Hello, I have a server plugin in the works that adds custom blocks and items to my spigot server. As such, I will need to build a client mod (which I'll design around Forge) to allow players to see and use these custom blocks and items. As a bonus, anyone with this client mod installed will be able to use these blocks by spawning them with commands or creative mode when playing SP, but that's beside the point... Anyways, for the question regarding this thread: how can I require users to have this client mod? For my server, I have a mixture of medieval, rusty tech, and clean tech (sci-fi) styled blocks. Many builds will be built with the latter two, but these will exclusively be staff builds. For player builds, they will be building using the medieval style blocks (unless they break and collect or otherwise acquire a "tech" style block). This presents an issue - if the user does not have the client mod, they will either see a purple block or they will see a big transparent hole. With Bukkit and Spigot, users who are banned, not whitelisted, or denied a connection for some reason do not see the server, but a screen with a dirt background, an error message, and an OK button. Is there a way that I can require the client mod to present something (certificate, guid, a dword (8 bytes), etc.) to prove to the server that the client mod is active on penalty of being denied a connection?