Devrandom

Devrandom is the name given to the kernel interface that provides random numbers to user space through special device files on Unix-like systems, typically /dev/random and /dev/urandom. These devices draw from the operating system’s entropy pool to supply bytes suitable for randomness-dependent tasks, including cryptography and security protocols.

The kernel gathers entropy from various sources, such as hardware events, timers, device I/O timings, and, when available, hardware random-number generators. This collected data feeds an entropy pool that is consumed by reads from the device files. /dev/random typically blocks when the pool lacks sufficient entropy, offering high-entropy output. In contrast, /dev/urandom provides bytes from a non-blocking pseudorandom number generator that reuses the pool when necessary, avoiding blocking but with a lower guarantee of fresh entropy if the pool is not well seeding.

Usage considerations: Many applications should prefer higher-level interfaces such as the getrandom system call or cryptographic libraries that rely on the operating system’s entropy source, rather than reading directly from /dev/random or /dev/urandom. This reduces the risk of blocking or misusing the interface. While the concepts are common across Unix-like systems, exact behavior and availability can vary by OS and kernel version.

Security and reliability: The quality of randomness depends on the entropy sources and the seeding state of the pool. Systems with hardware random-number generators can feed additional entropy to improve the pool’s strength. In environments requiring strict randomness, developers may rely on platform-provided APIs and ensure proper seeding and entropy management to avoid predictable outputs.