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DNS

🗺️ The Core Problem: Human-Readable vs. Machine Addresses

The fundamental problem DNS solves is the need to map human-friendly names (like Google.com) to machine-readable network addresses (IP addresses, e.g., 142.250.191.46).

  • The Analogy: The DNS acts like a phone book or a contact list on a mobile phone, where a person’s name (the domain name) is mapped to their phone number (the IP address).
  • Why it’s needed: It’s impractical and difficult for users to memorize the numeric IP address for every server they want to access.

🔄 How DNS Resolution Works (High-Level)

At its most basic, the process involves the client asking for the IP address and then using it to communicate.

  1. Client Request: A client (e.g., your browser) requests to access a domain name (such as google.com).

  2. DNS Query: The client sends a query to a recursive resolver (usually operated by your ISP or a public DNS like 8.8.8.8) to find the corresponding IP address.

  3. Recursive Resolution:

    • The resolver first contacts the Root DNS Servers, which direct it to the Top-Level Domain (TLD) servers (e.g., .com, .org).
    • The TLD server (operated by a Registry, such as Verisign for .com) then points the resolver to the Authoritative DNS servers responsible for that domain.
    • The Authoritative DNS server finally returns the actual IP address.

  4. DNS Response: The recursive resolver sends the IP address back to the client.

  5. Server Request: The client uses the IP to establish a connection with the target web server.

  6. Server Response: The web server responds with the requested content.

Demonstration: The nslookup google.com command shows this process by returning one or more IP addresses associated with the domain. Entering that IP directly in the browser typically leads to the same destination, confirming the mapping.

🌐 The Components of the DNS Ecosystem

DNS is a hierarchical and distributed system, governed by global coordination but operated by many independent organizations.

🧠 ICANN (Internet Corporation for Assigned Names and Numbers)

  • Role: The top-level, non-profit authority that oversees the global DNS system.
  • Function: It delegates the management of each TLD (like .com, .org, .cn) to trusted Registry Operators.
  • Goal: Ensures domain name uniqueness and stability across the Internet.

🏗️ Domain Registries (TLD Operators)

  • Role: These are organizations authorized by ICANN to operate specific Top-Level Domains (TLDs).

  • Examples:

    • .com and .net → Verisign
    • .org → Public Interest Registry
    • .cn → CNNIC (China Internet Network Information Center)

  • Responsibilities:

    • Maintain the official database of all domains under their TLD (e.g., all *.com names).
    • Run the TLD name servers, which tell the resolver which Authoritative DNS to ask next.

🌍 Domain Registrars

  • Role: Accredited by ICANN and connected to the registries, registrars sell domains to the public.

  • Examples: Namecheap, GoDaddy, Google Domains.

  • Process:

    • When you buy a domain, the registrar records your ownership in the Registry’s database.
    • You also configure which Authoritative DNS servers manage your domain (e.g., Cloudflare).

🧩 Authoritative DNS Providers

  • Role: The DNS servers that hold the actual DNS records (A, CNAME, MX, TXT, etc.) for your domain.
  • Examples: Cloudflare, AWS Route53, Google Cloud DNS, or even your hosting provider.
  • Function: When a resolver asks, “What’s the IP of mycoolapp.com?”, these servers give the final answer.

🛰️ Recursive Resolvers (Caching DNS)

  • Role: Intermediaries that perform the lookup process on behalf of the client.
  • Examples: Google Public DNS (8.8.8.8), Cloudflare DNS (1.1.1.1), or your ISP’s DNS.
  • Behavior: They cache results for efficiency, reducing global DNS traffic.

🧩 Internet Service Provider (ISP)

  • Role: Your ISP routes your DNS requests (often through its own recursive resolver) and provides network access.
  • Note: ISPs don’t manage domain ownership—they just help your request reach the broader DNS network.

🧭 The Hierarchical Relationship

[ ICANN ]
   ↓ (delegates)
[ Registry (e.g., Verisign for .com) ]
   ↓ (contracts with)
[ Registrar (e.g., Namecheap) ]
   ↓ (sets NS records)
[ Authoritative DNS (e.g., Cloudflare) ]
   ↓ (provides final records)
[ Recursive Resolver (e.g., 8.8.8.8) ]
   ↓ (answers client)
[ Client / Browser ]

💾 DNS Records and Caching

DNS Records

  • Definition: These are hosted on servers (often belonging to the domain registrar) and store the information needed to resolve a DNS request.
  • A Record (Address Record): A common type of record that maps a domain name directly to an IP address.

    Example: An A record for NeetCode.io points to a specific IP address belonging to the NeetCode.io server.

Caching

  • Concept: Since server IP addresses (especially for public servers) are usually static and don’t change often, clients can cache the resolved IP address on their local machine.
  • Benefit: Caching eliminates the need for a full DNS query every time a user visits the domain, speeding up subsequent access.

💻 The Server: The Destination

A server is the final destination for the request after DNS resolution.

  • Definition: A computer with a public IP address (often associated with a domain name).
  • Firewalls: Unlike typical home routers, a public server’s firewalls are explicitly configured to allow external public traffic to reach it.
  • Function: It receives requests, fulfills those requests, and sends the appropriate response back to the client.

🧱 Anatomy of a URL/Domain

A full web address (URL) is composed of several structured parts:

ComponentExample PortionDescription
Protocolhttps://Specifies the communication method (e.g., HTTP, HTTPS, or a local file protocol).
Subdomainwww. or domains.An optional prefix, often used to organize content or services within a primary domain (e.g., domains.google.com).
Primary Domain NamegoogleThe main, unique name that is purchased from a registrar.
Top Level Domain (TLD).com or .io or .jpThe final suffix of the domain, managed by ICANN.
Path/path/to/pageThe specific location or resource being requested on the server, often followed by query parameters.

Ownership: When purchasing a domain (e.g., google.com), you own the Primary Domain Name and control any Subdomains added to it.