Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.302
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.302 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.301 2020/01/25 22:36:22 djm Exp $
1.300 tedu 37: .Dd $Mdocdate: January 25 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
698: .Qq ssh -Q key .
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
781: .Qq ssh -Q key .
1.1 stevesk 782: .It Cm IgnoreRhosts
783: Specifies that
784: .Pa .rhosts
785: and
786: .Pa .shosts
787: files will not be used in
788: .Cm HostbasedAuthentication .
789: .Pp
790: .Pa /etc/hosts.equiv
791: and
792: .Pa /etc/shosts.equiv
793: are still used.
794: The default is
1.237 jmc 795: .Cm yes .
1.1 stevesk 796: .It Cm IgnoreUserKnownHosts
797: Specifies whether
1.52 jmc 798: .Xr sshd 8
1.1 stevesk 799: should ignore the user's
1.41 djm 800: .Pa ~/.ssh/known_hosts
1.1 stevesk 801: during
1.261 djm 802: .Cm HostbasedAuthentication
803: and use only the system-wide known hosts file
804: .Pa /etc/ssh/known_hosts .
1.1 stevesk 805: The default is
1.237 jmc 806: .Cm no .
1.129 djm 807: .It Cm IPQoS
808: Specifies the IPv4 type-of-service or DSCP class for the connection.
809: Accepted values are
1.237 jmc 810: .Cm af11 ,
811: .Cm af12 ,
812: .Cm af13 ,
813: .Cm af21 ,
814: .Cm af22 ,
815: .Cm af23 ,
816: .Cm af31 ,
817: .Cm af32 ,
818: .Cm af33 ,
819: .Cm af41 ,
820: .Cm af42 ,
821: .Cm af43 ,
822: .Cm cs0 ,
823: .Cm cs1 ,
824: .Cm cs2 ,
825: .Cm cs3 ,
826: .Cm cs4 ,
827: .Cm cs5 ,
828: .Cm cs6 ,
829: .Cm cs7 ,
830: .Cm ef ,
831: .Cm lowdelay ,
832: .Cm throughput ,
833: .Cm reliability ,
1.250 djm 834: a numeric value, or
835: .Cm none
836: to use the operating system default.
1.131 djm 837: This option may take one or two arguments, separated by whitespace.
1.129 djm 838: If one argument is specified, it is used as the packet class unconditionally.
839: If two values are specified, the first is automatically selected for
840: interactive sessions and the second for non-interactive sessions.
841: The default is
1.264 job 842: .Cm af21
1.265 jmc 843: (Low-Latency Data)
1.129 djm 844: for interactive sessions and
1.264 job 845: .Cm cs1
1.265 jmc 846: (Lower Effort)
1.129 djm 847: for non-interactive sessions.
1.171 jmc 848: .It Cm KbdInteractiveAuthentication
849: Specifies whether to allow keyboard-interactive authentication.
850: The argument to this keyword must be
1.237 jmc 851: .Cm yes
1.171 jmc 852: or
1.237 jmc 853: .Cm no .
1.171 jmc 854: The default is to use whatever value
855: .Cm ChallengeResponseAuthentication
856: is set to
857: (by default
1.237 jmc 858: .Cm yes ) .
1.1 stevesk 859: .It Cm KerberosAuthentication
1.24 markus 860: Specifies whether the password provided by the user for
1.1 stevesk 861: .Cm PasswordAuthentication
1.24 markus 862: will be validated through the Kerberos KDC.
1.1 stevesk 863: To use this option, the server needs a
864: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 865: The default is
1.237 jmc 866: .Cm no .
1.29 dtucker 867: .It Cm KerberosGetAFSToken
1.45 djm 868: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 869: an AFS token before accessing the user's home directory.
1.52 jmc 870: The default is
1.237 jmc 871: .Cm no .
1.1 stevesk 872: .It Cm KerberosOrLocalPasswd
1.52 jmc 873: If password authentication through Kerberos fails then
1.1 stevesk 874: the password will be validated via any additional local mechanism
875: such as
876: .Pa /etc/passwd .
1.52 jmc 877: The default is
1.237 jmc 878: .Cm yes .
1.1 stevesk 879: .It Cm KerberosTicketCleanup
880: Specifies whether to automatically destroy the user's ticket cache
881: file on logout.
1.52 jmc 882: The default is
1.237 jmc 883: .Cm yes .
1.127 djm 884: .It Cm KexAlgorithms
885: Specifies the available KEX (Key Exchange) algorithms.
886: Multiple algorithms must be comma-separated.
1.289 naddy 887: Alternately if the specified list begins with a
1.208 djm 888: .Sq +
889: character, then the specified methods will be appended to the default set
890: instead of replacing them.
1.289 naddy 891: If the specified list begins with a
1.242 djm 892: .Sq -
893: character, then the specified methods (including wildcards) will be removed
894: from the default set instead of replacing them.
1.290 naddy 895: If the specified list begins with a
896: .Sq ^
897: character, then the specified methods will be placed at the head of the
898: default set.
