What is a DNS Lookup (Request or query)?

About

A DNS lookup is a request to a DNS server in order to get DNS record from a DNS name

• a A query will get you a ipv4 address
• a AAAA query will get you the ipv6 address
• a TXT query will get your a TXT record
• …

Example of response known as replies

Each message ¹⁾ consists of:

• a header with:
  • ID: query identifier
  • QR: message type:
    • 0: query
    • 1: response
  • OPCODE: kind of query:
    • 0: standard query (QUERY)
    • 1: inverse query (IQUERY)
    • 2: a server status request (STATUS)
  • RCODE: Response code:
    • 0: No error
    • 1: Query Format error
    • 2: Server failure
    • 3: Name Error (the domain name referenced in the query does not exist)
    • 4: Query Type Not Implemented
    • 5: Refused (For Policy reason)
• and four sections:
  • question: the question for the name server composed of:
    • a query type (QTYPE),
    • a query class (QCLASS),
    • a query domain name (QNAME).
  • answer: the answer (in resource records (RR) format),
    • with a status code ²⁾
  • authority: the authoritative name server (in RR format),
  • and an additional space information (in RR format)

A header field (flags) controls the content of these four sections

Example of A record query against datacadamia.com

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56993
;; flags: qr rd ra ; qd: 1 an: 2 au: 0 ad: 1 

;; OPT PSEUDOSECTION: 
; EDNS: version: 0; flags: ; udp: 512
;; QUESTIONS:
;;	datacadamia.com., type = A, class = IN

;; ANSWERS:
datacadamia.com.	300	IN	A	188.114.96.0
datacadamia.com.	300	IN	A	188.114.97.0

;; AUTHORITY RECORDS:

;; ADDITIONAL RECORDS:


;; Message size: 76 bytes

Type

Forward

A forward lookup translate a DNS name to an IP. See What is a DNS Forward Lookup?

Reverse

A reverse lookup translate a an IP to a DNS name. See DNS - rDNS - Reverse DNS (lookup|resolution) - from IP to name