A Wall Behind A Wall: Emerging Regional Censorship in Chinacore

Since August 2023, Henan Province has operated its own TLS SNI-based and HTTP Host-based censorship middleboxes that inspect and block traffic exiting the province—a second filtering layer on top of the national GFW. The Henan Firewall is fingerprinted by a unique TCP RST+ACK injection carrying a fixed 10-byte payload (0x01 02 03 04 05 06 07 08 09 00), IP ID 0x0001, and an observed TTL of 58. Unlike the GFW, it injects resets only toward the client, performs no residual censorship, and requires no TCP handshake to trigger. Longitudinal testing (Nov 2023–Mar 2025, Tranco top 1M daily + 227M CZDS domains weekly) found the Henan Firewall blocked a cumulative 4.2 million domains—more than five times the GFW's cumulative blocklist—and at peak blocked ten times more domains than the GFW.

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The Henan Firewall only inspects traffic leaving Henan Province toward the rest of the world—it does not inspect domestic intra-China traffic nor inbound traffic entering the province. This contrasts with the GFW, which operates bidirectionally at China's national border. Measurement across seven CN cities (Beijing, Shanghai, Chongqing, Guangzhou, Nanjing, Chengdu, Zhengzhou) found no evidence of comparable provincial firewalls in the other six locations, making Henan the only documented province with an autonomous censorship layer as of March 2025. The Henan Firewall also uses the same blocklist for both HTTP Host-based and TLS SNI-based censorship, whereas the GFW maintains separate domain lists per protocol.

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The Henan Firewall is stateless in two exploitable ways: (1) it requires the TCP header to be exactly 20 bytes—enabling any TCP option (e.g., TCP Timestamps, which Windows disables by default) to bypass it entirely; (2) it does not perform TCP reassembly, so splitting a TLS ClientHello across two TCP segments such that the SNI extension straddles the boundary bypasses the censor. Both bypasses require only client-side changes and have already been implemented in Xray, GoodbyeDPI, and Shadowrocket. TLS record fragmentation (splitting the ClientHello across multiple TLS records within one TCP segment) also defeats both the Henan Firewall and the GFW, since neither performs TLS reassembly.

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