I've been trying to get more hands-on with our Firebox for troubleshooting. One common task is figuring out why specific traffic is being blocked.
I've settled on a basic workflow for packet captures that works for me. I start a capture on the suspected interface with a filter for the source IP. Then, I generate the traffic from the client machine (like pinging the blocked service). I stop the capture, download the .pcap, and open it in Wireshark locally. Looking for TCP RST or ICMP unreachable packets usually points me to the right rule. Does this approach sound about right, or is there a more efficient method? I'm still learning the built-in tools.
Your method is sound for basic connectivity issues. However, I'd suggest refining your capture filter from just the source IP to include the destination port and protocol as well. This dramatically reduces noise in the capture file, making analysis in Wireshark much faster when you're dealing with a busy interface.
Looking for RST packets is a good start, but don't neglect the firewall's own logs. The packet capture shows you the symptom on the wire, while the policy log entry will usually name the exact rule that caused the block. Correlating the timestamp from your pcap with a log search can give you the answer without needing to infer from the packets alone.
Have you tried using the CLI's `tcpdump` directly? It can be faster for iterative testing than the GUI workflow, letting you apply and adjust filters on the fly.
-- bb42
It's a solid starting workflow. The biggest efficiency gain you're missing is pre-filtering. Capture everything and you'll drown in packets.
Use the CLI to build your filter before you even start the capture. For example, if you suspect an outbound web block, run `tcpdump -ni interface0 host 192.168.1.100 and port 443` right on the box. This gives you immediate feedback in the terminal, often showing the RST as it happens. Saves the download-Wireshark loop for complex cases.
Also, trust but verify with the policy logs. A packet capture tells you *what* was dropped, but the Firebox logs name the exact rule. Always cross-reference the timestamps.
Show me the query.
It's a valid first-pass method, but you're creating extra steps for yourself. The download-Wireshark loop is only necessary when you need deep packet inspection. Most of the time, you just need to see if the SYN is getting a RST.
Skip the GUI and run a targeted tcpdump from the Firebox CLI for immediate feedback. Your current workflow adds latency when the answer is often right in the terminal. For example:
tcpdump -ni external 'host 10.1.1.50 and port 25'
That'll show you the blocking handshake in real time, no file transfer needed. Only grab the pcap when you need to decode application-layer weirdness, which is rarer than simple policy blocks.
APIs are not magic.
That's a good point about skipping the download step for simple blocks. Seeing RST in the terminal is much faster.
I've been hesitant to use the CLI because I'm not as comfortable there. Does tcpdump on the Firebox save the capture to a file by default, or do you have to add a flag for that? Just thinking about when you do need that pcap later.