The “Great Firewall” is the nickname for a group of laws, regulations, and technology used by the Chinese government to regulate internet usage and the reason you need a VPN when you travel to China. Some of the techniques used include:
Blocking VPNs and VPN websites — any VPN that isn’t government-approved is restricted from use, including access to the VPN website.
Deep Packet Inspection (DPI) — identifies data packets with VPN encryption and blocks them from accessing the internet.
URL Filtering — restricts specific URLs and blocks websites that contain specific unapproved keywords.
DNS Tampering — intercepts DNS requests for specific websites and blocks them.
China’s government blocks non-approved VPNs to maintain a high level of online restriction for its citizens. The Chinese government considers a lot of online content, like pornography and gambling, to be detrimental to the well-being of the country.
China’s regime also prohibits access to content that speaks poorly about the government and the country, which a VPN would allow access to. This means using one of the VPNs listed above is prohibited in China.