我正在尝试从 C 中的 DNS 服务器获取 MX 记录。我的问题是，每次我调用 sendto 或 recvfrom 时都会收到 Permission denied 错误。（IDE - Xcode 4，Mac OS X Lion）到目前为止还没有在 C 语言中做过任何事情，但我需要这个来完成作业。来自http://www.binarytides.com/blog/dns-query-code-in-c-with-linux-sockets/的“灵感” 到目前为止我的代码：
struct QUESTION
{
unsigned short qtype;
unsigned short qclass;
};
typedef struct
{
unsigned char *name;
struct QUESTION *ques;
} QUERY;
typedef struct
{
unsigned short type;
unsigned short _class;
unsigned int ttl;
unsigned short data_len;
} R_DATA;
int main(int argc, char *argv[])
{
int sockfd, i;
unsigned char buf[65536],*qname;
struct QUESTION *qinfo = NULL;
struct sockaddr_in dest;
struct dnsheader *dns = NULL;
sockfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
dest.sin_family=AF_INET;
dest.sin_port=htons(53);
dest.sin_addr.s_addr= inet_addr("ns1.sil.at");
srand((unsigned)time(NULL));
int16_t rand_number = (rand()%1000+1);
printf("generated random number : %i \n", rand_number);
dns = (struct dnsheader *)&buf;
dns->flags = STANDARD_QUERY_FLAGS;
dns->question_count = 1;
dns->transaction_id = rand_number;
dns->answer_rr_count = 0;
dns->additional_rr_count = 0;
dns->nameserver_rr_count = 0;
qname =(unsigned char*)&buf[sizeof(struct dnsheader)];
qinfo =(struct QUESTION*)&buf[sizeof(struct dnsheader) + (strlen((const char*)qname) + 1)]; //fill it
qinfo->qtype = htons( 15 ); // MX = 15
qinfo->qclass = htons(1);
printf("\nSending Packet...");
if( sendto(sockfd,(char*)buf,sizeof(struct dnsheader) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION),0,(struct sockaddr*)&dest,sizeof(dest)) < 0){
perror("\nsendto failed");
}
printf("\nDone\n");
i = sizeof dest;
printf("\nReceiving answer...");
if(recvfrom (sockfd,(char*)buf , 65536 , 0 , (struct sockaddr*)&dest , (socklen_t*)&i ) < 0){
perror("recvfrom failed");
}
printf("Done");
dns = (struct dnsheader*) buf;
print_dns_answers(buf, sizeof((char*)buf));
close(sockfd);
return 0;
}