DNS Resolution
Last updated
Last updated
DNS resolves because humans prefer to use domain names (like example.com), which are easy to remember, while machines communicate using IP addresses (like 192.0.2.1), which identify servers uniquely on the internet. DNS serves as a bridge between human-friendly domain names and machine-readable IP addresses.
Recursive DNS queries involve a DNS resolver handling the entire lookup process on behalf of the client. When a recursive query is made, the resolver:
Delegates the lookup: The resolver queries other DNS servers as needed to find the requested information.
Handles the work: It performs all necessary lookups and retrieves the information.
Reports back: The resolver provides the final result to the client.
Uses local cache: Future queries for the same information use the resolver's cached data to speed up the response.
Explain what happens when you type a URL into your web browser and hit Enter.
When you type a URL into your web browser and hit Enter, several steps occur behind the scenes to resolve that URL into an IP address:
Step 1 - Local DNS Cache: Your computer first checks its local DNS cache. This cache stores recently resolved domain names and their corresponding IP addresses. If the URL you entered has been recently visited, there's a good chance it's in the cache, saving time on DNS resolution.
Step 2 - Operating System: If the URL is not found in the local cache or has expired, your computer's operating system steps in. It sends a DNS resolution request to a DNS resolver, typically provided by your Internet Service Provider (ISP). This resolver is responsible for finding the IP address associated with the domain name.
Step 3 - Recursive Query: The DNS resolver doesn't have the IP address stored locally either. It then begins a recursive query, which involves multiple steps:
Step 3a. Root DNS Servers: The resolver starts by contacting the root DNS servers, the top-level servers in the DNS hierarchy. There are only 13 sets of root servers globally, distributed worldwide. The root servers don't know the specific IP address for the requested domain but can direct the resolver to the appropriate top-level domain (TLD) server.
Step 3b. TLD DNS Servers: The TLD servers handle domains like ".com," ".org," ".net," etc. The resolver contacts the relevant TLD server based on the domain's extension (e.g., ".com" for www.example.com). The TLD server provides information about which authoritative DNS server is responsible for the domain.
Step 3c. Authoritative DNS Server: The resolver contacts the authoritative DNS server for the domain (often managed by the domain's registrar or hosting provider). This server contains the actual IP address associated with the domain name.
Step 4 - Caching: Once the authoritative DNS server provides the IP address, the resolver caches this information to speed up future requests. Your computer's local DNS cache is also updated with this information.
Step 5 - IP Address Retrieval: The resolver returns the IP address to your computer's operating system, which in turn passes it to the web browser.
Step 6 - Establishing a Connection: With the IP address in hand, your web browser can now connect to the web server associated with the domain name. This process involves several steps, including routing through various network devices, but eventually, a connection is established.
Step 7 - Fetching Web Content: Once the connection is established, your web browser sends an HTTP request to the web server, asking for the specific webpage associated with the URL. The web server responds by sending the requested web content (HTML, images, scripts, etc.) back to your browser, which renders it for you to see and interact with.
The Domain Name System (DNS) is a vital component of the Internet’s infrastructure, allowing us to use easy-to-remember domain names instead of numeric IP addresses to access websites and services. When you enter a URL and hit Enter, a swift yet intricate process occurs behind the scenes to translate that URL into an IP address, making it possible to reach the desired content. Knowledge of how DNS operates is essential, as many competitive exams and interviews feature questions about this fundamental and often overlooked mechanism.
