IRONSIDES is an authoritative/recursive DNS server pair that is provably invulnerable to many of the problems that plague other servers. It achieves this property through the use of formal methods in its design, in particular the language Ada and the SPARK formal methods tool set. Code validated in this way is provably exception-free, contains no data flow errors, and terminates only in the ways that its programmers explicitly say that it can. These are very desirable properties from a computer security perspective.
The Ada-Europe 2013 paper (pdf) is pretty readable.