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What is a Domain Name System (DNS)? All You Need

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Have you ever wondered how typing a simple web address like www.google.com brings you to a specific website almost instantly? The answer lies in the Domain Name System (DNS), a foundational technology of the internet. That is what we have discussed briefly in this article.

Domain Name System translates easy-to-remember domain names into numerical IP addresses that computers use to identify each other on the network.

Understanding Domain Name System is essential for grasping how the internet functions behind the scenes, enabling billions of users worldwide to connect and interact daily. Let us read more about it in this article!

Table of contents

  1. What is a Domain Name System (DNS)?
  2. Domain Name System: Hierarchy and Components
  • DNS Resolver (Recursive Resolver)
  • DNS Caching
  • Root DNS Servers
  • Top-Level Domain (TLD) Servers
  • Authoritative Name Servers
  • Detailed DNS Resolution Process
    • Client Query
    • Recursive Resolver Query
    • Root Server Query
    • TLD Server Query
    • Authoritative Name Server Query
    • Returning the Response
    • Caching the Response
  • Types of DNS Queries
    • Recursive Query:
    • Iterative Query:
    • Non-Recursive Query:
  • DNS Security Considerations
    • DNS Spoofing (Cache Poisoning)
    • DNSSEC (DNS Security Extensions)
  • Conclusion

    • What is a Domain Name System (DNS)?

    The Domain Name System (DNS) is a fundamental component of the internet, responsible for translating human-readable domain names (www.google.com) into machine-readable IP addresses (172.217.14.206).

    This translation allows computers and devices to communicate across the internet using IP addresses while enabling humans to use easy-to-remember domain names.

    DNS operates in a hierarchical structure, involving multiple types of DNS servers and caches to efficiently resolve domain names. 

    Domain Name System: Hierarchy and Components

    The Domain Name System operates like a phonebook for the internet, and the process of resolving a domain name into an IP address involves several steps.

    1. DNS Resolver (Recursive Resolver)

    • The DNS Resolver is typically provided by the Internet Service Provider (ISP). It serves as the starting point for the DNS resolution process.
    • When you enter a domain name (www.example.com) in the browser, your device contacts the DNS resolver to find the corresponding IP address.
    • The resolver queries other DNS servers on behalf of the client until it finds the authoritative answer. The resolver is called “recursive” because it follows a recursive process, querying one server after another.

    2. DNS Caching

    DNS caching is a performance optimization mechanism that allows the DNS resolvers to temporarily store previously queried DNS records. This helps if the same domain is requested again, the resolver can return the cached IP address without querying upstream DNS servers, speeding up the resolution process.

    Caching can occur at multiple points:

    • Browser Cache: Browsers maintain their cache of recently visited domain names and corresponding IP addresses.
    • Operating System Cache: The OS, such as Windows or Linux, has its DNS cache to store results from previous queries.
    • Recursive DNS Resolver Cache: The resolver itself caches DNS responses to reduce the need for repeated queries to upstream servers.

    3. Root DNS Servers

    • At the top of the Domain Name System hierarchy are the Root DNS Servers. These are critical internet infrastructure components responsible for directing DNS resolvers to the appropriate Top-Level Domain (TLD) servers (such as .com, .net, .org, or country-specific TLDs like .uk or .jp ).
    • There are 13 sets of root DNS servers, operated by organizations like Verisign, ICANN, and others, but each set is distributed globally across many physical servers to provide resilience and reliability.

    4. Top-Level Domain (TLD) Servers

    • The TLD DNS Servers are responsible for specific top-level domains.
    • When a DNS resolver queries a root server, the root server doesn’t know the IP address of www.example.com, but it knows the TLD server responsible for .com domains. The resolver is then directed to the correct TLD server for .com.

    5. Authoritative Name Servers

    Authoritative Name Servers hold the DNS records that map domain names to IP addresses. These are the final steps in the DNS resolution process.

