Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.185
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.185 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.184 2014/12/21 23:35:14 jmc Exp $
1.184 jmc 37: .Dd $Mdocdate: December 21 2014 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.1 stevesk 123: .It Cm AllowTcpForwarding
124: Specifies whether TCP forwarding is permitted.
1.150 djm 125: The available options are
126: .Dq yes
127: or
128: .Dq all
129: to allow TCP forwarding,
130: .Dq no
131: to prevent all TCP forwarding,
132: .Dq local
133: to allow local (from the perspective of
1.151 jmc 134: .Xr ssh 1 )
135: forwarding only or
1.150 djm 136: .Dq remote
137: to allow remote forwarding only.
1.1 stevesk 138: The default is
139: .Dq yes .
140: Note that disabling TCP forwarding does not improve security unless
141: users are also denied shell access, as they can always install their
142: own forwarders.
1.175 millert 143: .It Cm AllowStreamLocalForwarding
144: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
145: The available options are
146: .Dq yes
147: or
148: .Dq all
149: to allow StreamLocal forwarding,
150: .Dq no
151: to prevent all StreamLocal forwarding,
152: .Dq local
153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
156: .Dq remote
157: to allow remote forwarding only.
158: The default is
159: .Dq yes .
160: Note that disabling StreamLocal forwarding does not improve security unless
161: users are also denied shell access, as they can always install their
162: own forwarders.
1.1 stevesk 163: .It Cm AllowUsers
164: This keyword can be followed by a list of user name patterns, separated
165: by spaces.
1.14 jmc 166: If specified, login is allowed only for user names that
1.1 stevesk 167: match one of the patterns.
168: Only user names are valid; a numerical user ID is not recognized.
169: By default, login is allowed for all users.
170: If the pattern takes the form USER@HOST then USER and HOST
171: are separately checked, restricting logins to particular
172: users from particular hosts.
1.54 jmc 173: The allow/deny directives are processed in the following order:
174: .Cm DenyUsers ,
175: .Cm AllowUsers ,
176: .Cm DenyGroups ,
177: and finally
178: .Cm AllowGroups .
1.49 jmc 179: .Pp
1.161 jmc 180: See PATTERNS in
1.49 jmc 181: .Xr ssh_config 5
182: for more information on patterns.
1.149 djm 183: .It Cm AuthenticationMethods
184: Specifies the authentication methods that must be successfully completed
185: for a user to be granted access.
186: This option must be followed by one or more comma-separated lists of
187: authentication method names.
188: Successful authentication requires completion of every method in at least
189: one of these lists.
190: .Pp
191: For example, an argument of
192: .Dq publickey,password publickey,keyboard-interactive
193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
196: so for this example, it would not be possible to attempt password or
197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
202: .Dq bsdauth ,
203: .Dq pam ,
204: or
205: .Dq skey ,
206: depending on the server configuration.
207: For example,
208: .Dq keyboard-interactive:bsdauth
209: would restrict keyboard interactive authentication to the
210: .Dq bsdauth
211: device.
1.185 ! djm 212: .Pp
! 213: If the
! 214: .Dq publickey
! 215: method is listed more than one,
! 216: .Xr sshd 8
! 217: verifies that keys that have been used successfully are not reused for
! 218: subsequent authentications.
! 219: For example, an
! 220: .Cm AuthenticationMethods
! 221: of
! 222: .Dq publickey,publickey
! 223: will require successful authentication using two different public keys.
1.181 djm 224: .Pp
1.149 djm 225: This option is only available for SSH protocol 2 and will yield a fatal
226: error if enabled if protocol 1 is also enabled.
227: Note that each authentication method listed should also be explicitly enabled
228: in the configuration.
229: The default is not to require multiple authentication; successful completion
230: of a single authentication method is sufficient.
1.146 djm 231: .It Cm AuthorizedKeysCommand
1.147 jmc 232: Specifies a program to be used to look up the user's public keys.
1.158 djm 233: The program must be owned by root and not writable by group or others.
234: It will be invoked with a single argument of the username
1.146 djm 235: being authenticated, and should produce on standard output zero or
1.161 jmc 236: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 237: .Xr sshd 8 ) .
1.146 djm 238: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
239: and authorize the user then public key authentication continues using the usual
240: .Cm AuthorizedKeysFile
241: files.
242: By default, no AuthorizedKeysCommand is run.
243: .It Cm AuthorizedKeysCommandUser
244: Specifies the user under whose account the AuthorizedKeysCommand is run.
245: It is recommended to use a dedicated user that has no other role on the host
246: than running authorized keys commands.
1.181 djm 247: If no user is specified then
248: .Cm AuthorizedKeysCommand
249: is ignored.
1.1 stevesk 250: .It Cm AuthorizedKeysFile
251: Specifies the file that contains the public keys that can be used
252: for user authentication.
1.124 djm 253: The format is described in the
1.161 jmc 254: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 255: section of
256: .Xr sshd 8 .
1.1 stevesk 257: .Cm AuthorizedKeysFile
258: may contain tokens of the form %T which are substituted during connection
1.52 jmc 259: setup.
1.17 jmc 260: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 261: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 262: %u is replaced by the username of that user.
263: After expansion,
264: .Cm AuthorizedKeysFile
265: is taken to be an absolute path or one relative to the user's home
266: directory.
1.133 jmc 267: Multiple files may be listed, separated by whitespace.
268: The default is
269: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 270: .It Cm AuthorizedPrincipalsFile
271: Specifies a file that lists principal names that are accepted for
272: certificate authentication.
273: When using certificates signed by a key listed in
274: .Cm TrustedUserCAKeys ,
275: this file lists names, one of which must appear in the certificate for it
276: to be accepted for authentication.
1.125 jmc 277: Names are listed one per line preceded by key options (as described
1.161 jmc 278: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 279: .Xr sshd 8 ) .
1.124 djm 280: Empty lines and comments starting with
1.121 djm 281: .Ql #
282: are ignored.
283: .Pp
284: .Cm AuthorizedPrincipalsFile
285: may contain tokens of the form %T which are substituted during connection
286: setup.
287: The following tokens are defined: %% is replaced by a literal '%',
288: %h is replaced by the home directory of the user being authenticated, and
289: %u is replaced by the username of that user.
290: After expansion,
291: .Cm AuthorizedPrincipalsFile
292: is taken to be an absolute path or one relative to the user's home
293: directory.
