套接字 - C 中的权限被拒绝

Question

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“权限被拒绝”错误。 （IDE - Xcode 4、Mac OS X Lion） 到目前为止，我还没有真正用 C 做过任何事情，但我需要这个来完成作业。 “灵感”来自 http://www. binarytides.com/blog/dns-query-code-in-c-with-linux-sockets/ 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; 
} 

Answer 1

inet_addr 函数采用字符串形式的 IP 地址，而不是主机名。尝试将 "ns1.sil.at" 替换为其 IP 地址 "213.129.232.1"

Answer 2

除了@joni-salonen的答案之外，您还需要绑定套接字。
使用bind函数调用。
它需要一个 struct sockaddr_in 来标识套接字的本地端。您可以使用系列 AF_INET、端口 0 和地址 INADDR_ANY 对其进行初始化。