Performing a Packet Capture¶
A packet capture may be performed within the pfSense® webGUI under Diagnostics > Packet Capture. The settings work the same as tcpdump. The capture can be viewed in the GUI or downloaded for later viewing with tcpdump or Wireshark.
Various filters may be added to restrict the scope of the capture, such as a specific Protocol, Host address, or Port (among others). The size of the capture may be adjusted as well. Often a few thousand packets are necessary to catch certain activity.
The Level of detail selector only controls the level of detail displayed in the pfSense webGUI for viewing the contents of a capture. It may be adjusted after a capture has been taken, to view the capture with more detail, adjust this value and click View Capture.
Click Start to start a capture. While a capture is running, a Stop button is also displayed to stop a capture in progress.
View Capture shows the contents of the previous capture.
Download Capture initiates a download of the capture file for viewing locally (or sending to a remote technician.)
tcpdump comes installed with pfSense software. It can be used over SSH or on the console in a shell. In this example a method of capturing traffic other than SSH, ARP, DNS and STP is highlighted. The capture will be directed to a file called Sniff_output in the current directory.
tcpdump -i em0 not port 22 and not port 53 and not arp and not stp >> Sniff_output
The -i is designating traffic from the em0 interface. In this example traffic from one of the subnets em0 connected to a pfSense instance is being grabbed.
A second method of sniffing traffic via a shell is with iftop, which is availabe in the package repository.
iftop allows designating the interface from which to grab traffic.
In this example em1 is the LAN Interface:
iftop -i em1
cf-in-f18.google.com => 192.168.1.245 0b <= 0b cf-in-f97.google.com => 192.168.1.245 0b <= 0b cf-in-f103.google.com => 192.168.1.245 0b <= 0b google.navigation.opendns.com => 192.168.1.245 0b <= 0b
pftop is a tool built into pfSense software that can monitor traffic/connections. This tool can be found in the webGUI under Diagnostics > pftop or by connecting to the pfSense firewall via SSH or the console.