Iptables – NAT and iptables

iptablesnat;

I am new to iptables and NAT. But I am working on it through various material both in online and books. I have a doubt regarding NAT implementation using iptables.

Let us suppose my network scenario is as shown below.

                  linuxbox2 (10.0.0.3)
                    |
                    |
linuxbox1-------- Switch ----- INTERNET
(10.0.0.2)        (194.160.1.1)

My Linux boxes 1 & 2 connect to the internet via switch and I have only one Public IP-Address(194.160.1.1). But the communication is a bit strange. box2 does not connect directly to outerworld via switch. But it is in vlan with box1. Hence traffice from box2 goes to box1 then travels down to switch to connect to outer world. We does natting at box1

Since we try to masquerade to the same public ip-address what should be the rules in iptable.

My question might be strange but I am not able to explain it clealry …

Started a new thread with more details and outputs at

Iptables and SNAT

Best Answer

If I understand your question well, Box 2 is in a VLAN with Box 1, and Box 1 is in another VLAN as well which has access to the Internet.

Assuming the first VLAN is e.g. VLAN 100 and the second VLAN is VLAN 200, I assume you configured VLANs correctly on Box 1 and thus you have two network interfaces for the two VLANs (typically eth0.100 and eth0.200).

In this case the solution is simple, on Box 1:

iptables -t nat -A POSTROUTING -o eth0.200 -j MASQUERADE

With this command Box 1 will NAT-masquerade packets to the Internet on VLAN 200. Thus when Box 2 will send packets to Box 1 on VLAN 100, Box 1 will forward them to VLAN 200.

Make sure IP forwarding is enabled on Box 1 (sysctl -w net.ipv4.ip_forward=1).

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