Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.179
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
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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
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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
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1.179 ! deraadt 36: .\" $OpenBSD: sshd_config.5,v 1.178 2014/09/09 09:45:36 djm Exp $
! 37: .Dd $Mdocdate: September 9 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.
1.149 djm 212: .Pp
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.
1.1 stevesk 235: .It Cm AuthorizedKeysFile
236: Specifies the file that contains the public keys that can be used
237: for user authentication.
1.124 djm 238: The format is described in the
1.161 jmc 239: AUTHORIZED_KEYS FILE FORMAT
1.124 djm 240: section of
241: .Xr sshd 8 .
1.1 stevesk 242: .Cm AuthorizedKeysFile
243: may contain tokens of the form %T which are substituted during connection
1.52 jmc 244: setup.
1.17 jmc 245: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 246: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 247: %u is replaced by the username of that user.
248: After expansion,
249: .Cm AuthorizedKeysFile
250: is taken to be an absolute path or one relative to the user's home
251: directory.
1.133 jmc 252: Multiple files may be listed, separated by whitespace.
253: The default is
254: .Dq .ssh/authorized_keys .ssh/authorized_keys2 .
1.121 djm 255: .It Cm AuthorizedPrincipalsFile
256: Specifies a file that lists principal names that are accepted for
257: certificate authentication.
258: When using certificates signed by a key listed in
259: .Cm TrustedUserCAKeys ,
260: this file lists names, one of which must appear in the certificate for it
261: to be accepted for authentication.
1.125 jmc 262: Names are listed one per line preceded by key options (as described
1.161 jmc 263: in AUTHORIZED_KEYS FILE FORMAT in
1.125 jmc 264: .Xr sshd 8 ) .
1.124 djm 265: Empty lines and comments starting with
1.121 djm 266: .Ql #
267: are ignored.
268: .Pp
269: .Cm AuthorizedPrincipalsFile
270: may contain tokens of the form %T which are substituted during connection
271: setup.
272: The following tokens are defined: %% is replaced by a literal '%',
273: %h is replaced by the home directory of the user being authenticated, and
274: %u is replaced by the username of that user.
275: After expansion,
276: .Cm AuthorizedPrincipalsFile
277: is taken to be an absolute path or one relative to the user's home
278: directory.
279: .Pp
1.138 djm 280: The default is
281: .Dq none ,
282: i.e. not to use a principals file \(en in this case, the username
1.121 djm 283: of the user must appear in a certificate's principals list for it to be
284: accepted.
285: Note that
286: .Cm AuthorizedPrincipalsFile
287: is only used when authentication proceeds using a CA listed in
288: .Cm TrustedUserCAKeys
289: and is not consulted for certification authorities trusted via
290: .Pa ~/.ssh/authorized_keys ,
291: though the
292: .Cm principals=
293: key option offers a similar facility (see
294: .Xr sshd 8
295: for details).
1.1 stevesk 296: .It Cm Banner
297: The contents of the specified file are sent to the remote user before
298: authentication is allowed.
1.78 djm 299: If the argument is
300: .Dq none
301: then no banner is displayed.
1.1 stevesk 302: This option is only available for protocol version 2.
303: By default, no banner is displayed.
304: .It Cm ChallengeResponseAuthentication
1.50 jmc 305: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 306: All authentication styles from
307: .Xr login.conf 5
308: are supported.
309: The default is
310: .Dq yes .
1.80 djm 311: .It Cm ChrootDirectory
1.113 stevesk 312: Specifies the pathname of a directory to
1.80 djm 313: .Xr chroot 2
314: to after authentication.
1.113 stevesk 315: All components of the pathname must be root-owned directories that are
1.80 djm 316: not writable by any other user or group.
1.106 stevesk 317: After the chroot,
318: .Xr sshd 8
319: changes the working directory to the user's home directory.
1.80 djm 320: .Pp
1.113 stevesk 321: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 322: the connecting user has been authenticated: %% is replaced by a literal '%',
323: %h is replaced by the home directory of the user being authenticated, and
324: %u is replaced by the username of that user.
325: .Pp
326: The
327: .Cm ChrootDirectory
328: must contain the necessary files and directories to support the
1.103 stevesk 329: user's session.
1.80 djm 330: For an interactive session this requires at least a shell, typically
331: .Xr sh 1 ,
332: and basic
333: .Pa /dev
334: nodes such as
335: .Xr null 4 ,
336: .Xr zero 4 ,
337: .Xr stdin 4 ,
338: .Xr stdout 4 ,
339: .Xr stderr 4 ,
340: .Xr tty 4
341: devices.
342: For file transfer sessions using
1.105 jmc 343: .Dq sftp ,
1.80 djm 344: no additional configuration of the environment is necessary if the
1.105 jmc 345: in-process sftp server is used,
1.176 schwarze 346: though sessions which use logging may require
1.104 stevesk 347: .Pa /dev/log
1.176 schwarze 348: inside the chroot directory on some operating systems (see
1.104 stevesk 349: .Xr sftp-server 8
1.81 jmc 350: for details).
1.80 djm 351: .Pp
352: The default is not to
353: .Xr chroot 2 .
1.1 stevesk 354: .It Cm Ciphers
355: Specifies the ciphers allowed for protocol version 2.
356: Multiple ciphers must be comma-separated.
1.167 djm 357: The supported ciphers are:
358: .Pp
1.173 naddy 359: .Bl -item -compact -offset indent
360: .It
361: 3des-cbc
362: .It
363: aes128-cbc
364: .It
365: aes192-cbc
366: .It
367: aes256-cbc
368: .It
369: aes128-ctr
370: .It
371: aes192-ctr
372: .It
373: aes256-ctr
374: .It
375: aes128-gcm@openssh.com
376: .It
377: aes256-gcm@openssh.com
378: .It
379: arcfour
380: .It
381: arcfour128
382: .It
383: arcfour256
384: .It
385: blowfish-cbc
386: .It
387: cast128-cbc
388: .It
389: chacha20-poly1305@openssh.com
390: .El
1.167 djm 391: .Pp
1.52 jmc 392: The default is:
1.173 naddy 393: .Bd -literal -offset indent
394: aes128-ctr,aes192-ctr,aes256-ctr,
1.153 markus 395: aes128-gcm@openssh.com,aes256-gcm@openssh.com,
1.173 naddy 396: chacha20-poly1305@openssh.com
1.1 stevesk 397: .Ed
1.167 djm 398: .Pp
399: The list of available ciphers may also be obtained using the
400: .Fl Q
401: option of
402: .Xr ssh 1 .
1.1 stevesk 403: .It Cm ClientAliveCountMax
1.48 jmc 404: Sets the number of client alive messages (see below) which may be
1.1 stevesk 405: sent without
1.52 jmc 406: .Xr sshd 8
1.17 jmc 407: receiving any messages back from the client.
408: If this threshold is reached while client alive messages are being sent,
1.52 jmc 409: sshd will disconnect the client, terminating the session.
1.17 jmc 410: It is important to note that the use of client alive messages is very
411: different from
1.27 markus 412: .Cm TCPKeepAlive
1.17 jmc 413: (below).
414: The client alive messages are sent through the encrypted channel
415: and therefore will not be spoofable.
416: The TCP keepalive option enabled by
1.27 markus 417: .Cm TCPKeepAlive
1.17 jmc 418: is spoofable.
419: The client alive mechanism is valuable when the client or
1.1 stevesk 420: server depend on knowing when a connection has become inactive.
421: .Pp
1.17 jmc 422: The default value is 3.
423: If
1.1 stevesk 424: .Cm ClientAliveInterval
1.48 jmc 425: (see below) is set to 15, and
1.1 stevesk 426: .Cm ClientAliveCountMax
1.52 jmc 427: is left at the default, unresponsive SSH clients
1.1 stevesk 428: will be disconnected after approximately 45 seconds.
1.57 markus 429: This option applies to protocol version 2 only.
1.42 djm 430: .It Cm ClientAliveInterval
431: Sets a timeout interval in seconds after which if no data has been received
432: from the client,
1.52 jmc 433: .Xr sshd 8
1.42 djm 434: will send a message through the encrypted
435: channel to request a response from the client.
436: The default
437: is 0, indicating that these messages will not be sent to the client.
438: This option applies to protocol version 2 only.
1.3 markus 439: .It Cm Compression
1.44 markus 440: Specifies whether compression is allowed, or delayed until
441: the user has authenticated successfully.
1.3 markus 442: The argument must be
1.44 markus 443: .Dq yes ,
444: .Dq delayed ,
1.3 markus 445: or
446: .Dq no .
447: The default is
1.44 markus 448: .Dq delayed .
1.1 stevesk 449: .It Cm DenyGroups
450: This keyword can be followed by a list of group name patterns, separated
451: by spaces.
452: Login is disallowed for users whose primary group or supplementary
453: group list matches one of the patterns.
454: Only group names are valid; a numerical group ID is not recognized.
455: By default, login is allowed for all groups.
1.54 jmc 456: The allow/deny directives are processed in the following order:
457: .Cm DenyUsers ,
458: .Cm AllowUsers ,
459: .Cm DenyGroups ,
460: and finally
461: .Cm AllowGroups .
1.49 jmc 462: .Pp
1.161 jmc 463: See PATTERNS in
1.49 jmc 464: .Xr ssh_config 5
465: for more information on patterns.
1.1 stevesk 466: .It Cm DenyUsers
467: This keyword can be followed by a list of user name patterns, separated
468: by spaces.
469: Login is disallowed for user names that match one of the patterns.
470: Only user names are valid; a numerical user ID is not recognized.
471: By default, login is allowed for all users.
472: If the pattern takes the form USER@HOST then USER and HOST
473: are separately checked, restricting logins to particular
474: users from particular hosts.
1.54 jmc 475: The allow/deny directives are processed in the following order:
476: .Cm DenyUsers ,
477: .Cm AllowUsers ,
478: .Cm DenyGroups ,
479: and finally
480: .Cm AllowGroups .
1.49 jmc 481: .Pp
1.161 jmc 482: See PATTERNS in
1.49 jmc 483: .Xr ssh_config 5
484: for more information on patterns.
1.67 dtucker 485: .It Cm ForceCommand
486: Forces the execution of the command specified by
487: .Cm ForceCommand ,
1.84 djm 488: ignoring any command supplied by the client and
489: .Pa ~/.ssh/rc
490: if present.
1.67 dtucker 491: The command is invoked by using the user's login shell with the -c option.
492: This applies to shell, command, or subsystem execution.
493: It is most useful inside a
494: .Cm Match
495: block.
496: The command originally supplied by the client is available in the
497: .Ev SSH_ORIGINAL_COMMAND
498: environment variable.
1.82 djm 499: Specifying a command of
500: .Dq internal-sftp
501: will force the use of an in-process sftp server that requires no support
502: files when used with
503: .Cm ChrootDirectory .
1.1 stevesk 504: .It Cm GatewayPorts
505: Specifies whether remote hosts are allowed to connect to ports
506: forwarded for the client.
507: By default,
1.52 jmc 508: .Xr sshd 8
1.15 jmc 509: binds remote port forwardings to the loopback address.
510: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 511: .Cm GatewayPorts
1.52 jmc 512: can be used to specify that sshd
1.39 djm 513: should allow remote port forwardings to bind to non-loopback addresses, thus
514: allowing other hosts to connect.
515: The argument may be
516: .Dq no
517: to force remote port forwardings to be available to the local host only,
1.1 stevesk 518: .Dq yes
1.39 djm 519: to force remote port forwardings to bind to the wildcard address, or
520: .Dq clientspecified
521: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 522: The default is
523: .Dq no .
1.23 markus 524: .It Cm GSSAPIAuthentication
1.25 markus 525: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 526: The default is
1.23 markus 527: .Dq no .
528: Note that this option applies to protocol version 2 only.
