问题描述：

在linux下使用C语言和raw socket来简单实现ipv4下的udp数据包的发送

参考资源：

http://www.pdbuchan.com/rawsock/rawsock.html

解决方案：

下面的代码拷贝于"参考资源"中的udp4.c文件。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>           // close()
#include <string.h>           // strcpy, memset(), and memcpy()

#include <netdb.h>            // struct addrinfo
#include <sys/types.h>        // needed for socket(), uint8_t, uint16_t, uint32_t
#include <sys/socket.h>       // needed for socket()
#include <netinet/in.h>       // IPPROTO_RAW, IPPROTO_IP, IPPROTO_UDP, INET_ADDRSTRLEN
#include <netinet/ip.h>       // struct ip and IP_MAXPACKET (which is 65535)
#include <netinet/udp.h>      // struct udphdr
#include <arpa/inet.h>        // inet_pton() and inet_ntop()
#include <sys/ioctl.h>        // macro ioctl is defined
#include <bits/ioctls.h>      // defines values for argument "request" of ioctl.
#include <net/if.h>           // struct ifreq

#include <errno.h>            // errno, perror()

// Define some constants.
#define IP4_HDRLEN 20         // IPv4 header length
#define UDP_HDRLEN  8         // UDP header length, excludes data

// Function prototypes
uint16_t checksum (uint16_t *, int);
uint16_t udp4_checksum (struct ip, struct udphdr, uint8_t *, int);
char *allocate_strmem (int);
uint8_t *allocate_ustrmem (int);
int *allocate_intmem (int);

int
main (int argc, char **argv)
{
  int status, datalen, sd, *ip_flags;
  const int on = 1;
  char *interface, *target, *src_ip, *dst_ip;
  struct ip iphdr;
  struct udphdr udphdr;
  uint8_t *data, *packet;
  struct addrinfo hints, *res;
  struct sockaddr_in *ipv4, sin;
  struct ifreq ifr;
  void *tmp;

  // Allocate memory for various arrays.
  data = allocate_ustrmem (IP_MAXPACKET);
  packet = allocate_ustrmem (IP_MAXPACKET);
  interface = allocate_strmem (40);
  target = allocate_strmem (40);
  src_ip = allocate_strmem (INET_ADDRSTRLEN);
  dst_ip = allocate_strmem (INET_ADDRSTRLEN);
  ip_flags = allocate_intmem (4);

  // Interface to send packet through.
  //-指定需要发送数据的网卡接口
  strcpy (interface, "eth0");

  // Submit request for a socket descriptor to look up interface.
  if ((sd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
    perror ("socket() failed to get socket descriptor for using ioctl() ");
    exit (EXIT_FAILURE);
  }

  // Use ioctl() to look up interface index which we will use to
  // bind socket descriptor sd to specified interface with setsockopt() since
  // none of the other arguments of sendto() specify which interface to use.
  memset (&ifr, 0, sizeof (ifr));
  snprintf (ifr.ifr_name, sizeof (ifr.ifr_name), "%s", interface);
  if (ioctl (sd, SIOCGIFINDEX, &ifr) < 0) {
    perror ("ioctl() failed to find interface ");
    return (EXIT_FAILURE);
  }
  close (sd);
  printf ("Index for interface %s is %i\n", interface, ifr.ifr_ifindex);

  // Source IPv4 address: you need to fill this out
  //-指定IP的源地址
  strcpy (src_ip, "192.168.1.132");

  // Destination URL or IPv4 address: you need to fill this out
  //-指定IP的目的地址
  strcpy (target, "www.google.com");

  // Fill out hints for getaddrinfo().
  memset (&hints, 0, sizeof (struct addrinfo));
  hints.ai_family = AF_INET;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags = hints.ai_flags | AI_CANONNAME;

  // Resolve target using getaddrinfo().
  if ((status = getaddrinfo (target, NULL, &hints, &res)) != 0) {
    fprintf (stderr, "getaddrinfo() failed: %s\n", gai_strerror (status));
    exit (EXIT_FAILURE);
  }
  
  ipv4 = (struct sockaddr_in *) res->ai_addr;
  tmp = &(ipv4->sin_addr);
  if (inet_ntop (AF_INET, tmp, dst_ip, INET_ADDRSTRLEN) == NULL) {
    status = errno;
    fprintf (stderr, "inet_ntop() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }
  freeaddrinfo (res);

