A squabble between a group fighting spam and a Dutch company that hosts websites said to be sending spam has escalated into one of the largest computer attacks on the Internet, causing widespread congestion and jamming crucial infrastructure.

Millions of ordinary Internet users have experienced delays in services or could not reach a particular website for a short time.

However, for the Internet engineers who run the global network, the problem is more worrisome. The attacks are becoming increasingly powerful, and computer security experts worry that if they continue to escalate, people may not be able to reach basic Internet services, like e-mail and online banking.

The dispute started when the spam-fighting group, called Spamhaus, added the Dutch company Cyberbunker to its blacklist, which is used by e-mail providers to weed out spam.

Cyberbunker, named for its headquarters, a five-story former Nato bunker, offers hosting services to any website “except child porn and anything related to terrorism”, according to its website.

A spokesman for Spamhaus, which is based in Europe, said the attacks began March 19 but had not stopped the group from distributing its blacklist.

Patrick Gilmore, chief architect at Akamai Networks, a digital content provider, saidSpamhaus’ role was to generate a list of Internet spammers.

Of Cyberbunker, he added: “These guys are just mad. To be frank, they got caught. They think they should be allowed to spam.”

Large data streams

Mr. Gilmore said that the attacks, which are generated by swarms of computers called botnets, concentrate data streams that are larger than the Internet connections of entire countries. He likened the technique, which uses a long-known flaw in the Internet’s basic plumbing, to using a machine gun to spray an entire crowd when the intent is to kill one person.

The attacks were first mentioned publicly last week by Cloudflare, an Internet security firm in Silicon Valley that was trying to defend against the attacks and as a result became a target.

“These things are essentially like nuclear bombs,” said Matthew Prince, chief executive of Cloudflare. “It’s so easy to cause so much damage.”

The so-called denial of service, or DDoS, attacks have reached previously unknown magnitudes, growing to a data stream of 300 billion bits per second. “It is a real number,” said Mr. Gilmore. “It is the largest publicly announced DDoS attack in the history of the Internet.”

Retaliation against a vigilante

Spamhaus, one of the most prominent groups tracking spammers on the Internet, uses volunteers to identify spammers and has been described as a vigilante group.

In the past, blacklisted sites have retaliated against Spamhaus with denial-of-service attacks, in which they flood Spamhaus with traffic requests from personal computers until it falls offline. But in recent weeks, the attackers hit back with a far more powerful strike that exploited Internet’s core infrastructure, called the Domain Name System, or DNS.

That system functions like a telephone switchboard for the Internet. It translates the names of websites like Facebook.com or Google.com into a string of numbers that the Internet’s underlying technology can understand. Millions of computer servers around the world perform the actual translation.

In the latest incident, attackers sent messages, masquerading as ones coming from Spamhaus, to those machines, which were then amplified drastically by the servers, causing torrents of data to be aimed back at the Spamhaus computers.

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, “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 deputised Spamhaus to determine what goes and does not go on the Internet,” said Mr. Kamphuis. “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, computer security experts say, because those servers cannot be shut off without halting the Internet.

Potential security flaw

''The No. 1 rule of the Internet is that it has to work," said Dan Kaminsky, a security researcher who pointed out the inherent vulnerabilities of the Domain Name System years ago. "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. New York Times News Service