I scanned through the 25-page unclassified report released a couple of days ago. I found no technical details, which I'm afraid means that the question devolves to whether you trust the FBI, CIA, and the NSA, (plus the non-technical arguments about Putin's motivations in the report, which I found reasonably compelling).
It seems to me that it would be difficult to get all three agencies to agree that Russia was behind the DNC email hack if that weren't true, so I suspect it's probably true, but not with great certainty.
That lack of technical details is pretty much all I need to hear. The whole thing smells of politics. The media have certainly been trying to conflate all of it in an attempt to make people think Trumps victory is invalid - and even worse, a plot by Putin.
If this is true, I really have to wonder why there is no evidence of this. Gov't already had their chance to release evidence in their 25 page JAR report, but after reading it, I found 0 evidence of Russia being responsible for the DNC hack.
It seems to me that it would be difficult to get all three agencies to agree that Russia was behind the DNC email hack if that weren't true, so I suspect it's probably true, but not with great certainty.