What Action Does a DNS Server Take If It Does Not Have an Entry for a Requested URL?

The Cisco 200-301 exam format consists of a single exam required to earn the CCNA certification. It includes a mix of multiple-choice, drag-and-drop, simulation, and testlet questions. The exam lasts 120 minutes and covers topics like networking fundamentals, IP services, security fundamentals, automation, and programmability. It is available in English and Japanese, and candidates must register through Pearson VUE.

Tech Professionals

11 April 2025

Cisco

What Action Does a DNS Server Take If It Does Not Have an Entry for a Requested URL?

Introduction To Cisco 200-301 Exam Format

The internet, a vast and intricate network connecting billions of devices, relies on a seemingly invisible yet crucial system to function: the Domain Name System (DNS). Every time you type a website address (a URL) into your browser, a complex process is initiated behind the scenes to translate that human-readable name into a numerical IP address that computers understand. But what happens when the DNS server you initially query doesn't have the answer.

This is a fundamental question for anyone seeking to understand the intricacies of network communication, and it's a concept thoroughly covered in resources like Study4Pass, especially relevant for those preparing for the Cisco 200-301 exam format.

Understanding the DNS resolution process, including how servers handle missing entries, is a key objective for anyone aiming to achieve the CCNA certification. Study4Pass provides comprehensive materials and practice tests designed to solidify your grasp on these essential networking concepts. Let's delve into the fascinating journey a DNS query takes when the initial server comes up empty-handed.

The Initial Query: A Polite Request

When you type a URL like `www.example.com` into your browser, the first step is a DNS query. Your computer, configured with one or more DNS server addresses (usually provided by your Internet Service Provider or manually set), sends a request to its designated primary DNS server. This request essentially asks, "What is the IP address associated with `www.example.com`?"

This initial query is typically a recursive query. This means the requesting client (your computer) expects the DNS server to provide a definitive answer. The client doesn't want to be told to go ask someone else; it wants the IP address.

Scenario 1: The Lucky Hit - The Authoritative Answer

Ideally, the local DNS server will have the answer cached. DNS servers maintain a cache of recently resolved domain names and their corresponding IP addresses to improve efficiency and reduce network traffic. If the record for `www.example.com` is in its cache and hasn't expired (TTL - Time To Live), the DNS server can immediately respond to your computer with the correct IP address. This is the fastest and most efficient scenario.

However, if the record is not in the cache or has expired, the DNS server needs to find the answer elsewhere. This is where the more complex process begins, and it directly addresses our central question: what happens when the DNS server doesn't have the entry?

Scenario 2: The Referral Journey - Iterative Queries

When the local DNS server doesn't have the requested record, and it's configured to perform recursive resolution (which is the default behavior for most client-facing DNS resolvers), it embarks on a journey to find the authoritative DNS server for the requested domain. This journey involves a series of iterative queries to different levels of the DNS hierarchy.

Here's a breakdown of the steps involved:

1. Querying the Root Servers: The local DNS server first contacts one of the root name servers. These servers are at the top of the DNS hierarchy and are responsible for knowing the location of the Top-Level Domain (TLD) name servers. For our example of `www.example.com`, the TLD is `.com`. The root server doesn't know the IP address of `www.example.com` directly, but it knows which servers are authoritative for the `.com` domain. It responds to the local DNS server with a referral, pointing it to the appropriate `.com` TLD name servers.

2. Querying the TLD Name Servers: Next, the local DNS server contacts one of the `.com` TLD name servers provided in the referral. These servers maintain information about all the second-level domains within the `.com` zone, such as `example.com`. Again, the TLD server might not know the specific IP address for `www.example.com`, but it knows which servers are authoritative for the `example.com` domain. It responds with another referral, this time pointing to the authoritative name servers for `example.com`.

3. Querying the Authoritative Name Servers: Finally, the local DNS server contacts one of the authoritative name servers for `example.com`. These are the servers that hold the actual DNS records for the `example.com` domain, including the A record (which maps a hostname to an IPv4 address) for `www.example.com`. If the record exists, the authoritative name server will respond with the IP address. This is the authoritative answer.

4. Caching the Response: Once the local DNS server receives the authoritative answer, it caches this information for a period determined by the TTL value associated with the record. This ensures that subsequent requests for the same domain name can be resolved much faster.

5. Responding to the Client: Finally, the local DNS server sends the resolved IP address back to your computer, allowing your browser to establish a connection with the web server hosting `www.example.com`.

Scenario 3: The Unsuccessful Search - Negative Responses

What happens if, at any point in this iterative process, the requested domain name does not exist? In this case, the authoritative name server for the relevant zone will respond with a negative response. There are different types of negative responses:

NXDOMAIN (Non-Existent Domain): This response indicates that the requested domain name itself does not exist. For example, if you typed `www.nonexistentdomain123.com`, the authoritative name servers for the `.com` TLD would eventually respond with NXDOMAIN.

NODATA (No Data): This response indicates that the domain name exists, but the specific record type requested (e.g., an A record for an IPv4 address) does not exist for that domain name. For example, if a domain has MX records (for mail exchange) but no A record for the bare domain name, a query specifically for the A record might result in a NODATA response.

When the local DNS server receives a negative response, it caches this negative result for a certain period (also determined by a TTL value, often referred to as negative TTL) to prevent repeated failed lookups for the same non-existent domain. It then relays this negative response back to your computer, and your browser will typically display an error message indicating that the website cannot be found.

The Role of Recursive Resolvers

It's important to distinguish between recursive resolvers (like the DNS servers provided by your ISP) and authoritative name servers (which hold the actual DNS records for specific domains). Recursive resolvers handle the task of traversing the DNS hierarchy on behalf of clients, performing iterative queries until they find the authoritative answer or a negative response. Authoritative name servers, on the other hand, only respond to queries for the domains they are responsible for.

Study4Pass and the Cisco 200-301 Exam Format

Understanding the intricacies of DNS resolution, including how DNS servers handle missing entries and the difference between recursive and iterative queries, is crucial for the Cisco 200-301 Certification. The exam often tests your knowledge of fundamental networking protocols and services, and DNS is a cornerstone of internet communication.

Study4Pass provides comprehensive study materials, including detailed explanations, diagrams, and practice questions that cover all aspects of DNS. By utilizing their resources, you can gain a thorough understanding of:

The DNS hierarchy (root, TLD, authoritative name servers).
The different types of DNS queries (recursive and iterative).
The process of DNS resolution when a record is found and when it's not.
Different types of DNS records (A, MX, CNAME, NS, etc.).
DNS caching and its impact on performance.
Troubleshooting common DNS issues.

The Cisco 200-301 exam format emphasizes practical knowledge, so understanding the real-world implications of DNS behavior, such as what happens when a DNS server doesn't have an entry, is essential for success. Study4Pass helps bridge the gap between theoretical knowledge and practical application, preparing you not just for the exam but also for real-world networking scenarios.

Conclusion:

When a DNS server doesn't have an entry for a requested URL, it doesn't simply give up. Instead, a well-orchestrated process of iterative queries begins, traversing the DNS hierarchy from the root servers down to the authoritative name servers for the specific domain. If the domain exists, the authoritative server will provide the IP address. If the domain doesn't exist or the requested record type is not present, a negative response is returned.