We recently had a case of our silent post reception suddenly disappearing, and I wanted to share the experience here in case this helps others.
Since all modern browsers now support the SNI specification, where you can now host multiple SSL sites under a single IP address (Name-based virtual hosts using a single IP) this presents the possibility of not only using a single IP but also SSL certificates that can support multiple domains.
We recently updated our hosting to take advantage of this, and all modern browsers support it. However, we discovered that the Authorize.net Silent Post feature is sensitive to any kind of problem with your SSL certificate if your URL is an https (ssl) one, and will silently FAIL - you won't get any activity in your log to indicate that Auth.net TRIED to communicate with your silent post URL, and Auth.net will not have any indictation either. Problems that could cause this are a self-signed SSL certificate, expried cert, non-matching cert, etc (classic issues) - but it looks like any kind of SNI support is also missing from Auth.net's Silent Post technology. Whatever tool (curl?) is being used, doesn't support SNI, as once we eliminated this from the equation, we began receiving our posts again.
Workarounds:
1) Provide a non-ssl silent post URL, if possible, or
2) Use non-SNI technology (i.e., dedicated IP and single-domain cert) for your website, until Auth.net updates their code to support SNI.
