Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.234
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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1.234 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.233 2016/09/21 01:34:45 djm Exp $
! 37: .Dd $Mdocdate: September 21 2016 $
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.
1.219 jmc 73: The
1.197 dtucker 74: .Ev TERM
1.199 jmc 75: environment variable is always sent whenever the client
1.204 djm 76: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 77: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 78: .Ql *
1.30 djm 79: and
80: .Ql \&? .
1.31 djm 81: Multiple environment variables may be separated by whitespace or spread
1.30 djm 82: across multiple
83: .Cm AcceptEnv
84: directives.
1.31 djm 85: Be warned that some environment variables could be used to bypass restricted
1.30 djm 86: user environments.
87: For this reason, care should be taken in the use of this directive.
88: The default is not to accept any environment variables.
1.37 djm 89: .It Cm AddressFamily
90: Specifies which address family should be used by
1.53 jmc 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
93: .Dq any ,
94: .Dq inet
1.52 jmc 95: (use IPv4 only), or
1.37 djm 96: .Dq inet6
97: (use IPv6 only).
98: The default is
99: .Dq any .
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
105: .Dq yes .
106: Note that disabling agent forwarding does not improve security
107: unless users are also denied shell access, as they can always install
108: their own forwarders.
1.1 stevesk 109: .It Cm AllowGroups
110: This keyword can be followed by a list of group name patterns, separated
111: by spaces.
112: If specified, login is allowed only for users whose primary
113: group or supplementary group list matches one of the patterns.
114: Only group names are valid; a numerical group ID is not recognized.
115: By default, login is allowed for all groups.
1.54 jmc 116: The allow/deny directives are processed in the following order:
117: .Cm DenyUsers ,
118: .Cm AllowUsers ,
119: .Cm DenyGroups ,
120: and finally
121: .Cm AllowGroups .
1.49 jmc 122: .Pp
1.161 jmc 123: See PATTERNS in
1.49 jmc 124: .Xr ssh_config 5
125: for more information on patterns.
1.231 jmc 126: .It Cm AllowStreamLocalForwarding
127: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 128: The available options are
129: .Dq yes
130: or
131: .Dq all
1.231 jmc 132: to allow StreamLocal forwarding,
1.150 djm 133: .Dq no
1.231 jmc 134: to prevent all StreamLocal forwarding,
1.150 djm 135: .Dq local
136: to allow local (from the perspective of
1.151 jmc 137: .Xr ssh 1 )
138: forwarding only or
1.150 djm 139: .Dq remote
140: to allow remote forwarding only.
1.1 stevesk 141: The default is
142: .Dq yes .
1.231 jmc 143: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 144: users are also denied shell access, as they can always install their
145: own forwarders.
1.231 jmc 146: .It Cm AllowTcpForwarding
147: Specifies whether TCP forwarding is permitted.
1.175 millert 148: The available options are
149: .Dq yes
150: or
151: .Dq all
1.231 jmc 152: to allow TCP forwarding,
1.175 millert 153: .Dq no
1.231 jmc 154: to prevent all TCP forwarding,
1.175 millert 155: .Dq local
156: to allow local (from the perspective of
157: .Xr ssh 1 )
158: forwarding only or
159: .Dq remote
160: to allow remote forwarding only.
161: The default is
162: .Dq yes .
1.231 jmc 163: Note that disabling TCP forwarding does not improve security unless
1.175 millert 164: users are also denied shell access, as they can always install their
165: own forwarders.
1.1 stevesk 166: .It Cm AllowUsers
167: This keyword can be followed by a list of user name patterns, separated
168: by spaces.
1.14 jmc 169: If specified, login is allowed only for user names that
1.1 stevesk 170: match one of the patterns.
171: Only user names are valid; a numerical user ID is not recognized.
172: By default, login is allowed for all users.
173: If the pattern takes the form USER@HOST then USER and HOST
174: are separately checked, restricting logins to particular
175: users from particular hosts.
1.222 jmc 176: HOST criteria may additionally contain addresses to match in CIDR
177: address/masklen format.
1.54 jmc 178: The allow/deny directives are processed in the following order:
179: .Cm DenyUsers ,
180: .Cm AllowUsers ,
181: .Cm DenyGroups ,
182: and finally
183: .Cm AllowGroups .
1.49 jmc 184: .Pp
1.161 jmc 185: See PATTERNS in
1.49 jmc 186: .Xr ssh_config 5
187: for more information on patterns.
1.149 djm 188: .It Cm AuthenticationMethods
189: Specifies the authentication methods that must be successfully completed
190: for a user to be granted access.
191: This option must be followed by one or more comma-separated lists of
1.224 djm 192: authentication method names, or by the single string
193: .Dq any
194: to indicate the default behaviour of accepting any single authentication
1.225 jmc 195: method.
1.224 djm 196: if the default is overridden, then successful authentication requires
197: completion of every method in at least one of these lists.
1.149 djm 198: .Pp
199: For example, an argument of
200: .Dq publickey,password publickey,keyboard-interactive
201: would require the user to complete public key authentication, followed by
202: either password or keyboard interactive authentication.
203: Only methods that are next in one or more lists are offered at each stage,
204: so for this example, it would not be possible to attempt password or
205: keyboard-interactive authentication before public key.
1.157 markus 206: .Pp
207: For keyboard interactive authentication it is also possible to
208: restrict authentication to a specific device by appending a
209: colon followed by the device identifier
210: .Dq bsdauth ,
211: .Dq pam ,
212: or
213: .Dq skey ,
214: depending on the server configuration.
215: For example,
216: .Dq keyboard-interactive:bsdauth
217: would restrict keyboard interactive authentication to the
218: .Dq bsdauth
219: device.
1.185 djm 220: .Pp
221: If the
222: .Dq publickey
1.187 jmc 223: method is listed more than once,
1.185 djm 224: .Xr sshd 8
225: verifies that keys that have been used successfully are not reused for
226: subsequent authentications.
227: For example, an
228: .Cm AuthenticationMethods
229: of
230: .Dq publickey,publickey
231: will require successful authentication using two different public keys.
1.181 djm 232: .Pp
1.149 djm 233: Note that each authentication method listed should also be explicitly enabled
234: in the configuration.
1.224 djm 235: The default
236: .Dq any
237: is not to require multiple authentication; successful completion
1.149 djm 238: of a single authentication method is sufficient.
1.146 djm 239: .It Cm AuthorizedKeysCommand
1.147 jmc 240: Specifies a program to be used to look up the user's public keys.
1.201 djm 241: The program must be owned by root, not writable by group or others and
242: specified by an absolute path.
243: .Pp
244: Arguments to
245: .Cm AuthorizedKeysCommand
246: may be provided using the following tokens, which will be expanded
247: at runtime: %% is replaced by a literal '%', %u is replaced by the
248: username being authenticated, %h is replaced by the home directory
249: of the user being authenticated, %t is replaced with the key type
250: offered for authentication, %f is replaced with the fingerprint of
251: the key, and %k is replaced with the key being offered for authentication.
252: If no arguments are specified then the username of the target user
253: will be supplied.
254: .Pp
255: The program should produce on standard output zero or
1.161 jmc 256: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 257: .Xr sshd 8 ) .
1.146 djm 258: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
259: and authorize the user then public key authentication continues using the usual
260: .Cm AuthorizedKeysFile
261: files.
262: By default, no AuthorizedKeysCommand is run.
263: .It Cm AuthorizedKeysCommandUser
264: Specifies the user under whose account the AuthorizedKeysCommand is run.
