Troubleshoot Connection Failed Errors
A connection failure can happen for several reasons.
Common causes:
- The backend is not running.
- The browser is connecting to the wrong port.
- The server and client use incompatible Socket.IO versions.
- Cross-origin access is not configured correctly.
- A reverse proxy is not forwarding WebSocket upgrade requests.
- Authentication middleware rejects the connection.
- A network firewall blocks the connection.
Diagnostic steps:
- Check the backend terminal.
- Inspect the browser Network tab.
- Look for failed Engine.IO requests.
- Inspect the connect_error event.
- Verify the server URL.
- Verify the authentication payload.
Remember that the initial Socket.IO transport may use HTTP long-polling before upgrading to WebSocket. A failed polling request can indicate an HTTP, proxy, or origin configuration issue.
Common Connection Problems
Connection refused
Server unavailable
Check whether the backend is running and listening on the expected port.
CORS error
Origin mismatch
Configure the exact frontend origin on the Socket.IO server.
Invalid namespace
Incorrect namespace
Verify the namespace path used by the client.
Authentication error
Invalid credentials
Verify the token and authentication middleware.
Key Takeaways
Inspect both browser and server logs.
Verify origins, ports, and authentication.
Check polling and WebSocket transport requests.