TCP Handshake in Code

Socket Programming Example (C)

// Server-side code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main() {
    int server_fd, new_socket;
    struct sockaddr_in address;
    int addrlen = sizeof(address);
    
    // Creating socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }
    
    // Setting socket options
    int opt = 1;
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    
    // Binding socket to port 8080
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(8080);
    
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    
    // Listening for connections (TCP handshake happens here)
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    
    // Accepting a connection (completes the handshake)
    if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
        perror("accept");
        exit(EXIT_FAILURE);
    }
    
    // Connection established, can send/receive data
    
    return 0;
}

Client-side Code Example

// Client-side code
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>

int main() {
    int sock = 0;
    struct sockaddr_in serv_addr;
    
    // Creating socket
    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        printf("\n Socket creation error \n");
        return -1;
    }
    
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(8080);
    
    // Convert IPv4 address from text to binary
    if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
        printf("\nInvalid address/ Address not supported \n");
        return -1;
    }
    
    // Connect initiates the TCP handshake
    if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        printf("\nConnection Failed \n");
        return -1;
    }
    
    // Connection established, can send/receive data
    
    return 0;
}

TCP Packet Structure

TCP Header Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |       |C|E|U|A|P|R|S|F|                               |
   | Offset| Res. |W|C|R|C|S|S|Y|I|            Window             |
   |       |       |R|E|G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                

Key TCP Flags for Handshake

• SYN (Synchronize): Initiates a connection
• ACK (Acknowledge): Acknowledges received data
• FIN (Finish): No more data from sender
• RST (Reset): Reset the connection
• PSH (Push): Push data to the application immediately
• URG (Urgent): Urgent data present

Handshake Packet Configuration

• SYN packet: SYN=1, ACK=0
• SYN-ACK packet: SYN=1, ACK=1
• ACK packet: SYN=0, ACK=1

Security Implications of TCP Handshake

SYN Flood Attack

A SYN flood is a form of denial-of-service attack where an attacker sends a sequence of SYN packets but never completes the handshake. This leaves connections in a half-open state and can exhaust server resources.

TCP Sequence Prediction

If an attacker can predict the sequence numbers used in TCP connections, they can potentially hijack sessions or inject data into existing connections.

TCP Reset Attack

An attacker can disrupt existing TCP connections by sending forged RST packets if they can guess or intercept valid sequence numbers.

Protection Mechanisms

• SYN cookies: A technique to prevent SYN flood attacks by avoiding resource allocation until the handshake completes
• Random Initial Sequence Numbers (ISN): Makes sequence prediction more difficult
• TCP Authentication Option (TCP-AO): Authenticates TCP segments to prevent forgery
• Firewall protection: Limiting SYN packets from a single source