How to connect to the Docker host machine from within a Docker container?

Prerequisites

Make sure to have:

• Java 11 installed
• Spring Boot CLI installed (make sure not to install version 3 or later, since the run command has been removed since 3.x)
• Docker installed

Blogpost

Small codebase with blogpost: How to connect to the Docker host from inside a Docker container?

Run the most simple Spring Boot web application

spring run helloWorld.groovy

A Spring Boot application will start at port 8080 on your host: http://localhost:8080

Start and attach to the Ubuntu container

docker run -it --add-host=host.docker.internal:host-gateway ubuntu bash

You have to specify the run flag --add-host=host.docker.internal:host-gateway (so just executing docker run -it ubuntu bash) otherwise your Ubuntu container can't resolve host.docker.internal and you will get an error like:

curl: (6) Could not resolve host: host.docker.internal

Install curl (inside the Ubuntu container):

apt update && apt install curl -y

Reach to Spring Boot application that is running on the host

curl http://host.docker.internal:8080

Response:

Hello world container. You are able to reach the Docker host!

Docker compose

Example config using Docker compose

version: "3.8"
services:

  ubuntu:
    image: ubuntu
    container_name: ubuntu
    extra_hosts:
      - "host.docker.internal:host-gateway"
    command: sleep infinity

Start the Ubuntu container using Docker compose:

docker-compose up -d

Attach to the Ubuntu container started using Docker compose:

docker exec -it ubuntu bash

Shut down the Ubuntu container:

docker-compose down -v