RFC7719
Internet Engineering Task Force (IETF) P. Hoffman Request for Comments: 7719 ICANN Category: Informational A. Sullivan ISSN: 2070-1721 Dyn
K. Fujiwara
JPRS
December 2015
DNS Terminology
Abstract
The DNS is defined in literally dozens of different RFCs. The terminology used 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
This document is not an Internet Standards Track specification; it is published for informational purposes.
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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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/rfc7719.
Copyright Notice
Copyright (c) 2015 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.
Contents
Introduction
The Domain Name System (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.
Most of the definitions here are 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. 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.
It is important to note that, during the development of this document, it became clear that some DNS-related terms are interpreted quite differently by different DNS experts. Further, some terms that are defined in early DNS RFCs now have definitions that are generally agreed to, but that are different from the original definitions. Therefore, the authors intend to follow this document with a substantial revision in the not-distant future. That revision will probably have more in-depth discussion of some terms as well as new terms; it will also update some of the RFCs with new definitions.
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.
Other organizations sometimes define DNS-related terms their own way. For example, the W3C defines "domain" at https://specs.webplatform.org/url/webspecs/develop/.
Note that there is no single consistent definition of "the DNS". It can be considered to be some combination of the following: a commonly used naming scheme for objects on the Internet; a distributed database representing the names and certain properties of these objects; an architecture providing distributed maintenance, resilience, and loose coherency for this database; and a simple query-response protocol (as mentioned below) implementing this architecture.
Capitalization in DNS terms is often inconsistent among RFCs and various DNS practitioners. The capitalization used in this document is a best guess at current practices, and is not meant to indicate that other capitalization styles are wrong or archaic. In some cases, multiple styles of capitalization are used for the same term due to quoting from different RFCs.
Names
Domain name: Section 3.1 of RFC1034 talks of "the domain name
space" as a tree structure. "Each node has a label, which is zero to 63 octets in length. ... The domain name of a node is the list of the labels on the path from the node to the root of the tree. ... To simplify implementations, the total number of octets that represent a domain name (i.e., the sum of all label octets and label lengths) is limited to 255." Any label in a domain name can contain any octet value.
Fully qualified domain name (FQDN): This is often just a clear way
of saying the same thing as "domain name of a node", as outlined above. However, the term is ambiguous. Strictly speaking, a fully qualified domain name would include every label, including the final, zero-length label of the root: such a name would be written "www.example.net." (note the terminating dot). But because every name eventually shares the common root, names are often written relative to the root (such as "www.example.net") and are still called "fully qualified". This term first appeared in RFC819. In this document, names are often written relative to the root.
The need for the term "fully qualified domain name" comes from the existence of partially qualified domain names, which are names where some of the right-most names are left off and are understood only by context.
Label: The identifier of an individual node in the sequence of nodes
identified by a fully qualified domain name.
Host name: This term and its equivalent, "hostname", have been
widely used but are not defined in RFC1034, RFC1035, RFC1123, or RFC2181. The DNS was originally deployed into the Host Tables environment as outlined in RFC952, and it is likely that the term followed informally from the definition there. Over time, the definition seems to have shifted. "Host name" is often meant to be a domain name that follows the rules in Section 3.5 of RFC1034, the "preferred name syntax". Note that any label in a domain name can contain any octet value; hostnames are generally considered to be domain names where every label follows the rules in the "preferred name syntax", with the amendment that labels can start with ASCII digits (this amendment comes from Section 2.1 of RFC1123).
People also sometimes use the term hostname to refer to just the first label of an FQDN, such as "printer" in "printer.admin.example.com". (Sometimes this is formalized in
configuration in operating systems.) In addition, people sometimes use this term to describe any name that refers to a machine, and those might include labels that do not conform to the "preferred name syntax".
TLD: A Top-Level Domain, meaning a zone that is one layer below the
root, such as "com" or "jp". There is nothing special, from the point of view of the DNS, about TLDs. Most of them are also delegation-centric zones, and there are significant policy issues around their operation. TLDs are often divided into sub-groups such as Country Code Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the division is a matter of policy, and beyond the scope of this document.
IDN: The common abbreviation for "Internationalized Domain Name".
The IDNA protocol is the standard mechanism for handling domain names with non-ASCII characters in applications in the DNS. The current standard, normally called "IDNA2008", is defined in RFC5890, RFC5891, RFC5892, RFC5893, and RFC5894. These documents define many IDN-specific terms such as "LDH label", "A-label", and "U-label". RFC6365 defines more terms that relate to internationalization (some of which relate to IDNs), and RFC6055 has a much more extensive discussion of IDNs, including some new terminology.
Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by seeing if the subdomain's name ends with the containing domain's name." (Quoted from RFC1034, Section 3.1). For example, in the host name "nnn.mmm.example.com", both "mmm.example.com" and "nnn.mmm.example.com" are subdomains of "example.com".
Alias: The owner of a CNAME resource record, or a subdomain of the
owner of a DNAME resource record RFC6672. See also "canonical name".
