Examples of using raw sockets (c, linux, raw socket)

udp_to_local.c

/*
    An example of using raw sockets.

    You can capture packets by tcpdump:
        tcpdump -X -s0 -i lo -p udp
 */

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <linux/ip.h>
#include <linux/udp.h>

#define PCKT_LEN 8192

unsigned short csum(unsigned short *buf, int nwords)
{
  unsigned long sum;
  for(sum=0; nwords>0; nwords--)
    sum += *buf++;
  sum = (sum >> 16) + (sum &0xffff);
  sum += (sum >> 16);
  return (unsigned short)(~sum);
}

int main(int argc, char const *argv[])
{
  if (argc != 5) {
    printf("Error: Invalid parameters!\n");
    printf("Usage: %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
    exit(1);
  }

  u_int16_t src_port, dst_port;
  u_int32_t src_addr, dst_addr;
  src_addr = inet_addr(argv[1]);
  dst_addr = inet_addr(argv[3]);
  src_port = atoi(argv[2]);
  dst_port = atoi(argv[4]);

  int sd;
  char buffer[PCKT_LEN];
  struct iphdr *ip = (struct iphdr *) buffer;
  struct udphdr *udp = (struct udphdr *) (buffer + sizeof(struct iphdr));

  struct sockaddr_in sin;
  int one = 1;
  const int *val = &one;

  memset(buffer, 0, PCKT_LEN);

  // create a raw socket with UDP protocol
  sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
  if (sd < 0) {
    perror("socket() error");
    exit(2);
  }
  printf("OK: a raw socket is created.\n");

  // inform the kernel do not fill up the packet structure, we will build our own
  if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0) {
    perror("setsockopt() error");
    exit(2);
  }
  printf("OK: socket option IP_HDRINCL is set.\n");

  sin.sin_family = AF_INET;
  sin.sin_port = htons(dst_port);
  sin.sin_addr.s_addr = inet_addr("127.0.0.1");

  // fabricate the IP header
  ip->ihl      = 5;
  ip->version  = 4;
  ip->tos      = 16; // low delay
  ip->tot_len  = sizeof(struct iphdr) + sizeof(struct udphdr);
  ip->id       = htons(54321);
  ip->ttl      = 64; // hops
  ip->protocol = 17; // UDP
  // source IP address, can use spoofed address here
  ip->saddr = src_addr;
  ip->daddr = dst_addr;

  // fabricate the UDP header
  udp->source = htons(src_port);
  // destination port number
  udp->dest = htons(dst_port);
  udp->len = htons(sizeof(struct udphdr));

  // calculate the checksum for integrity
  ip->check = csum((unsigned short *)buffer,
                   sizeof(struct iphdr) + sizeof(struct udphdr));

  if (sendto(sd, buffer, ip->tot_len, 0,
             (struct sockaddr *)&sin, sizeof(sin)) < 0)
  {
    perror("sendto()");
    exit(3);
  }
  printf("OK: one packet is sent.\n");

  close(sd);
  return 0;
}

udp_to_remote.c

/*
    An example of using raw sockets.

    You can capture packets by tcpdump:
        tcpdump -X -s0 -i eth0 -p udp
 */

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <linux/ip.h>
#include <linux/udp.h>

#define PCKT_LEN 8192

unsigned short csum(unsigned short *buf, int nwords)
{
  unsigned long sum;
  for(sum=0; nwords>0; nwords--)
    sum += *buf++;
  sum = (sum >> 16) + (sum &0xffff);
  sum += (sum >> 16);
  return (unsigned short)(~sum);
}

int main(int argc, char const *argv[])
{
  if (argc != 5) {
    printf("Error: Invalid parameters!\n");
    printf("Usage: %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
    exit(1);
  }

  u_int16_t src_port, dst_port;
  u_int32_t src_addr, dst_addr;
  src_addr = inet_addr(argv[1]);
  dst_addr = inet_addr(argv[3]);
  src_port = atoi(argv[2]);
  dst_port = atoi(argv[4]);

  int sd;
  char buffer[PCKT_LEN];
  struct iphdr *ip = (struct iphdr *) buffer;
  struct udphdr *udp = (struct udphdr *) (buffer + sizeof(struct iphdr));

  struct sockaddr_in sin;
  int one = 1;
  const int *val = &one;

  memset(buffer, 0, PCKT_LEN);

  // create a raw socket with UDP protocol
  sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
  if (sd < 0) {
    perror("socket() error");
    exit(2);
  }
  printf("OK: a raw socket is created.\n");

  // inform the kernel do not fill up the packet structure, we will build our own
  if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0) {
    perror("setsockopt() error");
    exit(2);
  }
  printf("OK: socket option IP_HDRINCL is set.\n");

  sin.sin_family = AF_INET;
  sin.sin_port = htons(dst_port);
  sin.sin_addr.s_addr = dst_addr;

  // fabricate the IP header
  ip->ihl      = 5;
  ip->version  = 4;
  ip->tos      = 16; // low delay
  ip->tot_len  = sizeof(struct iphdr) + sizeof(struct udphdr);
  ip->id       = htons(54321);
  ip->ttl      = 64; // hops
  ip->protocol = 17; // UDP
  // source IP address, can use spoofed address here
  ip->saddr = src_addr;
  ip->daddr = dst_addr;

  // fabricate the UDP header
  udp->source = htons(src_port);
  // destination port number
  udp->dest = htons(dst_port);
  udp->len = htons(sizeof(struct udphdr));

  // calculate the checksum for integrity
  ip->check = csum((unsigned short *)buffer,
                   sizeof(struct iphdr) + sizeof(struct udphdr));

  if (sendto(sd, buffer, ip->tot_len, 0,
             (struct sockaddr *)&sin, sizeof(sin)) < 0)
  {
    perror("sendto()");
    exit(3);
  }
  printf("OK: one packet is sent.\n");

  close(sd);
  return 0;
}