Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.308
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,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35: .\"
1.308 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.307 2020/02/07 03:54:44 dtucker Exp $
! 37: .Dd $Mdocdate: February 7 2020 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
1.295 jmc 42: .Nd OpenSSH daemon configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.53 jmc 44: .Xr sshd 8
1.1 stevesk 45: reads configuration data from
46: .Pa /etc/ssh/sshd_config
47: (or the file specified with
48: .Fl f
49: on the command line).
50: The file contains keyword-argument pairs, one per line.
1.254 benno 51: For each keyword, the first obtained value will be used.
1.1 stevesk 52: Lines starting with
53: .Ql #
54: and empty lines are interpreted as comments.
1.56 dtucker 55: Arguments may optionally be enclosed in double quotes
56: .Pq \&"
57: in order to represent arguments containing spaces.
1.1 stevesk 58: .Pp
59: The possible
60: keywords and their meanings are as follows (note that
61: keywords are case-insensitive and arguments are case-sensitive):
62: .Bl -tag -width Ds
1.30 djm 63: .It Cm AcceptEnv
64: Specifies what environment variables sent by the client will be copied into
65: the session's
66: .Xr environ 7 .
67: See
68: .Cm SendEnv
1.273 djm 69: and
70: .Cm SetEnv
1.30 djm 71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.219 jmc 74: The
1.197 dtucker 75: .Ev TERM
1.275 djm 76: environment variable is always accepted whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
1.237 jmc 94: .Cm any
95: (the default),
96: .Cm inet
1.52 jmc 97: (use IPv4 only), or
1.237 jmc 98: .Cm inet6
1.37 djm 99: (use IPv6 only).
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
1.237 jmc 105: .Cm yes .
1.89 jmc 106: Note that disabling agent forwarding does not improve security
107: unless users are also denied shell access, as they can always install
108: their own forwarders.
1.1 stevesk 109: .It Cm AllowGroups
110: This keyword can be followed by a list of group name patterns, separated
111: by spaces.
112: If specified, login is allowed only for users whose primary
113: group or supplementary group list matches one of the patterns.
114: Only group names are valid; a numerical group ID is not recognized.
115: By default, login is allowed for all groups.
1.301 djm 116: The allow/deny groups directives are processed in the following order:
1.54 jmc 117: .Cm DenyGroups ,
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.231 jmc 123: .It Cm AllowStreamLocalForwarding
124: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 125: The available options are
1.237 jmc 126: .Cm yes
127: (the default)
1.150 djm 128: or
1.237 jmc 129: .Cm all
1.231 jmc 130: to allow StreamLocal forwarding,
1.237 jmc 131: .Cm no
1.231 jmc 132: to prevent all StreamLocal forwarding,
1.237 jmc 133: .Cm local
1.150 djm 134: to allow local (from the perspective of
1.151 jmc 135: .Xr ssh 1 )
136: forwarding only or
1.237 jmc 137: .Cm remote
1.150 djm 138: to allow remote forwarding only.
1.231 jmc 139: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 140: users are also denied shell access, as they can always install their
141: own forwarders.
1.231 jmc 142: .It Cm AllowTcpForwarding
143: Specifies whether TCP forwarding is permitted.
1.175 millert 144: The available options are
1.237 jmc 145: .Cm yes
146: (the default)
1.175 millert 147: or
1.237 jmc 148: .Cm all
1.231 jmc 149: to allow TCP forwarding,
1.237 jmc 150: .Cm no
1.231 jmc 151: to prevent all TCP forwarding,
1.237 jmc 152: .Cm local
1.175 millert 153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
1.237 jmc 156: .Cm remote
1.175 millert 157: to allow remote forwarding only.
1.231 jmc 158: Note that disabling TCP forwarding does not improve security unless
1.175 millert 159: users are also denied shell access, as they can always install their
160: own forwarders.
1.1 stevesk 161: .It Cm AllowUsers
162: This keyword can be followed by a list of user name patterns, separated
163: by spaces.
1.14 jmc 164: If specified, login is allowed only for user names that
1.1 stevesk 165: match one of the patterns.
166: Only user names are valid; a numerical user ID is not recognized.
167: By default, login is allowed for all users.
168: If the pattern takes the form USER@HOST then USER and HOST
169: are separately checked, restricting logins to particular
170: users from particular hosts.
1.222 jmc 171: HOST criteria may additionally contain addresses to match in CIDR
172: address/masklen format.
1.301 djm 173: The allow/deny users directives are processed in the following order:
1.54 jmc 174: .Cm DenyUsers ,
1.301 djm 175: .Cm AllowUsers .
1.49 jmc 176: .Pp
1.161 jmc 177: See PATTERNS in
1.49 jmc 178: .Xr ssh_config 5
179: for more information on patterns.
1.149 djm 180: .It Cm AuthenticationMethods
181: Specifies the authentication methods that must be successfully completed
182: for a user to be granted access.
1.281 djm 183: This option must be followed by one or more lists of comma-separated
1.224 djm 184: authentication method names, or by the single string
1.237 jmc 185: .Cm any
1.224 djm 186: to indicate the default behaviour of accepting any single authentication
1.225 jmc 187: method.
1.237 jmc 188: If the default is overridden, then successful authentication requires
1.224 djm 189: completion of every method in at least one of these lists.
1.149 djm 190: .Pp
1.237 jmc 191: For example,
192: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 196: so for this example it would not be possible to attempt password or
1.149 djm 197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
1.237 jmc 202: .Cm bsdauth ,
203: .Cm pam ,
1.157 markus 204: or
1.237 jmc 205: .Cm skey ,
1.157 markus 206: depending on the server configuration.
207: For example,
1.237 jmc 208: .Qq keyboard-interactive:bsdauth
1.157 markus 209: would restrict keyboard interactive authentication to the
1.237 jmc 210: .Cm bsdauth
1.157 markus 211: device.
1.185 djm 212: .Pp
1.237 jmc 213: If the publickey method is listed more than once,
1.185 djm 214: .Xr sshd 8
215: verifies that keys that have been used successfully are not reused for
216: subsequent authentications.
1.237 jmc 217: For example,
218: .Qq publickey,publickey
219: requires successful authentication using two different public keys.
1.181 djm 220: .Pp
1.149 djm 221: Note that each authentication method listed should also be explicitly enabled
222: in the configuration.
1.251 djm 223: .Pp
224: The available authentication methods are:
225: .Qq gssapi-with-mic ,
226: .Qq hostbased ,
227: .Qq keyboard-interactive ,
228: .Qq none
229: (used for access to password-less accounts when
1.266 jmc 230: .Cm PermitEmptyPasswords
1.251 djm 231: is enabled),
232: .Qq password
233: and
234: .Qq publickey .
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: Arguments to
240: .Cm AuthorizedKeysCommand
1.235 jmc 241: accept the tokens described in the
242: .Sx TOKENS
243: section.
244: If no arguments are specified then the username of the target user is used.
1.201 djm 245: .Pp
246: The program should produce on standard output zero or
1.235 jmc 247: more lines of authorized_keys output (see
248: .Sx AUTHORIZED_KEYS
249: in
1.147 jmc 250: .Xr sshd 8 ) .
1.235 jmc 251: If a key supplied by
252: .Cm AuthorizedKeysCommand
253: does not successfully authenticate
1.146 djm 254: and authorize the user then public key authentication continues using the usual
255: .Cm AuthorizedKeysFile
256: files.
1.237 jmc 257: By default, no
258: .Cm AuthorizedKeysCommand
259: is run.
1.146 djm 260: .It Cm AuthorizedKeysCommandUser
1.237 jmc 261: Specifies the user under whose account the
262: .Cm AuthorizedKeysCommand
263: is run.
1.146 djm 264: It is recommended to use a dedicated user that has no other role on the host
265: than running authorized keys commands.
1.186 djm 266: If
1.181 djm 267: .Cm AuthorizedKeysCommand
1.186 djm 268: is specified but
269: .Cm AuthorizedKeysCommandUser
270: is not, then
271: .Xr sshd 8
272: will refuse to start.
1.1 stevesk 273: .It Cm AuthorizedKeysFile
1.235 jmc 274: Specifies the file that contains the public keys used for user authentication.
1.285 schwarze 275: The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
1.124 djm 276: .Xr sshd 8 .
1.235 jmc 277: Arguments to
1.1 stevesk 278: .Cm AuthorizedKeysFile
1.235 jmc 279: accept the tokens described in the
280: .Sx TOKENS
281: section.
1.1 stevesk 282: After expansion,
283: .Cm AuthorizedKeysFile
284: is taken to be an absolute path or one relative to the user's home
285: directory.
1.133 jmc 286: Multiple files may be listed, separated by whitespace.
1.212 djm 287: Alternately this option may be set to
1.237 jmc 288: .Cm none
1.212 djm 289: to skip checking for user keys in files.
