Why Can't My Client Connect to My WebSocket Server Inside a Docker Container?

Troubleshooting Dockerizing a WebSocket Server

Issue: Difficulty establishing a WebSocket connection from a client outside a Docker container to a server within the container.

Your code uses http.Server{Addr: "localhost:8000"}, which limits server listening to the loopback interface (127.0.0.1) within the container. Since the client is external to the container, it cannot connect to this address.

Solution:

To resolve this issue, specify the server listening address as http.Server{Addr: ":8000"}, which instructs the server to listen on all available IP addresses within the container.

Additional Information:

When ports are exposed in a Docker container, Docker creates iptables rules to forward traffic from the host machine to the container. These rules ensure that requests reach the correct IP address and port within the container. You can view these rules using the commands:

iptables -n -L
iptables -t nat -n -L

By adjusting the listen address, your server can now accept WebSocket connections from clients outside the Docker container, enabling communication between them.

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