So what steps can companies take to combat such attacks?
Perhaps most important is learning to recognize when an attack is taking place. Many organizations donít know what their query load is, so arenít even aware of when theyíre under attack. By using the statistics support built into the DNS software BIND, administrators can analyze their data for query rates, socket errors and other attack indicators. Even if itís not clear exactly what an attack looks like, monitoring DNS statistics will establish a baseline from which trends and anomalies can quickly be identified.
An organizationís internet-facing infrastructure should also be scrutinized for single points of failure not only in external authoritative name servers, but also in switch and router interactions, firewalls, and connections to the Internet. Once identified, the business should then consider whether these vulnerabilities can be effectively eliminated.
External authoritative name servers should be broadly geographically distributed wherever possible which will not only help to avoid single points of failure, but will also provide the added advantage of improving response time performance for their closest customers.
And, in the face of the huge number of responses resulting from a DDoS attack, itís worth considering overproviding existing infrastructure, a process that is both inexpensive and easy to trial prior to an incident.
Cloud-based DNS providers run name servers of their own in data centers around the world. These can be configured as secondaries for an organizationís own, with data loaded from a master name server designated and managed in-house. Itís worth noting, though, that most of these providers bill for the number of queries received, which will of course increase significantly during a DNS attack.
As well as configuring their DNS infrastructures to resist DDoS attacks, organizations should also ensure they donít become unwitting accomplices in DDoS attacks against others.
Unless the company is one of the very few that runs an open recursive name server, it can limit DNS queries to those IP addresses on its internal networks, thereby making sure that only authorized users have access to its recursive name servers.
And for those that run authoritative name servers, Response Rate Limiting (RRL), incorporated into BIND name servers, makes it difficult for attackers to amplify queries, stopping responses being sent to a single IP address at any rate higher than a pre-programmed threshold.
By understanding how DDoS attacks exploit DNS servers, and recognizing the signs, organizations can take measures to lower the threat on their own infrastructure, and avoid becoming complicit in attacks on others.
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