Support hairpin NAT

This re-applies commit b39d02b with additional iptables rules to solve the issue with containers routing back into themselves.

The previous issue with this attempt was that the DNAT rule would send traffic back into the container it came from. When this happens you have 2 issues.
1) reverse path filtering. The container is going to see the traffic coming in from the outside and it's going to have a source address of itself. So reverse path filtering will kick in and drop the packet.
2) direct return mismatch. Assuming you turned reverse path filtering off, when the packet comes back in, it's goign to have a source address of itself, thus when the reply traffic is sent, it's going to have a source address of itself. But the original packet was sent to the host IP address, so the traffic will be dropped because it's coming from an address which the original traffic was not sent to (and likely with an incorrect port as well).

The solution to this is to masquerade the traffic when it gets routed back into the origin container. However for this to work you need to enable hairpin mode on the bridge port, otherwise the kernel will just drop the traffic.
The hairpin mode set is part of libcontainer, while the MASQ change is part of docker.

This reverts commit 63c303eecd.

Docker-DCO-1.1-Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com> (github: phemmer)

pkg/iptables/iptables.go

@@ -73,7 +73,6 @@ func (c *Chain) Forward(action Action, ip net.IP, port int, proto, dest_addr str
 		"-p", proto,
 		"-d", daddr,
 		"--dport", strconv.Itoa(port),
-		"!", "-i", c.Bridge,
 		"-j", "DNAT",
 		"--to-destination", net.JoinHostPort(dest_addr, strconv.Itoa(dest_port))); err != nil {
 		return err
@@ -97,6 +96,17 @@ func (c *Chain) Forward(action Action, ip net.IP, port int, proto, dest_addr str
 		return fmt.Errorf("Error iptables forward: %s", output)
 	}
 
+	if output, err := Raw("-t", "nat", string(fAction), "POSTROUTING",
+		"-p", proto,
+		"-s", dest_addr,
+		"-d", dest_addr,
+		"--dport", strconv.Itoa(dest_port),
+		"-j", "MASQUERADE"); err != nil {
+		return err
+	} else if len(output) != 0 {
+		return fmt.Errorf("Error iptables forward: %s", output)
+	}
+
 	return nil
 }