Definition
The Great Firewall (officially part of the 'Golden Shield Project') is the technical apparatus China uses to control internet traffic between mainland China and the rest of the world. It combines DNS spoofing of foreign domains, IP blocking of services like Google, Facebook, and Twitter, deep packet inspection that identifies and blocks VPN handshakes, and active probing — where the firewall, on detecting suspected VPN traffic, sends crafted packets to the suspected server and blocks it if it responds in characteristic ways.
Why It Is Hard for VPNs
Most consumer VPN protocols (OpenVPN, WireGuard, IKEv2) are easily fingerprinted by DPI and blocked outright. The Great Firewall maintains and updates its own database of VPN server IPs and protocol signatures, often within hours of new servers coming online. To work in China, a VPN must use obfuscation that disguises its traffic as ordinary HTTPS or an unrecognizable protocol, run servers on IP ranges not yet known to the firewall, and update both regularly.
Working Approaches
Protocols and tools that have had at least intermittent success include Shadowsocks, V2Ray, Trojan, Hysteria, Cloak, and Stealth/Camouflage modes from major VPN providers. None work permanently; all are subject to rapid blocking. Ahead of major political events or anniversaries, the Great Firewall typically tightens further, and VPN reliability inside China drops sharply.