System Design

Domain name system


Source: DNS security presentation

A Domain Name System (DNS) translates a domain name such as to an IP address.

DNS is hierarchical, with a few authoritative servers at the top level. Your router or ISP provides information about which DNS server(s) to contact when doing a lookup. Lower level DNS servers cache mappings, which could become stale due to DNS propagation delays. DNS results can also be cached by your browser or OS for a certain period of time, determined by the time to live (TTL).

  • NS record (name server) – Specifies the DNS servers for your domain/subdomain.
  • MX record (mail exchange) – Specifies the mail servers for accepting messages.
  • A record (address) – Points a name to an IP address.
  • CNAME (canonical) – Points a name to another name or CNAME ( to or to an Arecord.

Services such as CloudFlare and Route 53 provide managed DNS services. Some DNS services can route traffic through various methods:

  • Weighted round robin
    • Prevent traffic from going to servers under maintenance
    • Balance between varying cluster sizes
    • A/B testing
  • Latency-based
  • Geolocation-based

Disadvantage(s): DNS

  • Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
  • DNS server management could be complex, although they are generally managed by governments, ISPs, and large companies.
  • DNS services have recently come under DDoS attack, preventing users from accessing websites such as Twitter without knowing Twitter’s IP address(es).

Source(s) and further reading

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