The Elephant In The Room With Preventing DDOS Attacks

DDOS attacks have been a regular fixture in infosec news for some time now. Primarily, those attacks have been using open DNS resolvers, though recently NTP flared up as a service of choice. The community made dramatic improvements in the number of NTP servers which were susceptible to being used in DDOS attacks in a pretty short amount of time. However, both open resolvers and NTP continue to be a problem. And there are likely to be other services targeted in the future, like SNMP.

One common theme is that these services are UDP-based, and so it’s trivial to spoof a source IP address and get significant traffic amplification directed toward the victims of these DDOS attacks.

While I think it’s necessary to focus on addressing the open resolver problem, NTP and similar issues, I’m very surprised that we, as a community, are not pushing to have ISPs implement a very basic control that would dramatically restrict these kinds of attacks: simple source address egress filtering.

Yes, this likely puts additional load on routers or firewalls, but it’s pretty basic hygiene to not allow packets out of a network with a source address which that ISP does not announce for. I am sure there are some edge case exceptions, such as with asymmetric routing, but it should be manageable between the customer and the ISP.

So, each time we hear about a DDOS attack and ponder the pool of poorly configured DNS servers, I propose that we should also be pondering the ISPs who allow traffic out of their networks with a source address that is clearly spoofed.

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