    Each domain (example.com) has its own set of authoritative DNS servers that store Resource Records (RRs), such as:

    • A Record: Maps a domain name to an IPv4 address.
    • AAAA Record: Maps a domain name to an IPv6 address.
    • MX Record: Specifies the mail servers for email handling.
    • CNAME Record: A canonical name that aliases one domain to another.
    • NS Record: Specifies the authoritative DNS servers for the domain.
    • TXT Record: Can store arbitrary text, often used for email verification or domain verification purposes.

    For example, for (www.example.com), the authoritative name server would return the A record with the IP address (93.184.216.34).

    MDN

    Detailed DNS Resolution Process

    1. Client Query

    • The DNS process begins when a user enters a domain name (www.example.com) into their web browser.
    • The browser first checks its internal DNS cache to see if it has the IP address. If not, it forwards the request to the operating system’s DNS client.

    2. Recursive Resolver Query

    • The OS checks its own DNS cache. If cached information is not found, the OS sends the DNS query to a recursive DNS resolver.
    • The resolver will first check its local cache. If it has a cached response from a previous query, it returns the IP address to the client. If not, it proceeds to query external DNS servers.

    3. Root Server Query

    • The recursive resolver sends a query to one of the root DNS servers. The root server doesn’t know the IP address of www.example.com, but it knows which TLD DNS server is responsible for .com domains.
    • The root server responds with a referral to the appropriate .com TLD DNS server.

    4. TLD Server Query

    • Next, the recursive resolver queries the .com TLD DNS server. Again, the TLD server doesn’t know the exact IP address for www.example.com, but it knows the authoritative name server for example.com.
    • The TLD server responds with a referral to the authoritative DNS server for it.

    5. Authoritative Name Server Query

    • Finally, the recursive resolver sends a query to the authoritative name server for example.com.
    • The authoritative server responds with the A record (IPv4) or AAAA record (IPv6) that contains the IP address for example.com.

    6. Returning the Response

    • The recursive resolver now has the IP address and returns it to the client’s device.
    • The client device can now initiate communication with the web server located at that IP address (e.g., opening a TCP connection to download the requested webpage).

    7. Caching the Response

    The IP address is cached at multiple levels:

    • In the recursive resolver’s cache.
    • In the OS-level cache of the user’s device.
    • In the browser’s cache.
    • This caching ensures faster resolution if the same domain is queried again within a certain period (determined by the TTL – Time to Live of the DNS records).

    Types of DNS Queries

    1. Recursive Query:

    • A recursive query is when the DNS client requests the resolver to obtain the IP address for a domain, no matter how many DNS servers it has to query.
    • The client expects either an answer (IP address) or an error message (if the domain cannot be found).

    2. Iterative Query:

    • In an iterative query, the DNS server (recursive resolver) returns the best information it has. This could be an IP address or a referral to another DNS server that is closer to the authoritative source.
    • Each DNS server responds with the next server to query, and the client itself follows the chain until it finds the authoritative name server.

    3. Non-Recursive Query:

    A non-recursive query occurs when the Domain Name System resolver already knows the answer (either from its cache or because it is the authoritative server for the domain). In this case, the resolver immediately responds with the IP address.

    DNS Security Considerations

    1. DNS Spoofing (Cache Poisoning)

    DNS Spoofing is an attack where a malicious actor injects false DNS records into the cache of a DNS resolver. This can lead users to visit fake websites designed for phishing, malware distribution, or other malicious purposes.

    2. DNSSEC (DNS Security Extensions)

    • To protect against such attacks, DNSSEC (Domain Name System Security Extensions) was introduced. DNSSEC ensures that DNS responses are authenticated and have not been tampered with.
    • DNSSEC uses digital signatures to verify that the Domain Name System records received come from a legitimate source, protecting users from being redirected to malicious sites.

    Thus, the Domain Name System plays a critical role in the functioning of the internet by ensuring smooth and efficient domain name resolution.

    MDN

    Conclusion

    In conclusion, from hierarchical name servers to caching mechanisms and different query types, the Domain Name System infrastructure is designed for speed, reliability, and security.

    However, as cyber threats like DNS spoofing and cache poisoning continue to evolve, implementing security measures such as DNSSEC has become increasingly important.

    As the internet grows, Domain Name System will continue to evolve, adapting to new technological advancements and security challenges to maintain a stable and secure online environment.

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