Factbox: How cyber activists bring down a website

(Reuters) - Cyber attacks in retaliation for attempts to block the WikiLeaks website have already hit the websites of credit-card giants MasterCard and Visa.

Using distributed denial-of-service (DDoS) attacks, hundreds of cyber activists have joined forces and temporarily disabled computer servers by bombarding them with requests.

On Thursday, supporters of WikiLeaks were plotting attacks on other perceived enemies of the publisher, which has angered U.S. authorities by starting to release details of 250,000 confidential diplomatic cables.

Here are details of how they go about bringing down a website:

-- The weapon of choice is a piece of software named a "Low Orbit Ion Cannon" (LOIC) which was developed to help Internet security experts test the vulnerability of a website to a DDoS attack. The LOIC is available for download on the Internet.

-- The LOIC can be controlled centrally by an administrator in an Internet Relay Chat (IRC) channel, a type of computer chat room, which can seize control of a network of computers whose combined power is used in a DDoS attack.

-- The attack is aimed at the target website and when the LOICs are activated they flood the website with a deluge of data requests at the same time.

-- The DDoS attack prevents the overloaded server from responding to legitimate requests and slows down the website to a crawl or shuts it down totally.

-- The attacks are coordinated in the IRC channel and on Thursday, around 3,000 people were active on the channel at one stage.

-- The current situation has some historical parallels to a decade ago, when, in February, 2000, several of the biggest U.S. ecommerce and media sites came under attack in denial-of-service attacks. Targets included Amazon.com, eBay, E-Trade, Buy.com and CNN, the news site. The ecommerce sites endured substantial losses during the outages, at a time when the Internet shopping phenomenon remained in its infancy.

(Reporting by Marius Bosch and Eric Auchard)