Oracle Text

Add R for each attacking creature you control. Until end of turn, attacking creatures you control gain "R: This creature gets +1/+0 until end of turn."

Card Rulings

11/24/2014 Even though it can generate mana, Dragonrage isn’t a mana ability. It uses the stack and can be responded to.
11/24/2014 Only creatures that are attacking as Dragonrage resolves will count toward how much mana is generated, and only those creatures will gain the “firebreathing” activated ability. In other words, casting Dragonrage before you’ve declared attackers usually won’t do anything.
11/24/2014 Remember that unused mana empties from players’ mana pools at the end of each step and phase. For example, if you cast Dragonrage during your declare attackers step, except in very rare cases, the mana Dragonrage generates won’t last until the declare blockers step or the postcombat main phase.