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Independent Submission B. Manning
Request for Comments: 6804 November 2012
Category: Historic
ISSN: 2070-1721
DISCOVER: Supporting Multicast DNS Queries
Abstract
This document describes the DISCOVER opcode, an experimental
extension to the Domain Name System (DNS) to use multicast queries
for resource discovery. This opcode was tested in experiments run
during 1995 and 1996 for the Topology Based Domain Search (TBDS)
project. This project is no longer active and there are no current
plans to restart it. TBDS was the first known use of multicast
transport for DNS. A client multicasts a DNS query using the
DISCOVER opcode and processes the multiple responses that may result.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for the historical record.
This document defines a Historic Document for the Internet community.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6804.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
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1. Introduction
The TBDS project developed extensions to existing network services to
enable software for clients and servers of an application to become
more resilient to changes in topology by dynamically sensing changes
and switching between client/server and peer-peer methods for both
end-system-to-server and server-to-server communications.
The first existing network service to be investigated was the Domain
Name Systems (DNS), which is used to map symbolic Internet names to
numeric Internet addresses. Based upon a hierarchical tree
structure, the DNS relies upon uninterrupted connectivity of nodes to
a special set of static, manually configured root servers. To
improve the robustness and availability of the DNS service, TBDS
developed and defined enhancements that enable nodes to map names to
numbers without the need for uninterrupted connectivity to the
Internet root servers. These techniques were automated, allowing
transition between connected and unconnected operations to be done
without direct human intervention.
These enhancements to the DNS server code are based on the open
source BIND to support reception and processing of multicast packets.
Proof-of-concept modifications to BIND 8.1.2 were made to show that
multicast awareness could be added to BIND. An analysis was made of
the existing DNS code deployment and the schedule of new feature
deployment so that we could synchronize TBDS with a more appropriate
code base. Testing identified a race condition due to overloading
the semantics of the DNS opcode that was used to communicate to
servers.
This race condition was explored within the IETF regarding use of
existing DNS opcodes. Discussion within the team and with others in
the IETF led to the idea that we needed a new opcode that would not
overload the semantics of existing opcodes. The original DNS design
specification presumes that few clients exist that would share common
DNS data. To correct this problem, a new opcode was designed to
disambiguate TBDS requests from normal nameserver requests.
In the standard Domain Name System (DNS) [1] [2], queries are always
unicast using the QUERY opcode. The TBDS research project [5],
funded under DARPA grant F30602-99-1-0523, explored the use of
multicast DNS [1] [2] queries for resource discovery by autonomous,
mobile nodes in disconnected networks. The operations model is
covered in the TBDS documentation. Multicast queries may return
multiple replies, while the standard DNS QUERY operation (see
Sections 3.7, 4.3, and 5 of RFC 1034 [1]; and Section 4.1.1 of RFC
1035 [2]) expects a single reply. Instead of extending the QUERY
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opcode, the project developed and tested a new query operation,
DISCOVER, that was designed to accommodate multiple responses from a
multicast query. The ability to accept multiple replies provides a
basis for discrimination of man-in-the-middle attacks, which succeed
by being the first to respond. Use of DISCOVER requires the use of
caching in the receiver, so the ephemeral nature of stub resolvers is
precluded.
This memo documents the processing rules for DISCOVER, for possible
incorporation in a future revision of the DNS specification.
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 [3].
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 per
responder.
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 the
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 for
QUERY.
DISCOVER works like QUERY, with the following exceptions:
1. The Question section of a DISCOVER operation contains
<QNAME=zonename,QTYPE=SOA> tuples, if the section is present.
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; 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.
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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 ip6.arpa domain.
o DISCOVER whether any in-scope server(s) are authoritative for
this zone.
If so, query these authoritative servers for local
in-addr/ip6 names.
o If not, DISCOVER whether there are recursive servers available.
If so, query these recursive servers for local in-addr/ip6
names.
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 Fully Qualified Domain Name (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
Times To Live (TTLs).
3.2. Performing Service Lookups
To lookup a service name using DISCOVER, the following steps may be
used:
o Use DISCOVER as outlined in Section 3.1 to perform
gethostbyaddr() and then gethostbyname() on one's own link-
local address. This gives a list of local authoritative
servers.
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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
_service._transport.*.parentdomain.
This is a change to the definition provided in RFC 1034 [1]. A
wildcard label ("*") in the QNAME used in a DNS message with
the opcode DISCOVER should be evaluated with special rules: the
wildcard should 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 an 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
preceding underscore.
s_aliases The names returned in the SRV Resource Records (RRs)
in replies to the query.
s_port The port number in the SRV RRs replies to the query.
