What is the difference between localhost and 127.0.0.1?

Often, we view localhost and 127.0.0.1 as interchangeable ways of referring to the local machine, but today I want to highlight a time when there's an important distinction between them.

When connecting to a MySQL service, if you specify the host as localhost it will attempt to connect via a Unix socket, something like /tmp/mysql.sock.

But if you specify 127.0.0.1, it instead uses TCP/IP to connect locally using port 3306 (or whatever you have configured).

When does this matter? Let's say you spin up a MySQL server in Docker. It could be for local development or for running a CI pipeline.

In this case, the MySQL socket is not exposed to the host machine, so a connection to localhost will fail. A connection to 127.0.0.1 will work though, assuming your exposing the right port in the Docker configuration.

I like to just keep things consistent, and always use the explicit 127.0.0.1 address.

Here to help,

Joel

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