It’s just a common saying “the problem is always DNS”. When you have a weird networking issue, the problem always seems to be DNS. Either some misconfigured DNS entry or DHCP giving you the wrong server address, etc…

And then the problem is confounded by the fact that, ironically, DNS works so well that we don’t think of it as a primary point of failure. So inevitably, when there is a DNS problem, it’s the last thing we check. This just reinforces the idea that the problem is always DNS… because in those long, hard to troubleshoot instances… the problem was DNS.