Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.205
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: .\"
1.205 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.204 2015/06/05 03:44:14 djm Exp $
! 37: .Dd $Mdocdate: June 5 2015 $
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.
1.197 dtucker 73: Note that environment passing is only supported for protocol 2, and
74: that the
75: .Ev TERM
1.199 jmc 76: environment variable is always sent whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
94: .Dq any ,
95: .Dq inet
1.52 jmc 96: (use IPv4 only), or
1.37 djm 97: .Dq inet6
98: (use IPv6 only).
99: The default is
100: .Dq any .
1.89 jmc 101: .It Cm AllowAgentForwarding
102: Specifies whether
103: .Xr ssh-agent 1
104: forwarding is permitted.
105: The default is
106: .Dq yes .
107: Note that disabling agent forwarding does not improve security
108: unless users are also denied shell access, as they can always install
109: their own forwarders.
1.1 stevesk 110: .It Cm AllowGroups
111: This keyword can be followed by a list of group name patterns, separated
112: by spaces.
113: If specified, login is allowed only for users whose primary
114: group or supplementary group list matches one of the patterns.
115: Only group names are valid; a numerical group ID is not recognized.
116: By default, login is allowed for all groups.
1.54 jmc 117: The allow/deny directives are processed in the following order:
118: .Cm DenyUsers ,
119: .Cm AllowUsers ,
120: .Cm DenyGroups ,
121: and finally
122: .Cm AllowGroups .
1.49 jmc 123: .Pp
1.161 jmc 124: See PATTERNS in
1.49 jmc 125: .Xr ssh_config 5
126: for more information on patterns.
1.1 stevesk 127: .It Cm AllowTcpForwarding
128: Specifies whether TCP forwarding is permitted.
1.150 djm 129: The available options are
130: .Dq yes
131: or
132: .Dq all
133: to allow TCP forwarding,
134: .Dq no
135: to prevent all TCP forwarding,
136: .Dq local
137: to allow local (from the perspective of
1.151 jmc 138: .Xr ssh 1 )
139: forwarding only or
1.150 djm 140: .Dq remote
141: to allow remote forwarding only.
1.1 stevesk 142: The default is
143: .Dq yes .
144: Note that disabling TCP forwarding does not improve security unless
145: users are also denied shell access, as they can always install their
146: own forwarders.
1.175 millert 147: .It Cm AllowStreamLocalForwarding
148: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
149: The available options are
150: .Dq yes
151: or
152: .Dq all
153: to allow StreamLocal forwarding,
154: .Dq no
155: to prevent all StreamLocal forwarding,
156: .Dq local
157: to allow local (from the perspective of
158: .Xr ssh 1 )
159: forwarding only or
160: .Dq remote
161: to allow remote forwarding only.
162: The default is
163: .Dq yes .
164: Note that disabling StreamLocal forwarding does not improve security unless
165: users are also denied shell access, as they can always install their
166: own forwarders.
1.1 stevesk 167: .It Cm AllowUsers
168: This keyword can be followed by a list of user name patterns, separated
169: by spaces.
1.14 jmc 170: If specified, login is allowed only for user names that
1.1 stevesk 171: match one of the patterns.
172: Only user names are valid; a numerical user ID is not recognized.
173: By default, login is allowed for all users.
174: If the pattern takes the form USER@HOST then USER and HOST
175: are separately checked, restricting logins to particular
176: users from particular hosts.
1.54 jmc 177: The allow/deny directives are processed in the following order:
178: .Cm DenyUsers ,
179: .Cm AllowUsers ,
180: .Cm DenyGroups ,
181: and finally
182: .Cm AllowGroups .
1.49 jmc 183: .Pp
1.161 jmc 184: See PATTERNS in
1.49 jmc 185: .Xr ssh_config 5
186: for more information on patterns.
1.149 djm 187: .It Cm AuthenticationMethods
188: Specifies the authentication methods that must be successfully completed
189: for a user to be granted access.
190: This option must be followed by one or more comma-separated lists of
191: authentication method names.
192: Successful authentication requires completion of every method in at least
193: one of these lists.
194: .Pp
195: For example, an argument of
196: .Dq publickey,password publickey,keyboard-interactive
197: would require the user to complete public key authentication, followed by
198: either password or keyboard interactive authentication.
199: Only methods that are next in one or more lists are offered at each stage,
200: so for this example, it would not be possible to attempt password or
201: keyboard-interactive authentication before public key.
1.157 markus 202: .Pp
203: For keyboard interactive authentication it is also possible to
204: restrict authentication to a specific device by appending a
205: colon followed by the device identifier
206: .Dq bsdauth ,
207: .Dq pam ,
208: or
209: .Dq skey ,
210: depending on the server configuration.
211: For example,
212: .Dq keyboard-interactive:bsdauth
213: would restrict keyboard interactive authentication to the
214: .Dq bsdauth
215: device.
1.185 djm 216: .Pp
217: If the
218: .Dq publickey
1.187 jmc 219: method is listed more than once,
1.185 djm 220: .Xr sshd 8
221: verifies that keys that have been used successfully are not reused for
222: subsequent authentications.
223: For example, an
224: .Cm AuthenticationMethods
225: of
226: .Dq publickey,publickey
227: will require successful authentication using two different public keys.
1.181 djm 228: .Pp
1.149 djm 229: This option is only available for SSH protocol 2 and will yield a fatal
230: error if enabled if protocol 1 is also enabled.
231: Note that each authentication method listed should also be explicitly enabled
232: in the configuration.
233: The default is not to require multiple authentication; successful completion
234: of a single authentication method is sufficient.
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: .Pp
240: Arguments to
241: .Cm AuthorizedKeysCommand
242: may be provided using the following tokens, which will be expanded
243: at runtime: %% is replaced by a literal '%', %u is replaced by the
244: username being authenticated, %h is replaced by the home directory
245: of the user being authenticated, %t is replaced with the key type
246: offered for authentication, %f is replaced with the fingerprint of
247: the key, and %k is replaced with the key being offered for authentication.
248: If no arguments are specified then the username of the target user
249: will be supplied.
250: .Pp
251: The program should produce on standard output zero or
1.161 jmc 252: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 253: .Xr sshd 8 ) .
1.146 djm 254: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
255: and authorize the user then public key authentication continues using the usual
256: .Cm AuthorizedKeysFile
257: files.
258: By default, no AuthorizedKeysCommand is run.
259: .It Cm AuthorizedKeysCommandUser
260: Specifies the user under whose account the AuthorizedKeysCommand is run.
261: It is recommended to use a dedicated user that has no other role on the host
262: than running authorized keys commands.
1.186 djm 263: If
1.181 djm 264: .Cm AuthorizedKeysCommand
1.186 djm 265: is specified but
266: .Cm AuthorizedKeysCommandUser
267: is not, then
268: .Xr sshd 8
269: will refuse to start.
1.1 stevesk 270: .It Cm AuthorizedKeysFile
271: Specifies the file that contains the public keys that can be used
272: for user authentication.
1.124 djm 273: The format is described in the
1.161 jmc 274: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 275: section of
276: .Xr sshd 8 .
1.1 stevesk 277: .Cm AuthorizedKeysFile
278: may contain tokens of the form %T which are substituted during connection
1.52 jmc 279: setup.
