In order to see why, we’ll need to understand how the TCP protocol works. However, since TCP reset packets are part of the TCP protocol itself, they cannot be validated using these higher-level protocols.ĭespite the ease with which spoofed segments can be sent, crafting the right spoofed segment and executing a successful TCP reset attack can still be challenging. All a receiver can do is to take the source IP address and port inside a packet or segment at face value, and where possible use higher-level protocols, such as TLS, to verify the sender’s identity. Internet service providers are supposed to refuse to transit IP packets that claim to have come from a clearly-spoofed IP address, but such verification is anecdotally said to be patchy. There is an extension to IP which does provide authentication, called IPSec. Sending spoofed TCP segments is in one sense easy, since neither TCP nor IP comes with any built-in way to verify a sender’s identity. Short-lived connections, for example those used to transmit small webpages, will typically have already fulfilled their purpose by the time an attacker is able to attempt to reset them. Fortunately, because it takes the attacker time to assemble and send their spoofed packet, reset attacks are only really effective against long-lived connections. The victims can create a new TCP connection in an attempt to resume their communications, but the attacker may be able to reset this new connection too. If a fake reset segment is crafted correctly, the receiver will accept it as valid and close their side of the connection, preventing the connection from being used to exchange further information. In normal, non-nefarious operations, computers send TCP reset segments whenever they receive unexpected TCP traffic and they want its sender to stop sending it.Ī TCP reset attack exploits this mechanism to trick victims into prematurely closing TCP connections by sending them fake reset segments. These messages are called TCP reset segments. In a TCP reset attack, an attacker kills a connection between two victims by sending one or both of them fake messages telling them to stop using the connection immediately. One such tool that they use is the TCP reset attack. To do this, the GFW needs tools that are capable of killing already-established connections. For example, they may want to generally allow traffic to a news website, but to censor specific videos containing banned keywords. Or it could be because they want to analyze the data exchanged over a connection and use this information to decide whether to allow or block it. This could be because they want to perform slow, out-of-band analysis on the connection, such as correlating it with other activity. However, the GFW may sometimes also want to allow a connection to be made, but to then kill it halfway through. To prevent users from even connecting to forbidden servers, the GFW uses techniques like DNS pollution and IP blocking (both stories for another time). ![]() The GFW actively blocks and kills connections to servers inside and outside of the country, as well as passively monitoring internet traffic for proscribed content. The Great Firewall (GFW) is a collection of systems and techniques used by the Chinese government to censor the internet for users inside China. How is the TCP reset attack used in the Great Firewall?
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