RANDOM(4) | Linux Programmer's Manual | RANDOM(4) |
NAME
random, urandom - kernel random number source devicesSYNOPSIS
#include <linux/random.h>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 unclassified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead.
Writing to /dev/random or /dev/urandom will update the entropy pool with the data written, but this will not result in a higher entropy count. This means that it will impact the contents read from both files, but it will not make reads from /dev/random faster.
Usage
If you are unsure about whether you should use /dev/random or /dev/urandom, then probably you want to use the latter. As a general rule, /dev/urandom should be used for everything except long-lived GPG/SSL/SSH keys.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
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
# 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. The semantics of this file vary across kernel versions:
- Linux 2.4:
- This file gives the size of the entropy pool in bytes. Normally, this file will have the value 512, but it is writable, and can be changed to any value for which an algorithm is available. The choices are 32, 64, 128, 256, 512, 1024, or 2048.
- Linux 2.6:
- This file is read-only, and gives the size of the entropy pool in bits. It contains the value 4096.
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.
ioctl(2) interface
The following ioctl(2) requests are defined on file descriptors connected to either /dev/random or /dev/urandom. All requests performed will interact with the input entropy pool impacting both /dev/random and /dev/urandom. The CAP_SYS_ADMIN capability is required for all requests except RNDGETENTCNT.- RNDGETENTCNT
- Retrieve the entropy count of the input pool, the contents will be the same as the entropy_avail file under proc. The result will be stored in the int pointed to by the argument.
- RNDADDTOENTCNT
- Increment or decrement the entropy count of the input pool by the value pointed to by the argument.
- RNDGETPOOL
- Removed in Linux 2.6.9.
- RNDADDENTROPY
- Add some additional entropy to the input pool, incrementing the entropy count. This differs from writing to /dev/random or /dev/urandom, which only adds some data but does not increment the entropy count. The following structure is used:
-
struct rand_pool_info {
int entropy_count;
int buf_size;
__u32 buf[0];
};
- Here entropy_count is the value added to (or subtracted from) the entropy count, and buf is the buffer of size buf_size which gets added to the entropy pool.
- RNDZAPENTCNT, RNDCLEARPOOL
- Zero the entropy count of all pools and add some system data (such as wall clock) to the pools.
FILES
/dev/randomSEE ALSO
mknod(1)COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.2013-03-15 | Linux |