linux-kernel - Re: Seeding /dev/random not working

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Message-ID: <de6d2b4f0705290817g2b92f031re29c52ed9f3264a7@mail.gmail.com>
Date:	Tue, 29 May 2007 16:17:24 +0100
From:	"M Macnair" <mmacnair@...il.com>
To:	"Pavel Machek" <pavel@....cz>
Cc:	"Theodore Tso" <tytso@....edu>, linux-kernel@...r.kernel.org
Subject: Re: Seeding /dev/random not working

On 5/29/07, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
> > > I have two embedded boards (one ARM, one PowerPC), running two
> > > different versions of 2.6.  They have no hard drives, keyboards or
> > > mice.  They each have a NIC, but I understand these make no
> > > contribution to the entropy pool.
> > >
> > >     if [ -f $random_seed ]; then
> > >             cat $random_seed >/dev/urandom  # should seed the pool
> > >     fi
> > >     dd if=/dev/urandom of=$random_seed count=1 2>/dev/null # save some
> > > data from urandom for next boot
> > >
> > > I have rebooted my boards many times, and after each boot I read the
> > > contents of $random_seed.  Whilst it does not happen every time, the
> > > contents of $random_seed are /often the same/.  To give you a feel:
> > > rebooted 11 times, got a total of 3 different outputs.
> >
> > Ok, so this is telling me a couple of things.  First of all, if you're
> > only getting three outputs, it means that you don't have any
> > peripherals feeding entropy into the system from the boot sequence.
> > Without any hard drives, keyboards or mice, and a NIC whose device
> > driver hasn't been configured to feed entropy, you're definitely
> > hosed.
>
> Can we get at least time-of-boot from rtc clock to the pool? We really
> should not be getting identical outputs...

Ah yes that is one thing I forgot to mention - when you turn these
boards off, you really do turn them off - no battery means no clock.
They wake up every time in 1970.

I am planning on providing my own entropy to this system by feeding
/dev/random (which leads to the problem of it not working...).
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