Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.309
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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1.309 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.308 2020/04/17 03:23:13 djm Exp $
! 37: .Dd $Mdocdate: April 17 2020 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
1.295 jmc 42: .Nd OpenSSH daemon configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.53 jmc 44: .Xr sshd 8
1.1 stevesk 45: reads configuration data from
46: .Pa /etc/ssh/sshd_config
47: (or the file specified with
48: .Fl f
49: on the command line).
50: The file contains keyword-argument pairs, one per line.
1.254 benno 51: For each keyword, the first obtained value will be used.
1.1 stevesk 52: Lines starting with
53: .Ql #
54: and empty lines are interpreted as comments.
1.56 dtucker 55: Arguments may optionally be enclosed in double quotes
56: .Pq \&"
57: in order to represent arguments containing spaces.
1.1 stevesk 58: .Pp
59: The possible
60: keywords and their meanings are as follows (note that
61: keywords are case-insensitive and arguments are case-sensitive):
62: .Bl -tag -width Ds
1.30 djm 63: .It Cm AcceptEnv
64: Specifies what environment variables sent by the client will be copied into
65: the session's
66: .Xr environ 7 .
67: See
68: .Cm SendEnv
1.273 djm 69: and
70: .Cm SetEnv
1.30 djm 71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.219 jmc 74: The
1.197 dtucker 75: .Ev TERM
1.275 djm 76: environment variable is always accepted whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
1.237 jmc 94: .Cm any
95: (the default),
96: .Cm inet
1.52 jmc 97: (use IPv4 only), or
1.237 jmc 98: .Cm inet6
1.37 djm 99: (use IPv6 only).
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
1.237 jmc 105: .Cm yes .
1.89 jmc 106: Note that disabling agent forwarding does not improve security
107: unless users are also denied shell access, as they can always install
108: their own forwarders.
1.1 stevesk 109: .It Cm AllowGroups
110: This keyword can be followed by a list of group name patterns, separated
111: by spaces.
112: If specified, login is allowed only for users whose primary
113: group or supplementary group list matches one of the patterns.
114: Only group names are valid; a numerical group ID is not recognized.
115: By default, login is allowed for all groups.
1.301 djm 116: The allow/deny groups directives are processed in the following order:
1.54 jmc 117: .Cm DenyGroups ,
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.231 jmc 123: .It Cm AllowStreamLocalForwarding
124: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 125: The available options are
1.237 jmc 126: .Cm yes
127: (the default)
1.150 djm 128: or
1.237 jmc 129: .Cm all
1.231 jmc 130: to allow StreamLocal forwarding,
1.237 jmc 131: .Cm no
1.231 jmc 132: to prevent all StreamLocal forwarding,
1.237 jmc 133: .Cm local
1.150 djm 134: to allow local (from the perspective of
1.151 jmc 135: .Xr ssh 1 )
136: forwarding only or
1.237 jmc 137: .Cm remote
1.150 djm 138: to allow remote forwarding only.
1.231 jmc 139: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 140: users are also denied shell access, as they can always install their
141: own forwarders.
1.231 jmc 142: .It Cm AllowTcpForwarding
143: Specifies whether TCP forwarding is permitted.
1.175 millert 144: The available options are
1.237 jmc 145: .Cm yes
146: (the default)
1.175 millert 147: or
1.237 jmc 148: .Cm all
1.231 jmc 149: to allow TCP forwarding,
1.237 jmc 150: .Cm no
1.231 jmc 151: to prevent all TCP forwarding,
1.237 jmc 152: .Cm local
1.175 millert 153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
1.237 jmc 156: .Cm remote
1.175 millert 157: to allow remote forwarding only.
1.231 jmc 158: Note that disabling TCP forwarding does not improve security unless
1.175 millert 159: users are also denied shell access, as they can always install their
160: own forwarders.
1.1 stevesk 161: .It Cm AllowUsers
162: This keyword can be followed by a list of user name patterns, separated
163: by spaces.
1.14 jmc 164: If specified, login is allowed only for user names that
1.1 stevesk 165: match one of the patterns.
166: Only user names are valid; a numerical user ID is not recognized.
167: By default, login is allowed for all users.
168: If the pattern takes the form USER@HOST then USER and HOST
169: are separately checked, restricting logins to particular
170: users from particular hosts.
1.222 jmc 171: HOST criteria may additionally contain addresses to match in CIDR
172: address/masklen format.
1.301 djm 173: The allow/deny users directives are processed in the following order:
1.54 jmc 174: .Cm DenyUsers ,
1.301 djm 175: .Cm AllowUsers .
1.49 jmc 176: .Pp
1.161 jmc 177: See PATTERNS in
1.49 jmc 178: .Xr ssh_config 5
179: for more information on patterns.
1.149 djm 180: .It Cm AuthenticationMethods
181: Specifies the authentication methods that must be successfully completed
182: for a user to be granted access.
1.281 djm 183: This option must be followed by one or more lists of comma-separated
1.224 djm 184: authentication method names, or by the single string
1.237 jmc 185: .Cm any
1.224 djm 186: to indicate the default behaviour of accepting any single authentication
1.225 jmc 187: method.
1.237 jmc 188: If the default is overridden, then successful authentication requires
1.224 djm 189: completion of every method in at least one of these lists.
1.149 djm 190: .Pp
1.237 jmc 191: For example,
192: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 196: so for this example it would not be possible to attempt password or
1.149 djm 197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
1.237 jmc 202: .Cm bsdauth ,
203: .Cm pam ,
1.157 markus 204: or
1.237 jmc 205: .Cm skey ,
1.157 markus 206: depending on the server configuration.
207: For example,
1.237 jmc 208: .Qq keyboard-interactive:bsdauth
1.157 markus 209: would restrict keyboard interactive authentication to the
1.237 jmc 210: .Cm bsdauth
1.157 markus 211: device.
1.185 djm 212: .Pp
1.237 jmc 213: If the publickey method is listed more than once,
1.185 djm 214: .Xr sshd 8
215: verifies that keys that have been used successfully are not reused for
216: subsequent authentications.
1.237 jmc 217: For example,
218: .Qq publickey,publickey
219: requires successful authentication using two different public keys.
1.181 djm 220: .Pp
1.149 djm 221: Note that each authentication method listed should also be explicitly enabled
222: in the configuration.
1.251 djm 223: .Pp
224: The available authentication methods are:
225: .Qq gssapi-with-mic ,
226: .Qq hostbased ,
227: .Qq keyboard-interactive ,
228: .Qq none
229: (used for access to password-less accounts when
1.266 jmc 230: .Cm PermitEmptyPasswords
1.251 djm 231: is enabled),
232: .Qq password
233: and
234: .Qq publickey .
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: Arguments to
240: .Cm AuthorizedKeysCommand
1.235 jmc 241: accept the tokens described in the
242: .Sx TOKENS
243: section.
244: If no arguments are specified then the username of the target user is used.
1.201 djm 245: .Pp
246: The program should produce on standard output zero or
1.235 jmc 247: more lines of authorized_keys output (see
248: .Sx AUTHORIZED_KEYS
249: in
1.147 jmc 250: .Xr sshd 8 ) .
1.235 jmc 251: If a key supplied by
252: .Cm AuthorizedKeysCommand
253: does not successfully authenticate
1.146 djm 254: and authorize the user then public key authentication continues using the usual
255: .Cm AuthorizedKeysFile
256: files.
1.237 jmc 257: By default, no
258: .Cm AuthorizedKeysCommand
259: is run.
1.146 djm 260: .It Cm AuthorizedKeysCommandUser
1.237 jmc 261: Specifies the user under whose account the
262: .Cm AuthorizedKeysCommand
263: is run.
1.146 djm 264: It is recommended to use a dedicated user that has no other role on the host
265: than running authorized keys commands.
1.186 djm 266: If
1.181 djm 267: .Cm AuthorizedKeysCommand
1.186 djm 268: is specified but
269: .Cm AuthorizedKeysCommandUser
270: is not, then
271: .Xr sshd 8
272: will refuse to start.
1.1 stevesk 273: .It Cm AuthorizedKeysFile
1.235 jmc 274: Specifies the file that contains the public keys used for user authentication.
1.285 schwarze 275: The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
1.124 djm 276: .Xr sshd 8 .
1.235 jmc 277: Arguments to
1.1 stevesk 278: .Cm AuthorizedKeysFile
1.235 jmc 279: accept the tokens described in the
280: .Sx TOKENS
281: section.
1.1 stevesk 282: After expansion,
283: .Cm AuthorizedKeysFile
284: is taken to be an absolute path or one relative to the user's home
285: directory.
1.133 jmc 286: Multiple files may be listed, separated by whitespace.
1.212 djm 287: Alternately this option may be set to
1.237 jmc 288: .Cm none
1.212 djm 289: to skip checking for user keys in files.