529: .It Cm GSSAPICleanupCredentials
530: Specifies whether to automatically destroy the user's credentials cache
531: on logout.
532: The default is
533: .Dq yes .
534: Note that this option applies to protocol version 2 only.
1.1 stevesk 535: .It Cm HostbasedAuthentication
536: Specifies whether rhosts or /etc/hosts.equiv authentication together
537: with successful public key client host authentication is allowed
1.50 jmc 538: (host-based authentication).
1.1 stevesk 539: This option is similar to
540: .Cm RhostsRSAAuthentication
541: and applies to protocol version 2 only.
1.70 dtucker 542: The default is
543: .Dq no .
544: .It Cm HostbasedUsesNameFromPacketOnly
545: Specifies whether or not the server will attempt to perform a reverse
546: name lookup when matching the name in the
547: .Pa ~/.shosts ,
548: .Pa ~/.rhosts ,
549: and
550: .Pa /etc/hosts.equiv
551: files during
552: .Cm HostbasedAuthentication .
553: A setting of
554: .Dq yes
555: means that
556: .Xr sshd 8
557: uses the name supplied by the client rather than
558: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 559: The default is
560: .Dq no .
1.117 djm 561: .It Cm HostCertificate
562: Specifies a file containing a public host certificate.
563: The certificate's public key must match a private host key already specified
564: by
565: .Cm HostKey .
566: The default behaviour of
567: .Xr sshd 8
568: is not to load any certificates.
1.1 stevesk 569: .It Cm HostKey
570: Specifies a file containing a private host key
571: used by SSH.
572: The default is
573: .Pa /etc/ssh/ssh_host_key
574: for protocol version 1, and
1.126 djm 575: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 576: .Pa /etc/ssh/ssh_host_ecdsa_key ,
577: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 578: and
1.1 stevesk 579: .Pa /etc/ssh/ssh_host_rsa_key
580: for protocol version 2.
581: Note that
1.52 jmc 582: .Xr sshd 8
1.1 stevesk 583: will refuse to use a file if it is group/world-accessible.
584: It is possible to have multiple host key files.
585: .Dq rsa1
586: keys are used for version 1 and
1.126 djm 587: .Dq dsa ,
1.169 naddy 588: .Dq ecdsa ,
589: .Dq ed25519
1.1 stevesk 590: or
591: .Dq rsa
592: are used for version 2 of the SSH protocol.
1.162 markus 593: It is also possible to specify public host key files instead.
594: In this case operations on the private key will be delegated
595: to an
596: .Xr ssh-agent 1 .
597: .It Cm HostKeyAgent
598: Identifies the UNIX-domain socket used to communicate
599: with an agent that has access to the private host keys.
600: If
601: .Dq SSH_AUTH_SOCK
602: is specified, the location of the socket will be read from the
603: .Ev SSH_AUTH_SOCK
604: environment variable.
1.1 stevesk 605: .It Cm IgnoreRhosts
606: Specifies that
607: .Pa .rhosts
608: and
609: .Pa .shosts
610: files will not be used in
611: .Cm RhostsRSAAuthentication
612: or
613: .Cm HostbasedAuthentication .
614: .Pp
615: .Pa /etc/hosts.equiv
616: and
617: .Pa /etc/shosts.equiv
618: are still used.
619: The default is
620: .Dq yes .
621: .It Cm IgnoreUserKnownHosts
622: Specifies whether
1.52 jmc 623: .Xr sshd 8
1.1 stevesk 624: should ignore the user's
1.41 djm 625: .Pa ~/.ssh/known_hosts
1.1 stevesk 626: during
627: .Cm RhostsRSAAuthentication
628: or
629: .Cm HostbasedAuthentication .
630: The default is
631: .Dq no .
1.129 djm 632: .It Cm IPQoS
633: Specifies the IPv4 type-of-service or DSCP class for the connection.
634: Accepted values are
635: .Dq af11 ,
636: .Dq af12 ,
637: .Dq af13 ,
1.136 djm 638: .Dq af21 ,
1.129 djm 639: .Dq af22 ,
640: .Dq af23 ,
641: .Dq af31 ,
642: .Dq af32 ,
643: .Dq af33 ,
644: .Dq af41 ,
645: .Dq af42 ,
646: .Dq af43 ,
647: .Dq cs0 ,
648: .Dq cs1 ,
649: .Dq cs2 ,
650: .Dq cs3 ,
651: .Dq cs4 ,
652: .Dq cs5 ,
653: .Dq cs6 ,
654: .Dq cs7 ,
655: .Dq ef ,
656: .Dq lowdelay ,
657: .Dq throughput ,
658: .Dq reliability ,
659: or a numeric value.
1.131 djm 660: This option may take one or two arguments, separated by whitespace.
1.129 djm 661: If one argument is specified, it is used as the packet class unconditionally.
662: If two values are specified, the first is automatically selected for
663: interactive sessions and the second for non-interactive sessions.
664: The default is
665: .Dq lowdelay
666: for interactive sessions and
667: .Dq throughput
668: for non-interactive sessions.
1.171 jmc 669: .It Cm KbdInteractiveAuthentication
670: Specifies whether to allow keyboard-interactive authentication.
671: The argument to this keyword must be
672: .Dq yes
673: or
674: .Dq no .
675: The default is to use whatever value
676: .Cm ChallengeResponseAuthentication
677: is set to
678: (by default
679: .Dq yes ) .
1.1 stevesk 680: .It Cm KerberosAuthentication
1.24 markus 681: Specifies whether the password provided by the user for
1.1 stevesk 682: .Cm PasswordAuthentication
1.24 markus 683: will be validated through the Kerberos KDC.
1.1 stevesk 684: To use this option, the server needs a
685: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 686: The default is
1.29 dtucker 687: .Dq no .
688: .It Cm KerberosGetAFSToken
1.45 djm 689: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 690: an AFS token before accessing the user's home directory.
1.52 jmc 691: The default is
1.1 stevesk 692: .Dq no .
