DuckDuckGo and Google use different querystrings for site-based searches. Do the same search on Google and DDG, then look at the URL in the address bar to see the difference.

You should now be able to see what needs to be changed in NNF to make the search function work with DDG.

Google is the best search engin

:: Exemple code

% html
<form id="search" method="get" action="https://duckduckgo.com/">
		<input type="hidden" name="sites" value="your.site"/>
		<input type="hidden" name="k8" value="#222"/>
		<input type="hidden" name="k9" value="#D51920"/>
		<input type="hidden" name="kt" value="c"/>
		<input id="query" name="q" maxlength="255" placeholder="Buscar&hellip;"/>
		<input type="submit" value="DuckDuckGo Search" style="visibility: hidden;" />
		<input id="go" type="image" src="/themes/greyscale/img/go.png" width="20" height="20" alt="Search">
</form>
%

Replace your.site for you real domain.

These changes can be made in */nnf/themes/greyscale/index.html* and */nnf/themes/greyscale/thread.html*.

For more details see DDG url parameters.
Link: https://duckduckgo.com/params