“We’ve now seen an attack that begins to illustrate the full extent of the problem,” Mr. Prince wrote in a blog post.

Closing an open resolver, unfortunately, is not as simple as flipping a switch or downloading some software. Finding out if your home cable box is an open resolver, for instance, requires you to call your cable company and tell them that you do not want to be running an open resolver — a tough request when most of the world’s population does not even know what an open resolver is.

Recent efforts have been made to increase awareness of the issue. Computer security experts have recently started “naming and shaming” the operators of open resolvers. The DNS Measurement Factory, one such group, published a survey of top offenders by network, and more recently the Open Resolver Project published a full list of the 27 million open servers online.

The campaign is making slow progress; thousands dropped off those lists in the last few months.

But Dr. Vixie calls the open resolvers just the low-hanging fruit. Even if they were all fixed tomorrow, there are other types of servers that could just as easily be used to amplify an attack, a fact that hackers are eager to point out.

“The guys doing the attack indeed use open resolvers, but those are not needed for this type of attack,” Mr. Kamphuis said in an online interview with The New York Times earlier this week.

Indeed, there are other servers that amplify attacks — including machines called Simple Network Management Protocol (SNMP) servers — albeit by a significantly smaller magnitude. Dr. Vixie and others have been working on what is called response rate limiting technology, a potential solution to the amplification problem. That technology helps servers decipher between unusual requests and normal traffic, but engineers still need to fine-tune it in such a way that it can be used without slowing Internet speeds.