RANDOM(4) BSD Kernel Interfaces Manual RANDOM(4)
random -- the entropy device
The random device returns an endless supply of random bytes when read.
It also accepts and reads data as any ordinary (and willing) file, but
discards data written to it. The device will probe for certain hardware
entropy sources, and use these in preference to the fallback, which is a
generator implemented in software.
If the device is using the software generator, writing data to random
would 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 software generator reseed itself. This can
be used for extra security, as it immediately introduces any/all new
entropy into the PRNG. The hardware generators will generate sufficient
quantities of entropy, and will therefore ignore user-supplied input.
The software random device may be controlled with sysctl(8).
To see the current settings of the software random device, use the com-
which results in something like:
(These would not be seen if a hardware generator is present.)
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 pro-
cesses the harvest queue to tsleep(9) briefly after that many events have
been processed. This helps prevent the random device from being so com-
pute-bound that it takes over all processing ability. A value of 0
(zero) 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 accepted.
The kern.random.sys.harvest.ethernet variable is used to select LAN traf-
fic as an entropy source. A 0 (zero) value means that LAN traffic is not
considered as an entropy source. Set the variable to 1 (one) 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 includes
PPP, SLIP and all tun0 traffic.) A 0 (zero) value means such traffic is
not considered as an entropy source. Set the variable to 1 (one) if you
wish to use it for entropy harvesting.
The kern.random.sys.harvest.interrupt variable is used to select hardware
interrupts as an entropy source. A 0 (zero) value means interrupts are
not considered as an entropy source. Set the variable to 1 (one) if you
wish to use them for entropy harvesting. All interrupt harvesting 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 contain:
Internal sysctl(3) handlers force the above variables into the stated
The use of randomness in the field of computing is a rather subtle issue
because randomness means different things to different people. Consider
generating a password randomly, simulating a coin tossing experiment or
choosing a random back-off period when a server does not respond. Each
of these tasks requires random numbers, but the random numbers in each
case have different requirements.
Generation of passwords, session keys and the like requires cryptographic
randomness. A cryptographic random number generator should be designed
so that its output is difficult to guess, even if a lot of auxiliary
information is known (such as when it was seeded, subsequent or previous
output, and so on). On FreeBSD, seeding for cryptographic random number
generators is provided by the random device, which provides real random-
ness. The arc4random(3) library call provides a pseudo-random sequence
which is generally reckoned to be suitable for simple cryptographic use.
The OpenSSL library also provides functions for managing randomness via
functions such as RAND_bytes(3) and RAND_add(3). Note that OpenSSL uses
the random device for seeding automatically.
Randomness for simulation is required in engineering or scientific soft-
ware and games. The first requirement of these applications is that the
random numbers produced conform to some well-known, usually uniform, dis-
tribution. The sequence of numbers should also appear numerically uncor-
related, as simulation often assumes independence of its random inputs.
Often it is desirable to reproduce the results of a simulation exactly,
so that if the generator is seeded in the same way, it should produce the
same results. A peripheral concern for simulation is the speed of a ran-
dom number generator.
Another issue in simulation is the size of the state associated with the
random number generator, and how frequently it repeats itself. For exam-
ple, a program which shuffles a pack of cards should have 52! possible
outputs, which requires the random number generator to have 52! starting
states. This means the seed should have at least log_2(52!) ~ 226 bits
of state if the program is to stand a chance of outputting all possible
sequences, and the program needs some unbiased way of generating these
bits. Again, the random device could be used for seeding here, but in
practice, smaller seeds are usually considered acceptable.
FreeBSD provides two families of functions which are considered suitable
for simulation. The random(3) family of functions provides a random
integer between 0 to (2**31)-1. The functions srandom(3), initstate(3)
and setstate(3) are provided for deterministically setting the state of
the generator and the function srandomdev(3) is provided for setting the
state via the random device. The drand48(3) family of functions are also
provided, which provide random floating point numbers in various ranges.
Randomness that is used for collision avoidance (for example, in certain
network protocols) has slightly different semantics again. It is usually
expected that the numbers will be uniform, as this produces the lowest
chances of collision. Here again, the seeding of the generator is very
important, as it is required that different instances of the generator
produce independent sequences. However, the guessability or repro-
ducibility of the sequence is unimportant, unlike the previous cases.
One final consideration for the seeding of random number generators is a
bootstrapping problem. In some cases, it may be difficult to find enough
randomness to seed a random number generator until a system is fully
operational, but the system requires random numbers to become fully oper-
ational. There is no substitute for careful thought here, but the
FreeBSD random device, which is based on the Yarrow system, should be of
some help in this area.
FreeBSD does also provide the traditional rand(3) library call, for com-
patibility purposes. However, it is known to be poor for simulation and
absolutely unsuitable for cryptographic purposes, so its use is discour-
arc4random(3), drand48(3), rand(3), RAND_add(3), RAND_bytes(3),
A random device appeared in FreeBSD 2.2. The early version was taken
from Theodore Ts'o's entropy driver for Linux. The current software
implementation, 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. The only hardware implementation currently is for the
VIA C3 Nehemiah (stepping 3 or greater) CPU. More will be added in the
The author gratefully acknowledges significant assistance from VIA Tech-
BSD October 3, 2004 BSD