Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.182
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.
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29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.182 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.181 2014/12/11 05:25:06 djm Exp $
! 37: .Dd $Mdocdate: December 11 2014 $
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.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.89 jmc 97: .It Cm AllowAgentForwarding
98: Specifies whether
99: .Xr ssh-agent 1
100: forwarding is permitted.
101: The default is
102: .Dq yes .
103: Note that disabling agent forwarding does not improve security
104: unless users are also denied shell access, as they can always install
105: their own forwarders.
1.1 stevesk 106: .It Cm AllowGroups
107: This keyword can be followed by a list of group name patterns, separated
108: by spaces.
109: If specified, login is allowed only for users whose primary
110: group or supplementary group list matches one of the patterns.
111: Only group names are valid; a numerical group ID is not recognized.
112: By default, login is allowed for all groups.
1.54 jmc 113: The allow/deny directives are processed in the following order:
114: .Cm DenyUsers ,
115: .Cm AllowUsers ,
116: .Cm DenyGroups ,
117: and finally
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.1 stevesk 123: .It Cm AllowTcpForwarding
124: Specifies whether TCP forwarding is permitted.
1.150 djm 125: The available options are
126: .Dq yes
127: or
128: .Dq all
129: to allow TCP forwarding,
130: .Dq no
131: to prevent all TCP forwarding,
132: .Dq local
133: to allow local (from the perspective of
1.151 jmc 134: .Xr ssh 1 )
135: forwarding only or
1.150 djm 136: .Dq remote
137: to allow remote forwarding only.
1.1 stevesk 138: The default is
139: .Dq yes .
140: Note that disabling TCP forwarding does not improve security unless
141: users are also denied shell access, as they can always install their
142: own forwarders.
1.175 millert 143: .It Cm AllowStreamLocalForwarding
144: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
145: The available options are
146: .Dq yes
147: or
148: .Dq all
149: to allow StreamLocal forwarding,
150: .Dq no
151: to prevent all StreamLocal forwarding,
152: .Dq local
153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
156: .Dq remote
157: to allow remote forwarding only.
158: The default is
159: .Dq yes .
160: Note that disabling StreamLocal forwarding does not improve security unless
161: users are also denied shell access, as they can always install their
162: own forwarders.
1.1 stevesk 163: .It Cm AllowUsers
164: This keyword can be followed by a list of user name patterns, separated
165: by spaces.
1.14 jmc 166: If specified, login is allowed only for user names that
1.1 stevesk 167: match one of the patterns.
168: Only user names are valid; a numerical user ID is not recognized.
169: By default, login is allowed for all users.
170: If the pattern takes the form USER@HOST then USER and HOST
171: are separately checked, restricting logins to particular
172: users from particular hosts.
1.54 jmc 173: The allow/deny directives are processed in the following order:
174: .Cm DenyUsers ,
175: .Cm AllowUsers ,
176: .Cm DenyGroups ,
177: and finally
178: .Cm AllowGroups .
1.49 jmc 179: .Pp
1.161 jmc 180: See PATTERNS in
1.49 jmc 181: .Xr ssh_config 5
182: for more information on patterns.
1.149 djm 183: .It Cm AuthenticationMethods
184: Specifies the authentication methods that must be successfully completed
185: for a user to be granted access.
186: This option must be followed by one or more comma-separated lists of
187: authentication method names.
188: Successful authentication requires completion of every method in at least
189: one of these lists.
190: .Pp
191: For example, an argument of
192: .Dq publickey,password publickey,keyboard-interactive
193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
196: so for this example, it would not be possible to attempt password or
197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
202: .Dq bsdauth ,
203: .Dq pam ,
204: or
205: .Dq skey ,
206: depending on the server configuration.
207: For example,
208: .Dq keyboard-interactive:bsdauth
209: would restrict keyboard interactive authentication to the
210: .Dq bsdauth
211: device.
1.181 djm 212: .Pp
1.149 djm 213: This option is only available for SSH protocol 2 and will yield a fatal
214: error if enabled if protocol 1 is also enabled.
215: Note that each authentication method listed should also be explicitly enabled
216: in the configuration.
217: The default is not to require multiple authentication; successful completion
218: of a single authentication method is sufficient.
1.146 djm 219: .It Cm AuthorizedKeysCommand
1.147 jmc 220: Specifies a program to be used to look up the user's public keys.
1.158 djm 221: The program must be owned by root and not writable by group or others.
222: It will be invoked with a single argument of the username
1.146 djm 223: being authenticated, and should produce on standard output zero or
1.161 jmc 224: more lines of authorized_keys output (see AUTHORIZED_KEYS in
1.147 jmc 225: .Xr sshd 8 ) .
1.146 djm 226: If a key supplied by AuthorizedKeysCommand does not successfully authenticate
227: and authorize the user then public key authentication continues using the usual
228: .Cm AuthorizedKeysFile
229: files.
230: By default, no AuthorizedKeysCommand is run.
231: .It Cm AuthorizedKeysCommandUser
232: Specifies the user under whose account the AuthorizedKeysCommand is run.
233: It is recommended to use a dedicated user that has no other role on the host
234: than running authorized keys commands.
1.181 djm 235: If no user is specified then
236: .Cm AuthorizedKeysCommand
237: is ignored.
1.1 stevesk 238: .It Cm AuthorizedKeysFile
239: Specifies the file that contains the public keys that can be used
240: for user authentication.
1.124 djm 241: The format is described in the
1.161 jmc 242: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 243: section of
244: .Xr sshd 8 .
1.1 stevesk 245: .Cm AuthorizedKeysFile
246: may contain tokens of the form %T which are substituted during connection
1.52 jmc 247: setup.
1.17 jmc 248: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 249: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 250: %u is replaced by the username of that user.
251: After expansion,
252: .Cm AuthorizedKeysFile
253: is taken to be an absolute path or one relative to the user's home
254: directory.
1.133 jmc 255: Multiple files may be listed, separated by whitespace.
256: The default is
257: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 258: .It Cm AuthorizedPrincipalsFile
259: Specifies a file that lists principal names that are accepted for
260: certificate authentication.
261: When using certificates signed by a key listed in
262: .Cm TrustedUserCAKeys ,
263: this file lists names, one of which must appear in the certificate for it
264: to be accepted for authentication.
1.125 jmc 265: Names are listed one per line preceded by key options (as described
1.161 jmc 266: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 267: .Xr sshd 8 ) .
1.124 djm 268: Empty lines and comments starting with
1.121 djm 269: .Ql #
270: are ignored.
