Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.299
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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1.299 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.298 2020/01/21 06:09:56 dtucker Exp $
! 37: .Dd $Mdocdate: January 21 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.
1.54 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
1.298 dtucker 935: .It
936: sntrup4591761x25519-sha512@tinyssh.org
1.173 naddy 937: .El
938: .Pp
939: The default is:
1.170 dtucker 940: .Bd -literal -offset indent
1.234 djm 941: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 942: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
943: diffie-hellman-group-exchange-sha256,
1.263 djm 944: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
945: diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
1.170 dtucker 946: .Ed
1.188 djm 947: .Pp
1.237 jmc 948: The list of available key exchange algorithms may also be obtained using
949: .Qq ssh -Q kex .
1.1 stevesk 950: .It Cm ListenAddress
951: Specifies the local addresses
1.52 jmc 952: .Xr sshd 8
1.1 stevesk 953: should listen on.
954: The following forms may be used:
955: .Pp
956: .Bl -item -offset indent -compact
957: .It
958: .Cm ListenAddress
959: .Sm off
1.259 jmc 960: .Ar hostname | address
1.1 stevesk 961: .Sm on
1.260 jmc 962: .Op Cm rdomain Ar domain
1.1 stevesk 963: .It
964: .Cm ListenAddress
965: .Sm off
1.259 jmc 966: .Ar hostname : port
1.1 stevesk 967: .Sm on
1.260 jmc 968: .Op Cm rdomain Ar domain
1.256 djm 969: .It
970: .Cm ListenAddress
971: .Sm off
1.259 jmc 972: .Ar IPv4_address : port
1.256 djm 973: .Sm on
1.260 jmc 974: .Op Cm rdomain Ar domain
1.1 stevesk 975: .It
976: .Cm ListenAddress
977: .Sm off
1.259 jmc 978: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 979: .Sm on
1.260 jmc 980: .Op Cm rdomain Ar domain
1.1 stevesk 981: .El
982: .Pp
1.256 djm 983: The optional
984: .Cm rdomain
985: qualifier requests
986: .Xr sshd 8
987: listen in an explicit routing domain.
1.1 stevesk 988: If
989: .Ar port
990: is not specified,
1.200 dtucker 991: sshd will listen on the address and all
1.1 stevesk 992: .Cm Port
1.17 jmc 993: options specified.
1.256 djm 994: The default is to listen on all local addresses on the current default
995: routing domain.
1.15 jmc 996: Multiple
1.1 stevesk 997: .Cm ListenAddress
1.17 jmc 998: options are permitted.
1.256 djm 999: For more information on routing domains, see
1.259 jmc 1000: .Xr rdomain 4 .
1.1 stevesk 1001: .It Cm LoginGraceTime
1002: The server disconnects after this time if the user has not
1003: successfully logged in.
1004: If the value is 0, there is no time limit.
1.12 stevesk 1005: The default is 120 seconds.
1.1 stevesk 1006: .It Cm LogLevel
1007: Gives the verbosity level that is used when logging messages from
1.53 jmc 1008: .Xr sshd 8 .
1.1 stevesk 1009: The possible values are:
1.52 jmc 1010: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1011: The default is INFO.
1012: DEBUG and DEBUG1 are equivalent.
1013: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1014: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1015: .It Cm MACs
1016: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1017: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1018: Multiple algorithms must be comma-separated.
1.289 naddy 1019: If the specified list begins with a
1.208 djm 1020: .Sq +
1021: character, then the specified algorithms will be appended to the default set
1022: instead of replacing them.
1.289 naddy 1023: If the specified list begins with a
1.242 djm 1024: .Sq -
1025: character, then the specified algorithms (including wildcards) will be removed
1026: from the default set instead of replacing them.
1.290 naddy 1027: If the specified list begins with a
1028: .Sq ^
1029: character, then the specified algorithms will be placed at the head of the
1030: default set.
1.208 djm 1031: .Pp
1.152 markus 1032: The algorithms that contain
1.237 jmc 1033: .Qq -etm
1.152 markus 1034: calculate the MAC after encryption (encrypt-then-mac).
1035: These are considered safer and their use recommended.
