Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.297
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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1.297 ! naddy 36: .\" $OpenBSD: sshd_config.5,v 1.296 2019/12/19 15:09:30 naddy Exp $
! 37: .Dd $Mdocdate: December 19 2019 $
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.
1.54 jmc 116: The allow/deny directives are processed in the following order:
117: .Cm DenyUsers ,
118: .Cm AllowUsers ,
119: .Cm DenyGroups ,
120: and finally
121: .Cm AllowGroups .
1.49 jmc 122: .Pp
1.161 jmc 123: See PATTERNS in
1.49 jmc 124: .Xr ssh_config 5
125: for more information on patterns.
1.231 jmc 126: .It Cm AllowStreamLocalForwarding
127: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 128: The available options are
1.237 jmc 129: .Cm yes
130: (the default)
1.150 djm 131: or
1.237 jmc 132: .Cm all
1.231 jmc 133: to allow StreamLocal forwarding,
1.237 jmc 134: .Cm no
1.231 jmc 135: to prevent all StreamLocal forwarding,
1.237 jmc 136: .Cm local
1.150 djm 137: to allow local (from the perspective of
1.151 jmc 138: .Xr ssh 1 )
139: forwarding only or
1.237 jmc 140: .Cm remote
1.150 djm 141: to allow remote forwarding only.
1.231 jmc 142: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 143: users are also denied shell access, as they can always install their
144: own forwarders.
1.231 jmc 145: .It Cm AllowTcpForwarding
146: Specifies whether TCP forwarding is permitted.
1.175 millert 147: The available options are
1.237 jmc 148: .Cm yes
149: (the default)
1.175 millert 150: or
1.237 jmc 151: .Cm all
1.231 jmc 152: to allow TCP forwarding,
1.237 jmc 153: .Cm no
1.231 jmc 154: to prevent all TCP forwarding,
1.237 jmc 155: .Cm local
1.175 millert 156: to allow local (from the perspective of
157: .Xr ssh 1 )
158: forwarding only or
1.237 jmc 159: .Cm remote
1.175 millert 160: to allow remote forwarding only.
1.231 jmc 161: Note that disabling TCP forwarding does not improve security unless
1.175 millert 162: users are also denied shell access, as they can always install their
163: own forwarders.
1.1 stevesk 164: .It Cm AllowUsers
165: This keyword can be followed by a list of user name patterns, separated
166: by spaces.
1.14 jmc 167: If specified, login is allowed only for user names that
1.1 stevesk 168: match one of the patterns.
169: Only user names are valid; a numerical user ID is not recognized.
170: By default, login is allowed for all users.
171: If the pattern takes the form USER@HOST then USER and HOST
172: are separately checked, restricting logins to particular
173: users from particular hosts.
1.222 jmc 174: HOST criteria may additionally contain addresses to match in CIDR
175: address/masklen format.
1.54 jmc 176: The allow/deny directives are processed in the following order:
177: .Cm DenyUsers ,
178: .Cm AllowUsers ,
179: .Cm DenyGroups ,
180: and finally
181: .Cm AllowGroups .
1.49 jmc 182: .Pp
1.161 jmc 183: See PATTERNS in
1.49 jmc 184: .Xr ssh_config 5
185: for more information on patterns.
1.149 djm 186: .It Cm AuthenticationMethods
187: Specifies the authentication methods that must be successfully completed
188: for a user to be granted access.
1.281 djm 189: This option must be followed by one or more lists of comma-separated
1.224 djm 190: authentication method names, or by the single string
1.237 jmc 191: .Cm any
1.224 djm 192: to indicate the default behaviour of accepting any single authentication
1.225 jmc 193: method.
1.237 jmc 194: If the default is overridden, then successful authentication requires
1.224 djm 195: completion of every method in at least one of these lists.
1.149 djm 196: .Pp
1.237 jmc 197: For example,
198: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 199: would require the user to complete public key authentication, followed by
200: either password or keyboard interactive authentication.
201: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 202: so for this example it would not be possible to attempt password or
1.149 djm 203: keyboard-interactive authentication before public key.
1.157 markus 204: .Pp
205: For keyboard interactive authentication it is also possible to
206: restrict authentication to a specific device by appending a
207: colon followed by the device identifier
1.237 jmc 208: .Cm bsdauth ,
209: .Cm pam ,
1.157 markus 210: or
1.237 jmc 211: .Cm skey ,
1.157 markus 212: depending on the server configuration.
213: For example,
1.237 jmc 214: .Qq keyboard-interactive:bsdauth
1.157 markus 215: would restrict keyboard interactive authentication to the
1.237 jmc 216: .Cm bsdauth
1.157 markus 217: device.
1.185 djm 218: .Pp
1.237 jmc 219: If the publickey method is listed more than once,
1.185 djm 220: .Xr sshd 8
221: verifies that keys that have been used successfully are not reused for
222: subsequent authentications.
1.237 jmc 223: For example,
224: .Qq publickey,publickey
225: requires successful authentication using two different public keys.
1.181 djm 226: .Pp
1.149 djm 227: Note that each authentication method listed should also be explicitly enabled
228: in the configuration.
1.251 djm 229: .Pp
230: The available authentication methods are:
231: .Qq gssapi-with-mic ,
232: .Qq hostbased ,
233: .Qq keyboard-interactive ,
234: .Qq none
235: (used for access to password-less accounts when
1.266 jmc 236: .Cm PermitEmptyPasswords
1.251 djm 237: is enabled),
238: .Qq password
239: and
240: .Qq publickey .
1.146 djm 241: .It Cm AuthorizedKeysCommand
1.147 jmc 242: Specifies a program to be used to look up the user's public keys.
1.201 djm 243: The program must be owned by root, not writable by group or others and
244: specified by an absolute path.
245: Arguments to
246: .Cm AuthorizedKeysCommand
1.235 jmc 247: accept the tokens described in the
248: .Sx TOKENS
249: section.
250: If no arguments are specified then the username of the target user is used.
1.201 djm 251: .Pp
252: The program should produce on standard output zero or
1.235 jmc 253: more lines of authorized_keys output (see
254: .Sx AUTHORIZED_KEYS
255: in
1.147 jmc 256: .Xr sshd 8 ) .
1.235 jmc 257: If a key supplied by
258: .Cm AuthorizedKeysCommand
259: does not successfully authenticate
1.146 djm 260: and authorize the user then public key authentication continues using the usual
261: .Cm AuthorizedKeysFile
262: files.
1.237 jmc 263: By default, no
264: .Cm AuthorizedKeysCommand
265: is run.
1.146 djm 266: .It Cm AuthorizedKeysCommandUser
1.237 jmc 267: Specifies the user under whose account the
268: .Cm AuthorizedKeysCommand
269: is run.
1.146 djm 270: It is recommended to use a dedicated user that has no other role on the host
271: than running authorized keys commands.
1.186 djm 272: If
1.181 djm 273: .Cm AuthorizedKeysCommand
1.186 djm 274: is specified but
275: .Cm AuthorizedKeysCommandUser
276: is not, then
277: .Xr sshd 8
278: will refuse to start.
1.1 stevesk 279: .It Cm AuthorizedKeysFile
1.235 jmc 280: Specifies the file that contains the public keys used for user authentication.
1.285 schwarze 281: The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
1.124 djm 282: .Xr sshd 8 .
1.235 jmc 283: Arguments to
1.1 stevesk 284: .Cm AuthorizedKeysFile
1.235 jmc 285: accept the tokens described in the
286: .Sx TOKENS
287: section.
1.1 stevesk 288: After expansion,
289: .Cm AuthorizedKeysFile
290: is taken to be an absolute path or one relative to the user's home
291: directory.
1.133 jmc 292: Multiple files may be listed, separated by whitespace.
1.212 djm 293: Alternately this option may be set to
1.237 jmc 294: .Cm none
1.212 djm 295: to skip checking for user keys in files.
1.133 jmc 296: The default is
1.237 jmc 297: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 298: .It Cm AuthorizedPrincipalsCommand
299: Specifies a program to be used to generate the list of allowed
300: certificate principals as per
301: .Cm AuthorizedPrincipalsFile .
302: The program must be owned by root, not writable by group or others and
303: specified by an absolute path.
304: Arguments to
305: .Cm AuthorizedPrincipalsCommand
1.235 jmc 306: accept the tokens described in the
307: .Sx TOKENS
308: section.
309: If no arguments are specified then the username of the target user is used.
1.202 djm 310: .Pp
311: The program should produce on standard output zero or
312: more lines of
313: .Cm AuthorizedPrincipalsFile
314: output.
