Beyond attacking CloudFlare's direct peers, the attackers also attacked the core IX infrastructure on the London Internet Exchange (LINX), the Amsterdam Internet Exchange (AMS-IX), the Frankfurt Internet Exchange (DE-CIX), and the Hong Kong Internet Exchange (HKIX). From our perspective, the attacks had the largest effect on LINX which caused impact over the exchange and LINX's systems that monitor the exchange, as visible through the drop in traffic recorded by their monitoring systems. (Corrected: see below for original phrasing.)
The congestion impacted many of the networks on the IXs, including CloudFlare's. As problems were detected on the IX, we would route traffic around them. However, several London-based CloudFlare users reported intermittent issues over the last several days. This is the root cause of those problems.
The attacks also exposed some vulnerabilities in the architecture of some IXs. We, along with many other network security experts, worked with the team at LINX to better secure themselves. In doing so, we developed a list of best practices for any IX in order to make them less vulnerable to attacks.
Two specific suggestions to limit attacks like this involve making it more difficult to attack the IP addresses that members of the IX use to interchange traffic between each other. We are working with IXs to ensure that: 1) these IP addresses should not be announced as routable across the public Internet; and 2) packets destined to these IP addresses should only be permitted from other IX IP addresses. We've been very impressed with the team at LINX and how quickly they've worked to implement these changes and add additional security to their IX and are hopeful other IXs will quickly follow their lead.