Sorry, I might have mislead you slightly, I'm not using 4.2 myself, so I don't know for sure exactly what the options are but historically Tiki has had two methods of searching:
- MySQL search using the features of the database
- Tiki-built search
1 is better, and if that is what you're using then you shouldn't have any problems searching for "DNS". I even tried it in one of my Tiki's (5.2) and was able to successfully search for that term on a page I'd updated only moments earlier.
However, if you're using 2 then this requires an index to be created periodically to allow searching, and if you try to search for terms on pages that haven't been re-indexed recently then some search terms will not yield results. Normally the indexing is done automatically but it might not be done adequately frequently in some case in which case there are options to refresh the search indexes yourself (you should see those somewhere on the search admin page).
Just to be clear, we're talking about this page:
The very first option in 3.x is "Database Search" which is number 1 of what I'm talking about. Number 2 is underneath it and curiously checked permanently apparently in 3.x. A little further down you should see links about refreshing the indexes. You could try them, but before you do perhaps try clearing your tiki caches on tiki-admin_system.php.