Re: Entropy Pool Contents

Lennart Sorensen
2006-11-23T20:54:40Z

On Thu, Nov 23, 2006 at 12:54:03AM +0100, Gunter Ohrner wrote:
> (PEBKAC warning. I'm probably doing something dump. I just don't know
> what...)
>
> I seem to have an entropy pool on a headless machine which is not nearly
> empty (a common problem in this case, I know), but completely empty and
> stuck in this state...
>
> Hornburg:~# cat /proc/sys/kernel/random/entropy_avail
> 0
> Hornburg:~# fuser /dev/urandom
> Hornburg:~# lsof | grep random
> Hornburg:~# cat /proc/sys/kernel/random/entropy_avail
> 0
> Hornburg:~# dd if=/dev/hdf of=/dev/urandom bs=512 count=1
> 1+0 records in
> 1+0 records out
> 512 bytes transferred in 0.016268 seconds (31473 bytes/sec)
> Hornburg:~# dd if=/dev/hdf of=/dev/random bs=512 count=1
> 1+0 records in
> 1+0 records out
> 512 bytes transferred in 0.031943 seconds (16029 bytes/sec)
> Hornburg:~# cat /proc/sys/kernel/random/entropy_avail
> 0
> Hornburg:~# fuser /dev/urandom
> Hornburg:~# fuser /dev/random
> Hornburg:~# lsof | grep random
> Hornburg:~# cat /proc/sys/kernel/random/poolsize
> 4096
> Hornburg:~#
>
> Also causing disk activities doesn't help at all. (Two disks on a Promise
> PDC20268 controller.)
>
> The system runs a rather ancient Debian Sarge 2.4 kernel:
> Linux Hornburg 2.4.27-3-386 #1 Thu Sep 14 08:44:58 UTC 2006 i486 GNU/Linux
>
> However as the machine itself is also ancient, the 2.4 seems like a good
> match. And also 2.4 ought to have a refilling entropy pool, doesn't it?
>
> Maybe someone can shed some light on what's happening here...

Only some devices/drivers generate entropy data. Some network drivers,
mouse, keyboard. None of the disk drivers are appear to do so. Serial
ports do not in general either. On my headless systems I patched
pcnet32 and the 8250 driver to generate entropy since otherwise I tended
to run out very quickly.

--
Len Sorensen