Linux Firewalls, Third Edition

Private Versus Public Network Services

One of the easiest ways to inadvertently allow uninvited intrusions is to allow outside access to local services that are designed only for LAN use. Some services, if offered locally, should never cross the boundary between your LAN and the Internet beyond. Some of these services annoy your neighbors, some provide information you'd be better off keeping to yourself, and some represent glaring security holes if they're available outside your LAN.

Some of the earliest network services, the r-* based commands in particular, were designed for local sharing and ease-of-access across multiple lab machines in a trusted environment. Some of the later services were intended for Internet access, but they were designed ...