Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.236
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: .\"
1.236 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.235 2016/09/22 19:19:01 jmc Exp $
1.235 jmc 37: .Dd $Mdocdate: September 22 2016 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.219 jmc 73: The
1.197 dtucker 74: .Ev TERM
1.199 jmc 75: environment variable is always sent whenever the client
1.204 djm 76: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 77: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 78: .Ql *
1.30 djm 79: and
80: .Ql \&? .
1.31 djm 81: Multiple environment variables may be separated by whitespace or spread
1.30 djm 82: across multiple
83: .Cm AcceptEnv
84: directives.
1.31 djm 85: Be warned that some environment variables could be used to bypass restricted
1.30 djm 86: user environments.
87: For this reason, care should be taken in the use of this directive.
88: The default is not to accept any environment variables.
1.37 djm 89: .It Cm AddressFamily
90: Specifies which address family should be used by
1.53 jmc 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
93: .Dq any ,
94: .Dq inet
1.52 jmc 95: (use IPv4 only), or
1.37 djm 96: .Dq inet6
97: (use IPv6 only).
98: The default is
99: .Dq any .
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
105: .Dq yes .
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
129: .Dq yes
130: or
131: .Dq all
1.231 jmc 132: to allow StreamLocal forwarding,
1.150 djm 133: .Dq no
1.231 jmc 134: to prevent all StreamLocal forwarding,
1.150 djm 135: .Dq local
136: to allow local (from the perspective of
1.151 jmc 137: .Xr ssh 1 )
138: forwarding only or
1.150 djm 139: .Dq remote
140: to allow remote forwarding only.
1.1 stevesk 141: The default is
142: .Dq yes .
1.231 jmc 143: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 144: users are also denied shell access, as they can always install their
145: own forwarders.
1.231 jmc 146: .It Cm AllowTcpForwarding
147: Specifies whether TCP forwarding is permitted.
1.175 millert 148: The available options are
149: .Dq yes
150: or
151: .Dq all
1.231 jmc 152: to allow TCP forwarding,
1.175 millert 153: .Dq no
1.231 jmc 154: to prevent all TCP forwarding,
1.175 millert 155: .Dq local
156: to allow local (from the perspective of
157: .Xr ssh 1 )
158: forwarding only or
159: .Dq remote
160: to allow remote forwarding only.
161: The default is
162: .Dq yes .
1.231 jmc 163: Note that disabling TCP forwarding does not improve security unless
1.175 millert 164: users are also denied shell access, as they can always install their
165: own forwarders.
1.1 stevesk 166: .It Cm AllowUsers
167: This keyword can be followed by a list of user name patterns, separated
168: by spaces.
1.14 jmc 169: If specified, login is allowed only for user names that
1.1 stevesk 170: match one of the patterns.
171: Only user names are valid; a numerical user ID is not recognized.
172: By default, login is allowed for all users.
173: If the pattern takes the form USER@HOST then USER and HOST
174: are separately checked, restricting logins to particular
175: users from particular hosts.
1.222 jmc 176: HOST criteria may additionally contain addresses to match in CIDR
177: address/masklen format.
1.54 jmc 178: The allow/deny directives are processed in the following order:
179: .Cm DenyUsers ,
180: .Cm AllowUsers ,
181: .Cm DenyGroups ,
182: and finally
183: .Cm AllowGroups .
1.49 jmc 184: .Pp
1.161 jmc 185: See PATTERNS in
1.49 jmc 186: .Xr ssh_config 5
187: for more information on patterns.
1.149 djm 188: .It Cm AuthenticationMethods
189: Specifies the authentication methods that must be successfully completed
190: for a user to be granted access.
191: This option must be followed by one or more comma-separated lists of
1.224 djm 192: authentication method names, or by the single string
193: .Dq any
194: to indicate the default behaviour of accepting any single authentication
1.225 jmc 195: method.
1.224 djm 196: if the default is overridden, then successful authentication requires
197: completion of every method in at least one of these lists.
1.149 djm 198: .Pp
199: For example, an argument of
200: .Dq publickey,password publickey,keyboard-interactive
201: would require the user to complete public key authentication, followed by
202: either password or keyboard interactive authentication.
203: Only methods that are next in one or more lists are offered at each stage,
204: so for this example, it would not be possible to attempt password or
205: keyboard-interactive authentication before public key.
1.157 markus 206: .Pp
207: For keyboard interactive authentication it is also possible to
208: restrict authentication to a specific device by appending a
209: colon followed by the device identifier
210: .Dq bsdauth ,
211: .Dq pam ,
212: or
213: .Dq skey ,
214: depending on the server configuration.
215: For example,
216: .Dq keyboard-interactive:bsdauth
217: would restrict keyboard interactive authentication to the
218: .Dq bsdauth
219: device.
1.185 djm 220: .Pp
221: If the
222: .Dq publickey
1.187 jmc 223: method is listed more than once,
1.185 djm 224: .Xr sshd 8
225: verifies that keys that have been used successfully are not reused for
226: subsequent authentications.
227: For example, an
228: .Cm AuthenticationMethods
229: of
230: .Dq publickey,publickey
231: will require successful authentication using two different public keys.
1.181 djm 232: .Pp
1.149 djm 233: Note that each authentication method listed should also be explicitly enabled
234: in the configuration.
1.224 djm 235: The default
236: .Dq any
237: is not to require multiple authentication; successful completion
1.149 djm 238: of a single authentication method is sufficient.
1.146 djm 239: .It Cm AuthorizedKeysCommand
1.147 jmc 240: Specifies a program to be used to look up the user's public keys.
1.201 djm 241: The program must be owned by root, not writable by group or others and
242: specified by an absolute path.
243: Arguments to
244: .Cm AuthorizedKeysCommand
1.235 jmc 245: accept the tokens described in the
246: .Sx TOKENS
247: section.
248: If no arguments are specified then the username of the target user is used.
1.201 djm 249: .Pp
250: The program should produce on standard output zero or
1.235 jmc 251: more lines of authorized_keys output (see
252: .Sx AUTHORIZED_KEYS
253: in
1.147 jmc 254: .Xr sshd 8 ) .
1.235 jmc 255: If a key supplied by
256: .Cm AuthorizedKeysCommand
257: does not successfully authenticate
1.146 djm 258: and authorize the user then public key authentication continues using the usual
259: .Cm AuthorizedKeysFile
260: files.
261: By default, no AuthorizedKeysCommand is run.
262: .It Cm AuthorizedKeysCommandUser
263: Specifies the user under whose account the AuthorizedKeysCommand is run.
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.
1.124 djm 275: The format is described in the
1.235 jmc 276: .Sx AUTHORIZED_KEYS FILE FORMAT
1.124 djm 277: section of
278: .Xr sshd 8 .
1.235 jmc 279: Arguments to
1.1 stevesk 280: .Cm AuthorizedKeysFile
1.235 jmc 281: accept the tokens described in the
282: .Sx TOKENS
283: section.
1.1 stevesk 284: After expansion,
285: .Cm AuthorizedKeysFile
286: is taken to be an absolute path or one relative to the user's home
287: directory.
1.133 jmc 288: Multiple files may be listed, separated by whitespace.
1.212 djm 289: Alternately this option may be set to
290: .Dq none
291: to skip checking for user keys in files.
