Enabling support for websockets in Synology DSM reverse proxy is easy:

1. Open Control Panel > Application Portal
2. Change to the Reverse Proxy tab
3. Select the proxy rule for which you want to enable Websockets and click on Edit
4. Change to the Custom Headers tab
5. Add two entries in the list:
   • Name: “Upgrade”, Value: “$http_upgrade”
   • Name: “Connection”, Value: “$connection_upgrade”