We're seeing an issue with our Front Door configuration where requests from the Facebook crawler are consistently seeing a 503 Service Unavailable error for all requests.
This seems to be related to the combination of request headers used:
User-Agent: facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php) Accept-Encoding: deflate, gzip Range: bytes=0-524288 Connection: close
Or in curl:
curl -v --compressed -H "Range: bytes=0-524288" -H "Connection: close" -A "facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)" "$URL"
Requesting the page without the Accept-Encoding correctly returns a 206 Partial Content response.
Sending the full request to the origin/backend host directly (Azure App Service) also returns a valid response with the expected page content.
I've seen (and replied to) the same issue reported here: https://feedback.azure.com/forums/217313-networking/suggestions/38617117