linux-kernel - Re: Why does reading from /dev/urandom deplete entropy so much?

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Message-ID: <20071210163736.GA6107@ucw.cz>
Date:	Mon, 10 Dec 2007 16:37:37 +0000
From:	Pavel Machek <pavel@....cz>
To:	Theodore Tso <tytso@....edu>, Mike McGrath <mmcgrath@...hat.com>,
	Matt Mackall <mpm@...enic.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ray Lee <ray@...rabbit.org>, Adrian Bunk <bunk@...nel.org>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

On Wed 2007-12-05 09:49:34, Theodore Tso wrote:
> On Wed, Dec 05, 2007 at 08:26:19AM -0600, Mike McGrath wrote:
> >
> > Ok, whats going on here is an issue with how the smolt RPM installs the 
> > UUID and how Fedora's Live CD does an install.  It's a complete false alarm 
> > on the kernel side, sorry for the confusion.
> 
> BTW, You may be better off using "uuidgen -t" to generate the UUID in
> the smolt RPM, since that will use 12 bits of randomness from
> /dev/random, plus the MAC, address and timestamp.  So even if there is
> zero randomness in /dev/random, and the time is January 1, 1970, at
> least the MAC will contribute some uniqueness to the UUID.

I thought that /dev/random blocks when 0 entropy is available...? I'd
expect uuid generation to block, not to issue duplicates.

-- 
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