What does "no source of entropy found" or "could not open entropy source foo" mean?

The server requires a source of entropy -- i.e., random numbers -- to perform certain cryptographic operations for DNSSEC or to generate keys for TSIG. These messages indicate that you have no source of entropy. On systems that use /dev/random or an equivalent device to provide access to the kernel's random number generator, that will be used by default. A source of entropy can also be defined using the random-device option in named.conf, or specified at compile time using configure --with-randomdev.