What does VPN Passthrough and IPSec NAT-T mean?
Often the protocol used to compose VPNs, for example IPSec, encrypts the payload of IP packets and authenticate them via a checksum on the header. A router that applies the NAT or PAT on packets must be able to inspect payload content, since the TCP/UDP ports are located here and are used to track connections in the NAT table. Obviously, if the payload is encrypted this is not possible. On the other hand, the NAT/PAT routers change packet headers, since they must translate the source/destination addresses. This ensures the authentication of IPSec fails. VPN Passthrough techniques are a solution to these problems for use of VPNs on networks subject to NAT and port forwarding, also found in more economical router ranges. There is no standard for these methods and they can vary from one router model to another. NAT Traversal or for short NAT-T, instead represents a standard and solves the aforementioned problems by encapsulating the packets already encrypted by IPSec in a UDP tunnel on po
