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Glossary

Definitions of key DNS terms used in RFC 1035.

Core Terms​

ANCOUNT​

Answer Count - Field in the DNS message header specifying the number of RRs in the answer section.

ARCOUNT​

Additional Record Count - Field in the DNS message header specifying the number of RRs in the additional section.

Authoritative Answer (AA)​

Authoritative Answer - Flag in the response indicating that the response comes from an authoritative name server.

AXFR​

Complete Zone Transfer - QTYPE value (252) requesting the transfer of an entire zone.

Message Format Terms​

Header - The 12-byte fixed portion of a DNS message containing control information.

Message Compression​

Message Compression - Technique using pointers to eliminate duplication of domain names.

Opcode​

Operation Code - 4-bit field specifying the query type (QUERY, IQUERY, STATUS).

QDCOUNT​

Question Count - Field specifying the number of entries in the question section.

QNAME​

Query Name - Domain name in the question section.

QTYPE​

Query Type - Specifies the requested RR type in the question section.

QCLASS​

Query Class - Specifies the requested RR class in the question section.

Response Codes​

NOERROR (0)​

No Error - Query completed successfully.

FORMERR (1)​

Format Error - Server unable to interpret the query.

SERVFAIL (2)​

Server Failure - Server problem preventing query processing.

NXDOMAIN (3)​

Name Does Not Exist - The queried domain name does not exist.

NOTIMP (4)​

Not Implemented - Server does not support the requested query type.

REFUSED (5)​

Refused - Server refused the query for policy reasons.

Flags​

QR​

Query/Response Flag - 0=Query, 1=Response.

RD​

Recursion Desired - Client requests recursive query.

RA​

Recursion Available - Server supports recursive queries.

TC​

Truncation - Message truncated due to length limitations.

RR Types​

A (1)​

Address Record - IPv4 address.

NS (2)​

Name Server Record - Specifies authoritative name server.

CNAME (5)​

Canonical Name Record - Alias.

SOA (6)​

Start of Authority Record - Zone management information.

PTR (12)​

Pointer Record - Used for reverse DNS.

MX (15)​

Mail Exchange Record - Mail server.

TXT (16)​

Text Record - Arbitrary text data.

UDP​

User Datagram Protocol - Default transport protocol for DNS, 512-byte limit.

TCP​

Transmission Control Protocol - Used for large messages and zone transfers.

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