Linux下構造UDP未經處理資料報－DNS報文樣本C語言實現

這幾天構造一個DNS報文，查了好些資料，好象許多朋友對UDP報文的校正和老是出問題，自己試了一些例子代碼，也有問題。所以花了一些時間，做了一個成功的例子，在Linux下GCC通過並驗證成功。

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

unsigned short checksum(unsigned short *buffer, int size)
{
    unsigned long cksum=0;

    while (size > 1)
    {
        cksum += *buffer++;
  size  -= sizeof(unsigned short);  
    }
    if (size)
    {
        cksum += *(unsigned char *)buffer;  
    }
    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >>16);

    return (unsigned short)(~cksum);
}

void CalculateCheckSum(
    void    *iphdr,
    struct udphdr *udphdr,
    char    *payload,
    int      payloadlen)
{
    struct iphdr  *v4hdr=NULL;
    unsigned long zero=0;
    char          buf[1000],
                 *ptr=NULL;
    int           chksumlen=0,
                  i;
   
    ptr = buf;

    v4hdr = (struct iphdr *)iphdr;

    // Include the source and destination IP addresses
    memcpy(ptr, &v4hdr->saddr,  sizeof(v4hdr->saddr)); 
    ptr += sizeof(v4hdr->saddr);
    chksumlen += sizeof(v4hdr->saddr);

    memcpy(ptr, &v4hdr->daddr, sizeof(v4hdr->daddr));
    ptr += sizeof(v4hdr->daddr);
    chksumlen += sizeof(v4hdr->daddr);
   
    // Include the 8 bit zero field
    memcpy(ptr, &zero, 1);
    ptr++;
    chksumlen += 1;

    // Protocol
    memcpy(ptr, &v4hdr->protocol, sizeof(v4hdr->protocol));
    ptr += sizeof(v4hdr->protocol);
    chksumlen += sizeof(v4hdr->protocol);

    // UDP length
    memcpy(ptr, &udphdr->len, sizeof(udphdr->len));
    ptr += sizeof(udphdr->len);
    chksumlen += sizeof(udphdr->len);
   
    // UDP source port
    memcpy(ptr, &udphdr->source, sizeof(udphdr->source));
    ptr += sizeof(udphdr->source);
    chksumlen += sizeof(udphdr->source);

    // UDP destination port
    memcpy(ptr, &udphdr->dest, sizeof(udphdr->dest));
    ptr += sizeof(udphdr->dest);
    chksumlen += sizeof(udphdr->dest);

    // UDP length again
    memcpy(ptr, &udphdr->len, sizeof(udphdr->len));
    ptr += sizeof(udphdr->len);
    chksumlen += sizeof(udphdr->len);
  
    // 16-bit UDP checksum, zero
    memcpy(ptr, &zero, sizeof(unsigned short));
    ptr += sizeof(unsigned short);
    chksumlen += sizeof(unsigned short);

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

    // pad to next 16-bit boundary
    for(i=0 ; i < payloadlen%2 ; i++, ptr++)
    {
        printf("pad one byte/n");
        *ptr = 0;
        ptr++;
        chksumlen++;
    }

    // Compute the checksum and put it in the UDP header
    udphdr->check = checksum((unsigned short *)buf, chksumlen);

    return;
}

void main()
{

 int sock;
 unsigned int buffer_size = sizeof(struct iphdr) + sizeof(struct udphdr);

 char DNS_Data[] = "/x71/x79/x81/x80/x00/x01"
                "/x00/x02/x00/x04/x00/x04/x03/x77/x77/x77/x03/x61/x62/x63/x03/x63"
                "/x6f/x6d/x00/x00/x01/x00/x01/xc0/x0c/x00/x05/x00/x01/x00/x00/x02"
                "/xe8/x00/x02/xc0/x10/xc0/x10/x00/x01/x00/x01/x00/x00/x02/xe9/x00"
                "/x04/x0a/xb5/x84/xfa/xc0/x10/x00/x02/x00/x01/x00/x00/xda/xeb/x00"
                "/x0d/x06/x73/x65/x6e/x73/x30/x31/x03/x64/x69/x67/xc0/x14/xc0/x10"
                "/x00/x02/x00/x01/x00/x00/xda/xeb/x00/x09/x06/x73/x65/x6e/x73/x30"
                "/x32/xc0/x4e/xc0/x10/x00/x02/x00/x01/x00/x00/xda/xeb/x00/x09/x06"
                "/x6f/x72/x6e/x73/x30/x31/xc0/x4e/xc0/x10/x00/x02/x00/x01/x00/x00"
                "/xda/xeb/x00/x09/x06/x6f/x72/x6e/x73/x30/x32/xc0/x4e/xc0/x75/x00"
                "/x01/x00/x01/x00/x00/x7a/x36/x00/x04/x0a/xbb/xbd/x2c/xc0/x8a/x00"
                "/x01/x00/x01/x00/x00/x1b/x96/x00/x04/x0a/xbb/xbe/x2c/xc0/x47/x00"
                "/x01/x00/x01/x00/x00/x92/xb1/x00/x04/x0a/xb5/x86/x10/xc0/x60/x00"
                "/x01/x00/x01/x00/x00/x92/xb1/x00/x04/x0a/xb5/x87/xc7";
 
 buffer_size += sizeof(DNS_Data);

 unsigned char buffer[buffer_size];
 memset (buffer, 0, buffer_size);

 struct iphdr *ip = (struct iphdr *)buffer;
 struct udphdr *udp = (struct udphdr *)(buffer + sizeof(struct iphdr));

 if ((sock = socket(AF_INET,SOCK_RAW,IPPROTO_UDP)) == -1) {

  perror("socket()");
  exit(EXIT_FAILURE);
 }

 int o = 1;
 if (setsockopt(sock,IPPROTO_IP,IP_HDRINCL,&o,sizeof(o)) == -1) {

  perror("setsockopt()");
  exit(EXIT_FAILURE);
 }

 ip->version = 4;
 ip->ihl = 5;
 ip->id = htonl(random());
 ip->saddr = inet_addr("1.0.0.1");
 ip->daddr = inet_addr("10.0.0.63");
 ip->ttl = 255;
 ip->protocol = IPPROTO_UDP;
 ip->tot_len = buffer_size;
 ip->check = 0;

 udp->source = htons(53);
 udp->dest = htons(1234);
 udp->len = htons(buffer_size - sizeof(struct iphdr));
 udp->check = 0;

 struct sockaddr_in addr;
 addr.sin_family = AF_INET;
 addr.sin_port = udp->source;
 addr.sin_addr.s_addr = ip->saddr;

 memcpy(buffer+sizeof(struct iphdr) + sizeof(struct udphdr),DNS_Data,sizeof(DNS_Data)); 
 CalculateCheckSum(ip,udp,DNS_Data,sizeof(DNS_Data));

 if ((sendto(sock, buffer, buffer_size, 0, (struct sockaddr*)&addr,
        sizeof(struct sockaddr_in))) == -1) {

  perror("send()");
  exit(1);
 }
 else
  printf("OK/n");
}

 

上面代碼構造了一個DNS應答報文。用抓包程式抓下來驗證正確。