Internet Engineering Task Force C. Perkins
INTERNET DRAFT E. Guttman
Sun Microsystems
16 December 1998
DHCP Options for Service Location Protocol
draft-ietf-dhc-slp-06.txt
Status of This Memo
This document is a submission by 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. Another
option provides an assignment of scope for configuration of SLP User
and Service Agents.
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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, Version 2 [3] need to
obtain the address of Directory Agents and Scope configuration. The
Service Location Protocol (SLP) provides a default configuration
for Scopes and Directory Agents may be discovered using multicast
or broadcast. It is useful in a larger deployment to be able
to configure SLP Agents using DHCP, so as to centralize the
administration and to deploy SLP in networks where multicast routing
is not available.
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 [1].
2. Introduction
The DHCP options described below are used to configure Agents using
the Service Location Protocol, Version 2 [3].
The SLP Directory Agent option is used to configure User Agents and
Service Agents with the location of Directory Agents in the network.
The SLP Scope Option takes precedence over both default and static
scope configuration of SLP agents.
3. SLP Directory Agent Option
This option specifies the location of one or more SLP Directory
Agents.
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 = 78 | Length | a1 | a2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| a3 | a4 | a1 | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SLP Directory Agent Option specifies a list of IP addresses
for Directory Agents. Directory Agents MUST be listed in order of
preference, if there is an order of preference.
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The address of the Directory Agent is given in network byte order.
The length of the option MUST always be divisible by 4 and has a
minimum length of 4.
The Directory Agents listed in this option MUST be configured with
the a non-empty subset of the scope list that the Agent receiving the
Directory Agent Option is configured with. See the notes below.
The SLPv2 specification [3] defines how to use this option.
4. SLP Service Scope Option
The scope list is a comma delimited list which indicates the scopes
that a SLP Agent is configured to use.
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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 = 79 | Length | MANDATORY | <Scope List>...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length indicates the number of bytes which follow. Since the
Scope-List String is encoded using UTF-8 [4] characters, it may be
the cast that the Length is not the same as the number of characters
in the Scope-List String. The Length value must include one for the
'MANDATORY' byte.
The 'MANDATORY' byte determines whether SLP Agents override their
static configuration for scopes with the <Scope List> string
provided by the option. This allows DHCP administrators to
implement a policy of assigning a set of scopes to Agents for service
provision. If the MANDATORY byte is zero, static configuration takes
precedence over the DHCP provided scope list. If the MANDATORY byte
is nonzero, the <Scope List> provided in this option MUST be used by
the SLP Agent.
The Scope List String syntax and usage are defined in the SLPv2
specification [3].
4.1. Zero Length Scope-List String Configuration
A SLP Service Scope Option which indicates a Length of 1 (in other
words, omitting the <Scope List> string entirely) validly configures
the SLP User Agent to use "User Selectable Scopes."
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The SLP Agent will use the aggregated list of scopes of all known
DAs. If no DAs are known, the UA will use SA discovery to determine
the list of scopes on the network, as defined in [3].
Note that this configuration is tantamount to removing all
centralized control of the scope configuration of hosts on the
network. This makes it possible for every User Agent to see every
service. This may not be desirable as users may not be able to or
desire to decide which services are appropriate for them.
5. Security Considerations
If a malicious host is able to insert fraudulent information in
DHCPOFFER packets sent to a prospective SLP Agent then the SLP Agent
will be unable to obtain service, or may unwittingly be directed to
use the incorrect services.
Many opportunities for denial of service exist. A service agent
could find that it might rely on fraudulent or otherwise malicious
directory agents to advertise its services. DHCPOFFERs could prevent
the regular SLP framework from functioning by directing clients to
not use multicast, to use nonexistent directory agents and so on.
These difficulties are 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.
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6. Full Copyright Statement
Copyright (C) The Internet Society (1997). 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."
References
[1] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[2] R. Droms. Dynamic Host Configuration Protocol. RFC 2131, March
1997.
[3] E. Guttman, C. Perkins, J. Veizades, and M. Day. Service
Location Protocol version 2. draft-ietf-svrloc-protocol-v2-11.txt,
October 1998. (work in progress).
[4] F. Yergeau. UTF-8, a transformation format of unicode and ISO
10646. RFC 2279, October 1998.
Author's Address
Questions about this memo can be directed to:
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Charles E. Perkins Erik Guttman
Technology Development Group Technology Development Group
Mail Stop MPK15-214 Mail Stop UFRA02
Sun Microsystems, Inc. Sun Microsystems, Inc.
15 Network Circle Bahnstr. 2
Menlo Park, CA 94025 74915 Waibstadt, Germany
phone: +1 650-786-6464 phone: +49 7263 911 701
fax: +1 650-786-6445 or: +1 650 786 5992
email: Charles.Perkins@Sun.Com Erik.Guttman@Sun.Com
Web: http://www.svrloc.org/~charliep
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