We have has a little trouble with a small box that acts as a VPN end-point and mail relay for our network, caused by the available entropy for /dev/random being too low (which causes TLS connection attempts by exim to fail).

The machine doesn't do anything else, so the normal feed into the entropy pool (interrupt timings from things like disk access) is not enough. As a quick hack I've set a looping script that reads from /dev/hda at a couple of Mbyte/sec which keeps it topped up. Other than buying a hardware RNG, is there a clean way of piping data for entry from elsewhere, such as a copy of the data our file server uses for its entropy source? I've spotted several tips for using rng-tools to feed it from /dev/urandom on the same machine but that "feels dirty".

Also, is it possible to increase the maximum pool size? It currently seems to max out at 3585.