693: .It Cm KerberosOrLocalPasswd
1.52 jmc 694: If password authentication through Kerberos fails then
1.1 stevesk 695: the password will be validated via any additional local mechanism
696: such as
697: .Pa /etc/passwd .
1.52 jmc 698: The default is
1.1 stevesk 699: .Dq yes .
700: .It Cm KerberosTicketCleanup
701: Specifies whether to automatically destroy the user's ticket cache
702: file on logout.
1.52 jmc 703: The default is
1.1 stevesk 704: .Dq yes .
1.127 djm 705: .It Cm KexAlgorithms
706: Specifies the available KEX (Key Exchange) algorithms.
707: Multiple algorithms must be comma-separated.
1.173 naddy 708: The supported algorithms are:
709: .Pp
710: .Bl -item -compact -offset indent
711: .It
712: curve25519-sha256@libssh.org
713: .It
714: diffie-hellman-group1-sha1
715: .It
716: diffie-hellman-group14-sha1
717: .It
718: diffie-hellman-group-exchange-sha1
719: .It
720: diffie-hellman-group-exchange-sha256
721: .It
722: ecdh-sha2-nistp256
723: .It
724: ecdh-sha2-nistp384
725: .It
726: ecdh-sha2-nistp521
727: .El
728: .Pp
729: The default is:
1.170 dtucker 730: .Bd -literal -offset indent
731: curve25519-sha256@libssh.org,
732: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
733: diffie-hellman-group-exchange-sha256,
1.173 naddy 734: diffie-hellman-group14-sha1
1.170 dtucker 735: .Ed
1.1 stevesk 736: .It Cm KeyRegenerationInterval
737: In protocol version 1, the ephemeral server key is automatically regenerated
738: after this many seconds (if it has been used).
739: The purpose of regeneration is to prevent
740: decrypting captured sessions by later breaking into the machine and
741: stealing the keys.
742: The key is never stored anywhere.
743: If the value is 0, the key is never regenerated.
744: The default is 3600 (seconds).
745: .It Cm ListenAddress
746: Specifies the local addresses
1.52 jmc 747: .Xr sshd 8
1.1 stevesk 748: should listen on.
749: The following forms may be used:
750: .Pp
751: .Bl -item -offset indent -compact
752: .It
753: .Cm ListenAddress
754: .Sm off
755: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
756: .Sm on
757: .It
758: .Cm ListenAddress
759: .Sm off
760: .Ar host No | Ar IPv4_addr No : Ar port
761: .Sm on
762: .It
763: .Cm ListenAddress
764: .Sm off
765: .Oo
766: .Ar host No | Ar IPv6_addr Oc : Ar port
767: .Sm on
768: .El
769: .Pp
770: If
771: .Ar port
772: is not specified,
1.52 jmc 773: sshd will listen on the address and all prior
1.1 stevesk 774: .Cm Port
1.17 jmc 775: options specified.
776: The default is to listen on all local addresses.
1.15 jmc 777: Multiple
1.1 stevesk 778: .Cm ListenAddress
1.17 jmc 779: options are permitted.
780: Additionally, any
1.1 stevesk 781: .Cm Port
1.52 jmc 782: options must precede this option for non-port qualified addresses.
1.1 stevesk 783: .It Cm LoginGraceTime
784: The server disconnects after this time if the user has not
785: successfully logged in.
786: If the value is 0, there is no time limit.
1.12 stevesk 787: The default is 120 seconds.
1.1 stevesk 788: .It Cm LogLevel
789: Gives the verbosity level that is used when logging messages from
1.53 jmc 790: .Xr sshd 8 .
1.1 stevesk 791: The possible values are:
1.52 jmc 792: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 793: The default is INFO.
794: DEBUG and DEBUG1 are equivalent.
795: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
796: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 797: .It Cm MACs
798: Specifies the available MAC (message authentication code) algorithms.
799: The MAC algorithm is used in protocol version 2
800: for data integrity protection.
801: Multiple algorithms must be comma-separated.
1.152 markus 802: The algorithms that contain
803: .Dq -etm
804: calculate the MAC after encryption (encrypt-then-mac).
805: These are considered safer and their use recommended.
1.173 naddy 806: The supported MACs are:
807: .Pp
808: .Bl -item -compact -offset indent
809: .It
810: hmac-md5
811: .It
812: hmac-md5-96
813: .It
814: hmac-ripemd160
815: .It
816: hmac-sha1
817: .It
818: hmac-sha1-96
819: .It
820: hmac-sha2-256
821: .It
822: hmac-sha2-512
823: .It
824: umac-64@openssh.com
825: .It
826: umac-128@openssh.com
827: .It
828: hmac-md5-etm@openssh.com
829: .It
830: hmac-md5-96-etm@openssh.com
831: .It
832: hmac-ripemd160-etm@openssh.com
833: .It
834: hmac-sha1-etm@openssh.com
835: .It
836: hmac-sha1-96-etm@openssh.com
837: .It
838: hmac-sha2-256-etm@openssh.com
839: .It
840: hmac-sha2-512-etm@openssh.com
841: .It
842: umac-64-etm@openssh.com
843: .It
844: umac-128-etm@openssh.com
845: .El
846: .Pp
1.52 jmc 847: The default is:
1.77 jmc 848: .Bd -literal -offset indent
1.152 markus 849: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
850: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.173 naddy 851: umac-64@openssh.com,umac-128@openssh.com,
852: hmac-sha2-256,hmac-sha2-512
1.77 jmc 853: .Ed
1.60 dtucker 854: .It Cm Match
1.61 jmc 855: Introduces a conditional block.
1.65 dtucker 856: If all of the criteria on the
1.60 dtucker 857: .Cm Match
1.65 dtucker 858: line are satisfied, the keywords on the following lines override those
859: set in the global section of the config file, until either another
1.60 dtucker 860: .Cm Match
1.65 dtucker 861: line or the end of the file.
1.172 djm 862: If a keyword appears in multiple
863: .Cm Match
1.177 sobrado 864: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 865: applied.
