RANDOM
Section: Linux Programmer's Manual (4)
Updated: 2017-09-15
Index
Return to Main Contents
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 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.
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 being 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 cryptographic 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 entropy 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 environmental 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.
Instead, 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 contents
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 during 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 (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.
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 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/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
NOTES
For an overview and comparison of the various interfaces that
can be used to obtain randomness, see
random(7).
BUGS
During early boot time, reads from
/dev/urandom
may return data prior to the entropy pool being initialized.
SEE ALSO
mknod(1),
getrandom(2),
random(7)
RFC 1750, "Randomness Recommendations for Security"
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Usage
-
- Configuration
-
- /proc interfaces
-
- ioctl(2) interface
-
- FILES
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
|
|
- Running on 
-
:
: 