RANDOM
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 bits of 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 from /dev/random will block
until additional environmental noise is gathered.
A read from the
/dev/urandom device will not block
waiting for more entropy.
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.
Configuration
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 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 random number generator..."
random_seed=/var/run/random-seed
# Carry a random seed from start-up to start-up
# Load and then save the whole entropy pool
if [ -f $random_seed ]; then
cat $random_seed >/dev/urandom
else
touch $random_seed
fi
chmod 600 $random_seed
poolfile=/proc/sys/kernel/random/poolsize
[ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
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
# Save the whole entropy pool
echo "Saving random seed..."
random_seed=/var/run/random-seed
touch $random_seed
chmod 600 $random_seed
poolfile=/proc/sys/kernel/random/poolsize
[ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
/proc Interface
The files in the directory
/proc/sys/kernel/random
(present since 2.3.16) provide an additional interface to the
/dev/random
device.
The read-only file
entropy_avail
gives the available entropy.
Normally, this will be 4096 (bits),
a full entropy pool.
The file
poolsize
gives the size of the entropy pool.
Normally, this will be 512 (bytes).
It can be changed to any value for which an algorithm is available.
Currently the choices are 32, 64, 128, 256, 512, 1024, 2048.
The file
read_wakeup_threshold
contains the number of bits of entropy required for waking up processes
that sleep waiting for entropy from
R /dev/random .
The default is 64.
The file
write_wakeup_threshold
contains the number of bits of entropy below which we wake up
processes that do a
select(2)
or
poll(2)
for write access to
R /dev/random .
These values can be changed by writing to the files.
The read-only files
uuid
and
boot_id
contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.
The former is generated afresh for each read, the latter was
generated once.
FILES
/dev/random
/dev/urandom
SEE ALSO
mknod (1)
RFC 1750, "Randomness Recommendations for Security"