Indicating Resolver Support of DNSSEC
Network Working Group D. Conrad
Request for Comments: 3225 Nominum, Inc.
Category: Standards Track December 2001
Indicating Resolver Support of DNSSEC
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 (C) The Internet Society (2001). All Rights Reserved.
In order to deploy DNSSEC (Domain Name System Security Extensions)
operationally, DNSSEC aware servers should only perform automatic
inclusion of DNSSEC RRs when there is an explicit indication that the
resolver can understand those RRs. This document proposes the use of
a bit in the EDNS0 header to provide that explicit indication and
describes the necessary protocol changes to implement that
DNSSEC [RFC2535] has been specified to provide data integrity and
authentication to security aware resolvers and applications through
the use of cryptographic digital signatures. However, as DNSSEC is
deployed, non-DNSSEC-aware clients will likely query DNSSEC-aware
servers. In such situations, the DNSSEC-aware server (responding to
a request for data in a signed zone) will respond with SIG, KEY,
and/or NXT records. For reasons described in the subsequent section,
such responses can have significant negative operational impacts for
the DNS infrastructure.
This document discusses a method to avoid these negative impacts,
namely DNSSEC-aware servers should only respond with SIG, KEY, and/or
NXT RRs when there is an explicit indication from the resolver that
it can understand those RRs.
For the purposes of this document, "DNSSEC security RRs" are
considered RRs of type SIG, KEY, or NXT.
Conrad Standards Track [Page 1]
RFC 3225 Indicating Resolver Support of DNSSEC December 2001
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 [RFC2119].
Initially, as DNSSEC is deployed, the vast majority of queries will
be from resolvers that are not DNSSEC aware and thus do not
understand or support the DNSSEC security RRs. When a query from
such a resolver is received for a DNSSEC signed zone, the DNSSEC
specification indicates the nameserver must respond with the
appropriate DNSSEC security RRs. As DNS UDP datagrams are limited to
512 bytes [RFC1035], responses including DNSSEC security RRs have a
high probability of resulting in a truncated response being returned
and the resolver retrying the query using TCP.
TCP DNS queries result in significant overhead due to connection
setup and teardown. Operationally, the impact of these TCP queries
will likely be quite detrimental in terms of increased network
traffic (typically five packets for a single query/response instead
of two), increased latency resulting from the additional round trip
times, increased incidences of queries failing due to timeouts, and
significantly increased load on nameservers.
In addition, in preliminary and experimental deployment of DNSSEC,
there have been reports of non-DNSSEC aware resolvers being unable to
handle responses which contain DNSSEC security RRs, resulting in the
resolver failing (in the worst case) or entire responses being
ignored (in the better case).
Given these operational implications, explicitly notifying the
nameserver that the client is prepared to receive (if not understand)
DNSSEC security RRs would be prudent.
Client-side support of DNSSEC is assumed to be binary -- either the
client is willing to receive all DNSSEC security RRs or it is not
willing to accept any. As such, a single bit is sufficient to
indicate client-side DNSSEC support. As effective use of DNSSEC
implies the need of EDNS0 [RFC2671], bits in the "classic" (non-EDNS
enhanced DNS header) are scarce, and there may be situations in which
non-compliant caching or forwarding servers inappropriately copy data
from classic headers as queries are passed on to authoritative
servers, the use of a bit from the EDNS0 header is proposed.
An alternative approach would be to use the existence of an EDNS0
header as an implicit indication of client-side support of DNSSEC.
This approach was not chosen as there may be applications in which
EDNS0 is supported but in which the use of DNSSEC is inappropriate.
Conrad Standards Track [Page 2]
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