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<title>s6-linux-utils: the s6-fillurandompool program</title>
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<a href="index.html">s6-linux-utils</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
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<h1> The <tt>s6-fillurandompool</tt> program </h1>
<p>
<tt>s6-fillurandompool</tt> blocks until the machine's
<tt>/dev/urandom</tt> entropy pool is filled up. Then it exits.
</p>
<h2> Interface </h2>
<pre>
s6-fillurandompool
</pre>
<h2> Rationale </h2>
<p>
For some reason, Linux has <em>two</em> separate entropy pools: one for
<tt>/dev/random</tt> and one for <tt>/dev/urandom</tt>.
</p>
<p>
Reading from <tt>/dev/random</tt> blocks when its entropy pool is
not full enough, so it will never return weak random data. (Reading
from <tt>/dev/random</tt> is overkill anyway, and
<a href="http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/">you
should not be doing it.</a>)
</p>
<p>
However, reading from <tt>/dev/urandom</tt> (which
<a href="http://www.2uo.de/myths-about-urandom/">you should be doing</a>)
will not block, even though the entropy pool may not have been
initialized yet. That's the only insecure thing about it: at boot time,
<tt>/dev/urandom</tt> may return weak random data, until its entropy
pool has filled up.
</p>
<p>
<tt>s6-fillurandompool</tt> is meant to address this issue. Call it once
early on in your boot scripts, before you need any serious random data;
when it exits, the <tt>/dev/urandom</tt> pool has been properly initialized,
and it is now safe to read from <tt>/dev/urandom</tt> every time you need
random data, until the machine shuts down.
</p>
<h2> Notes </h2>
<ul>
<li> <tt>s6-fillurandompool</tt> will only work on a Linux kernel version
3.17 or later: this is when the
<a href="http://man7.org/linux/man-pages/man2/getrandom.2.html"><tt>getrandom()</tt></a>
system call, which it internally uses, has been implemented. </li>
</ul>
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