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RFC 2485 PROPOSED STANDARD AUTHENTICATION · DHCP · CONFIGURATION

DHCP Option for The Open Group's User Authentication Protocol

A DHCP option that provides URLs for User Authentication Protocol servers.

On rfc-editor.org ↗
PUBLISHED
January 1999
AUTHORS
S. Drach
STREAM
IETF
DHCP OPTION CODE
98
DEFAULT HTTP/HTTPS PORTS
80 and 443
DEFAULT SERVICE PATH
/uap
DOI
10.17487/RFC2485

RFC text

Contents
Original text · RFC Editor View on rfc-editor.org ↗ 
Network Working Group                                           S. Drach
Request for Comments: 2485                              Sun Microsystems
Category: Standards Track                                   January 1999



     DHCP Option for The Open Group's User Authentication Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

   This document defines a DHCP [1] option that contains a list of
   pointers to User Authentication Protocol servers that provide user
   authentication services for clients that conform to The Open Group
   Network Computing Client Technical Standard [2].

Introduction

   The Open Group Network Computing Client Technical Standard, a product
   of The Open Group's Network Computing Working Group (NCWG), defines a
   network computing client user authentication facility named the User
   Authentication Protocol (UAP).

   UAP provides two levels of authentication, basic and secure.  Basic
   authentication uses the Basic Authentication mechanism defined in the
   HTTP 1.1 [3] specification.  Secure authentication is simply basic
   authentication encapsulated in an SSLv3 [4] session.

   In both cases, a UAP client needs to obtain the IP address and port
   of the UAP service.  Additional path information may be required,
   depending on the implementation of the service.  A URL [5] is an
   excellent mechanism for encapsulation of this information since many
   UAP servers will be implemented as components within legacy HTTP/SSL
   servers.

   Most UAP clients have no local state and are configured when booted
   through DHCP.  No existing DHCP option [6] has a data field that
   contains a URL.  Option 72 contains a list of IP addresses for WWW
   servers, but it is not adequate since a port and/or path can not be
   specified.  Hence there is a need for an option that contains a list
   of URLs.

User Authentication Protocol Option

   This option specifies a list of URLs, each pointing to a user
   authentication service that is capable of processing authentication
   requests encapsulated in the User Authentication Protocol (UAP).  UAP
   servers can accept either HTTP 1.1 or SSLv3 connections.  If the list
   includes a URL that does not contain a port component, the normal
   default port is assumed (i.e., port 80 for http and port 443 for
   https).  If the list includes a URL that does not contain a path
   component, the path /uap is assumed.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |    Length     |   URL list
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Code            98

      Length          The length of the data field (i.e., URL list) in
                      bytes.

      URL list        A list of one or more URLs separated by the ASCII
                      space character (0x20).

References

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

   [2]  Technical Standard: Network Computing Client, The Open Group,
        Document Number C801, October 1998.

   [3]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
        Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
        2068, January 1997.

   [4]  Freier, A., Karlton, P., and P. Kocher, "The SSL Protocol,
        Version 3.0", Netscape Communications Corp., November 1996.
        Standards Information Base, The Open Group,
        http://www.db.opengroup.org/sib.htm#SSL_3.

   [5]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
        Resource Locators (URL)", RFC 1738, December 1994.

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

Security Considerations

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

   The User Authentication Protocol does not have a means to detect
   whether or not the client is communicating with a rogue
   authentication service that the client contacted because it received
   a forged or otherwise compromised UAP option from a DHCP service
   whose security was compromised.  Even secure authentication does not
   provide relief from this type of attack.  This security exposure is
   mitigated by the environmental assumptions documented in the Network
   Computing Client Technical Standard.

Author's Address

   Steve Drach
   Sun Microsystems, Inc.
   901 San Antonio Road
   Palo Alto, CA 94303

   Phone: (650) 960-1300
   EMail: drach@sun.com

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.

ISO Plain Language

On this page
PROBLEMA UAP client configured through DHCP needs an authentication-service address that can include a port and path, not only a server IP address.
MECHANISMThe DHCP option carries one or more URLs for UAP services, separated by ASCII spaces.
THE SECURITY CATCHClients can receive a list of authentication-service locations, but DHCP does not authenticate the source of that list and a forged option can redirect a client.

§1Locating a UAP Authentication Service

The User Authentication Protocol (UAP) lets a client authenticate using a basic mechanism or a secure form carried in an SSLv3 session. In either case, the client needs the service's IP address and port, and some implementations may also need a path.

Many UAP clients have little local configuration and receive settings through DHCP when they boot. Existing DHCP options could provide web-server addresses but not a full URL with port and path, so the RFC defines an option carrying URLs.

A client needs to know where its authentication service is located. This DHCP option can provide the full URL, including a port or path when needed.

§1The URL List Option

The option contains one or more URLs for servers that process UAP authentication requests. URLs are separated by the ASCII space character, and the option uses code 98.

If a URL omits its port, the default is 80 for HTTP or 443 for HTTPS. If it omits its path, the assumed path is /uap.

The option gives the client a list of service URLs. The RFC defines default ports and a default path when those parts are left out.

§1A Forged DHCP Option Can Mislead a Client

The RFC notes that DHCP provides no authentication or security mechanism in the described setting. A forged or compromised option could direct a client to a rogue UAP service.

The document says that even secure UAP authentication does not protect against this kind of redirection. Its discussion points to environmental assumptions in the referenced Network Computing Client standard as the mitigation.

A client cannot rely on this option to prove that a server is trustworthy. If an attacker changes the DHCP response, the client may contact a rogue authentication service.