The sys admin's daily grind: Dnstop

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Article from Issue 185/2016

In last month's issue, Charly sent the lean pdnsd DNS cache down the catwalk. To see whether pdnsd really does the work expected of it, he now puts dnstop through infinite rounds in the name of names.

Most distributions include dnstop. If you prefer to build it yourself, you will find the source code online [1], but make sure you download and build the matching Libpcap [2] first. I launched the tool on the computer hosting my DNS cache with the following command:

dnstop -l 3 eth0

The -l 3 parameter tells dnstop to explore name requests up to the third level. For a request like, com is the first or top-level domain, linux-magazine is thus the second-level domain, and www is the third level.

When I press the 1 button on a computer running the command listed above, I can see which top-level domains are most frequently queried (Figure 1). What I am interested in here is which device on my network is asking for .xyz domains – but hey ho, if I press 2 or 3, I can extend the view to include the second and third levels.

Figure 1: Pressing the 1 key displays statistics with the requested top-level domains. It comes as little surprise that .com tops the list, but who is looking for .xyz?

Frequent and Rare Resource Records

Pressing T takes you to another practical statistic. It shows you what resource record types are most frequently requested. It is unsurprising to see requests for A (IPv4) and AAAA records (IPv6) topping the list (Figure 2). Well back in the field, is the A6 record, which comes from the early days of IPv6 and is about as widespread as gas streetlamps today. Other fairly sparsely represented records include DNSKEY, which come from DNSSEC (DNS Security). In contrast to A6 IPv6, DNSSEC is increasing steadily but still not well established.

Figure 2: Pressing T shows you the Resource Record overview. The A records typical of IPv4 have a two-thirds majority.

Pressing R (for Result) shows you how many requests were successful. In my short observation period, this was all of them, thankfully:

code       Count      %
------- --------- ------
Noerror     23987  100.0

If I use dnstop for evaluations at work – and then save them somewhere – I need to think about data protection. To avoid problems from the outset, I tend to launch the tool with -a for "anonymize." Then, dnstop replaces the client IP addresses with consecutive numbers, while all the other evaluations work as expected.

Charly Kühnast

Charly Kühnast is a Unix operating system administrator at the Data Center in Moers, Germany. His tasks include firewall and DMZ security and availability. He divides his leisure time into hot, wet, and eastern sectors, where he enjoys cooking, freshwater aquariums, and learning Japanese, respectively.

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