Platform & DevOps·Traffic into and inside the cluster
the port is open in the application and the connection still hangs, because a rule two layers below never allowed it.
Security group
Also calledcloud firewall, instance firewall, network ACL
A stateful firewall attached to cloud resources, allowing traffic by port, protocol and source. It is the layer people forget when debugging connectivity, because it fails by dropping rather than refusing, so the symptom is a timeout with nothing in any log. Referencing other groups rather than address ranges is what keeps the rules meaningful as instances come and go.