IPERF

Hi

I've installed iperf as server in an Ubuntu machine, and I was testing the use of iperf as client in Alpine in diskless mode.
I'm having a problem in using the iperf as client (in Alpine) with the -r flag, to do TX and RX tests separately.
The TX test is done successfully but when it tries to start the RX the server side ( in Ubuntu, says connection refused )
I booted the PC where I was using the Alpine pen, using his "real" OS ( also Ubuntu ), and tryed iperf in it, also as client, to test the Ubuntu server side, and this test was successful.
All testes were made in machines that are directly connected to the internet, with a public IP address, no routers.

Any ideas why this is happening ?

Regards,
António