Firewalls… love them or hate them, we all rely on firewalls to both protect and route our internet traffic. In the case of the web hosting industry, one of the most widely used firewalls is CSF (Config Server Firewall). While this product is reliable, it is far from perfect.
With the release of the 3.7+ Linux Kernel, the ipconntrack module was added, however older builds of CSF are not compatible with this new module. This can potentially cause machines to appear offline. To determine if you are running an older version of CSF, you may run the following command via SSH or OpenVZ console.
As of this writing, the latest build of CSF is 5.78 (cPanel). If you happen to have an older version of CSF installed, you may update CSF by running the following commands via SSH or the OpenVZ Console
service csf stop
service csf start
This should immediately restore connectivity to your server. CSF has released documented more on this fix in their official changelog.
As always, feel free to contact us if you need any assistance.