I was having a problem with sandbox and the poodle upgrades (disabling SSLv3 on an older server). I ran a ubuntu distribution upgrade last night, as well as upgrading php to 5.5 and lighttpd to the latest stable release.
Since the upgrade (which would have included openssl upgrade), my IPN now fails completely. It runs through the 8 retries and then fails - no http response code is provided, it's just blank.
I'm sure there are no php errors on the notifier script. Where should I be looking next in server error logs to see if the IPN is hitting the server at all? Or is there another approach I should take to debugging this?
In my case, the IPN was failing due to a totally unrelated DNS configuration change I had made, likely making the URL unreachable to Paypal. I deleted the configuration change and everything fired correctly.