How DNS works

Without DNS, we would only be able to visit any website directly via IP address, no human-readable anymore

DNS stands for Domain Name System
Its purpose is to resolve and translate human-readable website name to IPv4 or IPv6 address
It’s basically a large Database which resides on various servers around the world, that contains the names and IP address of various host/domains.
Without DNS, we would only be able to visit any website directly via IP address, no human-readable anymore
DNS works on both the TCP and UDP Protocols, port 53

Whenever you visit a domain such as github.com, the browser’s journey begins »>

Step 1: Request information

TODO: Type github.com to address bar of Chrome, it’s a new request for browser

Step 2: DNS cache on Chrome

Step 3: DNS cache on OS

TODO: OS receive request from chrome, then check its DNS cache, see mDNSResponder on MacOS
True: Return IP of github.com for browser
False: OS didn’t know it before same as browser, OS call the Resolver (Recursive DNS servers)

Step 4: The Resolver on ISP (Internet Service Provider)

TODO: local OS sends a DNS Query to the Resolver by using UDP Protocol over Port 53, the Resolver will check its cache to find IP for github.com
True: Return IP address for OS
False: ISP didn’t know it before same as our OS. ISP will ask the Root server

Step 5: The ROOT servers - see https://www.iana.org/domains/root/servers

Fact: We have 13 root server for DNS around the world. They don’t know the answer, but they know where to find it.
TODO: Root look at the first part of request, reading from right to left .com <–
Direct our request to Top-Level Domain (TLD) name servers for .com, it’s Verisign TLD
ISP will store TLD information, no need ask the root again.

Step 6: The TLD nameservers

TODO: The TLD nameservers review the next part of our request - github
Direct our query to the nameservers responsible for this specific domain
These Authoritative nameservers are responsible for knowing all the information about a specific domain, which are stored in DNS records

Step 7: The Authoritative nameservers

TODO: The Resolver (ISP) retrieves the A record for github.com from the authoritative nameservers and stores the record in ISP’s local cache
More keywords: time-to-live value, Domain Registrar, types of records

Step 8: Receive the answer

TODO: Resolver returns the A record back to OS
OS stores the record in its cache, reads the IP address then passes information to Chrome
Chrome stores the record in its cache

This entire process, from start to finish, takes only milliseconds to complete.

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