Running a Flask App on 0.0.0.0 But It Still Shows 127.0.0.1

While working on a Flask application, I encountered a perplexing issue. I wanted to make my app accessible externally by running it on 0.0.0.0, but despite setting the host accordingly, the server still displayed that it was running on http://127.0.0.1:8081. In this blog post, I'll walk through the problem, the steps I took to solve it, and explain why this behavior occurs.

The Problem

I had a Flask application that I wanted to run on all available network interfaces so that it could be accessed from other devices on the network. To do this, I set the host parameter in the app.run() method to 0.0.0.0:

if __name__ == '__main__':
    app.run(port=8081, debug=True, host="0.0.0.0")

After running the application, I expected it to indicate that it was running on http://0.0.0.0:8081. However, the console output was as follows:

* Running on all addresses (0.0.0.0)
* Running on <http://127.0.0.1:8081>
* Running on http://<my_server_ip>:8081

This was confusing because it seemed like the server was still bound to 127.0.0.1 (localhost), and I wasn't sure if it was accessible externally.

Investigating the Issue

To understand what was happening, I needed to dive deeper into how Flask's development server works and what the console messages actually mean.

Understanding Flask's Console Output

When you run a Flask app with app.run(host='0.0.0.0'), Flask binds the server to all available network interfaces. This means the app is accessible via any IP address assigned to the machine.

The console output includes helpful messages:

* Running on all addresses (0.0.0.0)
* Running on <http://127.0.0.1:8081>
* Running on http://<my_server_ip>:8081

Here, http://127.0.0.1:8081 is provided as a convenience, indicating that the app is accessible on the local machine. The key line is:

* Running on all addresses (0.0.0.0)

This line confirms that the server is listening on all interfaces.

Testing External Access

To verify that the app was indeed accessible externally, I tried accessing it from another device on the same network using the server's IP address:

http://<my_server_ip>:8081

Sure enough, the app was accessible, confirming that it was running on 0.0.0.0 despite the console output emphasizing 127.0.0.1.

Understanding 0.0.0.0 vs. 127.0.0.1

• 0.0.0.0: This is a special IP address that tells the server to listen on all available network interfaces.
• 127.0.0.1: This is the loopback address (localhost), which is only accessible from the local machine.

When you set host='0.0.0.0', Flask's development server binds to all interfaces, but it still includes 127.0.0.1 in the console output for convenience.

The Solution

The key realization was that the Flask development server was indeed running on all interfaces, even though the console output might suggest otherwise. Here's what I did to confirm and resolve the issue:

1. Verified the Server Binding: Ensured that app.run(host='0.0.0.0', port=8081) was correctly set.
2. Ignored the 127.0.0.1 Message: Understood that the console output includes 127.0.0.1 for convenience and does not mean the server is restricted to localhost.
3. Tested External Access: Accessed the app from another device using the server's IP address to confirm external accessibility.
4. Used a Production Server (Optional): For a production environment, it's recommended to use a WSGI server like Gunicorn. This wasn't necessary to solve the immediate issue, but it's a good practice for deployment.

Updated Code

Here's the simplified code:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello, World!"

if __name__ == '__main__':
    app.run(port=8081, debug=True, host="0.0.0.0")

Additional Considerations

Using a Production WSGI Server

While the Flask development server is suitable for testing and development, it's not designed for production use. For deploying your application, consider using a production-grade WSGI server like Gunicorn:

1. Install Gunicorn:

   pip install gunicorn

2. Run Your App with Gunicorn:

   gunicorn -w 4 -b 0.0.0.0:8081 app:app

   • w 4 specifies the number of worker processes.
   • app:app refers to your application module and Flask app object.

This approach ensures better performance and security for production environments.

Conclusion

The confusion arose from misinterpreting the console output when running the Flask development server. Even though the output emphasizes 127.0.0.1, the server is bound to all interfaces when host='0.0.0.0' is set. By testing external access, I confirmed that the app was accessible from other devices.

Key Takeaways

• Setting host='0.0.0.0': This allows your Flask app to be accessible on all network interfaces.
• Console Output: The inclusion of 127.0.0.1 in the console messages does not limit the server to localhost.
• Testing External Access: Always verify by accessing the app from an external device.
• Consider Using a Production Server: For deployment, use a WSGI server like Gunicorn for better performance and security.

References

• Flask Documentation: Running the Development Server
• Working with Gunicorn