You do not get **** on your knob, that was one of the things I was worried about when I first started doing it and it's never happened. It is possible, but unlikely.
As far as condoms are concerned you can still catch infections, but if you are with a trusted partner ect ect usual rules apply. What you must make sure is that you don't go from anal sex to normal sex without cleaning up between, keep some wet wipes handy or use a condom and whip it off before changing positions. It's ok to go from frontal to anal, but not the other way because of the bacteria that could be transferred.