dnat & snat

 67 static struct nf_hook_ops nf_nat_ipv4_ops[] __read_mostly = {
 68     /* Before packet filtering, change destination */
 69     {
 70         .hook       = iptable_nat_ipv4_in,
 71         .pf     = NFPROTO_IPV4,
 72         .hooknum    = NF_INET_PRE_ROUTING,
 73         .priority   = NF_IP_PRI_NAT_DST,
 74     },
 75     /* After packet filtering, change source */
 76     {
 77         .hook       = iptable_nat_ipv4_out,
 78         .pf     = NFPROTO_IPV4,
 79         .hooknum    = NF_INET_POST_ROUTING,
 80         .priority   = NF_IP_PRI_NAT_SRC,
 81     },
 82     /* Before packet filtering, change destination */
 83     {
 84         .hook       = iptable_nat_ipv4_local_fn,
 85         .pf     = NFPROTO_IPV4,
 86         .hooknum    = NF_INET_LOCAL_OUT,
 87         .priority   = NF_IP_PRI_NAT_DST,
 88     },
 89     /* After packet filtering, change source */
 90     {
 91         .hook       = iptable_nat_ipv4_fn,
 92         .pf     = NFPROTO_IPV4,
 93         .hooknum    = NF_INET_LOCAL_IN,
 94         .priority   = NF_IP_PRI_NAT_SRC,
 95     },
 96 };
 97 

iptables深入解析-nat篇

http://blog.chinaunix.net/uid-20786208-id-5145525.html