Joker.com currently experiences massive distributed denial of service attacks against nameservers," the registrar says in an advisory on its home page. "This affects DNS resolution of Joker.com itself, and also domains which make use of Joker.com nameservers. We are very sorry for this issue, but we are working hard for a permanent solution."
More than 550,000 domains are registered with Joker, which is based in Germany. Any of those domains that use Joker's DNS servers are likely to be affected.