Local area networking has become a widely used means for organizations to share distributed computing resources. Internet sites often use the TCP/IP protocol suite and UNIX for local area networking purposes, because in addition to providing standard local area network services, UNIX and TCP/IP offer methods for centralizing the management of users and resources. This aids greatly in reducing the amount of work and overhead involved in managing user accounts and making distributed resources available to users. It can also be practical to use the same protocols and services for wide area networking as well as for local area networking.
But, two factors now make using TCP/IP for local area networking an increasingly risky business: a number of the TCP/IP services are inherently flawed and vulnerable to exploitation, and the tremendous growth of the Internet has increased greatly the likelihood of such exploitation. Crackers often roam the Internet searching for unprotected sites; misconfigured systems as well as use of insecure protocols make the cracker's job much easier [Bel92]. Two of the TCP/IP services most often used in local area networking, NIS (Network Information Services) and NFS (Network File System), are easily exploited; crackers can use weaknesses in NIS and NFS to read and write files, learn user information, capture passwords, and gain privileged access.
Kerberos and Secure RPC are effective means for reducing risks and vulnerabilities on local area TCP/IP networks, however they suffer from the disadvantages of requiring modified network daemon programs on all participating hosts. For many sites, the most practical method for securing access to systems and use of inherently vulnerable services is to use a Secure Gateway, or firewall system. A firewall system resides at an Internet gateway (or any subnet gateway) and blocks certain protocols and services from entering or exiting the protected subnet. A firewall system can also restrict access to hosts, log important network activity, and prevent information about internal systems and users from leaking out to the rest of the Internet.