The commands are of the form:

--multiplay=in | out,Hz,destination address,destination port
--callsign=a_unique_name


Below are some examples of startup commands that demonstrate the use of the 
multiplayer facilities.

For two players on a local network or across the internet:
----------------------------------------------------------
Player1:
--multiplay=out,10,192.168.0.3,5500 --multiplay=in,10,192.168.0.2,5501 
--callsign=player1

Player2:
--multiplay=out,10,192.168.0.2,5501 --multiplay=in,10,192.168.0.3,5500 
--callsign=player2


For multiple players on a local network:
----------------------------------------
Player1:
--multiplay=out,10,255.255.255.255,5500 
--multiplay=in,10,255.255.255.255,5500 --callsign=player1

Playern:
--multiplay=out,10,255.255.255.255,5500 
--multiplay=in,10,255.255.255.255,5500 --callsign=playern

Note that the callsign is used to identify each player in a multiplayer game 
so the callsigns must be unique. The multiplayer code ignores packets that 
are sent back to itself, as would occur with broadcasting when the rx and tx 
ports are the same.


Multiple players sending to a single player:
--------------------------------------------
Player1:
--multiplay=out,10,192.168.0.2,5500 --callsign=player1

Player2:
--multiplay=out,10,192.168.0.2,5500 --callsign=player2

Player3:
--multiplay=out,10,192.168.0.2,5500 --callsign=player3

Player4 (rx only):
--multiplay=in,10,192.168.0.2,5500 --callsign=player4

This demonstrates that it is possible to have multiple instances of 
Flightgear that send to a single instance that displays all the traffic. This 
is the sort of implementation that we are considering for use as a tower 
visual simulator.


For use with a server:
----------------------
Oliver Schroeder has created a server for multiplayer flightgear use.
The server acts as a packet forwarding mechanism. When it
receives a packet, it sends it to all other active players.
Future versions might be more scalable, only forwarding information
on the planes in the receiving player's vicinity.

Check out the server homepage <http://www.o-schroeder.de/fg_server/>
for the current status. You can either download the server for
some local use, or join the developers on the existing servers.

Pigeon <http://pigeond.net> has created a web page monitoring
two such servers, showing the traffic in a Google map environment.
See <http://pigeond.net/flightgear/fg_server_map.html>.

Options needed to enable multiplayer game with a server:
Player1:
--multiplay=out,10,serveraddress,5002 --multiplay=in,10,myaddress,5002
--callsign=player1

Player2:
--multiplay=out,10,serveraddress,5002 --multiplay=in,10,myaddress,5002
--callsign=player2

...

PlayerN:
--multiplay=out,10,serveraddress,5002 --multiplay=in,10,myaddress,5002
--callsign=playerN

Note that if every player using a particular server, such as one of those
listed on the Pigeon's page, needs to have a unique callsign, not
already in use on that server.

If you are sitting behind a NAT'ting firewall, then you need to forward
the incoming traffic on the firewall outer (visible to the internet)
address arriving at the UDP port you use (5002 in the case above)
over to your private LAN address. In this case, use your PRIVATE LAN address
as <myaddress>. Example (if your private LAN address is 10.0.0.1,
in order to play on pigeond.net):

fgfs --multiplay=in,10,10.0.0.1,5002 --multiplay=out,10,pigeond.net,5002
	--callsign=...UNIQUE callsign here...

If you and the server are in the same address space (i.e., both have a public
IP address or both are on the same private LAN), you hopefully don't need to
mess with any firewalls.

If you don't see other players playing on the same server in your flightgear,
check that you have followed the above router configuration guidelines.  Check
that you don't have any LOCAL firewall running on your computer preventing the
flightgear network traffic flow.

Finally, use ethereal(1) or tethereal(1) to capture the UDP traffic on the port
that you are using, and see if you observe both incoming and outgoing packets.

It's a good idea to talk to the IRC channel #flightgear on irc.flightgear.org
while flying on one of the public servers. Also, it makes sense for every user
on the same server to use the same weather setup, e.g., the real weather
METAR feed, selected by setting to true the real-world-weather-fetch and
control-fdm-atmosphere properties.

Further reading (a must if you have a problem):
-----------------------------------------------
[1] The flightgear server homepage <http://www.o-schroeder.de/fg_server/>
[2] The flightgear wiki multiplayer howto <http://www.seedwiki.com/wiki/flight_gear/flightgear_multiplayer_documentation.cfm>
[3] If everything else fails, ask for help on
the IRC channel #flightgear on irc.flightgear.org