# Domain Name System (DNS) ## What is the domain name system?

DNS is a distributed directory system that translates human-readable domain names into machine-readable IP addresses

- Acts like the internet's address book - Enables users to access websites using memorable domain names (like `www.example.com`) instead of numeric IP addresses (like `93.184.216.34`) - When you enter a URL into your browser, DNS performs the translation from domain name to IP address automatically - This allows your computer to connect to the correct server hosting the website ### Why DNS is essential - **User-friendly navigation**: Without DNS, we would need to memorize IP addresses for every website we visit - Imagine having to remember `172.217.169.46` instead of typing `www.google.com` - **Automated management**: DNS servers handle all the mapping work, removing the burden from users and system administrators - **Dynamic updates**: When domains are newly registered or servers change IP addresses, DNS records must be updated through **DNS propagation**

DNS propagation is the process of updating DNS records across the internet when a domain's IP address changes

## Components of DNS DNS operates through a hierarchy of specialized servers, each playing a specific role in the address resolution process. ### DNS resolver - The first point of contact in the DNS lookup process - Usually provided by your Internet Service Provider (ISP) or third-party services like Google DNS or Cloudflare DNS - Acts as an intermediary between your device and other DNS servers - Caches frequently accessed domain-to-IP mappings to speed up future requests ### Root servers - The DNS resolver queries a root server to identify the appropriate top-level domain (TLD) - Root servers direct queries to the correct TLD server based on domain extensions like `.com`, `.org`, or `.edu` - There are 13 sets of root servers distributed globally for redundancy and performance ### TLD (Top-Level Domain) servers - The root server directs the resolver to a TLD server - TLD servers store information about all domains using a specific extension - Examples: `.com` TLD servers know about all `.com` domains - The TLD server provides the IP address of the domain's authoritative DNS server ### Authoritative DNS servers - These servers hold the definitive DNS records for specific domains - The TLD server responds with the address of the domain's authoritative DNS server - The resolver then queries this authoritative server for the actual IP address of the requested domain - Authoritative servers provide the final answer in the DNS lookup chain ## Benefits of DNS | Benefit | Description | |---------|-------------| | **Human-friendly naming** | Users can remember and use meaningful domain names instead of numerical IP addresses | | **Centralized management** | DNS servers automatically handle the complex task of mapping domain names to IP addresses | | **Scalability** | The distributed nature of DNS allows it to handle billions of domain name queries daily | | **Flexibility** | IP addresses can change without affecting domain names, making infrastructure updates transparent to users | ## DNS lookup process When you type a URL into a web browser, the following sequence occurs: ### Step-by-step breakdown 1. **URL Entry** - User enters a website URL (e.g., `www.musicstreaming.com`) into the web browser 2. **Local cache check** - The computer checks its local cache to see if it already has the IP address from a previous visit - If found, the process skips directly to requesting the web page 3. **DNS query initiated** - The web browser sends a query to a DNS resolver (typically hosted by your ISP) - The query requests translation of the domain name into an IP address 4. **DNS resolver cache check** - The DNS resolver checks its own cache for the IP address of the requested domain - If not found, the resolver proceeds to query DNS root servers 5. **Root server query** - The root server examines the domain extension (like `.com`, `.org`) - It directs the resolver to the appropriate TLD server for that extension 6. **TLD server query** - The resolver queries the TLD server for the domain - The TLD server responds with the IP address of the domain's authoritative DNS server 7. **Authoritative server query** - The resolver queries the authoritative DNS server - This server holds the actual DNS records for the requested domain 8. **Retrieve IP address** - The authoritative DNS server responds with the IP address for the requested domain - The resolver caches this information for future requests 9. **Request the web page** - The web browser sends an HTTP or HTTPS request to the retrieved IP address - This request asks the server to provide the website content 10. **Server response** - The server at the given IP address processes the request - It sends back the data for the web page (HTML, CSS, JavaScript, images, etc.) 11. **Render the web page** - The web browser interprets and renders the received data - The website appears on your screen

This entire process typically completes in milliseconds, making web browsing seamless and efficient

## Key protocols involved While DNS handles the name-to-address translation, other protocols work alongside it: | Protocol | Role | |----------|------| | **DNS** | Translates human-friendly URLs into computer-friendly IP addresses | | **HTTP/HTTPS** | Defines how web page requests and responses are formatted and transmitted | | **TCP/IP** | Handles the actual transmission of data packets across the network |

The multi-step DNS lookup process introduces potential security vulnerabilities, including DNS spoofing and cache poisoning attacks

## DNS records and caching ### Caching at multiple levels - **Browser cache**: Stores recently accessed domain-to-IP mappings - **Operating system cache**: Maintains DNS records at the system level - **Resolver cache**: DNS resolvers cache results to reduce lookup times - **TTL (Time To Live)**: Each DNS record has a TTL value specifying how long it can be cached before requiring refresh ### Common DNS record types | Record Type | Purpose | |-------------|---------| | **A record** | Maps a domain name to an IPv4 address | | **AAAA record** | Maps a domain name to an IPv6 address | | **CNAME record** | Creates an alias from one domain name to another | | **MX record** | Specifies mail servers for handling email | | **NS record** | Identifies authoritative name servers for a domain | ## Worked example **Question**: A student wants to access their school's website by typing the URL into a browser. Explain the role that DNS plays in loading the website onto the student's screen. **Marks**: 4 ### How to approach this question - For 4 marks, you need to explain the DNS lookup process rather than just stating what DNS does - Focus on the sequence of queries between different DNS servers - Include details about caching and the hierarchical structure of DNS ### Example answer that would achieve full marks The DNS (Domain Name System) is responsible for finding the IP address associated with the school website's domain name. When the student enters the URL, their browser sends a query to a DNS resolver, which first checks its cache to see if it has previously looked up this domain. If not, the resolver contacts a root name server, which directs it to the appropriate top-level domain (TLD) server based on the domain extension (like `.edu` for the school). The TLD server then provides the resolver with the address of the authoritative name server for that specific school domain. Finally, the resolver queries this authoritative server, which returns the actual IP address of the school's web server, allowing the browser to request and load the website.