I'm trying to setup a Xen virtualization on Ubuntu 16.04, and Xen 4.6.

My plan is to have 1 private network, as well as virtual servers with public IP addresses which should be accessible externally (as well as connected to the private network). My hosting provider gives me 4 additional public IPs.

For this, I've setup two bridges:

# brctl show
bridge name     bridge id               STP enabled     interfaces
xenbr0          8000.0025907784d4       no              eth0
xenbr1          8000.36829b44377f       no              dummy0

xenbr0 has the public static IP, and xenbr1 a private one (

I've then configured the virtual machine as follows

 'ip=<virtual server public ip>,mac=02:00:00:c1:fb:49,bridge=xenbr0',

I can connect to the address just fine, and that machine has access to the internet correctly configured through the gateway. So everything is good there.

I then add the public IP in /etc/network/interfaces on the virtual server

auto eth0
iface eth0 inet static
    address <public ip>
    network <public ip substituting .0 at the end>
    broadcast <public ip>
    hwaddress ether 02:00:00:c1:fb:49

And the internal network (which has the gateway)

iface eth1 inet static

Now the route looks like this

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UG    0      0        0 eth1   *        U     0      0        0 eth1

I can ping the IP from the server itself obviously - but I can't connect to it externally, or from the xen server itself.

What am I doing wrong?

  • Thank you for the reply - I've added the details of the 2nd internal network above, which has the default gateway. I've tried the netmask as you suggest, but I only have a single IP on this range assigned to the server / interface, so I think is correct. I still can't access that VM remotely by its public IP – Kevin Blake Jun 12 '16 at 9:25
  • The .255 was provided by my hosting provider. I also have eth1 configured (with a netmask where my outbound routing is configured (that's where the .1 and .6 local addresses are). eth0 doesn't belong to the network. – Kevin Blake Jun 13 '16 at 6:39
  • Ah! My bad! I misread your question :-o I assumed the .255 was on the private side. – garethTheRed Jun 13 '16 at 6:46
  • 1
    The default line of your routing table doesn't look right. As it stands, everything is routed through the private network. The Gateway should be the ISP IP address and the Iface should be eth0 so that Internet traffic is routed via the ISP. (assuming I've read your question correctly) – garethTheRed Jun 13 '16 at 7:06
  • I think you're onto something there. To simplify things I went back to basics and removed the private network altogether. Then added static routes to the xen host, and the xen hosts gateway. route add x.x.x.94 dev eth0 \ route add x.x.x.254 dev eth0 \ route add default gateway x.x.x.254 Now everything seems to be working, and I can access either server remotely. Thank you for helping out - much appreciated! This has been driving me a little nuts :) – Kevin Blake Jun 13 '16 at 18:00

My mistake appears to be routing all of the traffic over the private network, and via eth1 - xenbr1 - dummy0. eth1 should be used for the private network only, and eth0 used for general traffic.

So from eth1 on the virtual machine, remove the gateway in /etc/network/interfaces:

iface eth1 inet static

And change eth0 to include static routes to both the host machine, and host's gateway (I'm certain this can be done for the whole range, rather than the 2 IPs individually, to further simplify this).

iface eth0 inet static
    address 129.x.x.199
    broadcast 129.x.x.199
    up route add 23.x.x.94 dev eth0
    up route add 23.x.x.254 dev eth0
    up route add default gw 23.x.x.94
    down route del default gw 23.x.x.254
    down route del 23.x.x.94 dev eth0
    down route del 23.x.x.254 dev eth0

Note I've also removed the gateway definition from this one - until the static routes are added, the gateway is inaccessible (which leaves the network unable to start).

Now all internet traffic is routed through the eth0-xenbr0-eth0 bridge, and private traffic routed through eth1-xenbr1-dummy0.

The final route table looks like this:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         23.x.x.254         UG    0      0        0 eth0
23.x.x.94 *      UH    0      0        0 eth0
23.x.x.254 *      UH    0      0        0 eth0   *        U     0      0        0 eth0   *        U     0      0        0 eth1

Thank you to garethTheRed for your assistance!

  • I ran into a similar issue where when I configure the xenbr1 the external ip become inaccessible. Your answer is showing me some hope. Can you clarify the component dummy0? What is it and why is it required? Thanks – jazaman Jul 19 '17 at 19:23

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