Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.336
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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1.336 ! dtucker 36: .\" $OpenBSD: sshd_config.5,v 1.335 2021/09/03 05:25:50 dtucker Exp $
! 37: .Dd $Mdocdate: September 3 2021 $
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
1.334 djm 381: ssh-ed25519,ecdsa-sha2-nistp256,
382: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
383: sk-ssh-ed25519@openssh.com,
384: sk-ecdsa-sha2-nistp256@openssh.com,
1.330 djm 385: rsa-sha2-512,rsa-sha2-256
1.282 djm 386: .Ed
1.334 djm 387: .Pp
388: If the specified list begins with a
389: .Sq +
390: character, then the specified algorithms will be appended to the default set
391: instead of replacing them.
392: If the specified list begins with a
393: .Sq -
394: character, then the specified algorithms (including wildcards) will be removed
395: from the default set instead of replacing them.
1.282 djm 396: .Pp
397: Certificates signed using other algorithms will not be accepted for
398: public key or host-based authentication.
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.
1.235 jmc 410: Arguments to
411: .Cm ChrootDirectory
412: accept the tokens described in the
413: .Sx TOKENS
414: section.
1.80 djm 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.
1.237 jmc 433: For file transfer sessions using SFTP
434: no additional configuration of the environment is necessary if the in-process
435: sftp-server is used,
1.176 schwarze 436: though sessions which use logging may require
1.104 stevesk 437: .Pa /dev/log
1.176 schwarze 438: inside the chroot directory on some operating systems (see
1.104 stevesk 439: .Xr sftp-server 8
1.81 jmc 440: for details).
1.190 deraadt 441: .Pp
1.191 jmc 442: For safety, it is very important that the directory hierarchy be
1.190 deraadt 443: prevented from modification by other processes on the system (especially
444: those outside the jail).
445: Misconfiguration can lead to unsafe environments which
446: .Xr sshd 8
447: cannot detect.
1.80 djm 448: .Pp
1.215 djm 449: The default is
1.237 jmc 450: .Cm none ,
1.215 djm 451: indicating not to
1.80 djm 452: .Xr chroot 2 .
1.1 stevesk 453: .It Cm Ciphers
1.219 jmc 454: Specifies the ciphers allowed.
1.1 stevesk 455: Multiple ciphers must be comma-separated.
1.289 naddy 456: If the specified list begins with a
1.208 djm 457: .Sq +
458: character, then the specified ciphers will be appended to the default set
459: instead of replacing them.
1.289 naddy 460: If the specified list begins with a
1.242 djm 461: .Sq -
462: character, then the specified ciphers (including wildcards) will be removed
463: from the default set instead of replacing them.
1.290 naddy 464: If the specified list begins with a
465: .Sq ^
466: character, then the specified ciphers will be placed at the head of the
467: default set.
1.208 djm 468: .Pp
1.167 djm 469: The supported ciphers are:
470: .Pp
1.173 naddy 471: .Bl -item -compact -offset indent
472: .It
473: 3des-cbc
474: .It
475: aes128-cbc
476: .It
477: aes192-cbc
478: .It
479: aes256-cbc
480: .It
481: aes128-ctr
482: .It
483: aes192-ctr
484: .It
485: aes256-ctr
486: .It
487: aes128-gcm@openssh.com
488: .It
489: aes256-gcm@openssh.com
490: .It
491: chacha20-poly1305@openssh.com
492: .El
1.167 djm 493: .Pp
1.52 jmc 494: The default is:
1.173 naddy 495: .Bd -literal -offset indent
1.211 jmc 496: chacha20-poly1305@openssh.com,
1.173 naddy 497: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 498: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 499: .Ed
1.167 djm 500: .Pp
1.237 jmc 501: The list of available ciphers may also be obtained using
502: .Qq ssh -Q cipher .
1.1 stevesk 503: .It Cm ClientAliveCountMax
1.237 jmc 504: Sets the number of client alive messages which may be sent without
1.52 jmc 505: .Xr sshd 8
1.17 jmc 506: receiving any messages back from the client.
507: If this threshold is reached while client alive messages are being sent,
1.52 jmc 508: sshd will disconnect the client, terminating the session.
1.17 jmc 509: It is important to note that the use of client alive messages is very
510: different from
1.237 jmc 511: .Cm TCPKeepAlive .
1.17 jmc 512: The client alive messages are sent through the encrypted channel
513: and therefore will not be spoofable.
514: The TCP keepalive option enabled by
1.27 markus 515: .Cm TCPKeepAlive
1.17 jmc 516: is spoofable.
517: The client alive mechanism is valuable when the client or
1.288 dtucker 518: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 519: .Pp
1.17 jmc 520: The default value is 3.
521: If
1.1 stevesk 522: .Cm ClientAliveInterval
1.237 jmc 523: is set to 15, and
1.1 stevesk 524: .Cm ClientAliveCountMax
1.52 jmc 525: is left at the default, unresponsive SSH clients
1.1 stevesk 526: will be disconnected after approximately 45 seconds.
1.302 djm 527: Setting a zero
528: .Cm ClientAliveCountMax
529: disables connection termination.
1.42 djm 530: .It Cm ClientAliveInterval
531: Sets a timeout interval in seconds after which if no data has been received
532: from the client,
1.52 jmc 533: .Xr sshd 8
1.42 djm 534: will send a message through the encrypted
535: channel to request a response from the client.
536: The default
537: is 0, indicating that these messages will not be sent to the client.
1.3 markus 538: .It Cm Compression
1.236 djm 539: Specifies whether compression is enabled after
1.44 markus 540: the user has authenticated successfully.
1.3 markus 541: The argument must be
1.237 jmc 542: .Cm yes ,
543: .Cm delayed
1.236 djm 544: (a legacy synonym for
1.237 jmc 545: .Cm yes )
1.3 markus 546: or
1.237 jmc 547: .Cm no .
1.3 markus 548: The default is
1.237 jmc 549: .Cm yes .
1.1 stevesk 550: .It Cm DenyGroups
551: This keyword can be followed by a list of group name patterns, separated
552: by spaces.
553: Login is disallowed for users whose primary group or supplementary
554: group list matches one of the patterns.
555: Only group names are valid; a numerical group ID is not recognized.
556: By default, login is allowed for all groups.
1.301 djm 557: The allow/deny groups directives are processed in the following order:
1.54 jmc 558: .Cm DenyGroups ,
559: .Cm AllowGroups .
1.49 jmc 560: .Pp
1.161 jmc 561: See PATTERNS in
1.49 jmc 562: .Xr ssh_config 5
563: for more information on patterns.
1.1 stevesk 564: .It Cm DenyUsers
565: This keyword can be followed by a list of user name patterns, separated
566: by spaces.
567: Login is disallowed for user names that match one of the patterns.
568: Only user names are valid; a numerical user ID is not recognized.
569: By default, login is allowed for all users.
570: If the pattern takes the form USER@HOST then USER and HOST
571: are separately checked, restricting logins to particular
572: users from particular hosts.
1.222 jmc 573: HOST criteria may additionally contain addresses to match in CIDR
574: address/masklen format.
1.301 djm 575: The allow/deny users directives are processed in the following order:
1.54 jmc 576: .Cm DenyUsers ,
1.301 djm 577: .Cm AllowUsers .
1.49 jmc 578: .Pp
1.161 jmc 579: See PATTERNS in
1.49 jmc 580: .Xr ssh_config 5
581: for more information on patterns.
1.239 djm 582: .It Cm DisableForwarding
583: Disables all forwarding features, including X11,
584: .Xr ssh-agent 1 ,
585: TCP and StreamLocal.
586: This option overrides all other forwarding-related options and may
587: simplify restricted configurations.
