Resolving names and IPs

03 Jul 2012 Here is two examples of name resolving in linux.. I use getaddrinfo() and getnameinfo() respectively..


getaddrinfo: given a name retuns all translated IPs, one per line

/*
 * File: getaddrinfo.c
 * Compile: gcc getaddrinfo.c -o getaddrinfo
 * Usage: ./getaddrinfo FQN
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main(int argc, char **argv)
{
        int error;

        struct addrinfo saddr, *psaddr, *ptr;

        memset(&saddr, '\0', sizeof(saddr));
        saddr.ai_family = AF_INET;


        saddr.ai_socktype = SOCK_STREAM;

        error = getaddrinfo(argv[1], NULL, &saddr, &psaddr);
        if (error) {
                fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(error));
                exit(EXIT_FAILURE);
        }

        for (ptr = psaddr; ptr; ptr = ptr->ai_next) {
                puts(inet_ntoa(((struct sockaddr_in *)    
                               ptr->ai_addr)->sin_addr));
        }

        return 0;
}

getnameinfo: Given an IP returns the name that first resolves to it. 

/*
 * File: getnameinfo.c
 * Compile: gcc getnameinfo.c -o getnameinfo
 * Usage: ./getnameinfo IP
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>

static char hostname[255];

int main(int argc, char **argv)
{
        int error;
        struct sockaddr_in saddr_in;



        memset(&saddr_in, '\0', sizeof(saddr_in));
        saddr_in.sin_family = AF_INET;
        error = inet_aton(argv[1], &saddr_in.sin_addr);
        if (error == 0) {
                perror("inet_aton");
                exit(EXIT_FAILURE);
        }

        error = getnameinfo((struct sockaddr *)&saddr_in, sizeof(saddr_in),
                            hostname, sizeof(hostname),
                            NULL, 0, NI_NAMEREQD);
        if (error) {
                fprintf(stderr, "getnameinfo: %s\n", gai_strerror(error));
                exit(EXIT_FAILURE);
        }

        puts(hostname);
        return 0;
}

Cheers