The forward fuselage section including the cockpit of MH17 separated at the weakest point. Exactly were the forward section is joined to the middle section the break happened mid air as seen by photos of debris. The information used for this post has been taken from mh17.webtalk.ru.
When the missile exploded very close to the left cockpit window, the rooftop of the cockpit was blown away. Pieces where found in Petropavlivka. See this post for some photos.
The photos below show the separation at the rightside of the fuselage. The upper photo shows parts of the belly including the cheatline. This part is indicated by the black lines. Hardly visible is the cargo door.
The debris showing the Malaysian flag was found at a different location than where the forward fuselage section was found. A clear cut running vertical can be seen.
The lefthand side of the fuselage shows the same location of the break. The two pieces were photographed at a different location than where the cockpit was found.
The forward fuselage found near Rozsypne is indicated by the lighter colour in the picture below.
This photo of an unpainted Boeing 777 clearly shows the forward fuselage section. The darker coloured green matches the section of MH17 found at Rozsypne.