1.133 jmc 292: The default is
293: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 294: .It Cm AuthorizedPrincipalsCommand
295: Specifies a program to be used to generate the list of allowed
296: certificate principals as per
297: .Cm AuthorizedPrincipalsFile .
298: The program must be owned by root, not writable by group or others and
299: specified by an absolute path.
300: Arguments to
301: .Cm AuthorizedPrincipalsCommand
1.235 jmc 302: accept the tokens described in the
303: .Sx TOKENS
304: section.
305: If no arguments are specified then the username of the target user is used.
1.202 djm 306: .Pp
307: The program should produce on standard output zero or
308: more lines of
309: .Cm AuthorizedPrincipalsFile
310: output.
311: If either
312: .Cm AuthorizedPrincipalsCommand
313: or
314: .Cm AuthorizedPrincipalsFile
315: is specified, then certificates offered by the client for authentication
316: must contain a principal that is listed.
1.235 jmc 317: By default, no
318: .Cm AuthorizedPrincipalsCommand
319: is run.
1.202 djm 320: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 321: Specifies the user under whose account the
322: .Cm AuthorizedPrincipalsCommand
323: is run.
1.202 djm 324: It is recommended to use a dedicated user that has no other role on the host
325: than running authorized principals commands.
326: If
327: .Cm AuthorizedPrincipalsCommand
328: is specified but
329: .Cm AuthorizedPrincipalsCommandUser
330: is not, then
331: .Xr sshd 8
332: will refuse to start.
1.121 djm 333: .It Cm AuthorizedPrincipalsFile
334: Specifies a file that lists principal names that are accepted for
335: certificate authentication.
336: When using certificates signed by a key listed in
337: .Cm TrustedUserCAKeys ,
338: this file lists names, one of which must appear in the certificate for it
339: to be accepted for authentication.
1.235 jmc 340: Names are listed one per line preceded by key options (as described in
341: .Sx AUTHORIZED_KEYS FILE FORMAT
342: in
1.125 jmc 343: .Xr sshd 8 ) .
1.124 djm 344: Empty lines and comments starting with
1.121 djm 345: .Ql #
346: are ignored.
347: .Pp
1.235 jmc 348: Arguments to
1.121 djm 349: .Cm AuthorizedPrincipalsFile
1.235 jmc 350: accept the tokens described in the
351: .Sx TOKENS
352: section.
1.121 djm 353: After expansion,
354: .Cm AuthorizedPrincipalsFile
1.235 jmc 355: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 356: The default is
357: .Dq none ,
358: i.e. not to use a principals file \(en in this case, the username
1.121 djm 359: of the user must appear in a certificate's principals list for it to be
360: accepted.
1.235 jmc 361: .Pp
1.121 djm 362: Note that
363: .Cm AuthorizedPrincipalsFile
364: is only used when authentication proceeds using a CA listed in
365: .Cm TrustedUserCAKeys
366: and is not consulted for certification authorities trusted via
367: .Pa ~/.ssh/authorized_keys ,
368: though the
369: .Cm principals=
370: key option offers a similar facility (see
371: .Xr sshd 8
372: for details).
1.1 stevesk 373: .It Cm Banner
374: The contents of the specified file are sent to the remote user before
375: authentication is allowed.
1.78 djm 376: If the argument is
377: .Dq none
378: then no banner is displayed.
1.1 stevesk 379: By default, no banner is displayed.
380: .It Cm ChallengeResponseAuthentication
1.50 jmc 381: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 382: All authentication styles from
383: .Xr login.conf 5
384: are supported.
385: The default is
386: .Dq yes .
1.80 djm 387: .It Cm ChrootDirectory
1.113 stevesk 388: Specifies the pathname of a directory to
1.80 djm 389: .Xr chroot 2
390: to after authentication.
1.190 deraadt 391: At session startup
392: .Xr sshd 8
393: checks that all components of the pathname are root-owned directories
394: which are not writable by any other user or group.
1.106 stevesk 395: After the chroot,
396: .Xr sshd 8
397: changes the working directory to the user's home directory.
1.235 jmc 398: Arguments to
399: .Cm ChrootDirectory
400: accept the tokens described in the
401: .Sx TOKENS
402: section.
1.80 djm 403: .Pp
404: The
405: .Cm ChrootDirectory
406: must contain the necessary files and directories to support the
1.103 stevesk 407: user's session.
1.80 djm 408: For an interactive session this requires at least a shell, typically
409: .Xr sh 1 ,
410: and basic
411: .Pa /dev
412: nodes such as
413: .Xr null 4 ,
414: .Xr zero 4 ,
415: .Xr stdin 4 ,
416: .Xr stdout 4 ,
417: .Xr stderr 4 ,
1.180 jmc 418: and
1.80 djm 419: .Xr tty 4
420: devices.
421: For file transfer sessions using
1.105 jmc 422: .Dq sftp ,
1.80 djm 423: no additional configuration of the environment is necessary if the
1.105 jmc 424: in-process sftp server is used,
1.176 schwarze 425: though sessions which use logging may require
1.104 stevesk 426: .Pa /dev/log
1.176 schwarze 427: inside the chroot directory on some operating systems (see
1.104 stevesk 428: .Xr sftp-server 8
1.81 jmc 429: for details).
1.190 deraadt 430: .Pp
1.191 jmc 431: For safety, it is very important that the directory hierarchy be
1.190 deraadt 432: prevented from modification by other processes on the system (especially
433: those outside the jail).
434: Misconfiguration can lead to unsafe environments which
435: .Xr sshd 8
436: cannot detect.
1.80 djm 437: .Pp
1.215 djm 438: The default is
439: .Dq none ,
440: indicating not to
1.80 djm 441: .Xr chroot 2 .
1.1 stevesk 442: .It Cm Ciphers
1.219 jmc 443: Specifies the ciphers allowed.
1.1 stevesk 444: Multiple ciphers must be comma-separated.
1.208 djm 445: If the specified value begins with a
446: .Sq +
447: character, then the specified ciphers will be appended to the default set
448: instead of replacing them.
449: .Pp
1.167 djm 450: The supported ciphers are:
451: .Pp
1.173 naddy 452: .Bl -item -compact -offset indent
453: .It
454: 3des-cbc
455: .It
456: aes128-cbc
457: .It
458: aes192-cbc
459: .It
460: aes256-cbc
461: .It
462: aes128-ctr
463: .It
464: aes192-ctr
465: .It
466: aes256-ctr
467: .It
468: aes128-gcm@openssh.com
469: .It
470: aes256-gcm@openssh.com
471: .It
472: arcfour
473: .It
474: arcfour128
475: .It
476: arcfour256
477: .It
478: blowfish-cbc
479: .It
480: cast128-cbc
481: .It
482: chacha20-poly1305@openssh.com
483: .El
1.167 djm 484: .Pp
1.52 jmc 485: The default is:
1.173 naddy 486: .Bd -literal -offset indent
1.211 jmc 487: chacha20-poly1305@openssh.com,
1.173 naddy 488: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 489: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 490: .Ed
1.167 djm 491: .Pp
492: The list of available ciphers may also be obtained using the
493: .Fl Q
494: option of
1.188 djm 495: .Xr ssh 1
496: with an argument of
497: .Dq cipher .
1.1 stevesk 498: .It Cm ClientAliveCountMax
1.48 jmc 499: Sets the number of client alive messages (see below) which may be
1.1 stevesk 500: sent without
1.52 jmc 501: .Xr sshd 8
1.17 jmc 502: receiving any messages back from the client.
503: If this threshold is reached while client alive messages are being sent,
1.52 jmc 504: sshd will disconnect the client, terminating the session.
1.17 jmc 505: It is important to note that the use of client alive messages is very
506: different from
1.27 markus 507: .Cm TCPKeepAlive
1.17 jmc 508: (below).
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
1.1 stevesk 515: server depend on knowing when a connection has become inactive.
516: .Pp
1.17 jmc 517: The default value is 3.
518: If
1.1 stevesk 519: .Cm ClientAliveInterval
1.48 jmc 520: (see below) 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.
1.42 djm 524: .It Cm ClientAliveInterval
525: Sets a timeout interval in seconds after which if no data has been received
526: from the client,
1.52 jmc 527: .Xr sshd 8
1.42 djm 528: will send a message through the encrypted
529: channel to request a response from the client.
530: The default
531: is 0, indicating that these messages will not be sent to the client.
1.3 markus 532: .It Cm Compression
1.236 ! djm 533: Specifies whether compression is enabled after
1.44 markus 534: the user has authenticated successfully.
1.3 markus 535: The argument must be
1.44 markus 536: .Dq yes ,
1.236 ! djm 537: .Dq delayed
! 538: (a legacy synonym for
! 539: .Dq yes )
1.3 markus 540: or
541: .Dq no .
542: The default is
1.236 ! djm 543: .Dq yes .
1.1 stevesk 544: .It Cm DenyGroups
545: This keyword can be followed by a list of group name patterns, separated
546: by spaces.
547: Login is disallowed for users whose primary group or supplementary
548: group list matches one of the patterns.
549: Only group names are valid; a numerical group ID is not recognized.
550: By default, login is allowed for all groups.
1.54 jmc 551: The allow/deny directives are processed in the following order:
552: .Cm DenyUsers ,
553: .Cm AllowUsers ,
554: .Cm DenyGroups ,
555: and finally
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.
1.54 jmc 572: The allow/deny directives are processed in the following order:
573: .Cm DenyUsers ,
574: .Cm AllowUsers ,
575: .Cm DenyGroups ,
576: and finally
577: .Cm AllowGroups .
1.49 jmc 578: .Pp
1.161 jmc 579: See PATTERNS in
1.49 jmc 580: .Xr ssh_config 5
581: for more information on patterns.
1.183 djm 582: .It Cm FingerprintHash
583: Specifies the hash algorithm used when logging key fingerprints.
584: Valid options are:
585: .Dq md5
586: and
587: .Dq sha256 .
588: The default is
589: .Dq sha256 .
1.67 dtucker 590: .It Cm ForceCommand
591: Forces the execution of the command specified by
592: .Cm ForceCommand ,
1.84 djm 593: ignoring any command supplied by the client and
594: .Pa ~/.ssh/rc
595: if present.
1.67 dtucker 596: The command is invoked by using the user's login shell with the -c option.
597: This applies to shell, command, or subsystem execution.
598: It is most useful inside a
599: .Cm Match
600: block.
601: The command originally supplied by the client is available in the
602: .Ev SSH_ORIGINAL_COMMAND
603: environment variable.
1.82 djm 604: Specifying a command of
605: .Dq internal-sftp
606: will force the use of an in-process sftp server that requires no support
607: files when used with
608: .Cm ChrootDirectory .
1.215 djm 609: The default is
610: .Dq none .
1.1 stevesk 611: .It Cm GatewayPorts
612: Specifies whether remote hosts are allowed to connect to ports
613: forwarded for the client.
614: By default,
1.52 jmc 615: .Xr sshd 8
1.15 jmc 616: binds remote port forwardings to the loopback address.
617: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 618: .Cm GatewayPorts
1.52 jmc 619: can be used to specify that sshd
1.39 djm 620: should allow remote port forwardings to bind to non-loopback addresses, thus
621: allowing other hosts to connect.
622: The argument may be
623: .Dq no
624: to force remote port forwardings to be available to the local host only,
1.1 stevesk 625: .Dq yes
1.39 djm 626: to force remote port forwardings to bind to the wildcard address, or
627: .Dq clientspecified
628: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 629: The default is
630: .Dq no .
1.23 markus 631: .It Cm GSSAPIAuthentication
1.25 markus 632: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 633: The default is
1.23 markus 634: .Dq no .
635: .It Cm GSSAPICleanupCredentials
636: Specifies whether to automatically destroy the user's credentials cache
637: on logout.
638: The default is
639: .Dq yes .
1.203 djm 640: .It Cm GSSAPIStrictAcceptorCheck
641: Determines whether to be strict about the identity of the GSSAPI acceptor
642: a client authenticates against.
643: If set to
644: .Dq yes
645: then the client must authenticate against the
646: .Pa host
647: service on the current hostname.
648: If set to
649: .Dq no
650: then the client may authenticate against any service key stored in the
651: machine's default store.
652: This facility is provided to assist with operation on multi homed machines.
653: The default is
654: .Dq yes .
1.189 djm 655: .It Cm HostbasedAcceptedKeyTypes
656: Specifies the key types that will be accepted for hostbased authentication
657: as a comma-separated pattern list.
1.208 djm 658: Alternately if the specified value begins with a
659: .Sq +
660: character, then the specified key types will be appended to the default set
661: instead of replacing them.
1.206 markus 662: The default for this option is:
663: .Bd -literal -offset 3n
664: ecdsa-sha2-nistp256-cert-v01@openssh.com,
665: ecdsa-sha2-nistp384-cert-v01@openssh.com,
666: ecdsa-sha2-nistp521-cert-v01@openssh.com,
667: ssh-ed25519-cert-v01@openssh.com,
668: ssh-rsa-cert-v01@openssh.com,
669: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 670: ssh-ed25519,ssh-rsa
1.206 markus 671: .Ed
672: .Pp
1.189 djm 673: The
674: .Fl Q
675: option of
676: .Xr ssh 1
677: may be used to list supported key types.
1.1 stevesk 678: .It Cm HostbasedAuthentication
679: Specifies whether rhosts or /etc/hosts.equiv authentication together
680: with successful public key client host authentication is allowed
1.50 jmc 681: (host-based authentication).
1.70 dtucker 682: The default is
683: .Dq no .
684: .It Cm HostbasedUsesNameFromPacketOnly
685: Specifies whether or not the server will attempt to perform a reverse
686: name lookup when matching the name in the
687: .Pa ~/.shosts ,
688: .Pa ~/.rhosts ,
689: and
690: .Pa /etc/hosts.equiv
691: files during
692: .Cm HostbasedAuthentication .
693: A setting of
694: .Dq yes
695: means that
696: .Xr sshd 8
697: uses the name supplied by the client rather than
698: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 699: The default is
700: .Dq no .
1.117 djm 701: .It Cm HostCertificate
702: Specifies a file containing a public host certificate.
703: The certificate's public key must match a private host key already specified
704: by
705: .Cm HostKey .
706: The default behaviour of
707: .Xr sshd 8
708: is not to load any certificates.
1.1 stevesk 709: .It Cm HostKey
710: Specifies a file containing a private host key
711: used by SSH.
