Netcat anomaly

[Rick Stevens]

On 06/26/2014 09:53 AM, CLOSE Dave issued this missive:
> I have a Fedora 20 machine which is receiving UDP broadcast packets at
> regular intervals on a strange high port. No program is presently
> listening for these packets. If I run, "tcpdump -i eth0 port 29531", I
> see each of the packets arriving just as I expect. Note, the packets are
> not empty and contain mostly ASCII characters.
>
> But if I then run, "nc -lu 29531", I don't see anything! Why not? What
> obvious thing am I missing?
>
> This same operation works better (but still not as I expect) on Fedora
> 14. NC shows one packet arriving but then doesn't show any more.
>
> Running NC under strace on both machines, I see F14 NC seems to use
> poll(2). It outputs one packet then hangs on poll. F20 NC seems to use
> select(2). It hangs on the first call.
>
> SELinux and the firewall are disabled on both machines.

You may need to use "nc -lu --recv-only 29531" so the system doesn't
try to reply to the packet. Perhaps it's better to use wireshark or
tcpdump to copy the data to a file and examine it.
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