Netcat anomaly

[Rick Stevens]

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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- Rick Stevens, Systems Engineer, AllDigital    ricks at alldigital.com -
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