Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.331
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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1.331 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.330 2021/04/03 06:55:52 djm Exp $
! 37: .Dd $Mdocdate: April 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
1.330 djm 381: ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.331 ! jmc 382: sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,
1.330 djm 383: rsa-sha2-512,rsa-sha2-256
1.282 djm 384: .Ed
385: .Pp
386: Certificates signed using other algorithms will not be accepted for
387: public key or host-based authentication.
1.1 stevesk 388: .It Cm ChallengeResponseAuthentication
1.50 jmc 389: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 390: All authentication styles from
391: .Xr login.conf 5
392: are supported.
393: The default is
1.237 jmc 394: .Cm yes .
1.80 djm 395: .It Cm ChrootDirectory
1.113 stevesk 396: Specifies the pathname of a directory to
1.80 djm 397: .Xr chroot 2
398: to after authentication.
1.190 deraadt 399: At session startup
400: .Xr sshd 8
401: checks that all components of the pathname are root-owned directories
402: which are not writable by any other user or group.
1.106 stevesk 403: After the chroot,
404: .Xr sshd 8
405: changes the working directory to the user's home directory.
1.235 jmc 406: Arguments to
407: .Cm ChrootDirectory
408: accept the tokens described in the
409: .Sx TOKENS
410: section.
1.80 djm 411: .Pp
412: The
413: .Cm ChrootDirectory
414: must contain the necessary files and directories to support the
1.103 stevesk 415: user's session.
1.80 djm 416: For an interactive session this requires at least a shell, typically
417: .Xr sh 1 ,
418: and basic
419: .Pa /dev
420: nodes such as
421: .Xr null 4 ,
422: .Xr zero 4 ,
423: .Xr stdin 4 ,
424: .Xr stdout 4 ,
425: .Xr stderr 4 ,
1.180 jmc 426: and
1.80 djm 427: .Xr tty 4
428: devices.
1.237 jmc 429: For file transfer sessions using SFTP
430: no additional configuration of the environment is necessary if the in-process
431: sftp-server is used,
1.176 schwarze 432: though sessions which use logging may require
1.104 stevesk 433: .Pa /dev/log
1.176 schwarze 434: inside the chroot directory on some operating systems (see
1.104 stevesk 435: .Xr sftp-server 8
1.81 jmc 436: for details).
1.190 deraadt 437: .Pp
1.191 jmc 438: For safety, it is very important that the directory hierarchy be
1.190 deraadt 439: prevented from modification by other processes on the system (especially
440: those outside the jail).
441: Misconfiguration can lead to unsafe environments which
442: .Xr sshd 8
443: cannot detect.
1.80 djm 444: .Pp
1.215 djm 445: The default is
1.237 jmc 446: .Cm none ,
1.215 djm 447: indicating not to
1.80 djm 448: .Xr chroot 2 .
1.1 stevesk 449: .It Cm Ciphers
1.219 jmc 450: Specifies the ciphers allowed.
1.1 stevesk 451: Multiple ciphers must be comma-separated.
1.289 naddy 452: If the specified list begins with a
1.208 djm 453: .Sq +
454: character, then the specified ciphers will be appended to the default set
455: instead of replacing them.
1.289 naddy 456: If the specified list begins with a
1.242 djm 457: .Sq -
458: character, then the specified ciphers (including wildcards) will be removed
459: from the default set instead of replacing them.
1.290 naddy 460: If the specified list begins with a
461: .Sq ^
462: character, then the specified ciphers will be placed at the head of the
463: default set.
1.208 djm 464: .Pp
1.167 djm 465: The supported ciphers are:
466: .Pp
1.173 naddy 467: .Bl -item -compact -offset indent
468: .It
469: 3des-cbc
470: .It
471: aes128-cbc
472: .It
473: aes192-cbc
474: .It
475: aes256-cbc
476: .It
477: aes128-ctr
478: .It
479: aes192-ctr
480: .It
481: aes256-ctr
482: .It
483: aes128-gcm@openssh.com
484: .It
485: aes256-gcm@openssh.com
486: .It
487: chacha20-poly1305@openssh.com
488: .El
1.167 djm 489: .Pp
1.52 jmc 490: The default is:
1.173 naddy 491: .Bd -literal -offset indent
1.211 jmc 492: chacha20-poly1305@openssh.com,
1.173 naddy 493: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 494: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 495: .Ed
1.167 djm 496: .Pp
1.237 jmc 497: The list of available ciphers may also be obtained using
498: .Qq ssh -Q cipher .
1.1 stevesk 499: .It Cm ClientAliveCountMax
1.237 jmc 500: Sets the number of client alive messages which may be sent without
1.52 jmc 501: .Xr sshd 8
1.17 jmc 502: receiving any messages back from the client.
503: If this threshold is reached while client alive messages are being sent,
1.52 jmc 504: sshd will disconnect the client, terminating the session.
1.17 jmc 505: It is important to note that the use of client alive messages is very
506: different from
1.237 jmc 507: .Cm TCPKeepAlive .
1.17 jmc 508: The client alive messages are sent through the encrypted channel
509: and therefore will not be spoofable.
510: The TCP keepalive option enabled by
1.27 markus 511: .Cm TCPKeepAlive
1.17 jmc 512: is spoofable.
513: The client alive mechanism is valuable when the client or
1.288 dtucker 514: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 515: .Pp
1.17 jmc 516: The default value is 3.
517: If
1.1 stevesk 518: .Cm ClientAliveInterval
1.237 jmc 519: is set to 15, and
1.1 stevesk 520: .Cm ClientAliveCountMax
1.52 jmc 521: is left at the default, unresponsive SSH clients
1.1 stevesk 522: will be disconnected after approximately 45 seconds.
1.302 djm 523: Setting a zero
524: .Cm ClientAliveCountMax
525: disables connection termination.
1.42 djm 526: .It Cm ClientAliveInterval
527: Sets a timeout interval in seconds after which if no data has been received
528: from the client,
1.52 jmc 529: .Xr sshd 8
1.42 djm 530: will send a message through the encrypted
531: channel to request a response from the client.
532: The default
533: is 0, indicating that these messages will not be sent to the client.
1.3 markus 534: .It Cm Compression
1.236 djm 535: Specifies whether compression is enabled after
1.44 markus 536: the user has authenticated successfully.
1.3 markus 537: The argument must be
1.237 jmc 538: .Cm yes ,
539: .Cm delayed
1.236 djm 540: (a legacy synonym for
1.237 jmc 541: .Cm yes )
1.3 markus 542: or
1.237 jmc 543: .Cm no .
1.3 markus 544: The default is
1.237 jmc 545: .Cm yes .
1.1 stevesk 546: .It Cm DenyGroups
547: This keyword can be followed by a list of group name patterns, separated
548: by spaces.
549: Login is disallowed for users whose primary group or supplementary
550: group list matches one of the patterns.
551: Only group names are valid; a numerical group ID is not recognized.
552: By default, login is allowed for all groups.
1.301 djm 553: The allow/deny groups directives are processed in the following order:
1.54 jmc 554: .Cm DenyGroups ,
555: .Cm AllowGroups .
1.49 jmc 556: .Pp
1.161 jmc 557: See PATTERNS in
1.49 jmc 558: .Xr ssh_config 5
559: for more information on patterns.
1.1 stevesk 560: .It Cm DenyUsers
561: This keyword can be followed by a list of user name patterns, separated
562: by spaces.
563: Login is disallowed for user names that match one of the patterns.
564: Only user names are valid; a numerical user ID is not recognized.
565: By default, login is allowed for all users.
566: If the pattern takes the form USER@HOST then USER and HOST
567: are separately checked, restricting logins to particular
568: users from particular hosts.
1.222 jmc 569: HOST criteria may additionally contain addresses to match in CIDR
570: address/masklen format.
