iptables流程简图

--> PREROUTING --> [ROUTE] --> FORWARD --> POSTROUTING -->
      mangle          |         mangle          ↑ mangle
       nat            |         filter          |  nat 
       raw            |                         |
                      ↓                         |
                    INPUT                     OUTPUT
                      |                         ↑  raw            
                      | mangle                  | mangle
                      | filter                  |  nat          
                      ↓  -------> LOCAL ------->| filter