1.17 jmc 280: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 281: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 282: %u is replaced by the username of that user.
283: After expansion,
284: .Cm AuthorizedKeysFile
285: is taken to be an absolute path or one relative to the user's home
286: directory.
1.133 jmc 287: Multiple files may be listed, separated by whitespace.
288: The default is
289: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 290: .It Cm AuthorizedPrincipalsCommand
291: Specifies a program to be used to generate the list of allowed
292: certificate principals as per
293: .Cm AuthorizedPrincipalsFile .
294: The program must be owned by root, not writable by group or others and
295: specified by an absolute path.
296: .Pp
297: Arguments to
298: .Cm AuthorizedPrincipalsCommand
299: may be provided using the following tokens, which will be expanded
300: at runtime: %% is replaced by a literal '%', %u is replaced by the
301: username being authenticated and %h is replaced by the home directory
302: of the user being authenticated.
303: .Pp
304: The program should produce on standard output zero or
305: more lines of
306: .Cm AuthorizedPrincipalsFile
307: output.
308: If either
309: .Cm AuthorizedPrincipalsCommand
310: or
311: .Cm AuthorizedPrincipalsFile
312: is specified, then certificates offered by the client for authentication
313: must contain a principal that is listed.
314: By default, no AuthorizedPrincipalsCommand is run.
315: .It Cm AuthorizedPrincipalsCommandUser
316: Specifies the user under whose account the AuthorizedPrincipalsCommand is run.
317: It is recommended to use a dedicated user that has no other role on the host
318: than running authorized principals commands.
319: If
320: .Cm AuthorizedPrincipalsCommand
321: is specified but
322: .Cm AuthorizedPrincipalsCommandUser
323: is not, then
324: .Xr sshd 8
325: will refuse to start.
1.121 djm 326: .It Cm AuthorizedPrincipalsFile
327: Specifies a file that lists principal names that are accepted for
328: certificate authentication.
329: When using certificates signed by a key listed in
330: .Cm TrustedUserCAKeys ,
331: this file lists names, one of which must appear in the certificate for it
332: to be accepted for authentication.
1.125 jmc 333: Names are listed one per line preceded by key options (as described
1.161 jmc 334: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 335: .Xr sshd 8 ) .
1.124 djm 336: Empty lines and comments starting with
1.121 djm 337: .Ql #
338: are ignored.
339: .Pp
340: .Cm AuthorizedPrincipalsFile
341: may contain tokens of the form %T which are substituted during connection
342: setup.
343: The following tokens are defined: %% is replaced by a literal '%',
344: %h is replaced by the home directory of the user being authenticated, and
345: %u is replaced by the username of that user.
346: After expansion,
347: .Cm AuthorizedPrincipalsFile
348: is taken to be an absolute path or one relative to the user's home
349: directory.
350: .Pp
1.138 djm 351: The default is
352: .Dq none ,
353: i.e. not to use a principals file \(en in this case, the username
1.121 djm 354: of the user must appear in a certificate's principals list for it to be
355: accepted.
356: Note that
357: .Cm AuthorizedPrincipalsFile
358: is only used when authentication proceeds using a CA listed in
359: .Cm TrustedUserCAKeys
360: and is not consulted for certification authorities trusted via
361: .Pa ~/.ssh/authorized_keys ,
362: though the
363: .Cm principals=
364: key option offers a similar facility (see
365: .Xr sshd 8
366: for details).
1.1 stevesk 367: .It Cm Banner
368: The contents of the specified file are sent to the remote user before
369: authentication is allowed.
1.78 djm 370: If the argument is
371: .Dq none
372: then no banner is displayed.
1.1 stevesk 373: This option is only available for protocol version 2.
374: By default, no banner is displayed.
375: .It Cm ChallengeResponseAuthentication
1.50 jmc 376: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 377: All authentication styles from
378: .Xr login.conf 5
379: are supported.
380: The default is
381: .Dq yes .
1.80 djm 382: .It Cm ChrootDirectory
1.113 stevesk 383: Specifies the pathname of a directory to
1.80 djm 384: .Xr chroot 2
385: to after authentication.
1.190 deraadt 386: At session startup
387: .Xr sshd 8
388: checks that all components of the pathname are root-owned directories
389: which are not writable by any other user or group.
1.106 stevesk 390: After the chroot,
391: .Xr sshd 8
392: changes the working directory to the user's home directory.
1.80 djm 393: .Pp
1.113 stevesk 394: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 395: the connecting user has been authenticated: %% is replaced by a literal '%',
396: %h is replaced by the home directory of the user being authenticated, and
397: %u is replaced by the username of that user.
398: .Pp
399: The
400: .Cm ChrootDirectory
401: must contain the necessary files and directories to support the
1.103 stevesk 402: user's session.
1.80 djm 403: For an interactive session this requires at least a shell, typically
404: .Xr sh 1 ,
405: and basic
406: .Pa /dev
407: nodes such as
408: .Xr null 4 ,
409: .Xr zero 4 ,
410: .Xr stdin 4 ,
411: .Xr stdout 4 ,
412: .Xr stderr 4 ,
1.180 jmc 413: and
1.80 djm 414: .Xr tty 4
415: devices.
416: For file transfer sessions using
1.105 jmc 417: .Dq sftp ,
1.80 djm 418: no additional configuration of the environment is necessary if the
1.105 jmc 419: in-process sftp server is used,
1.176 schwarze 420: though sessions which use logging may require
1.104 stevesk 421: .Pa /dev/log
1.176 schwarze 422: inside the chroot directory on some operating systems (see
1.104 stevesk 423: .Xr sftp-server 8
1.81 jmc 424: for details).
1.190 deraadt 425: .Pp
1.191 jmc 426: For safety, it is very important that the directory hierarchy be
1.190 deraadt 427: prevented from modification by other processes on the system (especially
428: those outside the jail).
429: Misconfiguration can lead to unsafe environments which
430: .Xr sshd 8
431: cannot detect.
1.80 djm 432: .Pp
433: The default is not to
434: .Xr chroot 2 .
1.1 stevesk 435: .It Cm Ciphers
436: Specifies the ciphers allowed for protocol version 2.
437: Multiple ciphers must be comma-separated.
1.167 djm 438: The supported ciphers are:
439: .Pp
1.173 naddy 440: .Bl -item -compact -offset indent
441: .It
442: 3des-cbc
443: .It
444: aes128-cbc
445: .It
446: aes192-cbc
447: .It
448: aes256-cbc
449: .It
450: aes128-ctr
451: .It
452: aes192-ctr
453: .It
454: aes256-ctr
455: .It
456: aes128-gcm@openssh.com
457: .It
458: aes256-gcm@openssh.com
459: .It
460: arcfour
461: .It
462: arcfour128
463: .It
464: arcfour256
465: .It
466: blowfish-cbc
467: .It
468: cast128-cbc
469: .It
470: chacha20-poly1305@openssh.com
471: .El
1.167 djm 472: .Pp
1.52 jmc 473: The default is:
1.173 naddy 474: .Bd -literal -offset indent
475: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 476: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 477: chacha20-poly1305@openssh.com
1.1 stevesk 478: .Ed
1.167 djm 479: .Pp
480: The list of available ciphers may also be obtained using the
481: .Fl Q
482: option of
1.188 djm 483: .Xr ssh 1
484: with an argument of
485: .Dq cipher .
