RANDOM(4) FreeBSD Kernel Interfaces Manual RANDOM(4)
NAME
random - the entropy device
SYNOPSIS
options RANDOM_LOADABLE
options RANDOM_ENABLE_ETHER
options RANDOM_ENABLE_UMA
DESCRIPTION
The random device returns an endless supply of random bytes when read.
The generator will start in an unseeded state, and will block reads until
it is seeded for the first time.
To provide prompt access to the random device at boot time, FreeBSD
automatically saves some entropy data in /boot/entropy for the loader(8)
to provide to the kernel. Additional entropy is regularly saved in
/var/db/entropy. This saved entropy is sufficient to unblock the random
device on devices with writeable media.
Embedded applications without writable media must determine their own
scheme for re-seeding the random device on boot, or accept that the
device will remain unseeded and block reads indefinitely. See SECURITY
CONSIDERATIONS for more detail.
In addition to read(2), the direct output of the abstract kernel entropy
device can be read with getrandom(2), getentropy(3), or the sysctl(8)
pseudo-variable kern.arandom.
To see the current settings of the software random device, use the
command line:
sysctl kern.random
which results in something like:
kern.random.block_seeded_status: 0
kern.random.fortuna.minpoolsize: 64
kern.random.harvest.mask_symbolic: ENABLEDSOURCE,[DISABLEDSOURCE],...,CACHED
kern.random.harvest.mask_bin: 00000010000000111011111
kern.random.harvest.mask: 66015
kern.random.use_chacha20_cipher: 0
kern.random.random_sources: 'Intel Secure Key RNG'
kern.random.initial_seeding.bypass_before_seeding: 1
kern.random.initial_seeding.read_random_bypassed_before_seeding: 0
kern.random.initial_seeding.arc4random_bypassed_before_seeding: 0
kern.random.initial_seeding.disable_bypass_warnings: 0
Other than kern.random.block_seeded_status,
kern.random.fortuna.minpoolsize, and kern.random.harvest.mask, all
settings are read-only via sysctl(8).
The kern.random.fortuna.minpoolsize sysctl is used to set the seed
threshold. A smaller number gives a faster seed, but a less secure one.
In practice, values between 64 and 256 are acceptable.
The kern.random.harvest.mask bitmask is used to select the possible
entropy sources. A 0 (zero) value means the corresponding source is not
considered as an entropy source. Set the bit to 1 (one) if you wish to
use that source. The kern.random.harvest.mask_bin and
kern.random.harvest.mask_symbolic sysctls can be used to confirm settings
in a human readable form. Disabled items in the latter are listed in
square brackets. See random_harvest(9) for more on the harvesting of
entropy.
The kern.random.nist_healthtest_enabled tunable can be used to enable the
entropy source health tests outlined in section 4 of NIST Special
Publication 800-90B. When enabled, all entropy sources will be subject
to the repetition count and adaptive proportion tests described in that
document. If one of the tests fails, the source will be disabled, i.e.,
all subsequent entropy samples from that source will be discarded. The
implementation performs startup testing, during which entropy sources are
discarded.
FILES
/dev/random
/dev/urandom
DIAGNOSTICS
The following tunables are related to initial seeding of the random
device:
kern.random.initial_seeding.bypass_before_seeding
Defaults to 1 (on). When set, the system will bypass the random
device prior to initial seeding. On is unsafe, but provides
availability on many systems that lack early sources of entropy, or
cannot load /boot/entropy sufficiently early in boot for random
consumers. When unset (0), the system will block read_random(9) and
arc4random(9) requests if and until the random device is initially
seeded.
kern.random.initial_seeding.disable_bypass_warnings
Defaults to 0 (off). When set non-zero, disables warnings in dmesg
when the random device is bypassed.
The following read-only sysctl(8) variables allow programmatic diagnostic
of whether random device bypass occurred during boot. If they are set
(non-zero), the specific functional unit bypassed the strong random
device output and either produced no output (read_random(9)) or seeded
itself with minimal, non-cryptographic entropy (arc4random(9)).
o kern.random.initial_seeding.read_random_bypassed_before_seeding
o kern.random.initial_seeding.arc4random_bypassed_before_seeding
SEE ALSO
getrandom(2), arc4random(3), getentropy(3), random(3), sysctl(8),
random(9)
Ferguson, Schneier, and Kohno, Cryptography Engineering, Wiley, ISBN
978-0-470-47424-2.
HISTORY
A random device appeared in FreeBSD 2.2. The implementation was changed
to the Yarrow algorithm in FreeBSD 5.0. In FreeBSD 11.0, the Fortuna
algorithm was introduced as the default. In FreeBSD 12.0, Yarrow was
removed entirely.
AUTHORS
The current random code was authored by Mark R V Murray, with significant
contributions from many people.
The Fortuna algorithm was designed by Niels Ferguson, Bruce Schneier, and
Tadayoshi Kohno.
CAVEATS
When options RANDOM_LOADABLE is enabled, the /dev/random device is not
created until an "algorithm module" is loaded. The only module built by
default is random_fortuna. Loadable random modules are less efficient
than their compiled-in equivalents. This is because some functions must
be locked against load and unload events, and also must be indirect calls
to allow for removal.
When options RANDOM_ENABLE_UMA is enabled, the /dev/random device will
obtain entropy from the zone allocator. This is a very high rate source
with significant performance impact. Therefore, it is disabled by
default.
When options RANDOM_ENABLE_ETHER is enabled, the random device will
obtain entropy from mbuf structures passing through the network stack.
This source is both extremely expensive and a poor source of entropy, so
it is disabled by default.
SECURITY CONSIDERATIONS
The initial seeding of random number generators is a bootstrapping
problem that needs very careful attention. When writable media is
available, the Fortuna paper describes a robust system for rapidly
reseeding the device.
In some embedded cases, it may be difficult to find enough randomness to
seed a random number generator until a system is fully operational. In
these cases, is the responsibility of the system architect to ensure that
blocking is acceptable, or that the random device is seeded. (This
advice does not apply to typical consumer systems.)
To emulate embedded systems, developers may set the
kern.random.block_seeded_status tunable to 1 to verify boot does not
require early availability of the random device.
FreeBSD 15.1-STABLE-HBSD August 28, 2025 RANDOM(4)