# Measuring the Centrality of DNS Infrastructure in the Wild
## 1 Introduction  
The Domain Name System (DNS) is designed to be a distributed system, despite the fact that the domain name space has a tree structure. However, due to the evolving Internet business model, centralization has emerged in various aspects of DNS [1], including the market share of the public DNS services [2, 3] and DNS traffic [4]. This arouses much concern since DNS is a critical component of the Internet infrastructure. The consequences caused by DNS centralization can be multifaceted [5]. Firstly, centralized DNS services often lead to an awkward situation where the failure of one primary provider can render the domain resolution process vulnerable. On 17 June 2021, the Akamai DNS outage left numerous top websites and online services inaccessible, including Google, Amazon, Steam, Cloudflare, and FedEx [6]. Secondly, the likelihood of accidental enterprise failures may increase due to centralized services, and the challenge of recovery may also escalate. Facebook experienced the most influential outage in the entire history of the Internet on 4 October 2021 [7], which was caused by a mistake in BGP updating that resulted in the authoritative DNS service outage. These cases serve as a reminder that centralized DNS services can potentially drag enterprises into a service black hole that increases the risk of a single-point failure and eavesdropping, increasing the concern of violating users' privacy. In light of the aforementioned threats, the European Union is planning to launchthe DNS4 EU Infrastructure Project, which aims to build DNS infrastructure of its own to mitigate the consolidation of DNS [8]. Previous studies have explored the concentration of DNS resolution traffic or DNS name servers. [PERSON] et al. [4; 9] analyzed the passive traffic of several public DNS providers, DNS root servers, and country-code top-level domains (ccTLDs). Moreover, [PERSON] et al. [10] studied the centralization of name servers using TLD zone files. However, the level of centralization within the DNS infrastructure itself remains largely unknown. In addition, most previous studies have relied on passive analysis methods, such as traffic analysis and zone file analysis. Researchers appear to be hindered by the complex architecture and the puzzling behaviors of DNS resolvers [11]. To meet the access demands of a large number of Internet end-users, public DNS providers deploy the multiple-layer resolver pool for load balancing, which is transparent to clients. Consequently, there exists an implicit collaborative relationship between resolvers located within the same resolver pool. [PERSON] et al. [12] proposed a taxonomy for the client-side DNS infrastructure, which includes FDNS (forwarding resolver), RDNS (recursive resolver), iRDNS (indirect RDNS), and dRDNS (direct RDNS). Since resolvers are generally hidden from Internet end-users, identifying the resolver pool structure poses a critical challenge in measuring the centrality of the client-side DNS infrastructure. To overcome the challenge and systematically evaluate the centrality of the DNS infrastructure, we propose a novel resolver pool measurement approach that can identify the implicit resolver pool structure of the client-side DNS infrastructure by a single probing point and can be implemented at a low cost. Furthermore, we conducted Internet-wide scanning on all routable IPv4 addresses using our approach. Our measurement results reveal that over 90% of forwarding resolvers are supported by less than 5% of indirect resolvers. In addition, to comprehensively explore the centrality of name servers, we analyzed zone files of 1138 generic Top-Level Domains (gTLDs). Our analysis shows that more than 98% of all domain names rely on a single name server provider, and the top 10 leading name server providers serve 48.5% of all domain names.