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 at a time when the Internet was basically an extended community of academicians and researchers. The Internet was a relatively open, safe place. As the Internet grew into a global network including general public access, it developed into a completely untrusted environment.

Lots of Linux network services are designed to provide local information about user accounts on the system, which programs are running and which resources are in use, system status, network status, and similar information from other machines connected over the network. Not all these informational services represent security holes in and of themselves. It's not that someone can use them directly to gain unauthorized access to your system. It's that they provide information about your system and user accounts that can be useful to someone who is looking for known vulnerabilities. They might also supply information such as usernames, addresses, phone numbers, and so forth, which you don't want to be readily available to everyone who asks.

Some of the more dangerous network services are designed to provide LAN access to shared filesystems and devices, such as a networked printer or fax machine.

Some services are difficult to configure correctly and some are difficult to configure securely. Entire books are devoted to configuring some of the more complicated Linux services. Specific service configuration is beyond the scope of this book.

Some services just don't make sense in a home or small-office setting. Some are intended to manage large networks, provide Internet routing service, provide large database informational services, support two-way encryption and authentication, and so forth.

Protecting Nonsecure Local Services

The easiest way to protect yourself is to not offer the service. But what if you need one of these services locally? Not all services can be protected adequately at the packet-filtering level. Multimedia services, such as RealAudio and instant messaging services, and UDP-based RPC services are notoriously difficult to secure at the packet-filtering level.

One way to safeguard your computer is to not host network services on the firewall machine that you don't intend for public use. If the service isn't available, there's nothing for a remote client to connect to. Let firewalls be firewalls.

A packet-filtering firewall doesn't offer complete security. Some programs require higher-level security measures than can be provided at the packet-filtering level. Some programs are too problematic to risk running on a firewall machine, even on a less secure residential host.

Small sites such as those in the home often won't have a supply of computers available to enforce access security policies by running private services on other machines. Compromises must be made, particularly for required services that are provided solely by Linux. Nevertheless, small sites with a LAN should not be running file sharing or other private LAN services on the firewall, such as Samba. The machine should not have unnecessary user accounts. Unneeded system software should be removed from the system. The machine should have no function other than that of a security gateway.

Selecting Services to Run

When all is said and done, only you can decide which services you need or want. The first step in securing your system is to decide which services and daemons you intend to run on the firewall machine, as well as behind the firewall in the private LAN. Each service has its own security considerations. When it comes to selecting services to run under Linux or any operating system, the general rule of thumb is to run only network services that you need and understand. It's important to understand a network service, what it does and who it's intended for, before you run itespecially on a machine connected directly to the Internet.