Re: [libvirt-users] virRandomBits - not very random

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).