Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.310
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.310 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.309 2020/04/17 03:30:05 djm Exp $
1.309 djm 37: .Dd $Mdocdate: April 17 2020 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
1.295 jmc 42: .Nd OpenSSH daemon configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.53 jmc 44: .Xr sshd 8
1.1 stevesk 45: reads configuration data from
46: .Pa /etc/ssh/sshd_config
47: (or the file specified with
48: .Fl f
49: on the command line).
50: The file contains keyword-argument pairs, one per line.
1.254 benno 51: For each keyword, the first obtained value will be used.
1.1 stevesk 52: Lines starting with
53: .Ql #
54: and empty lines are interpreted as comments.
1.56 dtucker 55: Arguments may optionally be enclosed in double quotes
56: .Pq \&"
57: in order to represent arguments containing spaces.
1.1 stevesk 58: .Pp
59: The possible
60: keywords and their meanings are as follows (note that
61: keywords are case-insensitive and arguments are case-sensitive):
62: .Bl -tag -width Ds
1.30 djm 63: .It Cm AcceptEnv
64: Specifies what environment variables sent by the client will be copied into
65: the session's
66: .Xr environ 7 .
67: See
68: .Cm SendEnv
1.273 djm 69: and
70: .Cm SetEnv
1.30 djm 71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.219 jmc 74: The
1.197 dtucker 75: .Ev TERM
1.275 djm 76: environment variable is always accepted whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
1.237 jmc 94: .Cm any
95: (the default),
96: .Cm inet
1.52 jmc 97: (use IPv4 only), or
1.237 jmc 98: .Cm inet6
1.37 djm 99: (use IPv6 only).
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
1.237 jmc 105: .Cm yes .
1.89 jmc 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.
1.301 djm 116: The allow/deny groups directives are processed in the following order:
1.54 jmc 117: .Cm DenyGroups ,
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.
1.231 jmc 123: .It Cm AllowStreamLocalForwarding
124: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 125: The available options are
1.237 jmc 126: .Cm yes
127: (the default)
1.150 djm 128: or
1.237 jmc 129: .Cm all
1.231 jmc 130: to allow StreamLocal forwarding,
1.237 jmc 131: .Cm no
1.231 jmc 132: to prevent all StreamLocal forwarding,
1.237 jmc 133: .Cm local
1.150 djm 134: to allow local (from the perspective of
1.151 jmc 135: .Xr ssh 1 )
136: forwarding only or
1.237 jmc 137: .Cm remote
1.150 djm 138: to allow remote forwarding only.
1.231 jmc 139: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 140: users are also denied shell access, as they can always install their
141: own forwarders.
1.231 jmc 142: .It Cm AllowTcpForwarding
143: Specifies whether TCP forwarding is permitted.
1.175 millert 144: The available options are
1.237 jmc 145: .Cm yes
146: (the default)
1.175 millert 147: or
1.237 jmc 148: .Cm all
1.231 jmc 149: to allow TCP forwarding,
1.237 jmc 150: .Cm no
1.231 jmc 151: to prevent all TCP forwarding,
1.237 jmc 152: .Cm local
1.175 millert 153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
1.237 jmc 156: .Cm remote
1.175 millert 157: to allow remote forwarding only.
1.231 jmc 158: Note that disabling TCP forwarding does not improve security unless
1.175 millert 159: users are also denied shell access, as they can always install their
160: own forwarders.
1.1 stevesk 161: .It Cm AllowUsers
162: This keyword can be followed by a list of user name patterns, separated
163: by spaces.
1.14 jmc 164: If specified, login is allowed only for user names that
1.1 stevesk 165: match one of the patterns.
166: Only user names are valid; a numerical user ID is not recognized.
167: By default, login is allowed for all users.
168: If the pattern takes the form USER@HOST then USER and HOST
169: are separately checked, restricting logins to particular
170: users from particular hosts.
1.222 jmc 171: HOST criteria may additionally contain addresses to match in CIDR
172: address/masklen format.
1.301 djm 173: The allow/deny users directives are processed in the following order:
1.54 jmc 174: .Cm DenyUsers ,
1.301 djm 175: .Cm AllowUsers .
1.49 jmc 176: .Pp
1.161 jmc 177: See PATTERNS in
1.49 jmc 178: .Xr ssh_config 5
179: for more information on patterns.
1.149 djm 180: .It Cm AuthenticationMethods
181: Specifies the authentication methods that must be successfully completed
182: for a user to be granted access.
1.281 djm 183: This option must be followed by one or more lists of comma-separated
1.224 djm 184: authentication method names, or by the single string
1.237 jmc 185: .Cm any
1.224 djm 186: to indicate the default behaviour of accepting any single authentication
1.225 jmc 187: method.
1.237 jmc 188: If the default is overridden, then successful authentication requires
1.224 djm 189: completion of every method in at least one of these lists.
1.149 djm 190: .Pp
1.237 jmc 191: For example,
192: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 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,
1.237 jmc 196: so for this example it would not be possible to attempt password or
1.149 djm 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
1.237 jmc 202: .Cm bsdauth ,
203: .Cm pam ,
1.157 markus 204: or
1.237 jmc 205: .Cm skey ,
1.157 markus 206: depending on the server configuration.
207: For example,
1.237 jmc 208: .Qq keyboard-interactive:bsdauth
1.157 markus 209: would restrict keyboard interactive authentication to the
1.237 jmc 210: .Cm bsdauth
1.157 markus 211: device.
1.185 djm 212: .Pp
1.237 jmc 213: If the publickey method is listed more than once,
1.185 djm 214: .Xr sshd 8
215: verifies that keys that have been used successfully are not reused for
216: subsequent authentications.
1.237 jmc 217: For example,
218: .Qq publickey,publickey
219: requires successful authentication using two different public keys.
1.181 djm 220: .Pp
1.149 djm 221: Note that each authentication method listed should also be explicitly enabled
222: in the configuration.
1.251 djm 223: .Pp
224: The available authentication methods are:
225: .Qq gssapi-with-mic ,
226: .Qq hostbased ,
227: .Qq keyboard-interactive ,
228: .Qq none
229: (used for access to password-less accounts when
1.266 jmc 230: .Cm PermitEmptyPasswords
1.251 djm 231: is enabled),
232: .Qq password
233: and
234: .Qq publickey .
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: Arguments to
240: .Cm AuthorizedKeysCommand
1.235 jmc 241: accept the tokens described in the
242: .Sx TOKENS
243: section.
244: If no arguments are specified then the username of the target user is used.
1.201 djm 245: .Pp
246: The program should produce on standard output zero or
1.235 jmc 247: more lines of authorized_keys output (see
248: .Sx AUTHORIZED_KEYS
249: in
1.147 jmc 250: .Xr sshd 8 ) .
1.235 jmc 251: .Cm AuthorizedKeysCommand
1.310 ! djm 252: is tried after the usual
1.146 djm 253: .Cm AuthorizedKeysFile
1.310 ! djm 254: files and will not be executed if a matching key is found there.
1.237 jmc 255: By default, no
256: .Cm AuthorizedKeysCommand
257: is run.
1.146 djm 258: .It Cm AuthorizedKeysCommandUser
1.237 jmc 259: Specifies the user under whose account the
260: .Cm AuthorizedKeysCommand
261: is run.
1.146 djm 262: It is recommended to use a dedicated user that has no other role on the host
263: than running authorized keys commands.
1.186 djm 264: If
1.181 djm 265: .Cm AuthorizedKeysCommand
1.186 djm 266: is specified but
267: .Cm AuthorizedKeysCommandUser
268: is not, then
269: .Xr sshd 8
270: will refuse to start.
1.1 stevesk 271: .It Cm AuthorizedKeysFile
1.235 jmc 272: Specifies the file that contains the public keys used for user authentication.
1.285 schwarze 273: The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
1.124 djm 274: .Xr sshd 8 .
1.235 jmc 275: Arguments to
1.1 stevesk 276: .Cm AuthorizedKeysFile
1.235 jmc 277: accept the tokens described in the
278: .Sx TOKENS
279: section.
1.1 stevesk 280: After expansion,
281: .Cm AuthorizedKeysFile
282: is taken to be an absolute path or one relative to the user's home
283: directory.
1.133 jmc 284: Multiple files may be listed, separated by whitespace.
1.212 djm 285: Alternately this option may be set to
1.237 jmc 286: .Cm none
1.212 djm 287: to skip checking for user keys in files.
1.133 jmc 288: The default is
1.237 jmc 289: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 290: .It Cm AuthorizedPrincipalsCommand
291: Specifies a program to be used to generate the list of allowed
292: certificate principals as per
293: .Cm AuthorizedPrincipalsFile .
294: The program must be owned by root, not writable by group or others and
295: specified by an absolute path.
296: Arguments to
297: .Cm AuthorizedPrincipalsCommand
1.235 jmc 298: accept the tokens described in the
299: .Sx TOKENS
300: section.
