I think you would be quite silly to just outright accept intelligence agency declarations of "it was Russia". As history shows, not only are these people frequently ignorant to technical realities, but political reasons at every single layer of these organizations obscure and pervade the truth.
Of course it's not just intelligence agencies saying this. There's plenty of independent investigations [0] and they are pointing the same direction. There's the metadata on the leaked files indicating that at least the metadata was modified with a cyrillic computer. There's reports from two separate security firms implicating the same two Russian based actors. There's the fact that "Guccifier 2.0" had no online pres…
https://medium.com/@jeffreycarr/can-facts-slow-the-dnc-breac...
The IP-address (176.31.112.10) that links the DNC hack to the Bundestag hack is a machine in France (appearantly) controlled by a Pakistani hosting company. This article says the machine was closed because of abuse over a year ago. The material in the DNC hack is just one month old:
https://netzpolitik.org/2015/digital-attack-on-german-parlia...