How does the browser resolve DNS?

Have you ever wondered how your browser resolves a domain name when you hit Enter on a URL? Here’s how it works, at a high level.

1. Your browser first looks in its own cache for the address. If it doesn’t find it, it checks the OS cache, and finally the ISP’s cache.

2. If the name isn’t cached, the next step is the /etc/hosts file. If there’s an IP address there, the browser tries to load the page from that server.

3. If there’s no entry in the hosts file, the resolver reads /etc/resolv.conf to find a nameserver. That nameserver will either have the answer cached, or it will use the root hints file containing the IP addresses of the root servers.

   You can run the dig command on its own to fetch the root hints for your configured resolver.

4. The root servers point to the root zone, managed by IANA (the Internet Assigned Numbers Authority).

5. The root zone database contains the top-level domains. For example, .com is a top-level domain delegated to VeriSign - so those are nameservers operated by VeriSign.

6. Those nameservers hold the records for the domain you’re trying to reach, and return its IP address.

And with the IP address in hand, your browser can finally open a connection and request the page.

• Internet
• Networking