用C语言手撕一个DNS客户端

03 77 77 77 03 31 36 33 03 63 6f 6d

#include <stdio.h>#include <string.h>#include <stdlib.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#include <time.h>#include <arpa/nameser.h>#include <sys/errno.h>#include <resolv.h>#include <string.h>#include <netdb.h>#include "dnsClient.h"/*利用res_mkquery函数，生成一个DNS报文 */int create_dns_request(const char *domain, unsigned char *request, int rlen){    int len = strlen(domain);    int request_size = res_mkquery(ns_o_query, domain, ns_c_in, ns_t_a, NULL, 0, NULL, request, rlen);    if (request_size < 0)    {        perror("res_mkquery");        return -1;    }    return request_size;}/*打印dns报文，用于调试 */void print_dns_request(unsigned char *request, int request_size){    printf("DNS Request:n");    int i;    for (i = 0; i < request_size; i++)    {        printf("%02x ", request[i]);        if ((i + 1) % 16 == 0)        {            printf("n");        }    }    printf("n");}/* 利用ns_initparse函数解析dns回复的报文消息*/void parse_dns_response(const unsigned char *response, int response_size){    printf("DBS Response:n");    ns_msg handle;    if (ns_initparse(response, response_size, &handle) < 0)    {        fprintf(stderr, "ns_initparse errorn");        return;    }    int answer_count = ns_msg_count(handle, ns_s_an);    int i;    for (i = 0; i < answer_count; i++)    {        ns_rr rr;        if (ns_parserr(&handle, ns_s_an, i, &rr) < 0)        {            fprintf(stderr, "ns_parserr errorn");            return;        }        // ns_rr_type(rr) == ns_t_a && ns_rr_class(rr) == ns_c_in &&        if (ns_rr_rdlen(rr) == 4)        {            struct in_addr ip_address;            memcpy(&ip_address, ns_rr_rdata(rr), sizeof(struct in_addr));            printf("%sn", inet_ntoa(ip_address));        }    }}/*向dns服务器发送报文 */int dns_client_commit(const char *domain){    struct sockaddr_in servaddr;    unsigned char request[1024] = {0};    int length = create_dns_request(domain, (unsigned char *)request, 1024);    // print_dns_request(request, length);    servaddr.sin_family = AF_INET;    servaddr.sin_port = htons(DNS_SERVER_PORT);    servaddr.sin_addr.s_addr = inet_addr(DNS_SERVER_IP);    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);    if (sockfd < 0)    {        perror("socket");        return -1;    }    int ret = connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));    if (ret != 0)    {        perror("connect");        return -1;    }    int slen = sendto(sockfd, request, length, 0, NULL, 0);    if (slen < 0)    {        printf("error:%dn", errno);        perror("sendto");        return -2;    }    char response[1024] = {0};    struct sockaddr_in addr;    size_t addr_len = sizeof(struct sockaddr_in);    int n = recvfrom(sockfd, response, sizeof(response), 0, (struct sockaddr *)&addr, (socklen_t *)&addr_len);    parse_dns_response((unsigned char *)response, n);    return n;}/*利用getaddrinfo函数获取IP */void simple_convert_domain_ip(const char *domain){    struct addrinfo hints, *res, *p;    int status;    char ip[INET6_ADDRSTRLEN];    memset(&hints, 0, sizeof(hints));    hints.ai_family = AF_UNSPEC;    hints.ai_socktype = SOCK_STREAM;    if ((status = getaddrinfo(domain, NULL, &hints, &res)) != 0)    {        perror("getaddrinfo");        return;    }    printf("nIP address for %s:n", domain);    for (p = res; p != NULL; p = p->ai_next)    {        void *addr;        char *ip_version;        if (p->ai_family == AF_INET)        {            struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;            addr = &(ipv4->sin_addr);            ip_version = "IPv4";        }        else        {            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;            addr = &(ipv6->sin6_addr);            ip_version = "IPv6";        }        inet_ntop(p->ai_family, addr, ip, sizeof(ip));        printf("%s: %sn", ip_version, ip);    }    freeaddrinfo(res);    return;}

#include <stdio.h>#include <stdlib.h>#include "dnsClient.h"
int main(int argc, char *argv[]){    if (argc < 2)        return -1;    dns_client_commit(argv[1]);    simple_convert_domain_ip(argv[1]);    exit(EXIT_SUCCESS);}