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

Reviving an ancient thread: On 11/04/2014 02:18 AM, Daniel P. Berrange wrote: > 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). Did anyone ever open a BZ to track this?  As far as I can tell, we still have a very predictable (meaning bad) seeding algorithm that permits large clusters to create collisions when their random number sequences sync up.