From www.google.com to 172.217.5.253: The Magic of DNS

How DNS actually works?

1. Introduction to DNS

• Purpose of DNS: DNS (Domain Name System) translates human-readable domain names (e.g., www.google.com) into IP addresses (e.g., 172.217.5.253) that computers use to communicate over the internet.
• Role of DNS in Browser Requests: When you type www.google.com in your browser, the browser doesn’t directly use the domain name to establish a TCP connection. Instead, it uses the IP address associated with that domain name.

2. DNS Records and Zones

• DNS Records: These are key-value pairs that map domain names to IP addresses or other resources.
  • A Record: Maps a domain name to an IPv4 address (e.g., www.google.com -> 172.217.5.253).
  • CNAME Record: Maps a domain name to another domain name (e.g., www.google.com -> cname.google.com).
  • MX Record: Specifies mail servers responsible for accepting email messages on behalf of a domain.
  • TXT Record: Used for text data, often for SPF, DKIM, etc.
• DNS Zone: A DNS zone contains the DNS records for a specific domain (e.g., google.com). It is managed by an authoritative name server.
• Hosted Zone: In services like AWS Route 53, a hosted zone is a collection of DNS records for a specific domain.

3. Authoritative Name Servers

• Role: Authoritative name servers are responsible for storing and providing DNS records for a specific zone.
• Example: If ns1.google.com is an authoritative name server for google.com, it will provide the IP address for www.google.com when queried.
• Multiple Name Servers: Domains typically have multiple authoritative name servers for redundancy and fault tolerance.

4. DNS Resolvers

• Role: DNS resolvers are responsible for initiating and managing the DNS query process on behalf of the client (e.g., your browser).
• Location: DNS resolvers can be located at the ISP level, on your router, or even on your local machine.
• Popular Public DNS Resolvers:
  • Google DNS: 8.8.8.8 and 8.8.4.4
  • Cloudflare DNS: 1.1.1.1
• Caching: DNS resolvers cache DNS records to improve resolution speed and reduce load on authoritative name servers.

5. DNS Resolution Process

• Step-by-Step Resolution:
  1. Client Request: Your browser sends a DNS query to the DNS resolver (e.g., your router).
  2. DNS Resolver Checks Cache: If the IP address for www.google.com is cached, it returns it immediately.
  3. Root Name Servers: If not cached, the resolver queries one of the 13 root name servers (e.g., a.root-servers.net).
     • Anycast: Root name servers use anycast to distribute queries across multiple physical servers with the same IP address.
  4. TLD Name Servers: The root name server responds with the IP address of a TLD (Top-Level Domain) name server for .com.
  5. Authoritative Name Servers: The TLD name server responds with the IP address of an authoritative name server for google.com.
  6. DNS Record Retrieval: The authoritative name server for google.com provides the IP address for www.google.com.
  7. Resolver Caches and Returns IP: The resolver caches the IP address and returns it to the client.
  8. TCP Connection Establishment: The browser uses the IP address to establish a TCP connection with the server.

6. Hierarchical Structure of DNS

• Root Name Servers: 13 logically defined root name servers that form the root of the DNS hierarchy.
• TLD Name Servers: Handle domains within a specific TLD (e.g., .com, .org, .net).
• Authoritative Name Servers: Handle domains within a specific zone (e.g., google.com).

7. Anycast in DNS

• Definition: Anycast is a networking technique where a single IP address is shared among multiple servers in different locations.
• Benefits:
  • Load Distribution: Queries are distributed to the nearest server.
  • Fault Tolerance: If one server fails, queries are routed to another server with the same IP address.

8. Caching in DNS

• Resolver Caching: DNS resolvers cache DNS records for a certain period (TTL - Time to Live).
• Client Caching: Clients (e.g., browsers) also cache DNS records to reduce the number of DNS queries.
• TTL: The time a DNS record is cached before it needs to be refreshed.

9. Example DNS Resolution

• Scenario: Resolving www.google.com to an IP address.
• Process:
  1. Client Query: Browser requests www.google.com.
  2. Resolver Query: Resolver queries root name server.
  3. Root Name Server Response: Points to .com TLD name server.
  4. TLD Name Server Response: Points to ns1.google.com (authoritative name server for google.com).
  5. Authoritative Name Server Response: Provides IP address for www.google.com.
  6. Resolver Caches IP: Resolver caches the IP address and returns it to the client.
  7. TCP Connection: Browser connects to the IP address.

Conclusion

The DNS resolution process is a critical component of the internet, enabling human-readable domain names to be translated into machine-readable IP addresses. Understanding the roles of DNS resolvers, authoritative name servers, and the hierarchical structure of DNS is essential for managing and troubleshooting DNS-related issues. The use of anycast and caching mechanisms ensures that DNS resolution is both efficient and scalable.

for more resources :

• https://www.cloudflare.com/learning/dns/what-is-dns/
• https://www.geeksforgeeks.org/working-of-domain-name-system-dns-server/
• https://aws.amazon.com/route53/what-is-dns/
• https://howdns.works/
• https://www.youtube.com/watch?v=g_gKI2HCElk thanks arpit! :)