1.133 jmc 290: The default is
1.237 jmc 291: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 292: .It Cm AuthorizedPrincipalsCommand
293: Specifies a program to be used to generate the list of allowed
294: certificate principals as per
295: .Cm AuthorizedPrincipalsFile .
296: The program must be owned by root, not writable by group or others and
297: specified by an absolute path.
298: Arguments to
299: .Cm AuthorizedPrincipalsCommand
1.235 jmc 300: accept the tokens described in the
301: .Sx TOKENS
302: section.
303: If no arguments are specified then the username of the target user is used.
1.202 djm 304: .Pp
305: The program should produce on standard output zero or
306: more lines of
307: .Cm AuthorizedPrincipalsFile
308: output.
309: If either
310: .Cm AuthorizedPrincipalsCommand
311: or
312: .Cm AuthorizedPrincipalsFile
313: is specified, then certificates offered by the client for authentication
314: must contain a principal that is listed.
1.235 jmc 315: By default, no
316: .Cm AuthorizedPrincipalsCommand
317: is run.
1.202 djm 318: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 319: Specifies the user under whose account the
320: .Cm AuthorizedPrincipalsCommand
321: is run.
1.202 djm 322: It is recommended to use a dedicated user that has no other role on the host
323: than running authorized principals commands.
324: If
325: .Cm AuthorizedPrincipalsCommand
326: is specified but
327: .Cm AuthorizedPrincipalsCommandUser
328: is not, then
329: .Xr sshd 8
330: will refuse to start.
1.121 djm 331: .It Cm AuthorizedPrincipalsFile
332: Specifies a file that lists principal names that are accepted for
333: certificate authentication.
334: When using certificates signed by a key listed in
335: .Cm TrustedUserCAKeys ,
336: this file lists names, one of which must appear in the certificate for it
337: to be accepted for authentication.
1.235 jmc 338: Names are listed one per line preceded by key options (as described in
339: .Sx AUTHORIZED_KEYS FILE FORMAT
340: in
1.125 jmc 341: .Xr sshd 8 ) .
1.124 djm 342: Empty lines and comments starting with
1.121 djm 343: .Ql #
344: are ignored.
345: .Pp
1.235 jmc 346: Arguments to
1.121 djm 347: .Cm AuthorizedPrincipalsFile
1.235 jmc 348: accept the tokens described in the
349: .Sx TOKENS
350: section.
1.121 djm 351: After expansion,
352: .Cm AuthorizedPrincipalsFile
1.235 jmc 353: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 354: The default is
1.237 jmc 355: .Cm none ,
1.138 djm 356: i.e. not to use a principals file \(en in this case, the username
1.121 djm 357: of the user must appear in a certificate's principals list for it to be
358: accepted.
1.235 jmc 359: .Pp
1.121 djm 360: Note that
361: .Cm AuthorizedPrincipalsFile
362: is only used when authentication proceeds using a CA listed in
363: .Cm TrustedUserCAKeys
364: and is not consulted for certification authorities trusted via
365: .Pa ~/.ssh/authorized_keys ,
366: though the
367: .Cm principals=
368: key option offers a similar facility (see
369: .Xr sshd 8
370: for details).
1.1 stevesk 371: .It Cm Banner
372: The contents of the specified file are sent to the remote user before
373: authentication is allowed.
1.78 djm 374: If the argument is
1.237 jmc 375: .Cm none
1.78 djm 376: then no banner is displayed.
1.1 stevesk 377: By default, no banner is displayed.
1.282 djm 378: .It Cm CASignatureAlgorithms
379: Specifies which algorithms are allowed for signing of certificates
380: by certificate authorities (CAs).
381: The default is:
382: .Bd -literal -offset indent
1.287 dtucker 383: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.282 djm 384: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
385: .Ed
386: .Pp
387: Certificates signed using other algorithms will not be accepted for
388: public key or host-based authentication.
1.1 stevesk 389: .It Cm ChallengeResponseAuthentication
1.50 jmc 390: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 391: All authentication styles from
392: .Xr login.conf 5
393: are supported.
394: The default is
1.237 jmc 395: .Cm yes .
1.80 djm 396: .It Cm ChrootDirectory
1.113 stevesk 397: Specifies the pathname of a directory to
1.80 djm 398: .Xr chroot 2
399: to after authentication.
1.190 deraadt 400: At session startup
401: .Xr sshd 8
402: checks that all components of the pathname are root-owned directories
403: which are not writable by any other user or group.
1.106 stevesk 404: After the chroot,
405: .Xr sshd 8
406: changes the working directory to the user's home directory.
1.235 jmc 407: Arguments to
408: .Cm ChrootDirectory
409: accept the tokens described in the
410: .Sx TOKENS
411: section.
1.80 djm 412: .Pp
413: The
414: .Cm ChrootDirectory
415: must contain the necessary files and directories to support the
1.103 stevesk 416: user's session.
1.80 djm 417: For an interactive session this requires at least a shell, typically
418: .Xr sh 1 ,
419: and basic
420: .Pa /dev
421: nodes such as
422: .Xr null 4 ,
423: .Xr zero 4 ,
424: .Xr stdin 4 ,
425: .Xr stdout 4 ,
426: .Xr stderr 4 ,
1.180 jmc 427: and
1.80 djm 428: .Xr tty 4
429: devices.
1.237 jmc 430: For file transfer sessions using SFTP
431: no additional configuration of the environment is necessary if the in-process
432: sftp-server is used,
1.176 schwarze 433: though sessions which use logging may require
1.104 stevesk 434: .Pa /dev/log
1.176 schwarze 435: inside the chroot directory on some operating systems (see
1.104 stevesk 436: .Xr sftp-server 8
1.81 jmc 437: for details).
1.190 deraadt 438: .Pp
1.191 jmc 439: For safety, it is very important that the directory hierarchy be
1.190 deraadt 440: prevented from modification by other processes on the system (especially
441: those outside the jail).
442: Misconfiguration can lead to unsafe environments which
443: .Xr sshd 8
444: cannot detect.
1.80 djm 445: .Pp
1.215 djm 446: The default is
1.237 jmc 447: .Cm none ,
1.215 djm 448: indicating not to
1.80 djm 449: .Xr chroot 2 .
1.1 stevesk 450: .It Cm Ciphers
1.219 jmc 451: Specifies the ciphers allowed.
1.1 stevesk 452: Multiple ciphers must be comma-separated.
1.289 naddy 453: If the specified list begins with a
1.208 djm 454: .Sq +
455: character, then the specified ciphers will be appended to the default set
456: instead of replacing them.
1.289 naddy 457: If the specified list begins with a
1.242 djm 458: .Sq -
459: character, then the specified ciphers (including wildcards) will be removed
460: from the default set instead of replacing them.
1.290 naddy 461: If the specified list begins with a
462: .Sq ^
463: character, then the specified ciphers will be placed at the head of the
464: default set.
1.208 djm 465: .Pp
1.167 djm 466: The supported ciphers are:
467: .Pp
1.173 naddy 468: .Bl -item -compact -offset indent
469: .It
470: 3des-cbc
471: .It
472: aes128-cbc
473: .It
474: aes192-cbc
475: .It
476: aes256-cbc
477: .It
478: aes128-ctr
479: .It
480: aes192-ctr
481: .It
482: aes256-ctr
483: .It
484: aes128-gcm@openssh.com
485: .It
486: aes256-gcm@openssh.com
487: .It
488: chacha20-poly1305@openssh.com
489: .El
1.167 djm 490: .Pp
1.52 jmc 491: The default is:
1.173 naddy 492: .Bd -literal -offset indent
1.211 jmc 493: chacha20-poly1305@openssh.com,
1.173 naddy 494: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 495: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 496: .Ed
1.167 djm 497: .Pp
1.237 jmc 498: The list of available ciphers may also be obtained using
499: .Qq ssh -Q cipher .
1.1 stevesk 500: .It Cm ClientAliveCountMax
1.237 jmc 501: Sets the number of client alive messages which may be sent without
1.52 jmc 502: .Xr sshd 8
1.17 jmc 503: receiving any messages back from the client.
504: If this threshold is reached while client alive messages are being sent,
1.52 jmc 505: sshd will disconnect the client, terminating the session.
1.17 jmc 506: It is important to note that the use of client alive messages is very
507: different from
1.237 jmc 508: .Cm TCPKeepAlive .
1.17 jmc 509: The client alive messages are sent through the encrypted channel
510: and therefore will not be spoofable.
511: The TCP keepalive option enabled by
1.27 markus 512: .Cm TCPKeepAlive
1.17 jmc 513: is spoofable.
514: The client alive mechanism is valuable when the client or
1.288 dtucker 515: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 516: .Pp
1.17 jmc 517: The default value is 3.
518: If
1.1 stevesk 519: .Cm ClientAliveInterval
1.237 jmc 520: is set to 15, and
1.1 stevesk 521: .Cm ClientAliveCountMax
1.52 jmc 522: is left at the default, unresponsive SSH clients
1.1 stevesk 523: will be disconnected after approximately 45 seconds.
