Understanding TCP Headers With FIN Flag Set and Without ACK Flag Set

Figure 61 shows TCP segments with the FIN control flag set (to signal the conclusion of a session and terminate the connection). Normally, TCP segments with the FIN flag set also have the ACK flag set (to acknowledge the previous packet received). Because a TCP header with the FIN flag set but not the ACK flag is anomalous TCP behavior, there is no uniform response to this. The OS might respond by sending a TCP segment with the RST flag set. Another might completely ignore it. The victim's response can provide the attacker with a clue as to its OS. (Other purposes for sending a TCP segment with the FIN flag set are to evade detection while performing address and port scans and to evade defenses on guard for a SYN flood by performing a FIN flood instead.)

Note: Vendors have interpreted RFC 793, Transmission Control Protocol, variously when designing their TCP/IP implementations. When a TCP segment arrives with the FIN flag set but not the ACK flag, some implementations send RST segments, while others drop the packet without sending an RST.

When you enable this screen option, Junos OS checks if the FIN flag is set but not the ACK flag in TCP headers. If it discovers a packet with such a header, it drops the packet.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices