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RANDOM(4) Linux Programmer's Manual RANDOM(4)

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. The file /dev/random has major device
number 1 and minor device number 8. The file /dev/urandom has major device number 1 and mi‐
nor 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.

Linux 3.17 and later provides the simpler and safer getrandom(2) interface which requires no
special files; see the getrandom(2) manual page for details.

When read, the /dev/urandom device returns random bytes using a pseudorandom number generator
seeded from the entropy pool. Reads from this device do not block (i.e., the CPU is not
yielded), but can incur an appreciable delay when requesting large amounts of data.

When read during early boot time, /dev/urandom may return data prior to the entropy pool be‐
ing initialized. If this is of concern in your application, use getrandom(2) or /dev/random
instead.

The /dev/random device is a legacy interface which dates back to a time where the crypto‐
graphic primitives used in the implementation of /dev/urandom were not widely trusted. It
will return random bytes only within the estimated number of bits of fresh noise in the en‐
tropy pool, blocking if necessary. /dev/random is suitable for applications that need high
quality randomness, and can afford indeterminate delays.

When the entropy pool is empty, reads from /dev/random will block until additional environ‐
mental noise is gathered. If open(2) is called for /dev/random with the O_NONBLOCK flag, a
subsequent read(2) will not block if the requested number of bytes is not available. In‐
stead, the available bytes are returned. If no byte is available, read(2) will return -1 and
errno will be set to EAGAIN.

The O_NONBLOCK flag has no effect when opening /dev/urandom. When calling read(2) for the
device /dev/urandom, reads of up to 256 bytes will return as many bytes as are requested and
will not be interrupted by a signal handler. Reads with a buffer over this limit may return
less than the requested number of bytes or fail with the error EINTR, if interrupted by a
signal handler.

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 con‐
tents read from both files, but it will not make reads from /dev/random faster.

Usage
The /dev/random interface is considered a legacy interface, and /dev/urandom is preferred and
sufficient in all use cases, with the exception of applications which require randomness dur‐
ing early boot time; for these applications, getrandom(2) must be used instead, because it
will block until the entropy pool is initialized.

If a seed file is saved across reboots as recommended below, the output is cryptographically
secure against attackers without local root access as soon as it is reloaded in the boot se‐
quence, and perfectly adequate for network encryption session keys. (All major Linux distri‐
butions have saved the seed file across reboots since 2000 at least.) 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.

Configuration
If your system does not have /dev/random and /dev/urandom created already, they can be cre‐
ated 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 interfaces
The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide additional
information about the /dev/random device:

entropy_avail
This read-only file gives the available entropy, in bits. This will be a number in
the range 0 to 4096.

poolsize
This file 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 and later:
This file is read-only, and gives the size of the entropy pool in bits. It
contains the value 4096.

read_wakeup_threshold
This file contains the number of bits of entropy required for waking up processes that
sleep waiting for entropy from /dev/random. The default is 64.

write_wakeup_threshold
This file 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.

uuid and boot_id
These read-only files 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/ran‐
dom or /dev/urandom. All requests performed will interact with the input entropy pool im‐
pacting 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 en‐
tropy_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

NOTES
For an overview and comparison of the various interfaces that can be used to obtain random‐
ness, see random(7).

BUGS
During early boot time, reads from /dev/urandom may return data prior to the entropy pool be‐
ing initialized.

SEE ALSO
mknod(1), getrandom(2), random(7)

RFC 1750, "Randomness Recommendations for Security"

COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be found
at https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 RANDOM(4)