294: .Pp
1.138 djm 295: The default is
296: .Dq none ,
297: i.e. not to use a principals file \(en in this case, the username
1.121 djm 298: of the user must appear in a certificate's principals list for it to be
299: accepted.
300: Note that
301: .Cm AuthorizedPrincipalsFile
302: is only used when authentication proceeds using a CA listed in
303: .Cm TrustedUserCAKeys
304: and is not consulted for certification authorities trusted via
305: .Pa ~/.ssh/authorized_keys ,
306: though the
307: .Cm principals=
308: key option offers a similar facility (see
309: .Xr sshd 8
310: for details).
1.1 stevesk 311: .It Cm Banner
312: The contents of the specified file are sent to the remote user before
313: authentication is allowed.
1.78 djm 314: If the argument is
315: .Dq none
316: then no banner is displayed.
1.1 stevesk 317: This option is only available for protocol version 2.
318: By default, no banner is displayed.
319: .It Cm ChallengeResponseAuthentication
1.50 jmc 320: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 321: All authentication styles from
322: .Xr login.conf 5
323: are supported.
324: The default is
325: .Dq yes .
1.80 djm 326: .It Cm ChrootDirectory
1.113 stevesk 327: Specifies the pathname of a directory to
1.80 djm 328: .Xr chroot 2
329: to after authentication.
1.113 stevesk 330: All components of the pathname must be root-owned directories that are
1.80 djm 331: not writable by any other user or group.
1.106 stevesk 332: After the chroot,
333: .Xr sshd 8
334: changes the working directory to the user's home directory.
1.80 djm 335: .Pp
1.113 stevesk 336: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 337: the connecting user has been authenticated: %% is replaced by a literal '%',
338: %h is replaced by the home directory of the user being authenticated, and
339: %u is replaced by the username of that user.
340: .Pp
341: The
342: .Cm ChrootDirectory
343: must contain the necessary files and directories to support the
1.103 stevesk 344: user's session.
1.80 djm 345: For an interactive session this requires at least a shell, typically
346: .Xr sh 1 ,
347: and basic
348: .Pa /dev
349: nodes such as
350: .Xr null 4 ,
351: .Xr zero 4 ,
352: .Xr stdin 4 ,
353: .Xr stdout 4 ,
354: .Xr stderr 4 ,
1.180 jmc 355: and
1.80 djm 356: .Xr tty 4
357: devices.
358: For file transfer sessions using
1.105 jmc 359: .Dq sftp ,
1.80 djm 360: no additional configuration of the environment is necessary if the
1.105 jmc 361: in-process sftp server is used,
1.176 schwarze 362: though sessions which use logging may require
1.104 stevesk 363: .Pa /dev/log
1.176 schwarze 364: inside the chroot directory on some operating systems (see
1.104 stevesk 365: .Xr sftp-server 8
1.81 jmc 366: for details).
1.80 djm 367: .Pp
368: The default is not to
369: .Xr chroot 2 .
1.1 stevesk 370: .It Cm Ciphers
371: Specifies the ciphers allowed for protocol version 2.
372: Multiple ciphers must be comma-separated.
1.167 djm 373: The supported ciphers are:
374: .Pp
1.173 naddy 375: .Bl -item -compact -offset indent
376: .It
377: 3des-cbc
378: .It
379: aes128-cbc
380: .It
381: aes192-cbc
382: .It
383: aes256-cbc
384: .It
385: aes128-ctr
386: .It
387: aes192-ctr
388: .It
389: aes256-ctr
390: .It
391: aes128-gcm@openssh.com
392: .It
393: aes256-gcm@openssh.com
394: .It
395: arcfour
396: .It
397: arcfour128
398: .It
399: arcfour256
400: .It
401: blowfish-cbc
402: .It
403: cast128-cbc
404: .It
405: chacha20-poly1305@openssh.com
406: .El
1.167 djm 407: .Pp
1.52 jmc 408: The default is:
1.173 naddy 409: .Bd -literal -offset indent
410: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 411: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 412: chacha20-poly1305@openssh.com
1.1 stevesk 413: .Ed
1.167 djm 414: .Pp
415: The list of available ciphers may also be obtained using the
416: .Fl Q
417: option of
418: .Xr ssh 1 .
1.1 stevesk 419: .It Cm ClientAliveCountMax
1.48 jmc 420: Sets the number of client alive messages (see below) which may be
1.1 stevesk 421: sent without
1.52 jmc 422: .Xr sshd 8
1.17 jmc 423: receiving any messages back from the client.
424: If this threshold is reached while client alive messages are being sent,
1.52 jmc 425: sshd will disconnect the client, terminating the session.
1.17 jmc 426: It is important to note that the use of client alive messages is very
427: different from
1.27 markus 428: .Cm TCPKeepAlive
1.17 jmc 429: (below).
430: The client alive messages are sent through the encrypted channel
431: and therefore will not be spoofable.
432: The TCP keepalive option enabled by
1.27 markus 433: .Cm TCPKeepAlive
1.17 jmc 434: is spoofable.
435: The client alive mechanism is valuable when the client or
1.1 stevesk 436: server depend on knowing when a connection has become inactive.
437: .Pp
1.17 jmc 438: The default value is 3.
439: If
1.1 stevesk 440: .Cm ClientAliveInterval
1.48 jmc 441: (see below) is set to 15, and
1.1 stevesk 442: .Cm ClientAliveCountMax
1.52 jmc 443: is left at the default, unresponsive SSH clients
1.1 stevesk 444: will be disconnected after approximately 45 seconds.
1.57 markus 445: This option applies to protocol version 2 only.
1.42 djm 446: .It Cm ClientAliveInterval
447: Sets a timeout interval in seconds after which if no data has been received
448: from the client,
1.52 jmc 449: .Xr sshd 8
1.42 djm 450: will send a message through the encrypted
451: channel to request a response from the client.
452: The default
453: is 0, indicating that these messages will not be sent to the client.
454: This option applies to protocol version 2 only.
1.3 markus 455: .It Cm Compression
1.44 markus 456: Specifies whether compression is allowed, or delayed until
457: the user has authenticated successfully.
1.3 markus 458: The argument must be
1.44 markus 459: .Dq yes ,
460: .Dq delayed ,
1.3 markus 461: or
462: .Dq no .
463: The default is
1.44 markus 464: .Dq delayed .
1.1 stevesk 465: .It Cm DenyGroups
466: This keyword can be followed by a list of group name patterns, separated
467: by spaces.
468: Login is disallowed for users whose primary group or supplementary
469: group list matches one of the patterns.
470: Only group names are valid; a numerical group ID is not recognized.
