Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.201
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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1.201 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.200 2015/04/29 03:48:56 dtucker Exp $
! 37: .Dd $Mdocdate: April 29 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 .
1.121 djm 290: .It Cm AuthorizedPrincipalsFile
291: Specifies a file that lists principal names that are accepted for
292: certificate authentication.
293: When using certificates signed by a key listed in
294: .Cm TrustedUserCAKeys ,
295: this file lists names, one of which must appear in the certificate for it
296: to be accepted for authentication.
1.125 jmc 297: Names are listed one per line preceded by key options (as described
1.161 jmc 298: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 299: .Xr sshd 8 ) .
1.124 djm 300: Empty lines and comments starting with
1.121 djm 301: .Ql #
302: are ignored.
303: .Pp
304: .Cm AuthorizedPrincipalsFile
305: may contain tokens of the form %T which are substituted during connection
306: setup.
307: The following tokens are defined: %% is replaced by a literal '%',
308: %h is replaced by the home directory of the user being authenticated, and
309: %u is replaced by the username of that user.
310: After expansion,
311: .Cm AuthorizedPrincipalsFile
312: is taken to be an absolute path or one relative to the user's home
313: directory.
314: .Pp
1.138 djm 315: The default is
316: .Dq none ,
317: i.e. not to use a principals file \(en in this case, the username
1.121 djm 318: of the user must appear in a certificate's principals list for it to be
319: accepted.
320: Note that
321: .Cm AuthorizedPrincipalsFile
322: is only used when authentication proceeds using a CA listed in
323: .Cm TrustedUserCAKeys
324: and is not consulted for certification authorities trusted via
325: .Pa ~/.ssh/authorized_keys ,
326: though the
327: .Cm principals=
328: key option offers a similar facility (see
329: .Xr sshd 8
330: for details).
1.1 stevesk 331: .It Cm Banner
332: The contents of the specified file are sent to the remote user before
333: authentication is allowed.
1.78 djm 334: If the argument is
335: .Dq none
336: then no banner is displayed.
1.1 stevesk 337: This option is only available for protocol version 2.
338: By default, no banner is displayed.
339: .It Cm ChallengeResponseAuthentication
1.50 jmc 340: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 341: All authentication styles from
342: .Xr login.conf 5
343: are supported.
344: The default is
345: .Dq yes .
1.80 djm 346: .It Cm ChrootDirectory
1.113 stevesk 347: Specifies the pathname of a directory to
1.80 djm 348: .Xr chroot 2
349: to after authentication.
1.190 deraadt 350: At session startup
351: .Xr sshd 8
352: checks that all components of the pathname are root-owned directories
353: which are not writable by any other user or group.
1.106 stevesk 354: After the chroot,
355: .Xr sshd 8
356: changes the working directory to the user's home directory.
1.80 djm 357: .Pp
1.113 stevesk 358: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 359: the connecting user has been authenticated: %% is replaced by a literal '%',
360: %h is replaced by the home directory of the user being authenticated, and
361: %u is replaced by the username of that user.
362: .Pp
363: The
364: .Cm ChrootDirectory
365: must contain the necessary files and directories to support the
1.103 stevesk 366: user's session.
1.80 djm 367: For an interactive session this requires at least a shell, typically
368: .Xr sh 1 ,
369: and basic
370: .Pa /dev
371: nodes such as
372: .Xr null 4 ,
373: .Xr zero 4 ,
374: .Xr stdin 4 ,
375: .Xr stdout 4 ,
376: .Xr stderr 4 ,
1.180 jmc 377: and
1.80 djm 378: .Xr tty 4
379: devices.
380: For file transfer sessions using
1.105 jmc 381: .Dq sftp ,
1.80 djm 382: no additional configuration of the environment is necessary if the
1.105 jmc 383: in-process sftp server is used,
1.176 schwarze 384: though sessions which use logging may require
1.104 stevesk 385: .Pa /dev/log
1.176 schwarze 386: inside the chroot directory on some operating systems (see
1.104 stevesk 387: .Xr sftp-server 8
1.81 jmc 388: for details).
1.190 deraadt 389: .Pp
1.191 jmc 390: For safety, it is very important that the directory hierarchy be
1.190 deraadt 391: prevented from modification by other processes on the system (especially
392: those outside the jail).
393: Misconfiguration can lead to unsafe environments which
394: .Xr sshd 8
395: cannot detect.
1.80 djm 396: .Pp
397: The default is not to
398: .Xr chroot 2 .
1.1 stevesk 399: .It Cm Ciphers
400: Specifies the ciphers allowed for protocol version 2.
401: Multiple ciphers must be comma-separated.
1.167 djm 402: The supported ciphers are:
403: .Pp
1.173 naddy 404: .Bl -item -compact -offset indent
405: .It
406: 3des-cbc
407: .It
408: aes128-cbc
409: .It
410: aes192-cbc
411: .It
412: aes256-cbc
413: .It
414: aes128-ctr
415: .It
416: aes192-ctr
417: .It
418: aes256-ctr
419: .It
420: aes128-gcm@openssh.com
421: .It
422: aes256-gcm@openssh.com
423: .It
424: arcfour
425: .It
426: arcfour128
427: .It
428: arcfour256
429: .It
430: blowfish-cbc
431: .It
432: cast128-cbc
433: .It
434: chacha20-poly1305@openssh.com
435: .El
1.167 djm 436: .Pp
1.52 jmc 437: The default is:
1.173 naddy 438: .Bd -literal -offset indent
439: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 440: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 441: chacha20-poly1305@openssh.com
1.1 stevesk 442: .Ed
1.167 djm 443: .Pp
444: The list of available ciphers may also be obtained using the
445: .Fl Q
446: option of
1.188 djm 447: .Xr ssh 1
448: with an argument of
449: .Dq cipher .
1.1 stevesk 450: .It Cm ClientAliveCountMax
1.48 jmc 451: Sets the number of client alive messages (see below) which may be
1.1 stevesk 452: sent without
1.52 jmc 453: .Xr sshd 8
1.17 jmc 454: receiving any messages back from the client.