315: If either
316: .Cm AuthorizedPrincipalsCommand
317: or
318: .Cm AuthorizedPrincipalsFile
319: is specified, then certificates offered by the client for authentication
320: must contain a principal that is listed.
1.235 jmc 321: By default, no
322: .Cm AuthorizedPrincipalsCommand
323: is run.
1.202 djm 324: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 325: Specifies the user under whose account the
326: .Cm AuthorizedPrincipalsCommand
327: is run.
1.202 djm 328: It is recommended to use a dedicated user that has no other role on the host
329: than running authorized principals commands.
330: If
331: .Cm AuthorizedPrincipalsCommand
332: is specified but
333: .Cm AuthorizedPrincipalsCommandUser
334: is not, then
335: .Xr sshd 8
336: will refuse to start.
1.121 djm 337: .It Cm AuthorizedPrincipalsFile
338: Specifies a file that lists principal names that are accepted for
339: certificate authentication.
340: When using certificates signed by a key listed in
341: .Cm TrustedUserCAKeys ,
342: this file lists names, one of which must appear in the certificate for it
343: to be accepted for authentication.
1.235 jmc 344: Names are listed one per line preceded by key options (as described in
345: .Sx AUTHORIZED_KEYS FILE FORMAT
346: in
1.125 jmc 347: .Xr sshd 8 ) .
1.124 djm 348: Empty lines and comments starting with
1.121 djm 349: .Ql #
350: are ignored.
351: .Pp
1.235 jmc 352: Arguments to
1.121 djm 353: .Cm AuthorizedPrincipalsFile
1.235 jmc 354: accept the tokens described in the
355: .Sx TOKENS
356: section.
1.121 djm 357: After expansion,
358: .Cm AuthorizedPrincipalsFile
1.235 jmc 359: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 360: The default is
1.237 jmc 361: .Cm none ,
1.138 djm 362: i.e. not to use a principals file \(en in this case, the username
1.121 djm 363: of the user must appear in a certificate's principals list for it to be
364: accepted.
1.235 jmc 365: .Pp
1.121 djm 366: Note that
367: .Cm AuthorizedPrincipalsFile
368: is only used when authentication proceeds using a CA listed in
369: .Cm TrustedUserCAKeys
370: and is not consulted for certification authorities trusted via
371: .Pa ~/.ssh/authorized_keys ,
372: though the
373: .Cm principals=
374: key option offers a similar facility (see
375: .Xr sshd 8
376: for details).
1.1 stevesk 377: .It Cm Banner
378: The contents of the specified file are sent to the remote user before
379: authentication is allowed.
1.78 djm 380: If the argument is
1.237 jmc 381: .Cm none
1.78 djm 382: then no banner is displayed.
1.1 stevesk 383: By default, no banner is displayed.
1.282 djm 384: .It Cm CASignatureAlgorithms
385: Specifies which algorithms are allowed for signing of certificates
386: by certificate authorities (CAs).
387: The default is:
388: .Bd -literal -offset indent
1.287 dtucker 389: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.282 djm 390: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
391: .Ed
392: .Pp
393: Certificates signed using other algorithms will not be accepted for
394: public key or host-based authentication.
1.1 stevesk 395: .It Cm ChallengeResponseAuthentication
1.50 jmc 396: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 397: All authentication styles from
398: .Xr login.conf 5
399: are supported.
400: The default is
1.237 jmc 401: .Cm yes .
1.80 djm 402: .It Cm ChrootDirectory
1.113 stevesk 403: Specifies the pathname of a directory to
1.80 djm 404: .Xr chroot 2
405: to after authentication.
1.190 deraadt 406: At session startup
407: .Xr sshd 8
408: checks that all components of the pathname are root-owned directories
409: which are not writable by any other user or group.
1.106 stevesk 410: After the chroot,
411: .Xr sshd 8
412: changes the working directory to the user's home directory.
1.235 jmc 413: Arguments to
414: .Cm ChrootDirectory
415: accept the tokens described in the
416: .Sx TOKENS
417: section.
1.80 djm 418: .Pp
419: The
420: .Cm ChrootDirectory
421: must contain the necessary files and directories to support the
1.103 stevesk 422: user's session.
1.80 djm 423: For an interactive session this requires at least a shell, typically
424: .Xr sh 1 ,
425: and basic
426: .Pa /dev
427: nodes such as
428: .Xr null 4 ,
429: .Xr zero 4 ,
430: .Xr stdin 4 ,
431: .Xr stdout 4 ,
432: .Xr stderr 4 ,
1.180 jmc 433: and
1.80 djm 434: .Xr tty 4
435: devices.
1.237 jmc 436: For file transfer sessions using SFTP
437: no additional configuration of the environment is necessary if the in-process
438: sftp-server is used,
1.176 schwarze 439: though sessions which use logging may require
1.104 stevesk 440: .Pa /dev/log
1.176 schwarze 441: inside the chroot directory on some operating systems (see
1.104 stevesk 442: .Xr sftp-server 8
1.81 jmc 443: for details).
1.190 deraadt 444: .Pp
1.191 jmc 445: For safety, it is very important that the directory hierarchy be
1.190 deraadt 446: prevented from modification by other processes on the system (especially
447: those outside the jail).
448: Misconfiguration can lead to unsafe environments which
449: .Xr sshd 8
450: cannot detect.
1.80 djm 451: .Pp
1.215 djm 452: The default is
1.237 jmc 453: .Cm none ,
1.215 djm 454: indicating not to
1.80 djm 455: .Xr chroot 2 .
1.1 stevesk 456: .It Cm Ciphers
1.219 jmc 457: Specifies the ciphers allowed.
1.1 stevesk 458: Multiple ciphers must be comma-separated.
1.289 naddy 459: If the specified list begins with a
1.208 djm 460: .Sq +
461: character, then the specified ciphers will be appended to the default set
462: instead of replacing them.
1.289 naddy 463: If the specified list begins with a
1.242 djm 464: .Sq -
465: character, then the specified ciphers (including wildcards) will be removed
466: from the default set instead of replacing them.
1.290 naddy 467: If the specified list begins with a
468: .Sq ^
469: character, then the specified ciphers will be placed at the head of the
470: default set.
1.208 djm 471: .Pp
1.167 djm 472: The supported ciphers are:
473: .Pp
1.173 naddy 474: .Bl -item -compact -offset indent
475: .It
476: 3des-cbc
477: .It
478: aes128-cbc
479: .It
480: aes192-cbc
481: .It
482: aes256-cbc
483: .It
484: aes128-ctr
485: .It
486: aes192-ctr
487: .It
488: aes256-ctr
489: .It
490: aes128-gcm@openssh.com
491: .It
492: aes256-gcm@openssh.com
493: .It
494: chacha20-poly1305@openssh.com
495: .El
1.167 djm 496: .Pp
1.52 jmc 497: The default is:
1.173 naddy 498: .Bd -literal -offset indent
1.211 jmc 499: chacha20-poly1305@openssh.com,
1.173 naddy 500: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 501: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 502: .Ed
1.167 djm 503: .Pp
1.237 jmc 504: The list of available ciphers may also be obtained using
505: .Qq ssh -Q cipher .
1.1 stevesk 506: .It Cm ClientAliveCountMax
1.237 jmc 507: Sets the number of client alive messages which may be sent without
1.52 jmc 508: .Xr sshd 8
1.17 jmc 509: receiving any messages back from the client.
510: If this threshold is reached while client alive messages are being sent,
1.52 jmc 511: sshd will disconnect the client, terminating the session.
1.17 jmc 512: It is important to note that the use of client alive messages is very
513: different from
1.237 jmc 514: .Cm TCPKeepAlive .
1.17 jmc 515: The client alive messages are sent through the encrypted channel
516: and therefore will not be spoofable.
517: The TCP keepalive option enabled by
1.27 markus 518: .Cm TCPKeepAlive
1.17 jmc 519: is spoofable.
520: The client alive mechanism is valuable when the client or
1.288 dtucker 521: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 522: .Pp
1.17 jmc 523: The default value is 3.
524: If
1.1 stevesk 525: .Cm ClientAliveInterval
1.237 jmc 526: is set to 15, and
1.1 stevesk 527: .Cm ClientAliveCountMax
1.52 jmc 528: is left at the default, unresponsive SSH clients
1.1 stevesk 529: will be disconnected after approximately 45 seconds.
1.42 djm 530: .It Cm ClientAliveInterval
531: Sets a timeout interval in seconds after which if no data has been received
532: from the client,
1.52 jmc 533: .Xr sshd 8
1.42 djm 534: will send a message through the encrypted
535: channel to request a response from the client.
536: The default
537: is 0, indicating that these messages will not be sent to the client.