1.302 djm 524: Setting a zero
525: .Cm ClientAliveCountMax
526: disables connection termination.
1.42 djm 527: .It Cm ClientAliveInterval
528: Sets a timeout interval in seconds after which if no data has been received
529: from the client,
1.52 jmc 530: .Xr sshd 8
1.42 djm 531: will send a message through the encrypted
532: channel to request a response from the client.
533: The default
534: is 0, indicating that these messages will not be sent to the client.
1.3 markus 535: .It Cm Compression
1.236 djm 536: Specifies whether compression is enabled after
1.44 markus 537: the user has authenticated successfully.
1.3 markus 538: The argument must be
1.237 jmc 539: .Cm yes ,
540: .Cm delayed
1.236 djm 541: (a legacy synonym for
1.237 jmc 542: .Cm yes )
1.3 markus 543: or
1.237 jmc 544: .Cm no .
1.3 markus 545: The default is
1.237 jmc 546: .Cm yes .
1.1 stevesk 547: .It Cm DenyGroups
548: This keyword can be followed by a list of group name patterns, separated
549: by spaces.
550: Login is disallowed for users whose primary group or supplementary
551: group list matches one of the patterns.
552: Only group names are valid; a numerical group ID is not recognized.
553: By default, login is allowed for all groups.
1.301 djm 554: The allow/deny groups directives are processed in the following order:
1.54 jmc 555: .Cm DenyGroups ,
556: .Cm AllowGroups .
1.49 jmc 557: .Pp
1.161 jmc 558: See PATTERNS in
1.49 jmc 559: .Xr ssh_config 5
560: for more information on patterns.
1.1 stevesk 561: .It Cm DenyUsers
562: This keyword can be followed by a list of user name patterns, separated
563: by spaces.
564: Login is disallowed for user names that match one of the patterns.
565: Only user names are valid; a numerical user ID is not recognized.
566: By default, login is allowed for all users.
567: If the pattern takes the form USER@HOST then USER and HOST
568: are separately checked, restricting logins to particular
569: users from particular hosts.
1.222 jmc 570: HOST criteria may additionally contain addresses to match in CIDR
571: address/masklen format.
1.301 djm 572: The allow/deny users directives are processed in the following order:
1.54 jmc 573: .Cm DenyUsers ,
1.301 djm 574: .Cm AllowUsers .
1.49 jmc 575: .Pp
1.161 jmc 576: See PATTERNS in
1.49 jmc 577: .Xr ssh_config 5
578: for more information on patterns.
1.239 djm 579: .It Cm DisableForwarding
580: Disables all forwarding features, including X11,
581: .Xr ssh-agent 1 ,
582: TCP and StreamLocal.
583: This option overrides all other forwarding-related options and may
584: simplify restricted configurations.
1.246 djm 585: .It Cm ExposeAuthInfo
1.253 jmc 586: Writes a temporary file containing a list of authentication methods and
1.246 djm 587: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 588: The location of the file is exposed to the user session through the
1.248 djm 589: .Ev SSH_USER_AUTH
1.247 jmc 590: environment variable.
1.253 jmc 591: The default is
592: .Cm no .
1.183 djm 593: .It Cm FingerprintHash
594: Specifies the hash algorithm used when logging key fingerprints.
595: Valid options are:
1.237 jmc 596: .Cm md5
1.183 djm 597: and
1.237 jmc 598: .Cm sha256 .
1.183 djm 599: The default is
1.237 jmc 600: .Cm sha256 .
1.67 dtucker 601: .It Cm ForceCommand
602: Forces the execution of the command specified by
603: .Cm ForceCommand ,
1.84 djm 604: ignoring any command supplied by the client and
605: .Pa ~/.ssh/rc
606: if present.
1.67 dtucker 607: The command is invoked by using the user's login shell with the -c option.
608: This applies to shell, command, or subsystem execution.
609: It is most useful inside a
610: .Cm Match
611: block.
612: The command originally supplied by the client is available in the
613: .Ev SSH_ORIGINAL_COMMAND
614: environment variable.
1.82 djm 615: Specifying a command of
1.237 jmc 616: .Cm internal-sftp
617: will force the use of an in-process SFTP server that requires no support
1.82 djm 618: files when used with
619: .Cm ChrootDirectory .
1.215 djm 620: The default is
1.237 jmc 621: .Cm none .
1.1 stevesk 622: .It Cm GatewayPorts
623: Specifies whether remote hosts are allowed to connect to ports
624: forwarded for the client.
625: By default,
1.52 jmc 626: .Xr sshd 8
1.15 jmc 627: binds remote port forwardings to the loopback address.
628: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 629: .Cm GatewayPorts
1.52 jmc 630: can be used to specify that sshd
1.39 djm 631: should allow remote port forwardings to bind to non-loopback addresses, thus
632: allowing other hosts to connect.
633: The argument may be
1.237 jmc 634: .Cm no
1.39 djm 635: to force remote port forwardings to be available to the local host only,
1.237 jmc 636: .Cm yes
1.39 djm 637: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 638: .Cm clientspecified
1.39 djm 639: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 640: The default is
1.237 jmc 641: .Cm no .
1.23 markus 642: .It Cm GSSAPIAuthentication
1.25 markus 643: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 644: The default is
1.237 jmc 645: .Cm no .
1.23 markus 646: .It Cm GSSAPICleanupCredentials
647: Specifies whether to automatically destroy the user's credentials cache
648: on logout.
649: The default is
1.237 jmc 650: .Cm yes .
1.203 djm 651: .It Cm GSSAPIStrictAcceptorCheck
652: Determines whether to be strict about the identity of the GSSAPI acceptor
653: a client authenticates against.
654: If set to
1.237 jmc 655: .Cm yes
656: then the client must authenticate against the host
1.203 djm 657: service on the current hostname.
658: If set to
1.237 jmc 659: .Cm no
1.203 djm 660: then the client may authenticate against any service key stored in the
661: machine's default store.
662: This facility is provided to assist with operation on multi homed machines.
663: The default is
1.237 jmc 664: .Cm yes .
1.189 djm 665: .It Cm HostbasedAcceptedKeyTypes
666: Specifies the key types that will be accepted for hostbased authentication
1.280 djm 667: as a list of comma-separated patterns.
1.289 naddy 668: Alternately if the specified list begins with a
1.208 djm 669: .Sq +
670: character, then the specified key types will be appended to the default set
671: instead of replacing them.
1.289 naddy 672: If the specified list begins with a
1.242 djm 673: .Sq -
674: character, then the specified key types (including wildcards) will be removed
675: from the default set instead of replacing them.
1.290 naddy 676: If the specified list begins with a
677: .Sq ^
678: character, then the specified key types will be placed at the head of the
679: default set.
1.206 markus 680: The default for this option is:
681: .Bd -literal -offset 3n
682: ecdsa-sha2-nistp256-cert-v01@openssh.com,
683: ecdsa-sha2-nistp384-cert-v01@openssh.com,
684: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 685: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 686: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 687: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 688: rsa-sha2-512-cert-v01@openssh.com,
689: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 690: ssh-rsa-cert-v01@openssh.com,
691: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 692: sk-ecdsa-sha2-nistp256@openssh.com,
693: ssh-ed25519,sk-ssh-ed25519@openssh.com,
694: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 695: .Ed
696: .Pp
1.237 jmc 697: The list of available key types may also be obtained using
1.307 dtucker 698: .Qq ssh -Q HostbasedAcceptedKeyTypes .
1.1 stevesk 699: .It Cm HostbasedAuthentication
700: Specifies whether rhosts or /etc/hosts.equiv authentication together
701: with successful public key client host authentication is allowed
1.50 jmc 702: (host-based authentication).
1.70 dtucker 703: The default is
1.237 jmc 704: .Cm no .
1.70 dtucker 705: .It Cm HostbasedUsesNameFromPacketOnly
706: Specifies whether or not the server will attempt to perform a reverse
707: name lookup when matching the name in the
708: .Pa ~/.shosts ,
709: .Pa ~/.rhosts ,
710: and
711: .Pa /etc/hosts.equiv
712: files during
713: .Cm HostbasedAuthentication .
714: A setting of
1.237 jmc 715: .Cm yes
1.70 dtucker 716: means that
717: .Xr sshd 8
718: uses the name supplied by the client rather than
719: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 720: The default is
1.237 jmc 721: .Cm no .
1.117 djm 722: .It Cm HostCertificate
723: Specifies a file containing a public host certificate.
724: The certificate's public key must match a private host key already specified
725: by
726: .Cm HostKey .
727: The default behaviour of
728: .Xr sshd 8
729: is not to load any certificates.
1.1 stevesk 730: .It Cm HostKey
731: Specifies a file containing a private host key
732: used by SSH.