455: If this threshold is reached while client alive messages are being sent,
1.52 jmc 456: sshd will disconnect the client, terminating the session.
1.17 jmc 457: It is important to note that the use of client alive messages is very
458: different from
1.27 markus 459: .Cm TCPKeepAlive
1.17 jmc 460: (below).
461: The client alive messages are sent through the encrypted channel
462: and therefore will not be spoofable.
463: The TCP keepalive option enabled by
1.27 markus 464: .Cm TCPKeepAlive
1.17 jmc 465: is spoofable.
466: The client alive mechanism is valuable when the client or
1.1 stevesk 467: server depend on knowing when a connection has become inactive.
468: .Pp
1.17 jmc 469: The default value is 3.
470: If
1.1 stevesk 471: .Cm ClientAliveInterval
1.48 jmc 472: (see below) is set to 15, and
1.1 stevesk 473: .Cm ClientAliveCountMax
1.52 jmc 474: is left at the default, unresponsive SSH clients
1.1 stevesk 475: will be disconnected after approximately 45 seconds.
1.57 markus 476: This option applies to protocol version 2 only.
1.42 djm 477: .It Cm ClientAliveInterval
478: Sets a timeout interval in seconds after which if no data has been received
479: from the client,
1.52 jmc 480: .Xr sshd 8
1.42 djm 481: will send a message through the encrypted
482: channel to request a response from the client.
483: The default
484: is 0, indicating that these messages will not be sent to the client.
485: This option applies to protocol version 2 only.
1.3 markus 486: .It Cm Compression
1.44 markus 487: Specifies whether compression is allowed, or delayed until
488: the user has authenticated successfully.
1.3 markus 489: The argument must be
1.44 markus 490: .Dq yes ,
491: .Dq delayed ,
1.3 markus 492: or
493: .Dq no .
494: The default is
1.44 markus 495: .Dq delayed .
1.1 stevesk 496: .It Cm DenyGroups
497: This keyword can be followed by a list of group name patterns, separated
498: by spaces.
499: Login is disallowed for users whose primary group or supplementary
500: group list matches one of the patterns.
501: Only group names are valid; a numerical group ID is not recognized.
502: By default, login is allowed for all groups.
1.54 jmc 503: The allow/deny directives are processed in the following order:
504: .Cm DenyUsers ,
505: .Cm AllowUsers ,
506: .Cm DenyGroups ,
507: and finally
508: .Cm AllowGroups .
1.49 jmc 509: .Pp
1.161 jmc 510: See PATTERNS in
1.49 jmc 511: .Xr ssh_config 5
512: for more information on patterns.
1.1 stevesk 513: .It Cm DenyUsers
514: This keyword can be followed by a list of user name patterns, separated
515: by spaces.
516: Login is disallowed for user names that match one of the patterns.
517: Only user names are valid; a numerical user ID is not recognized.
518: By default, login is allowed for all users.
519: If the pattern takes the form USER@HOST then USER and HOST
520: are separately checked, restricting logins to particular
521: users from particular hosts.
1.54 jmc 522: The allow/deny directives are processed in the following order:
523: .Cm DenyUsers ,
524: .Cm AllowUsers ,
525: .Cm DenyGroups ,
526: and finally
527: .Cm AllowGroups .
1.49 jmc 528: .Pp
1.161 jmc 529: See PATTERNS in
1.49 jmc 530: .Xr ssh_config 5
531: for more information on patterns.
1.183 djm 532: .It Cm FingerprintHash
533: Specifies the hash algorithm used when logging key fingerprints.
534: Valid options are:
535: .Dq md5
536: and
537: .Dq sha256 .
538: The default is
539: .Dq sha256 .
1.67 dtucker 540: .It Cm ForceCommand
541: Forces the execution of the command specified by
542: .Cm ForceCommand ,
1.84 djm 543: ignoring any command supplied by the client and
544: .Pa ~/.ssh/rc
545: if present.
1.67 dtucker 546: The command is invoked by using the user's login shell with the -c option.
547: This applies to shell, command, or subsystem execution.
548: It is most useful inside a
549: .Cm Match
550: block.
551: The command originally supplied by the client is available in the
552: .Ev SSH_ORIGINAL_COMMAND
553: environment variable.
1.82 djm 554: Specifying a command of
555: .Dq internal-sftp
556: will force the use of an in-process sftp server that requires no support
557: files when used with
558: .Cm ChrootDirectory .
1.1 stevesk 559: .It Cm GatewayPorts
560: Specifies whether remote hosts are allowed to connect to ports
561: forwarded for the client.
562: By default,
1.52 jmc 563: .Xr sshd 8
1.15 jmc 564: binds remote port forwardings to the loopback address.
565: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 566: .Cm GatewayPorts
1.52 jmc 567: can be used to specify that sshd
1.39 djm 568: should allow remote port forwardings to bind to non-loopback addresses, thus
569: allowing other hosts to connect.
570: The argument may be
571: .Dq no
572: to force remote port forwardings to be available to the local host only,
1.1 stevesk 573: .Dq yes
1.39 djm 574: to force remote port forwardings to bind to the wildcard address, or
575: .Dq clientspecified
576: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 577: The default is
578: .Dq no .
1.23 markus 579: .It Cm GSSAPIAuthentication
1.25 markus 580: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 581: The default is
1.23 markus 582: .Dq no .
583: Note that this option applies to protocol version 2 only.
584: .It Cm GSSAPICleanupCredentials
585: Specifies whether to automatically destroy the user's credentials cache
586: on logout.
587: The default is
588: .Dq yes .
589: Note that this option applies to protocol version 2 only.
1.189 djm 590: .It Cm HostbasedAcceptedKeyTypes
591: Specifies the key types that will be accepted for hostbased authentication
592: as a comma-separated pattern list.
593: The default
594: .Dq *
595: will allow all key types.
596: The
597: .Fl Q
598: option of
599: .Xr ssh 1
600: may be used to list supported key types.
1.1 stevesk 601: .It Cm HostbasedAuthentication
602: Specifies whether rhosts or /etc/hosts.equiv authentication together
603: with successful public key client host authentication is allowed
1.50 jmc 604: (host-based authentication).