301: If no arguments are specified then the username of the target user is used.
1.202 djm 302: .Pp
303: The program should produce on standard output zero or
304: more lines of
305: .Cm AuthorizedPrincipalsFile
306: output.
307: If either
308: .Cm AuthorizedPrincipalsCommand
309: or
310: .Cm AuthorizedPrincipalsFile
311: is specified, then certificates offered by the client for authentication
312: must contain a principal that is listed.
1.235 jmc 313: By default, no
314: .Cm AuthorizedPrincipalsCommand
315: is run.
1.202 djm 316: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 317: Specifies the user under whose account the
318: .Cm AuthorizedPrincipalsCommand
319: is run.
1.202 djm 320: It is recommended to use a dedicated user that has no other role on the host
321: than running authorized principals commands.
322: If
323: .Cm AuthorizedPrincipalsCommand
324: is specified but
325: .Cm AuthorizedPrincipalsCommandUser
326: is not, then
327: .Xr sshd 8
328: will refuse to start.
1.121 djm 329: .It Cm AuthorizedPrincipalsFile
330: Specifies a file that lists principal names that are accepted for
331: certificate authentication.
332: When using certificates signed by a key listed in
333: .Cm TrustedUserCAKeys ,
334: this file lists names, one of which must appear in the certificate for it
335: to be accepted for authentication.
1.235 jmc 336: Names are listed one per line preceded by key options (as described in
337: .Sx AUTHORIZED_KEYS FILE FORMAT
338: in
1.125 jmc 339: .Xr sshd 8 ) .
1.124 djm 340: Empty lines and comments starting with
1.121 djm 341: .Ql #
342: are ignored.
343: .Pp
1.235 jmc 344: Arguments to
1.121 djm 345: .Cm AuthorizedPrincipalsFile
1.235 jmc 346: accept the tokens described in the
347: .Sx TOKENS
348: section.
1.121 djm 349: After expansion,
350: .Cm AuthorizedPrincipalsFile
1.235 jmc 351: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 352: The default is
1.237 jmc 353: .Cm none ,
1.138 djm 354: i.e. not to use a principals file \(en in this case, the username
1.121 djm 355: of the user must appear in a certificate's principals list for it to be
356: accepted.
1.235 jmc 357: .Pp
1.121 djm 358: Note that
359: .Cm AuthorizedPrincipalsFile
360: is only used when authentication proceeds using a CA listed in
361: .Cm TrustedUserCAKeys
362: and is not consulted for certification authorities trusted via
363: .Pa ~/.ssh/authorized_keys ,
364: though the
365: .Cm principals=
366: key option offers a similar facility (see
367: .Xr sshd 8
368: for details).
1.1 stevesk 369: .It Cm Banner
370: The contents of the specified file are sent to the remote user before
371: authentication is allowed.
1.78 djm 372: If the argument is
1.237 jmc 373: .Cm none
1.78 djm 374: then no banner is displayed.
1.1 stevesk 375: By default, no banner is displayed.
1.282 djm 376: .It Cm CASignatureAlgorithms
377: Specifies which algorithms are allowed for signing of certificates
378: by certificate authorities (CAs).
379: The default is:
380: .Bd -literal -offset indent
1.287 dtucker 381: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.282 djm 382: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
383: .Ed
384: .Pp
385: Certificates signed using other algorithms will not be accepted for
386: public key or host-based authentication.
1.1 stevesk 387: .It Cm ChallengeResponseAuthentication
1.50 jmc 388: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 389: All authentication styles from
390: .Xr login.conf 5
391: are supported.
392: The default is
1.237 jmc 393: .Cm yes .
1.80 djm 394: .It Cm ChrootDirectory
1.113 stevesk 395: Specifies the pathname of a directory to
1.80 djm 396: .Xr chroot 2
397: to after authentication.
1.190 deraadt 398: At session startup
399: .Xr sshd 8
400: checks that all components of the pathname are root-owned directories
401: which are not writable by any other user or group.
1.106 stevesk 402: After the chroot,
403: .Xr sshd 8
404: changes the working directory to the user's home directory.
1.235 jmc 405: Arguments to
406: .Cm ChrootDirectory
407: accept the tokens described in the
408: .Sx TOKENS
409: section.
1.80 djm 410: .Pp
411: The
412: .Cm ChrootDirectory
413: must contain the necessary files and directories to support the
1.103 stevesk 414: user's session.
1.80 djm 415: For an interactive session this requires at least a shell, typically
416: .Xr sh 1 ,
417: and basic
418: .Pa /dev
419: nodes such as
420: .Xr null 4 ,
421: .Xr zero 4 ,
422: .Xr stdin 4 ,
423: .Xr stdout 4 ,
424: .Xr stderr 4 ,
1.180 jmc 425: and
1.80 djm 426: .Xr tty 4
427: devices.
1.237 jmc 428: For file transfer sessions using SFTP
429: no additional configuration of the environment is necessary if the in-process
430: sftp-server is used,
1.176 schwarze 431: though sessions which use logging may require
1.104 stevesk 432: .Pa /dev/log
1.176 schwarze 433: inside the chroot directory on some operating systems (see
1.104 stevesk 434: .Xr sftp-server 8
1.81 jmc 435: for details).
1.190 deraadt 436: .Pp
1.191 jmc 437: For safety, it is very important that the directory hierarchy be
1.190 deraadt 438: prevented from modification by other processes on the system (especially
439: those outside the jail).
440: Misconfiguration can lead to unsafe environments which
441: .Xr sshd 8
442: cannot detect.
1.80 djm 443: .Pp
1.215 djm 444: The default is
1.237 jmc 445: .Cm none ,
1.215 djm 446: indicating not to
1.80 djm 447: .Xr chroot 2 .
1.1 stevesk 448: .It Cm Ciphers
1.219 jmc 449: Specifies the ciphers allowed.
1.1 stevesk 450: Multiple ciphers must be comma-separated.
1.289 naddy 451: If the specified list begins with a
1.208 djm 452: .Sq +
453: character, then the specified ciphers will be appended to the default set
454: instead of replacing them.
1.289 naddy 455: If the specified list begins with a
1.242 djm 456: .Sq -
457: character, then the specified ciphers (including wildcards) will be removed
458: from the default set instead of replacing them.
1.290 naddy 459: If the specified list begins with a
460: .Sq ^
461: character, then the specified ciphers will be placed at the head of the
462: default set.
1.208 djm 463: .Pp
1.167 djm 464: The supported ciphers are:
465: .Pp
1.173 naddy 466: .Bl -item -compact -offset indent
467: .It
468: 3des-cbc
469: .It
470: aes128-cbc
471: .It
472: aes192-cbc
473: .It
474: aes256-cbc
475: .It
476: aes128-ctr
477: .It
478: aes192-ctr
479: .It
480: aes256-ctr
481: .It
482: aes128-gcm@openssh.com
483: .It
484: aes256-gcm@openssh.com
485: .It
486: chacha20-poly1305@openssh.com
487: .El
1.167 djm 488: .Pp
1.52 jmc 489: The default is:
1.173 naddy 490: .Bd -literal -offset indent
1.211 jmc 491: chacha20-poly1305@openssh.com,
1.173 naddy 492: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 493: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 494: .Ed
1.167 djm 495: .Pp
1.237 jmc 496: The list of available ciphers may also be obtained using
497: .Qq ssh -Q cipher .
1.1 stevesk 498: .It Cm ClientAliveCountMax
1.237 jmc 499: Sets the number of client alive messages which may be sent without
1.52 jmc 500: .Xr sshd 8
1.17 jmc 501: receiving any messages back from the client.
502: If this threshold is reached while client alive messages are being sent,
1.52 jmc 503: sshd will disconnect the client, terminating the session.
1.17 jmc 504: It is important to note that the use of client alive messages is very
505: different from
1.237 jmc 506: .Cm TCPKeepAlive .
1.17 jmc 507: The client alive messages are sent through the encrypted channel
508: and therefore will not be spoofable.
509: The TCP keepalive option enabled by
1.27 markus 510: .Cm TCPKeepAlive
1.17 jmc 511: is spoofable.
512: The client alive mechanism is valuable when the client or
1.288 dtucker 513: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 514: .Pp
1.17 jmc 515: The default value is 3.
516: If
1.1 stevesk 517: .Cm ClientAliveInterval
1.237 jmc 518: is set to 15, and
1.1 stevesk 519: .Cm ClientAliveCountMax
1.52 jmc 520: is left at the default, unresponsive SSH clients
1.1 stevesk 521: will be disconnected after approximately 45 seconds.
1.302 djm 522: Setting a zero
523: .Cm ClientAliveCountMax
524: disables connection termination.
1.42 djm 525: .It Cm ClientAliveInterval
526: Sets a timeout interval in seconds after which if no data has been received
527: from the client,
1.52 jmc 528: .Xr sshd 8
1.42 djm 529: will send a message through the encrypted
530: channel to request a response from the client.