1.1 stevesk 486: .It Cm ClientAliveCountMax
1.48 jmc 487: Sets the number of client alive messages (see below) which may be
1.1 stevesk 488: sent without
1.52 jmc 489: .Xr sshd 8
1.17 jmc 490: receiving any messages back from the client.
491: If this threshold is reached while client alive messages are being sent,
1.52 jmc 492: sshd will disconnect the client, terminating the session.
1.17 jmc 493: It is important to note that the use of client alive messages is very
494: different from
1.27 markus 495: .Cm TCPKeepAlive
1.17 jmc 496: (below).
497: The client alive messages are sent through the encrypted channel
498: and therefore will not be spoofable.
499: The TCP keepalive option enabled by
1.27 markus 500: .Cm TCPKeepAlive
1.17 jmc 501: is spoofable.
502: The client alive mechanism is valuable when the client or
1.1 stevesk 503: server depend on knowing when a connection has become inactive.
504: .Pp
1.17 jmc 505: The default value is 3.
506: If
1.1 stevesk 507: .Cm ClientAliveInterval
1.48 jmc 508: (see below) is set to 15, and
1.1 stevesk 509: .Cm ClientAliveCountMax
1.52 jmc 510: is left at the default, unresponsive SSH clients
1.1 stevesk 511: will be disconnected after approximately 45 seconds.
1.57 markus 512: This option applies to protocol version 2 only.
1.42 djm 513: .It Cm ClientAliveInterval
514: Sets a timeout interval in seconds after which if no data has been received
515: from the client,
1.52 jmc 516: .Xr sshd 8
1.42 djm 517: will send a message through the encrypted
518: channel to request a response from the client.
519: The default
520: is 0, indicating that these messages will not be sent to the client.
521: This option applies to protocol version 2 only.
1.3 markus 522: .It Cm Compression
1.44 markus 523: Specifies whether compression is allowed, or delayed until
524: the user has authenticated successfully.
1.3 markus 525: The argument must be
1.44 markus 526: .Dq yes ,
527: .Dq delayed ,
1.3 markus 528: or
529: .Dq no .
530: The default is
1.44 markus 531: .Dq delayed .
1.1 stevesk 532: .It Cm DenyGroups
533: This keyword can be followed by a list of group name patterns, separated
534: by spaces.
535: Login is disallowed for users whose primary group or supplementary
536: group list matches one of the patterns.
537: Only group names are valid; a numerical group ID is not recognized.
538: By default, login is allowed for all groups.
1.54 jmc 539: The allow/deny directives are processed in the following order:
540: .Cm DenyUsers ,
541: .Cm AllowUsers ,
542: .Cm DenyGroups ,
543: and finally
544: .Cm AllowGroups .
1.49 jmc 545: .Pp
1.161 jmc 546: See PATTERNS in
1.49 jmc 547: .Xr ssh_config 5
548: for more information on patterns.
1.1 stevesk 549: .It Cm DenyUsers
550: This keyword can be followed by a list of user name patterns, separated
551: by spaces.
552: Login is disallowed for user names that match one of the patterns.
553: Only user names are valid; a numerical user ID is not recognized.
554: By default, login is allowed for all users.
555: If the pattern takes the form USER@HOST then USER and HOST
556: are separately checked, restricting logins to particular
557: users from particular hosts.
1.54 jmc 558: The allow/deny directives are processed in the following order:
559: .Cm DenyUsers ,
560: .Cm AllowUsers ,
561: .Cm DenyGroups ,
562: and finally
563: .Cm AllowGroups .
1.49 jmc 564: .Pp
1.161 jmc 565: See PATTERNS in
1.49 jmc 566: .Xr ssh_config 5
567: for more information on patterns.
1.183 djm 568: .It Cm FingerprintHash
569: Specifies the hash algorithm used when logging key fingerprints.
570: Valid options are:
571: .Dq md5
572: and
573: .Dq sha256 .
574: The default is
575: .Dq sha256 .
1.67 dtucker 576: .It Cm ForceCommand
577: Forces the execution of the command specified by
578: .Cm ForceCommand ,
1.84 djm 579: ignoring any command supplied by the client and
580: .Pa ~/.ssh/rc
581: if present.
1.67 dtucker 582: The command is invoked by using the user's login shell with the -c option.
583: This applies to shell, command, or subsystem execution.
584: It is most useful inside a
585: .Cm Match
586: block.
587: The command originally supplied by the client is available in the
588: .Ev SSH_ORIGINAL_COMMAND
589: environment variable.
1.82 djm 590: Specifying a command of
591: .Dq internal-sftp
592: will force the use of an in-process sftp server that requires no support
593: files when used with
594: .Cm ChrootDirectory .
1.1 stevesk 595: .It Cm GatewayPorts
596: Specifies whether remote hosts are allowed to connect to ports
597: forwarded for the client.
598: By default,
1.52 jmc 599: .Xr sshd 8
1.15 jmc 600: binds remote port forwardings to the loopback address.
601: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 602: .Cm GatewayPorts
1.52 jmc 603: can be used to specify that sshd
1.39 djm 604: should allow remote port forwardings to bind to non-loopback addresses, thus
605: allowing other hosts to connect.
606: The argument may be
607: .Dq no
608: to force remote port forwardings to be available to the local host only,
1.1 stevesk 609: .Dq yes
1.39 djm 610: to force remote port forwardings to bind to the wildcard address, or
611: .Dq clientspecified
612: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 613: The default is
614: .Dq no .
1.23 markus 615: .It Cm GSSAPIAuthentication
1.25 markus 616: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 617: The default is
1.23 markus 618: .Dq no .
619: Note that this option applies to protocol version 2 only.
620: .It Cm GSSAPICleanupCredentials
621: Specifies whether to automatically destroy the user's credentials cache
622: on logout.
623: The default is
624: .Dq yes .
625: Note that this option applies to protocol version 2 only.
1.203 djm 626: .It Cm GSSAPIStrictAcceptorCheck
627: Determines whether to be strict about the identity of the GSSAPI acceptor
628: a client authenticates against.
629: If set to
630: .Dq yes
631: then the client must authenticate against the
632: .Pa host
633: service on the current hostname.
634: If set to
635: .Dq no
636: then the client may authenticate against any service key stored in the
637: machine's default store.
638: This facility is provided to assist with operation on multi homed machines.
639: The default is
640: .Dq yes .
1.189 djm 641: .It Cm HostbasedAcceptedKeyTypes
642: Specifies the key types that will be accepted for hostbased authentication
643: as a comma-separated pattern list.
644: The default
645: .Dq *
646: will allow all key types.
647: The
648: .Fl Q
649: option of
650: .Xr ssh 1
651: may be used to list supported key types.
1.1 stevesk 652: .It Cm HostbasedAuthentication
653: Specifies whether rhosts or /etc/hosts.equiv authentication together
654: with successful public key client host authentication is allowed
1.50 jmc 655: (host-based authentication).
1.1 stevesk 656: This option is similar to
657: .Cm RhostsRSAAuthentication
658: and applies to protocol version 2 only.
1.70 dtucker 659: The default is
660: .Dq no .
