random(4) Kernel Interfaces 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 genera-
tor. 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 de-
vice number 9.
The random number generator gathers environmental noise from device dri-
vers 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) inter-
face which requires no special files; see the getrandom(2) manual page
for details.
When read, the /dev/urandom device returns random bytes using a pseudo-
random 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 ap-
preciable 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 ap-
plication, 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. Since Linux 5.6, the O_NON-
BLOCK flag is ignored as /dev/random will no longer block except during
early boot process. In earlier versions, 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 re-
turn -1 and errno will be set to EAGAIN.
The O_NONBLOCK flag has no effect when opening /dev/urandom. When call-
ing 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 in-
terrupted 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 be-
fore Linux 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 ex-
ception 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, 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. (All major Linux distributions
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 avail-
able.
Configuration
If your system does not have /dev/random and /dev/urandom created al-
ready, 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 en-
tropy pool may be in a fairly predictable state. This reduces the ac-
tual 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 appro-
priate 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 dur-
ing 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 Linux
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 al-
gorithm 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/ran-
dom. 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 con-
nected 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"
Linux man-pages 6.9.1 2024-05-02 random(4)
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