Amplified DNS common Attack

Amplified DNS Attack, general information, and fixes.

How I got my hands into this.

Well, the step by step on how I met this new challenge. You may find the general info here ( is external)), don’t get me wrong, the solution is not that big deal, yet it can get a bit tricky if you have a complicated network with our limitations.

You may find our network diagram on the site.

Well, everything started while I was working. On one of the networks, I noticed a constant 120 KByte/s, so I head up to the server and started trying to filter its source from the logs, thankfully. As I began using Munin, we were logging the queries on the Bind Server. After “tail -f /var/log/bind” I noticed many questions for, strangely asking for all the records on that domain.

So to that moment, I thought, Research TIME!.

Well yeah, so the attack is old as time itself, but I did not know as it never happened to us, so we never cared to follow this kind of threat.

Well going over this we found:?

1234.089099 -> my_ip_address DNS Standard query ANY isc.org4.090428 -> my_ip_address DNS Standard query ANY isc.org4.098280 -> my_ip_address DNS Standard query ANY

After this happened, we started noticing this has been going for a while, as you can see at the blog post, so here’s the fix!.

Depending on your scenario here are some options:

1.On windows (Turn off recursive queries).

On Linux (two ways).


As some may know, iptables are extensive and a robust way to protect a standard server without external help, so adding these two lines to your INPUT chains should get it fixed right away.

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

That should match ( another way is:

iptables -A INPUT -p udp -m string –string “isc?org?” –algo bm –to 65535 -j DROP

Remember, if another attack appears, check the packets with Wireshark or similar.


Turn off recursive queries for networks outside your own with ACL’s.


On Bind 9


acl vpn_nets {;;
acl external_nets {
acl local_nets {;;

Add this at the beginning of the configuration file.

This should not happen anymore, and you should be outside the red zone.


Sr. Software Engineer with over 10 years of experience. Hobbist photographer and mechanic. Tinkering soul in an endeavor to better understand this world. Love traveling, drinking coffee, and investments.

You may also like

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: