Are Jack Russells friendly with strangers?

Not usually. In fact, they can be possessive of “their” people and property and can be aggressive toward strangers. This is not true of all Jack Russells, however. Some are happy to receive visitors, while others would just as soon chew up anyone who sets foot on their property.

Isn’t being unfriendly toward strangers a good thing? I don’t want someone to break into my house.
Jack Russells make good watchdogs, meaning they will bark long and loud if someone is approaching your house. That’s good because they can alert you to an approaching stranger long before you would hear or see that person yourself. But Jacks don’t always distinguish between someone you want in your house and someone you don’t. Your Jack Russell is just as likely to bark like a maniac at your Uncle Fred from Iowa who is coming up the driveway to ring your doorbell as he would be at a criminal who is scoping out your house for a robbery. Know that this protective attitude about Uncle Fred may continue even after your uncle has come into your
home with your permission. If your Jack Russell bites Uncle Fred or anyone else who comes to your home with your permission, you will be both embarrassed and legally liable.