I'm starting work on a client mod and I have a need to verify some bytes on login. When a player joines I think they send about 40 bytes of information about themselfs to the server, I want to add about 20 more bytes to the end of that and use it as an identification. How does one change the bytes in Bukkit, I'm prepared to change the original bukkit class files if it is requred. Thanks.
Check out plugins like Spout or DenySpout DenySpout source: https://gist.github.com/2880544 For how to modify packets.