1.173 naddy 1036: The supported MACs are:
1037: .Pp
1038: .Bl -item -compact -offset indent
1039: .It
1040: hmac-md5
1041: .It
1042: hmac-md5-96
1043: .It
1044: hmac-sha1
1045: .It
1046: hmac-sha1-96
1047: .It
1048: hmac-sha2-256
1049: .It
1050: hmac-sha2-512
1051: .It
1052: umac-64@openssh.com
1053: .It
1054: umac-128@openssh.com
1055: .It
1056: hmac-md5-etm@openssh.com
1057: .It
1058: hmac-md5-96-etm@openssh.com
1059: .It
1060: hmac-sha1-etm@openssh.com
1061: .It
1062: hmac-sha1-96-etm@openssh.com
1063: .It
1064: hmac-sha2-256-etm@openssh.com
1065: .It
1066: hmac-sha2-512-etm@openssh.com
1067: .It
1068: umac-64-etm@openssh.com
1069: .It
1070: umac-128-etm@openssh.com
1071: .El
1072: .Pp
1.52 jmc 1073: The default is:
1.77 jmc 1074: .Bd -literal -offset indent
1.152 markus 1075: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1076: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1077: hmac-sha1-etm@openssh.com,
1.173 naddy 1078: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1079: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1080: .Ed
1.188 djm 1081: .Pp
1.237 jmc 1082: The list of available MAC algorithms may also be obtained using
1083: .Qq ssh -Q mac .
1.60 dtucker 1084: .It Cm Match
1.61 jmc 1085: Introduces a conditional block.
1.65 dtucker 1086: If all of the criteria on the
1.60 dtucker 1087: .Cm Match
1.65 dtucker 1088: line are satisfied, the keywords on the following lines override those
1089: set in the global section of the config file, until either another
1.60 dtucker 1090: .Cm Match
1.65 dtucker 1091: line or the end of the file.
1.172 djm 1092: If a keyword appears in multiple
1093: .Cm Match
1.177 sobrado 1094: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1095: applied.
1.91 djm 1096: .Pp
1.61 jmc 1097: The arguments to
1.60 dtucker 1098: .Cm Match
1.163 dtucker 1099: are one or more criteria-pattern pairs or the single token
1100: .Cm All
1101: which matches all criteria.
1.60 dtucker 1102: The available criteria are
1103: .Cm User ,
1.69 dtucker 1104: .Cm Group ,
1.60 dtucker 1105: .Cm Host ,
1.139 dtucker 1106: .Cm LocalAddress ,
1107: .Cm LocalPort ,
1.258 djm 1108: .Cm RDomain ,
1.60 dtucker 1109: and
1.258 djm 1110: .Cm Address
1111: (with
1112: .Cm RDomain
1113: representing the
1114: .Xr rdomain 4
1.284 jmc 1115: on which the connection was received).
1.258 djm 1116: .Pp
1.91 djm 1117: The match patterns may consist of single entries or comma-separated
1118: lists and may use the wildcard and negation operators described in the
1.237 jmc 1119: .Sx PATTERNS
1120: section of
1.92 djm 1121: .Xr ssh_config 5 .
1.91 djm 1122: .Pp
1123: The patterns in an
1124: .Cm Address
1125: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1126: address/masklen format,
1127: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1128: Note that the mask length provided must be consistent with the address -
1129: it is an error to specify a mask length that is too long for the address
1.93 jmc 1130: or one with bits set in this host portion of the address.
1.237 jmc 1131: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1132: .Pp
1.60 dtucker 1133: Only a subset of keywords may be used on the lines following a
1134: .Cm Match
1135: keyword.
1136: Available keywords are
1.142 jmc 1137: .Cm AcceptEnv ,
1.99 okan 1138: .Cm AllowAgentForwarding ,
1.142 jmc 1139: .Cm AllowGroups ,
1.193 djm 1140: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1141: .Cm AllowTcpForwarding ,
1.141 markus 1142: .Cm AllowUsers ,
1.149 djm 1143: .Cm AuthenticationMethods ,
1.146 djm 1144: .Cm AuthorizedKeysCommand ,
1145: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1146: .Cm AuthorizedKeysFile ,
1.214 djm 1147: .Cm AuthorizedPrincipalsCommand ,
1148: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1149: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1150: .Cm Banner ,
1.85 djm 1151: .Cm ChrootDirectory ,
1.238 markus 1152: .Cm ClientAliveCountMax ,
1153: .Cm ClientAliveInterval ,
1.141 markus 1154: .Cm DenyGroups ,
1155: .Cm DenyUsers ,
1.67 dtucker 1156: .Cm ForceCommand ,
1.194 djm 1157: .Cm GatewayPorts ,
1.193 djm 1158: .Cm GSSAPIAuthentication ,
1.189 djm 1159: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1160: .Cm HostbasedAuthentication ,
1.123 djm 1161: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1162: .Cm IPQoS ,
1.74 jmc 1163: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1164: .Cm KerberosAuthentication ,
1.245 djm 1165: .Cm LogLevel ,
1.95 dtucker 1166: .Cm MaxAuthTries ,
1.94 dtucker 1167: .Cm MaxSessions ,
1.72 dtucker 1168: .Cm PasswordAuthentication ,
1.97 djm 1169: .Cm PermitEmptyPasswords ,
1.271 djm 1170: .Cm PermitListen ,
1.66 dtucker 1171: .Cm PermitOpen ,
1.79 dtucker 1172: .Cm PermitRootLogin ,
1.164 djm 1173: .Cm PermitTTY ,
1.123 djm 1174: .Cm PermitTunnel ,
1.174 djm 1175: .Cm PermitUserRC ,
1.189 djm 1176: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1177: .Cm PubkeyAuthentication ,
1.159 dtucker 1178: .Cm RekeyLimit ,
1.193 djm 1179: .Cm RevokedKeys ,
1.257 djm 1180: .Cm RDomain ,
1.274 djm 1181: .Cm SetEnv ,
1.193 djm 1182: .Cm StreamLocalBindMask ,
1183: .Cm StreamLocalBindUnlink ,
1184: .Cm TrustedUserCAKeys ,
1.66 dtucker 1185: .Cm X11DisplayOffset ,
1.101 djm 1186: .Cm X11Forwarding
1.60 dtucker 1187: and
1.286 jmc 1188: .Cm X11UseLocalhost .
