I just upgraded a MacMini to Ventura 13.2.1 and DNS resolution failed. Browsers wouldn’t open and pinging a domain was failing. However, nslookup and dig continued to work fine…which was the clue as those tools don’t use mDNSResponder.
To cut a long story short (and I won’t bitch about how painful and useless was my call to Apple Support) I discovered that if I kill -9 the mDNSResponder process and its helper process, they immediately restart and DNS works properly. This is 100% reproducible (I shut down/restarted the Mac Mini four times)
I’ve seen numerous reports of this problem online - I don’t know what are the circumstances that make this happen as it clearly works for many users. In any case, here’s how to fix the problem.
In a terminal window, run the command ps aux | grep -i mdns and you’ll get a result like the following – the actual process id numbers will of course be different for each user
Then type the following, replacing those two numbers with the numbers displayed when you run the ps command above
sudo kill -9 284 275
You’ll be prompted for your password, type it in.
That fixes it…until the next time you start your Mac.
Obviously, I don’t know why this problem occurs but I speculate that the mDNSResponder process is starting before the computer has an active IP address available and the mDNSResponder is just hanging.