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Darren Newton
Asked: 2011-03-13 13:36:11 +0800 CST

Root cannot access /dev/urandom

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I am trying to generate a GPG key, and I cannot generate enough entropy. So I installed rng-tools and tried following these instructions: https://serverfault.com/questions/214605/gpg-not-enough-entropy

When I am logged in as root, and try to run rngd -r /dev/urandom I get the following error: can't open /dev/random: Permission denied

I find this disturbing as I am root.

This is Ubuntu on a virtual server (via Parallels I believe.)

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1 Answers

  1. /dev/random or /dev/urandom? The error is the former but the command is the latter, and rngd source suggests they should be the same.

    In any case, /dev/random is the correct one; /dev/urandom doesn't have a kernel interface for adding entropy, which is why the open fails (not even root can force the kernel to allow writes to a non-writable device, although arguably it should produce a more appropriate error like EINVAL or maybe EPERM). The kernel will feed /dev/urandom from /dev/random automatically.

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