661: .It Cm HostbasedUsesNameFromPacketOnly
662: Specifies whether or not the server will attempt to perform a reverse
663: name lookup when matching the name in the
664: .Pa ~/.shosts ,
665: .Pa ~/.rhosts ,
666: and
667: .Pa /etc/hosts.equiv
668: files during
669: .Cm HostbasedAuthentication .
670: A setting of
671: .Dq yes
672: means that
673: .Xr sshd 8
674: uses the name supplied by the client rather than
675: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 676: The default is
677: .Dq no .
1.117 djm 678: .It Cm HostCertificate
679: Specifies a file containing a public host certificate.
680: The certificate's public key must match a private host key already specified
681: by
682: .Cm HostKey .
683: The default behaviour of
684: .Xr sshd 8
685: is not to load any certificates.
1.1 stevesk 686: .It Cm HostKey
687: Specifies a file containing a private host key
688: used by SSH.
689: The default is
690: .Pa /etc/ssh/ssh_host_key
691: for protocol version 1, and
1.126 djm 692: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 693: .Pa /etc/ssh/ssh_host_ecdsa_key ,
694: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 695: and
1.1 stevesk 696: .Pa /etc/ssh/ssh_host_rsa_key
697: for protocol version 2.
698: Note that
1.52 jmc 699: .Xr sshd 8
1.1 stevesk 700: will refuse to use a file if it is group/world-accessible.
701: It is possible to have multiple host key files.
702: .Dq rsa1
703: keys are used for version 1 and
1.126 djm 704: .Dq dsa ,
1.169 naddy 705: .Dq ecdsa ,
706: .Dq ed25519
1.1 stevesk 707: or
708: .Dq rsa
709: are used for version 2 of the SSH protocol.
1.162 markus 710: It is also possible to specify public host key files instead.
711: In this case operations on the private key will be delegated
712: to an
713: .Xr ssh-agent 1 .
714: .It Cm HostKeyAgent
715: Identifies the UNIX-domain socket used to communicate
716: with an agent that has access to the private host keys.
717: If
718: .Dq SSH_AUTH_SOCK
719: is specified, the location of the socket will be read from the
720: .Ev SSH_AUTH_SOCK
721: environment variable.
1.1 stevesk 722: .It Cm IgnoreRhosts
723: Specifies that
724: .Pa .rhosts
725: and
726: .Pa .shosts
727: files will not be used in
728: .Cm RhostsRSAAuthentication
729: or
730: .Cm HostbasedAuthentication .
731: .Pp
732: .Pa /etc/hosts.equiv
733: and
734: .Pa /etc/shosts.equiv
735: are still used.
736: The default is
737: .Dq yes .
738: .It Cm IgnoreUserKnownHosts
739: Specifies whether
1.52 jmc 740: .Xr sshd 8
1.1 stevesk 741: should ignore the user's
1.41 djm 742: .Pa ~/.ssh/known_hosts
1.1 stevesk 743: during
744: .Cm RhostsRSAAuthentication
745: or
746: .Cm HostbasedAuthentication .
747: The default is
748: .Dq no .
1.129 djm 749: .It Cm IPQoS
750: Specifies the IPv4 type-of-service or DSCP class for the connection.
751: Accepted values are
752: .Dq af11 ,
753: .Dq af12 ,
754: .Dq af13 ,
1.136 djm 755: .Dq af21 ,
1.129 djm 756: .Dq af22 ,
757: .Dq af23 ,
758: .Dq af31 ,
759: .Dq af32 ,
760: .Dq af33 ,
761: .Dq af41 ,
762: .Dq af42 ,
763: .Dq af43 ,
764: .Dq cs0 ,
765: .Dq cs1 ,
766: .Dq cs2 ,
767: .Dq cs3 ,
768: .Dq cs4 ,
769: .Dq cs5 ,
770: .Dq cs6 ,
771: .Dq cs7 ,
772: .Dq ef ,
773: .Dq lowdelay ,
774: .Dq throughput ,
775: .Dq reliability ,
776: or a numeric value.
1.131 djm 777: This option may take one or two arguments, separated by whitespace.
1.129 djm 778: If one argument is specified, it is used as the packet class unconditionally.
779: If two values are specified, the first is automatically selected for
780: interactive sessions and the second for non-interactive sessions.
781: The default is
782: .Dq lowdelay
783: for interactive sessions and
784: .Dq throughput
785: for non-interactive sessions.
1.171 jmc 786: .It Cm KbdInteractiveAuthentication
787: Specifies whether to allow keyboard-interactive authentication.
788: The argument to this keyword must be
789: .Dq yes
790: or
791: .Dq no .
792: The default is to use whatever value
793: .Cm ChallengeResponseAuthentication
794: is set to
795: (by default
796: .Dq yes ) .
1.1 stevesk 797: .It Cm KerberosAuthentication
1.24 markus 798: Specifies whether the password provided by the user for
1.1 stevesk 799: .Cm PasswordAuthentication
1.24 markus 800: will be validated through the Kerberos KDC.
1.1 stevesk 801: To use this option, the server needs a
802: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 803: The default is
1.29 dtucker 804: .Dq no .
805: .It Cm KerberosGetAFSToken
1.45 djm 806: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 807: an AFS token before accessing the user's home directory.
1.52 jmc 808: The default is
1.1 stevesk 809: .Dq no .
810: .It Cm KerberosOrLocalPasswd
1.52 jmc 811: If password authentication through Kerberos fails then
1.1 stevesk 812: the password will be validated via any additional local mechanism
813: such as
814: .Pa /etc/passwd .
1.52 jmc 815: The default is
1.1 stevesk 816: .Dq yes .
817: .It Cm KerberosTicketCleanup
818: Specifies whether to automatically destroy the user's ticket cache
819: file on logout.
1.52 jmc 820: The default is
1.1 stevesk 821: .Dq yes .
1.127 djm 822: .It Cm KexAlgorithms
823: Specifies the available KEX (Key Exchange) algorithms.
824: Multiple algorithms must be comma-separated.
1.173 naddy 825: The supported algorithms are:
826: .Pp
827: .Bl -item -compact -offset indent
828: .It
829: curve25519-sha256@libssh.org
830: .It
831: diffie-hellman-group1-sha1
832: .It
833: diffie-hellman-group14-sha1
834: .It
835: diffie-hellman-group-exchange-sha1
836: .It
837: diffie-hellman-group-exchange-sha256
838: .It
839: ecdh-sha2-nistp256
840: .It
841: ecdh-sha2-nistp384
842: .It
843: ecdh-sha2-nistp521
844: .El
845: .Pp
846: The default is:
1.170 dtucker 847: .Bd -literal -offset indent
848: curve25519-sha256@libssh.org,
849: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
850: diffie-hellman-group-exchange-sha256,
1.173 naddy 851: diffie-hellman-group14-sha1
1.170 dtucker 852: .Ed
1.188 djm 853: .Pp
854: The list of available key exchange algorithms may also be obtained using the
855: .Fl Q
856: option of
857: .Xr ssh 1
858: with an argument of
859: .Dq kex .
1.1 stevesk 860: .It Cm KeyRegenerationInterval
861: In protocol version 1, the ephemeral server key is automatically regenerated
862: after this many seconds (if it has been used).
863: The purpose of regeneration is to prevent
864: decrypting captured sessions by later breaking into the machine and
865: stealing the keys.
866: The key is never stored anywhere.
