Although GUI-based Wireshark provides equally powerful filtering/dissecting capabilities via a more user-friendly interface, its relatively high memory footprint (for buffering packets) and GUI-based operations make Wireshark unsuitable when you are troubleshooting directly from remote headless servers. The main power of tcpdump comes from its (1) flexible packet filtering rules and (2) versatile protocol dissection capability. Tcpdump is a command-line tool packet sniffing that allows you to capture network packets based on packet filtering rules, interpret captured packet content, and display the result in a human-readable format. While required tools may vary depending on the types of network problems you are dealing with, there are a set of essential tools that every network administrator must be familiar with, and tcpdump is definitely one of them. The display filter syntax to filter out addresses between 192.168.1.1 – 192.168.1.255 would be ip.addr=192.168.1.0/24 and if you are comfortable with IP subnetting, you can alter the /24 to change the range.When it comes to network troubleshooting and monitoring, what types of tools you are using make a world of difference. However, if the addresses are contiguous or in the same subnet, you might be able to get away with a subnet filter. One time-consuming approach would be to literally type out all the addresses you want to filter on. In this video, I respond to a question from one of my readers who wanted to create a display filter for many IP addresses. In either case, you will need to use a display filter to narrow the traffic down. Even when you have a capture filter, it may be too generic. You may not know what to focus on when you capture packets, resulting in no capture filter. A display filter is configured after you have captured your packets. A capture filter is configured prior to starting your capture and affects what packets are captured. Note that in Wireshark, display and capture filter syntax are completely different. In this video, I review the two most common filters in Wireshark.
One of the keys to being an effective network troubleshooter when using a protocol analyzer is the ability to see patterns, which is where filters come into play.
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