FreeBSD, p0f, and USB pcap

If, like me, you had a working FreeBSD system with amavisd-new and p0f set up, you might find that p0f suddenly stopped working at some point. The cause for this is that it tries to use pcap on the USB bus, which it cannot do. The solution is to put hw.usb.no_pf=1 in your /boot/loader.conf.