Controlling access to network services can be a challenge. Firewalls are useful for controlling access in and out of a particular network, but they can be difficult to configure. TCP wrappers and xinetd control access to services by hostname and IP addresses. In addition, these tools also include logging and utilization management capabilities that are easy to configure.
Many modern network services, such as SSH, Telnet, and FTP, make use of TCP wrappers, a program that is designed to stand between an incoming request and the requested service. TCP wrappers is installed by default with a server-class installation of Red Hat Linux, providing many advantages over running a variety of different services, each with their own access control methods.
The idea behind TCP wrappers is that, rather than allowing an incoming client connection to communicate directly with a network service daemon running as a separate process on a server system, the target of the request is "wrapped" by another program, allowing a greater degree of access control and logging of who is attempting to use the service.
The functionality behind TCP wrappers is provided by libwrap.a, a library that network services, such as xinetd, sshd, and portmap, are compiled against. Additional network services, even networking programs you may write, can be compiled again libwrap.a to provide this functionality. Red Hat Linux bundles the necessary TCP wrapper programs and library in the tcp_wrappers-<version> RPM file.
When someone attempts to access a network service using TCP wrappers, a small wrapper program reports the name of the service requested and the client's host information. The wrapper program does not directly send any information back to the client, and after the access control directives are satisfied, the wrapper gets out of the way, not placing any additional overhead on the communication between the client and server.
TCP wrappers provide two basic advantages over other network service control techniques:
The connecting client is unaware that TCP wrappers are in use. Legitimate users will not notice anything different, and attackers never receive any additional information about why their attempted connections failed.
TCP wrappers operate in a manner that is separate from the applications the wrapper program protects. This allows many applications to share a common set of configuration files for simpler management.