In this case, we’ll be comparing the contents of the ToS byte (ip) with the hex value of 0xfc. In other words, you only get back a 1 when both binary values being compared are 1s. When comparing 2 binary numbers with AND, you should know that 0 AND 0 returns 0, 0 AND 1 returns 0, 1 AND 0 returns 0, and 1 AND 1 returns 1. The ToS byte includes 6 bits for the differentiated services code point per RFC2474 and 2 bits for explicit congestion notification, per RFC3168. If you check out RFC791, the second byte of the IP packet is the Type of Service (ToS) byte. Instead, we’ve selected ip, referring to the second byte (offset of 1). If we’d had ip, we’d be interested in the first byte (no offset).

Here, we’ve identified the protocol as IP, with an offset of 1. In the PCAP filter language, the bit in brackets defines which part of the protocol you’re interested in. “ip” – the second byte of the IP packet.In our expression, we’re using “not” because we want to ignore packets where the condition “ip & 0xfc = 0x0” is true. Here, we’re saying that the packet must be both “ip” and “the condition in parentheses following the and.” The capture filter I ended up writing to capture only IP packets with a non-zero DSCP field is as follows. While researching, I found this page from the Wireshark blog that got me some of the way to my goal. This seemed more efficient than using a display filter, since I wasn’t certain I’d find any packets like this on my home lab network where I was performing the capture. I wanted the Wireshark to capture IP packets with a non-zero DSCP value. In this post, I am going to focus on a capture filter I created to solve a specific problem. Capture filters are less intuitive, as they are cryptic when compared to display filters. Capture filters use a syntax of byte offsets, hex values, and masks coupled with booleans to filter. Display filters aren’t that hard to write once you’ve created a few. Display filters use a syntax of boolean operators and fields that intuitively describe what you’re filtering on. Display filters are used when you’ve captured everything, but need to cut through the noise to analyze specific packets or flows.Ĭapture filters and display filters are created using different syntaxes.

Capture filters only keep copies of packets that match the filter. In Wireshark, there are capture filters and display filters.

Tshark capture filter