
It depends what country you live in as to whether what you suggest is legal. In the UK the Guard Dogs Act says:
(1) A person shall not use or permit the use of a guard dog at any premises unless a person ('the handler') who is capable of controlling the dog is present on the premises and the dog is under the control of the handler at all times while it is secured so that it is not at liberty to go freely about the premises.
(2) The handler of a guard dog shall keep the dog under his control at all times while it is being used as a guard dog at any premises except:
(a) while another handler has control over the dog; or
(b) while the dog is secured so that it is not at liberty to go freely about the premises.
(1) A person shall not use or permit the use of a guard dog at any premises unless a notice containing a warning that a guard dog is present is clearly exhibited at each entrance to the premises.
If you have a dog which is known to be aggressive it would have to be muzzled if it were ever likely to meet strangers.