on 10-Oct-2019 (Thu)

On Linux/Unix/Mac platforms, you might also see a loopback interface that can be selected to capture packets being sent between applications on the same machine. However, in most cases, you'll only be interested in capturing packets from a network interface

The goal is to identify the active interface that will be used to communicate with the Internet when you open a browser and navigate to a website. If you have a wired local area network connection and the interface is enabled, that's probably the active interface, but you might also have a wireless interface that is enabled and you may or may not be the primary interface. The most reliable indicator of the active network interface is that it will have greater number of steadily increasing packets with a corresponding active number of packets/s (which will vary over time). Another possible indicator is if an interface has an IP address assigned and others do not. If you're still unsure, open a browser window and navigate to one of your favorite websites and watch the packets and packets/s counters to identify the interface that shows the greatest increase in activity.

Filtering out the noise Somewhere in your packet capture, there are packets involved with loading the Wireshark home page—but how do you find and view just those packets out of all the background noise? The simplest and most reliable method is to determine the IP address of the Wireshark website and filter out all the packets except those flowing between that IP address and the IP address of your workstation by using a display filter. The best approach—and the one that you'll likely use as a first step for most of your post-capture analysis work in future—is to investigate a list of all the conversations by IP address and/or hostnames, sorted by the most active nodes, and identify your target hostname, website name, or IP address from this list. From the Wireshark menu, select Conversations from the Statistics menu, and in the Conversations window that opens, select the IPv4 tab at the top. You'll see a list of network conversations identified by Address A and Address B, with columns for total Packets, Bytes, Packets A→B, Bytes A→B, Packets A←B, and Bytes A←B. Scrolling over to the right-hand side of this window, there are Relative Start values. These are the times when each particular conversation was first observed in the capture, relative to the start of the capture in seconds. The next column is Duration, which is how long this conversation persisted in the capture (first to last packet seen).

We want to sort the list of conversations to get the busiest ones—called the Top Talkers in network jargon—at the top of the list. Click on the Bytes column header and then click on it again.

We will need to resolve the IP addresses from our list to hostnames or website addresses, and this can be done from within Wireshark by turning on Network Name Resolution and trying to get hostnames and/or website addresses resolved for those IP addresses using reverse DNS queries (using what is known as a pointer (PTR) DNS record type). If you just installed or started Wireshark, the Name Resolution option may not be turned on by default.

This is usually a good thing, as Wireshark can create traffic of its own by transmitting the DNS queries trying to resolve all the IP addresses that it comes across during the capture, and you don't really want that going on during a capture. However, the Name Resolution option can be very helpful to resolve IP addresses to proper hostnames after a capture is complete.

To enable Name Resolution, navigate to View | Name Resolution | Enable for Network Layer (click to turn on the checkmark) and make sure Use External Network Name Resolver is enabled as well. Wireshark will attempt to resolve all the IP addresses in the capture to their hostname or website address, and the resolved names will then appear (replacing the previous IP addresses) in the packet list as well as the Conversations window. Note that the Name Resolution option at the bottom of the Conversations window must be enabled as well (it usually is by default), and this setting affects whether resolved names or IP addresses appear in the Conversations window (if Name Resolution is enabled in the Wireshark main screen), as shown in the following screenshot: