Dog biting

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My dog is a mix maltses & poodle. He is about two years and never bites. But now I find that he has been biting people in my yard and acting aggressive to other people visiting. This started about two months ago. Hes outside most of the day and when in the house he is confined to a 5x5 area due to he like to go to the bathroom in the house when we are not looking. Anyone have some suggestions.