Why Can't I Connect to My WebSocket Server Inside a Docker Container from Outside?

Troubleshooting Dockerization of a WebSocket Server

Issue:

Difficulty connecting to a WebSocket server running in a Docker container when accessing it from outside the container.

Server Code:

// server.go
func RootHandler(w http.ResponseWriter, r *http.Request) {
    ... (WebSocket handling code) ...
}

Dockerfile:

FROM golang:1.11.4-alpine3.8
... (Build and expose port commands) ...

Expected Behavior:

Connecting to the WebSocket server from outside the container should print "connected" on the client side.

Actual Error:

The client panics with the error: "panic: read tcp [::1]:60328->[::1]:8000: read: connection reset by peer."

Cause:

The server is listening on localhost (127.0.0.1) within the container, which is not accessible from outside the container.

Solution:

To resolve this issue, change the listen address of the server in server.go to ":8000" instead of "localhost:8000." This way, the server will listen on all IP addresses of the container.

// server.go
func RootHandler(w http.ResponseWriter, r *http.Request) {
    ... (WebSocket handling code) ...
}

// main()
server := http.Server{Addr: ":8000"}
... (Rest of server setup) ...

Additional Information:

• Docker exposes ports by creating iptables rules to forward traffic to the container's IP address.
• Docker containers typically get IP addresses within the docker0 network interface.
• When connecting from outside the container, traffic is forwarded to the container's IP address, which may not match the listen address of the server.

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