Network Working Group P. Hoffman Internet-Draft VPN Consortium Intended status: Best Current Practice A. Sullivan Expires: July 23, 2015 Dyn K. Fujiwara JPRS January 19, 2015 DNS Terminology draft-hoffman-dns-terminology-01 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 July 23, 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 July 23, 2015 [Page 1] Internet-Draft DNS Terminology January 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 Message Format . . . . . . . . . . . . . . . . . . . . . 4 4. Response Codes . . . . . . . . . . . . . . . . . . . . . . . 4 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 5 6. DNS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 9 9. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 10 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 11. Security Considerations . . . . . . . . . . . . . . . . . . . 12 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 13.1. Normative References . . . . . . . . . . . . . . . . . . 13 13.2. Informative References . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction The DNS is a simple query-response protocol whose messages in both directions have the same format. The protocol and message format are defined in [RFC1034] and [RFC1035]. These RFCs defined some terms, but later documents defined others. Some of the terms from RFCs 1034 and 1035 now have somewhat different meanings than they did in 1987. This document collects a wide variety of DNS-related terms. Some of them have been precisely defined in earlier RFCs, some have been loosely defined in earlier RFCs, and some are not defined in any earlier RFC at all. The definitions here are believed to be the consensus definition of the DNS community, both protocol developers and operators. Some of the definitions differ from earlier RFCs, and those differences are noted. The terms are organized loosely by topic. Some definitions are for new terms for things that are commonly talked about in the DNS community but that never had terms defined for them. In this document, where the consensus definition is the same as the one in an RFC, that RFC is quoted. Where the consensus definition has changed somewhat, the RFC is mentioned but the new stand-alone definition is given. Hoffman, et al. Expires July 23, 2015 [Page 2] Internet-Draft DNS Terminology January 2015 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 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. (Note: the formatting of this early draft is a bit funky. It will improve in later drafts. Bikeshedding the format, as compared to the content, of this draft is probably premature.) 2. Names Domain name -- Section 3.1 of RFC 1034 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". Host name -- This term and its equivalent, "hostname", have been widely used but are not defined in RFC 1034, 1035, 1123, or 2181. 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 RFC 1034, 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 RFC 1123). People also sometimes use the term hostname to refer to just the first label of an FQDN. In addition, people sometimes use this term Hoffman, et al. Expires July 23, 2015 [Page 3] Internet-Draft DNS Terminology January 2015 to describe any name that refers to a machine, and those might include labels that do not conform to the "preferred name syntax". 3. DNS Message Format Header -- The first 12 octets of a DNS message. Many of the fields and flags in the header diagram in section 4.1.1 of RFC 1035 are referred to by their names in that diagram. For example, the response codes are called "RCODEs", and the authoritative answer bit is often called "the AA flag" or "the AA bit". RCODEs are covered in Section 4. TTL -- The "time to live" of a resource record. A TTL value is an unsigned number, with a minimum value of 0, and a maximum value of 2147483647. That is, a maximum of 2^31 - 1. When transmitted, the TTL is encoded in the less significant 31 bits of the 32 bit TTL field, with the most significant, or sign, bit set to zero. (Quoted from [RFC2181], section 8) (Note that RFC 1035 erroneously stated that this is a signed integer; it is fixed in an erratum.) The TTL "specifies the time interval that the resource record may be cached before the source of the information should again be consulted". (Quoted from RFC 1035, section 3.2.1) Also: "the time interval (in seconds) that the resource record may be cached before it should be discarded". (Quoted from RFC 1035, section 4.1.3). Despite being defined for a resource record, the TTL of every resource record in an RRset is required to be the same (RFC2181, section 5.2). Glue records -- Resource records which are not part of the authoritative data, and are address resource records for the servers listed in the message. They contain data that allows access to name servers for subzones. (Definition from RFC 1034, section 4.2.1) Referrals -- Data from the authority section of a non-authoritative answer. RFC 1035 section 2.1 defines "authoritative" data. However, referrals at zone cuts are not authoritative. Referrals may be a zone cut NS resource records and their glue. NS records on the parent side of a zone cut are an authoritative delegation, but are not treated as authoritative data by the client. [[ A more complete and precise definition will be needed here. ]] 4. Response Codes Some of response codes that are defined in RFC 1035 have gotten their own shorthand names. Some common response code (RCODE) names that appear without reference to the numeric value are "FORMERR", "SERVFAIL", and "NXDOMAIN". All of the RCODEs are listed at Hoffman, et al. Expires July 23, 2015 [Page 4] Internet-Draft DNS Terminology January 2015 http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml, although that site uses mixed-case capitalization, while most documents use all-caps. NODATA -- This is