how to decode part of a message as IPv4 with a custom dissector?

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So what is the format of the packet on the network? Is the Ethernet type of the packet 0x0800 (for IPv4) or something for your custom protocol? If it's 0x0800, what is the protocol type in the IPv4 header - a standard value for a protocol running on top of IPv4, or a custom value for your custom protocol? And where is the IPv4 payload?

Thanks for the reply. The Ethernet type is the standard 0x0800 for IPv4. The protocol type in the IPv4 header is a custom value for the custom protocol. The original packet is like this: [Frame][Ethernet header][IPv4 header] [custom protocol] [payload]

What I am trying to do is insert a created IPv4 header (after I have dissected the custom protocol) between the custom protocol and payload, then pass the created IPv4 header and payload to be decoded by the IPv4 protocol.

So I think I would have to create a new tvb then use call_dissector to pass it along to the IPv4 dissector. What I am not sure is how to stitch together this new tvb with the created header+ original payload.

You cannot stitch them together - a dissector takes the whole tvb it gets, processes the header part and invokes sub-dissectors to handle the payload - no pointers to other buffers can be used. So you have to copy your created IPv4payload right after your created IPv4header into the newly created tvb from the original one, effectively creating a new packet for the ip (IPv4) dissector to handle.

So is the "Created IPv4 header" different from the actual IPv4 header? If so, in what ways is it different?

What I wrote above is relevant to your case if you actually create the IPv4 header using your Custom header dissector. If the octets of the second IP header are already present in the raw packet, following the Custom header, in correct format, then you don't need to create a second tvb at all - you merely invoke the ip dissector on the rest of the tvb which your Custom dissector has been given.