471: By default, login is allowed for all groups.
1.54 jmc 472: The allow/deny directives are processed in the following order:
473: .Cm DenyUsers ,
474: .Cm AllowUsers ,
475: .Cm DenyGroups ,
476: and finally
477: .Cm AllowGroups .
1.49 jmc 478: .Pp
1.161 jmc 479: See PATTERNS in
1.49 jmc 480: .Xr ssh_config 5
481: for more information on patterns.
1.1 stevesk 482: .It Cm DenyUsers
483: This keyword can be followed by a list of user name patterns, separated
484: by spaces.
485: Login is disallowed for user names that match one of the patterns.
486: Only user names are valid; a numerical user ID is not recognized.
487: By default, login is allowed for all users.
488: If the pattern takes the form USER@HOST then USER and HOST
489: are separately checked, restricting logins to particular
490: users from particular hosts.
1.54 jmc 491: The allow/deny directives are processed in the following order:
492: .Cm DenyUsers ,
493: .Cm AllowUsers ,
494: .Cm DenyGroups ,
495: and finally
496: .Cm AllowGroups .
1.49 jmc 497: .Pp
1.161 jmc 498: See PATTERNS in
1.49 jmc 499: .Xr ssh_config 5
500: for more information on patterns.
1.183 djm 501: .It Cm FingerprintHash
502: Specifies the hash algorithm used when logging key fingerprints.
503: Valid options are:
504: .Dq md5
505: and
506: .Dq sha256 .
507: The default is
508: .Dq sha256 .
1.67 dtucker 509: .It Cm ForceCommand
510: Forces the execution of the command specified by
511: .Cm ForceCommand ,
1.84 djm 512: ignoring any command supplied by the client and
513: .Pa ~/.ssh/rc
514: if present.
1.67 dtucker 515: The command is invoked by using the user's login shell with the -c option.
516: This applies to shell, command, or subsystem execution.
517: It is most useful inside a
518: .Cm Match
519: block.
520: The command originally supplied by the client is available in the
521: .Ev SSH_ORIGINAL_COMMAND
522: environment variable.
1.82 djm 523: Specifying a command of
524: .Dq internal-sftp
525: will force the use of an in-process sftp server that requires no support
526: files when used with
527: .Cm ChrootDirectory .
1.1 stevesk 528: .It Cm GatewayPorts
529: Specifies whether remote hosts are allowed to connect to ports
530: forwarded for the client.
531: By default,
1.52 jmc 532: .Xr sshd 8
1.15 jmc 533: binds remote port forwardings to the loopback address.
534: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 535: .Cm GatewayPorts
1.52 jmc 536: can be used to specify that sshd
1.39 djm 537: should allow remote port forwardings to bind to non-loopback addresses, thus
538: allowing other hosts to connect.
539: The argument may be
540: .Dq no
541: to force remote port forwardings to be available to the local host only,
1.1 stevesk 542: .Dq yes
1.39 djm 543: to force remote port forwardings to bind to the wildcard address, or
544: .Dq clientspecified
545: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 546: The default is
547: .Dq no .
1.23 markus 548: .It Cm GSSAPIAuthentication
1.25 markus 549: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 550: The default is
1.23 markus 551: .Dq no .
552: Note that this option applies to protocol version 2 only.
553: .It Cm GSSAPICleanupCredentials
554: Specifies whether to automatically destroy the user's credentials cache
555: on logout.
556: The default is
557: .Dq yes .
558: Note that this option applies to protocol version 2 only.
1.1 stevesk 559: .It Cm HostbasedAuthentication
560: Specifies whether rhosts or /etc/hosts.equiv authentication together
561: with successful public key client host authentication is allowed
1.50 jmc 562: (host-based authentication).
1.1 stevesk 563: This option is similar to
564: .Cm RhostsRSAAuthentication
565: and applies to protocol version 2 only.
1.70 dtucker 566: The default is
567: .Dq no .
568: .It Cm HostbasedUsesNameFromPacketOnly
569: Specifies whether or not the server will attempt to perform a reverse
570: name lookup when matching the name in the
571: .Pa ~/.shosts ,
572: .Pa ~/.rhosts ,
573: and
574: .Pa /etc/hosts.equiv
575: files during
576: .Cm HostbasedAuthentication .
577: A setting of
578: .Dq yes
579: means that
580: .Xr sshd 8
581: uses the name supplied by the client rather than
582: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 583: The default is
584: .Dq no .
1.117 djm 585: .It Cm HostCertificate
586: Specifies a file containing a public host certificate.
587: The certificate's public key must match a private host key already specified
588: by
589: .Cm HostKey .
590: The default behaviour of
591: .Xr sshd 8
592: is not to load any certificates.
1.1 stevesk 593: .It Cm HostKey
594: Specifies a file containing a private host key
595: used by SSH.
596: The default is
597: .Pa /etc/ssh/ssh_host_key
598: for protocol version 1, and
1.126 djm 599: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 600: .Pa /etc/ssh/ssh_host_ecdsa_key ,
601: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 602: and
1.1 stevesk 603: .Pa /etc/ssh/ssh_host_rsa_key
604: for protocol version 2.
605: Note that
1.52 jmc 606: .Xr sshd 8
1.1 stevesk 607: will refuse to use a file if it is group/world-accessible.
608: It is possible to have multiple host key files.
609: .Dq rsa1
610: keys are used for version 1 and
1.126 djm 611: .Dq dsa ,
1.169 naddy 612: .Dq ecdsa ,
613: .Dq ed25519
1.1 stevesk 614: or
615: .Dq rsa
616: are used for version 2 of the SSH protocol.
1.162 markus 617: It is also possible to specify public host key files instead.
618: In this case operations on the private key will be delegated
619: to an
620: .Xr ssh-agent 1 .
621: .It Cm HostKeyAgent
622: Identifies the UNIX-domain socket used to communicate
623: with an agent that has access to the private host keys.
624: If
625: .Dq SSH_AUTH_SOCK
626: is specified, the location of the socket will be read from the
627: .Ev SSH_AUTH_SOCK
628: environment variable.
1.1 stevesk 629: .It Cm IgnoreRhosts
630: Specifies that
631: .Pa .rhosts
632: and
633: .Pa .shosts
634: files will not be used in
635: .Cm RhostsRSAAuthentication
636: or
637: .Cm HostbasedAuthentication .
638: .Pp
639: .Pa /etc/hosts.equiv
640: and
641: .Pa /etc/shosts.equiv
642: are still used.
