How Does DNS Work in CDN Acceleration - 面试题

CDN (Content Delivery Network) accelerates content delivery by deploying edge nodes globally and caching website content on servers closest to users. DNS plays the intelligent scheduling core role in CDN, determining which edge node users access.

How CDN Works

Traditional Access vs CDN Access

Traditional Access:

shell
User (Beijing) → Origin Server (USA) → Returns content
     ↓
  High latency, slow speed

CDN Acceleration:

shell
User (Beijing) → DNS Intelligent Scheduling → CDN Beijing Node → Returns cached content
     ↓
  Low latency, fast speed

Role of DNS in CDN

1. CNAME Integration

Users point their domain to the CNAME address provided by CDN:

dns
; User domain configuration
www.example.com.    600    IN    CNAME    www.example.com.cdn-provider.com.

2. Intelligent DNS Resolution

CDN's DNS server decides which edge node IP to return based on multiple factors:

shell
User queries www.example.com
         ↓
    CNAME to CDN domain
         ↓
    CDN DNS Server
         ↓
    Intelligent Scheduling Algorithm
         ↓
    Returns optimal node IP

CDN DNS Scheduling Strategies

1. Geographic Scheduling

Principle: Return the nearest CDN node based on user's geographic location

shell
Beijing user → Returns Beijing CDN node IP
Shanghai user → Returns Shanghai CDN node IP
Guangzhou user → Returns Guangzhou CDN node IP
US user → Returns US CDN node IP

Implementation:

Determine location based on user DNS query source IP
Use GeoIP database
Return geographically closest node

2. ISP Scheduling

Principle: Return nodes from the same ISP based on user's ISP

shell
China Telecom user → Returns China Telecom line CDN node
China Unicom user → Returns China Unicom line CDN node
China Mobile user → Returns China Mobile line CDN node

Advantages:

Avoid cross-ISP access
Reduce network latency
Improve transmission speed

3. Load Balancing Scheduling

Principle: Allocate users to lower-load nodes based on node load conditions

shell
Node A: 80% load → Reduce allocation
Node B: 50% load → Normal allocation
Node C: 30% load → Priority allocation

Monitoring Metrics:

CPU usage
Bandwidth usage
Concurrent connections
Response time

4. Health Check Scheduling

Principle: Automatically detect node health status and remove faulty nodes

shell
Node A: Healthy ✅ → Normal service
Node B: Faulty ❌ → Automatically removed
Node C: Healthy ✅ → Normal service

Health Check Methods:

HTTP status code check
TCP port check
Response time check

CDN DNS Resolution Process Details

Complete Resolution Process

shell
1. User enters www.example.com
         ↓
2. Local DNS query
         ↓
3. Returns CNAME: www.example.com.cdn-provider.com
         ↓
4. Queries CDN DNS server
         ↓
5. CDN DNS analyzes user source IP
         ↓
6. Applies scheduling strategy (geographic + ISP + load)
         ↓
7. Returns optimal CDN node IP (e.g., 203.0.113.10)
         ↌
8. User accesses CDN node to get content

First Visit vs Cached Visit

First Visit (Not Cached):

shell
User → CDN Node → Origin pull from source → Cache content → Return to user

Cached Visit:

shell
User → CDN Node (directly returns cached content)

CDN DNS Configuration Examples

Scenario 1: Full Site Acceleration

dns
; Main domain CNAME to CDN
www.example.com.    600    IN    CNAME    example.cdn-provider.com.

; Static resources domain CNAME to CDN
static.example.com. 600    IN    CNAME    static.example.cdn-provider.com.

; Images domain CNAME to CDN
img.example.com.    600    IN    CNAME    img.example.cdn-provider.com.

Scenario 2: Dynamic/Static Separation

dns
; Dynamic content (not through CDN)
www.example.com.    600    IN    A        192.0.2.1

; Static content (using CDN)
static.example.com. 600    IN    CNAME    static.example.cdn-provider.com.
css.example.com.    600    IN    CNAME    static.example.cdn-provider.com.
js.example.com.     600    IN    CNAME    static.example.cdn-provider.com.