265: It is recommended to use a dedicated user that has no other role on the host
266: than running authorized keys commands.
1.186 djm 267: If
1.181 djm 268: .Cm AuthorizedKeysCommand
1.186 djm 269: is specified but
270: .Cm AuthorizedKeysCommandUser
271: is not, then
272: .Xr sshd 8
273: will refuse to start.
1.1 stevesk 274: .It Cm AuthorizedKeysFile
275: Specifies the file that contains the public keys that can be used
276: for user authentication.
1.124 djm 277: The format is described in the
1.161 jmc 278: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 279: section of
280: .Xr sshd 8 .
1.1 stevesk 281: .Cm AuthorizedKeysFile
282: may contain tokens of the form %T which are substituted during connection
1.52 jmc 283: setup.
1.17 jmc 284: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 285: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 286: %u is replaced by the username of that user.
287: After expansion,
288: .Cm AuthorizedKeysFile
289: is taken to be an absolute path or one relative to the user's home
290: directory.
1.133 jmc 291: Multiple files may be listed, separated by whitespace.
1.212 djm 292: Alternately this option may be set to
293: .Dq none
294: to skip checking for user keys in files.
1.133 jmc 295: The default is
296: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 297: .It Cm AuthorizedPrincipalsCommand
298: Specifies a program to be used to generate the list of allowed
299: certificate principals as per
300: .Cm AuthorizedPrincipalsFile .
301: The program must be owned by root, not writable by group or others and
302: specified by an absolute path.
303: .Pp
304: Arguments to
305: .Cm AuthorizedPrincipalsCommand
306: may be provided using the following tokens, which will be expanded
1.232 djm 307: at runtime:
308: %% is replaced by a literal '%',
1.233 djm 309: %F with the fingerprint of the CA key,
1.232 djm 310: %f is replaced with certificate fingerprint,
311: %K is replaced with the base-64 encoded CA key.
1.233 djm 312: %k is replaced with the full base-64 encoded certificate,
313: %h is replaced with the home directory of the user being authenticated,
314: %i is replaced with key ID in the certificate,
315: %s is replaced with the serial number of the certificate,
316: %T with the type of the CA key,
317: %t is replaced with type of the certificate being offered, and
318: %u is replaced by the username being authenticated,
1.232 djm 319: If no arguments are specified then the username of the target user
320: will be supplied.
1.202 djm 321: .Pp
322: The program should produce on standard output zero or
323: more lines of
324: .Cm AuthorizedPrincipalsFile
325: output.
326: If either
327: .Cm AuthorizedPrincipalsCommand
328: or
329: .Cm AuthorizedPrincipalsFile
330: is specified, then certificates offered by the client for authentication
331: must contain a principal that is listed.
332: By default, no AuthorizedPrincipalsCommand is run.
333: .It Cm AuthorizedPrincipalsCommandUser
334: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
335: It is recommended to use a dedicated user that has no other role on the host
336: than running authorized principals commands.
337: If
338: .Cm AuthorizedPrincipalsCommand
339: is specified but
340: .Cm AuthorizedPrincipalsCommandUser
341: is not, then
342: .Xr sshd 8
343: will refuse to start.
1.121 djm 344: .It Cm AuthorizedPrincipalsFile
345: Specifies a file that lists principal names that are accepted for
346: certificate authentication.
347: When using certificates signed by a key listed in
348: .Cm TrustedUserCAKeys ,
349: this file lists names, one of which must appear in the certificate for it
350: to be accepted for authentication.
1.125 jmc 351: Names are listed one per line preceded by key options (as described
1.161 jmc 352: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 353: .Xr sshd 8 ) .
1.124 djm 354: Empty lines and comments starting with
1.121 djm 355: .Ql #
356: are ignored.
357: .Pp
358: .Cm AuthorizedPrincipalsFile
359: may contain tokens of the form %T which are substituted during connection
360: setup.
361: The following tokens are defined: %% is replaced by a literal '%',
362: %h is replaced by the home directory of the user being authenticated, and
363: %u is replaced by the username of that user.
364: After expansion,
365: .Cm AuthorizedPrincipalsFile
366: is taken to be an absolute path or one relative to the user's home
367: directory.
368: .Pp
1.138 djm 369: The default is
370: .Dq none ,
371: i.e. not to use a principals file \(en in this case, the username
1.121 djm 372: of the user must appear in a certificate's principals list for it to be
373: accepted.
374: Note that
375: .Cm AuthorizedPrincipalsFile
376: is only used when authentication proceeds using a CA listed in
377: .Cm TrustedUserCAKeys
378: and is not consulted for certification authorities trusted via
379: .Pa ~/.ssh/authorized_keys ,
380: though the
381: .Cm principals=
382: key option offers a similar facility (see
383: .Xr sshd 8
384: for details).
1.1 stevesk 385: .It Cm Banner
386: The contents of the specified file are sent to the remote user before
387: authentication is allowed.
1.78 djm 388: If the argument is
389: .Dq none
390: then no banner is displayed.
1.1 stevesk 391: By default, no banner is displayed.
392: .It Cm ChallengeResponseAuthentication
1.50 jmc 393: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 394: All authentication styles from
395: .Xr login.conf 5
396: are supported.
397: The default is
398: .Dq yes .
1.80 djm 399: .It Cm ChrootDirectory
1.113 stevesk 400: Specifies the pathname of a directory to
1.80 djm 401: .Xr chroot 2
402: to after authentication.
1.190 deraadt 403: At session startup
404: .Xr sshd 8
405: checks that all components of the pathname are root-owned directories
406: which are not writable by any other user or group.
1.106 stevesk 407: After the chroot,
408: .Xr sshd 8
409: changes the working directory to the user's home directory.
1.80 djm 410: .Pp
1.113 stevesk 411: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 412: the connecting user has been authenticated: %% is replaced by a literal '%',
413: %h is replaced by the home directory of the user being authenticated, and
414: %u is replaced by the username of that user.
415: .Pp
416: The
417: .Cm ChrootDirectory
418: must contain the necessary files and directories to support the
1.103 stevesk 419: user's session.
1.80 djm 420: For an interactive session this requires at least a shell, typically
421: .Xr sh 1 ,
422: and basic
423: .Pa /dev
424: nodes such as
425: .Xr null 4 ,
426: .Xr zero 4 ,
427: .Xr stdin 4 ,
428: .Xr stdout 4 ,
429: .Xr stderr 4 ,
1.180 jmc 430: and
1.80 djm 431: .Xr tty 4
432: devices.
433: For file transfer sessions using
1.105 jmc 434: .Dq sftp ,
1.80 djm 435: no additional configuration of the environment is necessary if the
1.105 jmc 436: in-process sftp server is used,
1.176 schwarze 437: though sessions which use logging may require
1.104 stevesk 438: .Pa /dev/log
1.176 schwarze 439: inside the chroot directory on some operating systems (see
1.104 stevesk 440: .Xr sftp-server 8
1.81 jmc 441: for details).
1.190 deraadt 442: .Pp
1.191 jmc 443: For safety, it is very important that the directory hierarchy be
1.190 deraadt 444: prevented from modification by other processes on the system (especially
445: those outside the jail).
446: Misconfiguration can lead to unsafe environments which
447: .Xr sshd 8
448: cannot detect.
1.80 djm 449: .Pp
1.215 djm 450: The default is
451: .Dq none ,
452: indicating not to
1.80 djm 453: .Xr chroot 2 .
1.1 stevesk 454: .It Cm Ciphers
1.219 jmc 455: Specifies the ciphers allowed.
