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Comment by sweis | Hacker News Reader

Regarding entropy sources for guest virtual machines, the host can expose a random number generator device to the VM. This is an option in KVM: http://libvirt.org/formatdomain.html#elementsRng

Modern Intel x86 processors now have a hardware RNG built in, so even if the host boots without any devices, you have a source of entropy.

(Incidentally, KVM defaults to /dev/random which might create a DoS vulnerability if a guest exhausts entropy.)

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Interesting. Do you know if any cloud providers do this? (DigitalOcean uses KVM so it would be a possibility.)

Exposing hardware RNGs to guest VMs would be great; entropy in guests is pretty lousy in general, not just at first boot (though obviously at first boot it's at its very worst).

We've run into the /dev/random problem when doing heavy SSL traffic. We had to pipe in sources of entropy. It was a janky solution, but wasn't as janky as it might sound.

If you need more entropy for SSL traffic than a headless server that sees very little I/O load has available, I've found http://www.vanheusden.com/te/ useful in the past. The ~800 bits/sec is produced in our tests isn't massive, but it was considerably than the machines we used it on could generate from the kernel's usual sources on these machines. It claims to produce valid results in VMs too, and given the way it works it might be perfectly valid to scale is some way simply by running more than one copy.

If you want more than that in a cheap Heath Robinson manner (Americans: think Rube Goldberg if you are unaware of Heath, their work came from very similar inspirations) then many SoC solutions have a built-in RNG of sufficient quality for general cryptographic use and some solutions expose them easily. If you enable the relevant module a Raspberry Pi can provide up to ~550,000 bits/sec when asked to, for instance, which if one-off cost is a factor beats paying a few hundred dollars or more for a USB device providing the same sort of rate.

Another solution that we are happy with at least is http://www.issihosts.com/haveged/

Based on the HAVEGE research http://www.irisa.fr/caps/projects/hipsor/

Are you referring to the hardware RNG in TPMs? I thought they were very low bandwidth in terms of entropy, would that be a problem on first boot?

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