Scenario 3: Multi-CDN Vendors

dns
; Primary CDN
www.example.com.    600    IN    CNAME    example.cdn1-provider.com.

; Backup CDN (through weight or geographic distribution)
; Some DNS services support intelligent distribution

CDN DNS TTL Settings

Impact of TTL on CDN

TTL Setting	Advantages	Disadvantages
Short TTL (60-300s)	Fast failover, flexible scheduling	Increased DNS queries, origin pressure
Long TTL (3600s+)	Reduced DNS queries, better caching	Slow failover, inflexible scheduling

Recommended Configuration

dns
; Main domain - Medium TTL
www.example.com.    600    IN    CNAME    example.cdn-provider.com.

; Static resources - Longer TTL
static.example.com. 3600   IN    CNAME    static.example.cdn-provider.com.

Common CDN DNS Issues

Issue 1: DNS Cache Causes Scheduling Failure

Symptoms:

Users scheduled to faulty nodes
Some users still access old nodes after node change

Solution:

Lower TTL value
Wait for cache expiration
Use CDN's force refresh feature

Issue 2: Cross-ISP Access Slow

Symptoms:

China Telecom users accessing China Unicom CDN nodes
High latency, slow speed

Solution:

Ensure CDN supports ISP scheduling
Check if DNS configuration is correct
Contact CDN provider for optimization

Issue 3: Origin IP Exposure

Symptoms:

Direct ping to origin domain gets real IP
Attackers bypass CDN to directly attack origin

Solution:

Origin only allows CDN IP range access
Use firewall to restrict direct access
Separate origin domain and access domain

DNS Configuration for Major CDN Providers

Cloudflare

dns
; Change NS records to Cloudflare
example.com.    86400    IN    NS    lara.ns.cloudflare.com.
example.com.    86400    IN    NS    greg.ns.cloudflare.com.

Characteristics:

Takes over entire domain DNS
Automatic CDN acceleration
Supports CNAME Flattening

Alibaba Cloud CDN

dns
; CNAME integration
www.example.com.    600    IN    CNAME    www.example.com.w.kunlunar.com.

Characteristics:

Supports intelligent scheduling (geographic + ISP)
Rich domestic nodes

AWS CloudFront

dns
; CNAME integration
www.example.com.    600    IN    CNAME    d1234abcd5678.cloudfront.net.

Characteristics:

Global node distribution
Integrated with AWS ecosystem

CDN DNS Optimization Tips

1. Use HTTP DNS

Bypass local DNS, directly get optimal node through HTTP:

shell
Client → HTTP DNS Service → Returns optimal CDN IP

Advantages:

Avoid DNS hijacking
More precise scheduling
Real-time updates

2. Preheating and Refreshing

Preheating: Push popular content to CDN nodes before peak hours Refreshing: Actively clear CDN cache, force origin pull

3. Monitoring and Analysis

Monitor node response times
Analyze user geographic distribution
Optimize scheduling strategies

Common Interview Questions

Q: How does CDN know user location?

Through the source IP address of user DNS queries
Use GeoIP database to query IP's corresponding geographic location
Return the nearest CDN node

Q: Why is CDN CNAME record TTL usually set shorter?

Facilitates fast failover
Supports dynamic scheduling strategies
Adapts to node scaling
But increases DNS query volume

Q: If CDN node fails, how does DNS switch?

CDN's health check system detects node failure
Automatically removes the node from DNS resolution results
Users get healthy node IP on next query
Automatically adds back after recovery

Summary

Aspect	Description
Core Function	Intelligently schedule users to optimal CDN nodes
Scheduling Strategies	Geographic location, ISP, load, health status
Integration Method	CNAME points to CDN-provided domain
TTL Settings	Balance scheduling flexibility and DNS query volume
Optimization Directions	HTTP DNS, preheating/refreshing, monitoring/analysis

DNS is the "brain" of CDN. Through intelligent scheduling algorithms, it ensures users always access the optimal edge node, thereby achieving acceleration effects.