1.33 dtucker 1189: .It Cm MaxAuthTries
1190: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1191: connection.
1192: Once the number of failures reaches half this value,
1193: additional failures are logged.
1194: The default is 6.
1.90 djm 1195: .It Cm MaxSessions
1.216 djm 1196: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1197: sessions permitted per network connection.
1198: Multiple sessions may be established by clients that support connection
1199: multiplexing.
1200: Setting
1201: .Cm MaxSessions
1202: to 1 will effectively disable session multiplexing, whereas setting it to 0
1203: will prevent all shell, login and subsystem sessions while still permitting
1204: forwarding.
1.90 djm 1205: The default is 10.
1.1 stevesk 1206: .It Cm MaxStartups
1207: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1208: SSH daemon.
1.1 stevesk 1209: Additional connections will be dropped until authentication succeeds or the
1210: .Cm LoginGraceTime
1211: expires for a connection.
1.156 dtucker 1212: The default is 10:30:100.
1.1 stevesk 1213: .Pp
1214: Alternatively, random early drop can be enabled by specifying
1215: the three colon separated values
1.237 jmc 1216: start:rate:full (e.g. "10:30:60").
1.53 jmc 1217: .Xr sshd 8
1.237 jmc 1218: will refuse connection attempts with a probability of rate/100 (30%)
1219: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1220: The probability increases linearly and all connection attempts
1.237 jmc 1221: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1222: .It Cm PasswordAuthentication
1223: Specifies whether password authentication is allowed.
1224: The default is
1.237 jmc 1225: .Cm yes .
1.1 stevesk 1226: .It Cm PermitEmptyPasswords
1227: When password authentication is allowed, it specifies whether the
1228: server allows login to accounts with empty password strings.
1229: The default is
1.237 jmc 1230: .Cm no .
1.271 djm 1231: .It Cm PermitListen
1232: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1233: The listen specification must be one of the following forms:
1234: .Pp
1235: .Bl -item -offset indent -compact
1236: .It
1237: .Cm PermitListen
1238: .Sm off
1.276 djm 1239: .Ar port
1.271 djm 1240: .Sm on
1241: .It
1242: .Cm PermitListen
1243: .Sm off
1.276 djm 1244: .Ar host : port
1.271 djm 1245: .Sm on
1246: .El
1247: .Pp
1248: Multiple permissions may be specified by separating them with whitespace.
1249: An argument of
1250: .Cm any
1251: can be used to remove all restrictions and permit any listen requests.
1252: An argument of
1253: .Cm none
1254: can be used to prohibit all listen requests.
1255: The host name may contain wildcards as described in the PATTERNS section in
1256: .Xr ssh_config 5 .
1257: The wildcard
1258: .Sq *
1259: can also be used in place of a port number to allow all ports.
1260: By default all port forwarding listen requests are permitted.
1.272 jmc 1261: Note that the
1.271 djm 1262: .Cm GatewayPorts
1263: option may further restrict which addresses may be listened on.
1.276 djm 1264: Note also that
1265: .Xr ssh 1
1266: will request a listen host of
1267: .Dq localhost
1.283 dtucker 1268: if no listen host was specifically requested, and this name is
1.277 jmc 1269: treated differently to explicit localhost addresses of
1.276 djm 1270: .Dq 127.0.0.1
1271: and
1272: .Dq ::1 .
