Wireshark for Security Professionals. Parker Jeff T.

id="x_5_i138"> Figure 1-5: ARP packet Opcode

Usually a single expression is not specific enough to narrow down the stream of packets you are looking for when dealing with larger packet captures, as is the case with Figure 1-5. To locate the exact set of packets you want to see, you can combine expressions by logical operators. Table 1-2 shows the available operators. The symbol and English-word operator can be used interchangeably according to personal preference.

Table 1-2: Logical Operators

      Source: http://www.wireshark.org/docs/wsug_html_chunked/ChWorkBuildDisplayFilterSection.html

       Building Display Filters Interactively

      To quickly gain experience at building filters, you can use the graphical interface of Wireshark and the various context menus to build filters interactively. Start by right-clicking on a section of a packet that interests you, and then select Apply as Filter

Selected to filter the packet list by the selected variable. For example, selecting the source IP address field and applying a filter to it is a good way to start quickly narrowing down the packets you are interested in.

      After filtering for this particular IP address, you might want to add a destination port to the filter to only see traffic from this host to port 80. This can also be done in the GUI without throwing away the current filter by right-clicking the source port in the Packet Details pane and selecting Apply as Filter

Selected to combine the new filter with the old one using and. The GUI also lists other combinations, such as or, not, and so on. Additionally, you can use the Prepare as Filter context menu to create the filter without actually applying it to your Packet List pane.

Figure 1-6 shows an example of the display filter code after selecting two items: ARP protocol packets and the source MAC address.

Figure 1-6: Filter results of ARP from a source address

      After selecting ARP to apply as a filter, only ARP protocol packets from various systems were displayed in the Packet List pane. Subsequently selecting a source MAC (SamsungE_e1:ad:3c) as a filter expression, the display filter was amended to become arp.src.hw_mac == c4:57:6e:e1:ad:3c.

Figure 1-7 shows how complex filter statements can be built using this technique. As you can see in the status bar, Wireshark might suggest adding parentheses or suggest the User Guide. In upcoming chapters we will build and use many filters; this is just to show that filters can certainly grow past one or two functions.

Figure 1-7: Complex display filter example

      You can always use the context menus to edit the filter in the Filter bar after you start it. If building them interactively, make sure you are aware of the filters Wireshark applies for you by noting what syntax was inserted in the Filter bar.

      Building filters interactively provides a great way to understand the most commonly used filter fields and protocols. This will pay off when dealing with advanced Wireshark use cases in the future.

      Summary

      Congratulations on finishing the first chapter. It's a fairly light chapter, as we haven't begun actually working with the application yet. Given the belief that new Wireshark users are commonly surprised by the fast-growing number of packets, the book aims to nip overwhelm before it happens. The two big areas to cover before actually using Wireshark are the GUI and filters.

      We provided a general overview of the GUI, focusing on its layout and the reasoning behind it. The layout is divided into three panes: Packet List, Packet Details, and Packet Bytes. The panes present packet data at different levels of detail and serve to help the user drill down to individual bytes.