1.229 naddy 733: The defaults are
1.169 naddy 734: .Pa /etc/ssh/ssh_host_ecdsa_key ,
735: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 736: and
1.229 naddy 737: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 738: .Pp
1.1 stevesk 739: Note that
1.52 jmc 740: .Xr sshd 8
1.206 markus 741: will refuse to use a file if it is group/world-accessible
742: and that the
743: .Cm HostKeyAlgorithms
744: option restricts which of the keys are actually used by
745: .Xr sshd 8 .
746: .Pp
1.1 stevesk 747: It is possible to have multiple host key files.
1.162 markus 748: It is also possible to specify public host key files instead.
749: In this case operations on the private key will be delegated
750: to an
751: .Xr ssh-agent 1 .
752: .It Cm HostKeyAgent
753: Identifies the UNIX-domain socket used to communicate
754: with an agent that has access to the private host keys.
1.223 markus 755: If the string
1.237 jmc 756: .Qq SSH_AUTH_SOCK
1.162 markus 757: is specified, the location of the socket will be read from the
758: .Ev SSH_AUTH_SOCK
759: environment variable.
1.206 markus 760: .It Cm HostKeyAlgorithms
1.219 jmc 761: Specifies the host key algorithms
1.206 markus 762: that the server offers.
763: The default for this option is:
764: .Bd -literal -offset 3n
765: ecdsa-sha2-nistp256-cert-v01@openssh.com,
766: ecdsa-sha2-nistp384-cert-v01@openssh.com,
767: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 768: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 769: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 770: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 771: rsa-sha2-512-cert-v01@openssh.com,
772: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 773: ssh-rsa-cert-v01@openssh.com,
774: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 775: sk-ecdsa-sha2-nistp256@openssh.com,
776: ssh-ed25519,sk-ssh-ed25519@openssh.com,
777: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 778: .Ed
779: .Pp
1.237 jmc 780: The list of available key types may also be obtained using
1.307 dtucker 781: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 782: .It Cm IgnoreRhosts
1.309 ! djm 783: Specifies whether to ignore per-user
1.1 stevesk 784: .Pa .rhosts
785: and
786: .Pa .shosts
1.309 ! djm 787: files during
1.1 stevesk 788: .Cm HostbasedAuthentication .
1.309 ! djm 789: The system-wide
1.1 stevesk 790: .Pa /etc/hosts.equiv
791: and
792: .Pa /etc/shosts.equiv
1.309 ! djm 793: are still used regardless of this setting.
! 794: .Pp
! 795: Accepted values are
! 796: .Cm yes
! 797: (the default) to ignore all per-user files,
! 798: .Cm shosts-only
! 799: to allow the use of
! 800: .Pa .shosts
! 801: but to ignore
! 802: .Pa .rhosts
! 803: or
! 804: .Cm no
! 805: to allow both
! 806: .Pa .shosts
! 807: and
! 808: .Pa rhosts.
1.1 stevesk 809: .It Cm IgnoreUserKnownHosts
810: Specifies whether
1.52 jmc 811: .Xr sshd 8
1.1 stevesk 812: should ignore the user's
1.41 djm 813: .Pa ~/.ssh/known_hosts
1.1 stevesk 814: during
1.261 djm 815: .Cm HostbasedAuthentication
816: and use only the system-wide known hosts file
817: .Pa /etc/ssh/known_hosts .
1.1 stevesk 818: The default is
1.304 djm 819: .Dq no .
820: .It Cm Include
821: Include the specified configuration file(s).
1.305 jmc 822: Multiple pathnames may be specified and each pathname may contain
1.304 djm 823: .Xr glob 7
824: wildcards.
825: Files without absolute paths are assumed to be in
826: .Pa /etc/ssh .
1.305 jmc 827: An
1.304 djm 828: .Cm Include
829: directive may appear inside a
830: .Cm Match
831: block
832: to perform conditional inclusion.
1.129 djm 833: .It Cm IPQoS
834: Specifies the IPv4 type-of-service or DSCP class for the connection.
835: Accepted values are
1.237 jmc 836: .Cm af11 ,
837: .Cm af12 ,
838: .Cm af13 ,
839: .Cm af21 ,
840: .Cm af22 ,
841: .Cm af23 ,
842: .Cm af31 ,
843: .Cm af32 ,
844: .Cm af33 ,
845: .Cm af41 ,
846: .Cm af42 ,
847: .Cm af43 ,
848: .Cm cs0 ,
849: .Cm cs1 ,
850: .Cm cs2 ,
851: .Cm cs3 ,
852: .Cm cs4 ,
853: .Cm cs5 ,
854: .Cm cs6 ,
855: .Cm cs7 ,
856: .Cm ef ,
1.303 djm 857: .Cm le ,
1.237 jmc 858: .Cm lowdelay ,
859: .Cm throughput ,
860: .Cm reliability ,
1.250 djm 861: a numeric value, or
862: .Cm none
863: to use the operating system default.
1.131 djm 864: This option may take one or two arguments, separated by whitespace.
1.129 djm 865: If one argument is specified, it is used as the packet class unconditionally.
866: If two values are specified, the first is automatically selected for
867: interactive sessions and the second for non-interactive sessions.
868: The default is
1.264 job 869: .Cm af21
1.265 jmc 870: (Low-Latency Data)
1.129 djm 871: for interactive sessions and
1.264 job 872: .Cm cs1
1.265 jmc 873: (Lower Effort)
1.129 djm 874: for non-interactive sessions.
1.171 jmc 875: .It Cm KbdInteractiveAuthentication
876: Specifies whether to allow keyboard-interactive authentication.
877: The argument to this keyword must be
1.237 jmc 878: .Cm yes
1.171 jmc 879: or
1.237 jmc 880: .Cm no .
1.171 jmc 881: The default is to use whatever value
882: .Cm ChallengeResponseAuthentication
883: is set to
884: (by default
1.237 jmc 885: .Cm yes ) .
1.1 stevesk 886: .It Cm KerberosAuthentication
1.24 markus 887: Specifies whether the password provided by the user for
1.1 stevesk 888: .Cm PasswordAuthentication
1.24 markus 889: will be validated through the Kerberos KDC.
1.1 stevesk 890: To use this option, the server needs a
891: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 892: The default is
1.237 jmc 893: .Cm no .
1.29 dtucker 894: .It Cm KerberosGetAFSToken
1.45 djm 895: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 896: an AFS token before accessing the user's home directory.
1.52 jmc 897: The default is
1.237 jmc 898: .Cm no .
1.1 stevesk 899: .It Cm KerberosOrLocalPasswd
1.52 jmc 900: If password authentication through Kerberos fails then
1.1 stevesk 901: the password will be validated via any additional local mechanism
902: such as
903: .Pa /etc/passwd .
1.52 jmc 904: The default is
1.237 jmc 905: .Cm yes .
1.1 stevesk 906: .It Cm KerberosTicketCleanup
907: Specifies whether to automatically destroy the user's ticket cache
908: file on logout.
1.52 jmc 909: The default is
1.237 jmc 910: .Cm yes .
1.127 djm 911: .It Cm KexAlgorithms
912: Specifies the available KEX (Key Exchange) algorithms.
913: Multiple algorithms must be comma-separated.
1.289 naddy 914: Alternately if the specified list begins with a
1.208 djm 915: .Sq +
916: character, then the specified methods will be appended to the default set
917: instead of replacing them.
1.289 naddy 918: If the specified list begins with a
1.242 djm 919: .Sq -
920: character, then the specified methods (including wildcards) will be removed
921: from the default set instead of replacing them.
1.290 naddy 922: If the specified list begins with a
923: .Sq ^
924: character, then the specified methods will be placed at the head of the
925: default set.
1.173 naddy 926: The supported algorithms are:
927: .Pp
928: .Bl -item -compact -offset indent
929: .It
1.234 djm 930: curve25519-sha256
931: .It
1.173 naddy 932: curve25519-sha256@libssh.org
933: .It
934: diffie-hellman-group1-sha1
935: .It
936: diffie-hellman-group14-sha1
937: .It
1.263 djm 938: diffie-hellman-group14-sha256
939: .It
940: diffie-hellman-group16-sha512
941: .It
942: diffie-hellman-group18-sha512
943: .It
1.173 naddy 944: diffie-hellman-group-exchange-sha1
945: .It
946: diffie-hellman-group-exchange-sha256
947: .It
948: ecdh-sha2-nistp256
949: .It
950: ecdh-sha2-nistp384
951: .It
952: ecdh-sha2-nistp521
1.298 dtucker 953: .It
954: sntrup4591761x25519-sha512@tinyssh.org
1.173 naddy 955: .El
956: .Pp
957: The default is:
1.170 dtucker 958: .Bd -literal -offset indent
1.234 djm 959: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 960: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
961: diffie-hellman-group-exchange-sha256,
1.263 djm 962: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 963: diffie-hellman-group14-sha256
1.170 dtucker 964: .Ed
1.188 djm 965: .Pp
1.237 jmc 966: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 967: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 968: .It Cm ListenAddress
969: Specifies the local addresses
1.52 jmc 970: .Xr sshd 8
1.1 stevesk 971: should listen on.
