lua dissector, need to set pinfo.in_error_pkt but cannot

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I tried copying the tvbrange out to a bytearray, and using that bytearray as :tvb calling the ip dissector:

        local ipheader = buffer:bytes(some_offset, header_len)
    local iptvb = ipheader:tvb("IP Header")
    subtree:add(iptvb(0, -1), "foo")
    Dissector.get("ip"):call(iptvb(0, -1):tvb(), pinfo, tree)

I can see the "foo" tab (I made this for debug purpose) has the right 20 bytes, but the ip header dissection is once again complaining about the total length being higher (in this case 48) than the actual length ( = 20). The ip dissector wants to continue onwards with a tcp dissector, which of course is not provided.

What if you fudge/correct the ip.len bytes in the copied TVB to be 20?

Har har, I thought of that and tried it.

ipheader:set_index(3, 20)

That actually fixes the length error.

But of course, the dissector now outputs an "untruth" in the length field. And the length is not always twenty, I would need to check for options present and IPv6, and so on and on.

You could go the other way and pad the TVB with bogus TCP header/payload but that might be getting off in the weeds.

Bit of a drag, yes, especially when the IP header sometimes contains a jumbo frame length.

I am surprised that no one has come up with this kind of "embedded" use of an internal dissector before in lua. What is the technical reason for the read-only on that pinfo flag? Hmmm.