First, some computer security background from a very long and detailed analysis:
So the starting point is that the systems were compromised, and almost certainly compromised by several different intruders, all of whom but one (Guccifer) remain unnamed in the unclassified report claiming that the Russians did it. In other words, there is no uncertainty as to the compromise other than who did it, and enormous uncertainty as to that.
And so, on to the report. It is a 27 page PDF, so it's actually a quick read. It's quicker even than you might think based on its thickness when you consider that 18 pages are things like cover sheets, table of contents, background about the investigation (Yay FBI! Yay Intelligence Community!), discussions about how they don't disclose sources and methods, a long discussion of open source Russian media (especially RT television programming), and "This page intentionally left blank".
So there are only 4 pages that you need to read. Three are "Summary/findings", and so do not have anything got back up their claims. The meat of the report, therefore, are the pages numbered 2-5. From a computer/network security perspective, these are entirely unpersuasive that the Russians (and more specifically, Vladimir Putin) was behind the hacks. Here are the topics that those pages discuss:
- Putin ordered campaign to influence US election (likely true, although may not have been Putin himself)
- Russian campaign was multifaceted (you'd certainly think so)
- Cyber espionage has been going on against US political organizations (well, duh)
- Public disclosures of Russian-collected data says that the GRU (Russian Military Intelligence) ran the "Guccifer 2.0" persona and gave the data to Wikileaks. No evidence is given to support this.
- Russian intrusions into State and local electoral boards did not access vote tallying computers.
- Russia has a propaganda effort and uses Russian media (especially RT) to get its message out (again, duh)
- Influence effort was "boldest yet" in US (whatever)
- Election operation signals "new normal" in Russian influence efforts (whatever)
And so, back to the second link in this post which discusses how things work in the real world:
Now there may be classified evidence that is compelling but which is suppressed to protect sources and methods. These wouldn't be IP address metadata from NSA, because the hop into Russia will almost certainly not be the final leg (indeed, it might be a hop before one to China,or Israel, both of whom have excellent cyber exploit capabilities). It might be CIA intel from inside the Russian government, but that is unlikely to have detailed information on GRU technical operations (or maybe it does, in which case it's very classified and nobody will tell us about this, maybe ever).
And so we're back to trust us. That's pretty weak.
My take is that several state actors certainly hacked Hillary's email server for years and years, and silently read all her communications. Probably more than one state actor penetrated the DNC email system for several years. It's plausible than an insider leaked the DNC emails - some BertieBro IT Admin type who saw how the sausage was being made and who was smart enough to cover his tracks while pointing clues towards Russia.
Bottom line, this is a tale told by an idiot; full of sound and fury and signifying nothing. We know that something happened, but we don't know who did it, and what they say in the report doesn't change that.
If you're interested in the topic, I recommend that you click through to this analysis, and particularly the conclusion.