Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.187
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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1.187 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.186 2014/12/22 08:04:23 djm Exp $
1.186 djm 37: .Dd $Mdocdate: December 22 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
1.187 ! jmc 215: method is listed more than once,
1.185 djm 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.
1.186 djm 247: If
1.181 djm 248: .Cm AuthorizedKeysCommand
1.186 djm 249: is specified but
250: .Cm AuthorizedKeysCommandUser
251: is not, then
252: .Xr sshd 8
253: will refuse to start.
1.1 stevesk 254: .It Cm AuthorizedKeysFile
255: Specifies the file that contains the public keys that can be used
256: for user authentication.
1.124 djm 257: The format is described in the
1.161 jmc 258: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 259: section of
260: .Xr sshd 8 .
1.1 stevesk 261: .Cm AuthorizedKeysFile
262: may contain tokens of the form %T which are substituted during connection
1.52 jmc 263: setup.
1.17 jmc 264: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 265: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 266: %u is replaced by the username of that user.
267: After expansion,
268: .Cm AuthorizedKeysFile
269: is taken to be an absolute path or one relative to the user's home
270: directory.
1.133 jmc 271: Multiple files may be listed, separated by whitespace.
272: The default is
273: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 274: .It Cm AuthorizedPrincipalsFile
275: Specifies a file that lists principal names that are accepted for
276: certificate authentication.
277: When using certificates signed by a key listed in
278: .Cm TrustedUserCAKeys ,
279: this file lists names, one of which must appear in the certificate for it
280: to be accepted for authentication.
1.125 jmc 281: Names are listed one per line preceded by key options (as described
1.161 jmc 282: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 283: .Xr sshd 8 ) .
1.124 djm 284: Empty lines and comments starting with
1.121 djm 285: .Ql #
286: are ignored.
287: .Pp
288: .Cm AuthorizedPrincipalsFile
289: may contain tokens of the form %T which are substituted during connection
290: setup.
291: The following tokens are defined: %% is replaced by a literal '%',
292: %h is replaced by the home directory of the user being authenticated, and
293: %u is replaced by the username of that user.
294: After expansion,
295: .Cm AuthorizedPrincipalsFile
296: is taken to be an absolute path or one relative to the user's home
297: directory.
298: .Pp
1.138 djm 299: The default is
300: .Dq none ,
301: i.e. not to use a principals file \(en in this case, the username
1.121 djm 302: of the user must appear in a certificate's principals list for it to be
303: accepted.
304: Note that
305: .Cm AuthorizedPrincipalsFile
306: is only used when authentication proceeds using a CA listed in
307: .Cm TrustedUserCAKeys
308: and is not consulted for certification authorities trusted via
309: .Pa ~/.ssh/authorized_keys ,
310: though the
311: .Cm principals=
312: key option offers a similar facility (see
313: .Xr sshd 8
314: for details).
1.1 stevesk 315: .It Cm Banner
316: The contents of the specified file are sent to the remote user before
317: authentication is allowed.
1.78 djm 318: If the argument is
319: .Dq none
320: then no banner is displayed.
1.1 stevesk 321: This option is only available for protocol version 2.
322: By default, no banner is displayed.
323: .It Cm ChallengeResponseAuthentication
1.50 jmc 324: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 325: All authentication styles from
326: .Xr login.conf 5
327: are supported.
328: The default is
329: .Dq yes .
1.80 djm 330: .It Cm ChrootDirectory
1.113 stevesk 331: Specifies the pathname of a directory to
1.80 djm 332: .Xr chroot 2
333: to after authentication.
1.113 stevesk 334: All components of the pathname must be root-owned directories that are
1.80 djm 335: not writable by any other user or group.
1.106 stevesk 336: After the chroot,
337: .Xr sshd 8
338: changes the working directory to the user's home directory.
1.80 djm 339: .Pp
1.113 stevesk 340: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 341: the connecting user has been authenticated: %% is replaced by a literal '%',
342: %h is replaced by the home directory of the user being authenticated, and
343: %u is replaced by the username of that user.
344: .Pp
345: The
346: .Cm ChrootDirectory
347: must contain the necessary files and directories to support the
1.103 stevesk 348: user's session.
1.80 djm 349: For an interactive session this requires at least a shell, typically
350: .Xr sh 1 ,
351: and basic
352: .Pa /dev
353: nodes such as
354: .Xr null 4 ,
355: .Xr zero 4 ,
356: .Xr stdin 4 ,
357: .Xr stdout 4 ,
358: .Xr stderr 4 ,
1.180 jmc 359: and
1.80 djm 360: .Xr tty 4
361: devices.
362: For file transfer sessions using
1.105 jmc 363: .Dq sftp ,
1.80 djm 364: no additional configuration of the environment is necessary if the
1.105 jmc 365: in-process sftp server is used,
1.176 schwarze 366: though sessions which use logging may require
1.104 stevesk 367: .Pa /dev/log
1.176 schwarze 368: inside the chroot directory on some operating systems (see
1.104 stevesk 369: .Xr sftp-server 8
1.81 jmc 370: for details).
1.80 djm 371: .Pp
372: The default is not to
373: .Xr chroot 2 .
1.1 stevesk 374: .It Cm Ciphers
375: Specifies the ciphers allowed for protocol version 2.
376: Multiple ciphers must be comma-separated.
1.167 djm 377: The supported ciphers are:
378: .Pp
1.173 naddy 379: .Bl -item -compact -offset indent
380: .It
381: 3des-cbc
382: .It
383: aes128-cbc
384: .It
385: aes192-cbc
386: .It
387: aes256-cbc
388: .It
389: aes128-ctr
390: .It
391: aes192-ctr
392: .It
393: aes256-ctr
394: .It
395: aes128-gcm@openssh.com
396: .It
397: aes256-gcm@openssh.com
398: .It
399: arcfour
400: .It
401: arcfour128
402: .It
403: arcfour256
404: .It
405: blowfish-cbc
406: .It
407: cast128-cbc
408: .It
409: chacha20-poly1305@openssh.com
410: .El
1.167 djm 411: .Pp
1.52 jmc 412: The default is:
1.173 naddy 413: .Bd -literal -offset indent
414: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 415: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 416: chacha20-poly1305@openssh.com
1.1 stevesk 417: .Ed
1.167 djm 418: .Pp
419: The list of available ciphers may also be obtained using the
420: .Fl Q
421: option of
422: .Xr ssh 1 .