1.301 djm 571: The allow/deny users directives are processed in the following order:
1.54 jmc 572: .Cm DenyUsers ,
1.301 djm 573: .Cm AllowUsers .
1.49 jmc 574: .Pp
1.161 jmc 575: See PATTERNS in
1.49 jmc 576: .Xr ssh_config 5
577: for more information on patterns.
1.239 djm 578: .It Cm DisableForwarding
579: Disables all forwarding features, including X11,
580: .Xr ssh-agent 1 ,
581: TCP and StreamLocal.
582: This option overrides all other forwarding-related options and may
583: simplify restricted configurations.
1.246 djm 584: .It Cm ExposeAuthInfo
1.253 jmc 585: Writes a temporary file containing a list of authentication methods and
1.246 djm 586: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 587: The location of the file is exposed to the user session through the
1.248 djm 588: .Ev SSH_USER_AUTH
1.247 jmc 589: environment variable.
1.253 jmc 590: The default is
591: .Cm no .
1.183 djm 592: .It Cm FingerprintHash
593: Specifies the hash algorithm used when logging key fingerprints.
594: Valid options are:
1.237 jmc 595: .Cm md5
1.183 djm 596: and
1.237 jmc 597: .Cm sha256 .
1.183 djm 598: The default is
1.237 jmc 599: .Cm sha256 .
1.67 dtucker 600: .It Cm ForceCommand
601: Forces the execution of the command specified by
602: .Cm ForceCommand ,
1.84 djm 603: ignoring any command supplied by the client and
604: .Pa ~/.ssh/rc
605: if present.
1.67 dtucker 606: The command is invoked by using the user's login shell with the -c option.
607: This applies to shell, command, or subsystem execution.
608: It is most useful inside a
609: .Cm Match
610: block.
611: The command originally supplied by the client is available in the
612: .Ev SSH_ORIGINAL_COMMAND
613: environment variable.
1.82 djm 614: Specifying a command of
1.237 jmc 615: .Cm internal-sftp
616: will force the use of an in-process SFTP server that requires no support
1.82 djm 617: files when used with
618: .Cm ChrootDirectory .
1.215 djm 619: The default is
1.237 jmc 620: .Cm none .
1.1 stevesk 621: .It Cm GatewayPorts
622: Specifies whether remote hosts are allowed to connect to ports
623: forwarded for the client.
624: By default,
1.52 jmc 625: .Xr sshd 8
1.15 jmc 626: binds remote port forwardings to the loopback address.
627: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 628: .Cm GatewayPorts
1.52 jmc 629: can be used to specify that sshd
1.39 djm 630: should allow remote port forwardings to bind to non-loopback addresses, thus
631: allowing other hosts to connect.
632: The argument may be
1.237 jmc 633: .Cm no
1.39 djm 634: to force remote port forwardings to be available to the local host only,
1.237 jmc 635: .Cm yes
1.39 djm 636: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 637: .Cm clientspecified
1.39 djm 638: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 639: The default is
1.237 jmc 640: .Cm no .
1.23 markus 641: .It Cm GSSAPIAuthentication
1.25 markus 642: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 643: The default is
1.237 jmc 644: .Cm no .
1.23 markus 645: .It Cm GSSAPICleanupCredentials
646: Specifies whether to automatically destroy the user's credentials cache
647: on logout.
648: The default is
1.237 jmc 649: .Cm yes .
1.203 djm 650: .It Cm GSSAPIStrictAcceptorCheck
651: Determines whether to be strict about the identity of the GSSAPI acceptor
652: a client authenticates against.
653: If set to
1.237 jmc 654: .Cm yes
655: then the client must authenticate against the host
1.203 djm 656: service on the current hostname.
657: If set to
1.237 jmc 658: .Cm no
1.203 djm 659: then the client may authenticate against any service key stored in the
660: machine's default store.
661: This facility is provided to assist with operation on multi homed machines.
662: The default is
1.237 jmc 663: .Cm yes .
1.325 dtucker 664: .It Cm HostbasedAcceptedAlgorithms
1.327 djm 665: Specifies the signature algorithms that will be accepted for hostbased
666: authentication as a list of comma-separated patterns.
1.289 naddy 667: Alternately if the specified list begins with a
1.208 djm 668: .Sq +
1.327 djm 669: character, then the specified signature algorithms will be appended to
670: the default set instead of replacing them.
1.289 naddy 671: If the specified list begins with a
1.242 djm 672: .Sq -
1.327 djm 673: character, then the specified signature algorithms (including wildcards)
674: will be removed from the default set instead of replacing them.
1.290 naddy 675: If the specified list begins with a
676: .Sq ^
1.327 djm 677: character, then the specified signature algorithms will be placed at
678: the head of the default set.
1.206 markus 679: The default for this option is:
680: .Bd -literal -offset 3n
1.316 djm 681: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 682: ecdsa-sha2-nistp256-cert-v01@openssh.com,
683: ecdsa-sha2-nistp384-cert-v01@openssh.com,
684: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 685: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 686: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 687: rsa-sha2-512-cert-v01@openssh.com,
688: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 689: ssh-rsa-cert-v01@openssh.com,
1.316 djm 690: ssh-ed25519,
1.206 markus 691: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 692: sk-ssh-ed25519@openssh.com,
1.296 naddy 693: sk-ecdsa-sha2-nistp256@openssh.com,
694: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 695: .Ed
696: .Pp
1.327 djm 697: The list of available signature algorithms may also be obtained using
1.325 dtucker 698: .Qq ssh -Q HostbasedAcceptedAlgorithms .
699: This was formerly named HostbasedAcceptedKeyTypes.
1.1 stevesk 700: .It Cm HostbasedAuthentication
701: Specifies whether rhosts or /etc/hosts.equiv authentication together
702: with successful public key client host authentication is allowed
1.50 jmc 703: (host-based authentication).
1.70 dtucker 704: The default is
1.237 jmc 705: .Cm no .
1.70 dtucker 706: .It Cm HostbasedUsesNameFromPacketOnly
707: Specifies whether or not the server will attempt to perform a reverse
708: name lookup when matching the name in the
709: .Pa ~/.shosts ,
710: .Pa ~/.rhosts ,
711: and
712: .Pa /etc/hosts.equiv
713: files during
714: .Cm HostbasedAuthentication .
715: A setting of
1.237 jmc 716: .Cm yes
1.70 dtucker 717: means that
718: .Xr sshd 8
719: uses the name supplied by the client rather than
720: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 721: The default is
1.237 jmc 722: .Cm no .
1.117 djm 723: .It Cm HostCertificate
724: Specifies a file containing a public host certificate.
725: The certificate's public key must match a private host key already specified
726: by
727: .Cm HostKey .
728: The default behaviour of
729: .Xr sshd 8
730: is not to load any certificates.
1.1 stevesk 731: .It Cm HostKey
732: Specifies a file containing a private host key
733: used by SSH.
1.229 naddy 734: The defaults are
1.169 naddy 735: .Pa /etc/ssh/ssh_host_ecdsa_key ,
736: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 737: and
1.229 naddy 738: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 739: .Pp
1.1 stevesk 740: Note that
1.52 jmc 741: .Xr sshd 8
1.206 markus 742: will refuse to use a file if it is group/world-accessible
743: and that the
744: .Cm HostKeyAlgorithms
745: option restricts which of the keys are actually used by
746: .Xr sshd 8 .
747: .Pp
1.1 stevesk 748: It is possible to have multiple host key files.
1.162 markus 749: It is also possible to specify public host key files instead.
750: In this case operations on the private key will be delegated
751: to an
752: .Xr ssh-agent 1 .
753: .It Cm HostKeyAgent
754: Identifies the UNIX-domain socket used to communicate
755: with an agent that has access to the private host keys.