1.91 djm 866: .Pp
1.61 jmc 867: The arguments to
1.60 dtucker 868: .Cm Match
1.163 dtucker 869: are one or more criteria-pattern pairs or the single token
870: .Cm All
871: which matches all criteria.
1.60 dtucker 872: The available criteria are
873: .Cm User ,
1.69 dtucker 874: .Cm Group ,
1.60 dtucker 875: .Cm Host ,
1.139 dtucker 876: .Cm LocalAddress ,
877: .Cm LocalPort ,
1.60 dtucker 878: and
879: .Cm Address .
1.91 djm 880: The match patterns may consist of single entries or comma-separated
881: lists and may use the wildcard and negation operators described in the
1.161 jmc 882: PATTERNS section of
1.92 djm 883: .Xr ssh_config 5 .
1.91 djm 884: .Pp
885: The patterns in an
886: .Cm Address
887: criteria may additionally contain addresses to match in CIDR
1.93 jmc 888: address/masklen format, e.g.\&
1.91 djm 889: .Dq 192.0.2.0/24
890: or
891: .Dq 3ffe:ffff::/32 .
892: Note that the mask length provided must be consistent with the address -
893: it is an error to specify a mask length that is too long for the address
1.93 jmc 894: or one with bits set in this host portion of the address.
895: For example,
1.91 djm 896: .Dq 192.0.2.0/33
897: and
1.93 jmc 898: .Dq 192.0.2.0/8
1.91 djm 899: respectively.
900: .Pp
1.60 dtucker 901: Only a subset of keywords may be used on the lines following a
902: .Cm Match
903: keyword.
904: Available keywords are
1.142 jmc 905: .Cm AcceptEnv ,
1.99 okan 906: .Cm AllowAgentForwarding ,
1.142 jmc 907: .Cm AllowGroups ,
1.62 dtucker 908: .Cm AllowTcpForwarding ,
1.141 markus 909: .Cm AllowUsers ,
1.149 djm 910: .Cm AuthenticationMethods ,
1.146 djm 911: .Cm AuthorizedKeysCommand ,
912: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 913: .Cm AuthorizedKeysFile ,
1.123 djm 914: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 915: .Cm Banner ,
1.85 djm 916: .Cm ChrootDirectory ,
1.141 markus 917: .Cm DenyGroups ,
918: .Cm DenyUsers ,
1.67 dtucker 919: .Cm ForceCommand ,
1.142 jmc 920: .Cm GatewayPorts ,
1.141 markus 921: .Cm GSSAPIAuthentication ,
1.87 djm 922: .Cm HostbasedAuthentication ,
1.123 djm 923: .Cm HostbasedUsesNameFromPacketOnly ,
1.74 jmc 924: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 925: .Cm KerberosAuthentication ,
1.95 dtucker 926: .Cm MaxAuthTries ,
1.94 dtucker 927: .Cm MaxSessions ,
1.72 dtucker 928: .Cm PasswordAuthentication ,
1.97 djm 929: .Cm PermitEmptyPasswords ,
1.66 dtucker 930: .Cm PermitOpen ,
1.79 dtucker 931: .Cm PermitRootLogin ,
1.164 djm 932: .Cm PermitTTY ,
1.123 djm 933: .Cm PermitTunnel ,
1.174 djm 934: .Cm PermitUserRC ,
1.107 dtucker 935: .Cm PubkeyAuthentication ,
1.159 dtucker 936: .Cm RekeyLimit ,
1.142 jmc 937: .Cm RhostsRSAAuthentication ,
1.141 markus 938: .Cm RSAAuthentication ,
1.66 dtucker 939: .Cm X11DisplayOffset ,
1.101 djm 940: .Cm X11Forwarding
1.60 dtucker 941: and
1.102 djm 942: .Cm X11UseLocalHost .
1.33 dtucker 943: .It Cm MaxAuthTries
944: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 945: connection.
946: Once the number of failures reaches half this value,
947: additional failures are logged.
948: The default is 6.
1.90 djm 949: .It Cm MaxSessions
950: Specifies the maximum number of open sessions permitted per network connection.
951: The default is 10.
1.1 stevesk 952: .It Cm MaxStartups
953: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 954: SSH daemon.
1.1 stevesk 955: Additional connections will be dropped until authentication succeeds or the
956: .Cm LoginGraceTime
957: expires for a connection.
1.156 dtucker 958: The default is 10:30:100.
1.1 stevesk 959: .Pp
960: Alternatively, random early drop can be enabled by specifying
961: the three colon separated values
962: .Dq start:rate:full
1.51 jmc 963: (e.g. "10:30:60").
1.53 jmc 964: .Xr sshd 8
1.1 stevesk 965: will refuse connection attempts with a probability of
966: .Dq rate/100
967: (30%)
968: if there are currently
969: .Dq start
970: (10)
971: unauthenticated connections.
972: The probability increases linearly and all connection attempts
973: are refused if the number of unauthenticated connections reaches
974: .Dq full
975: (60).
976: .It Cm PasswordAuthentication
977: Specifies whether password authentication is allowed.
978: The default is
979: .Dq yes .
980: .It Cm PermitEmptyPasswords
981: When password authentication is allowed, it specifies whether the
982: server allows login to accounts with empty password strings.
983: The default is
984: .Dq no .
1.62 dtucker 985: .It Cm PermitOpen
986: Specifies the destinations to which TCP port forwarding is permitted.
987: The forwarding specification must be one of the following forms:
988: .Pp
989: .Bl -item -offset indent -compact
990: .It
991: .Cm PermitOpen
992: .Sm off
993: .Ar host : port
994: .Sm on
995: .It
996: .Cm PermitOpen
997: .Sm off
998: .Ar IPv4_addr : port
999: .Sm on
1000: .It
1001: .Cm PermitOpen
1002: .Sm off
1003: .Ar \&[ IPv6_addr \&] : port
1004: .Sm on
1005: .El
1006: .Pp
1.68 dtucker 1007: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1008: An argument of
1009: .Dq any
1010: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1011: An argument of
1012: .Dq none
1013: can be used to prohibit all forwarding requests.
