EXAMPLE FOR TCP `GENDER CHANGER´ USING SOCAT

See http://www.csnc.ch/static/download/publications/TCP-IP_GenderChanger_CSNC_V1.0.pdf
for a description of the problem.

Hosts:
   a server, blocked by a firewall
   a client outside the firewall ("outside-host")
   a firewall that allows arbitrary TCP connections from server to outside,
      port 80

1) Start the double client on the inside server
// every 10 seconds, it tries to establish a connection to the outside host.
// whenever it succeeds, it forks a sub process that connect to the internal
// service and starts to transfer data
$ socat -d -d -d -t5 tcp:outside-host:80,forever,intervall=10,fork tcp:localhost:80

2) Start double server on the outside client
// wait for a connection from a local client. whenever it accepted it, forks
// a subprocess that tries to bind to the socket where the inside double 
// client tries to connect (might need to wait for a previous process to 
// release the port) 
# socat -d -d -d tcp-l:80,reuseaddr,bind=127.0.0.1,fork tcp-l:80,bind=outside-host,reuseaddr,retry=10

3) Connect with outside client
$ mozilla http://127.0.0.1/