RS recommends the DB Bahn site saying
The Deutsche Bahn site is the most complete resource for train schedules throughout Europe. If your destination isn’t listed on the Deutsche Bahn site (and if you’ve spelled it correctly), it likely doesn’t have train service. But before giving up — especially for train travel in Spain and Italy — it’s a good idea to consult their national railway sites to double-check availability.
I couldn't find a direct train from Rennes to CDG so I checked on the SNCF site and it shows direct trains taking 3 hours. The DB Bahn site has a change in Paris to the RER and takes 3:36 - 3:46. This is for a test search for travel on 5/29/15 at 8am
I'm wondering if other people have seen trips not found on a DB Bahn search. In the past when I've been putting trips together I relied on DB Bahn to let me see what is possible, now I'm thinking I may have let some more convenient routes or times slip by.