You can "math" out the grammar. Treat it as an equality and use the "double negatives cancel" rule to flip the "not" modifying the 2 "is('s)" in that sentence a

You can "math" out the grammar. Treat it as an equality and use the "double negatives cancel" rule to flip the "not" modifying the 2 "is('s)" in that sentence and the title can be rewritten such that: "It’s not always DNS — unless it is. " Becomes : "It’s always DNS — unless it isn't." Ultimately they can both get interpreted as something like "It's DNS except if it isn't DNS." "DNS (NOT equal) (NOT DNS)" even. Not a super surprising statement. So wording that statement either way has the same meaning, however the way the author worded it for the article title matches the chronological order of troubleahooting events (at first seemed to not be DNS, but later it turned out it actually was).