linux - Why does dd only copy 128 bytes from /dev/random when I request more?

I'm trying to understand the output of the dd command. I tried

dd if=/dev/zero of=/dev/null bs=512 count=1

and got (as expected):

 1+0 records in
 1+0 records out
 512 bytes (512 B) copied, 2e-05 seconds, 26 MB/s

However when I tried

dd if=/dev/random of=/dev/null bs=512 count=1

I got

 0+1 records in
 0+1 records out
 128 bytes (128 B) copied, 0.00012 seconds, 1.1 MB/s

Why is it only copying 128 bytes?

Answer

You need to use /dev/urandom, or the "unblocking" random source.

/dev/random uses a kind of entropy pool to increase the randomness of the bit source. This method will only return as many random bits/bytes as can be returned based on the entropy pool's state at the time, so if a hardware random number generator is used, this can sometimes be a constant. From the Linux manpage:

The generator also keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool random numbers are created.

The /dev/urandom file keeps reusing the internal pool as-is to generate a number as long as you need. The side-effect of this is: do not use /dev/urandom for cryptographic purposes, as it is less random than the bits produced by /dev/random. See the manpage link above for details.