You mean an IP filter it's normally not concatenated together as "ipfilter".
There is the RFC which defines how the packet is supposed to be interpreted, it's pretty easy to read each word and extract the information from the IP headers, it just requires an FSM/counter to keep track of the position of the word and hence the contents of that word. The filtering part comes in when you have criteria that defines what you are going to do with the packet after you've examined the header, e.g. forward the packet payload, drop the packet, or whatever you need to do.
I've never seen any generic HDL code for this, I've always just written it myself for the specific application. If it isn't available (for free) on opencores then it's not likely something that will be available for free.