If your own dog occasionally has aggressive habits or you regularly meet aggressive dogs in public you may consider using a citronella spray to control them.
Citronella is a natural substance with a citrus smell that is derived from a combination of cultivated grasses. Citronella spray is a useful tool to have on-hand for many reasons including repelling airborne pests such as flies, gnats, mosquitoes and other biting insects. Dogs find the citrus smell of citronella unpleasant to say the least, so a quick spray in the dog's face can interrupt their aggression.
Citronella spray is safe for humans and pets alike and is unlikely to stain clothing, however, as it is oil based, citronella sprays and other products should be used with caution around clothing.
A citronella spray can also be used to control certain unwanted behaviors, such as incessant barking or the tendency to wander, a citronella spray may be useful for people who are strangers to a home with an aggressive pet, like a dog. A citronella spray can be aimed at the pet and sprayed near the eyes and mouth without causing much harm, but the strong scent will often deter or repel an aggressive pet.
If you wish to train your dog into not barking then a citronella collar is a better choice - it automatically releases the spray when the dog barks.