J'ai trouvé la réponse :

/dev/urandom is a compatibility nod to Linux. On Linux, /dev/urandom will
produce lower quality output if the entropy pool drains, while
/dev/random will prefer to block and wait for additional entropy to be
collected. With Yarrow, this choice and distinction is not necessary, and the two devices behave identically. You may use either.

From the RANDOM(4) BSD Kernel Interfaces Manual

/dev/random est la meme chose que /dev/urandom sur tout sauf Linux ou urandom est une version "amoindrie" de /dev/random