DNSext Working Group O. Sury
Internet-Draft CZ.NIC
Updates: 1034 (if approved) August 18, 2010
Intended status: Standards Track
Expires: February 19, 2011
CNAME at the zone apex
draft-sury-dnsext-cname-at-apex-00
Abstract
This document proposes a modification to CNAME record to coexist with
SOA and NS records at the zone apex. This proposal will improve
aliasing in the DNS system. The users are often forced to manually
add duplicate A, AAAA and MX records by copying data from the target
zone to the aliased zone. This forces zone owner to keep track of
target domain name since the mismatch in the data could cause
failures. This administrative burden will be eliminated by allowing
CNAME to coexist with NS and SOA resource records.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. CNAME at the apex . . . . . . . . . . . . . . . . . . . . . . . 4
4. Query processing . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Processing by Authoritative Servers . . . . . . . . . . . . 4
4.2. Processing by Recursive Servers . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
RFC 1033 [RFC1033] defines CNAME resource record for cases when there
are multiple names for single host. A CNAME resource record
identifies its owner name as an alias, and specifies the
corresponding canonical name in the RDATA section of the resource
record. If a CNAME resource record is present at a node, no other
data MUST be present; this ensures that the data for a canonical name
and its aliases cannot be different. This rule also insures that a
cached CNAME can be used without checking with an authoritative
server for other resource record types.
However there are already existing exceptions to this rule. RFC 4034
[RFC4034] defines exception to RRSIG and NSEC records, which MUST
exist for the same name as a CNAME resource record in a signed zone.
RFC 1034 [RFC1034] defines the data that defines the top node of the
zone. They are logically part of the authoritative data, the RRs
that describe the top node of the zone are especially important to
the zone's management. These RRs are of two types: name server RRs
that list, one per RR, all of the servers for the zone, and a single
SOA RR that describes zone management parameters.
The Start Of Authority (SOA) record designates the start of a zone.
It must be present in the zone apex.
The NS (Name Server) record lists the name of a machine the provides
domain service for a particular zone. The NS record is placed both
in the parent and the child zone and should be same. The NS
record(s) are also mandatory in the zone.
1.1. Terminology
All the basic terms used in this specification are defined in the
documents RFC 1033 [RFC1033], RFC 1034 [RFC1034].
1.2. Requirements Language
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 RFC 2119 [RFC2119].
2. Motivation
The aliasing in the DNS system is usually done by placing CNAME for
each individual record which needs to be aliased. There is one
notable exception - the zone apex which has to include the SOA and NS
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RRs. Because of that the zone owner is unable to place CNAME record
there and this can lead to several failure conditions. Either the
data in the apex is copied by hand and new administrative burden is
created to keep the data in the sync, or there is no alias in the
apex at all.
The aliases zone is prone to several types of errors when the copy
method is used. The target domain can change the data and while
subdomains (like www.example.net) still works the zone apex A record
(example.net) doesn't work. This could go overlooked for some time
if the zone owner doesn't do regular checks. Same condition can
happen with MX records causing failure to deliver email or it could
even lead to malicious use if the bad guys happen to own previous
manually copied MX records. Also there could be a semi-failures,
f.e. if the target zone adds AAAA record for IPv6, it will not be
copied to aliased zone automatically causing IPv6 resolution
failures.
3. CNAME at the apex
This proposal defines new rules for the CNAME record. RFC 1033
[RFC1034] defines that:
There must not be any other RRs associated with a nickname of the
same class.
This rule is changed with full compliance with DNSSEC RFCs to:
There must not be any other RRs with the same owner as the CNAME RR
with the exception of NS, SOA, DNSKEY, RRSIG and NSEC RRs.
4. Query processing
Because of the change of the existing behaviour in the CNAME
processing there is a need to add a signaling bit for the queries
issued by resolvers understanding CNAME at the zone apex.
4.1. Processing by Authoritative Servers
The authoritative server implementations MUST allow CNAME resource
record in the zone apex to coexist with NS, SOA, DNSKEY, RRSIG and
NSEC resource records. They MUST NOT allow any other resource record
types in the zone apex when the CNAME resource record is placed in
the zone apex.
The authoritative server will return an answer containing specific
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resource record type when asked for SOA, NS, DNSKEY, RRSIG or NSEC RR
type. The authoritative server will return an answer containing the
CNAME if any other RR type is requested in the query. If the query
contains the CNAME-at-apex signaling bit then the authoritative
server will use correct TTL in the zone for the requests in the zone
apex. If the query doesn't contain the CNAME-at-apex signaling bit
then the authoritative server will override TTL in the zone and will
return answer with 0 TTL for all resource requests in the zone. This
should prevent the resolvers to store the result in the cache and
thus breaking the internal rules for the CNAME.
4.2. Processing by Recursive Servers
The resolver compliant with this proposal will issue a query which
has the CNAME-at-apex signaling bit. Such resolver MUST NOT deny
CNAME if it already has other resource record in the cache with the
same owner with the SOA, NS, DNSKEY, RRSIG or NSEC resource record
type. It MUST NOT deny SOA, NS, DNSKEY, RRSIG or NSEC resource
records if it already has a CNAME resource record in the cache. The
compliant resolver SHOULD NOT deny CNAME in the case it has any other
RR type in the cache and it SHOULD NOT deny any other RR types if it
already has CNAME records in the cache.
The rules for the authoritative server is same for the compliant
resolver acting as an upstream cache.
5. Security Considerations
The author is not aware of any security consideration, but he is
aware that this proposal could create problems for the name servers
not following the Robustness principle - Be conservative in what you
send; be liberal in what you accept.
6. Normative References
[RFC1033] Lottor, M., "Domain administrators operations guide",
RFC 1033, November 1987.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
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RFC 4034, March 2005.
Author's Address
Ondrej Sury
CZ.NIC
Americka 23
120 00 Praha 2
CZ
Phone: +420 222 745 110
Email: ondrej.sury@nic.cz
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