I'm testing a website based on a CentOS server with NGINX and HHVM. The server regularly receives transactions / requests from an exteal provider and does some further processing with them.
The exteal provider is claiming that transactions are being sent to us but rejected by our server.
Having trawled through the nginx, syslog and php logs, I can find no record of these requests even hitting the server. Received requests appear in all of the aforementioned logs.
My first thought was that the firewall was dropping the requests for some reason, but firewalld is not ruing and there are no rules configured in iptables.
I contacted the VPS hosting provider to ask if a firewall at network level could be dropping the reqeusts. They advised that there is no exteal firewall and claimed that Webmin (which is also installed on the server) "can initiate blocks via iptables if it feels like it".
I have never heard of this before and I don't understand how that would even be possible with no iptables rules configured, but admittedly my knowledge of iptables and Webmin is fairly limited.
Even if it is possible, I don't understand how I can track which requests are being dropped, since Webmin does not seem to be logging them and iptables has no rules of any kind (let alone logging rules) configured.
Essentially I have now hit a brick wall so would love to hear suggestions on what to do next from those with more knowledge than I.