1.3 markus 538: .It Cm Compression
1.236 djm 539: Specifies whether compression is enabled after
1.44 markus 540: the user has authenticated successfully.
1.3 markus 541: The argument must be
1.237 jmc 542: .Cm yes ,
543: .Cm delayed
1.236 djm 544: (a legacy synonym for
1.237 jmc 545: .Cm yes )
1.3 markus 546: or
1.237 jmc 547: .Cm no .
1.3 markus 548: The default is
1.237 jmc 549: .Cm yes .
1.1 stevesk 550: .It Cm DenyGroups
551: This keyword can be followed by a list of group name patterns, separated
552: by spaces.
553: Login is disallowed for users whose primary group or supplementary
554: group list matches one of the patterns.
555: Only group names are valid; a numerical group ID is not recognized.
556: By default, login is allowed for all groups.
1.54 jmc 557: The allow/deny directives are processed in the following order:
558: .Cm DenyUsers ,
559: .Cm AllowUsers ,
560: .Cm DenyGroups ,
561: and finally
562: .Cm AllowGroups .
1.49 jmc 563: .Pp
1.161 jmc 564: See PATTERNS in
1.49 jmc 565: .Xr ssh_config 5
566: for more information on patterns.
1.1 stevesk 567: .It Cm DenyUsers
568: This keyword can be followed by a list of user name patterns, separated
569: by spaces.
570: Login is disallowed for user names that match one of the patterns.
571: Only user names are valid; a numerical user ID is not recognized.
572: By default, login is allowed for all users.
573: If the pattern takes the form USER@HOST then USER and HOST
574: are separately checked, restricting logins to particular
575: users from particular hosts.
1.222 jmc 576: HOST criteria may additionally contain addresses to match in CIDR
577: address/masklen format.
1.54 jmc 578: The allow/deny directives are processed in the following order:
579: .Cm DenyUsers ,
580: .Cm AllowUsers ,
581: .Cm DenyGroups ,
582: and finally
583: .Cm AllowGroups .
1.49 jmc 584: .Pp
1.161 jmc 585: See PATTERNS in
1.49 jmc 586: .Xr ssh_config 5
587: for more information on patterns.
1.239 djm 588: .It Cm DisableForwarding
589: Disables all forwarding features, including X11,
590: .Xr ssh-agent 1 ,
591: TCP and StreamLocal.
592: This option overrides all other forwarding-related options and may
593: simplify restricted configurations.
1.246 djm 594: .It Cm ExposeAuthInfo
1.253 jmc 595: Writes a temporary file containing a list of authentication methods and
1.246 djm 596: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 597: The location of the file is exposed to the user session through the
1.248 djm 598: .Ev SSH_USER_AUTH
1.247 jmc 599: environment variable.
1.253 jmc 600: The default is
601: .Cm no .
1.183 djm 602: .It Cm FingerprintHash
603: Specifies the hash algorithm used when logging key fingerprints.
604: Valid options are:
1.237 jmc 605: .Cm md5
1.183 djm 606: and
1.237 jmc 607: .Cm sha256 .
1.183 djm 608: The default is
1.237 jmc 609: .Cm sha256 .
1.67 dtucker 610: .It Cm ForceCommand
611: Forces the execution of the command specified by
612: .Cm ForceCommand ,
1.84 djm 613: ignoring any command supplied by the client and
614: .Pa ~/.ssh/rc
615: if present.
1.67 dtucker 616: The command is invoked by using the user's login shell with the -c option.
617: This applies to shell, command, or subsystem execution.
618: It is most useful inside a
619: .Cm Match
620: block.
621: The command originally supplied by the client is available in the
622: .Ev SSH_ORIGINAL_COMMAND
623: environment variable.
1.82 djm 624: Specifying a command of
1.237 jmc 625: .Cm internal-sftp
626: will force the use of an in-process SFTP server that requires no support
1.82 djm 627: files when used with
628: .Cm ChrootDirectory .
1.215 djm 629: The default is
1.237 jmc 630: .Cm none .
1.1 stevesk 631: .It Cm GatewayPorts
632: Specifies whether remote hosts are allowed to connect to ports
633: forwarded for the client.
634: By default,
1.52 jmc 635: .Xr sshd 8
1.15 jmc 636: binds remote port forwardings to the loopback address.
637: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 638: .Cm GatewayPorts
1.52 jmc 639: can be used to specify that sshd
1.39 djm 640: should allow remote port forwardings to bind to non-loopback addresses, thus
641: allowing other hosts to connect.
642: The argument may be
1.237 jmc 643: .Cm no
1.39 djm 644: to force remote port forwardings to be available to the local host only,
1.237 jmc 645: .Cm yes
1.39 djm 646: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 647: .Cm clientspecified
1.39 djm 648: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 649: The default is
1.237 jmc 650: .Cm no .
1.23 markus 651: .It Cm GSSAPIAuthentication
1.25 markus 652: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 653: The default is
1.237 jmc 654: .Cm no .
1.23 markus 655: .It Cm GSSAPICleanupCredentials
656: Specifies whether to automatically destroy the user's credentials cache
657: on logout.
658: The default is
1.237 jmc 659: .Cm yes .
1.203 djm 660: .It Cm GSSAPIStrictAcceptorCheck
661: Determines whether to be strict about the identity of the GSSAPI acceptor
662: a client authenticates against.
663: If set to
1.237 jmc 664: .Cm yes
665: then the client must authenticate against the host
1.203 djm 666: service on the current hostname.
667: If set to
1.237 jmc 668: .Cm no
1.203 djm 669: then the client may authenticate against any service key stored in the
670: machine's default store.
671: This facility is provided to assist with operation on multi homed machines.
672: The default is
1.237 jmc 673: .Cm yes .
1.189 djm 674: .It Cm HostbasedAcceptedKeyTypes
675: Specifies the key types that will be accepted for hostbased authentication
1.280 djm 676: as a list of comma-separated patterns.
1.289 naddy 677: Alternately if the specified list begins with a
1.208 djm 678: .Sq +
679: character, then the specified key types will be appended to the default set
680: instead of replacing them.
1.289 naddy 681: If the specified list begins with a
1.242 djm 682: .Sq -
683: character, then the specified key types (including wildcards) will be removed
684: from the default set instead of replacing them.
1.290 naddy 685: If the specified list begins with a
686: .Sq ^
687: character, then the specified key types will be placed at the head of the
688: default set.
1.206 markus 689: The default for this option is:
690: .Bd -literal -offset 3n
691: ecdsa-sha2-nistp256-cert-v01@openssh.com,
692: ecdsa-sha2-nistp384-cert-v01@openssh.com,
693: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 694: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 695: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 696: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 697: rsa-sha2-512-cert-v01@openssh.com,
698: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 699: ssh-rsa-cert-v01@openssh.com,
700: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 701: sk-ecdsa-sha2-nistp256@openssh.com,
702: ssh-ed25519,sk-ssh-ed25519@openssh.com,
703: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 704: .Ed
705: .Pp
1.237 jmc 706: The list of available key types may also be obtained using
707: .Qq ssh -Q key .
1.1 stevesk 708: .It Cm HostbasedAuthentication
709: Specifies whether rhosts or /etc/hosts.equiv authentication together
710: with successful public key client host authentication is allowed
1.50 jmc 711: (host-based authentication).
1.70 dtucker 712: The default is
1.237 jmc 713: .Cm no .
1.70 dtucker 714: .It Cm HostbasedUsesNameFromPacketOnly
715: Specifies whether or not the server will attempt to perform a reverse
716: name lookup when matching the name in the
717: .Pa ~/.shosts ,
718: .Pa ~/.rhosts ,
719: and
720: .Pa /etc/hosts.equiv
721: files during
722: .Cm HostbasedAuthentication .
723: A setting of
1.237 jmc 724: .Cm yes
1.70 dtucker 725: means that
726: .Xr sshd 8
727: uses the name supplied by the client rather than
728: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 729: The default is
1.237 jmc 730: .Cm no .
1.117 djm 731: .It Cm HostCertificate
732: Specifies a file containing a public host certificate.
733: The certificate's public key must match a private host key already specified
734: by
735: .Cm HostKey .
736: The default behaviour of
737: .Xr sshd 8
738: is not to load any certificates.
1.1 stevesk 739: .It Cm HostKey
740: Specifies a file containing a private host key
741: used by SSH.
1.229 naddy 742: The defaults are
1.169 naddy 743: .Pa /etc/ssh/ssh_host_ecdsa_key ,
744: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 745: and
1.229 naddy 746: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 747: .Pp
1.1 stevesk 748: Note that
1.52 jmc 749: .Xr sshd 8
1.206 markus 750: will refuse to use a file if it is group/world-accessible
751: and that the
752: .Cm HostKeyAlgorithms
753: option restricts which of the keys are actually used by
754: .Xr sshd 8 .
