Here is a long and interesting, pre-https era (2013), discussion about blank referrers in hotlinking prevention:
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webmasterworld.com ]
lucy24 noted in that thread: "But it is well-nigh impossible to block all referer-less requests without preventing some real humans from seeing your pictures".
Now my question was not, like much in that thread, about Google image search (which has since changed how it operates). But matrix_jan said it: "Personally I want all possible bots to index my images. I just don't want my images to get hotlinked." Therefore I have always made exceptions for G and B, like !^https://(www\.)?google\. and several variations of it (as I do not know what precisly is required nowadays.)
In [
webmasterworld.com ] (2012)
lucy24 said: "In fact that was my original reason for exempting blank referers in the hotlinking routine".
keyplyr: "I certainly would not block no referrer, unless it is combined in a rule with other specifics".
incrediBILL: "When it comes to images no referrer SHOULD be blocked to avoid hot-linking because the images should be referred from the page loading them".
jdMorgan on Sept 2, 2003: "Just be aware that many legitimate users will have a blank referrer - that is why it's allowed in most of the code you see here. Any user who comes through an ISP or corporate proxy, or who uses Norton Internet Security may have a blank referrer. You'll have to decide whether losing them is worth the cost of blocking blank referrers".
This has gone beyond https now, but I am confused. I have an image-heavy non-profit information site and in the hope of getting visits to pages I welcome indexing by image search engines. But due to some bandwidth considerations hotlinking from other people's sites ought to be avoided.
