Interior Railing Posts Attached to Rim Joist
Interior railing posts means they are attached to the interior of the rim joist, not the exterior. It's a another common building technique and requires notching the railing posts before attaching them to the inside of the deck joists and rim joists.
The deck boards are also notched so that a tight fit can be made around the base of the post. Be sure to rely on your level and shims when finalizing the fitting of the post.
Add Extra Blocking Between Joists
It is also wise to add extra blocking behind the rim or deck joist where the post attaches. This will stiffen the post to deck connection.
Follow the same procedure as for exterior mounted posts but this time, position the post against the inside edge of the rim joist. Use shims between the joist and the post to ensure level is attained. Screw the post against the joist.
Internally mounted posts require a bit more carpentry skill but in the end, offer a much more finished and subtle look.
Continous Top Rail Option
Also, since the internally attached posts lack the visual dominance of externally attached posts, you can play with more railing design options such as using a continuous top rail or allowing every second or third post to interrupt the top rail.
Internally attached wood posts tend to look great because they "grow" right out of the deck.
Annual Maintenance Still Required
But all wood posts, even when made from treated lumber, will rot in time. To slow the pace of rotting, they must be kept clean and maintained annually with cleaners and sealants.
Aside from regular maintenance there are also some post mounting brackets available that attempt to solve this problem.