1.223 markus 756: If the string
1.237 jmc 757: .Qq SSH_AUTH_SOCK
1.162 markus 758: is specified, the location of the socket will be read from the
759: .Ev SSH_AUTH_SOCK
760: environment variable.
1.206 markus 761: .It Cm HostKeyAlgorithms
1.327 djm 762: Specifies the host key signature algorithms
1.206 markus 763: that the server offers.
764: The default for this option is:
765: .Bd -literal -offset 3n
1.316 djm 766: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 767: ecdsa-sha2-nistp256-cert-v01@openssh.com,
768: ecdsa-sha2-nistp384-cert-v01@openssh.com,
769: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 770: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 771: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 772: rsa-sha2-512-cert-v01@openssh.com,
773: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 774: ssh-rsa-cert-v01@openssh.com,
1.316 djm 775: ssh-ed25519,
1.206 markus 776: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 777: sk-ssh-ed25519@openssh.com,
1.296 naddy 778: sk-ecdsa-sha2-nistp256@openssh.com,
779: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 780: .Ed
781: .Pp
1.327 djm 782: The list of available signature algorithms may also be obtained using
1.307 dtucker 783: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 784: .It Cm IgnoreRhosts
1.309 djm 785: Specifies whether to ignore per-user
1.1 stevesk 786: .Pa .rhosts
787: and
788: .Pa .shosts
1.309 djm 789: files during
1.1 stevesk 790: .Cm HostbasedAuthentication .
1.309 djm 791: The system-wide
1.1 stevesk 792: .Pa /etc/hosts.equiv
793: and
794: .Pa /etc/shosts.equiv
1.309 djm 795: are still used regardless of this setting.
796: .Pp
797: Accepted values are
798: .Cm yes
799: (the default) to ignore all per-user files,
800: .Cm shosts-only
801: to allow the use of
802: .Pa .shosts
803: but to ignore
804: .Pa .rhosts
805: or
806: .Cm no
807: to allow both
808: .Pa .shosts
809: and
1.311 jmc 810: .Pa rhosts .
1.1 stevesk 811: .It Cm IgnoreUserKnownHosts
812: Specifies whether
1.52 jmc 813: .Xr sshd 8
1.1 stevesk 814: should ignore the user's
1.41 djm 815: .Pa ~/.ssh/known_hosts
1.1 stevesk 816: during
1.261 djm 817: .Cm HostbasedAuthentication
818: and use only the system-wide known hosts file
819: .Pa /etc/ssh/known_hosts .
1.1 stevesk 820: The default is
1.304 djm 821: .Dq no .
822: .It Cm Include
823: Include the specified configuration file(s).
1.305 jmc 824: Multiple pathnames may be specified and each pathname may contain
1.304 djm 825: .Xr glob 7
1.312 djm 826: wildcards that will be expanded and processed in lexical order.
1.304 djm 827: Files without absolute paths are assumed to be in
828: .Pa /etc/ssh .
1.305 jmc 829: An
1.304 djm 830: .Cm Include
831: directive may appear inside a
832: .Cm Match
833: block
834: to perform conditional inclusion.
1.129 djm 835: .It Cm IPQoS
836: Specifies the IPv4 type-of-service or DSCP class for the connection.
837: Accepted values are
1.237 jmc 838: .Cm af11 ,
839: .Cm af12 ,
840: .Cm af13 ,
841: .Cm af21 ,
842: .Cm af22 ,
843: .Cm af23 ,
844: .Cm af31 ,
845: .Cm af32 ,
846: .Cm af33 ,
847: .Cm af41 ,
848: .Cm af42 ,
849: .Cm af43 ,
850: .Cm cs0 ,
851: .Cm cs1 ,
852: .Cm cs2 ,
853: .Cm cs3 ,
854: .Cm cs4 ,
855: .Cm cs5 ,
856: .Cm cs6 ,
857: .Cm cs7 ,
858: .Cm ef ,
1.303 djm 859: .Cm le ,
1.237 jmc 860: .Cm lowdelay ,
861: .Cm throughput ,
862: .Cm reliability ,
1.250 djm 863: a numeric value, or
864: .Cm none
865: to use the operating system default.
1.131 djm 866: This option may take one or two arguments, separated by whitespace.
1.129 djm 867: If one argument is specified, it is used as the packet class unconditionally.
868: If two values are specified, the first is automatically selected for
869: interactive sessions and the second for non-interactive sessions.
870: The default is
1.264 job 871: .Cm af21
1.265 jmc 872: (Low-Latency Data)
1.129 djm 873: for interactive sessions and
1.264 job 874: .Cm cs1
1.265 jmc 875: (Lower Effort)
1.129 djm 876: for non-interactive sessions.
1.171 jmc 877: .It Cm KbdInteractiveAuthentication
878: Specifies whether to allow keyboard-interactive authentication.
879: The argument to this keyword must be
1.237 jmc 880: .Cm yes
1.171 jmc 881: or
1.237 jmc 882: .Cm no .
1.171 jmc 883: The default is to use whatever value
884: .Cm ChallengeResponseAuthentication
885: is set to
886: (by default
1.237 jmc 887: .Cm yes ) .
1.1 stevesk 888: .It Cm KerberosAuthentication
1.24 markus 889: Specifies whether the password provided by the user for
1.1 stevesk 890: .Cm PasswordAuthentication
1.24 markus 891: will be validated through the Kerberos KDC.
1.1 stevesk 892: To use this option, the server needs a
893: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 894: The default is
1.237 jmc 895: .Cm no .
1.29 dtucker 896: .It Cm KerberosGetAFSToken
1.45 djm 897: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 898: an AFS token before accessing the user's home directory.
1.52 jmc 899: The default is
1.237 jmc 900: .Cm no .
1.1 stevesk 901: .It Cm KerberosOrLocalPasswd
1.52 jmc 902: If password authentication through Kerberos fails then
1.1 stevesk 903: the password will be validated via any additional local mechanism
904: such as
905: .Pa /etc/passwd .
1.52 jmc 906: The default is
1.237 jmc 907: .Cm yes .
1.1 stevesk 908: .It Cm KerberosTicketCleanup
909: Specifies whether to automatically destroy the user's ticket cache
910: file on logout.
1.52 jmc 911: The default is
1.237 jmc 912: .Cm yes .
1.127 djm 913: .It Cm KexAlgorithms
914: Specifies the available KEX (Key Exchange) algorithms.
915: Multiple algorithms must be comma-separated.
1.289 naddy 916: Alternately if the specified list begins with a
1.208 djm 917: .Sq +
918: character, then the specified methods will be appended to the default set
919: instead of replacing them.
1.289 naddy 920: If the specified list begins with a
1.242 djm 921: .Sq -
922: character, then the specified methods (including wildcards) will be removed
923: from the default set instead of replacing them.
1.290 naddy 924: If the specified list begins with a
925: .Sq ^
926: character, then the specified methods will be placed at the head of the
927: default set.
1.173 naddy 928: The supported algorithms are:
929: .Pp
930: .Bl -item -compact -offset indent
931: .It
1.234 djm 932: curve25519-sha256
933: .It
1.173 naddy 934: curve25519-sha256@libssh.org
935: .It
936: diffie-hellman-group1-sha1
937: .It
938: diffie-hellman-group14-sha1
939: .It
1.263 djm 940: diffie-hellman-group14-sha256
941: .It
942: diffie-hellman-group16-sha512
943: .It
944: diffie-hellman-group18-sha512
945: .It
1.173 naddy 946: diffie-hellman-group-exchange-sha1
947: .It
948: diffie-hellman-group-exchange-sha256
949: .It
950: ecdh-sha2-nistp256
951: .It
952: ecdh-sha2-nistp384
953: .It
954: ecdh-sha2-nistp521
1.298 dtucker 955: .It
1.319 djm 956: sntrup761x25519-sha512@openssh.com
1.173 naddy 957: .El
958: .Pp
959: The default is:
1.170 dtucker 960: .Bd -literal -offset indent
1.234 djm 961: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 962: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
963: diffie-hellman-group-exchange-sha256,
1.263 djm 964: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 965: diffie-hellman-group14-sha256
1.170 dtucker 966: .Ed
1.188 djm 967: .Pp
1.237 jmc 968: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 969: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 970: .It Cm ListenAddress
971: Specifies the local addresses
1.52 jmc 972: .Xr sshd 8
1.1 stevesk 973: should listen on.
