a.c

#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{ 
	struct addrinfo hints;
	memset(&hints, 0, sizeof(hints));
	hints.ai_family = AF_UNSPEC;
	hints.ai_socktype = SOCK_DGRAM;
	hints.ai_flags = AI_NUMERICHOST;
	
	struct addrinfo * addr_1111 = NULL;
	getaddrinfo("1.1.1.1", "53", &hints, &addr_1111);
	struct addrinfo * addr_8888 = NULL;
	getaddrinfo("8.8.8.8", "53", &hints, &addr_8888);
	
	int fd_1111 = socket(addr_1111->ai_family, addr_1111->ai_socktype, addr_1111->ai_protocol);
	int fd_8888 = socket(addr_8888->ai_family, addr_8888->ai_socktype, addr_8888->ai_protocol);
	
	connect(fd_8888, addr_8888->ai_addr, addr_8888->ai_addrlen);
	
	while (true)
	{
		static const uint8_t dns_google[] = {
			0x12,0x34, 1,0, 0,1, 0,0, 0,0, 0,0,
			6,'g','o','o','g','l','e',3,'c','o','m',0, 0,1, 0,1
		};
		static const uint8_t dns_cloudflare[] = {
			0x12,0x34, 1,0, 0,1, 0,0, 0,0, 0,0,
			10,'c','l','o','u','d','f','l','a','r','e',3,'c','o','m',0, 0,1, 0,1
		};
		
		sendto(fd_1111, dns_cloudflare, sizeof(dns_cloudflare), MSG_NOSIGNAL, addr_1111->ai_addr, addr_1111->ai_addrlen);
		sendto(fd_8888, dns_google,     sizeof(dns_google),     MSG_NOSIGNAL, addr_8888->ai_addr, addr_8888->ai_addrlen);
		
		usleep(1000000);
		for (int nsock=0;nsock<2;nsock++)
		{
			while (true)
			{
				char buf[1024];
				int ret = recvfrom(nsock ? fd_1111 : fd_8888, buf, sizeof(buf), MSG_DONTWAIT, NULL, NULL);
				if (ret < 0) break;
				
				for (int i=0;i<ret;i++)
					printf("%.2x ", buf[i]&0xff);
				puts("");
			}
		}
	}
}