RKN Alert is a free Chrome add-on developed by Maris that allows users to easily identify whether a website is included in the registry of blocked websites maintained by the Russian government's telecommunications watchdog, Roskomnadzor.

The add-on displays a colored icon to indicate whether a website or IP address is blocked. If the address or IP is listed in the registry, the icon for that particular tab will turn red. This simple and efficient add-on works quickly, providing users with instant information about blocked websites.

The RKN Alert add-on downloads the entire registry from the internet and periodically updates it to ensure accuracy. It is a valuable tool for users who want to stay informed about which websites are blocked by Roskomnadzor.

Please note that the developer, Maris, has expressed a desire for someone to share a database of permanently blocked websites, which currently stands at around 4000.