raw_tcp.c

// Manufacture TCP Packets
// Compile with
// gcc -o raw_tcp raw_tcp.c
// run as su
// sudo ./raw_tcp

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/ip.h> // For IP header
#include <netinet/tcp.h> // For TCP header

// Checksum calculation
unsigned short checksum(void *b, int len) {
    unsigned short *buf = b;
    unsigned int sum = 0;
    unsigned short result;

    for (sum = 0; len > 1; len -= 2)
        sum += *buf++;
    if (len == 1)
        sum += *(unsigned char *)buf;
    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    result = ~sum;
    return result;
}

int main() {
    int sock;
    char packet[4096]; // Buffer for packet
    struct sockaddr_in dest_addr;
    struct iphdr *ip_header;
    struct tcphdr *tcp_header;

    // Create a raw socket
    sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
    if (sock < 0) {
        perror("Socket creation failed");
        exit(1);
    }

    // Zero out the packet buffer
    memset(packet, 0, sizeof(packet));

    // Fill in IP Header
    ip_header = (struct iphdr *)packet;
    ip_header->ihl = 5; // Header length
    ip_header->version = 4; // IPv4
    ip_header->tos = 0; // Type of service
    ip_header->tot_len = sizeof(struct iphdr) + sizeof(struct tcphdr); // Total length
    ip_header->id = htonl(54321); // Random id
    ip_header->frag_off = 0; // No fragmentation
    ip_header->ttl = 255; // TTL
    ip_header->protocol = IPPROTO_TCP; // TCP
    ip_header->saddr = inet_addr("192.168.1.1"); // Source IP
    ip_header->daddr = inet_addr("192.168.1.2"); // Destination IP
    ip_header->check = checksum((unsigned short *)ip_header, sizeof(struct iphdr));

    // Fill in TCP Header
    tcp_header = (struct tcphdr *)(packet + sizeof(struct iphdr));
    tcp_header->source = htons(12345); // Source port
    tcp_header->dest = htons(80); // Destination port
    tcp_header->seq = htonl(0); // Sequence number
    tcp_header->ack_seq = 0; // Acknowledgment number
    tcp_header->doff = 5; // Header size
    tcp_header->syn = 1; // SYN flag
    tcp_header->window = htons(5840); // Window size
    tcp_header->check = 0; // Checksum (calculated later)
    tcp_header->urg_ptr = 0; // Urgent pointer

    // Pseudo header for checksum calculation
    struct pseudo_header {
        u_int32_t source_address;
        u_int32_t dest_address;
        u_int8_t placeholder;
        u_int8_t protocol;
        u_int16_t tcp_length;
        struct tcphdr tcp;
    } pseudo_hdr;

    pseudo_hdr.source_address = ip_header->saddr;
    pseudo_hdr.dest_address = ip_header->daddr;
    pseudo_hdr.placeholder = 0;
    pseudo_hdr.protocol = IPPROTO_TCP;
    pseudo_hdr.tcp_length = htons(sizeof(struct tcphdr));
    memcpy(&pseudo_hdr.tcp, tcp_header, sizeof(struct tcphdr));
    tcp_header->check = checksum((unsigned short *)&pseudo_hdr, sizeof(pseudo_hdr));

    // Set destination address
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_port = tcp_header->dest;
    dest_addr.sin_addr.s_addr = ip_header->daddr;

    // Send the packet
    if (sendto(sock, packet, ip_header->tot_len, 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) < 0) {
        perror("Send failed");
    } else {
        printf("Packet sent successfully\n");
    }

    close(sock);
    return 0;
}