271: .Pp
272: .Cm AuthorizedPrincipalsFile
273: may contain tokens of the form %T which are substituted during connection
274: setup.
275: The following tokens are defined: %% is replaced by a literal '%',
276: %h is replaced by the home directory of the user being authenticated, and
277: %u is replaced by the username of that user.
278: After expansion,
279: .Cm AuthorizedPrincipalsFile
280: is taken to be an absolute path or one relative to the user's home
281: directory.
282: .Pp
1.138 djm 283: The default is
284: .Dq none ,
285: i.e. not to use a principals file \(en in this case, the username
1.121 djm 286: of the user must appear in a certificate's principals list for it to be
287: accepted.
288: Note that
289: .Cm AuthorizedPrincipalsFile
290: is only used when authentication proceeds using a CA listed in
291: .Cm TrustedUserCAKeys
292: and is not consulted for certification authorities trusted via
293: .Pa ~/.ssh/authorized_keys ,
294: though the
295: .Cm principals=
296: key option offers a similar facility (see
297: .Xr sshd 8
298: for details).
1.1 stevesk 299: .It Cm Banner
300: The contents of the specified file are sent to the remote user before
301: authentication is allowed.
1.78 djm 302: If the argument is
303: .Dq none
304: then no banner is displayed.
1.1 stevesk 305: This option is only available for protocol version 2.
306: By default, no banner is displayed.
307: .It Cm ChallengeResponseAuthentication
1.50 jmc 308: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 309: All authentication styles from
310: .Xr login.conf 5
311: are supported.
312: The default is
313: .Dq yes .
1.80 djm 314: .It Cm ChrootDirectory
1.113 stevesk 315: Specifies the pathname of a directory to
1.80 djm 316: .Xr chroot 2
317: to after authentication.
1.113 stevesk 318: All components of the pathname must be root-owned directories that are
1.80 djm 319: not writable by any other user or group.
1.106 stevesk 320: After the chroot,
321: .Xr sshd 8
322: changes the working directory to the user's home directory.
1.80 djm 323: .Pp
1.113 stevesk 324: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 325: the connecting user has been authenticated: %% is replaced by a literal '%',
326: %h is replaced by the home directory of the user being authenticated, and
327: %u is replaced by the username of that user.
328: .Pp
329: The
330: .Cm ChrootDirectory
331: must contain the necessary files and directories to support the
1.103 stevesk 332: user's session.
1.80 djm 333: For an interactive session this requires at least a shell, typically
334: .Xr sh 1 ,
335: and basic
336: .Pa /dev
337: nodes such as
338: .Xr null 4 ,
339: .Xr zero 4 ,
340: .Xr stdin 4 ,
341: .Xr stdout 4 ,
342: .Xr stderr 4 ,
1.180 jmc 343: and
1.80 djm 344: .Xr tty 4
345: devices.
346: For file transfer sessions using
1.105 jmc 347: .Dq sftp ,
1.80 djm 348: no additional configuration of the environment is necessary if the
1.105 jmc 349: in-process sftp server is used,
1.176 schwarze 350: though sessions which use logging may require
1.104 stevesk 351: .Pa /dev/log
1.176 schwarze 352: inside the chroot directory on some operating systems (see
1.104 stevesk 353: .Xr sftp-server 8
1.81 jmc 354: for details).
1.80 djm 355: .Pp
356: The default is not to
357: .Xr chroot 2 .
1.1 stevesk 358: .It Cm Ciphers
359: Specifies the ciphers allowed for protocol version 2.
360: Multiple ciphers must be comma-separated.
1.167 djm 361: The supported ciphers are:
362: .Pp
1.173 naddy 363: .Bl -item -compact -offset indent
364: .It
365: 3des-cbc
366: .It
367: aes128-cbc
368: .It
369: aes192-cbc
370: .It
371: aes256-cbc
372: .It
373: aes128-ctr
374: .It
375: aes192-ctr
376: .It
377: aes256-ctr
378: .It
379: aes128-gcm@openssh.com
380: .It
381: aes256-gcm@openssh.com
382: .It
383: arcfour
384: .It
385: arcfour128
386: .It
387: arcfour256
388: .It
389: blowfish-cbc
390: .It
391: cast128-cbc
392: .It
393: chacha20-poly1305@openssh.com
394: .El
1.167 djm 395: .Pp
1.52 jmc 396: The default is:
1.173 naddy 397: .Bd -literal -offset indent
398: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 399: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 400: chacha20-poly1305@openssh.com
1.1 stevesk 401: .Ed
1.167 djm 402: .Pp
403: The list of available ciphers may also be obtained using the
404: .Fl Q
405: option of
406: .Xr ssh 1 .
1.1 stevesk 407: .It Cm ClientAliveCountMax
1.48 jmc 408: Sets the number of client alive messages (see below) which may be
1.1 stevesk 409: sent without
1.52 jmc 410: .Xr sshd 8
1.17 jmc 411: receiving any messages back from the client.
412: If this threshold is reached while client alive messages are being sent,
1.52 jmc 413: sshd will disconnect the client, terminating the session.
1.17 jmc 414: It is important to note that the use of client alive messages is very
415: different from
1.27 markus 416: .Cm TCPKeepAlive
1.17 jmc 417: (below).
418: The client alive messages are sent through the encrypted channel
419: and therefore will not be spoofable.
420: The TCP keepalive option enabled by
1.27 markus 421: .Cm TCPKeepAlive
1.17 jmc 422: is spoofable.
423: The client alive mechanism is valuable when the client or
1.1 stevesk 424: server depend on knowing when a connection has become inactive.
425: .Pp
1.17 jmc 426: The default value is 3.
427: If
1.1 stevesk 428: .Cm ClientAliveInterval
1.48 jmc 429: (see below) is set to 15, and
1.1 stevesk 430: .Cm ClientAliveCountMax
1.52 jmc 431: is left at the default, unresponsive SSH clients
1.1 stevesk 432: will be disconnected after approximately 45 seconds.
1.57 markus 433: This option applies to protocol version 2 only.
1.42 djm 434: .It Cm ClientAliveInterval
435: Sets a timeout interval in seconds after which if no data has been received
436: from the client,
1.52 jmc 437: .Xr sshd 8
1.42 djm 438: will send a message through the encrypted
439: channel to request a response from the client.
440: The default
441: is 0, indicating that these messages will not be sent to the client.
442: This option applies to protocol version 2 only.
1.3 markus 443: .It Cm Compression
1.44 markus 444: Specifies whether compression is allowed, or delayed until
445: the user has authenticated successfully.
1.3 markus 446: The argument must be
1.44 markus 447: .Dq yes ,
448: .Dq delayed ,
1.3 markus 449: or
450: .Dq no .
451: The default is
1.44 markus 452: .Dq delayed .
1.1 stevesk 453: .It Cm DenyGroups
454: This keyword can be followed by a list of group name patterns, separated
455: by spaces.
456: Login is disallowed for users whose primary group or supplementary
457: group list matches one of the patterns.
458: Only group names are valid; a numerical group ID is not recognized.
459: By default, login is allowed for all groups.
1.54 jmc 460: The allow/deny directives are processed in the following order:
461: .Cm DenyUsers ,
462: .Cm AllowUsers ,
463: .Cm DenyGroups ,
464: and finally
465: .Cm AllowGroups .
1.49 jmc 466: .Pp
1.161 jmc 467: See PATTERNS in
1.49 jmc 468: .Xr ssh_config 5
469: for more information on patterns.
1.1 stevesk 470: .It Cm DenyUsers
471: This keyword can be followed by a list of user name patterns, separated
472: by spaces.
473: Login is disallowed for user names that match one of the patterns.
474: Only user names are valid; a numerical user ID is not recognized.
475: By default, login is allowed for all users.
476: If the pattern takes the form USER@HOST then USER and HOST
477: are separately checked, restricting logins to particular
478: users from particular hosts.
1.54 jmc 479: The allow/deny directives are processed in the following order:
480: .Cm DenyUsers ,
481: .Cm AllowUsers ,
482: .Cm DenyGroups ,
483: and finally
484: .Cm AllowGroups .
1.49 jmc 485: .Pp
1.161 jmc 486: See PATTERNS in
1.49 jmc 487: .Xr ssh_config 5
488: for more information on patterns.
1.67 dtucker 489: .It Cm ForceCommand
490: Forces the execution of the command specified by
491: .Cm ForceCommand ,
1.84 djm 492: ignoring any command supplied by the client and
493: .Pa ~/.ssh/rc
494: if present.
1.67 dtucker 495: The command is invoked by using the user's login shell with the -c option.
496: This applies to shell, command, or subsystem execution.
497: It is most useful inside a
498: .Cm Match
499: block.
500: The command originally supplied by the client is available in the
501: .Ev SSH_ORIGINAL_COMMAND
502: environment variable.
1.82 djm 503: Specifying a command of
504: .Dq internal-sftp
505: will force the use of an in-process sftp server that requires no support
506: files when used with
507: .Cm ChrootDirectory .
1.1 stevesk 508: .It Cm GatewayPorts
509: Specifies whether remote hosts are allowed to connect to ports
510: forwarded for the client.
511: By default,
1.52 jmc 512: .Xr sshd 8
1.15 jmc 513: binds remote port forwardings to the loopback address.
514: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 515: .Cm GatewayPorts
1.52 jmc 516: can be used to specify that sshd
1.39 djm 517: should allow remote port forwardings to bind to non-loopback addresses, thus
518: allowing other hosts to connect.
519: The argument may be
520: .Dq no
521: to force remote port forwardings to be available to the local host only,
1.1 stevesk 522: .Dq yes
1.39 djm 523: to force remote port forwardings to bind to the wildcard address, or
524: .Dq clientspecified
525: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 526: The default is
527: .Dq no .
1.23 markus 528: .It Cm GSSAPIAuthentication
1.25 markus 529: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 530: The default is
1.23 markus 531: .Dq no .
532: Note that this option applies to protocol version 2 only.
533: .It Cm GSSAPICleanupCredentials
534: Specifies whether to automatically destroy the user's credentials cache
535: on logout.
536: The default is
537: .Dq yes .
538: Note that this option applies to protocol version 2 only.
1.1 stevesk 539: .It Cm HostbasedAuthentication
540: Specifies whether rhosts or /etc/hosts.equiv authentication together
541: with successful public key client host authentication is allowed
1.50 jmc 542: (host-based authentication).
1.1 stevesk 543: This option is similar to
544: .Cm RhostsRSAAuthentication
545: and applies to protocol version 2 only.
1.70 dtucker 546: The default is
547: .Dq no .
548: .It Cm HostbasedUsesNameFromPacketOnly
549: Specifies whether or not the server will attempt to perform a reverse
550: name lookup when matching the name in the
551: .Pa ~/.shosts ,
552: .Pa ~/.rhosts ,
553: and
554: .Pa /etc/hosts.equiv
555: files during
556: .Cm HostbasedAuthentication .
557: A setting of
558: .Dq yes
559: means that
560: .Xr sshd 8
561: uses the name supplied by the client rather than
562: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 563: The default is
564: .Dq no .
1.117 djm 565: .It Cm HostCertificate
566: Specifies a file containing a public host certificate.
567: The certificate's public key must match a private host key already specified
568: by
569: .Cm HostKey .
570: The default behaviour of
571: .Xr sshd 8
572: is not to load any certificates.
1.1 stevesk 573: .It Cm HostKey
574: Specifies a file containing a private host key
575: used by SSH.
576: The default is
577: .Pa /etc/ssh/ssh_host_key
578: for protocol version 1, and
1.126 djm 579: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 580: .Pa /etc/ssh/ssh_host_ecdsa_key ,
581: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 582: and
1.1 stevesk 583: .Pa /etc/ssh/ssh_host_rsa_key
584: for protocol version 2.
585: Note that
1.52 jmc 586: .Xr sshd 8
1.1 stevesk 587: will refuse to use a file if it is group/world-accessible.
588: It is possible to have multiple host key files.
589: .Dq rsa1
590: keys are used for version 1 and
1.126 djm 591: .Dq dsa ,
1.169 naddy 592: .Dq ecdsa ,
593: .Dq ed25519
1.1 stevesk 594: or
595: .Dq rsa
596: are used for version 2 of the SSH protocol.
1.162 markus 597: It is also possible to specify public host key files instead.
598: In this case operations on the private key will be delegated
599: to an
600: .Xr ssh-agent 1 .
601: .It Cm HostKeyAgent
602: Identifies the UNIX-domain socket used to communicate
603: with an agent that has access to the private host keys.
604: If
605: .Dq SSH_AUTH_SOCK
606: is specified, the location of the socket will be read from the
607: .Ev SSH_AUTH_SOCK
608: environment variable.
1.1 stevesk 609: .It Cm IgnoreRhosts
610: Specifies that
611: .Pa .rhosts
612: and
613: .Pa .shosts
614: files will not be used in
615: .Cm RhostsRSAAuthentication
616: or
617: .Cm HostbasedAuthentication .