1.62 dtucker 1273: .It Cm PermitOpen
1274: Specifies the destinations to which TCP port forwarding is permitted.
1275: The forwarding specification must be one of the following forms:
1276: .Pp
1277: .Bl -item -offset indent -compact
1278: .It
1279: .Cm PermitOpen
1280: .Sm off
1281: .Ar host : port
1282: .Sm on
1283: .It
1284: .Cm PermitOpen
1285: .Sm off
1286: .Ar IPv4_addr : port
1287: .Sm on
1288: .It
1289: .Cm PermitOpen
1290: .Sm off
1291: .Ar \&[ IPv6_addr \&] : port
1292: .Sm on
1293: .El
1294: .Pp
1.68 dtucker 1295: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1296: An argument of
1.237 jmc 1297: .Cm any
1.62 dtucker 1298: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1299: An argument of
1.237 jmc 1300: .Cm none
1.140 dtucker 1301: can be used to prohibit all forwarding requests.
1.227 jmc 1302: The wildcard
1.237 jmc 1303: .Sq *
1.299 ! djm 1304: can be used for host or port to allow all hosts or ports respectively.
! 1305: Otherwise, no pattern matching or address lookups are performed on supplied
! 1306: names.
1.63 jmc 1307: By default all port forwarding requests are permitted.
1.1 stevesk 1308: .It Cm PermitRootLogin
1.38 jmc 1309: Specifies whether root can log in using
1.1 stevesk 1310: .Xr ssh 1 .
1311: The argument must be
1.237 jmc 1312: .Cm yes ,
1313: .Cm prohibit-password ,
1314: .Cm forced-commands-only ,
1.1 stevesk 1315: or
1.237 jmc 1316: .Cm no .
1.1 stevesk 1317: The default is
1.237 jmc 1318: .Cm prohibit-password .
1.1 stevesk 1319: .Pp
1320: If this option is set to
1.237 jmc 1321: .Cm prohibit-password
1.255 jmc 1322: (or its deprecated alias,
1323: .Cm without-password ) ,
1.210 deraadt 1324: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1325: .Pp
1326: If this option is set to
1.237 jmc 1327: .Cm forced-commands-only ,
1.1 stevesk 1328: root login with public key authentication will be allowed,
1329: but only if the
1330: .Ar command
1331: option has been specified
1332: (which may be useful for taking remote backups even if root login is
1.17 jmc 1333: normally not allowed).
1334: All other authentication methods are disabled for root.
1.1 stevesk 1335: .Pp
1336: If this option is set to
1.237 jmc 1337: .Cm no ,
1.38 jmc 1338: root is not allowed to log in.
1.231 jmc 1339: .It Cm PermitTTY
1340: Specifies whether
1341: .Xr pty 4
1342: allocation is permitted.
1343: The default is
1.237 jmc 1344: .Cm yes .
1.46 reyk 1345: .It Cm PermitTunnel
1346: Specifies whether
1347: .Xr tun 4
1348: device forwarding is allowed.
1.47 reyk 1349: The argument must be
1.237 jmc 1350: .Cm yes ,
1351: .Cm point-to-point
1.58 stevesk 1352: (layer 3),
1.237 jmc 1353: .Cm ethernet
1.58 stevesk 1354: (layer 2), or
1.237 jmc 1355: .Cm no .
1.58 stevesk 1356: Specifying
1.237 jmc 1357: .Cm yes
1.58 stevesk 1358: permits both
1.237 jmc 1359: .Cm point-to-point
1.58 stevesk 1360: and
1.237 jmc 1361: .Cm ethernet .
1.46 reyk 1362: The default is
1.237 jmc 1363: .Cm no .
1.178 djm 1364: .Pp
1365: Independent of this setting, the permissions of the selected
1366: .Xr tun 4
1367: device must allow access to the user.
1.6 markus 1368: .It Cm PermitUserEnvironment
1369: Specifies whether
1370: .Pa ~/.ssh/environment
1.9 stevesk 1371: and
1.6 markus 1372: .Cm environment=
1373: options in
1374: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1375: are processed by
1.52 jmc 1376: .Xr sshd 8 .
1.278 djm 1377: Valid options are
1378: .Cm yes ,
1379: .Cm no
1380: or a pattern-list specifying which environment variable names to accept
1381: (for example
1382: .Qq LANG,LC_* ) .
1.6 markus 1383: The default is
1.237 jmc 1384: .Cm no .
1.9 stevesk 1385: Enabling environment processing may enable users to bypass access
1386: restrictions in some configurations using mechanisms such as
1387: .Ev LD_PRELOAD .
