Most programs which provide network services are susceptible to abuse. The Network File System (NFS) and commands which use the trusted hosts facility are particularly vulnerable. A system administrator should be aware of security risks associated with the use of each service. For systems that do not need to provide a specific network service, the system administrator may want to consider disabling the appropriate program. For example, a system administrator may want to disable the ftp and ftpd programs for a system that has no need for file transfer service.
This section describes ways to prevent vulnerabilities of common network access procedures from being exploited. For stronger security, the methods described in this section can be combined with the use of secure gateways (see sec. 10.3) or robust authentication methods (see sec. 10.4).
In addition to describing secure gateways, section 10.3 also describes a third-party package called the ``TCP Wrapper'' package. This package serves as a front end which provides access control for all services executed from the UNIX inet daemon process. The TCP Wrapper package can be used to determine whether a host requesting a telnet, ftp, or ``r'' command connection is authorized, to log the request, and then either to accept or reject the connection.
In order to provide an overall secure network, all systems using common network access procedures must be protected against the threats described in section 9.2. If individual systems do not protect themselves from exploitation of inherently flawed common network access procedures and a system is broken into, then other systems on the network may be compromised.