man 4 random has a very vague description of Linux kernel entropy sources:
The random number generator gathers environmental noise from device drivers and other sources into an entropy pool.
The paper Entropy transfers in the Linux Random Number Generator isn't much more specific, either. It lists:
These functinos are from
random.c, which includes following comment:
Sources of randomness from the environment include inter-keyboard timings, inter-interrupt timings from some interrupts, and other events which are both (a) non-deterministic and (b) hard for an outside observer to measure.
Further down, there is a function
add_hwgenerator_randomness(...) indicating support for hardware random number generators.
All those information are rather vague (or, in the case of the source code, require deep knowledge of the Linux kernel to understand). What are the actual entropy sources used, and does the Linux kernel support any hardware random number generators out-of-the-box?