what happens when white fang gets attacked

I don't think that White Fang himself was attacked by Indians. The Indian dogs are afraid of White fang because they were unable to beat him, even in groups. In Part 3 Mit-sah, Grey Beaver's son, is attacked by a group of young Indians. At first White Fang looks on. Apparently he thinks, in his doggy head, that this not his concern. When Mit-sah starts to lose, White Fang attacks and saves the son. Thus White Fang discovers a trade. In return for comfort and safety White Fang will give his loyalty to his Indian "flesh and blood" Gods. I hope this is the part of the story that you are referring to.