Solving ERR_NAME_NOT_RESOLVED in Docker Container HTTP Requests

Описание: Learn how to troubleshoot and fix HTTP request issues in Docker containers, specifically solving the `ERR_NAME_NOT_RESOLVED` error when connecting a React frontend to a Flask backend.
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This video is based on the question https://stackoverflow.com/q/71066611/ asked by the user 'Arty' ( https://stackoverflow.com/u/8586359/ ) and on the answer https://stackoverflow.com/a/71066739/ provided by the user 'Amila Senadheera' ( https://stackoverflow.com/u/8510405/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting HTTP Requests Between Docker Containers

Docker has revolutionized the way we deploy applications, creating isolated environments known as containers. However, communicating between these containers can sometimes pose challenges, especially when dealing with HTTP requests. One common issue developers face is the ERR_NAME_NOT_RESOLVED error when trying to connect a frontend to a backend. In this guide, we'll dissect this problem and walk through the solution step by step.

Understanding the Problem

In our scenario, we have two basic Docker containers:

A React frontend container.

A Flask backend container.

When the React application attempts to make a GET request to the Flask service, an error is thrown:
GET http://backend:5000/ net::ERR_NAME_NOT_RESOLVED. This indicates that the frontend cannot resolve the hostname backend to connect to the Flask service running on port 5000. Let’s dive into how we can resolve this issue.

Solution Overview

The root of the problem lies in how the frontend is referring to the backend service. To fix the error, we must update the REACT_APP_URL environment variable in the Docker Compose configuration and ensure the response is correctly handled. Here's how to do it:

Step 1: Update Docker Compose Configuration

In the docker-compose.yml file, change the REACT_APP_URL environment variable from backend to localhost. This is necessary because the React frontend needs to communicate with the exposed port on the host machine rather than using the internal Docker network hostname:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Modify Fetch Request in React

After updating the environment variable, you need to ensure that the response from the fetch request is properly parsed as JSON. Here’s an updated version of the relevant code snippet in your React component:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Running Your Application

With the changes applied, go ahead and rebuild your Docker containers to apply the new settings. You can do this with the following command:

[[See Video to Reveal this Text or Code Snippet]]

Make sure both containers are running after executing the command.

Conclusion

By updating the REACT_APP_URL to localhost and ensuring the response is parsed as JSON, you create a robust communication channel between your React frontend and Flask backend container. Docker’s flexibility makes it a powerful tool for development, but understanding networking nuances is crucial for seamless communication between containers.

Feel free to reach out with any further questions or troubleshooting tips you've learned along the way! Happy coding!