/dev/random

/dev/random is a non-deterministic random number generator.

It is slower, but more secure, than /dev/urandom.

The entropy pool used by /dev/random should be initialized when the system is booted. When it shuts down, the entropy pool should then be stored somewhere to be used again for the next system start.

See http://www.linuxfromscratch.org/blfs/view/stable/postlfs/random.html

Apparently, /dev/random is created in drivers/char/mem.c.

$ dd if=/dev/random of=/tmp/random.data bs=1M count=1