Why does reading from /dev/urandom deplete entropy so much?

[Marc Haber <mh+linux-kernel@zugschlus.de>]

While debugging Exim4's GnuTLS interface, I recently found out that
reading from /dev/urandom depletes entropy as much as reading from
/dev/random would. This has somehow surprised me since I have always
believed that /dev/urandom has lower quality entropy than /dev/random,
but lots of it.

This also means that I can "sabotage" applications reading from
/dev/random just by continuously reading from /dev/urandom, even not
meaning to do any harm.

Before I file a bug on bugzilla, can I ask why /dev/urandom wasn't
implemented as a PRNG which is periodically (say, every 1024 bytes or
even more) seeded from /dev/random? That way, /dev/random has a much
higher chance of holding enough entropy for applications that really
need "good" entropy.

Entropy depletion has become an issue on Linux systems since the
network was removed from the group of entropy sources a few years ago.
Of the around 20 Linux systems that I currently have root privileges
on, only two have a hardware RNG.

What can I do to keep sufficiently high entropy pools even if I read
from /dev/urandom more than once in a while?

Greetings
Marc

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