In which field of the TCP / IP header can information be transmitted and how to encode it?

Question

Computer networks

There is a network diagram (with conditional IP addresses) client(192.168.0.10/24) -> Linux(gw 192.168.0.1/24) -> Internet(various devices that route packets) -> Linux(gw 192.168.1.1/24) - > client (192.168.1.10/24), Communication takes place in both directions, despite the fact that initially the network (customer's network) will not be known to configure Nata. We cannot know where the connection will come from.
The bottom line is that you need to connect from one network to another (I know that you can GRE and the like, I won’t go into the depths of the issue, but in essence)
here the question is how can I encode and in which field the route of the client packet (src_ip dst_ip) and so that later it can be restored using the algorithm. I thought that in the urgent field (but again it is used for telnet connection)

Answer 1

Look at TOS
https://tools.ietf.org/html/rfc791
I'll add a little, look at this concept https://zeltser.com/c2-dns-tunneling/
Everything is the same, only with TOS headers.
Of course, it will be terribly slow and possibly cut off by routers somewhere.
Another option is to go up to the application layer, but as I understand it, this is not your way.
A completely frostbitten version is a kind of morse code. That is, you modify the package so that it has 2 differences. And you work according to the protocol:
........ - 0x00
.......- - 0x01
......-. - 0x03
......-- - 0x04