I want to debug a network program when it encounters problems
(timeouted connections, delays, etc). So I need to be able to
configure an interface with things like "drop 50 % of packets directed
to port 80".

On FreeBSD, there is dummynet
(http://info.iet.unipi.it/~luigi/ip_dummynet/) which seems to do
everything I want but does not appear to be available on NetBSD.

On Linux, there is the "random" plugin of Netfilter
(http://www.netfilter.org/patch-o-matic/pom-base.html#pom-base-random),
less featureful but probably sufficient for me.

Any NetBSD solution?