643: The default is
644: .Dq yes .
645: .It Cm IgnoreUserKnownHosts
646: Specifies whether
1.52 jmc 647: .Xr sshd 8
1.1 stevesk 648: should ignore the user's
1.41 djm 649: .Pa ~/.ssh/known_hosts
1.1 stevesk 650: during
651: .Cm RhostsRSAAuthentication
652: or
653: .Cm HostbasedAuthentication .
654: The default is
655: .Dq no .
1.129 djm 656: .It Cm IPQoS
657: Specifies the IPv4 type-of-service or DSCP class for the connection.
658: Accepted values are
659: .Dq af11 ,
660: .Dq af12 ,
661: .Dq af13 ,
1.136 djm 662: .Dq af21 ,
1.129 djm 663: .Dq af22 ,
664: .Dq af23 ,
665: .Dq af31 ,
666: .Dq af32 ,
667: .Dq af33 ,
668: .Dq af41 ,
669: .Dq af42 ,
670: .Dq af43 ,
671: .Dq cs0 ,
672: .Dq cs1 ,
673: .Dq cs2 ,
674: .Dq cs3 ,
675: .Dq cs4 ,
676: .Dq cs5 ,
677: .Dq cs6 ,
678: .Dq cs7 ,
679: .Dq ef ,
680: .Dq lowdelay ,
681: .Dq throughput ,
682: .Dq reliability ,
683: or a numeric value.
1.131 djm 684: This option may take one or two arguments, separated by whitespace.
1.129 djm 685: If one argument is specified, it is used as the packet class unconditionally.
686: If two values are specified, the first is automatically selected for
687: interactive sessions and the second for non-interactive sessions.
688: The default is
689: .Dq lowdelay
690: for interactive sessions and
691: .Dq throughput
692: for non-interactive sessions.
1.171 jmc 693: .It Cm KbdInteractiveAuthentication
694: Specifies whether to allow keyboard-interactive authentication.
695: The argument to this keyword must be
696: .Dq yes
697: or
698: .Dq no .
699: The default is to use whatever value
700: .Cm ChallengeResponseAuthentication
701: is set to
702: (by default
703: .Dq yes ) .
1.1 stevesk 704: .It Cm KerberosAuthentication
1.24 markus 705: Specifies whether the password provided by the user for
1.1 stevesk 706: .Cm PasswordAuthentication
1.24 markus 707: will be validated through the Kerberos KDC.
1.1 stevesk 708: To use this option, the server needs a
709: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 710: The default is
1.29 dtucker 711: .Dq no .
712: .It Cm KerberosGetAFSToken
1.45 djm 713: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 714: an AFS token before accessing the user's home directory.
1.52 jmc 715: The default is
1.1 stevesk 716: .Dq no .
717: .It Cm KerberosOrLocalPasswd
1.52 jmc 718: If password authentication through Kerberos fails then
1.1 stevesk 719: the password will be validated via any additional local mechanism
720: such as
721: .Pa /etc/passwd .
1.52 jmc 722: The default is
1.1 stevesk 723: .Dq yes .
724: .It Cm KerberosTicketCleanup
725: Specifies whether to automatically destroy the user's ticket cache
726: file on logout.
1.52 jmc 727: The default is
1.1 stevesk 728: .Dq yes .
1.127 djm 729: .It Cm KexAlgorithms
730: Specifies the available KEX (Key Exchange) algorithms.
731: Multiple algorithms must be comma-separated.
1.173 naddy 732: The supported algorithms are:
733: .Pp
734: .Bl -item -compact -offset indent
735: .It
736: curve25519-sha256@libssh.org
737: .It
738: diffie-hellman-group1-sha1
739: .It
740: diffie-hellman-group14-sha1
741: .It
742: diffie-hellman-group-exchange-sha1
743: .It
744: diffie-hellman-group-exchange-sha256
745: .It
746: ecdh-sha2-nistp256
747: .It
748: ecdh-sha2-nistp384
749: .It
750: ecdh-sha2-nistp521
751: .El
752: .Pp
753: The default is:
1.170 dtucker 754: .Bd -literal -offset indent
755: curve25519-sha256@libssh.org,
756: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
757: diffie-hellman-group-exchange-sha256,
1.173 naddy 758: diffie-hellman-group14-sha1
1.170 dtucker 759: .Ed
1.1 stevesk 760: .It Cm KeyRegenerationInterval
761: In protocol version 1, the ephemeral server key is automatically regenerated
762: after this many seconds (if it has been used).
763: The purpose of regeneration is to prevent
764: decrypting captured sessions by later breaking into the machine and
765: stealing the keys.
766: The key is never stored anywhere.
767: If the value is 0, the key is never regenerated.
768: The default is 3600 (seconds).
769: .It Cm ListenAddress
770: Specifies the local addresses
1.52 jmc 771: .Xr sshd 8
1.1 stevesk 772: should listen on.
773: The following forms may be used:
774: .Pp
775: .Bl -item -offset indent -compact
776: .It
777: .Cm ListenAddress
778: .Sm off
1.180 jmc 779: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 780: .Sm on
781: .It
782: .Cm ListenAddress
783: .Sm off
1.180 jmc 784: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 785: .Sm on
786: .It
787: .Cm ListenAddress
788: .Sm off
789: .Oo
1.180 jmc 790: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 791: .Sm on
792: .El
793: .Pp
794: If
795: .Ar port
796: is not specified,
1.52 jmc 797: sshd will listen on the address and all prior
1.1 stevesk 798: .Cm Port
1.17 jmc 799: options specified.
800: The default is to listen on all local addresses.
1.15 jmc 801: Multiple
1.1 stevesk 802: .Cm ListenAddress
1.17 jmc 803: options are permitted.
804: Additionally, any
1.1 stevesk 805: .Cm Port
1.52 jmc 806: options must precede this option for non-port qualified addresses.
1.1 stevesk 807: .It Cm LoginGraceTime
808: The server disconnects after this time if the user has not
809: successfully logged in.
810: If the value is 0, there is no time limit.
1.12 stevesk 811: The default is 120 seconds.
1.1 stevesk 812: .It Cm LogLevel
813: Gives the verbosity level that is used when logging messages from
1.53 jmc 814: .Xr sshd 8 .
1.1 stevesk 815: The possible values are:
1.52 jmc 816: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 817: The default is INFO.
818: DEBUG and DEBUG1 are equivalent.
819: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
820: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 821: .It Cm MACs
822: Specifies the available MAC (message authentication code) algorithms.
