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 persists some entropy data in /boot/entropy for the loader 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.

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 item are listed in square brackets. See random_harvest(9) for more on the harvesting of entropy.

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)).

kern.random.initial_seeding.read_random_bypassed_before_seeding
kern.random.initial_seeding.arc4random_bypassed_before_seeding

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.