Hey, I’ve run into this exact Docker port mapping issue on Windows 10 before, and since your Flask app works fine inside the container, let’s walk through some targeted fixes and checks:

1. Fix the Docker Run Command Parameter Order #

First, I noticed a small but critical issue in your docker run command: the --name flag should come before the image name. Right now, you’re passing --name simple-flask-app as a command argument to the container instead of a Docker option, which might cause unexpected behavior even if the container seems to run.

Correct command:

docker run -d --name simple-flask-app -p 5000:5000 simple-flask-app

Verify the port mapping is active with:

docker ps

You should see 0.0.0.0:5000->5000/tcp or :::5000->5000/tcp in the PORTS column. If only 5000/tcp shows up, the mapping didn’t apply correctly.

2. Ensure Flask Binds to 0.0.0.0 (Not Just 127.0.0.1) #

Even though you can reach Flask inside the container via 127.0.0.1:5000, Flask defaults to binding only to the container’s local loopback (127.0.0.1). This means the app won’t accept connections from outside the container (including port-mapped traffic from your Windows host).

• Check your Dockerfile or Flask app code:
  • If using flask run, make sure the command includes --host=0.0.0.0, e.g.:

    CMD ["flask", "run", "--host=0.0.0.0", "--port=5000"]
    

  • If running a Python script directly, ensure your app.run() call has host='0.0.0.0':

    if __name__ == '__main__':
        app.run(host='0.0.0.0', port=5000)
    

• Rebuild the image and restart the container after making this change.

3. Check for Windows Firewall Interference #

Windows Defender Firewall often blocks Docker’s port-mapped traffic, especially if you didn’t grant Docker Desktop network access when prompted.

• Temporary test: Disable the firewall briefly and try curl http://127.0.0.1:5000 again. If it works, you need to add a permanent rule:
  1. Open Windows Defender Firewall > Advanced Settings > Inbound Rules
  2. Create a new rule: Allow TCP port 5000, or allow the docker-proxy.exe process to communicate through the firewall.

4. Troubleshoot Docker Desktop Network Settings #

Docker’s network backend (WSL 2 vs. Hyper-V) can sometimes cause port mapping glitches on Windows:

• Restart Docker Desktop: Right-click the Docker tray icon > Restart. This fixes many transient network stack issues.
• Switch network backend: Go to Docker Desktop Settings > Resources > Network, then toggle between WSL 2 and Hyper-V backend (you’ll need to restart Docker after this).
• Test from WSL terminal (if using WSL 2): If you have WSL enabled, try accessing localhost:5000 from your WSL distro’s terminal instead of Windows CMD/PowerShell—WSL 2’s localhost mapping can sometimes be more reliable.

5. Verify Port Isn’t Occupied by Another Windows Process #

Another program might be using port 5000 on your host, preventing Docker from binding to it.

Check for port conflicts with:

netstat -ano | findstr :5000

If you see a process ID (PID) listed, kill it with:

taskkill /F /PID <your-PID>

Or use a different port for mapping, e.g.:

docker run -d --name simple-flask-app -p 5001:5000 simple-flask-app

6. Check Docker Bridge Network Connectivity #

If accessing the container’s direct IP still fails, verify the Docker bridge network is functioning:

1. Get your container’s IP:

   docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' simple-flask-app
   

2. Ping this IP from your Windows host. If ping fails, reset Docker’s network:

   docker network prune
   docker system prune -f
   

   Then restart Docker and re-run your container.

内容的提问来源于stack exchange，提问作者Benedikt Schmeitz