On 5/25/2018 8:58 AM, Eric Blake wrote:
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.
I never did. We just switched to maintaining the mac ourselves, and not
letting libvirt generate it.
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