1.1 stevesk 423: .It Cm ClientAliveCountMax
1.48 jmc 424: Sets the number of client alive messages (see below) which may be
1.1 stevesk 425: sent without
1.52 jmc 426: .Xr sshd 8
1.17 jmc 427: receiving any messages back from the client.
428: If this threshold is reached while client alive messages are being sent,
1.52 jmc 429: sshd will disconnect the client, terminating the session.
1.17 jmc 430: It is important to note that the use of client alive messages is very
431: different from
1.27 markus 432: .Cm TCPKeepAlive
1.17 jmc 433: (below).
434: The client alive messages are sent through the encrypted channel
435: and therefore will not be spoofable.
436: The TCP keepalive option enabled by
1.27 markus 437: .Cm TCPKeepAlive
1.17 jmc 438: is spoofable.
439: The client alive mechanism is valuable when the client or
1.1 stevesk 440: server depend on knowing when a connection has become inactive.
441: .Pp
1.17 jmc 442: The default value is 3.
443: If
1.1 stevesk 444: .Cm ClientAliveInterval
1.48 jmc 445: (see below) is set to 15, and
1.1 stevesk 446: .Cm ClientAliveCountMax
1.52 jmc 447: is left at the default, unresponsive SSH clients
1.1 stevesk 448: will be disconnected after approximately 45 seconds.
1.57 markus 449: This option applies to protocol version 2 only.
1.42 djm 450: .It Cm ClientAliveInterval
451: Sets a timeout interval in seconds after which if no data has been received
452: from the client,
1.52 jmc 453: .Xr sshd 8
1.42 djm 454: will send a message through the encrypted
455: channel to request a response from the client.
456: The default
457: is 0, indicating that these messages will not be sent to the client.
458: This option applies to protocol version 2 only.
1.3 markus 459: .It Cm Compression
1.44 markus 460: Specifies whether compression is allowed, or delayed until
461: the user has authenticated successfully.
1.3 markus 462: The argument must be
1.44 markus 463: .Dq yes ,
464: .Dq delayed ,
1.3 markus 465: or
466: .Dq no .
467: The default is
1.44 markus 468: .Dq delayed .
1.1 stevesk 469: .It Cm DenyGroups
470: This keyword can be followed by a list of group name patterns, separated
471: by spaces.
472: Login is disallowed for users whose primary group or supplementary
473: group list matches one of the patterns.
474: Only group names are valid; a numerical group ID is not recognized.
475: By default, login is allowed for all groups.
1.54 jmc 476: The allow/deny directives are processed in the following order:
477: .Cm DenyUsers ,
478: .Cm AllowUsers ,
479: .Cm DenyGroups ,
480: and finally
481: .Cm AllowGroups .
1.49 jmc 482: .Pp
1.161 jmc 483: See PATTERNS in
1.49 jmc 484: .Xr ssh_config 5
485: for more information on patterns.
1.1 stevesk 486: .It Cm DenyUsers
487: This keyword can be followed by a list of user name patterns, separated
488: by spaces.
489: Login is disallowed for user names that match one of the patterns.
490: Only user names are valid; a numerical user ID is not recognized.
491: By default, login is allowed for all users.
492: If the pattern takes the form USER@HOST then USER and HOST
493: are separately checked, restricting logins to particular
494: users from particular hosts.
1.54 jmc 495: The allow/deny directives are processed in the following order:
496: .Cm DenyUsers ,
497: .Cm AllowUsers ,
498: .Cm DenyGroups ,
499: and finally
500: .Cm AllowGroups .
1.49 jmc 501: .Pp
1.161 jmc 502: See PATTERNS in
1.49 jmc 503: .Xr ssh_config 5
504: for more information on patterns.
1.183 djm 505: .It Cm FingerprintHash
506: Specifies the hash algorithm used when logging key fingerprints.
507: Valid options are:
508: .Dq md5
509: and
510: .Dq sha256 .
511: The default is
512: .Dq sha256 .
1.67 dtucker 513: .It Cm ForceCommand
514: Forces the execution of the command specified by
515: .Cm ForceCommand ,
1.84 djm 516: ignoring any command supplied by the client and
517: .Pa ~/.ssh/rc
518: if present.
1.67 dtucker 519: The command is invoked by using the user's login shell with the -c option.
520: This applies to shell, command, or subsystem execution.
521: It is most useful inside a
522: .Cm Match
523: block.
524: The command originally supplied by the client is available in the
525: .Ev SSH_ORIGINAL_COMMAND
526: environment variable.
1.82 djm 527: Specifying a command of
528: .Dq internal-sftp
529: will force the use of an in-process sftp server that requires no support
530: files when used with
531: .Cm ChrootDirectory .
1.1 stevesk 532: .It Cm GatewayPorts
533: Specifies whether remote hosts are allowed to connect to ports
534: forwarded for the client.
535: By default,
1.52 jmc 536: .Xr sshd 8
1.15 jmc 537: binds remote port forwardings to the loopback address.
538: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 539: .Cm GatewayPorts
1.52 jmc 540: can be used to specify that sshd
1.39 djm 541: should allow remote port forwardings to bind to non-loopback addresses, thus
542: allowing other hosts to connect.
543: The argument may be
544: .Dq no
545: to force remote port forwardings to be available to the local host only,
1.1 stevesk 546: .Dq yes
1.39 djm 547: to force remote port forwardings to bind to the wildcard address, or
548: .Dq clientspecified
549: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 550: The default is
551: .Dq no .
1.23 markus 552: .It Cm GSSAPIAuthentication
1.25 markus 553: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 554: The default is
1.23 markus 555: .Dq no .
556: Note that this option applies to protocol version 2 only.
557: .It Cm GSSAPICleanupCredentials
558: Specifies whether to automatically destroy the user's credentials cache
559: on logout.
560: The default is
561: .Dq yes .
562: Note that this option applies to protocol version 2 only.
1.1 stevesk 563: .It Cm HostbasedAuthentication
564: Specifies whether rhosts or /etc/hosts.equiv authentication together
565: with successful public key client host authentication is allowed
1.50 jmc 566: (host-based authentication).
1.1 stevesk 567: This option is similar to
568: .Cm RhostsRSAAuthentication
569: and applies to protocol version 2 only.
1.70 dtucker 570: The default is
571: .Dq no .
572: .It Cm HostbasedUsesNameFromPacketOnly
573: Specifies whether or not the server will attempt to perform a reverse
574: name lookup when matching the name in the
575: .Pa ~/.shosts ,
576: .Pa ~/.rhosts ,
577: and
578: .Pa /etc/hosts.equiv
579: files during
580: .Cm HostbasedAuthentication .
581: A setting of
582: .Dq yes
583: means that
584: .Xr sshd 8
585: uses the name supplied by the client rather than
586: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 587: The default is
588: .Dq no .
1.117 djm 589: .It Cm HostCertificate
590: Specifies a file containing a public host certificate.
591: The certificate's public key must match a private host key already specified
592: by
593: .Cm HostKey .
594: The default behaviour of
595: .Xr sshd 8
596: is not to load any certificates.
1.1 stevesk 597: .It Cm HostKey
598: Specifies a file containing a private host key
599: used by SSH.
600: The default is
601: .Pa /etc/ssh/ssh_host_key
602: for protocol version 1, and
1.126 djm 603: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 604: .Pa /etc/ssh/ssh_host_ecdsa_key ,
605: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 606: and
1.1 stevesk 607: .Pa /etc/ssh/ssh_host_rsa_key
608: for protocol version 2.
609: Note that
1.52 jmc 610: .Xr sshd 8
1.1 stevesk 611: will refuse to use a file if it is group/world-accessible.
612: It is possible to have multiple host key files.
613: .Dq rsa1
614: keys are used for version 1 and
1.126 djm 615: .Dq dsa ,
1.169 naddy 616: .Dq ecdsa ,
617: .Dq ed25519
1.1 stevesk 618: or
619: .Dq rsa
620: are used for version 2 of the SSH protocol.
1.162 markus 621: It is also possible to specify public host key files instead.
622: In this case operations on the private key will be delegated
623: to an
624: .Xr ssh-agent 1 .
625: .It Cm HostKeyAgent
626: Identifies the UNIX-domain socket used to communicate
627: with an agent that has access to the private host keys.
628: If
629: .Dq SSH_AUTH_SOCK
630: is specified, the location of the socket will be read from the
631: .Ev SSH_AUTH_SOCK
632: environment variable.
1.1 stevesk 633: .It Cm IgnoreRhosts
634: Specifies that
635: .Pa .rhosts
636: and
637: .Pa .shosts
638: files will not be used in
639: .Cm RhostsRSAAuthentication
640: or
641: .Cm HostbasedAuthentication .
642: .Pp
643: .Pa /etc/hosts.equiv
644: and
645: .Pa /etc/shosts.equiv
646: are still used.
647: The default is
648: .Dq yes .
649: .It Cm IgnoreUserKnownHosts
650: Specifies whether
1.52 jmc 651: .Xr sshd 8
1.1 stevesk 652: should ignore the user's
1.41 djm 653: .Pa ~/.ssh/known_hosts
1.1 stevesk 654: during
655: .Cm RhostsRSAAuthentication
656: or
657: .Cm HostbasedAuthentication .
658: The default is
659: .Dq no .
1.129 djm 660: .It Cm IPQoS
661: Specifies the IPv4 type-of-service or DSCP class for the connection.
662: Accepted values are
663: .Dq af11 ,
664: .Dq af12 ,
665: .Dq af13 ,
1.136 djm 666: .Dq af21 ,
1.129 djm 667: .Dq af22 ,
668: .Dq af23 ,
669: .Dq af31 ,
670: .Dq af32 ,
671: .Dq af33 ,
672: .Dq af41 ,
673: .Dq af42 ,
674: .Dq af43 ,
675: .Dq cs0 ,
676: .Dq cs1 ,
677: .Dq cs2 ,
678: .Dq cs3 ,
679: .Dq cs4 ,
680: .Dq cs5 ,
681: .Dq cs6 ,
682: .Dq cs7 ,
683: .Dq ef ,
684: .Dq lowdelay ,
685: .Dq throughput ,
686: .Dq reliability ,
687: or a numeric value.
1.131 djm 688: This option may take one or two arguments, separated by whitespace.
1.129 djm 689: If one argument is specified, it is used as the packet class unconditionally.
690: If two values are specified, the first is automatically selected for
691: interactive sessions and the second for non-interactive sessions.
692: The default is
693: .Dq lowdelay
694: for interactive sessions and
695: .Dq throughput
696: for non-interactive sessions.
1.171 jmc 697: .It Cm KbdInteractiveAuthentication
698: Specifies whether to allow keyboard-interactive authentication.
699: The argument to this keyword must be
700: .Dq yes
701: or
702: .Dq no .
703: The default is to use whatever value
704: .Cm ChallengeResponseAuthentication
705: is set to
706: (by default
707: .Dq yes ) .
1.1 stevesk 708: .It Cm KerberosAuthentication
1.24 markus 709: Specifies whether the password provided by the user for
1.1 stevesk 710: .Cm PasswordAuthentication
1.24 markus 711: will be validated through the Kerberos KDC.
1.1 stevesk 712: To use this option, the server needs a
713: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 714: The default is
1.29 dtucker 715: .Dq no .
716: .It Cm KerberosGetAFSToken
1.45 djm 717: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 718: an AFS token before accessing the user's home directory.
