自己实现gethostbyaddr

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/epoll.h>
#include <time.h>

#define BUF_SIZE 1024
#define SRV_PORT 53
#define DNS_FILE "/etc/resolv.conf"

typedef unsigned short U16;
//const char srv_ip[] = "208.67.222.222";

typedef struct _DNS_HDR
{
    U16 id;
    U16 tag;
    U16 numq;
    U16 numa;
    U16 numa1;
    U16 numa2;
}DNS_HDR;

typedef struct _DNS_QER
{
    U16 type;
    U16 classes;
}DNS_QER;

char* get_dns_ip(void)
{
    FILE* fp = NULL;
    static char addr[32] = {''};

    fp = fopen(DNS_FILE, "r");
    if(fp == NULL)
        return NULL;

    fscanf(fp, "nameserver %s", addr);

    fclose(fp);
    return addr;
}

char* strtrim(char* str)
{
    char*p = NULL;
    char*q = NULL;

    p = str;
    q = str + strlen(str) - 1;
    while(isspace(*p))
        p++;
    while(isspace(*q))
        q--;
    *(q + 1) = '';
    return p;
}

char* get_reverct_ip(char* ipaddr)
{
    if (ipaddr == NULL)
        return NULL;
    char ipstr[INET_ADDRSTRLEN] = {'',};
    static char retstr[32] = {0};
    char a[4][4];
    struct in_addr addr1, addr2;
    memset(&addr1, 0, sizeof(addr1));
    memset(&addr2, 0, sizeof(addr2));
    int i, j;
    char *p = NULL;
    for (i = 0; i < 4; i++)
    {
        for (j = 0; j < 4; j++)
            a[i][j] = '';
    }
    i = 0;
    p = strtok(ipaddr, ".");
    while (p)
    {
        strcpy(a[i++], p);
        p = strtok(NULL, ".");
    }
    sprintf(retstr, "%s.%s.%s.%s.in-addr.arpa", a[3],a[2],a[1],a[0]);

#if 0
    inet_pton(AF_INET, ipaddr, &addr1);
    addr2.s_addr = ntohl(addr1.s_addr);
    inet_ntop(AF_INET, &addr2, ipstr, sizeof(ipstr));
    sprintf(retstr, "%s.in-addr.arpa", ipstr);
#endif

    return retstr;
}

int main(int argc, char** argv)
{
    int clifd, len = 0, i,count=0;
    int ipv4 = 0, ipv6 = 0;
    socklen_t  socklen = 0;
    char buf[BUF_SIZE] = {''};
    char tmpip[32] = {''};
    char *p = NULL;
    char *ipaddr = NULL;
    char *ipaddr2 = NULL;
    time_t sendtime = 0, recvtime = 0, caltime = 0;
    struct sockaddr_in servaddr;
    struct sockaddr_in6 servaddr6;
    struct timeval pretime, aftertime, tv;
    memset(&tv, '', sizeof(tv));

    if (strchr(argv[1], '.'))
        ipv4 = 1;
    else if (strchr(argv[1], ':'))
        ipv6 = 1;
    else
        return -1;

    ipaddr = get_dns_ip();
    if (ipv4)
    {
        clifd = socket(AF_INET, SOCK_DGRAM, 0 );
        if (clifd < 0)
        {
            printf( " create socket error!\n " );
            return -1;
        }
        bzero(&servaddr, sizeof(servaddr));
        servaddr.sin_family = AF_INET;
        servaddr.sin_port = htons(SRV_PORT);
        inet_pton(AF_INET, ipaddr, &servaddr.sin_addr);
    }
    else if (ipv6)
    {
        clifd = socket(AF_INET6, SOCK_DGRAM, 0 );
        if (clifd < 0)
        {
            printf( " create socket error!\n " );
            return -1;
        }
        bzero(&servaddr6, sizeof(servaddr6));
        servaddr6.sin6_family = AF_INET6;
        servaddr6.sin6_port = htons(SRV_PORT);
        inet_pton(AF_INET6, ipaddr, &servaddr6.sin6_addr);
    }

    DNS_HDR *dnshdr = (DNS_HDR *)buf;
    DNS_QER *dnsqer = (DNS_QER *)(buf + sizeof(DNS_HDR));
    memset(buf, 0, BUF_SIZE);
    dnshdr->id = (U16)1;
    dnshdr->tag = htons(0x0100);
    dnshdr->numq = htons(1);
    dnshdr->numa = 0;

    strcpy(tmpip, argv[1]);
    ipaddr2 = get_reverct_ip(argv[1]);
    strncpy(buf + sizeof(DNS_HDR) + 1, ipaddr2, strlen(ipaddr2));
    printf(", len: %d\n", ipaddr2, strlen(ipaddr2));
    p = buf + sizeof(DNS_HDR) + 1;
    i = 0;
    while (p < (buf + sizeof(DNS_HDR) + 1 + strlen(ipaddr2)))
    {
        if ( *p == '.')
        {
            *(p - i - 1) = i;
            i = 0;
        }
        else
        {
            i++;
        }
        p++;
      }
    *(p - i - 1) = i;

    dnsqer = (DNS_QER *)(buf + sizeof(DNS_HDR) + 2 + strlen(ipaddr2));
    dnsqer->classes = htons(1);
    dnsqer->type = htons(0x0c);

    tv.tv_sec = 2;
    tv.tv_usec = 0;
    gettimeofday(&pretime, NULL);
    if (ipv4)
    {
        setsockopt(clifd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
        socklen = sizeof(struct sockaddr_in);
        len = sendto(clifd, buf, sizeof(DNS_HDR) + sizeof(DNS_QER) + strlen(ipaddr2) + 2, 0, (struct sockaddr *)&servaddr, socklen);
        if (len < 0)
        {
            printf("send error\n");
            return -1;
        }
        len = recvfrom(clifd, buf, BUF_SIZE, 0, (struct sockaddr *)&servaddr, &socklen);
        if (len < 0)
        {
            if (errno == EAGAIN)
            {
                printf("recvfrom timeout!\n");
            }
        }
    }
    else
    {
        setsockopt(clifd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
        socklen = sizeof(struct sockaddr_in6);
        len = sendto(clifd, buf, sizeof(DNS_HDR) + sizeof(DNS_QER) + strlen(ipaddr2) + 2, 0, (struct sockaddr *)&servaddr6, socklen);
        if (len < 0)
        {
            printf("send error\n");
            return -1;
        }
        len = recvfrom(clifd, buf, BUF_SIZE, 0, (struct sockaddr *)&servaddr6, &socklen);
        if (len < 0)
        {
            if (errno == EAGAIN)
            {
                fprintf(stdout, "recvfrom timeout!\n");
            }
        }
    }
    gettimeofday(&aftertime, NULL);
    fprintf(stdout, "time ==> %d \n", (aftertime.tv_sec - pretime.tv_sec)*1000 + (aftertime.tv_usec - pretime.tv_usec)/1000);

    printf("answer: %d\n", dnshdr->numa);
    if (dnshdr->numa == 0)
    {
        printf("ack error\n");
        return -1;
    }
    p = buf + len -21;
    printf("%s ==> %s\n", tmpip, p);
    close(clifd);
    return 0;
}