823: The MAC algorithm is used in protocol version 2
824: for data integrity protection.
825: Multiple algorithms must be comma-separated.
1.152 markus 826: The algorithms that contain
827: .Dq -etm
828: calculate the MAC after encryption (encrypt-then-mac).
829: These are considered safer and their use recommended.
1.173 naddy 830: The supported MACs are:
831: .Pp
832: .Bl -item -compact -offset indent
833: .It
834: hmac-md5
835: .It
836: hmac-md5-96
837: .It
838: hmac-ripemd160
839: .It
840: hmac-sha1
841: .It
842: hmac-sha1-96
843: .It
844: hmac-sha2-256
845: .It
846: hmac-sha2-512
847: .It
848: umac-64@openssh.com
849: .It
850: umac-128@openssh.com
851: .It
852: hmac-md5-etm@openssh.com
853: .It
854: hmac-md5-96-etm@openssh.com
855: .It
856: hmac-ripemd160-etm@openssh.com
857: .It
858: hmac-sha1-etm@openssh.com
859: .It
860: hmac-sha1-96-etm@openssh.com
861: .It
862: hmac-sha2-256-etm@openssh.com
863: .It
864: hmac-sha2-512-etm@openssh.com
865: .It
866: umac-64-etm@openssh.com
867: .It
868: umac-128-etm@openssh.com
869: .El
870: .Pp
1.52 jmc 871: The default is:
1.77 jmc 872: .Bd -literal -offset indent
1.152 markus 873: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
874: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 875: umac-64@openssh.com,umac-128@openssh.com,
876: hmac-sha2-256,hmac-sha2-512
1.77 jmc 877: .Ed
1.60 dtucker 878: .It Cm Match
1.61 jmc 879: Introduces a conditional block.
1.65 dtucker 880: If all of the criteria on the
1.60 dtucker 881: .Cm Match
1.65 dtucker 882: line are satisfied, the keywords on the following lines override those
883: set in the global section of the config file, until either another
1.60 dtucker 884: .Cm Match
1.65 dtucker 885: line or the end of the file.
1.172 djm 886: If a keyword appears in multiple
887: .Cm Match
1.177 sobrado 888: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 889: applied.
1.91 djm 890: .Pp
1.61 jmc 891: The arguments to
1.60 dtucker 892: .Cm Match
1.163 dtucker 893: are one or more criteria-pattern pairs or the single token
894: .Cm All
895: which matches all criteria.
1.60 dtucker 896: The available criteria are
897: .Cm User ,
1.69 dtucker 898: .Cm Group ,
1.60 dtucker 899: .Cm Host ,
1.139 dtucker 900: .Cm LocalAddress ,
901: .Cm LocalPort ,
1.60 dtucker 902: and
903: .Cm Address .
1.91 djm 904: The match patterns may consist of single entries or comma-separated
905: lists and may use the wildcard and negation operators described in the
1.161 jmc 906: PATTERNS section of
1.92 djm 907: .Xr ssh_config 5 .
1.91 djm 908: .Pp
909: The patterns in an
910: .Cm Address
911: criteria may additionally contain addresses to match in CIDR
1.93 jmc 912: address/masklen format, e.g.\&
1.91 djm 913: .Dq 192.0.2.0/24
914: or
915: .Dq 3ffe:ffff::/32 .
916: Note that the mask length provided must be consistent with the address -
917: it is an error to specify a mask length that is too long for the address
1.93 jmc 918: or one with bits set in this host portion of the address.
919: For example,
1.91 djm 920: .Dq 192.0.2.0/33
921: and
1.93 jmc 922: .Dq 192.0.2.0/8
1.91 djm 923: respectively.
924: .Pp
1.60 dtucker 925: Only a subset of keywords may be used on the lines following a
926: .Cm Match
927: keyword.
928: Available keywords are
1.142 jmc 929: .Cm AcceptEnv ,
1.99 okan 930: .Cm AllowAgentForwarding ,
1.142 jmc 931: .Cm AllowGroups ,
1.62 dtucker 932: .Cm AllowTcpForwarding ,
1.141 markus 933: .Cm AllowUsers ,
1.149 djm 934: .Cm AuthenticationMethods ,
1.146 djm 935: .Cm AuthorizedKeysCommand ,
936: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 937: .Cm AuthorizedKeysFile ,
1.123 djm 938: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 939: .Cm Banner ,
1.85 djm 940: .Cm ChrootDirectory ,
1.141 markus 941: .Cm DenyGroups ,
942: .Cm DenyUsers ,
1.67 dtucker 943: .Cm ForceCommand ,
1.142 jmc 944: .Cm GatewayPorts ,
1.141 markus 945: .Cm GSSAPIAuthentication ,
1.87 djm 946: .Cm HostbasedAuthentication ,
1.123 djm 947: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 948: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 949: .Cm KerberosAuthentication ,
1.95 dtucker 950: .Cm MaxAuthTries ,
1.94 dtucker 951: .Cm MaxSessions ,
1.72 dtucker 952: .Cm PasswordAuthentication ,
1.97 djm 953: .Cm PermitEmptyPasswords ,
1.66 dtucker 954: .Cm PermitOpen ,
1.79 dtucker 955: .Cm PermitRootLogin ,
1.164 djm 956: .Cm PermitTTY ,
1.123 djm 957: .Cm PermitTunnel ,
1.174 djm 958: .Cm PermitUserRC ,
1.107 dtucker 959: .Cm PubkeyAuthentication ,
1.159 dtucker 960: .Cm RekeyLimit ,
1.142 jmc 961: .Cm RhostsRSAAuthentication ,
1.141 markus 962: .Cm RSAAuthentication ,
1.66 dtucker 963: .Cm X11DisplayOffset ,
1.101 djm 964: .Cm X11Forwarding
1.60 dtucker 965: and
1.102 djm 966: .Cm X11UseLocalHost .
1.33 dtucker 967: .It Cm MaxAuthTries
968: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 969: connection.
970: Once the number of failures reaches half this value,
971: additional failures are logged.
972: The default is 6.
1.90 djm 973: .It Cm MaxSessions
974: Specifies the maximum number of open sessions permitted per network connection.
975: The default is 10.
1.1 stevesk 976: .It Cm MaxStartups
977: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 978: SSH daemon.
1.1 stevesk 979: Additional connections will be dropped until authentication succeeds or the
980: .Cm LoginGraceTime
981: expires for a connection.
1.156 dtucker 982: The default is 10:30:100.
1.1 stevesk 983: .Pp
984: Alternatively, random early drop can be enabled by specifying
985: the three colon separated values
986: .Dq start:rate:full
1.51 jmc 987: (e.g. "10:30:60").
1.53 jmc 988: .Xr sshd 8
1.1 stevesk 989: will refuse connection attempts with a probability of
990: .Dq rate/100
991: (30%)
992: if there are currently
993: .Dq start
994: (10)
995: unauthenticated connections.
996: The probability increases linearly and all connection attempts
997: are refused if the number of unauthenticated connections reaches
998: .Dq full
999: (60).
1000: .It Cm PasswordAuthentication
1001: Specifies whether password authentication is allowed.
1002: The default is
1003: .Dq yes .
1004: .It Cm PermitEmptyPasswords
1005: When password authentication is allowed, it specifies whether the
1006: server allows login to accounts with empty password strings.
1007: The default is
1008: .Dq no .
1.62 dtucker 1009: .It Cm PermitOpen
1010: Specifies the destinations to which TCP port forwarding is permitted.
1011: The forwarding specification must be one of the following forms:
1012: .Pp
1013: .Bl -item -offset indent -compact
1014: .It
1015: .Cm PermitOpen
1016: .Sm off
1017: .Ar host : port
1018: .Sm on
1019: .It
1020: .Cm PermitOpen
1021: .Sm off
1022: .Ar IPv4_addr : port
1023: .Sm on
1024: .It
1025: .Cm PermitOpen
1026: .Sm off
1027: .Ar \&[ IPv6_addr \&] : port
1028: .Sm on
1029: .El
1030: .Pp
1.68 dtucker 1031: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1032: An argument of
1033: .Dq any
1034: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1035: An argument of
1036: .Dq none
1037: can be used to prohibit all forwarding requests.
1.63 jmc 1038: By default all port forwarding requests are permitted.
1.1 stevesk 1039: .It Cm PermitRootLogin
1.38 jmc 1040: Specifies whether root can log in using
1.1 stevesk 1041: .Xr ssh 1 .
1042: The argument must be
1043: .Dq yes ,
1044: .Dq without-password ,
1.52 jmc 1045: .Dq forced-commands-only ,
1.1 stevesk 1046: or
1047: .Dq no .
1048: The default is
1049: .Dq yes .
1050: .Pp
1051: If this option is set to
1.52 jmc 1052: .Dq without-password ,
1.1 stevesk 1053: password authentication is disabled for root.
1054: .Pp
1055: If this option is set to
1.52 jmc 1056: .Dq forced-commands-only ,
1.1 stevesk 1057: root login with public key authentication will be allowed,
1058: but only if the
1059: .Ar command
1060: option has been specified
1061: (which may be useful for taking remote backups even if root login is
1.17 jmc 1062: normally not allowed).
1063: All other authentication methods are disabled for root.
1.1 stevesk 1064: .Pp
1065: If this option is set to
1.52 jmc 1066: .Dq no ,
1.38 jmc 1067: root is not allowed to log in.
1.46 reyk 1068: .It Cm PermitTunnel
1069: Specifies whether
1070: .Xr tun 4
1071: device forwarding is allowed.
1.47 reyk 1072: The argument must be
1073: .Dq yes ,
1.58 stevesk 1074: .Dq point-to-point
1075: (layer 3),
1076: .Dq ethernet
1077: (layer 2), or
1.47 reyk 1078: .Dq no .
1.58 stevesk 1079: Specifying
1080: .Dq yes
1081: permits both
1082: .Dq point-to-point
1083: and
1084: .Dq ethernet .
1.46 reyk 1085: The default is
1086: .Dq no .
1.178 djm 1087: .Pp
1088: Independent of this setting, the permissions of the selected
1089: .Xr tun 4
1090: device must allow access to the user.
1.164 djm 1091: .It Cm PermitTTY
1092: Specifies whether
1.165 jmc 1093: .Xr pty 4
1.164 djm 1094: allocation is permitted.
1095: The default is
1096: .Dq yes .
1.6 markus 1097: .It Cm PermitUserEnvironment
1098: Specifies whether
1099: .Pa ~/.ssh/environment
1.9 stevesk 1100: and
1.6 markus 1101: .Cm environment=
1102: options in
1103: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1104: are processed by
1.52 jmc 1105: .Xr sshd 8 .
1.6 markus 1106: The default is
1107: .Dq no .
1.9 stevesk 1108: Enabling environment processing may enable users to bypass access
1109: restrictions in some configurations using mechanisms such as
1110: .Ev LD_PRELOAD .
1.174 djm 1111: .It Cm PermitUserRC
1112: Specifies whether any
1113: .Pa ~/.ssh/rc
1114: file is executed.
1115: The default is
1116: .Dq yes .
1.1 stevesk 1117: .It Cm PidFile
1.4 stevesk 1118: Specifies the file that contains the process ID of the
1.53 jmc 1119: SSH daemon.
1.1 stevesk 1120: The default is
1121: .Pa /var/run/sshd.pid .
1122: .It Cm Port
1123: Specifies the port number that
1.52 jmc 1124: .Xr sshd 8
1.1 stevesk 1125: listens on.
1126: The default is 22.
1127: Multiple options of this type are permitted.
1128: See also
1129: .Cm ListenAddress .
1130: .It Cm PrintLastLog
1131: Specifies whether
1.52 jmc 1132: .Xr sshd 8
1.36 jaredy 1133: should print the date and time of the last user login when a user logs
1134: in interactively.
1.1 stevesk 1135: The default is
1136: .Dq yes .
1137: .It Cm PrintMotd
1138: Specifies whether
1.52 jmc 1139: .Xr sshd 8
1.1 stevesk 1140: should print
1141: .Pa /etc/motd
1142: when a user logs in interactively.
1143: (On some systems it is also printed by the shell,
1144: .Pa /etc/profile ,
1145: or equivalent.)
1146: The default is
1147: .Dq yes .
1148: .It Cm Protocol
1149: Specifies the protocol versions
1.52 jmc 1150: .Xr sshd 8
1.5 stevesk 1151: supports.
1.1 stevesk 1152: The possible values are
1.52 jmc 1153: .Sq 1
1.1 stevesk 1154: and
1.52 jmc 1155: .Sq 2 .
1.1 stevesk 1156: Multiple versions must be comma-separated.
1157: The default is
1.109 jmc 1158: .Sq 2 .
1.5 stevesk 1159: Note that the order of the protocol list does not indicate preference,
1160: because the client selects among multiple protocol versions offered
1161: by the server.