1.173 naddy 899: The supported algorithms are:
900: .Pp
901: .Bl -item -compact -offset indent
902: .It
1.234 djm 903: curve25519-sha256
904: .It
1.173 naddy 905: curve25519-sha256@libssh.org
906: .It
907: diffie-hellman-group1-sha1
908: .It
909: diffie-hellman-group14-sha1
910: .It
1.263 djm 911: diffie-hellman-group14-sha256
912: .It
913: diffie-hellman-group16-sha512
914: .It
915: diffie-hellman-group18-sha512
916: .It
1.173 naddy 917: diffie-hellman-group-exchange-sha1
918: .It
919: diffie-hellman-group-exchange-sha256
920: .It
921: ecdh-sha2-nistp256
922: .It
923: ecdh-sha2-nistp384
924: .It
925: ecdh-sha2-nistp521
1.298 dtucker 926: .It
927: sntrup4591761x25519-sha512@tinyssh.org
1.173 naddy 928: .El
929: .Pp
930: The default is:
1.170 dtucker 931: .Bd -literal -offset indent
1.234 djm 932: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 933: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
934: diffie-hellman-group-exchange-sha256,
1.263 djm 935: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 936: diffie-hellman-group14-sha256
1.170 dtucker 937: .Ed
1.188 djm 938: .Pp
1.237 jmc 939: The list of available key exchange algorithms may also be obtained using
940: .Qq ssh -Q kex .
1.1 stevesk 941: .It Cm ListenAddress
942: Specifies the local addresses
1.52 jmc 943: .Xr sshd 8
1.1 stevesk 944: should listen on.
945: The following forms may be used:
946: .Pp
947: .Bl -item -offset indent -compact
948: .It
949: .Cm ListenAddress
950: .Sm off
1.259 jmc 951: .Ar hostname | address
1.1 stevesk 952: .Sm on
1.260 jmc 953: .Op Cm rdomain Ar domain
1.1 stevesk 954: .It
955: .Cm ListenAddress
956: .Sm off
1.259 jmc 957: .Ar hostname : port
1.1 stevesk 958: .Sm on
1.260 jmc 959: .Op Cm rdomain Ar domain
1.256 djm 960: .It
961: .Cm ListenAddress
962: .Sm off
1.259 jmc 963: .Ar IPv4_address : port
1.256 djm 964: .Sm on
1.260 jmc 965: .Op Cm rdomain Ar domain
1.1 stevesk 966: .It
967: .Cm ListenAddress
968: .Sm off
1.259 jmc 969: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 970: .Sm on
1.260 jmc 971: .Op Cm rdomain Ar domain
1.1 stevesk 972: .El
973: .Pp
1.256 djm 974: The optional
975: .Cm rdomain
976: qualifier requests
977: .Xr sshd 8
978: listen in an explicit routing domain.
1.1 stevesk 979: If
980: .Ar port
981: is not specified,
1.200 dtucker 982: sshd will listen on the address and all
1.1 stevesk 983: .Cm Port
1.17 jmc 984: options specified.
1.256 djm 985: The default is to listen on all local addresses on the current default
986: routing domain.
1.15 jmc 987: Multiple
1.1 stevesk 988: .Cm ListenAddress
1.17 jmc 989: options are permitted.
1.256 djm 990: For more information on routing domains, see
1.259 jmc 991: .Xr rdomain 4 .
1.1 stevesk 992: .It Cm LoginGraceTime
993: The server disconnects after this time if the user has not
994: successfully logged in.
995: If the value is 0, there is no time limit.
1.12 stevesk 996: The default is 120 seconds.
1.1 stevesk 997: .It Cm LogLevel
998: Gives the verbosity level that is used when logging messages from
1.53 jmc 999: .Xr sshd 8 .
1.1 stevesk 1000: The possible values are:
1.52 jmc 1001: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1002: The default is INFO.
1003: DEBUG and DEBUG1 are equivalent.
1004: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1005: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1006: .It Cm MACs
1007: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1008: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1009: Multiple algorithms must be comma-separated.
1.289 naddy 1010: If the specified list begins with a
1.208 djm 1011: .Sq +
1012: character, then the specified algorithms will be appended to the default set
1013: instead of replacing them.
1.289 naddy 1014: If the specified list begins with a
1.242 djm 1015: .Sq -
1016: character, then the specified algorithms (including wildcards) will be removed
1017: from the default set instead of replacing them.
1.290 naddy 1018: If the specified list begins with a
1019: .Sq ^
1020: character, then the specified algorithms will be placed at the head of the
1021: default set.
1.208 djm 1022: .Pp
1.152 markus 1023: The algorithms that contain
1.237 jmc 1024: .Qq -etm
1.152 markus 1025: calculate the MAC after encryption (encrypt-then-mac).
1026: These are considered safer and their use recommended.
1.173 naddy 1027: The supported MACs are:
1028: .Pp
1029: .Bl -item -compact -offset indent
1030: .It
1031: hmac-md5
1032: .It
1033: hmac-md5-96
1034: .It
1035: hmac-sha1
1036: .It
1037: hmac-sha1-96
1038: .It
1039: hmac-sha2-256
1040: .It
1041: hmac-sha2-512
1042: .It
1043: umac-64@openssh.com
1044: .It
1045: umac-128@openssh.com
1046: .It
1047: hmac-md5-etm@openssh.com
1048: .It
1049: hmac-md5-96-etm@openssh.com
1050: .It
1051: hmac-sha1-etm@openssh.com
1052: .It
1053: hmac-sha1-96-etm@openssh.com
1054: .It
1055: hmac-sha2-256-etm@openssh.com
1056: .It
1057: hmac-sha2-512-etm@openssh.com
1058: .It
1059: umac-64-etm@openssh.com
1060: .It
1061: umac-128-etm@openssh.com
1062: .El
1063: .Pp
1.52 jmc 1064: The default is:
1.77 jmc 1065: .Bd -literal -offset indent
1.152 markus 1066: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1067: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1068: hmac-sha1-etm@openssh.com,
1.173 naddy 1069: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1070: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1071: .Ed
1.188 djm 1072: .Pp
1.237 jmc 1073: The list of available MAC algorithms may also be obtained using
1074: .Qq ssh -Q mac .
