How to use custom protocol with text2pcap?

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The extent to which text2pcap "allows" protocols such as TCP, UDP, etc. is that it can take data for a protocol running on top of TCP, UDP, IP, etc. and construct its own link-layer, IP, TCP, UDP, etc. headers.

text2pcap does not have a general, extensible mechanism to add custom protocols to that list.

If you were to construct headers for your custom protocol and add them to the data payload, and put that into a hex dump file, and if your custom protocol runs atop a standard link-layer protocol that text2pcap knows how to construct, or runs atop IP with a custom protocol number, you could get text2pcap to turn that into a pcap file.

You cannot, however, get it to construct headers for your custom protocol, unless you modify it by hand to allow it to do so.