How to modify input-chain to simulate "lost packets"?

["Stefan Neufeind" <stefan@neufeind.net>]

Hi,

this is not directly ISP-related but maybe someone has already done 
some sort of thing like this for simultion or otherwise has an idea:

How could I achieve to randomly drop UDP-packets on the input-chain 
of a server? My problem is that I need to test a few tools which 
shall still work is some UDP-packets are lost ... those will be 
retransmitted.

So what I basically need is something like:
- drop every 17th (or other) packet on the input chain
OR
- drop aprox. 2% of traffic but vary a bit

Anybody got an idea how to achieve this? Thank you for your help.
  Stefan