1.60 dtucker 1075: .It Cm Match
1.61 jmc 1076: Introduces a conditional block.
1.65 dtucker 1077: If all of the criteria on the
1.60 dtucker 1078: .Cm Match
1.65 dtucker 1079: line are satisfied, the keywords on the following lines override those
1080: set in the global section of the config file, until either another
1.60 dtucker 1081: .Cm Match
1.65 dtucker 1082: line or the end of the file.
1.172 djm 1083: If a keyword appears in multiple
1084: .Cm Match
1.177 sobrado 1085: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1086: applied.
1.91 djm 1087: .Pp
1.61 jmc 1088: The arguments to
1.60 dtucker 1089: .Cm Match
1.163 dtucker 1090: are one or more criteria-pattern pairs or the single token
1091: .Cm All
1092: which matches all criteria.
1.60 dtucker 1093: The available criteria are
1094: .Cm User ,
1.69 dtucker 1095: .Cm Group ,
1.60 dtucker 1096: .Cm Host ,
1.139 dtucker 1097: .Cm LocalAddress ,
1098: .Cm LocalPort ,
1.258 djm 1099: .Cm RDomain ,
1.60 dtucker 1100: and
1.258 djm 1101: .Cm Address
1102: (with
1103: .Cm RDomain
1104: representing the
1105: .Xr rdomain 4
1.284 jmc 1106: on which the connection was received).
1.258 djm 1107: .Pp
1.91 djm 1108: The match patterns may consist of single entries or comma-separated
1109: lists and may use the wildcard and negation operators described in the
1.237 jmc 1110: .Sx PATTERNS
1111: section of
1.92 djm 1112: .Xr ssh_config 5 .
1.91 djm 1113: .Pp
1114: The patterns in an
1115: .Cm Address
1116: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1117: address/masklen format,
1118: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1119: Note that the mask length provided must be consistent with the address -
1120: it is an error to specify a mask length that is too long for the address
1.93 jmc 1121: or one with bits set in this host portion of the address.
1.237 jmc 1122: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1123: .Pp
1.60 dtucker 1124: Only a subset of keywords may be used on the lines following a
1125: .Cm Match
1126: keyword.
1127: Available keywords are
1.142 jmc 1128: .Cm AcceptEnv ,
1.99 okan 1129: .Cm AllowAgentForwarding ,
1.142 jmc 1130: .Cm AllowGroups ,
1.193 djm 1131: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1132: .Cm AllowTcpForwarding ,
1.141 markus 1133: .Cm AllowUsers ,
1.149 djm 1134: .Cm AuthenticationMethods ,
1.146 djm 1135: .Cm AuthorizedKeysCommand ,
1136: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1137: .Cm AuthorizedKeysFile ,
1.214 djm 1138: .Cm AuthorizedPrincipalsCommand ,
1139: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1140: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1141: .Cm Banner ,
1.85 djm 1142: .Cm ChrootDirectory ,
1.238 markus 1143: .Cm ClientAliveCountMax ,
1144: .Cm ClientAliveInterval ,
1.141 markus 1145: .Cm DenyGroups ,
1146: .Cm DenyUsers ,
1.67 dtucker 1147: .Cm ForceCommand ,
1.194 djm 1148: .Cm GatewayPorts ,
1.193 djm 1149: .Cm GSSAPIAuthentication ,
1.189 djm 1150: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1151: .Cm HostbasedAuthentication ,
1.123 djm 1152: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1153: .Cm IPQoS ,
1.74 jmc 1154: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1155: .Cm KerberosAuthentication ,
1.245 djm 1156: .Cm LogLevel ,
1.95 dtucker 1157: .Cm MaxAuthTries ,
1.94 dtucker 1158: .Cm MaxSessions ,
1.72 dtucker 1159: .Cm PasswordAuthentication ,
1.97 djm 1160: .Cm PermitEmptyPasswords ,
1.271 djm 1161: .Cm PermitListen ,
1.66 dtucker 1162: .Cm PermitOpen ,
1.79 dtucker 1163: .Cm PermitRootLogin ,
1.164 djm 1164: .Cm PermitTTY ,
1.123 djm 1165: .Cm PermitTunnel ,
1.174 djm 1166: .Cm PermitUserRC ,
1.189 djm 1167: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1168: .Cm PubkeyAuthentication ,
1.159 dtucker 1169: .Cm RekeyLimit ,
1.193 djm 1170: .Cm RevokedKeys ,
1.257 djm 1171: .Cm RDomain ,
1.274 djm 1172: .Cm SetEnv ,
1.193 djm 1173: .Cm StreamLocalBindMask ,
1174: .Cm StreamLocalBindUnlink ,
1175: .Cm TrustedUserCAKeys ,
1.66 dtucker 1176: .Cm X11DisplayOffset ,
1.101 djm 1177: .Cm X11Forwarding
1.60 dtucker 1178: and
1.286 jmc 1179: .Cm X11UseLocalhost .