  // UDP data
  // 构造UDP的数据
  datalen = 4;
  data[0] = 'T';
  data[1] = 'e';
  data[2] = 's';
  data[3] = 't';

  // IPv4 header
  // 构造ip报文
  // IPv4 header length (4 bits): Number of 32-bit words in header = 5
  iphdr.ip_hl = IP4_HDRLEN / sizeof (uint32_t);

  // Internet Protocol version (4 bits): IPv4
  iphdr.ip_v = 4;

  // Type of service (8 bits)
  iphdr.ip_tos = 0;

  // Total length of datagram (16 bits): IP header + UDP header + datalen
  iphdr.ip_len = htons (IP4_HDRLEN + UDP_HDRLEN + datalen);

  // ID sequence number (16 bits): unused, since single datagram
  iphdr.ip_id = htons (0);

  // Flags, and Fragmentation offset (3, 13 bits): 0 since single datagram

  // Zero (1 bit)
  ip_flags[0] = 0;

  // Do not fragment flag (1 bit)
  ip_flags[1] = 0;

  // More fragments following flag (1 bit)
  ip_flags[2] = 0;

  // Fragmentation offset (13 bits)
  ip_flags[3] = 0;

  iphdr.ip_off = htons ((ip_flags[0] << 15)
                      + (ip_flags[1] << 14)
                      + (ip_flags[2] << 13)
                      +  ip_flags[3]);

  // Time-to-Live (8 bits): default to maximum value
  iphdr.ip_ttl = 255;

  // Transport layer protocol (8 bits): 17 for UDP
  iphdr.ip_p = IPPROTO_UDP;

  // Source IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, src_ip, &(iphdr.ip_src))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // Destination IPv4 address (32 bits)
  if ((status = inet_pton (AF_INET, dst_ip, &(iphdr.ip_dst))) != 1) {
    fprintf (stderr, "inet_pton() failed.\nError message: %s", strerror (status));
    exit (EXIT_FAILURE);
  }

  // IPv4 header checksum (16 bits): set to 0 when calculating checksum
  iphdr.ip_sum = 0;
  iphdr.ip_sum = checksum ((uint16_t *) &iphdr, IP4_HDRLEN);

  // UDP header

  // Source port number (16 bits): pick a number
  //指定UDP报文源端口
  udphdr.source = htons (4950);

  // Destination port number (16 bits): pick a number
  //指定UDP报文的目的端口
  udphdr.dest = htons (4950);

  // Length of UDP datagram (16 bits): UDP header + UDP data
  udphdr.len = htons (UDP_HDRLEN + datalen);

  // UDP checksum (16 bits)
  udphdr.check = udp4_checksum (iphdr, udphdr, data, datalen);

  // Prepare packet.

  // First part is an IPv4 header.
  memcpy (packet, &iphdr, IP4_HDRLEN * sizeof (uint8_t));

  // Next part of packet is upper layer protocol header.
  memcpy ((packet + IP4_HDRLEN), &udphdr, UDP_HDRLEN * sizeof (uint8_t));

  // Finally, add the UDP data.
  memcpy (packet + IP4_HDRLEN + UDP_HDRLEN, data, datalen * sizeof (uint8_t));

  // The kernel is going to prepare layer 2 information (ethernet frame header) for us.
  // For that, we need to specify a destination for the kernel in order for it
  // to decide where to send the raw datagram. We fill in a struct in_addr with
  // the desired destination IP address, and pass this structure to the sendto() function.
  memset (&sin, 0, sizeof (struct sockaddr_in));
  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = iphdr.ip_dst.s_addr;

  // Submit request for a raw socket descriptor.
  if ((sd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
    perror ("socket() failed ");
    exit (EXIT_FAILURE);
  }

  // Set flag so socket expects us to provide IPv4 header.
  if (setsockopt (sd, IPPROTO_IP, IP_HDRINCL, &on, sizeof (on)) < 0) {
    perror ("setsockopt() failed to set IP_HDRINCL ");
    exit (EXIT_FAILURE);
  }

  // Bind socket to interface index.
  if (setsockopt (sd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof (ifr)) < 0) {
    perror ("setsockopt() failed to bind to interface ");
    exit (EXIT_FAILURE);
  }

  // Send packet.
  if (sendto (sd, packet, IP4_HDRLEN + UDP_HDRLEN + datalen, 0, (struct sockaddr *) &sin, sizeof (struct sockaddr)) < 0)  {
    perror ("sendto() failed ");
    exit (EXIT_FAILURE);
  }

