[AusNOG] Layer 7 - Distrusted Source (within a single AS) Distrusted Distention - Denial of Service Attack

[James Braunegg]

Dear All

Just thought I'd share some interesting, potentially frightful information with reference to DNS amplification request attacks we have observed.

We are now seeing 100's of targeted IP addresses within the same network AS targeted by 1000's of IP addresses (all of which are spoofed UDP packets) a network administrators nightmare.

Normally we see a DDoS attacks against specific /32 IP address although it would appear the tables are turning to have a more distributed attack towards the targeted network which hosts the /32 service which is being attacked.

What we have noticed however is all the attack traffic regardless of the source, distention, targeted URL or query has a common pattern matching signature of \50\fa\00\08\00\01\20 common to every packet generated from this substantial botnet which is frequently published on this amplification attack webpage. http://dnsamplificationattacks.blogspot.com.au/

This pattern is common both if you're receiving the attack or if your network is participating in the attack, so as long as you can filter each packet based on an exact hex format you have a chance on mitigating the attack traffic.

What's also interesting is whilst open DNS resolvers used to be the common source of DNS amplification older versions of bind are also susceptible to participate in an attack even if open resolving is turned off when a request comes through, as BIND prior to version 9.5 allows root hint servers to be returned even when a REFUSED response is sent. You can disable this by adding `additional_from_cache no;` into BIND's configuration, which has stopped sending root hint servers along with REFUSED status.

Hope this information is useful, happy to discuss in more detail if you're interested !

Kindest Regards

James Braunegg
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