Internet Engineering Task Force C. Perkins
INTERNET DRAFT E. Guttman
Sun Microsystems
09 October 1998
DHCP Options for Service Location Protocol
draft-ietf-dhc-slp-04.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
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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 [3] provides a framework
for passing configuration information to hosts on a TCP/IP network.
Entities using the Service Location Protocol, Version 2 [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.
The SLP Directory Agent option is used to configure User Agents and
Service Agents with the location of Directory Agents in the network.
These Directory Agents are assumed to support all of the scopes
supplied by the SLP Scope Option.
If the SLP Scope Option is absent, the scope string "default" is
used instead. If there is a scope string configured using local
configuration on the host that is used if no SLP Scope Option has
been sent. DHCP configuration takes precedence over the local
configuration of SLP scope lists. SLP Agents (be they Directory
Agents, User Agents or Service Agents) which use the SLP Directory
Agent Option MUST be configured with a scope.
3. SLP Directory Agent Option
This option specifies the location of one or more SLP Directory
Agents.
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| Code = 78 | Length | a1 | a2 |
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| a3 | a4 | a1 | ...
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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 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.
SLPv2 Service Agents which are configured using the SLP Directory
Agent Option MUST send a SrvRqst to the DAs in the DHCPOFFER. This
SLPv2 SrvRqst sets the Scope List to the value configured by the SLP
Scope Option if one was sent or "DEFAULT" otherwise. The service
type for the request is "service:directory-agent" and the predicate
is omitted. The reply will include the DA's attributes, scope list.
The SA MUST register all service with the DA which it advertises
which are advertised in one or more of the scopes in the DA's scope
list. The SA MUST register no faster than the "min-lifetime" and not
slower than the "max-lifetime" attributes of the DA. These attributes
are obtained in the DAAdvert solicited by the SA after it receives
the SLP Directory Agent Option DHCPOFFER.
SLPv2 User Agents which are configured using the SLP Directory Agent
Option MUST send their requests to the DAs listed, using the entire
list of scopes they are configured with.
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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| Code = 79 | Length | <Typed Scope List> String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length indicates the number of bytes which follow. Since the
Scope-List String is encoded using UTF8 characters, it may be the
cast that the Length is not the same as the number of characters in
the Scope-List String.
The minimum length is 0, and the maximum length is 256. This imposes
a limit on the size of the Scope-List String which can be delivered
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which does not exist in SLP. DHCP administrators will therefore have
to be careful to not configure very long scope names or very long
lists of scopes for any Agents in their network.
The Typed Scope List includes a list of scopes for the SLP Agent.
The list may be a list of scopes, such as "north,south,east,west".
The list may also include items which are 'typed scopes.' These
items indicate that SAs MUST advertise particular service types in
a scope other than that given in the scope list. UAs MUST issue
requests for services of these types in the scopes listed, or some
subset of the scopes. DAs ignore typed scopes in the SLP Service
Scope Option.
For example, "default,(service:printer:lpr=Math Department)"
indicates that SAs advertise all services in scope default except
for lpr printers, which are advertised in the Math Department scope.
UAs will request all services using the "default" scope, except lpr
printers, which are advertised in the "Math Department" scope. DAs
will simply be configured in the scope "default" since they ignore
the typed scope argument.
The grammar for the typed scope list is:
ts-list = ts-item / ts-item `,' ts-list
ts-item = scope-list / `(' srv-type `=' scope-list `)'
srv-type = ALPHA *srv-safe [ `.' 1*srv-safe ]
srv-safe = ALPHA / DIGIT / `+' / `-'
scope-list = scope-item / scope-item "," scope-list
scope-item = 1*safe-scope
safe-scope = ; Any character except rsvd-scope.
; Reserved characters must be escaped.
rsvd-scope = `(' / `)' / `,' / `\' / `!' / `<' / `=' / `>' / `~'
CTL / `;' / `*' / `+'
escaped = `$\backslash$' HEXDIGIT HEXDIGIT
This grammar follows ABNF rules [2]. Reserved characters used in
scope names must be escaped. Reserved characters in <srv-type> names
are not allowed. Note that <srv-type> names may include a Naming
Authority extension.
4.1. Zero Length Scope-List String Configuration
A SLP Service Scope Option which indicates a Length of 0 configures
the SLP Agent to use "User Selectable Scopes".
If this is done, the SLP Agent MUST NOT be configured using the SLP
Directory Agent Option.
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Instead, the SLP Agent will discover scopes using Directory Agent
discovery (or Service Agent Discovery) as defined in [4].
The SLP Agent will then use the aggregation of all scopes it
discovers on the network to configure its own scope list.
Note that this configuration is tantamount to removing all
centralized control of the configuration 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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References
[1] S. Bradner. Key Words for Use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[2] D. Crocker and P. Overell. Augmented BNF for Syntax
Specifications: ABNF. RFC 2234, November 1997.
[3] R. Droms. Dynamic Host Configuration Protocol. RFC 2131, March
1997.
[4] E. Guttman, C. Perkins, J. Veizades, and M. Day. Service
Location Protocol version 2. draft-ietf-svrloc-protocol-v2-04.txt,
March 1998. (work in progress).
Author's Address
Questions about this memo can be directed to:
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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