[libvirt-users] virRandomBits - not very random

Brian Rak brak at gameservers.com
Mon Nov 3 16:09:12 UTC 2014

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 
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.

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