DHCP Options for SLP: DHCP Options for Service Location Protocol
Two DHCP options that tell SLP agents where their Directory Agents are and which scopes to use.
Perkins, C. and E. Guttman, "DHCP Options for Service Location Protocol", RFC 2610, DOI 10.17487/RFC2610, June 1999, <https://www.rfc-editor.org/info/rfc2610>.
@misc{rfc2610,
series = {Request for Comments},
number = 2610,
howpublished = {RFC 2610},
publisher = {RFC Editor},
doi = {10.17487/RFC2610},
url = {https://www.rfc-editor.org/info/rfc2610},
author = {C. Perkins and E. Guttman},
title = {{DHCP Options for Service Location Protocol}},
pagetotal = 6,
year = 1999,
month = jun,
} RFC text
Contents
Network Working Group C. Perkins
Request for Comments: 2610 E. Guttman
Category: Standards Track Sun Microsystems
June 1999
DHCP Options for Service Location 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
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.
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] and
Service Location Protocol, Version 1 [4] 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] and Version 1 [4].
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 | Mandatory | a1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| a2 | a3 | a4 | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
The Length value must include one for the 'Mandatory' byte and
include four for each Directory Agent address which follows. Thus,
the Length minus one of the option MUST always be divisible by 4 and
has a minimum value of 5.
The address of the Directory Agent is given in network byte order.
The 'Mandatory' byte in the Directory Agent option may be set to
either 0 or 1. If it is set to 1, the SLP User Agent or Service
Agent so configured MUST NOT employ either active or passive
multicast discovery of Directory Agents.
Note that for backward compatibility with some deployed software the
Mandatory byte MUST NOT be set to any byte value for which the high
order bit (0x80) is set.
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.
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 = 79 | Length | Mandatory | <Scope List>...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length indicates the number of bytes which follow. Since the
Scope-List String is encoded using UTF-8 [5] 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 0, static configuration takes precedence over the
DHCP provided scope list. If the Mandatory byte is 1, 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."
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.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
1997.
[3] E. Guttman, C. Perkins, J. Veizades, and M. Day, "Service
Location Protocol version 2", Work in Progress.
[4] Veizades, J., Guttman, E., Perkins, C. and S. Kaplan, "Service
Location Protocol", RFC 2165, July 1997.
[5] Yergeau, F., "UTF-8, a transformation format of unicode and ISO
10646", RFC 2279, October 1998.
Authors' Addresses
Charles E. Perkins
Technology Development Group
Mail Stop MPK15-214
Sun Microsystems, Inc.
15 Network Circle
Menlo Park, CA 94025
Phone: +1 650-786-6464
Fax: +1 650-786-6445
EMail: Charles.Perkins@Sun.Com
Web: http://www.svrloc.org/~charliep
Erik Guttman
Technology Development Group
Mail Stop UFRA02
Sun Microsystems, Inc.
Bahnstr. 2
74915 Waibstadt, Germany
Phone: +49 7263 911 701
or: +1 650 786 5992
EMail: Erik.Guttman@Sun.Com
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AI ELI5
On this page
§1Why Configure SLP with DHCP
The Service Location Protocol (SLP), in versions 1 and 2, lets programs find network services. SLP agents need the addresses of Directory Agents (DAs) and a scope configuration. SLP has defaults for scopes, and DAs can be found by multicast or broadcast.
The RFC argues that in a larger deployment it is useful to configure SLP agents through DHCP. That centralizes administration and lets SLP be deployed in networks where multicast routing is not available. It defines two DHCP options for the job, which configure agents using both SLP version 2 and version 1.
Instead of every computer shouting to find the service directory, the network just tells it when it joins.
§2The Two Options
The SLP Directory Agent option (code 78) tells User Agents and Service Agents where the Directory Agents are. The SLP Service Scope option (code 79) assigns scopes. The scope option takes precedence over both default and static scope configuration of SLP agents.
Both options carry a Mandatory byte right after the length. It decides how firmly the option binds the agent, as the next sections describe.
There are two notes on the sheet: one with the directory's address, one with the groups you belong to.
§3SLP Directory Agent Option
Option 78 holds a list of IPv4 addresses of Directory Agents, in network byte order. They MUST be listed in order of preference if there is one. The Length counts one byte for Mandatory plus four for each address, so Length minus one MUST be divisible by 4 and the minimum Length is 5.
The Mandatory byte is 0 or 1. If it is 1, the agent MUST NOT use active or passive multicast discovery of Directory Agents. For backward compatibility with some deployed software, the byte MUST NOT have the high-order bit (0x80) set.
The listed DAs MUST be configured with a non-empty subset of the scope list that the receiving agent is configured with. How to use the option is defined in the SLPv2 specification.
It is a list of directory addresses, best one first. A flag can say "do not go looking anywhere else."
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
CODE | Code | 1 byte | 78 | Option code, 78. |
LENGTH | Length | 1 byte | One for Mandatory plus four per address; minimum 5. | |
MANDATORY | Mandatory | 1 byte | 0 or 1; if 1, no multicast discovery of DAs. | |
A1 | a1 | 1 byte | First byte of the first DA address; more addresses may follow. |
§4SLP Service Scope Option
Option 79 carries a comma-delimited scope list, encoded in UTF-8. Because UTF-8 characters can take more than one byte, Length may differ from the number of characters. Length includes one byte for Mandatory.
The Mandatory byte decides whether the DHCP scope list overrides the agent's static configuration. If it is 0, static configuration takes precedence. If it is 1, the scope list in the option MUST be used. This lets DHCP administrators enforce a policy of which scopes agents provide services in. The syntax and usage of the Scope List string are defined in the SLPv2 specification.
Empty scope list
A Length of 1, with no scope string at all, validly configures the User Agent to use "User Selectable Scopes." The agent then uses the combined scope lists of all known DAs, or, if none are known, uses SA discovery to learn the scopes on the network. The RFC warns this is tantamount to removing all centralized control of scopes, so every User Agent can see every service, which may not be desirable.
The note lists which groups you belong to. A flag decides whether this note beats what was already written on your computer.
§5Security Considerations
If a malicious host can insert false information into DHCPOFFER packets sent to an SLP agent, the agent may be unable to get service or may be steered to the wrong services. The RFC lists many chances for denial of service: a service agent may rely on fraudulent Directory Agents, and DHCPOFFERs could tell clients not to use multicast or to use nonexistent Directory Agents.
The RFC says these difficulties come from a larger problem: in common DHCP deployments, securing or authenticating any information from a DHCP server (or client) is not possible.
If someone sneaks a fake note onto the sheet, you might be sent to the wrong place. The network cannot yet tell real notes from fake ones.