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NAME
random, urandom - kernel random number source devices
SYNOPSIS
#include <linux/random.h>
int ioctl(fd, RNDrequest, param);
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 return random bytes only 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. If open(2) is called for
/dev/random with the flag O_NONBLOCK, a subsequent read(2) will not block if the requested number of
bytes is not available. Instead, the available bytes are returned. If no byte is available, read(2)
will return -1 and errno will be set to EAGAIN.
A read from the /dev/urandom device will not block waiting for more entropy. If there is not sufficient
entropy, a pseudorandom number generator is used to create the requested bytes. As a result, in this
case 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. O_NONBLOCK has no effect when opening /dev/urandom. When calling read(2) for the
device /dev/urandom, signals will not be handled until after the requested random bytes have been
generated.
Since Linux 3.16, a read(2) from /dev/urandom will return at most 32 MB. A read(2) from /dev/random will
return at most 512 bytes (340 bytes on Linux kernels before version 2.6.12).
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.
If a seed file is saved across reboots as recommended below (all major Linux distributions have done this
since 2000 at least), the output is cryptographically secure against attackers without local root access
as soon as it is reloaded in the boot sequence, and perfectly adequate for network encryption session
keys. Since reads from /dev/random may block, users will usually want to open it in nonblocking mode (or
perform a read with timeout), and provide some sort of user notification if the desired entropy is not
immediately available.
The kernel random-number generator is designed to produce a small amount of high-quality seed material to
seed a cryptographic pseudo-random number generator (CPRNG). It is designed for security, not speed, and
is poorly suited to generating large amounts of random data. Users should be very economical in the
amount of seed material that they read from /dev/urandom (and /dev/random); unnecessarily reading large
quantities of data from this device will have a negative impact on other users of the device.
The amount of seed material required to generate a cryptographic key equals the effective key size of the
key. For example, a 3072-bit RSA or Diffie-Hellman private key has an effective key size of 128 bits (it
requires about 2^128 operations to break) so a key generator needs only 128 bits (16 bytes) of seed
material from /dev/random.
While some safety margin above that minimum is reasonable, as a guard against flaws in the CPRNG
algorithm, no cryptographic primitive available today can hope to promise more than 256 bits of security,
so if any program reads more than 256 bits (32 bytes) from the kernel random pool per invocation, or per
reasonable reseed interval (not less than one minute), that should be taken as a sign that its
cryptography is not skillfully implemented.
Configuration
If your system does not have /dev/random and /dev/urandom created already, they can be created with the
following commands:
mknod -m 666 /dev/random c 1 8
mknod -m 666 /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 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 $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$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 $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
In the above examples, we assume Linux 2.6.0 or later, where /proc/sys/kernel/random/poolsize returns the
size of the entropy pool in bits (see below).
/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/random
/dev/urandom
SEE ALSO
getrandom(2), mknod(1)
RFC 1750, "Randomness Recommendations for Security"
COLOPHON
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