1.63 jmc 1014: By default all port forwarding requests are permitted.
1.1 stevesk 1015: .It Cm PermitRootLogin
1.38 jmc 1016: Specifies whether root can log in using
1.1 stevesk 1017: .Xr ssh 1 .
1018: The argument must be
1019: .Dq yes ,
1020: .Dq without-password ,
1.52 jmc 1021: .Dq forced-commands-only ,
1.1 stevesk 1022: or
1023: .Dq no .
1024: The default is
1025: .Dq yes .
1026: .Pp
1027: If this option is set to
1.52 jmc 1028: .Dq without-password ,
1.1 stevesk 1029: password authentication is disabled for root.
1030: .Pp
1031: If this option is set to
1.52 jmc 1032: .Dq forced-commands-only ,
1.1 stevesk 1033: root login with public key authentication will be allowed,
1034: but only if the
1035: .Ar command
1036: option has been specified
1037: (which may be useful for taking remote backups even if root login is
1.17 jmc 1038: normally not allowed).
1039: All other authentication methods are disabled for root.
1.1 stevesk 1040: .Pp
1041: If this option is set to
1.52 jmc 1042: .Dq no ,
1.38 jmc 1043: root is not allowed to log in.
1.46 reyk 1044: .It Cm PermitTunnel
1045: Specifies whether
1046: .Xr tun 4
1047: device forwarding is allowed.
1.47 reyk 1048: The argument must be
1049: .Dq yes ,
1.58 stevesk 1050: .Dq point-to-point
1051: (layer 3),
1052: .Dq ethernet
1053: (layer 2), or
1.47 reyk 1054: .Dq no .
1.58 stevesk 1055: Specifying
1056: .Dq yes
1057: permits both
1058: .Dq point-to-point
1059: and
1060: .Dq ethernet .
1.46 reyk 1061: The default is
1062: .Dq no .
1.178 djm 1063: .Pp
1064: Independent of this setting, the permissions of the selected
1065: .Xr tun 4
1066: device must allow access to the user.
1.164 djm 1067: .It Cm PermitTTY
1068: Specifies whether
1.165 jmc 1069: .Xr pty 4
1.164 djm 1070: allocation is permitted.
1071: The default is
1072: .Dq yes .
1.6 markus 1073: .It Cm PermitUserEnvironment
1074: Specifies whether
1075: .Pa ~/.ssh/environment
1.9 stevesk 1076: and
1.6 markus 1077: .Cm environment=
1078: options in
1079: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1080: are processed by
1.52 jmc 1081: .Xr sshd 8 .
1.6 markus 1082: The default is
1083: .Dq no .
1.9 stevesk 1084: Enabling environment processing may enable users to bypass access
1085: restrictions in some configurations using mechanisms such as
1086: .Ev LD_PRELOAD .
1.174 djm 1087: .It Cm PermitUserRC
1088: Specifies whether any
1089: .Pa ~/.ssh/rc
1090: file is executed.
1091: The default is
1092: .Dq yes .
1.1 stevesk 1093: .It Cm PidFile
1.4 stevesk 1094: Specifies the file that contains the process ID of the
1.53 jmc 1095: SSH daemon.
1.1 stevesk 1096: The default is
1097: .Pa /var/run/sshd.pid .
1098: .It Cm Port
1099: Specifies the port number that
1.52 jmc 1100: .Xr sshd 8
1.1 stevesk 1101: listens on.
1102: The default is 22.
1103: Multiple options of this type are permitted.
1104: See also
1105: .Cm ListenAddress .
1106: .It Cm PrintLastLog
1107: Specifies whether
1.52 jmc 1108: .Xr sshd 8
1.36 jaredy 1109: should print the date and time of the last user login when a user logs
1110: in interactively.
1.1 stevesk 1111: The default is
1112: .Dq yes .
1113: .It Cm PrintMotd
1114: Specifies whether
1.52 jmc 1115: .Xr sshd 8
1.1 stevesk 1116: should print
1117: .Pa /etc/motd
1118: when a user logs in interactively.
1119: (On some systems it is also printed by the shell,
1120: .Pa /etc/profile ,
1121: or equivalent.)
1122: The default is
1123: .Dq yes .
1124: .It Cm Protocol
1125: Specifies the protocol versions
1.52 jmc 1126: .Xr sshd 8
1.5 stevesk 1127: supports.
1.1 stevesk 1128: The possible values are
1.52 jmc 1129: .Sq 1
1.1 stevesk 1130: and
1.52 jmc 1131: .Sq 2 .
1.1 stevesk 1132: Multiple versions must be comma-separated.
1133: The default is
1.109 jmc 1134: .Sq 2 .
1.5 stevesk 1135: Note that the order of the protocol list does not indicate preference,
1136: because the client selects among multiple protocol versions offered
1137: by the server.
1138: Specifying
1139: .Dq 2,1
1140: is identical to
1141: .Dq 1,2 .
1.1 stevesk 1142: .It Cm PubkeyAuthentication
1143: Specifies whether public key authentication is allowed.
1144: The default is
1145: .Dq yes .
1146: Note that this option applies to protocol version 2 only.
1.159 dtucker 1147: .It Cm RekeyLimit
1148: Specifies the maximum amount of data that may be transmitted before the
1149: session key is renegotiated, optionally followed a maximum amount of
1150: time that may pass before the session key is renegotiated.
1151: The first argument is specified in bytes and may have a suffix of
1152: .Sq K ,
1153: .Sq M ,
1154: or
1155: .Sq G
1156: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1157: The default is between
1158: .Sq 1G
1159: and
1160: .Sq 4G ,
1161: depending on the cipher.
1162: The optional second value is specified in seconds and may use any of the
1163: units documented in the
1164: .Sx TIME FORMATS
1.160 jmc 1165: section.
