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© Rakhesh Sasidharan

CentOS NAT

My home lab setup is such that everything runs in my laptop within VMware Workstation (currently version 11) on a Windows 8.1 OS. Hyper-V might be a better choice here for performance but I am quite happy with VMware Workstation and it does what I want. Specifically – VMware allows for nested virtualization, so I can install a hypervisor such as ESXi or Hyper-V within VMware Workstation and run VMs in that! Cool, isn’t it?

VMware Workstation also natively supports installing ESXi within it as a VM. I can’t do that if I were using Hyper-V instead.

Finally, VMware Workstation has some laptop lab level benefits in that it easily lets you configure network such as having a NAT network up and running quickly. I can do the same with Hyper-V but that requires a bit more configuring (expected, because Hyper-V is meant for a different purpose – I should really be comparing Hyper-V with ESXi but that’s a comparison I can’t even make here).

Within VMware Workstation I have a Windows environment setup where I play with AD, IPv6, DirectAccess, etc. This has a Corporate Network spread over two sites. I also have some CentOS VMs that act as routers – to provide routing between the two Corporate Network sites above, for instance – and also act as NAT routers for remote use. Yes, my Workstation network also has a fake Internet and some fake homes which is a mix of CentOS VMs acting as routers/ NAT and Server 2012 for DNS (root zones, Teredo, etc).

The CentOS VM that acts as a router between the Corporate Networks can also do real Internet routing. For this I have an interface that’s connected to the NAT network of VMware Workstation. I chose to NAT this interface because back when I created this lab I used to hook up the laptop to a network with static IP. I had only one IPv4 address to use so couldn’t afford to bridge this interface because I had no IPv4 address to assign it.

Because of the NAT interface the CentOS VM itself can access the real Internet. But what about other VMs that forward to this for their Internet routing? You would think that simply enabling packet forwarding on this VM is sufficient – but that won’t do. This is because packet forwarding for when forwarding between networks but in this case the external network does not know anything about my virtual networks that are behind a NAT (of VMware Workstation) so simply forwarding won’t work. So what you need to do apart from forwarding is also set up NAT on the CentOS VM so as far as the external network is concerned everything is coming from the CentOS VM (via its interface that is NAT’d with VMware Workstation).

I have done all this in the past but today I needed to revisit this after some time and forgot what exactly I had done. So this post is just a reminder to myself on what needs to be done.

First, enable packet forwarding in the OS. Add the following line to /etc/sysctl.conf:

Reboot the VM or run the following to load it now itself:

Now modify the firewall rules to allow packet forwarding as well as NAT (aka MASQUERADE in iptables):

Lines 1-6 and 16 are the relevant ones here. I have a few extra like allow ICMP and SSH but those don’t matter for what I am doing here.