Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.235
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: .\"
1.235 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.234 2016/09/22 17:55:13 djm Exp $
! 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
1.44 markus 533: Specifies whether compression is allowed, or delayed until
534: the user has authenticated successfully.
1.3 markus 535: The argument must be
1.44 markus 536: .Dq yes ,
537: .Dq delayed ,
1.3 markus 538: or
539: .Dq no .
540: The default is
1.44 markus 541: .Dq delayed .
1.1 stevesk 542: .It Cm DenyGroups
543: This keyword can be followed by a list of group name patterns, separated
544: by spaces.
545: Login is disallowed for users whose primary group or supplementary
546: group list matches one of the patterns.
547: Only group names are valid; a numerical group ID is not recognized.
548: By default, login is allowed for all groups.
1.54 jmc 549: The allow/deny directives are processed in the following order:
550: .Cm DenyUsers ,
551: .Cm AllowUsers ,
552: .Cm DenyGroups ,
553: and finally
554: .Cm AllowGroups .
1.49 jmc 555: .Pp
1.161 jmc 556: See PATTERNS in
1.49 jmc 557: .Xr ssh_config 5
558: for more information on patterns.
1.1 stevesk 559: .It Cm DenyUsers
560: This keyword can be followed by a list of user name patterns, separated
561: by spaces.
562: Login is disallowed for user names that match one of the patterns.
563: Only user names are valid; a numerical user ID is not recognized.
564: By default, login is allowed for all users.
565: If the pattern takes the form USER@HOST then USER and HOST
566: are separately checked, restricting logins to particular
567: users from particular hosts.
1.222 jmc 568: HOST criteria may additionally contain addresses to match in CIDR
569: address/masklen format.
1.54 jmc 570: The allow/deny directives are processed in the following order:
571: .Cm DenyUsers ,
572: .Cm AllowUsers ,
573: .Cm DenyGroups ,
574: and finally
575: .Cm AllowGroups .
1.49 jmc 576: .Pp
1.161 jmc 577: See PATTERNS in
1.49 jmc 578: .Xr ssh_config 5
579: for more information on patterns.
1.183 djm 580: .It Cm FingerprintHash
581: Specifies the hash algorithm used when logging key fingerprints.
582: Valid options are:
583: .Dq md5
584: and
585: .Dq sha256 .
586: The default is
587: .Dq sha256 .
1.67 dtucker 588: .It Cm ForceCommand
589: Forces the execution of the command specified by
590: .Cm ForceCommand ,
1.84 djm 591: ignoring any command supplied by the client and
592: .Pa ~/.ssh/rc
593: if present.
1.67 dtucker 594: The command is invoked by using the user's login shell with the -c option.
595: This applies to shell, command, or subsystem execution.
596: It is most useful inside a
597: .Cm Match
598: block.
599: The command originally supplied by the client is available in the
600: .Ev SSH_ORIGINAL_COMMAND
601: environment variable.
1.82 djm 602: Specifying a command of
603: .Dq internal-sftp
604: will force the use of an in-process sftp server that requires no support
605: files when used with
606: .Cm ChrootDirectory .
1.215 djm 607: The default is
608: .Dq none .
1.1 stevesk 609: .It Cm GatewayPorts
610: Specifies whether remote hosts are allowed to connect to ports
611: forwarded for the client.
612: By default,
1.52 jmc 613: .Xr sshd 8
1.15 jmc 614: binds remote port forwardings to the loopback address.
615: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 616: .Cm GatewayPorts
1.52 jmc 617: can be used to specify that sshd
1.39 djm 618: should allow remote port forwardings to bind to non-loopback addresses, thus
619: allowing other hosts to connect.
620: The argument may be
621: .Dq no
622: to force remote port forwardings to be available to the local host only,
1.1 stevesk 623: .Dq yes
1.39 djm 624: to force remote port forwardings to bind to the wildcard address, or
625: .Dq clientspecified
626: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 627: The default is
628: .Dq no .
1.23 markus 629: .It Cm GSSAPIAuthentication
1.25 markus 630: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 631: The default is
1.23 markus 632: .Dq no .
633: .It Cm GSSAPICleanupCredentials
634: Specifies whether to automatically destroy the user's credentials cache
635: on logout.
636: The default is
637: .Dq yes .
1.203 djm 638: .It Cm GSSAPIStrictAcceptorCheck
639: Determines whether to be strict about the identity of the GSSAPI acceptor
640: a client authenticates against.
641: If set to
642: .Dq yes
643: then the client must authenticate against the
644: .Pa host
645: service on the current hostname.
646: If set to
647: .Dq no
648: then the client may authenticate against any service key stored in the
649: machine's default store.
650: This facility is provided to assist with operation on multi homed machines.
651: The default is
652: .Dq yes .
1.189 djm 653: .It Cm HostbasedAcceptedKeyTypes
654: Specifies the key types that will be accepted for hostbased authentication
655: as a comma-separated pattern list.
1.208 djm 656: Alternately if the specified value begins with a
657: .Sq +
658: character, then the specified key types will be appended to the default set
659: instead of replacing them.
1.206 markus 660: The default for this option is:
661: .Bd -literal -offset 3n
662: ecdsa-sha2-nistp256-cert-v01@openssh.com,
663: ecdsa-sha2-nistp384-cert-v01@openssh.com,
664: ecdsa-sha2-nistp521-cert-v01@openssh.com,
665: ssh-ed25519-cert-v01@openssh.com,
666: ssh-rsa-cert-v01@openssh.com,
667: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 668: ssh-ed25519,ssh-rsa
1.206 markus 669: .Ed
670: .Pp
1.189 djm 671: The
672: .Fl Q
673: option of
674: .Xr ssh 1
675: may be used to list supported key types.
