I've just been reading this stuff . . interesting when you get down to the 'radar, aircraft recognition' parts
http://www.whathappenedtoflightmh17....hot-down-mh17/
Civilian aircraft do not carry equipment to respond to a IFF interogation. An exception are airliners from Canada and the US. So IFF is primarily for military aircraft.
In case an Ukraine BUK was used by separatists the operator would target military aircraft of Ukraine. However a missile would not lock on a friendly aircraft because of IFF. The missile could only lock on non friendly targets like MH17. This for sure must have been noticed by the operator. The operator can only launch on a friendly aircraft by overruling IFF notice.
In case a Russian BUK would have been used, both for an Ukraine military aircraft and MH17 would appear to the operator as enemy aircraft.
Not that it makes any difference, just wondering how you could mistake a civilian airliner for a legitimate target with all the technology available. How someone decided it was a target is beyond me . . murder most foul