Minecraft wiki entry for Nether_survival, associated text: The player's first resource should be their flint and steel. If for some reason the player does not have one, relighting the obsidian portal frame becomes much more difficult. If the portal was exposed enough to get hit by a ghast fireball, the player could persuade a ghast to hit it with another fireball by getting into the line-of-sight long enough to shoot the fireball. If the player dodges the fireball quickly enough and it hits the obsidian of the portal, the portal gets re-lit.