/dev/urandom
Linux's device for cryptographically secure random numbers — the one you should use, whatever old man pages claim.
The persistent myth says /dev/random is "more secure" because it blocks when entropy runs low. It is not: once the pool has been seeded, urandom's output is equally unpredictable, and the blocking has caused far more outages than it has prevented attacks.
The only genuine problem is early boot before the pool has filled. getrandom(2) handles that correctly by blocking only then. Since Linux 5.6 /dev/random behaves essentially the same way.