1.229 naddy 712: The defaults are
1.126 djm 713: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 714: .Pa /etc/ssh/ssh_host_ecdsa_key ,
715: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 716: and
1.229 naddy 717: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 718: .Pp
1.1 stevesk 719: Note that
1.52 jmc 720: .Xr sshd 8
1.206 markus 721: will refuse to use a file if it is group/world-accessible
722: and that the
723: .Cm HostKeyAlgorithms
724: option restricts which of the keys are actually used by
725: .Xr sshd 8 .
726: .Pp
1.1 stevesk 727: It is possible to have multiple host key files.
1.162 markus 728: It is also possible to specify public host key files instead.
729: In this case operations on the private key will be delegated
730: to an
731: .Xr ssh-agent 1 .
732: .It Cm HostKeyAgent
733: Identifies the UNIX-domain socket used to communicate
734: with an agent that has access to the private host keys.
1.223 markus 735: If the string
1.162 markus 736: .Dq SSH_AUTH_SOCK
737: is specified, the location of the socket will be read from the
738: .Ev SSH_AUTH_SOCK
739: environment variable.
1.206 markus 740: .It Cm HostKeyAlgorithms
1.219 jmc 741: Specifies the host key algorithms
1.206 markus 742: that the server offers.
743: The default for this option is:
744: .Bd -literal -offset 3n
745: ecdsa-sha2-nistp256-cert-v01@openssh.com,
746: ecdsa-sha2-nistp384-cert-v01@openssh.com,
747: ecdsa-sha2-nistp521-cert-v01@openssh.com,
748: ssh-ed25519-cert-v01@openssh.com,
749: ssh-rsa-cert-v01@openssh.com,
750: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 751: ssh-ed25519,ssh-rsa
1.206 markus 752: .Ed
753: .Pp
754: The list of available key types may also be obtained using the
755: .Fl Q
756: option of
757: .Xr ssh 1
758: with an argument of
759: .Dq key .
1.1 stevesk 760: .It Cm IgnoreRhosts
761: Specifies that
762: .Pa .rhosts
763: and
764: .Pa .shosts
765: files will not be used in
766: .Cm HostbasedAuthentication .
767: .Pp
768: .Pa /etc/hosts.equiv
769: and
770: .Pa /etc/shosts.equiv
771: are still used.
772: The default is
773: .Dq yes .
774: .It Cm IgnoreUserKnownHosts
775: Specifies whether
1.52 jmc 776: .Xr sshd 8
1.1 stevesk 777: should ignore the user's
1.41 djm 778: .Pa ~/.ssh/known_hosts
1.1 stevesk 779: during
780: .Cm HostbasedAuthentication .
781: The default is
782: .Dq no .
1.129 djm 783: .It Cm IPQoS
784: Specifies the IPv4 type-of-service or DSCP class for the connection.
785: Accepted values are
786: .Dq af11 ,
787: .Dq af12 ,
788: .Dq af13 ,
1.136 djm 789: .Dq af21 ,
1.129 djm 790: .Dq af22 ,
791: .Dq af23 ,
792: .Dq af31 ,
793: .Dq af32 ,
794: .Dq af33 ,
795: .Dq af41 ,
796: .Dq af42 ,
797: .Dq af43 ,
798: .Dq cs0 ,
799: .Dq cs1 ,
800: .Dq cs2 ,
801: .Dq cs3 ,
802: .Dq cs4 ,
803: .Dq cs5 ,
804: .Dq cs6 ,
805: .Dq cs7 ,
806: .Dq ef ,
807: .Dq lowdelay ,
808: .Dq throughput ,
809: .Dq reliability ,
810: or a numeric value.
1.131 djm 811: This option may take one or two arguments, separated by whitespace.
1.129 djm 812: If one argument is specified, it is used as the packet class unconditionally.
813: If two values are specified, the first is automatically selected for
814: interactive sessions and the second for non-interactive sessions.
815: The default is
816: .Dq lowdelay
817: for interactive sessions and
818: .Dq throughput
819: for non-interactive sessions.
1.171 jmc 820: .It Cm KbdInteractiveAuthentication
821: Specifies whether to allow keyboard-interactive authentication.
822: The argument to this keyword must be
823: .Dq yes
824: or
825: .Dq no .
826: The default is to use whatever value
827: .Cm ChallengeResponseAuthentication
828: is set to
829: (by default
830: .Dq yes ) .
1.1 stevesk 831: .It Cm KerberosAuthentication
1.24 markus 832: Specifies whether the password provided by the user for
1.1 stevesk 833: .Cm PasswordAuthentication
1.24 markus 834: will be validated through the Kerberos KDC.
1.1 stevesk 835: To use this option, the server needs a
836: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 837: The default is
1.29 dtucker 838: .Dq no .
839: .It Cm KerberosGetAFSToken
1.45 djm 840: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 841: an AFS token before accessing the user's home directory.
1.52 jmc 842: The default is
1.1 stevesk 843: .Dq no .
844: .It Cm KerberosOrLocalPasswd
1.52 jmc 845: If password authentication through Kerberos fails then
1.1 stevesk 846: the password will be validated via any additional local mechanism
847: such as
848: .Pa /etc/passwd .
1.52 jmc 849: The default is
1.1 stevesk 850: .Dq yes .
851: .It Cm KerberosTicketCleanup
852: Specifies whether to automatically destroy the user's ticket cache
853: file on logout.
1.52 jmc 854: The default is
1.1 stevesk 855: .Dq yes .
1.127 djm 856: .It Cm KexAlgorithms
857: Specifies the available KEX (Key Exchange) algorithms.
858: Multiple algorithms must be comma-separated.
1.208 djm 859: Alternately if the specified value begins with a
860: .Sq +
861: character, then the specified methods will be appended to the default set
862: instead of replacing them.
1.173 naddy 863: The supported algorithms are:
864: .Pp
865: .Bl -item -compact -offset indent
866: .It
1.234 djm 867: curve25519-sha256
868: .It
1.173 naddy 869: curve25519-sha256@libssh.org
870: .It
871: diffie-hellman-group1-sha1
872: .It
873: diffie-hellman-group14-sha1
874: .It
875: diffie-hellman-group-exchange-sha1
876: .It
877: diffie-hellman-group-exchange-sha256
878: .It
879: ecdh-sha2-nistp256
880: .It
881: ecdh-sha2-nistp384
882: .It
883: ecdh-sha2-nistp521
884: .El
885: .Pp
886: The default is:
1.170 dtucker 887: .Bd -literal -offset indent
1.234 djm 888: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 889: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
890: diffie-hellman-group-exchange-sha256,
1.173 naddy 891: diffie-hellman-group14-sha1
1.170 dtucker 892: .Ed
1.188 djm 893: .Pp
894: The list of available key exchange algorithms may also be obtained using the
895: .Fl Q
896: option of
897: .Xr ssh 1
898: with an argument of
899: .Dq kex .
1.1 stevesk 900: .It Cm ListenAddress
901: Specifies the local addresses
1.52 jmc 902: .Xr sshd 8
1.1 stevesk 903: should listen on.
