Technical Analysis of China's Great Firewall Unconditional Port 443 Block on August 20, 2025
By
kotri
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Summary
This technical report documents a significant internet disruption event where China's Great Firewall (GFW) implemented an unconditional block on TCP port 443 for approximately 74 minutes on August 20, 2025. The GFW injected forged TCP RST+ACK packets to disrupt all connections on this port, causing widespread internet connectivity issues between China and the rest of the world. The report provides detailed measurements and analysis of this temporary but extensive blocking event, examining the technical mechanisms and impact of the GFW's anomalous behavior.
Key quotes
· 5 pulledBetween approximately 00:34 and 01:48 (Beijing Time, UTC+8) on August 20, 2025, the Great Firewall of China (GFW) exhibited anomalous behavior by unconditionally injecting forged TCP RST+ACK packets to disrupt all connections on TCP port 443
This incident caused massive disruption of the Internet connections between China and the rest of the world
The unconditional RST+ACK injection affected all connections regardless of content or destination
Our measurements show this was a temporary but widespread blocking event affecting the entire Chinese internet infrastructure
This report documents our measurements and analysis of this temporary, widespread blocking event
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