Domain Name System(DNS) is a hierarchical and distributed name service that provides a naming system for computers, resources and other resources on the IP network.

DNS delegates the responsibility of assigning domain names and mapping those names to Internet Resources by designating authoritative name servers for each Domain.

Domain Names

Domain Name consists of one or more parts Technically called Labels delimited by dots. The right most label conveys the top-level domain. The hierarchy of domain descends from right to left. A Label may contain 0 to 63 characters as the length is only allowed to 6 bits. The full domain name may not exceed the length of 253 characters

DNS Protocol

DNS Protocol uses 2 types of DNS messages , queries and responses, both have the same format.

Each message consists of a header and 4 sections: question,answer,authority and additional spaces. A header field controls the content of the these 4 sections.

header

The header section consists of the following fields: Identification, Flags, Number of answers, Number of authority resource records(RR) , and Number of additional RRs. Each field is 16 bits long.

Question section

question

Resource Record

DNS specifies a database of information elements for network resources. The types of information elements are categorized and organized with a list of DNS record types.

RR

The record Types:

  • A Address record
  • AAAA ipv6 address record
  • TXT Text record readable by human

Command line

dig (Domain Information Groper) a flexible tool to interrogate DSN name servers. It performs DNS lookup and displays the answers returned from the name servers.

dig www.google.com

; <<>> DiG 9.10.6 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43900
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.google.com.			IN	A

;; ANSWER SECTION:
www.google.com.		69	IN	A	172.217.25.196

;; Query time: 1 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Tue Jan 28 15:16:56 +07 2025
;; MSG SIZE  rcvd: 59