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I have a server. It's configured with a public IP, and I have /29 subnet assigned via a network bridge.

KVM is installed and works as intended. I can create and boot a VM, but when it comes to defining network access things start to go wrong.

If I manually enter the IP for the Guest (which comes from the /29). I enter the host IP as the gateway, and then I get the message that the gateway is unreachable.

Here's the contents of /etc/network/interfaces

auto lo
iface lo inet loopback
iface lo inet6 loopback

auto br0
iface br0 inet static
  address 144.76.15.98
  netmask 255.255.255.224
  gateway 144.76.15.97
  pointopoint 144.76.15.97
  bridge_ports eth0
  bridge_stp off
  bridge_maxwait 0
  bridge_fd 0
  up route add -host 136.243.185.120 dev br0
  up route add -host 136.243.185.121 dev br0
  up route add -host 136.243.185.122 dev br0
  up route add -host 136.243.185.123 dev br0
  up route add -host 136.243.185.124 dev br0
  up route add -host 136.243.185.125 dev br0
  up route add -host 136.243.185.126 dev br0
  up route add -host 136.243.185.127 dev br0

What am I doing wrong?

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1 Answer 1

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2

It depends what’s behind your eth0 interface, but it looks wrong to bridge your external network with your virtual network, given that they use different IP addresses.

I think things would work much better if you set up your host computer as a router between eth0 and br0.

Things might also be easier if you used your 136.243.185.120/29 as a subnet rather than route it IP by IP.

You would have something like this:

auto eth0
iface eth0 inet static
  address 144.76.15.98
  netmask 255.255.255.224
  gateway 144.76.15.97

auto br0
iface br0 inet static
  address 136.243.185.121
  netmask 255.255.255.248
  bridge_ports eth0
  bridge_stp off
  bridge_maxwait 0
  bridge_fd 0

You’d also have to enable packet forwarding with sysctl net.ipv4.ip_forward=1.

If you can’t afford to “loose” 3 addresses in your 136.243.185.120/29 network, you can configure br0 with address 144.76.15.98, netmask 255.255.255.255 and manually route your 8 addresses to that interface, as you did.

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  • Thanks for the suggestions. The box in question is hosted with Hetzner and I followed this guide: wiki.hetzner.de/index.php/… But I'll try your suggestion tonight. I'd rather lose some IP's to get this working, than feel an overwhelming urge to throw the box out the proverbial window.
    – Thomas
    Nov 21, 2017 at 14:51
  • I just had a look at that tutorial, and it looks more like my second suggestion than like your config: in the host, the physical interface is not part of the bridge, and the bridge is configured with the same IP as the external interface, with netmask 255.255.255.255.
    – user2233709
    Nov 21, 2017 at 22:12
  • The problem is that when I follow the above suggestions, I end up with VMs that return "gateway unreachable". This occurs when I try to install Ubuntu on the guest and I end up having to configure the network manually (because obviously, no DHCP)
    – Thomas
    Nov 22, 2017 at 12:06
  • What configuration did you try for the host? (Subnet or routes to the individual hosts?) And how did you configure the VM’s network?
    – user2233709
    Nov 22, 2017 at 19:44
  • I've set it up with individual routes. I have a VM up and running, but can't get the host/guest connection working on a non-lan ip. I've configured the VM with two nics; one running 10.0.0.x and one running the above /29. But it ain't taking. But I'll get to the bottom of this. :D
    – Thomas
    Nov 22, 2017 at 21:02

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