What is DoS (Denial of Service) Attack? How to Prevent it?

What is DoS (Denial of Service) Attack? A denial-of-service (DoS) is any type of attack where the attackers (hackers) attempt to prevent legitimate users from accessing the service. In a DoS attack, the attacker usually sends excessive messages asking the network or server to authenticate requests that have invalid return addresses. The network or server will not be able to find the return address of the attacker when sending the authentication approval, causing the server to wait before closing the connection. When the server closes the connection, the attacker sends more authentication messages with invalid return addresses. Hence, the process of authentication and server wait will begin again, keeping the network or server busy.

A DoS attack can be done in a several ways. The basic types of DoS attack include:

• Flooding the network to prevent legitimate network traffic
• Disrupting the connections between two machines, thus preventing access to a service
• Preventing a particular individual from accessing a service.
• Disrupting a service to a specific system or individual
• Disrupting the state of information, such resetting of TCP sessions

Another variant of the DoS is the smurf attack. This involves emails with automatic responses. If someone emails hundreds of email messages with a fake return email address to hundreds of people in an organization with an autoresponder on in their email, the initial sent messages can become thousands sent to the fake email address. If that fake email address actually belongs to someone, this can overwhelm that person’s account.

DoS attacks can cause the following problems:

• Ineffective services
• Inaccessible services
• Interruption of network traffic
• Connection interference

So what can you do to protect yourself against DDoS attacks?

1 ) Identify a DDoS Attack Early

If you run your own servers, then you need to be able to identify when you are under attack. That’s because the sooner you can establish that problems with your website are due to a DDoS attack, the sooner you can start to do something about it.

To be in a position to do this, it’s a good idea to familiarize yourself with your typical inbound traffic profile; the more you know about what your normal traffic looks like, the easier it is to spot when its profile changes. Most DDoS attacks start as sharp spikes in traffic, and it’s helpful to be able to tell the difference between a sudden surge of legitimate visitors and the start of a DDoS attack.

It’s also a good idea to nominate a DDoS leader in your company who is responsible for acting should you come under attack.

2) Overprovision Bandwidth

It generally makes sense to have more bandwidth available to your Web server than you ever think you are likely to need. That way, you can accommodate sudden and unexpected surges in traffic that could be a result of an advertising campaign, a special offer or even a mention of your company in the media.

Even if you overprovision by 100 percent — or 500 percent – that likely won’t stop a DDoS attack. But it may give you a few extra minutes to act before your resources are overwhelmed.

3) Defend at Network Perimeter (if You Run Your Own Web Server)

There are a few technical measures that can be taken to partially mitigate the effect of an attack — especially in the first minutes — and some of these are quite simple. For example, you can:

rate limit your router to prevent your Web server being overwhelmed
add filters to tell your router to drop packets from obvious sources of attack
timeout half-open connections more aggressively
drop spoofed or malformed packages
set lower SYN, ICMP, and UDP flood drop thresholds
But the truth is that while these steps have been effective in the past, DDoS attacks are now usually too large for these measures to have any significant effect. Again, the most you can hope for is that they will buy you a little time as an attack ramps up.

4) Call Your ISP or Hosting Provider

The next step is to call your ISP (or hosting provider if you do not host your own web server), tell them you are under attack and ask for help. Keep emergency contacts for your ISP or hosting provider readily available, so you can do this quickly. Depending on the strength of the attack, the ISP or hoster may already have detected it, or they may themselves start to be overwhelmed by the attack.

You stand a better chance of withstanding a DDoS attack if your Web server is located in a hosting center than if you run it yourself. That’s because its data center will likely have far higher bandwidth links and higher capacity routers than your company has itself, and its staff will probably have more experience dealing with attacks. Having your Web server located with a hoster will also keep DDoS traffic aimed at your Web server off your corporate LAN, so at least that part of your business — including email and possibly voice over IP services — should operate normally during an attack.

If an attack is large enough, the first thing a hosting company or ISP is likely to do is “null route” your traffic — which results in packets destined for your Web server being dropped before they arrive.

“It can be very costly for a hosting company to allow a DDoS on to their network because it consumes a lot of bandwidth and can affect other customers, so the first thing we might do is black hole you for a while,” says Liam Enticknap, a network operations engineer at PEER 1 hosting.

Tim Pat Dufficy, managing director of ISP and hosting company ServerSpace, agrees. “The first thing we do when we see a customer under attack is log on to our routers and stop the traffic getting on to our network,” he says. “That takes about two minutes to propagate globally using BGP (border gateway protocol) and then traffic falls off.”

If that was the end of the story, then the DDoS attack would be successful. To get the website back online, your ISP or hosting company may divert traffic to a “scrubber” where the malicious packets can be removed before the legitimate ones are be sent on to your Web server. “We use our experience, and various tools, to understand how the traffic to your site has changed from what it was receiving before and to identify malicious packets,” explains Enticknap.

He says PEER 1 has the capacity to take in, scrub and send on very high levels of traffic — as much as 20Gbps. But with levels of traffic comparable to those experienced by Spamhaus, even this scrubbing effort would likely be overwhelmed.

Do have a DDoS plan in place with your ISP or hoster so that it can begin mitigation or divert your traffic to a mitigation specialist with the minimum delay.