Intermittent packet loss in Mac OS X host

[indigospring]

This has been ongoing over all recent revisions of Virtualbox 3.1.x and 3.2.x, OS X Server 10.6.x (up to 10.6.4)

Running Virtualbox on a Mac OS X Server host causes intermittent UDP packet loss for the host.

The major symptom is that other clients on the network are periodically unable to successfully resolve names via DNS when trying to contact Apple supplied BIND named running on the host. The intermittent nature is why it has taken so long to isolate Virtualbox (more likely the network kext installed by StartupItem on the host) as the cause.

Clients intermittently report no DNS server can be found when trying to resolve well-known domains (eg www.google.com). Server logs for named (when extended debug logs enabled) report repeatedly "Success resolving /foo/ after reducing the advertised EDNS UDP packet size to 512 octets" The client is actually timing out while the server fails to respond to that particular name lookup (other lookups for other domains continue as normal).

It looks like some UDP packets are being dropped. I've only noticed it for DNS,

Using "dig" to investigate will often "unclog" things and clients can again resolve the problem domain for a while. (Using nslookup does not, it just reports the same timeout)

If it makes a difference, the Guest(s) were using bridged networking over the host's ethernet interface. The guest does not seem to exhibit any problems apart from the same issues resolving DNS using named running on the host.