Inetd is the master server for the group of Internet Services. Table 1 lists the various Internet Services. All the services make use of two configuration files
Services file
Protocols file
The Services file has an official name and an alias, along with the port number and protocol for each and every service. If you are interested in adding a new service to your system, you will have to edit this file.

Here are a couple of entries from this file:

Here, clearly, the first column specifies a service and the second and the third columns correspond to the port number and the protocol respectively. The fourth column is the alias name for the service.

The Protocols file contains information about the protocols known to the system. Let us have a look at this file's contents

Here, the first column is the name of the protocol, the second is the port number, and the third corresponds to the alias.

The master server coordinates the use of individual network services on your system. When Inetd is running, it listens for the connection requests for the various services listed in the configuration file and, when a request arrives, starts the appropriate server and record information about the connection request. The Internet super server allows a single process to be waiting that can service multiple requests, instead of having one process for each potential service. This reduces the number of processes running on the system.
You, as system manager can determine which Internet Services are available to your users by making appropriate settings in the inetd configuration file. It is the responsibility of this Internet daemon to check whether the requesting node had the permission to use the service.

Some of the services this daemon provides include

Each of these services can be contacted using either TCP or UDP. The TCP servers for these internal functions are handled as iterative servers if the amount of time to serve the request is fixed, or as concurrent servers if the amount of time to handle the request depends on the request itself.

ServiceDescription

Table 1: Internet Services

Let us have a look at the inetd configuration file. The Internet daemon uses the configuration data by reading the file /etc/inetd.conf. This file is read during the startup. Also, you have to instruct the running inetd to reread this file after making changes so that the changes will take effect. Every entry in the configuration file has a common format and each of the fields has a specific purpose. Each line of this file contains the following fields.

An optional security file is associated with inetd that controls which nodes have access to Internet services. This security file checks whether the node making the request has the necessary permissions. If not, it prevents inetd from starting that service. In other words, the individual entries in the security file will determine the nodes that are allowed / disallowed for a particular service. This security file is accessible by the file name /usr/adm/inetd.sec

Here is an entry from this file

The telnet entry denies the host with IP address a.b.c.d (for example 192.68.111.111) access to your system using telnet.

Thus the word allow or deny in the second column determines which hosts have access rights to access a particular service. One may make use of the wildcard characters in any of the address fields to specify the access for a group of hosts or networks. For example, the following line allows all hosts with network address starting with 5 to use telnet

But remember that you cannot use this wildcard character in combination with more that one integer . This is why the following entry in the security file generates an error message

One can also use the range indicator in any of the address fields to indicate which hosts or networks in a group have access to the specified services. Thus the entry

in the security file denies hosts in subnet 2 through 4 of network 8 access to telnet. ENS