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

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Date:	Tue, 04 Dec 2007 16:12:51 -0600
From:	Mike McGrath <mmcgrath@...hat.com>
To:	Theodore Tso <tytso@....edu>, Mike McGrath <mmcgrath@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matt Mackall <mpm@...enic.com>, 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?

Theodore Tso wrote:
> On Tue, Dec 04, 2007 at 02:48:12PM -0600, Mike McGrath wrote:
>   
>> Alan Cox wrote:
>>     
>>>> Here's the top 5:
>>>>
>>>>    266 28caf2c3-9766-4fe1-9e4c-d6b0ba8a0132
>>>>    336 810e7126-1c69-4aff-b8b1-9db0fa8aa15a
>>>>    402 c8dbb9d3-a9bd-4ba6-b92e-4a294ba5a95f
>>>>    884 06e84493-e024-44b1-9b32-32d78af04039
>>>>    931 e2b67e1d-e325-4740-b938-795addb45280
>>>>
>>>> The left number is times this month someone has submitted a profile with
>>>> that UUID.  If we take the last one as an example has come from over 800
>>>> IP's in the last 20 days.  It seems very unlikely that one person would
>>>> find his way to 800 different IP's this month.  Let me know if you'd
>>>> like more.
>>>>     
>>>>         
>> Background - Smolt runs this during its install:
>>
>> /bin/cat /proc/sys/kernel/random/uuid > /etc/sysconfig/hw-uuid
>>
>> For most users this would be run by the RPM %post scripts during install 
>> from anaconda. For some reason there are some UUID's (like those listed 
>> above) that come up more often then it seems they should if they are truly 
>> random.
>>     
>
> Would this be by any chance using kickstart where there is no user
> interaction, and no way of gathering entropy during the install process?
> The random number generator isn't *magic* you know....
>   

This is certainly possible but I think its unlikely. Not many people 
know about smolt. Most of our profiles come from prompting people during 
firstboot which gets skipped when people are running kickstart. I'll 
make sure to follow up with people that report a duplicate.

-Mike
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