Canonical name: A CNAME resource record "identifies its owner name
as an alias, and specifies the corresponding canonical name in the RDATA section of the RR." (Quoted from RFC1034, Section 3.6.2) This usage of the word "canonical" is related to the mathematical concept of "canonical form".
CNAME: "It is traditional to refer to the owner of a CNAME record as
'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of 'canonical name', and the owner of a CNAME record is an alias, not a canonical name." (Quoted from RFC2181, Section 10.1.1)
Public suffix: "A domain that is controlled by a public registry."
(Quoted from RFC6265, Section 5.3) A common definition for this term is a domain under which subdomains can be registered, and on which HTTP cookies (RFC6265) should not be set. There is no indication in a domain name whether it is a public suffix; that can only be determined by outside means. In fact, both a domain and a subdomain of that domain can be public suffixes. At the time this document is published, the IETF DBOUND Working Group [DBOUND] is dealing with issues concerning public suffixes.
There is nothing inherent in a domain name to indicate whether it is a public suffix. One resource for identifying public suffixes is the Public Suffix List (PSL) maintained by Mozilla (http://publicsuffix.org/).
For example, at the time this document is published, the "com.au" domain is listed as a public suffix in the PSL. (Note that this example might change in the future.)
Note that the term "public suffix" is controversial in the DNS community for many reasons, and may be significantly changed in the future. One example of the difficulty of calling a domain a public suffix is that designation can change over time as the registration policy for the zone changes, such as the case of the "uk" TLD around the time this document is published.
DNS Header and Response Codes
The header of a DNS message is its first 12 octets. Many of the fields and flags in the header diagram in Sections 4.1.1 through 4.1.3 of RFC1035 are referred to by their names in that diagram. For example, the response codes are called "RCODEs", the data for a record is called the "RDATA", and the authoritative answer bit is often called "the AA flag" or "the AA bit".
Some of response codes that are defined in RFC1035 have gotten their own shorthand names. Some common response code names that appear without reference to the numeric value are "FORMERR", "SERVFAIL", and "NXDOMAIN" (the latter of which is also referred to as "Name Error"). All of the RCODEs are listed at http://www.iana.org/assignments/dns-parameters, although that site uses mixed-case capitalization, while most documents use all-caps.
NODATA: "A pseudo RCODE which indicates that the name is valid for
the given class, but there are no records of the given type. A NODATA response has to be inferred from the answer." (Quoted from RFC2308, Section 1.) "NODATA is indicated by an answer with the RCODE set to NOERROR and no relevant answers in the answer
section. The authority section will contain an SOA record, or there will be no NS records there." (Quoted from RFC2308, Section 2.2.) Note that referrals have a similar format to NODATA replies; RFC2308 explains how to distinguish them.
The term "NXRRSET" is sometimes used as a synonym for NODATA. However, this is a mistake, given that NXRRSET is a specific error code defined in RFC2136.
Negative response: A response that indicates that a particular RRset
does not exist, or whose RCODE indicates the nameserver cannot answer. Sections 2 and 7 of RFC2308 describe the types of negative responses in detail.
Referrals: Data from the authority section of a non-authoritative
answer. RFC1035 Section 2.1 defines "authoritative" data. However, referrals at zone cuts (defined in Section 6) are not authoritative. Referrals may be zone cut NS resource records and their glue records. NS records on the parent side of a zone cut are an authoritative delegation, but are normally not treated as authoritative data. In general, a referral is a way for a server to send an answer saying that the server does not know the answer, but knows where the query should be directed in order to get an answer. Historically, many authoritative servers answered with a referral to the root zone when queried for a name for which they were not authoritative, but this practice has declined.
Resource Records
RR: An acronym for resource record. (RFC1034, Section 3.6.)
RRset: A set of resource records with the same label, class and
type, but with different data. (Definition from RFC2181) Also spelled RRSet in some documents. As a clarification, "same label" in this definition means "same owner name". In addition, RFC2181 states that "the TTLs of all RRs in an RRSet must be the same". (This definition is definitely not the same as "the response one gets to a query for QTYPE=ANY", which is an unfortunate misunderstanding.)
EDNS: The extension mechanisms for DNS, defined in RFC6891.
Sometimes called "EDNS0" or "EDNS(0)" to indicate the version number. EDNS allows DNS clients and servers to specify message sizes larger than the original 512 octet limit, to expand the response code space, and potentially to carry additional options that affect the handling of a DNS query.
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. (Definition from RFC6891, Section 6.1.1) It is used by EDNS.
Owner: The domain name where a RR is found (RFC1034, Section 3.6).
Often appears in the term "owner name".
SOA field names: DNS documents, including the definitions here,
often refer to the fields in the RDATA of an SOA resource record by field name. Those fields are defined in Section 3.3.13 of RFC1035. The names (in the order they appear in the SOA RDATA) are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the meaning of MINIMUM field is updated in Section 4 of RFC2308; the new definition is that the MINIMUM field is only "the TTL to be used for negative responses". This document tends to use field names instead of terms that describe the fields.
TTL: The maximum "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 RFC1035 erroneously stated that this is a signed integer; that was fixed by RFC2181.)
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 RFC1035, Section 3.2.1) Also: "the time interval (in seconds) that the resource record may be cached before it should be discarded". (Quoted from RFC1035, 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).
The reason that the TTL is the maximum time to live is that a cache operator might decide to shorten the time to live for operational purposes, such as if there is a policy to disallow TTL values over a certain number. Also, if a value is flushed from the cache when its value is still positive, the value effectively becomes zero. Some servers are known to ignore the TTL on some RRsets (such as when the authoritative data has a very short TTL) even though this is against the advice in RFC 1035.
There is also the concept of a "default TTL" for a zone, which can be a configuration parameter in the server software. This is often expressed by a default for the entire server, and a default for a zone using the $TTL directive in a zone file. The $TTL directive was added to the master file format by RFC2308.
Class independent: A resource record type whose syntax and semantics
are the same for every DNS class. A resource record type that is not class independent has different meanings depending on the DNS class of the record, or the meaning is undefined for classes other than IN (class 1, the Internet).
DNS Servers and Clients
This section defines the terms used for the systems that act as DNS clients, DNS servers, or both.
Resolver: A program "that extract[s] information from name servers
in response to client requests." (Quoted from RFC1034, Section 2.4) "The resolver is located on the same machine as the program that requests the resolver's services, but it may need to consult name servers on other hosts." (Quoted from RFC1034, 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 Section 5.3.1 of RFC1034. They are fully defined in Section 6.1.3.1 of RFC1123.
Iterative mode: A resolution mode of a server that receives DNS
queries and responds with a referral to another server. Section 2.3 of RFC1034 describes this as "The server refers the client to another server and lets the client pursue the query". A resolver that works in iterative mode is sometimes called an "iterative resolver".
Recursive mode: A resolution mode of a server that receives DNS
queries and either responds to those queries from a local cache or sends queries to other servers in order to get the final answers to the original queries. Section 2.3 of RFC1034 describes this as "The first server pursues the query for the client at another server". A server operating in recursive mode 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). Systems operating in this mode are commonly called "recursive servers". Sometimes they are called "recursive resolvers". While strictly the difference between these is that one of them sends queries to another recursive server and the other does not, in practice it is not possible to know in advance whether the server that one is querying will also perform recursion; both terms can be observed in use interchangeably.
Full resolver: This term is used in RFC1035, 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 RFC1035. This term is not properly defined in any RFC.
Full-service resolver: Section 6.1.3.1 of RFC1123 defines this
term to mean a resolver that acts in recursive mode with a cache (and meets other requirements).
Priming: The mechanism used by a resolver to determine where to send
queries before there is anything in the resolver's cache. Priming is most often done from a configuration setting that contains a list of authoritative servers for the root zone.
Negative caching: "The storage of knowledge that something does not
exist, cannot give an answer, or does not give an answer." (Quoted from RFC2308, Section 1)
Authoritative server: "A server that knows the content of a DNS zone
from local knowledge, and thus can answer queries about that zone without needing to query other servers." (Quoted from RFC2182, Section 2.) It is 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. Authoritative servers also provide "referrals", usually to child zones delegated from them; these referrals have the AA bit set to 0 and come with referral data in the Authority and (if needed) the Additional sections.
Authoritative-only server: A name server that only serves
authoritative data and ignores requests for recursion. It will "not normally generate any queries of its own. Instead, it answers non-recursive queries from iterative resolvers looking for information in zones it serves." (Quoted from RFC4697, Section 2.4)
Zone transfer: The act of a client requesting a copy of a zone and
an authoritative server sending the needed information. (See
Section 6 for a description of zones.) There are two common
standard ways to do zone transfers: the AXFR ("Authoritative
Transfer") mechanism to copy the full zone (described in
RFC5936, and the IXFR ("Incremental Transfer") mechanism to copy
only parts of the zone that have changed (described in RFC1995).
Many systems use non-standard methods for zone transfer outside
the DNS protocol.
Secondary server: "An authoritative server which uses zone transfer
to retrieve the zone" (Quoted from RFC1996, Section 2.1). RFC2182 describes secondary servers in detail. Although early DNS RFCs such as RFC1996 referred to this as a "slave", the current common usage has shifted to calling it a "secondary". Secondary servers are also discussed in RFC1034.
Slave server: See secondary server.
Primary server: "Any authoritative server configured to be the
source of zone transfer for one or more [secondary] servers" (Quoted from RFC1996, Section 2.1) or, more specifically, "an authoritative server configured to be the source of AXFR or IXFR data for one or more [secondary] servers" (Quoted from RFC2136). Although early DNS RFCs such as RFC1996 referred to this as a "master", the current common usage has shifted to "primary". Primary servers are also discussed in RFC1034.
Master server: See primary server.
