netcode

#include <stdint.h>

#define IP_HEADER_MIN_SIZE 20
#define IP_HEADER_MAX_SIZE 60
struct ip_header
{
    unsigned int version:4;
    unsigned int ihl:4;
    uint8_t tos;
    uint16_t tot_len;
    uint16_t id;
    uint16_t frag_off;
    uint8_t ttl;
    uint8_t protocol;
    uint16_t check;
    uint32_t saddr;
    uint32_t daddr;
    // IP options starts here
};

struct udp_header
{
    uint16_t uh_sport;
    uint16_t uh_dport;
    uint16_t uh_ulen;
    uint16_t uh_sum;
    // UDP payload starts here
};

/* Build a UDP/IP packet and store it into buffer passed as parameter */
void new_packet(char *buf, char *options, int optsize, int udp_udp_size) {
  int i;
  int ip_header_size = optsize + sizeof(struct ip_header); /* compute total IP header size, from option size */
  struct ip_header *ip = (void*) buf;

  for (i = 0 ; i < optsize; i++)
    buf[i + sizeof(struct ip_header)] = options[i];

  ip->ihl = ip_header_size >> 2; /* put correct header size in ip header */
  struct udp_header *udp = (void*)buf + ip_header_size; /* udp headers starts immediately after ip header end */
  udp->uh_ulen = udp_udp_size; /* put udp payload size in udp header */

  // Initialization of irrelevant fields (source/destination IP, ports, etc) have been omitted.
}

void raw_socket_write(char *data, int size) {
  // Write to raw socket (omitted)
}

void send_packet(char *buf) {
  struct ip_header *h1 = (void*) buf;
  int header_size = h1->ihl << 2; /* get ip header size */
  struct udp_header *h2 = (void*) buf + header_size; /* udp header starts immediately after ip header end */
  int udp_size = h2->uh_ulen; /* get udp payload size from udp header */
  int i;

  uint32_t checksum = 0;

  // compute UDP checksum
  for (i = 0; i < udp_size; i++) {
    // Actual checksum computation omitted
  }
  h2->uh_sum = checksum;

  raw_socket_write(buf, header_size + udp_size);
}

#define PAYLOAD_MIN_SIZE 16
#define PAYLOAD_MAX_SIZE 1024
int send_request_over_udp(char *options, int optsize, int udpsize) {
  char buf[1024];
  int i;
  if ((optsize + sizeof(struct ip_header) >= IP_HEADER_MIN_SIZE) &&
      (optsize + sizeof(struct ip_header) <= IP_HEADER_MAX_SIZE) &&
      (udpsize + sizeof(struct udp_header) >= PAYLOAD_MIN_SIZE) &&
      (udpsize + sizeof(struct udp_header) <= PAYLOAD_MAX_SIZE))
  {
    /* sizes are in acceptable range */
    new_packet(buf, options, optsize, udpsize);

    // Create packet payload here (omitted)

    // ...

    send_packet(buf);
  } else {
    return -1; // Error
  }
}

int main(void) {
  char options[1024];
  int optsize, udpsize;
  send_request_over_udp(options, optsize, udpsize);
}