Desktop Virtualization and IP addressing issue

My client’s problem

A bank, one of my clients, bought new hardware for their desktops some time ago. They have to run Windows 2000 on the branches because there is an critical application that doesn’t work on new versions. After they received the computers (around 700 units), they found that Windows 2000 isn’t compatible with the hardware. They are really new machines and the vendor doesn’t provide drivers for this old version of the Microsoft operating system. Porting the application to a new version is difficult and specially requires a lot of time.  Old computers are breaking from time to time and provisions for new hardware is urgent.

My client’s solution

The only solution that they found was to run Windows 2000 in the new machine virtualized. They install Linux, KVM and run the end user operating system over there. Hardware abstraction of KVM solves the problem, and Windows never sees the real hardware. This workaround works perfect. This may not be the best solution, but the other ones requires a lot of time.

The IP addressing problem

After finding this solution, they faced a new problem. Addressing. They use /24 subnets in the branches and big ones have more than 100 of desktops running. If they deploy, the virtualized desktop will double the required IPs per branch. One option is to change the IP addressing to support more IPs per branch, but that’s another big modification that requires time (IPs hardcoded in some apps, routers and firewalls configuration, etc, etc). It isn’t an option.

Linux hosts requires IP address because support techies will need to access to fix issues.

The solution to the IP addressing issue

The first measure to fix this issue was to configure every Linux with and IP addresss within the range 169.254.0.0/16, a special network range used for local communication between computers in the same network segment. All the branches will use the same subnet for the hosts. Connection between computers of every network branch is solved with this address, but connections from headquarters are impossible. This network isn’t routable.

So, another problem appears… how are support techies able to access the Linux hosts from the headquarters?

KVM uses a bridge to connect virtual machines to the physical network. With ebtables and iptables I’ve found a trick that permits connections to port 22 of the host using the IP address of the virtual machine. Let’s say that the VM uses the address 10.60.130.100 which is a valid address in the bank network. VM also has it’s own MAC address, for example 52:54:00:bf:57:bb. Have a look at this ebtables rule: 

ebtables -t nat -A PREROUTING -p arp --arp-opcode 1 --arp-ip-dst 10.60.130.100 -j arpreply --arpreply-mac 52:54:00:bf:57:bb

 This rule captures all the ARP requests asking for the IP address of Windows, generating a reply with the MAC address of the VM. So, ARP requests will be replied whether the Windows is running or not. This allows the packets to go through Linux always.

Now check this rule: 

iptables -t nat -A PREROUTING -i virbr0 -p tcp -d 10.60.130.100 --dport 22 -j REDIRECT

This is a typical REDIRECT rule, all the packets that have the IP address of the Windows machine and destination port 22 will be redirected to the Linux host.

Looks easy, right? But there are more work that needs to be done. In the default gateway of your network, you have to insert these rules:

 

iptables -t nat -A POSTROUTING -o eth0 -d 10.60.130.0/24 -p tcp --dport 22 -j SNAT --to 169.254.0.1 
iptables -t nat -A POSTROUTING -o ! eth0 -s 169.254.0.0/24 -o 10.0.0.0/8 -j SNAT --to 10.60.130.254

The first one is because the workstation only accepts traffic from local link network and the second one is to allow the machine to communicate with the rest of the world.

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