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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:

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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Feeding /dev/random from /dev/urandom is really not a good idea. /dev/urandom is derived from the same entropy pool as /dev/random, so you are not generating any additional entropy using this method. rngd is designed for feeding /dev/random from a trusted, hardware source. /dev/urandom is neither a trusted nor hardware source of new entropy. – ændrük Mar 12 '11 at 22:54
That aside, I don't get any error message when running sudo rngd -r /dev/urandom, so I agree that there is some kind of permissions or device issue here. – ændrük Mar 13 '11 at 0:05

1 Answer 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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I tried rngd -r /dev/random and got the same error. – Darren Newton Mar 12 '11 at 22:10
In that case I'd check that it's the right device node and its permissions (ls -l /dev/random) and then check /var/log/messages to see if the random driver is complaining. – geekosaur Mar 12 '11 at 22:18
The error message is not inconsistent. -r specifies the hardware entropy source and -o specifies the sink. When undefined, -o defaults to /dev/random. – ændrük Mar 12 '11 at 22:47
/var/log/messages is empty. – Darren Newton Mar 13 '11 at 14:55

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