1.1 stevesk 605: This option is similar to
606: .Cm RhostsRSAAuthentication
607: and applies to protocol version 2 only.
1.70 dtucker 608: The default is
609: .Dq no .
610: .It Cm HostbasedUsesNameFromPacketOnly
611: Specifies whether or not the server will attempt to perform a reverse
612: name lookup when matching the name in the
613: .Pa ~/.shosts ,
614: .Pa ~/.rhosts ,
615: and
616: .Pa /etc/hosts.equiv
617: files during
618: .Cm HostbasedAuthentication .
619: A setting of
620: .Dq yes
621: means that
622: .Xr sshd 8
623: uses the name supplied by the client rather than
624: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 625: The default is
626: .Dq no .
1.117 djm 627: .It Cm HostCertificate
628: Specifies a file containing a public host certificate.
629: The certificate's public key must match a private host key already specified
630: by
631: .Cm HostKey .
632: The default behaviour of
633: .Xr sshd 8
634: is not to load any certificates.
1.1 stevesk 635: .It Cm HostKey
636: Specifies a file containing a private host key
637: used by SSH.
638: The default is
639: .Pa /etc/ssh/ssh_host_key
640: for protocol version 1, and
1.126 djm 641: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 642: .Pa /etc/ssh/ssh_host_ecdsa_key ,
643: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 644: and
1.1 stevesk 645: .Pa /etc/ssh/ssh_host_rsa_key
646: for protocol version 2.
647: Note that
1.52 jmc 648: .Xr sshd 8
1.1 stevesk 649: will refuse to use a file if it is group/world-accessible.
650: It is possible to have multiple host key files.
651: .Dq rsa1
652: keys are used for version 1 and
1.126 djm 653: .Dq dsa ,
1.169 naddy 654: .Dq ecdsa ,
655: .Dq ed25519
1.1 stevesk 656: or
657: .Dq rsa
658: are used for version 2 of the SSH protocol.
1.162 markus 659: It is also possible to specify public host key files instead.
660: In this case operations on the private key will be delegated
661: to an
662: .Xr ssh-agent 1 .
663: .It Cm HostKeyAgent
664: Identifies the UNIX-domain socket used to communicate
665: with an agent that has access to the private host keys.
666: If
667: .Dq SSH_AUTH_SOCK
668: is specified, the location of the socket will be read from the
669: .Ev SSH_AUTH_SOCK
670: environment variable.
1.1 stevesk 671: .It Cm IgnoreRhosts
672: Specifies that
673: .Pa .rhosts
674: and
675: .Pa .shosts
676: files will not be used in
677: .Cm RhostsRSAAuthentication
678: or
679: .Cm HostbasedAuthentication .
680: .Pp
681: .Pa /etc/hosts.equiv
682: and
683: .Pa /etc/shosts.equiv
684: are still used.
685: The default is
686: .Dq yes .
687: .It Cm IgnoreUserKnownHosts
688: Specifies whether
1.52 jmc 689: .Xr sshd 8
1.1 stevesk 690: should ignore the user's
1.41 djm 691: .Pa ~/.ssh/known_hosts
1.1 stevesk 692: during
693: .Cm RhostsRSAAuthentication
694: or
695: .Cm HostbasedAuthentication .
696: The default is
697: .Dq no .
1.129 djm 698: .It Cm IPQoS
699: Specifies the IPv4 type-of-service or DSCP class for the connection.
700: Accepted values are
701: .Dq af11 ,
702: .Dq af12 ,
703: .Dq af13 ,
1.136 djm 704: .Dq af21 ,
1.129 djm 705: .Dq af22 ,
706: .Dq af23 ,
707: .Dq af31 ,
708: .Dq af32 ,
709: .Dq af33 ,
710: .Dq af41 ,
711: .Dq af42 ,
712: .Dq af43 ,
713: .Dq cs0 ,
714: .Dq cs1 ,
715: .Dq cs2 ,
716: .Dq cs3 ,
717: .Dq cs4 ,
718: .Dq cs5 ,
719: .Dq cs6 ,
720: .Dq cs7 ,
721: .Dq ef ,
722: .Dq lowdelay ,
723: .Dq throughput ,
724: .Dq reliability ,
725: or a numeric value.
1.131 djm 726: This option may take one or two arguments, separated by whitespace.
1.129 djm 727: If one argument is specified, it is used as the packet class unconditionally.
728: If two values are specified, the first is automatically selected for
729: interactive sessions and the second for non-interactive sessions.
730: The default is
731: .Dq lowdelay
732: for interactive sessions and
733: .Dq throughput
734: for non-interactive sessions.
1.171 jmc 735: .It Cm KbdInteractiveAuthentication
736: Specifies whether to allow keyboard-interactive authentication.
737: The argument to this keyword must be
738: .Dq yes
739: or
740: .Dq no .
741: The default is to use whatever value
742: .Cm ChallengeResponseAuthentication
743: is set to
744: (by default
745: .Dq yes ) .
1.1 stevesk 746: .It Cm KerberosAuthentication
1.24 markus 747: Specifies whether the password provided by the user for
1.1 stevesk 748: .Cm PasswordAuthentication
1.24 markus 749: will be validated through the Kerberos KDC.
1.1 stevesk 750: To use this option, the server needs a
751: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 752: The default is
1.29 dtucker 753: .Dq no .
754: .It Cm KerberosGetAFSToken
1.45 djm 755: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 756: an AFS token before accessing the user's home directory.
1.52 jmc 757: The default is
1.1 stevesk 758: .Dq no .
759: .It Cm KerberosOrLocalPasswd
1.52 jmc 760: If password authentication through Kerberos fails then
1.1 stevesk 761: the password will be validated via any additional local mechanism
762: such as
763: .Pa /etc/passwd .
1.52 jmc 764: The default is
1.1 stevesk 765: .Dq yes .
766: .It Cm KerberosTicketCleanup
767: Specifies whether to automatically destroy the user's ticket cache
768: file on logout.
1.52 jmc 769: The default is
1.1 stevesk 770: .Dq yes .
