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

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Message-ID: <p73veebe10p.fsf@bingen.suse.de>
Date:	30 May 2007 00:08:22 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	M Macnair <mmacnair@...il.com>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Seeding /dev/random not working

Matt Mackall <mpm@...enic.com> writes:

> On Tue, May 29, 2007 at 05:44:37PM +0100, M Macnair wrote:
> > On 29 May 2007 18:58:59 +0200, Andi Kleen <andi@...stfloor.org> wrote:
> > >"M Macnair" <mmacnair@...il.com> writes:
> > >>
> > >> Many distros ship with an init script that saves and restores the
> > >> entropy pool on startup and shutdown.  The bit that interests me that
> > >> is called on startup is (my comments):
> > >>       if [ -f $random_seed ]; then
> > >>               cat $random_seed >/dev/urandom  # should seed the pool
> > >.OA
> > >Writing doesn't actually work; to get real accounted entropy for 
> > >/dev/random
> > >you need to use a special ioctl. I ran into this problem some years ago
> > >and ended up writing http://www.muc.de/~ak/rndfeed.c
> > >
> > >-Andi
> > 
> > If this doesn't work, then it seems to me as though all the
> > debian-esque distros that use equivalents of the above script are
> > wasting their time, and the man page recommending that technique (man
> > 4 random) is also wrong.  Is that interpretation correct?
> 
> Andi is incorrect. Writing does work and everything you write is mixed

Note I wrote accounted entropy above.

> into the pool. It's just not counted as entropy credit.

This means everything using /dev/random blocks.  For me that
includes "does not work".

> This is as intended.

If the intention was to get everybody from stopping /dev/random
and moving them to /dev/urandom I guess it works well. Congratulations.

-Andi
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