1.52 jmc 719: The default is
1.1 stevesk 720: .Dq no .
721: .It Cm KerberosOrLocalPasswd
1.52 jmc 722: If password authentication through Kerberos fails then
1.1 stevesk 723: the password will be validated via any additional local mechanism
724: such as
725: .Pa /etc/passwd .
1.52 jmc 726: The default is
1.1 stevesk 727: .Dq yes .
728: .It Cm KerberosTicketCleanup
729: Specifies whether to automatically destroy the user's ticket cache
730: file on logout.
1.52 jmc 731: The default is
1.1 stevesk 732: .Dq yes .
1.127 djm 733: .It Cm KexAlgorithms
734: Specifies the available KEX (Key Exchange) algorithms.
735: Multiple algorithms must be comma-separated.
1.173 naddy 736: The supported algorithms are:
737: .Pp
738: .Bl -item -compact -offset indent
739: .It
740: curve25519-sha256@libssh.org
741: .It
742: diffie-hellman-group1-sha1
743: .It
744: diffie-hellman-group14-sha1
745: .It
746: diffie-hellman-group-exchange-sha1
747: .It
748: diffie-hellman-group-exchange-sha256
749: .It
750: ecdh-sha2-nistp256
751: .It
752: ecdh-sha2-nistp384
753: .It
754: ecdh-sha2-nistp521
755: .El
756: .Pp
757: The default is:
1.170 dtucker 758: .Bd -literal -offset indent
759: curve25519-sha256@libssh.org,
760: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
761: diffie-hellman-group-exchange-sha256,
1.173 naddy 762: diffie-hellman-group14-sha1
1.170 dtucker 763: .Ed
1.1 stevesk 764: .It Cm KeyRegenerationInterval
765: In protocol version 1, the ephemeral server key is automatically regenerated
766: after this many seconds (if it has been used).
767: The purpose of regeneration is to prevent
768: decrypting captured sessions by later breaking into the machine and
769: stealing the keys.
770: The key is never stored anywhere.
771: If the value is 0, the key is never regenerated.
772: The default is 3600 (seconds).
773: .It Cm ListenAddress
774: Specifies the local addresses
1.52 jmc 775: .Xr sshd 8
1.1 stevesk 776: should listen on.
777: The following forms may be used:
778: .Pp
779: .Bl -item -offset indent -compact
780: .It
781: .Cm ListenAddress
782: .Sm off
1.180 jmc 783: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 784: .Sm on
785: .It
786: .Cm ListenAddress
787: .Sm off
1.180 jmc 788: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 789: .Sm on
790: .It
791: .Cm ListenAddress
792: .Sm off
793: .Oo
1.180 jmc 794: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 795: .Sm on
796: .El
797: .Pp
798: If
799: .Ar port
800: is not specified,
1.52 jmc 801: sshd will listen on the address and all prior
1.1 stevesk 802: .Cm Port
1.17 jmc 803: options specified.
804: The default is to listen on all local addresses.
1.15 jmc 805: Multiple
1.1 stevesk 806: .Cm ListenAddress
1.17 jmc 807: options are permitted.
808: Additionally, any
1.1 stevesk 809: .Cm Port
1.52 jmc 810: options must precede this option for non-port qualified addresses.
1.1 stevesk 811: .It Cm LoginGraceTime
812: The server disconnects after this time if the user has not
813: successfully logged in.
814: If the value is 0, there is no time limit.
1.12 stevesk 815: The default is 120 seconds.
1.1 stevesk 816: .It Cm LogLevel
817: Gives the verbosity level that is used when logging messages from
1.53 jmc 818: .Xr sshd 8 .
1.1 stevesk 819: The possible values are:
1.52 jmc 820: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 821: The default is INFO.
822: DEBUG and DEBUG1 are equivalent.
823: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
824: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 825: .It Cm MACs
826: Specifies the available MAC (message authentication code) algorithms.
827: The MAC algorithm is used in protocol version 2
828: for data integrity protection.
829: Multiple algorithms must be comma-separated.
1.152 markus 830: The algorithms that contain
831: .Dq -etm
832: calculate the MAC after encryption (encrypt-then-mac).
833: These are considered safer and their use recommended.
1.173 naddy 834: The supported MACs are:
835: .Pp
836: .Bl -item -compact -offset indent
837: .It
838: hmac-md5
839: .It
840: hmac-md5-96
841: .It
842: hmac-ripemd160
843: .It
844: hmac-sha1
845: .It
846: hmac-sha1-96
847: .It
848: hmac-sha2-256
849: .It
850: hmac-sha2-512
851: .It
852: umac-64@openssh.com
853: .It
854: umac-128@openssh.com
855: .It
856: hmac-md5-etm@openssh.com
857: .It
858: hmac-md5-96-etm@openssh.com
859: .It
860: hmac-ripemd160-etm@openssh.com
861: .It
862: hmac-sha1-etm@openssh.com
863: .It
864: hmac-sha1-96-etm@openssh.com
865: .It
866: hmac-sha2-256-etm@openssh.com
867: .It
868: hmac-sha2-512-etm@openssh.com
869: .It
870: umac-64-etm@openssh.com
871: .It
872: umac-128-etm@openssh.com
873: .El
874: .Pp
1.52 jmc 875: The default is:
1.77 jmc 876: .Bd -literal -offset indent
1.152 markus 877: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
878: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 879: umac-64@openssh.com,umac-128@openssh.com,
880: hmac-sha2-256,hmac-sha2-512
1.77 jmc 881: .Ed
1.60 dtucker 882: .It Cm Match
1.61 jmc 883: Introduces a conditional block.
1.65 dtucker 884: If all of the criteria on the
1.60 dtucker 885: .Cm Match
1.65 dtucker 886: line are satisfied, the keywords on the following lines override those
887: set in the global section of the config file, until either another
1.60 dtucker 888: .Cm Match
1.65 dtucker 889: line or the end of the file.