618: .Pp
619: .Pa /etc/hosts.equiv
620: and
621: .Pa /etc/shosts.equiv
622: are still used.
623: The default is
624: .Dq yes .
625: .It Cm IgnoreUserKnownHosts
626: Specifies whether
1.52 jmc 627: .Xr sshd 8
1.1 stevesk 628: should ignore the user's
1.41 djm 629: .Pa ~/.ssh/known_hosts
1.1 stevesk 630: during
631: .Cm RhostsRSAAuthentication
632: or
633: .Cm HostbasedAuthentication .
634: The default is
635: .Dq no .
1.129 djm 636: .It Cm IPQoS
637: Specifies the IPv4 type-of-service or DSCP class for the connection.
638: Accepted values are
639: .Dq af11 ,
640: .Dq af12 ,
641: .Dq af13 ,
1.136 djm 642: .Dq af21 ,
1.129 djm 643: .Dq af22 ,
644: .Dq af23 ,
645: .Dq af31 ,
646: .Dq af32 ,
647: .Dq af33 ,
648: .Dq af41 ,
649: .Dq af42 ,
650: .Dq af43 ,
651: .Dq cs0 ,
652: .Dq cs1 ,
653: .Dq cs2 ,
654: .Dq cs3 ,
655: .Dq cs4 ,
656: .Dq cs5 ,
657: .Dq cs6 ,
658: .Dq cs7 ,
659: .Dq ef ,
660: .Dq lowdelay ,
661: .Dq throughput ,
662: .Dq reliability ,
663: or a numeric value.
1.131 djm 664: This option may take one or two arguments, separated by whitespace.
1.129 djm 665: If one argument is specified, it is used as the packet class unconditionally.
666: If two values are specified, the first is automatically selected for
667: interactive sessions and the second for non-interactive sessions.
668: The default is
669: .Dq lowdelay
670: for interactive sessions and
671: .Dq throughput
672: for non-interactive sessions.
1.171 jmc 673: .It Cm KbdInteractiveAuthentication
674: Specifies whether to allow keyboard-interactive authentication.
675: The argument to this keyword must be
676: .Dq yes
677: or
678: .Dq no .
679: The default is to use whatever value
680: .Cm ChallengeResponseAuthentication
681: is set to
682: (by default
683: .Dq yes ) .
1.1 stevesk 684: .It Cm KerberosAuthentication
1.24 markus 685: Specifies whether the password provided by the user for
1.1 stevesk 686: .Cm PasswordAuthentication
1.24 markus 687: will be validated through the Kerberos KDC.
1.1 stevesk 688: To use this option, the server needs a
689: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 690: The default is
1.29 dtucker 691: .Dq no .
692: .It Cm KerberosGetAFSToken
1.45 djm 693: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 694: an AFS token before accessing the user's home directory.
1.52 jmc 695: The default is
1.1 stevesk 696: .Dq no .
697: .It Cm KerberosOrLocalPasswd
1.52 jmc 698: If password authentication through Kerberos fails then
1.1 stevesk 699: the password will be validated via any additional local mechanism
700: such as
701: .Pa /etc/passwd .
1.52 jmc 702: The default is
1.1 stevesk 703: .Dq yes .
704: .It Cm KerberosTicketCleanup
705: Specifies whether to automatically destroy the user's ticket cache
706: file on logout.
1.52 jmc 707: The default is
1.1 stevesk 708: .Dq yes .
1.127 djm 709: .It Cm KexAlgorithms
710: Specifies the available KEX (Key Exchange) algorithms.
711: Multiple algorithms must be comma-separated.
1.173 naddy 712: The supported algorithms are:
713: .Pp
714: .Bl -item -compact -offset indent
715: .It
716: curve25519-sha256@libssh.org
717: .It
718: diffie-hellman-group1-sha1
719: .It
720: diffie-hellman-group14-sha1
721: .It
722: diffie-hellman-group-exchange-sha1
723: .It
724: diffie-hellman-group-exchange-sha256
725: .It
726: ecdh-sha2-nistp256
727: .It
728: ecdh-sha2-nistp384
729: .It
730: ecdh-sha2-nistp521
731: .El
732: .Pp
733: The default is:
1.170 dtucker 734: .Bd -literal -offset indent
735: curve25519-sha256@libssh.org,
736: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
737: diffie-hellman-group-exchange-sha256,
1.173 naddy 738: diffie-hellman-group14-sha1
1.170 dtucker 739: .Ed
1.1 stevesk 740: .It Cm KeyRegenerationInterval
741: In protocol version 1, the ephemeral server key is automatically regenerated
742: after this many seconds (if it has been used).
743: The purpose of regeneration is to prevent
744: decrypting captured sessions by later breaking into the machine and
745: stealing the keys.
746: The key is never stored anywhere.
747: If the value is 0, the key is never regenerated.
748: The default is 3600 (seconds).
749: .It Cm ListenAddress
750: Specifies the local addresses
1.52 jmc 751: .Xr sshd 8
1.1 stevesk 752: should listen on.
753: The following forms may be used:
754: .Pp
755: .Bl -item -offset indent -compact
756: .It
757: .Cm ListenAddress
758: .Sm off
1.180 jmc 759: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 760: .Sm on
761: .It
762: .Cm ListenAddress
763: .Sm off
1.180 jmc 764: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 765: .Sm on
766: .It
767: .Cm ListenAddress
768: .Sm off
769: .Oo
1.180 jmc 770: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 771: .Sm on
772: .El
773: .Pp
774: If
775: .Ar port
776: is not specified,
1.52 jmc 777: sshd will listen on the address and all prior
1.1 stevesk 778: .Cm Port
1.17 jmc 779: options specified.
780: The default is to listen on all local addresses.
1.15 jmc 781: Multiple
1.1 stevesk 782: .Cm ListenAddress
1.17 jmc 783: options are permitted.
784: Additionally, any
1.1 stevesk 785: .Cm Port
1.52 jmc 786: options must precede this option for non-port qualified addresses.
1.1 stevesk 787: .It Cm LoginGraceTime
788: The server disconnects after this time if the user has not
789: successfully logged in.
790: If the value is 0, there is no time limit.
1.12 stevesk 791: The default is 120 seconds.
1.1 stevesk 792: .It Cm LogLevel
793: Gives the verbosity level that is used when logging messages from
1.53 jmc 794: .Xr sshd 8 .