867: If the value is 0, the key is never regenerated.
868: The default is 3600 (seconds).
869: .It Cm ListenAddress
870: Specifies the local addresses
1.52 jmc 871: .Xr sshd 8
1.1 stevesk 872: should listen on.
873: The following forms may be used:
874: .Pp
875: .Bl -item -offset indent -compact
876: .It
877: .Cm ListenAddress
878: .Sm off
1.180 jmc 879: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 880: .Sm on
881: .It
882: .Cm ListenAddress
883: .Sm off
1.180 jmc 884: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 885: .Sm on
886: .It
887: .Cm ListenAddress
888: .Sm off
889: .Oo
1.180 jmc 890: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 891: .Sm on
892: .El
893: .Pp
894: If
895: .Ar port
896: is not specified,
1.200 dtucker 897: sshd will listen on the address and all
1.1 stevesk 898: .Cm Port
1.17 jmc 899: options specified.
900: The default is to listen on all local addresses.
1.15 jmc 901: Multiple
1.1 stevesk 902: .Cm ListenAddress
1.17 jmc 903: options are permitted.
1.1 stevesk 904: .It Cm LoginGraceTime
905: The server disconnects after this time if the user has not
906: successfully logged in.
907: If the value is 0, there is no time limit.
1.12 stevesk 908: The default is 120 seconds.
1.1 stevesk 909: .It Cm LogLevel
910: Gives the verbosity level that is used when logging messages from
1.53 jmc 911: .Xr sshd 8 .
1.1 stevesk 912: The possible values are:
1.52 jmc 913: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 914: The default is INFO.
915: DEBUG and DEBUG1 are equivalent.
916: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
917: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 918: .It Cm MACs
919: Specifies the available MAC (message authentication code) algorithms.
920: The MAC algorithm is used in protocol version 2
921: for data integrity protection.
922: Multiple algorithms must be comma-separated.
1.152 markus 923: The algorithms that contain
924: .Dq -etm
925: calculate the MAC after encryption (encrypt-then-mac).
926: These are considered safer and their use recommended.
1.173 naddy 927: The supported MACs are:
928: .Pp
929: .Bl -item -compact -offset indent
930: .It
931: hmac-md5
932: .It
933: hmac-md5-96
934: .It
935: hmac-ripemd160
936: .It
937: hmac-sha1
938: .It
939: hmac-sha1-96
940: .It
941: hmac-sha2-256
942: .It
943: hmac-sha2-512
944: .It
945: umac-64@openssh.com
946: .It
947: umac-128@openssh.com
948: .It
949: hmac-md5-etm@openssh.com
950: .It
951: hmac-md5-96-etm@openssh.com
952: .It
953: hmac-ripemd160-etm@openssh.com
954: .It
955: hmac-sha1-etm@openssh.com
956: .It
957: hmac-sha1-96-etm@openssh.com
958: .It
959: hmac-sha2-256-etm@openssh.com
960: .It
961: hmac-sha2-512-etm@openssh.com
962: .It
963: umac-64-etm@openssh.com
964: .It
965: umac-128-etm@openssh.com
966: .El
967: .Pp
1.52 jmc 968: The default is:
1.77 jmc 969: .Bd -literal -offset indent
1.152 markus 970: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
971: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 972: umac-64@openssh.com,umac-128@openssh.com,
973: hmac-sha2-256,hmac-sha2-512
1.77 jmc 974: .Ed
1.188 djm 975: .Pp
976: The list of available MAC algorithms may also be obtained using the
977: .Fl Q
978: option of
979: .Xr ssh 1
980: with an argument of
981: .Dq mac .
1.60 dtucker 982: .It Cm Match
1.61 jmc 983: Introduces a conditional block.
1.65 dtucker 984: If all of the criteria on the
1.60 dtucker 985: .Cm Match
1.65 dtucker 986: line are satisfied, the keywords on the following lines override those
987: set in the global section of the config file, until either another
1.60 dtucker 988: .Cm Match
1.65 dtucker 989: line or the end of the file.
1.172 djm 990: If a keyword appears in multiple
991: .Cm Match
1.177 sobrado 992: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 993: applied.
1.91 djm 994: .Pp
1.61 jmc 995: The arguments to
1.60 dtucker 996: .Cm Match
1.163 dtucker 997: are one or more criteria-pattern pairs or the single token
998: .Cm All
999: which matches all criteria.
1.60 dtucker 1000: The available criteria are
1001: .Cm User ,
1.69 dtucker 1002: .Cm Group ,
1.60 dtucker 1003: .Cm Host ,
1.139 dtucker 1004: .Cm LocalAddress ,
1005: .Cm LocalPort ,
1.60 dtucker 1006: and
1007: .Cm Address .
1.91 djm 1008: The match patterns may consist of single entries or comma-separated
1009: lists and may use the wildcard and negation operators described in the
1.161 jmc 1010: PATTERNS section of
1.92 djm 1011: .Xr ssh_config 5 .
1.91 djm 1012: .Pp
1013: The patterns in an
1014: .Cm Address
1015: criteria may additionally contain addresses to match in CIDR
1.93 jmc 1016: address/masklen format, e.g.\&
1.91 djm 1017: .Dq 192.0.2.0/24
1018: or
1019: .Dq 3ffe:ffff::/32 .
1020: Note that the mask length provided must be consistent with the address -
1021: it is an error to specify a mask length that is too long for the address
1.93 jmc 1022: or one with bits set in this host portion of the address.
1023: For example,
1.91 djm 1024: .Dq 192.0.2.0/33
1025: and
1.93 jmc 1026: .Dq 192.0.2.0/8
1.91 djm 1027: respectively.
1028: .Pp
1.60 dtucker 1029: Only a subset of keywords may be used on the lines following a
1030: .Cm Match
1031: keyword.
1032: Available keywords are
1.142 jmc 1033: .Cm AcceptEnv ,
1.99 okan 1034: .Cm AllowAgentForwarding ,
1.142 jmc 1035: .Cm AllowGroups ,
1.193 djm 1036: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1037: .Cm AllowTcpForwarding ,
1.141 markus 1038: .Cm AllowUsers ,
1.149 djm 1039: .Cm AuthenticationMethods ,
1.146 djm 1040: .Cm AuthorizedKeysCommand ,
1041: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1042: .Cm AuthorizedKeysFile ,
1.123 djm 1043: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1044: .Cm Banner ,
1.85 djm 1045: .Cm ChrootDirectory ,
1.141 markus 1046: .Cm DenyGroups ,
1047: .Cm DenyUsers ,
1.67 dtucker 1048: .Cm ForceCommand ,
1.194 djm 1049: .Cm GatewayPorts ,
1.193 djm 1050: .Cm GSSAPIAuthentication ,
1.189 djm 1051: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1052: .Cm HostbasedAuthentication ,
1.123 djm 1053: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1054: .Cm IPQoS ,
1.74 jmc 1055: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1056: .Cm KerberosAuthentication ,
1.95 dtucker 1057: .Cm MaxAuthTries ,
1.94 dtucker 1058: .Cm MaxSessions ,
1.72 dtucker 1059: .Cm PasswordAuthentication ,
1.97 djm 1060: .Cm PermitEmptyPasswords ,
1.66 dtucker 1061: .Cm PermitOpen ,
1.79 dtucker 1062: .Cm PermitRootLogin ,
1.164 djm 1063: .Cm PermitTTY ,
1.123 djm 1064: .Cm PermitTunnel ,
1.174 djm 1065: .Cm PermitUserRC ,
1.189 djm 1066: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1067: .Cm PubkeyAuthentication ,
1.159 dtucker 1068: .Cm RekeyLimit ,
1.193 djm 1069: .Cm RevokedKeys ,
1.142 jmc 1070: .Cm RhostsRSAAuthentication ,
1.194 djm 1071: .Cm RSAAuthentication ,
1.193 djm 1072: .Cm StreamLocalBindMask ,
1073: .Cm StreamLocalBindUnlink ,
1074: .Cm TrustedUserCAKeys ,
1.66 dtucker 1075: .Cm X11DisplayOffset ,
1.101 djm 1076: .Cm X11Forwarding
1.60 dtucker 1077: and
1.102 djm 1078: .Cm X11UseLocalHost .
