SAN FRANCISCO — Major websites were inaccessible to some East Coast users in the United States Friday morning and to people across the country in the early afternoon after a company that serves as an internet switchboard said it was under attack.
Dyn, whose servers monitor and reroute internet traffic, said it began experiencing what security experts call a distributed denial-of-service attack just after 7 a.m. The company, based in Manchester, N.H., said it had fended off the assault by 9:30 a.m. But by 11:52 a.m., Dyn said it was again under attack.
A distributed denial-of-service attack, or DDoS, occurs when hackers flood the servers that run a target’s site with internet traffic until it stumbles or collapses under the load. Such attacks are common, but there is evidence they are becoming more powerful, more sophisticated and increasingly aimed at core internet infrastructure providers.
Going after companies like Dyn can cause far more damage than aiming at a single website.
Dyn is one of a number of outfits that host the Domain Name System, or DNS, which functions as a switchboard for the internet. The DNS translates user-friendly web addresses like fbi.gov into numerical addresses that allow computers to speak to one another. Without the DNS servers operated by internet service providers, the internet could not operate.
In this case, the attack was aimed at the Dyn infrastructure that supports internet connections. While the attack did not affect the websites themselves, it blocked or slowed users trying to gain access to those sites.
Kyle York, Dyn’s chief strategist, said in an interview Friday morning during a lull in the attacks that the assaults on its servers were complex.
“This was not your everyday DDos attack,” Mr. York said. “The nature and source of the attack is still under investigation. We will be updating our users as soon as we learn more.”
Mr. York said his company and others that host the core parts of the internet’s infrastructure were targets for a growing number of more powerful DDoS attacks.
“The number and types of attacks, the duration of attacks and the complexity of these attacks are all on the rise,” Mr. York said.
In its most recent DDoS trends report, Verisign, a registrar for many internet sites that has a unique perspective into this type of attack activity, reported a 75 percent increase in DDoS attacks from April through June of this year, compared to the same period last year.
The attacks were not only more frequent, they were bigger and more sophisticated. The typical attack more than doubled in size. What’s more, the attackers were simultaneously using different methods to attack the company’s servers, making them harder to stop.
The most frequent targets, by far, were businesses that provide internet infrastructure services like Dyn.
“DNS has often been neglected in terms of its security and availability,” Richard Meeus, vice president of technology at Nsfocus, a network security firm, wrote in an email. “It is treated as if it will always be there in the same way that water comes out of the tap.”
Last month, Bruce Schneier, a security expert and blogger, wrote on the Lawfare blog that someone had been probing the defenses of companies that run crucial pieces of the internet.
“These probes take the form of precisely calibrated attacks designed to determine exactly how well the companies can defend themselves, and what would be required to take them down,” Mr. Schneier wrote. “We don’t know who is doing this, but it feels like a large nation-state. China and Russia would be my first guesses.”
It is too early to determine who was behind Friday’s attacks, but it is this type of DDoS attack that has election officials concerned. They are worried that an attack could keep citizens from submitting votes.
Thirty-one states and the District of Columbia allow internet voting for overseas military and civilians. Alaska allows any Alaskan citizens to do so. Barbara Simons, the co-author of the book “Broken Ballots: Will Your Vote Count?” and a member of the board of advisers to the Election Assistance Commission, the federal body that oversees voting technology standards, said she had been losing sleep over just this prospect.
“A DDoS attack could certainly impact these votes and make a big difference in swing states,” Dr. Simons said on Friday. “This is a strong argument for why we should not allow voters to send their voted ballots over the internet.”
This month the director of national intelligence, James Clapper, and the Department of Homeland Security accused Russia of hacking the Democratic National Committee, apparently in an effort to affect the presidential election. There has been intense speculation about whether President Obama has ordered the National Security Agency to conduct a retaliatory attack and the potential backlash this might cause from Russia.
Gillian M. Christensen, deputy press secretary for the Department of Homeland Security, said the agency was investigating “all potential causes” of the attack on Friday.
Vice President Joseph R. Biden Jr. said on the NBC News program “Meet the Press” this month that the United States was prepared to respond to Russia’s election attacks in kind. “We’re sending a message,” Mr. Biden said. “We have the capacity to do it.”
But technology providers in the United States could suffer blowback. As Dyn fell under recurring attacks on Friday, Mr. York, the chief strategist, said such assaults were the reason so many companies are pushing at least parts of their infrastructure to cloud computing networks, to decentralize their systems and make them harder to attack.
“It’s a total wild, wild west out there,” Mr. York said.