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RANDOM(4) 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 un- til it is seeded for the first time. To provide prompt access to the random device at boot time, FreeBSD au- tomatically 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 ran- dom 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 de- vice 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 en- tropy 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 com- mand 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 set- tings 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 en- tropy 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 set- tings in a human readable form. Disabled items in the latter are listed in square brackets. See random_harvest(9) for more on the har- vesting 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 dis- carded. The implementation performs startup testing, during which en- tropy sources are discarded. FILES /dev/random /dev/urandom DIAGNOSTICS The following tunables are related to initial seeding of the random de- vice: kern.random.initial_seeding.bypass_before_seeding Defaults to 1 (on). When set, the system will bypass the random de- vice prior to initial seeding. On is unsafe, but provides availabil- ity 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 diagnos- tic 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)). • kern.random.initial_seeding.read_random_bypassed_before_seeding • 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 signifi- cant 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 effi- cient than their compiled-in equivalents. This is because some func- tions 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 ob- tain 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 re- seeding 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 15.0 August 28, 2025 RANDOM(4)
NAME | SYNOPSIS | DESCRIPTION | FILES | DIAGNOSTICS | SEE ALSO | HISTORY | AUTHORS | CAVEATS | SECURITY CONSIDERATIONS
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