OpenStack nova network multihost with external gateway. Solving a metadata issue in Kilo. The power of ebtables.

In a recent project we found the issue that the metadata service running on the compute node was not reachable by the VMs running on it.

In the project we use Ubuntu 14.04.3 and vanilla OpenStack (kilo). The implementation uses nova network (with VLANs) in multihost mode with an external gateway for each VLAN. The latter can be achieved by:

  • configuring nova in /etc/nova/nova.conf with the option:


  • and by supplying a file /etc/dnsmasq-nova.conf where you give the external gateway configuration for the different networks (demo-net, in the following example):


In our configuration the VMs are not able to reach the metadata service when being provisioned. In other words is not properly DNATed to hypervisor_IP:8775 for the VM.

The underlying reason is that since the VM has a different gateway (the external gateway) than the hypervisors VLAN IP, there will be no IP routing (it is only a L2 not an L3) so the  PREROUTING chain in table NAT will not be traversed by these packets.

To solve this issue we force these packets to traverse the IP routing with an ebtables rule like:

ebtables -t nat -I PREROUTING -p ipv4  –ip-dst –ip-protocol 6 –ip-dport 80 -j redirect –redirect-target ACCEPT

Enjoy the ebtables! Find more examples and documentation in here.

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