I have three questions related to entropy on UNIX systems:I check entropy on Linux using: cat /proc/sys/kernel/random/entropy_avail. Is this the standard place with information about available entropy defined in POSIX?What is the correct available I should expect? I heard that entropy should be equal or more than 100 and that there may be a problem if entropy is constantly below 100.Is this entropy used by /dev/random or does it also has anything to do with /dev/urandom?
What is the correct available entropy on UNIX systems?
linux;random
/dev/random is not standardized. POSIX doesn't provide any way of generating cryptographically secure random data and doesn't have any notion of entropy.The Linux kernel's entropy calculation corresponds to an information-theoretic model of entropy which is not relevant to practical use. The only case where this is relevant is on a new device which has never had time to accumulate entropy (this includes live distributions; installed systems save their entropy from one boot to the next). Apart from this situation, there is always enough entropy, because entropy does not deplete. Since Linux's /dev/random blocks when it thinks it doesn't have enough entropy, use /dev/urandom, which never blocks. Using /dev/urandom is good for everything including generating cryptographic keys (except, as mentioned above, on a freshly minted device).In summary:No, this is not standard.You don't care.Use /dev/urandom.Many, but not all unix systems have /dev/urandom and /dev/random. See the Wikipedia page for a more detailed discussion.