In NetFlow records, which flags indicate that an HTTP connection was stopped by a security appliance, such as a firewall, before it could be fully established?

The choice indicating that an HTTP connection was stopped by a security appliance, such as a firewall, is the RST flag. The RST (Reset) flag is used in TCP communication to indicate that a connection should be immediately terminated. When a security appliance, like a firewall, intervenes and prevents a connection from being fully established, it can send a TCP reset packet to the sender. This effectively tells the sender that the connection cannot be completed, thereby stopping any further communication between the two endpoints.

In the context of TCP handshakes, each of the other flags—ACK, SYN ACK, and PSH, ACK—plays a role in the normal establishment and management of connections. The ACK flag indicates acknowledgment of receipt of a packet, while the SYN ACK flag signals that a response is received during the connection establishment phase. The PSH, ACK flag is used to indicate that the sender has pushed data to the receiving end, suggesting that data transfer is in progress. However, none of these flags imply an immediate termination of a connection like the RST flag does. Thus, the identification of the RST flag as the indicator of a connection being halted by a security appliance is correct and is crucial in packet analysis for security-related assessments.