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Versions: 01 02 03 04 05                                                
   Network Working Group
   Internet Draft                             Senthil K Balasubramanian
   Expires: December 2006                                        Intoto
                                                      Michael Alexander
                                                         Gustaf Neumann
                                           Wirtschaftsuniversitaet Wien
                                                              June 2006

                DHCP Option for Proxy Server Configuration

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document defines a new Dynamic Host Configuration Protocol
   DHCP) option, which can be used to configure the TCP/IP host's
   Proxy Server configuration for standard protocols like HTTP,FTP,
   NNTP,SOCKS, Gopher, SLL and etc.  Proxy Server provides controlled
   and efficient access to the Internet by access control mechanism
   for different types of user requests and caching frequently accessed
   information (Web pages and possibly files that might have been

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   downloaded using FTP and other protocols).

1.  Terminologies Used

   DHCP Client: A DHCP [RFC-2131] client is an Internet host that
        uses DHCP to obtain configuration information such as a
        network address.

   DHCP Server: A DHCP server [RFC-2131] is an Internet host that
        returns configuration parameters to DHCP clients.

   Proxy Server: In an enterprise network that connects to Internet, a
        proxy server is a server that acts as an intermediary between a
        workstation user and the Internet so that the enterprise can
        ensure security and administrative control. A Proxy server MAY
        provide caching services or be associated with or part of a
        gateway server that separates the enterprise network from
        the outside network (usually the Internet) and a firewall
        that protects the enterprise network from outside intrusion.

   PAC        :  PAC stands for Proxy Auto-Configuration. It is a file
        that contains information about the proxy servers to be used.
        Most of the browsers support Proxy Auto Configuration File
        format to specify the proxies to be used.

   URI        : Uniform Resource Identifier [RFC-3986]. A Uniform
        Resource Identifier is a formatted string that serves as an
        identifier for a resource, typically on the Internet. The
        formatted string comprises a name or address that can be used
        to refer to a internet resource. URIs in common practice include
        Uniform Resource Locators(URLs). The following are the examples
        of URI:


        In addition to identifying a resource, the URI also provides a
        means of acting upon or obtaining a representation of the
        resource by describing its primary access mechanism. For
        example, http://www.ietf.org is a URI that identifies a resource
        and implies that the resource is accessible through HTTP

2.  Introduction

   The Dynamic Host Configuration Protocol [RFC-2131] provides a
   framework for passing configuration information to hosts on a
   TCP/IP network.  This document describes a DHCP configuration
   option that can be used to inform a DHCP client of the IP

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   addresses and properties of one or more proxy services that are
   either available to it or that must be used in order to access
   internet services, for example through a coporate firewall.

   The following diagram depicts the typical setup of a proxy server
   providing proxy services to clients on a network that is protected
   by a firewall

   +---------------------------+                +-----------+
   |                           |                |Remote HTTP|
   |                           |     HTTP       |Server     |
   |  +------------+        +-------------+<--->+-----------+
   |  | Clients    |        |Proxy Server |
   |  | Inside the |<------>|    +        | FTP +-----------+
   |  | Firewall   |        |Firewall     |<--->|Remote FTP |
   |  +------------+        +-------------+     |Server     |
   |                           |  ^             +-----------+
   |                           |  |
   |                           |  |             +-----------+
   +---------------------------+  |  NNTP       |Remote NNTP|
                                  +------------>|Server     |

   The primary use of proxies is to allow access to the World Wide Web
   from within a firewall. A proxy service typically runs on firewall
   machine. It waits for a request from inside the firewall, forwards
   the request to the remote server outside the firewall, reads the
   response and then sends it back to the client. Usually, all the
   clients use the same proxy within a given network, which helps in
   efficient caching of documents that are requested by a number of
   clients. Similarly, proxies can provide document caching functions
   on the outside Internet.

   A proxy server can increase network security and user productivity
   by filtering content and controlling both internal and external
   access to information. Also, it provides several other
   functionalities that are not discussed here.

3.  Requirements terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in [RFC 2119].

4.  Proxy Server Configuration Option

   This document defines a new DHCP Option called the Proxy Server
   Configuration Option. The format of the Proxy Server configuration
   option is:

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         Code  Len   Proxy Server Configuration Field
         |  TBD |   N  |  i1  | i2   | i3   | i4   |      |  iN  |

   Code is TBD and will be assigned by IANA according to [RFC-2939].
   The length N gives the total number of octets in the proxy server
   configuration field.

   The Proxy server configuration field consists of SubOpt/Length/Value
   tuples for each sub-option, encoded in the following manner:

        SubOpt Len     Sub-option Value
         |  1   |   N  |  s1  |  s2  |  s3  |  s4  |      |  sN  |

        SubOpt Len     Sub-option Value
         |  2   |   N  |  i1  |  i2  |  i3  |  i4  |      |  iN  |

   The length N of the DHCP Proxy Server Information Option shall
   include all bytes of the sub-option code/length/value tuples. The
   length N of the sub-options shall be the number of octets in only
   that sub-option's value field. The sub-option need not appear  in
   sub-option code order. No pad sub-option is defined, and the proxy
   server configuration field shall NOT be terminated with a 255 sub-
   option. The initial assignment of DHCP Proxy Server Sub-options is as

   |DHCP Proxy Server | Sub Option Description |
   |Sub Option Code   |                        |
   |      1           | PAC URI                |
   |      2           | MD5 Digest of PAC URI  |

5.  Proxy Server Configuration Sub Options

5.1   PAC URI Sub Option and MD5 Digest Sub Option of PAC URI

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   The PAC URI Sub Option specifies an URI in the UTF-8[STD-63] format.
   The length of the sub-option is actually depends on the scheme used
   to specify an URI. However, it MUST be restricted to a maximum of 255
   octets. The DHCP Server MAY compute MD5[MD5] digest on the PAC URI
   configured and send the value in MD5 Digest Sub Option(Sub Option 2).
   The DHCP Client on receiving this sub-option MUST compute MD5 digest
   on the URI received in the PAC URI Sub Option and match it with the
   digest present in MD5 Digest Sub Option. If the calculated MD5 digest
   doesn't match with the received MD5 digest, then the DHCP client MUST
   drop this configuration. The PAC URI Sub-Option is as follows:

        SubOpt Len     PAC URI Sub-Option
       |  1  |  N  |  c1 | c2  | c3  |      |  cN  |

        SubOpt Len     MD5 Digest Sub Option
       |  2  |  N  |  c1 | c2  | c3  |      |  cN  |

   The MD5 sub option is OPTIONAL and DHCP Server MAY send it if it
   calculates MD5 digest on the PAC URI.

6.  Option Usage

   The DHCP Proxy server configuration option MUST always carry PAC URI
   Sub Option. However MD5 Digest of PAC URI Sub Option is OPTIONAL.
   This is provided for additional security. If the length of the Proxy
   server configuration option exceeds the maximum permissible within a
   single option (255 octets), then the Configuration Option MUST be
   represented in the DHCP message as specified in [RFC-3396].

7.  Security Considerations

   The DHCP Options defined here allow an intruder DHCP server to
   misdirect a client, causing it to access a nonexistent or
   malicious proxy server. This allows for a denial of service or
   man-in-the-middle attacks. The latter security consideration is
   a well known property of the DCHP protocol; this option does not
   create any additional risk of such attacks.

   DHCP provides an authentication mechanism, as described in
   [RFC-3118], which may be used if authentication is required.

8.  IANA Considerations

   IANA is requested to assign an option code to the Proxy Server
   Configuration Option and protocol numbers for the SSL and RDF

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9.  Normative References

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

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

   [RFC-3396] Lemon, T. and S. Cheshire, "Encoding Long DHCP Options",
                             RFC 3396, November 2002.

   [STD63]    Yergeau, F., "UTF-8, a transformation format of
              ISO 10646", STD 63, RFC 3629, November 2003.

   [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm",
              RFC 1321, April 1992.

10.   Informative References

   [RFC-3118] Droms, R.  and W.  Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC-2939] Droms, R., "Procedures and IANA Guidelines for
              Definition of New DHCP Options and Message Types", BCP 43,
              RFC 2939, September 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 3986,
              January 2005.

11.   Acknowledgments

   Thanks to the DHC Working Group for their time and input into the
   specification. In particular, thanks to (in alphabetical order)
   Bernie Volz, Ralph Droms, Robert Elz, Stig Venaas and Ted Lemon
   for their thorough review.

Author's Addresses

   Senthil Kumar Balasubramanian
   Intoto Software (I) Pvt Ltd.,
   New No 5, Old No 3, First Street,
   Nandanam Extension,
   Chennai, India

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   Phone: +91 44 5211 2783/4/5
   Email: ksenthil@intoto.com

   Michael Alexander
   Wirtschaftsuniversitaet Wien
   Augasse 2-6
   A-1090 Vienna, Austria
   Phone: +43 31336 4467
   Email: malexand@wu-wien.ac.at

   Gustaf Neumann
   Wirtschaftsuniversitaet Wien
   Augasse 2-6
   A-1090 Vienna, Austria
   Phone: +43 31336 4671
   Email: neumann@wu-wien.ac.at

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   This document and the information contained herein are provided on an

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Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
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