1.174 djm 1388: .It Cm PermitUserRC
1389: Specifies whether any
1390: .Pa ~/.ssh/rc
1391: file is executed.
1392: The default is
1.237 jmc 1393: .Cm yes .
1.1 stevesk 1394: .It Cm PidFile
1.4 stevesk 1395: Specifies the file that contains the process ID of the
1.195 dtucker 1396: SSH daemon, or
1.237 jmc 1397: .Cm none
1.195 dtucker 1398: to not write one.
1.1 stevesk 1399: The default is
1400: .Pa /var/run/sshd.pid .
1401: .It Cm Port
1402: Specifies the port number that
1.52 jmc 1403: .Xr sshd 8
1.1 stevesk 1404: listens on.
1405: The default is 22.
1406: Multiple options of this type are permitted.
1407: See also
1408: .Cm ListenAddress .
1409: .It Cm PrintLastLog
1410: Specifies whether
1.52 jmc 1411: .Xr sshd 8
1.36 jaredy 1412: should print the date and time of the last user login when a user logs
1413: in interactively.
1.1 stevesk 1414: The default is
1.237 jmc 1415: .Cm yes .
1.1 stevesk 1416: .It Cm PrintMotd
1417: Specifies whether
1.52 jmc 1418: .Xr sshd 8
1.1 stevesk 1419: should print
1420: .Pa /etc/motd
1421: when a user logs in interactively.
1422: (On some systems it is also printed by the shell,
1423: .Pa /etc/profile ,
1424: or equivalent.)
1425: The default is
1.237 jmc 1426: .Cm yes .
1.189 djm 1427: .It Cm PubkeyAcceptedKeyTypes
1428: Specifies the key types that will be accepted for public key authentication
1.280 djm 1429: as a list of comma-separated patterns.
1.289 naddy 1430: Alternately if the specified list begins with a
1.208 djm 1431: .Sq +
1432: character, then the specified key types will be appended to the default set
1433: instead of replacing them.
1.289 naddy 1434: If the specified list begins with a
1.242 djm 1435: .Sq -
1436: character, then the specified key types (including wildcards) will be removed
1437: from the default set instead of replacing them.
1.290 naddy 1438: If the specified list begins with a
1439: .Sq ^
1440: character, then the specified key types will be placed at the head of the
1441: default set.
1.206 markus 1442: The default for this option is:
1443: .Bd -literal -offset 3n
1444: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1445: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1446: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1447: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1448: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1449: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1450: rsa-sha2-512-cert-v01@openssh.com,
1451: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1452: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1453: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1454: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1455: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1456: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1457: .Ed
1458: .Pp
1.237 jmc 1459: The list of available key types may also be obtained using
1460: .Qq ssh -Q key .
1.293 djm 1461: .It Cm PubkeyAuthOptions
1462: Sets one or more public key authentication options.
1463: Two option keywords are currently supported:
1.294 jmc 1464: .Cm none
1465: (the default; indicating no additional options are enabled)
1.293 djm 1466: and
1467: .Cm touch-required .
1468: .Pp
1469: The
1470: .Cm touch-required
1.297 naddy 1471: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1472: (i.e.\&
1.293 djm 1473: .Cm ecdsa-sk
1474: or
1475: .Cm ed25519-sk )
1476: to always require the signature to attest that a physically present user
1.297 naddy 1477: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1478: By default,
1479: .Xr sshd 8
1.297 naddy 1480: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1481: The
1482: .Cm touch-required
1483: flag disables this override.
1.297 naddy 1484: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1485: .It Cm PubkeyAuthentication
1486: Specifies whether public key authentication is allowed.
1487: The default is
1.237 jmc 1488: .Cm yes .
1.159 dtucker 1489: .It Cm RekeyLimit
1490: Specifies the maximum amount of data that may be transmitted before the
1491: session key is renegotiated, optionally followed a maximum amount of
1492: time that may pass before the session key is renegotiated.
1493: The first argument is specified in bytes and may have a suffix of
1494: .Sq K ,
1495: .Sq M ,
1496: or
1497: .Sq G
1498: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1499: The default is between
1500: .Sq 1G
1501: and
1502: .Sq 4G ,
1503: depending on the cipher.
1504: The optional second value is specified in seconds and may use any of the
1505: units documented in the
1506: .Sx TIME FORMATS
1.160 jmc 1507: section.
1.159 dtucker 1508: The default value for
1509: .Cm RekeyLimit
1510: is
1.237 jmc 1511: .Cm default none ,
1.159 dtucker 1512: which means that rekeying is performed after the cipher's default amount
1513: of data has been sent or received and no time based rekeying is done.
