A variant in the French defence advance variation Consider the following beginning, quite typical of the advance variation of the French defense:

[fen ""]

1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Qb6 6.a3


On what I have seen on the web, the recommended black's next moves are 
6. ... Nh6, or 6. ... Bd7. But what about 6. ... f6 ? Lately this has been played 
regularly against me with the white pieces, and I don't know what to do. Any idea?

Thanks,

Joël
Actually, the GM who taught me (Vladimir Kosyrev) plays c4! closing the structure then Na5 then Bd7 then Nb3, after Nxb3, you pin the N on b3 with Ba4! and you can leave the pin until the queen moves, of course castle long and f5 to break through on the king side

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"] 
1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a3 c4 7. Nbd2 Na5! 8. g3 Bd7 9. Bg2 Nb3 10. Nxb3 Ba4!


Feel free to look up Vladimir Kosyrevs french games in this position after 6... c4!