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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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