904: The following forms may be used:
905: .Pp
906: .Bl -item -offset indent -compact
907: .It
908: .Cm ListenAddress
909: .Sm off
1.180 jmc 910: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 911: .Sm on
912: .It
913: .Cm ListenAddress
914: .Sm off
1.180 jmc 915: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 916: .Sm on
917: .It
918: .Cm ListenAddress
919: .Sm off
920: .Oo
1.180 jmc 921: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 922: .Sm on
923: .El
924: .Pp
925: If
926: .Ar port
927: is not specified,
1.200 dtucker 928: sshd will listen on the address and all
1.1 stevesk 929: .Cm Port
1.17 jmc 930: options specified.
931: The default is to listen on all local addresses.
1.15 jmc 932: Multiple
1.1 stevesk 933: .Cm ListenAddress
1.17 jmc 934: options are permitted.
1.1 stevesk 935: .It Cm LoginGraceTime
936: The server disconnects after this time if the user has not
937: successfully logged in.
938: If the value is 0, there is no time limit.
1.12 stevesk 939: The default is 120 seconds.
1.1 stevesk 940: .It Cm LogLevel
941: Gives the verbosity level that is used when logging messages from
1.53 jmc 942: .Xr sshd 8 .
1.1 stevesk 943: The possible values are:
1.52 jmc 944: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 945: The default is INFO.
946: DEBUG and DEBUG1 are equivalent.
947: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
948: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 949: .It Cm MACs
950: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 951: The MAC algorithm is used for data integrity protection.
1.1 stevesk 952: Multiple algorithms must be comma-separated.
1.208 djm 953: If the specified value begins with a
954: .Sq +
955: character, then the specified algorithms will be appended to the default set
956: instead of replacing them.
957: .Pp
1.152 markus 958: The algorithms that contain
959: .Dq -etm
960: calculate the MAC after encryption (encrypt-then-mac).
961: These are considered safer and their use recommended.
1.173 naddy 962: The supported MACs are:
963: .Pp
964: .Bl -item -compact -offset indent
965: .It
966: hmac-md5
967: .It
968: hmac-md5-96
969: .It
970: hmac-ripemd160
971: .It
972: hmac-sha1
973: .It
974: hmac-sha1-96
975: .It
976: hmac-sha2-256
977: .It
978: hmac-sha2-512
979: .It
980: umac-64@openssh.com
981: .It
982: umac-128@openssh.com
983: .It
984: hmac-md5-etm@openssh.com
985: .It
986: hmac-md5-96-etm@openssh.com
987: .It
988: hmac-ripemd160-etm@openssh.com
989: .It
990: hmac-sha1-etm@openssh.com
991: .It
992: hmac-sha1-96-etm@openssh.com
993: .It
994: hmac-sha2-256-etm@openssh.com
995: .It
996: hmac-sha2-512-etm@openssh.com
997: .It
998: umac-64-etm@openssh.com
999: .It
1000: umac-128-etm@openssh.com
1001: .El
1002: .Pp
1.52 jmc 1003: The default is:
1.77 jmc 1004: .Bd -literal -offset indent
1.152 markus 1005: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1006: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1007: hmac-sha1-etm@openssh.com,
1.173 naddy 1008: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1009: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1010: .Ed
1.188 djm 1011: .Pp
1012: The list of available MAC algorithms may also be obtained using the
1013: .Fl Q
1014: option of
1015: .Xr ssh 1
1016: with an argument of
1017: .Dq mac .
1.60 dtucker 1018: .It Cm Match
1.61 jmc 1019: Introduces a conditional block.
1.65 dtucker 1020: If all of the criteria on the
1.60 dtucker 1021: .Cm Match
1.65 dtucker 1022: line are satisfied, the keywords on the following lines override those
1023: set in the global section of the config file, until either another
1.60 dtucker 1024: .Cm Match
1.65 dtucker 1025: line or the end of the file.
1.172 djm 1026: If a keyword appears in multiple
1027: .Cm Match
1.177 sobrado 1028: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1029: applied.
1.91 djm 1030: .Pp
1.61 jmc 1031: The arguments to
1.60 dtucker 1032: .Cm Match
1.163 dtucker 1033: are one or more criteria-pattern pairs or the single token
1034: .Cm All
1035: which matches all criteria.
1.60 dtucker 1036: The available criteria are
1037: .Cm User ,
1.69 dtucker 1038: .Cm Group ,
1.60 dtucker 1039: .Cm Host ,
1.139 dtucker 1040: .Cm LocalAddress ,
1041: .Cm LocalPort ,
1.60 dtucker 1042: and
1043: .Cm Address .
1.91 djm 1044: The match patterns may consist of single entries or comma-separated
1045: lists and may use the wildcard and negation operators described in the
1.161 jmc 1046: PATTERNS section of
1.92 djm 1047: .Xr ssh_config 5 .
1.91 djm 1048: .Pp
1049: The patterns in an
1050: .Cm Address
1051: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1052: address/masklen format, e.g.\&
1.91 djm 1053: .Dq 192.0.2.0/24
1054: or
1.228 jca 1055: .Dq 2001:db8::/32 .
1.91 djm 1056: Note that the mask length provided must be consistent with the address -
1057: it is an error to specify a mask length that is too long for the address
1.93 jmc 1058: or one with bits set in this host portion of the address.
1059: For example,
1.91 djm 1060: .Dq 192.0.2.0/33
1061: and
1.93 jmc 1062: .Dq 192.0.2.0/8
1.91 djm 1063: respectively.
1064: .Pp
1.60 dtucker 1065: Only a subset of keywords may be used on the lines following a
1066: .Cm Match
1067: keyword.
1068: Available keywords are
1.142 jmc 1069: .Cm AcceptEnv ,
1.99 okan 1070: .Cm AllowAgentForwarding ,
1.142 jmc 1071: .Cm AllowGroups ,
1.193 djm 1072: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1073: .Cm AllowTcpForwarding ,
1.141 markus 1074: .Cm AllowUsers ,
1.149 djm 1075: .Cm AuthenticationMethods ,
1.146 djm 1076: .Cm AuthorizedKeysCommand ,
1077: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1078: .Cm AuthorizedKeysFile ,
1.214 djm 1079: .Cm AuthorizedPrincipalsCommand ,
1080: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1081: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1082: .Cm Banner ,
1.85 djm 1083: .Cm ChrootDirectory ,
1.141 markus 1084: .Cm DenyGroups ,
1085: .Cm DenyUsers ,
1.67 dtucker 1086: .Cm ForceCommand ,
1.194 djm 1087: .Cm GatewayPorts ,
1.193 djm 1088: .Cm GSSAPIAuthentication ,
1.189 djm 1089: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1090: .Cm HostbasedAuthentication ,
1.123 djm 1091: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1092: .Cm IPQoS ,
1.74 jmc 1093: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1094: .Cm KerberosAuthentication ,
1.95 dtucker 1095: .Cm MaxAuthTries ,
1.94 dtucker 1096: .Cm MaxSessions ,
1.72 dtucker 1097: .Cm PasswordAuthentication ,
1.97 djm 1098: .Cm PermitEmptyPasswords ,
1.66 dtucker 1099: .Cm PermitOpen ,
1.79 dtucker 1100: .Cm PermitRootLogin ,
1.164 djm 1101: .Cm PermitTTY ,
1.123 djm 1102: .Cm PermitTunnel ,
1.174 djm 1103: .Cm PermitUserRC ,
1.189 djm 1104: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1105: .Cm PubkeyAuthentication ,
1.159 dtucker 1106: .Cm RekeyLimit ,
1.193 djm 1107: .Cm RevokedKeys ,
1108: .Cm StreamLocalBindMask ,
1109: .Cm StreamLocalBindUnlink ,
1110: .Cm TrustedUserCAKeys ,
1.66 dtucker 1111: .Cm X11DisplayOffset ,
1.101 djm 1112: .Cm X11Forwarding
1.60 dtucker 1113: and
1.102 djm 1114: .Cm X11UseLocalHost .
