FreeBSD
The FreeBSD operating system implements a 256-bit variant of the Yarrow algorithm, intended to provide a cryptographically secure pseudorandom stream—this replaced a previous Linux style random device. Unlike the Linux /dev/random, the FreeBSD /dev/random device never blocks. Its behavior is similar to the Linux /dev/urandom, and /dev/urandom on FreeBSD is linked to /dev/random.
Yarrow is based on the assumptions that modern PRNGs are very secure if their internal state is unknown to an attacker, and that they are better understood than the estimation of entropy. Whilst entropy pool based methods are completely secure if implemented correctly, if they overestimate their entropy they may become less secure than well-seeded PRNGs. In some cases an attacker may have a considerable amount of control over the entropy, for example a diskless server may get almost all of it from the network—rendering it potentially vulnerable to man-in-the-middle attacks. Yarrow places a lot of emphasis on avoiding any pool compromise and on recovering from it as quickly as possible. It is regularly reseeded; on a system with small amount of network and disk activity, this is done after a fraction of a second.
FreeBSD also provides support for hardware random number generators, which will replace Yarrow when present.
Read more about this topic: /dev/random