Why Can't I Access My Dockerized Flask App From Outside the Container?

Deploying a Minimal Flask App in Docker: Tackling Server Connection Issues

When deploying a Flask application in Docker, you might encounter situations where the server runs but remains inaccessible from outside the container. This issue could result from incorrect configuration or networking concerns.

One common cause is the default configuration of the Flask development server. By default, Flask binds to the localhost interface (127.0.0.1), making the application only accessible within the container. To resolve this, you can explicitly specify the host interface to bind to.

In your provided Flask code, the issue can be addressed by modifying the run() method to bind to all interfaces (0.0.0.0) rather than localhost. Update the last line of your index.py script accordingly:

if __name__ == '__main__':
    app.run(host='0.0.0.0')

This modification will allow the server to listen on all network interfaces on the host, making it accessible externally.

If the problem persists, additional checks may be necessary. Ensure that the container's network is configured correctly and accessible from the host. Additionally, check the host machine's firewall settings to confirm if it allows incoming connections on the port where your Flask application is listening (typically port 5000).

Once these checks are complete, you should be able to access your Flask application from outside the Docker container. Remember, proper configuration of the host interface and networking settings is crucial for ensuring seamless communication between the container and external clients.

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