1.1 stevesk 795: The possible values are:
1.52 jmc 796: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 797: The default is INFO.
798: DEBUG and DEBUG1 are equivalent.
799: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
800: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 801: .It Cm MACs
802: Specifies the available MAC (message authentication code) algorithms.
803: The MAC algorithm is used in protocol version 2
804: for data integrity protection.
805: Multiple algorithms must be comma-separated.
1.152 markus 806: The algorithms that contain
807: .Dq -etm
808: calculate the MAC after encryption (encrypt-then-mac).
809: These are considered safer and their use recommended.
1.173 naddy 810: The supported MACs are:
811: .Pp
812: .Bl -item -compact -offset indent
813: .It
814: hmac-md5
815: .It
816: hmac-md5-96
817: .It
818: hmac-ripemd160
819: .It
820: hmac-sha1
821: .It
822: hmac-sha1-96
823: .It
824: hmac-sha2-256
825: .It
826: hmac-sha2-512
827: .It
828: umac-64@openssh.com
829: .It
830: umac-128@openssh.com
831: .It
832: hmac-md5-etm@openssh.com
833: .It
834: hmac-md5-96-etm@openssh.com
835: .It
836: hmac-ripemd160-etm@openssh.com
837: .It
838: hmac-sha1-etm@openssh.com
839: .It
840: hmac-sha1-96-etm@openssh.com
841: .It
842: hmac-sha2-256-etm@openssh.com
843: .It
844: hmac-sha2-512-etm@openssh.com
845: .It
846: umac-64-etm@openssh.com
847: .It
848: umac-128-etm@openssh.com
849: .El
850: .Pp
1.52 jmc 851: The default is:
1.77 jmc 852: .Bd -literal -offset indent
1.152 markus 853: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
854: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 855: umac-64@openssh.com,umac-128@openssh.com,
856: hmac-sha2-256,hmac-sha2-512
1.77 jmc 857: .Ed
1.60 dtucker 858: .It Cm Match
1.61 jmc 859: Introduces a conditional block.
1.65 dtucker 860: If all of the criteria on the
1.60 dtucker 861: .Cm Match
1.65 dtucker 862: line are satisfied, the keywords on the following lines override those
863: set in the global section of the config file, until either another
1.60 dtucker 864: .Cm Match
1.65 dtucker 865: line or the end of the file.
1.172 djm 866: If a keyword appears in multiple
867: .Cm Match
1.177 sobrado 868: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 869: applied.
1.91 djm 870: .Pp
1.61 jmc 871: The arguments to
1.60 dtucker 872: .Cm Match
1.163 dtucker 873: are one or more criteria-pattern pairs or the single token
874: .Cm All
875: which matches all criteria.
1.60 dtucker 876: The available criteria are
877: .Cm User ,
1.69 dtucker 878: .Cm Group ,
1.60 dtucker 879: .Cm Host ,
1.139 dtucker 880: .Cm LocalAddress ,
881: .Cm LocalPort ,
1.60 dtucker 882: and
883: .Cm Address .
1.91 djm 884: The match patterns may consist of single entries or comma-separated
885: lists and may use the wildcard and negation operators described in the
1.161 jmc 886: PATTERNS section of
1.92 djm 887: .Xr ssh_config 5 .
1.91 djm 888: .Pp
889: The patterns in an
890: .Cm Address
891: criteria may additionally contain addresses to match in CIDR
1.93 jmc 892: address/masklen format, e.g.\&
1.91 djm 893: .Dq 192.0.2.0/24
894: or
895: .Dq 3ffe:ffff::/32 .
896: Note that the mask length provided must be consistent with the address -
897: it is an error to specify a mask length that is too long for the address
1.93 jmc 898: or one with bits set in this host portion of the address.
899: For example,
1.91 djm 900: .Dq 192.0.2.0/33
901: and
1.93 jmc 902: .Dq 192.0.2.0/8
1.91 djm 903: respectively.
904: .Pp
1.60 dtucker 905: Only a subset of keywords may be used on the lines following a
906: .Cm Match
907: keyword.
908: Available keywords are
1.142 jmc 909: .Cm AcceptEnv ,
1.99 okan 910: .Cm AllowAgentForwarding ,
1.142 jmc 911: .Cm AllowGroups ,
1.62 dtucker 912: .Cm AllowTcpForwarding ,
1.141 markus 913: .Cm AllowUsers ,
1.149 djm 914: .Cm AuthenticationMethods ,
1.146 djm 915: .Cm AuthorizedKeysCommand ,
916: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 917: .Cm AuthorizedKeysFile ,
1.123 djm 918: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 919: .Cm Banner ,
1.85 djm 920: .Cm ChrootDirectory ,
1.141 markus 921: .Cm DenyGroups ,
922: .Cm DenyUsers ,
1.67 dtucker 923: .Cm ForceCommand ,
1.142 jmc 924: .Cm GatewayPorts ,
1.141 markus 925: .Cm GSSAPIAuthentication ,
1.87 djm 926: .Cm HostbasedAuthentication ,
1.123 djm 927: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 928: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 929: .Cm KerberosAuthentication ,
1.95 dtucker 930: .Cm MaxAuthTries ,
1.94 dtucker 931: .Cm MaxSessions ,
1.72 dtucker 932: .Cm PasswordAuthentication ,
1.97 djm 933: .Cm PermitEmptyPasswords ,
1.66 dtucker 934: .Cm PermitOpen ,
1.79 dtucker 935: .Cm PermitRootLogin ,
1.164 djm 936: .Cm PermitTTY ,
1.123 djm 937: .Cm PermitTunnel ,
1.174 djm 938: .Cm PermitUserRC ,
1.107 dtucker 939: .Cm PubkeyAuthentication ,
1.159 dtucker 940: .Cm RekeyLimit ,
1.142 jmc 941: .Cm RhostsRSAAuthentication ,
1.141 markus 942: .Cm RSAAuthentication ,
1.66 dtucker 943: .Cm X11DisplayOffset ,
1.101 djm 944: .Cm X11Forwarding
1.60 dtucker 945: and
1.102 djm 946: .Cm X11UseLocalHost .
1.33 dtucker 947: .It Cm MaxAuthTries
948: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 949: connection.
950: Once the number of failures reaches half this value,
951: additional failures are logged.
952: The default is 6.
1.90 djm 953: .It Cm MaxSessions
954: Specifies the maximum number of open sessions permitted per network connection.
955: The default is 10.
1.1 stevesk 956: .It Cm MaxStartups
957: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 958: SSH daemon.