755: .Pp
1.1 stevesk 756: It is possible to have multiple host key files.
1.162 markus 757: It is also possible to specify public host key files instead.
758: In this case operations on the private key will be delegated
759: to an
760: .Xr ssh-agent 1 .
761: .It Cm HostKeyAgent
762: Identifies the UNIX-domain socket used to communicate
763: with an agent that has access to the private host keys.
1.223 markus 764: If the string
1.237 jmc 765: .Qq SSH_AUTH_SOCK
1.162 markus 766: is specified, the location of the socket will be read from the
767: .Ev SSH_AUTH_SOCK
768: environment variable.
1.206 markus 769: .It Cm HostKeyAlgorithms
1.219 jmc 770: Specifies the host key algorithms
1.206 markus 771: that the server offers.
772: The default for this option is:
773: .Bd -literal -offset 3n
774: ecdsa-sha2-nistp256-cert-v01@openssh.com,
775: ecdsa-sha2-nistp384-cert-v01@openssh.com,
776: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 777: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 778: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 779: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 780: rsa-sha2-512-cert-v01@openssh.com,
781: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 782: ssh-rsa-cert-v01@openssh.com,
783: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 784: sk-ecdsa-sha2-nistp256@openssh.com,
785: ssh-ed25519,sk-ssh-ed25519@openssh.com,
786: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 787: .Ed
788: .Pp
1.237 jmc 789: The list of available key types may also be obtained using
790: .Qq ssh -Q key .
1.1 stevesk 791: .It Cm IgnoreRhosts
792: Specifies that
793: .Pa .rhosts
794: and
795: .Pa .shosts
796: files will not be used in
797: .Cm HostbasedAuthentication .
798: .Pp
799: .Pa /etc/hosts.equiv
800: and
801: .Pa /etc/shosts.equiv
802: are still used.
803: The default is
1.237 jmc 804: .Cm yes .
1.1 stevesk 805: .It Cm IgnoreUserKnownHosts
806: Specifies whether
1.52 jmc 807: .Xr sshd 8
1.1 stevesk 808: should ignore the user's
1.41 djm 809: .Pa ~/.ssh/known_hosts
1.1 stevesk 810: during
1.261 djm 811: .Cm HostbasedAuthentication
812: and use only the system-wide known hosts file
813: .Pa /etc/ssh/known_hosts .
1.1 stevesk 814: The default is
1.237 jmc 815: .Cm no .
1.129 djm 816: .It Cm IPQoS
817: Specifies the IPv4 type-of-service or DSCP class for the connection.
818: Accepted values are
1.237 jmc 819: .Cm af11 ,
820: .Cm af12 ,
821: .Cm af13 ,
822: .Cm af21 ,
823: .Cm af22 ,
824: .Cm af23 ,
825: .Cm af31 ,
826: .Cm af32 ,
827: .Cm af33 ,
828: .Cm af41 ,
829: .Cm af42 ,
830: .Cm af43 ,
831: .Cm cs0 ,
832: .Cm cs1 ,
833: .Cm cs2 ,
834: .Cm cs3 ,
835: .Cm cs4 ,
836: .Cm cs5 ,
837: .Cm cs6 ,
838: .Cm cs7 ,
839: .Cm ef ,
840: .Cm lowdelay ,
841: .Cm throughput ,
842: .Cm reliability ,
1.250 djm 843: a numeric value, or
844: .Cm none
845: to use the operating system default.
1.131 djm 846: This option may take one or two arguments, separated by whitespace.
1.129 djm 847: If one argument is specified, it is used as the packet class unconditionally.
848: If two values are specified, the first is automatically selected for
849: interactive sessions and the second for non-interactive sessions.
850: The default is
1.264 job 851: .Cm af21
1.265 jmc 852: (Low-Latency Data)
1.129 djm 853: for interactive sessions and
1.264 job 854: .Cm cs1
1.265 jmc 855: (Lower Effort)
1.129 djm 856: for non-interactive sessions.
1.171 jmc 857: .It Cm KbdInteractiveAuthentication
858: Specifies whether to allow keyboard-interactive authentication.
859: The argument to this keyword must be
1.237 jmc 860: .Cm yes
1.171 jmc 861: or
1.237 jmc 862: .Cm no .
1.171 jmc 863: The default is to use whatever value
864: .Cm ChallengeResponseAuthentication
865: is set to
866: (by default
1.237 jmc 867: .Cm yes ) .
1.1 stevesk 868: .It Cm KerberosAuthentication
1.24 markus 869: Specifies whether the password provided by the user for
1.1 stevesk 870: .Cm PasswordAuthentication
1.24 markus 871: will be validated through the Kerberos KDC.
1.1 stevesk 872: To use this option, the server needs a
873: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 874: The default is
1.237 jmc 875: .Cm no .
1.29 dtucker 876: .It Cm KerberosGetAFSToken
1.45 djm 877: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 878: an AFS token before accessing the user's home directory.
1.52 jmc 879: The default is
1.237 jmc 880: .Cm no .
1.1 stevesk 881: .It Cm KerberosOrLocalPasswd
1.52 jmc 882: If password authentication through Kerberos fails then
1.1 stevesk 883: the password will be validated via any additional local mechanism
884: such as
885: .Pa /etc/passwd .
1.52 jmc 886: The default is
1.237 jmc 887: .Cm yes .
1.1 stevesk 888: .It Cm KerberosTicketCleanup
889: Specifies whether to automatically destroy the user's ticket cache
890: file on logout.
1.52 jmc 891: The default is
1.237 jmc 892: .Cm yes .
1.127 djm 893: .It Cm KexAlgorithms
894: Specifies the available KEX (Key Exchange) algorithms.
895: Multiple algorithms must be comma-separated.
1.289 naddy 896: Alternately if the specified list begins with a
1.208 djm 897: .Sq +
898: character, then the specified methods will be appended to the default set
899: instead of replacing them.
1.289 naddy 900: If the specified list begins with a
1.242 djm 901: .Sq -
902: character, then the specified methods (including wildcards) will be removed
903: from the default set instead of replacing them.
1.290 naddy 904: If the specified list begins with a
905: .Sq ^
906: character, then the specified methods will be placed at the head of the
907: default set.
1.173 naddy 908: The supported algorithms are:
909: .Pp
910: .Bl -item -compact -offset indent
911: .It
1.234 djm 912: curve25519-sha256
913: .It
1.173 naddy 914: curve25519-sha256@libssh.org
915: .It
916: diffie-hellman-group1-sha1
917: .It
918: diffie-hellman-group14-sha1
919: .It
1.263 djm 920: diffie-hellman-group14-sha256
921: .It
922: diffie-hellman-group16-sha512
923: .It
924: diffie-hellman-group18-sha512
925: .It
1.173 naddy 926: diffie-hellman-group-exchange-sha1
927: .It
928: diffie-hellman-group-exchange-sha256
929: .It
930: ecdh-sha2-nistp256
931: .It
932: ecdh-sha2-nistp384
933: .It
934: ecdh-sha2-nistp521
935: .El
936: .Pp
937: The default is:
1.170 dtucker 938: .Bd -literal -offset indent
1.234 djm 939: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 940: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
941: diffie-hellman-group-exchange-sha256,
1.263 djm 942: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
943: diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
1.170 dtucker 944: .Ed
1.188 djm 945: .Pp
1.237 jmc 946: The list of available key exchange algorithms may also be obtained using
947: .Qq ssh -Q kex .
1.1 stevesk 948: .It Cm ListenAddress
949: Specifies the local addresses
1.52 jmc 950: .Xr sshd 8
1.1 stevesk 951: should listen on.
952: The following forms may be used:
953: .Pp
954: .Bl -item -offset indent -compact
955: .It
956: .Cm ListenAddress
957: .Sm off
1.259 jmc 958: .Ar hostname | address
1.1 stevesk 959: .Sm on
1.260 jmc 960: .Op Cm rdomain Ar domain
1.1 stevesk 961: .It
962: .Cm ListenAddress
963: .Sm off
1.259 jmc 964: .Ar hostname : port
1.1 stevesk 965: .Sm on
1.260 jmc 966: .Op Cm rdomain Ar domain
1.256 djm 967: .It
968: .Cm ListenAddress
969: .Sm off
1.259 jmc 970: .Ar IPv4_address : port
1.256 djm 971: .Sm on
1.260 jmc 972: .Op Cm rdomain Ar domain
1.1 stevesk 973: .It
974: .Cm ListenAddress
975: .Sm off
1.259 jmc 976: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 977: .Sm on
1.260 jmc 978: .Op Cm rdomain Ar domain
1.1 stevesk 979: .El
980: .Pp
1.256 djm 981: The optional
982: .Cm rdomain
983: qualifier requests
984: .Xr sshd 8
985: listen in an explicit routing domain.
