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Need to Know: Denial of Service

Twitter and Facebook were hit by a denial of service attack yesterday - but what exactly does this mean?

Firewalls will prevent basic attacks, but not more complex ones. Special packet filters can also be used to drop out the attacking traffic, while switches and routers can be manipulated to help limit the effectiveness of a DoS hit depending on the style of attack.

Automated traffic analysis systems and other security software can also be used. Because there are so many types of DoS attacks, there's no easy fix, however.

In fact, the best method to stop DoS attacks is to shut down botnets, which are made up of people's computers which have been compromised.

Attackers use those computers to access sites or to send spam to drive traffic, so if we all prevent our computers from going zombie by keeping our PCs safe from trojans and other malware, there will be fewer botnets out there to use.

As Cluley said: "Scan your PC with an up-to-date anti-virus, make sure you have the latest security patches and run a decent firewall. That way you'll be a good internet citizen and ensuring you're not contributing to the problem."

How common are they?

A DoS attack is a pretty common way of attacking a site. They are frequently used for political reasons it's a pretty scary way to make a point, but it's often unclear what the point is.

Indeed, the Twitter attacks are currently being attributed to a Russian spat with a Georgian blogger.

Earlier this year, DoS attacks took down American and South Korean government sites. Reports suggested some of the attacks originated in the UK, too.

Even the BBC has been a target, after getting hit by a five-hour long attack last year.

One classic DoS attack came with the MyDoom worm, which was programmed to infect computers to be used in an attack against SCO. The firm offered a $250,000 bounty for the capture of the attacker.

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