1.133 jmc 290: The default is
1.237 jmc 291: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 292: .It Cm AuthorizedPrincipalsCommand
293: Specifies a program to be used to generate the list of allowed
294: certificate principals as per
295: .Cm AuthorizedPrincipalsFile .
296: The program must be owned by root, not writable by group or others and
297: specified by an absolute path.
298: Arguments to
299: .Cm AuthorizedPrincipalsCommand
1.235 jmc 300: accept the tokens described in the
301: .Sx TOKENS
302: section.
303: If no arguments are specified then the username of the target user is used.
1.202 djm 304: .Pp
305: The program should produce on standard output zero or
306: more lines of
307: .Cm AuthorizedPrincipalsFile
308: output.
309: If either
310: .Cm AuthorizedPrincipalsCommand
311: or
312: .Cm AuthorizedPrincipalsFile
313: is specified, then certificates offered by the client for authentication
314: must contain a principal that is listed.
1.235 jmc 315: By default, no
316: .Cm AuthorizedPrincipalsCommand
317: is run.
1.202 djm 318: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 319: Specifies the user under whose account the
320: .Cm AuthorizedPrincipalsCommand
321: is run.
1.202 djm 322: It is recommended to use a dedicated user that has no other role on the host
323: than running authorized principals commands.
324: If
325: .Cm AuthorizedPrincipalsCommand
326: is specified but
327: .Cm AuthorizedPrincipalsCommandUser
328: is not, then
329: .Xr sshd 8
330: will refuse to start.
1.121 djm 331: .It Cm AuthorizedPrincipalsFile
332: Specifies a file that lists principal names that are accepted for
333: certificate authentication.
334: When using certificates signed by a key listed in
335: .Cm TrustedUserCAKeys ,
336: this file lists names, one of which must appear in the certificate for it
337: to be accepted for authentication.
1.235 jmc 338: Names are listed one per line preceded by key options (as described in
339: .Sx AUTHORIZED_KEYS FILE FORMAT
340: in
1.125 jmc 341: .Xr sshd 8 ) .
1.124 djm 342: Empty lines and comments starting with
1.121 djm 343: .Ql #
344: are ignored.
345: .Pp
1.235 jmc 346: Arguments to
1.121 djm 347: .Cm AuthorizedPrincipalsFile
1.235 jmc 348: accept the tokens described in the
349: .Sx TOKENS
350: section.
1.121 djm 351: After expansion,
352: .Cm AuthorizedPrincipalsFile
1.235 jmc 353: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 354: The default is
1.237 jmc 355: .Cm none ,
1.138 djm 356: i.e. not to use a principals file \(en in this case, the username
1.121 djm 357: of the user must appear in a certificate's principals list for it to be
358: accepted.
1.235 jmc 359: .Pp
1.121 djm 360: Note that
361: .Cm AuthorizedPrincipalsFile
362: is only used when authentication proceeds using a CA listed in
363: .Cm TrustedUserCAKeys
364: and is not consulted for certification authorities trusted via
365: .Pa ~/.ssh/authorized_keys ,
366: though the
367: .Cm principals=
368: key option offers a similar facility (see
369: .Xr sshd 8
370: for details).
1.1 stevesk 371: .It Cm Banner
372: The contents of the specified file are sent to the remote user before
373: authentication is allowed.
1.78 djm 374: If the argument is
1.237 jmc 375: .Cm none
1.78 djm 376: then no banner is displayed.
1.1 stevesk 377: By default, no banner is displayed.
1.282 djm 378: .It Cm CASignatureAlgorithms
379: Specifies which algorithms are allowed for signing of certificates
380: by certificate authorities (CAs).
381: The default is:
382: .Bd -literal -offset indent
1.287 dtucker 383: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.282 djm 384: ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
385: .Ed
386: .Pp
387: Certificates signed using other algorithms will not be accepted for
388: public key or host-based authentication.
1.1 stevesk 389: .It Cm ChallengeResponseAuthentication
1.50 jmc 390: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 391: All authentication styles from
392: .Xr login.conf 5
393: are supported.
394: The default is
1.237 jmc 395: .Cm yes .
1.80 djm 396: .It Cm ChrootDirectory
1.113 stevesk 397: Specifies the pathname of a directory to
1.80 djm 398: .Xr chroot 2
399: to after authentication.
1.190 deraadt 400: At session startup
401: .Xr sshd 8
402: checks that all components of the pathname are root-owned directories
403: which are not writable by any other user or group.
1.106 stevesk 404: After the chroot,
405: .Xr sshd 8
406: changes the working directory to the user's home directory.
1.235 jmc 407: Arguments to
408: .Cm ChrootDirectory
409: accept the tokens described in the
410: .Sx TOKENS
411: section.
1.80 djm 412: .Pp
413: The
414: .Cm ChrootDirectory
415: must contain the necessary files and directories to support the
1.103 stevesk 416: user's session.
1.80 djm 417: For an interactive session this requires at least a shell, typically
418: .Xr sh 1 ,
419: and basic
420: .Pa /dev
421: nodes such as
422: .Xr null 4 ,
423: .Xr zero 4 ,
424: .Xr stdin 4 ,
425: .Xr stdout 4 ,
426: .Xr stderr 4 ,
1.180 jmc 427: and
1.80 djm 428: .Xr tty 4
429: devices.
1.237 jmc 430: For file transfer sessions using SFTP
431: no additional configuration of the environment is necessary if the in-process
432: sftp-server is used,
1.176 schwarze 433: though sessions which use logging may require
1.104 stevesk 434: .Pa /dev/log
1.176 schwarze 435: inside the chroot directory on some operating systems (see
1.104 stevesk 436: .Xr sftp-server 8
1.81 jmc 437: for details).
1.190 deraadt 438: .Pp
1.191 jmc 439: For safety, it is very important that the directory hierarchy be
1.190 deraadt 440: prevented from modification by other processes on the system (especially
441: those outside the jail).
442: Misconfiguration can lead to unsafe environments which
443: .Xr sshd 8
444: cannot detect.
1.80 djm 445: .Pp
1.215 djm 446: The default is
1.237 jmc 447: .Cm none ,
1.215 djm 448: indicating not to
1.80 djm 449: .Xr chroot 2 .
1.1 stevesk 450: .It Cm Ciphers
1.219 jmc 451: Specifies the ciphers allowed.
1.1 stevesk 452: Multiple ciphers must be comma-separated.
1.289 naddy 453: If the specified list begins with a
1.208 djm 454: .Sq +
455: character, then the specified ciphers will be appended to the default set
456: instead of replacing them.
1.289 naddy 457: If the specified list begins with a
1.242 djm 458: .Sq -
459: character, then the specified ciphers (including wildcards) will be removed
460: from the default set instead of replacing them.
1.290 naddy 461: If the specified list begins with a
462: .Sq ^
463: character, then the specified ciphers will be placed at the head of the
464: default set.
1.208 djm 465: .Pp
1.167 djm 466: The supported ciphers are:
467: .Pp
1.173 naddy 468: .Bl -item -compact -offset indent
469: .It
470: 3des-cbc
471: .It
472: aes128-cbc
473: .It
474: aes192-cbc
475: .It
476: aes256-cbc
477: .It
478: aes128-ctr
479: .It
480: aes192-ctr
481: .It
482: aes256-ctr
483: .It
484: aes128-gcm@openssh.com
485: .It
486: aes256-gcm@openssh.com
487: .It
488: chacha20-poly1305@openssh.com
489: .El
1.167 djm 490: .Pp
1.52 jmc 491: The default is:
1.173 naddy 492: .Bd -literal -offset indent
1.211 jmc 493: chacha20-poly1305@openssh.com,
1.173 naddy 494: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 495: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 496: .Ed
1.167 djm 497: .Pp
1.237 jmc 498: The list of available ciphers may also be obtained using
499: .Qq ssh -Q cipher .
1.1 stevesk 500: .It Cm ClientAliveCountMax
1.237 jmc 501: Sets the number of client alive messages which may be sent without
1.52 jmc 502: .Xr sshd 8
1.17 jmc 503: receiving any messages back from the client.
504: If this threshold is reached while client alive messages are being sent,
1.52 jmc 505: sshd will disconnect the client, terminating the session.
1.17 jmc 506: It is important to note that the use of client alive messages is very
507: different from
1.237 jmc 508: .Cm TCPKeepAlive .
1.17 jmc 509: The client alive messages are sent through the encrypted channel
510: and therefore will not be spoofable.
511: The TCP keepalive option enabled by
1.27 markus 512: .Cm TCPKeepAlive
1.17 jmc 513: is spoofable.
514: The client alive mechanism is valuable when the client or
1.288 dtucker 515: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 516: .Pp
1.17 jmc 517: The default value is 3.
