There seems to be a man page an ipfw but I can't find what package I have to install to use ipfw / DummyNet. Any hints?
ipfw is the freebsd firewall package.
Dummynet however is available for Linux and must be usable somehow from iptables. I haven't yet found a tutorial on how to use it with linux. I'm sure it'll exist somewhere.
Looks like you have to compile it yourself.
See README file that comes with the dummynet source.