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Spamhaus vs. “The Largest DDos Attack in History”

Anti-spam organization Spamhaus has recovered from what may possibly be the largest DDoS (Distributed Denial of Service) attack in history. A massive 300Gbps (Gigabits-per-second) attack was thrown against Spamhaus’ website in an attempt to bring their systems to their knees, but the anti-spam organization was able to recover from the attack and get its core services back up and running.

Spamhaus supplies lists of IP addresses for servers and computers on the net linked to the distribution of spam. The blacklists supplied by the not-for-profit organization are used by ISPs, large corporations and spam filtering vendors to block the worst sources of junk mail before other spam filtering measures are brought into play.

Spammers, of course, hate this practice so it’s no big surprise that Spamhaus gets threatened, sued, and attacked regularly. The latest run of attacks began on 18 March with a 10Gbps packet flood that saturated Spamhaus’ connection to the rest of the Internet and knocked its site offline. Spamhaus’s blocklists are distributed via DNS and widely mirrored in order to ensure that it is resilient to attacks. The website, however, was unreachable and the blacklists weren’t getting updated.

The largest source of attack traffic against Spamhaus came from DNS reflection, launched through Open DNS resolvers rather than directly via compromised networks. Spamhaus turned to CloudFlare for help and the content delivery firm was able to mitigate attacks that reached a peak of 75Gbps.

Things remained calm for a few days before kicking off again with even greater intensity – to the extent that collateral damage was seen against services such as Netflix. Spamhaus’ site remains available at the time of writing on Wednesday. Steve Linford, chief executive for Spamhaus, told the BBC that the scale of the attack was unprecedented.

A blog post by CloudFlare, written last week before the latest run of attacks, explains the mechanism of the attack against Spamhaus and how it can be used to amplify UDP (universal datagram protocol) packet floods.

The basic technique of a DNS reflection attack is to send a request for a large DNS zone file with the source IP address spoofed to be the intended victim to a large number of open DNS resolvers. The resolvers then respond to the request, sending the large DNS zone answer to the intended victim. The attackers’ requests themselves are only a fraction of the size of the responses, meaning the attacker can effectively amplify their attack to many times the size of the bandwidth resources they themselves control.

In the Spamhaus case, the attacker was sending requests for the DNS zone file for ripe.net to open DNS resolvers. The attacker spoofed the CloudFlare IPs we’d issued for Spamhaus as the source in their DNS requests. The open resolvers responded with DNS zone file, generating collectively approximately 75Gbps of attack traffic. The requests were likely approximately 36 bytes long (e.g. dig ANY ripe.net @X.X.X.X +edns=0 +bufsize=4096, where X.X.X.X is replaced with the IP address of an open DNS resolver) and the response was approximately 3,000 bytes, translating to a 100x amplification factor.

 

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