1.1 stevesk 456: Multiple ciphers must be comma-separated.
1.208 djm 457: If the specified value begins with a
458: .Sq +
459: character, then the specified ciphers will be appended to the default set
460: instead of replacing them.
461: .Pp
1.167 djm 462: The supported ciphers are:
463: .Pp
1.173 naddy 464: .Bl -item -compact -offset indent
465: .It
466: 3des-cbc
467: .It
468: aes128-cbc
469: .It
470: aes192-cbc
471: .It
472: aes256-cbc
473: .It
474: aes128-ctr
475: .It
476: aes192-ctr
477: .It
478: aes256-ctr
479: .It
480: aes128-gcm@openssh.com
481: .It
482: aes256-gcm@openssh.com
483: .It
484: arcfour
485: .It
486: arcfour128
487: .It
488: arcfour256
489: .It
490: blowfish-cbc
491: .It
492: cast128-cbc
493: .It
494: chacha20-poly1305@openssh.com
495: .El
1.167 djm 496: .Pp
1.52 jmc 497: The default is:
1.173 naddy 498: .Bd -literal -offset indent
1.211 jmc 499: chacha20-poly1305@openssh.com,
1.173 naddy 500: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 501: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 502: .Ed
1.167 djm 503: .Pp
504: The list of available ciphers may also be obtained using the
505: .Fl Q
506: option of
1.188 djm 507: .Xr ssh 1
508: with an argument of
509: .Dq cipher .
1.1 stevesk 510: .It Cm ClientAliveCountMax
1.48 jmc 511: Sets the number of client alive messages (see below) which may be
1.1 stevesk 512: sent without
1.52 jmc 513: .Xr sshd 8
1.17 jmc 514: receiving any messages back from the client.
515: If this threshold is reached while client alive messages are being sent,
1.52 jmc 516: sshd will disconnect the client, terminating the session.
1.17 jmc 517: It is important to note that the use of client alive messages is very
518: different from
1.27 markus 519: .Cm TCPKeepAlive
1.17 jmc 520: (below).
521: The client alive messages are sent through the encrypted channel
522: and therefore will not be spoofable.
523: The TCP keepalive option enabled by
1.27 markus 524: .Cm TCPKeepAlive
1.17 jmc 525: is spoofable.
526: The client alive mechanism is valuable when the client or
1.1 stevesk 527: server depend on knowing when a connection has become inactive.
528: .Pp
1.17 jmc 529: The default value is 3.
530: If
1.1 stevesk 531: .Cm ClientAliveInterval
1.48 jmc 532: (see below) is set to 15, and
1.1 stevesk 533: .Cm ClientAliveCountMax
1.52 jmc 534: is left at the default, unresponsive SSH clients
1.1 stevesk 535: will be disconnected after approximately 45 seconds.
1.42 djm 536: .It Cm ClientAliveInterval
537: Sets a timeout interval in seconds after which if no data has been received
538: from the client,
1.52 jmc 539: .Xr sshd 8
1.42 djm 540: will send a message through the encrypted
541: channel to request a response from the client.
542: The default
543: is 0, indicating that these messages will not be sent to the client.
1.3 markus 544: .It Cm Compression
1.44 markus 545: Specifies whether compression is allowed, or delayed until
546: the user has authenticated successfully.
1.3 markus 547: The argument must be
1.44 markus 548: .Dq yes ,
549: .Dq delayed ,
1.3 markus 550: or
551: .Dq no .
552: The default is
1.44 markus 553: .Dq delayed .
1.1 stevesk 554: .It Cm DenyGroups
555: This keyword can be followed by a list of group name patterns, separated
556: by spaces.
557: Login is disallowed for users whose primary group or supplementary
558: group list matches one of the patterns.
559: Only group names are valid; a numerical group ID is not recognized.
560: By default, login is allowed for all groups.
1.54 jmc 561: The allow/deny directives are processed in the following order:
562: .Cm DenyUsers ,
563: .Cm AllowUsers ,
564: .Cm DenyGroups ,
565: and finally
566: .Cm AllowGroups .
1.49 jmc 567: .Pp
1.161 jmc 568: See PATTERNS in
1.49 jmc 569: .Xr ssh_config 5
570: for more information on patterns.
1.1 stevesk 571: .It Cm DenyUsers
572: This keyword can be followed by a list of user name patterns, separated
573: by spaces.
574: Login is disallowed for user names that match one of the patterns.
575: Only user names are valid; a numerical user ID is not recognized.
576: By default, login is allowed for all users.
577: If the pattern takes the form USER@HOST then USER and HOST
578: are separately checked, restricting logins to particular
579: users from particular hosts.
1.222 jmc 580: HOST criteria may additionally contain addresses to match in CIDR
581: address/masklen format.
1.54 jmc 582: The allow/deny directives are processed in the following order:
583: .Cm DenyUsers ,
584: .Cm AllowUsers ,
585: .Cm DenyGroups ,
586: and finally
587: .Cm AllowGroups .
1.49 jmc 588: .Pp
1.161 jmc 589: See PATTERNS in
1.49 jmc 590: .Xr ssh_config 5
591: for more information on patterns.
1.183 djm 592: .It Cm FingerprintHash
593: Specifies the hash algorithm used when logging key fingerprints.
594: Valid options are:
595: .Dq md5
596: and
597: .Dq sha256 .
598: The default is
599: .Dq sha256 .
1.67 dtucker 600: .It Cm ForceCommand
601: Forces the execution of the command specified by
602: .Cm ForceCommand ,
1.84 djm 603: ignoring any command supplied by the client and
604: .Pa ~/.ssh/rc
605: if present.
1.67 dtucker 606: The command is invoked by using the user's login shell with the -c option.
607: This applies to shell, command, or subsystem execution.
608: It is most useful inside a
609: .Cm Match
610: block.
611: The command originally supplied by the client is available in the
612: .Ev SSH_ORIGINAL_COMMAND
613: environment variable.
1.82 djm 614: Specifying a command of
615: .Dq internal-sftp
616: will force the use of an in-process sftp server that requires no support
617: files when used with
618: .Cm ChrootDirectory .
1.215 djm 619: The default is
620: .Dq none .
1.1 stevesk 621: .It Cm GatewayPorts
622: Specifies whether remote hosts are allowed to connect to ports
623: forwarded for the client.
624: By default,
1.52 jmc 625: .Xr sshd 8
1.15 jmc 626: binds remote port forwardings to the loopback address.
627: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 628: .Cm GatewayPorts
1.52 jmc 629: can be used to specify that sshd
1.39 djm 630: should allow remote port forwardings to bind to non-loopback addresses, thus
631: allowing other hosts to connect.
632: The argument may be
633: .Dq no
634: to force remote port forwardings to be available to the local host only,
1.1 stevesk 635: .Dq yes
1.39 djm 636: to force remote port forwardings to bind to the wildcard address, or
637: .Dq clientspecified
638: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 639: The default is
640: .Dq no .
1.23 markus 641: .It Cm GSSAPIAuthentication
1.25 markus 642: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 643: The default is
1.23 markus 644: .Dq no .
645: .It Cm GSSAPICleanupCredentials
646: Specifies whether to automatically destroy the user's credentials cache
647: on logout.
648: The default is
649: .Dq yes .
1.203 djm 650: .It Cm GSSAPIStrictAcceptorCheck
651: Determines whether to be strict about the identity of the GSSAPI acceptor
652: a client authenticates against.
653: If set to
654: .Dq yes
655: then the client must authenticate against the
656: .Pa host
657: service on the current hostname.
