Internet Engineering Task Force C. Perkins
INTERNET DRAFT Sun Microsystems
24 April 1997
DHCP Options for Service Location Protocol
draft-ietf-dhc-slp-02.txt
Status of This Memo
This document is a submission to the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF). Comments
should be submitted to the dhcp-v4@bucknell.edu mailing list.
Distribution of this memo is unlimited.
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Abstract
The Dynamic Host Configuration Protocol provides a framework for
passing configuration information to hosts on a TCP/IP network.
Entities using the Service Location Protocol need to find out the
address of Directory Agents in order to transact messages. In
certain other instances they may need to discover the correct scope
to be used in conjunction with the service attributes which are
exchanged using the Service Location Protocol.
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1. Introduction
The Dynamic Host Configuration Protocol [2] provides a framework
for passing configuration information to hosts on a TCP/IP network.
Entities using the Service Location Protocol [3] need to find out
the address of Directory Agents in order to transact messages. In
certain other instances they may need to discover the correct scope
to be used in conjunction with the service attributes which are
exchanged using the Service Location Protocol.
The scope MAY be denoted in any standardized character set. Values
for character encoding can be found in IANA's database
http://www.isi.edu/in-notes/iana/assignments/character-sets
and have the values referred by the MIBEnum value. Note that in some
character sets, each character may require two or more octets of data
for its representation.
Note that each option listed below MAY be included multiple times in
the same DHCPOFFER or DHCPREQUEST. If so, then the options SHOULD be
included in order of decreasing preference.
2. Directory Agent Option
This option requests or specifies a Directory Agent (DA), along with
zero or more scopes supported by that DA.
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 |D|F|M|S| rsv | DA Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Directory Agent (variable length) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Char Encoding | Service Scope (variable length)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code 78
Length (variable) The length of the option.
D If the 'D' bit is set, the Directory Agent field is
present.
F If the 'F' bit is set, the Directory Agent is indicated
by including its variable length host name or Fully
Qualified Domain Name (FQDN) instead of its 4 octet IP
address.
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M If the 'M' bit is set, the Directory Agent address is
the only one that may be used, and multicast methods for
discovering Directory Agents MUST NOT be used.
S If the 'S' bit is set, the scope is present, encoded in
the indicated character set.
rsv reserved; ignored upon reception; MUST be sent as zero
DA Length The length (in octets) of the Directory Agent field.
Directory Agent
The Fully Qualified Domain Name (FQDN), host name, or IP
address of the Directory Agent.
Char Encoding
The standardized encoding for the characters denoting the
scope.
scope The characters denoting the scope.
In order to simplify administration of the configuration of Directory
Agents for Service Location Protocol clients, the Directory Agent
can be indicated by presenting its FQDN or host name instead of its
IP address. This allows renumbering to proceed more smoothly [1].
When the FQDN or host name is used, the server sets the 'F' bit. The
host name can be distinguished from the FQDN by the presence of a '.'
character. In any case, the DA length field is set to be the length
of the Directory Agent field. When the 'F' bit is not set, the DA
Length MUST be 4.
Note that more than one Directory Agent option may be present in a
DHCP message. Each such option may have the same or different scope.
The client may request any Directory Agent with a particular scope,
by including the Directory Agent option in a DHCP Request message
with no Directory Agent address included (the 'D' bit set to zero),
and the characters denoting the scope. The length of the scope is
only indicated implicitly by the overall length of the option.
3. Service Scope Option
This option indicates a scope that should be used by a Service Agent
(SA) [3], when responding to Service Request messages as specified by
the Service Location Protocol.
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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Char Encoding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Scope ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code 79
Length (variable) The length of the option.
Char Encoding
The standardized encoding for the characters denoting the
scope.
scope the characters denoting the scope.
Note that more than one Service Scope option may be present in a DHCP
message. The length of the scope is only indicated implicitly by the
overall length of the option.
4. Security Considerations
If a malicious host is able to insert fraudulent information in
DHCPOFFER packets sent to a prospective client of the Service
Location Protocol, then the client will be unable to obtain service,
and vulnerable to disclosing information to unauthorized service
agents. Likewise, a service agent would find that it might rely on
fraudulent or otherwise malicious directory agents to advertise its
services. Many opportunities for denial of service exist.
This difficulty is inherited from the much larger and more serious
problem, viz. securing or authenticating any information whatsoever
from a DHCP server (or client!) is not possible in common DHCP
deployments.
5. Acknowledgements
Thanks to Erik Guttman for his helpful suggestions in the creation of
this draft.
References
[1] B. Carpenter and Y. Rekhter. Renumbering needs work. RFC 1900,
February 1996.
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[2] Ralph Droms. Dynamic Host Configuration Protocol. RFC 1541,
October 1993.
[3] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol, April 1997. draft-ietf-svrloc-protocol-17.txt
(work in progress).
Author's Address
Questions about this memo can be directed to:
Charles E. Perkins
Sun Microsystems
2550 Garcia Avenue
Mountain View, CA 94043
Phone: +1 415 336 7153
Fax: +1 415 336 0670
EMail: charliep@acm.org
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