Is the dog najis (unclean), and does touching it invalidate ablution, and is there a difference between a farm dog and a wild dog?

A dog is entirely impure, and there is no difference between a guard dog and any other dog. A dog touching a person's body or clothes does not invalidate ablution (Wudu'), but it is obligatory to wash whatever it touched seven times, one of them with earth, if the dog was wet.