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 com-
mands:
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 interac-
tion, 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 information 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"