When Spamhaus requested aid from CloudFlare, the attackers began to focus their digital ire on the companies that provide data connections for both Spamhaus and CloudFlare.

Questioned about the attacks, Sven Olaf Kamphuis, an Internet activist who said he was a spokesman for the attackers, said in an online message that, “We are aware that this is one of the largest DDoS attacks the world had publicly seen.” Mr. Kamphuis said Cyberbunker was retaliating against Spamhaus for “abusing their influence.”

“Nobody ever deputized Spamhaus to determine what goes and does not go on the Internet,” Mr. Kamphuis said. “They worked themselves into that position by pretending to fight spam.”

A typical denial-of-service attack tends to affect only a small number of networks. But in the case of a Domain Name System flood attack, data packets are aimed at the victim from servers all over the world. Such attacks cannot easily be stopped, experts say, because those servers cannot be shut off without halting the Internet.

“The No. 1 rule of the Internet is that it has to work,” said Dan Kaminsky, a security researcher who years ago pointed out the inherent vulnerabilities of the Domain Name System. “You can’t stop a DNS flood by shutting down those servers because those machines have to be open and public by default. The only way to deal with this problem is to find the people doing it and arrest them.”

The heart of the problem, according to several Internet engineers, is that many large Internet service providers have not set up their networks to make sure that traffic leaving their networks is actually coming from their own users. The potential security flaw has long been known by Internet security specialists, but it has only recently been exploited in a way that threatens the Internet infrastructure.

An engineer at one of the largest Internet communications firms said the attacks in recent days have been as many as five times larger than what was seen recently in attacks against major American banks. He said the attacks were not large enough to saturate the company’s largest routers, but they had overwhelmed important equipment.