Dog is territorial

Hi! I need help with my dog's behaviour. She's usually quite friendly and social with both humans and other dogs. But the past few days we've been on vacation at my parents' house in the countryside and she's become unusually "territorial". Whenever a stranger comes into the garden she growls and barks and lunges at them. I really don't know what to do. I want her to be protective but not aggressive. Any advice would be helpful. Thanks!