1.246 djm 588: .It Cm ExposeAuthInfo
1.253 jmc 589: Writes a temporary file containing a list of authentication methods and
1.246 djm 590: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 591: The location of the file is exposed to the user session through the
1.248 djm 592: .Ev SSH_USER_AUTH
1.247 jmc 593: environment variable.
1.253 jmc 594: The default is
595: .Cm no .
1.183 djm 596: .It Cm FingerprintHash
597: Specifies the hash algorithm used when logging key fingerprints.
598: Valid options are:
1.237 jmc 599: .Cm md5
1.183 djm 600: and
1.237 jmc 601: .Cm sha256 .
1.183 djm 602: The default is
1.237 jmc 603: .Cm sha256 .
1.67 dtucker 604: .It Cm ForceCommand
605: Forces the execution of the command specified by
606: .Cm ForceCommand ,
1.84 djm 607: ignoring any command supplied by the client and
608: .Pa ~/.ssh/rc
609: if present.
1.67 dtucker 610: The command is invoked by using the user's login shell with the -c option.
611: This applies to shell, command, or subsystem execution.
612: It is most useful inside a
613: .Cm Match
614: block.
615: The command originally supplied by the client is available in the
616: .Ev SSH_ORIGINAL_COMMAND
617: environment variable.
1.82 djm 618: Specifying a command of
1.237 jmc 619: .Cm internal-sftp
620: will force the use of an in-process SFTP server that requires no support
1.82 djm 621: files when used with
622: .Cm ChrootDirectory .
1.215 djm 623: The default is
1.237 jmc 624: .Cm none .
1.1 stevesk 625: .It Cm GatewayPorts
626: Specifies whether remote hosts are allowed to connect to ports
627: forwarded for the client.
628: By default,
1.52 jmc 629: .Xr sshd 8
1.15 jmc 630: binds remote port forwardings to the loopback address.
631: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 632: .Cm GatewayPorts
1.52 jmc 633: can be used to specify that sshd
1.39 djm 634: should allow remote port forwardings to bind to non-loopback addresses, thus
635: allowing other hosts to connect.
636: The argument may be
1.237 jmc 637: .Cm no
1.39 djm 638: to force remote port forwardings to be available to the local host only,
1.237 jmc 639: .Cm yes
1.39 djm 640: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 641: .Cm clientspecified
1.39 djm 642: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 643: The default is
1.237 jmc 644: .Cm no .
1.23 markus 645: .It Cm GSSAPIAuthentication
1.25 markus 646: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 647: The default is
1.237 jmc 648: .Cm no .
1.23 markus 649: .It Cm GSSAPICleanupCredentials
650: Specifies whether to automatically destroy the user's credentials cache
651: on logout.
652: The default is
1.237 jmc 653: .Cm yes .
1.203 djm 654: .It Cm GSSAPIStrictAcceptorCheck
655: Determines whether to be strict about the identity of the GSSAPI acceptor
656: a client authenticates against.
657: If set to
1.237 jmc 658: .Cm yes
659: then the client must authenticate against the host
1.203 djm 660: service on the current hostname.
661: If set to
1.237 jmc 662: .Cm no
1.203 djm 663: then the client may authenticate against any service key stored in the
664: machine's default store.
665: This facility is provided to assist with operation on multi homed machines.
666: The default is
1.237 jmc 667: .Cm yes .
1.325 dtucker 668: .It Cm HostbasedAcceptedAlgorithms
1.327 djm 669: Specifies the signature algorithms that will be accepted for hostbased
670: authentication as a list of comma-separated patterns.
1.289 naddy 671: Alternately if the specified list begins with a
1.208 djm 672: .Sq +
1.327 djm 673: character, then the specified signature algorithms will be appended to
674: the default set instead of replacing them.
1.289 naddy 675: If the specified list begins with a
1.242 djm 676: .Sq -
1.327 djm 677: character, then the specified signature algorithms (including wildcards)
678: will be removed from the default set instead of replacing them.
1.290 naddy 679: If the specified list begins with a
680: .Sq ^
1.327 djm 681: character, then the specified signature algorithms will be placed at
682: the head of the default set.
1.206 markus 683: The default for this option is:
684: .Bd -literal -offset 3n
1.316 djm 685: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 686: ecdsa-sha2-nistp256-cert-v01@openssh.com,
687: ecdsa-sha2-nistp384-cert-v01@openssh.com,
688: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 689: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 690: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 691: rsa-sha2-512-cert-v01@openssh.com,
692: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 693: ssh-rsa-cert-v01@openssh.com,
1.316 djm 694: ssh-ed25519,
1.206 markus 695: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 696: sk-ssh-ed25519@openssh.com,
1.296 naddy 697: sk-ecdsa-sha2-nistp256@openssh.com,
698: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 699: .Ed
700: .Pp
1.327 djm 701: The list of available signature algorithms may also be obtained using
1.325 dtucker 702: .Qq ssh -Q HostbasedAcceptedAlgorithms .
703: This was formerly named HostbasedAcceptedKeyTypes.
1.1 stevesk 704: .It Cm HostbasedAuthentication
705: Specifies whether rhosts or /etc/hosts.equiv authentication together
706: with successful public key client host authentication is allowed
1.50 jmc 707: (host-based authentication).
1.70 dtucker 708: The default is
1.237 jmc 709: .Cm no .
1.70 dtucker 710: .It Cm HostbasedUsesNameFromPacketOnly
711: Specifies whether or not the server will attempt to perform a reverse
712: name lookup when matching the name in the
713: .Pa ~/.shosts ,
714: .Pa ~/.rhosts ,
715: and
716: .Pa /etc/hosts.equiv
717: files during
718: .Cm HostbasedAuthentication .
719: A setting of
1.237 jmc 720: .Cm yes
1.70 dtucker 721: means that
722: .Xr sshd 8
723: uses the name supplied by the client rather than
724: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 725: The default is
1.237 jmc 726: .Cm no .
1.117 djm 727: .It Cm HostCertificate
728: Specifies a file containing a public host certificate.
729: The certificate's public key must match a private host key already specified
730: by
731: .Cm HostKey .
732: The default behaviour of
733: .Xr sshd 8
734: is not to load any certificates.
1.1 stevesk 735: .It Cm HostKey
736: Specifies a file containing a private host key
737: used by SSH.
1.229 naddy 738: The defaults are
1.169 naddy 739: .Pa /etc/ssh/ssh_host_ecdsa_key ,
740: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 741: and
1.229 naddy 742: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 743: .Pp
1.1 stevesk 744: Note that
1.52 jmc 745: .Xr sshd 8
1.206 markus 746: will refuse to use a file if it is group/world-accessible
747: and that the
748: .Cm HostKeyAlgorithms
749: option restricts which of the keys are actually used by
750: .Xr sshd 8 .
751: .Pp
1.1 stevesk 752: It is possible to have multiple host key files.
1.162 markus 753: It is also possible to specify public host key files instead.
754: In this case operations on the private key will be delegated
755: to an
756: .Xr ssh-agent 1 .
757: .It Cm HostKeyAgent
758: Identifies the UNIX-domain socket used to communicate
759: with an agent that has access to the private host keys.
1.223 markus 760: If the string
1.237 jmc 761: .Qq SSH_AUTH_SOCK
1.162 markus 762: is specified, the location of the socket will be read from the
763: .Ev SSH_AUTH_SOCK
764: environment variable.
1.206 markus 765: .It Cm HostKeyAlgorithms
1.327 djm 766: Specifies the host key signature algorithms
1.206 markus 767: that the server offers.
768: The default for this option is:
769: .Bd -literal -offset 3n
1.316 djm 770: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 771: ecdsa-sha2-nistp256-cert-v01@openssh.com,
772: ecdsa-sha2-nistp384-cert-v01@openssh.com,
773: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 774: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 775: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 776: rsa-sha2-512-cert-v01@openssh.com,
777: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 778: ssh-rsa-cert-v01@openssh.com,
1.316 djm 779: ssh-ed25519,
1.206 markus 780: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 781: sk-ssh-ed25519@openssh.com,
1.296 naddy 782: sk-ecdsa-sha2-nistp256@openssh.com,
783: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 784: .Ed
785: .Pp
1.327 djm 786: The list of available signature algorithms may also be obtained using
1.307 dtucker 787: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 788: .It Cm IgnoreRhosts
1.309 djm 789: Specifies whether to ignore per-user
1.1 stevesk 790: .Pa .rhosts
791: and
792: .Pa .shosts
1.309 djm 793: files during
1.1 stevesk 794: .Cm HostbasedAuthentication .
1.309 djm 795: The system-wide
1.1 stevesk 796: .Pa /etc/hosts.equiv
797: and
798: .Pa /etc/shosts.equiv
1.309 djm 799: are still used regardless of this setting.
800: .Pp
801: Accepted values are
802: .Cm yes
803: (the default) to ignore all per-user files,
804: .Cm shosts-only
805: to allow the use of
806: .Pa .shosts
807: but to ignore
808: .Pa .rhosts
809: or
810: .Cm no
811: to allow both
812: .Pa .shosts
813: and
1.311 jmc 814: .Pa rhosts .
1.1 stevesk 815: .It Cm IgnoreUserKnownHosts
816: Specifies whether
1.52 jmc 817: .Xr sshd 8
1.1 stevesk 818: should ignore the user's
1.41 djm 819: .Pa ~/.ssh/known_hosts
1.1 stevesk 820: during
1.261 djm 821: .Cm HostbasedAuthentication
822: and use only the system-wide known hosts file
823: .Pa /etc/ssh/known_hosts .
1.1 stevesk 824: The default is
1.304 djm 825: .Dq no .
826: .It Cm Include
827: Include the specified configuration file(s).
1.305 jmc 828: Multiple pathnames may be specified and each pathname may contain
1.304 djm 829: .Xr glob 7
1.312 djm 830: wildcards that will be expanded and processed in lexical order.
1.304 djm 831: Files without absolute paths are assumed to be in
832: .Pa /etc/ssh .
1.305 jmc 833: An
1.304 djm 834: .Cm Include
835: directive may appear inside a
836: .Cm Match
837: block
838: to perform conditional inclusion.
1.129 djm 839: .It Cm IPQoS
840: Specifies the IPv4 type-of-service or DSCP class for the connection.
841: Accepted values are
1.237 jmc 842: .Cm af11 ,
843: .Cm af12 ,
844: .Cm af13 ,
845: .Cm af21 ,
846: .Cm af22 ,
847: .Cm af23 ,
848: .Cm af31 ,
849: .Cm af32 ,
850: .Cm af33 ,
851: .Cm af41 ,
852: .Cm af42 ,
853: .Cm af43 ,
854: .Cm cs0 ,
855: .Cm cs1 ,
856: .Cm cs2 ,
857: .Cm cs3 ,
858: .Cm cs4 ,
859: .Cm cs5 ,
860: .Cm cs6 ,
861: .Cm cs7 ,
862: .Cm ef ,
1.303 djm 863: .Cm le ,
1.237 jmc 864: .Cm lowdelay ,
865: .Cm throughput ,
866: .Cm reliability ,
1.250 djm 867: a numeric value, or
868: .Cm none
869: to use the operating system default.
1.131 djm 870: This option may take one or two arguments, separated by whitespace.
1.129 djm 871: If one argument is specified, it is used as the packet class unconditionally.
872: If two values are specified, the first is automatically selected for
873: interactive sessions and the second for non-interactive sessions.
874: The default is
1.264 job 875: .Cm af21
1.265 jmc 876: (Low-Latency Data)
1.129 djm 877: for interactive sessions and
1.264 job 878: .Cm cs1
1.265 jmc 879: (Lower Effort)
1.129 djm 880: for non-interactive sessions.
1.171 jmc 881: .It Cm KbdInteractiveAuthentication
882: Specifies whether to allow keyboard-interactive authentication.
1.332 dtucker 883: All authentication styles from
884: .Xr login.conf 5
885: are supported.
886: The default is
887: .Cm yes .
1.171 jmc 888: The argument to this keyword must be
1.237 jmc 889: .Cm yes
1.171 jmc 890: or
1.237 jmc 891: .Cm no .
1.171 jmc 892: .Cm ChallengeResponseAuthentication
1.332 dtucker 893: is a deprecated alias for this.
1.1 stevesk 894: .It Cm KerberosAuthentication
1.24 markus 895: Specifies whether the password provided by the user for
1.1 stevesk 896: .Cm PasswordAuthentication
1.24 markus 897: will be validated through the Kerberos KDC.
1.1 stevesk 898: To use this option, the server needs a
899: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 900: The default is
1.237 jmc 901: .Cm no .
1.29 dtucker 902: .It Cm KerberosGetAFSToken
1.45 djm 903: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 904: an AFS token before accessing the user's home directory.
1.52 jmc 905: The default is
1.237 jmc 906: .Cm no .
1.1 stevesk 907: .It Cm KerberosOrLocalPasswd
1.52 jmc 908: If password authentication through Kerberos fails then
1.1 stevesk 909: the password will be validated via any additional local mechanism
910: such as
911: .Pa /etc/passwd .
1.52 jmc 912: The default is
1.237 jmc 913: .Cm yes .
1.1 stevesk 914: .It Cm KerberosTicketCleanup
915: Specifies whether to automatically destroy the user's ticket cache
916: file on logout.
1.52 jmc 917: The default is
1.237 jmc 918: .Cm yes .
1.127 djm 919: .It Cm KexAlgorithms
920: Specifies the available KEX (Key Exchange) algorithms.
921: Multiple algorithms must be comma-separated.
1.289 naddy 922: Alternately if the specified list begins with a
1.208 djm 923: .Sq +
1.335 dtucker 924: character, then the specified algorithms will be appended to the default set
1.208 djm 925: instead of replacing them.
1.289 naddy 926: If the specified list begins with a
1.242 djm 927: .Sq -
1.335 dtucker 928: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 929: from the default set instead of replacing them.
1.290 naddy 930: If the specified list begins with a
931: .Sq ^
1.335 dtucker 932: character, then the specified algorithms will be placed at the head of the
1.290 naddy 933: default set.
1.173 naddy 934: The supported algorithms are:
935: .Pp
936: .Bl -item -compact -offset indent
937: .It
1.234 djm 938: curve25519-sha256
939: .It
1.173 naddy 940: curve25519-sha256@libssh.org
941: .It
942: diffie-hellman-group1-sha1
943: .It
944: diffie-hellman-group14-sha1
945: .It
1.263 djm 946: diffie-hellman-group14-sha256
947: .It
948: diffie-hellman-group16-sha512
949: .It
950: diffie-hellman-group18-sha512
951: .It
1.173 naddy 952: diffie-hellman-group-exchange-sha1
953: .It
954: diffie-hellman-group-exchange-sha256
955: .It
956: ecdh-sha2-nistp256
957: .It
958: ecdh-sha2-nistp384
959: .It
960: ecdh-sha2-nistp521
1.298 dtucker 961: .It
1.319 djm 962: sntrup761x25519-sha512@openssh.com
1.173 naddy 963: .El
964: .Pp
965: The default is:
1.170 dtucker 966: .Bd -literal -offset indent
1.234 djm 967: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 968: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
969: diffie-hellman-group-exchange-sha256,
1.263 djm 970: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 971: diffie-hellman-group14-sha256
1.170 dtucker 972: .Ed
1.188 djm 973: .Pp
1.237 jmc 974: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 975: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 976: .It Cm ListenAddress
977: Specifies the local addresses
1.52 jmc 978: .Xr sshd 8
1.1 stevesk 979: should listen on.
980: The following forms may be used:
981: .Pp
982: .Bl -item -offset indent -compact
983: .It
984: .Cm ListenAddress
985: .Sm off
1.259 jmc 986: .Ar hostname | address
1.1 stevesk 987: .Sm on
1.260 jmc 988: .Op Cm rdomain Ar domain
1.1 stevesk 989: .It
990: .Cm ListenAddress
991: .Sm off
1.259 jmc 992: .Ar hostname : port
1.1 stevesk 993: .Sm on
1.260 jmc 994: .Op Cm rdomain Ar domain
1.256 djm 995: .It
996: .Cm ListenAddress
997: .Sm off
1.259 jmc 998: .Ar IPv4_address : port
1.256 djm 999: .Sm on
1.260 jmc 1000: .Op Cm rdomain Ar domain
1.1 stevesk 1001: .It
1002: .Cm ListenAddress
1003: .Sm off
1.259 jmc 1004: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 1005: .Sm on
1.260 jmc 1006: .Op Cm rdomain Ar domain
1.1 stevesk 1007: .El
1008: .Pp
1.256 djm 1009: The optional
1010: .Cm rdomain
1011: qualifier requests
1012: .Xr sshd 8
1013: listen in an explicit routing domain.
1.1 stevesk 1014: If
1015: .Ar port
1016: is not specified,
1.200 dtucker 1017: sshd will listen on the address and all
1.1 stevesk 1018: .Cm Port
1.17 jmc 1019: options specified.
1.256 djm 1020: The default is to listen on all local addresses on the current default
1021: routing domain.
1.15 jmc 1022: Multiple
1.1 stevesk 1023: .Cm ListenAddress
1.17 jmc 1024: options are permitted.
1.256 djm 1025: For more information on routing domains, see
1.259 jmc 1026: .Xr rdomain 4 .
1.1 stevesk 1027: .It Cm LoginGraceTime
1028: The server disconnects after this time if the user has not
1029: successfully logged in.
1030: If the value is 0, there is no time limit.
1.12 stevesk 1031: The default is 120 seconds.
1.1 stevesk 1032: .It Cm LogLevel
1033: Gives the verbosity level that is used when logging messages from
1.53 jmc 1034: .Xr sshd 8 .
1.1 stevesk 1035: The possible values are:
1.52 jmc 1036: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1037: The default is INFO.
1038: DEBUG and DEBUG1 are equivalent.
1039: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1040: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.317 djm 1041: .It Cm LogVerbose
1042: Specify one or more overrides to LogLevel.
1043: An override consists of a pattern lists that matches the source file, function
1044: and line number to force detailed logging for.
1045: For example, an override pattern of:
1046: .Bd -literal -offset indent
1047: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1048: .Ed
1049: .Pp
1050: would enable detailed logging for line 1000 of
1.318 jmc 1051: .Pa kex.c ,
1.317 djm 1052: everything in the
1053: .Fn kex_exchange_identification
1054: function, and all code in the
1055: .Pa packet.c
1056: file.
1057: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1058: .It Cm MACs
1059: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1060: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1061: Multiple algorithms must be comma-separated.
1.289 naddy 1062: If the specified list begins with a
1.208 djm 1063: .Sq +
1064: character, then the specified algorithms will be appended to the default set
1065: instead of replacing them.
1.289 naddy 1066: If the specified list begins with a
1.242 djm 1067: .Sq -
1068: character, then the specified algorithms (including wildcards) will be removed
1069: from the default set instead of replacing them.
1.290 naddy 1070: If the specified list begins with a
1071: .Sq ^
1072: character, then the specified algorithms will be placed at the head of the
1073: default set.
1.208 djm 1074: .Pp
1.152 markus 1075: The algorithms that contain
1.237 jmc 1076: .Qq -etm
1.152 markus 1077: calculate the MAC after encryption (encrypt-then-mac).
1078: These are considered safer and their use recommended.
1.173 naddy 1079: The supported MACs are:
1080: .Pp
1081: .Bl -item -compact -offset indent
1082: .It
1083: hmac-md5
1084: .It
1085: hmac-md5-96
1086: .It
1087: hmac-sha1
1088: .It
1089: hmac-sha1-96
1090: .It
1091: hmac-sha2-256
1092: .It
1093: hmac-sha2-512
1094: .It
1095: umac-64@openssh.com
1096: .It
1097: umac-128@openssh.com
1098: .It
1099: hmac-md5-etm@openssh.com
1100: .It
1101: hmac-md5-96-etm@openssh.com
1102: .It
1103: hmac-sha1-etm@openssh.com
1104: .It
1105: hmac-sha1-96-etm@openssh.com
1106: .It
1107: hmac-sha2-256-etm@openssh.com
1108: .It
1109: hmac-sha2-512-etm@openssh.com
1110: .It
1111: umac-64-etm@openssh.com
1112: .It
1113: umac-128-etm@openssh.com
1114: .El
1115: .Pp
1.52 jmc 1116: The default is:
1.77 jmc 1117: .Bd -literal -offset indent
1.152 markus 1118: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1119: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1120: hmac-sha1-etm@openssh.com,
1.173 naddy 1121: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1122: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1123: .Ed
1.188 djm 1124: .Pp
1.237 jmc 1125: The list of available MAC algorithms may also be obtained using
1126: .Qq ssh -Q mac .
1.60 dtucker 1127: .It Cm Match
1.61 jmc 1128: Introduces a conditional block.
1.65 dtucker 1129: If all of the criteria on the
1.60 dtucker 1130: .Cm Match
1.65 dtucker 1131: line are satisfied, the keywords on the following lines override those
1132: set in the global section of the config file, until either another
1.60 dtucker 1133: .Cm Match
1.65 dtucker 1134: line or the end of the file.
1.172 djm 1135: If a keyword appears in multiple
1136: .Cm Match
1.177 sobrado 1137: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1138: applied.
1.91 djm 1139: .Pp
1.61 jmc 1140: The arguments to
1.60 dtucker 1141: .Cm Match
1.163 dtucker 1142: are one or more criteria-pattern pairs or the single token
1143: .Cm All
1144: which matches all criteria.
1.60 dtucker 1145: The available criteria are
1146: .Cm User ,
1.69 dtucker 1147: .Cm Group ,
1.60 dtucker 1148: .Cm Host ,
1.139 dtucker 1149: .Cm LocalAddress ,
1150: .Cm LocalPort ,
1.258 djm 1151: .Cm RDomain ,
1.60 dtucker 1152: and
1.258 djm 1153: .Cm Address
1154: (with
1155: .Cm RDomain
1156: representing the
1157: .Xr rdomain 4
1.284 jmc 1158: on which the connection was received).
1.258 djm 1159: .Pp
1.91 djm 1160: The match patterns may consist of single entries or comma-separated
1161: lists and may use the wildcard and negation operators described in the
1.237 jmc 1162: .Sx PATTERNS
1163: section of
1.92 djm 1164: .Xr ssh_config 5 .
1.91 djm 1165: .Pp
1166: The patterns in an
1167: .Cm Address
1168: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1169: address/masklen format,
1170: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1171: Note that the mask length provided must be consistent with the address -
1172: it is an error to specify a mask length that is too long for the address
1.93 jmc 1173: or one with bits set in this host portion of the address.
1.237 jmc 1174: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1175: .Pp
1.60 dtucker 1176: Only a subset of keywords may be used on the lines following a
1177: .Cm Match
1178: keyword.
1179: Available keywords are
1.142 jmc 1180: .Cm AcceptEnv ,
1.99 okan 1181: .Cm AllowAgentForwarding ,
1.142 jmc 1182: .Cm AllowGroups ,
1.193 djm 1183: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1184: .Cm AllowTcpForwarding ,
1.141 markus 1185: .Cm AllowUsers ,
1.149 djm 1186: .Cm AuthenticationMethods ,
1.146 djm 1187: .Cm AuthorizedKeysCommand ,
1188: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1189: .Cm AuthorizedKeysFile ,
1.214 djm 1190: .Cm AuthorizedPrincipalsCommand ,
1191: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1192: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1193: .Cm Banner ,
1.336 ! dtucker 1194: .Cm CASignatureAlgorithms ,
1.85 djm 1195: .Cm ChrootDirectory ,
1.238 markus 1196: .Cm ClientAliveCountMax ,
1197: .Cm ClientAliveInterval ,
1.141 markus 1198: .Cm DenyGroups ,
1199: .Cm DenyUsers ,
1.320 djm 1200: .Cm DisableForwarding ,
1.336 ! dtucker 1201: .Cm ExposeAuthInfo ,
1.67 dtucker 1202: .Cm ForceCommand ,
1.194 djm 1203: .Cm GatewayPorts ,
1.193 djm 1204: .Cm GSSAPIAuthentication ,
1.325 dtucker 1205: .Cm HostbasedAcceptedAlgorithms ,
1.87 djm 1206: .Cm HostbasedAuthentication ,
1.123 djm 1207: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1208: .Cm IgnoreRhosts ,
1.306 naddy 1209: .Cm Include ,
1.193 djm 1210: .Cm IPQoS ,
1.74 jmc 1211: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1212: .Cm KerberosAuthentication ,
1.245 djm 1213: .Cm LogLevel ,
1.95 dtucker 1214: .Cm MaxAuthTries ,
1.94 dtucker 1215: .Cm MaxSessions ,
1.72 dtucker 1216: .Cm PasswordAuthentication ,
1.97 djm 1217: .Cm PermitEmptyPasswords ,
1.271 djm 1218: .Cm PermitListen ,
1.66 dtucker 1219: .Cm PermitOpen ,
1.79 dtucker 1220: .Cm PermitRootLogin ,
1.164 djm 1221: .Cm PermitTTY ,
1.123 djm 1222: .Cm PermitTunnel ,
1.174 djm 1223: .Cm PermitUserRC ,
1.324 dtucker 1224: .Cm PubkeyAcceptedAlgorithms ,
1.107 dtucker 1225: .Cm PubkeyAuthentication ,
1.336 ! dtucker 1226: .Cm PubkeyAuthOptions ,
1.159 dtucker 1227: .Cm RekeyLimit ,
1.193 djm 1228: .Cm RevokedKeys ,
1.257 djm 1229: .Cm RDomain ,
1.274 djm 1230: .Cm SetEnv ,
1.193 djm 1231: .Cm StreamLocalBindMask ,
1232: .Cm StreamLocalBindUnlink ,
1233: .Cm TrustedUserCAKeys ,
1.66 dtucker 1234: .Cm X11DisplayOffset ,
1.101 djm 1235: .Cm X11Forwarding
1.60 dtucker 1236: and
1.286 jmc 1237: .Cm X11UseLocalhost .
1.33 dtucker 1238: .It Cm MaxAuthTries
1239: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1240: connection.
1241: Once the number of failures reaches half this value,
1242: additional failures are logged.
1243: The default is 6.
1.90 djm 1244: .It Cm MaxSessions
1.216 djm 1245: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1246: sessions permitted per network connection.
1247: Multiple sessions may be established by clients that support connection
1248: multiplexing.
1249: Setting
1250: .Cm MaxSessions
1251: to 1 will effectively disable session multiplexing, whereas setting it to 0
1252: will prevent all shell, login and subsystem sessions while still permitting
1253: forwarding.
1.90 djm 1254: The default is 10.
1.1 stevesk 1255: .It Cm MaxStartups
1256: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1257: SSH daemon.
1.1 stevesk 1258: Additional connections will be dropped until authentication succeeds or the
1259: .Cm LoginGraceTime
1260: expires for a connection.
1.156 dtucker 1261: The default is 10:30:100.
1.1 stevesk 1262: .Pp
1263: Alternatively, random early drop can be enabled by specifying
1264: the three colon separated values
1.237 jmc 1265: start:rate:full (e.g. "10:30:60").
1.53 jmc 1266: .Xr sshd 8
1.237 jmc 1267: will refuse connection attempts with a probability of rate/100 (30%)
1268: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1269: The probability increases linearly and all connection attempts
1.237 jmc 1270: are refused if the number of unauthenticated connections reaches full (60).
1.329 dtucker 1271: .It Cm ModuliFile
1272: Specifies the
1273: .Xr moduli 5
1274: file that contains the Diffie-Hellman groups used for the
1275: .Dq diffie-hellman-group-exchange-sha1
1276: and
1277: .Dq diffie-hellman-group-exchange-sha256
1278: key exchange methods.
1279: The default is
1280: .Pa /etc/moduli .
1.1 stevesk 1281: .It Cm PasswordAuthentication
1282: Specifies whether password authentication is allowed.
1283: The default is
1.237 jmc 1284: .Cm yes .
1.1 stevesk 1285: .It Cm PermitEmptyPasswords
1286: When password authentication is allowed, it specifies whether the
1287: server allows login to accounts with empty password strings.
1288: The default is
1.237 jmc 1289: .Cm no .
1.271 djm 1290: .It Cm PermitListen
1291: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1292: The listen specification must be one of the following forms:
1293: .Pp
1294: .Bl -item -offset indent -compact
1295: .It
1296: .Cm PermitListen
1297: .Sm off
1.276 djm 1298: .Ar port
1.271 djm 1299: .Sm on
1300: .It
1301: .Cm PermitListen
1302: .Sm off
1.276 djm 1303: .Ar host : port
1.271 djm 1304: .Sm on
1305: .El
1306: .Pp
1307: Multiple permissions may be specified by separating them with whitespace.
1308: An argument of
1309: .Cm any
1310: can be used to remove all restrictions and permit any listen requests.
1311: An argument of
1312: .Cm none
1313: can be used to prohibit all listen requests.
1314: The host name may contain wildcards as described in the PATTERNS section in
1315: .Xr ssh_config 5 .
1316: The wildcard
1317: .Sq *
1318: can also be used in place of a port number to allow all ports.
1319: By default all port forwarding listen requests are permitted.
1.272 jmc 1320: Note that the
1.271 djm 1321: .Cm GatewayPorts
1322: option may further restrict which addresses may be listened on.
1.276 djm 1323: Note also that
1324: .Xr ssh 1
1325: will request a listen host of
1326: .Dq localhost
1.283 dtucker 1327: if no listen host was specifically requested, and this name is
1.277 jmc 1328: treated differently to explicit localhost addresses of
1.276 djm 1329: .Dq 127.0.0.1
1330: and
1331: .Dq ::1 .
1.62 dtucker 1332: .It Cm PermitOpen
1333: Specifies the destinations to which TCP port forwarding is permitted.
1334: The forwarding specification must be one of the following forms:
1335: .Pp
1336: .Bl -item -offset indent -compact
1337: .It
1338: .Cm PermitOpen
1339: .Sm off
1340: .Ar host : port
1341: .Sm on
1342: .It
1343: .Cm PermitOpen
1344: .Sm off
1345: .Ar IPv4_addr : port
1346: .Sm on
1347: .It
1348: .Cm PermitOpen
1349: .Sm off
1350: .Ar \&[ IPv6_addr \&] : port
1351: .Sm on
1352: .El
1353: .Pp
1.68 dtucker 1354: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1355: An argument of
1.237 jmc 1356: .Cm any
1.62 dtucker 1357: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1358: An argument of
1.237 jmc 1359: .Cm none
1.140 dtucker 1360: can be used to prohibit all forwarding requests.
1.227 jmc 1361: The wildcard
1.237 jmc 1362: .Sq *
1.299 djm 1363: can be used for host or port to allow all hosts or ports respectively.
1364: Otherwise, no pattern matching or address lookups are performed on supplied
1365: names.
1.63 jmc 1366: By default all port forwarding requests are permitted.
1.1 stevesk 1367: .It Cm PermitRootLogin
1.38 jmc 1368: Specifies whether root can log in using
1.1 stevesk 1369: .Xr ssh 1 .
1370: The argument must be
1.237 jmc 1371: .Cm yes ,
1372: .Cm prohibit-password ,
1373: .Cm forced-commands-only ,
1.1 stevesk 1374: or
1.237 jmc 1375: .Cm no .
1.1 stevesk 1376: The default is
1.237 jmc 1377: .Cm prohibit-password .
1.1 stevesk 1378: .Pp
1379: If this option is set to
1.237 jmc 1380: .Cm prohibit-password
1.255 jmc 1381: (or its deprecated alias,
1382: .Cm without-password ) ,
1.210 deraadt 1383: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1384: .Pp
1385: If this option is set to
1.237 jmc 1386: .Cm forced-commands-only ,
1.1 stevesk 1387: root login with public key authentication will be allowed,
1388: but only if the
1389: .Ar command
1390: option has been specified
1391: (which may be useful for taking remote backups even if root login is
1.17 jmc 1392: normally not allowed).
1393: All other authentication methods are disabled for root.
1.1 stevesk 1394: .Pp
1395: If this option is set to
1.237 jmc 1396: .Cm no ,
1.38 jmc 1397: root is not allowed to log in.
1.231 jmc 1398: .It Cm PermitTTY
1399: Specifies whether
1400: .Xr pty 4
1401: allocation is permitted.
1402: The default is
1.237 jmc 1403: .Cm yes .
1.46 reyk 1404: .It Cm PermitTunnel
1405: Specifies whether
1406: .Xr tun 4
1407: device forwarding is allowed.
1.47 reyk 1408: The argument must be
1.237 jmc 1409: .Cm yes ,
1410: .Cm point-to-point
1.58 stevesk 1411: (layer 3),
1.237 jmc 1412: .Cm ethernet
1.58 stevesk 1413: (layer 2), or
1.237 jmc 1414: .Cm no .
1.58 stevesk 1415: Specifying
1.237 jmc 1416: .Cm yes
1.58 stevesk 1417: permits both
1.237 jmc 1418: .Cm point-to-point
1.58 stevesk 1419: and
1.237 jmc 1420: .Cm ethernet .
1.46 reyk 1421: The default is
1.237 jmc 1422: .Cm no .
1.178 djm 1423: .Pp
1424: Independent of this setting, the permissions of the selected
1425: .Xr tun 4
1426: device must allow access to the user.
1.6 markus 1427: .It Cm PermitUserEnvironment
1428: Specifies whether
1429: .Pa ~/.ssh/environment
1.9 stevesk 1430: and
1.6 markus 1431: .Cm environment=
1432: options in
1433: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1434: are processed by
1.52 jmc 1435: .Xr sshd 8 .
1.278 djm 1436: Valid options are
1437: .Cm yes ,
1438: .Cm no
1439: or a pattern-list specifying which environment variable names to accept
1440: (for example
1441: .Qq LANG,LC_* ) .
1.6 markus 1442: The default is
1.237 jmc 1443: .Cm no .
1.9 stevesk 1444: Enabling environment processing may enable users to bypass access
1445: restrictions in some configurations using mechanisms such as
1446: .Ev LD_PRELOAD .
1.174 djm 1447: .It Cm PermitUserRC
1448: Specifies whether any
1449: .Pa ~/.ssh/rc
1450: file is executed.
1451: The default is
1.237 jmc 1452: .Cm yes .
1.321 dtucker 1453: .It Cm PerSourceMaxStartups
1454: Specifies the number of unauthenticated connections allowed from a
1455: given source address, or
1456: .Dq none
1457: if there is no limit.
1458: This limit is applied in addition to
1459: .Cm MaxStartups ,
1460: whichever is lower.
1461: The default is
1462: .Cm none .
1463: .It Cm PerSourceNetBlockSize
1464: Specifies the number of bits of source address that are grouped together
1465: for the purposes of applying PerSourceMaxStartups limits.
1466: Values for IPv4 and optionally IPv6 may be specified, separated by a colon.
1467: The default is
1.322 jmc 1468: .Cm 32:128 ,
1.321 dtucker 1469: which means each address is considered individually.
1.328 djm 1470: .It Cm PidFile
1471: Specifies the file that contains the process ID of the
1472: SSH daemon, or
1473: .Cm none
1474: to not write one.
1475: The default is
1476: .Pa /var/run/sshd.pid .
1.1 stevesk 1477: .It Cm Port
1478: Specifies the port number that
1.52 jmc 1479: .Xr sshd 8
1.1 stevesk 1480: listens on.
1481: The default is 22.
1482: Multiple options of this type are permitted.
1483: See also
1484: .Cm ListenAddress .
1485: .It Cm PrintLastLog
1486: Specifies whether
1.52 jmc 1487: .Xr sshd 8
1.36 jaredy 1488: should print the date and time of the last user login when a user logs
1489: in interactively.
1.1 stevesk 1490: The default is
1.237 jmc 1491: .Cm yes .
1.1 stevesk 1492: .It Cm PrintMotd
1493: Specifies whether
1.52 jmc 1494: .Xr sshd 8
1.1 stevesk 1495: should print
1496: .Pa /etc/motd
1497: when a user logs in interactively.
1498: (On some systems it is also printed by the shell,
1499: .Pa /etc/profile ,
1500: or equivalent.)
1501: The default is
1.237 jmc 1502: .Cm yes .
1.324 dtucker 1503: .It Cm PubkeyAcceptedAlgorithms
1504: Specifies the signature algorithms that will be accepted for public key
1505: authentication as a list of comma-separated patterns.
1.289 naddy 1506: Alternately if the specified list begins with a
1.208 djm 1507: .Sq +
1.324 dtucker 1508: character, then the specified algorithms will be appended to the default set
1.208 djm 1509: instead of replacing them.
1.289 naddy 1510: If the specified list begins with a
1.242 djm 1511: .Sq -
1.324 dtucker 1512: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 1513: from the default set instead of replacing them.
1.290 naddy 1514: If the specified list begins with a
1515: .Sq ^
1.324 dtucker 1516: character, then the specified algorithms will be placed at the head of the
1.290 naddy 1517: default set.
1.206 markus 1518: The default for this option is:
1519: .Bd -literal -offset 3n
1.316 djm 1520: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 1521: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1522: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1523: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 1524: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 1525: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 1526: rsa-sha2-512-cert-v01@openssh.com,
1527: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1528: ssh-rsa-cert-v01@openssh.com,
1.316 djm 1529: ssh-ed25519,
1.296 naddy 1530: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 1531: sk-ssh-ed25519@openssh.com,
1.291 naddy 1532: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1533: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1534: .Ed
1535: .Pp
1.326 djm 1536: The list of available signature algorithms may also be obtained using
1.324 dtucker 1537: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.293 djm 1538: .It Cm PubkeyAuthOptions
1539: Sets one or more public key authentication options.
1.313 djm 1540: The supported keywords are:
1.294 jmc 1541: .Cm none
1.313 djm 1542: (the default; indicating no additional options are enabled),
1543: .Cm touch-required
1.293 djm 1544: and
1.313 djm 1545: .Cm verify-required .
1.293 djm 1546: .Pp
1547: The
1548: .Cm touch-required
1.297 naddy 1549: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1550: (i.e.\&
1.293 djm 1551: .Cm ecdsa-sk
1552: or
1553: .Cm ed25519-sk )
1554: to always require the signature to attest that a physically present user
1.297 naddy 1555: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1556: By default,
1557: .Xr sshd 8
1.297 naddy 1558: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1559: The
1560: .Cm touch-required
1561: flag disables this override.
1.313 djm 1562: .Pp
1563: The
1564: .Cm verify-required
1.315 jmc 1565: option requires a FIDO key signature attest that the user was verified,
1.314 jmc 1566: e.g. via a PIN.
1.313 djm 1567: .Pp
1568: Neither the
1569: .Cm touch-required
1570: or
1571: .Cm verify-required
1.314 jmc 1572: options have any effect for other, non-FIDO, public key types.
1.1 stevesk 1573: .It Cm PubkeyAuthentication
1574: Specifies whether public key authentication is allowed.
1575: The default is
1.237 jmc 1576: .Cm yes .
1.159 dtucker 1577: .It Cm RekeyLimit
1578: Specifies the maximum amount of data that may be transmitted before the
1.323 rob 1579: session key is renegotiated, optionally followed by a maximum amount of
1.159 dtucker 1580: time that may pass before the session key is renegotiated.
1581: The first argument is specified in bytes and may have a suffix of
1582: .Sq K ,
1583: .Sq M ,
1584: or
1585: .Sq G
1586: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1587: The default is between
1588: .Sq 1G
1589: and
1590: .Sq 4G ,
1591: depending on the cipher.
1592: The optional second value is specified in seconds and may use any of the
1593: units documented in the
1594: .Sx TIME FORMATS
1.160 jmc 1595: section.
1.159 dtucker 1596: The default value for
1597: .Cm RekeyLimit
1598: is
1.237 jmc 1599: .Cm default none ,
1.159 dtucker 1600: which means that rekeying is performed after the cipher's default amount
1601: of data has been sent or received and no time based rekeying is done.
1.118 djm 1602: .It Cm RevokedKeys
1.195 dtucker 1603: Specifies revoked public keys file, or
1.237 jmc 1604: .Cm none
1.195 dtucker 1605: to not use one.
1.118 djm 1606: Keys listed in this file will be refused for public key authentication.
1607: Note that if this file is not readable, then public key authentication will
1608: be refused for all users.
1.154 djm 1609: Keys may be specified as a text file, listing one public key per line, or as
1610: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1611: .Xr ssh-keygen 1 .
1.161 jmc 1612: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1613: .Xr ssh-keygen 1 .
1.257 djm 1614: .It Cm RDomain
1615: Specifies an explicit routing domain that is applied after authentication
1616: has completed.
1.333 jmc 1617: The user session, as well as any forwarded or listening IP sockets,
1.259 jmc 1618: will be bound to this
1.257 djm 1619: .Xr rdomain 4 .
1620: If the routing domain is set to
1621: .Cm \&%D ,
1.259 jmc 1622: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1623: .It Cm SecurityKeyProvider
1.297 naddy 1624: Specifies a path to a library that will be used when loading
1625: FIDO authenticator-hosted keys, overriding the default of using
1626: the built-in USB HID support.
1.274 djm 1627: .It Cm SetEnv
1628: Specifies one or more environment variables to set in child sessions started
1629: by
1630: .Xr sshd 8
1631: as
1632: .Dq NAME=VALUE .
1633: The environment value may be quoted (e.g. if it contains whitespace
1634: characters).
1635: Environment variables set by
1636: .Cm SetEnv
1637: override the default environment and any variables specified by the user
1638: via
1639: .Cm AcceptEnv
1640: or
1641: .Cm PermitUserEnvironment .
1.175 millert 1642: .It Cm StreamLocalBindMask
1643: Sets the octal file creation mode mask
1644: .Pq umask
1645: used when creating a Unix-domain socket file for local or remote
1646: port forwarding.
1647: This option is only used for port forwarding to a Unix-domain socket file.
1648: .Pp
1649: The default value is 0177, which creates a Unix-domain socket file that is
1650: readable and writable only by the owner.
1651: Note that not all operating systems honor the file mode on Unix-domain
1652: socket files.
1653: .It Cm StreamLocalBindUnlink
1654: Specifies whether to remove an existing Unix-domain socket file for local
1655: or remote port forwarding before creating a new one.
1656: If the socket file already exists and
1657: .Cm StreamLocalBindUnlink
1658: is not enabled,
1659: .Nm sshd
1660: will be unable to forward the port to the Unix-domain socket file.
1661: This option is only used for port forwarding to a Unix-domain socket file.
1662: .Pp
1663: The argument must be
1.237 jmc 1664: .Cm yes
1.175 millert 1665: or
1.237 jmc 1666: .Cm no .
1.175 millert 1667: The default is
1.237 jmc 1668: .Cm no .
1.1 stevesk 1669: .It Cm StrictModes
1670: Specifies whether
1.52 jmc 1671: .Xr sshd 8
1.1 stevesk 1672: should check file modes and ownership of the
1673: user's files and home directory before accepting login.
1674: This is normally desirable because novices sometimes accidentally leave their
1675: directory or files world-writable.
1676: The default is
1.237 jmc 1677: .Cm yes .
1.112 djm 1678: Note that this does not apply to
1679: .Cm ChrootDirectory ,
1680: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1681: .It Cm Subsystem
1.51 jmc 1682: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1683: Arguments should be a subsystem name and a command (with optional arguments)
1684: to execute upon subsystem request.
1.80 djm 1685: .Pp
1.1 stevesk 1686: The command
1.237 jmc 1687: .Cm sftp-server
1688: implements the SFTP file transfer subsystem.
1.80 djm 1689: .Pp
1690: Alternately the name
1.237 jmc 1691: .Cm internal-sftp
1692: implements an in-process SFTP server.
1.80 djm 1693: This may simplify configurations using
1694: .Cm ChrootDirectory
1695: to force a different filesystem root on clients.
1696: .Pp
1.1 stevesk 1697: By default no subsystems are defined.
1698: .It Cm SyslogFacility
1699: Gives the facility code that is used when logging messages from
1.53 jmc 1700: .Xr sshd 8 .
1.1 stevesk 1701: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1702: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1703: The default is AUTH.
1.27 markus 1704: .It Cm TCPKeepAlive
1705: Specifies whether the system should send TCP keepalive messages to the
1706: other side.
1707: If they are sent, death of the connection or crash of one
1708: of the machines will be properly noticed.
1709: However, this means that
1710: connections will die if the route is down temporarily, and some people
1711: find it annoying.
1712: On the other hand, if TCP keepalives are not sent,
1713: sessions may hang indefinitely on the server, leaving
1.237 jmc 1714: .Qq ghost
1.27 markus 1715: users and consuming server resources.
1716: .Pp
1717: The default is
1.237 jmc 1718: .Cm yes
1.27 markus 1719: (to send TCP keepalive messages), and the server will notice
1720: if the network goes down or the client host crashes.
1721: This avoids infinitely hanging sessions.
1722: .Pp
1723: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1724: .Cm no .
1.118 djm 1725: .It Cm TrustedUserCAKeys
1726: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1727: trusted to sign user certificates for authentication, or
1.237 jmc 1728: .Cm none
1.195 dtucker 1729: to not use one.
1.119 jmc 1730: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1731: .Ql #
1732: are allowed.
1733: If a certificate is presented for authentication and has its signing CA key
1734: listed in this file, then it may be used for authentication for any user
1735: listed in the certificate's principals list.
1736: Note that certificates that lack a list of principals will not be permitted
1737: for authentication using
1738: .Cm TrustedUserCAKeys .
1.161 jmc 1739: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1740: .Xr ssh-keygen 1 .
