Why do snippets often return 403 when sharing to FB and VK on VPS?

Question

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Sites on VPS, on different cms WP and OC. php works "like an apache module". A direct link opens a picture, but in og markup, when forming a snippet, it often doesn’t pull up and, for example, FB writes 403. But not always. Sometimes instead of 403 it's just an empty space instead of a picture. In VK, the snippet is just a title and a description. One entry from the site is rummaged normally, the other is not. Problems started after moving to VPS. Everything worked on shared.

Answer 1

In my case, IPv6 is not configured on the server, and all these services knock on it and get 403. Removed AAAA records from domains and everything is ok.