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IETF RFC 6604
Last modified on Friday, April 13th, 2012
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Internet Engineering Task Force (IETF) D. Eastlake 3rd
Request for Comments: 6604 Huawei
Updates: 1035, 2308, 2672 April 2012
Category: Standards Track
ISSN: 2070-1721
xNAME RCODE and Status Bits Clarification
 Abstract
The Domain Name System (DNS) has long provided means, such as the
CNAME (Canonical Name), whereby a DNS query can be redirected to a
different name. A DNS response header has an RCODE (Response Code)
field, used for indicating errors, and response status bits. This
document clarifies, in the case of such redirected queries, how the
RCODE and status bits correspond to the initial query cycle (where
the CNAME or the like was detected) and subsequent or final query
cycles.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in 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/RFC 6604.
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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Eastlake Standards Track PAGE 1 
RFC 6604 xNAME RCODE Clarification April 2012
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................3
2. Restatement of Status Bits and What They Mean ...................3
2.1. The Authoritative Answer Bit ...............................3
2.2. The Authentic Data Bit .....................................3
3. RCODE Clarification .............................................3
4. Security Considerations .........................................4
5. References ......................................................4
5.1. Normative References .......................................4
5.2. Informative References .....................................5
1. Introduction
The Domain Name System (DNS) has long provided means, such as the
CNAME (Canonical Name [RFC 1035]) and DNAME [RFC 2672] RRs (Resource
Records), whereby a DNS query can be redirected to a different name.
In particular, CNAME normally causes a query to its owner name to be
redirected, while DNAME normally causes a query to any lower-level
name to be redirected. There has been a proposal for another
redirection RR. In addition, as specified in [RFC 2672], redirection
through a DNAME also results in the synthesis of a CNAME RR in the
response. In this document, we will refer to all RRs causing such
redirection as xNAME RRs.
xNAME RRs can be explicitly retrieved by querying for the xNAME type.
When a different type is queried and an xNAME RR is encountered, the
xNAME RR (and possibly a synthesized CNAME) is added to the answer in
the response, DNS Security Extensions (DNSSEC) [RFC 4035] RRs
applicable to the xNAME RR may be added to the response, and the
query is restarted with the name to which it was redirected.
An xNAME may redirect a query to a name at which there is another
xNAME and so on. In this document, we use "xNAME chain" to refer to
a series of one or more xNAMEs each of which refers to another xNAME
except the last, which refers to a non-xNAME or results in an error.
A DNS response header has an RCODE (Response Code) field, used for
indicating errors, and status bits that indicate whether an answer is
authoritative and/or authentic. This document clarifies, in the case
of such redirected queries, how the RCODE and status bits correspond
to the initial query cycle (where the (first) xNAME was detected) and
subsequent or final query cycles.
Eastlake Standards Track PAGE 2
RFC 6604 xNAME RCODE Clarification April 2012
1.1. Conventions Used in This Document
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].
2. Restatement of Status Bits and What They Mean
There are two status bits returned in query responses for which a
question could arise as to how, in the case of an xNAME chain, they
relate to the first, possible intermediate, and/or last queries, as
below. Note that the following is unchanged from [RFC 1035] and
[RFC 4035]. The meaning of these bits is simply restated here for
clarity, because of observations of released implementations that did
not follow these meanings.
2.1. The Authoritative Answer Bit
The AA, or Authoritative Answer bit, in the DNS response header
indicates that the answer returned is from a DNS server authoritative
for the zone containing that answer. For an xNAME chain, this
"authoritative" status could be different for each answer in that
chain.
[RFC 1035] states that the AA bit is to be set based on whether the
server providing the answer with the first owner name in the answer
section is authoritative. This specification of the AA bit has not
been changed.
2.2. The Authentic Data Bit
The AD, or Authentic Data bit, indicates that the response returned
is authentic according to the dictates of DNSSEC [RFC 4035].
[RFC 4035] unambiguously states that the AD bit is to be set in a DNS
response header only if the DNSSEC-enabled server believes all RRs in
the answer and authority sections of that response to be authentic.
This specification of the AD bit has not been changed.
3. RCODE Clarification
The RCODE field in a DNS query response header is non-zero to
indicate an error. Section 4.3.2 of [RFC 1034] has a resolution
algorithm that includes CNAME processing but has been found to be
unclear concerning the ultimate setting of RCODE in the case of such
redirection. Section 2.1 of [RFC 2308] implies that the RCODE should
be set based on the last query cycle in the case of an xNAME chain,
but Section 2.2.1 of [RFC 2308] says that some servers don't do that!
Eastlake Standards Track PAGE 3
RFC 6604 xNAME RCODE Clarification April 2012
When there is an xNAME chain, the RCODE field is set as follows:
When an xNAME chain is followed, all but the last query cycle
necessarily had no error. The RCODE in the ultimate DNS response
MUST BE set based on the final query cycle leading to that
response. If the xNAME chain was terminated by an error, it will
be that error code. If the xNAME chain terminated without error,
it will be zero.
4. Security Considerations
The AA header flag bit is not protected by DNSSEC [RFC 4033]. To
secure it, secure communications are needed between the querying
resolver and the DNS server. Such security can be provided by DNS
transaction security, either TSIG [RFC 2845] or SIG(0) [RFC 2931].
An AD header flag bit and the RCODE in a response are not, in
general, protected by DNSSEC, so the same conditions as stated in the
previous paragraph generally apply to them; however, this is not
always true. In particular, if the following apply, then the AD bit
and an NXDOMAIN RCODE are protected by DNSSEC in the sense that the
querier can calculate whether they are correct:
1. The zone where an NXDOMAIN RCODE occurs or all the zones where the
data whose authenticity would be indicated by the AD flag bit are
signed zones.
2. The query or queries involved indicate that DNSSEC RRs are OK in
responses.
3. The responses providing these indications are from servers that
include the additional DNSSEC RRs required by DNSSEC.
4. The querier has appropriate trust anchor(s) and appropriately
validates and processes the DNSSEC RRs in the response.
5. References
5.1. Normative References
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Eastlake Standards Track PAGE 4
RFC 6604 xNAME RCODE Clarification April 2012
[RFC 2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC 4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
5.2. Informative References
[RFC 2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for
DNS (TSIG)", RFC 2845, May 2000.
[RFC 2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
Author's Address
Donald E. Eastlake 3rd
Huawei R&D USA
155 Beaver Street
Milford, MA 01757
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Eastlake Standards Track PAGE 5
RFC TOTAL SIZE: 10790 bytes
PUBLICATION DATE: Friday, April 13th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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