How to rebuild a hydraulic cylinder

Pictured is the hydraulic cylinder on my tractor and she leaks out the back around the piston like crazy.

Could someone advise what is done to repair this leak? Is there a packing or some kind of O ring?

It looks like the control valve is attached to the cylinder head with a couple of bolts, The first thing I would do is take it loose and then pull the cylinder off of the machine. Once off the machine take the tie rods apart so the ends come off of the cylinder and your left with the tail end, the hollow cylinder tube, and the head with the rod sticking through it. Once you get this far you'll need to take either the piston end or the other end off of the rod and take it out of the head.
From there change the rod seals in the head, the seals on the piston, and the orings between the tail, head, and the cylinder tube. Looking at it there is probably one or two between the control block and the head so go ahead and change it too.
On a cylinder like that you can probably take everything to your local hydraulic shop and have the match everything up.
With the new parts on hand go ahead and install everything and begin reassembly. Make sure when you slide the piston back into the tube you don't cut the seal, and the same goes for putting the rod back through the seals in thr head. To reassemble hook all 4 tie rods back up and tighten them evenly til tight. Reinstall the control block and your back in business. Good luck, and if you run into problems post again and ask.

Here is a breakdown of how they are built