socat udp client server

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http://stackoverflow.com – Following http://jdimpson.livejournal.com/6812.html, I've socat UDP client-server set as following: socat as a UDP server on port 11111. socat STDIO UDP-LISTEN:11111 and then as a UDP client. socat - UDP:localhost:11111 Now the question is that when I write from client, server gets the data via UDP. But UDP is unidirectional. Why does client receives data when I write something in server and how does it reaches client? via UDP? (HowTos)