1.1 stevesk 676: .It Cm HostbasedAuthentication
677: Specifies whether rhosts or /etc/hosts.equiv authentication together
678: with successful public key client host authentication is allowed
1.50 jmc 679: (host-based authentication).
1.70 dtucker 680: The default is
681: .Dq no .
682: .It Cm HostbasedUsesNameFromPacketOnly
683: Specifies whether or not the server will attempt to perform a reverse
684: name lookup when matching the name in the
685: .Pa ~/.shosts ,
686: .Pa ~/.rhosts ,
687: and
688: .Pa /etc/hosts.equiv
689: files during
690: .Cm HostbasedAuthentication .
691: A setting of
692: .Dq yes
693: means that
694: .Xr sshd 8
695: uses the name supplied by the client rather than
696: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 697: The default is
698: .Dq no .
1.117 djm 699: .It Cm HostCertificate
700: Specifies a file containing a public host certificate.
701: The certificate's public key must match a private host key already specified
702: by
703: .Cm HostKey .
704: The default behaviour of
705: .Xr sshd 8
706: is not to load any certificates.
1.1 stevesk 707: .It Cm HostKey
708: Specifies a file containing a private host key
709: used by SSH.
1.229 naddy 710: The defaults are
1.126 djm 711: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 712: .Pa /etc/ssh/ssh_host_ecdsa_key ,
713: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 714: and
1.229 naddy 715: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 716: .Pp
1.1 stevesk 717: Note that
1.52 jmc 718: .Xr sshd 8
1.206 markus 719: will refuse to use a file if it is group/world-accessible
720: and that the
721: .Cm HostKeyAlgorithms
722: option restricts which of the keys are actually used by
723: .Xr sshd 8 .
724: .Pp
1.1 stevesk 725: It is possible to have multiple host key files.
1.162 markus 726: It is also possible to specify public host key files instead.
727: In this case operations on the private key will be delegated
728: to an
729: .Xr ssh-agent 1 .
730: .It Cm HostKeyAgent
731: Identifies the UNIX-domain socket used to communicate
732: with an agent that has access to the private host keys.
1.223 markus 733: If the string
1.162 markus 734: .Dq SSH_AUTH_SOCK
735: is specified, the location of the socket will be read from the
736: .Ev SSH_AUTH_SOCK
737: environment variable.
1.206 markus 738: .It Cm HostKeyAlgorithms
1.219 jmc 739: Specifies the host key algorithms
1.206 markus 740: that the server offers.
741: The default for this option is:
742: .Bd -literal -offset 3n
743: ecdsa-sha2-nistp256-cert-v01@openssh.com,
744: ecdsa-sha2-nistp384-cert-v01@openssh.com,
745: ecdsa-sha2-nistp521-cert-v01@openssh.com,
746: ssh-ed25519-cert-v01@openssh.com,
747: ssh-rsa-cert-v01@openssh.com,
748: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 749: ssh-ed25519,ssh-rsa
1.206 markus 750: .Ed
751: .Pp
752: The list of available key types may also be obtained using the
753: .Fl Q
754: option of
755: .Xr ssh 1
756: with an argument of
757: .Dq key .
1.1 stevesk 758: .It Cm IgnoreRhosts
759: Specifies that
760: .Pa .rhosts
761: and
762: .Pa .shosts
763: files will not be used in
764: .Cm HostbasedAuthentication .
765: .Pp
766: .Pa /etc/hosts.equiv
767: and
768: .Pa /etc/shosts.equiv
769: are still used.
770: The default is
771: .Dq yes .
772: .It Cm IgnoreUserKnownHosts
773: Specifies whether
1.52 jmc 774: .Xr sshd 8
1.1 stevesk 775: should ignore the user's
1.41 djm 776: .Pa ~/.ssh/known_hosts
1.1 stevesk 777: during
778: .Cm HostbasedAuthentication .
779: The default is
780: .Dq no .
1.129 djm 781: .It Cm IPQoS
782: Specifies the IPv4 type-of-service or DSCP class for the connection.
783: Accepted values are
784: .Dq af11 ,
785: .Dq af12 ,
786: .Dq af13 ,
1.136 djm 787: .Dq af21 ,
1.129 djm 788: .Dq af22 ,
789: .Dq af23 ,
790: .Dq af31 ,
791: .Dq af32 ,
792: .Dq af33 ,
793: .Dq af41 ,
794: .Dq af42 ,
795: .Dq af43 ,
796: .Dq cs0 ,
797: .Dq cs1 ,
798: .Dq cs2 ,
799: .Dq cs3 ,
800: .Dq cs4 ,
801: .Dq cs5 ,
802: .Dq cs6 ,
803: .Dq cs7 ,
804: .Dq ef ,
805: .Dq lowdelay ,
806: .Dq throughput ,
807: .Dq reliability ,
808: or a numeric value.
1.131 djm 809: This option may take one or two arguments, separated by whitespace.
1.129 djm 810: If one argument is specified, it is used as the packet class unconditionally.
811: If two values are specified, the first is automatically selected for
812: interactive sessions and the second for non-interactive sessions.
813: The default is
814: .Dq lowdelay
815: for interactive sessions and
816: .Dq throughput
817: for non-interactive sessions.
