linux-kernel - Re: dd if=/dev/random of=data.bin bs=512 count=1

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Date:	Sat, 23 May 2009 14:38:07 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Jon <jg@...k.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: dd if=/dev/random of=data.bin bs=512 count=1

On Sat, 2009-05-23 at 20:12 +0100, Jon wrote:
> Hello KOSAKI,
> 
> Thank you for your reply.
> 
> KOSAKI Motohiro wrote:
> >> Hello
> >>
> >> Issuing this command, I expected to get a file 4096 bytes in size.
> >> Could someone let me know if I have missed something..? Or is there a
> >> problem with using /dev/random this way?
> >>
> >> if I have a count=10 it hangs forever (several hours).
> >>
> >> Changing to use /dev/urandom and it generates the file ok.
> > 
> > you already have the answer. /dev/random might blocked, /dev/urandom doesn't.
> 
> Is this a "feature" of /dev/random ? ..it does not seem very useful to 
> have it able to block for so long.

Yes. The theory is that it will be secure even in the event of a
catastrophic failure of the cryptographic compression function. But
don't worry about that; use /dev/urandom.

-- 
http://selenic.com : development and support for Mercurial and Linux


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