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System Design

Domain Name System (DNS)

A guide to how domain names are translated into IP addresses, a fundamental process of the internet.

Table of Contents

What is DNS?

The Domain Name System (DNS) is the phonebook of the internet. It translates human-readable domain names (e.g., www.google.com) into machine-readable IP addresses (e.g., 172.217.12.142). This allows browsers and other applications to locate and connect to the correct servers on the internet.

How DNS Works: The Resolution Process

When you type a domain name into your browser, a series of steps occur to find the corresponding IP address:

  1. Browser/OS Cache: Your browser and operating system first check their own caches to see if they already have the IP address for the requested domain.
  2. Recursive Resolver: If the IP is not in the local cache, the request is sent to a recursive resolver (usually provided by your ISP). The resolver’s job is to find the IP address on your behalf.
  3. Root Nameserver: The resolver queries a root nameserver, which directs it to the appropriate Top-Level Domain (TLD) nameserver (e.g., the .com nameserver).
  4. TLD Nameserver: The TLD nameserver responds with the address of the authoritative nameserver for the requested domain.
  5. Authoritative Nameserver: This is the final authority for the domain. It holds the actual DNS records and returns the IP address to the recursive resolver.
  6. Resolver to Client: The recursive resolver sends the IP address back to your browser, which can now establish a connection with the server.

DNS Hierarchy and Zones

The DNS database is organized in a hierarchical tree-like structure called the domain namespace. A zone is a portion of this namespace that is managed by a specific organization or administrator. Zone files, which contain the DNS records for a zone, are maintained on DNS servers.

  • Primary Server: The main server for a zone, where all updates are made.
  • Secondary Server: A read-only copy of a primary zone, used for redundancy and load balancing.

DNS Query Types

Recursive Query

In a recursive query, the DNS client requires the server to provide a complete answer to the query, either by returning the requested IP address or an error message. The server will do all the work of contacting other nameservers to find the answer.

Note: This is the type of query a client (e.g., your computer) sends to a recursive resolver.

Iterative Query

In an iterative query, the DNS server is not required to provide a complete answer. If the server doesn’t know the answer, it will return a referral to another nameserver that might know. The client is then responsible for querying that server.

Note: This is the type of query that DNS servers use to communicate with each other.

Common DNS Record Types

RecordNameDescription
AAddressPoints a domain or subdomain to an IPv4 address.
AAAAIPv6 AddressPoints a domain or subdomain to an IPv6 address.
CNAMECanonical NameMaps one domain name to another. Useful for aliasing.
MXMail ExchangeSpecifies the mail servers for accepting email messages.
NSName ServerSpecifies the authoritative nameservers for a domain.

DNS-Based Routing Policies

DNS can be used for more than just simple domain-to-IP mapping. Advanced DNS services can route traffic based on various policies:

  • Weighted Round Robin: Distributes traffic across multiple servers based on assigned weights. Useful for A/B testing, blue-green deployments, and balancing load between clusters of different sizes.
  • Latency-Based Routing: Routes users to the server with the lowest latency for them. This is determined by measuring the network distance between the user and different server locations.
  • Geolocation-Based Routing: Routes users based on their geographic location. Useful for localizing content, restricting content distribution, and ensuring users are consistently routed to the same endpoint.

DNS Caching

To improve performance and reduce the load on DNS servers, DNS responses are cached at various levels:

  • Browser Cache: The browser itself caches DNS records for a short period.
  • Operating System Cache: The OS maintains its own cache of DNS records.
  • Recursive Resolver Cache: ISPs and other resolver services cache DNS records to speed up responses for their users.

Trade-off: Caching improves performance, but it can also lead to stale data if a DNS record is updated before the cache expires (due to propagation delays).

DNS Security Considerations

DNS is a critical part of the internet infrastructure, making it a target for attacks.

  • DDoS Attacks: DNS servers are often targeted by Distributed Denial-of-Service (DDoS) attacks, which can make websites and services unavailable.
  • DNS Spoofing/Cache Poisoning: An attacker can corrupt a DNS cache, causing it to return an incorrect IP address and redirecting users to a malicious site.
  • Amazon Route 53: A highly available and scalable cloud DNS web service from AWS.
  • Cloudflare DNS: A free, fast, and privacy-focused DNS service.

Summary

DNS is a fundamental and often overlooked component of the internet. It not only translates domain names into IP addresses but also plays a crucial role in routing traffic, improving performance, and ensuring the availability of web services. Understanding the DNS resolution process, record types, and routing policies is essential for anyone involved in system design or web development.