658: If set to
659: .Dq no
660: then the client may authenticate against any service key stored in the
661: machine's default store.
662: This facility is provided to assist with operation on multi homed machines.
663: The default is
664: .Dq yes .
1.189 djm 665: .It Cm HostbasedAcceptedKeyTypes
666: Specifies the key types that will be accepted for hostbased authentication
667: as a comma-separated pattern list.
1.208 djm 668: Alternately if the specified value begins with a
669: .Sq +
670: character, then the specified key types will be appended to the default set
671: instead of replacing them.
1.206 markus 672: The default for this option is:
673: .Bd -literal -offset 3n
674: ecdsa-sha2-nistp256-cert-v01@openssh.com,
675: ecdsa-sha2-nistp384-cert-v01@openssh.com,
676: ecdsa-sha2-nistp521-cert-v01@openssh.com,
677: ssh-ed25519-cert-v01@openssh.com,
678: ssh-rsa-cert-v01@openssh.com,
679: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 680: ssh-ed25519,ssh-rsa
1.206 markus 681: .Ed
682: .Pp
1.189 djm 683: The
684: .Fl Q
685: option of
686: .Xr ssh 1
687: may be used to list supported key types.
1.1 stevesk 688: .It Cm HostbasedAuthentication
689: Specifies whether rhosts or /etc/hosts.equiv authentication together
690: with successful public key client host authentication is allowed
1.50 jmc 691: (host-based authentication).
1.70 dtucker 692: The default is
693: .Dq no .
694: .It Cm HostbasedUsesNameFromPacketOnly
695: Specifies whether or not the server will attempt to perform a reverse
696: name lookup when matching the name in the
697: .Pa ~/.shosts ,
698: .Pa ~/.rhosts ,
699: and
700: .Pa /etc/hosts.equiv
701: files during
702: .Cm HostbasedAuthentication .
703: A setting of
704: .Dq yes
705: means that
706: .Xr sshd 8
707: uses the name supplied by the client rather than
708: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 709: The default is
710: .Dq no .
1.117 djm 711: .It Cm HostCertificate
712: Specifies a file containing a public host certificate.
713: The certificate's public key must match a private host key already specified
714: by
715: .Cm HostKey .
716: The default behaviour of
717: .Xr sshd 8
718: is not to load any certificates.
1.1 stevesk 719: .It Cm HostKey
720: Specifies a file containing a private host key
721: used by SSH.
1.229 naddy 722: The defaults are
1.126 djm 723: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 724: .Pa /etc/ssh/ssh_host_ecdsa_key ,
725: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 726: and
1.229 naddy 727: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 728: .Pp
1.1 stevesk 729: Note that
1.52 jmc 730: .Xr sshd 8
1.206 markus 731: will refuse to use a file if it is group/world-accessible
732: and that the
733: .Cm HostKeyAlgorithms
734: option restricts which of the keys are actually used by
735: .Xr sshd 8 .
736: .Pp
1.1 stevesk 737: It is possible to have multiple host key files.
1.162 markus 738: It is also possible to specify public host key files instead.
739: In this case operations on the private key will be delegated
740: to an
741: .Xr ssh-agent 1 .
742: .It Cm HostKeyAgent
743: Identifies the UNIX-domain socket used to communicate
744: with an agent that has access to the private host keys.
1.223 markus 745: If the string
1.162 markus 746: .Dq SSH_AUTH_SOCK
747: is specified, the location of the socket will be read from the
748: .Ev SSH_AUTH_SOCK
749: environment variable.
1.206 markus 750: .It Cm HostKeyAlgorithms
1.219 jmc 751: Specifies the host key algorithms
1.206 markus 752: that the server offers.
753: The default for this option is:
754: .Bd -literal -offset 3n
755: ecdsa-sha2-nistp256-cert-v01@openssh.com,
756: ecdsa-sha2-nistp384-cert-v01@openssh.com,
757: ecdsa-sha2-nistp521-cert-v01@openssh.com,
758: ssh-ed25519-cert-v01@openssh.com,
759: ssh-rsa-cert-v01@openssh.com,
760: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 761: ssh-ed25519,ssh-rsa
1.206 markus 762: .Ed
763: .Pp
764: The list of available key types may also be obtained using the
765: .Fl Q
766: option of
767: .Xr ssh 1
768: with an argument of
769: .Dq key .
1.1 stevesk 770: .It Cm IgnoreRhosts
771: Specifies that
772: .Pa .rhosts
773: and
774: .Pa .shosts
775: files will not be used in
776: .Cm HostbasedAuthentication .
777: .Pp
778: .Pa /etc/hosts.equiv
779: and
780: .Pa /etc/shosts.equiv
781: are still used.
782: The default is
783: .Dq yes .
784: .It Cm IgnoreUserKnownHosts
785: Specifies whether
1.52 jmc 786: .Xr sshd 8
1.1 stevesk 787: should ignore the user's
1.41 djm 788: .Pa ~/.ssh/known_hosts
1.1 stevesk 789: during
790: .Cm HostbasedAuthentication .
791: The default is
792: .Dq no .
1.129 djm 793: .It Cm IPQoS
794: Specifies the IPv4 type-of-service or DSCP class for the connection.
795: Accepted values are
796: .Dq af11 ,
797: .Dq af12 ,
798: .Dq af13 ,
1.136 djm 799: .Dq af21 ,
1.129 djm 800: .Dq af22 ,
801: .Dq af23 ,
802: .Dq af31 ,
803: .Dq af32 ,
804: .Dq af33 ,
805: .Dq af41 ,
806: .Dq af42 ,
807: .Dq af43 ,
808: .Dq cs0 ,
809: .Dq cs1 ,
810: .Dq cs2 ,
811: .Dq cs3 ,
812: .Dq cs4 ,
813: .Dq cs5 ,
814: .Dq cs6 ,
815: .Dq cs7 ,
816: .Dq ef ,
817: .Dq lowdelay ,
818: .Dq throughput ,
819: .Dq reliability ,
820: or a numeric value.
1.131 djm 821: This option may take one or two arguments, separated by whitespace.
1.129 djm 822: If one argument is specified, it is used as the packet class unconditionally.
823: If two values are specified, the first is automatically selected for
824: interactive sessions and the second for non-interactive sessions.
825: The default is
826: .Dq lowdelay
827: for interactive sessions and
828: .Dq throughput
829: for non-interactive sessions.
1.171 jmc 830: .It Cm KbdInteractiveAuthentication
831: Specifies whether to allow keyboard-interactive authentication.
832: The argument to this keyword must be
833: .Dq yes
834: or
835: .Dq no .
836: The default is to use whatever value
837: .Cm ChallengeResponseAuthentication
838: is set to
839: (by default
840: .Dq yes ) .
1.1 stevesk 841: .It Cm KerberosAuthentication
1.24 markus 842: Specifies whether the password provided by the user for
1.1 stevesk 843: .Cm PasswordAuthentication
1.24 markus 844: will be validated through the Kerberos KDC.
1.1 stevesk 845: To use this option, the server needs a
846: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 847: The default is
1.29 dtucker 848: .Dq no .
849: .It Cm KerberosGetAFSToken
1.45 djm 850: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 851: an AFS token before accessing the user's home directory.
1.52 jmc 852: The default is
1.1 stevesk 853: .Dq no .
854: .It Cm KerberosOrLocalPasswd
1.52 jmc 855: If password authentication through Kerberos fails then
1.1 stevesk 856: the password will be validated via any additional local mechanism
857: such as
858: .Pa /etc/passwd .