1.33 dtucker 1079: .It Cm MaxAuthTries
1080: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1081: connection.
1082: Once the number of failures reaches half this value,
1083: additional failures are logged.
1084: The default is 6.
1.90 djm 1085: .It Cm MaxSessions
1086: Specifies the maximum number of open sessions permitted per network connection.
1087: The default is 10.
1.1 stevesk 1088: .It Cm MaxStartups
1089: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1090: SSH daemon.
1.1 stevesk 1091: Additional connections will be dropped until authentication succeeds or the
1092: .Cm LoginGraceTime
1093: expires for a connection.
1.156 dtucker 1094: The default is 10:30:100.
1.1 stevesk 1095: .Pp
1096: Alternatively, random early drop can be enabled by specifying
1097: the three colon separated values
1098: .Dq start:rate:full
1.51 jmc 1099: (e.g. "10:30:60").
1.53 jmc 1100: .Xr sshd 8
1.1 stevesk 1101: will refuse connection attempts with a probability of
1102: .Dq rate/100
1103: (30%)
1104: if there are currently
1105: .Dq start
1106: (10)
1107: unauthenticated connections.
1108: The probability increases linearly and all connection attempts
1109: are refused if the number of unauthenticated connections reaches
1110: .Dq full
1111: (60).
1112: .It Cm PasswordAuthentication
1113: Specifies whether password authentication is allowed.
1114: The default is
1115: .Dq yes .
1116: .It Cm PermitEmptyPasswords
1117: When password authentication is allowed, it specifies whether the
1118: server allows login to accounts with empty password strings.
1119: The default is
1120: .Dq no .
1.62 dtucker 1121: .It Cm PermitOpen
1122: Specifies the destinations to which TCP port forwarding is permitted.
1123: The forwarding specification must be one of the following forms:
1124: .Pp
1125: .Bl -item -offset indent -compact
1126: .It
1127: .Cm PermitOpen
1128: .Sm off
1129: .Ar host : port
1130: .Sm on
1131: .It
1132: .Cm PermitOpen
1133: .Sm off
1134: .Ar IPv4_addr : port
1135: .Sm on
1136: .It
1137: .Cm PermitOpen
1138: .Sm off
1139: .Ar \&[ IPv6_addr \&] : port
1140: .Sm on
1141: .El
1142: .Pp
1.68 dtucker 1143: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1144: An argument of
1145: .Dq any
1146: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1147: An argument of
1148: .Dq none
1149: can be used to prohibit all forwarding requests.
1.63 jmc 1150: By default all port forwarding requests are permitted.
1.1 stevesk 1151: .It Cm PermitRootLogin
1.38 jmc 1152: Specifies whether root can log in using
1.1 stevesk 1153: .Xr ssh 1 .
1154: The argument must be
1155: .Dq yes ,
1156: .Dq without-password ,
1.52 jmc 1157: .Dq forced-commands-only ,
1.1 stevesk 1158: or
1159: .Dq no .
1160: The default is
1.196 djm 1161: .Dq no .
1.1 stevesk 1162: .Pp
1163: If this option is set to
1.52 jmc 1164: .Dq without-password ,
1.1 stevesk 1165: password authentication is disabled for root.
1166: .Pp
1167: If this option is set to
1.52 jmc 1168: .Dq forced-commands-only ,
1.1 stevesk 1169: root login with public key authentication will be allowed,
1170: but only if the
1171: .Ar command
1172: option has been specified
1173: (which may be useful for taking remote backups even if root login is
1.17 jmc 1174: normally not allowed).
1175: All other authentication methods are disabled for root.
1.1 stevesk 1176: .Pp
1177: If this option is set to
1.52 jmc 1178: .Dq no ,
1.38 jmc 1179: root is not allowed to log in.
1.46 reyk 1180: .It Cm PermitTunnel
1181: Specifies whether
1182: .Xr tun 4
1183: device forwarding is allowed.
1.47 reyk 1184: The argument must be
1185: .Dq yes ,
1.58 stevesk 1186: .Dq point-to-point
1187: (layer 3),
1188: .Dq ethernet
1189: (layer 2), or
1.47 reyk 1190: .Dq no .
1.58 stevesk 1191: Specifying
1192: .Dq yes
1193: permits both
1194: .Dq point-to-point
1195: and
1196: .Dq ethernet .
1.46 reyk 1197: The default is
1198: .Dq no .
1.178 djm 1199: .Pp
1200: Independent of this setting, the permissions of the selected
1201: .Xr tun 4
1202: device must allow access to the user.
1.164 djm 1203: .It Cm PermitTTY
1204: Specifies whether
1.165 jmc 1205: .Xr pty 4
1.164 djm 1206: allocation is permitted.
1207: The default is
1208: .Dq yes .
1.6 markus 1209: .It Cm PermitUserEnvironment
1210: Specifies whether
1211: .Pa ~/.ssh/environment
1.9 stevesk 1212: and
1.6 markus 1213: .Cm environment=
1214: options in
1215: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1216: are processed by
1.52 jmc 1217: .Xr sshd 8 .
1.6 markus 1218: The default is
1219: .Dq no .
1.9 stevesk 1220: Enabling environment processing may enable users to bypass access
1221: restrictions in some configurations using mechanisms such as
1222: .Ev LD_PRELOAD .
1.174 djm 1223: .It Cm PermitUserRC
1224: Specifies whether any
1225: .Pa ~/.ssh/rc
1226: file is executed.
1227: The default is
1228: .Dq yes .
1.1 stevesk 1229: .It Cm PidFile
1.4 stevesk 1230: Specifies the file that contains the process ID of the
1.195 dtucker 1231: SSH daemon, or
1232: .Dq none
1233: to not write one.
1.1 stevesk 1234: The default is
1235: .Pa /var/run/sshd.pid .
1236: .It Cm Port
1237: Specifies the port number that
1.52 jmc 1238: .Xr sshd 8
1.1 stevesk 1239: listens on.
1240: The default is 22.
1241: Multiple options of this type are permitted.
1242: See also
1243: .Cm ListenAddress .
1244: .It Cm PrintLastLog
1245: Specifies whether
1.52 jmc 1246: .Xr sshd 8
1.36 jaredy 1247: should print the date and time of the last user login when a user logs
1248: in interactively.
1.1 stevesk 1249: The default is
1250: .Dq yes .
