The information below is for an old version of the document.
This is an older version of an Internet-Draft that was ultimately published as RFC 7719.
|Authors||Paul E. Hoffman , Andrew Sullivan , Kazunori Fujiwara|
|RFC stream||Internet Engineering Task Force (IETF)|
GENART Telechat review (of -04) Ready with Issues
OPSDIR Last Call review (of -03) Has Issues
GENART Last Call review (of -03) On the Right Track
|Additional resources||Mailing list discussion|
|Stream||WG state||In WG Last Call|
|Document shepherd||Tim Wicinski|
|IESG||IESG state||I-D Exists|
|Send notices to||"Tim Wicinski" <firstname.lastname@example.org>|
Network Working Group P. Hoffman Internet-Draft VPN Consortium Intended status: Best Current Practice A. Sullivan Expires: November 27, 2015 Dyn K. Fujiwara JPRS May 26, 2015 DNS Terminology draft-ietf-dnsop-dns-terminology-02 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 27, 2015. 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 Hoffman, et al. Expires November 27, 2015 [Page 1] Internet-Draft DNS Terminology May 2015 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. DNS Header and Response Codes . . . . . . . . . . . . . . . . 5 4. Resource Records . . . . . . . . . . . . . . . . . . . . . . 6 5. DNS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7. Registration Model . . . . . . . . . . . . . . . . . . . . . 14 8. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 15 9. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 17 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 11. Security Considerations . . . . . . . . . . . . . . . . . . . 19 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 13.1. Normative References . . . . . . . . . . . . . . . . . . 20 13.2. Informative References . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. 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. 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 Hoffman, et al. Expires November 27, 2015 [Page 2] Internet-Draft DNS Terminology May 2015 has changed somewhat, the RFC is mentioned but the new stand-alone definition is given. 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 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 between RFCs and between 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. 2. 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." 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 name would include every label, including the final, zero-length label of the root zone: 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 [RFC1206]. 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. Hoffman, et al. Expires November 27, 2015 [Page 3] Internet-Draft DNS Terminology May 2015 Label: The identifier of an individual node in the sequence of nodes that comprise 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 [RFC0952], 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 any 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. 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 "ccTLDs", "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". IDNs are the current standard mechanism for handling domain names with non-ASCII characters in applications. The current standard, normally called "IDNA2008", is defined in [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. 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". Hoffman, et al. Expires November 27, 2015 [Page 4] Internet-Draft DNS Terminology May 2015 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 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 or not it is a public suffix; that can only be determined by outside means. The IETF DBOUND Working Group [DBOUND] deals with issues with public suffixes. For example, at the time this document is published, .com.au is considered a public suffix, but .au is not. (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 zone around the time this document is published. 3. DNS Header and Response Codes The header of a DNS message is first 12 octets. Many of the fields and flags in the header diagram in section 4.1.1 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" (sometimes also called "RRdata"), 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". All of the RCODEs are listed at http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml, 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 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], Hoffman, et al. Expires November 27, 2015 [Page 5] Internet-Draft DNS Terminology May 2015 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 which 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 are not authoritative. Referrals may be a 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 by the client. 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. Zone transfer: The act of a client requesting a copy of a zone and an authoritative server sending the needed information. 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. 4. Resource Records RR: A short form 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". 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 Hoffman, et al. Expires November 27, 2015 [Page 6] Internet-Draft DNS Terminology May 2015 sizes larger than the original 512 octet limit, to expand the response code space, and to potentially 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 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". 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; 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 [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 not allow 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 do not honor the TTL on an RRset from the authoritative servers, such as when when the authoritative data has a very short TTL. Hoffman, et al. Expires November 27, 2015 [Page 7] Internet-Draft DNS Terminology May 2015 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]. 5. 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 8 for those definitions. Resolver: A program that extracts 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 188.8.131.52 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 Hoffman, et al. Expires November 27, 2015 [Page 8] Internet-Draft DNS Terminology May 2015 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]. Full-service resolver: Section 184.108.40.206 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 DNS 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 Section 1 of [RFC2308]) 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. 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". 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 Hoffman, et al. Expires November 27, 2015 [Page 9] Internet-Draft DNS Terminology May 2015 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". Master server: See primary server. Primary master: The primary master is named in the zone's SOA MNAME field and optionally by an NS resource record. (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." Stealth server: This is the same as a slave server except that it is not listed in an NS resource record 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. Hoffman, et al. Expires November 27, 2015 [Page 10] Internet-Draft DNS Terminology May 2015 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 or not 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". 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 or not 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". Child-centric resolver: A DNS resolver that, instead of serving the NS RRset and glue records that it obtained from the parent of a zone, serves data from the authoritative servers for that zone. The term "child-centric" is meant as the opposite of "parent- centric", which means a resolver that simply serves the NS RRset and glue records for a zone that it obtained from the zone's parent, without checking the authoritative servers for that zone. 6. Zones This section defines terms that are used when discussing zones that are being served or retrieved. Hoffman, et al. Expires November 27, 2015 [Page 11] Internet-Draft DNS Terminology May 2015 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 [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 [RFC0882] as "the name server that has authority over the place in the domain name space that will hold the new domain". 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.) (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.". 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". 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" instead of "apex". Delegation: The process by which a separate zone is created in the name space beneath the apex of a given domain. Delegation happens Hoffman, et al. Expires November 27, 2015 [Page 12] Internet-Draft DNS Terminology May 2015 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 the name of which is either subordinate to or (rarely) the same as the zone origin. In-bailiwick name servers require glue in their parent zone. (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 Section 4.2.1 of [RFC1034]) 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 Hoffman, et al. Expires November 27, 2015 [Page 13] Internet-Draft DNS Terminology May 2015 NS RRs below the zone cut. This reveals the ambiguity in the notion of authoritative data, because the parent-size NS records authoritatively indicate the delegation, even though they are not themselves authoritative data. Root zone: The zone whose origin 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 which consists mostly of delegations to child zones. This term is used in contrast to a zone which 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. 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 bound 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) It is often to deliver malware. Because the addresses change so rapidly, it is difficult to definitively find 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. 7. 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 Hoffman, et al. Expires November 27, 2015 [Page 14] Internet-Draft DNS Terminology May 2015 formally, whoever decides what data goes into a zone is the registry for that zone. 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 be 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. 8. General DNSSEC Most DNSSEC terms are defined in [RFC4033], [RFC4034], and [RFC4035]. The terms that have caused confusion in the DNS community are highlighted here. DNSSEC-aware and DNSSEC-unaware: Section 2 of [RFC4033] defines many types of resolvers and validators. In specific, the terms "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'" are all defined. (Note that the term "validating resolver", which is used in some places in those documents, is nevertheless not defined in that section.) 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 Hoffman, et al. Expires November 27, 2015 [Page 15] Internet-Draft DNS Terminology May 2015 publication of [RFC6840], NSEC records are no longer required for signed zones: a signed zone might include NSEC3 records instead. 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] adding an additional situation when 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 4034) NSEC3: The NSEC3 resource record is quite different than the NSEC resource record. Like the NSEC record, the NSEC3 record also provides authenticated denial of existence; however, NSEC3 records mitigates against zone enumeration and support Opt-Out. NSEC3 resource records are defined in [RFC5155]. Opt-out: The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations. (Quoted from [RFC5155], section 220.127.116.11.) 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 call "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.) Hoffman, et al. Expires November 27, 2015 [Page 16] Internet-Draft DNS Terminology May 2015 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. 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 RRdata 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. (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) 9. 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. Section 5 of [RFC4033] says: Hoffman, et al. Expires November 27, 2015 [Page 17] Internet-Draft DNS Terminology May 2015 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: Hoffman, et al. Expires November 27, 2015 [Page 18] Internet-Draft DNS Terminology May 2015 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 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. IANA Considerations This document has no effect on IANA registries. 11. Security Considerations These definitions do not change any security considerations for the DNS. 12. 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, and many others in the DNSOP Working Group have helped shape this document. Hoffman, et al. Expires November 27, 2015 [Page 19] Internet-Draft DNS Terminology May 2015 13. References 13.1. Normative References [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", RFC 882, November 1983. [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. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC1206] Malkin, G. and A. Marine, "FYI on Questions and Answers: Answers to commonly asked "new Internet user" questions", RFC 1206, February 1991. [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996. [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection and Operation of Secondary DNS Servers", BCP 16, RFC 2182, July 1997. [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, March 1998. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. Hoffman, et al. Expires November 27, 2015 [Page 20] Internet-Draft DNS Terminology May 2015 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name System", RFC 4592, July 2006. [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008. [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", STD 69, RFC 5730, August 2009. [RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol (AXFR)", RFC 5936, June 2010. [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, "Recommendations for the Remediation of Bots in ISP Networks", RFC 6561, March 2012. [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the DNS", RFC 6672, June 2012. [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, December 2012. [RFC6840] Weiler, S. and D. Blacka, "Clarifications and Implementation Notes for DNS Security (DNSSEC)", RFC 6840, February 2013. [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A Framework for DNSSEC Policies and DNSSEC Practice Statements", RFC 6841, January 2013. [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. 13.2. Informative References [DBOUND] "DBOUND Working Group", 2015, <https://datatracker.ietf.org/wg/dbound/charter/>. [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet host table specification", RFC 952, October 1985. Hoffman, et al. Expires November 27, 2015 [Page 21] Internet-Draft DNS Terminology May 2015 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August 1996. [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, September 2004. [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", RFC 4641, September 2006. [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security (DNSSEC) Opt-In", RFC 4956, July 2007. [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, August 2009. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. [RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, August 2010. [RFC5892] Faltstrom, P., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)", RFC 5892, August 2010. [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, August 2010. [RFC5894] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, August 2010. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011. Authors' Addresses Paul Hoffman VPN Consortium 127 Segre Place Santa Cruz, CA 95060 USA Email: email@example.com Hoffman, et al. Expires November 27, 2015 [Page 22] Internet-Draft DNS Terminology May 2015 Andrew Sullivan Dyn 150 Dow St, Tower 2 Manchester, NH 1604 USA Email: firstname.lastname@example.org 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@example.com Hoffman, et al. Expires November 27, 2015 [Page 23]