TheAlanWang

Notes

DNS

2 min read

DNS (Domain Naming System)

Definition: Domain Name System is a distributed, hierarchical naming system that translates human-readable domain names (like www.google.com) into IP addresses (like 142.250.x.x) and other records (e.g., MX, CNAME), so clients can locate services on the Internet.

DNS Lookup

When your computer needs to resolve a domain name (for example, www.google.com) and no cached result is available, it performs a recursive DNS lookup.

Your computer does not contact many DNS servers by itself. Instead, it sends the request to a recursive resolver (such as your ISP’s DNS server or a public resolver like 8.8.8.8). The resolver does the work on your behalf:

  1. Query the Root DNS servers
    The resolver asks: “Which servers handle the .com domain?”
    The root servers reply with a list of TLD (.com) DNS servers.
  2. Query the .com TLD DNS servers
    The resolver asks: “Which authoritative DNS servers are responsible for google.com?”
    The TLD servers reply with the authoritative name servers for google.com.
  3. Query the Authoritative DNS server
    The resolver asks: “What is the IP address (A/AAAA record) for www.google.com?”
    The authoritative server replies with one or more IP addresses.
  4. Return and cache the result
    The resolver returns the IP address to your computer and caches the result according to its TTL (Time To Live), so future lookups are faster.

nslookup Tool

alanwang@Alans-MacBook-Pro ~ % nslookup www.google.com
Server:		2600:1700:65a0:10a0::1
Address:	2600:1700:65a0:10a0::1#53
 
Non-authoritative answer:
Name:	www.google.com
Address: 64.233.185.106
Name:	www.google.com
Address: 64.233.185.147

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