The good news: This question has not been asked 80 times in the past week.
The bad news: It's no use sitting back and waiting for Lucy24 to answer it.
Question: When using mod_setenvif, is there any way to distinguish between a header that's absent and a header that's empty? (I guess the same applies to mod_rewrite, but I'm currently in setenvif.)
Backstory: I found a blocked request from an IP range that I would never, ever block.* After poring over logs and headers, I found that the request had sent such-and-such header, as humans do-- but it was empty. Bad news, because the absence of this particular header is a strong robot marker. But I can't find any way to say "it's present but empty". (Weird corollary discovery: Seznam Preview also sends an empty header. Wonder if there's any way to find out why?)
* In fact I feel so strongly about it that I've now poked a vigorous hole for mod_setenvif, and will later see about doing the same for mod_rewrite.
