Client use is illegal only..As for your second question all you have to do is to put your IP Adress inside "properties" files in the "config" folder of both login and gameserver
where it says:
ExternalIPAddress: "your WAN IP here" (you can find it here: www.whatismyip.com)
InternalIPAddress: "your LAN IP here" (you can find it: start=>run then type "ipconfig")
After that, open ports 2106,7777,9104 in your router,
then edit the l2.ini file of the lineage 2 system folder, add your IP, and share it with your friends!
(GR: K oti katalaves katalaves)