Running tcpdump without specifying an interface can cause it to listen on the any pseudo-interface, capturing traffic from all interfaces. On a busy Security Gateway, this processes a massive number of packets, leading to high CPU utilization. The -i flag allows you to specify a single network interface (e.g., tcpdump -i eth0). This restricts the capture to only the traffic passing through that specific interface, significantly reducing the packet processing load and thereby lowering CPU usage. This is a fundamental technique for controlling the scope and performance impact of tcpdump.

A. o: The -o flag is not a standard, valid option for the tcpdump command. It is a valid flag for the Check Point fw monitor command to specify an output file.

B. O: The -O flag explicitly disables the packet-matching code optimizer in tcpdump. This would likely increase CPU usage, making the performance problem worse, not better.

C. I: The -I flag in tcpdump is used to put an interface into "monitor mode," a feature typically used only for 802.11 wireless interfaces and is not a performance optimization tool for gateway traffic.

1. Check Point SK30583, "How to use tcpdump": This article details the usage of tcpdump on Check Point appliances. It lists -i as the option to "Listen on a particular interface." This directly supports the use of -i to limit the scope of the capture.

2. tcpdump(8) Linux man page: The official manual for tcpdump confirms the function of the flags.

-i interface: "Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback)..." On some systems, the any device can be used to capture from all interfaces.

-O: "Do not run the packet-matching code optimizer."

-I: "Put the interface in 'monitor mode'; this is supported only on IEEE 802.11 Wi-Fi interfaces..."

3. Check Point SK162334, "'fw monitor' command": This article describes the flags for the fw monitor utility, which is often confused with tcpdump. It shows that -I is a valid flag for fw monitor (specifying an inspection point), explaining the likely source of confusion for option C. It also shows -o as the flag for an output file, explaining the confusion for option A.