What actually happens? You get a search results page that says no results found? I'm kind of at a loss at this point. Sorry.
Just for kicks I created a page called "DNS" on my 3.x Wiki and put in a sentence or two on the page (threw in the "DNS" term in there again for good measure). The search found it without any problems, so it's definitely not some kind of "reserved word".
Have you tried searching directly from the search page: tiki-searchindex.php? (I don't know how you're currently initiating the search).