netfilter五個hook點

netfilter五個hook點分别是：

NF_INIT_PRE_ROUTING

NF_INIT_LOCAL_IN

NF_INIT_FORWARD

NF_INIT_LOCAL_OUT

NF_INIT_POST_ROUTING

大緻流程：

                                                                                                ip_local_deliver -->LOCAL IN-->ip_local_deliver_finish

                                                                                             /

ip_rcv --> PRE ROUTING --> ip_rcv_finish -- >ip_forword

                                                                                             \

                                                                                                ip_forword -->FORWORD--> ip_forward_finish--> ip_finish_output --> ip_output -->POST ROUTING

__ip_local_out --> LOCAL OUT --> ip_output -->POST ROUTING