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Network Working Group P. Mockapetris Request for Comments: 1035 ISI November 1987 Obsoletes: RFCs 882, 883, 973

DOMAIN NAMES – IMPLEMENTATION AND SPECIFICATION

1. STATUS OF THIS MEMO

This RFC describes the details of the domain system and protocol, and assumes that the reader is familiar with the concepts discussed in a companion RFC, “Domain Names – Concepts and Facilities” [RFC-1034].

The domain system is a mixture of functions and data types which are an official protocol and functions and data types which are still experimental. Since the domain system is intentionally extensible, new data types and experimental behavior should always be expected in parts of the system beyond the official protocol. The official protocol parts include standard queries, responses and the Internet class RR data formats (e.g., host addresses). Since the previous RFC set, several definitions have changed, so some previous definitions are obsolete.

Experimental or obsolete features are clearly marked in these RFCs, and such information should be used with caution.

The reader is especially cautioned not to depend on the values which appear in examples to be current or complete, since their purpose is primarily pedagogical. Distribution of this memo is unlimited.

Table of Contents

1. STATUS OF THIS MEMO 1 2. INTRODUCTION 3 2.1. Overview 3 2.2. Common configurations 4 2.3. Conventions 7 2.3.1. Preferred name syntax 7 2.3.2. Data Transmission Order 8 2.3.3. Character Case 9 2.3.4. Size limits 10 3. DOMAIN NAME SPACE AND RR DEFINITIONS 10 3.1. Name space definitions 10 3.2. RR definitions 11 3.2.1. Format 11 3.2.2. TYPE values 12 3.2.3. QTYPE values 12 3.2.4. CLASS values 13

Mockapetris [Page 1] RFC 1035 Domain Implementation and Specification November 1987

3.2.5. QCLASS values 13 3.3. Standard RRs 13 3.3.1. CNAME RDATA format 14 3.3.2. HINFO RDATA format 14 3.3.3. MB RDATA format (EXPERIMENTAL) 14 3.3.4. MD RDATA format (Obsolete) 15 3.3.5. MF RDATA format (Obsolete) 15 3.3.6. MG RDATA format (EXPERIMENTAL) 16 3.3.7. MINFO RDATA format (EXPERIMENTAL) 16 3.3.8. MR RDATA format (EXPERIMENTAL) 17 3.3.9. MX RDATA format 17 3.3.10. NULL RDATA format (EXPERIMENTAL) 17 3.3.11. NS RDATA format 18 3.3.12. PTR RDATA format 18 3.3.13. SOA RDATA format 19 3.3.14. TXT RDATA format 20 3.4. ARPA Internet specific RRs 20 3.4.1. A RDATA format 20 3.4.2. WKS RDATA format 21 3.5. IN-ADDR.ARPA domain 22 3.6. Defining new types, classes, and special namespaces 24 4. MESSAGES 25 4.1. Format 25 4.1.1. Header section format 26 4.1.2. Question section format 28 4.1.3. Resource record format 29 4.1.4. Message compression 30 4.2. Transport 32 4.2.1. UDP usage 32 4.2.2. TCP usage 32 5. MASTER FILES 33 5.1. Format 33 5.2. Use of master files to define zones 35 5.3. Master file example 36 6. NAME SERVER IMPLEMENTATION 37 6.1. Architecture 37 6.1.1. Control 37 6.1.2. Database 37 6.1.3. Time 39 6.2. Standard query processing 39 6.3. Zone refresh and reload processing 39 6.4. Inverse queries (Optional) 40 6.4.1. The contents of inverse queries and responses 40 6.4.2. Inverse query and response example 41 6.4.3. Inverse query processing 42

Mockapetris [Page 2] RFC 1035 Domain Implementation and Specification November 1987

6.5. Completion queries and responses 42 7. RESOLVER IMPLEMENTATION 43 7.1. Transforming a user request into a query 43 7.2. Sending the queries 44 7.3. Processing responses 46 7.4. Using the cache 47 8. MAIL SUPPORT 47 8.1. Mail exchange binding 48 8.2. Mailbox binding (Experimental) 48 9. REFERENCES and BIBLIOGRAPHY 50 Index 54

2. INTRODUCTION

2.1. Overview

The goal of domain names is to provide a mechanism for naming resources in such a way that the names are usable in different hosts, networks, protocol families, internets, and administrative organizations.

From the user’s point of view, domain names are useful as arguments to a local agent, called a resolver, which retrieves information associated with the domain name. Thus a user might ask for the host address or mail information associated with a particular domain name. To enable the user to request a particular type of information, an appropriate query type is passed to the resolver with the domain name. To the user, the domain tree is a single information space; the resolver is responsible for hiding the distribution of data among name servers from the user.

From the resolver’s point of view, the database that makes up the domain space is distributed among various name servers. Different parts of the domain space are stored in different name servers, although a particular data item will be stored redundantly in two or more name servers. The resolver starts with knowledge of at least one name server. When the resolver processes a user query it asks a known name server for the information; in return, the resolver either receives the desired information or a referral to another name server. Using these referrals, resolvers learn the identities and contents of other name servers. Resolvers are responsible for dealing with the distribution of the domain space and dealing with the effects of name server failure by consulting redundant databases in other servers.

Name servers manage two kinds of data. The first kind of data held in sets called zones; each zone is the complete database for a particular “pruned” subtree of the domain space. This data is called
authoritative. A name server periodically checks to make sure that its zones are up to date, and if not, obtains a new copy of updated zones

Mockapetris [Page 3] RFC 1035 Domain Implementation and Specification November 1987

from master files stored locally or in another name server. The second kind of data is cached data which was acquired by a local resolver. This data may be incomplete, but improves the performance of the retrieval process when non-local data is repeatedly accessed. Cached data is eventually discarded by a timeout mechanism.

This functional structure isolates the problems of user interface, failure recovery, and distribution in the resolvers and isolates the database update and refresh problems in the name servers.

2.2. Common configurations

A host can participate in the domain name system in a number of ways, depending on whether the host runs programs that retrieve information from the domain system, name servers that answer queries from other hosts, or various combinations of both functions. The simplest, and perhaps most typical, configuration is shown below:

Local Host | Foreign |
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