libiptc (Extra headers and libraries of iptables)

It enables programs to communicate with netfilter bypassing iptables

Iptables can be used to build internet firewalls based on stateless
and stateful packet filtering, use NAT and masquerading for sharing
internet access if you don't have enough public IP addresses, use NAT
to implement transparent proxies, aid the tc and iproute2 systems
used to build sophisticated QoS and policy routers, do further packet
manipulation (mangling) like altering the TOS/DSCP/ECN bits of the IP
header, and much more.