1.171 jmc 818: .It Cm KbdInteractiveAuthentication
819: Specifies whether to allow keyboard-interactive authentication.
820: The argument to this keyword must be
821: .Dq yes
822: or
823: .Dq no .
824: The default is to use whatever value
825: .Cm ChallengeResponseAuthentication
826: is set to
827: (by default
828: .Dq yes ) .
1.1 stevesk 829: .It Cm KerberosAuthentication
1.24 markus 830: Specifies whether the password provided by the user for
1.1 stevesk 831: .Cm PasswordAuthentication
1.24 markus 832: will be validated through the Kerberos KDC.
1.1 stevesk 833: To use this option, the server needs a
834: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 835: The default is
1.29 dtucker 836: .Dq no .
837: .It Cm KerberosGetAFSToken
1.45 djm 838: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 839: an AFS token before accessing the user's home directory.
1.52 jmc 840: The default is
1.1 stevesk 841: .Dq no .
842: .It Cm KerberosOrLocalPasswd
1.52 jmc 843: If password authentication through Kerberos fails then
1.1 stevesk 844: the password will be validated via any additional local mechanism
845: such as
846: .Pa /etc/passwd .
1.52 jmc 847: The default is
1.1 stevesk 848: .Dq yes .
849: .It Cm KerberosTicketCleanup
850: Specifies whether to automatically destroy the user's ticket cache
851: file on logout.
1.52 jmc 852: The default is
1.1 stevesk 853: .Dq yes .
1.127 djm 854: .It Cm KexAlgorithms
855: Specifies the available KEX (Key Exchange) algorithms.
856: Multiple algorithms must be comma-separated.
1.208 djm 857: Alternately if the specified value begins with a
858: .Sq +
859: character, then the specified methods will be appended to the default set
860: instead of replacing them.
1.173 naddy 861: The supported algorithms are:
862: .Pp
863: .Bl -item -compact -offset indent
864: .It
1.234 djm 865: curve25519-sha256
866: .It
1.173 naddy 867: curve25519-sha256@libssh.org
868: .It
869: diffie-hellman-group1-sha1
870: .It
871: diffie-hellman-group14-sha1
872: .It
873: diffie-hellman-group-exchange-sha1
874: .It
875: diffie-hellman-group-exchange-sha256
876: .It
877: ecdh-sha2-nistp256
878: .It
879: ecdh-sha2-nistp384
880: .It
881: ecdh-sha2-nistp521
882: .El
883: .Pp
884: The default is:
1.170 dtucker 885: .Bd -literal -offset indent
1.234 djm 886: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 887: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
888: diffie-hellman-group-exchange-sha256,
1.173 naddy 889: diffie-hellman-group14-sha1
1.170 dtucker 890: .Ed
1.188 djm 891: .Pp
892: The list of available key exchange algorithms may also be obtained using the
893: .Fl Q
894: option of
895: .Xr ssh 1
896: with an argument of
897: .Dq kex .
1.1 stevesk 898: .It Cm ListenAddress
899: Specifies the local addresses
1.52 jmc 900: .Xr sshd 8
1.1 stevesk 901: should listen on.
902: The following forms may be used:
903: .Pp
904: .Bl -item -offset indent -compact
905: .It
906: .Cm ListenAddress
907: .Sm off
1.180 jmc 908: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 909: .Sm on
910: .It
911: .Cm ListenAddress
912: .Sm off
1.180 jmc 913: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 914: .Sm on
915: .It
916: .Cm ListenAddress
917: .Sm off
918: .Oo
1.180 jmc 919: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 920: .Sm on
921: .El
922: .Pp
923: If
924: .Ar port
925: is not specified,
1.200 dtucker 926: sshd will listen on the address and all
1.1 stevesk 927: .Cm Port
1.17 jmc 928: options specified.
929: The default is to listen on all local addresses.
1.15 jmc 930: Multiple
1.1 stevesk 931: .Cm ListenAddress
1.17 jmc 932: options are permitted.
1.1 stevesk 933: .It Cm LoginGraceTime
934: The server disconnects after this time if the user has not
935: successfully logged in.
936: If the value is 0, there is no time limit.
1.12 stevesk 937: The default is 120 seconds.
1.1 stevesk 938: .It Cm LogLevel
939: Gives the verbosity level that is used when logging messages from
1.53 jmc 940: .Xr sshd 8 .
1.1 stevesk 941: The possible values are:
1.52 jmc 942: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 943: The default is INFO.
944: DEBUG and DEBUG1 are equivalent.
945: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
946: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 947: .It Cm MACs
948: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 949: The MAC algorithm is used for data integrity protection.
1.1 stevesk 950: Multiple algorithms must be comma-separated.
1.208 djm 951: If the specified value begins with a
952: .Sq +
953: character, then the specified algorithms will be appended to the default set
954: instead of replacing them.
955: .Pp
1.152 markus 956: The algorithms that contain
957: .Dq -etm
958: calculate the MAC after encryption (encrypt-then-mac).
959: These are considered safer and their use recommended.
