What is an open resolver?

Question

Domain Name System

There is a small VPS on linode, the hoster sent the following letter:

It has come to our attention that your Linode is running an open resolver. This is a concern because this configuration means your Linode could be utilized in a DNS amplification attack. We kindly ask that you investigate this matter as soon as possible and take appropriate action.
If you are running a resolver as part of your networking configuration, you will likely want to adjust your configuration so that only authorized requests are accepted. If you are intentionally running an open resolver, please let us know why this is so, and what steps you are taking to mitigate abuse so we can note your account appropriately.
We would appreciate an update on this matter as soon as possible. If we do not hear from you in a timely manner we will be forced to take appropriate steps to ensure the integrity of our network..

Please tell me how to solve the problem.

Answer 1

Here is a description of the solution to your problem blogerator.ru/page/dns-amplification-ddos-otrazhen... if you need not only a solution but also an understanding of what it is, read the first part of this article.

Answer 2

This means that your server is running a DNS that accepts recursive requests from the Internet.
If you do not have any zone spinning on it, feel free to block incoming dns with a firewall.
And set the limit of recursive requests only from certain addresses.

Answer 3

What is an open resolver?

In this context, a DNS server that answers recursive queries to any host. If a malicious host spoofs the source IP address and forms a request that has a significant response size, then it can generate (using your server) a significant amount of traffic to an arbitrary target. This is often used to carry out DDoS attacks (a technique called DNS Amplification).

Please tell me how to solve the problem.

Find a DNS server on your VPS. Then change its settings accordingly ( example , another example ) or deactivate it if it is not needed.