Something Crazy about localhost: Unveiling the Inner Workings

Ever wonder when you type localhost into your browser, you might take for granted that it magically knows to connect your local computer. However, there’s a fascinating mechanism behind how localhost works.

There were a bunch of questions I had in my mind.

1. How does it map to an IP address?
2. Does it function similarly to DNS?

Let’s dive into the details of localhost

Localhost is a hostname that refers to the current device used to access it. The name resolves to the IP address by default as 127.0.0.1 for IPv4 and ::1 for IPv6, pointing to the loopback network interface.

Wait what’s a loopback Network Interface?

The loopback interface is a kind of special network interface used by the local computer to send network traffic to itself. Imagine it’s like a virtual network device that software on your computer uses to test network applications without needing access to a physical network

• IPv4 -> 127.0.0.1
• IPv6 -> ::1

That’s great, So how does the loopback interface know this information ?

Mapping of Localhost to IP

The Mapping varies based on the operating system.

• Mac / Linux System -> /etc/hosts
• Windows: C:\Windows\System32\drivers\etc\hosts

Now you can play around with this hosts file and see how it’s mapping magically.

Does Localhost Work Like DNS?

While both localhost and DNS (Domain Name System) serve the purpose of resolving hostnames to IP addresses, they operate quite differently.

Hosts File: The resolution of localhost is handled locally through the hosts file. When you access localhost, the operating system reads the hosts file and maps localhost to 127.0.0.1 or ::1.

DNS: DNS is a hierarchical and decentralized naming system used for resolving domain names to IP addresses on the Internet. When you type a domain name (e.g., example.com) into your browser, the DNS resolver contacts multiple DNS servers to find the corresponding IP address.

Playing around with localhost using Python

import socket
print(socket.getaddrinfo('localhost', 0))

output:

[(<AddressFamily.AF_INET6: 30>, <SocketKind.SOCK_DGRAM: 2>, 17, '', ('::1', 0, 0, 0)), (<AddressFamily.AF_INET6: 30>, <SocketKind.SOCK_STREAM: 1>, 6, '', ('::1', 0, 0, 0)), (<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_DGRAM: 2>, 17, '', ('127.0.0.1', 0)), (<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_STREAM: 1>, 6, '', ('127.0.0.1', 0))]

Noticed the localhost carries both IPv4 and IPv6. So by using the IP address directly would be more efficient which would reduce the lookup. Interesting.

Conclusion

The simplicity and reliability of localhost mask the sophisticated mechanisms working behind the scenes. By mapping to 127.0.0.1 or ::1 through the hosts file, localhost provides an essential tool for developers and system administrators.

What did we learn

1. IP address directly would be more efficient which would reduce the lookup
2. IP addresses are just present within a hosts file.
3. DNS and local mapping of IP’s are different

Thanks for reading my first blog about something we use daily.