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 gener‐
ator. File /dev/random has major device number 1 and minor device num‐
ber 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 liter‐
ature, 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 infor‐
mation 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 algo‐
rithm 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
/dev/random. The default is 64. The file write_wakeup_threshold con‐
tains the number of bits of entropy below which we wake up processes
that do a select() or poll() for write access to /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
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 2003-10-25 RANDOM(4)