1.33 dtucker 1115: .It Cm MaxAuthTries
1116: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1117: connection.
1118: Once the number of failures reaches half this value,
1119: additional failures are logged.
1120: The default is 6.
1.90 djm 1121: .It Cm MaxSessions
1.216 djm 1122: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1123: sessions permitted per network connection.
1124: Multiple sessions may be established by clients that support connection
1125: multiplexing.
1126: Setting
1127: .Cm MaxSessions
1128: to 1 will effectively disable session multiplexing, whereas setting it to 0
1129: will prevent all shell, login and subsystem sessions while still permitting
1130: forwarding.
1.90 djm 1131: The default is 10.
1.1 stevesk 1132: .It Cm MaxStartups
1133: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1134: SSH daemon.
1.1 stevesk 1135: Additional connections will be dropped until authentication succeeds or the
1136: .Cm LoginGraceTime
1137: expires for a connection.
1.156 dtucker 1138: The default is 10:30:100.
1.1 stevesk 1139: .Pp
1140: Alternatively, random early drop can be enabled by specifying
1141: the three colon separated values
1142: .Dq start:rate:full
1.51 jmc 1143: (e.g. "10:30:60").
1.53 jmc 1144: .Xr sshd 8
1.1 stevesk 1145: will refuse connection attempts with a probability of
1146: .Dq rate/100
1147: (30%)
1148: if there are currently
1149: .Dq start
1150: (10)
1151: unauthenticated connections.
1152: The probability increases linearly and all connection attempts
1153: are refused if the number of unauthenticated connections reaches
1154: .Dq full
1155: (60).
1156: .It Cm PasswordAuthentication
1157: Specifies whether password authentication is allowed.
1158: The default is
1159: .Dq yes .
1160: .It Cm PermitEmptyPasswords
1161: When password authentication is allowed, it specifies whether the
1162: server allows login to accounts with empty password strings.
1163: The default is
1164: .Dq no .
1.62 dtucker 1165: .It Cm PermitOpen
1166: Specifies the destinations to which TCP port forwarding is permitted.
1167: The forwarding specification must be one of the following forms:
1168: .Pp
1169: .Bl -item -offset indent -compact
1170: .It
1171: .Cm PermitOpen
1172: .Sm off
1173: .Ar host : port
1174: .Sm on
1175: .It
1176: .Cm PermitOpen
1177: .Sm off
1178: .Ar IPv4_addr : port
1179: .Sm on
1180: .It
1181: .Cm PermitOpen
1182: .Sm off
1183: .Ar \&[ IPv6_addr \&] : port
1184: .Sm on
1185: .El
1186: .Pp
1.68 dtucker 1187: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1188: An argument of
1189: .Dq any
1190: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1191: An argument of
1192: .Dq none
1193: can be used to prohibit all forwarding requests.
1.227 jmc 1194: The wildcard
1.226 dtucker 1195: .Dq *
1.227 jmc 1196: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1197: By default all port forwarding requests are permitted.
1.1 stevesk 1198: .It Cm PermitRootLogin
1.38 jmc 1199: Specifies whether root can log in using
1.1 stevesk 1200: .Xr ssh 1 .
1201: The argument must be
1202: .Dq yes ,
1.210 deraadt 1203: .Dq prohibit-password ,
1.1 stevesk 1204: .Dq without-password ,
1.52 jmc 1205: .Dq forced-commands-only ,
1.1 stevesk 1206: or
1207: .Dq no .
1208: The default is
1.210 deraadt 1209: .Dq prohibit-password .
1.1 stevesk 1210: .Pp
1211: If this option is set to
1.210 deraadt 1212: .Dq prohibit-password
1213: or
1.52 jmc 1214: .Dq without-password ,
1.210 deraadt 1215: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1216: .Pp
1217: If this option is set to
1.52 jmc 1218: .Dq forced-commands-only ,
1.1 stevesk 1219: root login with public key authentication will be allowed,
1220: but only if the
1221: .Ar command
1222: option has been specified
1223: (which may be useful for taking remote backups even if root login is
1.17 jmc 1224: normally not allowed).
1225: All other authentication methods are disabled for root.
1.1 stevesk 1226: .Pp
1227: If this option is set to
1.52 jmc 1228: .Dq no ,
1.38 jmc 1229: root is not allowed to log in.
1.231 jmc 1230: .It Cm PermitTTY
1231: Specifies whether
1232: .Xr pty 4
1233: allocation is permitted.
1234: The default is
1235: .Dq yes .
1.46 reyk 1236: .It Cm PermitTunnel
1237: Specifies whether
1238: .Xr tun 4
1239: device forwarding is allowed.
1.47 reyk 1240: The argument must be
1241: .Dq yes ,
1.58 stevesk 1242: .Dq point-to-point
1243: (layer 3),
1244: .Dq ethernet
1245: (layer 2), or
1.47 reyk 1246: .Dq no .
1.58 stevesk 1247: Specifying
1248: .Dq yes
1249: permits both
1250: .Dq point-to-point
1251: and
1252: .Dq ethernet .
1.46 reyk 1253: The default is
1254: .Dq no .
1.178 djm 1255: .Pp
1256: Independent of this setting, the permissions of the selected
1257: .Xr tun 4
1258: device must allow access to the user.
1.6 markus 1259: .It Cm PermitUserEnvironment
1260: Specifies whether
1261: .Pa ~/.ssh/environment
1.9 stevesk 1262: and
1.6 markus 1263: .Cm environment=
1264: options in
1265: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1266: are processed by
1.52 jmc 1267: .Xr sshd 8 .
1.6 markus 1268: The default is
1269: .Dq no .
1.9 stevesk 1270: Enabling environment processing may enable users to bypass access
1271: restrictions in some configurations using mechanisms such as
1272: .Ev LD_PRELOAD .
1.174 djm 1273: .It Cm PermitUserRC
1274: Specifies whether any
1275: .Pa ~/.ssh/rc
1276: file is executed.
1277: The default is
1278: .Dq yes .
1.1 stevesk 1279: .It Cm PidFile
1.4 stevesk 1280: Specifies the file that contains the process ID of the
1.195 dtucker 1281: SSH daemon, or
1282: .Dq none
1283: to not write one.
1.1 stevesk 1284: The default is
1285: .Pa /var/run/sshd.pid .
1286: .It Cm Port
1287: Specifies the port number that
1.52 jmc 1288: .Xr sshd 8
1.1 stevesk 1289: listens on.
1290: The default is 22.
1291: Multiple options of this type are permitted.
1292: See also
1293: .Cm ListenAddress .
1294: .It Cm PrintLastLog
1295: Specifies whether
1.52 jmc 1296: .Xr sshd 8
1.36 jaredy 1297: should print the date and time of the last user login when a user logs
1298: in interactively.
1.1 stevesk 1299: The default is
1300: .Dq yes .
