[libvirt-users] virRandomBits - not very random
Daniel P. Berrange
berrange at redhat.com
Tue Nov 4 08:18:28 UTC 2014
On Mon, Nov 03, 2014 at 11:09:12AM -0500, Brian Rak wrote:
> I just ran into an issue where I had about 30 guests get duplicate mac
> addresses assigned. These were scattered across 30 different machines.
>
> Some debugging revealed that:
>
> 1) All the host machines were restarted within a couple seconds of each
> other
> 2) All the host machines had fairly similar libvirtd pids (within ~100 PIDs
> of each other)
> 3) Libvirt seeds the RNG using 'time(NULL) ^ getpid()'
>
> This perfectly explains why I saw so many duplicate mac addresses.
>
> Why is the RNG seed such a predictable value? Surely there has to be a
> better source of a random seed then the timestamp and the pid?
>
> The PID seems to me to be a very bad source of any randomness. I just ran a
> test across 60 of our hosts. 43 of them shared their PID with at least one
> other machine.
We should probably seed it with data from /dev/urandom, and/or the new
Linux getrandom() syscall (or BSD equivalent).
Regards,
Daniel
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