Network Working Group Glenn Stump, IBM
INTERNET DRAFT Ralph Droms, Bucknell University
Obsoletes: draft-ietf-dhc-userclass-02.txt November 1998
Expires May 1999
The User Class Option for DHCP
<draft-ietf-dhc-userclass-03.txt>
Status of this Memo
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Abstract
This option is used by a DHCP client to optionally identify the type
or category of user or applications it represents. The information
contained in this option is an NVT ASCII text object that represents
the user class of which the client is a member.
1. 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 RFC 2119 [3].
2. 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.
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o "DHCP server"
A DHCP server of "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.
3. User Class Information
This option is used by a DHCP client to optionally identify the type
or category of user or applications it represents. The information
contained in this option is an NVT ASCII text object that represents
the user class of which the client is a member.
This option is a DHCP option [1, 2].
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, an identifier may specify that
a particular DHCP client is a member of the class "accounting
auditors", which have special service needs such as a particular
database server.
Servers not equipped to interpret the user class specified by a
client MUST ignore it (although it may be reported). Otherwise,
servers SHOULD respond with the set of options corresponding to the
user class specified by the client. Further, if the server responds
with the set of options corresponding to the given user class, it
MUST return this option (with the given user class value) to the
client.
Clients which do not receive information for the user class requested
SHOULD make an attempt to operate without it, although they may do so
(and may announce they are doing so) in a degraded mode.
The code for this option is TBD. The minimum length for this option
is two.
Code Len text1
+-----+-----+-----+-----+-----
| TBD | N | c1 | c2 | ...
+-----+-----+-----+-----+-----
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DISCUSSION: Simulating Multiple User Classes
Although the user class option field only permits one NVT string,
the working group envisions that multiple classes can be simulated
by creating combination classes which map into a single class NVT
string. For example, suppose a site desires to create multiple
logical user classes, including:
"mobile" -- These hosts receive short lease times
and are assumed to dynamically update
their own DNS records
"engineer" -- These hosts are assigned a high-
performance NFS file server
For the above two classes, then, a combination class could look
something like:
"mobeng" -- hosts of this mobile-engineer combination
class get assigned a high-performance
file server and a short lease time, and
a DNS proxy A record update is not attempted
on their behalf.
Thus, by mapping combinations of classes into single class names,
you can effectively implement multiple user classing at a site
using only the single NVT string field.
DISCUSSION: Serving Competing Option Values
When servicing a request from a client of a particular user class,
a DHCP server makes decisions about what collection of options to
include in its response. These decisions are expected to consider
options and values designated for the client host by virtue of its
subnet/location, vendor class, user class, or client id.
In cases where multiple option values are possible for return to
the client due to multiple, overlapping "affiliations", DHCP
server policy may dictate which values take precedence over
others. A DHCP server implementation SHOULD provide flexibility
in specifying DHCP option precedence policy so that DHCP
administrators can customize a DHCP system to best suit their
network environments.
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If flexibility in a server's option precedence policy is not
implemented by a vendor (or is perhaps implemented but not
exercised by an administrator), a recommended default policy is
that option values of specific affiliations override those of less
specific. That is, an option value designated for a specific
client -- sometimes known as a "reserved binding" -- SHOULD
override option values designated for the client's user or vendor
class, which SHOULD override option values designated for the
client's vendor class, which SHOULD override option values for the
client's subnet.
4. Security Considerations
DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed is section 7 of the
protocol specification [1].
5. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[2] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119, March 1997.
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6. Author Information
Glenn Stump
IBM Networking Software
P.O. Box 12195
RTP, NC 27709
Phone: (919) 301-4277
email: stumpga@us.ibm.com
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
email: droms@bucknell.edu
7. Expiration
This document will expire on May 31, 1999.
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