Understanding DNS, Intelligent DNS, EDNS, and HTTP-DNS in Qunar Architecture

What is DNS – DNS (Domain Name System) translates human‑readable domain names into IP addresses, similar to how phone numbers map to contacts.

DNS Resolution Process – A user’s request is sent to the locally configured DNS server (local DNS), which performs a series of recursive queries to external DNS servers and finally returns the resolved IP address to the user.

Qunar‑DNS Structure – The architecture consists of DNSdb (management system), dnsmaster (master‑slave synchronization to local DNS and nameserver), DNSmasq (high‑performance cache), LocalDNS (in‑datacenter recursive resolver), and Nameserver (provides intelligent DNS, EDNS, and HTTP‑DNS services).

Intelligent DNS – By returning multiple possible answers for the same domain, intelligent DNS solves problems such as uneven access speed, traffic imbalance across data centers, and single‑point failures.

Automatic Failover – When a data‑center’s outbound link fails, the system automatically switches the domain to another data‑center without affecting users, achieving seamless failover.

EDNS (Extension DNS) – EDNS adds extra fields to DNS packets, allowing the system to distinguish domestic from overseas requests, which reduces unnecessary overseas CDN traffic.

HTTP‑DNS – HTTP‑DNS encapsulates DNS queries inside HTTP requests, requiring client support; it helps avoid DNS hijacking. Qunar’s HTTP‑DNS flow forwards requests from the front‑end cluster to the intelligent DNS layer, which then processes and returns the response, falling back to local DNS if needed.

Summary – DNS translates domain names to IPs; intelligent DNS provides optimal, independent resolution and automatic failover; EDNS ensures domestic users stay domestic, saving CDN bandwidth; HTTP‑DNS prevents DNS hijacking.