COE 518 Operating Systems. Lab 10

Transcription

1 COE 518 Operating Systems Lab 10 Table of Contents 1. Required Reading 2. Lab Description 3. Lab Objectives 4. Exercises 5. Deliverable 1. Required Reading 2. Lab Description You have already implemented interprocess communication using pipes and message queues. In previous labs, the two communicating processes were located on the same machine. In this lab, you will learn to communicate between processes which can be located on different machines across the network. You will create a TCP/IP client/server structure. The server listens on a port for connections from clients and a client sends a message to the server. The server reads the message from the client and responds accordingly. 3. Lab Objectives Learn how to communicate between processes using socket programming E. Bagheri (2013) K. Clowes (2014)

2 4. Exercises Hacking the password server In the context of this exercise, we build a server which is responsible for keeping a password. This server receives text and responds by checking whether the text is equivalent to the password or not. For security reasons the actual password is not in the code. Instead, server keeps the MD5 hash of the password in the code and verifies the correctness of the password by comparing the MD5 hash of the string sent by the client to the MD5 hash of the password. If they are same, a match has been found. This server will be listening on port 5000 and accepts connections on this socket from clients. When a client connects to the server, it sends a text to the server and the server reads text from the socket. The server responds with one character, which is T when the sent text is the correct password or F if the text is not the password. The goal in this lab is enabling communication between the server and the clients using sockets in the way described above and also developing a client which tries to find the correct password by trying random passwords. 1. Download the code server.c client.c and md5.h The md5.h file includes the function for MD5 hashing and is only used as a utility library during this lab. Each of the placeholders should be replaced with one or more C instructions in order to complete the program. The required library files have been added but you may need to include more libraries if you follow a different approach. The general approach in order to setup a connection between the clients and the server has been shown in the figure below. The server defines a socket and binds it to a port. Binding to a port allows the server to listen for incoming connection requests on that port. The server sleeps until a connection request is made (the server sleeps on the accept command). The client creates a socket and tries to connect to the server using that socket. After that, the client and server can communicate through their own sockets using send and recv functions. After the communication between the server and the client finishes, the connection needs to be closed by closing the sockets. Try to read the incomplete program from the code templates and understand it. In the server code file, handle_connection function gets a connection identifier and handles that incoming connection and verify_password function checks whether the password sent to it as an input is correct by comparing MD5 hash of it with MD5 hash of the actual password. The password is correct if its MD5 hash is equal to the MD5 hash of the actual password. In the client code, try_a_password

3 function creates a connection to the server and tries a random password to see if it is correct. If the password is correct, it prints the password. Assume that we already know that the correct password has three capital letters and the letters are selected from the A to J. The create_random_password function creates a random password satisfying those rules. The connection between client/server 2. Let s start by completing the server code first. Each socket is identified by an integer. Replace placeholder A with the code that declares the integer variable that keeps the socket and creates a socket which the server is going to use to bind to a port and listen to it and communicate with the client. The socket can be created by proper call to the socket function. This socket which is going to bind to a port and accept connections from clients is called master socket. Note that in the initialization of the socket we are using an IP v4 address and the TCP/IP protocol. 3. The socket should be bound to a socket address. The socket address is defined using struct variable of type sockaddr_in. Three properties of this variable should be initialized in order to define the bind address: sin_family, sin_addr.s_addr and sin_port. Replace placeholder B with declaration and initialization of the struct variable keeping the socket address and initialize it considering that we are using IP v4 addresses and it should accept all inbound connection requests on port Replace placeholder C with the proper call to the bind function so that the server could listen to this port. 5. Replace placeholder D with the call to the listen function. This makes the server E. Bagheri (2013) K. Clowes (2014)

4 start listening on the socket. The server should be able to accept 100,000 simultaneous connections. 6. The server waits for a connection from a client; it accepts the connection request sent on the socket address that has been defined for. Accepting a connection request by the server creates a connection. Like sockets, each connection has a unique integer ID. Considering that multiple clients can connect to the server master socket, the server uses connection ids to distinguish between these connections when it sends and receives data to them. In the template code, the variable conn_id stores the connection ID for the current connection. Replace placeholder E with a proper call to the accept function so the server accepts incoming connections and places the connection ID in the conn_id variable. 7. Once the connection is accepted, the server creates a thread running the handle_connection function. This function gets the connection id and handles the input text from the client, by checking whether it is the correct password or not, and replying to the client. Replace F with the required code that checks if accepting the connection was successful and if so creates a thread running handle_connection function sending the connection id as parameter. Make sure that you detach the thread considering that each process can have a limited number of attached thread. 8. Reading from the connection can be done using the recv function. Replace placeholder G with proper call to recv function so it reads from the connection and puts the read characters in recv_buffer and the number of read characters in the n variable. Make sure the receive function has finished correctly by checking its return value. 9. After reading from the connection, it should be ensured that no problem has occurred while reading from the connection. If the reading is successful, it should verify if the read string is the password using the verify_password function. The server should write T or F on the socket using send function according to the result of verify_password functions. Make sure the send function has finished correctly by checking its return value. Replace H with the codes preforming these actions. 10. Close the connection after reading has finished with proper call to the close function in placeholder I. 11. The server is now complete, now we need to complete the client program. We start by completing the try_a_password function, which creates a random password and checks if it is, corrects by sending it to the server. Similar to the server program, a socket is needed in order to create a connection: Replace placeholder J with proper

5 call to the socket function. 12. In order to make a connection using a socket you need to define a socket address. Declare and initialize the socket address for the socket in placeholder K. The difference here is that you should give the address of the specific server address (in the case that the server runs locally, it is ) to the client. In this program, the server address is passed to the program as the program s only argument. The server address is stored in binary form in the sin_addr.s_addr field of the socket address. Therefore you should use inet_pton function in order to convert the server address from string to binary form. The sin_family and sin_port are the same as the one we used for the server because the client also uses port 5000 to connect to the server and uses the IP V4 standard. 13. Use the socket you made to connect to the server. Replace L with proper call to connect function to connect the server process. Check the return value of the connect function in order to make sure the connection has been successful. 14. After connecting to the server, the client writes the random password into the socket using send function. Replace placeholder M with proper call to the send function. Make sure the send function has finished correctly by checking its return value. 15. After the password has been sent to the server, the program should wait for the server s response. It should receive the response and check if the server response is T. If the response is T, the program should print what the password is. Replace placeholder N with code performing these actions. 16. Replace placeholder O by calling the close function in order to close the socket after writing has been completed. 17. The main program should call the try_a_password function iteratively until it finds out what the password is. Replace P with a loop that performs this action. Make sure that the program stops after it found the password. Once the client and server programs are complete, run the server program first and then run the client program. The server program does not show anything but the client program prints the random passwords which it tried but was not correct until it finds the correct password. Include in your report what is the correct password. 5. Deliverables E. Bagheri (2013) K. Clowes (2014)

6 You should complete the exercises described above. Then, you will prepare and submit the results in one single zip file containing the following two items: 2. A zip archive with all of your source code files. This will be a zip file, named as follows: YourName_[Lab#]_Code.zip.