DNSSEC DO bit: Indicating Resolver Support of DNSSEC
A bit in an EDNS0 query tells a DNS server whether the resolver can accept DNSSEC records.
Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225, DOI 10.17487/RFC3225, December 2001, <https://www.rfc-editor.org/info/rfc3225>.
@misc{rfc3225,
series = {Request for Comments},
number = 3225,
howpublished = {RFC 3225},
publisher = {RFC Editor},
doi = {10.17487/RFC3225},
url = {https://www.rfc-editor.org/info/rfc3225},
author = {D. Conrad},
title = {{Indicating Resolver Support of DNSSEC}},
pagetotal = 6,
year = 2001,
month = dec,
} RFC text
Contents
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 Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
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
notification.
1. Introduction
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.
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].
2. Rationale
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.
3. Protocol Changes
The mechanism chosen for the explicit notification of the ability of
the client to accept (if not understand) DNSSEC security RRs is using
the most significant bit of the Z field on the EDNS0 OPT header in
the query. This bit is referred to as the "DNSSEC OK" (DO) bit. In
the context of the EDNS0 OPT meta-RR, the DO bit is the first bit of
the third and fourth bytes of the "extended RCODE and flags" portion
of the EDNS0 OPT meta-RR, structured as follows:
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO| Z |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Setting the DO bit to one in a query indicates to the server that the
resolver is able to accept DNSSEC security RRs. The DO bit cleared
(set to zero) indicates the resolver is unprepared to handle DNSSEC
security RRs and those RRs MUST NOT be returned in the response
(unless DNSSEC security RRs are explicitly queried for). The DO bit
of the query MUST be copied in the response.
More explicitly, DNSSEC-aware nameservers MUST NOT insert SIG, KEY,
or NXT RRs to authenticate a response as specified in [RFC2535]
unless the DO bit was set on the request. Security records that
match an explicit SIG, KEY, NXT, or ANY query, or are part of the
zone data for an AXFR or IXFR query, are included whether or not the
DO bit was set.
A recursive DNSSEC-aware server MUST set the DO bit on recursive
requests, regardless of the status of the DO bit on the initiating
resolver request. If the initiating resolver request does not have
the DO bit set, the recursive DNSSEC-aware server MUST remove DNSSEC
security RRs before returning the data to the client, however cached
data MUST NOT be modified.
In the event a server returns a NOTIMP, FORMERR or SERVFAIL response
to a query that has the DO bit set, the resolver SHOULD NOT expect
DNSSEC security RRs and SHOULD retry the query without EDNS0 in
accordance with section 5.3 of [RFC2671].
Security Considerations
The absence of DNSSEC data in response to a query with the DO bit set
MUST NOT be taken to mean no security information is available for
that zone as the response may be forged or a non-forged response of
an altered (DO bit cleared) query.
IANA Considerations
EDNS0 [RFC2671] defines 16 bits as extended flags in the OPT record,
these bits are encoded into the TTL field of the OPT record (RFC2671
section 4.6).
This document reserves one of these bits as the OK bit. It is
requested that the left most bit be allocated. Thus the USE of the
OPT record TTL field would look like
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO| Z |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Acknowledgements
This document is based on a rough draft by Bob Halley with input from
Olafur Gudmundsson, Andreas Gustafsson, Brian Wellington, Randy Bush,
Rob Austein, Steve Bellovin, and Erik Nordmark.
References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
Author's Address
David Conrad
Nominum Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 381 6003
EMail: david.conrad@nominum.com
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ISO Plain Language
On this page
§1Why Resolver Support Must Be Explicit
DNSSEC-aware servers may include SIG, KEY or NXT records when answering for a signed zone. RFC 3225 calls these DNSSEC security records. A resolver that cannot process them should not receive them automatically.
The RFC assumes resolver support is binary: a resolver either accepts all DNSSEC security records or none. It therefore uses one bit to tell the server whether the resolver is prepared to receive them.
A DNSSEC-aware server needs to know whether a resolver can receive DNSSEC records. One flag bit provides that signal.
§2The Cost of Extra DNS Records
Classic DNS over UDP is limited to 512 bytes. Adding DNSSEC records makes responses more likely to be truncated, after which a resolver may retry using TCP. The RFC says this typically takes five packets for a query and response instead of two.
These retries add delay and traffic, increase the chance of timeouts and load nameservers. The RFC also reports that some early non-DNSSEC-aware resolvers failed or ignored responses containing security records.
Security records can make an answer too large for a classic UDP response. The resolver may need a longer TCP exchange, which uses more packets and can take longer.
§3Where the DO Bit Is Located
The DNSSEC OK (DO) bit is the most significant bit of the Z field in the EDNS0 OPT record. The RFC places it in the extended RCODE and flags portion of that record, rather than in the classic DNS header where bits are scarce.
The RFC does not treat the presence of EDNS0 by itself as evidence of DNSSEC support. An application may use EDNS0 for other purposes and still be unable to handle DNSSEC security records.
The DO bit is in the extra EDNS0 record, not the crowded classic DNS header. Using EDNS0 alone does not mean a resolver accepts DNSSEC records.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
EXTENDED-RCODE | Extended RCODE | 1 byte | Extended response code. | |
VERSION | Version | 1 byte | EDNS version. | |
DO | DO | 1 bits | 1 | Resolver can accept DNSSEC security records. |
Z | Z | 15 bits | Remaining flag bits. |
§3How Resolvers and Servers Use DO
When DO is set to one in a query, the resolver can accept DNSSEC security records. When it is zero, the server MUST NOT return SIG, KEY or NXT records unless they were explicitly queried for. The server copies the query's DO bit into the response.
A recursive DNSSEC-aware server MUST set DO on its own recursive queries. If the client did not set DO, the recursive server removes security records before replying, without changing cached data. If a DO query receives NOTIMP, FORMERR or SERVFAIL, the resolver SHOULD retry without EDNS0.
With DO set, the resolver asks to receive DNSSEC records. A recursive server can request those records upstream and remove them before replying to a client that did not ask for them.
§5Interpreting Missing Security Records
If a response to a query with DO set contains no DNSSEC records, the resolver MUST NOT assume that the zone has no security information. An attacker may have forged the response or changed the query by clearing its DO bit.
The RFC reserves the leftmost bit of the EDNS0 extended flags as the DNSSEC OK bit. It defines the signaling behavior but does not make the absence of records proof that a zone is unsigned.
Missing DNSSEC records do not prove that a zone has no security information. A response may have been forged, or the request may have been changed in transit.