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Network Programming

- - - 1. Client-Server Model
    - 2. Socket Address Structures
    - 3. Robust Error Handling
    - 4. HTTP Protocol Basics
    - 5. Concurrency with `fork()`
    - 6. DNS Resolution with `getaddrinfo()`
  - Compilation & Testing
  - Summary
  - Multiple-Choice Questions
  - Answers and Explanations

1. Client-Server Model

Key Concepts:

Clients request services, servers provide services.
Communication via sockets over TCP/IP or UDP.
Servers handle multiple clients via concurrency (forking/threading).

Code Example: Simple TCP Echo Server

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>#define PORT 8080
#define BUFFER_SIZE 1024int main() {int server_fd, new_socket;struct sockaddr_in address;int addrlen = sizeof(address);char buffer[BUFFER_SIZE] = {0};// Create socketif ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {perror("socket failed");exit(EXIT_FAILURE);}// Configure addressaddress.sin_family = AF_INET;address.sin_addr.s_addr = INADDR_ANY;address.sin_port = htons(PORT);// Bind socket to portif (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {perror("bind failed");exit(EXIT_FAILURE);}// Listen for connectionsif (listen(server_fd, 3) < 0) {perror("listen failed");exit(EXIT_FAILURE);}printf("Server listening on port %d...\n", PORT);// Accept incoming connectionif ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {perror("accept failed");exit(EXIT_FAILURE);}// Echo received datassize_t bytes_read;while ((bytes_read = read(new_socket, buffer, BUFFER_SIZE)) > 0) {write(new_socket, buffer, bytes_read);memset(buffer, 0, BUFFER_SIZE);}close(new_socket);close(server_fd);return 0;
}

Test Client (Use telnet or nc):

$ telnet localhost 8080
Trying 127.0.0.1...
Connected to localhost.
Hello, Server!  # Client sends
Hello, Server!  # Server echoes back

2. Socket Address Structures

Key Concepts:

sockaddr_in for IPv4, sockaddr_in6 for IPv6.
Use htons()/htonl() for byte ordering (host-to-network).
INADDR_ANY binds to all interfaces.

Code Snippet: Address Initialization

struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(8080);
server_addr.sin_addr.s_addr = htonl(INADDR_ANY);  // Bind to all interfaces

3. Robust Error Handling

Key Concepts:

Check return values of all system calls (socket, bind, listen, etc.).
Use perror() to print human-readable errors.
Graceful shutdown on failure.

Code Example: Error Handling in bind()

if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {perror("bind failed");close(sockfd);  // Cleanup socketexit(EXIT_FAILURE);
}

4. HTTP Protocol Basics

Key Concepts:

HTTP is a text-based protocol over TCP.
Requests: GET /path HTTP/1.1\r\nHost: ...\r\n\r\n.
Responses: HTTP/1.1 200 OK\r\nContent-Type: ...\r\n\r\n<body>.

Code Example: Minimal HTTP Server

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>#define PORT 8080
#define RESPONSE "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\nHello, World!\r\n"int main() {int server_fd, new_socket;struct sockaddr_in address;int addrlen = sizeof(address);// Create socket, bind, listen (same as previous example)printf("HTTP Server listening on port %d...\n", PORT);if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {perror("accept failed");exit(EXIT_FAILURE);}// Send HTTP responsesend(new_socket, RESPONSE, strlen(RESPONSE), 0);printf("Response sent\n");close(new_socket);close(server_fd);return 0;
}

Test with curl:

$ curl http://localhost:8080
Hello, World!

5. Concurrency with `fork()`

Key Concepts:

Servers handle multiple clients by forking a child process per connection.
Parent process continues listening; child handles the client.

Code Snippet: Forking Server

while (1) {int client_sock = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);if (client_sock < 0) {perror("accept failed");continue;}pid_t pid = fork();if (pid == 0) {  // Child processclose(server_fd);  // Child doesn't need listenerhandle_client(client_sock);exit(0);  // Terminate child after handling} else if (pid > 0) {close(client_sock);  // Parent doesn't need client socket} else {perror("fork failed");}
}

Test with Multiple Clients:

$ telnet localhost 8080 &  # Start multiple clients in background
$ telnet localhost 8080 &

6. DNS Resolution with `getaddrinfo()`

Key Concepts:

Converts hostnames (e.g., “google.com”) to IP addresses.
Handles IPv4/IPv6 transparently.

Code Example: Resolve Hostname

#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <arpa/inet.h>int main() {struct addrinfo hints, *res, *p;char ipstr[INET6_ADDRSTRLEN];memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // IPv4 or IPv6if (getaddrinfo("google.com", NULL, &hints, &res) != 0) {perror("getaddrinfo failed");exit(EXIT_FAILURE);}// Iterate through resultsfor (p = res; p != NULL; p = p->ai_next) {void *addr;if (p->ai_family == AF_INET) {  // IPv4struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;addr = &(ipv4->sin_addr);} else {  // IPv6struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;addr = &(ipv6->sin6_addr);}inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);printf("IP: %s\n", ipstr);}freeaddrinfo(res);return 0;
}

Output:

IP: 142.250.191.46
IP: 2607:f8b0:4005:80a::200e

Compilation & Testing

Build Commands:

# For server.c
gcc server.c -o server# For DNS resolver
gcc dns_resolver.c -o dns_resolver

Run:

./server  # Start the server in one terminal
./dns_resolver  # Resolve hostnames in another

Summary

TCP Sockets: Use socket(), bind(), listen(), accept() for servers; connect() for clients.
HTTP: Text-based protocol; responses include status line, headers, and body.
Concurrency: Forking allows handling multiple clients simultaneously.
Error Handling: Check all system calls; use perror() and exit() on failure.
DNS: Use getaddrinfo() to resolve hostnames to IP addresses.

Multiple-Choice Questions

Question 1: Socket API Fundamentals
Which of the following are required steps to set up a TCP server socket?
A) Call socket with SOCK_STREAM and AF_INET
B) Bind the socket to a specific port using bind
C) Use listen to mark the socket as passive
D) Call accept to wait for client connections

Question 2: Address Conversion
Which functions correctly convert human-readable IP addresses to binary network byte order?
A) inet_ntoa
B) inet_pton
C) getaddrinfo
D) htons

Question 3: HTTP Protocol
Which HTTP request methods are idempotent?
A) GET
B) POST
C) PUT
D) DELETE

Question 4: TCP vs. UDP
Which statements about TCP and UDP are true?
A) TCP guarantees in-order delivery; UDP does not.
B) UDP is connection-oriented.
C) TCP uses sequence numbers to detect packet loss.
D) UDP is suitable for real-time video streaming.

Question 5: Concurrent Servers
Which concurrency models allow a server to handle multiple clients simultaneously?
A) Single-threaded iterative processing
B) Forking a child process per client
C) Using threads with a shared listening socket
D) I/O multiplexing with select or epoll

Question 6: Socket Options
Which socket options are relevant for reusing ports to avoid “Address already in use” errors?
A) SO_REUSEADDR
B) SO_KEEPALIVE
C) SO_LINGER
D) SO_RCVBUF

Question 7: Error Handling
Which errors might occur when using blocking I/O on sockets?
A) EINTR (interrupted system call)
B) ECONNREFUSED
C) ENOMEM (out of memory)
D) EAGAIN (resource temporarily unavailable)

Question 8: URI Parsing
When parsing an HTTP request URI, which components are mandatory?
A) Hostname
B) Port number
C) Path
D) Query string

Question 9: Web Content Generation
Which techniques generate dynamic web content?
A) Returning a static HTML file
B) Executing a CGI program
C) Using server-side templates (e.g., Jinja2)
D) Serving precompiled binary data

Question 10: Protocol-Independent Programming
Which functions support protocol-independent server code?
A) gethostbyname
B) getaddrinfo
C) inet_ntop
D) strerror

Question 11: HTTP Headers
Which HTTP headers are required in a valid HTTP/1.1 response?
A) Content-Length
B) Connection
C) Date
D) Server

Question 12: Robust I/O
Which issues does the RIO package address?
A) Partial reads/writes due to signal interrupts
B) Buffer overflow vulnerabilities
C) Network latency optimization
D) Short counts in I/O operations

Question 13: Client-Server Communication
Which steps are required for a client to connect to a server using TCP?
A) Call socket with SOCK_DGRAM
B) Use connect to establish a connection
C) Call bind to specify the client port
D) Use getaddrinfo to resolve the server address

Question 14: HTTP Status Codes
Which status codes indicate client-side errors?
A) 200 OK
B) 404 Not Found
C) 500 Internal Server Error
D) 400 Bad Request

Question 15: Secure Programming
Which practices mitigate buffer overflow attacks in network programs?
A) Using strcpy for string operations
B) Validating input lengths before copying
C) Employing bounds-checked functions like strncpy
D) Disabling stack canaries

Answers and Explanations

Question 1
Correct Answers: B, C, D
Explanation:

A) Incorrect. While SOCK_STREAM is required for TCP, AF_INET restricts the server to IPv4. Protocol-independent servers use AF_UNSPEC.
B) Correct. bind associates the socket with a port.
C) Correct. listen configures the socket to accept incoming connections.
D) Correct. accept blocks until a client connects.

Question 2
Correct Answers: B, C
Explanation:

A) inet_ntoa converts binary to string, not vice versa.
B) inet_pton converts IPv4/IPv6 addresses to binary.
C) getaddrinfo resolves hostnames to struct addrinfo, including binary addresses.
D) htons converts port numbers to network byte order, not IP addresses.

Question 3
Correct Answers: A, C, D
Explanation:

Idempotent methods produce the same result after multiple identical requests.
GET, PUT, and DELETE are idempotent; POST is not.

Question 4
Correct Answers: A, C, D
Explanation:

B) Incorrect. UDP is connectionless.
D) Correct. UDP’s lower latency suits real-time applications.

Question 5
Correct Answers: B, C, D
Explanation:

A) Iterative servers process clients sequentially, not concurrently.
B/C/D) Forking, threading, and I/O multiplexing enable concurrency.

Question 6
Correct Answer: A
Explanation:

SO_REUSEADDR allows reuse of ports in TIME_WAIT state. Other options are unrelated.

Question 7
Correct Answers: A, B
Explanation:

EINTR occurs if a signal interrupts a blocking call. ECONNREFUSED happens if the server rejects the connection.
EAGAIN is non-blocking I/O specific.

Question 8
Correct Answer: C
Explanation:

The path is mandatory (e.g., GET /index.html). Hostname is required in HTTP/1.1 but parsed from the Host header, not the URI.

Question 9
Correct Answers: B, C
Explanation:

A/D) Static content is not dynamic.
B/C) CGI and templates generate dynamic responses.

Question 10
Correct Answers: B, C
Explanation:

getaddrinfo and inet_ntop handle IPv4/IPv6 transparently. gethostbyname is IPv4-only.

Question 11
Correct Answers: None (Trick Question)
Explanation:

HTTP/1.1 requires Date and Connection headers, but these are often enforced by servers, not clients. Content-Length is required only for non-chunked responses.

Question 12
Correct Answers: A, D
Explanation: