DNS is the hierarchical way of associating domain names with IP addresses. It includes the network of databases and computers, together with the Application layer service. DNS is relying on multiple databases spread out across the globe, instead of using single file.
DNS can be divided to the components for effectiveness:
Resolvers are the clients that require domain name information. HTTP is resolver client built in TCP/IP applications. When user wants to load the website, HTTP client starts resolver service. Already visited web-sites information is stored locally on the computer in DNS cache. It is the first place where resolver would check from. If resolver fails to get information from local database, it will send request to the name server.
Name server (DNS server) is the database server that stores data about associated IP addresses. If it does not have information about requested domain, the resolver will use DNS server higher in hierarchy. The process repeats until information is not found. Example of DNS is shown in Figure 1.
Namespace is a database of all the IP associations, as described before. It is a concept of how the servers share DNS information and communicate with each other.
Figure 1. Domain name resolution.
Hosts can be arranged in zones, for easier management. Zone consists of one or several domains. Information about members of DNS zone is store in zone file, that contains resource records. Each resource record is a single record of information of the DNS database.
There are multiple types of resource records (shown in Table 1), but most common type is address resource record. It contains information about the name of the record, its class, type of resource record, length of time the record should be stored, data length field ensuring no data lost, and, finally, data. The example of address resource record is shown in Figure 2.
Type
Name
Description
A
Address record
A host’s IPv4 address
AAAA
Address record
A host’s IPv6 address
CNAME
Canonical name record
Another name for the host
MX
Mail exchange record
Identifies a mail server
PTR
Pointer record
Points to a canonical name
Table 1. Common DNS record types.
Figure 2. Examples of the DNS address resource records.
Resource records are updated automatically, without human factor involved. Usually, client machine receives DNS server information together with IP address from the local service provider. In rare situations then DNS records need to be updated manually, the ipconfig /registerdns command is issued, as shown in Figure 3. That command forces updating of clients DNS information.
Figure 3. Example of “registerdns” command execution.
