VTI Tunnel Interface with strongSwan

tutorial #ipsec, #strongswan

I successfully managed to get Linux VTI (Virtual Tunnel Interface) working with strongSwan. By using VTI it is no longer needed to rely on the routing policy database, making understanding and maintaining routes easier. Also with VTI you can see the cleartext traffic on the VTI interface itself. It was confusing to see actual tunnel traffic before using tcpdump using the standard policy database setup. (There are ulog/nflog hacks to see cleartext traffic in both direction though, similar to BSD pflog.)

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