Add support for GRE decapsulation

[IvanNardi]

It would be nice to have GRE de-tunneling capability in ndpiReader (like GTP or CAPWAP).
Pcap example: gre_sip.zip

[leonn]

Well de-tunneling capability is not hard to implement, but it brings more discussions like:

• as GRE tunnels can have many layers, what to do with that info? just get it de-tunneled and discard the inner layers info? thins may lead to less visibility of what is actually happening
• well GRE types vary and those have their own implementations to be covered (LCC-SLL/WCCP/ERSPAN etc) to provide a full de-tunneling capability

I think this need to be clarified before we can move forward

[torres-miguel]

Hi, I would like to contribute to this issue. However, I can see that further clarification was needed. Has it been solved somehow by now?

[IvanNardi]

Hi, I would like to contribute to this issue. However, I can see that further clarification was needed. Has it been solved somehow by now?

Let's start simple: try detunneling (i.e strip outer ip header and pass to nDPI the inner one) the trace attached above and gre.zip and gre_erspan.zip. We can add complexity later if/when required/necessary (expecially since this code is in the example and not in the library itself)

[IvanNardi]

Added GRE v0 and v1 on f1193d5
TODO: ERSPAN