Some applications have criteria which affect the type of PRNG which is needed. For instance, later on we will discuss IP datagram IDs and DNS  query-IDs, both of these issues have qualities which make it extremely desirable to have a PRNG which makes efforts to avoid emitting repetitions (thus ruling out use of a true-random source).
Many other operating systems also have random number device drivers and other related mechanisms, but largely make no use of them. Some such systems even provide such support only as optional device drivers, therefore discouraging use (i.e., reliance). OpenBSD deviates by actually using these mechanisms in numerous ways. A few major interfaces or techniques are used:
Each of these, and their uses in OpenBSD, will be covered in the following sections.