1251: .It Cm PrintMotd
1252: Specifies whether
1.52 jmc 1253: .Xr sshd 8
1.1 stevesk 1254: should print
1255: .Pa /etc/motd
1256: when a user logs in interactively.
1257: (On some systems it is also printed by the shell,
1258: .Pa /etc/profile ,
1259: or equivalent.)
1260: The default is
1261: .Dq yes .
1262: .It Cm Protocol
1263: Specifies the protocol versions
1.52 jmc 1264: .Xr sshd 8
1.5 stevesk 1265: supports.
1.1 stevesk 1266: The possible values are
1.52 jmc 1267: .Sq 1
1.1 stevesk 1268: and
1.52 jmc 1269: .Sq 2 .
1.1 stevesk 1270: Multiple versions must be comma-separated.
1271: The default is
1.109 jmc 1272: .Sq 2 .
1.5 stevesk 1273: Note that the order of the protocol list does not indicate preference,
1274: because the client selects among multiple protocol versions offered
1275: by the server.
1276: Specifying
1277: .Dq 2,1
1278: is identical to
1279: .Dq 1,2 .
1.189 djm 1280: .It Cm PubkeyAcceptedKeyTypes
1281: Specifies the key types that will be accepted for public key authentication
1282: as a comma-separated pattern list.
1283: The default
1284: .Dq *
1285: will allow all key types.
1286: The
1287: .Fl Q
1288: option of
1289: .Xr ssh 1
1290: may be used to list supported key types.
1.1 stevesk 1291: .It Cm PubkeyAuthentication
1292: Specifies whether public key authentication is allowed.
1293: The default is
1294: .Dq yes .
1295: Note that this option applies to protocol version 2 only.
1.159 dtucker 1296: .It Cm RekeyLimit
1297: Specifies the maximum amount of data that may be transmitted before the
1298: session key is renegotiated, optionally followed a maximum amount of
1299: time that may pass before the session key is renegotiated.
1300: The first argument is specified in bytes and may have a suffix of
1301: .Sq K ,
1302: .Sq M ,
1303: or
1304: .Sq G
1305: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1306: The default is between
1307: .Sq 1G
1308: and
1309: .Sq 4G ,
1310: depending on the cipher.
1311: The optional second value is specified in seconds and may use any of the
1312: units documented in the
1313: .Sx TIME FORMATS
1.160 jmc 1314: section.
1.159 dtucker 1315: The default value for
1316: .Cm RekeyLimit
1317: is
1318: .Dq default none ,
1319: which means that rekeying is performed after the cipher's default amount
1320: of data has been sent or received and no time based rekeying is done.
1321: This option applies to protocol version 2 only.
1.118 djm 1322: .It Cm RevokedKeys
1.195 dtucker 1323: Specifies revoked public keys file, or
1324: .Dq none
1325: to not use one.
1.118 djm 1326: Keys listed in this file will be refused for public key authentication.
1327: Note that if this file is not readable, then public key authentication will
1328: be refused for all users.
1.154 djm 1329: Keys may be specified as a text file, listing one public key per line, or as
1330: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1331: .Xr ssh-keygen 1 .
1.161 jmc 1332: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1333: .Xr ssh-keygen 1 .
1.1 stevesk 1334: .It Cm RhostsRSAAuthentication
1335: Specifies whether rhosts or /etc/hosts.equiv authentication together
1336: with successful RSA host authentication is allowed.
1337: The default is
1338: .Dq no .
1339: This option applies to protocol version 1 only.
1340: .It Cm RSAAuthentication
1341: Specifies whether pure RSA authentication is allowed.
1342: The default is
1343: .Dq yes .
1344: This option applies to protocol version 1 only.
1345: .It Cm ServerKeyBits
1346: Defines the number of bits in the ephemeral protocol version 1 server key.
1.205 ! djm 1347: The default and minimum value is 1024.
1.175 millert 1348: .It Cm StreamLocalBindMask
1349: Sets the octal file creation mode mask
1350: .Pq umask
1351: used when creating a Unix-domain socket file for local or remote
1352: port forwarding.
1353: This option is only used for port forwarding to a Unix-domain socket file.
1354: .Pp
1355: The default value is 0177, which creates a Unix-domain socket file that is
1356: readable and writable only by the owner.
1357: Note that not all operating systems honor the file mode on Unix-domain
1358: socket files.
1359: .It Cm StreamLocalBindUnlink
1360: Specifies whether to remove an existing Unix-domain socket file for local
1361: or remote port forwarding before creating a new one.
1362: If the socket file already exists and
1363: .Cm StreamLocalBindUnlink
1364: is not enabled,
1365: .Nm sshd
1366: will be unable to forward the port to the Unix-domain socket file.
1367: This option is only used for port forwarding to a Unix-domain socket file.
1368: .Pp
1369: The argument must be
1370: .Dq yes
1371: or
1372: .Dq no .
1373: The default is
1374: .Dq no .
1.1 stevesk 1375: .It Cm StrictModes
1376: Specifies whether
1.52 jmc 1377: .Xr sshd 8
1.1 stevesk 1378: should check file modes and ownership of the
1379: user's files and home directory before accepting login.
1380: This is normally desirable because novices sometimes accidentally leave their
1381: directory or files world-writable.
1382: The default is
1383: .Dq yes .
1.112 djm 1384: Note that this does not apply to
1385: .Cm ChrootDirectory ,
1386: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1387: .It Cm Subsystem
1.51 jmc 1388: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1389: Arguments should be a subsystem name and a command (with optional arguments)
1390: to execute upon subsystem request.
1.80 djm 1391: .Pp
1.1 stevesk 1392: The command
1393: .Xr sftp-server 8
1394: implements the
1395: .Dq sftp
1396: file transfer subsystem.
1.80 djm 1397: .Pp
1398: Alternately the name
1399: .Dq internal-sftp
1400: implements an in-process
1401: .Dq sftp
1402: server.
1403: This may simplify configurations using
1404: .Cm ChrootDirectory
1405: to force a different filesystem root on clients.
1406: .Pp
1.1 stevesk 1407: By default no subsystems are defined.
1408: Note that this option applies to protocol version 2 only.
1409: .It Cm SyslogFacility
1410: Gives the facility code that is used when logging messages from
1.53 jmc 1411: .Xr sshd 8 .
1.1 stevesk 1412: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1413: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1414: The default is AUTH.
1.27 markus 1415: .It Cm TCPKeepAlive
1416: Specifies whether the system should send TCP keepalive messages to the
1417: other side.
1418: If they are sent, death of the connection or crash of one
1419: of the machines will be properly noticed.
1420: However, this means that
1421: connections will die if the route is down temporarily, and some people
1422: find it annoying.
1423: On the other hand, if TCP keepalives are not sent,
1424: sessions may hang indefinitely on the server, leaving
1425: .Dq ghost
1426: users and consuming server resources.
1427: .Pp
1428: The default is
1429: .Dq yes
1430: (to send TCP keepalive messages), and the server will notice
1431: if the network goes down or the client host crashes.
1432: This avoids infinitely hanging sessions.
1433: .Pp
1434: To disable TCP keepalive messages, the value should be set to
1435: .Dq no .
1.118 djm 1436: .It Cm TrustedUserCAKeys
1437: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1438: trusted to sign user certificates for authentication, or
1439: .Dq none
1440: to not use one.
