cm AuthenticationMethods
225: of
226: .Dq publickey,publickey
227: will require successful authentication using two different public keys.
1.181 djm 228: .Pp
1.149 djm 229: This option is only available for SSH protocol 2 and will yield a fatal
230: error if enabled if protocol 1 is also enabled.
231: Note that each authentication method listed should also be explicitly enabled
232: in the configuration.
233: The default is not to require multiple authentication; successful completion
234: of a single authentication method is sufficient.
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: .Pp
240: Arguments to
241: .Cm AuthorizedKeysCommand
242: may be provided using the following tokens, which will be expanded
243: at runtime: %% is replaced by a literal '%', %u is replaced by the
244: username being authenticated, %h is replaced by the home directory
245: of the user being authenticated, %t is replaced with the key type
246: offered for authentication, %f is replaced with the fingerprint of
247: the key, and %k is replaced with the key being offered for authentication.
248: If no arguments are specified then the username of the target user
249: will be supplied.
250: .Pp
251: The program should produce on standard output zero or
1.161 jmc 252: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 253: .Xr sshd 8 ) .
1.146 djm 254: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
255: and authorize the user then public key authentication continues using the usual
256: .Cm AuthorizedKeysFile
257: files.
258: By default, no AuthorizedKeysCommand is run.
259: .It Cm AuthorizedKeysCommandUser
260: Specifies the user under whose account the AuthorizedKeysCommand is run.
261: It is recommended to use a dedicated user that has no other role on the host
262: than running authorized keys commands.
1.186 djm 263: If
1.181 djm 264: .Cm AuthorizedKeysCommand
1.186 djm 265: is specified but
266: .Cm AuthorizedKeysCommandUser
267: is not, then
268: .Xr sshd 8
269: will refuse to start.
1.1 stevesk 270: .It Cm AuthorizedKeysFile
271: Specifies the file that contains the public keys that can be used
272: for user authentication.
1.124 djm 273: The format is described in the
1.161 jmc 274: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 275: section of
276: .Xr sshd 8 .
1.1 stevesk 277: .Cm AuthorizedKeysFile
278: may contain tokens of the form %T which are substituted during connection
1.52 jmc 279: setup.
1.17 jmc 280: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 281: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 282: %u is replaced by the username of that user.
283: After expansion,
284: .Cm AuthorizedKeysFile
285: is taken to be an absolute path or one relative to the user's home
286: directory.
1.133 jmc 287: Multiple files may be listed, separated by whitespace.
288: The default is
289: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 ! djm 290: .It Cm AuthorizedPrincipalsCommand
! 291: Specifies a program to be used to generate the list of allowed
! 292: certificate principals as per
! 293: .Cm AuthorizedPrincipalsFile .
! 294: The program must be owned by root, not writable by group or others and
! 295: specified by an absolute path.
! 296: .Pp
! 297: Arguments to
! 298: .Cm AuthorizedPrincipalsCommand
! 299: may be provided using the following tokens, which will be expanded
! 300: at runtime: %% is replaced by a literal '%', %u is replaced by the
! 301: username being authenticated and %h is replaced by the home directory
! 302: of the user being authenticated.
! 303: .Pp
! 304: The program should produce on standard output zero or
! 305: more lines of
! 306: .Cm AuthorizedPrincipalsFile
! 307: output.
! 308: If either
! 309: .Cm AuthorizedPrincipalsCommand
! 310: or
! 311: .Cm AuthorizedPrincipalsFile
! 312: is specified, then certificates offered by the client for authentication
! 313: must contain a principal that is listed.
! 314: By default, no AuthorizedPrincipalsCommand is run.
! 315: .It Cm AuthorizedPrincipalsCommandUser
! 316: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
! 317: It is recommended to use a dedicated user that has no other role on the host
! 318: than running authorized principals commands.
! 319: If
! 320: .Cm AuthorizedPrincipalsCommand
! 321: is specified but
! 322: .Cm AuthorizedPrincipalsCommandUser
! 323: is not, then
! 324: .Xr sshd 8
! 325: will refuse to start.
1.121 djm 326: .It Cm AuthorizedPrincipalsFile
327: Specifies a file that lists principal names that are accepted for
328: certificate authentication.
329: When using certificates signed by a key listed in
330: .Cm TrustedUserCAKeys ,
331: this file lists names, one of which must appear in the certificate for it
332: to be accepted for authentication.
1.125 jmc 333: Names are listed one per line preceded by key options (as described
1.161 jmc 334: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 335: .Xr sshd 8 ) .
1.124 djm 336: Empty lines and comments starting with
1.121 djm 337: .Ql #
338: are ignored.
339: .Pp
340: .Cm AuthorizedPrincipalsFile
341: may contain tokens of the form %T which are substituted during connection
342: setup.
343: The following tokens are defined: %% is replaced by a literal '%',
344: %h is replaced by the home directory of the user being authenticated, and
345: %u is replaced by the username of that user.
346: After expansion,
347: .Cm AuthorizedPrincipalsFile
348: is taken to be an absolute path or one relative to the user's home
349: directory.
350: .Pp
1.138 djm 351: The default is
352: .Dq none ,
353: i.e. not to use a principals file \(en in this case, the username
1.121 djm 354: of the user must appear in a certificate's principals list for it to be
355: accepted.
356: Note that
357: .Cm AuthorizedPrincipalsFile
358: is only used when authentication proceeds using a CA listed in
359: .Cm TrustedUserCAKeys
360: and is not consulted for certification authorities trusted via
361: .Pa ~/.ssh/authorized_keys ,
362: though the
363: .Cm principals=
364: key option offers a similar facility (see
365: .Xr sshd 8
366: for details).
1.1 stevesk 367: .It Cm Banner
368: The contents of the specified file are sent to the remote user before
369: authentication is allowed.
1.78 djm 370: If the argument is
371: .Dq none
372: then no banner is displayed.
1.1 stevesk 373: This option is only available for protocol version 2.
374: By default, no banner is displayed.
375: .It Cm ChallengeResponseAuthentication
1.50 jmc 376: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 377: All authentication styles from
378: .Xr login.conf 5
379: are supported.
380: The default is
381: .Dq yes .
1.80 djm 382: .It Cm ChrootDirectory
1.113 stevesk 383: Specifies the pathname of a directory to
1.80 djm 384: .Xr chroot 2
385: to after authentication.
1.190 deraadt 386: At session startup
387: .Xr sshd 8
388: checks that all components of the pathname are root-owned directories
389: which are not writable by any other user or group.
1.106 stevesk 390: After the chroot,
391: .Xr sshd 8
392: changes the working directory to the user's home directory.
1.80 djm 393: .Pp
1.113 stevesk 394: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 395: the connecting user has been authenticated: %% is replaced by a literal '%',
396: %h is replaced by the home directory of the user being authenticated, and
397: %u is replaced by the username of that user.
398: .Pp
399: The
400: .Cm ChrootDirectory
401: must contain the necessary files and directories to support the
1.103 stevesk 402: user's session.
1.80 djm 403: For an interactive session this requires at least a shell, typically
404: .Xr sh 1 ,
405: and basic
406: .Pa /dev
407: nodes such as
408: .Xr null 4 ,
409: .Xr zero 4 ,
410: .Xr stdin 4 ,
411: .Xr stdout 4 ,
412: .Xr stderr 4 ,
1.180 jmc 413: and
1.80 djm 414: .Xr tty 4
415: devices.
416: For file transfer sessions using
1.105 jmc 417: .Dq sftp ,
1.80 djm 418: no additional configuration of the environment is necessary if the
1.105 jmc 419: in-process sftp server is used,
1.176 schwarze 420: though sessions which use logging may require
1.104 stevesk 421: .Pa /dev/log
1.176 schwarze 422: inside the chroot directory on some operating systems (see
1.104 stevesk 423: .Xr sftp-server 8
1.81 jmc 424: for details).
1.190 deraadt 425: .Pp
1.191 jmc 426: For safety, it is very important that the directory hierarchy be
1.190 deraadt 427: prevented from modification by other processes on the system (especially
428: those outside the jail).
429: Misconfiguration can lead to unsafe environments which
430: .Xr sshd 8
431: cannot detect.
1.80 djm 432: .Pp
433: The default is not to
434: .Xr chroot 2 .
1.1 stevesk 435: .It Cm Ciphers
436: Specifies the ciphers allowed for protocol version 2.
437: Multiple ciphers must be comma-separated.
1.167 djm 438: The supported ciphers are:
439: .Pp
1.173 naddy 440: .Bl -item -compact -offset indent
441: .It
442: 3des-cbc
443: .It
444: aes128-cbc
445: .It
446: aes192-cbc
447: .It
448: aes256-cbc
449: .It
450: aes128-ctr
451: .It
452: aes192-ctr
453: .It
454: aes256-ctr
455: .It
456: aes128-gcm@openssh.com
457: .It
458: aes256-gcm@openssh.com
459: .It
460: arcfour
461: .It
462: arcfour128
463: .It
464: arcfour256
465: .It
466: blowfish-cbc
467: .It
468: cast128-cbc
469: .It
470: chacha20-poly1305@openssh.com
471: .El
1.167 djm 472: .Pp
1.52 jmc 473: The default is:
1.173 naddy 474: .Bd -literal -offset indent
475: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 476: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 477: chacha20-poly1305@openssh.com
1.1 stevesk 478: .Ed
1.167 djm 479: .Pp
480: The list of available ciphers may also be obtained using the
481: .Fl Q
482: option of
1.188 djm 483: .Xr ssh 1
484: with an argument of
485: .Dq cipher .
1.1 stevesk 486: .It Cm ClientAliveCountMax
1.48 jmc 487: Sets the number of client alive messages (see below) which may be
1.1 stevesk 488: sent without
1.52 jmc 489: .Xr sshd 8
1.17 jmc 490: receiving any messages back from the client.
491: If this threshold is reached while client alive messages are being sent,
1.52 jmc 492: sshd will disconnect the client, terminating the session.
1.17 jmc 493: It is important to note that the use of client alive messages is very
494: different from
1.27 markus 495: .Cm TCPKeepAlive
1.17 jmc 496: (below).
497: The client alive messages are sent through the encrypted channel
498: and therefore will not be spoofable.
499: The TCP keepalive option enabled by
1.27 markus 500: .Cm TCPKeepAlive
1.17 jmc 501: is spoofable.
502: The client alive mechanism is valuable when the client or
1.1 stevesk 503: server depend on knowing when a connection has become inactive.
504: .Pp
1.17 jmc 505: The default value is 3.
506: If
1.1 stevesk 507: .Cm ClientAliveInterval
1.48 jmc 508: (see below) is set to 15, and
1.1 stevesk 509: .Cm ClientAliveCountMax
1.52 jmc 510: is left at the default, unresponsive SSH clients
1.1 stevesk 511: will be disconnected after approximately 45 seconds.
1.57 markus 512: This option applies to protocol version 2 only.
1.42 djm 513: .It Cm ClientAliveInterval
514: Sets a timeout interval in seconds after which if no data has been received
515: from the client,
1.52 jmc 516: .Xr sshd 8
1.42 djm 517: will send a message through the encrypted
518: channel to request a response from the client.
519: The default
520: is 0, indicating that these messages will not be sent to the client.
521: This option applies to protocol version 2 only.
1.3 markus 522: .It Cm Compression
1.44 markus 523: Specifies whether compression is allowed, or delayed until
524: the user has authenticated successfully.
1.3 markus 525: The argument must be
1.44 markus 526: .Dq yes ,
527: .Dq delayed ,
1.3 markus 528: or
529: .Dq no .
530: The default is
1.44 markus 531: .Dq delayed .
1.1 stevesk 532: .It Cm DenyGroups
533: This keyword can be followed by a list of group name patterns, separated
534: by spaces.
535: Login is disallowed for users whose primary group or supplementary
536: group list matches one of the patterns.
537: Only group names are valid; a numerical group ID is not recognized.
538: By default, login is allowed for all groups.
1.54 jmc 539: The allow/deny directives are processed in the following order:
540: .Cm DenyUsers ,
541: .Cm AllowUsers ,
542: .Cm DenyGroups ,
543: and finally
544: .Cm AllowGroups .
1.49 jmc 545: .Pp
1.161 jmc 546: See PATTERNS in
1.49 jmc 547: .Xr ssh_config 5
548: for more information on patterns.
1.1 stevesk 549: .It Cm DenyUsers
550: This keyword can be followed by a list of user name patterns, separated
551: by spaces.
552: Login is disallowed for user names that match one of the patterns.
553: Only user names are valid; a numerical user ID is not recognized.
554: By default, login is allowed for all users.
555: If the pattern takes the form USER@HOST then USER and HOST
556: are separately checked, restricting logins to particular
557: users from particular hosts.
1.54 jmc 558: The allow/deny directives are processed in the following order:
559: .Cm DenyUsers ,
560: .Cm AllowUsers ,
561: .Cm DenyGroups ,
562: and finally
563: .Cm AllowGroups .
1.49 jmc 564: .Pp
1.161 jmc 565: See PATTERNS in
1.49 jmc 566: .Xr ssh_config 5
567: for more information on patterns.
1.183 djm 568: .It Cm FingerprintHash
569: Specifies the hash algorithm used when logging key fingerprints.
570: Valid options are:
571: .Dq md5
572: and
573: .Dq sha256 .
574: The default is
575: .Dq sha256 .
1.67 dtucker 576: .It Cm ForceCommand
577: Forces the execution of the command specified by
578: .Cm ForceCommand ,
1.84 djm 579: ignoring any command supplied by the client and
580: .Pa ~/.ssh/rc
581: if present.
1.67 dtucker 582: The command is invoked by using the user's login shell with the -c option.
583: This applies to shell, command, or subsystem execution.
584: It is most useful inside a
585: .Cm Match
586: block.
587: The command originally supplied by the client is available in the
588: .Ev SSH_ORIGINAL_COMMAND
589: environment variable.
1.82 djm 590: Specifying a command of
591: .Dq internal-sftp
592: will force the use of an in-process sftp server that requires no support
593: files when used with
594: .Cm ChrootDirectory .
1.1 stevesk 595: .It Cm GatewayPorts
596: Specifies whether remote hosts are allowed to connect to ports
597: forwarded for the client.
598: By default,
1.52 jmc 599: .Xr sshd 8
1.15 jmc 600: binds remote port forwardings to the loopback address.
601: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 602: .Cm GatewayPorts
1.52 jmc 603: can be used to specify that sshd
1.39 djm 604: should allow remote port forwardings to bind to non-loopback addresses, thus
605: allowing other hosts to connect.
606: The argument may be
607: .Dq no
608: to force remote port forwardings to be available to the local host only,
1.1 stevesk 609: .Dq yes
1.39 djm 610: to force remote port forwardings to bind to the wildcard address, or
611: .Dq clientspecified
612: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 613: The default is
614: .Dq no .
1.23 markus 615: .It Cm GSSAPIAuthentication
1.25 markus 616: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 617: The default is
1.23 markus 618: .Dq no .
619: Note that this option applies to protocol version 2 only.
620: .It Cm GSSAPICleanupCredentials
621: Specifies whether to automatically destroy the user's credentials cache
622: on logout.
623: The default is
624: .Dq yes .
625: Note that this option applies to protocol version 2 only.
1.189 djm 626: .It Cm HostbasedAcceptedKeyTypes
627: Specifies the key types that will be accepted for hostbased authentication
628: as a comma-separated pattern list.
629: The default
630: .Dq *
631: will allow all key types.
632: The
633: .Fl Q
634: option of
635: .Xr ssh 1
636: may be used to list supported key types.
1.1 stevesk 637: .It Cm HostbasedAuthentication
638: Specifies whether rhosts or /etc/hosts.equiv authentication together
639: with successful public key client host authentication is allowed
1.50 jmc 640: (host-based authentication).
1.1 stevesk 641: This option is similar to
642: .Cm RhostsRSAAuthentication
643: and applies to protocol version 2 only.
1.70 dtucker 644: The default is
645: .Dq no .
646: .It Cm HostbasedUsesNameFromPacketOnly
647: Specifies whether or not the server will attempt to perform a reverse
648: name lookup when matching the name in the
649: .Pa ~/.shosts ,
650: .Pa ~/.rhosts ,
651: and
652: .Pa /etc/hosts.equiv
653: files during
654: .Cm HostbasedAuthentication .
655: A setting of
656: .Dq yes
657: means that
658: .Xr sshd 8
659: uses the name supplied by the client rather than
660: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 661: The default is
662: .Dq no .
1.117 djm 663: .It Cm HostCertificate
664: Specifies a file containing a public host certificate.
665: The certificate's public key must match a private host key already specified
666: by
667: .Cm HostKey .
668: The default behaviour of
669: .Xr sshd 8
670: is not to load any certificates.
1.1 stevesk 671: .It Cm HostKey
672: Specifies a file containing a private host key
673: used by SSH.
674: The default is
675: .Pa /etc/ssh/ssh_host_key
676: for protocol version 1, and
1.126 djm 677: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 678: .Pa /etc/ssh/ssh_host_ecdsa_key ,
679: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 680: and
1.1 stevesk 681: .Pa /etc/ssh/ssh_host_rsa_key
682: for protocol version 2.
683: Note that
1.52 jmc 684: .Xr sshd 8
1.1 stevesk 685: will refuse to use a file if it is group/world-accessible.
686: It is possible to have multiple host key files.
687: .Dq rsa1
688: keys are used for version 1 and
1.126 djm 689: .Dq dsa ,
1.169 naddy 690: .Dq ecdsa ,
691: .Dq ed25519
1.1 stevesk 692: or
693: .Dq rsa
694: are used for version 2 of the SSH protocol.
1.162 markus 695: It is also possible to specify public host key files instead.
696: In this case operations on the private key will be delegated
697: to an
698: .Xr ssh-agent 1 .
699: .It Cm HostKeyAgent
700: Identifies the UNIX-domain socket used to communicate
701: with an agent that has access to the private host keys.
702: If
703: .Dq SSH_AUTH_SOCK
704: is specified, the location of the socket will be read from the
705: .Ev SSH_AUTH_SOCK
706: environment variable.
1.1 stevesk 707: .It Cm IgnoreRhosts
708: Specifies that
709: .Pa .rhosts
710: and
711: .Pa .shosts
712: files will not be used in
713: .Cm RhostsRSAAuthentication
714: or
715: .Cm HostbasedAuthentication .
716: .Pp
717: .Pa /etc/hosts.equiv
718: and
719: .Pa /etc/shosts.equiv
720: are still used.
721: The default is
722: .Dq yes .
723: .It Cm IgnoreUserKnownHosts
724: Specifies whether
1.52 jmc 725: .Xr sshd 8
1.1 stevesk 726: should ignore the user's
1.41 djm 727: .Pa ~/.ssh/known_hosts
1.1 stevesk 728: during
729: .Cm RhostsRSAAuthentication
730: or
731: .Cm HostbasedAuthentication .
732: The default is
733: .Dq no .
1.129 djm 734: .It Cm IPQoS
735: Specifies the IPv4 type-of-service or DSCP class for the connection.
