GbDns is an open source recursive-only caching DNS server for Windows, designed to operate securely behind a NAT firewall that does not randomize ports. Each cache entry has a "Confidence" value, which is intended to be a rough indication of whether the entry can be trusted. New entries in the cache typically require an extra DNS request, to obtain a suitable confidence level. The program is deliberately extremely simple : there are no configuration options, and installation is via a simple Windows Installer file.