FreeBSD Manual Pages
RANDOM(4) Kernel Interfaces Manual RANDOM(4) NAME random -- the entropy device DESCRIPTION The random device accepts and reads data as any ordinary (and willing) file, but throws away any data written to it, and returns an endless supply of random bytes when read. The only purpose of writing data to random is to perturb the internal state. This perturbation of the internal state is the only userland method of introducing extra entropy into the device. If the writer has superuser privilege, then closing the device after writing will make the internal generator reseed itself. This can be used for extra secu- rity, as it immediately introduces any/all new entropy into the PRNG. The random device can be controlled with sysctl(8). To see the devices' current settings, use the command line: sysctl kern.random which results in something like: kern.random.sys.seeded: 1 kern.random.sys.burst: 20 kern.random.sys.harvest.ethernet: 0 kern.random.sys.harvest.point_to_point: 0 kern.random.sys.harvest.interrupt: 0 kern.random.yarrow.gengateinterval: 10 kern.random.yarrow.bins: 10 kern.random.yarrow.fastthresh: 100 kern.random.yarrow.slowthresh: 160 kern.random.yarrow.slowoverthresh: 2 All settings are read/write. The kern.random.sys.seeded variable indicates whether or not the random device is in an acceptably secure state as a result of reseeding. If set to 0, the device will block (on read) until the next reseed (which can be from an explicit write, or as a result of entropy harvesting). A reseed will set the value to 1 (non-blocking). The kern.random.sys.burst variable instructs the kernel thread that processes the harvest queue to tsleep(9) briefly after that many events have been processed. This helps prevent the random device from being so compute-bound that it takes over all processing ability. A value of zero (0) is treated as infinity, and will only allow the kernel to pause if the queue is empty. Only values in the range [0..20] are ac- cepted. The kern.random.sys.harvest.ethernet variable is used to select LAN traffic as an entropy source. A zero (0) value means that LAN traffic is not considered as an entropy source. Set the variable to one (1) if you wish to use LAN traffic for entropy harvesting. The kern.random.sys.harvest.point_to_point variable is used to select serial line traffic as an entropy source. (Serial line traffic in- cludes PPP, SLIP and all tun0 traffic.) A zero (0) value means such traffic is not considered as an entropy source. Set the variable to one (1) if you wish to use it for entropy harvesting. The kern.random.sys.harvest.interrupt variable is used to select hard- ware interrupts as an entropy source. A zero (0) value means inter- rupts are not considered as an entropy source. Set the variable to one (1) if you wish to use them for entropy harvesting. All interrupt har- vesting is setup by the individual device drivers. The other variables are explained in the paper describing the Yarrow algorithm at http://www.counterpane.com/yarrow.html. These variables are all limited in terms of the values they may con- tain: kern.random.yarrow.gengateinterval [4..64] kern.random.yarrow.bins [2..16] kern.random.yarrow.fastthresh [64..256] kern.random.yarrow.slowthresh [64..256] kern.random.yarrow.slowoverthresh [1..5] Internal sysctl(3) handlers force the above variables into the stated ranges. FILES /dev/random SEE ALSO sysctl(8) HISTORY A random device appeared in FreeBSD 2.2. The early version was taken from Theodore Ts'o's entropy driver for Linux. The current implementa- tion, introduced in FreeBSD 5.0, is a complete rewrite by Mark R V Murray, and is an implementation of the Yarrow algorithm by Bruce Schneier, et al. FreeBSD 5.0 February 10, 2001 RANDOM(4)
NAME | DESCRIPTION | FILES | SEE ALSO | HISTORY
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=random&sektion=4&manpath=FreeBSD+5.0-RELEASE>