When you send a UDP datagram with nc, like so:
  echo foo | nc -u 10.0.0.123 2001
the new version of nc included with FC5 sometimes eats 100% of the CPU.

- If you send it to another listening computer (nc -l -u 2001), it returns
immediately to the command line.  This is as expected.
- If you send it to a non-listening (but existing) computer, it hangs, and nc
eats 100% of the CPU time.
- Finally, if you send it to a non-existent computer, it hangs but does not eat
CPU time.

In the tables below, I manipulated the first two variables, and the last two
variables are the results.  I ran 'nc -u -l -p 2001' to make the remote host listen.

This is nc 1.84 on FC5:

 host   host         eat
exists listen  hang  CPU
  Y      Y      N     N
  Y      N      Y     Y
  N      -      Y     N


Contrast this with nc 1.10 on an FC3 installation:

 host   host         eat
exists listen  hang  CPU
  Y      Y      N     N
  Y      N      N     N
  N      -      Y     N

In all cases, the target host was on the same subnet.  I don't know if it would
matter if the target was on a different subnet.


I personally don't understand why it should hang at all -- shouldn't it just
shoot out the UDP datagram and return?  But certainly the 100% CPU use is wrong.