1.33 dtucker 1180: .It Cm MaxAuthTries
1181: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1182: connection.
1183: Once the number of failures reaches half this value,
1184: additional failures are logged.
1185: The default is 6.
1.90 djm 1186: .It Cm MaxSessions
1.216 djm 1187: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1188: sessions permitted per network connection.
1189: Multiple sessions may be established by clients that support connection
1190: multiplexing.
1191: Setting
1192: .Cm MaxSessions
1193: to 1 will effectively disable session multiplexing, whereas setting it to 0
1194: will prevent all shell, login and subsystem sessions while still permitting
1195: forwarding.
1.90 djm 1196: The default is 10.
1.1 stevesk 1197: .It Cm MaxStartups
1198: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1199: SSH daemon.
1.1 stevesk 1200: Additional connections will be dropped until authentication succeeds or the
1201: .Cm LoginGraceTime
1202: expires for a connection.
1.156 dtucker 1203: The default is 10:30:100.
1.1 stevesk 1204: .Pp
1205: Alternatively, random early drop can be enabled by specifying
1206: the three colon separated values
1.237 jmc 1207: start:rate:full (e.g. "10:30:60").
1.53 jmc 1208: .Xr sshd 8
1.237 jmc 1209: will refuse connection attempts with a probability of rate/100 (30%)
1210: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1211: The probability increases linearly and all connection attempts
1.237 jmc 1212: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1213: .It Cm PasswordAuthentication
1214: Specifies whether password authentication is allowed.
1215: The default is
1.237 jmc 1216: .Cm yes .
1.1 stevesk 1217: .It Cm PermitEmptyPasswords
1218: When password authentication is allowed, it specifies whether the
1219: server allows login to accounts with empty password strings.
1220: The default is
1.237 jmc 1221: .Cm no .
1.271 djm 1222: .It Cm PermitListen
1223: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1224: The listen specification must be one of the following forms:
1225: .Pp
1226: .Bl -item -offset indent -compact
1227: .It
1228: .Cm PermitListen
1229: .Sm off
1.276 djm 1230: .Ar port
1.271 djm 1231: .Sm on
1232: .It
1233: .Cm PermitListen
1234: .Sm off
1.276 djm 1235: .Ar host : port
1.271 djm 1236: .Sm on
1237: .El
1238: .Pp
1239: Multiple permissions may be specified by separating them with whitespace.
1240: An argument of
1241: .Cm any
1242: can be used to remove all restrictions and permit any listen requests.
1243: An argument of
1244: .Cm none
1245: can be used to prohibit all listen requests.
1246: The host name may contain wildcards as described in the PATTERNS section in
1247: .Xr ssh_config 5 .
1248: The wildcard
1249: .Sq *
1250: can also be used in place of a port number to allow all ports.
1251: By default all port forwarding listen requests are permitted.
1.272 jmc 1252: Note that the
1.271 djm 1253: .Cm GatewayPorts
1254: option may further restrict which addresses may be listened on.
1.276 djm 1255: Note also that
1256: .Xr ssh 1
1257: will request a listen host of
1258: .Dq localhost
1.283 dtucker 1259: if no listen host was specifically requested, and this name is
1.277 jmc 1260: treated differently to explicit localhost addresses of
1.276 djm 1261: .Dq 127.0.0.1
1262: and
1263: .Dq ::1 .
1.62 dtucker 1264: .It Cm PermitOpen
1265: Specifies the destinations to which TCP port forwarding is permitted.
1266: The forwarding specification must be one of the following forms:
1267: .Pp
1268: .Bl -item -offset indent -compact
1269: .It
1270: .Cm PermitOpen
1271: .Sm off
1272: .Ar host : port
1273: .Sm on
1274: .It
1275: .Cm PermitOpen
1276: .Sm off
1277: .Ar IPv4_addr : port
1278: .Sm on
1279: .It
1280: .Cm PermitOpen
1281: .Sm off
1282: .Ar \&[ IPv6_addr \&] : port
1283: .Sm on
1284: .El
1285: .Pp
1.68 dtucker 1286: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1287: An argument of
1.237 jmc 1288: .Cm any
1.62 dtucker 1289: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1290: An argument of
1.237 jmc 1291: .Cm none
1.140 dtucker 1292: can be used to prohibit all forwarding requests.
1.227 jmc 1293: The wildcard
1.237 jmc 1294: .Sq *
1.299 djm 1295: can be used for host or port to allow all hosts or ports respectively.
1296: Otherwise, no pattern matching or address lookups are performed on supplied
1297: names.
1.63 jmc 1298: By default all port forwarding requests are permitted.
1.1 stevesk 1299: .It Cm PermitRootLogin
1.38 jmc 1300: Specifies whether root can log in using
1.1 stevesk 1301: .Xr ssh 1 .
1302: The argument must be
1.237 jmc 1303: .Cm yes ,
1304: .Cm prohibit-password ,
1305: .Cm forced-commands-only ,
1.1 stevesk 1306: or
1.237 jmc 1307: .Cm no .
