TP n°1 TCP client/server with “ident” protocol

Subject:

After studying the server-side then client-side implementation of the “ident” protocol (defined by RFC 1413 and normally using port 113 in TCP) implement an ident client in a mini server.

For the server port of our application, we will choose a free port number on the Unix server. As an example, I would take the value 2000, but if you are around twenty students doing the same practical work, each one can take a value between 2001 and 2020 (guided by the teacher's instructions).

For the IP address of the machine, I will use here 127.0.0.1 but you will replace it with the IP address of the Unix server indicated by the teacher.

Question 1:

On the Unix server, which you use for the lab, launch 3 command line interfaces (unix shell) from 3 different windows:

• From the first window, use the command " nc -l -p 2000 " (see the nc tool man for more information), to connect the keyboard/screen couple to the server-side bidirectional channel waiting on port 2000.
• From the second window, use the command " telnet 127.0.0.1 2000 " (see the man of the telnet tool for more information), to connect the keyboard/screen couple to the bidirectional channel on the client side by specifying the server to contact by its IP address and its port.

Note: From here, you can communicate between the two windows: all messages sent from one window are displayed on the other.

• From the third window, we will be able to examine the parameters of the bidirectional communication channel and query the ident server by indicating the parameters retrieved. To do this, we will issue the following commands:
• Run the command “ netstat -nap ” to see all the IP sockets used as well as the name of the program that uses them each time. You can thus find the port number used by the communication channel on the client side (on the server side, it is port 2000); either “ yyyyy ” this port number. You can also use the command “netstat -nap | grep 2000 ” to see only the rows that match our socket.

(yyyyy is 52266 in the example above)

• Pass the command “ (echo 'yyyyy, 2000'; sleep 5) | telnet 127.0.0.1 113 ».

Example reply

• Pass the command “ (echo '2000, yyyyy'; sleep 5) | telnet 127.0.0.1 113 ”.

By comparing with what is written in RFC 1413 available on the internet, students can analyze the results obtained in the third window.

Students can start the experiment again with their partner who will place the command " telnet 127.0.0.1 2000 ". And they will find that the ident server does not respond the same when crossing the source and destination port numbers.

Question 2:

Create, in C language, a program that accepts server connections on port 2000 and then displays the login of the account that used the client-side call socket by querying the server ident.