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Analysis of the GFW's Unconditional Port 443 Block on August 20, 2025
The Great Firewall of China (GFW) conducted a large-scale, unconditional block targeting TCP port 443 on August 20, 2025. This report documents the measurements and analysis we conducted of that event.
Previous research has established that different components, such as those responsible for HTTP Host-based and TLS SNI-based filtering, exhibit unique packet-level fingerprints when injecting TCP RST packets. We used the same destination IP address so that our probe packets would be more likely to traverse the same network path and interact with the same set of censorship middleboxes, allowing for a consistent fingerprint analysis. IP FlagIP IDIP TTLTCP Relative Sequence NumberTCP FlagsTCP Window SizeDon’t Fragment40305 (0x9D71)961RST+ACK2072Don’t Fragment39808 (0x9B80)971RST+ACK2073Don’t Fragment38891 (0x97E3)981RST+ACK2074 Table 1: Characteristics of Unconditionally Injected TCP RST Packets
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