1.1 stevesk 959: Additional connections will be dropped until authentication succeeds or the
960: .Cm LoginGraceTime
961: expires for a connection.
1.156 dtucker 962: The default is 10:30:100.
1.1 stevesk 963: .Pp
964: Alternatively, random early drop can be enabled by specifying
965: the three colon separated values
966: .Dq start:rate:full
1.51 jmc 967: (e.g. "10:30:60").
1.53 jmc 968: .Xr sshd 8
1.1 stevesk 969: will refuse connection attempts with a probability of
970: .Dq rate/100
971: (30%)
972: if there are currently
973: .Dq start
974: (10)
975: unauthenticated connections.
976: The probability increases linearly and all connection attempts
977: are refused if the number of unauthenticated connections reaches
978: .Dq full
979: (60).
980: .It Cm PasswordAuthentication
981: Specifies whether password authentication is allowed.
982: The default is
983: .Dq yes .
984: .It Cm PermitEmptyPasswords
985: When password authentication is allowed, it specifies whether the
986: server allows login to accounts with empty password strings.
987: The default is
988: .Dq no .
1.62 dtucker 989: .It Cm PermitOpen
990: Specifies the destinations to which TCP port forwarding is permitted.
991: The forwarding specification must be one of the following forms:
992: .Pp
993: .Bl -item -offset indent -compact
994: .It
995: .Cm PermitOpen
996: .Sm off
997: .Ar host : port
998: .Sm on
999: .It
1000: .Cm PermitOpen
1001: .Sm off
1002: .Ar IPv4_addr : port
1003: .Sm on
1004: .It
1005: .Cm PermitOpen
1006: .Sm off
1007: .Ar \&[ IPv6_addr \&] : port
1008: .Sm on
1009: .El
1010: .Pp
1.68 dtucker 1011: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1012: An argument of
1013: .Dq any
1014: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1015: An argument of
1016: .Dq none
1017: can be used to prohibit all forwarding requests.
1.63 jmc 1018: By default all port forwarding requests are permitted.
1.1 stevesk 1019: .It Cm PermitRootLogin
1.38 jmc 1020: Specifies whether root can log in using
1.1 stevesk 1021: .Xr ssh 1 .
1022: The argument must be
1023: .Dq yes ,
1024: .Dq without-password ,
1.52 jmc 1025: .Dq forced-commands-only ,
1.1 stevesk 1026: or
1027: .Dq no .
1028: The default is
1029: .Dq yes .
1030: .Pp
1031: If this option is set to
1.52 jmc 1032: .Dq without-password ,
1.1 stevesk 1033: password authentication is disabled for root.
1034: .Pp
1035: If this option is set to
1.52 jmc 1036: .Dq forced-commands-only ,
1.1 stevesk 1037: root login with public key authentication will be allowed,
1038: but only if the
1039: .Ar command
1040: option has been specified
1041: (which may be useful for taking remote backups even if root login is
1.17 jmc 1042: normally not allowed).
1043: All other authentication methods are disabled for root.
1.1 stevesk 1044: .Pp
1045: If this option is set to
1.52 jmc 1046: .Dq no ,
1.38 jmc 1047: root is not allowed to log in.
1.46 reyk 1048: .It Cm PermitTunnel
1049: Specifies whether
1050: .Xr tun 4
1051: device forwarding is allowed.
1.47 reyk 1052: The argument must be
1053: .Dq yes ,
1.58 stevesk 1054: .Dq point-to-point
1055: (layer 3),
1056: .Dq ethernet
1057: (layer 2), or
1.47 reyk 1058: .Dq no .
1.58 stevesk 1059: Specifying
1060: .Dq yes
1061: permits both
1062: .Dq point-to-point
1063: and
1064: .Dq ethernet .
1.46 reyk 1065: The default is
1066: .Dq no .
1.178 djm 1067: .Pp
1068: Independent of this setting, the permissions of the selected
1069: .Xr tun 4
1070: device must allow access to the user.
1.164 djm 1071: .It Cm PermitTTY
1072: Specifies whether
1.165 jmc 1073: .Xr pty 4
1.164 djm 1074: allocation is permitted.
1075: The default is
1076: .Dq yes .
1.6 markus 1077: .It Cm PermitUserEnvironment
1078: Specifies whether
1079: .Pa ~/.ssh/environment
1.9 stevesk 1080: and
1.6 markus 1081: .Cm environment=
1082: options in
1083: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1084: are processed by
1.52 jmc 1085: .Xr sshd 8 .
1.6 markus 1086: The default is
1087: .Dq no .
1.9 stevesk 1088: Enabling environment processing may enable users to bypass access
1089: restrictions in some configurations using mechanisms such as
1090: .Ev LD_PRELOAD .
1.174 djm 1091: .It Cm PermitUserRC
1092: Specifies whether any
1093: .Pa ~/.ssh/rc
1094: file is executed.
1095: The default is
1096: .Dq yes .
1.1 stevesk 1097: .It Cm PidFile
1.4 stevesk 1098: Specifies the file that contains the process ID of the
1.53 jmc 1099: SSH daemon.
1.1 stevesk 1100: The default is
1101: .Pa /var/run/sshd.pid .
1102: .It Cm Port
1103: Specifies the port number that
1.52 jmc 1104: .Xr sshd 8
1.1 stevesk 1105: listens on.
1106: The default is 22.
1107: Multiple options of this type are permitted.
1108: See also
1109: .Cm ListenAddress .
1110: .It Cm PrintLastLog
1111: Specifies whether
1.52 jmc 1112: .Xr sshd 8
1.36 jaredy 1113: should print the date and time of the last user login when a user logs
1114: in interactively.
1.1 stevesk 1115: The default is
1116: .Dq yes .
1117: .It Cm PrintMotd
1118: Specifies whether
1.52 jmc 1119: .Xr sshd 8
1.1 stevesk 1120: should print
1121: .Pa /etc/motd
1122: when a user logs in interactively.
1123: (On some systems it is also printed by the shell,
1124: .Pa /etc/profile ,
1125: or equivalent.)
1126: The default is
1127: .Dq yes .
1128: .It Cm Protocol
1129: Specifies the protocol versions
1.52 jmc 1130: .Xr sshd 8
1.5 stevesk 1131: supports.
1.1 stevesk 1132: The possible values are
1.52 jmc 1133: .Sq 1
1.1 stevesk 1134: and
1.52 jmc 1135: .Sq 2 .
1.1 stevesk 1136: Multiple versions must be comma-separated.
1137: The default is
1.109 jmc 1138: .Sq 2 .