  // Close socket descriptor.
  close (sd);

  // Free allocated memory.
  free (data);
  free (packet);
  free (interface);
  free (target);
  free (src_ip);
  free (dst_ip);
  free (ip_flags);

  return (EXIT_SUCCESS);
}

// Checksum function
uint16_t
checksum (uint16_t *addr, int len)
{
  int nleft = len;
  int sum = 0;
  uint16_t *w = addr;
  uint16_t answer = 0;

  while (nleft > 1) {
    sum += *w++;
    nleft -= sizeof (uint16_t);
  }

  if (nleft == 1) {
    *(uint8_t *) (&answer) = *(uint8_t *) w;
    sum += answer;
  }

  sum = (sum >> 16) + (sum & 0xFFFF);
  sum += (sum >> 16);
  answer = ~sum;
  return (answer);
}

// Build IPv4 UDP pseudo-header and call checksum function.
uint16_t
udp4_checksum (struct ip iphdr, struct udphdr udphdr, uint8_t *payload, int payloadlen)
{
  char buf[IP_MAXPACKET];
  char *ptr;
  int chksumlen = 0;
  int i;

  ptr = &buf[0];  // ptr points to beginning of buffer buf

  // Copy source IP address into buf (32 bits)
  memcpy (ptr, &iphdr.ip_src.s_addr, sizeof (iphdr.ip_src.s_addr));
  ptr += sizeof (iphdr.ip_src.s_addr);
  chksumlen += sizeof (iphdr.ip_src.s_addr);

  // Copy destination IP address into buf (32 bits)
  memcpy (ptr, &iphdr.ip_dst.s_addr, sizeof (iphdr.ip_dst.s_addr));
  ptr += sizeof (iphdr.ip_dst.s_addr);
  chksumlen += sizeof (iphdr.ip_dst.s_addr);

  // Copy zero field to buf (8 bits)
  *ptr = 0; ptr++;
  chksumlen += 1;

  // Copy transport layer protocol to buf (8 bits)
  memcpy (ptr, &iphdr.ip_p, sizeof (iphdr.ip_p));
  ptr += sizeof (iphdr.ip_p);
  chksumlen += sizeof (iphdr.ip_p);

  // Copy UDP length to buf (16 bits)
  memcpy (ptr, &udphdr.len, sizeof (udphdr.len));
  ptr += sizeof (udphdr.len);
  chksumlen += sizeof (udphdr.len);

  // Copy UDP source port to buf (16 bits)
  memcpy (ptr, &udphdr.source, sizeof (udphdr.source));
  ptr += sizeof (udphdr.source);
  chksumlen += sizeof (udphdr.source);

  // Copy UDP destination port to buf (16 bits)
  memcpy (ptr, &udphdr.dest, sizeof (udphdr.dest));
  ptr += sizeof (udphdr.dest);
  chksumlen += sizeof (udphdr.dest);

  // Copy UDP length again to buf (16 bits)
  memcpy (ptr, &udphdr.len, sizeof (udphdr.len));
  ptr += sizeof (udphdr.len);
  chksumlen += sizeof (udphdr.len);

  // Copy UDP checksum to buf (16 bits)
  // Zero, since we don't know it yet
  *ptr = 0; ptr++;
  *ptr = 0; ptr++;
  chksumlen += 2;

  // Copy payload to buf
  memcpy (ptr, payload, payloadlen);
  ptr += payloadlen;
  chksumlen += payloadlen;

  // Pad to the next 16-bit boundary
  for (i=0; i<payloadlen%2; i++, ptr++) {
    *ptr = 0;
    ptr++;
    chksumlen++;
  }

  return checksum ((uint16_t *) buf, chksumlen);
}

// Allocate memory for an array of chars.
char *
allocate_strmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_strmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (char *) malloc (len * sizeof (char));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (char));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_strmem().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of unsigned chars.
uint8_t *
allocate_ustrmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_ustrmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (uint8_t *) malloc (len * sizeof (uint8_t));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (uint8_t));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_ustrmem().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of ints.
int *
allocate_intmem (int len)
{
  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_intmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (int *) malloc (len * sizeof (int));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (int));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array allocate_intmem().\n");
    exit (EXIT_FAILURE);
  }
}

补充：

在"参考资源"中有很多raw socket相关的例子(tcp,ipv6,udp6,tcp6...)