518: If
1.1 stevesk 519: .Cm ClientAliveInterval
1.237 jmc 520: is set to 15, and
1.1 stevesk 521: .Cm ClientAliveCountMax
1.52 jmc 522: is left at the default, unresponsive SSH clients
1.1 stevesk 523: will be disconnected after approximately 45 seconds.
1.302 djm 524: Setting a zero
525: .Cm ClientAliveCountMax
526: disables connection termination.
1.42 djm 527: .It Cm ClientAliveInterval
528: Sets a timeout interval in seconds after which if no data has been received
529: from the client,
1.52 jmc 530: .Xr sshd 8
1.42 djm 531: will send a message through the encrypted
532: channel to request a response from the client.
533: The default
534: is 0, indicating that these messages will not be sent to the client.
1.3 markus 535: .It Cm Compression
1.236 djm 536: Specifies whether compression is enabled after
1.44 markus 537: the user has authenticated successfully.
1.3 markus 538: The argument must be
1.237 jmc 539: .Cm yes ,
540: .Cm delayed
1.236 djm 541: (a legacy synonym for
1.237 jmc 542: .Cm yes )
1.3 markus 543: or
1.237 jmc 544: .Cm no .
1.3 markus 545: The default is
1.237 jmc 546: .Cm yes .
1.1 stevesk 547: .It Cm DenyGroups
548: This keyword can be followed by a list of group name patterns, separated
549: by spaces.
550: Login is disallowed for users whose primary group or supplementary
551: group list matches one of the patterns.
552: Only group names are valid; a numerical group ID is not recognized.
553: By default, login is allowed for all groups.
1.301 djm 554: The allow/deny groups directives are processed in the following order:
1.54 jmc 555: .Cm DenyGroups ,
556: .Cm AllowGroups .
1.49 jmc 557: .Pp
1.161 jmc 558: See PATTERNS in
1.49 jmc 559: .Xr ssh_config 5
560: for more information on patterns.
1.1 stevesk 561: .It Cm DenyUsers
562: This keyword can be followed by a list of user name patterns, separated
563: by spaces.
564: Login is disallowed for user names that match one of the patterns.
565: Only user names are valid; a numerical user ID is not recognized.
566: By default, login is allowed for all users.
567: If the pattern takes the form USER@HOST then USER and HOST
568: are separately checked, restricting logins to particular
569: users from particular hosts.
1.222 jmc 570: HOST criteria may additionally contain addresses to match in CIDR
571: address/masklen format.
1.301 djm 572: The allow/deny users directives are processed in the following order:
1.54 jmc 573: .Cm DenyUsers ,
1.301 djm 574: .Cm AllowUsers .
1.49 jmc 575: .Pp
1.161 jmc 576: See PATTERNS in
1.49 jmc 577: .Xr ssh_config 5
578: for more information on patterns.
1.239 djm 579: .It Cm DisableForwarding
580: Disables all forwarding features, including X11,
581: .Xr ssh-agent 1 ,
582: TCP and StreamLocal.
583: This option overrides all other forwarding-related options and may
584: simplify restricted configurations.
1.246 djm 585: .It Cm ExposeAuthInfo
1.253 jmc 586: Writes a temporary file containing a list of authentication methods and
1.246 djm 587: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 588: The location of the file is exposed to the user session through the
1.248 djm 589: .Ev SSH_USER_AUTH
1.247 jmc 590: environment variable.
1.253 jmc 591: The default is
592: .Cm no .
1.183 djm 593: .It Cm FingerprintHash
594: Specifies the hash algorithm used when logging key fingerprints.
595: Valid options are:
1.237 jmc 596: .Cm md5
1.183 djm 597: and
1.237 jmc 598: .Cm sha256 .
1.183 djm 599: The default is
1.237 jmc 600: .Cm sha256 .
1.67 dtucker 601: .It Cm ForceCommand
602: Forces the execution of the command specified by
603: .Cm ForceCommand ,
1.84 djm 604: ignoring any command supplied by the client and
605: .Pa ~/.ssh/rc
606: if present.
1.67 dtucker 607: The command is invoked by using the user's login shell with the -c option.
608: This applies to shell, command, or subsystem execution.
609: It is most useful inside a
610: .Cm Match
611: block.
612: The command originally supplied by the client is available in the
613: .Ev SSH_ORIGINAL_COMMAND
614: environment variable.
1.82 djm 615: Specifying a command of
1.237 jmc 616: .Cm internal-sftp
617: will force the use of an in-process SFTP server that requires no support
1.82 djm 618: files when used with
619: .Cm ChrootDirectory .
1.215 djm 620: The default is
1.237 jmc 621: .Cm none .
1.1 stevesk 622: .It Cm GatewayPorts
623: Specifies whether remote hosts are allowed to connect to ports
624: forwarded for the client.
625: By default,
1.52 jmc 626: .Xr sshd 8
1.15 jmc 627: binds remote port forwardings to the loopback address.
628: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 629: .Cm GatewayPorts
1.52 jmc 630: can be used to specify that sshd
1.39 djm 631: should allow remote port forwardings to bind to non-loopback addresses, thus
632: allowing other hosts to connect.
633: The argument may be
1.237 jmc 634: .Cm no
1.39 djm 635: to force remote port forwardings to be available to the local host only,
1.237 jmc 636: .Cm yes
1.39 djm 637: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 638: .Cm clientspecified
1.39 djm 639: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 640: The default is
1.237 jmc 641: .Cm no .
1.23 markus 642: .It Cm GSSAPIAuthentication
1.25 markus 643: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 644: The default is
1.237 jmc 645: .Cm no .
1.23 markus 646: .It Cm GSSAPICleanupCredentials
647: Specifies whether to automatically destroy the user's credentials cache
648: on logout.
649: The default is
1.237 jmc 650: .Cm yes .
1.203 djm 651: .It Cm GSSAPIStrictAcceptorCheck
652: Determines whether to be strict about the identity of the GSSAPI acceptor
653: a client authenticates against.
654: If set to
1.237 jmc 655: .Cm yes
656: then the client must authenticate against the host
1.203 djm 657: service on the current hostname.
658: If set to
1.237 jmc 659: .Cm no
1.203 djm 660: then the client may authenticate against any service key stored in the
661: machine's default store.
662: This facility is provided to assist with operation on multi homed machines.
663: The default is
1.237 jmc 664: .Cm yes .
1.189 djm 665: .It Cm HostbasedAcceptedKeyTypes
666: Specifies the key types that will be accepted for hostbased authentication
1.280 djm 667: as a list of comma-separated patterns.
1.289 naddy 668: Alternately if the specified list begins with a
1.208 djm 669: .Sq +
670: character, then the specified key types will be appended to the default set
671: instead of replacing them.
1.289 naddy 672: If the specified list begins with a
1.242 djm 673: .Sq -
674: character, then the specified key types (including wildcards) will be removed
675: from the default set instead of replacing them.
1.290 naddy 676: If the specified list begins with a
677: .Sq ^
678: character, then the specified key types will be placed at the head of the
679: default set.
1.206 markus 680: The default for this option is:
681: .Bd -literal -offset 3n
682: ecdsa-sha2-nistp256-cert-v01@openssh.com,
683: ecdsa-sha2-nistp384-cert-v01@openssh.com,
684: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 685: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 686: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 687: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 688: rsa-sha2-512-cert-v01@openssh.com,
689: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 690: ssh-rsa-cert-v01@openssh.com,
691: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 692: sk-ecdsa-sha2-nistp256@openssh.com,
693: ssh-ed25519,sk-ssh-ed25519@openssh.com,
694: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 695: .Ed
696: .Pp
1.237 jmc 697: The list of available key types may also be obtained using
1.307 dtucker 698: .Qq ssh -Q HostbasedAcceptedKeyTypes .
1.1 stevesk 699: .It Cm HostbasedAuthentication
700: Specifies whether rhosts or /etc/hosts.equiv authentication together
701: with successful public key client host authentication is allowed
1.50 jmc 702: (host-based authentication).
1.70 dtucker 703: The default is
1.237 jmc 704: .Cm no .
1.70 dtucker 705: .It Cm HostbasedUsesNameFromPacketOnly
706: Specifies whether or not the server will attempt to perform a reverse
707: name lookup when matching the name in the
708: .Pa ~/.shosts ,
709: .Pa ~/.rhosts ,
710: and
711: .Pa /etc/hosts.equiv
712: files during
713: .Cm HostbasedAuthentication .
714: A setting of
1.237 jmc 715: .Cm yes
1.70 dtucker 716: means that
717: .Xr sshd 8
718: uses the name supplied by the client rather than
719: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 720: The default is
1.237 jmc 721: .Cm no .
1.117 djm 722: .It Cm HostCertificate
723: Specifies a file containing a public host certificate.
724: The certificate's public key must match a private host key already specified
725: by
726: .Cm HostKey .
727: The default behaviour of
728: .Xr sshd 8
729: is not to load any certificates.
