Platform: CentOS 6 x86_64

I knew when I implemented my split DNS solution years ago that I wanted to try to keep one recursive and open to use for queries. I wanted this more for my convenience (testing, and I could remember my address) when working on issues outside my own network. I knew at the time that I was taking a risk. It seemed like it was more about resources than security. I have two DNSs, but only the one was open to queries without restriction. Well, after probably about 10 years running like this, I finally had a need to add some ACLs and close up my convenient access. I was hit by several very long flurries of requests for Because I have such limited bandwidth, it was quickly pretty obvious that something was wrong.

Initially, I looked to my gateway server. And this stumped me for a while, because I detected nothing wrong or any unusual traffic volume. However, a quick trace and look at the DNS logs on my secondary server revealed the requests. They would go on for hours and hours, but they would stop once in a while for a few hours before starting up again.

My initial approach was to block the IP address the queries were coming from, and hope to preserve my convenient access to my own recursive lookup server. I also found this interesting solution using iptables to block any requests to

iptables -A INPUT -p udp -m string --hex-string "|03697363036f726700|" --algo bm --to 65535 -j DROP

But this does not seem practical to somebody who uses services provided by the Internet Systems Consortium, and does actually find myself on the site once in a while. While this solution does not really work for me in this case, it certainly enlightened me to adding this to my arsenal for some other situations. All you need to do is convert the URL to hex and create your iptables statement.

After watching the requests continue to come in despite being dropped at my firewall, I decided that it finally come to an end. I gave in an have added the ACLs to my external DNS servers.

In the /var/named/chroot/etc/named.conf:
acl "AllowToQuery" {
// Add subnets I trust to use my DNS for queries.
// Added the ACL after DDOS attacked - repeated queries for
allow-query { AllowToQuery; };

The allow-query statement was added to my external view, since I am running a split DNS. Having the ACLs setup this way eventually caused the request to stop.