What is the criterion for acquiring a guard dog in cities and apartment buildings in our present time, with the change in security means and the proliferation of dogs unsuitable for guarding?

The permitted uses for dogs, explicitly mentioned in texts, are for hunting, farming, and guarding livestock. Scholars have differed on the permissibility of keeping a dog for guarding homes. The preponderant opinion is that it is permissible out of necessity, provided there is a genuine need and not merely a false claim. It is preferable for a guard dog to be kept outside the house because it defiles vessels, furnishings, and clothes, and because angels refrain from entering a house in which there is a dog, even if keeping it is permissible.