1.1 stevesk 730: .It Cm HostKey
731: Specifies a file containing a private host key
732: used by SSH.
1.229 naddy 733: The defaults are
1.169 naddy 734: .Pa /etc/ssh/ssh_host_ecdsa_key ,
735: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 736: and
1.229 naddy 737: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 738: .Pp
1.1 stevesk 739: Note that
1.52 jmc 740: .Xr sshd 8
1.206 markus 741: will refuse to use a file if it is group/world-accessible
742: and that the
743: .Cm HostKeyAlgorithms
744: option restricts which of the keys are actually used by
745: .Xr sshd 8 .
746: .Pp
1.1 stevesk 747: It is possible to have multiple host key files.
1.162 markus 748: It is also possible to specify public host key files instead.
749: In this case operations on the private key will be delegated
750: to an
751: .Xr ssh-agent 1 .
752: .It Cm HostKeyAgent
753: Identifies the UNIX-domain socket used to communicate
754: with an agent that has access to the private host keys.
1.223 markus 755: If the string
1.237 jmc 756: .Qq SSH_AUTH_SOCK
1.162 markus 757: is specified, the location of the socket will be read from the
758: .Ev SSH_AUTH_SOCK
759: environment variable.
1.206 markus 760: .It Cm HostKeyAlgorithms
1.219 jmc 761: Specifies the host key algorithms
1.206 markus 762: that the server offers.
763: The default for this option is:
764: .Bd -literal -offset 3n
765: ecdsa-sha2-nistp256-cert-v01@openssh.com,
766: ecdsa-sha2-nistp384-cert-v01@openssh.com,
767: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 768: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.206 markus 769: ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 770: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 771: rsa-sha2-512-cert-v01@openssh.com,
772: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 773: ssh-rsa-cert-v01@openssh.com,
774: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.296 naddy 775: sk-ecdsa-sha2-nistp256@openssh.com,
776: ssh-ed25519,sk-ssh-ed25519@openssh.com,
777: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 778: .Ed
779: .Pp
1.237 jmc 780: The list of available key types may also be obtained using
1.307 dtucker 781: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 782: .It Cm IgnoreRhosts
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
1.304 djm 806: .Dq no .
807: .It Cm Include
808: Include the specified configuration file(s).
1.305 jmc 809: Multiple pathnames may be specified and each pathname may contain
1.304 djm 810: .Xr glob 7
811: wildcards.
812: Files without absolute paths are assumed to be in
813: .Pa /etc/ssh .
1.305 jmc 814: An
1.304 djm 815: .Cm Include
816: directive may appear inside a
817: .Cm Match
818: block
819: to perform conditional inclusion.
1.129 djm 820: .It Cm IPQoS
821: Specifies the IPv4 type-of-service or DSCP class for the connection.
822: Accepted values are
1.237 jmc 823: .Cm af11 ,
824: .Cm af12 ,
825: .Cm af13 ,
826: .Cm af21 ,
827: .Cm af22 ,
828: .Cm af23 ,
829: .Cm af31 ,
830: .Cm af32 ,
831: .Cm af33 ,
832: .Cm af41 ,
833: .Cm af42 ,
834: .Cm af43 ,
835: .Cm cs0 ,
836: .Cm cs1 ,
837: .Cm cs2 ,
838: .Cm cs3 ,
839: .Cm cs4 ,
840: .Cm cs5 ,
841: .Cm cs6 ,
842: .Cm cs7 ,
843: .Cm ef ,
1.303 djm 844: .Cm le ,
1.237 jmc 845: .Cm lowdelay ,
846: .Cm throughput ,
847: .Cm reliability ,
1.250 djm 848: a numeric value, or
849: .Cm none
850: to use the operating system default.
1.131 djm 851: This option may take one or two arguments, separated by whitespace.
1.129 djm 852: If one argument is specified, it is used as the packet class unconditionally.
853: If two values are specified, the first is automatically selected for
854: interactive sessions and the second for non-interactive sessions.
855: The default is
1.264 job 856: .Cm af21
1.265 jmc 857: (Low-Latency Data)
1.129 djm 858: for interactive sessions and
1.264 job 859: .Cm cs1
1.265 jmc 860: (Lower Effort)
1.129 djm 861: for non-interactive sessions.
1.171 jmc 862: .It Cm KbdInteractiveAuthentication
863: Specifies whether to allow keyboard-interactive authentication.
864: The argument to this keyword must be
1.237 jmc 865: .Cm yes
1.171 jmc 866: or
1.237 jmc 867: .Cm no .
1.171 jmc 868: The default is to use whatever value
869: .Cm ChallengeResponseAuthentication
870: is set to
871: (by default
1.237 jmc 872: .Cm yes ) .
1.1 stevesk 873: .It Cm KerberosAuthentication
1.24 markus 874: Specifies whether the password provided by the user for
1.1 stevesk 875: .Cm PasswordAuthentication
1.24 markus 876: will be validated through the Kerberos KDC.
1.1 stevesk 877: To use this option, the server needs a
878: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 879: The default is
1.237 jmc 880: .Cm no .
1.29 dtucker 881: .It Cm KerberosGetAFSToken
1.45 djm 882: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 883: an AFS token before accessing the user's home directory.
1.52 jmc 884: The default is
1.237 jmc 885: .Cm no .
1.1 stevesk 886: .It Cm KerberosOrLocalPasswd
1.52 jmc 887: If password authentication through Kerberos fails then
1.1 stevesk 888: the password will be validated via any additional local mechanism
889: such as
890: .Pa /etc/passwd .
1.52 jmc 891: The default is
1.237 jmc 892: .Cm yes .
1.1 stevesk 893: .It Cm KerberosTicketCleanup
894: Specifies whether to automatically destroy the user's ticket cache
895: file on logout.
1.52 jmc 896: The default is
1.237 jmc 897: .Cm yes .
1.127 djm 898: .It Cm KexAlgorithms
899: Specifies the available KEX (Key Exchange) algorithms.
900: Multiple algorithms must be comma-separated.
1.289 naddy 901: Alternately if the specified list begins with a
1.208 djm 902: .Sq +
903: character, then the specified methods will be appended to the default set
904: instead of replacing them.
1.289 naddy 905: If the specified list begins with a
1.242 djm 906: .Sq -
907: character, then the specified methods (including wildcards) will be removed
908: from the default set instead of replacing them.
1.290 naddy 909: If the specified list begins with a
910: .Sq ^
911: character, then the specified methods will be placed at the head of the
912: default set.
1.173 naddy 913: The supported algorithms are:
914: .Pp
915: .Bl -item -compact -offset indent
916: .It
1.234 djm 917: curve25519-sha256
918: .It
1.173 naddy 919: curve25519-sha256@libssh.org
920: .It
921: diffie-hellman-group1-sha1
922: .It
923: diffie-hellman-group14-sha1
924: .It
1.263 djm 925: diffie-hellman-group14-sha256
926: .It
927: diffie-hellman-group16-sha512
928: .It
929: diffie-hellman-group18-sha512
930: .It
1.173 naddy 931: diffie-hellman-group-exchange-sha1
932: .It
933: diffie-hellman-group-exchange-sha256
934: .It
935: ecdh-sha2-nistp256
936: .It
937: ecdh-sha2-nistp384
938: .It
939: ecdh-sha2-nistp521
1.298 dtucker 940: .It
941: sntrup4591761x25519-sha512@tinyssh.org
1.173 naddy 942: .El
943: .Pp
944: The default is:
1.170 dtucker 945: .Bd -literal -offset indent
1.234 djm 946: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 947: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
948: diffie-hellman-group-exchange-sha256,
1.263 djm 949: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 950: diffie-hellman-group14-sha256
1.170 dtucker 951: .Ed
1.188 djm 952: .Pp
1.237 jmc 953: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 954: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 955: .It Cm ListenAddress
956: Specifies the local addresses
1.52 jmc 957: .Xr sshd 8
1.1 stevesk 958: should listen on.
959: The following forms may be used:
960: .Pp
961: .Bl -item -offset indent -compact
962: .It
963: .Cm ListenAddress
964: .Sm off
1.259 jmc 965: .Ar hostname | address
1.1 stevesk 966: .Sm on
1.260 jmc 967: .Op Cm rdomain Ar domain
1.1 stevesk 968: .It
969: .Cm ListenAddress
970: .Sm off
1.259 jmc 971: .Ar hostname : port
1.1 stevesk 972: .Sm on
1.260 jmc 973: .Op Cm rdomain Ar domain
1.256 djm 974: .It
975: .Cm ListenAddress
976: .Sm off
1.259 jmc 977: .Ar IPv4_address : port
1.256 djm 978: .Sm on
1.260 jmc 979: .Op Cm rdomain Ar domain
1.1 stevesk 980: .It
981: .Cm ListenAddress
982: .Sm off
1.259 jmc 983: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 984: .Sm on
1.260 jmc 985: .Op Cm rdomain Ar domain
1.1 stevesk 986: .El
987: .Pp
1.256 djm 988: The optional
989: .Cm rdomain
990: qualifier requests
991: .Xr sshd 8
992: listen in an explicit routing domain.
1.1 stevesk 993: If
994: .Ar port
995: is not specified,
1.200 dtucker 996: sshd will listen on the address and all
1.1 stevesk 997: .Cm Port
1.17 jmc 998: options specified.
1.256 djm 999: The default is to listen on all local addresses on the current default
1000: routing domain.
1.15 jmc 1001: Multiple
1.1 stevesk 1002: .Cm ListenAddress
1.17 jmc 1003: options are permitted.
1.256 djm 1004: For more information on routing domains, see
1.259 jmc 1005: .Xr rdomain 4 .
1.1 stevesk 1006: .It Cm LoginGraceTime
1007: The server disconnects after this time if the user has not
1008: successfully logged in.
1009: If the value is 0, there is no time limit.
1.12 stevesk 1010: The default is 120 seconds.
1.1 stevesk 1011: .It Cm LogLevel
1012: Gives the verbosity level that is used when logging messages from
1.53 jmc 1013: .Xr sshd 8 .
1.1 stevesk 1014: The possible values are:
1.52 jmc 1015: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1016: The default is INFO.
1017: DEBUG and DEBUG1 are equivalent.
1018: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1019: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 1020: .It Cm MACs
1021: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1022: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1023: Multiple algorithms must be comma-separated.
1.289 naddy 1024: If the specified list begins with a
1.208 djm 1025: .Sq +
1026: character, then the specified algorithms will be appended to the default set
1027: instead of replacing them.
1.289 naddy 1028: If the specified list begins with a
1.242 djm 1029: .Sq -
1030: character, then the specified algorithms (including wildcards) will be removed
1031: from the default set instead of replacing them.
1.290 naddy 1032: If the specified list begins with a
1033: .Sq ^
1034: character, then the specified algorithms will be placed at the head of the
1035: default set.
1.208 djm 1036: .Pp
1.152 markus 1037: The algorithms that contain
1.237 jmc 1038: .Qq -etm
1.152 markus 1039: calculate the MAC after encryption (encrypt-then-mac).
1040: These are considered safer and their use recommended.
1.173 naddy 1041: The supported MACs are:
1042: .Pp
1043: .Bl -item -compact -offset indent
1044: .It
1045: hmac-md5
1046: .It
1047: hmac-md5-96
1048: .It
1049: hmac-sha1
1050: .It
1051: hmac-sha1-96
1052: .It
1053: hmac-sha2-256
1054: .It
1055: hmac-sha2-512
1056: .It
1057: umac-64@openssh.com
1058: .It
1059: umac-128@openssh.com
1060: .It
1061: hmac-md5-etm@openssh.com
1062: .It
1063: hmac-md5-96-etm@openssh.com
1064: .It
1065: hmac-sha1-etm@openssh.com
1066: .It
1067: hmac-sha1-96-etm@openssh.com
1068: .It
1069: hmac-sha2-256-etm@openssh.com
1070: .It
1071: hmac-sha2-512-etm@openssh.com
1072: .It
1073: umac-64-etm@openssh.com
1074: .It
1075: umac-128-etm@openssh.com
1076: .El
1077: .Pp
1.52 jmc 1078: The default is:
1.77 jmc 1079: .Bd -literal -offset indent
1.152 markus 1080: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1081: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1082: hmac-sha1-etm@openssh.com,
1.173 naddy 1083: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1084: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1085: .Ed
1.188 djm 1086: .Pp
1.237 jmc 1087: The list of available MAC algorithms may also be obtained using
1088: .Qq ssh -Q mac .
1.60 dtucker 1089: .It Cm Match
1.61 jmc 1090: Introduces a conditional block.
1.65 dtucker 1091: If all of the criteria on the
1.60 dtucker 1092: .Cm Match
1.65 dtucker 1093: line are satisfied, the keywords on the following lines override those
1094: set in the global section of the config file, until either another
1.60 dtucker 1095: .Cm Match
1.65 dtucker 1096: line or the end of the file.
1.172 djm 1097: If a keyword appears in multiple
1098: .Cm Match
1.177 sobrado 1099: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1100: applied.
1.91 djm 1101: .Pp
1.61 jmc 1102: The arguments to
1.60 dtucker 1103: .Cm Match
1.163 dtucker 1104: are one or more criteria-pattern pairs or the single token
1105: .Cm All
1106: which matches all criteria.
1.60 dtucker 1107: The available criteria are
1108: .Cm User ,
1.69 dtucker 1109: .Cm Group ,
1.60 dtucker 1110: .Cm Host ,
1.139 dtucker 1111: .Cm LocalAddress ,
1112: .Cm LocalPort ,
1.258 djm 1113: .Cm RDomain ,
1.60 dtucker 1114: and
1.258 djm 1115: .Cm Address
1116: (with
1117: .Cm RDomain
1118: representing the
1119: .Xr rdomain 4
1.284 jmc 1120: on which the connection was received).
1.258 djm 1121: .Pp
1.91 djm 1122: The match patterns may consist of single entries or comma-separated
1123: lists and may use the wildcard and negation operators described in the
1.237 jmc 1124: .Sx PATTERNS
1125: section of
1.92 djm 1126: .Xr ssh_config 5 .
1.91 djm 1127: .Pp
1128: The patterns in an
1129: .Cm Address
1130: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1131: address/masklen format,
1132: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1133: Note that the mask length provided must be consistent with the address -
1134: it is an error to specify a mask length that is too long for the address
1.93 jmc 1135: or one with bits set in this host portion of the address.
1.237 jmc 1136: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1137: .Pp
1.60 dtucker 1138: Only a subset of keywords may be used on the lines following a
1139: .Cm Match
1140: keyword.
1141: Available keywords are
1.142 jmc 1142: .Cm AcceptEnv ,
1.99 okan 1143: .Cm AllowAgentForwarding ,
1.142 jmc 1144: .Cm AllowGroups ,
1.193 djm 1145: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1146: .Cm AllowTcpForwarding ,
1.141 markus 1147: .Cm AllowUsers ,
1.149 djm 1148: .Cm AuthenticationMethods ,
1.146 djm 1149: .Cm AuthorizedKeysCommand ,
1150: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1151: .Cm AuthorizedKeysFile ,
1.214 djm 1152: .Cm AuthorizedPrincipalsCommand ,
1153: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1154: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1155: .Cm Banner ,
1.85 djm 1156: .Cm ChrootDirectory ,
1.238 markus 1157: .Cm ClientAliveCountMax ,
1158: .Cm ClientAliveInterval ,
1.141 markus 1159: .Cm DenyGroups ,
1160: .Cm DenyUsers ,
1.67 dtucker 1161: .Cm ForceCommand ,
1.194 djm 1162: .Cm GatewayPorts ,
1.193 djm 1163: .Cm GSSAPIAuthentication ,
1.189 djm 1164: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1165: .Cm HostbasedAuthentication ,
1.123 djm 1166: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 ! djm 1167: .Cm IgnoreRhosts ,
1.306 naddy 1168: .Cm Include ,
1.193 djm 1169: .Cm IPQoS ,
1.74 jmc 1170: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1171: .Cm KerberosAuthentication ,
1.245 djm 1172: .Cm LogLevel ,
1.95 dtucker 1173: .Cm MaxAuthTries ,
1.94 dtucker 1174: .Cm MaxSessions ,
1.72 dtucker 1175: .Cm PasswordAuthentication ,
1.97 djm 1176: .Cm PermitEmptyPasswords ,
1.271 djm 1177: .Cm PermitListen ,
1.66 dtucker 1178: .Cm PermitOpen ,
1.79 dtucker 1179: .Cm PermitRootLogin ,
1.164 djm 1180: .Cm PermitTTY ,
1.123 djm 1181: .Cm PermitTunnel ,
1.174 djm 1182: .Cm PermitUserRC ,
1.189 djm 1183: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1184: .Cm PubkeyAuthentication ,
1.159 dtucker 1185: .Cm RekeyLimit ,
1.193 djm 1186: .Cm RevokedKeys ,
1.257 djm 1187: .Cm RDomain ,
1.274 djm 1188: .Cm SetEnv ,
1.193 djm 1189: .Cm StreamLocalBindMask ,
1190: .Cm StreamLocalBindUnlink ,
1191: .Cm TrustedUserCAKeys ,
1.66 dtucker 1192: .Cm X11DisplayOffset ,
1.101 djm 1193: .Cm X11Forwarding
1.60 dtucker 1194: and
1.286 jmc 1195: .Cm X11UseLocalhost .
1.33 dtucker 1196: .It Cm MaxAuthTries
1197: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1198: connection.
