It is haram for a traveller to break his fast when: 1- he is travelling but the distance does not reach the distance at which it becomes permissible to shorten the prayers, 2- his journey is not permissible, 3- he is travelling in order to break the fast, and 4- he is travelling and wants to break the fast before leaving the houses of his village or city.

The majority of scholars view that fasting whilst traveling is permissible. If the traveler fasts, it counts and he does not have to make it up. For more, please see the detailed answer