1.119 jmc 1441: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1442: .Ql #
1443: are allowed.
1444: If a certificate is presented for authentication and has its signing CA key
1445: listed in this file, then it may be used for authentication for any user
1446: listed in the certificate's principals list.
1447: Note that certificates that lack a list of principals will not be permitted
1448: for authentication using
1449: .Cm TrustedUserCAKeys .
1.161 jmc 1450: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1451: .Xr ssh-keygen 1 .
1.18 markus 1452: .It Cm UseDNS
1453: Specifies whether
1.52 jmc 1454: .Xr sshd 8
1.40 jmc 1455: should look up the remote host name and check that
1.18 markus 1456: the resolved host name for the remote IP address maps back to the
1457: very same IP address.
1458: The default is
1.192 deraadt 1459: .Dq no .
1.1 stevesk 1460: .It Cm UseLogin
1461: Specifies whether
1462: .Xr login 1
1463: is used for interactive login sessions.
1464: The default is
1465: .Dq no .
1466: Note that
1467: .Xr login 1
1468: is never used for remote command execution.
1469: Note also, that if this is enabled,
1470: .Cm X11Forwarding
1471: will be disabled because
1472: .Xr login 1
1473: does not know how to handle
1474: .Xr xauth 1
1.15 jmc 1475: cookies.
1476: If
1.1 stevesk 1477: .Cm UsePrivilegeSeparation
1478: is specified, it will be disabled after authentication.
1479: .It Cm UsePrivilegeSeparation
1480: Specifies whether
1.52 jmc 1481: .Xr sshd 8
1.2 stevesk 1482: separates privileges by creating an unprivileged child process
1.15 jmc 1483: to deal with incoming network traffic.
1484: After successful authentication, another process will be created that has
1485: the privilege of the authenticated user.
1486: The goal of privilege separation is to prevent privilege
1.1 stevesk 1487: escalation by containing any corruption within the unprivileged processes.
1488: The default is
1489: .Dq yes .
1.134 djm 1490: If
1491: .Cm UsePrivilegeSeparation
1492: is set to
1493: .Dq sandbox
1494: then the pre-authentication unprivileged process is subject to additional
1495: restrictions.
1.137 djm 1496: .It Cm VersionAddendum
1497: Optionally specifies additional text to append to the SSH protocol banner
1498: sent by the server upon connection.
1499: The default is
1500: .Dq none .
1.1 stevesk 1501: .It Cm X11DisplayOffset
1502: Specifies the first display number available for
1.52 jmc 1503: .Xr sshd 8 Ns 's
1.1 stevesk 1504: X11 forwarding.
1.52 jmc 1505: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1506: The default is 10.
1507: .It Cm X11Forwarding
1508: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1509: The argument must be
1510: .Dq yes
1511: or
1512: .Dq no .
1.1 stevesk 1513: The default is
1514: .Dq no .
1.13 stevesk 1515: .Pp
1516: When X11 forwarding is enabled, there may be additional exposure to
1517: the server and to client displays if the
1.52 jmc 1518: .Xr sshd 8
1.13 stevesk 1519: proxy display is configured to listen on the wildcard address (see
1520: .Cm X11UseLocalhost
1.52 jmc 1521: below), though this is not the default.
1.13 stevesk 1522: Additionally, the authentication spoofing and authentication data
1523: verification and substitution occur on the client side.
1524: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1525: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1526: forwarding (see the warnings for
1527: .Cm ForwardX11
1528: in
1.19 jmc 1529: .Xr ssh_config 5 ) .
1.13 stevesk 1530: A system administrator may have a stance in which they want to
1531: protect clients that may expose themselves to attack by unwittingly
1532: requesting X11 forwarding, which can warrant a
1533: .Dq no
1534: setting.
1535: .Pp
1536: Note that disabling X11 forwarding does not prevent users from
1537: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1538: X11 forwarding is automatically disabled if
1539: .Cm UseLogin
1540: is enabled.
1541: .It Cm X11UseLocalhost
1542: Specifies whether
1.52 jmc 1543: .Xr sshd 8
1.1 stevesk 1544: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1545: the wildcard address.
1546: By default,
1.52 jmc 1547: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1548: hostname part of the
1549: .Ev DISPLAY
1550: environment variable to
1551: .Dq localhost .
1.8 stevesk 1552: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1553: However, some older X11 clients may not function with this
1554: configuration.
1555: .Cm X11UseLocalhost
1556: may be set to
1557: .Dq no
1558: to specify that the forwarding server should be bound to the wildcard
1559: address.
1560: The argument must be
1561: .Dq yes
1562: or
1563: .Dq no .
1564: The default is
1565: .Dq yes .
1566: .It Cm XAuthLocation
1.11 stevesk 1567: Specifies the full pathname of the
1.1 stevesk 1568: .Xr xauth 1
1.195 dtucker 1569: program, or
1570: .Dq none
1571: to not use one.
1.1 stevesk 1572: The default is
1573: .Pa /usr/X11R6/bin/xauth .
1574: .El
1.55 jmc 1575: .Sh TIME FORMATS
1.53 jmc 1576: .Xr sshd 8
1.1 stevesk 1577: command-line arguments and configuration file options that specify time
1578: may be expressed using a sequence of the form:
1579: .Sm off
1.7 stevesk 1580: .Ar time Op Ar qualifier ,
1.1 stevesk 1581: .Sm on
1582: where
1583: .Ar time
1584: is a positive integer value and
1585: .Ar qualifier
1586: is one of the following:
1587: .Pp
1588: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1589: .It Aq Cm none
1.1 stevesk 1590: seconds
1591: .It Cm s | Cm S
1592: seconds
1593: .It Cm m | Cm M
1594: minutes
1595: .It Cm h | Cm H
1596: hours
1597: .It Cm d | Cm D
1598: days
1599: .It Cm w | Cm W
1600: weeks
1601: .El
1602: .Pp
1603: Each member of the sequence is added together to calculate
1604: the total time value.
1605: .Pp
1606: Time format examples:
1607: .Pp
1608: .Bl -tag -width Ds -compact -offset indent
1609: .It 600
1610: 600 seconds (10 minutes)
1611: .It 10m
1612: 10 minutes
1613: .It 1h30m
1614: 1 hour 30 minutes (90 minutes)
1615: .El
1616: .Sh FILES
1617: .Bl -tag -width Ds
1618: .It Pa /etc/ssh/sshd_config
1619: Contains configuration data for
1.53 jmc 1620: .Xr sshd 8 .
1.1 stevesk 1621: This file should be writable by root only, but it is recommended
1622: (though not necessary) that it be world-readable.
1623: .El
1.19 jmc 1624: .Sh SEE ALSO
1625: .Xr sshd 8
1.1 stevesk 1626: .Sh AUTHORS
1627: OpenSSH is a derivative of the original and free
1628: ssh 1.2.12 release by Tatu Ylonen.
1629: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1630: Theo de Raadt and Dug Song
1631: removed many bugs, re-added newer features and
1632: created OpenSSH.
1633: Markus Friedl contributed the support for SSH
1634: protocol versions 1.5 and 2.0.
1635: Niels Provos and Markus Friedl contributed support
1636: for privilege separation.
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd_config.5 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh