"The Great Chinese Firewall" — The large-scale Internet censorship system implemented by Beijing seems to have discovered a problem. According to The Register, this led to the country's disconnection from most of the world's Internet.
The glitch was observed by an activist group called the Great Firewall Report, which claims that the glitch affected all traffic on TCP port 443, the standard port used for Internet connections.
"Approximately between 00:34 and 01:48 (Beijing time) on August 20, 2025, the Great Chinese Firewall demonstrated abnormal behavior, unconditionally introducing fake TCP RST+ACK packets to interrupt all connections on TCP port 443," the group wrote in a published message.
In computer networks, communication via TCP (Transmission Control Protocol) is carried out through packets containing a "header" with various "flags" (check marks) that indicate the connection status. Two of these flags are: RST (Reset), signaling the need to close the connection immediately. It is used in case of problems (for example, if the server rejects the connection); ACK (Acknowledgement) — confirms receipt of data.
Almost all packets after the connection is established contain an ACK. Sending TCP RST+ACK packets is a way to suddenly drop the connection, and firewalls or internet censorship use this method to block traffic. The failure of the Internet has led to the unavailability of not only websites outside of China. Due to the outage, Chinese internet users were unable to access most websites hosted outside of China.
The incident also blocked other services using port 443, as many services need to interact with servers or information sources outside of China for operational reasons. For example, Apple and Tesla use this port to connect to external servers that provide some of their main services.
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