How Domain Name System Work | System Design

DNS is internet directory, which translates human readable domain names such as unisala.com to a machine readable ip address. This is the reason why when you type unisala.com you can access this website hosted on a server.

According to google:

The Domain Name System (DNS) protocol is an important part of the web's infrastructure, serving as the Internet's phone book: every time you visit a website, your computer performs a DNS lookup.

There are different type of DNS servers in DNS hierarchy, each serving a different purpose.

1. When a browser makes a DNS query, it's asking for a resolver. DNS resolver could be from:
2. Internet service provider
3. Popular DNS like cloudfare 1.1.1.1
4. Google's DNS 8.8.8.8
5. If the DNS resolver does not have the answer in it's cache, it finds the right authoritative name-server and ask it
6. The authoritative name-server is the one that holds the answer
7. When we are updating a domain's DNS records, we are updating it's authoritative name-server.

How does the DNS resolver find the authoritative name-server?

1. There are 3 main levels of authoritative DNS servers:
2. Root nameservers: Store ip address of TLD name-servers
3. There are 13 root nameservers, with ip address assigned to it
4. There are many physical servers behind each ip address.
5. Through the magic of anycast, we get routed to closest one.
6. Top Level Domain (TLD): Store Ip Address of Authoritative Name-servers
7. TLD names are .com, .edu, and also country code like .np
8. Authoritative Name-servers: This provides the authoritative answer to DNS queries.

When we register a domain, the registar host the authoritative namerservers by default, but we can change them to others.

Cloud providers like AWS & cloudfare run authoritative nameservers.

This image illustrates how a DNS query is made from browswer when you enter google.com in the browser and how browser brings you the google.com app in your computer.

1. When you enter google.com browser 1st checks it's cache.
2. if no answer if found , it makes an Operating system call to try to get the answer.
3. If no answer if found, it reaches out to DNS resolver.
4. DNS resolver checks it's cache too, if the answer is not there, or cache has expired. It reaches out the root name server
5. Root nameserver return the ip address for those .com TLD nameserver
6. DNS resolver then reaches out to the .com TLD name server
7. .com TLD name server then returns the authoritative name server
8. Finally, DNS resolver reaches out to the google.com authoritative name-server
9. Authoritative name server returns the ip address of the google.com.
10. This DNs resolver then returns the ip address of google.com to the operating system,
11. then the OS returns the ip address to the browser
12. Browser then downloads the file from google.com as an website to interact.

Updating DNS records for a live high traffic production system:

TTL : Time to live for a cached record.

For example if unisala.com running on a server

192.168.123.132

i decide to upgrade the server for example move it to a new server with higher specs.

new ip address is:

192.168.123.104

Now i want all the user accessing unisala.com to go to this new server ip address.

1. DNS propagation is slow because there is TTL on each DNS record.
2. Some of the default TTLs are pretty long, also not every DNS resolver is a good citizen (there are some buggy once that don't honor TTL)

To mitigate the risk there are two practical steps to take:

1. Since we are updating the ip address,we will update the ttl of current record to short time i.e 60 secs, well in advance before we update ip.
2. This gives enough time to all the DNS servers to receive a shortened TTL
3. This will allow the actual record update to take effect based on the new shortened TTL.
4. Second leave the server running on the old ip address for a while
5. Only decommission the old server when traffic dies down to acceptable level.
6. Because some DNS resolvers don't honor the TTL, this could take a bit of time and patience.

DNS is what translates unisala.com -> 192.168.123.132, making it a critical component of the internet backbone.

This note is derived from ByteByteGo. All the credit goes to it.

#ByteByteGo #SystemDesign #UnderstandingHowInternetWorks.