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, 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 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.
