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Networking Fundamentals · Intermediate

RST Flag

The TCP reset flag that immediately aborts a connection or rejects a connection attempt to a closed port.

Definition

A host sends RST when it receives a segment for a port with no listening application or for a connection that does not exist. Firewalls can also send RSTs to reject traffic actively. No acknowledgment of a RST is sent.

Example

Connecting to TCP port 81 on a server with nothing listening returns a RST, shown as "connection refused."

Exam tip

"Connection refused" means an RST came back; "connection timed out" usually means packets were dropped.

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