TCP Wrappers was a popular access control mechanism in earlier decades: https://en.wikipedia.org/wiki/TCP_Wrappers https://en.wikipedia.org/wiki/TCP_Wrappers I

TCP Wrappers was a popular access control mechanism in earlier decades: https://en.wikipedia.org/wiki/TCP_Wrappers https://en.wikipedia.org/wiki/TCP_Wrappers It does a number of things in userspace (with a simple minimalist code base) that people might nowadays more often do in a firewall.