1.52 jmc 859: The default is
1.1 stevesk 860: .Dq yes .
861: .It Cm KerberosTicketCleanup
862: Specifies whether to automatically destroy the user's ticket cache
863: file on logout.
1.52 jmc 864: The default is
1.1 stevesk 865: .Dq yes .
1.127 djm 866: .It Cm KexAlgorithms
867: Specifies the available KEX (Key Exchange) algorithms.
868: Multiple algorithms must be comma-separated.
1.208 djm 869: Alternately if the specified value begins with a
870: .Sq +
871: character, then the specified methods will be appended to the default set
872: instead of replacing them.
1.173 naddy 873: The supported algorithms are:
874: .Pp
875: .Bl -item -compact -offset indent
876: .It
1.234 ! djm 877: curve25519-sha256
! 878: .It
1.173 naddy 879: curve25519-sha256@libssh.org
880: .It
881: diffie-hellman-group1-sha1
882: .It
883: diffie-hellman-group14-sha1
884: .It
885: diffie-hellman-group-exchange-sha1
886: .It
887: diffie-hellman-group-exchange-sha256
888: .It
889: ecdh-sha2-nistp256
890: .It
891: ecdh-sha2-nistp384
892: .It
893: ecdh-sha2-nistp521
894: .El
895: .Pp
896: The default is:
1.170 dtucker 897: .Bd -literal -offset indent
1.234 ! djm 898: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 899: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
900: diffie-hellman-group-exchange-sha256,
1.173 naddy 901: diffie-hellman-group14-sha1
1.170 dtucker 902: .Ed
1.188 djm 903: .Pp
904: The list of available key exchange algorithms may also be obtained using the
905: .Fl Q
906: option of
907: .Xr ssh 1
908: with an argument of
909: .Dq kex .
1.1 stevesk 910: .It Cm ListenAddress
911: Specifies the local addresses
1.52 jmc 912: .Xr sshd 8
1.1 stevesk 913: should listen on.
914: The following forms may be used:
915: .Pp
916: .Bl -item -offset indent -compact
917: .It
918: .Cm ListenAddress
919: .Sm off
1.180 jmc 920: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 921: .Sm on
922: .It
923: .Cm ListenAddress
924: .Sm off
1.180 jmc 925: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 926: .Sm on
927: .It
928: .Cm ListenAddress
929: .Sm off
930: .Oo
1.180 jmc 931: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 932: .Sm on
933: .El
934: .Pp
935: If
936: .Ar port
937: is not specified,
1.200 dtucker 938: sshd will listen on the address and all
1.1 stevesk 939: .Cm Port
1.17 jmc 940: options specified.
941: The default is to listen on all local addresses.
1.15 jmc 942: Multiple
1.1 stevesk 943: .Cm ListenAddress
1.17 jmc 944: options are permitted.
1.1 stevesk 945: .It Cm LoginGraceTime
946: The server disconnects after this time if the user has not
947: successfully logged in.
948: If the value is 0, there is no time limit.
1.12 stevesk 949: The default is 120 seconds.
1.1 stevesk 950: .It Cm LogLevel
951: Gives the verbosity level that is used when logging messages from
1.53 jmc 952: .Xr sshd 8 .
1.1 stevesk 953: The possible values are:
1.52 jmc 954: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 955: The default is INFO.
956: DEBUG and DEBUG1 are equivalent.
957: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
958: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 959: .It Cm MACs
960: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 961: The MAC algorithm is used for data integrity protection.
1.1 stevesk 962: Multiple algorithms must be comma-separated.
1.208 djm 963: If the specified value begins with a
964: .Sq +
965: character, then the specified algorithms will be appended to the default set
966: instead of replacing them.
967: .Pp
1.152 markus 968: The algorithms that contain
969: .Dq -etm
970: calculate the MAC after encryption (encrypt-then-mac).
971: These are considered safer and their use recommended.
1.173 naddy 972: The supported MACs are:
973: .Pp
974: .Bl -item -compact -offset indent
975: .It
976: hmac-md5
977: .It
978: hmac-md5-96
979: .It
980: hmac-ripemd160
981: .It
982: hmac-sha1
983: .It
984: hmac-sha1-96
985: .It
986: hmac-sha2-256
987: .It
988: hmac-sha2-512
989: .It
990: umac-64@openssh.com
991: .It
992: umac-128@openssh.com
993: .It
994: hmac-md5-etm@openssh.com
995: .It
996: hmac-md5-96-etm@openssh.com
997: .It
998: hmac-ripemd160-etm@openssh.com
999: .It
1000: hmac-sha1-etm@openssh.com
1001: .It
1002: hmac-sha1-96-etm@openssh.com
1003: .It
1004: hmac-sha2-256-etm@openssh.com
1005: .It
1006: hmac-sha2-512-etm@openssh.com
1007: .It
1008: umac-64-etm@openssh.com
1009: .It
1010: umac-128-etm@openssh.com
1011: .El
1012: .Pp
1.52 jmc 1013: The default is:
1.77 jmc 1014: .Bd -literal -offset indent
1.152 markus 1015: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1016: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1017: hmac-sha1-etm@openssh.com,
1.173 naddy 1018: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1019: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1020: .Ed
1.188 djm 1021: .Pp
1022: The list of available MAC algorithms may also be obtained using the
1023: .Fl Q
1024: option of
1025: .Xr ssh 1
1026: with an argument of
1027: .Dq mac .
1.60 dtucker 1028: .It Cm Match
1.61 jmc 1029: Introduces a conditional block.
1.65 dtucker 1030: If all of the criteria on the
1.60 dtucker 1031: .Cm Match
1.65 dtucker 1032: line are satisfied, the keywords on the following lines override those
1033: set in the global section of the config file, until either another
1.60 dtucker 1034: .Cm Match
1.65 dtucker 1035: line or the end of the file.
1.172 djm 1036: If a keyword appears in multiple
1037: .Cm Match
1.177 sobrado 1038: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1039: applied.
1.91 djm 1040: .Pp
1.61 jmc 1041: The arguments to
1.60 dtucker 1042: .Cm Match
1.163 dtucker 1043: are one or more criteria-pattern pairs or the single token
1044: .Cm All
1045: which matches all criteria.
1.60 dtucker 1046: The available criteria are
1047: .Cm User ,
1.69 dtucker 1048: .Cm Group ,
1.60 dtucker 1049: .Cm Host ,
1.139 dtucker 1050: .Cm LocalAddress ,
1051: .Cm LocalPort ,
1.60 dtucker 1052: and
1053: .Cm Address .
1.91 djm 1054: The match patterns may consist of single entries or comma-separated
1055: lists and may use the wildcard and negation operators described in the
1.161 jmc 1056: PATTERNS section of
1.92 djm 1057: .Xr ssh_config 5 .
1.91 djm 1058: .Pp
1059: The patterns in an
1060: .Cm Address
1061: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1062: address/masklen format, e.g.\&
1.91 djm 1063: .Dq 192.0.2.0/24
1064: or
1.228 jca 1065: .Dq 2001:db8::/32 .
1.91 djm 1066: Note that the mask length provided must be consistent with the address -
1067: it is an error to specify a mask length that is too long for the address
1.93 jmc 1068: or one with bits set in this host portion of the address.
1069: For example,
1.91 djm 1070: .Dq 192.0.2.0/33
1071: and
1.93 jmc 1072: .Dq 192.0.2.0/8
1.91 djm 1073: respectively.
1074: .Pp
1.60 dtucker 1075: Only a subset of keywords may be used on the lines following a
1076: .Cm Match
1077: keyword.
1078: Available keywords are
1.142 jmc 1079: .Cm AcceptEnv ,
1.99 okan 1080: .Cm AllowAgentForwarding ,
1.142 jmc 1081: .Cm AllowGroups ,
1.193 djm 1082: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1083: .Cm AllowTcpForwarding ,
1.141 markus 1084: .Cm AllowUsers ,
1.149 djm 1085: .Cm AuthenticationMethods ,
1.146 djm 1086: .Cm AuthorizedKeysCommand ,
1087: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1088: .Cm AuthorizedKeysFile ,
1.214 djm 1089: .Cm AuthorizedPrincipalsCommand ,
1090: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1091: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1092: .Cm Banner ,
1.85 djm 1093: .Cm ChrootDirectory ,
1.141 markus 1094: .Cm DenyGroups ,
1095: .Cm DenyUsers ,
1.67 dtucker 1096: .Cm ForceCommand ,
1.194 djm 1097: .Cm GatewayPorts ,
1.193 djm 1098: .Cm GSSAPIAuthentication ,
1.189 djm 1099: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1100: .Cm HostbasedAuthentication ,
1.123 djm 1101: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1102: .Cm IPQoS ,
1.74 jmc 1103: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1104: .Cm KerberosAuthentication ,
1.95 dtucker 1105: .Cm MaxAuthTries ,
1.94 dtucker 1106: .Cm MaxSessions ,
1.72 dtucker 1107: .Cm PasswordAuthentication ,
1.97 djm 1108: .Cm PermitEmptyPasswords ,
1.66 dtucker 1109: .Cm PermitOpen ,
1.79 dtucker 1110: .Cm PermitRootLogin ,
1.164 djm 1111: .Cm PermitTTY ,
1.123 djm 1112: .Cm PermitTunnel ,
1.174 djm 1113: .Cm PermitUserRC ,
1.189 djm 1114: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1115: .Cm PubkeyAuthentication ,
1.159 dtucker 1116: .Cm RekeyLimit ,
1.193 djm 1117: .Cm RevokedKeys ,
1118: .Cm StreamLocalBindMask ,
1119: .Cm StreamLocalBindUnlink ,
1120: .Cm TrustedUserCAKeys ,
1.66 dtucker 1121: .Cm X11DisplayOffset ,
1.101 djm 1122: .Cm X11Forwarding
1.60 dtucker 1123: and
1.102 djm 1124: .Cm X11UseLocalHost .
1.33 dtucker 1125: .It Cm MaxAuthTries
1126: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1127: connection.
1128: Once the number of failures reaches half this value,
1129: additional failures are logged.
1130: The default is 6.
1.90 djm 1131: .It Cm MaxSessions
1.216 djm 1132: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1133: sessions permitted per network connection.
1134: Multiple sessions may be established by clients that support connection
1135: multiplexing.
1136: Setting
1137: .Cm MaxSessions
1138: to 1 will effectively disable session multiplexing, whereas setting it to 0
1139: will prevent all shell, login and subsystem sessions while still permitting
1140: forwarding.
1.90 djm 1141: The default is 10.
1.1 stevesk 1142: .It Cm MaxStartups
1143: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1144: SSH daemon.
1.1 stevesk 1145: Additional connections will be dropped until authentication succeeds or the
1146: .Cm LoginGraceTime
1147: expires for a connection.
1.156 dtucker 1148: The default is 10:30:100.
1.1 stevesk 1149: .Pp
1150: Alternatively, random early drop can be enabled by specifying
1151: the three colon separated values
1152: .Dq start:rate:full
1.51 jmc 1153: (e.g. "10:30:60").
1.53 jmc 1154: .Xr sshd 8
1.1 stevesk 1155: will refuse connection attempts with a probability of
1156: .Dq rate/100
1157: (30%)
1158: if there are currently
1159: .Dq start
1160: (10)
1161: unauthenticated connections.
1162: The probability increases linearly and all connection attempts
1163: are refused if the number of unauthenticated connections reaches
1164: .Dq full
1165: (60).
1166: .It Cm PasswordAuthentication
1167: Specifies whether password authentication is allowed.
1168: The default is
1169: .Dq yes .
1170: .It Cm PermitEmptyPasswords
1171: When password authentication is allowed, it specifies whether the
1172: server allows login to accounts with empty password strings.
1173: The default is
1174: .Dq no .
1.62 dtucker 1175: .It Cm PermitOpen
1176: Specifies the destinations to which TCP port forwarding is permitted.
1177: The forwarding specification must be one of the following forms:
1178: .Pp
1179: .Bl -item -offset indent -compact
1180: .It
1181: .Cm PermitOpen
1182: .Sm off
1183: .Ar host : port
1184: .Sm on
1185: .It
1186: .Cm PermitOpen
1187: .Sm off
1188: .Ar IPv4_addr : port
1189: .Sm on
1190: .It
1191: .Cm PermitOpen
1192: .Sm off
1193: .Ar \&[ IPv6_addr \&] : port
1194: .Sm on
1195: .El
1196: .Pp
1.68 dtucker 1197: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1198: An argument of
1199: .Dq any
1200: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1201: An argument of
1202: .Dq none
1203: can be used to prohibit all forwarding requests.
1.227 jmc 1204: The wildcard
1.226 dtucker 1205: .Dq *
1.227 jmc 1206: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1207: By default all port forwarding requests are permitted.
1.1 stevesk 1208: .It Cm PermitRootLogin
1.38 jmc 1209: Specifies whether root can log in using
1.1 stevesk 1210: .Xr ssh 1 .
1211: The argument must be
1212: .Dq yes ,
1.210 deraadt 1213: .Dq prohibit-password ,
1.1 stevesk 1214: .Dq without-password ,
1.52 jmc 1215: .Dq forced-commands-only ,
1.1 stevesk 1216: or
1217: .Dq no .
1218: The default is
1.210 deraadt 1219: .Dq prohibit-password .
1.1 stevesk 1220: .Pp
1221: If this option is set to
1.210 deraadt 1222: .Dq prohibit-password
1223: or
1.52 jmc 1224: .Dq without-password ,
1.210 deraadt 1225: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1226: .Pp
1227: If this option is set to
1.52 jmc 1228: .Dq forced-commands-only ,
1.1 stevesk 1229: root login with public key authentication will be allowed,
1230: but only if the
1231: .Ar command
1232: option has been specified
1233: (which may be useful for taking remote backups even if root login is
1.17 jmc 1234: normally not allowed).
1235: All other authentication methods are disabled for root.
1.1 stevesk 1236: .Pp
1237: If this option is set to
1.52 jmc 1238: .Dq no ,
1.38 jmc 1239: root is not allowed to log in.
1.231 jmc 1240: .It Cm PermitTTY
1241: Specifies whether
1242: .Xr pty 4
1243: allocation is permitted.
1244: The default is
1245: .Dq yes .
1.46 reyk 1246: .It Cm PermitTunnel
1247: Specifies whether
1248: .Xr tun 4
1249: device forwarding is allowed.
1.47 reyk 1250: The argument must be
1251: .Dq yes ,
1.58 stevesk 1252: .Dq point-to-point
1253: (layer 3),
1254: .Dq ethernet
1255: (layer 2), or
1.47 reyk 1256: .Dq no .
1.58 stevesk 1257: Specifying
1258: .Dq yes
1259: permits both
1260: .Dq point-to-point
1261: and
1262: .Dq ethernet .
1.46 reyk 1263: The default is
1264: .Dq no .
1.178 djm 1265: .Pp
1266: Independent of this setting, the permissions of the selected
1267: .Xr tun 4
1268: device must allow access to the user.
1.6 markus 1269: .It Cm PermitUserEnvironment
1270: Specifies whether
1271: .Pa ~/.ssh/environment
1.9 stevesk 1272: and
1.6 markus 1273: .Cm environment=
1274: options in
1275: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1276: are processed by
1.52 jmc 1277: .Xr sshd 8 .
1.6 markus 1278: The default is
1279: .Dq no .
1.9 stevesk 1280: Enabling environment processing may enable users to bypass access
1281: restrictions in some configurations using mechanisms such as
1282: .Ev LD_PRELOAD .
1.174 djm 1283: .It Cm PermitUserRC
1284: Specifies whether any
1285: .Pa ~/.ssh/rc
1286: file is executed.
1287: The default is
1288: .Dq yes .
1.1 stevesk 1289: .It Cm PidFile
1.4 stevesk 1290: Specifies the file that contains the process ID of the
1.195 dtucker 1291: SSH daemon, or
1292: .Dq none
1293: to not write one.
1.1 stevesk 1294: The default is
1295: .Pa /var/run/sshd.pid .
1296: .It Cm Port
1297: Specifies the port number that
1.52 jmc 1298: .Xr sshd 8
1.1 stevesk 1299: listens on.
1300: The default is 22.
1301: Multiple options of this type are permitted.
1302: See also
1303: .Cm ListenAddress .
1304: .It Cm PrintLastLog
1305: Specifies whether
1.52 jmc 1306: .Xr sshd 8
1.36 jaredy 1307: should print the date and time of the last user login when a user logs
1308: in interactively.
1.1 stevesk 1309: The default is
1310: .Dq yes .
1311: .It Cm PrintMotd
1312: Specifies whether
1.52 jmc 1313: .Xr sshd 8
1.1 stevesk 1314: should print
1315: .Pa /etc/motd
1316: when a user logs in interactively.
1317: (On some systems it is also printed by the shell,
1318: .Pa /etc/profile ,
1319: or equivalent.)
1320: The default is
1321: .Dq yes .
1.189 djm 1322: .It Cm PubkeyAcceptedKeyTypes
1323: Specifies the key types that will be accepted for public key authentication
1324: as a comma-separated pattern list.
1.208 djm 1325: Alternately if the specified value begins with a
1326: .Sq +
1327: character, then the specified key types will be appended to the default set
1328: instead of replacing them.
1.206 markus 1329: The default for this option is:
1330: .Bd -literal -offset 3n
1331: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1332: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1333: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1334: ssh-ed25519-cert-v01@openssh.com,
1335: ssh-rsa-cert-v01@openssh.com,
1336: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1337: ssh-ed25519,ssh-rsa
1.206 markus 1338: .Ed
1339: .Pp
1.189 djm 1340: The
1341: .Fl Q
1342: option of
1343: .Xr ssh 1
1344: may be used to list supported key types.
1.1 stevesk 1345: .It Cm PubkeyAuthentication
1346: Specifies whether public key authentication is allowed.
1347: The default is
1348: .Dq yes .
1.159 dtucker 1349: .It Cm RekeyLimit
1350: Specifies the maximum amount of data that may be transmitted before the
1351: session key is renegotiated, optionally followed a maximum amount of
1352: time that may pass before the session key is renegotiated.
1353: The first argument is specified in bytes and may have a suffix of
1354: .Sq K ,
1355: .Sq M ,
1356: or
1357: .Sq G
1358: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1359: The default is between
1360: .Sq 1G
1361: and
1362: .Sq 4G ,
1363: depending on the cipher.
1364: The optional second value is specified in seconds and may use any of the
1365: units documented in the
1366: .Sx TIME FORMATS
1.160 jmc 1367: section.
1.159 dtucker 1368: The default value for
1369: .Cm RekeyLimit
1370: is
1371: .Dq default none ,
1372: which means that rekeying is performed after the cipher's default amount
1373: of data has been sent or received and no time based rekeying is done.
1.118 djm 1374: .It Cm RevokedKeys
1.195 dtucker 1375: Specifies revoked public keys file, or
1376: .Dq none
1377: to not use one.
1.118 djm 1378: Keys listed in this file will be refused for public key authentication.
1379: Note that if this file is not readable, then public key authentication will
1380: be refused for all users.
1.154 djm 1381: Keys may be specified as a text file, listing one public key per line, or as
1382: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1383: .Xr ssh-keygen 1 .
1.161 jmc 1384: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1385: .Xr ssh-keygen 1 .
1.175 millert 1386: .It Cm StreamLocalBindMask
1387: Sets the octal file creation mode mask
1388: .Pq umask
1389: used when creating a Unix-domain socket file for local or remote
1390: port forwarding.
1391: This option is only used for port forwarding to a Unix-domain socket file.
1392: .Pp
1393: The default value is 0177, which creates a Unix-domain socket file that is
1394: readable and writable only by the owner.
1395: Note that not all operating systems honor the file mode on Unix-domain
1396: socket files.
1397: .It Cm StreamLocalBindUnlink
1398: Specifies whether to remove an existing Unix-domain socket file for local
1399: or remote port forwarding before creating a new one.
1400: If the socket file already exists and
1401: .Cm StreamLocalBindUnlink
1402: is not enabled,
1403: .Nm sshd
1404: will be unable to forward the port to the Unix-domain socket file.
1405: This option is only used for port forwarding to a Unix-domain socket file.
1406: .Pp
1407: The argument must be
1408: .Dq yes
1409: or
1410: .Dq no .
1411: The default is
1412: .Dq no .
1.1 stevesk 1413: .It Cm StrictModes
1414: Specifies whether
1.52 jmc 1415: .Xr sshd 8
1.1 stevesk 1416: should check file modes and ownership of the
1417: user's files and home directory before accepting login.
1418: This is normally desirable because novices sometimes accidentally leave their
1419: directory or files world-writable.
1420: The default is
1421: .Dq yes .
1.112 djm 1422: Note that this does not apply to
1423: .Cm ChrootDirectory ,
1424: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1425: .It Cm Subsystem
1.51 jmc 1426: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1427: Arguments should be a subsystem name and a command (with optional arguments)
1428: to execute upon subsystem request.
1.80 djm 1429: .Pp
1.1 stevesk 1430: The command
1431: .Xr sftp-server 8
1432: implements the
1433: .Dq sftp
1434: file transfer subsystem.
1.80 djm 1435: .Pp
1436: Alternately the name
1437: .Dq internal-sftp
1438: implements an in-process
1439: .Dq sftp
1440: server.
1441: This may simplify configurations using
1442: .Cm ChrootDirectory
1443: to force a different filesystem root on clients.
1444: .Pp
1.1 stevesk 1445: By default no subsystems are defined.
1446: .It Cm SyslogFacility
1447: Gives the facility code that is used when logging messages from
1.53 jmc 1448: .Xr sshd 8 .
1.1 stevesk 1449: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1450: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1451: The default is AUTH.
1.27 markus 1452: .It Cm TCPKeepAlive
1453: Specifies whether the system should send TCP keepalive messages to the
1454: other side.
1455: If they are sent, death of the connection or crash of one
1456: of the machines will be properly noticed.
1457: However, this means that
1458: connections will die if the route is down temporarily, and some people
1459: find it annoying.