1199: Once the number of failures reaches half this value,
1200: additional failures are logged.
1201: The default is 6.
1.90 djm 1202: .It Cm MaxSessions
1.216 djm 1203: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1204: sessions permitted per network connection.
1205: Multiple sessions may be established by clients that support connection
1206: multiplexing.
1207: Setting
1208: .Cm MaxSessions
1209: to 1 will effectively disable session multiplexing, whereas setting it to 0
1210: will prevent all shell, login and subsystem sessions while still permitting
1211: forwarding.
1.90 djm 1212: The default is 10.
1.1 stevesk 1213: .It Cm MaxStartups
1214: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1215: SSH daemon.
1.1 stevesk 1216: Additional connections will be dropped until authentication succeeds or the
1217: .Cm LoginGraceTime
1218: expires for a connection.
1.156 dtucker 1219: The default is 10:30:100.
1.1 stevesk 1220: .Pp
1221: Alternatively, random early drop can be enabled by specifying
1222: the three colon separated values
1.237 jmc 1223: start:rate:full (e.g. "10:30:60").
1.53 jmc 1224: .Xr sshd 8
1.237 jmc 1225: will refuse connection attempts with a probability of rate/100 (30%)
1226: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1227: The probability increases linearly and all connection attempts
1.237 jmc 1228: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1229: .It Cm PasswordAuthentication
1230: Specifies whether password authentication is allowed.
1231: The default is
1.237 jmc 1232: .Cm yes .
1.1 stevesk 1233: .It Cm PermitEmptyPasswords
1234: When password authentication is allowed, it specifies whether the
1235: server allows login to accounts with empty password strings.
1236: The default is
1.237 jmc 1237: .Cm no .
1.271 djm 1238: .It Cm PermitListen
1239: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1240: The listen specification must be one of the following forms:
1241: .Pp
1242: .Bl -item -offset indent -compact
1243: .It
1244: .Cm PermitListen
1245: .Sm off
1.276 djm 1246: .Ar port
1.271 djm 1247: .Sm on
1248: .It
1249: .Cm PermitListen
1250: .Sm off
1.276 djm 1251: .Ar host : port
1.271 djm 1252: .Sm on
1253: .El
1254: .Pp
1255: Multiple permissions may be specified by separating them with whitespace.
1256: An argument of
1257: .Cm any
1258: can be used to remove all restrictions and permit any listen requests.
1259: An argument of
1260: .Cm none
1261: can be used to prohibit all listen requests.
1262: The host name may contain wildcards as described in the PATTERNS section in
1263: .Xr ssh_config 5 .
1264: The wildcard
1265: .Sq *
1266: can also be used in place of a port number to allow all ports.
1267: By default all port forwarding listen requests are permitted.
1.272 jmc 1268: Note that the
1.271 djm 1269: .Cm GatewayPorts
1270: option may further restrict which addresses may be listened on.
1.276 djm 1271: Note also that
1272: .Xr ssh 1
1273: will request a listen host of
1274: .Dq localhost
1.283 dtucker 1275: if no listen host was specifically requested, and this name is
1.277 jmc 1276: treated differently to explicit localhost addresses of
1.276 djm 1277: .Dq 127.0.0.1
1278: and
1279: .Dq ::1 .
1.62 dtucker 1280: .It Cm PermitOpen
1281: Specifies the destinations to which TCP port forwarding is permitted.
1282: The forwarding specification must be one of the following forms:
1283: .Pp
1284: .Bl -item -offset indent -compact
1285: .It
1286: .Cm PermitOpen
1287: .Sm off
1288: .Ar host : port
1289: .Sm on
1290: .It
1291: .Cm PermitOpen
1292: .Sm off
1293: .Ar IPv4_addr : port
1294: .Sm on
1295: .It
1296: .Cm PermitOpen
1297: .Sm off
1298: .Ar \&[ IPv6_addr \&] : port
1299: .Sm on
1300: .El
1301: .Pp
1.68 dtucker 1302: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1303: An argument of
1.237 jmc 1304: .Cm any
1.62 dtucker 1305: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1306: An argument of
1.237 jmc 1307: .Cm none
1.140 dtucker 1308: can be used to prohibit all forwarding requests.
1.227 jmc 1309: The wildcard
1.237 jmc 1310: .Sq *
1.299 djm 1311: can be used for host or port to allow all hosts or ports respectively.
1312: Otherwise, no pattern matching or address lookups are performed on supplied
1313: names.
1.63 jmc 1314: By default all port forwarding requests are permitted.
1.1 stevesk 1315: .It Cm PermitRootLogin
1.38 jmc 1316: Specifies whether root can log in using
1.1 stevesk 1317: .Xr ssh 1 .
1318: The argument must be
1.237 jmc 1319: .Cm yes ,
1320: .Cm prohibit-password ,
1321: .Cm forced-commands-only ,
1.1 stevesk 1322: or
1.237 jmc 1323: .Cm no .
1.1 stevesk 1324: The default is
1.237 jmc 1325: .Cm prohibit-password .
1.1 stevesk 1326: .Pp
1327: If this option is set to
1.237 jmc 1328: .Cm prohibit-password
1.255 jmc 1329: (or its deprecated alias,
1330: .Cm without-password ) ,
1.210 deraadt 1331: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1332: .Pp
1333: If this option is set to
1.237 jmc 1334: .Cm forced-commands-only ,
1.1 stevesk 1335: root login with public key authentication will be allowed,
1336: but only if the
1337: .Ar command
1338: option has been specified
1339: (which may be useful for taking remote backups even if root login is
1.17 jmc 1340: normally not allowed).
1341: All other authentication methods are disabled for root.
1.1 stevesk 1342: .Pp
1343: If this option is set to
1.237 jmc 1344: .Cm no ,
1.38 jmc 1345: root is not allowed to log in.
1.231 jmc 1346: .It Cm PermitTTY
1347: Specifies whether
1348: .Xr pty 4
1349: allocation is permitted.
1350: The default is
1.237 jmc 1351: .Cm yes .
1.46 reyk 1352: .It Cm PermitTunnel
1353: Specifies whether
1354: .Xr tun 4
1355: device forwarding is allowed.
1.47 reyk 1356: The argument must be
1.237 jmc 1357: .Cm yes ,
1358: .Cm point-to-point
1.58 stevesk 1359: (layer 3),
1.237 jmc 1360: .Cm ethernet
1.58 stevesk 1361: (layer 2), or
1.237 jmc 1362: .Cm no .
1.58 stevesk 1363: Specifying
1.237 jmc 1364: .Cm yes
1.58 stevesk 1365: permits both
1.237 jmc 1366: .Cm point-to-point
1.58 stevesk 1367: and
1.237 jmc 1368: .Cm ethernet .
1.46 reyk 1369: The default is
1.237 jmc 1370: .Cm no .
1.178 djm 1371: .Pp
1372: Independent of this setting, the permissions of the selected
1373: .Xr tun 4
1374: device must allow access to the user.
1.6 markus 1375: .It Cm PermitUserEnvironment
1376: Specifies whether
1377: .Pa ~/.ssh/environment
1.9 stevesk 1378: and
1.6 markus 1379: .Cm environment=
1380: options in
1381: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1382: are processed by
1.52 jmc 1383: .Xr sshd 8 .
1.278 djm 1384: Valid options are
1385: .Cm yes ,
1386: .Cm no
1387: or a pattern-list specifying which environment variable names to accept
1388: (for example
1389: .Qq LANG,LC_* ) .
1.6 markus 1390: The default is
1.237 jmc 1391: .Cm no .
1.9 stevesk 1392: Enabling environment processing may enable users to bypass access
1393: restrictions in some configurations using mechanisms such as
1394: .Ev LD_PRELOAD .
1.174 djm 1395: .It Cm PermitUserRC
1396: Specifies whether any
1397: .Pa ~/.ssh/rc
1398: file is executed.
1399: The default is
1.237 jmc 1400: .Cm yes .
1.1 stevesk 1401: .It Cm PidFile
1.4 stevesk 1402: Specifies the file that contains the process ID of the
1.195 dtucker 1403: SSH daemon, or
1.237 jmc 1404: .Cm none
1.195 dtucker 1405: to not write one.
1.1 stevesk 1406: The default is
1407: .Pa /var/run/sshd.pid .
1408: .It Cm Port
1409: Specifies the port number that
1.52 jmc 1410: .Xr sshd 8
1.1 stevesk 1411: listens on.
1412: The default is 22.
1413: Multiple options of this type are permitted.
1414: See also
1415: .Cm ListenAddress .
1416: .It Cm PrintLastLog
1417: Specifies whether
1.52 jmc 1418: .Xr sshd 8
1.36 jaredy 1419: should print the date and time of the last user login when a user logs
1420: in interactively.
1.1 stevesk 1421: The default is
1.237 jmc 1422: .Cm yes .
1.1 stevesk 1423: .It Cm PrintMotd
1424: Specifies whether
1.52 jmc 1425: .Xr sshd 8
1.1 stevesk 1426: should print
1427: .Pa /etc/motd
1428: when a user logs in interactively.
1429: (On some systems it is also printed by the shell,
1430: .Pa /etc/profile ,
1431: or equivalent.)
1432: The default is
1.237 jmc 1433: .Cm yes .
1.189 djm 1434: .It Cm PubkeyAcceptedKeyTypes
1435: Specifies the key types that will be accepted for public key authentication
1.280 djm 1436: as a list of comma-separated patterns.
1.289 naddy 1437: Alternately if the specified list begins with a
1.208 djm 1438: .Sq +
1439: character, then the specified key types will be appended to the default set
1440: instead of replacing them.
1.289 naddy 1441: If the specified list begins with a
1.242 djm 1442: .Sq -
1443: character, then the specified key types (including wildcards) will be removed
1444: from the default set instead of replacing them.
1.290 naddy 1445: If the specified list begins with a
1446: .Sq ^
1447: character, then the specified key types will be placed at the head of the
1448: default set.
1.206 markus 1449: The default for this option is:
1450: .Bd -literal -offset 3n
1451: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1452: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1453: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.296 naddy 1454: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1455: ssh-ed25519-cert-v01@openssh.com,
1.292 djm 1456: sk-ssh-ed25519-cert-v01@openssh.com,
1.291 naddy 1457: rsa-sha2-512-cert-v01@openssh.com,
1458: rsa-sha2-256-cert-v01@openssh.com,
1.206 markus 1459: ssh-rsa-cert-v01@openssh.com,
1.296 naddy 1460: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.291 naddy 1461: sk-ecdsa-sha2-nistp256@openssh.com,
1.296 naddy 1462: ssh-ed25519,sk-ssh-ed25519@openssh.com,
1463: rsa-sha2-512,rsa-sha2-256,ssh-rsa
1.206 markus 1464: .Ed
1465: .Pp
1.237 jmc 1466: The list of available key types may also be obtained using
1.307 dtucker 1467: .Qq ssh -Q PubkeyAcceptedKeyTypes .
1.293 djm 1468: .It Cm PubkeyAuthOptions
1469: Sets one or more public key authentication options.
1470: Two option keywords are currently supported:
1.294 jmc 1471: .Cm none
1472: (the default; indicating no additional options are enabled)
1.293 djm 1473: and
1474: .Cm touch-required .
1475: .Pp
1476: The
1477: .Cm touch-required
1.297 naddy 1478: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1479: (i.e.\&
1.293 djm 1480: .Cm ecdsa-sk
1481: or
1482: .Cm ed25519-sk )
1483: to always require the signature to attest that a physically present user
1.297 naddy 1484: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1485: By default,
1486: .Xr sshd 8
1.297 naddy 1487: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1488: The
1489: .Cm touch-required
1490: flag disables this override.
1.297 naddy 1491: This option has no effect for other, non-authenticator public key types.
1.1 stevesk 1492: .It Cm PubkeyAuthentication
1493: Specifies whether public key authentication is allowed.
1494: The default is
1.237 jmc 1495: .Cm yes .
1.159 dtucker 1496: .It Cm RekeyLimit
1497: Specifies the maximum amount of data that may be transmitted before the
1498: session key is renegotiated, optionally followed a maximum amount of
1499: time that may pass before the session key is renegotiated.
1500: The first argument is specified in bytes and may have a suffix of
1501: .Sq K ,
1502: .Sq M ,
1503: or
1504: .Sq G
1505: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1506: The default is between
1507: .Sq 1G
1508: and
1509: .Sq 4G ,
1510: depending on the cipher.
1511: The optional second value is specified in seconds and may use any of the
1512: units documented in the
1513: .Sx TIME FORMATS
1.160 jmc 1514: section.
1.159 dtucker 1515: The default value for
1516: .Cm RekeyLimit
1517: is
1.237 jmc 1518: .Cm default none ,
1.159 dtucker 1519: which means that rekeying is performed after the cipher's default amount
1520: of data has been sent or received and no time based rekeying is done.
1.118 djm 1521: .It Cm RevokedKeys
1.195 dtucker 1522: Specifies revoked public keys file, or
1.237 jmc 1523: .Cm none
1.195 dtucker 1524: to not use one.
1.118 djm 1525: Keys listed in this file will be refused for public key authentication.
1526: Note that if this file is not readable, then public key authentication will
1527: be refused for all users.
1.154 djm 1528: Keys may be specified as a text file, listing one public key per line, or as
1529: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1530: .Xr ssh-keygen 1 .
1.161 jmc 1531: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1532: .Xr ssh-keygen 1 .
1.257 djm 1533: .It Cm RDomain
1534: Specifies an explicit routing domain that is applied after authentication
1535: has completed.
1.259 jmc 1536: The user session, as well and any forwarded or listening IP sockets,
1537: will be bound to this
1.257 djm 1538: .Xr rdomain 4 .
1539: If the routing domain is set to
1540: .Cm \&%D ,
1.259 jmc 1541: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1542: .It Cm SecurityKeyProvider
1.297 naddy 1543: Specifies a path to a library that will be used when loading
1544: FIDO authenticator-hosted keys, overriding the default of using
1545: the built-in USB HID support.
1.274 djm 1546: .It Cm SetEnv
1547: Specifies one or more environment variables to set in child sessions started
1548: by
1549: .Xr sshd 8
1550: as
1551: .Dq NAME=VALUE .
1552: The environment value may be quoted (e.g. if it contains whitespace
1553: characters).
1554: Environment variables set by
1555: .Cm SetEnv
1556: override the default environment and any variables specified by the user
1557: via
1558: .Cm AcceptEnv
1559: or
1560: .Cm PermitUserEnvironment .
1.175 millert 1561: .It Cm StreamLocalBindMask
1562: Sets the octal file creation mode mask
1563: .Pq umask
1564: used when creating a Unix-domain socket file for local or remote
1565: port forwarding.
1566: This option is only used for port forwarding to a Unix-domain socket file.
1567: .Pp
1568: The default value is 0177, which creates a Unix-domain socket file that is
1569: readable and writable only by the owner.
1570: Note that not all operating systems honor the file mode on Unix-domain
1571: socket files.
1572: .It Cm StreamLocalBindUnlink
1573: Specifies whether to remove an existing Unix-domain socket file for local
1574: or remote port forwarding before creating a new one.
1575: If the socket file already exists and
1576: .Cm StreamLocalBindUnlink
1577: is not enabled,
1578: .Nm sshd
1579: will be unable to forward the port to the Unix-domain socket file.
1580: This option is only used for port forwarding to a Unix-domain socket file.
1581: .Pp
1582: The argument must be
1.237 jmc 1583: .Cm yes
1.175 millert 1584: or
1.237 jmc 1585: .Cm no .
1.175 millert 1586: The default is
1.237 jmc 1587: .Cm no .
1.1 stevesk 1588: .It Cm StrictModes
1589: Specifies whether
1.52 jmc 1590: .Xr sshd 8
1.1 stevesk 1591: should check file modes and ownership of the
1592: user's files and home directory before accepting login.
1593: This is normally desirable because novices sometimes accidentally leave their
1594: directory or files world-writable.
1595: The default is
1.237 jmc 1596: .Cm yes .
1.112 djm 1597: Note that this does not apply to
1598: .Cm ChrootDirectory ,
1599: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1600: .It Cm Subsystem
1.51 jmc 1601: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1602: Arguments should be a subsystem name and a command (with optional arguments)
1603: to execute upon subsystem request.
1.80 djm 1604: .Pp
1.1 stevesk 1605: The command
1.237 jmc 1606: .Cm sftp-server
1607: implements the SFTP file transfer subsystem.
1.80 djm 1608: .Pp
1609: Alternately the name
1.237 jmc 1610: .Cm internal-sftp
1611: implements an in-process SFTP server.
1.80 djm 1612: This may simplify configurations using
1613: .Cm ChrootDirectory
1614: to force a different filesystem root on clients.
1615: .Pp
1.1 stevesk 1616: By default no subsystems are defined.
1617: .It Cm SyslogFacility
1618: Gives the facility code that is used when logging messages from
1.53 jmc 1619: .Xr sshd 8 .
1.1 stevesk 1620: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1621: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1622: The default is AUTH.
1.27 markus 1623: .It Cm TCPKeepAlive
1624: Specifies whether the system should send TCP keepalive messages to the
1625: other side.
1626: If they are sent, death of the connection or crash of one
1627: of the machines will be properly noticed.
1628: However, this means that
1629: connections will die if the route is down temporarily, and some people
1630: find it annoying.
1631: On the other hand, if TCP keepalives are not sent,
1632: sessions may hang indefinitely on the server, leaving
1.237 jmc 1633: .Qq ghost
1.27 markus 1634: users and consuming server resources.
