Gatekeeper
also: firewall, access control, gate, checkpoint
A security mechanism or system that controls and monitors access to a network, system, or resource by evaluating incoming requests against defined policies before allowing them through.
A gatekeeper acts as a checkpoint that stands between an untrusted source and a protected resource. It examines traffic or requests, applies security rules, and decides whether to permit or deny access based on configured policies.
Common examples include firewalls that gate network traffic, authentication systems that gate user access, and SELinux policies that gate process capabilities. For instance, a firewall gatekeeper might block all incoming traffic on port 22 except from specific IP addresses, effectively controlling SSH access to a server.
The gatekeeper concept is fundamental to the principle of least privilege: only explicitly allowed traffic or access is permitted, while everything else is denied by default.