The SOURCE for truly random numbers in the kernel is external hardware

I note that the entropy key's network entropy daemon actually relies on the kernel's entropy estimation: in particular, it relies on it so that it knows when not to bother sending in more entropy. This matters because extracting the entropy over USB is relatively CPU-expensive (it can eat a few percent of CPU time on slow machines), so it is best not to throw too much entropy into the pool unless people are using what you throw in. The key generates enough entropy to fill the pool in a couple of seconds, after all... and by the standards of modern entropy sources that is *slow*.