1.127 djm 771: .It Cm KexAlgorithms
772: Specifies the available KEX (Key Exchange) algorithms.
773: Multiple algorithms must be comma-separated.
1.173 naddy 774: The supported algorithms are:
775: .Pp
776: .Bl -item -compact -offset indent
777: .It
778: curve25519-sha256@libssh.org
779: .It
780: diffie-hellman-group1-sha1
781: .It
782: diffie-hellman-group14-sha1
783: .It
784: diffie-hellman-group-exchange-sha1
785: .It
786: diffie-hellman-group-exchange-sha256
787: .It
788: ecdh-sha2-nistp256
789: .It
790: ecdh-sha2-nistp384
791: .It
792: ecdh-sha2-nistp521
793: .El
794: .Pp
795: The default is:
1.170 dtucker 796: .Bd -literal -offset indent
797: curve25519-sha256@libssh.org,
798: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
799: diffie-hellman-group-exchange-sha256,
1.173 naddy 800: diffie-hellman-group14-sha1
1.170 dtucker 801: .Ed
1.188 djm 802: .Pp
803: The list of available key exchange algorithms may also be obtained using the
804: .Fl Q
805: option of
806: .Xr ssh 1
807: with an argument of
808: .Dq kex .
1.1 stevesk 809: .It Cm KeyRegenerationInterval
810: In protocol version 1, the ephemeral server key is automatically regenerated
811: after this many seconds (if it has been used).
812: The purpose of regeneration is to prevent
813: decrypting captured sessions by later breaking into the machine and
814: stealing the keys.
815: The key is never stored anywhere.
816: If the value is 0, the key is never regenerated.
817: The default is 3600 (seconds).
818: .It Cm ListenAddress
819: Specifies the local addresses
1.52 jmc 820: .Xr sshd 8
1.1 stevesk 821: should listen on.
822: The following forms may be used:
823: .Pp
824: .Bl -item -offset indent -compact
825: .It
826: .Cm ListenAddress
827: .Sm off
1.180 jmc 828: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 829: .Sm on
830: .It
831: .Cm ListenAddress
832: .Sm off
1.180 jmc 833: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 834: .Sm on
835: .It
836: .Cm ListenAddress
837: .Sm off
838: .Oo
1.180 jmc 839: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 840: .Sm on
841: .El
842: .Pp
843: If
844: .Ar port
845: is not specified,
1.200 dtucker 846: sshd will listen on the address and all
1.1 stevesk 847: .Cm Port
1.17 jmc 848: options specified.
849: The default is to listen on all local addresses.
1.15 jmc 850: Multiple
1.1 stevesk 851: .Cm ListenAddress
1.17 jmc 852: options are permitted.
1.1 stevesk 853: .It Cm LoginGraceTime
854: The server disconnects after this time if the user has not
855: successfully logged in.
856: If the value is 0, there is no time limit.
1.12 stevesk 857: The default is 120 seconds.
1.1 stevesk 858: .It Cm LogLevel
859: Gives the verbosity level that is used when logging messages from
1.53 jmc 860: .Xr sshd 8 .
1.1 stevesk 861: The possible values are:
1.52 jmc 862: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 863: The default is INFO.
864: DEBUG and DEBUG1 are equivalent.
865: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
866: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 867: .It Cm MACs
868: Specifies the available MAC (message authentication code) algorithms.
869: The MAC algorithm is used in protocol version 2
870: for data integrity protection.
871: Multiple algorithms must be comma-separated.
1.152 markus 872: The algorithms that contain
873: .Dq -etm
874: calculate the MAC after encryption (encrypt-then-mac).
875: These are considered safer and their use recommended.
1.173 naddy 876: The supported MACs are:
877: .Pp
878: .Bl -item -compact -offset indent
879: .It
880: hmac-md5
881: .It
882: hmac-md5-96
883: .It
884: hmac-ripemd160
885: .It
886: hmac-sha1
887: .It
888: hmac-sha1-96
889: .It
890: hmac-sha2-256
891: .It
892: hmac-sha2-512
893: .It
894: umac-64@openssh.com
895: .It
896: umac-128@openssh.com
897: .It
898: hmac-md5-etm@openssh.com
899: .It
900: hmac-md5-96-etm@openssh.com
901: .It
902: hmac-ripemd160-etm@openssh.com
903: .It
904: hmac-sha1-etm@openssh.com
905: .It
906: hmac-sha1-96-etm@openssh.com
907: .It
908: hmac-sha2-256-etm@openssh.com
909: .It
910: hmac-sha2-512-etm@openssh.com
911: .It
912: umac-64-etm@openssh.com
913: .It
914: umac-128-etm@openssh.com
915: .El
916: .Pp
1.52 jmc 917: The default is:
1.77 jmc 918: .Bd -literal -offset indent
1.152 markus 919: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
920: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 921: umac-64@openssh.com,umac-128@openssh.com,
922: hmac-sha2-256,hmac-sha2-512
1.77 jmc 923: .Ed
1.188 djm 924: .Pp
925: The list of available MAC algorithms may also be obtained using the
926: .Fl Q
927: option of
928: .Xr ssh 1
929: with an argument of
930: .Dq mac .
1.60 dtucker 931: .It Cm Match
1.61 jmc 932: Introduces a conditional block.
1.65 dtucker 933: If all of the criteria on the
1.60 dtucker 934: .Cm Match
1.65 dtucker 935: line are satisfied, the keywords on the following lines override those
936: set in the global section of the config file, until either another
1.60 dtucker 937: .Cm Match
1.65 dtucker 938: line or the end of the file.
1.172 djm 939: If a keyword appears in multiple
940: .Cm Match
1.177 sobrado 941: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 942: applied.
1.91 djm 943: .Pp
1.61 jmc 944: The arguments to
1.60 dtucker 945: .Cm Match
1.163 dtucker 946: are one or more criteria-pattern pairs or the single token
947: .Cm All
948: which matches all criteria.
1.60 dtucker 949: The available criteria are
950: .Cm User ,
1.69 dtucker 951: .Cm Group ,
1.60 dtucker 952: .Cm Host ,
1.139 dtucker 953: .Cm LocalAddress ,
954: .Cm LocalPort ,
1.60 dtucker 955: and
956: .Cm Address .
1.91 djm 957: The match patterns may consist of single entries or comma-separated
958: lists and may use the wildcard and negation operators described in the
1.161 jmc 959: PATTERNS section of
1.92 djm 960: .Xr ssh_config 5 .
1.91 djm 961: .Pp
962: The patterns in an
963: .Cm Address
964: criteria may additionally contain addresses to match in CIDR
1.93 jmc 965: address/masklen format, e.g.\&
1.91 djm 966: .Dq 192.0.2.0/24
967: or
968: .Dq 3ffe:ffff::/32 .
969: Note that the mask length provided must be consistent with the address -
970: it is an error to specify a mask length that is too long for the address
1.93 jmc 971: or one with bits set in this host portion of the address.
972: For example,
1.91 djm 973: .Dq 192.0.2.0/33
974: and
1.93 jmc 975: .Dq 192.0.2.0/8
1.91 djm 976: respectively.
977: .Pp
1.60 dtucker 978: Only a subset of keywords may be used on the lines following a
979: .Cm Match
980: keyword.
981: Available keywords are
1.142 jmc 982: .Cm AcceptEnv ,
1.99 okan 983: .Cm AllowAgentForwarding ,
1.142 jmc 984: .Cm AllowGroups ,
1.193 djm 985: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 986: .Cm AllowTcpForwarding ,
1.141 markus 987: .Cm AllowUsers ,
1.149 djm 988: .Cm AuthenticationMethods ,
1.146 djm 989: .Cm AuthorizedKeysCommand ,
990: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 991: .Cm AuthorizedKeysFile ,
1.123 djm 992: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 993: .Cm Banner ,
1.85 djm 994: .Cm ChrootDirectory ,
1.141 markus 995: .Cm DenyGroups ,
996: .Cm DenyUsers ,
1.67 dtucker 997: .Cm ForceCommand ,
1.194 djm 998: .Cm GatewayPorts ,
1.193 djm 999: .Cm GSSAPIAuthentication ,
1.189 djm 1000: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1001: .Cm HostbasedAuthentication ,
1.123 djm 1002: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1003: .Cm IPQoS ,
1.74 jmc 1004: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1005: .Cm KerberosAuthentication ,
1.95 dtucker 1006: .Cm MaxAuthTries ,
1.94 dtucker 1007: .Cm MaxSessions ,
1.72 dtucker 1008: .Cm PasswordAuthentication ,
1.97 djm 1009: .Cm PermitEmptyPasswords ,
1.66 dtucker 1010: .Cm PermitOpen ,
1.79 dtucker 1011: .Cm PermitRootLogin ,
1.164 djm 1012: .Cm PermitTTY ,
1.123 djm 1013: .Cm PermitTunnel ,
1.174 djm 1014: .Cm PermitUserRC ,
1.189 djm 1015: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1016: .Cm PubkeyAuthentication ,
1.159 dtucker 1017: .Cm RekeyLimit ,
1.193 djm 1018: .Cm RevokedKeys ,
1.142 jmc 1019: .Cm RhostsRSAAuthentication ,
1.194 djm 1020: .Cm RSAAuthentication ,
1.193 djm 1021: .Cm StreamLocalBindMask ,
1022: .Cm StreamLocalBindUnlink ,
1023: .Cm TrustedUserCAKeys ,
1.66 dtucker 1024: .Cm X11DisplayOffset ,
1.101 djm 1025: .Cm X11Forwarding
1.60 dtucker 1026: and
1.102 djm 1027: .Cm X11UseLocalHost .
1.33 dtucker 1028: .It Cm MaxAuthTries
1029: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1030: connection.
1031: Once the number of failures reaches half this value,
1032: additional failures are logged.
1033: The default is 6.
1.90 djm 1034: .It Cm MaxSessions
1035: Specifies the maximum number of open sessions permitted per network connection.
1036: The default is 10.
1.1 stevesk 1037: .It Cm MaxStartups
1038: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1039: SSH daemon.
1.1 stevesk 1040: Additional connections will be dropped until authentication succeeds or the
1041: .Cm LoginGraceTime
1042: expires for a connection.
1.156 dtucker 1043: The default is 10:30:100.
1.1 stevesk 1044: .Pp
1045: Alternatively, random early drop can be enabled by specifying
1046: the three colon separated values
1047: .Dq start:rate:full
1.51 jmc 1048: (e.g. "10:30:60").
1.53 jmc 1049: .Xr sshd 8
1.1 stevesk 1050: will refuse connection attempts with a probability of
1051: .Dq rate/100
1052: (30%)
1053: if there are currently
1054: .Dq start
1055: (10)
1056: unauthenticated connections.
1057: The probability increases linearly and all connection attempts
1058: are refused if the number of unauthenticated connections reaches
1059: .Dq full
1060: (60).
1061: .It Cm PasswordAuthentication
1062: Specifies whether password authentication is allowed.
1063: The default is
1064: .Dq yes .
1065: .It Cm PermitEmptyPasswords
1066: When password authentication is allowed, it specifies whether the
1067: server allows login to accounts with empty password strings.
1068: The default is
1069: .Dq no .
1.62 dtucker 1070: .It Cm PermitOpen
1071: Specifies the destinations to which TCP port forwarding is permitted.
1072: The forwarding specification must be one of the following forms:
1073: .Pp
1074: .Bl -item -offset indent -compact
1075: .It
1076: .Cm PermitOpen
1077: .Sm off
1078: .Ar host : port
1079: .Sm on
1080: .It
1081: .Cm PermitOpen
1082: .Sm off
1083: .Ar IPv4_addr : port
1084: .Sm on
1085: .It
1086: .Cm PermitOpen
1087: .Sm off
1088: .Ar \&[ IPv6_addr \&] : port
1089: .Sm on
1090: .El
1091: .Pp
1.68 dtucker 1092: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1093: An argument of
1094: .Dq any
1095: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1096: An argument of
1097: .Dq none
1098: can be used to prohibit all forwarding requests.
1.63 jmc 1099: By default all port forwarding requests are permitted.
1.1 stevesk 1100: .It Cm PermitRootLogin
1.38 jmc 1101: Specifies whether root can log in using
1.1 stevesk 1102: .Xr ssh 1 .
1103: The argument must be
1104: .Dq yes ,
1105: .Dq without-password ,
1.52 jmc 1106: .Dq forced-commands-only ,
1.1 stevesk 1107: or
1108: .Dq no .
1109: The default is
1.196 djm 1110: .Dq no .
1.1 stevesk 1111: .Pp
1112: If this option is set to
1.52 jmc 1113: .Dq without-password ,
1.1 stevesk 1114: password authentication is disabled for root.
1115: .Pp
1116: If this option is set to
1.52 jmc 1117: .Dq forced-commands-only ,
1.1 stevesk 1118: root login with public key authentication will be allowed,
1119: but only if the
1120: .Ar command
1121: option has been specified
1122: (which may be useful for taking remote backups even if root login is
1.17 jmc 1123: normally not allowed).
1124: All other authentication methods are disabled for root.
1.1 stevesk 1125: .Pp
1126: If this option is set to
1.52 jmc 1127: .Dq no ,
1.38 jmc 1128: root is not allowed to log in.
1.46 reyk 1129: .It Cm PermitTunnel
1130: Specifies whether
1131: .Xr tun 4
1132: device forwarding is allowed.
1.47 reyk 1133: The argument must be
1134: .Dq yes ,
1.58 stevesk 1135: .Dq point-to-point
1136: (layer 3),
1137: .Dq ethernet
1138: (layer 2), or
1.47 reyk 1139: .Dq no .
1.58 stevesk 1140: Specifying
1141: .Dq yes
1142: permits both
1143: .Dq point-to-point
1144: and
1145: .Dq ethernet .
1.46 reyk 1146: The default is
1147: .Dq no .
1.178 djm 1148: .Pp
1149: Independent of this setting, the permissions of the selected
1150: .Xr tun 4
1151: device must allow access to the user.
1.164 djm 1152: .It Cm PermitTTY
1153: Specifies whether
1.165 jmc 1154: .Xr pty 4
1.164 djm 1155: allocation is permitted.
1156: The default is
1157: .Dq yes .
1.6 markus 1158: .It Cm PermitUserEnvironment
1159: Specifies whether
1160: .Pa ~/.ssh/environment
1.9 stevesk 1161: and
1.6 markus 1162: .Cm environment=
1163: options in
1164: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1165: are processed by
1.52 jmc 1166: .Xr sshd 8 .
1.6 markus 1167: The default is
1168: .Dq no .
1.9 stevesk 1169: Enabling environment processing may enable users to bypass access
1170: restrictions in some configurations using mechanisms such as
1171: .Ev LD_PRELOAD .
1.174 djm 1172: .It Cm PermitUserRC
1173: Specifies whether any
1174: .Pa ~/.ssh/rc
1175: file is executed.
1176: The default is
1177: .Dq yes .
1.1 stevesk 1178: .It Cm PidFile
1.4 stevesk 1179: Specifies the file that contains the process ID of the
1.195 dtucker 1180: SSH daemon, or
1181: .Dq none
1182: to not write one.
1.1 stevesk 1183: The default is
1184: .Pa /var/run/sshd.pid .
1185: .It Cm Port
1186: Specifies the port number that
1.52 jmc 1187: .Xr sshd 8
1.1 stevesk 1188: listens on.
1189: The default is 22.
1190: Multiple options of this type are permitted.
1191: See also
1192: .Cm ListenAddress .
1193: .It Cm PrintLastLog
1194: Specifies whether
1.52 jmc 1195: .Xr sshd 8
1.36 jaredy 1196: should print the date and time of the last user login when a user logs
1197: in interactively.
1.1 stevesk 1198: The default is
1199: .Dq yes .
1200: .It Cm PrintMotd
1201: Specifies whether
1.52 jmc 1202: .Xr sshd 8
1.1 stevesk 1203: should print
1204: .Pa /etc/motd
1205: when a user logs in interactively.
1206: (On some systems it is also printed by the shell,
1207: .Pa /etc/profile ,
1208: or equivalent.)
1209: The default is
1210: .Dq yes .
1211: .It Cm Protocol
1212: Specifies the protocol versions
1.52 jmc 1213: .Xr sshd 8
1.5 stevesk 1214: supports.
1.1 stevesk 1215: The possible values are
1.52 jmc 1216: .Sq 1
1.1 stevesk 1217: and
1.52 jmc 1218: .Sq 2 .
1.1 stevesk 1219: Multiple versions must be comma-separated.
1220: The default is
1.109 jmc 1221: .Sq 2 .
1.5 stevesk 1222: Note that the order of the protocol list does not indicate preference,
1223: because the client selects among multiple protocol versions offered
1224: by the server.
1225: Specifying
1226: .Dq 2,1
1227: is identical to
1228: .Dq 1,2 .
1.189 djm 1229: .It Cm PubkeyAcceptedKeyTypes
1230: Specifies the key types that will be accepted for public key authentication
1231: as a comma-separated pattern list.
1232: The default
1233: .Dq *
1234: will allow all key types.
1235: The
1236: .Fl Q
1237: option of
1238: .Xr ssh 1
1239: may be used to list supported key types.
1.1 stevesk 1240: .It Cm PubkeyAuthentication
1241: Specifies whether public key authentication is allowed.
1242: The default is
1243: .Dq yes .
1244: Note that this option applies to protocol version 2 only.
1.159 dtucker 1245: .It Cm RekeyLimit
1246: Specifies the maximum amount of data that may be transmitted before the
1247: session key is renegotiated, optionally followed a maximum amount of
1248: time that may pass before the session key is renegotiated.
1249: The first argument is specified in bytes and may have a suffix of
1250: .Sq K ,
1251: .Sq M ,
1252: or
1253: .Sq G
1254: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1255: The default is between
1256: .Sq 1G
1257: and
1258: .Sq 4G ,
1259: depending on the cipher.
1260: The optional second value is specified in seconds and may use any of the
1261: units documented in the
1262: .Sx TIME FORMATS
1.160 jmc 1263: section.