1.1 stevesk 986: If
987: .Ar port
988: is not specified,
1.200 dtucker 989: sshd will listen on the address and all
1.1 stevesk 990: .Cm Port
1.17 jmc 991: options specified.
1.256 djm 992: The default is to listen on all local addresses on the current default
993: routing domain.
1.15 jmc 994: Multiple
1.1 stevesk 995: .Cm ListenAddress
1.17 jmc 996: options are permitted.
1.256 djm 997: For more information on routing domains, see
1.259 jmc 998: .Xr rdomain 4 .
1.1 stevesk 999: .It Cm LoginGraceTime
1000: The server disconnects after this time if the user has not
1001: successfully logged in.
1002: If the value is 0, there is no time limit.
1.12 stevesk 1003: The default is 120 seconds.
1.1 stevesk 1004: .It Cm LogLevel
1005: Gives the verbosity level that is used when logging messages from
1.53 jmc 1006: .Xr sshd 8 .
1.1 stevesk 1007: The possible values are:
1.52 jmc 1008: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1009: The default is INFO.
1010: DEBUG and DEBUG1 are equivalent.
1011: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1012: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1013: .It Cm MACs
1014: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1015: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1016: Multiple algorithms must be comma-separated.
1.289 naddy 1017: If the specified list begins with a
1.208 djm 1018: .Sq +
1019: character, then the specified algorithms will be appended to the default set
1020: instead of replacing them.
1.289 naddy 1021: If the specified list begins with a
1.242 djm 1022: .Sq -
1023: character, then the specified algorithms (including wildcards) will be removed
1024: from the default set instead of replacing them.
1.290 naddy 1025: If the specified list begins with a
1026: .Sq ^
1027: character, then the specified algorithms will be placed at the head of the
1028: default set.
1.208 djm 1029: .Pp
1.152 markus 1030: The algorithms that contain
1.237 jmc 1031: .Qq -etm
1.152 markus 1032: calculate the MAC after encryption (encrypt-then-mac).
1033: These are considered safer and their use recommended.
1.173 naddy 1034: The supported MACs are:
1035: .Pp
1036: .Bl -item -compact -offset indent
1037: .It
1038: hmac-md5
1039: .It
1040: hmac-md5-96
1041: .It
1042: hmac-sha1
1043: .It
1044: hmac-sha1-96
1045: .It
1046: hmac-sha2-256
1047: .It
1048: hmac-sha2-512
1049: .It
1050: umac-64@openssh.com
1051: .It
1052: umac-128@openssh.com
1053: .It
1054: hmac-md5-etm@openssh.com
1055: .It
1056: hmac-md5-96-etm@openssh.com
1057: .It
1058: hmac-sha1-etm@openssh.com
1059: .It
1060: hmac-sha1-96-etm@openssh.com
1061: .It
1062: hmac-sha2-256-etm@openssh.com
1063: .It
1064: hmac-sha2-512-etm@openssh.com
1065: .It
1066: umac-64-etm@openssh.com
1067: .It
1068: umac-128-etm@openssh.com
1069: .El
1070: .Pp
1.52 jmc 1071: The default is:
1.77 jmc 1072: .Bd -literal -offset indent
1.152 markus 1073: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1074: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1075: hmac-sha1-etm@openssh.com,
1.173 naddy 1076: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1077: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1078: .Ed
1.188 djm 1079: .Pp
1.237 jmc 1080: The list of available MAC algorithms may also be obtained using
1081: .Qq ssh -Q mac .
1.60 dtucker 1082: .It Cm Match
1.61 jmc 1083: Introduces a conditional block.
1.65 dtucker 1084: If all of the criteria on the
1.60 dtucker 1085: .Cm Match
1.65 dtucker 1086: line are satisfied, the keywords on the following lines override those
1087: set in the global section of the config file, until either another
1.60 dtucker 1088: .Cm Match
1.65 dtucker 1089: line or the end of the file.
1.172 djm 1090: If a keyword appears in multiple
1091: .Cm Match
1.177 sobrado 1092: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1093: applied.
1.91 djm 1094: .Pp
1.61 jmc 1095: The arguments to
1.60 dtucker 1096: .Cm Match
1.163 dtucker 1097: are one or more criteria-pattern pairs or the single token
1098: .Cm All
1099: which matches all criteria.
1.60 dtucker 1100: The available criteria are
1101: .Cm User ,
1.69 dtucker 1102: .Cm Group ,
1.60 dtucker 1103: .Cm Host ,
1.139 dtucker 1104: .Cm LocalAddress ,
1105: .Cm LocalPort ,
1.258 djm 1106: .Cm RDomain ,
1.60 dtucker 1107: and
1.258 djm 1108: .Cm Address
1109: (with
1110: .Cm RDomain
1111: representing the
1112: .Xr rdomain 4
1.284 jmc 1113: on which the connection was received).
1.258 djm 1114: .Pp
1.91 djm 1115: The match patterns may consist of single entries or comma-separated
1116: lists and may use the wildcard and negation operators described in the
1.237 jmc 1117: .Sx PATTERNS
1118: section of
1.92 djm 1119: .Xr ssh_config 5 .
1.91 djm 1120: .Pp
1121: The patterns in an
1122: .Cm Address
1123: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1124: address/masklen format,
1125: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1126: Note that the mask length provided must be consistent with the address -
1127: it is an error to specify a mask length that is too long for the address
1.93 jmc 1128: or one with bits set in this host portion of the address.
1.237 jmc 1129: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1130: .Pp
1.60 dtucker 1131: Only a subset of keywords may be used on the lines following a
1132: .Cm Match
1133: keyword.
1134: Available keywords are
1.142 jmc 1135: .Cm AcceptEnv ,
1.99 okan 1136: .Cm AllowAgentForwarding ,
1.142 jmc 1137: .Cm AllowGroups ,
1.193 djm 1138: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1139: .Cm AllowTcpForwarding ,
1.141 markus 1140: .Cm AllowUsers ,
1.149 djm 1141: .Cm AuthenticationMethods ,
1.146 djm 1142: .Cm AuthorizedKeysCommand ,
1143: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1144: .Cm AuthorizedKeysFile ,
1.214 djm 1145: .Cm AuthorizedPrincipalsCommand ,
1146: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1147: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1148: .Cm Banner ,
1.85 djm 1149: .Cm ChrootDirectory ,
1.238 markus 1150: .Cm ClientAliveCountMax ,
1151: .Cm ClientAliveInterval ,
1.141 markus 1152: .Cm DenyGroups ,
1153: .Cm DenyUsers ,
1.67 dtucker 1154: .Cm ForceCommand ,
1.194 djm 1155: .Cm GatewayPorts ,
1.193 djm 1156: .Cm GSSAPIAuthentication ,
1.189 djm 1157: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1158: .Cm HostbasedAuthentication ,
1.123 djm 1159: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1160: .Cm IPQoS ,
1.74 jmc 1161: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1162: .Cm KerberosAuthentication ,
1.245 djm 1163: .Cm LogLevel ,
1.95 dtucker 1164: .Cm MaxAuthTries ,
1.94 dtucker 1165: .Cm MaxSessions ,
1.72 dtucker 1166: .Cm PasswordAuthentication ,
1.97 djm 1167: .Cm PermitEmptyPasswords ,
1.271 djm 1168: .Cm PermitListen ,
1.66 dtucker 1169: .Cm PermitOpen ,
1.79 dtucker 1170: .Cm PermitRootLogin ,
1.164 djm 1171: .Cm PermitTTY ,
1.123 djm 1172: .Cm PermitTunnel ,
1.174 djm 1173: .Cm PermitUserRC ,
1.189 djm 1174: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1175: .Cm PubkeyAuthentication ,
1.159 dtucker 1176: .Cm RekeyLimit ,
1.193 djm 1177: .Cm RevokedKeys ,
1.257 djm 1178: .Cm RDomain ,
1.274 djm 1179: .Cm SetEnv ,
1.193 djm 1180: .Cm StreamLocalBindMask ,
1181: .Cm StreamLocalBindUnlink ,
1182: .Cm TrustedUserCAKeys ,
1.66 dtucker 1183: .Cm X11DisplayOffset ,
1.101 djm 1184: .Cm X11Forwarding
1.60 dtucker 1185: and
1.286 jmc 1186: .Cm X11UseLocalhost .
1.33 dtucker 1187: .It Cm MaxAuthTries
1188: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1189: connection.
1190: Once the number of failures reaches half this value,
1191: additional failures are logged.
1192: The default is 6.
1.90 djm 1193: .It Cm MaxSessions
1.216 djm 1194: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1195: sessions permitted per network connection.
1196: Multiple sessions may be established by clients that support connection
1197: multiplexing.
1198: Setting
1199: .Cm MaxSessions
1200: to 1 will effectively disable session multiplexing, whereas setting it to 0
1201: will prevent all shell, login and subsystem sessions while still permitting
1202: forwarding.
1.90 djm 1203: The default is 10.
1.1 stevesk 1204: .It Cm MaxStartups
1205: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1206: SSH daemon.
1.1 stevesk 1207: Additional connections will be dropped until authentication succeeds or the
1208: .Cm LoginGraceTime
1209: expires for a connection.
1.156 dtucker 1210: The default is 10:30:100.
1.1 stevesk 1211: .Pp
1212: Alternatively, random early drop can be enabled by specifying
1213: the three colon separated values
1.237 jmc 1214: start:rate:full (e.g. "10:30:60").
1.53 jmc 1215: .Xr sshd 8
1.237 jmc 1216: will refuse connection attempts with a probability of rate/100 (30%)
1217: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1218: The probability increases linearly and all connection attempts
1.237 jmc 1219: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1220: .It Cm PasswordAuthentication
1221: Specifies whether password authentication is allowed.
1222: The default is
1.237 jmc 1223: .Cm yes .
1.1 stevesk 1224: .It Cm PermitEmptyPasswords
1225: When password authentication is allowed, it specifies whether the
1226: server allows login to accounts with empty password strings.
1227: The default is
1.237 jmc 1228: .Cm no .
1.271 djm 1229: .It Cm PermitListen
1230: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1231: The listen specification must be one of the following forms:
1232: .Pp
1233: .Bl -item -offset indent -compact
1234: .It
1235: .Cm PermitListen
1236: .Sm off
1.276 djm 1237: .Ar port
1.271 djm 1238: .Sm on
1239: .It
1240: .Cm PermitListen
1241: .Sm off
1.276 djm 1242: .Ar host : port
1.271 djm 1243: .Sm on
1244: .El
1245: .Pp
1246: Multiple permissions may be specified by separating them with whitespace.
1247: An argument of
1248: .Cm any
1249: can be used to remove all restrictions and permit any listen requests.
1250: An argument of
1251: .Cm none
1252: can be used to prohibit all listen requests.
1253: The host name may contain wildcards as described in the PATTERNS section in
1254: .Xr ssh_config 5 .
1255: The wildcard
1256: .Sq *
1257: can also be used in place of a port number to allow all ports.
1258: By default all port forwarding listen requests are permitted.
1.272 jmc 1259: Note that the
1.271 djm 1260: .Cm GatewayPorts
1261: option may further restrict which addresses may be listened on.
1.276 djm 1262: Note also that
1263: .Xr ssh 1
1264: will request a listen host of
1265: .Dq localhost
1.283 dtucker 1266: if no listen host was specifically requested, and this name is
1.277 jmc 1267: treated differently to explicit localhost addresses of
1.276 djm 1268: .Dq 127.0.0.1
1269: and
1270: .Dq ::1 .
1.62 dtucker 1271: .It Cm PermitOpen
1272: Specifies the destinations to which TCP port forwarding is permitted.
1273: The forwarding specification must be one of the following forms:
1274: .Pp
1275: .Bl -item -offset indent -compact
1276: .It
1277: .Cm PermitOpen
1278: .Sm off
1279: .Ar host : port
1280: .Sm on
1281: .It
1282: .Cm PermitOpen
1283: .Sm off
1284: .Ar IPv4_addr : port
1285: .Sm on
1286: .It
1287: .Cm PermitOpen
1288: .Sm off
1289: .Ar \&[ IPv6_addr \&] : port
1290: .Sm on
1291: .El
1292: .Pp
1.68 dtucker 1293: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1294: An argument of
1.237 jmc 1295: .Cm any
1.62 dtucker 1296: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1297: An argument of
1.237 jmc 1298: .Cm none
1.140 dtucker 1299: can be used to prohibit all forwarding requests.
1.227 jmc 1300: The wildcard
1.237 jmc 1301: .Sq *
1.227 jmc 1302: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1303: By default all port forwarding requests are permitted.
1.1 stevesk 1304: .It Cm PermitRootLogin
1.38 jmc 1305: Specifies whether root can log in using
1.1 stevesk 1306: .Xr ssh 1 .
1307: The argument must be
1.237 jmc 1308: .Cm yes ,
1309: .Cm prohibit-password ,
1310: .Cm forced-commands-only ,
1.1 stevesk 1311: or
1.237 jmc 1312: .Cm no .
1.1 stevesk 1313: The default is
1.237 jmc 1314: .Cm prohibit-password .
1.1 stevesk 1315: .Pp
1316: If this option is set to
1.237 jmc 1317: .Cm prohibit-password
1.255 jmc 1318: (or its deprecated alias,
1319: .Cm without-password ) ,
1.210 deraadt 1320: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1321: .Pp
1322: If this option is set to
1.237 jmc 1323: .Cm forced-commands-only ,
1.1 stevesk 1324: root login with public key authentication will be allowed,
1325: but only if the
1326: .Ar command
1327: option has been specified
1328: (which may be useful for taking remote backups even if root login is
1.17 jmc 1329: normally not allowed).
1330: All other authentication methods are disabled for root.
1.1 stevesk 1331: .Pp
1332: If this option is set to
1.237 jmc 1333: .Cm no ,
1.38 jmc 1334: root is not allowed to log in.
1.231 jmc 1335: .It Cm PermitTTY
1336: Specifies whether
1337: .Xr pty 4
1338: allocation is permitted.
1339: The default is
1.237 jmc 1340: .Cm yes .
1.46 reyk 1341: .It Cm PermitTunnel
1342: Specifies whether
1343: .Xr tun 4
1344: device forwarding is allowed.
1.47 reyk 1345: The argument must be
1.237 jmc 1346: .Cm yes ,
1347: .Cm point-to-point
1.58 stevesk 1348: (layer 3),
1.237 jmc 1349: .Cm ethernet
1.58 stevesk 1350: (layer 2), or
1.237 jmc 1351: .Cm no .
1.58 stevesk 1352: Specifying
1.237 jmc 1353: .Cm yes
1.58 stevesk 1354: permits both
1.237 jmc 1355: .Cm point-to-point
1.58 stevesk 1356: and
1.237 jmc 1357: .Cm ethernet .
1.46 reyk 1358: The default is
1.237 jmc 1359: .Cm no .
1.178 djm 1360: .Pp
1361: Independent of this setting, the permissions of the selected
1362: .Xr tun 4
1363: device must allow access to the user.
1.6 markus 1364: .It Cm PermitUserEnvironment
1365: Specifies whether
1366: .Pa ~/.ssh/environment
1.9 stevesk 1367: and
1.6 markus 1368: .Cm environment=
1369: options in
1370: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1371: are processed by
1.52 jmc 1372: .Xr sshd 8 .
1.278 djm 1373: Valid options are
1374: .Cm yes ,
1375: .Cm no
1376: or a pattern-list specifying which environment variable names to accept
1377: (for example
1378: .Qq LANG,LC_* ) .
1.6 markus 1379: The default is
1.237 jmc 1380: .Cm no .
1.9 stevesk 1381: Enabling environment processing may enable users to bypass access
1382: restrictions in some configurations using mechanisms such as
1383: .Ev LD_PRELOAD .
1.174 djm 1384: .It Cm PermitUserRC
1385: Specifies whether any
1386: .Pa ~/.ssh/rc
1387: file is executed.
1388: The default is
1.237 jmc 1389: .Cm yes .
1.1 stevesk 1390: .It Cm PidFile
1.4 stevesk 1391: Specifies the file that contains the process ID of the
1.195 dtucker 1392: SSH daemon, or
1.237 jmc 1393: .Cm none
1.195 dtucker 1394: to not write one.
1.1 stevesk 1395: The default is
1396: .Pa /var/run/sshd.pid .
1397: .It Cm Port
1398: Specifies the port number that
1.52 jmc 1399: .Xr sshd 8
1.1 stevesk 1400: listens on.
1401: The default is 22.
1402: Multiple options of this type are permitted.
1403: See also
1404: .Cm ListenAddress .
