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