Mr. Llama Mr. Llama - 5 months ago 26
Linux Question

/dev/random Extremely Slow?

Some background info: I was looking to run a script on a Red Hat server to read some data from /dev/random and use the Perl unpack() command to convert it to a hex string for usage later on (benchmarking database operations). I ran a few "head -1" on /dev/random and it seemed to be working out fine, but after calling it a few times, it would just kinda hang. After a few minutes, it would finally output a small block of text, then finish.

I switched to /dev/urandom (I really didn't want to, its slower and I don't need that quality of randomness) and it worked fine for the first two or three calls, then it too began hang.
I was wondering if it was the "head" command that was bombing it, so I tried doing some simple I/O using Perl, and it too was hanging.
As a last ditch effort, I used the "dd" command to dump some info out of it directly to a file instead of to the terminal. All I asked of it was 1mb of data, but it took 3 minutes to get ~400 bytes before I killed it.

I checked the process lists, CPU and memory were basically untouched. What exactly could cause /dev/random to crap out like this and what can I do to prevent/fix it in the future?

Edit: Thanks for the help guys! It seems that I had random and urandom mixed up. I've got the script up and running now. Looks like I learned something new today. :)

Tim Tim
Answer

On most Linux systems, /dev/random is powered from actual entropy gathered by the environment. If your system isn't delivering a large amount of data from /dev/random, it likely means that you're not generating enough environmental randomness to power it.

I'm not sure why you think /dev/urandom is "slower" or higher quality. It reuses an internal entropy pool to generate pseudorandomness - making it slightly lower quality - but it doesn't block. Generally, applications that don't require high-level or long-term cryptography can use /dev/urandom reliably.

Try waiting a little while then reading from /dev/urandom again. It's possible that you've exhausted the internal entropy pool reading so much from /dev/random, breaking both generators - allowing your system to create more entropy should replenish them.

See Wikipedia for more info about /dev/random and /dev/urandom.

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