ISC Stormcast For Tuesday, April 16th 2019

Odd DNS Requests that are Normal

Published: 2019-04-16
Last Updated: 2019-04-16 04:05:07 UTC
by Johannes Ullrich (Version: 1)
4 comment(s)

If you ever heard me talk about DNS, you will know that I am a big fan of monitoring DNS queries, and I think DNS query logs are the best "band for the buck" log source to detect anomalous behavior. Everything that happens on your network, good or bad, tends to be reflected in DNS.

But there are a couple common "odd" DNS request types that are often mistaken for malicious, or unusual but are actually quite normal. Here are my favorite once:

- Anti Malware Checks:

I got an example from Sophos Anti Virus here, but other vendors use a similar technique:

At first sight, you may mistake these requests for typical DNS covert channels. But they are actually associated with Sophos Antivirus. The reason for these queries is that Anti-Malware uses DNS to check if certain files are malicious. The software will send a hash of the file to the vendor and receive back an indication if the file is malicious or not. This will also allow the vendor to compile statistics on the popularity of certain software which will then often be used to compile risk scores (sorry... feed a machine learning AI engine that will protect you from 0-day attacks... or something like this if you read the vendor ads for various products like this). In some ways, this is an exfiltration activity. Just not malicious.

- Mail Servers

We all know that clients usually try to resolve A or AAAA records. But let's take a look at the snapshot below of the records types from a quick query log sample (collected via bro in this case):

The high percentage of PTR records may appear odd. In this case, however, the network includes a busy mail server. Mail servers, for anti-spam filtering, often resolve IP addresses to match forward and reverse resolution.

- Other .arpa hostnames

Talking about PTR records. Pretty much everybody reading this, probably knows about and and how it is used for reverse resolution. But these are not the only ".arpa" records you see. One record I see more and more is This record is used to detect if the host is on an IPv6 only network, and DNS64 is used to map IPv4 addresses to IPv6. This record should resolve to Only the A record exists. For a AAAA query, you will not get an answer unless your name servers (do to DNS64), is making one up. There are actually a few more .arpa hostnames but this is the one I usually see quite frequently.


When I saw this first, it looked like a typosquatting domain to me. But the company behind this domain is an active contributor to a number of open source projects, and in my case, it was their contribution of resources to that triggered the DNS requests.

Any odd DNS requests that you ran down to only find them to be harmless?




Johannes B. Ullrich, Ph.D. , Dean of Research, SANS Technology Institute

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