1.172 djm 890: If a keyword appears in multiple
891: .Cm Match
1.177 sobrado 892: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 893: applied.
1.91 djm 894: .Pp
1.61 jmc 895: The arguments to
1.60 dtucker 896: .Cm Match
1.163 dtucker 897: are one or more criteria-pattern pairs or the single token
898: .Cm All
899: which matches all criteria.
1.60 dtucker 900: The available criteria are
901: .Cm User ,
1.69 dtucker 902: .Cm Group ,
1.60 dtucker 903: .Cm Host ,
1.139 dtucker 904: .Cm LocalAddress ,
905: .Cm LocalPort ,
1.60 dtucker 906: and
907: .Cm Address .
1.91 djm 908: The match patterns may consist of single entries or comma-separated
909: lists and may use the wildcard and negation operators described in the
1.161 jmc 910: PATTERNS section of
1.92 djm 911: .Xr ssh_config 5 .
1.91 djm 912: .Pp
913: The patterns in an
914: .Cm Address
915: criteria may additionally contain addresses to match in CIDR
1.93 jmc 916: address/masklen format, e.g.\&
1.91 djm 917: .Dq 192.0.2.0/24
918: or
919: .Dq 3ffe:ffff::/32 .
920: Note that the mask length provided must be consistent with the address -
921: it is an error to specify a mask length that is too long for the address
1.93 jmc 922: or one with bits set in this host portion of the address.
923: For example,
1.91 djm 924: .Dq 192.0.2.0/33
925: and
1.93 jmc 926: .Dq 192.0.2.0/8
1.91 djm 927: respectively.
928: .Pp
1.60 dtucker 929: Only a subset of keywords may be used on the lines following a
930: .Cm Match
931: keyword.
932: Available keywords are
1.142 jmc 933: .Cm AcceptEnv ,
1.99 okan 934: .Cm AllowAgentForwarding ,
1.142 jmc 935: .Cm AllowGroups ,
1.62 dtucker 936: .Cm AllowTcpForwarding ,
1.141 markus 937: .Cm AllowUsers ,
1.149 djm 938: .Cm AuthenticationMethods ,
1.146 djm 939: .Cm AuthorizedKeysCommand ,
940: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 941: .Cm AuthorizedKeysFile ,
1.123 djm 942: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 943: .Cm Banner ,
1.85 djm 944: .Cm ChrootDirectory ,
1.141 markus 945: .Cm DenyGroups ,
946: .Cm DenyUsers ,
1.67 dtucker 947: .Cm ForceCommand ,
1.142 jmc 948: .Cm GatewayPorts ,
1.141 markus 949: .Cm GSSAPIAuthentication ,
1.87 djm 950: .Cm HostbasedAuthentication ,
1.123 djm 951: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 952: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 953: .Cm KerberosAuthentication ,
1.95 dtucker 954: .Cm MaxAuthTries ,
1.94 dtucker 955: .Cm MaxSessions ,
1.72 dtucker 956: .Cm PasswordAuthentication ,
1.97 djm 957: .Cm PermitEmptyPasswords ,
1.66 dtucker 958: .Cm PermitOpen ,
1.79 dtucker 959: .Cm PermitRootLogin ,
1.164 djm 960: .Cm PermitTTY ,
1.123 djm 961: .Cm PermitTunnel ,
1.174 djm 962: .Cm PermitUserRC ,
1.107 dtucker 963: .Cm PubkeyAuthentication ,
1.159 dtucker 964: .Cm RekeyLimit ,
1.142 jmc 965: .Cm RhostsRSAAuthentication ,
1.141 markus 966: .Cm RSAAuthentication ,
1.66 dtucker 967: .Cm X11DisplayOffset ,
1.101 djm 968: .Cm X11Forwarding
1.60 dtucker 969: and
1.102 djm 970: .Cm X11UseLocalHost .
1.33 dtucker 971: .It Cm MaxAuthTries
972: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 973: connection.
974: Once the number of failures reaches half this value,
975: additional failures are logged.
976: The default is 6.
1.90 djm 977: .It Cm MaxSessions
978: Specifies the maximum number of open sessions permitted per network connection.
979: The default is 10.
1.1 stevesk 980: .It Cm MaxStartups
981: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 982: SSH daemon.
1.1 stevesk 983: Additional connections will be dropped until authentication succeeds or the
984: .Cm LoginGraceTime
985: expires for a connection.
1.156 dtucker 986: The default is 10:30:100.
1.1 stevesk 987: .Pp
988: Alternatively, random early drop can be enabled by specifying
989: the three colon separated values
990: .Dq start:rate:full
1.51 jmc 991: (e.g. "10:30:60").
1.53 jmc 992: .Xr sshd 8
1.1 stevesk 993: will refuse connection attempts with a probability of
994: .Dq rate/100
995: (30%)
996: if there are currently
997: .Dq start
998: (10)
999: unauthenticated connections.
1000: The probability increases linearly and all connection attempts
1001: are refused if the number of unauthenticated connections reaches
1002: .Dq full
1003: (60).
1004: .It Cm PasswordAuthentication
1005: Specifies whether password authentication is allowed.
1006: The default is
1007: .Dq yes .
1008: .It Cm PermitEmptyPasswords
1009: When password authentication is allowed, it specifies whether the
1010: server allows login to accounts with empty password strings.
1011: The default is
1012: .Dq no .
1.62 dtucker 1013: .It Cm PermitOpen
1014: Specifies the destinations to which TCP port forwarding is permitted.
1015: The forwarding specification must be one of the following forms:
1016: .Pp
1017: .Bl -item -offset indent -compact
1018: .It
1019: .Cm PermitOpen
1020: .Sm off
1021: .Ar host : port
1022: .Sm on
1023: .It
1024: .Cm PermitOpen
1025: .Sm off
1026: .Ar IPv4_addr : port
1027: .Sm on
1028: .It
1029: .Cm PermitOpen
1030: .Sm off
1031: .Ar \&[ IPv6_addr \&] : port
1032: .Sm on
1033: .El
1034: .Pp
1.68 dtucker 1035: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1036: An argument of
1037: .Dq any
1038: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1039: An argument of
1040: .Dq none
1041: can be used to prohibit all forwarding requests.
