How to Connect to localhost from a Docker Container

Connecting to the host machine's localhost from inside a Docker container requires using special DNS names or networking configurations. Here are the main approaches:

1. Using host.docker.internal

On Windows and macOS, use the special DNS name host.docker.internal to access the host machine:

# Example connection string
mysql -h host.docker.internal -u root -p

2. Using host network mode

Run the container with host network mode to share the host's network namespace:

docker run --network host your-image

3. Using Docker Compose

In docker-compose.yml, configure the host connection:

services:
  web:
    image: nginx
    extra_hosts:
      - "host.docker.internal:host-gateway"

4. Using Linux-specific configuration

For Linux systems, add the host IP manually:

docker run --add-host=host.docker.internal:host-gateway your-image

When accessing services on the host machine, use the appropriate port numbers and ensure the host services are configured to accept external connections.

Common Connection Examples

# MySQL connection
mysql -h host.docker.internal -P 3306 -u user -p

# HTTP connection
curl http://host.docker.internal:8080

# MongoDB connection
mongodb://host.docker.internal:27017

Remember to configure your host services to listen on all interfaces (0.0.0.0) instead of just localhost (127.0.0.1) to allow container access.