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OpenFlow NAT Router



In this tutorial you will learn how to build a router for a network with a private address space that needs a one-to-many NAT (IP Masquerade) using OpenFlow. We will use the following network topology for this experiment. You will also learn how to take advantage of kernel L3 routing while using OVS . route topology


For this tutorial you need :


All the tools will already be installed at your nodes. For your reference we are going to use a Ryu controller.

Where to get help:

For any questions or problem with the tutorial please email


If you have already reserved the topology from a previous tutorial you can move to Execute.

1. Verify your Environment Setup:

This exercise assumes you have already setup your account at the GENI Portal. In particular ensure that:
  1. You can login to the GENI Portal
  2. You are a member of a GENI Project (there is at least one project listed under the ''Projects'' tab)
  3. You have setup your ssh keys (there is at least one key listed under the ''Profile->SSH Keys'' tab)

2. Setup the Topology:

  1. Login to the GENI Portal
  2. Reserve:
    1. the topology from an InstaGENI rack using the OpenFlow OVS all XEN RSpec (In Portal: "OpenFlow OVS all XEN"; URL:
    2. at a different InstaGENI rack reserve a XEN OpenFlow Controller RSpec (In Portal: "XEN OpenFlow Controller"; URL:


3. Test reachability before starting controller

3.1 Login to your hosts

To start our experiment we need to ssh into all the hosts (controller, ovs, host1, host2, host3). Depending on which tool and OS you are using there is a slightly different process for logging in. If you don't know how to SSH to your reserved hosts take a look in this page. Once you have logged in, follow the rest of the instructions.

3.1a Configure OVS

  1. Write down the interface names that correspond to the connections to your hosts (use ifconfig). The correspondence is:
    • h1_if: Interface with IP to host1 - ethX
    • h2_if: Interface with IP to host2 - ethY
    • h3_if: Interface with IP to host3 - ethZ
  2. In the OVS node run:
    wget ; chmod +x
    sudo ./ <h1_if> <h2_if> <h3_if> <controller_ip>
    The controller_ip is the public IP of your controller node, look at the end for a tip on how to get the IP of a node.
  3. Look around to see what the script did:
      sudo ovs-vsctl show

3.1b Configure hosts

The hosts in your topology are all in the same subnet, We will move host3 to a different subnet:

  1. host3: Assign to host3.
     sudo ifconfig eth1
  2. host1, host2: Setup routes at host1 and host1 to subnet:
     sudo route add -net netmask gw

3.2 Test reachability

  1. First we start a ping from ihost1 to host2, which should work since they are both inside the same LAN.
    host1:~$ ping -c 10
  1. Then we start a ping from host3 to host1, which should timeout as there is no routing information in its routing table. You can use route -n to verify that.
    host3:~$ ping -c 10
  1. Similarly, we cannot ping from host1 to host3.
  1. You can also use Netcat (nc) to test reachability of TCP and UDP. The behavior should be the same.

4 Start controller to enable NAT

4.1 Access a server from behind the NAT

You can try to write your own controller to implement NAT. However, we a provide you a functional controller.

  1. Download the NAT Ryu module. At your controller node run:
    cd /tmp/ryu/
    tar xvfz ryu-nat.tar.gz
  1. Start the controller on controller host:
     cd /tmp/ryu/; ./bin/ryu-manager

You should see output similar to following log after the switch is connected to the controller

loading app
loading app ryu.controller.dpset
loading app ryu.controller.ofp_handler
instantiating app ryu.controller.dpset of DPSet
instantiating app ryu.controller.ofp_handler of OFPHandler
instantiating app of NAT
switch connected <ryu.controller.controller.Datapath object at 0x2185210>
  1. On host3, we start a nc server:
    host3:~$ nc -l 6666

and we start a nc client on host1 to connect it:

host1:~$ nc 6666
  1. Now send message between each other and try the same thing between host3 and host2.
  1. On the terminal of controller, in which you started your controller, you should see a log similar to:
    Created mapping 31596 to 59997

Note that there should be only one log per connection, because the rest of the communication will re-use the mapping.

5 Handle ARP and ICMP

One of very common mistakes that people make, when writing OF controller, is forgetting to handle ARP and ICMP message and finding their controller does not work as expected.

5.1 ARP

As we mentioned before, we should insert rules into the OF switch that allow ARP packets to go through, probably after the switch is connected.

5.2 ICMP

Handling ARP is trivial as NAT does not involve ARP. However, it's not the case for ICMP. If you only process translation for TCP/UDP, you will find you cannot ping between host3 and host1 while nc is working properly. Handling ICMP is even not as straightforward as for TCP/UDP. Because for ICMP, you cannot get port information to bind with. Our provided solution makes use of ICMP echo identifier. You may come up with different approach involves ICMP sequence number or others.

  1. On host1, start a ping to host3.
    host1:~$ ping
  1. Do the same thing on host2.
    host2:~$ ping

You should see both pinging are working.

  1. On host3, use tcpdump to check the packets it receives.
    host3:~$ sudo tcpdump -i eth1 -n icmp

You should see it's receiving two groups of icmp packets, differentiated by id.

Make sure to delete the bridges especially if you are using the same topology for another tutorial.

  sudo ovs-vsctl del-br OVS1
  sudo ovs-vsctl del-br OVS2


6. Cleanup

After you are done with the exercise and you have captured everything requested for the writeup, you should release your resources so that other experimenters can use them. In order to cleanup your slice :
  1. In Portal, press the Delete button in the bottom of your canvas
  2. Select Delete at used managers and confirm your selection.
Wait and after a few moments all the resources will have been released and you will have an empty canvas again. Notice that your slice is still there. There is no way to delete a slice, it will be removed automatically after its expiration date, but remember that a slice is just an empty container so it doesn't take up any resources.


  • Remember that you can use “ifconfig” to determine which Ethernet interface (e.g., eth0) is bound to what IP address at each of the nodes.
  • In order to enable IP forwarding of packets on a node you have to execute the following command:
    sudo sh -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'