showip.c

/* show IP addresses for a host given on the command line */

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

int main(int argc, char *argv[])
{
	struct addrinfo hints, *res, *p;
	int status;
	char ipstr[INET6_ADDRSTRLEN];

	if (argc != 2) {
		fprintf(stderr,"usage: showip hostname\n");
		return 1;
	}

	memset(&hints, 0, sizeof(hints));
	hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
	hints.ai_socktype = SOCK_STREAM;

	if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
		fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
		return 2;
	}

	printf("IP addresses for %s:\n\n", argv[1]);

	for(p = res; p != NULL; p = p->ai_next) {
		void *addr;
		char *ipver;

		// get the pointer to the address itself
		// different fields in IPv4 and IPv6
		if (p->ai_family == AF_INET) { // IPv4
			struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
			addr = &(ipv4->sin_addr);
			ipver = "IPv4";
		} else { // IPv6
			struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
			addr = &(ipv6->sin6_addr);
			ipver = "IPv6";
		}

		// convert the IP to a string and print it
		inet_ntop(p->ai_family, addr, ipstr, sizeof(ipstr));
		printf(" %s: %s\n", ipver, ipstr);
	}

	freeaddrinfo(res); // free the linked list

	return 0;
}