As of this morning, Internet Service Provider Cogent Communications is also blocking 4Chan.org.
This afternoon, AT&T release an official statement explaining their actions.
"Beginning Friday, an AT&T customer was impacted by a denial-of-service attack stemming from IP addresses connected to img.4chan.org," the statement said. "To prevent this attack from disrupting service for the impacted AT&T customer, and to prevent the attack from spreading to impact our other customers, AT&T temporarily blocked access to the IP addresses in question for our customers. This action was in no way related to the content at img.4chan.org; our focus was on protecting our customers from malicious traffic.
"Overnight Sunday, after we determined the denial-of-service threat no longer existed, AT&T removed the block on the IP addresses in question. We will continue to monitor for denial-of-service activity and any malicious traffic to protect our customers."
For the uninitiated, 4Chan is an anarchic message board that draws highly skilled Internet users from around the world. These users exchange knowledge, jokes and images of all kinds. The board has spawned many famous Internet memes, including Rick-Rolling and lolcats. In May, 4Chan users bombarded YouTube with adult videos in a coordinated movement.
But in addition to general fun, 4Chan has also, according to many online reports, sired the shadowy Internet organization Anonymous, so named for the board's thousands of members, who all contribute to 4Chan anonymously, their identities marked only by a number. Anonymous achieved a level of mainstream fame in 2008 when it kicked off a worldwide protest against the Church of Scientology.
As of Sunday morning, AT&T had blocked access to many areas of 4Chan, including the site's infamous /b/ board. Different sections on 4Chan are designated with single letters. The /b/ board is dedicated to random discussion, and by association, much of 4Chan's edgiest content, including dirty jokes, controversial images and pornography.
AT&T briefly lifted the block Sunday afternoon before expanding the ban to the entire 4Chan domain 11 p.m. PST Sunday. In response, 4Chan members have started an organization called Project AT&T, whose purpose remains unknown, and a Twitter feed called 4Chan4ever. So far, 4Chan4ever only includes updates on the status of AT&T's ban, as well as preliminary planning for how 4Chan users will eventually retaliate.
Tech analyst David Murphy of PCWorld.com speculated that AT&T wasn't simply censoring an edgy website.
"Given the level of doublespeak surrounding this entire controversy, it's hard to think that the real reason for the temporary 4Chan AT&T block will see the light of day," he said. "My money's on some kind of DDoS-related issue — either the magnitude of attacks flowing into 4Chan's site, the Internet attacks of 4Chan users, or a forged DDoS attack that gave the appearance of starting from 4Chan. Of all the places on the Internet to target, it's just too perfect a storm to think that AT&T, out of the blue, would start a Sunday morning censorship campaign by targeting a site like 4Chan."
A DDoS attack is a "distributed denial of service" attack, which knocks out a website (or group of websites) for its intended users. Other unconfirmed online reports point toward possible attacks originating from — or purporting to originate from — 4Chan as well. Complaints about ACK scans, which look for vulnerabilities in computer systems, have been levied at users coming from 4Chan IP addresses.