972: The following forms may be used:
973: .Pp
974: .Bl -item -offset indent -compact
975: .It
976: .Cm ListenAddress
977: .Sm off
1.259 jmc 978: .Ar hostname | address
1.1 stevesk 979: .Sm on
1.260 jmc 980: .Op Cm rdomain Ar domain
1.1 stevesk 981: .It
982: .Cm ListenAddress
983: .Sm off
1.259 jmc 984: .Ar hostname : port
1.1 stevesk 985: .Sm on
1.260 jmc 986: .Op Cm rdomain Ar domain
1.256 djm 987: .It
988: .Cm ListenAddress
989: .Sm off
1.259 jmc 990: .Ar IPv4_address : port
1.256 djm 991: .Sm on
1.260 jmc 992: .Op Cm rdomain Ar domain
1.1 stevesk 993: .It
994: .Cm ListenAddress
995: .Sm off
1.259 jmc 996: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 997: .Sm on
1.260 jmc 998: .Op Cm rdomain Ar domain
1.1 stevesk 999: .El
1000: .Pp
1.256 djm 1001: The optional
1002: .Cm rdomain
1003: qualifier requests
1004: .Xr sshd 8
1005: listen in an explicit routing domain.
1.1 stevesk 1006: If
1007: .Ar port
1008: is not specified,
1.200 dtucker 1009: sshd will listen on the address and all
1.1 stevesk 1010: .Cm Port
1.17 jmc 1011: options specified.
1.256 djm 1012: The default is to listen on all local addresses on the current default
1013: routing domain.
1.15 jmc 1014: Multiple
1.1 stevesk 1015: .Cm ListenAddress
1.17 jmc 1016: options are permitted.
1.256 djm 1017: For more information on routing domains, see
1.259 jmc 1018: .Xr rdomain 4 .
1.1 stevesk 1019: .It Cm LoginGraceTime
1020: The server disconnects after this time if the user has not
1021: successfully logged in.
1022: If the value is 0, there is no time limit.
1.12 stevesk 1023: The default is 120 seconds.
1.1 stevesk 1024: .It Cm LogLevel
1025: Gives the verbosity level that is used when logging messages from
1.53 jmc 1026: .Xr sshd 8 .
1.1 stevesk 1027: The possible values are:
1.52 jmc 1028: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1029: The default is INFO.
1030: DEBUG and DEBUG1 are equivalent.
1031: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1032: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1033: .It Cm MACs
1034: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1035: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1036: Multiple algorithms must be comma-separated.
1.289 naddy 1037: If the specified list begins with a
1.208 djm 1038: .Sq +
1039: character, then the specified algorithms will be appended to the default set
1040: instead of replacing them.
1.289 naddy 1041: If the specified list begins with a
1.242 djm 1042: .Sq -
1043: character, then the specified algorithms (including wildcards) will be removed
1044: from the default set instead of replacing them.
1.290 naddy 1045: If the specified list begins with a
1046: .Sq ^
1047: character, then the specified algorithms will be placed at the head of the
1048: default set.
1.208 djm 1049: .Pp
1.152 markus 1050: The algorithms that contain
1.237 jmc 1051: .Qq -etm
1.152 markus 1052: calculate the MAC after encryption (encrypt-then-mac).
1053: These are considered safer and their use recommended.
1.173 naddy 1054: The supported MACs are:
1055: .Pp
1056: .Bl -item -compact -offset indent
1057: .It
1058: hmac-md5
1059: .It
1060: hmac-md5-96
1061: .It
1062: hmac-sha1
1063: .It
1064: hmac-sha1-96
1065: .It
1066: hmac-sha2-256
1067: .It
1068: hmac-sha2-512
1069: .It
1070: umac-64@openssh.com
1071: .It
1072: umac-128@openssh.com
1073: .It
1074: hmac-md5-etm@openssh.com
1075: .It
1076: hmac-md5-96-etm@openssh.com
1077: .It
1078: hmac-sha1-etm@openssh.com
1079: .It
1080: hmac-sha1-96-etm@openssh.com
1081: .It
1082: hmac-sha2-256-etm@openssh.com
1083: .It
1084: hmac-sha2-512-etm@openssh.com
1085: .It
1086: umac-64-etm@openssh.com
1087: .It
1088: umac-128-etm@openssh.com
1089: .El
1090: .Pp
1.52 jmc 1091: The default is:
1.77 jmc 1092: .Bd -literal -offset indent
1.152 markus 1093: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1094: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1095: hmac-sha1-etm@openssh.com,
1.173 naddy 1096: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1097: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1098: .Ed
1.188 djm 1099: .Pp
1.237 jmc 1100: The list of available MAC algorithms may also be obtained using
1101: .Qq ssh -Q mac .
1.60 dtucker 1102: .It Cm Match
1.61 jmc 1103: Introduces a conditional block.
1.65 dtucker 1104: If all of the criteria on the
1.60 dtucker 1105: .Cm Match
1.65 dtucker 1106: line are satisfied, the keywords on the following lines override those
1107: set in the global section of the config file, until either another
1.60 dtucker 1108: .Cm Match
1.65 dtucker 1109: line or the end of the file.
1.172 djm 1110: If a keyword appears in multiple
1111: .Cm Match
1.177 sobrado 1112: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1113: applied.
1.91 djm 1114: .Pp
1.61 jmc 1115: The arguments to
1.60 dtucker 1116: .Cm Match
1.163 dtucker 1117: are one or more criteria-pattern pairs or the single token
1118: .Cm All
1119: which matches all criteria.
1.60 dtucker 1120: The available criteria are
1121: .Cm User ,
1.69 dtucker 1122: .Cm Group ,
1.60 dtucker 1123: .Cm Host ,
1.139 dtucker 1124: .Cm LocalAddress ,
1125: .Cm LocalPort ,
1.258 djm 1126: .Cm RDomain ,
1.60 dtucker 1127: and
1.258 djm 1128: .Cm Address
1129: (with
1130: .Cm RDomain
1131: representing the
1132: .Xr rdomain 4
1.284 jmc 1133: on which the connection was received).
1.258 djm 1134: .Pp
1.91 djm 1135: The match patterns may consist of single entries or comma-separated
1136: lists and may use the wildcard and negation operators described in the
1.237 jmc 1137: .Sx PATTERNS
1138: section of
1.92 djm 1139: .Xr ssh_config 5 .
1.91 djm 1140: .Pp
1141: The patterns in an
1142: .Cm Address
1143: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1144: address/masklen format,
1145: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1146: Note that the mask length provided must be consistent with the address -
1147: it is an error to specify a mask length that is too long for the address
1.93 jmc 1148: or one with bits set in this host portion of the address.
1.237 jmc 1149: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1150: .Pp
1.60 dtucker 1151: Only a subset of keywords may be used on the lines following a
1152: .Cm Match
1153: keyword.
1154: Available keywords are
1.142 jmc 1155: .Cm AcceptEnv ,
1.99 okan 1156: .Cm AllowAgentForwarding ,
1.142 jmc 1157: .Cm AllowGroups ,
1.193 djm 1158: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1159: .Cm AllowTcpForwarding ,
1.141 markus 1160: .Cm AllowUsers ,
1.149 djm 1161: .Cm AuthenticationMethods ,
1.146 djm 1162: .Cm AuthorizedKeysCommand ,
1163: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1164: .Cm AuthorizedKeysFile ,
1.214 djm 1165: .Cm AuthorizedPrincipalsCommand ,
1166: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1167: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1168: .Cm Banner ,
1.85 djm 1169: .Cm ChrootDirectory ,
1.238 markus 1170: .Cm ClientAliveCountMax ,
1171: .Cm ClientAliveInterval ,
1.141 markus 1172: .Cm DenyGroups ,
1173: .Cm DenyUsers ,
1.67 dtucker 1174: .Cm ForceCommand ,
1.194 djm 1175: .Cm GatewayPorts ,
1.193 djm 1176: .Cm GSSAPIAuthentication ,
1.189 djm 1177: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1178: .Cm HostbasedAuthentication ,
1.123 djm 1179: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1180: .Cm IgnoreRhosts ,
1.306 naddy 1181: .Cm Include ,
1.193 djm 1182: .Cm IPQoS ,
1.74 jmc 1183: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1184: .Cm KerberosAuthentication ,
1.245 djm 1185: .Cm LogLevel ,
1.95 dtucker 1186: .Cm MaxAuthTries ,
1.94 dtucker 1187: .Cm MaxSessions ,
1.72 dtucker 1188: .Cm PasswordAuthentication ,
1.97 djm 1189: .Cm PermitEmptyPasswords ,
1.271 djm 1190: .Cm PermitListen ,
1.66 dtucker 1191: .Cm PermitOpen ,
1.79 dtucker 1192: .Cm PermitRootLogin ,
1.164 djm 1193: .Cm PermitTTY ,
1.123 djm 1194: .Cm PermitTunnel ,
1.174 djm 1195: .Cm PermitUserRC ,
1.189 djm 1196: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1197: .Cm PubkeyAuthentication ,
1.159 dtucker 1198: .Cm RekeyLimit ,
1.193 djm 1199: .Cm RevokedKeys ,
1.257 djm 1200: .Cm RDomain ,
1.274 djm 1201: .Cm SetEnv ,
1.193 djm 1202: .Cm StreamLocalBindMask ,
1203: .Cm StreamLocalBindUnlink ,
1204: .Cm TrustedUserCAKeys ,
1.66 dtucker 1205: .Cm X11DisplayOffset ,
1.101 djm 1206: .Cm X11Forwarding
1.60 dtucker 1207: and
1.286 jmc 1208: .Cm X11UseLocalhost .