1635: .Pp
1636: The default is
1.237 jmc 1637: .Cm yes
1.27 markus 1638: (to send TCP keepalive messages), and the server will notice
1639: if the network goes down or the client host crashes.
1640: This avoids infinitely hanging sessions.
1641: .Pp
1642: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1643: .Cm no .
1.118 djm 1644: .It Cm TrustedUserCAKeys
1645: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1646: trusted to sign user certificates for authentication, or
1.237 jmc 1647: .Cm none
1.195 dtucker 1648: to not use one.
1.119 jmc 1649: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1650: .Ql #
1651: are allowed.
1652: If a certificate is presented for authentication and has its signing CA key
1653: listed in this file, then it may be used for authentication for any user
1654: listed in the certificate's principals list.
1655: Note that certificates that lack a list of principals will not be permitted
1656: for authentication using
1657: .Cm TrustedUserCAKeys .
1.161 jmc 1658: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1659: .Xr ssh-keygen 1 .
1.18 markus 1660: .It Cm UseDNS
1661: Specifies whether
1.52 jmc 1662: .Xr sshd 8
1.207 djm 1663: should look up the remote host name, and to check that
1.18 markus 1664: the resolved host name for the remote IP address maps back to the
1665: very same IP address.
1.207 djm 1666: .Pp
1667: If this option is set to
1.237 jmc 1668: .Cm no
1.207 djm 1669: (the default) then only addresses and not host names may be used in
1.221 djm 1670: .Pa ~/.ssh/authorized_keys
1.207 djm 1671: .Cm from
1672: and
1.211 jmc 1673: .Nm
1.207 djm 1674: .Cm Match
1675: .Cm Host
1676: directives.
1.137 djm 1677: .It Cm VersionAddendum
1678: Optionally specifies additional text to append to the SSH protocol banner
1679: sent by the server upon connection.
1680: The default is
1.237 jmc 1681: .Cm none .
1.1 stevesk 1682: .It Cm X11DisplayOffset
1683: Specifies the first display number available for
1.52 jmc 1684: .Xr sshd 8 Ns 's
1.1 stevesk 1685: X11 forwarding.
1.52 jmc 1686: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1687: The default is 10.
1688: .It Cm X11Forwarding
1689: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1690: The argument must be
1.237 jmc 1691: .Cm yes
1.13 stevesk 1692: or
1.237 jmc 1693: .Cm no .
1.1 stevesk 1694: The default is
1.237 jmc 1695: .Cm no .
1.13 stevesk 1696: .Pp
1697: When X11 forwarding is enabled, there may be additional exposure to
1698: the server and to client displays if the
1.52 jmc 1699: .Xr sshd 8
1.13 stevesk 1700: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1701: .Cm X11UseLocalhost ) ,
1702: though this is not the default.
1.13 stevesk 1703: Additionally, the authentication spoofing and authentication data
1704: verification and substitution occur on the client side.
1705: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1706: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1707: forwarding (see the warnings for
1708: .Cm ForwardX11
1709: in
1.19 jmc 1710: .Xr ssh_config 5 ) .
1.13 stevesk 1711: A system administrator may have a stance in which they want to
1712: protect clients that may expose themselves to attack by unwittingly
1713: requesting X11 forwarding, which can warrant a
1.237 jmc 1714: .Cm no
1.13 stevesk 1715: setting.
1716: .Pp
1717: Note that disabling X11 forwarding does not prevent users from
1718: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1719: .It Cm X11UseLocalhost
1720: Specifies whether
1.52 jmc 1721: .Xr sshd 8
1.1 stevesk 1722: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1723: the wildcard address.
1724: By default,
1.52 jmc 1725: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1726: hostname part of the
1727: .Ev DISPLAY
1728: environment variable to
1.237 jmc 1729: .Cm localhost .
1.8 stevesk 1730: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1731: However, some older X11 clients may not function with this
1732: configuration.
1733: .Cm X11UseLocalhost
1734: may be set to
1.237 jmc 1735: .Cm no
1.1 stevesk 1736: to specify that the forwarding server should be bound to the wildcard
1737: address.
1738: The argument must be
1.237 jmc 1739: .Cm yes
1.1 stevesk 1740: or
1.237 jmc 1741: .Cm no .
1.1 stevesk 1742: The default is
1.237 jmc 1743: .Cm yes .
1.1 stevesk 1744: .It Cm XAuthLocation
1.11 stevesk 1745: Specifies the full pathname of the
1.1 stevesk 1746: .Xr xauth 1
1.195 dtucker 1747: program, or
1.237 jmc 1748: .Cm none
1.195 dtucker 1749: to not use one.
1.1 stevesk 1750: The default is
1751: .Pa /usr/X11R6/bin/xauth .
1752: .El
1.55 jmc 1753: .Sh TIME FORMATS
1.53 jmc 1754: .Xr sshd 8
1.1 stevesk 1755: command-line arguments and configuration file options that specify time
1756: may be expressed using a sequence of the form:
1757: .Sm off
1.7 stevesk 1758: .Ar time Op Ar qualifier ,
1.1 stevesk 1759: .Sm on
1760: where
1761: .Ar time
1762: is a positive integer value and
1763: .Ar qualifier
1764: is one of the following:
1765: .Pp
1766: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1767: .It Aq Cm none
1.1 stevesk 1768: seconds
1769: .It Cm s | Cm S
1770: seconds
1771: .It Cm m | Cm M
1772: minutes
1773: .It Cm h | Cm H
1774: hours
1775: .It Cm d | Cm D
1776: days
1777: .It Cm w | Cm W
1778: weeks
1779: .El
1780: .Pp
1781: Each member of the sequence is added together to calculate
1782: the total time value.
1783: .Pp
1784: Time format examples:
1785: .Pp
1786: .Bl -tag -width Ds -compact -offset indent
1787: .It 600
1788: 600 seconds (10 minutes)
1789: .It 10m
1790: 10 minutes
1791: .It 1h30m
1792: 1 hour 30 minutes (90 minutes)
1793: .El
1.235 jmc 1794: .Sh TOKENS
1795: Arguments to some keywords can make use of tokens,
1796: which are expanded at runtime:
1797: .Pp
1798: .Bl -tag -width XXXX -offset indent -compact
1799: .It %%
1800: A literal
1801: .Sq % .
1.257 djm 1802: .It \&%D
1803: The routing domain in which the incoming connection was received.
1.235 jmc 1804: .It %F
1805: The fingerprint of the CA key.
1806: .It %f
1807: The fingerprint of the key or certificate.
1808: .It %h
1809: The home directory of the user.
1810: .It %i
1811: The key ID in the certificate.
1812: .It %K
1813: The base64-encoded CA key.
1814: .It %k
1815: The base64-encoded key or certificate for authentication.
1816: .It %s
1817: The serial number of the certificate.
1818: .It \&%T
1819: The type of the CA key.
1820: .It %t
1821: The key or certificate type.
1.268 jmc 1822: .It \&%U
1823: The numeric user ID of the target user.
1.235 jmc 1824: .It %u
1825: The username.
1826: .El
1827: .Pp
1828: .Cm AuthorizedKeysCommand
1.270 jmc 1829: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1830: .Pp
1831: .Cm AuthorizedKeysFile
1.270 jmc 1832: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1833: .Pp
1834: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1835: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1836: .Pp
1837: .Cm AuthorizedPrincipalsFile
1.270 jmc 1838: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1839: .Pp
1840: .Cm ChrootDirectory
1.270 jmc 1841: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1842: .Pp
1843: .Cm RoutingDomain
1844: accepts the token %D.
1.1 stevesk 1845: .Sh FILES
1846: .Bl -tag -width Ds
1847: .It Pa /etc/ssh/sshd_config
1848: Contains configuration data for
1.53 jmc 1849: .Xr sshd 8 .
1.1 stevesk 1850: This file should be writable by root only, but it is recommended
1851: (though not necessary) that it be world-readable.
1852: .El
1.19 jmc 1853: .Sh SEE ALSO
1.237 jmc 1854: .Xr sftp-server 8 ,
1.19 jmc 1855: .Xr sshd 8
1.1 stevesk 1856: .Sh AUTHORS
1.237 jmc 1857: .An -nosplit
1.1 stevesk 1858: OpenSSH is a derivative of the original and free
1.237 jmc 1859: ssh 1.2.12 release by
1860: .An Tatu Ylonen .
1861: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1862: .An Theo de Raadt
1863: and
1864: .An Dug Song
1.1 stevesk 1865: removed many bugs, re-added newer features and
1866: created OpenSSH.
1.237 jmc 1867: .An Markus Friedl
1868: contributed the support for SSH protocol versions 1.5 and 2.0.
1869: .An Niels Provos
1870: and
1871: .An Markus Friedl
1872: contributed support for privilege separation.
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd_config.5 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh