[strongSwan] how does pluto and charon share port 500

[Martin Willi]

Hello Simon,

> I am running tests with User-Mode Linux. I face the problem that if I
> start both pluto and charon then charon can never establish connection.
> Tcpdump at both ends indicate the response does come back but charon
> just can't receive it. 

To run both pluto and charon in parallel, charon can't bind to the same
port as pluto. It therefore uses a raw socket to receive packets.

In some older releases, there was a bug which could explain the behavior
you describe: If more than one socket plugin gets registered, charon
can't receive packets. Double-check what socket plugins get loaded. To
run charon in parallel to pluto, make sure to load the socket-raw plugin
only.

Regards
Martin