Distributed Denial of Service attacks are becoming and increasingly common phenomenon with both Gov’t agencies, activists, individuals and business entities using the attack as a tool to further their goals. Evidence of this can be seen in the list below:

Along with the increasing occurrence of DDoS attacks, the power of such attacks is also increasing. Studies conducted in 2002 and again in 2009 showed an increase in the average size of large attacks from 400 Mbps to 49 Gbps. One might argue that this increase would be matched by target networks ability to handle bandwidth, however the study compared the attack from 2002 to be 1 fifth of Harvard’s network capability to 25 times Harvard in 2009. Additionally the paper noted that a 400 Mbps DDoS attack will still cause many networks to crash. The paper used in sourcing for these points is specific to Human Rights sites (a common target for DDoS attacks) and was compiled by Suckerman, E., Roberts, H., McGrady, R., York, J., Palfrey, J., 2010. A link to the article:  click here

Organized activist groups, particularly Anonymous have launched serveral well publicized DDoS attacks in the past 12 months particularly, Operation Payback in relation to companies boycotting WikiLeaks.

Despite the rise in DDoS attacks, three out of ten web hosting providers reported having no dedicated security staff. –  Danny McPherson et al., “Worldwide Infrastructure Security Report: Volume V, 2009 Report,” Arbor Networks, January 19,  2010, http://staging.arbornetworks.com/dmdocuments/ISR2009_EN.pdf.


A 2009 study identified a shift away from purley bandwidth based attack. – Danny McPherson et al., “Worldwide Infrastructure Security Report: Volume V, 2009 Report.” Additionally, most major network operators reported that DDoS attacks were usually mitigated within 1 hour, much of which came from the ability to call on upstream peers to disconnect attacking sub-nets.

DDoS attacks can be catagorized into:

Application attacks: Use software vulnerabilities to exhaust system resources.

Network Attack: saturate communication lines to the target.

Arbor’s 2009 report states that 45% of DDoS attacks were network attacks and 49% were application attacks.

Botnets and amplifiers are two key components of DDoS attacks. Botnets assist in braodening the range of IP address the attack is coming from, reducing detection and increasing collateral damange in mitigation. A botnet of several hundred thousand computer is not however sufficient to generate 49 Gbps of bandwidth. To up the bandwidth, amplifiers are used. An example of amplification is an attacker sending DNS requests to a DNS server with the source IP address of the target. The packet send to the DNS server by the attack is 1 / 76 the size of the packet send to the target. We can see that the attack has been sgnificantly amplified.

In essence, DDoS attackers use the distributing effect of a botnet in association with resource leverage such as DNS amplification to increase the potency of their attacks.

DNS amplification attack, source: 10networks.com

On a “normal” day, Arbor detects roughly 1300 DDoS attacks. – Arbor Networks, “Atlas Summary Report: Global Denial of Service,” accessed October 26, 2010,


The balance between reducing malicious traffic and service availability to genuine users is very difficult to effectivley maintain. The challenge for all network admins should be to keep this ratio as high as possible. Some simple mitigation methods are listed below, a more expansive review will be conducted in the next post. The legality and lack of collaboration between contries and companies is another key point needed for discussion in a wholistic mitigation strategy.

  • Avoiding ‘edge’ ISPs, ie: tier 3, small/inhouse hosting companies
  • Replacement of CMS sites withe static HTML content.
  • Adding aggressive caching
  • Use of DDoS resistent servers (ie: blogger cloud, EC2 cloud) or atleast have these servers as a backup
  • Clear communication and understanding of ISP SLAs.