1301: .It Cm PrintMotd
1302: Specifies whether
1.52 jmc 1303: .Xr sshd 8
1.1 stevesk 1304: should print
1305: .Pa /etc/motd
1306: when a user logs in interactively.
1307: (On some systems it is also printed by the shell,
1308: .Pa /etc/profile ,
1309: or equivalent.)
1310: The default is
1311: .Dq yes .
1.189 djm 1312: .It Cm PubkeyAcceptedKeyTypes
1313: Specifies the key types that will be accepted for public key authentication
1314: as a comma-separated pattern list.
1.208 djm 1315: Alternately if the specified value begins with a
1316: .Sq +
1317: character, then the specified key types will be appended to the default set
1318: instead of replacing them.
1.206 markus 1319: The default for this option is:
1320: .Bd -literal -offset 3n
1321: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1322: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1323: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1324: ssh-ed25519-cert-v01@openssh.com,
1325: ssh-rsa-cert-v01@openssh.com,
1326: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1327: ssh-ed25519,ssh-rsa
1.206 markus 1328: .Ed
1329: .Pp
1.189 djm 1330: The
1331: .Fl Q
1332: option of
1333: .Xr ssh 1
1334: may be used to list supported key types.
1.1 stevesk 1335: .It Cm PubkeyAuthentication
1336: Specifies whether public key authentication is allowed.
1337: The default is
1338: .Dq yes .
1.159 dtucker 1339: .It Cm RekeyLimit
1340: Specifies the maximum amount of data that may be transmitted before the
1341: session key is renegotiated, optionally followed a maximum amount of
1342: time that may pass before the session key is renegotiated.
1343: The first argument is specified in bytes and may have a suffix of
1344: .Sq K ,
1345: .Sq M ,
1346: or
1347: .Sq G
1348: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1349: The default is between
1350: .Sq 1G
1351: and
1352: .Sq 4G ,
1353: depending on the cipher.
1354: The optional second value is specified in seconds and may use any of the
1355: units documented in the
1356: .Sx TIME FORMATS
1.160 jmc 1357: section.
1.159 dtucker 1358: The default value for
1359: .Cm RekeyLimit
1360: is
1361: .Dq default none ,
1362: which means that rekeying is performed after the cipher's default amount
1363: of data has been sent or received and no time based rekeying is done.
1.118 djm 1364: .It Cm RevokedKeys
1.195 dtucker 1365: Specifies revoked public keys file, or
1366: .Dq none
1367: to not use one.
1.118 djm 1368: Keys listed in this file will be refused for public key authentication.
1369: Note that if this file is not readable, then public key authentication will
1370: be refused for all users.
1.154 djm 1371: Keys may be specified as a text file, listing one public key per line, or as
1372: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1373: .Xr ssh-keygen 1 .
1.161 jmc 1374: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1375: .Xr ssh-keygen 1 .
1.175 millert 1376: .It Cm StreamLocalBindMask
1377: Sets the octal file creation mode mask
1378: .Pq umask
1379: used when creating a Unix-domain socket file for local or remote
1380: port forwarding.
1381: This option is only used for port forwarding to a Unix-domain socket file.
1382: .Pp
1383: The default value is 0177, which creates a Unix-domain socket file that is
1384: readable and writable only by the owner.
1385: Note that not all operating systems honor the file mode on Unix-domain
1386: socket files.
1387: .It Cm StreamLocalBindUnlink
1388: Specifies whether to remove an existing Unix-domain socket file for local
1389: or remote port forwarding before creating a new one.
1390: If the socket file already exists and
1391: .Cm StreamLocalBindUnlink
1392: is not enabled,
1393: .Nm sshd
1394: will be unable to forward the port to the Unix-domain socket file.
1395: This option is only used for port forwarding to a Unix-domain socket file.
1396: .Pp
1397: The argument must be
1398: .Dq yes
1399: or
1400: .Dq no .
1401: The default is
1402: .Dq no .
1.1 stevesk 1403: .It Cm StrictModes
1404: Specifies whether
1.52 jmc 1405: .Xr sshd 8
1.1 stevesk 1406: should check file modes and ownership of the
1407: user's files and home directory before accepting login.
1408: This is normally desirable because novices sometimes accidentally leave their
1409: directory or files world-writable.
1410: The default is
1411: .Dq yes .
1.112 djm 1412: Note that this does not apply to
1413: .Cm ChrootDirectory ,
1414: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1415: .It Cm Subsystem
1.51 jmc 1416: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1417: Arguments should be a subsystem name and a command (with optional arguments)
1418: to execute upon subsystem request.
1.80 djm 1419: .Pp
1.1 stevesk 1420: The command
1421: .Xr sftp-server 8
1422: implements the
1423: .Dq sftp
1424: file transfer subsystem.
1.80 djm 1425: .Pp
1426: Alternately the name
1427: .Dq internal-sftp
1428: implements an in-process
1429: .Dq sftp
1430: server.
1431: This may simplify configurations using
1432: .Cm ChrootDirectory
1433: to force a different filesystem root on clients.
1434: .Pp
1.1 stevesk 1435: By default no subsystems are defined.
1436: .It Cm SyslogFacility
1437: Gives the facility code that is used when logging messages from
1.53 jmc 1438: .Xr sshd 8 .
1.1 stevesk 1439: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1440: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1441: The default is AUTH.
1.27 markus 1442: .It Cm TCPKeepAlive
1443: Specifies whether the system should send TCP keepalive messages to the
1444: other side.
1445: If they are sent, death of the connection or crash of one
1446: of the machines will be properly noticed.
1447: However, this means that
1448: connections will die if the route is down temporarily, and some people
1449: find it annoying.
1450: On the other hand, if TCP keepalives are not sent,
1451: sessions may hang indefinitely on the server, leaving
1452: .Dq ghost
1453: users and consuming server resources.
1454: .Pp
1455: The default is
1456: .Dq yes
1457: (to send TCP keepalive messages), and the server will notice
1458: if the network goes down or the client host crashes.
1459: This avoids infinitely hanging sessions.
1460: .Pp
1461: To disable TCP keepalive messages, the value should be set to
1462: .Dq no .
1.118 djm 1463: .It Cm TrustedUserCAKeys
1464: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1465: trusted to sign user certificates for authentication, or
1466: .Dq none
1467: to not use one.
1.119 jmc 1468: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1469: .Ql #
1470: are allowed.
1471: If a certificate is presented for authentication and has its signing CA key
1472: listed in this file, then it may be used for authentication for any user
1473: listed in the certificate's principals list.
1474: Note that certificates that lack a list of principals will not be permitted
1475: for authentication using
1476: .Cm TrustedUserCAKeys .
1.161 jmc 1477: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1478: .Xr ssh-keygen 1 .
1.18 markus 1479: .It Cm UseDNS
1480: Specifies whether
1.52 jmc 1481: .Xr sshd 8
1.207 djm 1482: should look up the remote host name, and to check that
1.18 markus 1483: the resolved host name for the remote IP address maps back to the
1484: very same IP address.
1.207 djm 1485: .Pp
1486: If this option is set to
1487: .Dq no
1488: (the default) then only addresses and not host names may be used in
1.221 djm 1489: .Pa ~/.ssh/authorized_keys
1.207 djm 1490: .Cm from
1491: and
1.211 jmc 1492: .Nm
1.207 djm 1493: .Cm Match
1494: .Cm Host
1495: directives.
