Explanation
The Capture w/Trace wizard in Cisco FMC allows you to capture packets on an FTD device and trace their path through the Snort engine. This can help you troubleshoot connectivity issues from an endpoint behind an FTD device and a public DNS server, as well as verify the Snort verdict for the DNS traffic. The Capture w/Trace wizard lets you specify the source and destination IP addresses, ports, and protocols for the packets you want to capture and trace, as well as the FTD device and interface where you want to perform the capture.
You can also apply filters to limit the capture size and duration. After you start the capture, you can ping the DNS server from the endpoint and then view the captured packets and their Snort verdicts in the FMC web interface2.
To use the Capture w/Trace wizard in Cisco FMC, you need to follow these steps2:
In the FMC web interface, navigate to Troubleshooting > Capture/Trace.
Click New Capture.
Choose an FTD device from the Device drop-down list.
Choose an interface from the Interface drop-down list.
Enter the source and destination IP addresses, ports, and protocols for the packets you want to capture and trace. For example, if you want to capture DNS queries from an endpoint with IP address 10.1.1.100 to a DNS server with IP address 8.8.8.8, you can enter these values:
Source IP: 10.1.1.100
Source Port: any
Destination IP: 8.8.8.8
Destination Port: 53
Protocol: UDP
Optionally, apply filters to limit the capture size and duration. For example, you can set the maximum number of packets to capture, the maximum capture file size, or the maximum capture time.
Click Start.
Ping the DNS server from the endpoint and wait for some packets to be captured.
Click Stop to stop the capture.
Click View Capture to see the captured packets and their Snort verdicts.
The other options are incorrect because:
Performing a Snort engine capture using tcpdump from the FTD CLI will not allow you to trace the path of the packets through the Snort engine or verify their Snort verdicts. Tcpdump is a command-line tool that can capture packets on an FTD device, but it does not provide any information about how Snort processes those packets or what actions Snort takes on them2.
Creating a Custom Workflow in Cisco FMC will not help you troubleshoot a connectivity issue from an endpoint behind an FTD device and a public DNS server. A Custom Workflow is a user-defined set of pages that display event data in different formats, such as tables, charts, maps, and so on. A Custom Workflow does not allow you to capture or trace packets on an FTD device3.
Running the system support firewall-engine-debug command from the FTD CLI will not allow you to simulate real DNS traffic on the FTD device or verify the Snort verdict for that traffic. The firewall-engine-debug command is a diagnostic tool that can generate synthetic packets and send them through the Snort engine on an FTD device. The synthetic packets are not real network traffic and do not affect any connections or policies on the FTD device4.