The GRE header is a standard size of 4 bytes.If this isn't the case or can't be relied upon, then everywhere I use 20 in the filter will need to be replaced with the IP header length calculation, which is (ip & 0x0f) * 4. The outer IP header is a standard 20 bytes.I don't know how accurate your leader wants the filter, but if we can make a few assumptions about the outer IP and GRE headers, then the filter isn't too complicated.
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