974: The following forms may be used:
975: .Pp
976: .Bl -item -offset indent -compact
977: .It
978: .Cm ListenAddress
979: .Sm off
1.259 jmc 980: .Ar hostname | address
1.1 stevesk 981: .Sm on
1.260 jmc 982: .Op Cm rdomain Ar domain
1.1 stevesk 983: .It
984: .Cm ListenAddress
985: .Sm off
1.259 jmc 986: .Ar hostname : port
1.1 stevesk 987: .Sm on
1.260 jmc 988: .Op Cm rdomain Ar domain
1.256 djm 989: .It
990: .Cm ListenAddress
991: .Sm off
1.259 jmc 992: .Ar IPv4_address : port
1.256 djm 993: .Sm on
1.260 jmc 994: .Op Cm rdomain Ar domain
1.1 stevesk 995: .It
996: .Cm ListenAddress
997: .Sm off
1.259 jmc 998: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 999: .Sm on
1.260 jmc 1000: .Op Cm rdomain Ar domain
1.1 stevesk 1001: .El
1002: .Pp
1.256 djm 1003: The optional
1004: .Cm rdomain
1005: qualifier requests
1006: .Xr sshd 8
1007: listen in an explicit routing domain.
1.1 stevesk 1008: If
1009: .Ar port
1010: is not specified,
1.200 dtucker 1011: sshd will listen on the address and all
1.1 stevesk 1012: .Cm Port
1.17 jmc 1013: options specified.
1.256 djm 1014: The default is to listen on all local addresses on the current default
1015: routing domain.
1.15 jmc 1016: Multiple
1.1 stevesk 1017: .Cm ListenAddress
1.17 jmc 1018: options are permitted.
1.256 djm 1019: For more information on routing domains, see
1.259 jmc 1020: .Xr rdomain 4 .
1.1 stevesk 1021: .It Cm LoginGraceTime
1022: The server disconnects after this time if the user has not
1023: successfully logged in.
1024: If the value is 0, there is no time limit.
1.12 stevesk 1025: The default is 120 seconds.
1.1 stevesk 1026: .It Cm LogLevel
1027: Gives the verbosity level that is used when logging messages from
1.53 jmc 1028: .Xr sshd 8 .
1.1 stevesk 1029: The possible values are:
1.52 jmc 1030: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1031: The default is INFO.
1032: DEBUG and DEBUG1 are equivalent.
1033: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1034: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.317 djm 1035: .It Cm LogVerbose
1036: Specify one or more overrides to LogLevel.
1037: An override consists of a pattern lists that matches the source file, function
1038: and line number to force detailed logging for.
1039: For example, an override pattern of:
1040: .Bd -literal -offset indent
1041: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1042: .Ed
1043: .Pp
1044: would enable detailed logging for line 1000 of
1.318 jmc 1045: .Pa kex.c ,
1.317 djm 1046: everything in the
1047: .Fn kex_exchange_identification
1048: function, and all code in the
1049: .Pa packet.c
1050: file.
1051: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1052: .It Cm MACs
1053: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1054: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1055: Multiple algorithms must be comma-separated.
1.289 naddy 1056: If the specified list begins with a
1.208 djm 1057: .Sq +
1058: character, then the specified algorithms will be appended to the default set
1059: instead of replacing them.
1.289 naddy 1060: If the specified list begins with a
1.242 djm 1061: .Sq -
1062: character, then the specified algorithms (including wildcards) will be removed
1063: from the default set instead of replacing them.
1.290 naddy 1064: If the specified list begins with a
1065: .Sq ^
1066: character, then the specified algorithms will be placed at the head of the
1067: default set.
1.208 djm 1068: .Pp
1.152 markus 1069: The algorithms that contain
1.237 jmc 1070: .Qq -etm
1.152 markus 1071: calculate the MAC after encryption (encrypt-then-mac).
1072: These are considered safer and their use recommended.
1.173 naddy 1073: The supported MACs are:
1074: .Pp
1075: .Bl -item -compact -offset indent
1076: .It
1077: hmac-md5
1078: .It
1079: hmac-md5-96
1080: .It
1081: hmac-sha1
1082: .It
1083: hmac-sha1-96
1084: .It
1085: hmac-sha2-256
1086: .It
1087: hmac-sha2-512
1088: .It
1089: umac-64@openssh.com
1090: .It
1091: umac-128@openssh.com
1092: .It
1093: hmac-md5-etm@openssh.com
1094: .It
1095: hmac-md5-96-etm@openssh.com
1096: .It
1097: hmac-sha1-etm@openssh.com
1098: .It
1099: hmac-sha1-96-etm@openssh.com
1100: .It
1101: hmac-sha2-256-etm@openssh.com
1102: .It
1103: hmac-sha2-512-etm@openssh.com
1104: .It
1105: umac-64-etm@openssh.com
1106: .It
1107: umac-128-etm@openssh.com
1108: .El
1109: .Pp
1.52 jmc 1110: The default is:
1.77 jmc 1111: .Bd -literal -offset indent
1.152 markus 1112: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1113: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1114: hmac-sha1-etm@openssh.com,
1.173 naddy 1115: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1116: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1117: .Ed
1.188 djm 1118: .Pp
1.237 jmc 1119: The list of available MAC algorithms may also be obtained using
1120: .Qq ssh -Q mac .
1.60 dtucker 1121: .It Cm Match
1.61 jmc 1122: Introduces a conditional block.
1.65 dtucker 1123: If all of the criteria on the
1.60 dtucker 1124: .Cm Match
1.65 dtucker 1125: line are satisfied, the keywords on the following lines override those
1126: set in the global section of the config file, until either another
1.60 dtucker 1127: .Cm Match
1.65 dtucker 1128: line or the end of the file.
1.172 djm 1129: If a keyword appears in multiple
1130: .Cm Match
1.177 sobrado 1131: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1132: applied.
1.91 djm 1133: .Pp
1.61 jmc 1134: The arguments to
1.60 dtucker 1135: .Cm Match
1.163 dtucker 1136: are one or more criteria-pattern pairs or the single token
1137: .Cm All
1138: which matches all criteria.
1.60 dtucker 1139: The available criteria are
1140: .Cm User ,
1.69 dtucker 1141: .Cm Group ,
1.60 dtucker 1142: .Cm Host ,
1.139 dtucker 1143: .Cm LocalAddress ,
1144: .Cm LocalPort ,
1.258 djm 1145: .Cm RDomain ,
1.60 dtucker 1146: and
1.258 djm 1147: .Cm Address
1148: (with
1149: .Cm RDomain
1150: representing the
1151: .Xr rdomain 4
1.284 jmc 1152: on which the connection was received).
1.258 djm 1153: .Pp
1.91 djm 1154: The match patterns may consist of single entries or comma-separated
1155: lists and may use the wildcard and negation operators described in the
1.237 jmc 1156: .Sx PATTERNS
1157: section of
1.92 djm 1158: .Xr ssh_config 5 .
1.91 djm 1159: .Pp
1160: The patterns in an
1161: .Cm Address
1162: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1163: address/masklen format,
1164: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1165: Note that the mask length provided must be consistent with the address -
1166: it is an error to specify a mask length that is too long for the address
1.93 jmc 1167: or one with bits set in this host portion of the address.
1.237 jmc 1168: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1169: .Pp
1.60 dtucker 1170: Only a subset of keywords may be used on the lines following a
1171: .Cm Match
1172: keyword.
1173: Available keywords are
1.142 jmc 1174: .Cm AcceptEnv ,
1.99 okan 1175: .Cm AllowAgentForwarding ,
1.142 jmc 1176: .Cm AllowGroups ,
1.193 djm 1177: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1178: .Cm AllowTcpForwarding ,
1.141 markus 1179: .Cm AllowUsers ,
1.149 djm 1180: .Cm AuthenticationMethods ,
1.146 djm 1181: .Cm AuthorizedKeysCommand ,
1182: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1183: .Cm AuthorizedKeysFile ,
1.214 djm 1184: .Cm AuthorizedPrincipalsCommand ,
1185: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1186: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1187: .Cm Banner ,
1.85 djm 1188: .Cm ChrootDirectory ,
1.238 markus 1189: .Cm ClientAliveCountMax ,
1190: .Cm ClientAliveInterval ,
1.141 markus 1191: .Cm DenyGroups ,
1192: .Cm DenyUsers ,
1.320 djm 1193: .Cm DisableForwarding ,
1.67 dtucker 1194: .Cm ForceCommand ,
1.194 djm 1195: .Cm GatewayPorts ,
1.193 djm 1196: .Cm GSSAPIAuthentication ,
1.325 dtucker 1197: .Cm HostbasedAcceptedAlgorithms ,
1.87 djm 1198: .Cm HostbasedAuthentication ,
1.123 djm 1199: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1200: .Cm IgnoreRhosts ,
1.306 naddy 1201: .Cm Include ,
1.193 djm 1202: .Cm IPQoS ,
1.74 jmc 1203: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1204: .Cm KerberosAuthentication ,
1.245 djm 1205: .Cm LogLevel ,
1.95 dtucker 1206: .Cm MaxAuthTries ,
1.94 dtucker 1207: .Cm MaxSessions ,
1.72 dtucker 1208: .Cm PasswordAuthentication ,
1.97 djm 1209: .Cm PermitEmptyPasswords ,
1.271 djm 1210: .Cm PermitListen ,
1.66 dtucker 1211: .Cm PermitOpen ,
1.79 dtucker 1212: .Cm PermitRootLogin ,
1.164 djm 1213: .Cm PermitTTY ,
1.123 djm 1214: .Cm PermitTunnel ,
1.174 djm 1215: .Cm PermitUserRC ,
1.324 dtucker 1216: .Cm PubkeyAcceptedAlgorithms ,
1.107 dtucker 1217: .Cm PubkeyAuthentication ,
1.159 dtucker 1218: .Cm RekeyLimit ,
1.193 djm 1219: .Cm RevokedKeys ,
1.257 djm 1220: .Cm RDomain ,
1.274 djm 1221: .Cm SetEnv ,
1.193 djm 1222: .Cm StreamLocalBindMask ,
1223: .Cm StreamLocalBindUnlink ,
1224: .Cm TrustedUserCAKeys ,
1.66 dtucker 1225: .Cm X11DisplayOffset ,
1.101 djm 1226: .Cm X11Forwarding
1.60 dtucker 1227: and
1.286 jmc 1228: .Cm X11UseLocalhost .
1.33 dtucker 1229: .It Cm MaxAuthTries
1230: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1231: connection.
1232: Once the number of failures reaches half this value,
1233: additional failures are logged.
1234: The default is 6.
1.90 djm 1235: .It Cm MaxSessions
1.216 djm 1236: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1237: sessions permitted per network connection.
1238: Multiple sessions may be established by clients that support connection
1239: multiplexing.
1240: Setting
1241: .Cm MaxSessions
1242: to 1 will effectively disable session multiplexing, whereas setting it to 0
1243: will prevent all shell, login and subsystem sessions while still permitting
1244: forwarding.
1.90 djm 1245: The default is 10.
1.1 stevesk 1246: .It Cm MaxStartups
1247: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1248: SSH daemon.
1.1 stevesk 1249: Additional connections will be dropped until authentication succeeds or the
1250: .Cm LoginGraceTime
1251: expires for a connection.
1.156 dtucker 1252: The default is 10:30:100.
1.1 stevesk 1253: .Pp
1254: Alternatively, random early drop can be enabled by specifying
1255: the three colon separated values
1.237 jmc 1256: start:rate:full (e.g. "10:30:60").
1.53 jmc 1257: .Xr sshd 8
1.237 jmc 1258: will refuse connection attempts with a probability of rate/100 (30%)
1259: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1260: The probability increases linearly and all connection attempts
1.237 jmc 1261: are refused if the number of unauthenticated connections reaches full (60).
1.329 dtucker 1262: .It Cm ModuliFile
1263: Specifies the
1264: .Xr moduli 5
1265: file that contains the Diffie-Hellman groups used for the
1266: .Dq diffie-hellman-group-exchange-sha1
1267: and
1268: .Dq diffie-hellman-group-exchange-sha256
1269: key exchange methods.
1270: The default is
1271: .Pa /etc/moduli .
1.1 stevesk 1272: .It Cm PasswordAuthentication
1273: Specifies whether password authentication is allowed.
1274: The default is
1.237 jmc 1275: .Cm yes .
1.1 stevesk 1276: .It Cm PermitEmptyPasswords
1277: When password authentication is allowed, it specifies whether the
1278: server allows login to accounts with empty password strings.
1279: The default is
1.237 jmc 1280: .Cm no .
1.271 djm 1281: .It Cm PermitListen
1282: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1283: The listen specification must be one of the following forms:
1284: .Pp
1285: .Bl -item -offset indent -compact
1286: .It
1287: .Cm PermitListen
1288: .Sm off
1.276 djm 1289: .Ar port
1.271 djm 1290: .Sm on
1291: .It
1292: .Cm PermitListen
1293: .Sm off
1.276 djm 1294: .Ar host : port
1.271 djm 1295: .Sm on
1296: .El
1297: .Pp
1298: Multiple permissions may be specified by separating them with whitespace.
1299: An argument of
1300: .Cm any
1301: can be used to remove all restrictions and permit any listen requests.
1302: An argument of
1303: .Cm none
1304: can be used to prohibit all listen requests.
1305: The host name may contain wildcards as described in the PATTERNS section in
1306: .Xr ssh_config 5 .
1307: The wildcard
1308: .Sq *
1309: can also be used in place of a port number to allow all ports.
1310: By default all port forwarding listen requests are permitted.
1.272 jmc 1311: Note that the
1.271 djm 1312: .Cm GatewayPorts
1313: option may further restrict which addresses may be listened on.
1.276 djm 1314: Note also that
1315: .Xr ssh 1
1316: will request a listen host of
1317: .Dq localhost
1.283 dtucker 1318: if no listen host was specifically requested, and this name is
1.277 jmc 1319: treated differently to explicit localhost addresses of
1.276 djm 1320: .Dq 127.0.0.1
1321: and
1322: .Dq ::1 .
1.62 dtucker 1323: .It Cm PermitOpen
1324: Specifies the destinations to which TCP port forwarding is permitted.
1325: The forwarding specification must be one of the following forms:
1326: .Pp
1327: .Bl -item -offset indent -compact
1328: .It
1329: .Cm PermitOpen
1330: .Sm off
1331: .Ar host : port
1332: .Sm on
1333: .It
1334: .Cm PermitOpen
1335: .Sm off
1336: .Ar IPv4_addr : port
1337: .Sm on
1338: .It
1339: .Cm PermitOpen
1340: .Sm off
1341: .Ar \&[ IPv6_addr \&] : port
1342: .Sm on
1343: .El
1344: .Pp
1.68 dtucker 1345: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1346: An argument of
1.237 jmc 1347: .Cm any
1.62 dtucker 1348: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1349: An argument of
1.237 jmc 1350: .Cm none
1.140 dtucker 1351: can be used to prohibit all forwarding requests.
