If it's absolutely critical that your users get service right now, it might be a good idea to at least prepare a migration to other DNS servers, like perhaps th

If it's absolutely critical that your users get service right now, it might be a good idea to at least prepare a migration to other DNS servers, like perhaps those of Linode. If the situation doesn't improve within an hour or so, it might be that they don't have a good way to deal with it, and the outtage might take long, depending on the depth of the DDoSers pockets.