Contents I Xtables 4 1 Nomenclature 4 2 Match extensions 5 3 Userspace plugin 17 4 Tricks and traps 27 5 Target extensions 34 II Connection Tracking 44 6 nf_conn structure 44 7 Layer-3 connection tracker 46 8 Layer-4 connection tracker 48 III Netfilter Core 53 9 Netfilter Hooks 53 IV Appendix 56 A F