1.63 jmc 1042: By default all port forwarding requests are permitted.
1.1 stevesk 1043: .It Cm PermitRootLogin
1.38 jmc 1044: Specifies whether root can log in using
1.1 stevesk 1045: .Xr ssh 1 .
1046: The argument must be
1047: .Dq yes ,
1048: .Dq without-password ,
1.52 jmc 1049: .Dq forced-commands-only ,
1.1 stevesk 1050: or
1051: .Dq no .
1052: The default is
1053: .Dq yes .
1054: .Pp
1055: If this option is set to
1.52 jmc 1056: .Dq without-password ,
1.1 stevesk 1057: password authentication is disabled for root.
1058: .Pp
1059: If this option is set to
1.52 jmc 1060: .Dq forced-commands-only ,
1.1 stevesk 1061: root login with public key authentication will be allowed,
1062: but only if the
1063: .Ar command
1064: option has been specified
1065: (which may be useful for taking remote backups even if root login is
1.17 jmc 1066: normally not allowed).
1067: All other authentication methods are disabled for root.
1.1 stevesk 1068: .Pp
1069: If this option is set to
1.52 jmc 1070: .Dq no ,
1.38 jmc 1071: root is not allowed to log in.
1.46 reyk 1072: .It Cm PermitTunnel
1073: Specifies whether
1074: .Xr tun 4
1075: device forwarding is allowed.
1.47 reyk 1076: The argument must be
1077: .Dq yes ,
1.58 stevesk 1078: .Dq point-to-point
1079: (layer 3),
1080: .Dq ethernet
1081: (layer 2), or
1.47 reyk 1082: .Dq no .
1.58 stevesk 1083: Specifying
1084: .Dq yes
1085: permits both
1086: .Dq point-to-point
1087: and
1088: .Dq ethernet .
1.46 reyk 1089: The default is
1090: .Dq no .
1.178 djm 1091: .Pp
1092: Independent of this setting, the permissions of the selected
1093: .Xr tun 4
1094: device must allow access to the user.
1.164 djm 1095: .It Cm PermitTTY
1096: Specifies whether
1.165 jmc 1097: .Xr pty 4
1.164 djm 1098: allocation is permitted.
1099: The default is
1100: .Dq yes .
1.6 markus 1101: .It Cm PermitUserEnvironment
1102: Specifies whether
1103: .Pa ~/.ssh/environment
1.9 stevesk 1104: and
1.6 markus 1105: .Cm environment=
1106: options in
1107: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1108: are processed by
1.52 jmc 1109: .Xr sshd 8 .
1.6 markus 1110: The default is
1111: .Dq no .
1.9 stevesk 1112: Enabling environment processing may enable users to bypass access
1113: restrictions in some configurations using mechanisms such as
1114: .Ev LD_PRELOAD .
1.174 djm 1115: .It Cm PermitUserRC
1116: Specifies whether any
1117: .Pa ~/.ssh/rc
1118: file is executed.
1119: The default is
1120: .Dq yes .
1.1 stevesk 1121: .It Cm PidFile
1.4 stevesk 1122: Specifies the file that contains the process ID of the
1.53 jmc 1123: SSH daemon.
1.1 stevesk 1124: The default is
1125: .Pa /var/run/sshd.pid .
1126: .It Cm Port
1127: Specifies the port number that
1.52 jmc 1128: .Xr sshd 8
1.1 stevesk 1129: listens on.
1130: The default is 22.
1131: Multiple options of this type are permitted.
1132: See also
1133: .Cm ListenAddress .
1134: .It Cm PrintLastLog
1135: Specifies whether
1.52 jmc 1136: .Xr sshd 8
1.36 jaredy 1137: should print the date and time of the last user login when a user logs
1138: in interactively.
1.1 stevesk 1139: The default is
1140: .Dq yes .
1141: .It Cm PrintMotd
1142: Specifies whether
1.52 jmc 1143: .Xr sshd 8
1.1 stevesk 1144: should print
1145: .Pa /etc/motd
1146: when a user logs in interactively.
1147: (On some systems it is also printed by the shell,
1148: .Pa /etc/profile ,
1149: or equivalent.)
1150: The default is
1151: .Dq yes .
1152: .It Cm Protocol
1153: Specifies the protocol versions
1.52 jmc 1154: .Xr sshd 8
1.5 stevesk 1155: supports.
1.1 stevesk 1156: The possible values are
1.52 jmc 1157: .Sq 1
1.1 stevesk 1158: and
1.52 jmc 1159: .Sq 2 .
1.1 stevesk 1160: Multiple versions must be comma-separated.
1161: The default is
1.109 jmc 1162: .Sq 2 .
1.5 stevesk 1163: Note that the order of the protocol list does not indicate preference,
1164: because the client selects among multiple protocol versions offered
1165: by the server.
1166: Specifying
1167: .Dq 2,1
1168: is identical to
1169: .Dq 1,2 .
1.1 stevesk 1170: .It Cm PubkeyAuthentication
1171: Specifies whether public key authentication is allowed.
1172: The default is
1173: .Dq yes .
1174: Note that this option applies to protocol version 2 only.
1.159 dtucker 1175: .It Cm RekeyLimit
1176: Specifies the maximum amount of data that may be transmitted before the
1177: session key is renegotiated, optionally followed a maximum amount of
1178: time that may pass before the session key is renegotiated.
1179: The first argument is specified in bytes and may have a suffix of
1180: .Sq K ,
1181: .Sq M ,
1182: or
1183: .Sq G
1184: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1185: The default is between
1186: .Sq 1G
1187: and
1188: .Sq 4G ,
1189: depending on the cipher.
1190: The optional second value is specified in seconds and may use any of the
1191: units documented in the
1192: .Sx TIME FORMATS
1.160 jmc 1193: section.