1.173 naddy 960: The supported MACs are:
961: .Pp
962: .Bl -item -compact -offset indent
963: .It
964: hmac-md5
965: .It
966: hmac-md5-96
967: .It
968: hmac-ripemd160
969: .It
970: hmac-sha1
971: .It
972: hmac-sha1-96
973: .It
974: hmac-sha2-256
975: .It
976: hmac-sha2-512
977: .It
978: umac-64@openssh.com
979: .It
980: umac-128@openssh.com
981: .It
982: hmac-md5-etm@openssh.com
983: .It
984: hmac-md5-96-etm@openssh.com
985: .It
986: hmac-ripemd160-etm@openssh.com
987: .It
988: hmac-sha1-etm@openssh.com
989: .It
990: hmac-sha1-96-etm@openssh.com
991: .It
992: hmac-sha2-256-etm@openssh.com
993: .It
994: hmac-sha2-512-etm@openssh.com
995: .It
996: umac-64-etm@openssh.com
997: .It
998: umac-128-etm@openssh.com
999: .El
1000: .Pp
1.52 jmc 1001: The default is:
1.77 jmc 1002: .Bd -literal -offset indent
1.152 markus 1003: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1004: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1005: hmac-sha1-etm@openssh.com,
1.173 naddy 1006: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1007: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1008: .Ed
1.188 djm 1009: .Pp
1010: The list of available MAC algorithms may also be obtained using the
1011: .Fl Q
1012: option of
1013: .Xr ssh 1
1014: with an argument of
1015: .Dq mac .
1.60 dtucker 1016: .It Cm Match
1.61 jmc 1017: Introduces a conditional block.
1.65 dtucker 1018: If all of the criteria on the
1.60 dtucker 1019: .Cm Match
1.65 dtucker 1020: line are satisfied, the keywords on the following lines override those
1021: set in the global section of the config file, until either another
1.60 dtucker 1022: .Cm Match
1.65 dtucker 1023: line or the end of the file.
1.172 djm 1024: If a keyword appears in multiple
1025: .Cm Match
1.177 sobrado 1026: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1027: applied.
1.91 djm 1028: .Pp
1.61 jmc 1029: The arguments to
1.60 dtucker 1030: .Cm Match
1.163 dtucker 1031: are one or more criteria-pattern pairs or the single token
1032: .Cm All
1033: which matches all criteria.
1.60 dtucker 1034: The available criteria are
1035: .Cm User ,
1.69 dtucker 1036: .Cm Group ,
1.60 dtucker 1037: .Cm Host ,
1.139 dtucker 1038: .Cm LocalAddress ,
1039: .Cm LocalPort ,
1.60 dtucker 1040: and
1041: .Cm Address .
1.91 djm 1042: The match patterns may consist of single entries or comma-separated
1043: lists and may use the wildcard and negation operators described in the
1.161 jmc 1044: PATTERNS section of
1.92 djm 1045: .Xr ssh_config 5 .
1.91 djm 1046: .Pp
1047: The patterns in an
1048: .Cm Address
1049: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1050: address/masklen format, e.g.\&
1.91 djm 1051: .Dq 192.0.2.0/24
1052: or
1.228 jca 1053: .Dq 2001:db8::/32 .
1.91 djm 1054: Note that the mask length provided must be consistent with the address -
1055: it is an error to specify a mask length that is too long for the address
1.93 jmc 1056: or one with bits set in this host portion of the address.
1057: For example,
1.91 djm 1058: .Dq 192.0.2.0/33
1059: and
1.93 jmc 1060: .Dq 192.0.2.0/8
1.91 djm 1061: respectively.
1062: .Pp
1.60 dtucker 1063: Only a subset of keywords may be used on the lines following a
1064: .Cm Match
1065: keyword.
1066: Available keywords are
1.142 jmc 1067: .Cm AcceptEnv ,
1.99 okan 1068: .Cm AllowAgentForwarding ,
1.142 jmc 1069: .Cm AllowGroups ,
1.193 djm 1070: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1071: .Cm AllowTcpForwarding ,
1.141 markus 1072: .Cm AllowUsers ,
1.149 djm 1073: .Cm AuthenticationMethods ,
1.146 djm 1074: .Cm AuthorizedKeysCommand ,
1075: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1076: .Cm AuthorizedKeysFile ,
1.214 djm 1077: .Cm AuthorizedPrincipalsCommand ,
1078: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1079: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1080: .Cm Banner ,
1.85 djm 1081: .Cm ChrootDirectory ,
1.141 markus 1082: .Cm DenyGroups ,
1083: .Cm DenyUsers ,
1.67 dtucker 1084: .Cm ForceCommand ,
1.194 djm 1085: .Cm GatewayPorts ,
1.193 djm 1086: .Cm GSSAPIAuthentication ,
1.189 djm 1087: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1088: .Cm HostbasedAuthentication ,
1.123 djm 1089: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1090: .Cm IPQoS ,
1.74 jmc 1091: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1092: .Cm KerberosAuthentication ,
1.95 dtucker 1093: .Cm MaxAuthTries ,
1.94 dtucker 1094: .Cm MaxSessions ,
1.72 dtucker 1095: .Cm PasswordAuthentication ,
1.97 djm 1096: .Cm PermitEmptyPasswords ,
1.66 dtucker 1097: .Cm PermitOpen ,
1.79 dtucker 1098: .Cm PermitRootLogin ,
1.164 djm 1099: .Cm PermitTTY ,
1.123 djm 1100: .Cm PermitTunnel ,
1.174 djm 1101: .Cm PermitUserRC ,
1.189 djm 1102: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1103: .Cm PubkeyAuthentication ,
1.159 dtucker 1104: .Cm RekeyLimit ,
1.193 djm 1105: .Cm RevokedKeys ,
1106: .Cm StreamLocalBindMask ,
1107: .Cm StreamLocalBindUnlink ,
1108: .Cm TrustedUserCAKeys ,
1.66 dtucker 1109: .Cm X11DisplayOffset ,
1.101 djm 1110: .Cm X11Forwarding
1.60 dtucker 1111: and
1.102 djm 1112: .Cm X11UseLocalHost .
1.33 dtucker 1113: .It Cm MaxAuthTries
1114: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1115: connection.
1116: Once the number of failures reaches half this value,
1117: additional failures are logged.
1118: The default is 6.
1.90 djm 1119: .It Cm MaxSessions
1.216 djm 1120: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1121: sessions permitted per network connection.
1122: Multiple sessions may be established by clients that support connection
1123: multiplexing.
1124: Setting
1125: .Cm MaxSessions
1126: to 1 will effectively disable session multiplexing, whereas setting it to 0
1127: will prevent all shell, login and subsystem sessions while still permitting
1128: forwarding.
1.90 djm 1129: The default is 10.
1.1 stevesk 1130: .It Cm MaxStartups
1131: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1132: SSH daemon.
1.1 stevesk 1133: Additional connections will be dropped until authentication succeeds or the
1134: .Cm LoginGraceTime
1135: expires for a connection.
1.156 dtucker 1136: The default is 10:30:100.
1.1 stevesk 1137: .Pp
1138: Alternatively, random early drop can be enabled by specifying
1139: the three colon separated values
1140: .Dq start:rate:full
1.51 jmc 1141: (e.g. "10:30:60").
1.53 jmc 1142: .Xr sshd 8
1.1 stevesk 1143: will refuse connection attempts with a probability of
1144: .Dq rate/100
1145: (30%)
1146: if there are currently
1147: .Dq start
1148: (10)
1149: unauthenticated connections.
1150: The probability increases linearly and all connection attempts
1151: are refused if the number of unauthenticated connections reaches
1152: .Dq full
1153: (60).
1154: .It Cm PasswordAuthentication
1155: Specifies whether password authentication is allowed.
1156: The default is
1157: .Dq yes .
1158: .It Cm PermitEmptyPasswords
1159: When password authentication is allowed, it specifies whether the
1160: server allows login to accounts with empty password strings.
1161: The default is
1162: .Dq no .
1.62 dtucker 1163: .It Cm PermitOpen
1164: Specifies the destinations to which TCP port forwarding is permitted.
1165: The forwarding specification must be one of the following forms:
1166: .Pp
1167: .Bl -item -offset indent -compact
1168: .It
1169: .Cm PermitOpen
1170: .Sm off
1171: .Ar host : port
1172: .Sm on
1173: .It
1174: .Cm PermitOpen
1175: .Sm off
1176: .Ar IPv4_addr : port
1177: .Sm on
1178: .It
1179: .Cm PermitOpen
1180: .Sm off
1181: .Ar \&[ IPv6_addr \&] : port
1182: .Sm on
1183: .El
1184: .Pp
1.68 dtucker 1185: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1186: An argument of
1187: .Dq any
1188: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1189: An argument of
1190: .Dq none
1191: can be used to prohibit all forwarding requests.
1.227 jmc 1192: The wildcard
1.226 dtucker 1193: .Dq *
1.227 jmc 1194: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1195: By default all port forwarding requests are permitted.
1.1 stevesk 1196: .It Cm PermitRootLogin
1.38 jmc 1197: Specifies whether root can log in using
1.1 stevesk 1198: .Xr ssh 1 .
1199: The argument must be
1200: .Dq yes ,
1.210 deraadt 1201: .Dq prohibit-password ,
1.1 stevesk 1202: .Dq without-password ,
1.52 jmc 1203: .Dq forced-commands-only ,
1.1 stevesk 1204: or
1205: .Dq no .
1206: The default is
1.210 deraadt 1207: .Dq prohibit-password .
1.1 stevesk 1208: .Pp
1209: If this option is set to
1.210 deraadt 1210: .Dq prohibit-password
1211: or
1.52 jmc 1212: .Dq without-password ,
1.210 deraadt 1213: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1214: .Pp
1215: If this option is set to
1.52 jmc 1216: .Dq forced-commands-only ,
1.1 stevesk 1217: root login with public key authentication will be allowed,
1218: but only if the
1219: .Ar command
1220: option has been specified
1221: (which may be useful for taking remote backups even if root login is
1.17 jmc 1222: normally not allowed).
1223: All other authentication methods are disabled for root.
1.1 stevesk 1224: .Pp
1225: If this option is set to
1.52 jmc 1226: .Dq no ,
1.38 jmc 1227: root is not allowed to log in.
1.231 jmc 1228: .It Cm PermitTTY
1229: Specifies whether
1230: .Xr pty 4
1231: allocation is permitted.
1232: The default is
1233: .Dq yes .
1.46 reyk 1234: .It Cm PermitTunnel
1235: Specifies whether
1236: .Xr tun 4
1237: device forwarding is allowed.
1.47 reyk 1238: The argument must be
1239: .Dq yes ,
1.58 stevesk 1240: .Dq point-to-point
1241: (layer 3),
1242: .Dq ethernet
1243: (layer 2), or
1.47 reyk 1244: .Dq no .
1.58 stevesk 1245: Specifying
1246: .Dq yes
1247: permits both
1248: .Dq point-to-point
1249: and
1250: .Dq ethernet .
1.46 reyk 1251: The default is
1252: .Dq no .
1.178 djm 1253: .Pp
1254: Independent of this setting, the permissions of the selected
1255: .Xr tun 4
1256: device must allow access to the user.
1.6 markus 1257: .It Cm PermitUserEnvironment
1258: Specifies whether
1259: .Pa ~/.ssh/environment
1.9 stevesk 1260: and
1.6 markus 1261: .Cm environment=
1262: options in
1263: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1264: are processed by
1.52 jmc 1265: .Xr sshd 8 .
1.6 markus 1266: The default is
1267: .Dq no .
1.9 stevesk 1268: Enabling environment processing may enable users to bypass access
1269: restrictions in some configurations using mechanisms such as
1270: .Ev LD_PRELOAD .
1.174 djm 1271: .It Cm PermitUserRC
1272: Specifies whether any
1273: .Pa ~/.ssh/rc
1274: file is executed.
1275: The default is
1276: .Dq yes .
1.1 stevesk 1277: .It Cm PidFile
1.4 stevesk 1278: Specifies the file that contains the process ID of the
1.195 dtucker 1279: SSH daemon, or
1280: .Dq none
1281: to not write one.
1.1 stevesk 1282: The default is
1283: .Pa /var/run/sshd.pid .
1284: .It Cm Port
1285: Specifies the port number that
1.52 jmc 1286: .Xr sshd 8
1.1 stevesk 1287: listens on.
1288: The default is 22.
1289: Multiple options of this type are permitted.
1290: See also
1291: .Cm ListenAddress .
1292: .It Cm PrintLastLog
1293: Specifies whether
1.52 jmc 1294: .Xr sshd 8
1.36 jaredy 1295: should print the date and time of the last user login when a user logs
1296: in interactively.
1.1 stevesk 1297: The default is
1298: .Dq yes .
1299: .It Cm PrintMotd
1300: Specifies whether
1.52 jmc 1301: .Xr sshd 8
1.1 stevesk 1302: should print
1303: .Pa /etc/motd
1304: when a user logs in interactively.
1305: (On some systems it is also printed by the shell,
1306: .Pa /etc/profile ,
1307: or equivalent.)
1308: The default is
1309: .Dq yes .
1.189 djm 1310: .It Cm PubkeyAcceptedKeyTypes
1311: Specifies the key types that will be accepted for public key authentication
1312: as a comma-separated pattern list.
1.208 djm 1313: Alternately if the specified value begins with a
1314: .Sq +
1315: character, then the specified key types will be appended to the default set
1316: instead of replacing them.
1.206 markus 1317: The default for this option is:
1318: .Bd -literal -offset 3n
1319: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1320: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1321: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1322: ssh-ed25519-cert-v01@openssh.com,
1323: ssh-rsa-cert-v01@openssh.com,
1324: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1325: ssh-ed25519,ssh-rsa
1.206 markus 1326: .Ed
1327: .Pp
1.189 djm 1328: The
1329: .Fl Q
1330: option of
1331: .Xr ssh 1
1332: may be used to list supported key types.
1.1 stevesk 1333: .It Cm PubkeyAuthentication
1334: Specifies whether public key authentication is allowed.
1335: The default is
1336: .Dq yes .
1.159 dtucker 1337: .It Cm RekeyLimit
1338: Specifies the maximum amount of data that may be transmitted before the
1339: session key is renegotiated, optionally followed a maximum amount of
1340: time that may pass before the session key is renegotiated.
1341: The first argument is specified in bytes and may have a suffix of
1342: .Sq K ,
1343: .Sq M ,
1344: or
1345: .Sq G
1346: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1347: The default is between
1348: .Sq 1G
1349: and
1350: .Sq 4G ,
1351: depending on the cipher.
1352: The optional second value is specified in seconds and may use any of the
1353: units documented in the
1354: .Sx TIME FORMATS
1.160 jmc 1355: section.
1.159 dtucker 1356: The default value for
1357: .Cm RekeyLimit
1358: is
1359: .Dq default none ,
1360: which means that rekeying is performed after the cipher's default amount
1361: of data has been sent or received and no time based rekeying is done.
1.118 djm 1362: .It Cm RevokedKeys
1.195 dtucker 1363: Specifies revoked public keys file, or
1364: .Dq none
1365: to not use one.
1.118 djm 1366: Keys listed in this file will be refused for public key authentication.
1367: Note that if this file is not readable, then public key authentication will
1368: be refused for all users.
1.154 djm 1369: Keys may be specified as a text file, listing one public key per line, or as
1370: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1371: .Xr ssh-keygen 1 .
1.161 jmc 1372: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1373: .Xr ssh-keygen 1 .
1.175 millert 1374: .It Cm StreamLocalBindMask
1375: Sets the octal file creation mode mask
1376: .Pq umask
1377: used when creating a Unix-domain socket file for local or remote
1378: port forwarding.
1379: This option is only used for port forwarding to a Unix-domain socket file.
1380: .Pp
1381: The default value is 0177, which creates a Unix-domain socket file that is
1382: readable and writable only by the owner.
1383: Note that not all operating systems honor the file mode on Unix-domain
1384: socket files.
1385: .It Cm StreamLocalBindUnlink
1386: Specifies whether to remove an existing Unix-domain socket file for local
1387: or remote port forwarding before creating a new one.
1388: If the socket file already exists and
1389: .Cm StreamLocalBindUnlink
1390: is not enabled,
1391: .Nm sshd
1392: will be unable to forward the port to the Unix-domain socket file.
1393: This option is only used for port forwarding to a Unix-domain socket file.
1394: .Pp
1395: The argument must be
1396: .Dq yes
1397: or
1398: .Dq no .
1399: The default is
1400: .Dq no .
1.1 stevesk 1401: .It Cm StrictModes
1402: Specifies whether
1.52 jmc 1403: .Xr sshd 8
1.1 stevesk 1404: should check file modes and ownership of the
1405: user's files and home directory before accepting login.
1406: This is normally desirable because novices sometimes accidentally leave their
1407: directory or files world-writable.
1408: The default is
1409: .Dq yes .
1.112 djm 1410: Note that this does not apply to
1411: .Cm ChrootDirectory ,
1412: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1413: .It Cm Subsystem
1.51 jmc 1414: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1415: Arguments should be a subsystem name and a command (with optional arguments)
1416: to execute upon subsystem request.
1.80 djm 1417: .Pp
1.1 stevesk 1418: The command
1419: .Xr sftp-server 8
1420: implements the
1421: .Dq sftp
1422: file transfer subsystem.
1.80 djm 1423: .Pp
1424: Alternately the name
1425: .Dq internal-sftp
1426: implements an in-process
1427: .Dq sftp
1428: server.
1429: This may simplify configurations using
1430: .Cm ChrootDirectory
1431: to force a different filesystem root on clients.
1432: .Pp
1.1 stevesk 1433: By default no subsystems are defined.
1434: .It Cm SyslogFacility
1435: Gives the facility code that is used when logging messages from
1.53 jmc 1436: .Xr sshd 8 .
1.1 stevesk 1437: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1438: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1439: The default is AUTH.
1.27 markus 1440: .It Cm TCPKeepAlive
1441: Specifies whether the system should send TCP keepalive messages to the
1442: other side.
1443: If they are sent, death of the connection or crash of one
1444: of the machines will be properly noticed.
1445: However, this means that
1446: connections will die if the route is down temporarily, and some people
1447: find it annoying.
1448: On the other hand, if TCP keepalives are not sent,
1449: sessions may hang indefinitely on the server, leaving
1450: .Dq ghost
1451: users and consuming server resources.
1452: .Pp
1453: The default is
1454: .Dq yes
1455: (to send TCP keepalive messages), and the server will notice
1456: if the network goes down or the client host crashes.
1457: This avoids infinitely hanging sessions.
1458: .Pp
1459: To disable TCP keepalive messages, the value should be set to
1460: .Dq no .
1.118 djm 1461: .It Cm TrustedUserCAKeys
1462: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1463: trusted to sign user certificates for authentication, or
1464: .Dq none
1465: to not use one.
1.119 jmc 1466: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1467: .Ql #
1468: are allowed.
1469: If a certificate is presented for authentication and has its signing CA key
1470: listed in this file, then it may be used for authentication for any user
1471: listed in the certificate's principals list.
1472: Note that certificates that lack a list of principals will not be permitted
1473: for authentication using
1474: .Cm TrustedUserCAKeys .
1.161 jmc 1475: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1476: .Xr ssh-keygen 1 .
1.18 markus 1477: .It Cm UseDNS
1478: Specifies whether
1.52 jmc 1479: .Xr sshd 8
1.207 djm 1480: should look up the remote host name, and to check that
1.18 markus 1481: the resolved host name for the remote IP address maps back to the
1482: very same IP address.
1.207 djm 1483: .Pp
1484: If this option is set to
1485: .Dq no
1486: (the default) then only addresses and not host names may be used in
1.221 djm 1487: .Pa ~/.ssh/authorized_keys
1.207 djm 1488: .Cm from
1489: and
1.211 jmc 1490: .Nm
1.207 djm 1491: .Cm Match
1492: .Cm Host
1493: directives.
1.1 stevesk 1494: .It Cm UsePrivilegeSeparation
1495: Specifies whether
1.52 jmc 1496: .Xr sshd 8
1.2 stevesk 1497: separates privileges by creating an unprivileged child process
1.15 jmc 1498: to deal with incoming network traffic.
1499: After successful authentication, another process will be created that has
1500: the privilege of the authenticated user.
1501: The goal of privilege separation is to prevent privilege
1.1 stevesk 1502: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1503: The argument must be
1504: .Dq yes ,
1505: .Dq no ,
1506: or
1507: .Dq sandbox .
1.134 djm 1508: If
1509: .Cm UsePrivilegeSeparation
1510: is set to
1511: .Dq sandbox
1512: then the pre-authentication unprivileged process is subject to additional
1513: restrictions.
1.213 sobrado 1514: The default is
1515: .Dq sandbox .
1.137 djm 1516: .It Cm VersionAddendum
1517: Optionally specifies additional text to append to the SSH protocol banner
1518: sent by the server upon connection.
1519: The default is
1520: .Dq none .
1.1 stevesk 1521: .It Cm X11DisplayOffset
1522: Specifies the first display number available for
1.52 jmc 1523: .Xr sshd 8 Ns 's
1.1 stevesk 1524: X11 forwarding.
1.52 jmc 1525: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1526: The default is 10.
1527: .It Cm X11Forwarding
1528: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1529: The argument must be
1530: .Dq yes
1531: or
1532: .Dq no .
1.1 stevesk 1533: The default is
1534: .Dq no .
1.13 stevesk 1535: .Pp
1536: When X11 forwarding is enabled, there may be additional exposure to
1537: the server and to client displays if the
1.52 jmc 1538: .Xr sshd 8
1.13 stevesk 1539: proxy display is configured to listen on the wildcard address (see
1540: .Cm X11UseLocalhost
1.52 jmc 1541: below), though this is not the default.
1.13 stevesk 1542: Additionally, the authentication spoofing and authentication data
1543: verification and substitution occur on the client side.
1544: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1545: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1546: forwarding (see the warnings for
1547: .Cm ForwardX11
1548: in
1.19 jmc 1549: .Xr ssh_config 5 ) .
1.13 stevesk 1550: A system administrator may have a stance in which they want to
1551: protect clients that may expose themselves to attack by unwittingly
1552: requesting X11 forwarding, which can warrant a
1553: .Dq no
1554: setting.
1555: .Pp
1556: Note that disabling X11 forwarding does not prevent users from
1557: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1558: .It Cm X11UseLocalhost
1559: Specifies whether
1.52 jmc 1560: .Xr sshd 8
1.1 stevesk 1561: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1562: the wildcard address.
1563: By default,
1.52 jmc 1564: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1565: hostname part of the
1566: .Ev DISPLAY
1567: environment variable to
1568: .Dq localhost .
1.8 stevesk 1569: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1570: However, some older X11 clients may not function with this
1571: configuration.
1572: .Cm X11UseLocalhost
1573: may be set to
1574: .Dq no
1575: to specify that the forwarding server should be bound to the wildcard
1576: address.
1577: The argument must be
1578: .Dq yes
1579: or
1580: .Dq no .
1581: The default is
1582: .Dq yes .
1583: .It Cm XAuthLocation
1.11 stevesk 1584: Specifies the full pathname of the
1.1 stevesk 1585: .Xr xauth 1
1.195 dtucker 1586: program, or
1587: .Dq none
1588: to not use one.
1.1 stevesk 1589: The default is
1590: .Pa /usr/X11R6/bin/xauth .
1591: .El
1.55 jmc 1592: .Sh TIME FORMATS
1.53 jmc 1593: .Xr sshd 8
1.1 stevesk 1594: command-line arguments and configuration file options that specify time
1595: may be expressed using a sequence of the form:
1596: .Sm off
1.7 stevesk 1597: .Ar time Op Ar qualifier ,
1.1 stevesk 1598: .Sm on
1599: where
1600: .Ar time
1601: is a positive integer value and
1602: .Ar qualifier
1603: is one of the following:
1604: .Pp
1605: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1606: .It Aq Cm none
1.1 stevesk 1607: seconds
1608: .It Cm s | Cm S
1609: seconds
1610: .It Cm m | Cm M
1611: minutes
1612: .It Cm h | Cm H
1613: hours
1614: .It Cm d | Cm D
1615: days
1616: .It Cm w | Cm W
1617: weeks
1618: .El
1619: .Pp
1620: Each member of the sequence is added together to calculate
1621: the total time value.
1622: .Pp
1623: Time format examples:
1624: .Pp
1625: .Bl -tag -width Ds -compact -offset indent
1626: .It 600
1627: 600 seconds (10 minutes)
1628: .It 10m
1629: 10 minutes
1630: .It 1h30m
1631: 1 hour 30 minutes (90 minutes)
1632: .El
1.235 ! jmc 1633: .Sh TOKENS
! 1634: Arguments to some keywords can make use of tokens,
! 1635: which are expanded at runtime:
! 1636: .Pp
! 1637: .Bl -tag -width XXXX -offset indent -compact
! 1638: .It %%
! 1639: A literal
! 1640: .Sq % .
! 1641: .It %F
! 1642: The fingerprint of the CA key.
! 1643: .It %f
! 1644: The fingerprint of the key or certificate.
! 1645: .It %h
! 1646: The home directory of the user.
! 1647: .It %i
! 1648: The key ID in the certificate.
! 1649: .It %K
! 1650: The base64-encoded CA key.
! 1651: .It %k
! 1652: The base64-encoded key or certificate for authentication.
! 1653: .It %s
! 1654: The serial number of the certificate.
! 1655: .It \&%T
! 1656: The type of the CA key.
! 1657: .It %t
! 1658: The key or certificate type.
! 1659: .It %u
! 1660: The username.
! 1661: .El
! 1662: .Pp
! 1663: .Cm AuthorizedKeysCommand
! 1664: accepts the tokens %%, %f, %h, %t, and %u.
! 1665: .Pp
! 1666: .Cm AuthorizedKeysFile
! 1667: accepts the tokens %%, %h, and %u.
! 1668: .Pp
! 1669: .Cm AuthorizedPrincipalsCommand
! 1670: accepts the tokens %%, %F, %f, %K, %k, %h, %i, %s, %T, %t, and %u.
! 1671: .Pp
! 1672: .Cm AuthorizedPrincipalsFile
! 1673: accepts the tokens %%, %h, and %u.
! 1674: .Pp
! 1675: .Cm ChrootDirectory
! 1676: accepts the tokens %%, %h, and %u.
1.1 stevesk 1677: .Sh FILES
1678: .Bl -tag -width Ds
1679: .It Pa /etc/ssh/sshd_config
1680: Contains configuration data for
1.53 jmc 1681: .Xr sshd 8 .
1.1 stevesk 1682: This file should be writable by root only, but it is recommended
1683: (though not necessary) that it be world-readable.
1684: .El
1.19 jmc 1685: .Sh SEE ALSO
1686: .Xr sshd 8
1.1 stevesk 1687: .Sh AUTHORS
1688: OpenSSH is a derivative of the original and free
1689: ssh 1.2.12 release by Tatu Ylonen.
1690: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1691: Theo de Raadt and Dug Song
1692: removed many bugs, re-added newer features and
1693: created OpenSSH.
1694: Markus Friedl contributed the support for SSH
1695: protocol versions 1.5 and 2.0.
1696: Niels Provos and Markus Friedl contributed support
1697: for privilege separation.
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