I've gotten C&C Generals working via LAN Lobby using the following settings.
The hosting pc has udp connections to all other players, players only have udp connection to the hosting pc.
TCP/UDP ports:
8086
8088
Enabled advanced settings:
Ethernet II
UDP
ARP
rest are unchecked.
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This setup requires everyone to be on the same subnet and have different ip addresses. Now everyone can see themselves as well as the Host in the LAN Lobby and everyone can join the game when the Host creates it.
The game may stand still at the loading screen for a while but it will work fine after some time.
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Problems:
Seems to be some packet duplication going on, when chatting ingame messages will often display twice. This was first set up with udp tunnels between all players and that resulted in massive duplication of packets.
Sometimes theres a lot of ARP traffic.
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Improvements ?