Recently I started asking myself questions about how DNS works. It turns out I forgot almost all I learned at university (damn frontend development!), so I wrote some notes to refresh my memory.

DNS stands for Domain Name Server. It is used by computer to find IP addresses of domains^[1].

When a browser needs an IP it asks a DNS Resolver. The resolver runs this algorithm:

• if it's not in cache, queries one of the 13 root nameservers. These IPs are hardcoded. The resolver will choose the fastest one.
• These root nameservers are nowadays replicated in hundreds of instances so every geographic region has a lot of those servers very close to the them. These nameservers are operated by different companies to ensure indipendence, transparency and adherence to standards.
• The root nameserver does not contain individual entries, but it contains the IPs of top domain nameservers . So, if you ask michelenasti.com , it will respond with .com nameservers (more than one, in order to be fault-resistant).
• the DNS Resolver now queries the .com nameserver and it will respond with michelenasti.com nameserver.
• the DNS Resolver asks michelenasti.com nameserver, that in my case is currently hosted on Netlify, and it will finally return the IP address of something ^[2] that will return my website.
• Finally, the DNS resolver responds back to the browser with the IP address.

Note: for every step, the DNS caches every result. Also, these results have a Time-To-Live (TTL) so if these results do change, they will be refetched after the time expires.

Question: why there are 13 root nameservers?

Since the #UDP spec says that datagrams less than 512 bytes should not be fragmented, DNS designers thought to restrict the number of root nameservers to 13 (32 * 13 = 416 bytes) leaving some space for future.

every address takes 32 bits = 4 bytes; However it seems addresses are hardcoded as 32 bytes. The error here is that in the packet there's not just the IP address but a whole record.

This is however historical and things have changed a lot. A description of how it currently works is here.

Question: Which one is chosen at any time?

BIND, the Open Source software that was powering up DNS in almost every operating system^[3], usually contacts all root nameservers and then chooses the one with least roundtrip time. source.

Question: When I connect to a network, what happens exactly? Why when I am at home i use my own DNS server and when I am at work I use the network's dns server?

#DHCP will give to clients, among other informations, also the nameserver for this subnet.

Question: What's the AnyCast protocol ?

Even though there are 13 IP addresses for root servers, in reality there are hundreds of them. Each IP is shared among same servers using AnyCast protocol, so there's a root server close to every subnet.

A description of Anycast is here.

Hope you have enjoyed my journey back to DNS world. Is there any other question you think I should add?

I want to thank Stéphane Bortzmeyer for suggesting some corrections to this article.