RFC3004 - The User Class Option for DHCP
Network Working Group G. Stump
Request for Comments: 3004 IBM
Category: Standards Track R. Droms
Cisco Systems
Y. Gu
R. Vyaghrapuri
A. Demirtjis
Microsoft
B. Beser
Pacific Broadband Communications
J. Privat
Northstream AB
November 2000
The User Class Option for DHCP
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 (2000). All Rights Reserved.
Abstract
This option is used by a Dynamic Host Configuration Protocol (DHCP)
client to optionally identify the type or category of user or
applications it represents. The information contained in this option
is an opaque field that represents the user class of which the client
is a member. Based on this class, a DHCP server selects the
appropriate address pool to assign an address to the client and the
appropriate configuration parameters. This option should be
configurable by a user.
1. IntrodUCtion
DHCP administrators may define specific user class identifiers to
convey information about a client's software configuration or about
its user's preferences. For example, the User Class option can be
used to configure all clients of people in the accounting department
with a different printer than clients of people in the marketing
department.
2. Requirements Terminology
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [3].
3. DHCP Terminology
o "DHCP client"
A DHCP client or "client" is an Internet host using DHCP to oBTain
configuration parameters such as a network address.
o "DHCP server"
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
o "binding"
A binding is a collection of configuration parameters, including at
least an IP address, associated with or "bound to" a DHCP client.
Bindings are managed by DHCP servers.
4. User Class option
This option is used by a DHCP client to optionally identify the type
or category of user or applications it represents. A DHCP server
uses the User Class option to choose the address pool it allocates an
address from and/or to select any other configuration option.
This option is a DHCP option [1, 2].
This option MAY carry multiple User Classes. Servers may interpret
the meanings of multiple class specifications in an implementation
dependent or configuration dependent manner, and so the use of
multiple classes by a DHCP client should be based on the specific
server implementation and configuration which will be used to process
that User class option.
The format of this option is as follows:
Code Len Value
+-----+-----+--------------------- . . . --+
77 N User Class Data ('Len' octets)
+-----+-----+--------------------- . . . --+
where Value consists of one or more instances of User Class Data.
Each instance of User Class Data is formatted as follows:
UC_Len_i User_Class_Data_i
+--------+------------------------ . . . --+
L_i Opaque-Data ('UC_Len_i' octets)
+--------+------------------------ . . . --+
Each User Class value (User_Class_Data_i) is indicated as an opaque
field. The value in UC_Len_i does not include the length field
itself and MUST be non-zero. Let m be the number of User Classes
carried in the option. The length of the option as specified in Len
must be the sum of the lengths of each of the class names plus m:
Len= UC_Len_1 + UC_Len_2 + ... + UC_Len_m + m. If any instances of
User Class Data are present, the minimum value of Len is two (Len =
UC_Len_1 + 1 = 1 + 1 = 2).
The Code for this option is 77.
A server that is not equipped to interpret any given user class
specified by a client MUST ignore it (although it may be reported).
If a server recognizes one or more user classes specified by the
client, but does not recognize one or more other user classes
specified by the client, the server MAY use the user classes it
recognizes.
DHCP clients implementing this option SHOULD allow users to enter one
or more user class values.
5. IANA Considerations
Option 77, which IANA has already assigned for this purpose, should
be used as the User Class Option for DHCP.
6. Security Considerations
DHCP currently provides no authentication or security mechanisms.
Potential eXPosures to attack are discussed is section 7 of the
protocol specification [1].
This lack of authentication mechanism means that a DHCP server cannot
check if a client or user is authorized to use a given User Class.
This introduces an obvious vulnerability when using the User Class
option. For example, if the User Class is used to give out a special
parameter (e.g., a particular database server), there is no way to
authenticate a client and it is therefore impossible to check if a
client is authorized to use this parameter.
7. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC2131, March
1997.
[2] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March 1997.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
8. Acknowledgments
This document is based on earlier drafts by Glenn Stump, Ralph Droms,
Ye Gu, Ramesh Vyaghrapuri and Burcak Beser. Thanks to Ted Lemon,
Steve Gonczi, Kim Kinnear, Bernie Volz, Richard Jones, Barr Hibbs and
Thomas Narten for their comments and suggestions.
9. Authors' Addresses
Glenn Stump
IBM Networking Software
P.O. Box 12195
RTP, NC 27709
Phone: 919 301 4277
EMail: stumpga@us.ibm.com
Ralph Droms
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
Phone: 978 244 4733
EMail: rdroms@cisco.com
Ye Gu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425 936 8601
EMail: yegu@microsoft.com
Ramesh Vyaghrapuri
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425 703 9581
EMail: rameshv@microsoft.com
Burcak Beser
Pacific Broadband Communications
3103 North 1st Street
San Jose, CA 95134
Phone: 408 468 6265
Email: Burcak@pacband.com
Ann Demirtjis
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
Phone: 425 705 2254
EMail: annd@microsoft.com
Jerome Privat
Northstream AB
Espace Beethoven 1
1200 Route des Lucioles
BP 302
06906 Sophia Antipolis Cedex
France
Phone: +33 4 97 23 40 45
Fax: +33 4 97 23 24 51
Mobile: +33 6 13 81 76 71
Email: jerome.privat@northstream.se
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