1.159 dtucker 1194: The default value for
1195: .Cm RekeyLimit
1196: is
1197: .Dq default none ,
1198: which means that rekeying is performed after the cipher's default amount
1199: of data has been sent or received and no time based rekeying is done.
1200: This option applies to protocol version 2 only.
1.118 djm 1201: .It Cm RevokedKeys
1.154 djm 1202: Specifies revoked public keys.
1.118 djm 1203: Keys listed in this file will be refused for public key authentication.
1204: Note that if this file is not readable, then public key authentication will
1205: be refused for all users.
1.154 djm 1206: Keys may be specified as a text file, listing one public key per line, or as
1207: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1208: .Xr ssh-keygen 1 .
1.161 jmc 1209: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1210: .Xr ssh-keygen 1 .
1.1 stevesk 1211: .It Cm RhostsRSAAuthentication
1212: Specifies whether rhosts or /etc/hosts.equiv authentication together
1213: with successful RSA host authentication is allowed.
1214: The default is
1215: .Dq no .
1216: This option applies to protocol version 1 only.
1217: .It Cm RSAAuthentication
1218: Specifies whether pure RSA authentication is allowed.
1219: The default is
1220: .Dq yes .
1221: This option applies to protocol version 1 only.
1222: .It Cm ServerKeyBits
1223: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1224: The minimum value is 512, and the default is 1024.
1.175 millert 1225: .It Cm StreamLocalBindMask
1226: Sets the octal file creation mode mask
1227: .Pq umask
1228: used when creating a Unix-domain socket file for local or remote
1229: port forwarding.
1230: This option is only used for port forwarding to a Unix-domain socket file.
1231: .Pp
1232: The default value is 0177, which creates a Unix-domain socket file that is
1233: readable and writable only by the owner.
1234: Note that not all operating systems honor the file mode on Unix-domain
1235: socket files.
1236: .It Cm StreamLocalBindUnlink
1237: Specifies whether to remove an existing Unix-domain socket file for local
1238: or remote port forwarding before creating a new one.
1239: If the socket file already exists and
1240: .Cm StreamLocalBindUnlink
1241: is not enabled,
1242: .Nm sshd
1243: will be unable to forward the port to the Unix-domain socket file.
1244: This option is only used for port forwarding to a Unix-domain socket file.
1245: .Pp
1246: The argument must be
1247: .Dq yes
1248: or
1249: .Dq no .
1250: The default is
1251: .Dq no .
1.1 stevesk 1252: .It Cm StrictModes
1253: Specifies whether
1.52 jmc 1254: .Xr sshd 8
1.1 stevesk 1255: should check file modes and ownership of the
1256: user's files and home directory before accepting login.
1257: This is normally desirable because novices sometimes accidentally leave their
1258: directory or files world-writable.
1259: The default is
1260: .Dq yes .
1.112 djm 1261: Note that this does not apply to
1262: .Cm ChrootDirectory ,
1263: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1264: .It Cm Subsystem
1.51 jmc 1265: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1266: Arguments should be a subsystem name and a command (with optional arguments)
1267: to execute upon subsystem request.
1.80 djm 1268: .Pp
1.1 stevesk 1269: The command
1270: .Xr sftp-server 8
1271: implements the
1272: .Dq sftp
1273: file transfer subsystem.
1.80 djm 1274: .Pp
1275: Alternately the name
1276: .Dq internal-sftp
1277: implements an in-process
1278: .Dq sftp
1279: server.
1280: This may simplify configurations using
1281: .Cm ChrootDirectory
1282: to force a different filesystem root on clients.
1283: .Pp
1.1 stevesk 1284: By default no subsystems are defined.
1285: Note that this option applies to protocol version 2 only.
1286: .It Cm SyslogFacility
1287: Gives the facility code that is used when logging messages from
1.53 jmc 1288: .Xr sshd 8 .
1.1 stevesk 1289: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1290: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1291: The default is AUTH.
1.27 markus 1292: .It Cm TCPKeepAlive
1293: Specifies whether the system should send TCP keepalive messages to the
1294: other side.
1295: If they are sent, death of the connection or crash of one
1296: of the machines will be properly noticed.
1297: However, this means that
1298: connections will die if the route is down temporarily, and some people
1299: find it annoying.
1300: On the other hand, if TCP keepalives are not sent,
1301: sessions may hang indefinitely on the server, leaving
1302: .Dq ghost
1303: users and consuming server resources.
1304: .Pp
1305: The default is
1306: .Dq yes
1307: (to send TCP keepalive messages), and the server will notice
1308: if the network goes down or the client host crashes.
1309: This avoids infinitely hanging sessions.
1310: .Pp
1311: To disable TCP keepalive messages, the value should be set to
1312: .Dq no .
1.118 djm 1313: .It Cm TrustedUserCAKeys
1314: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1315: trusted to sign user certificates for authentication.
1.119 jmc 1316: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1317: .Ql #
1318: are allowed.
1319: If a certificate is presented for authentication and has its signing CA key
1320: listed in this file, then it may be used for authentication for any user
1321: listed in the certificate's principals list.
1322: Note that certificates that lack a list of principals will not be permitted
1323: for authentication using
1324: .Cm TrustedUserCAKeys .
1.161 jmc 1325: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1326: .Xr ssh-keygen 1 .
1.18 markus 1327: .It Cm UseDNS
1328: Specifies whether
1.52 jmc 1329: .Xr sshd 8
1.40 jmc 1330: should look up the remote host name and check that
1.18 markus 1331: the resolved host name for the remote IP address maps back to the
1332: very same IP address.
1333: The default is
1334: .Dq yes .
1.1 stevesk 1335: .It Cm UseLogin
1336: Specifies whether
1337: .Xr login 1
1338: is used for interactive login sessions.
1339: The default is
1340: .Dq no .
1341: Note that
1342: .Xr login 1
1343: is never used for remote command execution.