1.33 dtucker 1209: .It Cm MaxAuthTries
1210: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1211: connection.
1212: Once the number of failures reaches half this value,
1213: additional failures are logged.
1214: The default is 6.
1.90 djm 1215: .It Cm MaxSessions
1.216 djm 1216: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1217: sessions permitted per network connection.
1218: Multiple sessions may be established by clients that support connection
1219: multiplexing.
1220: Setting
1221: .Cm MaxSessions
1222: to 1 will effectively disable session multiplexing, whereas setting it to 0
1223: will prevent all shell, login and subsystem sessions while still permitting
1224: forwarding.
1.90 djm 1225: The default is 10.
1.1 stevesk 1226: .It Cm MaxStartups
1227: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1228: SSH daemon.
1.1 stevesk 1229: Additional connections will be dropped until authentication succeeds or the
1230: .Cm LoginGraceTime
1231: expires for a connection.
1.156 dtucker 1232: The default is 10:30:100.
1.1 stevesk 1233: .Pp
1234: Alternatively, random early drop can be enabled by specifying
1235: the three colon separated values
1.237 jmc 1236: start:rate:full (e.g. "10:30:60").
1.53 jmc 1237: .Xr sshd 8
1.237 jmc 1238: will refuse connection attempts with a probability of rate/100 (30%)
1239: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1240: The probability increases linearly and all connection attempts
1.237 jmc 1241: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1242: .It Cm PasswordAuthentication
1243: Specifies whether password authentication is allowed.
1244: The default is
1.237 jmc 1245: .Cm yes .
1.1 stevesk 1246: .It Cm PermitEmptyPasswords
1247: When password authentication is allowed, it specifies whether the
1248: server allows login to accounts with empty password strings.
1249: The default is
1.237 jmc 1250: .Cm no .
1.271 djm 1251: .It Cm PermitListen
1252: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1253: The listen specification must be one of the following forms:
1254: .Pp
1255: .Bl -item -offset indent -compact
1256: .It
1257: .Cm PermitListen
1258: .Sm off
1.276 djm 1259: .Ar port
1.271 djm 1260: .Sm on
1261: .It
1262: .Cm PermitListen
1263: .Sm off
1.276 djm 1264: .Ar host : port
1.271 djm 1265: .Sm on
1266: .El
1267: .Pp
1268: Multiple permissions may be specified by separating them with whitespace.
1269: An argument of
1270: .Cm any
1271: can be used to remove all restrictions and permit any listen requests.
1272: An argument of
1273: .Cm none
1274: can be used to prohibit all listen requests.
1275: The host name may contain wildcards as described in the PATTERNS section in
1276: .Xr ssh_config 5 .
1277: The wildcard
1278: .Sq *
1279: can also be used in place of a port number to allow all ports.
1280: By default all port forwarding listen requests are permitted.
1.272 jmc 1281: Note that the
1.271 djm 1282: .Cm GatewayPorts
1283: option may further restrict which addresses may be listened on.
1.276 djm 1284: Note also that
1285: .Xr ssh 1
1286: will request a listen host of
1287: .Dq localhost
1.283 dtucker 1288: if no listen host was specifically requested, and this name is
1.277 jmc 1289: treated differently to explicit localhost addresses of
1.276 djm 1290: .Dq 127.0.0.1
1291: and
1292: .Dq ::1 .
1.62 dtucker 1293: .It Cm PermitOpen
1294: Specifies the destinations to which TCP port forwarding is permitted.
1295: The forwarding specification must be one of the following forms:
1296: .Pp
1297: .Bl -item -offset indent -compact
1298: .It
1299: .Cm PermitOpen
1300: .Sm off
1301: .Ar host : port
1302: .Sm on
1303: .It
1304: .Cm PermitOpen
1305: .Sm off
1306: .Ar IPv4_addr : port
1307: .Sm on
1308: .It
1309: .Cm PermitOpen
1310: .Sm off
1311: .Ar \&[ IPv6_addr \&] : port
1312: .Sm on
1313: .El
1314: .Pp
1.68 dtucker 1315: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1316: An argument of
1.237 jmc 1317: .Cm any
1.62 dtucker 1318: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1319: An argument of
1.237 jmc 1320: .Cm none
1.140 dtucker 1321: can be used to prohibit all forwarding requests.
1.227 jmc 1322: The wildcard
1.237 jmc 1323: .Sq *
1.299 djm 1324: can be used for host or port to allow all hosts or ports respectively.
1325: Otherwise, no pattern matching or address lookups are performed on supplied
1326: names.
1.63 jmc 1327: By default all port forwarding requests are permitted.
1.1 stevesk 1328: .It Cm PermitRootLogin
1.38 jmc 1329: Specifies whether root can log in using
1.1 stevesk 1330: .Xr ssh 1 .
1331: The argument must be
1.237 jmc 1332: .Cm yes ,
1333: .Cm prohibit-password ,
1334: .Cm forced-commands-only ,
1.1 stevesk 1335: or
1.237 jmc 1336: .Cm no .
1.1 stevesk 1337: The default is
1.237 jmc 1338: .Cm prohibit-password .
1.1 stevesk 1339: .Pp
1340: If this option is set to
1.237 jmc 1341: .Cm prohibit-password
1.255 jmc 1342: (or its deprecated alias,
1343: .Cm without-password ) ,
1.210 deraadt 1344: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1345: .Pp
1346: If this option is set to
1.237 jmc 1347: .Cm forced-commands-only ,
1.1 stevesk 1348: root login with public key authentication will be allowed,
1349: but only if the
1350: .Ar command
1351: option has been specified
1352: (which may be useful for taking remote backups even if root login is
1.17 jmc 1353: normally not allowed).
1354: All other authentication methods are disabled for root.
1.1 stevesk 1355: .Pp
1356: If this option is set to
1.237 jmc 1357: .Cm no ,
1.38 jmc 1358: root is not allowed to log in.
1.231 jmc 1359: .It Cm PermitTTY
1360: Specifies whether
1361: .Xr pty 4
1362: allocation is permitted.
1363: The default is
1.237 jmc 1364: .Cm yes .
1.46 reyk 1365: .It Cm PermitTunnel
1366: Specifies whether
1367: .Xr tun 4
1368: device forwarding is allowed.
1.47 reyk 1369: The argument must be
1.237 jmc 1370: .Cm yes ,
1371: .Cm point-to-point
1.58 stevesk 1372: (layer 3),
1.237 jmc 1373: .Cm ethernet
1.58 stevesk 1374: (layer 2), or
1.237 jmc 1375: .Cm no .
1.58 stevesk 1376: Specifying
1.237 jmc 1377: .Cm yes
1.58 stevesk 1378: permits both
1.237 jmc 1379: .Cm point-to-point
1.58 stevesk 1380: and
1.237 jmc 1381: .Cm ethernet .
1.46 reyk 1382: The default is
1.237 jmc 1383: .Cm no .
1.178 djm 1384: .Pp
1385: Independent of this setting, the permissions of the selected
1386: .Xr tun 4
1387: device must allow access to the user.
1.6 markus 1388: .It Cm PermitUserEnvironment
1389: Specifies whether
1390: .Pa ~/.ssh/environment
1.9 stevesk 1391: and
1.6 markus 1392: .Cm environment=
1393: options in
1394: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1395: are processed by
1.52 jmc 1396: .Xr sshd 8 .
1.278 djm 1397: Valid options are
1398: .Cm yes ,
1399: .Cm no
1400: or a pattern-list specifying which environment variable names to accept
1401: (for example
1402: .Qq LANG,LC_* ) .
1.6 markus 1403: The default is
1.237 jmc 1404: .Cm no .
1.9 stevesk 1405: Enabling environment processing may enable users to bypass access
1406: restrictions in some configurations using mechanisms such as
1407: .Ev LD_PRELOAD .
1.174 djm 1408: .It Cm PermitUserRC
1409: Specifies whether any
1410: .Pa ~/.ssh/rc
1411: file is executed.
1412: The default is
1.237 jmc 1413: .Cm yes .
1.1 stevesk 1414: .It Cm PidFile
1.4 stevesk 1415: Specifies the file that contains the process ID of the
1.195 dtucker 1416: SSH daemon, or
1.237 jmc 1417: .Cm none
1.195 dtucker 1418: to not write one.
