Наглядная схема прохождения пакета по цепочкам

                            Network
                    -----------+-----------
                               |
                  +--------------------------+
          +-------+-------+        +---------+---------+
          |    IPCHAINS   |        |      IPTABLES     |
          |     INPUT     |        |     PREROUTING    |
          +-------+-------+        | +-------+-------+ |
                  |                | |   conntrack   | |
                  |                | +-------+-------+ |
                  |                | |    mangle     | | <- MARK WRITE  
                  |                | +-------+-------+ |
                  |                | |      IMQ      | |
                  |                | +-------+-------+ |
                  |                | |      nat      | | <- DEST REWRITE
                  |                | +-------+-------+ |     DNAT or REDIRECT or DE-MASQUERADE
                  |                +---------+---------+
                  +------------+-------------+
                               |
                       +-------+-------+
                       |      QOS      |
                       |    INGRESS    |
                       +-------+-------+
                               |
         packet is for +-------+-------+ packet is for
          this machine |     INPUT     | another address
        +--------------+    ROUTING    +--------------+
        |              |    + PDBB     |              |
        |              +---------------+              |
+-------+-------+                                     |
|   IPTABLES    |                                     |
|     INPUT     |                                     |
| +-----+-----+ |                                     |
| |   mangle  | |                                     |
| +-----+-----+ |                                     |
| |   filter  | |                                     |
| +-----+-----+ |                                     |
+-------+-------+                                     |
        |                               +---------------------------+
+-------+-------+                       |                           |
|     Local     |               +-------+-------+           +-------+-------+
|    Process    |               |    IPCHAINS   |           |    IPTABLES   |
+-------+-------+               |    FORWARD    |           |    FORWARD    |
        |                       +-------+-------+           | +-----+-----+ |
+-------+-------+                       |                   | |  mangle   | | <- MARK WRITE
|    OUTPUT     |                       |                   | +-----+-----+ |
|    ROUTING    |                       |                   | |  filter   | |
+-------+-------+                       |                   | +-----+-----+ |
        |                               |                   +-------+-------+
+-------+-------+                       |                           |
|    IPTABLES   |                       +---------------------------+
|     OUTPUT    |                                     |
| +-----------+ |                                     |
| | conntrack | |                                     |
| +-----+-----+ |                                     |
| |   mangle  | | <- MARK WRITE                       |
| +-----+-----+ |                                     |
| |    nat    | | <-DEST REWRITE                      |
| +-----+-----+ |     DNAT or REDIRECT                |
| |   filter  | |                                     |
| +-----+-----+ |                                     |
+-------+-------+                                     |
        |                                             |
        +----------------------+----------------------+
                               |
                  +------------+------------+
                  |                         |
          +-------+-------+       +---------+---------+
          |    IPCHAINS   |       |      IPTABLES     |
          |     OUTPUT    |       |    POSTROUTING    |
          +-------+-------        | +-------+-------+ |
                  |               | |    mangle     | | <- MARK WRITE  
                  |               | +-------+-------+ |
                  |               | |      nat      | | <- SOURCE REWRITE
                  |               | +-------+-------+ |      SNAT or MASQUERADE
                  |               | |      IMQ      | |
                  |               | +-------+-------+ |
                  |               +---------+---------+
                  +------------+------------+
                               |
                        +------+------+
                        |     QOS     |
                        |    EGRESS   |
                        +------+------+
                               |
                    -----------+-----------
                            Network