It depends on the type of DOS attack being used by the attackers. If they are using a SYN attack you can change the source ip as you don't care to hear back. Bu

It depends on the type of DOS attack being used by the attackers. If they are using a SYN attack you can change the source ip as you don't care to hear back. But a SYN attack can be mitigated a bit easier once you know about it. If you are doing a more application specific attack, you need to establish a connection (assuming it is TCP), so you can't hide you IP in that case.