1.1 stevesk 1419: The default is
1420: .Pa /var/run/sshd.pid .
1421: .It Cm Port
1422: Specifies the port number that
1.52 jmc 1423: .Xr sshd 8
1.1 stevesk 1424: listens on.
1425: The default is 22.
1426: Multiple options of this type are permitted.
1427: See also
1428: .Cm ListenAddress .
1429: .It Cm PrintLastLog
1430: Specifies whether
1.52 jmc 1431: .Xr sshd 8
1.36 jaredy 1432: should print the date and time of the last user login when a user logs
1433: in interactively.
1.1 stevesk 1434: The default is
1.237 jmc 1435: .Cm yes .
1.1 stevesk 1436: .It Cm PrintMotd
1437: Specifies whether
1.52 jmc 1438: .Xr sshd 8
1.1 stevesk 1439: should print
1440: .Pa /etc/motd
1441: when a user logs in interactively.
1442: (On some systems it is also printed by the shell,
1443: .Pa /etc/profile ,
1444: or equivalent.)
1445: The default is
1.237 jmc 1446: .Cm yes .
1.189 djm 1447: .It Cm PubkeyAcceptedKeyTypes
1448: Specifies the key types that will be accepted for public key authentication
1.280 djm 1449: as a list of comma-separated patterns.
1.289 naddy 1450: Alternately if the specified list begins with a
1.208 djm 1451: .Sq +
1452: character, then the specified key types will be appended to the default set
1453: instead of replacing them.
1.289 naddy 1454: If the specified list begins with a
1.242 djm 1455: .Sq -
1456: character, then the specified key types (including wildcards) will be removed
1457: from the default set instead of replacing them.
1.290 naddy 1458: If the specified list begins with a
1459: .Sq ^
1460: character, then the specified key types will be placed at the head of the
1461: default set.
1.206 markus 1462: The default for this option is:
1463: .Bd -literal -offset 3n
1464: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1465: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1466: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1467: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1468: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1469: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1470: rsa-sha2-512-cert-v01@openssh.com,
1471: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1472: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1473: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1474: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1475: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1476: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1477: .Ed
1478: .Pp
1.237 jmc 1479: The list of available key types may also be obtained using
1.307 dtucker 1480: .Qq ssh -Q PubkeyAcceptedKeyTypes .
1.293 djm 1481: .It Cm PubkeyAuthOptions
1482: Sets one or more public key authentication options.
1483: Two option keywords are currently supported:
1.294 jmc 1484: .Cm none
1485: (the default; indicating no additional options are enabled)
1.293 djm 1486: and
1487: .Cm touch-required .
1488: .Pp
1489: The
1490: .Cm touch-required
1.297 naddy 1491: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1492: (i.e.\&
1.293 djm 1493: .Cm ecdsa-sk
1494: or
1495: .Cm ed25519-sk )
1496: to always require the signature to attest that a physically present user
1.297 naddy 1497: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1498: By default,
1499: .Xr sshd 8
1.297 naddy 1500: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1501: The
1502: .Cm touch-required
1503: flag disables this override.
1.297 naddy 1504: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1505: .It Cm PubkeyAuthentication
1506: Specifies whether public key authentication is allowed.
1507: The default is
1.237 jmc 1508: .Cm yes .
1.159 dtucker 1509: .It Cm RekeyLimit
1510: Specifies the maximum amount of data that may be transmitted before the
1511: session key is renegotiated, optionally followed a maximum amount of
1512: time that may pass before the session key is renegotiated.
1513: The first argument is specified in bytes and may have a suffix of
1514: .Sq K ,
1515: .Sq M ,
1516: or
1517: .Sq G
1518: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1519: The default is between
1520: .Sq 1G
1521: and
1522: .Sq 4G ,
1523: depending on the cipher.
1524: The optional second value is specified in seconds and may use any of the
1525: units documented in the
1526: .Sx TIME FORMATS
1.160 jmc 1527: section.
1.159 dtucker 1528: The default value for
1529: .Cm RekeyLimit
1530: is
1.237 jmc 1531: .Cm default none ,
1.159 dtucker 1532: which means that rekeying is performed after the cipher's default amount
1533: of data has been sent or received and no time based rekeying is done.
1.118 djm 1534: .It Cm RevokedKeys
1.195 dtucker 1535: Specifies revoked public keys file, or
1.237 jmc 1536: .Cm none
1.195 dtucker 1537: to not use one.
1.118 djm 1538: Keys listed in this file will be refused for public key authentication.
1539: Note that if this file is not readable, then public key authentication will
1540: be refused for all users.
1.154 djm 1541: Keys may be specified as a text file, listing one public key per line, or as
1542: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1543: .Xr ssh-keygen 1 .
1.161 jmc 1544: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1545: .Xr ssh-keygen 1 .
1.257 djm 1546: .It Cm RDomain
1547: Specifies an explicit routing domain that is applied after authentication
1548: has completed.
1.259 jmc 1549: The user session, as well and any forwarded or listening IP sockets,
1550: will be bound to this
1.257 djm 1551: .Xr rdomain 4 .
1552: If the routing domain is set to
1553: .Cm \&%D ,
1.259 jmc 1554: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1555: .It Cm SecurityKeyProvider
1.297 naddy 1556: Specifies a path to a library that will be used when loading
1557: FIDO authenticator-hosted keys, overriding the default of using
1558: the built-in USB HID support.
1.274 djm 1559: .It Cm SetEnv
1560: Specifies one or more environment variables to set in child sessions started
1561: by
1562: .Xr sshd 8
1563: as
1564: .Dq NAME=VALUE .
1565: The environment value may be quoted (e.g. if it contains whitespace
1566: characters).
1567: Environment variables set by
1568: .Cm SetEnv
1569: override the default environment and any variables specified by the user
1570: via
1571: .Cm AcceptEnv
1572: or
1573: .Cm PermitUserEnvironment .
1.175 millert 1574: .It Cm StreamLocalBindMask
1575: Sets the octal file creation mode mask
1576: .Pq umask
1577: used when creating a Unix-domain socket file for local or remote
1578: port forwarding.
1579: This option is only used for port forwarding to a Unix-domain socket file.
1580: .Pp
1581: The default value is 0177, which creates a Unix-domain socket file that is
1582: readable and writable only by the owner.
1583: Note that not all operating systems honor the file mode on Unix-domain
1584: socket files.
1585: .It Cm StreamLocalBindUnlink
1586: Specifies whether to remove an existing Unix-domain socket file for local
1587: or remote port forwarding before creating a new one.
1588: If the socket file already exists and
1589: .Cm StreamLocalBindUnlink
1590: is not enabled,
1591: .Nm sshd
1592: will be unable to forward the port to the Unix-domain socket file.
1593: This option is only used for port forwarding to a Unix-domain socket file.
1594: .Pp
1595: The argument must be
1.237 jmc 1596: .Cm yes
1.175 millert 1597: or
1.237 jmc 1598: .Cm no .
1.175 millert 1599: The default is
1.237 jmc 1600: .Cm no .
1.1 stevesk 1601: .It Cm StrictModes
1602: Specifies whether
1.52 jmc 1603: .Xr sshd 8
1.1 stevesk 1604: should check file modes and ownership of the
1605: user's files and home directory before accepting login.
1606: This is normally desirable because novices sometimes accidentally leave their
1607: directory or files world-writable.
1608: The default is
1.237 jmc 1609: .Cm yes .
1.112 djm 1610: Note that this does not apply to
1611: .Cm ChrootDirectory ,
1612: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1613: .It Cm Subsystem
1.51 jmc 1614: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1615: Arguments should be a subsystem name and a command (with optional arguments)
1616: to execute upon subsystem request.
1.80 djm 1617: .Pp
1.1 stevesk 1618: The command
1.237 jmc 1619: .Cm sftp-server
1620: implements the SFTP file transfer subsystem.
1.80 djm 1621: .Pp
1622: Alternately the name
1.237 jmc 1623: .Cm internal-sftp
1624: implements an in-process SFTP server.
1.80 djm 1625: This may simplify configurations using
1626: .Cm ChrootDirectory
1627: to force a different filesystem root on clients.
1628: .Pp
1.1 stevesk 1629: By default no subsystems are defined.
1630: .It Cm SyslogFacility
1631: Gives the facility code that is used when logging messages from
1.53 jmc 1632: .Xr sshd 8 .
1.1 stevesk 1633: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1634: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1635: The default is AUTH.
1.27 markus 1636: .It Cm TCPKeepAlive
1637: Specifies whether the system should send TCP keepalive messages to the
1638: other side.
1639: If they are sent, death of the connection or crash of one
1640: of the machines will be properly noticed.
1641: However, this means that
1642: connections will die if the route is down temporarily, and some people
1643: find it annoying.
