CODEWITHSUNDEEP
cmacosnetwork-programmingdnsip
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Reputation: 419

getnameinfo fails to perform a reverse DNS

Goal: return FQDN given the sockaddr

I'm trying to use getnameinfo to perform a reverse DNS lookup and return the FQDN for example.com, however the code below fails to print the FQDN with error EAI_NONAME (8). Prior to running the code below, I open example.com in the browser to make sure the DNS cache contains a mapping of domain_name to ip_address, but the code below seems to fail to return "example.com" as the FQDN. Am I missing something? I'm using macOS 10.14.4 and compiling the code with gcc "filename.c"

#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>

int main()
{
    const char* EXAMPLE_COM_IP_ADDR = "93.184.216.34"; // check this with `ping example.com`
    struct sockaddr_in sa = {0};
    char host[NI_MAXHOST] = "";
    char serv[NI_MAXSERV] = "";
    int rc = 0;

    // fill sockaddr_in with data - https://beej.us/guide/bgnet/html/multi/sockaddr_inman.html
    sa.sin_family = AF_INET;
    sa.sin_port = htons(80);
    rc = inet_pton(AF_INET, EXAMPLE_COM_IP_ADDR, &(sa.sin_addr));
    if (rc != 1)
        printf("inet_pton rc = %d\n", rc);

    // translate sockaddr to host + service - https://beej.us/guide/bgnet/html/multi/getnameinfoman.html
    printf("getnameinfo with NI_NAMEREQD flag:\n");
    rc = getnameinfo((struct sockaddr*)(&sa), sizeof(sa), host, sizeof(host), serv, sizeof(serv), NI_NAMEREQD);
    if (rc != 0)
        printf("getnameinfo rc = %d, err= %s\n", rc, gai_strerror(rc));
    printf("host: %s\n", host); // should print - "www.example.com"
    printf("serv: %s\n", serv); // should print - "http"

    return 0;
}

Upvotes: 2

Views: 925

Answers (1)

Antti Haapala -- Слава Україні
Antti Haapala -- Слава Україні

Reputation: 133939

The reverse lookup does not use the DNS cache. Instead it uses the rDNS lookup, i.e. the in-addr.arpa pseudo-domain. Well, it turns out that:

% host 93.184.216.34
Host 34.216.184.93.in-addr.arpa. not found: 3(NXDOMAIN)

There is no reverse PTR record for the IP address.

If you replace the address with 8.8.8.8, say, you get

% ./a.out           
getnameinfo with NI_NAMEREQD flag:
host: google-public-dns-a.google.com
serv: http

And this is only because:

% host 8.8.8.8      
8.8.8.8.in-addr.arpa domain name pointer google-public-dns-a.google.com.

Upvotes: 1

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