1.159 dtucker 1264: The default value for
1265: .Cm RekeyLimit
1266: is
1267: .Dq default none ,
1268: which means that rekeying is performed after the cipher's default amount
1269: of data has been sent or received and no time based rekeying is done.
1270: This option applies to protocol version 2 only.
1.118 djm 1271: .It Cm RevokedKeys
1.195 dtucker 1272: Specifies revoked public keys file, or
1273: .Dq none
1274: to not use one.
1.118 djm 1275: Keys listed in this file will be refused for public key authentication.
1276: Note that if this file is not readable, then public key authentication will
1277: be refused for all users.
1.154 djm 1278: Keys may be specified as a text file, listing one public key per line, or as
1279: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1280: .Xr ssh-keygen 1 .
1.161 jmc 1281: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1282: .Xr ssh-keygen 1 .
1.1 stevesk 1283: .It Cm RhostsRSAAuthentication
1284: Specifies whether rhosts or /etc/hosts.equiv authentication together
1285: with successful RSA host authentication is allowed.
1286: The default is
1287: .Dq no .
1288: This option applies to protocol version 1 only.
1289: .It Cm RSAAuthentication
1290: Specifies whether pure RSA authentication is allowed.
1291: The default is
1292: .Dq yes .
1293: This option applies to protocol version 1 only.
1294: .It Cm ServerKeyBits
1295: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1296: The minimum value is 512, and the default is 1024.
1.175 millert 1297: .It Cm StreamLocalBindMask
1298: Sets the octal file creation mode mask
1299: .Pq umask
1300: used when creating a Unix-domain socket file for local or remote
1301: port forwarding.
1302: This option is only used for port forwarding to a Unix-domain socket file.
1303: .Pp
1304: The default value is 0177, which creates a Unix-domain socket file that is
1305: readable and writable only by the owner.
1306: Note that not all operating systems honor the file mode on Unix-domain
1307: socket files.
1308: .It Cm StreamLocalBindUnlink
1309: Specifies whether to remove an existing Unix-domain socket file for local
1310: or remote port forwarding before creating a new one.
1311: If the socket file already exists and
1312: .Cm StreamLocalBindUnlink
1313: is not enabled,
1314: .Nm sshd
1315: will be unable to forward the port to the Unix-domain socket file.
1316: This option is only used for port forwarding to a Unix-domain socket file.
1317: .Pp
1318: The argument must be
1319: .Dq yes
1320: or
1321: .Dq no .
1322: The default is
1323: .Dq no .
1.1 stevesk 1324: .It Cm StrictModes
1325: Specifies whether
1.52 jmc 1326: .Xr sshd 8
1.1 stevesk 1327: should check file modes and ownership of the
1328: user's files and home directory before accepting login.
1329: This is normally desirable because novices sometimes accidentally leave their
1330: directory or files world-writable.
1331: The default is
1332: .Dq yes .
1.112 djm 1333: Note that this does not apply to
1334: .Cm ChrootDirectory ,
1335: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1336: .It Cm Subsystem
1.51 jmc 1337: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1338: Arguments should be a subsystem name and a command (with optional arguments)
1339: to execute upon subsystem request.
1.80 djm 1340: .Pp
1.1 stevesk 1341: The command
1342: .Xr sftp-server 8
1343: implements the
1344: .Dq sftp
1345: file transfer subsystem.
1.80 djm 1346: .Pp
1347: Alternately the name
1348: .Dq internal-sftp
1349: implements an in-process
1350: .Dq sftp
1351: server.
1352: This may simplify configurations using
1353: .Cm ChrootDirectory
1354: to force a different filesystem root on clients.
1355: .Pp
1.1 stevesk 1356: By default no subsystems are defined.
1357: Note that this option applies to protocol version 2 only.
1358: .It Cm SyslogFacility
1359: Gives the facility code that is used when logging messages from
1.53 jmc 1360: .Xr sshd 8 .
1.1 stevesk 1361: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1362: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1363: The default is AUTH.
1.27 markus 1364: .It Cm TCPKeepAlive
1365: Specifies whether the system should send TCP keepalive messages to the
1366: other side.
1367: If they are sent, death of the connection or crash of one
1368: of the machines will be properly noticed.
1369: However, this means that
1370: connections will die if the route is down temporarily, and some people
1371: find it annoying.
1372: On the other hand, if TCP keepalives are not sent,
1373: sessions may hang indefinitely on the server, leaving
1374: .Dq ghost
1375: users and consuming server resources.
1376: .Pp
1377: The default is
1378: .Dq yes
1379: (to send TCP keepalive messages), and the server will notice
1380: if the network goes down or the client host crashes.
1381: This avoids infinitely hanging sessions.
1382: .Pp
1383: To disable TCP keepalive messages, the value should be set to
1384: .Dq no .
1.118 djm 1385: .It Cm TrustedUserCAKeys
1386: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1387: trusted to sign user certificates for authentication, or
1388: .Dq none
1389: to not use one.
1.119 jmc 1390: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1391: .Ql #
1392: are allowed.
1393: If a certificate is presented for authentication and has its signing CA key
1394: listed in this file, then it may be used for authentication for any user
1395: listed in the certificate's principals list.
1396: Note that certificates that lack a list of principals will not be permitted
1397: for authentication using
1398: .Cm TrustedUserCAKeys .
1.161 jmc 1399: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1400: .Xr ssh-keygen 1 .
1.18 markus 1401: .It Cm UseDNS
1402: Specifies whether
1.52 jmc 1403: .Xr sshd 8
1.40 jmc 1404: should look up the remote host name and check that
1.18 markus 1405: the resolved host name for the remote IP address maps back to the
1406: very same IP address.
1407: The default is
1.192 deraadt 1408: .Dq no .
1.1 stevesk 1409: .It Cm UseLogin
1410: Specifies whether
1411: .Xr login 1
1412: is used for interactive login sessions.
1413: The default is
1414: .Dq no .
1415: Note that
1416: .Xr login 1
1417: is never used for remote command execution.