Primary master: "The primary master is named in the zone's SOA MNAME
field and optionally by an NS RR". (Quoted from RFC1996, Section 2.1). RFC2136 defines "primary master" as "Master server at the root of the AXFR/IXFR dependency graph. The primary master is named in the zone's SOA MNAME field and optionally by an NS RR. There is by definition only one primary master server per zone." The idea of a primary master is only used by RFC2136, and is considered archaic in other parts of the DNS.
Stealth server: This is "like a slave server except not listed in an
NS RR for the zone." (Quoted from RFC1996, Section 2.1)
Hidden master: A stealth server that is a master for zone transfers.
"In this arrangement, the master name server that processes the updates is unavailable to general hosts on the Internet; it is not listed in the NS RRset." (Quoted from RFC6781, Section 3.4.3.) An earlier RFC, RFC4641, said that the hidden master's name appears in the SOA RRs MNAME field, although in some setups, the name does not appear at all in the public DNS. A hidden master can be either a secondary or a primary master.
Forwarding: The process of one server sending a DNS query with the
RD bit set to 1 to another server to resolve that query. Forwarding is a function of a DNS resolver; it is different than simply blindly relaying queries.
RFC5625 does not give a specific definition for forwarding, but describes in detail what features a system that forwards need to support. Systems that forward are sometimes called "DNS proxies", but that term has not yet been defined (even in RFC5625).
Forwarder: Section 1 of RFC2308 describes a forwarder as "a
nameserver used to resolve queries instead of directly using the authoritative nameserver chain". RFC2308 further says "The forwarder typically either has better access to the internet, or maintains a bigger cache which may be shared amongst many resolvers." That definition appears to suggest that forwarders normally only query authoritative servers. In current use, however, forwarders often stand between stub resolvers and recursive servers. RFC2308 is silent on whether a forwarder is iterative-only or can be a full-service resolver.
Policy-implementing resolver: A resolver acting in recursive mode
that changes some of the answers that it returns based on policy criteria, such as to prevent access to malware sites or objectionable content. In general, a stub resolver has no idea whether upstream resolvers implement such policy or, if they do, the exact policy about what changes will be made. In some cases, the user of the stub resolver has selected the policy-implementing resolver with the explicit intention of using it to implement the policies. In other cases, policies are imposed without the user of the stub resolver being informed.
Open resolver: A full-service resolver that accepts and processes
queries from any (or nearly any) stub resolver. This is sometimes also called a "public resolver", although the term "public resolver" is used more with open resolvers that are meant to be open, as compared to the vast majority of open resolvers that are probably misconfigured to be open.
View: A configuration for a DNS server that allows it to provide
different answers depending on attributes of the query. Typically, views differ by the source IP address of a query, but can also be based on the destination IP address, the type of query (such as AXFR), whether it is recursive, and so on. Views are often used to provide more names or different addresses to queries from "inside" a protected network than to those "outside" that network. Views are not a standardized part of the DNS, but they are widely implemented in server software.
Passive DNS: A mechanism to collect large amounts of DNS data by
storing DNS responses from servers. Some of these systems also collect the DNS queries associated with the responses; this can raise privacy issues. Passive DNS databases can be used to answer historical questions about DNS zones such as which records were available for them at what times in the past. Passive DNS databases allow searching of the stored records on keys other than just the name, such as "find all names which have A records of a particular value".
Anycast: "The practice of making a particular service address
available in multiple, discrete, autonomous locations, such that datagrams sent are routed to one of several available locations." (Quoted from RFC4786, Section 2)
Zones
This section defines terms that are used when discussing zones that are being served or retrieved.
Zone: "Authoritative information is organized into units called
'zones', and these zones can be automatically distributed to the name servers which provide redundant service for the data in a zone." (Quoted from RFC1034, 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
RFC7344, Section 1.1) Earlier, "parent name server" was defined in RFC882 as "the name server that has authority over the place in the domain name space that will hold the new domain". (Note that RFC882 was obsoleted by RFC1034 and RFC1035.) RFC819 also has some description of the relationship between parents and children.
Origin:
(a) "The domain name that appears at the top of a zone (just below the cut that separates the zone from its parent). The name of the zone is the same as the name of the domain at the zone's origin." (Quoted from RFC2181, Section 6.) These days, this sense of "origin" and "apex" (defined below) are often used interchangeably.
(b) The domain name within which a given relative domain name appears in zone files. Generally seen in the context of "$ORIGIN", which is a control entry defined in RFC1035, 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.".
Apex: The point in the tree at an owner of an SOA and corresponding
authoritative NS RRset. This is also called the "zone apex". RFC4033 defines it as "the name at the child's side of a zone cut". The "apex" can usefully be thought of as a data-theoretic description of a tree structure, and "origin" is the name of the same concept when it is implemented in zone files. The distinction is not always maintained in use, however, and one can find uses that conflict subtly with this definition. RFC1034 uses the term "top node of the zone" as a synonym of "apex", but that term is not widely used. These days, the first sense of "origin" (above) and "apex" are often used interchangeably.
