Why Do TCP Connections Send RST Packets? Causes and Analysis Explained

What Is a TCP RST Packet?

According to RFC 793, a TCP RST packet is a segment with the RST flag set to 1, used to terminate an existing connection or reject a connection request when a TCP endpoint wants to reset the session immediately.

RST Packets in the Connection‑Establishment Phase

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

Server port not open : No service is listening on the requested port.

TCP connection limit reached : If tcp_abort_on_overflow=1 is set, the server sends RST when its connection queue is full.

TIME_WAIT state : A socket still in TIME_WAIT may cause a new connection attempt to be rejected with RST.

Malformed SYN packet : SYN carrying illegal flags (e.g., FIN, URG) triggers an immediate RST.

Firewall policy : ACLs that block the client IP result in RST packets.

When an RST appears at this stage, traffic should be examined on the server side to identify the root cause.

Client‑Initiated RST Packets

After the SYN/ACK exchange or even after the handshake completes, a client may send an RST, indicating a refusal or a port‑scan attempt. Such traffic typically reflects abnormal client behavior or active scanning.

RST Packets During Data Transfer

In the data‑transfer phase, either side can send RST when the connection encounters errors. Typical causes are:

Retransmission limit exceeded : Repeated retransmissions without acknowledgment lead the sender to reset the connection.

Long idle period : No data exchanged for a prolonged time (e.g., 120 seconds) may cause a timeout and an RST, possibly generated by load balancers or firewalls.

Analyzing the timing diagram helps determine whether the reset is due to an error or a timeout.

RST Packets Generated by Firewalls

When an ACL or security policy matches and the action is “Reject,” or when a bypass‑deployed security device cannot drop traffic directly, the device may send an RST to terminate the session. These RSTs are often “fake” RSTs that lack the ACK flag and may have a different IP TTL compared to the genuine server packets.

Distinguishing “True” and “Fake” RST Packets

True RST packets carry both RST and ACK flags, while fake RSTs carry only the RST flag. Comparing the IP TTL of RST and SYN/ACK packets can reveal different network origins, indicating middle‑box interference.

RST Packets in the Connection‑Termination Phase

During graceful termination, a FIN exchange is expected. However, load balancers or security devices may send an RST after receiving a FIN to close the session quickly, bypassing the TIME_WAIT state. This practice speeds up teardown but violates the TCP standard.

Conclusion

RST packets can appear at any stage of a TCP session—establishment, data transfer, security‑policy enforcement, or termination. Understanding the reasons behind each RST, examining sequence diagrams, flag combinations, and TTL values, enables effective network troubleshooting, performance optimization, and security hardening.