[vbox-dev] Static route or default gateway for NAT

Malcolm Clarke Malcolm.Clarke at brunel.ac.uk
Mon Oct 3 20:34:16 UTC 2016


Dear Maxine

The problem is that a packet can eminate from a VM on the hidden subnet and is correctly routed by the internal router to the NAT router (the internal server contains the NAT as its default router), and so the packet is sent to the public network. The NAT router will correctly create an entry in the mapping table with the return IP address. However when a packet returns from the public network, even though the NAT router can substitute the correct destination IP address for the hidden subnet, it does not have the routing information to deliver the packet to the internal router for it to be returned to the VM on the hidden network.

It would therefore require some simple mechanism to add static routes (or private side default gateway) to the NAT router.

Using a bridge will not work as that would require configuring "public" routers to deliver packets to the internal router. NAT is the best solution.

Regards

Malcolm

On 03/10/2016 13:58, Maxime Dor wrote:
Hi Malcom,

That is on purpose - being behind a NAT network means you want to hide any subnet connect to that network from the outside world.
Any outgoing connection will look like it came from the NAT Router "public" IP.
If you want to allow specific connections to be allowed in, you need to configure port forward - so far, I don't think I tell you anything new.

But if you need the "outside" world to know about "inside" networks, then NAT is not the right choice. You need to switch to a non-NAT solution like Bridged mode (or Host-Only with routing enabled on the host) and the "outside" world needs to know about those "inside" network with two possibilities:
- Static routes on all routers that need to deal with those subnets
- Internal routing protocol like RIP, EIGRP or OSPF that will auto-detect routes and populate routing tables of routers.

On 03/10/16 13:30, Malcolm Clarke wrote:
Dear Development Group

I am trying to demonstrate routing in a virtualised network created using VirtualBox with a FreeBSD server acting as router between 2 virtual networks. One network is set as NAT Network to allow access to outside world. However, although packets can be directed from the router to the NAT router for outward delivery, the NAT router does not know how to deliver the incoming packets for the "hidden" subnet.

I wonder if anyone has modified the NAT network to allow simple static routes or default gateway to support this cnfiguration.

I do not know the interest for this functionality and whether the work is justified for the use that would be made.

Regards

Malcolm


--
Malcolm Clarke BSc (Hons), PhD
Reader in Telemedicine and Data Communication Systems
T +44 (0) 1895 265053

Brunel University London
College of Engineering, Design and Physical Sciences
Department of Computer Science

HNZW011, Heinz Wolff Building, Kingston Lane, Uxbridge, Middlesex, UB8 3PH

www.brunel.ac.uk<http://www.brunel.ac.uk/>

Connect with the university on Linkedin, Twitter, Facebook




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--
Malcolm Clarke BSc (Hons), PhD
Reader in Telemedicine and Data Communication Systems
T +44 (0) 1895 265053

Brunel University London
College of Engineering, Design and Physical Sciences
Department of Computer Science

HNZW011, Heinz Wolff Building, Kingston Lane, Uxbridge, Middlesex, UB8 3PH

www.brunel.ac.uk<http://www.brunel.ac.uk/>

Connect with the university on Linkedin, Twitter, Facebook



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