1.159 dtucker 1166: The default value for
1167: .Cm RekeyLimit
1168: is
1169: .Dq default none ,
1170: which means that rekeying is performed after the cipher's default amount
1171: of data has been sent or received and no time based rekeying is done.
1172: This option applies to protocol version 2 only.
1.118 djm 1173: .It Cm RevokedKeys
1.154 djm 1174: Specifies revoked public keys.
1.118 djm 1175: Keys listed in this file will be refused for public key authentication.
1176: Note that if this file is not readable, then public key authentication will
1177: be refused for all users.
1.154 djm 1178: Keys may be specified as a text file, listing one public key per line, or as
1179: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1180: .Xr ssh-keygen 1 .
1.161 jmc 1181: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1182: .Xr ssh-keygen 1 .
1.1 stevesk 1183: .It Cm RhostsRSAAuthentication
1184: Specifies whether rhosts or /etc/hosts.equiv authentication together
1185: with successful RSA host authentication is allowed.
1186: The default is
1187: .Dq no .
1188: This option applies to protocol version 1 only.
1189: .It Cm RSAAuthentication
1190: Specifies whether pure RSA authentication is allowed.
1191: The default is
1192: .Dq yes .
1193: This option applies to protocol version 1 only.
1194: .It Cm ServerKeyBits
1195: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 1196: The minimum value is 512, and the default is 1024.
1.175 millert 1197: .It Cm StreamLocalBindMask
1198: Sets the octal file creation mode mask
1199: .Pq umask
1200: used when creating a Unix-domain socket file for local or remote
1201: port forwarding.
1202: This option is only used for port forwarding to a Unix-domain socket file.
1203: .Pp
1204: The default value is 0177, which creates a Unix-domain socket file that is
1205: readable and writable only by the owner.
1206: Note that not all operating systems honor the file mode on Unix-domain
1207: socket files.
1208: .It Cm StreamLocalBindUnlink
1209: Specifies whether to remove an existing Unix-domain socket file for local
1210: or remote port forwarding before creating a new one.
1211: If the socket file already exists and
1212: .Cm StreamLocalBindUnlink
1213: is not enabled,
1214: .Nm sshd
1215: will be unable to forward the port to the Unix-domain socket file.
1216: This option is only used for port forwarding to a Unix-domain socket file.
1217: .Pp
1218: The argument must be
1219: .Dq yes
1220: or
1221: .Dq no .
1222: The default is
1223: .Dq no .
1.1 stevesk 1224: .It Cm StrictModes
1225: Specifies whether
1.52 jmc 1226: .Xr sshd 8
1.1 stevesk 1227: should check file modes and ownership of the
1228: user's files and home directory before accepting login.
1229: This is normally desirable because novices sometimes accidentally leave their
1230: directory or files world-writable.
1231: The default is
1232: .Dq yes .
1.112 djm 1233: Note that this does not apply to
1234: .Cm ChrootDirectory ,
1235: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1236: .It Cm Subsystem
1.51 jmc 1237: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1238: Arguments should be a subsystem name and a command (with optional arguments)
1239: to execute upon subsystem request.
1.80 djm 1240: .Pp
1.1 stevesk 1241: The command
1242: .Xr sftp-server 8
1243: implements the
1244: .Dq sftp
1245: file transfer subsystem.
1.80 djm 1246: .Pp
1247: Alternately the name
1248: .Dq internal-sftp
1249: implements an in-process
1250: .Dq sftp
1251: server.
1252: This may simplify configurations using
1253: .Cm ChrootDirectory
1254: to force a different filesystem root on clients.
1255: .Pp
1.1 stevesk 1256: By default no subsystems are defined.
1257: Note that this option applies to protocol version 2 only.
1258: .It Cm SyslogFacility
1259: Gives the facility code that is used when logging messages from
1.53 jmc 1260: .Xr sshd 8 .
1.1 stevesk 1261: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1262: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1263: The default is AUTH.
1.27 markus 1264: .It Cm TCPKeepAlive
1265: Specifies whether the system should send TCP keepalive messages to the
1266: other side.
1267: If they are sent, death of the connection or crash of one
1268: of the machines will be properly noticed.
1269: However, this means that
1270: connections will die if the route is down temporarily, and some people
1271: find it annoying.
1272: On the other hand, if TCP keepalives are not sent,
1273: sessions may hang indefinitely on the server, leaving
1274: .Dq ghost
1275: users and consuming server resources.
1276: .Pp
1277: The default is
1278: .Dq yes
1279: (to send TCP keepalive messages), and the server will notice
1280: if the network goes down or the client host crashes.
1281: This avoids infinitely hanging sessions.
1282: .Pp
1283: To disable TCP keepalive messages, the value should be set to
1284: .Dq no .
1.118 djm 1285: .It Cm TrustedUserCAKeys
1286: Specifies a file containing public keys of certificate authorities that are
1.120 djm 1287: trusted to sign user certificates for authentication.
1.119 jmc 1288: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1289: .Ql #
1290: are allowed.
1291: If a certificate is presented for authentication and has its signing CA key
1292: listed in this file, then it may be used for authentication for any user
1293: listed in the certificate's principals list.
1294: Note that certificates that lack a list of principals will not be permitted
1295: for authentication using
1296: .Cm TrustedUserCAKeys .
1.161 jmc 1297: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1298: .Xr ssh-keygen 1 .
1.18 markus 1299: .It Cm UseDNS
1300: Specifies whether
1.52 jmc 1301: .Xr sshd 8
1.40 jmc 1302: should look up the remote host name and check that
1.18 markus 1303: the resolved host name for the remote IP address maps back to the
1304: very same IP address.
1305: The default is
1306: .Dq yes .
1.1 stevesk 1307: .It Cm UseLogin
1308: Specifies whether
1309: .Xr login 1
1310: is used for interactive login sessions.
1311: The default is
1312: .Dq no .
1313: Note that
1314: .Xr login 1
1315: is never used for remote command execution.
1316: Note also, that if this is enabled,
1317: .Cm X11Forwarding
1318: will be disabled because
1319: .Xr login 1
1320: does not know how to handle
1321: .Xr xauth 1
1.15 jmc 1322: cookies.
1323: If
1.1 stevesk 1324: .Cm UsePrivilegeSeparation
1325: is specified, it will be disabled after authentication.
1326: .It Cm UsePrivilegeSeparation
1327: Specifies whether
1.52 jmc 1328: .Xr sshd 8
1.2 stevesk 1329: separates privileges by creating an unprivileged child process
1.15 jmc 1330: to deal with incoming network traffic.
1331: After successful authentication, another process will be created that has
1332: the privilege of the authenticated user.
1333: The goal of privilege separation is to prevent privilege
1.1 stevesk 1334: escalation by containing any corruption within the unprivileged processes.
1335: The default is
1336: .Dq yes .
1.134 djm 1337: If
1338: .Cm UsePrivilegeSeparation
1339: is set to
1340: .Dq sandbox
1341: then the pre-authentication unprivileged process is subject to additional
1342: restrictions.
1.137 djm 1343: .It Cm VersionAddendum
1344: Optionally specifies additional text to append to the SSH protocol banner
1345: sent by the server upon connection.
1346: The default is
1347: .Dq none .
1.1 stevesk 1348: .It Cm X11DisplayOffset
1349: Specifies the first display number available for
1.52 jmc 1350: .Xr sshd 8 Ns 's
1.1 stevesk 1351: X11 forwarding.
1.52 jmc 1352: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1353: The default is 10.
1354: .It Cm X11Forwarding
1355: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1356: The argument must be
1357: .Dq yes
1358: or
1359: .Dq no .
1.1 stevesk 1360: The default is
1361: .Dq no .
1.13 stevesk 1362: .Pp
1363: When X11 forwarding is enabled, there may be additional exposure to
1364: the server and to client displays if the
1.52 jmc 1365: .Xr sshd 8
1.13 stevesk 1366: proxy display is configured to listen on the wildcard address (see
1367: .Cm X11UseLocalhost
1.52 jmc 1368: below), though this is not the default.
1.13 stevesk 1369: Additionally, the authentication spoofing and authentication data
1370: verification and substitution occur on the client side.
1371: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1372: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1373: forwarding (see the warnings for
1374: .Cm ForwardX11
1375: in
1.19 jmc 1376: .Xr ssh_config 5 ) .
1.13 stevesk 1377: A system administrator may have a stance in which they want to
1378: protect clients that may expose themselves to attack by unwittingly
1379: requesting X11 forwarding, which can warrant a
1380: .Dq no
1381: setting.
1382: .Pp
1383: Note that disabling X11 forwarding does not prevent users from
1384: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1385: X11 forwarding is automatically disabled if
1386: .Cm UseLogin
1387: is enabled.
1388: .It Cm X11UseLocalhost
1389: Specifies whether
1.52 jmc 1390: .Xr sshd 8
1.1 stevesk 1391: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1392: the wildcard address.
1393: By default,
1.52 jmc 1394: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1395: hostname part of the
1396: .Ev DISPLAY
1397: environment variable to
1398: .Dq localhost .
1.8 stevesk 1399: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1400: However, some older X11 clients may not function with this
1401: configuration.
1402: .Cm X11UseLocalhost
1403: may be set to
1404: .Dq no
1405: to specify that the forwarding server should be bound to the wildcard
1406: address.
1407: The argument must be
1408: .Dq yes
1409: or
1410: .Dq no .
1411: The default is
1412: .Dq yes .
1413: .It Cm XAuthLocation
1.11 stevesk 1414: Specifies the full pathname of the
1.1 stevesk 1415: .Xr xauth 1
1416: program.
1417: The default is
1418: .Pa /usr/X11R6/bin/xauth .
1419: .El
1.55 jmc 1420: .Sh TIME FORMATS
1.53 jmc 1421: .Xr sshd 8
1.1 stevesk 1422: command-line arguments and configuration file options that specify time
1423: may be expressed using a sequence of the form:
1424: .Sm off
1.7 stevesk 1425: .Ar time Op Ar qualifier ,
1.1 stevesk 1426: .Sm on
1427: where
1428: .Ar time
1429: is a positive integer value and
1430: .Ar qualifier
1431: is one of the following:
1432: .Pp
1433: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1434: .It Aq Cm none
1.1 stevesk 1435: seconds
1436: .It Cm s | Cm S
1437: seconds
1438: .It Cm m | Cm M
1439: minutes
1440: .It Cm h | Cm H
1441: hours
1442: .It Cm d | Cm D
1443: days
1444: .It Cm w | Cm W
1445: weeks
1446: .El
1447: .Pp
1448: Each member of the sequence is added together to calculate
1449: the total time value.
1450: .Pp
1451: Time format examples:
1452: .Pp
1453: .Bl -tag -width Ds -compact -offset indent
1454: .It 600
1455: 600 seconds (10 minutes)
1456: .It 10m
1457: 10 minutes
1458: .It 1h30m
1459: 1 hour 30 minutes (90 minutes)
1460: .El
1461: .Sh FILES
1462: .Bl -tag -width Ds
1463: .It Pa /etc/ssh/sshd_config
1464: Contains configuration data for
1.53 jmc 1465: .Xr sshd 8 .
1.1 stevesk 1466: This file should be writable by root only, but it is recommended
1467: (though not necessary) that it be world-readable.
1468: .El
1.19 jmc 1469: .Sh SEE ALSO
1470: .Xr sshd 8
1.1 stevesk 1471: .Sh AUTHORS
1472: OpenSSH is a derivative of the original and free
1473: ssh 1.2.12 release by Tatu Ylonen.
1474: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1475: Theo de Raadt and Dug Song
1476: removed many bugs, re-added newer features and
1477: created OpenSSH.
1478: Markus Friedl contributed the support for SSH
1479: protocol versions 1.5 and 2.0.
1480: Niels Provos and Markus Friedl contributed support
1481: for privilege separation.
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