736: Accepted values are
737: .Dq af11 ,
738: .Dq af12 ,
739: .Dq af13 ,
1.136 djm 740: .Dq af21 ,
1.129 djm 741: .Dq af22 ,
742: .Dq af23 ,
743: .Dq af31 ,
744: .Dq af32 ,
745: .Dq af33 ,
746: .Dq af41 ,
747: .Dq af42 ,
748: .Dq af43 ,
749: .Dq cs0 ,
750: .Dq cs1 ,
751: .Dq cs2 ,
752: .Dq cs3 ,
753: .Dq cs4 ,
754: .Dq cs5 ,
755: .Dq cs6 ,
756: .Dq cs7 ,
757: .Dq ef ,
758: .Dq lowdelay ,
759: .Dq throughput ,
760: .Dq reliability ,
761: or a numeric value.
1.131 djm 762: This option may take one or two arguments, separated by whitespace.
1.129 djm 763: If one argument is specified, it is used as the packet class unconditionally.
764: If two values are specified, the first is automatically selected for
765: interactive sessions and the second for non-interactive sessions.
766: The default is
767: .Dq lowdelay
768: for interactive sessions and
769: .Dq throughput
770: for non-interactive sessions.
1.171 jmc 771: .It Cm KbdInteractiveAuthentication
772: Specifies whether to allow keyboard-interactive authentication.
773: The argument to this keyword must be
774: .Dq yes
775: or
776: .Dq no .
777: The default is to use whatever value
778: .Cm ChallengeResponseAuthentication
779: is set to
780: (by default
781: .Dq yes ) .
1.1 stevesk 782: .It Cm KerberosAuthentication
1.24 markus 783: Specifies whether the password provided by the user for
1.1 stevesk 784: .Cm PasswordAuthentication
1.24 markus 785: will be validated through the Kerberos KDC.
1.1 stevesk 786: To use this option, the server needs a
787: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 788: The default is
1.29 dtucker 789: .Dq no .
790: .It Cm KerberosGetAFSToken
1.45 djm 791: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 792: an AFS token before accessing the user's home directory.
1.52 jmc 793: The default is
1.1 stevesk 794: .Dq no .
795: .It Cm KerberosOrLocalPasswd
1.52 jmc 796: If password authentication through Kerberos fails then
1.1 stevesk 797: the password will be validated via any additional local mechanism
798: such as
799: .Pa /etc/passwd .
1.52 jmc 800: The default is
1.1 stevesk 801: .Dq yes .
802: .It Cm KerberosTicketCleanup
803: Specifies whether to automatically destroy the user's ticket cache
804: file on logout.
1.52 jmc 805: The default is
1.1 stevesk 806: .Dq yes .
1.127 djm 807: .It Cm KexAlgorithms
808: Specifies the available KEX (Key Exchange) algorithms.
809: Multiple algorithms must be comma-separated.
1.173 naddy 810: The supported algorithms are:
811: .Pp
812: .Bl -item -compact -offset indent
813: .It
814: curve25519-sha256@libssh.org
815: .It
816: diffie-hellman-group1-sha1
817: .It
818: diffie-hellman-group14-sha1
819: .It
820: diffie-hellman-group-exchange-sha1
821: .It
822: diffie-hellman-group-exchange-sha256
823: .It
824: ecdh-sha2-nistp256
825: .It
826: ecdh-sha2-nistp384
827: .It
828: ecdh-sha2-nistp521
829: .El
830: .Pp
831: The default is:
1.170 dtucker 832: .Bd -literal -offset indent
833: curve25519-sha256@libssh.org,
834: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
835: diffie-hellman-group-exchange-sha256,
1.173 naddy 836: diffie-hellman-group14-sha1
1.170 dtucker 837: .Ed
1.188 djm 838: .Pp
839: The list of available key exchange algorithms may also be obtained using the
840: .Fl Q
841: option of
842: .Xr ssh 1
843: with an argument of
844: .Dq kex .
1.1 stevesk 845: .It Cm KeyRegenerationInterval
846: In protocol version 1, the ephemeral server key is automatically regenerated
847: after this many seconds (if it has been used).
848: The purpose of regeneration is to prevent
849: decrypting captured sessions by later breaking into the machine and
850: stealing the keys.
851: The key is never stored anywhere.
852: If the value is 0, the key is never regenerated.
853: The default is 3600 (seconds).
854: .It Cm ListenAddress
855: Specifies the local addresses
1.52 jmc 856: .Xr sshd 8
1.1 stevesk 857: should listen on.
858: The following forms may be used:
859: .Pp
860: .Bl -item -offset indent -compact
861: .It
862: .Cm ListenAddress
863: .Sm off
1.180 jmc 864: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 865: .Sm on
866: .It
867: .Cm ListenAddress
868: .Sm off
1.180 jmc 869: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 870: .Sm on
871: .It
872: .Cm ListenAddress
873: .Sm off
874: .Oo
1.180 jmc 875: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 876: .Sm on
877: .El
878: .Pp
879: If
880: .Ar port
881: is not specified,
1.200 dtucker 882: sshd will listen on the address and all
1.1 stevesk 883: .Cm Port
1.17 jmc 884: options specified.
885: The default is to listen on all local addresses.
1.15 jmc 886: Multiple
1.1 stevesk 887: .Cm ListenAddress
1.17 jmc 888: options are permitted.
1.1 stevesk 889: .It Cm LoginGraceTime
890: The server disconnects after this time if the user has not
891: successfully logged in.
892: If the value is 0, there is no time limit.
1.12 stevesk 893: The default is 120 seconds.
1.1 stevesk 894: .It Cm LogLevel
895: Gives the verbosity level that is used when logging messages from
1.53 jmc 896: .Xr sshd 8 .
1.1 stevesk 897: The possible values are:
1.52 jmc 898: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 899: The default is INFO.
900: DEBUG and DEBUG1 are equivalent.
901: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
902: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 903: .It Cm MACs
904: Specifies the available MAC (message authentication code) algorithms.
905: The MAC algorithm is used in protocol version 2
906: for data integrity protection.
907: Multiple algorithms must be comma-separated.
1.152 markus 908: The algorithms that contain
909: .Dq -etm
910: calculate the MAC after encryption (encrypt-then-mac).
911: These are considered safer and their use recommended.