Zone cut: The delimitation point between two zones where the origin
of one of the zones is the child of the other zone.
"Zones are delimited by 'zone cuts'. Each zone cut separates a 'child' zone (below the cut) from a 'parent' zone (above the cut). (Quoted from RFC2181, Section 6; note that this is barely an ostensive definition.) Section 4.2 of RFC1034 uses "cuts" as 'zone cut'."
Delegation: The process by which a separate zone is created in the
name space beneath the apex of a given domain. Delegation happens when an NS RRset is added in the parent zone for the child origin. Delegation inherently happens at a zone cut. The term is also commonly a noun: the new zone that is created by the act of delegating.
Glue records: "[Resource records] which are not part of the
authoritative data [of the zone], and are address resource records for the [name servers in subzones]. These RRs are only necessary if the name server's name is 'below' the cut, and are only used as part of a referral response." Without glue "we could be faced with the situation where the NS RRs tell us that in order to learn a name server's address, we should contact the server using the address we wish to learn." (Definition from RFC1034, Section 4.2.1)
A later definition is that glue "includes any record in a zone file that is not properly part of that zone, including nameserver records of delegated sub-zones (NS records), address records that accompany those NS records (A, AAAA, etc), and any other stray data that might appear" (RFC2181, Section 5.4.1). Although glue is sometimes used today with this wider definition in mind, the context surrounding the RFC2181 definition suggests it is intended to apply to the use of glue within the document itself and not necessarily beyond.
In-bailiwick:
(a) An adjective to describe a name server whose name is either subordinate to or (rarely) the same as the zone origin. In- bailiwick name servers require glue records in their parent zone (using the first of the definitions of "glue records" in the definition above).
(b) Data for which the server is either authoritative, or else authoritative for an ancestor of the owner name. This sense of the term normally is used when discussing the relevancy of glue records in a response. For example, the server for 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 records are in- bailiwick.
Out-of-bailiwick: The antonym of in-bailiwick.
Authoritative data: "All of the RRs attached to all of the nodes
from the top node of the zone down to leaf nodes or nodes above cuts around the bottom edge of the zone." (Quoted from RFC1034, Section 4.2.1) It is noted that this definition might inadvertently also include any NS records that appear in the zone, even those that might not truly be authoritative because there are identical NS RRs below the zone cut. This reveals the ambiguity
in the notion of authoritative data, because the parent-side NS records authoritatively indicate the delegation, even though they are not themselves authoritative data.
Root zone: The zone whose apex is the zero-length label. Also
sometimes called "the DNS root".
Empty non-terminals: "Domain names that own no resource records but
have subdomains that do." (Quoted from RFC4592, Section 2.2.2.) A typical example is in SRV records: in the name "_sip._tcp.example.com", it is likely that "_tcp.example.com" has no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV RRset.
Delegation-centric zone: A zone that consists mostly of delegations
to child zones. This term is used in contrast to a zone that might have some delegations to child zones, but also has many data resource records for the zone itself and/or for child zones. The term is used in RFC4956 and RFC5155, but is not defined there.
Wildcard: RFC1034 defined "wildcard", but in a way that turned out
to be confusing to implementers. Special treatment is given to RRs with owner names starting with the label "*". "Such RRs are called 'wildcards'. Wildcard RRs can be thought of as instructions for synthesizing RRs." (Quoted from RFC1034, Section 4.3.3) For an extended discussion of wildcards, including clearer definitions, see RFC4592.
Occluded name: "The addition of a delegation point via dynamic
update will render all subordinate domain names to be in a limbo, still part of the zone, but not available to the lookup process. The addition of a DNAME resource record has the same impact. The subordinate names are said to be 'occluded'." (Quoted from RFC5936, Section 3.5)
Fast flux DNS: This "occurs when a domain is found in DNS using A
records to multiple IP addresses, each of which has a very short Time-to-Live (TTL) value associated with it. This means that the domain resolves to varying IP addresses over a short period of time." (Quoted from RFC6561, Section 1.1.5, with typo corrected) It is often used to deliver malware. Because the addresses change so rapidly, it is difficult to ascertain all the hosts. It should be noted that the technique also works with AAAA records, but such use is not frequently observed on the Internet as of this writing.
Registration Model
Registry: The administrative operation of a zone that allows
registration of names within that zone. People often use this term to refer only to those organizations that perform registration in large delegation-centric zones (such as TLDs); but formally, whoever decides what data goes into a zone is the registry for that zone. This definition of "registry" is from a DNS point of view; for some zones, the policies that determine what can go in the zone are decided by superior zones and not the registry operator.
Registrant: An individual or organization on whose behalf a name in
a zone is registered by the registry. In many zones, the registry and the registrant may be the same entity, but in TLDs they often are not.
Registrar: A service provider that acts as a go-between for
registrants and registries. Not all registrations require a registrar, though it is common to have registrars involved in registrations in TLDs.
EPP: The Extensible Provisioning Protocol (EPP), which is commonly
used for communication of registration information between registries and registrars. EPP is defined in RFC5730.
WHOIS: A protocol specified in RFC3912, often used for querying
registry databases. WHOIS data is frequently used to associate registration data (such as zone management contacts) with domain names. The term "WHOIS data" is often used as a synonym for the registry database, even though that database may be served by different protocols, particularly RDAP. The WHOIS protocol is also used with IP address registry data.
RDAP: The Registration Data Access Protocol, defined in RFC7480,
RFC7481, RFC7482, RFC7483, RFC7484, and RFC7485. The RDAP protocol and data format are meant as a replacement for WHOIS.
DNS operator: An entity responsible for running DNS servers. For a
zone's authoritative servers, the registrant may act as their own DNS operator, or their registrar may do it on their behalf, or they may use a third-party operator. For some zones, the registry function is performed by the DNS operator plus other entities who decide about the allowed contents of the zone.
General DNSSEC
Most DNSSEC terms are defined in RFC4033, RFC4034, RFC4035, and RFC5155. The terms that have caused confusion in the DNS community are highlighted here.
DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
some RFCs, have not been formally defined. However, Section 2 of RFC4033 defines many types of resolvers and validators, including "non-validating security-aware stub resolver", "non- validating stub resolver", "security-aware name server", "security-aware recursive name server", "security-aware resolver", "security-aware stub resolver", and "security-oblivious 'anything'". (Note that the term "validating resolver", which is used in some places in DNSSEC-related documents, is also not defined.)
Signed zone: "A zone whose RRsets are signed and that contains
properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records." (Quoted from RFC4033, Section 2.) It has been noted in other contexts that the zone itself is not really signed, but all the relevant RRsets in the zone are signed. Nevertheless, if a zone that should be signed contains any RRsets that are not signed (or opted out), those RRsets will be treated as bogus, so the whole zone needs to be handled in some way.
It should also be noted that, since the publication of RFC6840, NSEC records are no longer required for signed zones: a signed zone might include NSEC3 records instead. RFC7129 provides additional background commentary and some context for the NSEC and NSEC3 mechanisms used by DNSSEC to provide authenticated denial- of-existence responses.
Unsigned zone: Section 2 of RFC4033 defines this as "a zone that
is not signed". Section 2 of RFC4035 defines this as "A zone that does not include these records [properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records] according to the rules in this section". There is an important note at the end of Section 5.2 of RFC4035 that defines an additional situation in which a zone is considered unsigned: "If the resolver does not support any of the algorithms listed in an authenticated DS RRset, then the resolver will not be able to verify the authentication path to the child zone. In this case, the resolver SHOULD treat the child zone as if it were unsigned."
NSEC: "The NSEC record allows a security-aware resolver to
authenticate a negative reply for either name or type non- existence with the same mechanisms used to authenticate other DNS replies." (Quoted from RFC4033, Section 3.2.) In short, an NSEC record provides authenticated denial of existence.
"The NSEC resource record lists two separate things: the next owner name (in the canonical ordering of the zone) that contains authoritative data or a delegation point NS RRset, and the set of RR types present at the NSEC RR's owner name." (Quoted from Section 4 of RFC 4034)
NSEC3: Like the NSEC record, the NSEC3 record also provides
authenticated denial of existence; however, NSEC3 records mitigate against zone enumeration and support Opt-Out. NSEC3 resource records are defined in RFC5155.
Note that RFC6840 says that RFC5155 "is now considered part of the DNS Security Document Family as described by Section 10 of RFC4033". This means that some of the definitions from earlier RFCs that only talk about NSEC records should probably be considered to be talking about both NSEC and NSEC3.
Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover
unsigned delegations." (Quoted from RFC5155, Section 3.1.2.1.) Opt-out tackles the high costs of securing a delegation to an insecure zone. When using Opt-Out, names that are an insecure delegation (and empty non-terminals that are only derived from insecure delegations) don't require an NSEC3 record or its corresponding RRSIG records. Opt-Out NSEC3 records are not able to prove or deny the existence of the insecure delegations. (Adapted from RFC7129, Section 5.1)
Zone enumeration: "The practice of discovering the full content of a
zone via successive queries." (Quoted from RFC5155, Section 1.3.) This is also sometimes called "zone walking". Zone enumeration is different from zone content guessing where the guesser uses a large dictionary of possible labels and sends successive queries for them, or matches the contents of NSEC3 records against such a dictionary.
Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY
RRset in a zone."(Quoted from RFC6781, Section 3.1)
Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
the RRsets in a zone that require signatures, other than the apex DNSKEY RRset." (Quoted from RFC6781, Section 3.1) Note that the roles KSK and ZSK are not mutually exclusive: a single key can be both KSK and ZSK at the same time. Also note that a ZSK is sometimes used to sign the apex DNSKEY RRset.
