Computer Networks Lab (CSC15206 : 2019:20) project

CHAT-SERVER (using socket programming)

It is a basic chat server programmed to handle multiple users (clients) using socket programming. You can download / upload files from / to the server and send messaages through the server to other connected clients.
In a multi-client chat server, N clients are connected to a server and send messages. In this program, one of the clients send messages to the server and it(server) will sent back the messages to all other connected clients(we can say broadcast). The code is implemented using C language, with a TCP connection.

SERVER

Working :

• In the server program, first is the establishment of connection to a port(socket creation and binding) using socket() and bind() functions.
• Server listens for incoming connections using listen() function as well as keep reading from the connections it already have connected to. Server accepts incoming connections using accept() function, sends a welcome message to the client and recieves option(what client wants to actually do) and username from every connection using send()/recv() functions respectively.
• select() function gives the power to monitor several sockets at the same time. It tells which ones are ready for reading, which are ready for writing, and which sockets have raised exceptions.

int select(int numfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

• Description of select() :

  • Select command allows to monitor multiple file descriptors, waiting until one of the file descriptors become active. For example, if there is some data to be read on one of the sockets select will provide that information.
  • Select works like an interrupt handler, which gets activated as soon as any file descriptor sends any data.
  • If we want to see if we can read from standard input and some socket descriptor, sockfd, just add the file descriptors 0 and sockfd to the set readfds. The parameter numfds should be set to the values of the highest file descriptor plus one.
  • When select() returns, readfds will be modified to reflect which of the file descriptors we selected which is ready for reading.

• After select(), it run through the existing connections looking for data to read. If we got one, new connections are handled and the new file descriptor is added to the master set by keeping track of the maximum file descriptor. If there is no need to handle new connection, handle data from a client.

Functionalities provided :

The server provides 3 functionalities to its users :

1. Download file from the server - sendFile() function in server-sendfile.h is called.
2. Upload file to the server - getFile() function in server-getfile.h is called.
3. Send message to other connected clients to the server - send_recv_msg() function in server-message.h is called.

Also a client can :

1. Disconnect from server - Connection of client with the server is terminated using close().
2. Close the server (if the client is the ADMIN CLINET) - All the connected clients to the server are disconnected first and then the server is closed using close() function and he exit().

Image showing functionalities provided at the client side by server

Server Log :

• File "server_log.txt" keeps record of all the activities like starting/closing of the server, connecting/disconnecting of clients, downloads/uploads of files by the users and errors occured if any.
• File operations are used to log (append) the information to the file after each process.

CLIENT

Working :

• In the client program, first is prompt to enter the username that will be used during communication and creation of the client socket descriptor using socket() function.
• Second is the establishment of connection to the server and running on the localhost. Connection is established by using connect() function.
• Then select() function is used for either reading or writing as in the server program. It sends message to the server from the keyboard input using stdin. If there is data in the recv_buf, it receives data using recv().
• Now if the socket in readfds is not '0' then it means we have to recieve message from server and print it to stdout which can be done by calling send_recv_msg() function in client-channel-selector.h else the client is prompted for options and the option and username is sent to the server and relevant data is recieved from the server by calling send_recv() function in client-channel-selector.h. The channel selector selects the process as per the option input by the client.

Functionalities available :

Every client is provided with 4 + 1* options options :

1. "1" : Download file from server - getFile() function in client-getfile.h is called
2. "2" : Upload file to server - sendFile() function in client-sendFile.h is called
3. "3" : Send message to other connected clients - send_recv_msg() function in client-message.h is called
4. "0" : Disconnect from server - client socket is closed using close() function at server side and then exit() at client side.
5. ** "4" : Close the server - (ONLY FOR A ADMIN CLIENT) - An ADMIN client can use option "4" to close the server. For doing so ADMIN client is prompted for a password if his username is stored as a ADMIN client username. If the password matches with the password stored at server then all the connected clients to server are disconnected with a message and the server is closed.

The specifications of options available are:-

* "1". Download file

The user is asked to input the name of the file to be downloaded from the server. If the file exists at the server the file is transferred otherwise an no transfer takes place and 'No File Present' error is displayed.

* "2". Upload file

The user is asked to input the name of the file which is to be uploaded to the server. If the file does not exist at client location, the request is terminated by an error message else if :

• A file is already present at server with same name : Client is given two options :
  • "0" Owerwrite the file present at server
  • "1 / any other number" Abort the upload
• No such file exists at server : File is uploaded to the server.

* "3". Send message

The user is asked to input message. The message is sent to all the remaining connected clients(a type of broadcast).

* "0". Disconnect from the server

The user is disconnected from the server and the user leaves the chat server.

* "4". Close the server (Only for ADMIN client)

Only an administrator which has the correct username and password can close the server for all users. To close the server for all users, server first authenticates the username. If the username matches it asks for a password. If the password is correct server gets closed for all the users otherwise an error message is displayed stating incorrect username/password.

Project running :

Requirements : a compiler to compile a C program. Eg. GCC compiler.

Running :

• Download the project. In a terminal first navigate to "SERVER" directory and compile the "server.c" program using the command :

gcc server.c

• Now run the executable :

./a.out

NOTE: The server needs to be started first before the clients.

• Now open other terminals and navigate to "CLIENTS" directory and compile the "client.c" program using the command :

gcc client.c

• Now run the executable :

./a.out

• Now do as prompted.

This is a simple chat - server program.