Shuts down the socket so that all other responses are rejected (port 'AAAA' record, but after the first response it cancels the queries and mDNSResponder queries for both the 'A' and Of particular relevance is a thread which Andy references, and I'm wondering if this issue with mDNSResponder, or something similar, still exists as I believe it could explain the problem I'm having. Whilst investigating the problem and searching the web, I came across a short thread in google groups: 10.6 DNS resolution, does not obey DNS server priority from DHCP that intrigued me. Whilst things are looking much better here, I am still getting a few of these packets periodically and I would like to fix the issue that is causing them, whilst I have the momentum. Over the last few hours, my DNS resolution seems stable and the number of these packets has reduced significantly. The symptom of the problem seemed to be that my HOST was generating a lot of ICMP Destination unreachable (Port unreachable) packets that were being sent back to my DNS server. I used Wireshark to help me debug the above problem. The server would sporadically resolve my local (internal access only) domains but later refuse to resolve them (only to recommence resolution some time later). I was having some local domain resolution problems with a DNS server I have setup inside a VirtualBox 'Host-Only' network that I think I've now fixed.

Icmp wireshark mac address