August 03, 2015
While most Internet users are not aware of the importance of DNS resolution in their overall web browsing experience, they often complain about shortage of bandwidth when experiencing sluggish Internet connectivity.
It is no surprise that most of the service providers also ignore this fact by redirecting their customer DNS traffic to public DNS servers.
I’ve seen this mostly happening to Google’s Public DNS servers. Doing a quick test, it turned out that choosing DNS Public DNS Servers is not a wise choice:
# ./dns-speed.sh wikipedia.org server-list.txt 10 18.104.22.168 average 163 min 139 max 230 ms 10 responses 22.214.171.124 average 673 min 174 max 5140 ms 10 responses 126.96.36.199 average 149 min 133 max 168 ms 10 responses 188.8.131.52 average 135 min 132 max 140 ms 10 responses 184.108.40.206 average 150 min 133 max 278 ms 10 responses 220.127.116.11 average 133 min 131 max 136 ms 10 responses 18.104.22.168 average 134 min 132 max 138 ms 10 responses 22.214.171.124 average 134 min 128 max 139 ms 10 responses
Two addresses (126.96.36.199 and 188.8.131.52) belong to Google and are widely in use by end users, while the rest of the addresses (4.2.2.x) belong to Level3 Communications. They are obviously using BGP Anycast to distribute their DNS resolvers around the world. Most DNS servers respond to a specifically crafted query to reveal their physical location:
# dig +short id.server TXT chaos @184.108.40.206 “answerx_on_cns3.frf1”
Any experienced network engineer could guess that this host should be located in Frankfurt, Germany, and a traceroute reveals a shorter path with lower latency to this host. However, Google Public DNS servers respond with an empty reply to ‘id.server’ requests, so it is not easy to say where the host is located:
# dig +short id.server TXT chaos @220.127.116.11
Running a traceroute from my network, it seems like this host is also located somewhere in Europe (given the latency), but it is 40ms farther from Level3 DNS servers. At the end, I turned to the RIPE Atlas service to measure the quality of access to public DNS servers from various networks in Iran. I chose to measure DNS connectivity from 50 different nodes in Iran (at various locations) toward 18.104.22.168 and 22.214.171.124, querying A records for the domain wikipedia.org. The output of these measurements can be found here:
You can see the detailed information on RIPE Atlas reports, but I put together a quick python script to calculate the average success rate and response time for both datasets. Following is the result for Google Public DNS:
./reader.py RIPE-Atlas-measurement-3458036.json Average Response Time: 177.360 ms Success Rate = 94%
And for Level3:
% ./reader.py RIPE-Atlas-measurement-3458037.json Average Response Time: 182.329 ms Success Rate = 94%
While the reachability of public DNS servers from both Google and Level3 are the same, I observed more variable response time from Google DNS servers compared to Level3. But in average they both performed similarly when it comes to reachability and response time. In the second part of this report, I will dig into reachability and performance of other public DNS servers, such as Verisign and OpenDNS and will also measure the performance of TIC public DNS resolvers which are widely used by local operators.