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
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.
Transport Related
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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