Sniffers, Packet Capture

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WebGUI Packet Captures

A packet capture may be performed within the 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 GUI 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

tcpdump comes installed with pfSense. 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 pfSense is being grabbed.

iftop

A second method of sniffing traffic via a shell is with iftop. The method of installing iftop is highlighted on this wiki page Add_Packages.

iftop allows designating the interface from which to grab traffic.

In this example em1 is the LAN Interface

iftop -i em1

Sample Output

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

pftop is a tool built into pfSense that can monitor traffic/connections. This tool can be found in the GUI under Diagnostics > pftop or by connecting to pfSense via SSH or the console.

This article is part of the How-To series.