1.118 djm 1514: .It Cm RevokedKeys
1.195 dtucker 1515: Specifies revoked public keys file, or
1.237 jmc 1516: .Cm none
1.195 dtucker 1517: to not use one.
1.118 djm 1518: Keys listed in this file will be refused for public key authentication.
1519: Note that if this file is not readable, then public key authentication will
1520: be refused for all users.
1.154 djm 1521: Keys may be specified as a text file, listing one public key per line, or as
1522: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1523: .Xr ssh-keygen 1 .
1.161 jmc 1524: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1525: .Xr ssh-keygen 1 .
1.257 djm 1526: .It Cm RDomain
1527: Specifies an explicit routing domain that is applied after authentication
1528: has completed.
1.259 jmc 1529: The user session, as well and any forwarded or listening IP sockets,
1530: will be bound to this
1.257 djm 1531: .Xr rdomain 4 .
1532: If the routing domain is set to
1533: .Cm \&%D ,
1.259 jmc 1534: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1535: .It Cm SecurityKeyProvider
1.297 naddy 1536: Specifies a path to a library that will be used when loading
1537: FIDO authenticator-hosted keys, overriding the default of using
1538: the built-in USB HID support.
1.274 djm 1539: .It Cm SetEnv
1540: Specifies one or more environment variables to set in child sessions started
1541: by
1542: .Xr sshd 8
1543: as
1544: .Dq NAME=VALUE .
1545: The environment value may be quoted (e.g. if it contains whitespace
1546: characters).
1547: Environment variables set by
1548: .Cm SetEnv
1549: override the default environment and any variables specified by the user
1550: via
1551: .Cm AcceptEnv
1552: or
1553: .Cm PermitUserEnvironment .
1.175 millert 1554: .It Cm StreamLocalBindMask
1555: Sets the octal file creation mode mask
1556: .Pq umask
1557: used when creating a Unix-domain socket file for local or remote
1558: port forwarding.
1559: This option is only used for port forwarding to a Unix-domain socket file.
1560: .Pp
1561: The default value is 0177, which creates a Unix-domain socket file that is
1562: readable and writable only by the owner.
1563: Note that not all operating systems honor the file mode on Unix-domain
1564: socket files.
1565: .It Cm StreamLocalBindUnlink
1566: Specifies whether to remove an existing Unix-domain socket file for local
1567: or remote port forwarding before creating a new one.
1568: If the socket file already exists and
1569: .Cm StreamLocalBindUnlink
1570: is not enabled,
1571: .Nm sshd
1572: will be unable to forward the port to the Unix-domain socket file.
1573: This option is only used for port forwarding to a Unix-domain socket file.
1574: .Pp
1575: The argument must be
1.237 jmc 1576: .Cm yes
1.175 millert 1577: or
1.237 jmc 1578: .Cm no .
1.175 millert 1579: The default is
1.237 jmc 1580: .Cm no .
1.1 stevesk 1581: .It Cm StrictModes
1582: Specifies whether
1.52 jmc 1583: .Xr sshd 8
1.1 stevesk 1584: should check file modes and ownership of the
1585: user's files and home directory before accepting login.
1586: This is normally desirable because novices sometimes accidentally leave their
1587: directory or files world-writable.
1588: The default is
1.237 jmc 1589: .Cm yes .
1.112 djm 1590: Note that this does not apply to
1591: .Cm ChrootDirectory ,
1592: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1593: .It Cm Subsystem
1.51 jmc 1594: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1595: Arguments should be a subsystem name and a command (with optional arguments)
1596: to execute upon subsystem request.
1.80 djm 1597: .Pp
1.1 stevesk 1598: The command
1.237 jmc 1599: .Cm sftp-server
1600: implements the SFTP file transfer subsystem.
1.80 djm 1601: .Pp
1602: Alternately the name
1.237 jmc 1603: .Cm internal-sftp
1604: implements an in-process SFTP server.
1.80 djm 1605: This may simplify configurations using
1606: .Cm ChrootDirectory
1607: to force a different filesystem root on clients.
1608: .Pp
1.1 stevesk 1609: By default no subsystems are defined.
1610: .It Cm SyslogFacility
1611: Gives the facility code that is used when logging messages from
1.53 jmc 1612: .Xr sshd 8 .
1.1 stevesk 1613: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1614: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1615: The default is AUTH.
1.27 markus 1616: .It Cm TCPKeepAlive
1617: Specifies whether the system should send TCP keepalive messages to the
1618: other side.
1619: If they are sent, death of the connection or crash of one
1620: of the machines will be properly noticed.
