KVM - nat or bridge. How I do that

Question

I have Debian Jessie server, and I have install apache server. From apache hosted several sites from different ip address. My server have resources for KVM. My question is - How I can set my free IP to guest KVM machine.. My free IP is eth0:3 -> 80.80.130.135 My configuration /etc/network/interfaces file is:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface

auto eth0
auto eth0:0
auto eth0:1
auto eth0:2
auto eth0:3

allow-hotplug eth0
iface eth0 inet static
    address 80.80.130.131
    netmask 255.255.255.192
    network 80.80.130.128
    broadcast 80.80.130.191
    gateway 80.80.130.1
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    dns-search localdomain

iface eth0:0 inet static
    address 80.80.130.132
    netmask 255.255.255.192
    network 80.80.130.128
    gateway 80.80.130.1
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    dns-search localdomain


iface eth0:1 inet static
    address 80.80.130.133
    netmask 255.255.255.192
    network 80.80.130.128
    broadcast 80.80.130.191
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    dns-search localdomain


iface eth0:2 inet static
    address 80.80.130.134
    netmask 255.255.255.192
    network 80.80.130.128
    broadcast 80.80.130.191
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    dns-search localdomain


iface eth0:3 inet static
    address 80.80.130.135
    netmask 255.255.255.192
    network 80.80.130.128
    broadcast 80.80.130.191
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    dns-search localdomain

Answer 1

1. Modify /etc/network/interfaces on your jessie host so that it uses a bridged interface for eth0.

e.g. delete your eth0 and eth0:* aliases definitions and add the following:

auto br0
iface br0 inet static
    address 80.80.130.131
    netmask 255.255.255.192
    network 80.80.130.128
    broadcast 80.80.130.191
    gateway 80.80.130.1
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 85.17.151.123 62.211.64.122
    post-up ip addr add 80.80.130.132 dev br0
    post-up ip addr add 80.80.130.133 dev br0
    post-up ip addr add 80.80.130.134 dev br0
    bridge_ports eth0
    bridge_stp off
    bridge_maxwait 1
    bridge_fd 1
    bridge_hello 2
    bridge_maxage 12

After rebooting or ifdown -a ; ifup -a you can configure your guest:

2. Configure the guest to use br0. You can do this with a GUI using virt-manager, or you can edit the XML file directly with virsh edit domainname, and change the interface definition to something like this:

<interface type='bridge'>
  <mac address='xx:xx:xx:xx:xx:xx'/>
  <source bridge='br0'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

3. Now you can either configure the guest to use the static IP 80.80.130.135 in /etc/network/interfaces (or whatever is appropriate for your VM if it's not another Debian system), or you can configure dnsmasq to allocate that IP to the VM's MAC address.