How to build a CDN (content distribution network) on a high-bandwidth VPS requires first understanding the specific core architecture. A complete CDN system consists of the following key parts: edge nodes, load balancers, source servers, and monitoring systems. Edge nodes are responsible for caching and distributing content, and are usually deployed on VPS servers around the world; load balancers intelligently distribute requests based on user geographic location and node load conditions; source servers store original content;
　　The core value of CDN usually lies in the geographical location edge nodes caching content, allowing users to obtain data from near to far nodes. High-bandwidth VPS is particularly suitable as a CDN node because it usually provides a network bandwidth of more than 1Gbps and the cost is lower than that of professional CDN services. Before implementation, at least three high-bandwidth VPSs with different nodes need to be prepared. The recommended configuration is 4-core CPU, 8GB memory, 500G SSD storage and 1Gbps bandwidth. Such a configuration can support an average daily traffic distribution of 10TB.
　　Network architecture design is the key to success. It is recommended to adopt a three-tier architecture: edge nodes are responsible for directly serving user requests, regional center nodes coordinate multiple edge nodes, and source stations store original content. Edge nodes synchronize cache data through a dedicated network. This design can ensure performance and effectively control costs. In actual deployment, Nginx or Apache Traffic Server can be used as cache servers. They both have perfect cache functions and rich module support.  
　　When configuring Nginx as an edge node, special attention should be paid to cache strategy optimization.

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=CDN_CACHE:100m inactive=7d max_size=50g use_temp_path=off;
server {
    listen 80;
    server_name cdn.example.com;
    
    location / {
        proxy_pass http://origin_server;
        proxy_cache CDN_CACHE;
        proxy_cache_valid 200 302 12h;
        proxy_cache_use_stale error timeout updating;
        add_header X-Cache-Status $upstream_cache_status;
    }
}

　　The cache policy should be set according to the content type. Static resources (such as images, CSS, JS) can be cached for a long time, 7-30 days is recommended; dynamic content requires a shorter cache time or real-time verification. By setting the Cache-Control header and Expires header reasonably, the cache behavior can be precisely controlled. The ETag and Last-Modified mechanisms should also be enabled to support conditional requests and reduce unnecessary data transmission.
　　Intelligent routing systems are critical to CDN performance. The simplest routing can be achieved through geographic resolution at the DNS level, but more sophisticated control requires the use of Anycast or dynamic routing based on performance detection. The following is an example of implementing geographic DNS through BIND;

$TTL 300
@ IN SOA ns1.example.com. admin.example.com. (2023070101 3600 900 604800 300)
@ IN NS ns1.example.com.
; Geographic routing records
www IN A 192.0.2.1 ; North American users
www IN A 203.0.113.2 ; Asian Users
www IN A 198.51.100.3 ; European Users

　　For services with dynamic content or that require session persistence, a load balancer can be deployed to distribute requests at the front end. Nginx's upstream module combined with health checks can build a robust load balancing system;

upstream backend {
    zone backend 64k;
    server edge1.example.com:80 weight=5;
    server edge2.example.com:80 weight=3;
    server edge3.example.com:80 weight=2;
    
    sticky cookie srv_id expires=1h domain=.example.com path=/;
}

server {
    location / {
        proxy_pass http://backend;
        health_check interval=5s uri=/healthcheck.html;
    }
}

　　Cache synchronization is a key challenge to ensure content consistency. For small CDNs, you can use rsync to synchronize regularly; large-scale deployments require more professional solutions, such as using a self-built P2P synchronization network or commercial synchronization tools.

#!/bin/bash
RSYNC_OPTS="-az --delete --exclude='*.tmp'"
LOG_FILE="/var/log/cdn_sync.log"
for EDGE_NODE in edge2 edge3 edge4; do
    rsync $RSYNC_OPTS /var/cache/nginx/ $EDGE_NODE:/var/cache/nginx/ >> $LOG_FILE 2>&1
done

　　Security protection cannot be ignored. Basic protection includes firewall rule configuration, rate limiting, and basic WAF rules. The following iptables rules can prevent common attacks:

iptables -N CDN_FILTER
iptables -A INPUT -p tcp --dport 80 -j CDN_FILTER
iptables -A CDN_FILTER -m connlimit --connlimit-above 100 -j DROP
iptables -A CDN_FILTER -m recent --name BAD_ACTORS --update --seconds 60 --hitcount 20 -j DROP

　　The monitoring system is the "eyes" of operation and maintenance. It is recommended to use Prometheus to collect indicators, Grafana to display data, and Alertmanager to process alarms. The following is the node monitoring configuration of Prometheus;

scrape_configs:
  - job_name: 'cdn_nodes'
    metrics_path: '/nginx_status'
    static_configs:
      - targets: ['edge1.example.com:9113', 'edge2.example.com:9113']
    relabel_configs:
      - source_labels: [__address__]
        target_label: region
        regex: 'edge(.+)\.example\.com'
        replacement: '$1'

　　Cost optimization requires continuous attention. By analyzing access logs, you can identify hot content and cache it first; adjusting the cache strategy can improve the hit rate; intelligent bandwidth allocation can avoid resource waste. The following Python script can analyze logs and generate optimization suggestions;

import pandas as pd
from collections import Counter
logs = pd.read_csv('/var/log/nginx/access.log', sep=' ', header=None)
top_files = Counter(logs[6]).most_common(100)
print("Recommended files to be cached first：")
for file, count in top_files:
    print(f"{file}: {count}Visits")

　　Performance tuning is a never-ending process. Kernel parameter tuning, TCP stack optimization, and file system selection will all affect the final performance. The following sysctl tuning is worth trying;

echo 'net.core.rmem_max=4194304' >> /etc/sysctl.conf
echo 'net.core.wmem_max=4194304' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_rmem="4096 87380 4194304"' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_wmem="4096 65536 4194304"' >> /etc/sysctl.conf
sysctl -p

　　一个成熟的自建CDN系统应该具备以下特征：95%以上的缓存命中率、平均响应时间低于50ms、支持自动扩展、具备完善的监控和告警机制。通过持续优化和调整，在大带宽VPS上搭建的CDN完全能够媲美商业CDN服务的性能，同时节省大量成本。