Why Does a TCP Connection Send RST Packets? Deep Dive into Causes and Analysis

Why are there RST packets in a TCP connection?

TCP RST packets are TCP segments with the RST flag set to 1. According to RFC 793, they are used to terminate an existing connection or reject a connection request. Either endpoint can send an RST to immediately reset the connection.

Analyzing RST packets during the connection establishment phase

During the three‑way handshake, a server may send an RST to refuse the connection. Common reasons include:

Server port not open : No service listening on the target port.

Server TCP connection limit reached : When the backlog is full and tcp_abort_on_overflow=1 is set.

TIME_WAIT state : A socket still in TIME_WAIT cannot accept a new connection.

Malformed SYN packet : SYN carries illegal flags such as FIN or URG.

Firewall policy : Client IP is blocked by an ACL.

When such RSTs appear, the issue should be investigated from the server side.

RST packets from the client side

A client can also send RST after receiving SYN/ACK or even after the handshake completes, indicating the client rejects the connection. These RSTs often accompany port‑scan activities.

RST packets during data transfer

Both sides may emit RST when the connection encounters errors. Typical causes are:

Excessive retransmissions : Repeated retransmits without acknowledgment lead to an RST.

Long idle period : No data exchange for a timeout period (e.g., 120 seconds) causes one side to reset the connection.

RST packets generated by intermediate security devices

When an ACL or security policy matches and the action is Reject, or when a bypass device cannot drop traffic, the device may send an RST to terminate the session. Such “bypass‑block” RSTs can be distinguished by their flag composition and TTL differences.

Identifying “real” vs. “fake” RST packets

True RST packets carry both RST and ACK flags, while fake RSTs often carry only the RST flag. Comparing the IP TTL of RST and SYN/ACK packets can reveal different network locations, helping to pinpoint middlebox involvement.

RST packets in the connection termination phase

During graceful teardown, a device may send an RST to accelerate closure, bypassing the normal FIN/ACK and TIME_WAIT sequence. This fast‑close method is non‑standard but common in load balancers and firewalls.

Understanding the timing and reasons for RST packets is essential for maintaining network stability and security.