In a recent upgrade (from Openstack Diablo on Ubuntu Lucid to Openstack Essex on Ubuntu Precise), we found that DNS packets were frequently (almost always) dropped on the bridge interface (br100). For our compute-node hosts, that's a Mellanox MT26428 using the mlx4_en driver module.

We've found two workarounds for this:

1. Use an old lucid kernel (e.g. 2.6.32-41-generic). This causes other problems, in particular the lack of cgroups and the old version of the kvm and kvm_amd modules (we suspect the kvm module version is the source of a bug we're seeing where occasionally a VM will use 100% CPU). We've been running with this for the last few months, but can't stay here forever.

2. With the newer Ubuntu Precise kernels (3.2.x), we've found that if we use sysctl to disable netfilter on bridge (see sysctl settings below) that DNS started working perfectly again. We thought this was the solution to our problem until we realised that turning off netfilter on the bridge interface will, of course, mean that the DNAT rule to redirect VM requests for the nova-api-metadata server (i.e. redirect packets destined for 169.254.169.254:80 to compute-node's-IP:8775) will be completely bypassed.

Long-story short: with 3.x kernels, we can have reliable networking and broken metadata service or we can have broken networking and a metadata service that would work fine if there were any VMs to service. We haven't yet found a way to have both.

Anyone seen this problem or anything like it before? got a fix? or a pointer in the right direction?

Our suspicion is that it's specific to the Mellanox driver, but we're not sure of that (we've tried several different versions of the mlx4_en driver, starting with the version built-in to the 3.2.x kernels all the way up to the latest 1.5.8.3 driver from the mellanox web site. The mlx4_en driver in the 3.5.x kernel from Quantal doesn't work at all)

BTW, our compute nodes have supermicro H8DGT motherboards with built-in mellanox NIC:

02:00.0 InfiniBand: Mellanox Technologies MT26428 [ConnectX VPI PCIe 2.0 5GT/s - IB QDR / 10GigE] (rev b0)

we're not using the other two NICs in the system, only the Mellanox and the IPMI card are connected.

Bridge netfilter sysctl settings:

net.bridge.bridge-nf-call-arptables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-ip6tables = 0

Since discovering this bridge-nf sysctl workaround, we've found a few pages on the net recommending exactly this (including Openstack's latest network troubleshooting page and a launchpad bug report that linked to this blog-post that has a great description of the problem and the solution)....it's easier to find stuff when you know what to search for :), but we haven't found anything on the DNAT issue that it causes.