1418: Note also, that if this is enabled,
1419: .Cm X11Forwarding
1420: will be disabled because
1421: .Xr login 1
1422: does not know how to handle
1423: .Xr xauth 1
1.15 jmc 1424: cookies.
1425: If
1.1 stevesk 1426: .Cm UsePrivilegeSeparation
1427: is specified, it will be disabled after authentication.
1428: .It Cm UsePrivilegeSeparation
1429: Specifies whether
1.52 jmc 1430: .Xr sshd 8
1.2 stevesk 1431: separates privileges by creating an unprivileged child process
1.15 jmc 1432: to deal with incoming network traffic.
1433: After successful authentication, another process will be created that has
1434: the privilege of the authenticated user.
1435: The goal of privilege separation is to prevent privilege
1.1 stevesk 1436: escalation by containing any corruption within the unprivileged processes.
1437: The default is
1438: .Dq yes .
1.134 djm 1439: If
1440: .Cm UsePrivilegeSeparation
1441: is set to
1442: .Dq sandbox
1443: then the pre-authentication unprivileged process is subject to additional
1444: restrictions.
1.137 djm 1445: .It Cm VersionAddendum
1446: Optionally specifies additional text to append to the SSH protocol banner
1447: sent by the server upon connection.
1448: The default is
1449: .Dq none .
1.1 stevesk 1450: .It Cm X11DisplayOffset
1451: Specifies the first display number available for
1.52 jmc 1452: .Xr sshd 8 Ns 's
1.1 stevesk 1453: X11 forwarding.
1.52 jmc 1454: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1455: The default is 10.
1456: .It Cm X11Forwarding
1457: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1458: The argument must be
1459: .Dq yes
1460: or
1461: .Dq no .
1.1 stevesk 1462: The default is
1463: .Dq no .
1.13 stevesk 1464: .Pp
1465: When X11 forwarding is enabled, there may be additional exposure to
1466: the server and to client displays if the
1.52 jmc 1467: .Xr sshd 8
1.13 stevesk 1468: proxy display is configured to listen on the wildcard address (see
1469: .Cm X11UseLocalhost
1.52 jmc 1470: below), though this is not the default.
1.13 stevesk 1471: Additionally, the authentication spoofing and authentication data
1472: verification and substitution occur on the client side.
1473: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1474: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1475: forwarding (see the warnings for
1476: .Cm ForwardX11
1477: in
1.19 jmc 1478: .Xr ssh_config 5 ) .
1.13 stevesk 1479: A system administrator may have a stance in which they want to
1480: protect clients that may expose themselves to attack by unwittingly
1481: requesting X11 forwarding, which can warrant a
1482: .Dq no
1483: setting.
1484: .Pp
1485: Note that disabling X11 forwarding does not prevent users from
1486: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1487: X11 forwarding is automatically disabled if
1488: .Cm UseLogin
1489: is enabled.
1490: .It Cm X11UseLocalhost
1491: Specifies whether
1.52 jmc 1492: .Xr sshd 8
1.1 stevesk 1493: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1494: the wildcard address.
1495: By default,
1.52 jmc 1496: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1497: hostname part of the
1498: .Ev DISPLAY
1499: environment variable to
1500: .Dq localhost .
1.8 stevesk 1501: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1502: However, some older X11 clients may not function with this
1503: configuration.
1504: .Cm X11UseLocalhost
1505: may be set to
1506: .Dq no
1507: to specify that the forwarding server should be bound to the wildcard
1508: address.
1509: The argument must be
1510: .Dq yes
1511: or
1512: .Dq no .
1513: The default is
1514: .Dq yes .
1515: .It Cm XAuthLocation
1.11 stevesk 1516: Specifies the full pathname of the
1.1 stevesk 1517: .Xr xauth 1
1.195 dtucker 1518: program, or
1519: .Dq none
1520: to not use one.
1.1 stevesk 1521: The default is
1522: .Pa /usr/X11R6/bin/xauth .
1523: .El
1.55 jmc 1524: .Sh TIME FORMATS
1.53 jmc 1525: .Xr sshd 8
1.1 stevesk 1526: command-line arguments and configuration file options that specify time
1527: may be expressed using a sequence of the form:
1528: .Sm off
1.7 stevesk 1529: .Ar time Op Ar qualifier ,
1.1 stevesk 1530: .Sm on
1531: where
1532: .Ar time
1533: is a positive integer value and
1534: .Ar qualifier
1535: is one of the following:
1536: .Pp
1537: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1538: .It Aq Cm none
1.1 stevesk 1539: seconds
1540: .It Cm s | Cm S
1541: seconds
1542: .It Cm m | Cm M
1543: minutes
1544: .It Cm h | Cm H
1545: hours
1546: .It Cm d | Cm D
1547: days
1548: .It Cm w | Cm W
1549: weeks
1550: .El
1551: .Pp
1552: Each member of the sequence is added together to calculate
1553: the total time value.
1554: .Pp
1555: Time format examples:
1556: .Pp
1557: .Bl -tag -width Ds -compact -offset indent
1558: .It 600
1559: 600 seconds (10 minutes)
1560: .It 10m
1561: 10 minutes
1562: .It 1h30m
1563: 1 hour 30 minutes (90 minutes)
1564: .El
1565: .Sh FILES
1566: .Bl -tag -width Ds
1567: .It Pa /etc/ssh/sshd_config
1568: Contains configuration data for
1.53 jmc 1569: .Xr sshd 8 .
1.1 stevesk 1570: This file should be writable by root only, but it is recommended
1571: (though not necessary) that it be world-readable.
1572: .El
1.19 jmc 1573: .Sh SEE ALSO
1574: .Xr sshd 8
1.1 stevesk 1575: .Sh AUTHORS
1576: OpenSSH is a derivative of the original and free
1577: ssh 1.2.12 release by Tatu Ylonen.
1578: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1579: Theo de Raadt and Dug Song
1580: removed many bugs, re-added newer features and
1581: created OpenSSH.
1582: Markus Friedl contributed the support for SSH
1583: protocol versions 1.5 and 2.0.
1584: Niels Provos and Markus Friedl contributed support
1585: for privilege separation.
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