I noticed that this hasn't been mentioned yet, so I'll bring it up here. While RF-based noise generators can be influenced by outside sources, using an RTL-SDR

I noticed that this hasn't been mentioned yet, so I'll bring it up here. While RF-based noise generators can be influenced by outside sources, using an RTL-SDR with https://github.com/pwarren/rtl-entropy https://github.com/pwarren/rtl-entropy can be a great way to get environmental RF noise. In actual reality, the generator won't be influenced, because it will require a strong signal, close by, likely transmitting illegally on the frequency you are listening to (of course, the attacker would need to know the frequency you're tuned to). Simple monitoring tools and scripts can be used to test the quality of the generator output. If the quality of the randomness begins to fail, as would happen with deliberate RF interference, then the script would stop outputting bits, and alert the administrator. Or, even better, switch to a different frequency, chosen at random by the kernel CSPRNG. What's great about using the RTL-SDR as an RNG, is these devices are cheap (about $20) and can output 3 MiBps, which vastly out performs any USB HWRNG that I have personally tested for the value (bits per second per dollar). FWIW, random.org uses RF noise as the basis for delivering random numbers to the Internet. Anyway, FYI.