Combined signing key (CSK): "In cases where the differentiation
between the KSK and ZSK is not made, i.e., where keys have the role of both KSK and ZSK, we talk about a Single-Type Signing Scheme." (Quoted from RFC6781, Section 3.1) This is sometimes called a "combined signing key" or CSK. It is operational practice, not protocol, that determines whether a particular key is a ZSK, a KSK, or a CSK.
Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
used to distinguish between keys that are intended to be used as the secure entry point into the zone when building chains of trust, i.e., they are (to be) pointed to by parental DS RRs or configured as a trust anchor. Therefore, it is suggested that the SEP flag be set on keys that are used as KSKs and not on keys that are used as ZSKs, while in those cases where a distinction between a KSK and ZSK is not made (i.e., for a Single-Type Signing Scheme), it is suggested that the SEP flag be set on all keys." (Quoted from RFC6781, Section 3.2.3.) Note that the SEP flag is only a hint, and its presence or absence may not be used to disqualify a given DNSKEY RR from use as a KSK or ZSK during validation.
DNSSEC Policy (DP): A statement that "sets forth the security
requirements and standards to be implemented for a DNSSEC-signed zone." (Quoted from RFC6841, Section 2)
DNSSEC Practice Statement (DPS): "A practices disclosure document
that may support and be a supplemental document to the DNSSEC Policy (if such exists), and it states how the management of a given zone implements procedures and controls at a high level." (Quoted from RFC6841, Section 2)
DNSSEC States
A validating resolver can determine that a response is in one of four states: secure, insecure, bogus, or indeterminate. These states are defined in RFC4033 and RFC4035, although the two definitions differ a bit. This document makes no effort to reconcile the two definitions, and takes no position as to whether they need to be reconciled.
Section 5 of RFC4033 says:
A validating resolver can determine the following 4 states:
Secure: The validating resolver has a trust anchor, has a chain
of trust, and is able to verify all the signatures in the
response.
Insecure: The validating resolver has a trust anchor, a chain
of trust, and, at some delegation point, signed proof of the
non-existence of a DS record. This indicates that subsequent
branches in the tree are provably insecure. A validating
resolver may have a local policy to mark parts of the domain
space as insecure.
Bogus: The validating resolver has a trust anchor and a secure
delegation indicating that subsidiary data is signed, but
the response fails to validate for some reason: missing
signatures, expired signatures, signatures with unsupported
algorithms, data missing that the relevant NSEC RR says
should be present, and so forth.
Indeterminate: There is no trust anchor that would indicate that a
specific portion of the tree is secure. This is the default
operation mode.
Section 4.3 of RFC4035 says:
A security-aware resolver must be able to distinguish between four cases:
Secure: An RRset for which the resolver is able to build a chain
of signed DNSKEY and DS RRs from a trusted security anchor to
the RRset. In this case, the RRset should be signed and is
subject to signature validation, as described above.
Insecure: An RRset for which the resolver knows that it has no
chain of signed DNSKEY and DS RRs from any trusted starting
point to the RRset. This can occur when the target RRset lies
in an unsigned zone or in a descendent [sic] of an unsigned
zone. In this case, the RRset may or may not be signed, but
the resolver will not be able to verify the signature.
Bogus: An RRset for which the resolver believes that it ought to
be able to establish a chain of trust but for which it is
unable to do so, either due to signatures that for some reason
fail to validate or due to missing data that the relevant
DNSSEC RRs indicate should be present. This case may indicate
an attack but may also indicate a configuration error or some
form of data corruption.
Indeterminate: An RRset for which the resolver is not able to
determine whether the RRset should be signed, as the resolver
is not able to obtain the necessary DNSSEC RRs. This can occur
when the security-aware resolver is not able to contact
security-aware name servers for the relevant zones.
10. Security Considerations
These definitions do not change any security considerations for the DNS.
11. References
11.1. Normative References
RFC882 Mockapetris, P., "Domain names: Concepts and facilities",
RFC 882, DOI 10.17487/RFC0882, November 1983, <http://www.rfc-editor.org/info/rfc882>.
RFC1034 Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, <http://www.rfc-editor.org/info/rfc1034>.
RFC1035 Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <http://www.rfc-editor.org/info/rfc1035>.
RFC1123 Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, DOI 10.17487/RFC1123, October 1989, <http://www.rfc-editor.org/info/rfc1123>.
RFC1996 Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, August 1996, <http://www.rfc-editor.org/info/rfc1996>.
RFC2136 Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.
RFC2181 Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, <http://www.rfc-editor.org/info/rfc2181>.
RFC2182 Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
and Operation of Secondary DNS Servers", BCP 16, RFC 2182, DOI 10.17487/RFC2182, July 1997, <http://www.rfc-editor.org/info/rfc2182>.
RFC2308 Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, <http://www.rfc-editor.org/info/rfc2308>.
RFC4033 Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
RFC4034 Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.
RFC4035 Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
RFC4592 Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006, <http://www.rfc-editor.org/info/rfc4592>.
RFC5155 Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<http://www.rfc-editor.org/info/rfc5155>.
