Domain Name System (DNS)

DNS is the phonebook of the internet. It translates human-readable domain names (like www.example.com) into IP addresses that computers use to identify each other.

How DNS Works

When you type a URL in your browser:

Browser Cache: First checks if the domain was recently resolved
OS Cache: Checks the operating system's DNS cache
Resolver: Contacts your ISP's DNS resolver
Root Server: If not cached, queries the root DNS server
TLD Server: Directs to Top-Level Domain server (.com, .org, etc.)
Authoritative Server: Returns the final IP address

DNS Record Types

Type	Purpose	Example
A	Maps domain to IPv4	`example.com → 93.184.216.34`
AAAA	Maps domain to IPv6	`example.com → 2606:2800:220:1:...`
CNAME	Alias to another domain	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com`
NS	Name server	`example.com → ns1.example.com`
TXT	Text records	SPF, DKIM verification

DNS Caching

DNS responses are cached at multiple levels to improve performance:

Code

interface DNSCache {
  ttl: number;        // Time-to-live in seconds
  record: DNSRecord;
  timestamp: Date;
}
 
// Typical TTL values
const TTL_VALUES = {
  short: 300,      // 5 minutes - frequently changing
  medium: 3600,    // 1 hour - standard
  long: 86400,     // 24 hours - stable records
};

DNS Load Balancing

DNS can distribute traffic across multiple servers:

Round Robin

Returns different IP addresses in rotation:

Query 1: example.com → 192.168.1.1
Query 2: example.com → 192.168.1.2
Query 3: example.com → 192.168.1.3

Geolocation-based

Returns IP based on user's location for lower latency.

Weighted

Assigns weights to different servers based on capacity.

DNS Security

Common Attacks

DNS Spoofing: Returning fake IP addresses
DNS Amplification: DDoS using DNS responses
Cache Poisoning: Injecting malicious records

Protection Mechanisms

DNSSEC: Cryptographic signatures for DNS records
DNS over HTTPS (DoH): Encrypts DNS queries
DNS over TLS (DoT): TLS encryption for DNS

Best Practices

Use multiple DNS providers for redundancy
Set appropriate TTLs based on change frequency
Implement DNSSEC for security
Monitor DNS performance and availability
Use anycast DNS for global distribution