rfc2563.txt

RFC 的详细文档！

                          |               |               |
                          |      Graceful shutdown        |
                          |               |               |
                          |               |               |
                          v               v               v


2.3.  DHCP Server Behavior

   When a DHCP server receives a DHCPDISCOVER, it MUST be processed as
   described in [DHCP], section 4.3.1.  However, if no address is chosen
   for the host, a few additional steps MUST be taken.

   If the DHCPDISCOVER does not contain the Auto-Configure option, it is
   not answered.

   If the DHCPDISCOVER contains the Auto-Configure option, and the site
   administrator has specified that Auto-Configuration should be
   disabled on the subnet the DHCPDISCOVER is originating from, or for
   the client originating the request, then a DHCPOFFER MUST be sent to
   the DHCP client.  This offer MUST be for the address 0x00000000, and
   the Auto-Configure option MUST be set to "DoNotAutoConfigure".





Troll                       Standards Track                     [Page 5]

RFC 2563             DHCP Auto-Configuration Option             May 1999


   If the site administrator allows auto-configuration on the
   originating subnet, the DHCPDISCOVER is not answered as before.

2.4.  Mixed Environments

   Environments containing a mixture of clients and servers that do and
   do not support the Auto-Configure option will not be a problem.
   Every DHCP transaction is between a Server and a Client, and the
   possible mixed scenarios between these two are listed below.

2.4.1.  Client Supports, Server Does Not

   If a DHCP client sends a request that contains the Auto-Configure
   tag, a DHCP server that does not know what this tag is will respond
   normally.  According to [DHCP] Section 4.3.1, the server MUST NOT
   return a value for that parameter.

   In this case, the server will either respond with a valid DHCPOFFER,
   or it will not respond at all.  In both cases, a DHCP client that
   supports this option will never care what the state of the option is,
   and may auto-configure.

2.4.2.  Servers Supports, Client Does Not

   If the Auto-Configure option is not present in the DHCPDISCOVER, the
   server will do nothing about it.  The client will auto-configure if
   it doesn't receive a response and believes that's what it should do.

   This scenario SHOULD not occur, as any stacks that implement an
   auto-configuration mechanism MUST implement this option as well.

2.5.  Interaction With Other DHCP Messages

   As this option only affects the initial IP address selection, it does
   not apply to subsequent DHCP messages.  If the DHCP client received a
   lease from a DHCP server, future DHCP messages (RENEW, INFORM, ACK,
   etc.) have no need to fall over into an auto- configuration state.

   If the DHCP client's lease expires, the client falls back into the
   INIT state, and the initial DHCPDISCOVER is sent as before.

2.5.1.  DHCPRELEASE Messages

   DHCPRELEASEs occur exactly as described in [DHCP], section 4.4.6.
   When a DHCP client is done with a lease, it MAY notify the server
   that it is finished.  For this to occur, the DHCP client already
   received a DHCP lease, and the state of Auto-Configuration on the
   local wire does not matter.



Troll                       Standards Track                     [Page 6]

RFC 2563             DHCP Auto-Configuration Option             May 1999


2.5.2.  DHCPDECLINE Messages

   A DHCPDECLINE is sent by the DHCP client when it determines the
   network address it is attempting to use is already in use.  As a
   network address has been tested, it must have been offered by the
   DHCP Server, and the state of Auto-Configuration on the local wire
   does not matter.

2.5.3.  DHCPINFORM Messages

   DHCPINFORMs should be handled as described in [DHCP], section 4.4.3.
   No changes are necessary.

2.6.  Message Option

   If the DHCP server would like to tell a client why it is not allowed
   to auto-configure, it MAY add the Message option to the response.
   This option is defined in [DHCPOPT], Section 9.9.

   If the DHCP client receives a response with the Message option set,
   it MUST provide this information to the administrator of the DHCP
   client.  How this information is provided is implementation
   dependant.

3.  Security Considerations

   DHCP per se currently provides no authentication or security
   mechanisms.  Potential exposures to attack are discussed in section 7
   of the DHCP protocol specification [DHCP].

   This mechanism does add one other potential attack.  Malicious users
   on a subnet may respond to all DHCP requests with responses telling
   DHCP clients that they should NOT auto-configure on the local wire.
   On a network where Auto-Configuration is required, this will cause
   all DHCP clients to not choose an address.

4.  Acknowledgments

   This idea started at a joint Common Solutions Group / Microsoft
   meeting at Microsoft in May, 1998.  The IP stacks in Win98 and NT5
   assign themselves an IP address (in a specific subnet) in the absence
   of a responding DHCP server, and this is causing headaches for many
   sites that actually rely on machines not getting IP addresses when
   the DHCP servers do not know them.

   Walter Wong proposed a solution that would allow the DHCP servers to
   tell clients not to do this.  His initial solution would not work
   without slight modifications to DHCP itself.  This document describes



Troll                       Standards Track                     [Page 7]

RFC 2563             DHCP Auto-Configuration Option             May 1999


   those modifications.

5.  IANA Considerations

   The IANA has assigned option number 116 for this option.

6.  References

   [DHCP]     Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [DHCPOPT]  Alexander, S. and  R. Droms, "DHCP  Options  and  BOOTP
              Vendor Extension", RFC 2132, March 1997.

   [IPv6SAC]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [KEYWORDS] Bradner, S., "Key  words  for  use  in  RFCs  to  Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

7.  Author's Address

   Ryan Troll
   @Home Network
   425 Broadway
   Redwood City, CA 94063

   Phone: [phone]
   EMail: [e-mail]






















Troll                       Standards Track                     [Page 8]

RFC 2563             DHCP Auto-Configuration Option             May 1999


8.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















Troll                       Standards Track                     [Page 9]