1.5 stevesk 1139: Note that the order of the protocol list does not indicate preference,
1140: because the client selects among multiple protocol versions offered
1141: by the server.
1142: Specifying
1143: .Dq 2,1
1144: is identical to
1145: .Dq 1,2 .
1.1 stevesk 1146: .It Cm PubkeyAuthentication
1147: Specifies whether public key authentication is allowed.
1148: The default is
1149: .Dq yes .
1150: Note that this option applies to protocol version 2 only.
1.159 dtucker 1151: .It Cm RekeyLimit
1152: Specifies the maximum amount of data that may be transmitted before the
1153: session key is renegotiated, optionally followed a maximum amount of
1154: time that may pass before the session key is renegotiated.
1155: The first argument is specified in bytes and may have a suffix of
1156: .Sq K ,
1157: .Sq M ,
1158: or
1159: .Sq G
1160: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1161: The default is between
1162: .Sq 1G
1163: and
1164: .Sq 4G ,
1165: depending on the cipher.
1166: The optional second value is specified in seconds and may use any of the
1167: units documented in the
1168: .Sx TIME FORMATS
1.160 jmc 1169: section.
1.159 dtucker 1170: The default value for
1171: .Cm RekeyLimit
1172: is
1173: .Dq default none ,
1174: which means that rekeying is performed after the cipher's default amount
1175: of data has been sent or received and no time based rekeying is done.
1176: This option applies to protocol version 2 only.
1.118 djm 1177: .It Cm RevokedKeys
1.154 djm 1178: Specifies revoked public keys.
1.118 djm 1179: Keys listed in this file will be refused for public key authentication.
1180: Note that if this file is not readable, then public key authentication will
1181: be refused for all users.
1.154 djm 1182: Keys may be specified as a text file, listing one public key per line, or as
1183: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1184: .Xr ssh-keygen 1 .
1.161 jmc 1185: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1186: .Xr ssh-keygen 1 .
1.1 stevesk 1187: .It Cm RhostsRSAAuthentication
1188: Specifies whether rhosts or /etc/hosts.equiv authentication together
1189: with successful RSA host authentication is allowed.
1190: The default is
1191: .Dq no .
1192: This option applies to protocol version 1 only.
1193: .It Cm RSAAuthentication
1194: Specifies whether pure RSA authentication is allowed.
1195: The default is
1196: .Dq yes .
1197: This option applies to protocol version 1 only.
1198: .It Cm ServerKeyBits
1199: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1200: The minimum value is 512, and the default is 1024.
1.175 millert 1201: .It Cm StreamLocalBindMask
1202: Sets the octal file creation mode mask
1203: .Pq umask
1204: used when creating a Unix-domain socket file for local or remote
1205: port forwarding.
1206: This option is only used for port forwarding to a Unix-domain socket file.
1207: .Pp
1208: The default value is 0177, which creates a Unix-domain socket file that is
1209: readable and writable only by the owner.
1210: Note that not all operating systems honor the file mode on Unix-domain
1211: socket files.
1212: .It Cm StreamLocalBindUnlink
1213: Specifies whether to remove an existing Unix-domain socket file for local
1214: or remote port forwarding before creating a new one.
1215: If the socket file already exists and
1216: .Cm StreamLocalBindUnlink
1217: is not enabled,
1218: .Nm sshd
1219: will be unable to forward the port to the Unix-domain socket file.
1220: This option is only used for port forwarding to a Unix-domain socket file.
1221: .Pp
1222: The argument must be
1223: .Dq yes
1224: or
1225: .Dq no .
1226: The default is
1227: .Dq no .
1.1 stevesk 1228: .It Cm StrictModes
1229: Specifies whether
1.52 jmc 1230: .Xr sshd 8
1.1 stevesk 1231: should check file modes and ownership of the
1232: user's files and home directory before accepting login.
1233: This is normally desirable because novices sometimes accidentally leave their
1234: directory or files world-writable.
1235: The default is
1236: .Dq yes .
1.112 djm 1237: Note that this does not apply to
1238: .Cm ChrootDirectory ,
1239: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1240: .It Cm Subsystem
1.51 jmc 1241: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1242: Arguments should be a subsystem name and a command (with optional arguments)
1243: to execute upon subsystem request.
1.80 djm 1244: .Pp
1.1 stevesk 1245: The command
1246: .Xr sftp-server 8
1247: implements the
1248: .Dq sftp
1249: file transfer subsystem.
1.80 djm 1250: .Pp
1251: Alternately the name
1252: .Dq internal-sftp
1253: implements an in-process
1254: .Dq sftp
1255: server.
1256: This may simplify configurations using
1257: .Cm ChrootDirectory
1258: to force a different filesystem root on clients.
1259: .Pp
1.1 stevesk 1260: By default no subsystems are defined.
1261: Note that this option applies to protocol version 2 only.
1262: .It Cm SyslogFacility
1263: Gives the facility code that is used when logging messages from
1.53 jmc 1264: .Xr sshd 8 .
1.1 stevesk 1265: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1266: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1267: The default is AUTH.
1.27 markus 1268: .It Cm TCPKeepAlive
1269: Specifies whether the system should send TCP keepalive messages to the
1270: other side.
1271: If they are sent, death of the connection or crash of one
1272: of the machines will be properly noticed.
1273: However, this means that
1274: connections will die if the route is down temporarily, and some people
1275: find it annoying.
1276: On the other hand, if TCP keepalives are not sent,
1277: sessions may hang indefinitely on the server, leaving
1278: .Dq ghost
1279: users and consuming server resources.
1280: .Pp
1281: The default is
1282: .Dq yes
1283: (to send TCP keepalive messages), and the server will notice
1284: if the network goes down or the client host crashes.
1285: This avoids infinitely hanging sessions.
1286: .Pp
1287: To disable TCP keepalive messages, the value should be set to
1288: .Dq no .
1.118 djm 1289: .It Cm TrustedUserCAKeys
1290: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1291: trusted to sign user certificates for authentication.
1.119 jmc 1292: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1293: .Ql #
1294: are allowed.
1295: If a certificate is presented for authentication and has its signing CA key
1296: listed in this file, then it may be used for authentication for any user
1297: listed in the certificate's principals list.
1298: Note that certificates that lack a list of principals will not be permitted
1299: for authentication using
1300: .Cm TrustedUserCAKeys .
1.161 jmc 1301: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1302: .Xr ssh-keygen 1 .
1.18 markus 1303: .It Cm UseDNS
1304: Specifies whether
1.52 jmc 1305: .Xr sshd 8
1.40 jmc 1306: should look up the remote host name and check that
1.18 markus 1307: the resolved host name for the remote IP address maps back to the
1308: very same IP address.
1309: The default is
1310: .Dq yes .
1.1 stevesk 1311: .It Cm UseLogin
1312: Specifies whether
1313: .Xr login 1
1314: is used for interactive login sessions.
1315: The default is
1316: .Dq no .
1317: Note that
1318: .Xr login 1
1319: is never used for remote command execution.
1320: Note also, that if this is enabled,
1321: .Cm X11Forwarding
1322: will be disabled because
1323: .Xr login 1
1324: does not know how to handle
1325: .Xr xauth 1
1.15 jmc 1326: cookies.
1327: If
1.1 stevesk 1328: .Cm UsePrivilegeSeparation
1329: is specified, it will be disabled after authentication.
1330: .It Cm UsePrivilegeSeparation
1331: Specifies whether
1.52 jmc 1332: .Xr sshd 8
1.2 stevesk 1333: separates privileges by creating an unprivileged child process
1.15 jmc 1334: to deal with incoming network traffic.
1335: After successful authentication, another process will be created that has
1336: the privilege of the authenticated user.
1337: The goal of privilege separation is to prevent privilege
1.1 stevesk 1338: escalation by containing any corruption within the unprivileged processes.
1339: The default is
1340: .Dq yes .
1.134 djm 1341: If
1342: .Cm UsePrivilegeSeparation
1343: is set to
1344: .Dq sandbox
1345: then the pre-authentication unprivileged process is subject to additional
1346: restrictions.
1.137 djm 1347: .It Cm VersionAddendum
1348: Optionally specifies additional text to append to the SSH protocol banner
1349: sent by the server upon connection.
1350: The default is
1351: .Dq none .
1.1 stevesk 1352: .It Cm X11DisplayOffset
1353: Specifies the first display number available for
1.52 jmc 1354: .Xr sshd 8 Ns 's
1.1 stevesk 1355: X11 forwarding.
1.52 jmc 1356: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1357: The default is 10.
1358: .It Cm X11Forwarding
1359: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1360: The argument must be
1361: .Dq yes
1362: or
1363: .Dq no .
1.1 stevesk 1364: The default is
1365: .Dq no .
1.13 stevesk 1366: .Pp
1367: When X11 forwarding is enabled, there may be additional exposure to
1368: the server and to client displays if the
1.52 jmc 1369: .Xr sshd 8
1.13 stevesk 1370: proxy display is configured to listen on the wildcard address (see
1371: .Cm X11UseLocalhost
1.52 jmc 1372: below), though this is not the default.
1.13 stevesk 1373: Additionally, the authentication spoofing and authentication data
1374: verification and substitution occur on the client side.
1375: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1376: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1377: forwarding (see the warnings for
1378: .Cm ForwardX11
1379: in
1.19 jmc 1380: .Xr ssh_config 5 ) .
1.13 stevesk 1381: A system administrator may have a stance in which they want to
1382: protect clients that may expose themselves to attack by unwittingly
1383: requesting X11 forwarding, which can warrant a
1384: .Dq no
1385: setting.
1386: .Pp
1387: Note that disabling X11 forwarding does not prevent users from
1388: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1389: X11 forwarding is automatically disabled if
1390: .Cm UseLogin
1391: is enabled.
1392: .It Cm X11UseLocalhost
1393: Specifies whether
1.52 jmc 1394: .Xr sshd 8
1.1 stevesk 1395: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1396: the wildcard address.
1397: By default,
1.52 jmc 1398: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1399: hostname part of the
1400: .Ev DISPLAY
1401: environment variable to
1402: .Dq localhost .
1.8 stevesk 1403: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1404: However, some older X11 clients may not function with this
1405: configuration.
1406: .Cm X11UseLocalhost
1407: may be set to
1408: .Dq no
1409: to specify that the forwarding server should be bound to the wildcard
1410: address.
1411: The argument must be
1412: .Dq yes
1413: or
1414: .Dq no .
1415: The default is
1416: .Dq yes .
1417: .It Cm XAuthLocation
1.11 stevesk 1418: Specifies the full pathname of the
1.1 stevesk 1419: .Xr xauth 1
1420: program.
1421: The default is
1422: .Pa /usr/X11R6/bin/xauth .
1423: .El
1.55 jmc 1424: .Sh TIME FORMATS
1.53 jmc 1425: .Xr sshd 8
1.1 stevesk 1426: command-line arguments and configuration file options that specify time
1427: may be expressed using a sequence of the form:
1428: .Sm off
1.7 stevesk 1429: .Ar time Op Ar qualifier ,
1.1 stevesk 1430: .Sm on
1431: where
1432: .Ar time
1433: is a positive integer value and
1434: .Ar qualifier
1435: is one of the following:
1436: .Pp
1437: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1438: .It Aq Cm none
1.1 stevesk 1439: seconds
1440: .It Cm s | Cm S
1441: seconds
1442: .It Cm m | Cm M
1443: minutes
1444: .It Cm h | Cm H
1445: hours
1446: .It Cm d | Cm D
1447: days
1448: .It Cm w | Cm W
1449: weeks
1450: .El
1451: .Pp
1452: Each member of the sequence is added together to calculate
1453: the total time value.
1454: .Pp
1455: Time format examples:
1456: .Pp
1457: .Bl -tag -width Ds -compact -offset indent
1458: .It 600
1459: 600 seconds (10 minutes)
1460: .It 10m
1461: 10 minutes
1462: .It 1h30m
1463: 1 hour 30 minutes (90 minutes)
1464: .El
1465: .Sh FILES
1466: .Bl -tag -width Ds
1467: .It Pa /etc/ssh/sshd_config
1468: Contains configuration data for
1.53 jmc 1469: .Xr sshd 8 .
1.1 stevesk 1470: This file should be writable by root only, but it is recommended
1471: (though not necessary) that it be world-readable.
1472: .El
1.19 jmc 1473: .Sh SEE ALSO
1474: .Xr sshd 8
1.1 stevesk 1475: .Sh AUTHORS
1476: OpenSSH is a derivative of the original and free
1477: ssh 1.2.12 release by Tatu Ylonen.
1478: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1479: Theo de Raadt and Dug Song
1480: removed many bugs, re-added newer features and
1481: created OpenSSH.
1482: Markus Friedl contributed the support for SSH
1483: protocol versions 1.5 and 2.0.
1484: Niels Provos and Markus Friedl contributed support
1485: for privilege separation.
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