1.227 jmc 1352: The wildcard
1.237 jmc 1353: .Sq *
1.299 djm 1354: can be used for host or port to allow all hosts or ports respectively.
1355: Otherwise, no pattern matching or address lookups are performed on supplied
1356: names.
1.63 jmc 1357: By default all port forwarding requests are permitted.
1.1 stevesk 1358: .It Cm PermitRootLogin
1.38 jmc 1359: Specifies whether root can log in using
1.1 stevesk 1360: .Xr ssh 1 .
1361: The argument must be
1.237 jmc 1362: .Cm yes ,
1363: .Cm prohibit-password ,
1364: .Cm forced-commands-only ,
1.1 stevesk 1365: or
1.237 jmc 1366: .Cm no .
1.1 stevesk 1367: The default is
1.237 jmc 1368: .Cm prohibit-password .
1.1 stevesk 1369: .Pp
1370: If this option is set to
1.237 jmc 1371: .Cm prohibit-password
1.255 jmc 1372: (or its deprecated alias,
1373: .Cm without-password ) ,
1.210 deraadt 1374: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1375: .Pp
1376: If this option is set to
1.237 jmc 1377: .Cm forced-commands-only ,
1.1 stevesk 1378: root login with public key authentication will be allowed,
1379: but only if the
1380: .Ar command
1381: option has been specified
1382: (which may be useful for taking remote backups even if root login is
1.17 jmc 1383: normally not allowed).
1384: All other authentication methods are disabled for root.
1.1 stevesk 1385: .Pp
1386: If this option is set to
1.237 jmc 1387: .Cm no ,
1.38 jmc 1388: root is not allowed to log in.
1.231 jmc 1389: .It Cm PermitTTY
1390: Specifies whether
1391: .Xr pty 4
1392: allocation is permitted.
1393: The default is
1.237 jmc 1394: .Cm yes .
1.46 reyk 1395: .It Cm PermitTunnel
1396: Specifies whether
1397: .Xr tun 4
1398: device forwarding is allowed.
1.47 reyk 1399: The argument must be
1.237 jmc 1400: .Cm yes ,
1401: .Cm point-to-point
1.58 stevesk 1402: (layer 3),
1.237 jmc 1403: .Cm ethernet
1.58 stevesk 1404: (layer 2), or
1.237 jmc 1405: .Cm no .
1.58 stevesk 1406: Specifying
1.237 jmc 1407: .Cm yes
1.58 stevesk 1408: permits both
1.237 jmc 1409: .Cm point-to-point
1.58 stevesk 1410: and
1.237 jmc 1411: .Cm ethernet .
1.46 reyk 1412: The default is
1.237 jmc 1413: .Cm no .
1.178 djm 1414: .Pp
1415: Independent of this setting, the permissions of the selected
1416: .Xr tun 4
1417: device must allow access to the user.
1.6 markus 1418: .It Cm PermitUserEnvironment
1419: Specifies whether
1420: .Pa ~/.ssh/environment
1.9 stevesk 1421: and
1.6 markus 1422: .Cm environment=
1423: options in
1424: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1425: are processed by
1.52 jmc 1426: .Xr sshd 8 .
1.278 djm 1427: Valid options are
1428: .Cm yes ,
1429: .Cm no
1430: or a pattern-list specifying which environment variable names to accept
1431: (for example
1432: .Qq LANG,LC_* ) .
1.6 markus 1433: The default is
1.237 jmc 1434: .Cm no .
1.9 stevesk 1435: Enabling environment processing may enable users to bypass access
1436: restrictions in some configurations using mechanisms such as
1437: .Ev LD_PRELOAD .
1.174 djm 1438: .It Cm PermitUserRC
1439: Specifies whether any
1440: .Pa ~/.ssh/rc
1441: file is executed.
1442: The default is
1.237 jmc 1443: .Cm yes .
1.321 dtucker 1444: .It Cm PerSourceMaxStartups
1445: Specifies the number of unauthenticated connections allowed from a
1446: given source address, or
1447: .Dq none
1448: if there is no limit.
1449: This limit is applied in addition to
1450: .Cm MaxStartups ,
1451: whichever is lower.
1452: The default is
1453: .Cm none .
1454: .It Cm PerSourceNetBlockSize
1455: Specifies the number of bits of source address that are grouped together
1456: for the purposes of applying PerSourceMaxStartups limits.
1457: Values for IPv4 and optionally IPv6 may be specified, separated by a colon.
1458: The default is
1.322 jmc 1459: .Cm 32:128 ,
1.321 dtucker 1460: which means each address is considered individually.
1.328 djm 1461: .It Cm PidFile
1462: Specifies the file that contains the process ID of the
1463: SSH daemon, or
1464: .Cm none
1465: to not write one.
1466: The default is
1467: .Pa /var/run/sshd.pid .
1.1 stevesk 1468: .It Cm Port
1469: Specifies the port number that
1.52 jmc 1470: .Xr sshd 8
1.1 stevesk 1471: listens on.
1472: The default is 22.
1473: Multiple options of this type are permitted.
1474: See also
1475: .Cm ListenAddress .
1476: .It Cm PrintLastLog
1477: Specifies whether
1.52 jmc 1478: .Xr sshd 8
1.36 jaredy 1479: should print the date and time of the last user login when a user logs
1480: in interactively.
1.1 stevesk 1481: The default is
1.237 jmc 1482: .Cm yes .
1.1 stevesk 1483: .It Cm PrintMotd
1484: Specifies whether
1.52 jmc 1485: .Xr sshd 8
1.1 stevesk 1486: should print
1487: .Pa /etc/motd
1488: when a user logs in interactively.
1489: (On some systems it is also printed by the shell,
1490: .Pa /etc/profile ,
1491: or equivalent.)
1492: The default is
1.237 jmc 1493: .Cm yes .
1.324 dtucker 1494: .It Cm PubkeyAcceptedAlgorithms
1495: Specifies the signature algorithms that will be accepted for public key
1496: authentication as a list of comma-separated patterns.
1.289 naddy 1497: Alternately if the specified list begins with a
1.208 djm 1498: .Sq +
1.324 dtucker 1499: character, then the specified algorithms will be appended to the default set
1.208 djm 1500: instead of replacing them.
1.289 naddy 1501: If the specified list begins with a
1.242 djm 1502: .Sq -
1.324 dtucker 1503: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 1504: from the default set instead of replacing them.
1.290 naddy 1505: If the specified list begins with a
1506: .Sq ^
1.324 dtucker 1507: character, then the specified algorithms will be placed at the head of the
1.290 naddy 1508: default set.
1.206 markus 1509: The default for this option is:
1510: .Bd -literal -offset 3n
1.316 djm 1511: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 1512: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1513: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1514: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 1515: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 1516: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 1517: rsa-sha2-512-cert-v01@openssh.com,
1518: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1519: ssh-rsa-cert-v01@openssh.com,
1.316 djm 1520: ssh-ed25519,
1.296 naddy 1521: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 1522: sk-ssh-ed25519@openssh.com,
1.291 naddy 1523: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1524: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1525: .Ed
1526: .Pp
1.326 djm 1527: The list of available signature algorithms may also be obtained using
1.324 dtucker 1528: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.293 djm 1529: .It Cm PubkeyAuthOptions
1530: Sets one or more public key authentication options.
1.313 djm 1531: The supported keywords are:
1.294 jmc 1532: .Cm none
1.313 djm 1533: (the default; indicating no additional options are enabled),
1534: .Cm touch-required
1.293 djm 1535: and
1.313 djm 1536: .Cm verify-required .