1.173 naddy 912: The supported MACs are:
913: .Pp
914: .Bl -item -compact -offset indent
915: .It
916: hmac-md5
917: .It
918: hmac-md5-96
919: .It
920: hmac-ripemd160
921: .It
922: hmac-sha1
923: .It
924: hmac-sha1-96
925: .It
926: hmac-sha2-256
927: .It
928: hmac-sha2-512
929: .It
930: umac-64@openssh.com
931: .It
932: umac-128@openssh.com
933: .It
934: hmac-md5-etm@openssh.com
935: .It
936: hmac-md5-96-etm@openssh.com
937: .It
938: hmac-ripemd160-etm@openssh.com
939: .It
940: hmac-sha1-etm@openssh.com
941: .It
942: hmac-sha1-96-etm@openssh.com
943: .It
944: hmac-sha2-256-etm@openssh.com
945: .It
946: hmac-sha2-512-etm@openssh.com
947: .It
948: umac-64-etm@openssh.com
949: .It
950: umac-128-etm@openssh.com
951: .El
952: .Pp
1.52 jmc 953: The default is:
1.77 jmc 954: .Bd -literal -offset indent
1.152 markus 955: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
956: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 957: umac-64@openssh.com,umac-128@openssh.com,
958: hmac-sha2-256,hmac-sha2-512
1.77 jmc 959: .Ed
1.188 djm 960: .Pp
961: The list of available MAC algorithms may also be obtained using the
962: .Fl Q
963: option of
964: .Xr ssh 1
965: with an argument of
966: .Dq mac .
1.60 dtucker 967: .It Cm Match
1.61 jmc 968: Introduces a conditional block.
1.65 dtucker 969: If all of the criteria on the
1.60 dtucker 970: .Cm Match
1.65 dtucker 971: line are satisfied, the keywords on the following lines override those
972: set in the global section of the config file, until either another
1.60 dtucker 973: .Cm Match
1.65 dtucker 974: line or the end of the file.
1.172 djm 975: If a keyword appears in multiple
976: .Cm Match
1.177 sobrado 977: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 978: applied.
1.91 djm 979: .Pp
1.61 jmc 980: The arguments to
1.60 dtucker 981: .Cm Match
1.163 dtucker 982: are one or more criteria-pattern pairs or the single token
983: .Cm All
984: which matches all criteria.
1.60 dtucker 985: The available criteria are
986: .Cm User ,
1.69 dtucker 987: .Cm Group ,
1.60 dtucker 988: .Cm Host ,
1.139 dtucker 989: .Cm LocalAddress ,
990: .Cm LocalPort ,
1.60 dtucker 991: and
992: .Cm Address .
1.91 djm 993: The match patterns may consist of single entries or comma-separated
994: lists and may use the wildcard and negation operators described in the
1.161 jmc 995: PATTERNS section of
1.92 djm 996: .Xr ssh_config 5 .
1.91 djm 997: .Pp
998: The patterns in an
999: .Cm Address
1000: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1001: address/masklen format, e.g.\&
1.91 djm 1002: .Dq 192.0.2.0/24
1003: or
1004: .Dq 3ffe:ffff::/32 .
1005: Note that the mask length provided must be consistent with the address -
1006: it is an error to specify a mask length that is too long for the address
1.93 jmc 1007: or one with bits set in this host portion of the address.
1008: For example,
1.91 djm 1009: .Dq 192.0.2.0/33
1010: and
1.93 jmc 1011: .Dq 192.0.2.0/8
1.91 djm 1012: respectively.
1013: .Pp
1.60 dtucker 1014: Only a subset of keywords may be used on the lines following a
1015: .Cm Match
1016: keyword.
1017: Available keywords are
1.142 jmc 1018: .Cm AcceptEnv ,
1.99 okan 1019: .Cm AllowAgentForwarding ,
1.142 jmc 1020: .Cm AllowGroups ,
1.193 djm 1021: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1022: .Cm AllowTcpForwarding ,
1.141 markus 1023: .Cm AllowUsers ,
1.149 djm 1024: .Cm AuthenticationMethods ,
1.146 djm 1025: .Cm AuthorizedKeysCommand ,
1026: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1027: .Cm AuthorizedKeysFile ,
1.123 djm 1028: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1029: .Cm Banner ,
1.85 djm 1030: .Cm ChrootDirectory ,
1.141 markus 1031: .Cm DenyGroups ,
1032: .Cm DenyUsers ,
1.67 dtucker 1033: .Cm ForceCommand ,
1.194 djm 1034: .Cm GatewayPorts ,
1.193 djm 1035: .Cm GSSAPIAuthentication ,
1.189 djm 1036: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1037: .Cm HostbasedAuthentication ,
1.123 djm 1038: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1039: .Cm IPQoS ,
1.74 jmc 1040: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1041: .Cm KerberosAuthentication ,
1.95 dtucker 1042: .Cm MaxAuthTries ,
1.94 dtucker 1043: .Cm MaxSessions ,
1.72 dtucker 1044: .Cm PasswordAuthentication ,
1.97 djm 1045: .Cm PermitEmptyPasswords ,
1.66 dtucker 1046: .Cm PermitOpen ,
1.79 dtucker 1047: .Cm PermitRootLogin ,
1.164 djm 1048: .Cm PermitTTY ,
1.123 djm 1049: .Cm PermitTunnel ,
1.174 djm 1050: .Cm PermitUserRC ,
1.189 djm 1051: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1052: .Cm PubkeyAuthentication ,
1.159 dtucker 1053: .Cm RekeyLimit ,
1.193 djm 1054: .Cm RevokedKeys ,
1.142 jmc 1055: .Cm RhostsRSAAuthentication ,
1.194 djm 1056: .Cm RSAAuthentication ,
1.193 djm 1057: .Cm StreamLocalBindMask ,
1058: .Cm StreamLocalBindUnlink ,
1059: .Cm TrustedUserCAKeys ,
1.66 dtucker 1060: .Cm X11DisplayOffset ,
1.101 djm 1061: .Cm X11Forwarding
1.60 dtucker 1062: and
1.102 djm 1063: .Cm X11UseLocalHost .
1.33 dtucker 1064: .It Cm MaxAuthTries
1065: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1066: connection.
1067: Once the number of failures reaches half this value,
1068: additional failures are logged.
1069: The default is 6.
1.90 djm 1070: .It Cm MaxSessions
1071: Specifies the maximum number of open sessions permitted per network connection.
1072: The default is 10.
1.1 stevesk 1073: .It Cm MaxStartups
1074: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1075: SSH daemon.
1.1 stevesk 1076: Additional connections will be dropped until authentication succeeds or the
1077: .Cm LoginGraceTime
1078: expires for a connection.
1.156 dtucker 1079: The default is 10:30:100.
1.1 stevesk 1080: .Pp
1081: Alternatively, random early drop can be enabled by specifying
1082: the three colon separated values
1083: .Dq start:rate:full
1.51 jmc 1084: (e.g. "10:30:60").
1.53 jmc 1085: .Xr sshd 8
1.1 stevesk 1086: will refuse connection attempts with a probability of
1087: .Dq rate/100
1088: (30%)
1089: if there are currently
1090: .Dq start
1091: (10)
1092: unauthenticated connections.
