This article will explain the series of processes that occur after a URL is entered into a browser and a query is initiated. The content is based on the book How the Internet Connects with some additional clarifications and expansions for better understanding.
DNS - From Domain Name to IP Address
When the browser receives a user’s URL request, the first step is to resolve the domain
name into an IP address. This is because domain names are easier for humans to remember
and are more readable, but computers need IP addresses to identify the location of
other machines on the network. When we enter a website in the browser and press the
Enter key, the browser will first call a method named gethostbyname from the system’s
Socket API (Socket is an interface provided by the operating system for network
communication. It offers a series of methods for interacting with the network protocol
stack within the OS). The gethostbyname method takes a domain name as a parameter,
sends a query to the DNS server, and returns the corresponding IP address. (In modern
systems, this method is gradually being replaced by getaddrinfo, which supports a
wider range of protocols.) The DNS server stores the mappings between domain names
and IP addresses, and its task is to handle query requests from clients. The query
parameters typically include three elements:
- Domain
- Class
- Record Type
First, the Domain is the parameter we input in the gethostbyname method. It is
important to note that this domain name is not limited to the one we input in the
browser. It can also include domain names used by services like mail servers (the part
after the @ symbol). Second, the Class parameter is used to identify the type of
network. Since the Internet is the most commonly used network today, the most typical
input for this parameter is IN, representing the Internet. The third parameter
indicates the type of data the client wants to retrieve. Here, we will introduce two
common types: the first is the A type, which represents an IP address, the DNS server
will return the IPv4 address for this type. The second type is the MX type used by
mail servers. In response to this type of query, the DNS server will return the domain
name of the mail server, its priority, and the corresponding IP address.
However, there are so many IP addresses and domain names globally that no single DNS server can handle all the requests worldwide. Therefore, DNS servers are distributed across the world.