1.293 djm 1537: .Pp
1538: The
1539: .Cm touch-required
1.297 naddy 1540: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1541: (i.e.\&
1.293 djm 1542: .Cm ecdsa-sk
1543: or
1544: .Cm ed25519-sk )
1545: to always require the signature to attest that a physically present user
1.297 naddy 1546: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1547: By default,
1548: .Xr sshd 8
1.297 naddy 1549: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1550: The
1551: .Cm touch-required
1552: flag disables this override.
1.313 djm 1553: .Pp
1554: The
1555: .Cm verify-required
1.315 jmc 1556: option requires a FIDO key signature attest that the user was verified,
1.314 jmc 1557: e.g. via a PIN.
1.313 djm 1558: .Pp
1559: Neither the
1560: .Cm touch-required
1561: or
1562: .Cm verify-required
1.314 jmc 1563: options have any effect for other, non-FIDO, public key types.
1.1 stevesk 1564: .It Cm PubkeyAuthentication
1565: Specifies whether public key authentication is allowed.
1566: The default is
1.237 jmc 1567: .Cm yes .
1.159 dtucker 1568: .It Cm RekeyLimit
1569: Specifies the maximum amount of data that may be transmitted before the
1.323 rob 1570: session key is renegotiated, optionally followed by a maximum amount of
1.159 dtucker 1571: time that may pass before the session key is renegotiated.
1572: The first argument is specified in bytes and may have a suffix of
1573: .Sq K ,
1574: .Sq M ,
1575: or
1576: .Sq G
1577: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1578: The default is between
1579: .Sq 1G
1580: and
1581: .Sq 4G ,
1582: depending on the cipher.
1583: The optional second value is specified in seconds and may use any of the
1584: units documented in the
1585: .Sx TIME FORMATS
1.160 jmc 1586: section.
1.159 dtucker 1587: The default value for
1588: .Cm RekeyLimit
1589: is
1.237 jmc 1590: .Cm default none ,
1.159 dtucker 1591: which means that rekeying is performed after the cipher's default amount
1592: of data has been sent or received and no time based rekeying is done.
1.118 djm 1593: .It Cm RevokedKeys
1.195 dtucker 1594: Specifies revoked public keys file, or
1.237 jmc 1595: .Cm none
1.195 dtucker 1596: to not use one.
1.118 djm 1597: Keys listed in this file will be refused for public key authentication.
1598: Note that if this file is not readable, then public key authentication will
1599: be refused for all users.
1.154 djm 1600: Keys may be specified as a text file, listing one public key per line, or as
1601: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1602: .Xr ssh-keygen 1 .
1.161 jmc 1603: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1604: .Xr ssh-keygen 1 .
1.257 djm 1605: .It Cm RDomain
1606: Specifies an explicit routing domain that is applied after authentication
1607: has completed.
1.259 jmc 1608: The user session, as well and any forwarded or listening IP sockets,
1609: will be bound to this
1.257 djm 1610: .Xr rdomain 4 .
1611: If the routing domain is set to
1612: .Cm \&%D ,
1.259 jmc 1613: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1614: .It Cm SecurityKeyProvider
1.297 naddy 1615: Specifies a path to a library that will be used when loading
1616: FIDO authenticator-hosted keys, overriding the default of using
1617: the built-in USB HID support.
1.274 djm 1618: .It Cm SetEnv
1619: Specifies one or more environment variables to set in child sessions started
1620: by
1621: .Xr sshd 8
1622: as
1623: .Dq NAME=VALUE .
1624: The environment value may be quoted (e.g. if it contains whitespace
1625: characters).
1626: Environment variables set by
1627: .Cm SetEnv
1628: override the default environment and any variables specified by the user
1629: via
1630: .Cm AcceptEnv
1631: or
1632: .Cm PermitUserEnvironment .
1.175 millert 1633: .It Cm StreamLocalBindMask
1634: Sets the octal file creation mode mask
1635: .Pq umask
1636: used when creating a Unix-domain socket file for local or remote
1637: port forwarding.
1638: This option is only used for port forwarding to a Unix-domain socket file.
1639: .Pp
1640: The default value is 0177, which creates a Unix-domain socket file that is
1641: readable and writable only by the owner.
1642: Note that not all operating systems honor the file mode on Unix-domain
1643: socket files.
1644: .It Cm StreamLocalBindUnlink
1645: Specifies whether to remove an existing Unix-domain socket file for local
1646: or remote port forwarding before creating a new one.
1647: If the socket file already exists and
1648: .Cm StreamLocalBindUnlink
1649: is not enabled,
1650: .Nm sshd
1651: will be unable to forward the port to the Unix-domain socket file.
1652: This option is only used for port forwarding to a Unix-domain socket file.
1653: .Pp
1654: The argument must be
1.237 jmc 1655: .Cm yes
1.175 millert 1656: or
1.237 jmc 1657: .Cm no .
1.175 millert 1658: The default is
1.237 jmc 1659: .Cm no .
1.1 stevesk 1660: .It Cm StrictModes
1661: Specifies whether
1.52 jmc 1662: .Xr sshd 8
1.1 stevesk 1663: should check file modes and ownership of the
1664: user's files and home directory before accepting login.
1665: This is normally desirable because novices sometimes accidentally leave their
1666: directory or files world-writable.
1667: The default is
1.237 jmc 1668: .Cm yes .
1.112 djm 1669: Note that this does not apply to
1670: .Cm ChrootDirectory ,
1671: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1672: .It Cm Subsystem
1.51 jmc 1673: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1674: Arguments should be a subsystem name and a command (with optional arguments)
1675: to execute upon subsystem request.
1.80 djm 1676: .Pp
1.1 stevesk 1677: The command
1.237 jmc 1678: .Cm sftp-server
1679: implements the SFTP file transfer subsystem.
1.80 djm 1680: .Pp
1681: Alternately the name
1.237 jmc 1682: .Cm internal-sftp
1683: implements an in-process SFTP server.
1.80 djm 1684: This may simplify configurations using
1685: .Cm ChrootDirectory
1686: to force a different filesystem root on clients.
1687: .Pp
1.1 stevesk 1688: By default no subsystems are defined.
1689: .It Cm SyslogFacility
1690: Gives the facility code that is used when logging messages from
1.53 jmc 1691: .Xr sshd 8 .
1.1 stevesk 1692: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1693: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1694: The default is AUTH.
1.27 markus 1695: .It Cm TCPKeepAlive
1696: Specifies whether the system should send TCP keepalive messages to the
1697: other side.
1698: If they are sent, death of the connection or crash of one
1699: of the machines will be properly noticed.
1700: However, this means that
1701: connections will die if the route is down temporarily, and some people
1702: find it annoying.
1703: On the other hand, if TCP keepalives are not sent,
1704: sessions may hang indefinitely on the server, leaving
1.237 jmc 1705: .Qq ghost
1.27 markus 1706: users and consuming server resources.
1707: .Pp
1708: The default is
1.237 jmc 1709: .Cm yes
1.27 markus 1710: (to send TCP keepalive messages), and the server will notice
1711: if the network goes down or the client host crashes.
1712: This avoids infinitely hanging sessions.
1713: .Pp
1714: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1715: .Cm no .
1.118 djm 1716: .It Cm TrustedUserCAKeys
1717: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1718: trusted to sign user certificates for authentication, or
1.237 jmc 1719: .Cm none
1.195 dtucker 1720: to not use one.
1.119 jmc 1721: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1722: .Ql #
1723: are allowed.
1724: If a certificate is presented for authentication and has its signing CA key
1725: listed in this file, then it may be used for authentication for any user
1726: listed in the certificate's principals list.
1727: Note that certificates that lack a list of principals will not be permitted
1728: for authentication using
1729: .Cm TrustedUserCAKeys .
1.161 jmc 1730: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1731: .Xr ssh-keygen 1 .
1.18 markus 1732: .It Cm UseDNS
1733: Specifies whether
1.52 jmc 1734: .Xr sshd 8
1.207 djm 1735: should look up the remote host name, and to check that
1.18 markus 1736: the resolved host name for the remote IP address maps back to the
1737: very same IP address.
1.207 djm 1738: .Pp
1739: If this option is set to
1.237 jmc 1740: .Cm no
1.207 djm 1741: (the default) then only addresses and not host names may be used in
1.221 djm 1742: .Pa ~/.ssh/authorized_keys
1.207 djm 1743: .Cm from
1744: and
1.211 jmc 1745: .Nm
1.207 djm 1746: .Cm Match
1747: .Cm Host
1748: directives.
1.137 djm 1749: .It Cm VersionAddendum
1750: Optionally specifies additional text to append to the SSH protocol banner
1751: sent by the server upon connection.
1752: The default is
1.237 jmc 1753: .Cm none .
1.1 stevesk 1754: .It Cm X11DisplayOffset
1755: Specifies the first display number available for
1.52 jmc 1756: .Xr sshd 8 Ns 's
1.1 stevesk 1757: X11 forwarding.
1.52 jmc 1758: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1759: The default is 10.
1760: .It Cm X11Forwarding
1761: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1762: The argument must be
1.237 jmc 1763: .Cm yes
1.13 stevesk 1764: or
1.237 jmc 1765: .Cm no .
1.1 stevesk 1766: The default is
1.237 jmc 1767: .Cm no .
1.13 stevesk 1768: .Pp
1769: When X11 forwarding is enabled, there may be additional exposure to
1770: the server and to client displays if the
1.52 jmc 1771: .Xr sshd 8
1.13 stevesk 1772: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1773: .Cm X11UseLocalhost ) ,
1774: though this is not the default.
1.13 stevesk 1775: Additionally, the authentication spoofing and authentication data
1776: verification and substitution occur on the client side.
1777: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1778: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1779: forwarding (see the warnings for
1780: .Cm ForwardX11
1781: in
1.19 jmc 1782: .Xr ssh_config 5 ) .
1.13 stevesk 1783: A system administrator may have a stance in which they want to
1784: protect clients that may expose themselves to attack by unwittingly
1785: requesting X11 forwarding, which can warrant a
1.237 jmc 1786: .Cm no
1.13 stevesk 1787: setting.
1788: .Pp
1789: Note that disabling X11 forwarding does not prevent users from
1790: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1791: .It Cm X11UseLocalhost
1792: Specifies whether
1.52 jmc 1793: .Xr sshd 8
1.1 stevesk 1794: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1795: the wildcard address.
1796: By default,
1.52 jmc 1797: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1798: hostname part of the
1799: .Ev DISPLAY
1800: environment variable to
1.237 jmc 1801: .Cm localhost .
1.8 stevesk 1802: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1803: However, some older X11 clients may not function with this
1804: configuration.
1805: .Cm X11UseLocalhost
1806: may be set to
1.237 jmc 1807: .Cm no
1.1 stevesk 1808: to specify that the forwarding server should be bound to the wildcard
1809: address.
1810: The argument must be
1.237 jmc 1811: .Cm yes
1.1 stevesk 1812: or
1.237 jmc 1813: .Cm no .
1.1 stevesk 1814: The default is
1.237 jmc 1815: .Cm yes .
1.1 stevesk 1816: .It Cm XAuthLocation
1.11 stevesk 1817: Specifies the full pathname of the
1.1 stevesk 1818: .Xr xauth 1
1.195 dtucker 1819: program, or
1.237 jmc 1820: .Cm none
1.195 dtucker 1821: to not use one.
1.1 stevesk 1822: The default is
1823: .Pa /usr/X11R6/bin/xauth .
1824: .El
1.55 jmc 1825: .Sh TIME FORMATS
1.53 jmc 1826: .Xr sshd 8
1.1 stevesk 1827: command-line arguments and configuration file options that specify time
1828: may be expressed using a sequence of the form:
1829: .Sm off
1.7 stevesk 1830: .Ar time Op Ar qualifier ,
1.1 stevesk 1831: .Sm on
1832: where
1833: .Ar time
1834: is a positive integer value and
1835: .Ar qualifier
1836: is one of the following:
1837: .Pp
1838: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1839: .It Aq Cm none
1.1 stevesk 1840: seconds
1841: .It Cm s | Cm S
1842: seconds
1843: .It Cm m | Cm M
1844: minutes
1845: .It Cm h | Cm H
1846: hours
1847: .It Cm d | Cm D
1848: days
1849: .It Cm w | Cm W
1850: weeks
1851: .El
1852: .Pp
1853: Each member of the sequence is added together to calculate
1854: the total time value.
1855: .Pp
1856: Time format examples:
1857: .Pp
1858: .Bl -tag -width Ds -compact -offset indent
1859: .It 600
1860: 600 seconds (10 minutes)
1861: .It 10m
1862: 10 minutes
1863: .It 1h30m
1864: 1 hour 30 minutes (90 minutes)
1865: .El
1.235 jmc 1866: .Sh TOKENS
1867: Arguments to some keywords can make use of tokens,
1868: which are expanded at runtime:
1869: .Pp
1870: .Bl -tag -width XXXX -offset indent -compact
1871: .It %%
1872: A literal
1873: .Sq % .
1.257 djm 1874: .It \&%D
1875: The routing domain in which the incoming connection was received.
1.235 jmc 1876: .It %F
1877: The fingerprint of the CA key.
1878: .It %f
1879: The fingerprint of the key or certificate.
1880: .It %h
1881: The home directory of the user.
1882: .It %i
1883: The key ID in the certificate.
1884: .It %K
1885: The base64-encoded CA key.
1886: .It %k
1887: The base64-encoded key or certificate for authentication.
1888: .It %s
1889: The serial number of the certificate.
1890: .It \&%T
1891: The type of the CA key.
1892: .It %t
1893: The key or certificate type.
1.268 jmc 1894: .It \&%U
1895: The numeric user ID of the target user.
1.235 jmc 1896: .It %u
1897: The username.
1898: .El
1899: .Pp
1900: .Cm AuthorizedKeysCommand
1.270 jmc 1901: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1902: .Pp
1903: .Cm AuthorizedKeysFile
1.270 jmc 1904: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1905: .Pp
1906: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1907: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1908: .Pp
1909: .Cm AuthorizedPrincipalsFile
1.270 jmc 1910: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1911: .Pp
1912: .Cm ChrootDirectory
1.270 jmc 1913: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1914: .Pp
1915: .Cm RoutingDomain
1916: accepts the token %D.
1.1 stevesk 1917: .Sh FILES
1918: .Bl -tag -width Ds
1919: .It Pa /etc/ssh/sshd_config
1920: Contains configuration data for
1.53 jmc 1921: .Xr sshd 8 .
1.1 stevesk 1922: This file should be writable by root only, but it is recommended
1923: (though not necessary) that it be world-readable.
1924: .El
1.19 jmc 1925: .Sh SEE ALSO
1.237 jmc 1926: .Xr sftp-server 8 ,
1.19 jmc 1927: .Xr sshd 8
1.1 stevesk 1928: .Sh AUTHORS
1.237 jmc 1929: .An -nosplit
1.1 stevesk 1930: OpenSSH is a derivative of the original and free
1.237 jmc 1931: ssh 1.2.12 release by
1932: .An Tatu Ylonen .
1933: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1934: .An Theo de Raadt
1935: and
1936: .An Dug Song
1.1 stevesk 1937: removed many bugs, re-added newer features and
1938: created OpenSSH.
1.237 jmc 1939: .An Markus Friedl
1940: contributed the support for SSH protocol versions 1.5 and 2.0.
1941: .An Niels Provos
1942: and
1943: .An Markus Friedl
1944: contributed support for privilege separation.
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