Setting up a service using inetd

Background

Setting up a service on a unix machine does not require a great deal of knowledge about writing client server applications. There are only two basic requirements:

root permissions on the machine where the service will be installed
an ability to write an application in ANY language which reads from STANDARD INPUT and writes to STANDARD OUTPUT which can even include simple unix shell commands

Then it becomes a simple matter of making a few entries in a few text files.

Granted, this approach is not very sophisticated and will only satisfy the requirements of the simplest services, it can still solve the requirements of numerous types of problems. The program you will write represents the service, and telnet (a common application available on any internet enabled machine) will serve as the client for your service. The use of telnet as the client side limits significantly what you are able to do in the client-server interaction, nonetheless, it provides a simple text interface to your service. As long as the client only needs to use keyboard entries for the interaction with the service (not a complicated protocol), telnet will work fine.

Writing the service

Once you have written the service program and tested it so that it works using the standard input and standard output devices. move on and set it up to run on the internet.

A sample service written as a unix script is included.

Setting up the service

step 1 - choosing a port number

Select a port number on which your service will run. The /etc/services file contains a list of the services and port numbers associated with those services. In choosing a port number, you need to be careful not to choose a number that conflicts with other services. In order to avoid going into a lot of details at this point, just choose a number over 10000 which is not already in use (listed in the file). Edit the /etc/services file to include your service. The format should be self-explanatory from the file but would look like this for a service named myservice at port 10001.

myservice 10001/tcp

tcp represents the protocol the interaction will use and tcp is the most reliable and simple one to use.

step 2 - configuring inetd for that service

An entry must be made in /etc/xinetd.conf and once again, the format follows the other entries in the file. Most of the following entries are a simple consequence of the previous service specifications. A few of them require a little investigation to fully understand, but detailed knowledge is not generally important; simply copy the entry. Replace all bold-faced entries appropriately for your service.

service myservice
{
        port = 10001
        socket_type = stream
        protocol = tcp
        user = root
        server = /sbin/myservice.sh
        type = UNLISTED
        wait = no
}

servicename is obvious
nobody is the name of the owner of the service as it runs. Using root is dangerous and should be used sparingly. nobody has no permissions or you could use your own username. The bottom line is that the service can do anything this user is allowed to do.
/PathNameToServer is the full path including the program name
ProgramName is the command name plus any commandline options for your service if any exist. YEs this IS somewhat redundant in that the command name is part of this AND the previous entry.

Example:

myservice stream tcp nowait dgame /home/dgame/service service

where the user is dgame and the program named service is in the root directory of dgame.

Making inetd reboot

You can force inetd to read the changes you just made to inetd.conf by

sending it a hangup signal OR
rebooting the machine

Generally the former is better.

Use the unix ps command to find the process number of inetd.
send (x)inetd a HANGUP signal

E.g. if (x)inetd is running on process 231

kill -HUP 231

Testing the service

Use telnet to test the service. Go to another machine and use telnet, but use the port number you set up in your /etc/services file. For example, to access the service on 137.155.2.199 at port 10001

telnet 137.155.2.199 10001

Your server application should come up and interact with the telnet client you are running.

Special Considerations

There are a few glitches associated with using shell scripts for your service:

Do not use the unix more command.
Be sure to include the !#/bin/sh ( or whatever command shell is appropriate for you ) on the first line of a script.
There may be some firewall issues which prevent the telnet access to your service, if this appears to be the problem, stop at this point and we will review the problem in class.
When reading input, you may need to use the following code to avoid getting some extraneous characters from the client.

read answer
.... `echo $answer | cut -c1` ......

instead of using $answer directly