The Bombay Stock Exchange benchmark Sensex today gained over 170 points in opening trade on buying by foreign funds. The 30-share index was trading at 17,633, up by 109 points when reports last came in. Similarly, the wide-based National Stock Exchange index Nifty rose by over 35 points to 5,267. Brokers said firming trend on the Asian markets following overnight rally in the US markets mainly triggered buying by foreign funds.