A stateful inspection firewall validates packets against an established connection state. The second packet of a TCP handshake (SYN+ACK) is only valid if the firewall has already processed the first packet (SYN) and created a session entry. In the scenario described, no session exists. Therefore, the SYN+ACK packet is considered an out-of-state or "first packet."

By default, a stateful firewall will drop an out-of-state SYN+ACK packet as it violates the TCP state machine. However, the firewall's ultimate action is governed by its configured security policy. Option D is a conditional statement: "If the firewall security policy permits packets to pass, the packets can pass." This is the most accurate statement because it is possible, though not default, to configure a security policy to allow out-of-state packets (e.g., to support asymmetric routing). The statement correctly frames the packet's fate as being contingent upon the policy.

A. A session table is created upon receiving a valid initial packet (TCP SYN) that is permitted by policy, not the second packet (SYN+ACK) when no session exists.

B. This option is irrelevant as it describes a different scenario where stateful detection is disabled, contradicting the premise of the question.

C. This is incorrect. The packet will be dropped under default stateful inspection rules because it is an out-of-state packet. It does not "must pass."

1. Huawei USG6000E & USG9500 Series V600R007C00 Product Documentation

"Configuration Guide - Security Policy": This guide details the packet forwarding process. It specifies that for a "first packet" of a data flow

the firewall checks the security policy. If permitted

a session is created. For subsequent packets

the firewall matches them against the session table. This confirms that a SYN+ACK without a session is treated as a "first packet."

2. Huawei USG6000E & USG9500 Series V600R007C00 Product Documentation

"Configuration Guide - Attack Defense": In the "IP/TCP/UDP Packet Check" section

it is stated that the firewall checks for TCP state machine compliance. "The firewall checks whether the sequence number and flag of a TCP packet are valid. If not

the firewall considers the packet as malformed and discards it." A SYN+ACK packet arriving before a SYN packet is a violation of the TCP state machine and would be discarded by default. This supports that the packet is dropped unless a specific policy overrides this behavior.