More Defenses Against Pseudo Random Subdomain Attacks (PRSD)

Last year we reported a new kind of DNS attack that we called the “Water Torture Attack”. This attack is also known as the Pseudo Random Subdomain Attack (PRSD, although we still like our name better).

In this attack, hackers send queries to open proxies around the world for random, non-existent subdomains of legitimate domains. For example:

These queries are forwarded to DNS resolvers at the upstream ISP. Although the attacks are intended to take down the authoritative servers for these legitimate domains, they have the side effect of dramatically increasing the load on ISP’s DNS resolvers to the point that they, too, can become overloaded and either slow down or crash.

Here are some additional steps that DNS operators can take, in addition to the steps we outlined in our previous blog, to protect their resolvers from these attacks:

1. “Prudently provision” their servers with enough RAM and query capacity. The attacks impact the resolver because it causes them to run out of critical system resources. By increasing these resources, the resolver may be able to sustain the higher recursive query loads.

2. Tune their configurations to maximize the number of simultaneous recursive queries allowed.

3. Automatically block IP addresses generating too many SERVFAIL responses, if possible. This capability is not available in many DNS resolvers, but is a new feature of our DNS Cache product.