If you read the LWN article on this and Mr. Reed's "RIP" comment, there is actually an answer. According to the revised copyright law (made effective in 1998 I believe), after Mr. Reed passes away, whenever that may be, we just have to wait 70 years before his code goes into the public domain.
So, don't worry.
Joy. What a bloody mess.
Unfortunately, and I really mean that, Mr. Reed seems to have the legalities behind him on this. So whoever that private individual was whom he states contacted him via email to tell him this, well, they were right. Copyright law for non-visual works grants exclusive rights to the author the rights of distribution, reproduction (making copies), and derivations (modifications).
Note that by exclusive rights, I mean exclusive rights given to the author, which some may confused with the use of exclusive rights in copyright agreements, which are rights of copyrighted works wholly transferred to another--a different issue.
Anyways, more to the point. The IPFilter license does not specify the last right, the right to modify. And the author retains those rights which are not specified (the rights are exclusive and inclusive, meaning that the rights are retained unless stated otherwise).
Some of you out there may have noticed that Mr. Reed's IPFilter license did not include the frequently seen statement "All rights reserved" in his copyright notice. You may have interpreted this as a potential loophole. Under current law, you don't need that, so throw that theory out the door. He retains the right to modify, as he clarified.
So far, I've really been talking US copyright law. But notice, that unlike crypto laws, copyright laws seem to extend from the author and go country to country, taking the shortest route. iow, even if copyright law in one country may somehow allow modifications, even if those other-country-legal modifications came to the US, the author (Mr. Reed) can claim copyright violations on US soil for US participants of that other code.
Translation: This situation utterly sucks.
While I agree with Mr. Reed's copyright claim, that's all I agree with. I do not agree with the sentiments that have come out of his clarification from him, particularly on this forum. And, in my personal opinion, the community has been misled. While Mr. Reed is under no direct obligation to enlighten us (users, developers, or project leaders all) of our mistake, he certainly did know of this limitation in the license, and, most importantly, he knew that the majority of the community generally was not clear on it. Even if project leaders or developers knew, *most* people did not.
To (loudly? clearly? finally?) inform us on this late certainly doesn't make Mr. Reed a monster or what not, but to consider the whole picture, it also isn't exactly the actions of a person concerned with the community. Combine that with what this license clarification means...
What does this clarification mean? To me, it seems that that if Mr. Reed does not port the code or grants someone else permission to port the code to a particular OS and the default IPFilter code does not work on that OS (even down to the version and incremental OS updates), it violates his copyright if someone distributes code to make IPFilter work. Even if done as a patch (a derivative work is a derivative work, regardless of form--you can see more on this if you look at fan extended stories/works of TV series or movies).
It doesn't matter if Mr. Reed drops his code deliberately or not--if he passes away, if he's behind keeping up, it's still a violation. Worse, he can reassure us all day that he will continue to provide updated code to all the BSDs, and then, one day, stop doing so. If an incompatibility or security hole comes up that he does not correct or grants permission to someone to correct, we're screwed.
iow, it would appear that he controls features, code updates, security fixes, which OS it gets ported to, which version of IPFilter runs under what OS version, etc. He could, if under the current license, even play around and do favorites--maybe he decides he likes one OS better than another, so he'll update that one faster. Or feels the FreeBSD community is treating him a better, so he'll implement the latest and greatest for them first, with a large wait before seeing the changes implement in "competing" OSs.
Note that I'm not saying that these things will come to pass or that my paranoia is accurate, but we also have no concrete and definitive knowledge under the current IPFilter license that they will not.
Unless there is a change in the IPFilter license, I think an OpenIPF project may be warranted. If and until it does, in the meantime, I would urge people to politely ask Mr. Reed to change his license to save us both time and prevent a duplication of effort. IPFilter is excellent software. Just the license stinks.
P.S. Note that this is not a failing of a BSD license. IPFilter's license simply is not a BSD license--the BSD license, both the original and the one without the advertising clause, clearly grants the right to modify code.
P.S.S. Copyright extends to pseudonyms (actually, they are granted very slightly more rights than individuals) and to all posted works, even if without a copyright notice given. Funny, eh? :/