Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Best Current Practice A. Sullivan
Expires: June 1, 2015 Dyn
K. Fujiwara
JPRS
November 28, 2014
DNS Terminology
draft-hoffman-dns-terminology-00
Abstract
The DNS is defined in literally dozens of different RFCs. The
terminology used in by implementers and developers of DNS protocols,
and by operators of DNS systems, has sometimes changed in the decades
since the DNS was first defined. This document gives current
definitions for many of the terms used in the DNS in a single
document.
Status of This Memo
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This Internet-Draft will expire on June 1, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DNS Message Format . . . . . . . . . . . . . . . . . . . . . 3
3. Resource Records . . . . . . . . . . . . . . . . . . . . . . 4
4. DNS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The DNS is a simple query-response protocol whose messages in both
directions have the same format. The protocol and message format are
defined in [RFC1034] and [RFC1035]. These RFCs defined some terms,
but later documents defined others. Some of the terms from RFCs 1034
and 1035 now have somewhat different meanings than they did in 1987.
This document collects a wide variety of DNS-related terms. Some of
them have been precisely defined in earlier RFCs, some have been
loosely defined in earlier RFCs, and some are not defined in any
earlier RFC at all.
The definitions here are believed to be the consensus definition of
the DNS community, both protocol developers and operators. Some of
the definitions differ from earlier RFCs, and those differences are
noted. The terms are organized loosely by topic. Some definitions
are for new terms for things that are commonly talked about in the
DNS community but that never had terms defined for them.
In this document, where the consensus definition is the same as the
one in an RFC, that RFC is quoted. Where the consensus definition
has changed somewhat, the RFC is mentioned but the new stand-alone
definition is given.
Note that capitalization in DNS terms is often inconsistent between
RFCs and between DNS practitioners. The capitalization used in this
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document is a best guess at current practices, and is not meant to
indicate that other capitalization styles are wrong or archaic.
(Note: the formatting of this early draft is a bit funky. It will
improve in later drafts. Bikeshedding the format, as compared to the
content, of this draft is probably premature.)
2. DNS Message Format
Header -- The first 12 octets of a DNS message. Many of the fields
and flags in the header diagram in section 4.1.1 of RFC 1035 are
referred to by their names in that diagram. For example, the
response codes are called "RCODEs", and the authoritative answer bit
is often called "the AA flag" or "the AA bit".
Some of response codes that are defined in RFC 1035 have gotten their
own shorthand names. Some common ones are:
FORMERR -- A response message whose header has an RCODE of 1
SERVFAIL -- A response message whose header has an RCODE of 2
NXDOMAIN -- A response message whose header has an RCODE of 3.
NXDOMAIN is defined as an official synonym for Name Error in
RFC 2308, section 1.
TTL -- The "time to live" of a resource record. A TTL value is an
unsigned number, with a minimum value of 0, and a maximum value of
2147483647. That is, a maximum of 2^31 - 1. When transmitted, the
TTL is encoded in the less significant 31 bits of the 32 bit TTL
field, with the most significant, or sign, bit set to zero. (Quoted
from [RFC2181], section 8) (Note that RFC 1035 erroneously stated
that this is a signed integer; it is fixed in an erratum.)
The TTL "specifies the time interval that the resource record may be
cached before the source of the information should again be
consulted". (Quoted from RFC 1035, section 3.2.1) Also: "the time
interval (in seconds) that the resource record may be cached before
it should be discarded". (Quoted from RFC 1035, section 4.1.3).
Despite being defined for a resource record, the TTL of every
resource record in an RRset is required to be the same (RFC2181,
section 5.2).
Glue records -- Resource records which are not part of the
authoritative data, and are address resource records for the servers
listed in the message. They contain data that allows access to name
servers for subzones. (Definition from RFC 1034, section 4.2.1)
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Referrals -- Data from the authority section of a non-authoritative
answer. RFC 1035 section 2.1 defines "authoritative" data. However,
referrals at zone cuts are not authoritative. Referrals may be a
zone cut NS resource records and their glue. NS records on the
parent side of a zone cut are an authoritative delegation, but are
not treated as authoritative data by the client. [[ A more complete
and precise definition will be needed here. ]]
3. Resource Records
RR -- A short form for resource record. (RFC 1034, section 3.6.)
RRset -- A set of resource records with the same label, class and
type, but with different data. (Definition from RFC 2181). Also
spelled RRSet in some documents.
OPT -- A pseudo-RR (sometimes called a meta-RR) that is used only to
contain control information pertaining to the question-and-answer
sequence of a specific transaction. contains control information
pertaining to the question-and-answer sequence of a specific
transaction. (Definition from [RFC6891], section 6.1.1)
Owner -- The domain name where a RR is found (RFC 1034, section 3.6).
Often appears in the term "owner name".
4. DNS Servers
This section defines the terms used for the systems that act as DNS
clients, DNS servers, or both. Some terms about servers describe
servers that do and do not use DNSSEC; see Section 6 for those
definitions.
Resolver -- Programs that interface user programs to domain name
servers. (Quoted from RFC 1034, section 5.1) A resolver performs
queries for a name, type, and class, and receives answers. The
logical function is called "resolution". In practice, the term is
usually referring to some specific type of resolver (some of which
are defined below), and understanding the use of the term depends on
understanding the context.
Stub resolver -- A resolver that cannot perform all resolution
itself. Stub resolvers generally depend on a recursive resolver to
undertake the actual resolution function. Stub resolvers are
discussed but never fully defined in RFC 1034, section 5.3.1.