If these port numbers do not match, one of the port
numbers is chosen, and only those names that
correspond are returned.
s_proto The transport protocol passed from the DNS process
using the "_transport" label, without the preceding
underscore.
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 special-
purpose software.
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DISCOVER usage for disconnected networks with no authoritative
servers can be achieved using the following conditions:
o Hosts run a "stub authoritative 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 server consist of a single
change to the method of use, and no change whatsoever to the current
format of DNS packets. Specifically, this extension allows UDP DNS
queries, as documented in RFC 1035, Sections 4.1.1, 4.1.2, and 4.2.1,
to be addressed to port 53 of statically assigned relative offset -4
within the range of multicast addresses defined as "administratively
scoped" by Section 9 of RFC 2365 [6]. Within the full /8 of
administratively scoped addresses, this corresponds to the
destination address 239.255.255.251. Until the Multicast-Scope Zone
Announcement Protocol (MZAP) or a similar protocol is implemented to
allow hosts to discover the extent of the local multicast scopes that
enclose them, it is anticipated that implementations will simply
utilize the destination address 239.255.255.251. Queries sent via
multicast MUST NOT request recursion.
In order to receive multicasted queries, DNS server implementations
MUST listen on the -4 offset to their local scope (as above, in the
absence of a method of determining the scope, this will be assumed to
be relative to the full /8 allocated for administratively scoped
multicast use, or 239.255.255.251) and respond via ordinary unicast
UDP to ONLY those queries for which they have a positive answer that
originated within a locally-configured zone file. That is, a server
MUST NOT answer a multicasted query with cached information that it
received from another server, nor may it request further resolution
from other servers on behalf of a multicasted query. A multicast-
capable server may, however, utilize multicast queries to perform
further resolution on behalf of queries received via ordinary
unicast. This is referred to as "proxy" operation. Multicast-
enabled DNS servers MUST answer multicasted queries non-
authoritatively. That is, when responding to a query that was
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received via multicast, they MUST NOT include an NS record that
contains data that resolves back to their own IP address and MUST NOT
set the AA bit.
Resolvers MUST anticipate receiving no replies to some multicasted
queries, in the event that no multicast-enabled DNS server
implementations are active within the local scope, or in the event
that no positive responses exist to the transmitted query. That is,
a query for the MX record for host.domain.com would go unanswered if
no local server was able to resolve that request, if no MX record
exists for host.domain.com, or if no local servers were capable of
receiving multicast queries. The resolver that initiated the query
MUST treat such non-response as a non-cacheable negative response.
Since this multicast transmission does not provide reliable delivery,
resolvers MAY repeat the transmission of a query in order to assure
themselves that is has been received by any hosts capable of
answering; however, any resolvers that repeat a query MUST increase
the interval by a factor of two between each repetition. It is more
likely, however, that any repeated queries will be performed under
the explicit direction of the application driving the query, rather
than autonomously by the resolver implementation.
It will often be the case that multicast queries will result in
responses from multiple servers. In the event that the multicast
query was generated via a current API such as gethostbyname, or as
the result of a proxy operation, the first response received must be
passed to the requesting application or host, and all subsequently
received responses must be discarded. Future multicast-aware APIs
that use DISCOVER should anticipate receiving multiple independent RR
sets in response to queries and using external heuristics for
selecting the most appropriate RR set.
Such 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
that exactly match QNAME's, not with zone names that are owned by
QNAME's.
4. IANA Considerations
At such time as this idea might be considered for a future addition
to the DNS protocol, IANA would need to assign a value for the
opcode.
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5. Security Considerations
The following paragraph on security considerations was written very
early in the use and exploration of IP multicast and, as such,
represents a fairly naive view on the type and scope of exploits that
are enabled through the use of IP multicast. A more up-to-date
understanding of multicast security considerations may be found in
RFC 5294 [4].
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. How
to scope multicast is not part of the DISCOVER processing rules. 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 DNS Security (DNSSEC), so possible interactions with DNSSEC-
aware/-capable servers are unknown.
6. Acknowledgments
This material was generated in discussions on the mdns mailing 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, and Erik Guttman were
active contributors. Suzanne Woolf was part of the original
implementation team and an invaluable sanity checker. Funding for
the RFC Editor function is currently provided by the Internet
Society.
7. References
7.1. Normative References
[1] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Savola, P. and J. Lingard, "Host Threats to Protocol Independent
Multicast (PIM)", RFC 5294, August 2008.
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7.2. Informative References
[5] Manning, B., "Topology Based Domain Search (TBDS)", Final
Report, June 2002,
<http://www.dtic.mil/docs/citations/ADA407598>.
[6] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365, July 1998.
Authors' Addresses
Bill Manning
PO 12317
Marina del Rey, CA. 90295
United States
EMail: bmanning@sfc.keio.ac.jp
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