1.1 stevesk 1308: The default is
1.237 jmc 1309: .Cm prohibit-password .
1.1 stevesk 1310: .Pp
1311: If this option is set to
1.237 jmc 1312: .Cm prohibit-password
1.255 jmc 1313: (or its deprecated alias,
1314: .Cm without-password ) ,
1.210 deraadt 1315: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1316: .Pp
1317: If this option is set to
1.237 jmc 1318: .Cm forced-commands-only ,
1.1 stevesk 1319: root login with public key authentication will be allowed,
1320: but only if the
1321: .Ar command
1322: option has been specified
1323: (which may be useful for taking remote backups even if root login is
1.17 jmc 1324: normally not allowed).
1325: All other authentication methods are disabled for root.
1.1 stevesk 1326: .Pp
1327: If this option is set to
1.237 jmc 1328: .Cm no ,
1.38 jmc 1329: root is not allowed to log in.
1.231 jmc 1330: .It Cm PermitTTY
1331: Specifies whether
1332: .Xr pty 4
1333: allocation is permitted.
1334: The default is
1.237 jmc 1335: .Cm yes .
1.46 reyk 1336: .It Cm PermitTunnel
1337: Specifies whether
1338: .Xr tun 4
1339: device forwarding is allowed.
1.47 reyk 1340: The argument must be
1.237 jmc 1341: .Cm yes ,
1342: .Cm point-to-point
1.58 stevesk 1343: (layer 3),
1.237 jmc 1344: .Cm ethernet
1.58 stevesk 1345: (layer 2), or
1.237 jmc 1346: .Cm no .
1.58 stevesk 1347: Specifying
1.237 jmc 1348: .Cm yes
1.58 stevesk 1349: permits both
1.237 jmc 1350: .Cm point-to-point
1.58 stevesk 1351: and
1.237 jmc 1352: .Cm ethernet .
1.46 reyk 1353: The default is
1.237 jmc 1354: .Cm no .
1.178 djm 1355: .Pp
1356: Independent of this setting, the permissions of the selected
1357: .Xr tun 4
1358: device must allow access to the user.
1.6 markus 1359: .It Cm PermitUserEnvironment
1360: Specifies whether
1361: .Pa ~/.ssh/environment
1.9 stevesk 1362: and
1.6 markus 1363: .Cm environment=
1364: options in
1365: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1366: are processed by
1.52 jmc 1367: .Xr sshd 8 .
1.278 djm 1368: Valid options are
1369: .Cm yes ,
1370: .Cm no
1371: or a pattern-list specifying which environment variable names to accept
1372: (for example
1373: .Qq LANG,LC_* ) .
1.6 markus 1374: The default is
1.237 jmc 1375: .Cm no .
1.9 stevesk 1376: Enabling environment processing may enable users to bypass access
1377: restrictions in some configurations using mechanisms such as
1378: .Ev LD_PRELOAD .
1.174 djm 1379: .It Cm PermitUserRC
1380: Specifies whether any
1381: .Pa ~/.ssh/rc
1382: file is executed.
1383: The default is
1.237 jmc 1384: .Cm yes .
1.1 stevesk 1385: .It Cm PidFile
1.4 stevesk 1386: Specifies the file that contains the process ID of the
1.195 dtucker 1387: SSH daemon, or
1.237 jmc 1388: .Cm none
1.195 dtucker 1389: to not write one.
1.1 stevesk 1390: The default is
1391: .Pa /var/run/sshd.pid .
1392: .It Cm Port
1393: Specifies the port number that
1.52 jmc 1394: .Xr sshd 8
1.1 stevesk 1395: listens on.
1396: The default is 22.
1397: Multiple options of this type are permitted.
1398: See also
1399: .Cm ListenAddress .
1400: .It Cm PrintLastLog
1401: Specifies whether
1.52 jmc 1402: .Xr sshd 8
1.36 jaredy 1403: should print the date and time of the last user login when a user logs
1404: in interactively.
1.1 stevesk 1405: The default is
1.237 jmc 1406: .Cm yes .
1.1 stevesk 1407: .It Cm PrintMotd
1408: Specifies whether
1.52 jmc 1409: .Xr sshd 8
1.1 stevesk 1410: should print
1411: .Pa /etc/motd
1412: when a user logs in interactively.
1413: (On some systems it is also printed by the shell,
1414: .Pa /etc/profile ,
1415: or equivalent.)
1416: The default is
1.237 jmc 1417: .Cm yes .
1.189 djm 1418: .It Cm PubkeyAcceptedKeyTypes
1419: Specifies the key types that will be accepted for public key authentication
1.280 djm 1420: as a list of comma-separated patterns.
1.289 naddy 1421: Alternately if the specified list begins with a
1.208 djm 1422: .Sq +
1423: character, then the specified key types will be appended to the default set
1424: instead of replacing them.
1.289 naddy 1425: If the specified list begins with a
1.242 djm 1426: .Sq -
1427: character, then the specified key types (including wildcards) will be removed
1428: from the default set instead of replacing them.
1.290 naddy 1429: If the specified list begins with a
1430: .Sq ^
1431: character, then the specified key types will be placed at the head of the
1432: default set.
1.206 markus 1433: The default for this option is:
1434: .Bd -literal -offset 3n
1435: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1436: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1437: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1438: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1439: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1440: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1441: rsa-sha2-512-cert-v01@openssh.com,
1442: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1443: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1444: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1445: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1446: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1447: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1448: .Ed
1449: .Pp
1.237 jmc 1450: The list of available key types may also be obtained using
1451: .Qq ssh -Q key .
1.293 djm 1452: .It Cm PubkeyAuthOptions
1453: Sets one or more public key authentication options.
1454: Two option keywords are currently supported:
1.294 jmc 1455: .Cm none
1456: (the default; indicating no additional options are enabled)
1.293 djm 1457: and
1458: .Cm touch-required .
1459: .Pp
1460: The
1461: .Cm touch-required
1.297 naddy 1462: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1463: (i.e.\&
1.293 djm 1464: .Cm ecdsa-sk
1465: or
1466: .Cm ed25519-sk )
1467: to always require the signature to attest that a physically present user
1.297 naddy 1468: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1469: By default,
1470: .Xr sshd 8
1.297 naddy 1471: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1472: The
1473: .Cm touch-required
1474: flag disables this override.
1.297 naddy 1475: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1476: .It Cm PubkeyAuthentication
1477: Specifies whether public key authentication is allowed.
1478: The default is
1.237 jmc 1479: .Cm yes .
1.159 dtucker 1480: .It Cm RekeyLimit
1481: Specifies the maximum amount of data that may be transmitted before the
1482: session key is renegotiated, optionally followed a maximum amount of
1483: time that may pass before the session key is renegotiated.
1484: The first argument is specified in bytes and may have a suffix of
1485: .Sq K ,
1486: .Sq M ,
1487: or
1488: .Sq G
1489: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1490: The default is between
1491: .Sq 1G
1492: and
1493: .Sq 4G ,
1494: depending on the cipher.
1495: The optional second value is specified in seconds and may use any of the
1496: units documented in the
1497: .Sx TIME FORMATS
1.160 jmc 1498: section.
1.159 dtucker 1499: The default value for
1500: .Cm RekeyLimit
1501: is
1.237 jmc 1502: .Cm default none ,
1.159 dtucker 1503: which means that rekeying is performed after the cipher's default amount
1504: of data has been sent or received and no time based rekeying is done.
1.118 djm 1505: .It Cm RevokedKeys
1.195 dtucker 1506: Specifies revoked public keys file, or
1.237 jmc 1507: .Cm none
1.195 dtucker 1508: to not use one.
1.118 djm 1509: Keys listed in this file will be refused for public key authentication.
1510: Note that if this file is not readable, then public key authentication will
1511: be refused for all users.
1.154 djm 1512: Keys may be specified as a text file, listing one public key per line, or as
1513: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1514: .Xr ssh-keygen 1 .
1.161 jmc 1515: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1516: .Xr ssh-keygen 1 .
1.257 djm 1517: .It Cm RDomain
1518: Specifies an explicit routing domain that is applied after authentication
1519: has completed.
1.259 jmc 1520: The user session, as well and any forwarded or listening IP sockets,
1521: will be bound to this
1.257 djm 1522: .Xr rdomain 4 .
1523: If the routing domain is set to
1524: .Cm \&%D ,
1.259 jmc 1525: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1526: .It Cm SecurityKeyProvider
1.297 naddy 1527: Specifies a path to a library that will be used when loading
1528: FIDO authenticator-hosted keys, overriding the default of using
1529: the built-in USB HID support.
1.274 djm 1530: .It Cm SetEnv
1531: Specifies one or more environment variables to set in child sessions started
1532: by
1533: .Xr sshd 8
1534: as
1535: .Dq NAME=VALUE .
1536: The environment value may be quoted (e.g. if it contains whitespace
1537: characters).
1538: Environment variables set by
1539: .Cm SetEnv
1540: override the default environment and any variables specified by the user
1541: via
1542: .Cm AcceptEnv
1543: or
1544: .Cm PermitUserEnvironment .
1.175 millert 1545: .It Cm StreamLocalBindMask
1546: Sets the octal file creation mode mask
1547: .Pq umask
1548: used when creating a Unix-domain socket file for local or remote
1549: port forwarding.
1550: This option is only used for port forwarding to a Unix-domain socket file.
1551: .Pp
1552: The default value is 0177, which creates a Unix-domain socket file that is
1553: readable and writable only by the owner.
1554: Note that not all operating systems honor the file mode on Unix-domain
1555: socket files.
1556: .It Cm StreamLocalBindUnlink
1557: Specifies whether to remove an existing Unix-domain socket file for local
1558: or remote port forwarding before creating a new one.
1559: If the socket file already exists and
1560: .Cm StreamLocalBindUnlink
1561: is not enabled,
1562: .Nm sshd
1563: will be unable to forward the port to the Unix-domain socket file.
1564: This option is only used for port forwarding to a Unix-domain socket file.
1565: .Pp
1566: The argument must be
1.237 jmc 1567: .Cm yes
1.175 millert 1568: or
1.237 jmc 1569: .Cm no .
1.175 millert 1570: The default is
1.237 jmc 1571: .Cm no .
1.1 stevesk 1572: .It Cm StrictModes
1573: Specifies whether
1.52 jmc 1574: .Xr sshd 8
1.1 stevesk 1575: should check file modes and ownership of the
1576: user's files and home directory before accepting login.
1577: This is normally desirable because novices sometimes accidentally leave their
1578: directory or files world-writable.
1579: The default is
1.237 jmc 1580: .Cm yes .
1.112 djm 1581: Note that this does not apply to
1582: .Cm ChrootDirectory ,
1583: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1584: .It Cm Subsystem
1.51 jmc 1585: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1586: Arguments should be a subsystem name and a command (with optional arguments)
1587: to execute upon subsystem request.
1.80 djm 1588: .Pp
1.1 stevesk 1589: The command
1.237 jmc 1590: .Cm sftp-server
1591: implements the SFTP file transfer subsystem.
1.80 djm 1592: .Pp
1593: Alternately the name
1.237 jmc 1594: .Cm internal-sftp
1595: implements an in-process SFTP server.
1.80 djm 1596: This may simplify configurations using
1597: .Cm ChrootDirectory
1598: to force a different filesystem root on clients.
1599: .Pp
1.1 stevesk 1600: By default no subsystems are defined.
1601: .It Cm SyslogFacility
1602: Gives the facility code that is used when logging messages from
1.53 jmc 1603: .Xr sshd 8 .
1.1 stevesk 1604: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1605: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1606: The default is AUTH.
1.27 markus 1607: .It Cm TCPKeepAlive
1608: Specifies whether the system should send TCP keepalive messages to the
1609: other side.
1610: If they are sent, death of the connection or crash of one
1611: of the machines will be properly noticed.
1612: However, this means that
1613: connections will die if the route is down temporarily, and some people
1614: find it annoying.
1615: On the other hand, if TCP keepalives are not sent,
1616: sessions may hang indefinitely on the server, leaving
1.237 jmc 1617: .Qq ghost
1.27 markus 1618: users and consuming server resources.
1619: .Pp
1620: The default is
1.237 jmc 1621: .Cm yes
1.27 markus 1622: (to send TCP keepalive messages), and the server will notice
1623: if the network goes down or the client host crashes.
1624: This avoids infinitely hanging sessions.
1625: .Pp
1626: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1627: .Cm no .
1.118 djm 1628: .It Cm TrustedUserCAKeys
1629: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1630: trusted to sign user certificates for authentication, or
1.237 jmc 1631: .Cm none
1.195 dtucker 1632: to not use one.
1.119 jmc 1633: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1634: .Ql #
1635: are allowed.
1636: If a certificate is presented for authentication and has its signing CA key
1637: listed in this file, then it may be used for authentication for any user
1638: listed in the certificate's principals list.
1639: Note that certificates that lack a list of principals will not be permitted
1640: for authentication using
1641: .Cm TrustedUserCAKeys .
1.161 jmc 1642: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1643: .Xr ssh-keygen 1 .
1.18 markus 1644: .It Cm UseDNS
1645: Specifies whether
1.52 jmc 1646: .Xr sshd 8
1.207 djm 1647: should look up the remote host name, and to check that
1.18 markus 1648: the resolved host name for the remote IP address maps back to the
1649: very same IP address.
1.207 djm 1650: .Pp
1651: If this option is set to
1.237 jmc 1652: .Cm no
1.207 djm 1653: (the default) then only addresses and not host names may be used in
1.221 djm 1654: .Pa ~/.ssh/authorized_keys
1.207 djm 1655: .Cm from
1656: and
1.211 jmc 1657: .Nm
1.207 djm 1658: .Cm Match
1659: .Cm Host
1660: directives.
1.137 djm 1661: .It Cm VersionAddendum
1662: Optionally specifies additional text to append to the SSH protocol banner
1663: sent by the server upon connection.
1664: The default is
1.237 jmc 1665: .Cm none .
1.1 stevesk 1666: .It Cm X11DisplayOffset
1667: Specifies the first display number available for
1.52 jmc 1668: .Xr sshd 8 Ns 's
1.1 stevesk 1669: X11 forwarding.
1.52 jmc 1670: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1671: The default is 10.
1672: .It Cm X11Forwarding
1673: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1674: The argument must be
1.237 jmc 1675: .Cm yes
1.13 stevesk 1676: or
1.237 jmc 1677: .Cm no .
1.1 stevesk 1678: The default is
1.237 jmc 1679: .Cm no .
1.13 stevesk 1680: .Pp
1681: When X11 forwarding is enabled, there may be additional exposure to
1682: the server and to client displays if the
1.52 jmc 1683: .Xr sshd 8
1.13 stevesk 1684: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1685: .Cm X11UseLocalhost ) ,
1686: though this is not the default.
1.13 stevesk 1687: Additionally, the authentication spoofing and authentication data
1688: verification and substitution occur on the client side.
1689: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1690: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1691: forwarding (see the warnings for
1692: .Cm ForwardX11
1693: in
1.19 jmc 1694: .Xr ssh_config 5 ) .
1.13 stevesk 1695: A system administrator may have a stance in which they want to
1696: protect clients that may expose themselves to attack by unwittingly
1697: requesting X11 forwarding, which can warrant a
1.237 jmc 1698: .Cm no
1.13 stevesk 1699: setting.
1700: .Pp
1701: Note that disabling X11 forwarding does not prevent users from
1702: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1703: .It Cm X11UseLocalhost
1704: Specifies whether
1.52 jmc 1705: .Xr sshd 8
1.1 stevesk 1706: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1707: the wildcard address.
