We post payment requests to PayPal on behalf of our clients, we have had this setup since 2010 and it has worked without fault up until about October 2016. From this point on we started receiving more frequent reports from our clients to say that the invoice was never updated as paid, which translates to a failure with the IPN, or rather the IPN is simply never called. 9 out of 10 IPN callbacks work fine although in the cases where it fails and the client provides us with access to their IPN logs, there is no HTTP response code and no record in our server logs that PayPal even hit the endpoint. We did move our service behind CloudFlare in October 2016 so we were wondering if this may be related? Having discussed this with CloudFlare they confirm that no PayPal IPN traffic was routed through their proxy. We also get an A grade on SSLLabs when testing SSL connectivity so I don't believe it can be related to the recent SSL requirements implemented by PayPal. Any ideas what could possibly be causing this intermittent issue?
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