What Is DNS?

Why Do We Need DNS?

On the Internet, a computer uniquely identifies itself when communicating with other computers by using an IP address, which is similar to a phone number. However, these all-digit identifiers are difficult to remember, and there are many of them on the Internet. For example, there are nearly 4.3 billion IPv4 addresses, each of which is a 32-bit binary number or a dotted decimal number (such as 192.168.1.1). The function of DNS is to translate human-memorable names into machine-recognizable IP addresses so that computers can connect to each other. The working principle of DNS is similar to that of a phonebook — both of them manage mappings between names and numbers. When making phone calls, we usually search for people by name rather than entering phone numbers directly. Likewise, we enter the URL — for example, https://www.huawei.com — in the browser when we want to open a web page or view a video on the Internet. A URL is actually a computer name, which is typically called a domain name.

Composition of a Domain Name

Initially, the domain names of devices consisted of a sequence of characters. At that time, all of the domain names formed a non-hierarchical domain name structure, which not only led to name conflicts but also complex management and maintenance. To address this, DNS domain names in a hierarchical tree structure were designed in the TCP/IP protocol stack. Each country that applies to join the Internet must register a top-level domain name with the NIC. A top-level domain typically represents either an organization type or a geographical location. For example, "cn" and "us" represent China and the United States, respectively. The following table describes common top-level domains. The NIC authorizes management agencies to classify top-level domains into level-2 domains. The agencies in charge of this can authorize subordinate agencies to continue classifying the domains. As a result, the Internet has a hierarchical domain name structure.

In terms of syntax, each domain name consists of labels, which are separated by dots (.). Take the domain name www.huawei.com as an example. Each part from right to left is described as follows:

• com: top-level domain name, which represents a commercial organization.
• huawei: level-2 domain name, which belongs to a company.
• www: level-3 domain name, which represents a common web page.

DNS Server, Client, and Relay Agent

DNS-related device roles on a network are DNS server, client, and relay agent.

DNS Domain Name Resolution Process

The process of obtaining an IP address based on a domain name is called domain name resolution. DNS domain name resolution is classified into the following types:

• Static domain name resolution

  Static domain name resolution requires a static domain name resolution table, which is manually created and holds mappings between commonly used domain names and IP addresses. A DNS client first searches the static domain name resolution table for a domain name to resolve it into an IP address. This improves the efficiency of domain name resolution.

• Dynamic domain name resolution

  Dynamic domain name resolution requires a dedicated DNS server. This server runs the domain name resolution program, maps domain names to IP addresses, and receives DNS requests from clients.

To speed up DNS queries, static domain name resolution is first used to resolve a domain name. If the resolution fails, dynamic domain name resolution is used.

As shown in the preceding figure, DNS works as follows:

1. When a user program (such as ping or Telnet) uses a domain name to access the network, it sends a DNS request to the resolver of the DNS client.

2. After receiving the request, the resolver first checks the local cache.

   • If the resolver finds the mapping entry for the domain name in the local cache, it directly returns the mapped IP address to the user program.
   • If the resolver does not find such a mapping entry in the local cache, it sends a request to the DNS server.
3. The DNS server checks whether the requested domain name is within a sub-domain it manages and then responds to the DNS client accordingly.

   • If the requested domain name is within a sub-domain it manages, the DNS server searches for the IP address corresponding to the domain name in its own database.
   • If the requested domain name is not within a sub-domain it manages, the DNS server forwards the request to upper-level DNS servers. After completing the resolution, the corresponding upper-level DNS server returns the result to the DNS client.

4. The resolver receives and resolves the response sent by the DNS server, and returns the result to the user program.

Dynamically resolved mappings between domain names and IP addresses are stored in the cache. If a domain name is searched for again, the DNS client obtains the corresponding IP address from the cache directly instead of sending a request to the DNS server. Mappings stored in the cache will expire and be deleted after a period to ensure that the latest mappings can be obtained from the DNS server.