DHCP unicast requests are not responding

[Eric Desrochers]

Issue:
We run nova-network in VLAN+multi_host mode on Kilo and notice that only one dnsmasq process (either the oldest or newest) on the hypervisor responds to unicast BOOTPREQUESTS. dhclient on VMs will retry until it eventually gives up and broadcasts the request, which is then responded to. Depending on the timing of the DHCP broadcast request, VMs can briefly lose connectivity as they attempt rebinding.

According to http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commitdiff;h=9380ba70d67db6b69f817d8e318de5ba1e990b12, it seems that passing "--interface" argument, in addition to "--bind-interfaces" is necessary for dnsmasq to work correctly in VLAN mode.

Reproduce steps:
1. Create two tenants
2. Create a VM under each tenant, forcing the VMs to run on a single hypervisor. I tested with a vanilla Ubuntu cloud image, but any other image that uses dhclient should also work.
3. On the hypervisor, run dhcpdump -i <bridge interface> for each tenant's bridge interface. On at least one of them, you should see unicast BOOTPREQUEST with no corresponding BOOTPREPLY. dnsmasq will reply when the request eventually hits 255.255.255.255.

Nova/Openstack/dnsmasq versions:
ii nova-api 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - API frontend
ii nova-common 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - common files
ii nova-compute 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - compute node base
ii nova-compute-libvirt 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - compute node libvirt support
ii nova-compute-qemu 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - compute node (QEmu)
ii nova-network 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - Network manager
ii nova-novncproxy 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute - NoVNC proxy
ii python-nova 1:2015.1.2-0ubuntu2~cloud0 all OpenStack Compute Python libraries
ii python-nova-adminclient 0.1.8-0ubuntu2 amd64 client for administering Openstack Nova
ii python-novaclient 1:2.22.0-0ubuntu2~cloud0 all client library for OpenStack Compute API
ii dnsmasq-base 2.68-1ubuntu0.1 amd64 Small caching DNS proxy and DHCP/TFTP server
ii dnsmasq-utils 2.68-1ubuntu0.1 amd64 Utilities for manipulating DHCP lease

[Eric Desrochers (slashd)]

Upstream bug for OpenStack Compute (nova):
https://bugs.launchpad.net/nova/+bug/1554227