redhat man page for random

RANDOM(4)						     Linux Programmer's Manual							 RANDOM(4)

NAME
       random, urandom - kernel random number source devices

DESCRIPTION
       The  character  special	files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel's random number
       generator.  File /dev/random has major device number 1 and minor device number 8.  File /dev/urandom has major device number  1	and  minor
       device number 9.

       The  random  number  generator  gathers environmental noise from device drivers and other sources into an entropy pool.	The generator also
       keeps an estimate of the number of bit of the noise in the entropy pool.  From this entropy pool random numbers are created.

       When read, the /dev/random device will only return random bytes within the  estimated  number  of  bits	of  noise  in  the  entropy  pool.
       /dev/random  should  be	suitable for uses that need very high quality randomness such as one-time pad or key generation.  When the entropy
       pool is empty, reads to /dev/random will block until additional environmental noise is gathered.

       When read, /dev/urandom device will return as many bytes as are requested.  As a result, if there is not sufficient entropy in the  entropy
       pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver.  Knowledge of how to
       do this is not available in the current non-classified literature, but it is theoretically possible that such an attack may exist.  If this
       is a concern in your application, use /dev/random instead.

CONFIGURING
       If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands:

	       mknod -m 644 /dev/random c 1 8
	       mknod -m 644 /dev/urandom c 1 9
	       chown root:root /dev/random /dev/urandom

       When  a	Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state.  This reduces the
       actual amount of noise in the entropy pool below the estimate.  In order to counteract this effect, it helps to carry entropy pool informa-
       tion  across  shut-downs  and start-ups.  To do this, add the following lines to an appropriate script which is run during the Linux system
       start-up sequence:

	    echo "Initializing kernel random number generator..."
	    # Initialize kernel random number generator with random seed
	    # from last shut-down (or start-up) to this start-up.  Load and
	    # then save 512 bytes, which is the size of the entropy pool.
	    if [ -f /var/random-seed ]; then
		 cat /var/random-seed >/dev/urandom
	    fi
	    dd if=/dev/urandom of=/var/random-seed count=1

       Also, add the following lines in an appropriate script which is run during the Linux system shutdown:

	    # Carry a random seed from shut-down to start-up for the random
	    # number generator.  Save 512 bytes, which is the size of the
	    # random number generator's entropy pool.
	    echo "Saving random seed..."
	    dd if=/dev/urandom of=/var/random-seed count=1

FILES
       /dev/random
       /dev/urandom

AUTHOR
       The kernel's random number generator was written by Theodore Ts'o (tytso@athena.mit.edu).

SEE ALSO
       mknod (1)
       RFC 1750, "Randomness Recommendations for Security"

Linux								    1997-08-01								 RANDOM(4)