# Measuring the Centrality of DNS Infrastructure in the Wild
## 5 Resolver Centrality Analysis
### Service Geo-Distribution  
In this section, we have endeavored to depict the geographical distribution of DNS services for public DNS providers and ISP DNS providers. We investigated Google Public DNS, OpenDNS, and Cloudflare DNS as public DNS providers and Iran Telecommunication DNS as the ISP DNS provider, respectively. We first identified the resolver pool for each DNS provider using the method described in the previous section (Section 4). Resolvers in the same pool serve the same set of FDNSes. So, we correlated FDNSes and iRDNSes, inferring the usage of certain public DNS services. We located matched FDNSes using MaxMind's IPGeoLite2 database and mapped it into OpenStreetMap to visualize their geo-distribution.  
We correlated with 4.22 million FDNSes forward DNS lookup to Google Public DNS (8.8.8.8 or 8.8.4.4), which distributed in 221 countries or or regions. Figure 9 shows the geo-distribution of these FDNSes in the form of heat map, where the density of FDNSes is represented by the darkness of color. From the figure, we observe that Google Public DNS is popular with Internet users globally, yet its distribution is uneven. For example, in Africa and Australia, it has less usage probably due to the less-developed economy, Internet access, or the dominant status of local DNS providers. Another two public DNS providers, OpenDNS and Cloudflare, showed similar global distribution, but they were apparently less popular than Google Public DNS. OpenDNS was more prevalent in US and Brazil, while Cloudflare was more popular in Europe and China.  
Figure 8: Sankey diagram of resolver pool and its geographic location for Google Public DNS.  
Now we turn our attention to ISP DNS. As a case study, we analyzed DNS operated by Iran Telecommunication. As mentioned in the previous section, we first identified the resolvers by discovering all the iRDNSes operated by Iran Telecommunication. Then we correlated the FDNSes that exist forwarding relationship with the resolver pool, which matches 1.04 million FDNSes. Different from Google Public DNS, whose customers are highly distributed across the world, although these FDNSes locate in 13 countries, 99.8% of these FDNSes are located in Iran, as shown in Figure 10. This indicates that the resolver pool's major customers are located in Iran, as well.  
**Summary:** In this section, we quantify the centrality of the client-side DNS infrastructure in terms of the forwarding relationship among different types of resolvers. We extend previous Internet-wide DNS studies and propose a novel and lightweight resolver pool discovery method. We gain new insights into the centrality of client-side DNS infrastructure through uncovering the consolidation situation among FDNSes, iRDNSes, and DNS providers. From the perspective of forwarders, we were surprised to find that 5% of indirect resolvers support 90% of forwarders. This implies that internet traffic is concentrated on a small portion of resolvers, significantly increasing the likelihood of resolution  
Figure 10: Geographic Distribution of FDNS backed by Iran Telecommunication.  
Figure 9: Geographic distribution of FDNS backed by Google Public DNS.  
failures. From the service provider's perspective, 40% of forwarders belong to five service providers, with Google alone serving more than 22% of forwarders. This indicates that the rise of the domain name market has led to an oligopoly of DNS service providers, which also contradicts the original design principles of DNS. We recommend that the internet community focuses on the centralization tendencies within DNS infrastructure and takes steps to alleviate the influence of service provider oligopolies on resolution services, such as promoting the community-driven deployment of public DNS.