In the lengthy description section, you will find the “ expr relop expr” text near the bottom especially helpful for decoding cryptic capture filter expressions. PCAP-FILTER, packet capture filter syntax from Wireshark.Good luck writing your Wireshark capture filters! More Reading Making sense of the capture filter syntax can be daunting, but walking through an example item by item helps bring clarity.įor a more in-depth understanding of the concepts I touched on here, read through the links below. But once in a while, a capture filter seems like a cleaner way to go. Most of the time, I use Wireshark to capture all packets and examine what I need using a display filter. ![]() That packet would be captured, because the result is not equal to 0x0. If the ToS byte is 00000000, what would the result of the AND operation be? Let’s think through a couple of examples. We want the value of the DSCP field specifically to be anything but zero, and we wrote an bitwise AND comparison to test for it. Now that we know what we’re looking at, we tell the capture filter what we want that value to be or not be. Then we tell it an offset of 1 so that it will start one byte in, i.e. Therefore, we use the protocol type “ip” to tell the capture filter where to start. How do we tell the capture filter to look at the ToS byte? We know that the ToS byte is the second byte in the IP header. To do it, we’re telling the capture filter to keep only those IPv4 packets that, when the ToS byte is AND’ed with 11111100, gets back something other than zero. We want to capture IPv4 packets with a non-zero value in the DSCP field. Now, let’s condense that breakdown into plain English.
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