Internet Engineering Task Force (IETF) D. Eastlake 3rd
Request for Comments: 6604 Huawei
Updates: 1035, 2308, 2672 April 2012
Category: Standards Track
xNAME RCODE and Status Bits Clarification
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
Status of This Memo
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Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. Restatement of Status Bits and What They Mean ...................32.1. The Authoritative Answer Bit ...............................32.2. The Authentic Data Bit .....................................33. RCODE Clarification .............................................34. Security Considerations .........................................45. References ......................................................45.1. Normative References .......................................45.2. Informative References .....................................51. Introduction
The Domain Name System (DNS) has long provided means, such as the
CNAME (Canonical Name [RFC1035]) and DNAME [RFC2672] 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 [RFC2672], 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) [RFC4035] 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.
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 [RFC2119].
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 [RFC1035] and
[RFC4035]. 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
[RFC1035] 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
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 [RFC4035].
[RFC4035] 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 [RFC1034] 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 [RFC2308] 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 [RFC2308] says that some servers don't do that!
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 [RFC4033]. 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 [RFC2845] or SIG(0) [RFC2931].
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
2. The query or queries involved indicate that DNSSEC RRs are OK in
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.
5.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", 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.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC4035] 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
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for
DNS (TSIG)", RFC 2845, May 2000.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
Donald E. Eastlake 3rd
Huawei R&D USA
155 Beaver Street
Milford, MA 01757