Transparent DNS Proxies: How ISPs Hijack 8.8.8.8

You manually configured 1.1.1.1 on your network adapter. You feel secure. But when you visit a blocked site, you still get the ISP landing page. How?

1. The Destination NAT Trick

Sophisticated ISPs configure their routers to intercept ANY packet heading to destination port 53 (UDP), regardless of the destination IP. They rewrite the packet header, redirect it to their own DNS server, get the result, rewrite the source IP back to "1.1.1.1" (spoofing), and send it to you.

Your computer thinks it talked to Cloudflare, but it actually talked to your ISP.

2. The Fix: DoH / DoT

This attack only works on unencrypted UDP. It fails against DNS over HTTPS (DoH) because the ISP cannot see inside the TLS packet to know it is a DNS request, nor can they spoof the SSL certificate of Cloudflare.