In my modified OpenVPN server process, as an intermediate node, I'd need to embed my custom 3 bytes data in the header of every IP packet and then in the same machine (after going through
tun) filter those packets by those custom 3 bytes.
Where's the best place in IPv4 header to put my 3 bytes? And how to retrieve it using iptables? Please note that I won't need those 3 bytes anymore after being processed by iptables.