Network Working Group Bill Manning
Expires: May 2006 Paul Vixie
16 November 2005
DISCOVER: Supporting Multicast DNS Queries
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This document describes the DISCOVER opcode, an experimental
extension to the Domain Name System (DNS) to use multicast queries
for resource discovery. A client multicasts a DNS query using the
DISCOVER opcode and processes the multiple responses that may
In the standard Domain Name System (DNS)  , queries are always
unicast using the QUERY opcode. The TBDS research project, funded
under DARPA grant F30602-99-1-0523, explored the use of multicast
DNS  queries for resource discovery. Multicast queries may
return multiple replies, while the standard DNS QUERY operation 
expects a single reply. Instead of extending the QUERY
opcode, the project developed and tested a new query operation,
DISCOVER, that is designed to accommodate multiple responses from a
multicast query. This memo documents the processing rules for
DISCOVER, for possible incorporation in a future revision of the DNS
2. DISCOVER Processing Rules
A requester will send a DISCOVER query message to a multicast
destination address, with some particular multicast scope. The
requester must be prepared to receive multiple replies from multiple
responders, although we expect that there will be a single reply
DISCOVER responses (i.e., response messages from DISCOVER queries)
have standard Answer, Authority, and Additional sections. For
example, the DISCOVER response is the same as the response to a
QUERY operation. Zero-content answers should not be sent, to avoid
badly formed or unfulfilled requests. Responses should be sent to
th unicast address of the requester, and the source address should
reflect the unicast address of the responder. DISCOVER responses
may echo the request's Question section or leave it blank, just as
DISCOVER works like QUERY, except:
1. The Question section of a DISCOVER operation contains
<QNAME=zonename,QTYPE=SOA> tuples, if the section is
Within TBDS, this structure was augmented with:
<QNAME=service,QTYPE=SRV>. While this worked, it would be
cleaner to ask the SRV question in a separate pass, and any
future work should take this into consideration.
2. If QDCOUNT equals 0, then only servers willing to do recursion
should answer; other servers must silently discard a DISCOVER
request with QDCOUNT equals 0.
3. if QDCOUNT is not equal to 0, then only servers that are
authoritative for the zones named by some QNAME should answer.
Hence, replies to DISCOVER queries will always be authoritative or
else have RA (Recursion Available) set.
3. Using DISCOVER Queries
3.1 Performing Host Lookups
To perform a hostname lookup using DISCOVER, a node could:
o Compute the zone name of the enclosing in-addr.arpa, ip6.int, or
o DISCOVER whether any in-scope server(s) are authoritative for
If so, query these authoritative servers for local
o If not, DISCOVER whether there are recursive servers available.
If so, query these recursive servers for local
The requester can determine from the replies whether there are
any DNS servers that are authoritative (or support recursion)
for the zone.
o Once the host's FQDN is known, repeat the process to
discover the closest enclosing authoritative server for
this local name.
o Cache all NS and A data learned in this process, respecting TTL's.
3.2 Performing Service Lookups
To lookup a service name using DISCOVER, the following steps may be
o Use DISCOVER as outlines in Section 3.1 to perform
gethostbyaddr() and then gethostbyname() on one's own
link-local address. This gives a list of local authoritative
o Assume that the closest enclosing zone for which an
authoritative server responds to an in-scope DISCOVER message is
this host's "parent domain", and compute the SRV name as
This is a change to the definition as defined in RFC 1034 .
A wildcard label ("*") in the QNAME used in a DNS message with
op-code DISCOVER should be evaluated with special rules: the
wildcardshould match any label for which the DNS server data is
authoritative. For example 'x.*.example.com.' would match
'x.y.example.com.' and 'x.yy.example.com.', provided that the
server was authoritative for 'example.com.'
o Finally, send a SRV query for this SRV name to the discovered
local authoritative servers, to complete the getservbyname() call.
This call returns a structure that can be populated by response
values, as follows:
s_name The name of the service, "_service" without the
s_aliases The names returned in the SRV RRs in replies
to the query.
s_port The port number in the SRV RRs replies to the
query. If these port numbers disagree - one
of the port numbers is chosen, and only those
names which correspond are returned.
s_proto The transport protocol from named by the
"_transport" label, without the preceding
3.3 Using DISCOVER for Disconnected Names
DISCOVER allows discovery of a host (for example, a printer offering
LPD services) whose DNS server answers authoritatively for a domain
name that hasn't been delegated to it, but is defined within some
local scope. Since DISCOVER is explicitly defined to discover
undelegated zones for tightly-scoped queries, this behavior isn't a
violation of DNS's coherency principles. Note that a responder to
DISCOVER might not be traditional DNS software, it could be
DISCOVER usage for disconnected networks with no authoritative
servers can be achieved using the following conditions.
o Hosts run a "stub server" that acts as though its FQDN
were a zone name.
o The computed SOA gives the host's FQDN as the MNAME, "." as
the ANAME, seconds-since-1Jan2000 as the SERIAL, and low
constants for EXPIRE and the other SOA timers.
o NS is used as the host's FQDN.
o The glue is computed as the host's link-local address, or
hosts may run a "DNS stub server" that acts as though its
FQDN were a zone name.
The rules governing the behavior of this stub server are given
elsewhere  .
Such stub servers should answer DISCOVER packets for its zone, and
will be found by the iterative "discover closest enclosing authority
server" by DISCOVER clients, in either the gethostbyname() or SRV
cases described above. Note that stub servers answer only with
zone names which exactly match QNAME's, not with zone names which
are owned by QNAME's.
4. IANA Considerations
The IANA will need to assign a numeric value for the DISCOVER opcode.
5. Security Considerations
No new security considerations are known to be introduced with a new
DNS query operation. However, using multicast for service discovery
has the potential for denial of service from flooding attacks. It
may also be possible to enable deliberate misconfiguration of
clients simply by running a malicious DNS server that falsely claims
to be authoritative for delegations. One possible way to mitigate
this threat is to use credentials, such as CERT resource records
within an RR set. The TBDS project took this approach. TBDS did
not directly utilize DNSSEC and so possible interactions with
DNSSEC aware/capable servers are unknown.
This material was generated in discussions on the mdns maili
list hosted by Zocalo in March 2000 and updated by discussions in
September/October 2003 on a closed mailing list. David Lawrence,
Scott Rose, Stuart Cheshire, Bill Woodcock, Erik Guttman were
active contributors. Suzanne Woolf was part of the original
implementation team and an invaluable sanity checker.
 Esibov, L., Aboba, B., Thaler, D., "Multicast DNS",
Work in Progress, November 2000.
 Woodcock, B., Manning, B., "Multicast Domain Name Service",
Work in Progress, August 2000.
 Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
 Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION",
RFC 1035, November 1987.
 QUERY opcode -- defined in section 3.7, 4.3, and section 5 of RFC
1034 and in section 4.1.1 of RFC 1035.
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