You can find here some information to start making server-side source modifications.

How rAthena works

rAthena has a multi-process architecture. It runs three different processes: login-server, char-server and map-server.

Login Server

Login server : receives the login packet, checks if account exists and if the password is correct. If all is OK, the connection is then passed to the char-server.

Char-Server

Char-server : controls all the character related, Guild, party, pet, inventory etc.

Map-Server

The last one is the map-server, which controls char and NPC position, chats, skills, exp etc.

Source Modifications

Before you start modifying rA, you should take a look first at the src/ folder. Remember, it's necessary to re-compile rAthena after a source modification. You can also find information about packets.