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Kim et al. J Surveill Secur Saf 2020;1:34-60 I http://dx.doi.org/10.20517/jsss.2020.14 Page 37

Figure 2. Domain Name System structure

2.1.2 DNS philosophy
Technically, DNS is a hierarchical name server system that uses a globally distributed database system that
holds information about each domain. The DNS information is stored in distributed DNS servers, and the
information can be searched at any time upon user request.

Figure 2 illustrates the hierarchical DNS structure via a common domain name. DNS begins with the
.(Root) domain at the top. .com is a TLD (Top Level Domain) whose parent is the .(Root) domain. .google
is an SLD (Second Level Domain) whose parent is the .com domain. Finally, .www (i.e., a web service) is a
subdomain of .google.com.

As the top level of DNS, Root name servers are a global network with 13 redundant servers located in
various countries, which manage all TLDs. The TLD comprises two types: the country code Top Level
Domain (ccTLD) and the general Top Level Domain (gTLD). The ccTLD stands for the country domain
name, such as .kr (South Korea) and the gTLD stands for the general domain type, such as .com (Company)
or .org (Organization). As the number of domains increased, the number of available TLDs became
insufficient, and ICANN announced a new set of TLDs in 2014. Currently, the number of TLD servers
around the world is approximately 1,500 (maintained by IANA). Such vertical tree structure enables DNS
not only to facilitate the management of each domain information but also to distribute numerous DNS
requests efficiently.

The process of translating IP addresses to corresponding domain names through DNS is called name
[1]
resolution or DNS resolution . DNS resolution begins with a client’s DNS request. Figure 3 illustrates how
a client obtains the IP address for a web server via DNS resolution, allowing it to receive web services.

(1) A client requests an IP address www.google.com from a local recursive DNS resolver.
(2) The recursive DNS resolver first checks the address translation in its local cache.
(3) If there is no information in the cache, the recursive DNS resolver requests the IP address of the TLD