Host names differ in and outside of docker

This is part of the Semicolon&Sons Code Diary - consisting of lessons learned on the job. You're in the docker category.

Last Updated: 2021-05-16

Be careful of different hostnames within the docker world vs. outside. For example, in a dockerized Laravel app, when I set DB_HOST in my .env file to mysql_docker (i.e. the container name of my container), it failed to connect to the DB when -- on my host machine -- I ran:

$ php artisan migrate

However it worked when I ran the same command from within the Docker container:

$ docker-compose exec -u root -T app php artisan migrate --force

In other words, host names based on container names (e.g. mysql_docker) are only available when you execute the code within the docker container. This makes sense and isn't surprising, but can prove confusing when you're in the heat of the moment and new to containers.

If I wanted the access the DB from outside docker, I should have used localnost IP address

DB_HOST=127.0.0.1
# The correct port must also be set!

Note too that the ports differ on each side. In docker compose I had the mysql ports as: 8306:3306. This means 8306 on 127.0.0.1 on my host machine, but 3306 on mysql_docker within any container.