1621: However, this means that
1622: connections will die if the route is down temporarily, and some people
1623: find it annoying.
1624: On the other hand, if TCP keepalives are not sent,
1625: sessions may hang indefinitely on the server, leaving
1.237 jmc 1626: .Qq ghost
1.27 markus 1627: users and consuming server resources.
1628: .Pp
1629: The default is
1.237 jmc 1630: .Cm yes
1.27 markus 1631: (to send TCP keepalive messages), and the server will notice
1632: if the network goes down or the client host crashes.
1633: This avoids infinitely hanging sessions.
1634: .Pp
1635: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1636: .Cm no .
1.118 djm 1637: .It Cm TrustedUserCAKeys
1638: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1639: trusted to sign user certificates for authentication, or
1.237 jmc 1640: .Cm none
1.195 dtucker 1641: to not use one.
1.119 jmc 1642: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1643: .Ql #
1644: are allowed.
1645: If a certificate is presented for authentication and has its signing CA key
1646: listed in this file, then it may be used for authentication for any user
1647: listed in the certificate's principals list.
1648: Note that certificates that lack a list of principals will not be permitted
1649: for authentication using
1650: .Cm TrustedUserCAKeys .
1.161 jmc 1651: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1652: .Xr ssh-keygen 1 .
1.18 markus 1653: .It Cm UseDNS
1654: Specifies whether
1.52 jmc 1655: .Xr sshd 8
1.207 djm 1656: should look up the remote host name, and to check that
1.18 markus 1657: the resolved host name for the remote IP address maps back to the
1658: very same IP address.
1.207 djm 1659: .Pp
1660: If this option is set to
1.237 jmc 1661: .Cm no
1.207 djm 1662: (the default) then only addresses and not host names may be used in
1.221 djm 1663: .Pa ~/.ssh/authorized_keys
1.207 djm 1664: .Cm from
1665: and
1.211 jmc 1666: .Nm
1.207 djm 1667: .Cm Match
1668: .Cm Host
1669: directives.
1.137 djm 1670: .It Cm VersionAddendum
1671: Optionally specifies additional text to append to the SSH protocol banner
1672: sent by the server upon connection.
1673: The default is
1.237 jmc 1674: .Cm none .
1.1 stevesk 1675: .It Cm X11DisplayOffset
1676: Specifies the first display number available for
1.52 jmc 1677: .Xr sshd 8 Ns 's
1.1 stevesk 1678: X11 forwarding.
1.52 jmc 1679: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1680: The default is 10.
1681: .It Cm X11Forwarding
1682: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1683: The argument must be
1.237 jmc 1684: .Cm yes
1.13 stevesk 1685: or
1.237 jmc 1686: .Cm no .
1.1 stevesk 1687: The default is
1.237 jmc 1688: .Cm no .
1.13 stevesk 1689: .Pp
1690: When X11 forwarding is enabled, there may be additional exposure to
1691: the server and to client displays if the
1.52 jmc 1692: .Xr sshd 8
1.13 stevesk 1693: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1694: .Cm X11UseLocalhost ) ,
1695: though this is not the default.
1.13 stevesk 1696: Additionally, the authentication spoofing and authentication data
1697: verification and substitution occur on the client side.
1698: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1699: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1700: forwarding (see the warnings for
1701: .Cm ForwardX11
1702: in
1.19 jmc 1703: .Xr ssh_config 5 ) .
1.13 stevesk 1704: A system administrator may have a stance in which they want to
1705: protect clients that may expose themselves to attack by unwittingly
1706: requesting X11 forwarding, which can warrant a
1.237 jmc 1707: .Cm no
1.13 stevesk 1708: setting.
1709: .Pp
1710: Note that disabling X11 forwarding does not prevent users from
1711: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1712: .It Cm X11UseLocalhost
1713: Specifies whether
1.52 jmc 1714: .Xr sshd 8
1.1 stevesk 1715: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1716: the wildcard address.
1717: By default,
1.52 jmc 1718: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1719: hostname part of the
1720: .Ev DISPLAY
1721: environment variable to
1.237 jmc 1722: .Cm localhost .
1.8 stevesk 1723: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1724: However, some older X11 clients may not function with this
1725: configuration.
1726: .Cm X11UseLocalhost
1727: may be set to
1.237 jmc 1728: .Cm no
1.1 stevesk 1729: to specify that the forwarding server should be bound to the wildcard
1730: address.
1731: The argument must be
1.237 jmc 1732: .Cm yes
1.1 stevesk 1733: or
1.237 jmc 1734: .Cm no .
1.1 stevesk 1735: The default is
1.237 jmc 1736: .Cm yes .
