Discovering IPv6-in-IPv4 Tunnels in the Internet

Tunnels are widely used to improve security and to expand networks without having to deploy native infrastructure, and play an important role in the migration to IPv6. In this paper we introduce a number of techniques to detect, and collect information about, IPv6-in-IPv4 tunnels. We also show how, once a tunnel has been discovered, it can be used as a "vantage point" to launch third-party tunnel-discovery explorations, scaling up the discovery process. We describe the Tunneltrace tool which implements the proposed techniques, and validate them by means of a wide experimentation on the 6bone tunneled network, on the Italian Academic and Research network, and through the test boxes deployed worldwide by the RIPE NCC as part of the Test Traffic Measurements Service. We assess to what extent 6bone registry information is coherent with the actual network topology, and we provide the first experimental results on the current distribution of IPv6-in-IPv4 tunnels in the Internet, showing that tunnels are very common: even the "native" networks we tested reach more than 60% of all IPv6 prefixes through tunnels.