1.1 stevesk 1496: .It Cm UsePrivilegeSeparation
1497: Specifies whether
1.52 jmc 1498: .Xr sshd 8
1.2 stevesk 1499: separates privileges by creating an unprivileged child process
1.15 jmc 1500: to deal with incoming network traffic.
1501: After successful authentication, another process will be created that has
1502: the privilege of the authenticated user.
1503: The goal of privilege separation is to prevent privilege
1.1 stevesk 1504: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1505: The argument must be
1506: .Dq yes ,
1507: .Dq no ,
1508: or
1509: .Dq sandbox .
1.134 djm 1510: If
1511: .Cm UsePrivilegeSeparation
1512: is set to
1513: .Dq sandbox
1514: then the pre-authentication unprivileged process is subject to additional
1515: restrictions.
1.213 sobrado 1516: The default is
1517: .Dq sandbox .
1.137 djm 1518: .It Cm VersionAddendum
1519: Optionally specifies additional text to append to the SSH protocol banner
1520: sent by the server upon connection.
1521: The default is
1522: .Dq none .
1.1 stevesk 1523: .It Cm X11DisplayOffset
1524: Specifies the first display number available for
1.52 jmc 1525: .Xr sshd 8 Ns 's
1.1 stevesk 1526: X11 forwarding.
1.52 jmc 1527: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1528: The default is 10.
1529: .It Cm X11Forwarding
1530: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1531: The argument must be
1532: .Dq yes
1533: or
1534: .Dq no .
1.1 stevesk 1535: The default is
1536: .Dq no .
1.13 stevesk 1537: .Pp
1538: When X11 forwarding is enabled, there may be additional exposure to
1539: the server and to client displays if the
1.52 jmc 1540: .Xr sshd 8
1.13 stevesk 1541: proxy display is configured to listen on the wildcard address (see
1542: .Cm X11UseLocalhost
1.52 jmc 1543: below), though this is not the default.
1.13 stevesk 1544: Additionally, the authentication spoofing and authentication data
1545: verification and substitution occur on the client side.
1546: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1547: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1548: forwarding (see the warnings for
1549: .Cm ForwardX11
1550: in
1.19 jmc 1551: .Xr ssh_config 5 ) .
1.13 stevesk 1552: A system administrator may have a stance in which they want to
1553: protect clients that may expose themselves to attack by unwittingly
1554: requesting X11 forwarding, which can warrant a
1555: .Dq no
1556: setting.
1557: .Pp
1558: Note that disabling X11 forwarding does not prevent users from
1559: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1560: .It Cm X11UseLocalhost
1561: Specifies whether
1.52 jmc 1562: .Xr sshd 8
1.1 stevesk 1563: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1564: the wildcard address.
1565: By default,
1.52 jmc 1566: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1567: hostname part of the
1568: .Ev DISPLAY
1569: environment variable to
1570: .Dq localhost .
1.8 stevesk 1571: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1572: However, some older X11 clients may not function with this
1573: configuration.
1574: .Cm X11UseLocalhost
1575: may be set to
1576: .Dq no
1577: to specify that the forwarding server should be bound to the wildcard
1578: address.
1579: The argument must be
1580: .Dq yes
1581: or
1582: .Dq no .
1583: The default is
1584: .Dq yes .
1585: .It Cm XAuthLocation
1.11 stevesk 1586: Specifies the full pathname of the
1.1 stevesk 1587: .Xr xauth 1
1.195 dtucker 1588: program, or
1589: .Dq none
1590: to not use one.
1.1 stevesk 1591: The default is
1592: .Pa /usr/X11R6/bin/xauth .
1593: .El
1.55 jmc 1594: .Sh TIME FORMATS
1.53 jmc 1595: .Xr sshd 8
1.1 stevesk 1596: command-line arguments and configuration file options that specify time
1597: may be expressed using a sequence of the form:
1598: .Sm off
1.7 stevesk 1599: .Ar time Op Ar qualifier ,
1.1 stevesk 1600: .Sm on
1601: where
1602: .Ar time
1603: is a positive integer value and
1604: .Ar qualifier
1605: is one of the following:
1606: .Pp
1607: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1608: .It Aq Cm none
1.1 stevesk 1609: seconds
1610: .It Cm s | Cm S
1611: seconds
1612: .It Cm m | Cm M
1613: minutes
1614: .It Cm h | Cm H
1615: hours
1616: .It Cm d | Cm D
1617: days
1618: .It Cm w | Cm W
1619: weeks
1620: .El
1621: .Pp
1622: Each member of the sequence is added together to calculate
1623: the total time value.
1624: .Pp
1625: Time format examples:
1626: .Pp
1627: .Bl -tag -width Ds -compact -offset indent
1628: .It 600
1629: 600 seconds (10 minutes)
1630: .It 10m
1631: 10 minutes
1632: .It 1h30m
1633: 1 hour 30 minutes (90 minutes)
1634: .El
1.235 jmc 1635: .Sh TOKENS
1636: Arguments to some keywords can make use of tokens,
1637: which are expanded at runtime:
1638: .Pp
1639: .Bl -tag -width XXXX -offset indent -compact
1640: .It %%
1641: A literal
1642: .Sq % .
1643: .It %F
1644: The fingerprint of the CA key.
1645: .It %f
1646: The fingerprint of the key or certificate.
1647: .It %h
1648: The home directory of the user.
1649: .It %i
1650: The key ID in the certificate.
1651: .It %K
1652: The base64-encoded CA key.
1653: .It %k
1654: The base64-encoded key or certificate for authentication.
1655: .It %s
1656: The serial number of the certificate.
1657: .It \&%T
1658: The type of the CA key.
1659: .It %t
1660: The key or certificate type.
1661: .It %u
1662: The username.
1663: .El
1664: .Pp
1665: .Cm AuthorizedKeysCommand
1666: accepts the tokens %%, %f, %h, %t, and %u.
1667: .Pp
1668: .Cm AuthorizedKeysFile
1669: accepts the tokens %%, %h, and %u.
1670: .Pp
1671: .Cm AuthorizedPrincipalsCommand
1672: accepts the tokens %%, %F, %f, %K, %k, %h, %i, %s, %T, %t, and %u.
1673: .Pp
1674: .Cm AuthorizedPrincipalsFile
1675: accepts the tokens %%, %h, and %u.
1676: .Pp
1677: .Cm ChrootDirectory
1678: accepts the tokens %%, %h, and %u.
1.1 stevesk 1679: .Sh FILES
1680: .Bl -tag -width Ds
1681: .It Pa /etc/ssh/sshd_config
1682: Contains configuration data for
1.53 jmc 1683: .Xr sshd 8 .
1.1 stevesk 1684: This file should be writable by root only, but it is recommended
1685: (though not necessary) that it be world-readable.
1686: .El
1.19 jmc 1687: .Sh SEE ALSO
1688: .Xr sshd 8
1.1 stevesk 1689: .Sh AUTHORS
1690: OpenSSH is a derivative of the original and free
1691: ssh 1.2.12 release by Tatu Ylonen.
1692: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1693: Theo de Raadt and Dug Song
1694: removed many bugs, re-added newer features and
1695: created OpenSSH.
1696: Markus Friedl contributed the support for SSH
1697: protocol versions 1.5 and 2.0.
1698: Niels Provos and Markus Friedl contributed support
1699: for privilege separation.
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