Iterative resolver -- A system that receives DNS queries and responds
with a referral to another server. RFC 1034 (section 2.3) describes
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this as, "The server refers the client to another server and lets the
client pursue the query."
Recursive resolver -- A system that receives DNS queries and either
responds to those queries from a local cache or sends queries to
authoritative servers in order to get the final answers to the
original queries. RFC 1034 (section 2.3) describes this as, "The
first server pursues the query for the client at another server."
Recursive resolvers may be thought of as having a name server side
(which is what answers the query) and a resolver side (which performs
the resolution function). A recursive resolver is responsible for
resolving domain names for clients by following the domain's
delegation chain, starting at the DNS root. These systems are also
commonly called "recursive servers".
Authoritative server -- A system that responds to DNS queries with
information about zones for which it has been configured to answer
with the AA flag in the response header set to 1. It is a server
that has authority over one or more DNS zones. Note that it is
possible for an authoritative server to respond to a query without
the parent zone delegating authority to that server.
DNS forwarder -- A system receives a DNS query, possibly changes the
query, sends the resulting query to a recursive resolver, receives
the response from a resolver, possibly changes the response, and
sends the resulting response to the stub resolver. Section 1 of
[RFC2308] describes a forwarder as "a nameserver used to resolve
queries instead of directly using the authoritative nameserver
chain". [RFC5625] does not give a specific definition for DNS
forwarder, but describes in detail what features they need to
support. The protocol interfaces for DNS forwarders are exactly the
same as those for recursive resolvers (for interactions with DNS
stubs) and as those for stub resolvers (for interactions with
recursive resolvers).
Full resolver -- This term is used in RFC 1035, but it is not defined
there. RFC 1123 defines a "full-service resolver" that may or may
not be what was intended by "full resolver" in RFC 1035. In the
vernacular, a full-service resolver is usually one that would be
suitable for use by a stub resolver.
Consensual policy-implementing resolver -- A resolver that changes
some answers it returns based on policy criteria, such as to prevent
access to malware sites. These policy criteria are agreed to by
systems that query this resolver through some out of band mechanism
(such as finding out about the resolver from a web site and reading
the policy).
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Non-consensual policy-implementing resolver -- A resolver that is not
a consensual policy-implementing resolver that changes the answers it
returns. The difference between this and a consensual policy-
implementing resolver is that users of this resolver are not expected
to know that there is a policy to change the answers it returns.
5. Zones
This section defines terms that are used when discussing zones that
are being served or retrieved.
Zone -- A unit of organization of authoritative data. Zones can be
automatically distributed to the name servers which provide redundant
service for the data in a zone. (Quoted from RFC 1034, section 2.4).
Child -- The entity on record that has the delegation of the domain
from the Parent. (Quoted from [RFC7344], section 1.1)
Parent -- The domain in which the Child is registered. (Quoted from
RFC 7344, section 1.1)
Zone cut -- The delimitation point between two zones where the origin
of one of the zones is the child of the other zone. (Section 6 of
RFC 2181 uses this term extensively, although never actually defines
it.)
In-bailiwick response -- A response in which the name server
answering is authoritative for an ancestor of the owner name in the
response. The term normally is used when discussing the relevancy of
glue records. For example, the parent zone example.com might reply
with glue records for ns.child.example.com. Because the
child.example.com zone is a descendant of the example.com zone, the
glue is in-bailiwick.
Out-of-bailiwick response -- A response in which the name server
answering is not authoritative for an ancestor of the owner name in
the response.
Origin -- 1. The domain name within which a given relative domain
name appears. Generally seen in the context of "$ORIGIN", which is a
control entry defined in RFC 1035, section 5.1, as part of the master
file format. For example, if the $ORIGIN is set to "example.org.",
then a master file line for "www" is in fact an entry for
"www.example.org.". 2. The domain name that appears at the top of a
zone, that is, the owner name of the apex records.
Authoritative data -- RRsets in a DNS response that has the AA bit in
the response header set to 1.
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Delegation -- The process by which a separate zone is created in the
name space beneath a given domain. Delegation happens when an NS
RRset is added in the parent zone for the child origin, and a
corresponding zone apex is created at the child origin.
Apex -- The SOA and NS RRsets at the origin of a zone. This is also
called the "zone apex".
Root zone -- The zone whose origin is the zero-length label. Also
sometimes called "the DNS root".
6. DNSSEC
This section will mostly be populated with direct quotes from RFC
4033. For some terms, there will be additional commentary.
[[ The four types of validation states ]]
[[ The many types of DNSSEC-aware and -unaware resolvers and
validators ]]
NSEC -- [[ Definition goes here ]]
NSEC3 -- [[ Definition goes here ]]
7. IANA Considerations
This document has no effect on IANA registries.
8. Security Considerations
These definitions do not change any security considerations for the
DNS.
9. Acknowledgements
The authors gratefully acknowledge all of the authors of DNS-related
RFCs that proceed this one. [[ More acks will go here as people point
out new terms to add and changes to the ones we have listed here. ]]
10. References
10.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
DNSSEC Delegation Trust Maintenance", RFC 7344, September
2014.
10.2. Informative References
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP
152, RFC 5625, August 2009.
Authors' Addresses
Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA 95060
USA
Email: paul.hoffman@vpnc.org
Andrew Sullivan
Dyn
150 Dow St, Tower 2
Manchester, NH 1604
USA
Email: asullivan@dyn.com
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Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
Email: fujiwara@jprs.co.jp
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