1.1 stevesk 1737: .It Cm XAuthLocation
1.11 stevesk 1738: Specifies the full pathname of the
1.1 stevesk 1739: .Xr xauth 1
1.195 dtucker 1740: program, or
1.237 jmc 1741: .Cm none
1.195 dtucker 1742: to not use one.
1.1 stevesk 1743: The default is
1744: .Pa /usr/X11R6/bin/xauth .
1745: .El
1.55 jmc 1746: .Sh TIME FORMATS
1.53 jmc 1747: .Xr sshd 8
1.1 stevesk 1748: command-line arguments and configuration file options that specify time
1749: may be expressed using a sequence of the form:
1750: .Sm off
1.7 stevesk 1751: .Ar time Op Ar qualifier ,
1.1 stevesk 1752: .Sm on
1753: where
1754: .Ar time
1755: is a positive integer value and
1756: .Ar qualifier
1757: is one of the following:
1758: .Pp
1759: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1760: .It Aq Cm none
1.1 stevesk 1761: seconds
1762: .It Cm s | Cm S
1763: seconds
1764: .It Cm m | Cm M
1765: minutes
1766: .It Cm h | Cm H
1767: hours
1768: .It Cm d | Cm D
1769: days
1770: .It Cm w | Cm W
1771: weeks
1772: .El
1773: .Pp
1774: Each member of the sequence is added together to calculate
1775: the total time value.
1776: .Pp
1777: Time format examples:
1778: .Pp
1779: .Bl -tag -width Ds -compact -offset indent
1780: .It 600
1781: 600 seconds (10 minutes)
1782: .It 10m
1783: 10 minutes
1784: .It 1h30m
1785: 1 hour 30 minutes (90 minutes)
1786: .El
1.235 jmc 1787: .Sh TOKENS
1788: Arguments to some keywords can make use of tokens,
1789: which are expanded at runtime:
1790: .Pp
1791: .Bl -tag -width XXXX -offset indent -compact
1792: .It %%
1793: A literal
1794: .Sq % .
1.257 djm 1795: .It \&%D
1796: The routing domain in which the incoming connection was received.
1.235 jmc 1797: .It %F
1798: The fingerprint of the CA key.
1799: .It %f
1800: The fingerprint of the key or certificate.
1801: .It %h
1802: The home directory of the user.
1803: .It %i
1804: The key ID in the certificate.
1805: .It %K
1806: The base64-encoded CA key.
1807: .It %k
1808: The base64-encoded key or certificate for authentication.
1809: .It %s
1810: The serial number of the certificate.
1811: .It \&%T
1812: The type of the CA key.
1813: .It %t
1814: The key or certificate type.
1.268 jmc 1815: .It \&%U
1816: The numeric user ID of the target user.
1.235 jmc 1817: .It %u
1818: The username.
1819: .El
1820: .Pp
1821: .Cm AuthorizedKeysCommand
1.270 jmc 1822: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1823: .Pp
1824: .Cm AuthorizedKeysFile
1.270 jmc 1825: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1826: .Pp
1827: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1828: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1829: .Pp
1830: .Cm AuthorizedPrincipalsFile
1.270 jmc 1831: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1832: .Pp
1833: .Cm ChrootDirectory
1.270 jmc 1834: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1835: .Pp
1836: .Cm RoutingDomain
1837: accepts the token %D.
1.1 stevesk 1838: .Sh FILES
1839: .Bl -tag -width Ds
1840: .It Pa /etc/ssh/sshd_config
1841: Contains configuration data for
1.53 jmc 1842: .Xr sshd 8 .
1.1 stevesk 1843: This file should be writable by root only, but it is recommended
1844: (though not necessary) that it be world-readable.
1845: .El
1.19 jmc 1846: .Sh SEE ALSO
1.237 jmc 1847: .Xr sftp-server 8 ,
1.19 jmc 1848: .Xr sshd 8
1.1 stevesk 1849: .Sh AUTHORS
1.237 jmc 1850: .An -nosplit
1.1 stevesk 1851: OpenSSH is a derivative of the original and free
1.237 jmc 1852: ssh 1.2.12 release by
1853: .An Tatu Ylonen .
1854: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1855: .An Theo de Raadt
1856: and
1857: .An Dug Song
1.1 stevesk 1858: removed many bugs, re-added newer features and
1859: created OpenSSH.
1.237 jmc 1860: .An Markus Friedl
1861: contributed the support for SSH protocol versions 1.5 and 2.0.
1862: .An Niels Provos
1863: and
1864: .An Markus Friedl
1865: contributed support for privilege separation.