RFC5730 Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009, <http://www.rfc-editor.org/info/rfc5730>.
RFC5936 Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, <http://www.rfc-editor.org/info/rfc5936>.
RFC6561 Livingood, J., Mody, N., and M. O'Reirdan,
"Recommendations for the Remediation of Bots in ISP
Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,
<http://www.rfc-editor.org/info/rfc6561>.
RFC6672 Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, <http://www.rfc-editor.org/info/rfc6672>.
RFC6781 Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, December 2012, <http://www.rfc-editor.org/info/rfc6781>.
RFC6840 Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840, DOI 10.17487/RFC6840, February 2013, <http://www.rfc-editor.org/info/rfc6840>.
RFC6841 Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
Framework for DNSSEC Policies and DNSSEC Practice
Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,
<http://www.rfc-editor.org/info/rfc6841>.
RFC6891 Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, <http://www.rfc-editor.org/info/rfc6891>.
RFC7344 Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
DNSSEC Delegation Trust Maintenance", RFC 7344, DOI 10.17487/RFC7344, September 2014, <http://www.rfc-editor.org/info/rfc7344>.
11.2. Informative References
[DBOUND] IETF, "Domain Boundaries (dbound) Working Group", 2015,
<https://datatracker.ietf.org/wg/dbound/charter/>.
RFC819 Su, Z. and J. Postel, "The Domain Naming Convention for
Internet User Applications", RFC 819, DOI 10.17487/RFC0819, August 1982, <http://www.rfc-editor.org/info/rfc819>.
RFC952 Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, DOI 10.17487/RFC0952, October 1985, <http://www.rfc-editor.org/info/rfc952>.
RFC1995 Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<http://www.rfc-editor.org/info/rfc1995>.
RFC3912 Daigle, L., "WHOIS Protocol Specification", RFC 3912,
DOI 10.17487/RFC3912, September 2004,
<http://www.rfc-editor.org/info/rfc3912>.
RFC4641 Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, DOI 10.17487/RFC4641, September 2006, <http://www.rfc-editor.org/info/rfc4641>.
RFC4697 Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697, October 2006, <http://www.rfc-editor.org/info/rfc4697>.
RFC4786 Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, December 2006, <http://www.rfc-editor.org/info/rfc4786>.
RFC4956 Arends, R., Kosters, M., and D. Blacka, "DNS Security
(DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July 2007, <http://www.rfc-editor.org/info/rfc4956>.
RFC5625 Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, <http://www.rfc-editor.org/info/rfc5625>.
RFC5890 Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
RFC5891 Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010, <http://www.rfc-editor.org/info/rfc5891>.
RFC5892 Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC5892, August 2010,
<http://www.rfc-editor.org/info/rfc5892>.
RFC5893 Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
<http://www.rfc-editor.org/info/rfc5893>.
RFC5894 Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
<http://www.rfc-editor.org/info/rfc5894>.
RFC6055 Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055, DOI 10.17487/RFC6055, February 2011, <http://www.rfc-editor.org/info/rfc6055>.
RFC6265 Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<http://www.rfc-editor.org/info/rfc6265>.
RFC6365 Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, <http://www.rfc-editor.org/info/rfc6365>.
RFC7129 Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, February 2014, <http://www.rfc-editor.org/info/rfc7129>.
RFC7480 Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
Registration Data Access Protocol (RDAP)", RFC 7480, DOI 10.17487/RFC7480, March 2015, <http://www.rfc-editor.org/info/rfc7480>.
RFC7481 Hollenbeck, S. and N. Kong, "Security Services for the
Registration Data Access Protocol (RDAP)", RFC 7481, DOI 10.17487/RFC7481, March 2015, <http://www.rfc-editor.org/info/rfc7481>.
RFC7482 Newton, A. and S. Hollenbeck, "Registration Data Access
Protocol (RDAP) Query Format", RFC 7482, DOI 10.17487/RFC7482, March 2015, <http://www.rfc-editor.org/info/rfc7482>.
RFC7483 Newton, A. and S. Hollenbeck, "JSON Responses for the
Registration Data Access Protocol (RDAP)", RFC 7483, DOI 10.17487/RFC7483, March 2015, <http://www.rfc-editor.org/info/rfc7483>.
RFC7484 Blanchet, M., "Finding the Authoritative Registration Data
(RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March 2015, <http://www.rfc-editor.org/info/rfc7484>.
RFC7485 Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,
"Inventory and Analysis of WHOIS Registration Objects",
RFC 7485, DOI 10.17487/RFC7485, March 2015,
<http://www.rfc-editor.org/info/rfc7485>.
Acknowledgements
The authors gratefully acknowledge all of the authors of DNS-related RFCs that proceed this one. Comments from Tony Finch, Stephane Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, John Klensin, David Black, and many others in the DNSOP Working Group have helped shape this document.
Authors' Addresses
Paul Hoffman ICANN
Email: [email protected]
Andrew Sullivan Dyn 150 Dow Street, Tower 2 Manchester, NH 03101 United States
Email: [email protected]
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: [email protected]
