@nadoo I notice the implementation is based on another repo. That repo has both UDP and TCP implementations, but the TCP implementation was never added to this repo.
https://github.com/KatelynHaworth/go-tproxy/blob/master/tproxy_tcp.go
Iptables docs mention TPROXY supports both udp and tcp: https://manpages.debian.org/bookworm/iptables/iptables-extensions.8.en.html#TPROXY