531: The default
532: is 0, indicating that these messages will not be sent to the client.
1.3 markus 533: .It Cm Compression
1.236 djm 534: Specifies whether compression is enabled after
1.44 markus 535: the user has authenticated successfully.
1.3 markus 536: The argument must be
1.237 jmc 537: .Cm yes ,
538: .Cm delayed
1.236 djm 539: (a legacy synonym for
1.237 jmc 540: .Cm yes )
1.3 markus 541: or
1.237 jmc 542: .Cm no .
1.3 markus 543: The default is
1.237 jmc 544: .Cm yes .
1.1 stevesk 545: .It Cm DenyGroups
546: This keyword can be followed by a list of group name patterns, separated
547: by spaces.
548: Login is disallowed for users whose primary group or supplementary
549: group list matches one of the patterns.
550: Only group names are valid; a numerical group ID is not recognized.
551: By default, login is allowed for all groups.
1.301 djm 552: The allow/deny groups directives are processed in the following order:
1.54 jmc 553: .Cm DenyGroups ,
554: .Cm AllowGroups .
1.49 jmc 555: .Pp
1.161 jmc 556: See PATTERNS in
1.49 jmc 557: .Xr ssh_config 5
558: for more information on patterns.
1.1 stevesk 559: .It Cm DenyUsers
560: This keyword can be followed by a list of user name patterns, separated
561: by spaces.
562: Login is disallowed for user names that match one of the patterns.
563: Only user names are valid; a numerical user ID is not recognized.
564: By default, login is allowed for all users.
565: If the pattern takes the form USER@HOST then USER and HOST
566: are separately checked, restricting logins to particular
567: users from particular hosts.
1.222 jmc 568: HOST criteria may additionally contain addresses to match in CIDR
569: address/masklen format.
1.301 djm 570: The allow/deny users directives are processed in the following order:
1.54 jmc 571: .Cm DenyUsers ,
1.301 djm 572: .Cm AllowUsers .
1.49 jmc 573: .Pp
1.161 jmc 574: See PATTERNS in
1.49 jmc 575: .Xr ssh_config 5
576: for more information on patterns.
1.239 djm 577: .It Cm DisableForwarding
578: Disables all forwarding features, including X11,
579: .Xr ssh-agent 1 ,
580: TCP and StreamLocal.
581: This option overrides all other forwarding-related options and may
582: simplify restricted configurations.
1.246 djm 583: .It Cm ExposeAuthInfo
1.253 jmc 584: Writes a temporary file containing a list of authentication methods and
1.246 djm 585: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 586: The location of the file is exposed to the user session through the
1.248 djm 587: .Ev SSH_USER_AUTH
1.247 jmc 588: environment variable.
1.253 jmc 589: The default is
590: .Cm no .
1.183 djm 591: .It Cm FingerprintHash
592: Specifies the hash algorithm used when logging key fingerprints.
593: Valid options are:
1.237 jmc 594: .Cm md5
1.183 djm 595: and
1.237 jmc 596: .Cm sha256 .
1.183 djm 597: The default is
1.237 jmc 598: .Cm sha256 .
1.67 dtucker 599: .It Cm ForceCommand
600: Forces the execution of the command specified by
601: .Cm ForceCommand ,
1.84 djm 602: ignoring any command supplied by the client and
603: .Pa ~/.ssh/rc
604: if present.
1.67 dtucker 605: The command is invoked by using the user's login shell with the -c option.
606: This applies to shell, command, or subsystem execution.
607: It is most useful inside a
608: .Cm Match
609: block.
610: The command originally supplied by the client is available in the
611: .Ev SSH_ORIGINAL_COMMAND
612: environment variable.
1.82 djm 613: Specifying a command of
1.237 jmc 614: .Cm internal-sftp
615: will force the use of an in-process SFTP server that requires no support
1.82 djm 616: files when used with
617: .Cm ChrootDirectory .
1.215 djm 618: The default is
1.237 jmc 619: .Cm none .
1.1 stevesk 620: .It Cm GatewayPorts
621: Specifies whether remote hosts are allowed to connect to ports
622: forwarded for the client.
623: By default,
1.52 jmc 624: .Xr sshd 8
1.15 jmc 625: binds remote port forwardings to the loopback address.
626: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 627: .Cm GatewayPorts
1.52 jmc 628: can be used to specify that sshd
1.39 djm 629: should allow remote port forwardings to bind to non-loopback addresses, thus
630: allowing other hosts to connect.
631: The argument may be
1.237 jmc 632: .Cm no
1.39 djm 633: to force remote port forwardings to be available to the local host only,
1.237 jmc 634: .Cm yes
1.39 djm 635: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 636: .Cm clientspecified
1.39 djm 637: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 638: The default is
1.237 jmc 639: .Cm no .
1.23 markus 640: .It Cm GSSAPIAuthentication
1.25 markus 641: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 642: The default is
1.237 jmc 643: .Cm no .
1.23 markus 644: .It Cm GSSAPICleanupCredentials
645: Specifies whether to automatically destroy the user's credentials cache
646: on logout.
647: The default is
1.237 jmc 648: .Cm yes .
1.203 djm 649: .It Cm GSSAPIStrictAcceptorCheck
650: Determines whether to be strict about the identity of the GSSAPI acceptor
651: a client authenticates against.
652: If set to
1.237 jmc 653: .Cm yes
654: then the client must authenticate against the host
1.203 djm 655: service on the current hostname.
656: If set to
1.237 jmc 657: .Cm no
1.203 djm 658: then the client may authenticate against any service key stored in the
659: machine's default store.
660: This facility is provided to assist with operation on multi homed machines.
661: The default is
1.237 jmc 662: .Cm yes .
1.189 djm 663: .It Cm HostbasedAcceptedKeyTypes
664: Specifies the key types that will be accepted for hostbased authentication
1.280 djm 665: as a list of comma-separated patterns.
1.289 naddy 666: Alternately if the specified list begins with a
1.208 djm 667: .Sq +
668: character, then the specified key types will be appended to the default set
669: instead of replacing them.
1.289 naddy 670: If the specified list begins with a
1.242 djm 671: .Sq -
672: character, then the specified key types (including wildcards) will be removed
673: from the default set instead of replacing them.
1.290 naddy 674: If the specified list begins with a
675: .Sq ^
676: character, then the specified key types will be placed at the head of the
677: default set.
1.206 markus 678: The default for this option is:
679: .Bd -literal -offset 3n
680: ecdsa-sha2-nistp256-cert-v01@openssh.com,
681: ecdsa-sha2-nistp384-cert-v01@openssh.com,
682: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 683: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 684: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 685: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 686: rsa-sha2-512-cert-v01@openssh.com,
687: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 688: ssh-rsa-cert-v01@openssh.com,
689: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 690: sk-ecdsa-sha2-nistp256@openssh.com,
691: ssh-ed25519,sk-ssh-ed25519@openssh.com,
692: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 693: .Ed
694: .Pp
1.237 jmc 695: The list of available key types may also be obtained using
1.307 dtucker 696: .Qq ssh -Q HostbasedAcceptedKeyTypes .
1.1 stevesk 697: .It Cm HostbasedAuthentication
698: Specifies whether rhosts or /etc/hosts.equiv authentication together
699: with successful public key client host authentication is allowed
1.50 jmc 700: (host-based authentication).
1.70 dtucker 701: The default is
1.237 jmc 702: .Cm no .
1.70 dtucker 703: .It Cm HostbasedUsesNameFromPacketOnly
704: Specifies whether or not the server will attempt to perform a reverse
705: name lookup when matching the name in the
706: .Pa ~/.shosts ,
707: .Pa ~/.rhosts ,
708: and
709: .Pa /etc/hosts.equiv
710: files during
711: .Cm HostbasedAuthentication .
712: A setting of
1.237 jmc 713: .Cm yes
1.70 dtucker 714: means that
715: .Xr sshd 8
716: uses the name supplied by the client rather than
717: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 718: The default is
1.237 jmc 719: .Cm no .
1.117 djm 720: .It Cm HostCertificate
721: Specifies a file containing a public host certificate.
722: The certificate's public key must match a private host key already specified
723: by
724: .Cm HostKey .
725: The default behaviour of
726: .Xr sshd 8
727: is not to load any certificates.
1.1 stevesk 728: .It Cm HostKey
729: Specifies a file containing a private host key
730: used by SSH.
1.229 naddy 731: The defaults are
1.169 naddy 732: .Pa /etc/ssh/ssh_host_ecdsa_key ,
733: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 734: and
1.229 naddy 735: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 736: .Pp
1.1 stevesk 737: Note that
1.52 jmc 738: .Xr sshd 8
1.206 markus 739: will refuse to use a file if it is group/world-accessible
740: and that the
741: .Cm HostKeyAlgorithms
742: option restricts which of the keys are actually used by
743: .Xr sshd 8 .
