Force local IP traffic to an external interface

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Force local IP traffic to an external interface – Managing your servers can streamline the performance of your team by allowing them to complete complex tasks faster. Plus, it can enable them to detect problems early on before they get out of hand and compromise your business. As a result, the risk of experiencing operational setbacks is drastically lower.

But the only way to make the most of your server management is to perform it correctly. And to help you do so, this article will share nine tips on improving your server management and fix some problem about linux, networking, command-line-interface, ip, iptables.

I have a machine with several interfaces that I can configure as I want, for instance:

  • eth1:
  • eth2:

I would like to forward all the traffic sent to one of these local addresses through the other interface. For instance, all requests to an iperf, ftp, http server at should be not just routed internally, but forwarded through eth2 (and the external network will take care of re-routing the packet to eth1).

I tried and looked at several commands, like iptables, ip route, etc… but nothing worked.

The closest behavior I could get was done with:

ip route change to dev eth2

which send all 192.168.1.x on eth2, except for which is still routed internally.
May be I could then do NAT forwarding of all traffic directed to fake on eth1, rerouted to internally? I am actually struggling with iptables, but it is too tough for me.

The goal of this setup is to do interface driver testing without using two PCs.

I am using Linux, but if you know how to do that with Windows, I’ll buy it!


The external network is just a crossover cable between eth1 and eth2.
Let’s say I have an http server on my machine.
Now I want to access this server from the same machine, but I want to force the TCP/IP traffic to go through this eth1/eth2 cable. How should I configure my interfaces for this?

Solution :

I expanded on caladona’s answer since I could not see response packets. For this example:

  1. On my local PC I have NIC’s on different subnets, 192.168.1/24, 192.168.2/24
  2. There is an external router/PC that has access to both subnets.
  3. I want to send bi-directional traffic over the NICs on the local PC.
  4. The configuration requires two unused IP addresses for each subnet.

Local PC iptable routes are set to SNAT and DNAT outgoing traffic to the ‘fake’ IP.

iptables -t nat -A POSTROUTING -d -s -j SNAT --to-source
iptables -t nat -A PREROUTING  -d -i eth0           -j DNAT --to-destination
iptables -t nat -A POSTROUTING -d -s -j SNAT --to-source
iptables -t nat -A PREROUTING  -d -i eth1           -j DNAT --to-destination

The rules do the following:

  1. Rewrite source to on outgoing packets
  2. Rewrite destination to on incoming packets
  3. Rewrite source to on outgoing packets
  4. Rewrite destination to on incoming packets

To summarize, the local system now can talk to a ‘virtual’ machine with addresses and

Next you have to force your local PC to use the external router to reach your fake IP. You do this by creating a direct route to the IP’s through via the router. You want to make sure that you force the packets onto the opposite of the destination subnet.

ip route via $ROUTER_2_SUBNET_IP 
ip route via $ROUTER_1_SUBNET_IP

Finally to make this all work, the external router needs to know how to reach the faked IPs on your local PC. You can do thins by turning on proxy ARPs on for your system.

echo 1 | sudo tee /proc/sys/net/ipv4/conf/all/proxy_arp
echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward

With this setup, you can now treat the fake IPs as a real system on your local PC. Sending data to .1 subnet will force packets out the .2 interface. Sending data to the .2 subnet will force packets out the .1 interface.


I successfully used the following on Linux to test throughput on a new dual-port 10Gbps card in “loopback” mode, that is, one port plugged directly into the other. This is all just a bit of voodoo just to force packets out the wire, but if you don’t, Linux will just short-circuit the traffic through the kernel (hence the OP’s question). In Casey’s answer above, I’m not sure if it was really necessary to have an external router or not above, but the following is completely self-contained. The two interfaces are eth2 and eth3.

Give IPs to the interfaces, and put them on separate networks:

ifconfig eth2
ifconfig eth3

Next we’ll set up a double NAT scenario: two new fake networks used to reach the other. On the way out, source NAT to your fake network. On the way in, fix the destination. And vice versa for the other network:

# nat source IP -> when going to
iptables -t nat -A POSTROUTING -s -d -j SNAT --to-source

# nat inbound ->
iptables -t nat -A PREROUTING -d -j DNAT --to-destination

# nat source IP -> when going to
iptables -t nat -A POSTROUTING -s -d -j SNAT --to-source

# nat inbound ->
iptables -t nat -A PREROUTING -d -j DNAT --to-destination

Now tell the system how to get to each fake network, and prepopulate the arp entries (be sure to substitute your MAC addresses, don’t use mine):

ip route add dev eth2
arp -i eth2 -s 00:1B:21:C1:F6:0F # eth3's mac address

ip route add dev eth3 
arp -i eth3 -s 00:1B:21:C1:F6:0E # eth2's mac address

This fools Linux enough to actually put packets onto the wire. For example:


goes out eth2, the source IP gets NATted to, and as it comes into eth3 the destination gets NATted to And the reply takes a similar journey.

Now use iperf to test throughput. Bind to the correct IPs, and be certain which IP you’re contacting (the other end’s fake address):

# server
./iperf -B -s

# client: your destination is the other end's fake address
./iperf -B -c -t 60 -i 10

Make sure traffic is really going out to the wire:

tcpdump -nn -i eth2 -c 500

You can also watch /proc/interrupts just to be absolutely sure the card is being used:

while true ; do egrep 'eth2|eth3' /proc/interrupts ; sleep 1 ; done

Anyhow, I found this post searching for how to do this, thanks for the Q&A guys, and hope this helps anyone else finding this post in the future.

As always – I’m a little late – but nowadays one could use network namespaces to isolate the interfaces and prevent any local forwarding (and fiddling with iptables :)).

Create namespaces (all done with required permissions, e.g. as root):

ip netns add ns_server
ip netns add ns_client

Note that the interfaces status / config must now be accessed within
the context of the assigned namespace – so they won’t appear if you run a naked ip link as this is run in the context of the default namespace.
Running a command within a namespace can be done using

ip netns exec <namespace-name> <command>

as prefix.

Now assign namespaces to interfaces, apply config and set interfaces up:

ip link set eth1 netns ns_server
ip netns exec ns_server ip addr add dev eth1
ip netns exec ns_server ip link set dev eth1 up
ip link set eth2 netns ns_client
ip netns exec ns_client ip addr add dev eth2
ip netns exec ns_client ip link set dev eth2 up

Now you can execute the applications within the namespace – for the iperf server run

ip netns exec ns_server iperf -s -B

and the client:

ip netns exec ns_client iperf -c -B

The traffic will now be send via the physical interfaces as the whole networkstack, interface, routing … is isolated by the namespaces so the kernel is not able to match the addresses used within the traffic with local (available) interfaces.

If you are done with your experiments simply delete the namespaces:

ip netns del <namespace-name>

The interfaces will be reassigned to the default namespace and all configuration done within the namespace disappears (e.g. no need to delete assigned IP addresses).

Ok, I finally succeeded in setting up my config.

The idea is to use another fake address, to force the route of this fake address to the interface 2 , then to translate the fake address with the real address 2 with NAT/iptables.

My set up is actually made of one router I can telnet between IF1 (interface 1) and IF2

In my set up, FAKE_ADDR and IF1_ADDR are on the same subnet.

ifconfig $IF1 $IF1_ADDR netmask
ifconfig $IF2 $IF2_ADDR netmask

iptables -t nat -A PREROUTING -d $FAKE_ADDR -i $IF2 -j DNAT --to-destination $IF2_ADDR
iptables -t nat -A POSTROUTING -s $IF2_ADDR -d $IF1_ADDR/24 -j SNAT --to-source $FAKE_ADDR

route add $FAKE_ADDR gw $ROUTER_ADDR

And on the router:

route add $FAKE_ADDR gw $IF2_ADDR

If I send something to FAKE_ADDR, pkt is forwarded through IF1 to the router, forwarded again to IF2, then FAKE_IP is replaced by IF2_ADDR. The packet is processed by the server, the result is send back to IF1_ADDR, from IF2_ADDR which is replaced by FAKE_ADDR.

May be it is possible to use a simpler configuration with just one crossover cable, but as I didnot tried I prefer to give my working solution.

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