1644: On the other hand, if TCP keepalives are not sent,
1645: sessions may hang indefinitely on the server, leaving
1.237 jmc 1646: .Qq ghost
1.27 markus 1647: users and consuming server resources.
1648: .Pp
1649: The default is
1.237 jmc 1650: .Cm yes
1.27 markus 1651: (to send TCP keepalive messages), and the server will notice
1652: if the network goes down or the client host crashes.
1653: This avoids infinitely hanging sessions.
1654: .Pp
1655: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1656: .Cm no .
1.118 djm 1657: .It Cm TrustedUserCAKeys
1658: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1659: trusted to sign user certificates for authentication, or
1.237 jmc 1660: .Cm none
1.195 dtucker 1661: to not use one.
1.119 jmc 1662: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1663: .Ql #
1664: are allowed.
1665: If a certificate is presented for authentication and has its signing CA key
1666: listed in this file, then it may be used for authentication for any user
1667: listed in the certificate's principals list.
1668: Note that certificates that lack a list of principals will not be permitted
1669: for authentication using
1670: .Cm TrustedUserCAKeys .
1.161 jmc 1671: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1672: .Xr ssh-keygen 1 .
1.18 markus 1673: .It Cm UseDNS
1674: Specifies whether
1.52 jmc 1675: .Xr sshd 8
1.207 djm 1676: should look up the remote host name, and to check that
1.18 markus 1677: the resolved host name for the remote IP address maps back to the
1678: very same IP address.
1.207 djm 1679: .Pp
1680: If this option is set to
1.237 jmc 1681: .Cm no
1.207 djm 1682: (the default) then only addresses and not host names may be used in
1.221 djm 1683: .Pa ~/.ssh/authorized_keys
1.207 djm 1684: .Cm from
1685: and
1.211 jmc 1686: .Nm
1.207 djm 1687: .Cm Match
1688: .Cm Host
1689: directives.
1.137 djm 1690: .It Cm VersionAddendum
1691: Optionally specifies additional text to append to the SSH protocol banner
1692: sent by the server upon connection.
1693: The default is
1.237 jmc 1694: .Cm none .
1.1 stevesk 1695: .It Cm X11DisplayOffset
1696: Specifies the first display number available for
1.52 jmc 1697: .Xr sshd 8 Ns 's
1.1 stevesk 1698: X11 forwarding.
1.52 jmc 1699: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1700: The default is 10.
1701: .It Cm X11Forwarding
1702: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1703: The argument must be
1.237 jmc 1704: .Cm yes
1.13 stevesk 1705: or
1.237 jmc 1706: .Cm no .
1.1 stevesk 1707: The default is
1.237 jmc 1708: .Cm no .
1.13 stevesk 1709: .Pp
1710: When X11 forwarding is enabled, there may be additional exposure to
1711: the server and to client displays if the
1.52 jmc 1712: .Xr sshd 8
1.13 stevesk 1713: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1714: .Cm X11UseLocalhost ) ,
1715: though this is not the default.
1.13 stevesk 1716: Additionally, the authentication spoofing and authentication data
1717: verification and substitution occur on the client side.
1718: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1719: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1720: forwarding (see the warnings for
1721: .Cm ForwardX11
1722: in
1.19 jmc 1723: .Xr ssh_config 5 ) .
1.13 stevesk 1724: A system administrator may have a stance in which they want to
1725: protect clients that may expose themselves to attack by unwittingly
1726: requesting X11 forwarding, which can warrant a
1.237 jmc 1727: .Cm no
1.13 stevesk 1728: setting.
1729: .Pp
1730: Note that disabling X11 forwarding does not prevent users from
1731: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1732: .It Cm X11UseLocalhost
1733: Specifies whether
1.52 jmc 1734: .Xr sshd 8
1.1 stevesk 1735: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1736: the wildcard address.
1737: By default,
1.52 jmc 1738: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1739: hostname part of the
1740: .Ev DISPLAY
1741: environment variable to
1.237 jmc 1742: .Cm localhost .
1.8 stevesk 1743: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1744: However, some older X11 clients may not function with this
1745: configuration.
1746: .Cm X11UseLocalhost
1747: may be set to
1.237 jmc 1748: .Cm no
1.1 stevesk 1749: to specify that the forwarding server should be bound to the wildcard
1750: address.
1751: The argument must be
1.237 jmc 1752: .Cm yes
1.1 stevesk 1753: or
1.237 jmc 1754: .Cm no .
1.1 stevesk 1755: The default is
1.237 jmc 1756: .Cm yes .
1.1 stevesk 1757: .It Cm XAuthLocation
1.11 stevesk 1758: Specifies the full pathname of the
1.1 stevesk 1759: .Xr xauth 1
1.195 dtucker 1760: program, or
1.237 jmc 1761: .Cm none
1.195 dtucker 1762: to not use one.
1.1 stevesk 1763: The default is
1764: .Pa /usr/X11R6/bin/xauth .
1765: .El
1.55 jmc 1766: .Sh TIME FORMATS
1.53 jmc 1767: .Xr sshd 8
1.1 stevesk 1768: command-line arguments and configuration file options that specify time
1769: may be expressed using a sequence of the form:
1770: .Sm off
1.7 stevesk 1771: .Ar time Op Ar qualifier ,
1.1 stevesk 1772: .Sm on
1773: where
1774: .Ar time
1775: is a positive integer value and
1776: .Ar qualifier
1777: is one of the following:
1778: .Pp
1779: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1780: .It Aq Cm none
1.1 stevesk 1781: seconds
1782: .It Cm s | Cm S
1783: seconds
1784: .It Cm m | Cm M
1785: minutes
1786: .It Cm h | Cm H
1787: hours
1788: .It Cm d | Cm D
1789: days
1790: .It Cm w | Cm W
1791: weeks
1792: .El
1793: .Pp
1794: Each member of the sequence is added together to calculate
1795: the total time value.
1796: .Pp
1797: Time format examples:
1798: .Pp
1799: .Bl -tag -width Ds -compact -offset indent
1800: .It 600
1801: 600 seconds (10 minutes)
1802: .It 10m
1803: 10 minutes
1804: .It 1h30m
1805: 1 hour 30 minutes (90 minutes)
1806: .El
1.235 jmc 1807: .Sh TOKENS
1808: Arguments to some keywords can make use of tokens,
1809: which are expanded at runtime:
1810: .Pp
1811: .Bl -tag -width XXXX -offset indent -compact
1812: .It %%
1813: A literal
1814: .Sq % .
1.257 djm 1815: .It \&%D
1816: The routing domain in which the incoming connection was received.
1.235 jmc 1817: .It %F
1818: The fingerprint of the CA key.
1819: .It %f
1820: The fingerprint of the key or certificate.
1821: .It %h
1822: The home directory of the user.
1823: .It %i
1824: The key ID in the certificate.
1825: .It %K
1826: The base64-encoded CA key.
1827: .It %k
1828: The base64-encoded key or certificate for authentication.
1829: .It %s
1830: The serial number of the certificate.
1831: .It \&%T
1832: The type of the CA key.
1833: .It %t
1834: The key or certificate type.
1.268 jmc 1835: .It \&%U
1836: The numeric user ID of the target user.
1.235 jmc 1837: .It %u
1838: The username.
1839: .El
1840: .Pp
1841: .Cm AuthorizedKeysCommand
1.270 jmc 1842: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1843: .Pp
1844: .Cm AuthorizedKeysFile
1.270 jmc 1845: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1846: .Pp
1847: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1848: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1849: .Pp
1850: .Cm AuthorizedPrincipalsFile
1.270 jmc 1851: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1852: .Pp
1853: .Cm ChrootDirectory
1.270 jmc 1854: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1855: .Pp
1856: .Cm RoutingDomain
1857: accepts the token %D.
1.1 stevesk 1858: .Sh FILES
1859: .Bl -tag -width Ds
1860: .It Pa /etc/ssh/sshd_config
1861: Contains configuration data for
1.53 jmc 1862: .Xr sshd 8 .
1.1 stevesk 1863: This file should be writable by root only, but it is recommended
1864: (though not necessary) that it be world-readable.
1865: .El
1.19 jmc 1866: .Sh SEE ALSO
1.237 jmc 1867: .Xr sftp-server 8 ,
1.19 jmc 1868: .Xr sshd 8
1.1 stevesk 1869: .Sh AUTHORS
1.237 jmc 1870: .An -nosplit
1.1 stevesk 1871: OpenSSH is a derivative of the original and free
1.237 jmc 1872: ssh 1.2.12 release by
1873: .An Tatu Ylonen .
1874: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1875: .An Theo de Raadt
1876: and
1877: .An Dug Song
1.1 stevesk 1878: removed many bugs, re-added newer features and
1879: created OpenSSH.
1.237 jmc 1880: .An Markus Friedl
1881: contributed the support for SSH protocol versions 1.5 and 2.0.
1882: .An Niels Provos
1883: and
1884: .An Markus Friedl
1885: contributed support for privilege separation.
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