1460: On the other hand, if TCP keepalives are not sent,
1461: sessions may hang indefinitely on the server, leaving
1462: .Dq ghost
1463: users and consuming server resources.
1464: .Pp
1465: The default is
1466: .Dq yes
1467: (to send TCP keepalive messages), and the server will notice
1468: if the network goes down or the client host crashes.
1469: This avoids infinitely hanging sessions.
1470: .Pp
1471: To disable TCP keepalive messages, the value should be set to
1472: .Dq no .
1.118 djm 1473: .It Cm TrustedUserCAKeys
1474: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1475: trusted to sign user certificates for authentication, or
1476: .Dq none
1477: to not use one.
1.119 jmc 1478: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1479: .Ql #
1480: are allowed.
1481: If a certificate is presented for authentication and has its signing CA key
1482: listed in this file, then it may be used for authentication for any user
1483: listed in the certificate's principals list.
1484: Note that certificates that lack a list of principals will not be permitted
1485: for authentication using
1486: .Cm TrustedUserCAKeys .
1.161 jmc 1487: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1488: .Xr ssh-keygen 1 .
1.18 markus 1489: .It Cm UseDNS
1490: Specifies whether
1.52 jmc 1491: .Xr sshd 8
1.207 djm 1492: should look up the remote host name, and to check that
1.18 markus 1493: the resolved host name for the remote IP address maps back to the
1494: very same IP address.
1.207 djm 1495: .Pp
1496: If this option is set to
1497: .Dq no
1498: (the default) then only addresses and not host names may be used in
1.221 djm 1499: .Pa ~/.ssh/authorized_keys
1.207 djm 1500: .Cm from
1501: and
1.211 jmc 1502: .Nm
1.207 djm 1503: .Cm Match
1504: .Cm Host
1505: directives.
1.1 stevesk 1506: .It Cm UsePrivilegeSeparation
1507: Specifies whether
1.52 jmc 1508: .Xr sshd 8
1.2 stevesk 1509: separates privileges by creating an unprivileged child process
1.15 jmc 1510: to deal with incoming network traffic.
1511: After successful authentication, another process will be created that has
1512: the privilege of the authenticated user.
1513: The goal of privilege separation is to prevent privilege
1.1 stevesk 1514: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1515: The argument must be
1516: .Dq yes ,
1517: .Dq no ,
1518: or
1519: .Dq sandbox .
1.134 djm 1520: If
1521: .Cm UsePrivilegeSeparation
1522: is set to
1523: .Dq sandbox
1524: then the pre-authentication unprivileged process is subject to additional
1525: restrictions.
1.213 sobrado 1526: The default is
1527: .Dq sandbox .
1.137 djm 1528: .It Cm VersionAddendum
1529: Optionally specifies additional text to append to the SSH protocol banner
1530: sent by the server upon connection.
1531: The default is
1532: .Dq none .
1.1 stevesk 1533: .It Cm X11DisplayOffset
1534: Specifies the first display number available for
1.52 jmc 1535: .Xr sshd 8 Ns 's
1.1 stevesk 1536: X11 forwarding.
1.52 jmc 1537: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1538: The default is 10.
1539: .It Cm X11Forwarding
1540: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1541: The argument must be
1542: .Dq yes
1543: or
1544: .Dq no .
1.1 stevesk 1545: The default is
1546: .Dq no .
1.13 stevesk 1547: .Pp
1548: When X11 forwarding is enabled, there may be additional exposure to
1549: the server and to client displays if the
1.52 jmc 1550: .Xr sshd 8
1.13 stevesk 1551: proxy display is configured to listen on the wildcard address (see
1552: .Cm X11UseLocalhost
1.52 jmc 1553: below), though this is not the default.
1.13 stevesk 1554: Additionally, the authentication spoofing and authentication data
1555: verification and substitution occur on the client side.
1556: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1557: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1558: forwarding (see the warnings for
1559: .Cm ForwardX11
1560: in
1.19 jmc 1561: .Xr ssh_config 5 ) .
1.13 stevesk 1562: A system administrator may have a stance in which they want to
1563: protect clients that may expose themselves to attack by unwittingly
1564: requesting X11 forwarding, which can warrant a
1565: .Dq no
1566: setting.
1567: .Pp
1568: Note that disabling X11 forwarding does not prevent users from
1569: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1570: .It Cm X11UseLocalhost
1571: Specifies whether
1.52 jmc 1572: .Xr sshd 8
1.1 stevesk 1573: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1574: the wildcard address.
1575: By default,
1.52 jmc 1576: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1577: hostname part of the
1578: .Ev DISPLAY
1579: environment variable to
1580: .Dq localhost .
1.8 stevesk 1581: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1582: However, some older X11 clients may not function with this
1583: configuration.
1584: .Cm X11UseLocalhost
1585: may be set to
1586: .Dq no
1587: to specify that the forwarding server should be bound to the wildcard
1588: address.
1589: The argument must be
1590: .Dq yes
1591: or
1592: .Dq no .
1593: The default is
1594: .Dq yes .
1595: .It Cm XAuthLocation
1.11 stevesk 1596: Specifies the full pathname of the
1.1 stevesk 1597: .Xr xauth 1
1.195 dtucker 1598: program, or
1599: .Dq none
1600: to not use one.
1.1 stevesk 1601: The default is
1602: .Pa /usr/X11R6/bin/xauth .
1603: .El
1.55 jmc 1604: .Sh TIME FORMATS
1.53 jmc 1605: .Xr sshd 8
1.1 stevesk 1606: command-line arguments and configuration file options that specify time
1607: may be expressed using a sequence of the form:
1608: .Sm off
1.7 stevesk 1609: .Ar time Op Ar qualifier ,
1.1 stevesk 1610: .Sm on
1611: where
1612: .Ar time
1613: is a positive integer value and
1614: .Ar qualifier
1615: is one of the following:
1616: .Pp
1617: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1618: .It Aq Cm none
1.1 stevesk 1619: seconds
1620: .It Cm s | Cm S
1621: seconds
1622: .It Cm m | Cm M
1623: minutes
1624: .It Cm h | Cm H
1625: hours
1626: .It Cm d | Cm D
1627: days
1628: .It Cm w | Cm W
1629: weeks
1630: .El
1631: .Pp
1632: Each member of the sequence is added together to calculate
1633: the total time value.
1634: .Pp
1635: Time format examples:
1636: .Pp
1637: .Bl -tag -width Ds -compact -offset indent
1638: .It 600
1639: 600 seconds (10 minutes)
1640: .It 10m
1641: 10 minutes
1642: .It 1h30m
1643: 1 hour 30 minutes (90 minutes)
1644: .El
1645: .Sh FILES
1646: .Bl -tag -width Ds
1647: .It Pa /etc/ssh/sshd_config
1648: Contains configuration data for
1.53 jmc 1649: .Xr sshd 8 .
1.1 stevesk 1650: This file should be writable by root only, but it is recommended
1651: (though not necessary) that it be world-readable.
1652: .El
1.19 jmc 1653: .Sh SEE ALSO
1654: .Xr sshd 8
1.1 stevesk 1655: .Sh AUTHORS
1656: OpenSSH is a derivative of the original and free
1657: ssh 1.2.12 release by Tatu Ylonen.
1658: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1659: Theo de Raadt and Dug Song
1660: removed many bugs, re-added newer features and
1661: created OpenSSH.
1662: Markus Friedl contributed the support for SSH
1663: protocol versions 1.5 and 2.0.
1664: Niels Provos and Markus Friedl contributed support
1665: for privilege separation.
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