The filter compiler uses a base offset for fetching data from the packet. Shouldn’t we be looking at both 26 and 30? We’re looking for the IP address at byte 30. What if we change the “and” to an “or”? (000) ldh We have to add the word “vlan” to our filter to get the right offsets, e.g. 1q, and adding a check to the filter code would add a lot of unnecessary overhead. Libpcap and WinPcap don’t know you’re using. The ethertype will be at offset 16 instead of 12, and the IP source address will be at offset 30 instead of 26. This is the minimum amount of checking required for that capture filter if you’re running IP over Ethernet.Ĩ02.1q inserts an extra four bytes in front of the ethertype, so this filter won’t do what you want. You just need to know that the first two lines look for the IP ethertype (0x800) starting at byte 12 and the next two lines look for the IP address 10.16.32.48 (0xa102030) starting at byte 26. The dump of our filter looks like this: (000) ldh We can do this by running tcpdump -d, which takes a filter, compiles it, and dumps out the result. Let’s take an up-close and personal look at the capture filter “ip src host 10.16.32.48”. Each user was having the same problem yet these are different network technologies - what do they have to do with each other?
A couple of questions have come up on the wireshark-users mailing list recently about using capture filters for MPLS and VLANs.
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