1405: .It Cm PrintLastLog
1406: Specifies whether
1.52 jmc 1407: .Xr sshd 8
1.36 jaredy 1408: should print the date and time of the last user login when a user logs
1409: in interactively.
1.1 stevesk 1410: The default is
1.237 jmc 1411: .Cm yes .
1.1 stevesk 1412: .It Cm PrintMotd
1413: Specifies whether
1.52 jmc 1414: .Xr sshd 8
1.1 stevesk 1415: should print
1416: .Pa /etc/motd
1417: when a user logs in interactively.
1418: (On some systems it is also printed by the shell,
1419: .Pa /etc/profile ,
1420: or equivalent.)
1421: The default is
1.237 jmc 1422: .Cm yes .
1.189 djm 1423: .It Cm PubkeyAcceptedKeyTypes
1424: Specifies the key types that will be accepted for public key authentication
1.280 djm 1425: as a list of comma-separated patterns.
1.289 naddy 1426: Alternately if the specified list begins with a
1.208 djm 1427: .Sq +
1428: character, then the specified key types will be appended to the default set
1429: instead of replacing them.
1.289 naddy 1430: If the specified list begins with a
1.242 djm 1431: .Sq -
1432: character, then the specified key types (including wildcards) will be removed
1433: from the default set instead of replacing them.
1.290 naddy 1434: If the specified list begins with a
1435: .Sq ^
1436: character, then the specified key types will be placed at the head of the
1437: default set.
1.206 markus 1438: The default for this option is:
1439: .Bd -literal -offset 3n
1440: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1441: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1442: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1443: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1444: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1445: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1446: rsa-sha2-512-cert-v01@openssh.com,
1447: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1448: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1449: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1450: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1451: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1452: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1453: .Ed
1454: .Pp
1.237 jmc 1455: The list of available key types may also be obtained using
1456: .Qq ssh -Q key .
1.293 djm 1457: .It Cm PubkeyAuthOptions
1458: Sets one or more public key authentication options.
1459: Two option keywords are currently supported:
1.294 jmc 1460: .Cm none
1461: (the default; indicating no additional options are enabled)
1.293 djm 1462: and
1463: .Cm touch-required .
1464: .Pp
1465: The
1466: .Cm touch-required
1.297 ! naddy 1467: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1468: (i.e.\&
1.293 djm 1469: .Cm ecdsa-sk
1470: or
1471: .Cm ed25519-sk )
1472: to always require the signature to attest that a physically present user
1.297 ! naddy 1473: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1474: By default,
1475: .Xr sshd 8
1.297 ! naddy 1476: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1477: The
1478: .Cm touch-required
1479: flag disables this override.
1.297 ! naddy 1480: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1481: .It Cm PubkeyAuthentication
1482: Specifies whether public key authentication is allowed.
1483: The default is
1.237 jmc 1484: .Cm yes .
1.159 dtucker 1485: .It Cm RekeyLimit
1486: Specifies the maximum amount of data that may be transmitted before the
1487: session key is renegotiated, optionally followed a maximum amount of
1488: time that may pass before the session key is renegotiated.
1489: The first argument is specified in bytes and may have a suffix of
1490: .Sq K ,
1491: .Sq M ,
1492: or
1493: .Sq G
1494: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1495: The default is between
1496: .Sq 1G
1497: and
1498: .Sq 4G ,
1499: depending on the cipher.
1500: The optional second value is specified in seconds and may use any of the
1501: units documented in the
1502: .Sx TIME FORMATS
1.160 jmc 1503: section.
1.159 dtucker 1504: The default value for
1505: .Cm RekeyLimit
1506: is
1.237 jmc 1507: .Cm default none ,
1.159 dtucker 1508: which means that rekeying is performed after the cipher's default amount
1509: of data has been sent or received and no time based rekeying is done.
1.118 djm 1510: .It Cm RevokedKeys
1.195 dtucker 1511: Specifies revoked public keys file, or
1.237 jmc 1512: .Cm none
1.195 dtucker 1513: to not use one.
1.118 djm 1514: Keys listed in this file will be refused for public key authentication.
1515: Note that if this file is not readable, then public key authentication will
1516: be refused for all users.
1.154 djm 1517: Keys may be specified as a text file, listing one public key per line, or as
1518: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1519: .Xr ssh-keygen 1 .
1.161 jmc 1520: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1521: .Xr ssh-keygen 1 .
1.257 djm 1522: .It Cm RDomain
1523: Specifies an explicit routing domain that is applied after authentication
1524: has completed.
1.259 jmc 1525: The user session, as well and any forwarded or listening IP sockets,
1526: will be bound to this
1.257 djm 1527: .Xr rdomain 4 .
1528: If the routing domain is set to
1529: .Cm \&%D ,
1.259 jmc 1530: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1531: .It Cm SecurityKeyProvider
1.297 ! naddy 1532: Specifies a path to a library that will be used when loading
! 1533: FIDO authenticator-hosted keys, overriding the default of using
! 1534: the built-in USB HID support.
1.274 djm 1535: .It Cm SetEnv
1536: Specifies one or more environment variables to set in child sessions started
1537: by
1538: .Xr sshd 8
1539: as
1540: .Dq NAME=VALUE .
1541: The environment value may be quoted (e.g. if it contains whitespace
1542: characters).
1543: Environment variables set by
1544: .Cm SetEnv
1545: override the default environment and any variables specified by the user
1546: via
1547: .Cm AcceptEnv
1548: or
1549: .Cm PermitUserEnvironment .
1.175 millert 1550: .It Cm StreamLocalBindMask
1551: Sets the octal file creation mode mask
1552: .Pq umask
1553: used when creating a Unix-domain socket file for local or remote
1554: port forwarding.
1555: This option is only used for port forwarding to a Unix-domain socket file.
1556: .Pp
1557: The default value is 0177, which creates a Unix-domain socket file that is
1558: readable and writable only by the owner.
1559: Note that not all operating systems honor the file mode on Unix-domain
1560: socket files.
1561: .It Cm StreamLocalBindUnlink
1562: Specifies whether to remove an existing Unix-domain socket file for local
1563: or remote port forwarding before creating a new one.
1564: If the socket file already exists and
1565: .Cm StreamLocalBindUnlink
1566: is not enabled,
1567: .Nm sshd
1568: will be unable to forward the port to the Unix-domain socket file.
1569: This option is only used for port forwarding to a Unix-domain socket file.
1570: .Pp
1571: The argument must be
1.237 jmc 1572: .Cm yes
1.175 millert 1573: or
1.237 jmc 1574: .Cm no .
1.175 millert 1575: The default is
1.237 jmc 1576: .Cm no .
1.1 stevesk 1577: .It Cm StrictModes
1578: Specifies whether
1.52 jmc 1579: .Xr sshd 8
1.1 stevesk 1580: should check file modes and ownership of the
1581: user's files and home directory before accepting login.
1582: This is normally desirable because novices sometimes accidentally leave their
1583: directory or files world-writable.
1584: The default is
1.237 jmc 1585: .Cm yes .
1.112 djm 1586: Note that this does not apply to
1587: .Cm ChrootDirectory ,
1588: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1589: .It Cm Subsystem
1.51 jmc 1590: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1591: Arguments should be a subsystem name and a command (with optional arguments)
1592: to execute upon subsystem request.
1.80 djm 1593: .Pp
1.1 stevesk 1594: The command
1.237 jmc 1595: .Cm sftp-server
1596: implements the SFTP file transfer subsystem.
1.80 djm 1597: .Pp
1598: Alternately the name
1.237 jmc 1599: .Cm internal-sftp
1600: implements an in-process SFTP server.
1.80 djm 1601: This may simplify configurations using
1602: .Cm ChrootDirectory
1603: to force a different filesystem root on clients.
1604: .Pp
1.1 stevesk 1605: By default no subsystems are defined.
1606: .It Cm SyslogFacility
1607: Gives the facility code that is used when logging messages from
1.53 jmc 1608: .Xr sshd 8 .
1.1 stevesk 1609: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1610: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1611: The default is AUTH.
1.27 markus 1612: .It Cm TCPKeepAlive
1613: Specifies whether the system should send TCP keepalive messages to the
1614: other side.
1615: If they are sent, death of the connection or crash of one
1616: of the machines will be properly noticed.
1617: However, this means that
1618: connections will die if the route is down temporarily, and some people
1619: find it annoying.
1620: On the other hand, if TCP keepalives are not sent,
1621: sessions may hang indefinitely on the server, leaving
1.237 jmc 1622: .Qq ghost
1.27 markus 1623: users and consuming server resources.
1624: .Pp
1625: The default is
1.237 jmc 1626: .Cm yes
1.27 markus 1627: (to send TCP keepalive messages), and the server will notice
1628: if the network goes down or the client host crashes.
