On 06/26/2014 11:21 AM, CLOSE Dave issued this missive:
I wrote:
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.
Rick Stevens answered:
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.
Thanks. But that doesn't make any obvious difference. Still no output.
If examining the packets were my only concern, tcpdump would do just
fine. But I actually have a program which would like to receive those
packets and doesn't. Using NC is part of the investigation to discover
why not. But NC doesn't seem to get the packets, either.
Quite odd. Seems to work on my machine. In one window, I did a
nc -lu 29531
and in another:
echo 'Hello there!' | nc -u localhost 29531
and the first window saw the message. Try the same and see if it works.
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- Make it idiot proof and someone will make a better idiot. -
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