I need help what is a canker sore? how will i know if i have it?

Ulcer with red halo: Canker sores are caused for unkown reasons and cold sores are caused from a viral infection. Both can look the same (white ulcer surrounded by a red halo; sizes vary) , but size and location can distinguish them. In a healthy individual, they usually go away in 10-14 days. You can go to a drugstore and check for different ointments that may help with the sensitivity.