Obtain hostnames from IP addresses

[Conrado Badenas <Conrado.Badenas@uv.es>]

Hello!

Does anyone know how to obtain hostnames (e.g., skywalker.termol.uv.es)
from IP addresses (e.g., 147.156.6.236). I've written this program:

#include <netdb.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>

int main() {
        int l;
        char addr[]="147.156.6.236";
        struct hostent *ph;
        
        l=strlen(addr);ph=gethostbyaddr(addr,l,AF_INET);
        if (ph!=NULL) fprintf(stderr,"%d:\t%s\n",i,ph->h_name);
        else          perror("gethostbyaddr()");
        return 0;
}

following the recommendations of a informatic friend of mine about using
function gethostbyaddr, but I obtain:

147.156.6.236: gethostbyaddr(): Address family not supported by protocol

With "man 3 gethostbyaddr" I read "The only valid address type is
currently AF_INET", and when reading /usr/include/sys/socket.h and
/usr/include/socketbits.h I see that AF_INET is 2 for the "IP protocol
family".

Thus, what is wrong?

Is there any program (like a filter: input address in stdin, output name
in stdout) that could do it? I've tried with nslookup but doesn't work
(and it worked last month).

-- 
Conrado Badenas (Assistant Lecturer)
Department of Thermodynamics. University of Valencia
c/. Doctor Moliner, 50       | e-m: Conrado.Badenas@uv.es
46100 Burjassot (Valencia)   | Phn: +34-63864350
SPAIN                        | Fax: +34-63983385