As far as I know getaddrinfo will only return A and AAAA records.
I understand we don’t want to add some dependency library for this. But since we only need to send a particular query I don’t think that’s necessary. From what I learned writing the seeder, DNS is refreshingly straightforward. Here’s some C++ to send and receive a NULL query to demonstrate.
#include <arpa/inet.h>
#include <cstring>
#include <iostream>
#include <netinet/in.h>
#include <sstream>
#include <string>
#include <sys/socket.h>
#include <unistd.h>
#include <vector>
struct DNSHeader {
uint16_t id;
uint16_t flags;
uint16_t q_count;
uint16_t ans_count;
uint16_t auth_count;
uint16_t add_count;
};
struct DNSQuestion {
std::vector<unsigned char> qname;
uint16_t qtype;
uint16_t qclass;
DNSQuestion(const std::string &domain, uint16_t type, uint16_t cls)
: qtype(type), qclass(cls) {
// Convert domain to DNS format: prefix parts with their length and
// end with null byte (e.g. dnsseed.21.ninja -> 7dnsseed2215ninja0)
std::stringstream ss(domain);
std::string segment;
while (getline(ss, segment, '.')) {
qname.push_back(static_cast<uint8_t>(segment.size()));
qname.insert(qname.end(), segment.begin(), segment.end());
}
qname.push_back(0);
}
std::vector<unsigned char> serialize() const {
std::vector<unsigned char> serialized;
serialized.insert(serialized.end(), qname.begin(), qname.end());
serialized.insert(
serialized.end(), reinterpret_cast<const unsigned char *>(&qtype),
reinterpret_cast<const unsigned char *>(&qtype) + sizeof(qtype));
serialized.insert(
serialized.end(), reinterpret_cast<const unsigned char *>(&qclass),
reinterpret_cast<const unsigned char *>(&qclass) + sizeof(qclass));
return serialized;
}
};
int main() {
const std::string domain = "dnsseed.21.ninja";
const std::string nameserver = "89.116.30.184";
// Prepare DNS query
std::vector<unsigned char> query;
DNSHeader header = {static_cast<uint16_t>(getpid() % 65536), htons(0x0100), htons(1), 0, 0, 0}; // 0x0100 for recursion desired
query.insert(query.end(), reinterpret_cast<unsigned char *>(&header), reinterpret_cast<unsigned char *>(&header) + sizeof(DNSHeader));
DNSQuestion question = {domain, htons(10), htons(1)}; // 10 for NULL record, 1 for IN class
auto serializedQuestion = question.serialize();
query.insert(query.end(), serializedQuestion.begin(), serializedQuestion.end());
int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
struct sockaddr_in dest = { .sin_family = AF_INET, .sin_port = htons(53), .sin_addr = {.s_addr = inet_addr(nameserver.c_str())}};
ssize_t bytes_sent = sendto(sock, query.data(), query.size(), 0, (struct sockaddr *)&dest, sizeof(dest));
printf("Sent query (size=%ld, header id=%d)\n", bytes_sent, reinterpret_cast<const DNSHeader *>(query.data())->id);
std::vector<unsigned char> response(512);
socklen_t addr_len = sizeof(dest);
ssize_t bytes_received = recvfrom(sock, response.data(), response.size(), 0, (struct sockaddr *)&dest, &addr_len);
printf("Received reply (size=%ld, header id=%d)\n", bytes_received, reinterpret_cast<const DNSHeader *>(response.data())->id);
close(sock);
return 0;
}