Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV
Using DNS SRV records to find SLP Directory Agents in other domains so services there can be discovered.
Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C., and W. Jerome, "Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV", RFC 3832, DOI 10.17487/RFC3832, July 2004, <https://www.rfc-editor.org/info/rfc3832>.
@misc{rfc3832,
series = {Request for Comments},
number = 3832,
howpublished = {RFC 3832},
publisher = {RFC Editor},
doi = {10.17487/RFC3832},
url = {https://www.rfc-editor.org/info/rfc3832},
author = {W. Zhao and H. Schulzrinne and E. Guttman and C. Bisdikian and W. Jerome},
title = {{Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV}},
pagetotal = 8,
year = 2004,
month = jul,
} RFC text
Contents
Network Working Group W. Zhao
Request for Comments: 3832 H. Schulzrinne
Category: Experimental Columbia University
E. Guttman
Sun Microsystems
C. Bisdikian
W. Jerome
IBM
July 2004
Remote Service Discovery in the
Service Location Protocol (SLP) via DNS SRV
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
Remote service discovery refers to discovering desired services in
given remote (i.e., non-local) DNS domains. This document describes
remote service discovery in the Service Location Protocol (SLP) via
DNS SRV. It defines the DNS SRV Resource Records for SLP Directory
Agent services, discusses various issues in using SLP and DNS SRV
together for remote service discovery, and gives the steps for
discovering desired services in remote DNS domains.
1. Introduction
This document describes remote service discovery in the Service
Location Protocol (SLP) [RFC2608] via DNS SRV [RFC2782]. We consider
remote service discovery as discovering desired services in given
remote DNS domains, and local service discovery as discovering
desired services within the local administrative domain.
SLP provides a scalable framework for local service discovery and
selection. In SLP, User Agents (UAs) discover desired services in
the local administrative domain by querying all Service Agents (SAs)
via multicast or querying a Directory Agent (DA) via unicast. To
query a DA using unicast, a UA needs to first learn about the DA via
DHCP, static configuration or multicast (listening for DAAdvert
multicast or issuing DA discovery SrvRqst multicast).
DNS SRV provides good support for remote service discovery. However,
if multiple servers are discovered via DNS SRV for a service, only
priority and weight can be used to make a selection. If additional
service properties (such as cost, speed and service quality) need to
be considered in the selection process, DNS SRV becomes insufficient.
We propose that using SLP and DNS SRV together can provide better
support for remote service discovery. First, a UA uses DNS SRV to
find SLP DAs at a remote DNS domain. Then, the UA uses SLP to query
one of those DAs to discover desired services. In this way, we can
avoid the limitations in using SLP and DNS SRV separately. On one
hand, without DNS SRV, an SLP UA needs to depend on static
configuration to learn about remote DAs because DHCP and multicast DA
discovery are not generally applicable beyond the local
administrative domain. On the other hand, without SLP, DNS SRV has
limited support for service selection.
In this document, we first define the DNS SRV Resource Records (RRs)
for SLP DA services, which are used to map a given DNS domain to
remotely accessible (i.e., accessible from the Internet) DAs in that
domain. Then, we discuss various issues in using SLP and DNS SRV
together for remote service discovery. Finally, we give the steps
for discovering services in remote DNS domains.
1.1. Notation Conventions
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 BCP 14, RFC 2119
[RFC2119].
2. The DNS SRV RRs for SLP DA services
According to RFC 2782 [RFC2782], the DNS SRV RRs for SLP DA services
are defined as follows:
_slpda._Proto.Name TTL Class SRV Priority Weight Port Target
where "slpda" is the symbolic name for SLP DA services, the Proto
field is either "tcp" or "udp", and the Target field is the domain
name of an SLP DA. Please refer to [RFC2782] for detailed
explanation of each field in DNS SRV RRs.
Next we show an example of using DNS SRV RRs to map a given DNS
domain to remotely accessible DAs in that domain. To discover
remotely accessible DAs in a remote domain (say, example.com), a UA
makes a DNS query [RFC1034,RFC1035] for QNAME=_slpda._tcp.example.com
(or QNAME=_slpda._udp.example.com), QCLASS=IN, and QTYPE=SRV. Then
the UA will receive a list of DNS SRV RRs in a DNS reply, which gives
all remotely accessible DAs in the domain example.com, such as:
;; Priority Weight Port Target
_slpda._tcp.example.com IN SRV 0 0 427 da1.example.com
_slpda._tcp.example.com IN SRV 0 0 427 da2.example.com
3. Remote Service Discovery in SLP via DNS SRV
SLP DAs can be discovered in two ways: (1) using the mechanisms
described in RFC 2608, and (2) using DNS SRV RRs as described in this
document. The second approach is useful for UAs to acquire service
information for remote DNS domains. For example, a mobile node
visiting a network (without the use of mobile IP) may want to obtain
information about services in its home network.
3.1. The DNS Domain of Interest for Remote Service Discovery
Using DNS SRV RRs to discover SLP DAs requires knowledge of the
domain where SLP DAs are registered. For remote service discovery,
it is assumed that the DNS domain of interest is known via a priori
knowledge. For example, a UA is configured with a domain name or the
user provides the domain name manually.
Note that there is no implied "search order" of DNS domains in
finding remote DAs. For instance, if a UA is looking for remote DAs
in the domain foo.bar.example.com, it SHOULD only look for
_slp._tcp.foo.bar.example.com (or _slp._udp.foo.bar.example.com), and
MUST NOT fall back to look for _slp._tcp.bar.example.com,
_slp._tcp.example.com, and so on.
3.2. SLP DAs for Remote Service Discovery
A UA discovers desired services in a given remote DNS domain by
unicasting requests to DAs in that domain. The UA uses remote DAs
according to these prioritized rules: (1) using DAs which it has been
configured with, and (2) using DAs which it has discovered via DNS
SRV.
3.3. SLP Scopes for Remote Service Discovery
As SLP scopes are intended to be used only within one administrative
domain, they are likely incomprehensible to users outside of the
administrative domain. Thus, any remotely accessible service MUST be
registered in the "default" scope, but it MAY be registered in other
scopes at the same time. Similarly, all DAs advertised via DNS SRV
MUST serve the "default" scope, but they MAY serve other scopes at
the same time. As a result, users wishing to discover services at a
remote DNS domain SHOULD only search the "default" scope.
4. Steps for Remote Service Discovery
The steps for a User Agent U to discover desired services in a remote
DNS domain D are as follows.
(1) U makes a DNS query for QNAME=_slpda._tcp.D (or
QNAME=_slpda._udp.D), QCLASS=IN, and QTYPE=SRV. Then, U gets a
list of DNS SRV RRs (referred to as L) in a DNS reply, which
gives all remotely accessible DAs in D.
(2) U selects a DA X from L based on the priority and weight
information per RFC 2782.
(3) U queries X in the "default" scope to discover desired services
in D.
Note that the services discovered in the above steps may not
necessarily be remotely accessible.
5. Security Considerations
To support remote service discovery, a domain exposes its service
information to the Internet. Standard SLP authentication SHOULD be
used to protect valuable service information. First, there is a risk
that any SA could advertise any service on a DA accessible from the
Internet. Such a DA SHOULD reject all registrations and
deregistrations that cannot be authenticated. Secondly, to avoid
disclosing unintended service information to remote users, a DA
accessible from the Internet SHOULD at least authenticate service
queries that are not in the "default" scope. In addition, the
security considerations for DNS SRV [RFC2782] apply to this document.
Also, the DNS security extensions [RFC 2535] SHOULD be used to
provide origin authentication and integrity protection for DNS data.
6. Applicability Statements
This specification describes remote service discovery in SLP via DNS
SRV. It facilitates discovering services at a remote DNS domain if
the domain name is known via a priori knowledge. However, it does
not intend to solve the problem of Internet-wide service discovery.
Users should be aware of two constraints in using DNS SRV to discover
SLP DAs: (1) they SHOULD only use DNS SRV to discover DAs in the
"default" scope, and (2) an administrator may choose to register only
a subset of all DAs in the "default" scope via DNS SRV. Thus, to
discover local DAs, implementations MUST use the standard SLP
mechanisms per RFC 2608. In addition, implementations supporting
this specification MAY use DNS SRV to discover local DAs in the
"default" scope.
As SLP scopes are not intended to be used outside the local
administrative domain, all remote service discovery in SLP SHOULD be
carried only in the "default" scope.
Note that the services discovered via DNS SRV and remote SLP DAs may
not necessarily be remotely accessible.
7. IANA Considerations
In the DNS SRV RRs for SLP DA services, the symbolic name for the
Service field is "slpda", supported protocols are "tcp" and "udp".
The following values have been registered with IANA:
Service Field Protocol Field Reference
------------- -------------- ---------
slpda tcp [RFC3832]
slpda udp [RFC3832]
8. Acknowledgments
The authors would like to thank Bernard Aboba, Kevin Arnold, Steven
Bellovin, Ted Hardie, James Kempf, Thomas Narten, Erik Nordmark, and
Jon Peterson for their valuable comments.
9. Normative References
[RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2 ", RFC 2608, June 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2535] Eastlake 3rd, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
10. Authors' Addresses
Weibin Zhao
Department of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
EMail: zwb@cs.columbia.edu
Henning Schulzrinne
Department of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
EMail: hgs@cs.columbia.edu
Erik Guttman
Sun Microsystems
Eichhoelzelstr. 7
74915 Waibstadt
Germany
EMail: Erik.Guttman@sun.com
Dr. Chatschik Bisdikian
IBM T. J. Watson Research Center
30 Saw Mill River Road, M/S 3S-B34
Hawthorne, NY 10532, USA
Phone: +1 914 784 7439
Fax: +1 914 784 6225
EMail: bisdik@us.ibm.com
William F. Jerome
IBM Corp.
Thomas J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532, USA
EMail: wfj@us.ibm.com
11. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
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AI ELI5
On this page
§1Local and Remote Service Discovery
The Service Location Protocol (SLP, RFC 2608) lets User Agents (UAs) find services in the local administrative domain, either by multicasting to Service Agents or by asking a unicast Directory Agent (DA). To use a DA, a UA first has to learn about it through DHCP, static configuration or multicast. The last two are not generally applicable beyond the local administrative domain.
Remote service discovery means finding services in a given non-local DNS domain. DNS SRV (RFC 2782) supports that, but when several servers are found it can select only by priority and weight, not by properties like cost or quality. The RFC therefore proposes combining them: use DNS SRV to find a remote DA, then use SLP to query it.
SLP is like asking the people on your street where the nearest printer is. This lets you look up the helper for a faraway street first, then ask that helper.
§2The SRV Record for SLP Directory Agents
The record follows RFC 2782 and has the form _slpda._Proto.Name TTL Class SRV Priority Weight Port Target. Here slpda is the symbolic service name, Proto is tcp or udp, and Target is the domain name of a DA.
To find DAs in example.com, a UA sends a DNS query for _slpda._tcp.example.com (or the _udp form), class IN, type SRV. The reply lists the remotely accessible DAs, for example two targets da1.example.com and da2.example.com on port 427 with priority 0 and weight 0.
The phone book has a special entry type that says "the helpers for this neighborhood are over here".
§3Domain, Scope and Applicability Rules
The remote domain must be known in advance, for instance by configuration or because the user types it in. There is no implied search order: for foo.bar.example.com a UA SHOULD only look under that name and MUST NOT fall back to bar.example.com or example.com.
SLP scopes are meant for one administrative domain and are likely meaningless to outsiders. So any remotely accessible service MUST be registered in the default scope (it MAY be in others too), and every DA advertised through DNS SRV MUST serve the default scope. Users searching a remote domain SHOULD search only the default scope. A UA uses DAs it has been configured with first, then DAs found via DNS SRV.
The RFC says this does not solve Internet-wide service discovery. An administrator may publish only some of the DAs through SRV, so local discovery MUST still use the standard SLP mechanisms, though an implementation MAY also use DNS SRV for local DAs in the default scope.
Outsiders do not know your street's private room names, so the front hall, called the default scope, is the only place visitors are sent.
§4Steps for Remote Discovery
To find services in remote domain D, a User Agent U does three things. It queries DNS for the _slpda SRV records of D, getting a list of remotely accessible DAs. It selects one DA from that list by priority and weight, as RFC 2782 describes. Then it queries that DA in the default scope for the desired services.
The RFC adds a caution: the services found this way are not necessarily remotely accessible.
First look up who the helper is, then pick one, then ask that helper your question.
- SRV query _slpda._tcp.DStep 1: ask for the DAs of domain D.
- List of SRV recordsRemotely accessible DAs in D.
- Service request, default scopeStep 3, after picking one DA by priority and weight.
§5Security Considerations
Supporting remote discovery exposes a domain's service information to the Internet. Standard SLP authentication SHOULD be used. An Internet-accessible DA SHOULD reject registrations and deregistrations it cannot authenticate, since otherwise any Service Agent could advertise any service. It SHOULD also authenticate queries outside the default scope. The security considerations of DNS SRV apply, and the DNS security extensions SHOULD be used for origin authentication and integrity protection of DNS data.
If strangers can look up your helper, the helper should check who is asking and who is adding things to the list.
§7IANA Registration
IANA registered the symbolic service name slpda for both tcp and udp, with this RFC as the reference. That is what makes the _slpda._tcp and _slpda._udp names usable.
The special phone-book entry name was officially reserved so nobody else uses it for something different.