1.18 markus 1741: .It Cm UseDNS
1742: Specifies whether
1.52 jmc 1743: .Xr sshd 8
1.207 djm 1744: should look up the remote host name, and to check that
1.18 markus 1745: the resolved host name for the remote IP address maps back to the
1746: very same IP address.
1.207 djm 1747: .Pp
1748: If this option is set to
1.237 jmc 1749: .Cm no
1.207 djm 1750: (the default) then only addresses and not host names may be used in
1.221 djm 1751: .Pa ~/.ssh/authorized_keys
1.207 djm 1752: .Cm from
1753: and
1.211 jmc 1754: .Nm
1.207 djm 1755: .Cm Match
1756: .Cm Host
1757: directives.
1.137 djm 1758: .It Cm VersionAddendum
1759: Optionally specifies additional text to append to the SSH protocol banner
1760: sent by the server upon connection.
1761: The default is
1.237 jmc 1762: .Cm none .
1.1 stevesk 1763: .It Cm X11DisplayOffset
1764: Specifies the first display number available for
1.52 jmc 1765: .Xr sshd 8 Ns 's
1.1 stevesk 1766: X11 forwarding.
1.52 jmc 1767: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1768: The default is 10.
1769: .It Cm X11Forwarding
1770: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1771: The argument must be
1.237 jmc 1772: .Cm yes
1.13 stevesk 1773: or
1.237 jmc 1774: .Cm no .
1.1 stevesk 1775: The default is
1.237 jmc 1776: .Cm no .
1.13 stevesk 1777: .Pp
1778: When X11 forwarding is enabled, there may be additional exposure to
1779: the server and to client displays if the
1.52 jmc 1780: .Xr sshd 8
1.13 stevesk 1781: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1782: .Cm X11UseLocalhost ) ,
1783: though this is not the default.
1.13 stevesk 1784: Additionally, the authentication spoofing and authentication data
1785: verification and substitution occur on the client side.
1786: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1787: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1788: forwarding (see the warnings for
1789: .Cm ForwardX11
1790: in
1.19 jmc 1791: .Xr ssh_config 5 ) .
1.13 stevesk 1792: A system administrator may have a stance in which they want to
1793: protect clients that may expose themselves to attack by unwittingly
1794: requesting X11 forwarding, which can warrant a
1.237 jmc 1795: .Cm no
1.13 stevesk 1796: setting.
1797: .Pp
1798: Note that disabling X11 forwarding does not prevent users from
1799: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1800: .It Cm X11UseLocalhost
1801: Specifies whether
1.52 jmc 1802: .Xr sshd 8
1.1 stevesk 1803: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1804: the wildcard address.
1805: By default,
1.52 jmc 1806: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1807: hostname part of the
1808: .Ev DISPLAY
1809: environment variable to
1.237 jmc 1810: .Cm localhost .
1.8 stevesk 1811: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1812: However, some older X11 clients may not function with this
1813: configuration.
1814: .Cm X11UseLocalhost
1815: may be set to
1.237 jmc 1816: .Cm no
1.1 stevesk 1817: to specify that the forwarding server should be bound to the wildcard
1818: address.
1819: The argument must be
1.237 jmc 1820: .Cm yes
1.1 stevesk 1821: or
1.237 jmc 1822: .Cm no .
1.1 stevesk 1823: The default is
1.237 jmc 1824: .Cm yes .
1.1 stevesk 1825: .It Cm XAuthLocation
1.11 stevesk 1826: Specifies the full pathname of the
1.1 stevesk 1827: .Xr xauth 1
1.195 dtucker 1828: program, or
1.237 jmc 1829: .Cm none
1.195 dtucker 1830: to not use one.
1.1 stevesk 1831: The default is
1832: .Pa /usr/X11R6/bin/xauth .
1833: .El
1.55 jmc 1834: .Sh TIME FORMATS
1.53 jmc 1835: .Xr sshd 8
1.1 stevesk 1836: command-line arguments and configuration file options that specify time
1837: may be expressed using a sequence of the form:
1838: .Sm off
1.7 stevesk 1839: .Ar time Op Ar qualifier ,
1.1 stevesk 1840: .Sm on
1841: where
1842: .Ar time
1843: is a positive integer value and
1844: .Ar qualifier
1845: is one of the following:
1846: .Pp
1847: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1848: .It Aq Cm none
1.1 stevesk 1849: seconds
1850: .It Cm s | Cm S
1851: seconds
1852: .It Cm m | Cm M
1853: minutes
1854: .It Cm h | Cm H
1855: hours
1856: .It Cm d | Cm D
1857: days
1858: .It Cm w | Cm W
1859: weeks
1860: .El
1861: .Pp
1862: Each member of the sequence is added together to calculate
1863: the total time value.
1864: .Pp
1865: Time format examples:
1866: .Pp
1867: .Bl -tag -width Ds -compact -offset indent
1868: .It 600
1869: 600 seconds (10 minutes)
1870: .It 10m
1871: 10 minutes
1872: .It 1h30m
1873: 1 hour 30 minutes (90 minutes)
1874: .El
1.235 jmc 1875: .Sh TOKENS
1876: Arguments to some keywords can make use of tokens,
1877: which are expanded at runtime:
1878: .Pp
1879: .Bl -tag -width XXXX -offset indent -compact
1880: .It %%
1881: A literal
1882: .Sq % .
1.257 djm 1883: .It \&%D
1884: The routing domain in which the incoming connection was received.
1.235 jmc 1885: .It %F
1886: The fingerprint of the CA key.
1887: .It %f
1888: The fingerprint of the key or certificate.
1889: .It %h
1890: The home directory of the user.
1891: .It %i
1892: The key ID in the certificate.
1893: .It %K
1894: The base64-encoded CA key.
1895: .It %k
1896: The base64-encoded key or certificate for authentication.
1897: .It %s
1898: The serial number of the certificate.
1899: .It \&%T
1900: The type of the CA key.
1901: .It %t
1902: The key or certificate type.
1.268 jmc 1903: .It \&%U
1904: The numeric user ID of the target user.
1.235 jmc 1905: .It %u
1906: The username.
1907: .El
1908: .Pp
1909: .Cm AuthorizedKeysCommand
1.270 jmc 1910: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1911: .Pp
1912: .Cm AuthorizedKeysFile
1.270 jmc 1913: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1914: .Pp
1915: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1916: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1917: .Pp
1918: .Cm AuthorizedPrincipalsFile
1.270 jmc 1919: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1920: .Pp
1921: .Cm ChrootDirectory
1.270 jmc 1922: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1923: .Pp
1924: .Cm RoutingDomain
1925: accepts the token %D.
1.1 stevesk 1926: .Sh FILES
1927: .Bl -tag -width Ds
1928: .It Pa /etc/ssh/sshd_config
1929: Contains configuration data for
1.53 jmc 1930: .Xr sshd 8 .
1.1 stevesk 1931: This file should be writable by root only, but it is recommended
1932: (though not necessary) that it be world-readable.
1933: .El
1.19 jmc 1934: .Sh SEE ALSO
1.237 jmc 1935: .Xr sftp-server 8 ,
1.19 jmc 1936: .Xr sshd 8
1.1 stevesk 1937: .Sh AUTHORS
1.237 jmc 1938: .An -nosplit
1.1 stevesk 1939: OpenSSH is a derivative of the original and free
1.237 jmc 1940: ssh 1.2.12 release by
1941: .An Tatu Ylonen .
1942: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1943: .An Theo de Raadt
1944: and
1945: .An Dug Song
1.1 stevesk 1946: removed many bugs, re-added newer features and
1947: created OpenSSH.
1.237 jmc 1948: .An Markus Friedl
1949: contributed the support for SSH protocol versions 1.5 and 2.0.
1950: .An Niels Provos
1951: and
1952: .An Markus Friedl
1953: contributed support for privilege separation.
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