744: .Pp
1.1 stevesk 745: It is possible to have multiple host key files.
1.162 markus 746: It is also possible to specify public host key files instead.
747: In this case operations on the private key will be delegated
748: to an
749: .Xr ssh-agent 1 .
750: .It Cm HostKeyAgent
751: Identifies the UNIX-domain socket used to communicate
752: with an agent that has access to the private host keys.
1.223 markus 753: If the string
1.237 jmc 754: .Qq SSH_AUTH_SOCK
1.162 markus 755: is specified, the location of the socket will be read from the
756: .Ev SSH_AUTH_SOCK
757: environment variable.
1.206 markus 758: .It Cm HostKeyAlgorithms
1.219 jmc 759: Specifies the host key algorithms
1.206 markus 760: that the server offers.
761: The default for this option is:
762: .Bd -literal -offset 3n
763: ecdsa-sha2-nistp256-cert-v01@openssh.com,
764: ecdsa-sha2-nistp384-cert-v01@openssh.com,
765: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 766: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 767: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 768: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 769: rsa-sha2-512-cert-v01@openssh.com,
770: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 771: ssh-rsa-cert-v01@openssh.com,
772: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 773: sk-ecdsa-sha2-nistp256@openssh.com,
774: ssh-ed25519,sk-ssh-ed25519@openssh.com,
775: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 776: .Ed
777: .Pp
1.237 jmc 778: The list of available key types may also be obtained using
1.307 dtucker 779: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 780: .It Cm IgnoreRhosts
1.309 djm 781: Specifies whether to ignore per-user
1.1 stevesk 782: .Pa .rhosts
783: and
784: .Pa .shosts
1.309 djm 785: files during
1.1 stevesk 786: .Cm HostbasedAuthentication .
1.309 djm 787: The system-wide
1.1 stevesk 788: .Pa /etc/hosts.equiv
789: and
790: .Pa /etc/shosts.equiv
1.309 djm 791: are still used regardless of this setting.
792: .Pp
793: Accepted values are
794: .Cm yes
795: (the default) to ignore all per-user files,
796: .Cm shosts-only
797: to allow the use of
798: .Pa .shosts
799: but to ignore
800: .Pa .rhosts
801: or
802: .Cm no
803: to allow both
804: .Pa .shosts
805: and
806: .Pa rhosts.
1.1 stevesk 807: .It Cm IgnoreUserKnownHosts
808: Specifies whether
1.52 jmc 809: .Xr sshd 8
1.1 stevesk 810: should ignore the user's
1.41 djm 811: .Pa ~/.ssh/known_hosts
1.1 stevesk 812: during
1.261 djm 813: .Cm HostbasedAuthentication
814: and use only the system-wide known hosts file
815: .Pa /etc/ssh/known_hosts .
1.1 stevesk 816: The default is
1.304 djm 817: .Dq no .
818: .It Cm Include
819: Include the specified configuration file(s).
1.305 jmc 820: Multiple pathnames may be specified and each pathname may contain
1.304 djm 821: .Xr glob 7
822: wildcards.
823: Files without absolute paths are assumed to be in
824: .Pa /etc/ssh .
1.305 jmc 825: An
1.304 djm 826: .Cm Include
827: directive may appear inside a
828: .Cm Match
829: block
830: to perform conditional inclusion.
1.129 djm 831: .It Cm IPQoS
832: Specifies the IPv4 type-of-service or DSCP class for the connection.
833: Accepted values are
1.237 jmc 834: .Cm af11 ,
835: .Cm af12 ,
836: .Cm af13 ,
837: .Cm af21 ,
838: .Cm af22 ,
839: .Cm af23 ,
840: .Cm af31 ,
841: .Cm af32 ,
842: .Cm af33 ,
843: .Cm af41 ,
844: .Cm af42 ,
845: .Cm af43 ,
846: .Cm cs0 ,
847: .Cm cs1 ,
848: .Cm cs2 ,
849: .Cm cs3 ,
850: .Cm cs4 ,
851: .Cm cs5 ,
852: .Cm cs6 ,
853: .Cm cs7 ,
854: .Cm ef ,
1.303 djm 855: .Cm le ,
1.237 jmc 856: .Cm lowdelay ,
857: .Cm throughput ,
858: .Cm reliability ,
1.250 djm 859: a numeric value, or
860: .Cm none
861: to use the operating system default.
1.131 djm 862: This option may take one or two arguments, separated by whitespace.
1.129 djm 863: If one argument is specified, it is used as the packet class unconditionally.
864: If two values are specified, the first is automatically selected for
865: interactive sessions and the second for non-interactive sessions.
866: The default is
1.264 job 867: .Cm af21
1.265 jmc 868: (Low-Latency Data)
1.129 djm 869: for interactive sessions and
1.264 job 870: .Cm cs1
1.265 jmc 871: (Lower Effort)
1.129 djm 872: for non-interactive sessions.
1.171 jmc 873: .It Cm KbdInteractiveAuthentication
874: Specifies whether to allow keyboard-interactive authentication.
875: The argument to this keyword must be
1.237 jmc 876: .Cm yes
1.171 jmc 877: or
1.237 jmc 878: .Cm no .
1.171 jmc 879: The default is to use whatever value
880: .Cm ChallengeResponseAuthentication
881: is set to
882: (by default
1.237 jmc 883: .Cm yes ) .
1.1 stevesk 884: .It Cm KerberosAuthentication
1.24 markus 885: Specifies whether the password provided by the user for
1.1 stevesk 886: .Cm PasswordAuthentication
1.24 markus 887: will be validated through the Kerberos KDC.
1.1 stevesk 888: To use this option, the server needs a
889: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 890: The default is
1.237 jmc 891: .Cm no .
1.29 dtucker 892: .It Cm KerberosGetAFSToken
1.45 djm 893: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 894: an AFS token before accessing the user's home directory.
1.52 jmc 895: The default is
1.237 jmc 896: .Cm no .
1.1 stevesk 897: .It Cm KerberosOrLocalPasswd
1.52 jmc 898: If password authentication through Kerberos fails then
1.1 stevesk 899: the password will be validated via any additional local mechanism
900: such as
901: .Pa /etc/passwd .
1.52 jmc 902: The default is
1.237 jmc 903: .Cm yes .
1.1 stevesk 904: .It Cm KerberosTicketCleanup
905: Specifies whether to automatically destroy the user's ticket cache
906: file on logout.
1.52 jmc 907: The default is
1.237 jmc 908: .Cm yes .
1.127 djm 909: .It Cm KexAlgorithms
910: Specifies the available KEX (Key Exchange) algorithms.
911: Multiple algorithms must be comma-separated.
1.289 naddy 912: Alternately if the specified list begins with a
1.208 djm 913: .Sq +
914: character, then the specified methods will be appended to the default set
915: instead of replacing them.
1.289 naddy 916: If the specified list begins with a
1.242 djm 917: .Sq -
918: character, then the specified methods (including wildcards) will be removed
919: from the default set instead of replacing them.
1.290 naddy 920: If the specified list begins with a
921: .Sq ^
922: character, then the specified methods will be placed at the head of the
923: default set.
1.173 naddy 924: The supported algorithms are:
925: .Pp
926: .Bl -item -compact -offset indent
927: .It
1.234 djm 928: curve25519-sha256
929: .It
1.173 naddy 930: curve25519-sha256@libssh.org
931: .It
932: diffie-hellman-group1-sha1
933: .It
934: diffie-hellman-group14-sha1
935: .It
1.263 djm 936: diffie-hellman-group14-sha256
937: .It
938: diffie-hellman-group16-sha512
939: .It
940: diffie-hellman-group18-sha512
941: .It
1.173 naddy 942: diffie-hellman-group-exchange-sha1
943: .It
944: diffie-hellman-group-exchange-sha256
945: .It
946: ecdh-sha2-nistp256
947: .It
948: ecdh-sha2-nistp384
949: .It
950: ecdh-sha2-nistp521
1.298 dtucker 951: .It
952: sntrup4591761x25519-sha512@tinyssh.org
1.173 naddy 953: .El
954: .Pp
955: The default is:
1.170 dtucker 956: .Bd -literal -offset indent
1.234 djm 957: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 958: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
959: diffie-hellman-group-exchange-sha256,
1.263 djm 960: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 961: diffie-hellman-group14-sha256
1.170 dtucker 962: .Ed
1.188 djm 963: .Pp
1.237 jmc 964: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 965: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 966: .It Cm ListenAddress
967: Specifies the local addresses
1.52 jmc 968: .Xr sshd 8
1.1 stevesk 969: should listen on.
970: The following forms may be used:
971: .Pp
972: .Bl -item -offset indent -compact
973: .It
974: .Cm ListenAddress
975: .Sm off
1.259 jmc 976: .Ar hostname | address
1.1 stevesk 977: .Sm on
1.260 jmc 978: .Op Cm rdomain Ar domain
1.1 stevesk 979: .It
980: .Cm ListenAddress
981: .Sm off
1.259 jmc 982: .Ar hostname : port
1.1 stevesk 983: .Sm on
1.260 jmc 984: .Op Cm rdomain Ar domain
1.256 djm 985: .It
986: .Cm ListenAddress
987: .Sm off
1.259 jmc 988: .Ar IPv4_address : port
1.256 djm 989: .Sm on
1.260 jmc 990: .Op Cm rdomain Ar domain
1.1 stevesk 991: .It
992: .Cm ListenAddress
993: .Sm off
1.259 jmc 994: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 995: .Sm on
1.260 jmc 996: .Op Cm rdomain Ar domain
1.1 stevesk 997: .El
998: .Pp
1.256 djm 999: The optional
1000: .Cm rdomain
1001: qualifier requests
1002: .Xr sshd 8
1003: listen in an explicit routing domain.
1.1 stevesk 1004: If
1005: .Ar port
1006: is not specified,
1.200 dtucker 1007: sshd will listen on the address and all
1.1 stevesk 1008: .Cm Port
1.17 jmc 1009: options specified.
1.256 djm 1010: The default is to listen on all local addresses on the current default
1011: routing domain.
1.15 jmc 1012: Multiple
1.1 stevesk 1013: .Cm ListenAddress
1.17 jmc 1014: options are permitted.
1.256 djm 1015: For more information on routing domains, see
1.259 jmc 1016: .Xr rdomain 4 .
1.1 stevesk 1017: .It Cm LoginGraceTime
1018: The server disconnects after this time if the user has not
1019: successfully logged in.
1020: If the value is 0, there is no time limit.
1.12 stevesk 1021: The default is 120 seconds.
1.1 stevesk 1022: .It Cm LogLevel
1023: Gives the verbosity level that is used when logging messages from
1.53 jmc 1024: .Xr sshd 8 .
1.1 stevesk 1025: The possible values are:
1.52 jmc 1026: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1027: The default is INFO.
1028: DEBUG and DEBUG1 are equivalent.
1029: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1030: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1031: .It Cm MACs
1032: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1033: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1034: Multiple algorithms must be comma-separated.
1.289 naddy 1035: If the specified list begins with a
1.208 djm 1036: .Sq +
1037: character, then the specified algorithms will be appended to the default set
1038: instead of replacing them.
1.289 naddy 1039: If the specified list begins with a
1.242 djm 1040: .Sq -
1041: character, then the specified algorithms (including wildcards) will be removed
1042: from the default set instead of replacing them.
1.290 naddy 1043: If the specified list begins with a
1044: .Sq ^
1045: character, then the specified algorithms will be placed at the head of the
1046: default set.
1.208 djm 1047: .Pp
1.152 markus 1048: The algorithms that contain
1.237 jmc 1049: .Qq -etm
1.152 markus 1050: calculate the MAC after encryption (encrypt-then-mac).
1051: These are considered safer and their use recommended.
1.173 naddy 1052: The supported MACs are:
1053: .Pp
1054: .Bl -item -compact -offset indent
1055: .It
1056: hmac-md5
1057: .It
1058: hmac-md5-96
1059: .It
1060: hmac-sha1
1061: .It
1062: hmac-sha1-96
1063: .It
1064: hmac-sha2-256
1065: .It
1066: hmac-sha2-512
1067: .It
1068: umac-64@openssh.com
1069: .It
1070: umac-128@openssh.com
1071: .It
1072: hmac-md5-etm@openssh.com
1073: .It
1074: hmac-md5-96-etm@openssh.com
1075: .It
1076: hmac-sha1-etm@openssh.com
1077: .It
1078: hmac-sha1-96-etm@openssh.com
1079: .It
1080: hmac-sha2-256-etm@openssh.com
1081: .It
1082: hmac-sha2-512-etm@openssh.com
1083: .It
1084: umac-64-etm@openssh.com
1085: .It
1086: umac-128-etm@openssh.com
1087: .El
1088: .Pp
1.52 jmc 1089: The default is:
1.77 jmc 1090: .Bd -literal -offset indent
1.152 markus 1091: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1092: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1093: hmac-sha1-etm@openssh.com,
1.173 naddy 1094: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1095: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1096: .Ed
1.188 djm 1097: .Pp
1.237 jmc 1098: The list of available MAC algorithms may also be obtained using
1099: .Qq ssh -Q mac .
1.60 dtucker 1100: .It Cm Match
1.61 jmc 1101: Introduces a conditional block.
1.65 dtucker 1102: If all of the criteria on the
1.60 dtucker 1103: .Cm Match
1.65 dtucker 1104: line are satisfied, the keywords on the following lines override those
1105: set in the global section of the config file, until either another
1.60 dtucker 1106: .Cm Match
1.65 dtucker 1107: line or the end of the file.
1.172 djm 1108: If a keyword appears in multiple
1109: .Cm Match
1.177 sobrado 1110: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1111: applied.
1.91 djm 1112: .Pp
1.61 jmc 1113: The arguments to
1.60 dtucker 1114: .Cm Match
1.163 dtucker 1115: are one or more criteria-pattern pairs or the single token
1116: .Cm All
1117: which matches all criteria.
1.60 dtucker 1118: The available criteria are
1119: .Cm User ,
1.69 dtucker 1120: .Cm Group ,
1.60 dtucker 1121: .Cm Host ,
1.139 dtucker 1122: .Cm LocalAddress ,
1123: .Cm LocalPort ,
1.258 djm 1124: .Cm RDomain ,
1.60 dtucker 1125: and
1.258 djm 1126: .Cm Address
1127: (with
1128: .Cm RDomain
1129: representing the
1130: .Xr rdomain 4
1.284 jmc 1131: on which the connection was received).
1.258 djm 1132: .Pp
1.91 djm 1133: The match patterns may consist of single entries or comma-separated
1134: lists and may use the wildcard and negation operators described in the
1.237 jmc 1135: .Sx PATTERNS
1136: section of
1.92 djm 1137: .Xr ssh_config 5 .
1.91 djm 1138: .Pp
1139: The patterns in an
1140: .Cm Address
1141: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1142: address/masklen format,
1143: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1144: Note that the mask length provided must be consistent with the address -
1145: it is an error to specify a mask length that is too long for the address
1.93 jmc 1146: or one with bits set in this host portion of the address.
1.237 jmc 1147: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1148: .Pp
1.60 dtucker 1149: Only a subset of keywords may be used on the lines following a
1150: .Cm Match
1151: keyword.
1152: Available keywords are
1.142 jmc 1153: .Cm AcceptEnv ,
1.99 okan 1154: .Cm AllowAgentForwarding ,
1.142 jmc 1155: .Cm AllowGroups ,
1.193 djm 1156: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1157: .Cm AllowTcpForwarding ,
1.141 markus 1158: .Cm AllowUsers ,
1.149 djm 1159: .Cm AuthenticationMethods ,
1.146 djm 1160: .Cm AuthorizedKeysCommand ,
1161: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1162: .Cm AuthorizedKeysFile ,
1.214 djm 1163: .Cm AuthorizedPrincipalsCommand ,
1164: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1165: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1166: .Cm Banner ,
1.85 djm 1167: .Cm ChrootDirectory ,
1.238 markus 1168: .Cm ClientAliveCountMax ,
1169: .Cm ClientAliveInterval ,
1.141 markus 1170: .Cm DenyGroups ,
1171: .Cm DenyUsers ,
1.67 dtucker 1172: .Cm ForceCommand ,
1.194 djm 1173: .Cm GatewayPorts ,
1.193 djm 1174: .Cm GSSAPIAuthentication ,
1.189 djm 1175: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1176: .Cm HostbasedAuthentication ,
1.123 djm 1177: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1178: .Cm IgnoreRhosts ,
1.306 naddy 1179: .Cm Include ,
1.193 djm 1180: .Cm IPQoS ,
1.74 jmc 1181: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1182: .Cm KerberosAuthentication ,
1.245 djm 1183: .Cm LogLevel ,
1.95 dtucker 1184: .Cm MaxAuthTries ,
1.94 dtucker 1185: .Cm MaxSessions ,
1.72 dtucker 1186: .Cm PasswordAuthentication ,
1.97 djm 1187: .Cm PermitEmptyPasswords ,
1.271 djm 1188: .Cm PermitListen ,
1.66 dtucker 1189: .Cm PermitOpen ,
1.79 dtucker 1190: .Cm PermitRootLogin ,
1.164 djm 1191: .Cm PermitTTY ,
1.123 djm 1192: .Cm PermitTunnel ,
1.174 djm 1193: .Cm PermitUserRC ,
1.189 djm 1194: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1195: .Cm PubkeyAuthentication ,
1.159 dtucker 1196: .Cm RekeyLimit ,
1.193 djm 1197: .Cm RevokedKeys ,
1.257 djm 1198: .Cm RDomain ,
1.274 djm 1199: .Cm SetEnv ,
1.193 djm 1200: .Cm StreamLocalBindMask ,
1201: .Cm StreamLocalBindUnlink ,
1202: .Cm TrustedUserCAKeys ,
1.66 dtucker 1203: .Cm X11DisplayOffset ,
1.101 djm 1204: .Cm X11Forwarding
1.60 dtucker 1205: and
1.286 jmc 1206: .Cm X11UseLocalhost .
1.33 dtucker 1207: .It Cm MaxAuthTries
1208: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1209: connection.
1210: Once the number of failures reaches half this value,
1211: additional failures are logged.
1212: The default is 6.
1.90 djm 1213: .It Cm MaxSessions
1.216 djm 1214: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1215: sessions permitted per network connection.
1216: Multiple sessions may be established by clients that support connection
1217: multiplexing.
1218: Setting
1219: .Cm MaxSessions
1220: to 1 will effectively disable session multiplexing, whereas setting it to 0
1221: will prevent all shell, login and subsystem sessions while still permitting
1222: forwarding.
1.90 djm 1223: The default is 10.
1.1 stevesk 1224: .It Cm MaxStartups
1225: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1226: SSH daemon.
1.1 stevesk 1227: Additional connections will be dropped until authentication succeeds or the
1228: .Cm LoginGraceTime
1229: expires for a connection.
1.156 dtucker 1230: The default is 10:30:100.
1.1 stevesk 1231: .Pp
1232: Alternatively, random early drop can be enabled by specifying
1233: the three colon separated values
1.237 jmc 1234: start:rate:full (e.g. "10:30:60").
1.53 jmc 1235: .Xr sshd 8
1.237 jmc 1236: will refuse connection attempts with a probability of rate/100 (30%)
1237: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1238: The probability increases linearly and all connection attempts
1.237 jmc 1239: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1240: .It Cm PasswordAuthentication
1241: Specifies whether password authentication is allowed.
1242: The default is
1.237 jmc 1243: .Cm yes .
1.1 stevesk 1244: .It Cm PermitEmptyPasswords
1245: When password authentication is allowed, it specifies whether the
1246: server allows login to accounts with empty password strings.
1247: The default is
1.237 jmc 1248: .Cm no .
1.271 djm 1249: .It Cm PermitListen
1250: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1251: The listen specification must be one of the following forms:
1252: .Pp
1253: .Bl -item -offset indent -compact
1254: .It
1255: .Cm PermitListen
1256: .Sm off
1.276 djm 1257: .Ar port
1.271 djm 1258: .Sm on
1259: .It
1260: .Cm PermitListen
1261: .Sm off
1.276 djm 1262: .Ar host : port
1.271 djm 1263: .Sm on
1264: .El
1265: .Pp
1266: Multiple permissions may be specified by separating them with whitespace.
1267: An argument of
1268: .Cm any
1269: can be used to remove all restrictions and permit any listen requests.
1270: An argument of
1271: .Cm none
1272: can be used to prohibit all listen requests.
1273: The host name may contain wildcards as described in the PATTERNS section in
1274: .Xr ssh_config 5 .
1275: The wildcard
1276: .Sq *
1277: can also be used in place of a port number to allow all ports.
1278: By default all port forwarding listen requests are permitted.
1.272 jmc 1279: Note that the
1.271 djm 1280: .Cm GatewayPorts
1281: option may further restrict which addresses may be listened on.
1.276 djm 1282: Note also that
1283: .Xr ssh 1
1284: will request a listen host of
1285: .Dq localhost
1.283 dtucker 1286: if no listen host was specifically requested, and this name is
1.277 jmc 1287: treated differently to explicit localhost addresses of
1.276 djm 1288: .Dq 127.0.0.1
1289: and
1290: .Dq ::1 .
1.62 dtucker 1291: .It Cm PermitOpen
1292: Specifies the destinations to which TCP port forwarding is permitted.
1293: The forwarding specification must be one of the following forms:
1294: .Pp
1295: .Bl -item -offset indent -compact
1296: .It
1297: .Cm PermitOpen
1298: .Sm off
1299: .Ar host : port
1300: .Sm on
1301: .It
1302: .Cm PermitOpen
1303: .Sm off
1304: .Ar IPv4_addr : port
1305: .Sm on
1306: .It
1307: .Cm PermitOpen
1308: .Sm off
1309: .Ar \&[ IPv6_addr \&] : port
1310: .Sm on
1311: .El
1312: .Pp
1.68 dtucker 1313: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1314: An argument of
1.237 jmc 1315: .Cm any
1.62 dtucker 1316: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1317: An argument of
1.237 jmc 1318: .Cm none
1.140 dtucker 1319: can be used to prohibit all forwarding requests.
1.227 jmc 1320: The wildcard
1.237 jmc 1321: .Sq *
1.299 djm 1322: can be used for host or port to allow all hosts or ports respectively.
1323: Otherwise, no pattern matching or address lookups are performed on supplied
1324: names.
1.63 jmc 1325: By default all port forwarding requests are permitted.
1.1 stevesk 1326: .It Cm PermitRootLogin
1.38 jmc 1327: Specifies whether root can log in using
1.1 stevesk 1328: .Xr ssh 1 .
1329: The argument must be
1.237 jmc 1330: .Cm yes ,
1331: .Cm prohibit-password ,
1332: .Cm forced-commands-only ,
1.1 stevesk 1333: or
1.237 jmc 1334: .Cm no .
1.1 stevesk 1335: The default is
1.237 jmc 1336: .Cm prohibit-password .
1.1 stevesk 1337: .Pp
1338: If this option is set to
1.237 jmc 1339: .Cm prohibit-password
1.255 jmc 1340: (or its deprecated alias,
1341: .Cm without-password ) ,
1.210 deraadt 1342: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1343: .Pp
1344: If this option is set to
1.237 jmc 1345: .Cm forced-commands-only ,
1.1 stevesk 1346: root login with public key authentication will be allowed,
1347: but only if the
1348: .Ar command
1349: option has been specified
1350: (which may be useful for taking remote backups even if root login is
1.17 jmc 1351: normally not allowed).
1352: All other authentication methods are disabled for root.
1.1 stevesk 1353: .Pp
1354: If this option is set to
1.237 jmc 1355: .Cm no ,
1.38 jmc 1356: root is not allowed to log in.
1.231 jmc 1357: .It Cm PermitTTY
1358: Specifies whether
1359: .Xr pty 4
1360: allocation is permitted.
1361: The default is
1.237 jmc 1362: .Cm yes .
1.46 reyk 1363: .It Cm PermitTunnel
1364: Specifies whether
1365: .Xr tun 4
1366: device forwarding is allowed.
1.47 reyk 1367: The argument must be
1.237 jmc 1368: .Cm yes ,
1369: .Cm point-to-point
1.58 stevesk 1370: (layer 3),
1.237 jmc 1371: .Cm ethernet
1.58 stevesk 1372: (layer 2), or
1.237 jmc 1373: .Cm no .
1.58 stevesk 1374: Specifying
1.237 jmc 1375: .Cm yes
1.58 stevesk 1376: permits both
1.237 jmc 1377: .Cm point-to-point
1.58 stevesk 1378: and
1.237 jmc 1379: .Cm ethernet .
1.46 reyk 1380: The default is
1.237 jmc 1381: .Cm no .
1.178 djm 1382: .Pp
1383: Independent of this setting, the permissions of the selected
1384: .Xr tun 4
1385: device must allow access to the user.
1.6 markus 1386: .It Cm PermitUserEnvironment
1387: Specifies whether
1388: .Pa ~/.ssh/environment
1.9 stevesk 1389: and
1.6 markus 1390: .Cm environment=
1391: options in
1392: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1393: are processed by
1.52 jmc 1394: .Xr sshd 8 .
1.278 djm 1395: Valid options are
1396: .Cm yes ,
1397: .Cm no
1398: or a pattern-list specifying which environment variable names to accept
1399: (for example
1400: .Qq LANG,LC_* ) .
1.6 markus 1401: The default is
1.237 jmc 1402: .Cm no .
1.9 stevesk 1403: Enabling environment processing may enable users to bypass access
1404: restrictions in some configurations using mechanisms such as
1405: .Ev LD_PRELOAD .
1.174 djm 1406: .It Cm PermitUserRC
1407: Specifies whether any
1408: .Pa ~/.ssh/rc
1409: file is executed.
1410: The default is
1.237 jmc 1411: .Cm yes .
1.1 stevesk 1412: .It Cm PidFile
1.4 stevesk 1413: Specifies the file that contains the process ID of the
1.195 dtucker 1414: SSH daemon, or
1.237 jmc 1415: .Cm none
1.195 dtucker 1416: to not write one.
1.1 stevesk 1417: The default is
1418: .Pa /var/run/sshd.pid .
1419: .It Cm Port
1420: Specifies the port number that
1.52 jmc 1421: .Xr sshd 8
1.1 stevesk 1422: listens on.
1423: The default is 22.
1424: Multiple options of this type are permitted.
1425: See also
1426: .Cm ListenAddress .
1427: .It Cm PrintLastLog
1428: Specifies whether
1.52 jmc 1429: .Xr sshd 8
1.36 jaredy 1430: should print the date and time of the last user login when a user logs
1431: in interactively.
1.1 stevesk 1432: The default is
1.237 jmc 1433: .Cm yes .
1.1 stevesk 1434: .It Cm PrintMotd
1435: Specifies whether
1.52 jmc 1436: .Xr sshd 8
1.1 stevesk 1437: should print
1438: .Pa /etc/motd
1439: when a user logs in interactively.
1440: (On some systems it is also printed by the shell,
1441: .Pa /etc/profile ,
1442: or equivalent.)
1443: The default is
1.237 jmc 1444: .Cm yes .
1.189 djm 1445: .It Cm PubkeyAcceptedKeyTypes
1446: Specifies the key types that will be accepted for public key authentication
1.280 djm 1447: as a list of comma-separated patterns.
1.289 naddy 1448: Alternately if the specified list begins with a
1.208 djm 1449: .Sq +
1450: character, then the specified key types will be appended to the default set
1451: instead of replacing them.
1.289 naddy 1452: If the specified list begins with a
1.242 djm 1453: .Sq -
1454: character, then the specified key types (including wildcards) will be removed
1455: from the default set instead of replacing them.
1.290 naddy 1456: If the specified list begins with a
1457: .Sq ^
1458: character, then the specified key types will be placed at the head of the
1459: default set.
1.206 markus 1460: The default for this option is:
1461: .Bd -literal -offset 3n
1462: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1463: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1464: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1465: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1466: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1467: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1468: rsa-sha2-512-cert-v01@openssh.com,
1469: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1470: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1471: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1472: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1473: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1474: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1475: .Ed
1476: .Pp
1.237 jmc 1477: The list of available key types may also be obtained using
1.307 dtucker 1478: .Qq ssh -Q PubkeyAcceptedKeyTypes .
1.293 djm 1479: .It Cm PubkeyAuthOptions
1480: Sets one or more public key authentication options.
1481: Two option keywords are currently supported:
1.294 jmc 1482: .Cm none
1483: (the default; indicating no additional options are enabled)
1.293 djm 1484: and
1485: .Cm touch-required .
1486: .Pp
1487: The
1488: .Cm touch-required
1.297 naddy 1489: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1490: (i.e.\&
1.293 djm 1491: .Cm ecdsa-sk
1492: or
1493: .Cm ed25519-sk )
1494: to always require the signature to attest that a physically present user
1.297 naddy 1495: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1496: By default,
1497: .Xr sshd 8
1.297 naddy 1498: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1499: The
1500: .Cm touch-required
1501: flag disables this override.
1.297 naddy 1502: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1503: .It Cm PubkeyAuthentication
1504: Specifies whether public key authentication is allowed.
1505: The default is
1.237 jmc 1506: .Cm yes .
1.159 dtucker 1507: .It Cm RekeyLimit
1508: Specifies the maximum amount of data that may be transmitted before the
1509: session key is renegotiated, optionally followed a maximum amount of
1510: time that may pass before the session key is renegotiated.
1511: The first argument is specified in bytes and may have a suffix of
1512: .Sq K ,
1513: .Sq M ,
1514: or
1515: .Sq G
1516: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1517: The default is between
1518: .Sq 1G
1519: and
1520: .Sq 4G ,
1521: depending on the cipher.
1522: The optional second value is specified in seconds and may use any of the
1523: units documented in the
1524: .Sx TIME FORMATS
1.160 jmc 1525: section.
1.159 dtucker 1526: The default value for
1527: .Cm RekeyLimit
1528: is
1.237 jmc 1529: .Cm default none ,
1.159 dtucker 1530: which means that rekeying is performed after the cipher's default amount
1531: of data has been sent or received and no time based rekeying is done.
1.118 djm 1532: .It Cm RevokedKeys
1.195 dtucker 1533: Specifies revoked public keys file, or
1.237 jmc 1534: .Cm none
1.195 dtucker 1535: to not use one.
1.118 djm 1536: Keys listed in this file will be refused for public key authentication.
1537: Note that if this file is not readable, then public key authentication will
1538: be refused for all users.
1.154 djm 1539: Keys may be specified as a text file, listing one public key per line, or as
1540: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1541: .Xr ssh-keygen 1 .
1.161 jmc 1542: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1543: .Xr ssh-keygen 1 .
1.257 djm 1544: .It Cm RDomain
1545: Specifies an explicit routing domain that is applied after authentication
1546: has completed.
1.259 jmc 1547: The user session, as well and any forwarded or listening IP sockets,
1548: will be bound to this
1.257 djm 1549: .Xr rdomain 4 .
1550: If the routing domain is set to
1551: .Cm \&%D ,
1.259 jmc 1552: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1553: .It Cm SecurityKeyProvider
1.297 naddy 1554: Specifies a path to a library that will be used when loading
1555: FIDO authenticator-hosted keys, overriding the default of using
1556: the built-in USB HID support.
1.274 djm 1557: .It Cm SetEnv
1558: Specifies one or more environment variables to set in child sessions started
1559: by
1560: .Xr sshd 8
1561: as
1562: .Dq NAME=VALUE .
1563: The environment value may be quoted (e.g. if it contains whitespace
1564: characters).
1565: Environment variables set by
1566: .Cm SetEnv
1567: override the default environment and any variables specified by the user
1568: via
1569: .Cm AcceptEnv
1570: or
1571: .Cm PermitUserEnvironment .
1.175 millert 1572: .It Cm StreamLocalBindMask
1573: Sets the octal file creation mode mask
1574: .Pq umask
1575: used when creating a Unix-domain socket file for local or remote
1576: port forwarding.
1577: This option is only used for port forwarding to a Unix-domain socket file.
1578: .Pp
1579: The default value is 0177, which creates a Unix-domain socket file that is
1580: readable and writable only by the owner.
1581: Note that not all operating systems honor the file mode on Unix-domain
1582: socket files.
1583: .It Cm StreamLocalBindUnlink
1584: Specifies whether to remove an existing Unix-domain socket file for local
1585: or remote port forwarding before creating a new one.
1586: If the socket file already exists and
1587: .Cm StreamLocalBindUnlink
1588: is not enabled,
1589: .Nm sshd
1590: will be unable to forward the port to the Unix-domain socket file.
1591: This option is only used for port forwarding to a Unix-domain socket file.
1592: .Pp
1593: The argument must be
1.237 jmc 1594: .Cm yes
1.175 millert 1595: or
1.237 jmc 1596: .Cm no .
1.175 millert 1597: The default is
1.237 jmc 1598: .Cm no .
1.1 stevesk 1599: .It Cm StrictModes
1600: Specifies whether
1.52 jmc 1601: .Xr sshd 8
1.1 stevesk 1602: should check file modes and ownership of the
1603: user's files and home directory before accepting login.
1604: This is normally desirable because novices sometimes accidentally leave their
1605: directory or files world-writable.
1606: The default is
1.237 jmc 1607: .Cm yes .
1.112 djm 1608: Note that this does not apply to
1609: .Cm ChrootDirectory ,
1610: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1611: .It Cm Subsystem
1.51 jmc 1612: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1613: Arguments should be a subsystem name and a command (with optional arguments)
1614: to execute upon subsystem request.
1.80 djm 1615: .Pp
1.1 stevesk 1616: The command
1.237 jmc 1617: .Cm sftp-server
1618: implements the SFTP file transfer subsystem.
1.80 djm 1619: .Pp
1620: Alternately the name
1.237 jmc 1621: .Cm internal-sftp
1622: implements an in-process SFTP server.
1.80 djm 1623: This may simplify configurations using
1624: .Cm ChrootDirectory
1625: to force a different filesystem root on clients.
1626: .Pp
1.1 stevesk 1627: By default no subsystems are defined.
1628: .It Cm SyslogFacility
1629: Gives the facility code that is used when logging messages from
1.53 jmc 1630: .Xr sshd 8 .
1.1 stevesk 1631: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1632: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1633: The default is AUTH.
1.27 markus 1634: .It Cm TCPKeepAlive
1635: Specifies whether the system should send TCP keepalive messages to the
1636: other side.
1637: If they are sent, death of the connection or crash of one
1638: of the machines will be properly noticed.
1639: However, this means that
1640: connections will die if the route is down temporarily, and some people
1641: find it annoying.
1642: On the other hand, if TCP keepalives are not sent,
1643: sessions may hang indefinitely on the server, leaving
1.237 jmc 1644: .Qq ghost
1.27 markus 1645: users and consuming server resources.
1646: .Pp
1647: The default is
1.237 jmc 1648: .Cm yes
1.27 markus 1649: (to send TCP keepalive messages), and the server will notice
1650: if the network goes down or the client host crashes.
1651: This avoids infinitely hanging sessions.
1652: .Pp
1653: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1654: .Cm no .
1.118 djm 1655: .It Cm TrustedUserCAKeys
1656: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1657: trusted to sign user certificates for authentication, or
1.237 jmc 1658: .Cm none
1.195 dtucker 1659: to not use one.
1.119 jmc 1660: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1661: .Ql #
1662: are allowed.
1663: If a certificate is presented for authentication and has its signing CA key
1664: listed in this file, then it may be used for authentication for any user
1665: listed in the certificate's principals list.
1666: Note that certificates that lack a list of principals will not be permitted
1667: for authentication using
1668: .Cm TrustedUserCAKeys .
1.161 jmc 1669: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1670: .Xr ssh-keygen 1 .
1.18 markus 1671: .It Cm UseDNS
1672: Specifies whether
1.52 jmc 1673: .Xr sshd 8
1.207 djm 1674: should look up the remote host name, and to check that
1.18 markus 1675: the resolved host name for the remote IP address maps back to the
1676: very same IP address.
1.207 djm 1677: .Pp
1678: If this option is set to
1.237 jmc 1679: .Cm no
1.207 djm 1680: (the default) then only addresses and not host names may be used in
1.221 djm 1681: .Pa ~/.ssh/authorized_keys
1.207 djm 1682: .Cm from
1683: and
1.211 jmc 1684: .Nm
1.207 djm 1685: .Cm Match
1686: .Cm Host
1687: directives.
1.137 djm 1688: .It Cm VersionAddendum
1689: Optionally specifies additional text to append to the SSH protocol banner
1690: sent by the server upon connection.
1691: The default is
1.237 jmc 1692: .Cm none .
1.1 stevesk 1693: .It Cm X11DisplayOffset
1694: Specifies the first display number available for
1.52 jmc 1695: .Xr sshd 8 Ns 's
1.1 stevesk 1696: X11 forwarding.
1.52 jmc 1697: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1698: The default is 10.
1699: .It Cm X11Forwarding
1700: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1701: The argument must be
1.237 jmc 1702: .Cm yes
1.13 stevesk 1703: or
1.237 jmc 1704: .Cm no .
1.1 stevesk 1705: The default is
1.237 jmc 1706: .Cm no .
1.13 stevesk 1707: .Pp
1708: When X11 forwarding is enabled, there may be additional exposure to
1709: the server and to client displays if the
1.52 jmc 1710: .Xr sshd 8
1.13 stevesk 1711: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1712: .Cm X11UseLocalhost ) ,
1713: though this is not the default.
1.13 stevesk 1714: Additionally, the authentication spoofing and authentication data
1715: verification and substitution occur on the client side.
1716: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1717: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1718: forwarding (see the warnings for
1719: .Cm ForwardX11
1720: in
1.19 jmc 1721: .Xr ssh_config 5 ) .
1.13 stevesk 1722: A system administrator may have a stance in which they want to
1723: protect clients that may expose themselves to attack by unwittingly
1724: requesting X11 forwarding, which can warrant a
1.237 jmc 1725: .Cm no
1.13 stevesk 1726: setting.
1727: .Pp
1728: Note that disabling X11 forwarding does not prevent users from
1729: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1730: .It Cm X11UseLocalhost
1731: Specifies whether
1.52 jmc 1732: .Xr sshd 8
1.1 stevesk 1733: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1734: the wildcard address.
1735: By default,
1.52 jmc 1736: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1737: hostname part of the
1738: .Ev DISPLAY
1739: environment variable to
1.237 jmc 1740: .Cm localhost .
1.8 stevesk 1741: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1742: However, some older X11 clients may not function with this
1743: configuration.
1744: .Cm X11UseLocalhost
1745: may be set to
1.237 jmc 1746: .Cm no
1.1 stevesk 1747: to specify that the forwarding server should be bound to the wildcard
1748: address.
1749: The argument must be
1.237 jmc 1750: .Cm yes
1.1 stevesk 1751: or
1.237 jmc 1752: .Cm no .
1.1 stevesk 1753: The default is
1.237 jmc 1754: .Cm yes .
1.1 stevesk 1755: .It Cm XAuthLocation
1.11 stevesk 1756: Specifies the full pathname of the
1.1 stevesk 1757: .Xr xauth 1
1.195 dtucker 1758: program, or
1.237 jmc 1759: .Cm none
1.195 dtucker 1760: to not use one.
1.1 stevesk 1761: The default is
1762: .Pa /usr/X11R6/bin/xauth .
1763: .El
1.55 jmc 1764: .Sh TIME FORMATS
1.53 jmc 1765: .Xr sshd 8
1.1 stevesk 1766: command-line arguments and configuration file options that specify time
1767: may be expressed using a sequence of the form:
1768: .Sm off
1.7 stevesk 1769: .Ar time Op Ar qualifier ,
1.1 stevesk 1770: .Sm on
1771: where
1772: .Ar time
1773: is a positive integer value and
1774: .Ar qualifier
1775: is one of the following:
1776: .Pp
1777: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1778: .It Aq Cm none
1.1 stevesk 1779: seconds
1780: .It Cm s | Cm S
1781: seconds
1782: .It Cm m | Cm M
1783: minutes
1784: .It Cm h | Cm H
1785: hours
1786: .It Cm d | Cm D
1787: days
1788: .It Cm w | Cm W
1789: weeks
1790: .El
1791: .Pp
1792: Each member of the sequence is added together to calculate
1793: the total time value.
1794: .Pp
1795: Time format examples:
1796: .Pp
1797: .Bl -tag -width Ds -compact -offset indent
1798: .It 600
1799: 600 seconds (10 minutes)
1800: .It 10m
1801: 10 minutes
1802: .It 1h30m
1803: 1 hour 30 minutes (90 minutes)
1804: .El
1.235 jmc 1805: .Sh TOKENS
1806: Arguments to some keywords can make use of tokens,
1807: which are expanded at runtime:
1808: .Pp
1809: .Bl -tag -width XXXX -offset indent -compact
1810: .It %%
1811: A literal
1812: .Sq % .
1.257 djm 1813: .It \&%D
1814: The routing domain in which the incoming connection was received.
1.235 jmc 1815: .It %F
1816: The fingerprint of the CA key.
1817: .It %f
1818: The fingerprint of the key or certificate.
1819: .It %h
1820: The home directory of the user.
1821: .It %i
1822: The key ID in the certificate.
1823: .It %K
1824: The base64-encoded CA key.
1825: .It %k
1826: The base64-encoded key or certificate for authentication.
1827: .It %s
1828: The serial number of the certificate.
1829: .It \&%T
1830: The type of the CA key.
1831: .It %t
1832: The key or certificate type.
1.268 jmc 1833: .It \&%U
1834: The numeric user ID of the target user.
1.235 jmc 1835: .It %u
1836: The username.
1837: .El
1838: .Pp
1839: .Cm AuthorizedKeysCommand
1.270 jmc 1840: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1841: .Pp
1842: .Cm AuthorizedKeysFile
1.270 jmc 1843: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1844: .Pp
1845: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1846: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1847: .Pp
1848: .Cm AuthorizedPrincipalsFile
1.270 jmc 1849: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1850: .Pp
1851: .Cm ChrootDirectory
1.270 jmc 1852: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1853: .Pp
1854: .Cm RoutingDomain
1855: accepts the token %D.
1.1 stevesk 1856: .Sh FILES
1857: .Bl -tag -width Ds
1858: .It Pa /etc/ssh/sshd_config
1859: Contains configuration data for
1.53 jmc 1860: .Xr sshd 8 .
1.1 stevesk 1861: This file should be writable by root only, but it is recommended
1862: (though not necessary) that it be world-readable.
1863: .El
1.19 jmc 1864: .Sh SEE ALSO
1.237 jmc 1865: .Xr sftp-server 8 ,
1.19 jmc 1866: .Xr sshd 8
1.1 stevesk 1867: .Sh AUTHORS
1.237 jmc 1868: .An -nosplit
1.1 stevesk 1869: OpenSSH is a derivative of the original and free
1.237 jmc 1870: ssh 1.2.12 release by
1871: .An Tatu Ylonen .
1872: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1873: .An Theo de Raadt
1874: and
1875: .An Dug Song
1.1 stevesk 1876: removed many bugs, re-added newer features and
1877: created OpenSSH.
1.237 jmc 1878: .An Markus Friedl
1879: contributed the support for SSH protocol versions 1.5 and 2.0.
1880: .An Niels Provos
1881: and
1882: .An Markus Friedl
1883: contributed support for privilege separation.
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