1162: Specifying
1163: .Dq 2,1
1164: is identical to
1165: .Dq 1,2 .
1.1 stevesk 1166: .It Cm PubkeyAuthentication
1167: Specifies whether public key authentication is allowed.
1168: The default is
1169: .Dq yes .
1170: Note that this option applies to protocol version 2 only.
1.159 dtucker 1171: .It Cm RekeyLimit
1172: Specifies the maximum amount of data that may be transmitted before the
1173: session key is renegotiated, optionally followed a maximum amount of
1174: time that may pass before the session key is renegotiated.
1175: The first argument is specified in bytes and may have a suffix of
1176: .Sq K ,
1177: .Sq M ,
1178: or
1179: .Sq G
1180: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1181: The default is between
1182: .Sq 1G
1183: and
1184: .Sq 4G ,
1185: depending on the cipher.
1186: The optional second value is specified in seconds and may use any of the
1187: units documented in the
1188: .Sx TIME FORMATS
1.160 jmc 1189: section.
1.159 dtucker 1190: The default value for
1191: .Cm RekeyLimit
1192: is
1193: .Dq default none ,
1194: which means that rekeying is performed after the cipher's default amount
1195: of data has been sent or received and no time based rekeying is done.
1196: This option applies to protocol version 2 only.
1.118 djm 1197: .It Cm RevokedKeys
1.154 djm 1198: Specifies revoked public keys.
1.118 djm 1199: Keys listed in this file will be refused for public key authentication.
1200: Note that if this file is not readable, then public key authentication will
1201: be refused for all users.
1.154 djm 1202: Keys may be specified as a text file, listing one public key per line, or as
1203: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1204: .Xr ssh-keygen 1 .
1.161 jmc 1205: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1206: .Xr ssh-keygen 1 .
1.1 stevesk 1207: .It Cm RhostsRSAAuthentication
1208: Specifies whether rhosts or /etc/hosts.equiv authentication together
1209: with successful RSA host authentication is allowed.
1210: The default is
1211: .Dq no .
1212: This option applies to protocol version 1 only.
1213: .It Cm RSAAuthentication
1214: Specifies whether pure RSA authentication is allowed.
1215: The default is
1216: .Dq yes .
1217: This option applies to protocol version 1 only.
1218: .It Cm ServerKeyBits
1219: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1220: The minimum value is 512, and the default is 1024.
1.175 millert 1221: .It Cm StreamLocalBindMask
1222: Sets the octal file creation mode mask
1223: .Pq umask
1224: used when creating a Unix-domain socket file for local or remote
1225: port forwarding.
1226: This option is only used for port forwarding to a Unix-domain socket file.
1227: .Pp
1228: The default value is 0177, which creates a Unix-domain socket file that is
1229: readable and writable only by the owner.
1230: Note that not all operating systems honor the file mode on Unix-domain
1231: socket files.
1232: .It Cm StreamLocalBindUnlink
1233: Specifies whether to remove an existing Unix-domain socket file for local
1234: or remote port forwarding before creating a new one.
1235: If the socket file already exists and
1236: .Cm StreamLocalBindUnlink
1237: is not enabled,
1238: .Nm sshd
1239: will be unable to forward the port to the Unix-domain socket file.
1240: This option is only used for port forwarding to a Unix-domain socket file.
1241: .Pp
1242: The argument must be
1243: .Dq yes
1244: or
1245: .Dq no .
1246: The default is
1247: .Dq no .
1.1 stevesk 1248: .It Cm StrictModes
1249: Specifies whether
1.52 jmc 1250: .Xr sshd 8
1.1 stevesk 1251: should check file modes and ownership of the
1252: user's files and home directory before accepting login.
1253: This is normally desirable because novices sometimes accidentally leave their
1254: directory or files world-writable.
1255: The default is
1256: .Dq yes .
1.112 djm 1257: Note that this does not apply to
1258: .Cm ChrootDirectory ,
1259: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1260: .It Cm Subsystem
1.51 jmc 1261: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1262: Arguments should be a subsystem name and a command (with optional arguments)
1263: to execute upon subsystem request.
1.80 djm 1264: .Pp
1.1 stevesk 1265: The command
1266: .Xr sftp-server 8
1267: implements the
1268: .Dq sftp
1269: file transfer subsystem.
1.80 djm 1270: .Pp
1271: Alternately the name
1272: .Dq internal-sftp
1273: implements an in-process
1274: .Dq sftp
1275: server.
1276: This may simplify configurations using
1277: .Cm ChrootDirectory
1278: to force a different filesystem root on clients.
1279: .Pp
1.1 stevesk 1280: By default no subsystems are defined.
1281: Note that this option applies to protocol version 2 only.
1282: .It Cm SyslogFacility
1283: Gives the facility code that is used when logging messages from
1.53 jmc 1284: .Xr sshd 8 .
1.1 stevesk 1285: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1286: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1287: The default is AUTH.
1.27 markus 1288: .It Cm TCPKeepAlive
1289: Specifies whether the system should send TCP keepalive messages to the
1290: other side.
1291: If they are sent, death of the connection or crash of one
1292: of the machines will be properly noticed.
1293: However, this means that
1294: connections will die if the route is down temporarily, and some people
1295: find it annoying.
1296: On the other hand, if TCP keepalives are not sent,
1297: sessions may hang indefinitely on the server, leaving
1298: .Dq ghost
1299: users and consuming server resources.
1300: .Pp
1301: The default is
1302: .Dq yes
1303: (to send TCP keepalive messages), and the server will notice
1304: if the network goes down or the client host crashes.
1305: This avoids infinitely hanging sessions.
1306: .Pp
1307: To disable TCP keepalive messages, the value should be set to
1308: .Dq no .
1.118 djm 1309: .It Cm TrustedUserCAKeys
1310: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1311: trusted to sign user certificates for authentication.
1.119 jmc 1312: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1313: .Ql #
1314: are allowed.
1315: If a certificate is presented for authentication and has its signing CA key
1316: listed in this file, then it may be used for authentication for any user
1317: listed in the certificate's principals list.
1318: Note that certificates that lack a list of principals will not be permitted
1319: for authentication using
1320: .Cm TrustedUserCAKeys .
1.161 jmc 1321: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1322: .Xr ssh-keygen 1 .
1.18 markus 1323: .It Cm UseDNS
1324: Specifies whether
1.52 jmc 1325: .Xr sshd 8
1.40 jmc 1326: should look up the remote host name and check that
1.18 markus 1327: the resolved host name for the remote IP address maps back to the
1328: very same IP address.