1093: The probability increases linearly and all connection attempts
1094: are refused if the number of unauthenticated connections reaches
1095: .Dq full
1096: (60).
1097: .It Cm PasswordAuthentication
1098: Specifies whether password authentication is allowed.
1099: The default is
1100: .Dq yes .
1101: .It Cm PermitEmptyPasswords
1102: When password authentication is allowed, it specifies whether the
1103: server allows login to accounts with empty password strings.
1104: The default is
1105: .Dq no .
1.62 dtucker 1106: .It Cm PermitOpen
1107: Specifies the destinations to which TCP port forwarding is permitted.
1108: The forwarding specification must be one of the following forms:
1109: .Pp
1110: .Bl -item -offset indent -compact
1111: .It
1112: .Cm PermitOpen
1113: .Sm off
1114: .Ar host : port
1115: .Sm on
1116: .It
1117: .Cm PermitOpen
1118: .Sm off
1119: .Ar IPv4_addr : port
1120: .Sm on
1121: .It
1122: .Cm PermitOpen
1123: .Sm off
1124: .Ar \&[ IPv6_addr \&] : port
1125: .Sm on
1126: .El
1127: .Pp
1.68 dtucker 1128: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1129: An argument of
1130: .Dq any
1131: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1132: An argument of
1133: .Dq none
1134: can be used to prohibit all forwarding requests.
1.63 jmc 1135: By default all port forwarding requests are permitted.
1.1 stevesk 1136: .It Cm PermitRootLogin
1.38 jmc 1137: Specifies whether root can log in using
1.1 stevesk 1138: .Xr ssh 1 .
1139: The argument must be
1140: .Dq yes ,
1141: .Dq without-password ,
1.52 jmc 1142: .Dq forced-commands-only ,
1.1 stevesk 1143: or
1144: .Dq no .
1145: The default is
1.196 djm 1146: .Dq no .
1.1 stevesk 1147: .Pp
1148: If this option is set to
1.52 jmc 1149: .Dq without-password ,
1.1 stevesk 1150: password authentication is disabled for root.
1151: .Pp
1152: If this option is set to
1.52 jmc 1153: .Dq forced-commands-only ,
1.1 stevesk 1154: root login with public key authentication will be allowed,
1155: but only if the
1156: .Ar command
1157: option has been specified
1158: (which may be useful for taking remote backups even if root login is
1.17 jmc 1159: normally not allowed).
1160: All other authentication methods are disabled for root.
1.1 stevesk 1161: .Pp
1162: If this option is set to
1.52 jmc 1163: .Dq no ,
1.38 jmc 1164: root is not allowed to log in.
1.46 reyk 1165: .It Cm PermitTunnel
1166: Specifies whether
1167: .Xr tun 4
1168: device forwarding is allowed.
1.47 reyk 1169: The argument must be
1170: .Dq yes ,
1.58 stevesk 1171: .Dq point-to-point
1172: (layer 3),
1173: .Dq ethernet
1174: (layer 2), or
1.47 reyk 1175: .Dq no .
1.58 stevesk 1176: Specifying
1177: .Dq yes
1178: permits both
1179: .Dq point-to-point
1180: and
1181: .Dq ethernet .
1.46 reyk 1182: The default is
1183: .Dq no .
1.178 djm 1184: .Pp
1185: Independent of this setting, the permissions of the selected
1186: .Xr tun 4
1187: device must allow access to the user.
1.164 djm 1188: .It Cm PermitTTY
1189: Specifies whether
1.165 jmc 1190: .Xr pty 4
1.164 djm 1191: allocation is permitted.
1192: The default is
1193: .Dq yes .
1.6 markus 1194: .It Cm PermitUserEnvironment
1195: Specifies whether
1196: .Pa ~/.ssh/environment
1.9 stevesk 1197: and
1.6 markus 1198: .Cm environment=
1199: options in
1200: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1201: are processed by
1.52 jmc 1202: .Xr sshd 8 .
1.6 markus 1203: The default is
1204: .Dq no .
1.9 stevesk 1205: Enabling environment processing may enable users to bypass access
1206: restrictions in some configurations using mechanisms such as
1207: .Ev LD_PRELOAD .
1.174 djm 1208: .It Cm PermitUserRC
1209: Specifies whether any
1210: .Pa ~/.ssh/rc
1211: file is executed.
1212: The default is
1213: .Dq yes .
1.1 stevesk 1214: .It Cm PidFile
1.4 stevesk 1215: Specifies the file that contains the process ID of the
1.195 dtucker 1216: SSH daemon, or
1217: .Dq none
1218: to not write one.
1.1 stevesk 1219: The default is
1220: .Pa /var/run/sshd.pid .
1221: .It Cm Port
1222: Specifies the port number that
1.52 jmc 1223: .Xr sshd 8
1.1 stevesk 1224: listens on.
1225: The default is 22.
1226: Multiple options of this type are permitted.
1227: See also
1228: .Cm ListenAddress .
1229: .It Cm PrintLastLog
1230: Specifies whether
1.52 jmc 1231: .Xr sshd 8
1.36 jaredy 1232: should print the date and time of the last user login when a user logs
1233: in interactively.
1.1 stevesk 1234: The default is
1235: .Dq yes .
1236: .It Cm PrintMotd
1237: Specifies whether
1.52 jmc 1238: .Xr sshd 8
1.1 stevesk 1239: should print
1240: .Pa /etc/motd
1241: when a user logs in interactively.
1242: (On some systems it is also printed by the shell,
1243: .Pa /etc/profile ,
1244: or equivalent.)
1245: The default is
1246: .Dq yes .
1247: .It Cm Protocol
1248: Specifies the protocol versions
1.52 jmc 1249: .Xr sshd 8
1.5 stevesk 1250: supports.
1.1 stevesk 1251: The possible values are
1.52 jmc 1252: .Sq 1
1.1 stevesk 1253: and
1.52 jmc 1254: .Sq 2 .
1.1 stevesk 1255: Multiple versions must be comma-separated.
1256: The default is
1.109 jmc 1257: .Sq 2 .
1.5 stevesk 1258: Note that the order of the protocol list does not indicate preference,
1259: because the client selects among multiple protocol versions offered
1260: by the server.
1261: Specifying
1262: .Dq 2,1
1263: is identical to
1264: .Dq 1,2 .
1.189 djm 1265: .It Cm PubkeyAcceptedKeyTypes
1266: Specifies the key types that will be accepted for public key authentication
1267: as a comma-separated pattern list.
1268: The default
1269: .Dq *
1270: will allow all key types.