1344: Note also, that if this is enabled,
1345: .Cm X11Forwarding
1346: will be disabled because
1347: .Xr login 1
1348: does not know how to handle
1349: .Xr xauth 1
1.15 jmc 1350: cookies.
1351: If
1.1 stevesk 1352: .Cm UsePrivilegeSeparation
1353: is specified, it will be disabled after authentication.
1354: .It Cm UsePrivilegeSeparation
1355: Specifies whether
1.52 jmc 1356: .Xr sshd 8
1.2 stevesk 1357: separates privileges by creating an unprivileged child process
1.15 jmc 1358: to deal with incoming network traffic.
1359: After successful authentication, another process will be created that has
1360: the privilege of the authenticated user.
1361: The goal of privilege separation is to prevent privilege
1.1 stevesk 1362: escalation by containing any corruption within the unprivileged processes.
1363: The default is
1364: .Dq yes .
1.134 djm 1365: If
1366: .Cm UsePrivilegeSeparation
1367: is set to
1368: .Dq sandbox
1369: then the pre-authentication unprivileged process is subject to additional
1370: restrictions.
1.137 djm 1371: .It Cm VersionAddendum
1372: Optionally specifies additional text to append to the SSH protocol banner
1373: sent by the server upon connection.
1374: The default is
1375: .Dq none .
1.1 stevesk 1376: .It Cm X11DisplayOffset
1377: Specifies the first display number available for
1.52 jmc 1378: .Xr sshd 8 Ns 's
1.1 stevesk 1379: X11 forwarding.
1.52 jmc 1380: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1381: The default is 10.
1382: .It Cm X11Forwarding
1383: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1384: The argument must be
1385: .Dq yes
1386: or
1387: .Dq no .
1.1 stevesk 1388: The default is
1389: .Dq no .
1.13 stevesk 1390: .Pp
1391: When X11 forwarding is enabled, there may be additional exposure to
1392: the server and to client displays if the
1.52 jmc 1393: .Xr sshd 8
1.13 stevesk 1394: proxy display is configured to listen on the wildcard address (see
1395: .Cm X11UseLocalhost
1.52 jmc 1396: below), though this is not the default.
1.13 stevesk 1397: Additionally, the authentication spoofing and authentication data
1398: verification and substitution occur on the client side.
1399: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1400: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1401: forwarding (see the warnings for
1402: .Cm ForwardX11
1403: in
1.19 jmc 1404: .Xr ssh_config 5 ) .
1.13 stevesk 1405: A system administrator may have a stance in which they want to
1406: protect clients that may expose themselves to attack by unwittingly
1407: requesting X11 forwarding, which can warrant a
1408: .Dq no
1409: setting.
1410: .Pp
1411: Note that disabling X11 forwarding does not prevent users from
1412: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1413: X11 forwarding is automatically disabled if
1414: .Cm UseLogin
1415: is enabled.
1416: .It Cm X11UseLocalhost
1417: Specifies whether
1.52 jmc 1418: .Xr sshd 8
1.1 stevesk 1419: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1420: the wildcard address.
1421: By default,
1.52 jmc 1422: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1423: hostname part of the
1424: .Ev DISPLAY
1425: environment variable to
1426: .Dq localhost .
1.8 stevesk 1427: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1428: However, some older X11 clients may not function with this
1429: configuration.
1430: .Cm X11UseLocalhost
1431: may be set to
1432: .Dq no
1433: to specify that the forwarding server should be bound to the wildcard
1434: address.
1435: The argument must be
1436: .Dq yes
1437: or
1438: .Dq no .
1439: The default is
1440: .Dq yes .
1441: .It Cm XAuthLocation
1.11 stevesk 1442: Specifies the full pathname of the
1.1 stevesk 1443: .Xr xauth 1
1444: program.
1445: The default is
1446: .Pa /usr/X11R6/bin/xauth .
1447: .El
1.55 jmc 1448: .Sh TIME FORMATS
1.53 jmc 1449: .Xr sshd 8
1.1 stevesk 1450: command-line arguments and configuration file options that specify time
1451: may be expressed using a sequence of the form:
1452: .Sm off
1.7 stevesk 1453: .Ar time Op Ar qualifier ,
1.1 stevesk 1454: .Sm on
1455: where
1456: .Ar time
1457: is a positive integer value and
1458: .Ar qualifier
1459: is one of the following:
1460: .Pp
1461: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1462: .It Aq Cm none
1.1 stevesk 1463: seconds
1464: .It Cm s | Cm S
1465: seconds
1466: .It Cm m | Cm M
1467: minutes
1468: .It Cm h | Cm H
1469: hours
1470: .It Cm d | Cm D
1471: days
1472: .It Cm w | Cm W
1473: weeks
1474: .El
1475: .Pp
1476: Each member of the sequence is added together to calculate
1477: the total time value.
1478: .Pp
1479: Time format examples:
1480: .Pp
1481: .Bl -tag -width Ds -compact -offset indent
1482: .It 600
1483: 600 seconds (10 minutes)
1484: .It 10m
1485: 10 minutes
1486: .It 1h30m
1487: 1 hour 30 minutes (90 minutes)
1488: .El
1489: .Sh FILES
1490: .Bl -tag -width Ds
1491: .It Pa /etc/ssh/sshd_config
1492: Contains configuration data for
1.53 jmc 1493: .Xr sshd 8 .
1.1 stevesk 1494: This file should be writable by root only, but it is recommended
1495: (though not necessary) that it be world-readable.
1496: .El
1.19 jmc 1497: .Sh SEE ALSO
1498: .Xr sshd 8
1.1 stevesk 1499: .Sh AUTHORS
1500: OpenSSH is a derivative of the original and free
1501: ssh 1.2.12 release by Tatu Ylonen.
1502: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1503: Theo de Raadt and Dug Song
1504: removed many bugs, re-added newer features and
1505: created OpenSSH.
1506: Markus Friedl contributed the support for SSH
1507: protocol versions 1.5 and 2.0.
1508: Niels Provos and Markus Friedl contributed support
1509: for privilege separation.
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