random, urandom — kernel random number source devices
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.
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
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 /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
/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.
mknod (1)
RFC 1750, "Randomness Recommendations for Security"
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