1271: The
1272: .Fl Q
1273: option of
1274: .Xr ssh 1
1275: may be used to list supported key types.
1.1 stevesk 1276: .It Cm PubkeyAuthentication
1277: Specifies whether public key authentication is allowed.
1278: The default is
1279: .Dq yes .
1280: Note that this option applies to protocol version 2 only.
1.159 dtucker 1281: .It Cm RekeyLimit
1282: Specifies the maximum amount of data that may be transmitted before the
1283: session key is renegotiated, optionally followed a maximum amount of
1284: time that may pass before the session key is renegotiated.
1285: The first argument is specified in bytes and may have a suffix of
1286: .Sq K ,
1287: .Sq M ,
1288: or
1289: .Sq G
1290: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1291: The default is between
1292: .Sq 1G
1293: and
1294: .Sq 4G ,
1295: depending on the cipher.
1296: The optional second value is specified in seconds and may use any of the
1297: units documented in the
1298: .Sx TIME FORMATS
1.160 jmc 1299: section.
1.159 dtucker 1300: The default value for
1301: .Cm RekeyLimit
1302: is
1303: .Dq default none ,
1304: which means that rekeying is performed after the cipher's default amount
1305: of data has been sent or received and no time based rekeying is done.
1306: This option applies to protocol version 2 only.
1.118 djm 1307: .It Cm RevokedKeys
1.195 dtucker 1308: Specifies revoked public keys file, or
1309: .Dq none
1310: to not use one.
1.118 djm 1311: Keys listed in this file will be refused for public key authentication.
1312: Note that if this file is not readable, then public key authentication will
1313: be refused for all users.
1.154 djm 1314: Keys may be specified as a text file, listing one public key per line, or as
1315: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1316: .Xr ssh-keygen 1 .
1.161 jmc 1317: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1318: .Xr ssh-keygen 1 .
1.1 stevesk 1319: .It Cm RhostsRSAAuthentication
1320: Specifies whether rhosts or /etc/hosts.equiv authentication together
1321: with successful RSA host authentication is allowed.
1322: The default is
1323: .Dq no .
1324: This option applies to protocol version 1 only.
1325: .It Cm RSAAuthentication
1326: Specifies whether pure RSA authentication is allowed.
1327: The default is
1328: .Dq yes .
1329: This option applies to protocol version 1 only.
1330: .It Cm ServerKeyBits
1331: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1332: The minimum value is 512, and the default is 1024.
1.175 millert 1333: .It Cm StreamLocalBindMask
1334: Sets the octal file creation mode mask
1335: .Pq umask
1336: used when creating a Unix-domain socket file for local or remote
1337: port forwarding.
1338: This option is only used for port forwarding to a Unix-domain socket file.
1339: .Pp
1340: The default value is 0177, which creates a Unix-domain socket file that is
1341: readable and writable only by the owner.
1342: Note that not all operating systems honor the file mode on Unix-domain
1343: socket files.
1344: .It Cm StreamLocalBindUnlink
1345: Specifies whether to remove an existing Unix-domain socket file for local
1346: or remote port forwarding before creating a new one.
1347: If the socket file already exists and
1348: .Cm StreamLocalBindUnlink
1349: is not enabled,
1350: .Nm sshd
1351: will be unable to forward the port to the Unix-domain socket file.
1352: This option is only used for port forwarding to a Unix-domain socket file.
1353: .Pp
1354: The argument must be
1355: .Dq yes
1356: or
1357: .Dq no .
1358: The default is
1359: .Dq no .
1.1 stevesk 1360: .It Cm StrictModes
1361: Specifies whether
1.52 jmc 1362: .Xr sshd 8
1.1 stevesk 1363: should check file modes and ownership of the
1364: user's files and home directory before accepting login.
1365: This is normally desirable because novices sometimes accidentally leave their
1366: directory or files world-writable.
1367: The default is
1368: .Dq yes .
1.112 djm 1369: Note that this does not apply to
1370: .Cm ChrootDirectory ,
1371: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1372: .It Cm Subsystem
1.51 jmc 1373: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1374: Arguments should be a subsystem name and a command (with optional arguments)
1375: to execute upon subsystem request.
1.80 djm 1376: .Pp
1.1 stevesk 1377: The command
1378: .Xr sftp-server 8
1379: implements the
1380: .Dq sftp
1381: file transfer subsystem.
1.80 djm 1382: .Pp
1383: Alternately the name
1384: .Dq internal-sftp
1385: implements an in-process
1386: .Dq sftp
1387: server.
1388: This may simplify configurations using
1389: .Cm ChrootDirectory
1390: to force a different filesystem root on clients.
1391: .Pp
1.1 stevesk 1392: By default no subsystems are defined.
1393: Note that this option applies to protocol version 2 only.
1394: .It Cm SyslogFacility
1395: Gives the facility code that is used when logging messages from
1.53 jmc 1396: .Xr sshd 8 .
1.1 stevesk 1397: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1398: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1399: The default is AUTH.
1.27 markus 1400: .It Cm TCPKeepAlive
1401: Specifies whether the system should send TCP keepalive messages to the
1402: other side.
1403: If they are sent, death of the connection or crash of one
1404: of the machines will be properly noticed.
1405: However, this means that
1406: connections will die if the route is down temporarily, and some people
1407: find it annoying.
1408: On the other hand, if TCP keepalives are not sent,
1409: sessions may hang indefinitely on the server, leaving
1410: .Dq ghost
1411: users and consuming server resources.
1412: .Pp
1413: The default is
1414: .Dq yes
1415: (to send TCP keepalive messages), and the server will notice
1416: if the network goes down or the client host crashes.
1417: This avoids infinitely hanging sessions.
1418: .Pp
1419: To disable TCP keepalive messages, the value should be set to
1420: .Dq no .
1.118 djm 1421: .It Cm TrustedUserCAKeys
1422: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1423: trusted to sign user certificates for authentication, or
1424: .Dq none
1425: to not use one.
1.119 jmc 1426: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1427: .Ql #
1428: are allowed.
1429: If a certificate is presented for authentication and has its signing CA key
1430: listed in this file, then it may be used for authentication for any user
1431: listed in the certificate's principals list.
1432: Note that certificates that lack a list of principals will not be permitted
1433: for authentication using
1434: .Cm TrustedUserCAKeys .
1.161 jmc 1435: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1436: .Xr ssh-keygen 1 .
1.18 markus 1437: .It Cm UseDNS
1438: Specifies whether
1.52 jmc 1439: .Xr sshd 8
1.40 jmc 1440: should look up the remote host name and check that
1.18 markus 1441: the resolved host name for the remote IP address maps back to the
1442: very same IP address.