1329: The default is
1330: .Dq yes .
1.1 stevesk 1331: .It Cm UseLogin
1332: Specifies whether
1333: .Xr login 1
1334: is used for interactive login sessions.
1335: The default is
1336: .Dq no .
1337: Note that
1338: .Xr login 1
1339: is never used for remote command execution.
1340: Note also, that if this is enabled,
1341: .Cm X11Forwarding
1342: will be disabled because
1343: .Xr login 1
1344: does not know how to handle
1345: .Xr xauth 1
1.15 jmc 1346: cookies.
1347: If
1.1 stevesk 1348: .Cm UsePrivilegeSeparation
1349: is specified, it will be disabled after authentication.
1350: .It Cm UsePrivilegeSeparation
1351: Specifies whether
1.52 jmc 1352: .Xr sshd 8
1.2 stevesk 1353: separates privileges by creating an unprivileged child process
1.15 jmc 1354: to deal with incoming network traffic.
1355: After successful authentication, another process will be created that has
1356: the privilege of the authenticated user.
1357: The goal of privilege separation is to prevent privilege
1.1 stevesk 1358: escalation by containing any corruption within the unprivileged processes.
1359: The default is
1360: .Dq yes .
1.134 djm 1361: If
1362: .Cm UsePrivilegeSeparation
1363: is set to
1364: .Dq sandbox
1365: then the pre-authentication unprivileged process is subject to additional
1366: restrictions.
1.137 djm 1367: .It Cm VersionAddendum
1368: Optionally specifies additional text to append to the SSH protocol banner
1369: sent by the server upon connection.
1370: The default is
1371: .Dq none .
1.1 stevesk 1372: .It Cm X11DisplayOffset
1373: Specifies the first display number available for
1.52 jmc 1374: .Xr sshd 8 Ns 's
1.1 stevesk 1375: X11 forwarding.
1.52 jmc 1376: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1377: The default is 10.
1378: .It Cm X11Forwarding
1379: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1380: The argument must be
1381: .Dq yes
1382: or
1383: .Dq no .
1.1 stevesk 1384: The default is
1385: .Dq no .
1.13 stevesk 1386: .Pp
1387: When X11 forwarding is enabled, there may be additional exposure to
1388: the server and to client displays if the
1.52 jmc 1389: .Xr sshd 8
1.13 stevesk 1390: proxy display is configured to listen on the wildcard address (see
1391: .Cm X11UseLocalhost
1.52 jmc 1392: below), though this is not the default.
1.13 stevesk 1393: Additionally, the authentication spoofing and authentication data
1394: verification and substitution occur on the client side.
1395: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1396: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1397: forwarding (see the warnings for
1398: .Cm ForwardX11
1399: in
1.19 jmc 1400: .Xr ssh_config 5 ) .
1.13 stevesk 1401: A system administrator may have a stance in which they want to
1402: protect clients that may expose themselves to attack by unwittingly
1403: requesting X11 forwarding, which can warrant a
1404: .Dq no
1405: setting.
1406: .Pp
1407: Note that disabling X11 forwarding does not prevent users from
1408: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1409: X11 forwarding is automatically disabled if
1410: .Cm UseLogin
1411: is enabled.
1412: .It Cm X11UseLocalhost
1413: Specifies whether
1.52 jmc 1414: .Xr sshd 8
1.1 stevesk 1415: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1416: the wildcard address.
1417: By default,
1.52 jmc 1418: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1419: hostname part of the
1420: .Ev DISPLAY
1421: environment variable to
1422: .Dq localhost .
1.8 stevesk 1423: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1424: However, some older X11 clients may not function with this
1425: configuration.
1426: .Cm X11UseLocalhost
1427: may be set to
1428: .Dq no
1429: to specify that the forwarding server should be bound to the wildcard
1430: address.
1431: The argument must be
1432: .Dq yes
1433: or
1434: .Dq no .
1435: The default is
1436: .Dq yes .
1437: .It Cm XAuthLocation
1.11 stevesk 1438: Specifies the full pathname of the
1.1 stevesk 1439: .Xr xauth 1
1440: program.
1441: The default is
1442: .Pa /usr/X11R6/bin/xauth .
1443: .El
1.55 jmc 1444: .Sh TIME FORMATS
1.53 jmc 1445: .Xr sshd 8
1.1 stevesk 1446: command-line arguments and configuration file options that specify time
1447: may be expressed using a sequence of the form:
1448: .Sm off
1.7 stevesk 1449: .Ar time Op Ar qualifier ,
1.1 stevesk 1450: .Sm on
1451: where
1452: .Ar time
1453: is a positive integer value and
1454: .Ar qualifier
1455: is one of the following:
1456: .Pp
1457: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1458: .It Aq Cm none
1.1 stevesk 1459: seconds
1460: .It Cm s | Cm S
1461: seconds
1462: .It Cm m | Cm M
1463: minutes
1464: .It Cm h | Cm H
1465: hours
1466: .It Cm d | Cm D
1467: days
1468: .It Cm w | Cm W
1469: weeks
1470: .El
1471: .Pp
1472: Each member of the sequence is added together to calculate
1473: the total time value.
1474: .Pp
1475: Time format examples:
1476: .Pp
1477: .Bl -tag -width Ds -compact -offset indent
1478: .It 600
1479: 600 seconds (10 minutes)
1480: .It 10m
1481: 10 minutes
1482: .It 1h30m
1483: 1 hour 30 minutes (90 minutes)
1484: .El
1485: .Sh FILES
1486: .Bl -tag -width Ds
1487: .It Pa /etc/ssh/sshd_config
1488: Contains configuration data for
1.53 jmc 1489: .Xr sshd 8 .
1.1 stevesk 1490: This file should be writable by root only, but it is recommended
1491: (though not necessary) that it be world-readable.
1492: .El
1.19 jmc 1493: .Sh SEE ALSO
1494: .Xr sshd 8
1.1 stevesk 1495: .Sh AUTHORS
1496: OpenSSH is a derivative of the original and free
1497: ssh 1.2.12 release by Tatu Ylonen.
1498: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1499: Theo de Raadt and Dug Song
1500: removed many bugs, re-added newer features and
1501: created OpenSSH.
1502: Markus Friedl contributed the support for SSH
1503: protocol versions 1.5 and 2.0.
1504: Niels Provos and Markus Friedl contributed support
1505: for privilege separation.
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