not an actual response code, but instead is the combination of an RCODE of 0 (NOERROR) and an Answer section that is empty. That is, it indicates that the response is no answer, but that there was not supposed to be one. Section 1 of [RFC2308] defines it as "a pseudo RCODE which indicates that the name is valid, for the given class, but are no records of the given type." 5. Resource Records RR -- A short form for resource record. (RFC 1034, section 3.6.) RRset -- A set of resource records with the same label, class and type, but with different data. (Definition from RFC 2181) Also spelled RRSet in some documents. As a clarification, "same label" in this definition means "same owner name". OPT -- A pseudo-RR (sometimes called a meta-RR) that is used only to contain control information pertaining to the question-and-answer sequence of a specific transaction. contains control information pertaining to the question-and-answer sequence of a specific transaction. (Definition from [RFC6891], section 6.1.1) Owner -- The domain name where a RR is found (RFC 1034, section 3.6). Often appears in the term "owner name". 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 RFC 1035. The names (in the order they appear in the SOA RDATA) are MNAME, RNAME, SERIAL, REFRESH, RETRY, and EXPIRE, MINIMUM. Note that the meaning of MINIMUM field is updated in Section 4 of RFC 2308; the new definition is that the MINIMUM field is only "the TTL to be used for negative responses". 6. 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. [[ There is a request to "first describe the iterative and recursive resolution processes, and mention the expected values of the RD,RA,AA bits. Then you can describe the distinctions between recursive and Hoffman, et al. Expires July 23, 2015 [Page 5] Internet-Draft DNS Terminology January 2015 iterative clients, and between recursive and authoritative servers, in terms of the roles they play in the different resolution processes." This would require the section to be quite different than the other sections in the document. ]] Resolver -- A program that extracts information from name servers in response to client requests. (Quoted from RFC 1034, section 2.4) It is a program that interfaces user programs to domain name servers. The resolver is located on the same machine as the program that requests the resolver's services. (Quoted from RFC 1034, section 5.1) A resolver performs queries for a name, type, and class, and receives answers. The logical function is called "resolution". In practice, the term is usually referring to some specific type of resolver (some of which are defined below), and understanding the use of the term depends on understanding the context. Stub resolver -- A resolver that cannot perform all resolution itself. Stub resolvers generally depend on a recursive resolver to undertake the actual resolution function. Stub resolvers are discussed but never fully defined in RFC 1034, section 5.3.1. Iterative mode -- A resolution mode of a server that receives DNS queries and responds with a referral to another server. Section 2.3 of RFC 1034 describes this as "The server refers the client to another server and lets the client pursue the query". 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 RFC 1034 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. Authoritative server -- A system that responds to DNS queries with information about zones for which it has been configured to answer with the AA flag in the response header set to 1. It is a server that has authority over one or more DNS zones. Note that it is possible for an authoritative server to respond to a query without the parent zone delegating authority to that server. Hoffman, et al. Expires July 23, 2015 [Page 6] Internet-Draft DNS Terminology January 2015 Slave -- An authoritative server which uses zone transfer to retrieve the zone. (Quoted from [RFC1996], section 2.1) Master -- Any authoritative server configured to be the source of zone transfer for one or more slave servers. (Quoted from RFC 1996, section 2.1) Primary master -- The primary master is named in the zone's SOA MNAME field and optionally by an NS resource record. (Quoted from RFC 1996, section 2.1) 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 RFC 1996, section 2.1) A stealth server is often actually a master for zone transfers, and in that case is called a "hidden master". DNS forwarder -- A system receives a DNS query, possibly changes the query, sends the resulting query to a recursive resolver, receives the response from a resolver, possibly changes the response, and sends the resulting response to the stub resolver. Section 1 of RFC 2308 describes a forwarder as "a nameserver used to resolve queries instead of directly using the authoritative nameserver chain". RFC further says "The forwarder typically either has better access to the internet, or maintains a bigger cache which may be shared amongst many resolvers." [RFC5625] does not give a specific definition for DNS forwarder, but describes in detail what features they need to support. The protocol interfaces for DNS forwarders are exactly the same as those for recursive resolvers (for interactions with DNS stubs) and as those for stub resolvers (for interactions with recursive resolvers). Full resolver -- This term is used in RFC 1035, but it is not defined there. RFC 1123 defines a "full-service resolver" that may or may not be what was intended by "full resolver" in RFC 1035. In the vernacular, a full-service resolver is usually one that would be suitable for use by a stub resolver. Consensual policy-implementing resolver -- A resolver that changes some answers it returns based on policy criteria, such as to prevent access to malware sites. These policy criteria are agreed to by systems that query this resolver through some out of band mechanism (such as finding out about the resolver from a web site and reading the policy). Non-consensual policy-implementing resolver -- A resolver that is not a consensual policy-implementing resolver that changes the answers it returns. The difference between this and a consensual policy- Hoffman, et al. Expires July 23, 2015 [Page 7] Internet-Draft DNS Terminology January 2015 implementing resolver is that users of this resolver are not expected to know that there is a policy to change the answers it returns. 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 RFC 2308) 7. Zones This section defines terms that are used when discussing zones that are being served or retrieved. Zone -- A unit of organization of authoritative data. Zones can be automatically distributed to the name servers which provide redundant service for the data in a zone. (Quoted from RFC 1034, section 2.4). Child -- The entity on record that has the delegation of the domain from the Parent. (Quoted from [RFC7344], section 1.1) Parent -- The domain in which the Child is registered. (Quoted from RFC 7344, section 1.1) 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". Zone cut -- The delimitation point between two zones where the origin of one of the zones is the child of the other zone. (Section 6 of RFC 2181 uses this term extensively, although never actually defines it.) Section 4.2 of RFC 1034 uses "cuts" as "zone cut". In-bailiwick response -- A response in which the name server answering is authoritative for an ancestor of the owner name in the response. The term normally is used when discussing the relevancy of glue records. For example, the parent zone example.com might reply with glue records for ns.child.example.com. Because the child.example.com zone is a descendant of the example.com zone, the glue is in-bailiwick. Out-of-bailiwick response -- A response in which the name server answering is not authoritative for an ancestor of the owner name in the response. Origin -- 1. The domain name within which a given relative domain name appears. Generally seen in the context of "$ORIGIN", which is a control entry defined in RFC 1035, section 5.1, as part of the master file format. For example, if the $ORIGIN is set to "example.org.", then a master file line for "www" is in fact an entry for "www.example.org.". 2. The domain name that appears at the top of a zone, that is, the owner name of the apex records. Hoffman, et al. Expires July 23, 2015 [Page 8] Internet-Draft DNS Terminology January 2015 Authoritative data -- RRsets in the Answer section of a DNS response that has the AA bit in the response header set to 1. Delegation -- The process by which a separate zone is created in the name space beneath a given domain. Delegation happens when an NS RRset is added in the parent zone for the child origin, and a corresponding zone apex is created at the child origin. Apex -- The SOA and NS RRsets at the origin of a zone. This is also called the "zone apex". Root zone -- The zone whose origin is the zero-length label. Also sometimes called "the DNS root". Empty non-terminal -- A domain name that has no RRsets, but has descendants that have RRsets. 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. Wildcard -- RFC 1034 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]. 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 RFC 4033 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. 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 RFC 4033, 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. 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 NSEC -- The NSEC resource record lists two separate things: the next owner name (in the canonical ordering of the zone) that contains Hoffman, et al. Expires July 23, 2015 [Page 9] Internet-Draft DNS Terminology January 2015 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. NSEC3 resource records are defined in [RFC5155]. Opt-out -- The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations. (Quoted from Section 3.1.2.1 of RFC 5155) 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 RFC 6841, 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 RFC 4033 and 4035, although the two definitions differ a bit. Section 5 of RFC 4033 says: Hoffman, et al. Expires July 23, 2015 [Page 10] Internet-Draft DNS Terminology January 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 RFC 4035 says: Hoffman, et al. Expires July 23, 2015 [Page 11] Internet-Draft DNS Terminology January 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, and others have helped shape this document. [[ More acks will go here as people point out new terms to add and changes to the ones we have listed here. ]] Hoffman, et al. Expires July 23, 2015 [Page 12] Internet-Draft DNS Terminology January 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. [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996. [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, March 1998. [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. [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. [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. Hoffman, et al. Expires July 23, 2015 [Page 13] Internet-Draft DNS Terminology January 2015 [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 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet host table specification", RFC 952, October 1985. [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, August 2009. Authors' Addresses Paul Hoffman VPN Consortium 127 Segre Place Santa Cruz, CA 95060 USA Email: paul.hoffman@vpnc.org Andrew Sullivan Dyn 150 Dow St, Tower 2 Manchester, NH 1604 USA Email: asullivan@dyn.com Kazunori Fujiwara Japan Registry Services Co., Ltd. Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda-ku, Tokyo 101-0065 Japan Phone: +81 3 5215 8451 Email: fujiwara@jprs.co.jp Hoffman, et al. Expires July 23, 2015 [Page 14]