1629: This avoids infinitely hanging sessions.
1630: .Pp
1631: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1632: .Cm no .
1.118 djm 1633: .It Cm TrustedUserCAKeys
1634: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1635: trusted to sign user certificates for authentication, or
1.237 jmc 1636: .Cm none
1.195 dtucker 1637: to not use one.
1.119 jmc 1638: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1639: .Ql #
1640: are allowed.
1641: If a certificate is presented for authentication and has its signing CA key
1642: listed in this file, then it may be used for authentication for any user
1643: listed in the certificate's principals list.
1644: Note that certificates that lack a list of principals will not be permitted
1645: for authentication using
1646: .Cm TrustedUserCAKeys .
1.161 jmc 1647: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1648: .Xr ssh-keygen 1 .
1.18 markus 1649: .It Cm UseDNS
1650: Specifies whether
1.52 jmc 1651: .Xr sshd 8
1.207 djm 1652: should look up the remote host name, and to check that
1.18 markus 1653: the resolved host name for the remote IP address maps back to the
1654: very same IP address.
1.207 djm 1655: .Pp
1656: If this option is set to
1.237 jmc 1657: .Cm no
1.207 djm 1658: (the default) then only addresses and not host names may be used in
1.221 djm 1659: .Pa ~/.ssh/authorized_keys
1.207 djm 1660: .Cm from
1661: and
1.211 jmc 1662: .Nm
1.207 djm 1663: .Cm Match
1664: .Cm Host
1665: directives.
1.137 djm 1666: .It Cm VersionAddendum
1667: Optionally specifies additional text to append to the SSH protocol banner
1668: sent by the server upon connection.
1669: The default is
1.237 jmc 1670: .Cm none .
1.1 stevesk 1671: .It Cm X11DisplayOffset
1672: Specifies the first display number available for
1.52 jmc 1673: .Xr sshd 8 Ns 's
1.1 stevesk 1674: X11 forwarding.
1.52 jmc 1675: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1676: The default is 10.
1677: .It Cm X11Forwarding
1678: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1679: The argument must be
1.237 jmc 1680: .Cm yes
1.13 stevesk 1681: or
1.237 jmc 1682: .Cm no .
1.1 stevesk 1683: The default is
1.237 jmc 1684: .Cm no .
1.13 stevesk 1685: .Pp
1686: When X11 forwarding is enabled, there may be additional exposure to
1687: the server and to client displays if the
1.52 jmc 1688: .Xr sshd 8
1.13 stevesk 1689: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1690: .Cm X11UseLocalhost ) ,
1691: though this is not the default.
1.13 stevesk 1692: Additionally, the authentication spoofing and authentication data
1693: verification and substitution occur on the client side.
1694: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1695: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1696: forwarding (see the warnings for
1697: .Cm ForwardX11
1698: in
1.19 jmc 1699: .Xr ssh_config 5 ) .
1.13 stevesk 1700: A system administrator may have a stance in which they want to
1701: protect clients that may expose themselves to attack by unwittingly
1702: requesting X11 forwarding, which can warrant a
1.237 jmc 1703: .Cm no
1.13 stevesk 1704: setting.
1705: .Pp
1706: Note that disabling X11 forwarding does not prevent users from
1707: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1708: .It Cm X11UseLocalhost
1709: Specifies whether
1.52 jmc 1710: .Xr sshd 8
1.1 stevesk 1711: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1712: the wildcard address.
1713: By default,
1.52 jmc 1714: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1715: hostname part of the
1716: .Ev DISPLAY
1717: environment variable to
1.237 jmc 1718: .Cm localhost .
1.8 stevesk 1719: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1720: However, some older X11 clients may not function with this
1721: configuration.
1722: .Cm X11UseLocalhost
1723: may be set to
1.237 jmc 1724: .Cm no
1.1 stevesk 1725: to specify that the forwarding server should be bound to the wildcard
1726: address.
1727: The argument must be
1.237 jmc 1728: .Cm yes
1.1 stevesk 1729: or
1.237 jmc 1730: .Cm no .
1.1 stevesk 1731: The default is
1.237 jmc 1732: .Cm yes .
1.1 stevesk 1733: .It Cm XAuthLocation
1.11 stevesk 1734: Specifies the full pathname of the
1.1 stevesk 1735: .Xr xauth 1
1.195 dtucker 1736: program, or
1.237 jmc 1737: .Cm none
1.195 dtucker 1738: to not use one.
1.1 stevesk 1739: The default is
1740: .Pa /usr/X11R6/bin/xauth .
1741: .El
1.55 jmc 1742: .Sh TIME FORMATS
1.53 jmc 1743: .Xr sshd 8
1.1 stevesk 1744: command-line arguments and configuration file options that specify time
1745: may be expressed using a sequence of the form:
1746: .Sm off
1.7 stevesk 1747: .Ar time Op Ar qualifier ,
1.1 stevesk 1748: .Sm on
1749: where
1750: .Ar time
1751: is a positive integer value and
1752: .Ar qualifier
1753: is one of the following:
1754: .Pp
1755: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1756: .It Aq Cm none
1.1 stevesk 1757: seconds
1758: .It Cm s | Cm S
1759: seconds
1760: .It Cm m | Cm M
1761: minutes
1762: .It Cm h | Cm H
1763: hours
1764: .It Cm d | Cm D
1765: days
1766: .It Cm w | Cm W
1767: weeks
1768: .El
1769: .Pp
1770: Each member of the sequence is added together to calculate
1771: the total time value.
1772: .Pp
1773: Time format examples:
1774: .Pp
1775: .Bl -tag -width Ds -compact -offset indent
1776: .It 600
1777: 600 seconds (10 minutes)
1778: .It 10m
1779: 10 minutes
1780: .It 1h30m
1781: 1 hour 30 minutes (90 minutes)
1782: .El
1.235 jmc 1783: .Sh TOKENS
1784: Arguments to some keywords can make use of tokens,
1785: which are expanded at runtime:
1786: .Pp
1787: .Bl -tag -width XXXX -offset indent -compact
1788: .It %%
1789: A literal
1790: .Sq % .
1.257 djm 1791: .It \&%D
1792: The routing domain in which the incoming connection was received.
1.235 jmc 1793: .It %F
1794: The fingerprint of the CA key.
1795: .It %f
1796: The fingerprint of the key or certificate.
1797: .It %h
1798: The home directory of the user.
1799: .It %i
1800: The key ID in the certificate.
1801: .It %K
1802: The base64-encoded CA key.
1803: .It %k
1804: The base64-encoded key or certificate for authentication.
1805: .It %s
1806: The serial number of the certificate.
1807: .It \&%T
1808: The type of the CA key.
1809: .It %t
1810: The key or certificate type.
1.268 jmc 1811: .It \&%U
1812: The numeric user ID of the target user.
1.235 jmc 1813: .It %u
1814: The username.
1815: .El
1816: .Pp
1817: .Cm AuthorizedKeysCommand
1.270 jmc 1818: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1819: .Pp
1820: .Cm AuthorizedKeysFile
1.270 jmc 1821: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1822: .Pp
1823: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1824: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1825: .Pp
1826: .Cm AuthorizedPrincipalsFile
1.270 jmc 1827: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1828: .Pp
1829: .Cm ChrootDirectory
1.270 jmc 1830: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1831: .Pp
1832: .Cm RoutingDomain
1833: accepts the token %D.
1.1 stevesk 1834: .Sh FILES
1835: .Bl -tag -width Ds
1836: .It Pa /etc/ssh/sshd_config
1837: Contains configuration data for
1.53 jmc 1838: .Xr sshd 8 .
1.1 stevesk 1839: This file should be writable by root only, but it is recommended
1840: (though not necessary) that it be world-readable.
1841: .El
1.19 jmc 1842: .Sh SEE ALSO
1.237 jmc 1843: .Xr sftp-server 8 ,
1.19 jmc 1844: .Xr sshd 8
1.1 stevesk 1845: .Sh AUTHORS
1.237 jmc 1846: .An -nosplit
1.1 stevesk 1847: OpenSSH is a derivative of the original and free
1.237 jmc 1848: ssh 1.2.12 release by
1849: .An Tatu Ylonen .
1850: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1851: .An Theo de Raadt
1852: and
1853: .An Dug Song
1.1 stevesk 1854: removed many bugs, re-added newer features and
1855: created OpenSSH.
1.237 jmc 1856: .An Markus Friedl
1857: contributed the support for SSH protocol versions 1.5 and 2.0.
1858: .An Niels Provos
1859: and
1860: .An Markus Friedl
1861: contributed support for privilege separation.
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