1708: By default,
1.52 jmc 1709: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1710: hostname part of the
1711: .Ev DISPLAY
1712: environment variable to
1.237 jmc 1713: .Cm localhost .
1.8 stevesk 1714: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1715: However, some older X11 clients may not function with this
1716: configuration.
1717: .Cm X11UseLocalhost
1718: may be set to
1.237 jmc 1719: .Cm no
1.1 stevesk 1720: to specify that the forwarding server should be bound to the wildcard
1721: address.
1722: The argument must be
1.237 jmc 1723: .Cm yes
1.1 stevesk 1724: or
1.237 jmc 1725: .Cm no .
1.1 stevesk 1726: The default is
1.237 jmc 1727: .Cm yes .
1.1 stevesk 1728: .It Cm XAuthLocation
1.11 stevesk 1729: Specifies the full pathname of the
1.1 stevesk 1730: .Xr xauth 1
1.195 dtucker 1731: program, or
1.237 jmc 1732: .Cm none
1.195 dtucker 1733: to not use one.
1.1 stevesk 1734: The default is
1735: .Pa /usr/X11R6/bin/xauth .
1736: .El
1.55 jmc 1737: .Sh TIME FORMATS
1.53 jmc 1738: .Xr sshd 8
1.1 stevesk 1739: command-line arguments and configuration file options that specify time
1740: may be expressed using a sequence of the form:
1741: .Sm off
1.7 stevesk 1742: .Ar time Op Ar qualifier ,
1.1 stevesk 1743: .Sm on
1744: where
1745: .Ar time
1746: is a positive integer value and
1747: .Ar qualifier
1748: is one of the following:
1749: .Pp
1750: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1751: .It Aq Cm none
1.1 stevesk 1752: seconds
1753: .It Cm s | Cm S
1754: seconds
1755: .It Cm m | Cm M
1756: minutes
1757: .It Cm h | Cm H
1758: hours
1759: .It Cm d | Cm D
1760: days
1761: .It Cm w | Cm W
1762: weeks
1763: .El
1764: .Pp
1765: Each member of the sequence is added together to calculate
1766: the total time value.
1767: .Pp
1768: Time format examples:
1769: .Pp
1770: .Bl -tag -width Ds -compact -offset indent
1771: .It 600
1772: 600 seconds (10 minutes)
1773: .It 10m
1774: 10 minutes
1775: .It 1h30m
1776: 1 hour 30 minutes (90 minutes)
1777: .El
1.235 jmc 1778: .Sh TOKENS
1779: Arguments to some keywords can make use of tokens,
1780: which are expanded at runtime:
1781: .Pp
1782: .Bl -tag -width XXXX -offset indent -compact
1783: .It %%
1784: A literal
1785: .Sq % .
1.257 djm 1786: .It \&%D
1787: The routing domain in which the incoming connection was received.
1.235 jmc 1788: .It %F
1789: The fingerprint of the CA key.
1790: .It %f
1791: The fingerprint of the key or certificate.
1792: .It %h
1793: The home directory of the user.
1794: .It %i
1795: The key ID in the certificate.
1796: .It %K
1797: The base64-encoded CA key.
1798: .It %k
1799: The base64-encoded key or certificate for authentication.
1800: .It %s
1801: The serial number of the certificate.
1802: .It \&%T
1803: The type of the CA key.
1804: .It %t
1805: The key or certificate type.
1.268 jmc 1806: .It \&%U
1807: The numeric user ID of the target user.
1.235 jmc 1808: .It %u
1809: The username.
1810: .El
1811: .Pp
1812: .Cm AuthorizedKeysCommand
1.270 jmc 1813: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1814: .Pp
1815: .Cm AuthorizedKeysFile
1.270 jmc 1816: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1817: .Pp
1818: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1819: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1820: .Pp
1821: .Cm AuthorizedPrincipalsFile
1.270 jmc 1822: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1823: .Pp
1824: .Cm ChrootDirectory
1.270 jmc 1825: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1826: .Pp
1827: .Cm RoutingDomain
1828: accepts the token %D.
1.1 stevesk 1829: .Sh FILES
1830: .Bl -tag -width Ds
1831: .It Pa /etc/ssh/sshd_config
1832: Contains configuration data for
1.53 jmc 1833: .Xr sshd 8 .
1.1 stevesk 1834: This file should be writable by root only, but it is recommended
1835: (though not necessary) that it be world-readable.
1836: .El
1.19 jmc 1837: .Sh SEE ALSO
1.237 jmc 1838: .Xr sftp-server 8 ,
1.19 jmc 1839: .Xr sshd 8
1.1 stevesk 1840: .Sh AUTHORS
1.237 jmc 1841: .An -nosplit
1.1 stevesk 1842: OpenSSH is a derivative of the original and free
1.237 jmc 1843: ssh 1.2.12 release by
1844: .An Tatu Ylonen .
1845: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1846: .An Theo de Raadt
1847: and
1848: .An Dug Song
1.1 stevesk 1849: removed many bugs, re-added newer features and
1850: created OpenSSH.
1.237 jmc 1851: .An Markus Friedl
1852: contributed the support for SSH protocol versions 1.5 and 2.0.
1853: .An Niels Provos
1854: and
1855: .An Markus Friedl
1856: contributed support for privilege separation.
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