DNScat is a DNS tunnel software written in Java by Tadeusz Pietraszek and licensed under the GPL open source license. It uses PPP to provide a VPN.
Other implementations of a DNS tunnel include OzymanDNS, iodine, DNS2TCP, NSTX, and TUNS. A ICMP tunnel works similar to a DNS tunnel.
The latest version is 0.02 from 27. August 2005.