Figure 32 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. For information about FIN scans, see FIN Scan.)
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. Some drop the packet without sending an RST.
Figure 32: TCP Header with FIN Flag Set
When you enable this SCREEN option, JUNOS software with enhanced services 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.