1443: The default is
1.192 deraadt 1444: .Dq no .
1.1 stevesk 1445: .It Cm UseLogin
1446: Specifies whether
1447: .Xr login 1
1448: is used for interactive login sessions.
1449: The default is
1450: .Dq no .
1451: Note that
1452: .Xr login 1
1453: is never used for remote command execution.
1454: Note also, that if this is enabled,
1455: .Cm X11Forwarding
1456: will be disabled because
1457: .Xr login 1
1458: does not know how to handle
1459: .Xr xauth 1
1.15 jmc 1460: cookies.
1461: If
1.1 stevesk 1462: .Cm UsePrivilegeSeparation
1463: is specified, it will be disabled after authentication.
1464: .It Cm UsePrivilegeSeparation
1465: Specifies whether
1.52 jmc 1466: .Xr sshd 8
1.2 stevesk 1467: separates privileges by creating an unprivileged child process
1.15 jmc 1468: to deal with incoming network traffic.
1469: After successful authentication, another process will be created that has
1470: the privilege of the authenticated user.
1471: The goal of privilege separation is to prevent privilege
1.1 stevesk 1472: escalation by containing any corruption within the unprivileged processes.
1473: The default is
1474: .Dq yes .
1.134 djm 1475: If
1476: .Cm UsePrivilegeSeparation
1477: is set to
1478: .Dq sandbox
1479: then the pre-authentication unprivileged process is subject to additional
1480: restrictions.
1.137 djm 1481: .It Cm VersionAddendum
1482: Optionally specifies additional text to append to the SSH protocol banner
1483: sent by the server upon connection.
1484: The default is
1485: .Dq none .
1.1 stevesk 1486: .It Cm X11DisplayOffset
1487: Specifies the first display number available for
1.52 jmc 1488: .Xr sshd 8 Ns 's
1.1 stevesk 1489: X11 forwarding.
1.52 jmc 1490: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1491: The default is 10.
1492: .It Cm X11Forwarding
1493: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1494: The argument must be
1495: .Dq yes
1496: or
1497: .Dq no .
1.1 stevesk 1498: The default is
1499: .Dq no .
1.13 stevesk 1500: .Pp
1501: When X11 forwarding is enabled, there may be additional exposure to
1502: the server and to client displays if the
1.52 jmc 1503: .Xr sshd 8
1.13 stevesk 1504: proxy display is configured to listen on the wildcard address (see
1505: .Cm X11UseLocalhost
1.52 jmc 1506: below), though this is not the default.
1.13 stevesk 1507: Additionally, the authentication spoofing and authentication data
1508: verification and substitution occur on the client side.
1509: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1510: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1511: forwarding (see the warnings for
1512: .Cm ForwardX11
1513: in
1.19 jmc 1514: .Xr ssh_config 5 ) .
1.13 stevesk 1515: A system administrator may have a stance in which they want to
1516: protect clients that may expose themselves to attack by unwittingly
1517: requesting X11 forwarding, which can warrant a
1518: .Dq no
1519: setting.
1520: .Pp
1521: Note that disabling X11 forwarding does not prevent users from
1522: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1523: X11 forwarding is automatically disabled if
1524: .Cm UseLogin
1525: is enabled.
1526: .It Cm X11UseLocalhost
1527: Specifies whether
1.52 jmc 1528: .Xr sshd 8
1.1 stevesk 1529: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1530: the wildcard address.
1531: By default,
1.52 jmc 1532: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1533: hostname part of the
1534: .Ev DISPLAY
1535: environment variable to
1536: .Dq localhost .
1.8 stevesk 1537: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1538: However, some older X11 clients may not function with this
1539: configuration.
1540: .Cm X11UseLocalhost
1541: may be set to
1542: .Dq no
1543: to specify that the forwarding server should be bound to the wildcard
1544: address.
1545: The argument must be
1546: .Dq yes
1547: or
1548: .Dq no .
1549: The default is
1550: .Dq yes .
1551: .It Cm XAuthLocation
1.11 stevesk 1552: Specifies the full pathname of the
1.1 stevesk 1553: .Xr xauth 1
1.195 dtucker 1554: program, or
1555: .Dq none
1556: to not use one.
1.1 stevesk 1557: The default is
1558: .Pa /usr/X11R6/bin/xauth .
1559: .El
1.55 jmc 1560: .Sh TIME FORMATS
1.53 jmc 1561: .Xr sshd 8
1.1 stevesk 1562: command-line arguments and configuration file options that specify time
1563: may be expressed using a sequence of the form:
1564: .Sm off
1.7 stevesk 1565: .Ar time Op Ar qualifier ,
1.1 stevesk 1566: .Sm on
1567: where
1568: .Ar time
1569: is a positive integer value and
1570: .Ar qualifier
1571: is one of the following:
1572: .Pp
1573: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1574: .It Aq Cm none
1.1 stevesk 1575: seconds
1576: .It Cm s | Cm S
1577: seconds
1578: .It Cm m | Cm M
1579: minutes
1580: .It Cm h | Cm H
1581: hours
1582: .It Cm d | Cm D
1583: days
1584: .It Cm w | Cm W
1585: weeks
1586: .El
1587: .Pp
1588: Each member of the sequence is added together to calculate
1589: the total time value.
1590: .Pp
1591: Time format examples:
1592: .Pp
1593: .Bl -tag -width Ds -compact -offset indent
1594: .It 600
1595: 600 seconds (10 minutes)
1596: .It 10m
1597: 10 minutes
1598: .It 1h30m
1599: 1 hour 30 minutes (90 minutes)
1600: .El
1601: .Sh FILES
1602: .Bl -tag -width Ds
1603: .It Pa /etc/ssh/sshd_config
1604: Contains configuration data for
1.53 jmc 1605: .Xr sshd 8 .
1.1 stevesk 1606: This file should be writable by root only, but it is recommended
1607: (though not necessary) that it be world-readable.
1608: .El
1.19 jmc 1609: .Sh SEE ALSO
1610: .Xr sshd 8
1.1 stevesk 1611: .Sh AUTHORS
1612: OpenSSH is a derivative of the original and free
1613: ssh 1.2.12 release by Tatu Ylonen.
1614: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1615: Theo de Raadt and Dug Song
1616: removed many bugs, re-added newer features and
1617: created OpenSSH.
1618: Markus Friedl contributed the support for SSH
1619: protocol versions 1.5 and 2.0.
1620: Niels Provos and Markus Friedl contributed support
1621: for privilege separation.
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