CSCI 3500: Studio 24

Layer 7: Application Layer

Application layer protocols allow different processes to communicate with one another in an orderly and meaningful fashion. By agreeing on a pre-arranged set of messages (the protocol) the two processes can communicate to achieve every internet-enabled application you can think of. Internet hostnames are translated to IP addresses with the DNS protocol. Web browsers recieve web pages with the HTTP protocol. Servers provide secure terminal access through the SSH protocol, and etcetera.

In this studio, you will:

1. Implement a basic file server

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers, and when finished submit your work via the git repository.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. In your previous studio you created a server and a client program where the server would listen for connections from a client, and then echo the contents of those messages to the output of the server. Your server would also listen for a specific message ("quit") which would cause the server to terminate. This demonstrated communication between a server and a client, but without accomplishing anything meaningful. We will modify your server and client so that the client can request a specific file be retrieved from the server.

   Modify your server so that it interprets incoming messages (other than "quit") as file names. It should attempt to open these files with the open() or fopen() functions. Check the return value: if you cannot open the file then return an error message to the client by writing to the socket (the one returned by accept() in the server). Otherwise, write the contents of the file to the socket. Once the server is done writing (either the error message or the file contents) you should terminate the server's end of the connection by calling the function shutdown() with option SHUT_RDWR (see man 2 shutdown for details).

   Similarly, modify your client so that after sending its request it will read() from the socket, printing everything it recieves to standard output. Once the call to read() returns zero (signifying there is nothing more to read from the socket) it should terminate.

   Hint: We had several studios early on (two and four) that involved reading and reprinting text files through streams. Feel free to use this code again.

3. Now, add one more special context word along with "quit". If the client sends the string "ls" then the server should execute the command ls and send the result back to the client. I would suggest you do this with the popen() function documented at man 3 popen. Note that this function returns a FILE* type, so you will need to use fgets() to read from the returned file pointer.

   Optional Enrichment Exercises

   1. Create a more complex protocol that involves telling the server whether to send or recive a file from the client. If the client wants to read a file then the server should behave as above. If the client wants to send a file then the server should write the contents from the scocket into the desried file. Now you can save and restore files on a remote server!

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