4.5. Available Network Services

While user access to administrative controls is an important issue for system administrators within an organization, keeping tabs on which network services is of paramount importance to anyone who installs and operates a Linux system.

Many services under Linux behave as network servers. If a network service is running on a machine, then a server application called a daemon is listening for connections on one or more network ports. Each of these servers should be treated as potential avenue of attack.

4.5.1. Risks To Services

Network services can pose many risks for Linux systems. Below is a list of some of the primary issues:

To limit exposure to attacks over the network all services that are unused should be turned off.

4.5.2. Identifying and Configuring Services

To enhance security, most network services installed with Red Hat Linux are turned off by default. There are, however some notable exceptions:

When determining whether or not to leave these services running, it is best to use common sense and err on the side of caution. For example, if you do not own a printer, do not leave cupsd running with the assumption that one day you might buy one. The same is true for portmap. If you do not mount NFS volumes or use NIS (the ypbind service), then portmap should be disabled.

Red Hat Linux ships with three programs designed to switch services on or off. They are the Services Configuration Tool (redhat-config-services), ntsysv, and chkconfig. For information on using these tools, see the chapter titled Controlling Access to Services in the Red Hat Linux Customization Guide.

Figure 4-3. Services Configuration Tool

If you are not sure what purpose a service has, the Services Configuration Tool has a description field, illustrated in Figure 4-3, that may be of some use.

But checking to see which network services are available to start at boot time is not enough. Good system administrators should also check which ports are open and listening. See Section 5.8 Verifying Which Ports Are Listening for more on this subject.

4.5.3. Insecure Services

Potentially, any network service is insecure. This is why turning unused services off is so important. Exploits for services are revealed and patched routinely. So it is important to keep packages associated with any network service updated. See Chapter 3 Security Updates for more information about this issue.

Some network protocols are inherently more insecure than others. These include any services which do the following things:

Examples of inherently insecure services includes the following:

All remote login and shell programs (rlogin, rsh, and telnet) should be avoided in favor of SSH. (see Section 4.7 Security Enhanced Communication Tools for more information about sshd).

FTP is not as inherently dangerous to the security of the system as remote shells, but FTP servers must be carefully configured and monitored to avoid problems. See Section 5.6 Securing FTP for more information on securing FTP servers.

Services which should be carefully implemented and behind a firewall include:

More information on securing network services is available in Chapter 5 Server Security.

The next section discusses tools available to set up a simple firewall.