CentOS – Using NAT with KVM Guests

Wednesday, July 17th, 2019

CentOS – Using NAT with KVM Guests

Please note that all commands in this guide must be run on the main HOST machine (the physical machine).  They should not be run on KVM guests (virtual machines).

If your server has a limited number of IPv4 addresses, it might be best to setup and run some virtual machines that are configured to use the default NAT network interface that KVM provides.  This will allow you to run multiple virtual machines that share the same IP address.  Think of it as setting up a home network with multiple devices with certain ports forwarded to specific devices for incoming connections.

First, create the virtual machines that are going to use NAT using virt-manager as you normally would.  In the virtual machine configuration wizard, assign the default NAT network interface named "virbr0".  After the virtual machines have been created, shut down the virtual machines.  Now, we'll assign these virtual machines static LAN IP addresses so that we can port forward certain ports and always have them reach the proper virtual machines. 

The first thing we need to do is get the MAC address of each virtual machine.  Write down the name of the virtual machine and its MAC address, as we'll need this information later on when we edit the NAT interface and assign static LAN IP addresses to our virtual machines.  Run this command to retrieve the MAC address for a specific virtual machine.

virsh dumpxml VM_NAME | grep -i '<mac'

Using the MAC address information from the VMs we want to use NAT with, edit the default NAT interface by running the below command. 

virsh net-edit default

If for some reason the NAT interface is not named default, you can find it by running the below command:

virsh net-list

After the <range /> entry, assign the static LAN IP addresses similar to the following:

<dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
      <host mac='52:54:00:ff:4a:2a' name='vm1' ip='192.168.122.13'/>
      <host mac='52:54:00:bb:35:67' name='vm2' ip='192.168.122.14'/>
      <host mac='52:54:00:aa:d9:f2' name='vm3' ip='192.168.122.15'/>
</dhcp>

Save the file with your desired values and quit the editor.  Restart the NAT interface by running the below commands:

virsh net-destroy default
virsh net-start default

Now, you'll need to setup your iptables port forwarding rules.  Adjust the below rules as necessary (changing the port numbers to the ones you want to use) and then save them so that they persist:

iptables -I FORWARD -o virbr0 -d 192.168.122.13 -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39989 -j DNAT --to 192.168.122.13:39989
iptables -I FORWARD -o virbr0 -d 192.168.122.14 -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39990 -j DNAT --to 192.168.122.14:39990
iptables -I FORWARD -o virbr0 -d 192.168.122.15 -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39991 -j DNAT --to 192.168.122.15:39991
iptables -t nat -A POSTROUTING -s 192.168.122.0/24 -j MASQUERADE
iptables -A FORWARD -o virbr0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i virbr0 -o br0 -j ACCEPT
iptables -A FORWARD -i virbr0 -o lo -j ACCEPT
iptables-save
service iptables save

Congrats, your virtual machines are now using NAT, have been assigned static LAN IP addresses, and iptables rules on the host server have been configured to port forward specific ports to each NAT VM.


Persistently Saving NAT Port Forward Rules

The only solution I found that would persistently save my NAT forwarding rules is to create a libvirt hook bash script as mentioned here

service iptables stop
iptables -F
service iptables save
service iptables start
mkdir -p  /etc/libvirt/hooks
nano  /etc/libvirt/hooks/qemu

The contents of the "/etc/libvirt/hooks/qemu" file should look similar to the following:

#!/bin/bash
# IMPORTANT: Change the "VM NAME" string to match your actual VM Name.
# In order to create rules to other VMs, just duplicate the below block and configure
# it accordingly.
if [ "${1}" = "vm1" ]; then   # Update the following variables to fit your setup
   GUEST_IP=192.168.122.13
   GUEST_PORT=39989
   HOST_PORT=39989   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
    /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
    /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
    /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
    /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
fi
if [ "${1}" = "vm2" ]; then   # Update the following variables to fit your setup
   GUEST_IP=192.168.122.14
   GUEST_PORT=39990
   HOST_PORT=39990   
   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
fi
if [ "${1}" = "vm3" ]; then   # Update the following variables to fit your setup
   GUEST_IP=192.168.122.15
   GUEST_PORT=39991
   HOST_PORT=39991
   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   fi
fi

Save and exit.  Make the script executable.

chmod +x /etc/libvirt/hooks/qemu

Reboot the host server.


Old Instructions for Persistent Saving (Non-Working)

If your iptables forwarding rules are not persisted after the host machine is rebooted or shutdown, run the following commands:

sudo -i
yum install -y iptables-services
systemctl stop firewalld
systemctl disable firewalld
systemctl enable iptables
nano /etc/sysconfig/iptables-config 

Change the below values to "yes":

IPTABLES_SAVE_ON_RESTART="yes"
IPTABLES_SAVE_ON_STOP="yes"

Save and exit.  Reboot the server.

If you're still having issues, try this (will clear your existing iptables rules):

iptables-save > iptables_bk
service iptables stop
iptables -F
<run iptables NAT rules here>
<run any other iptables rules you want>
service iptables save
service iptables start

More Detailed Guide

How to Make MATE Look Like Windows XP using the Luna Theme

Sunday, April 14th, 2013

Make MATE or GNOME2 Look Like Windows XP Using the Luna Theme

If you want your Linux installation to look like the original theme used in Windows XP, you can do that! This guide will walk you through the process of easily making any MATE or GNOME2 Desktop Environment look like the Windows XP GUI. The Luna Theme can be downloaded here and installed using our simple installation script. If you already have MATE installed or are already running GNOME2, skip to the Luna Theme install instructions.

Install MATE on Ubuntu:

Run the below commands for your matching Ubuntu version in a terminal to install MATE.  To find out which version of Ubuntu you're running, use this command:

lsb_release -a

For Ubuntu 12.04:

sudo add-apt-repository "deb http://packages.mate-desktop.org/repo/ubuntu precise main"
sudo apt-get update 
sudo apt-get --yes --quiet --allow-unauthenticated install mate-archive-keyring 
sudo apt-get update
sudo apt-get install -y mate-desktop-environment

For Ubuntu 14.04:

sudo apt-add-repository ppa:ubuntu-mate-dev/ppa
sudo apt-add-repository ppa:ubuntu-mate-dev/trusty-mate
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install -y mate-desktop-environment-extras

For Ubuntu 16.04:

sudo add-apt-repository ppa:ubuntu-mate-dev/xenial-mate
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install -y mate-desktop-environment

For Other Distributions (Distros):

http://wiki.mate-desktop.org/download

Installing the Luna Theme:

Our version of the Luna theme has been converted and ported over to GTK3, so it should work with all newer flavors of Linux running MATE while still working on older Linux installs running GNOME2.To install the Luna Theme which will make Linux look like Windows XP, run the following commands. The theme files will be downloaded and saved in your Downloads directory.

cd ~/Downloads
wget -O linux_xp_luna_theme_install.tar.gz http://dinofly.com/files/linux_xp_luna_theme_install.tar.gz
mkdir Luna
tar -zxvf linux_xp_luna_theme_install.tar.gz -C Luna
cd Luna
sudo rm -rf /usr/share/themes/Luna
rm -rf ~/.themes/Luna
sudo bash install.sh

Next, Right Click on the Desktop, and choose "Change Desktop Background".  Click on the "Themes" tab.  Select "Luna".  Click on the "Background" tab.  If you want the default XP wallpaper set as your background, click on the "Add" button.   Select your "Pictures" folder.  Select "luna_background.jpg".  Click "Open".  Click on "Close" to change it. 

Now, MATE or GNOME2 looks like XP!  Enjoy!  This theme was copied from Ylmf OS 3.0.