getaddrinfo(3) と getnameinfo(3)

addrinfo.c

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

enum {
	Ip2Addr = 0,
	Addr2Ip,
};

static void
ip2host(const char *target)
{
	struct sockaddr_in sa = { 0 };
	char buf[NI_MAXHOST] = { 0 };
	int ret = 0;

	sa.sin_family = AF_INET;
	inet_pton(sa.sin_family, target, &sa.sin_addr);

	ret = getnameinfo((const struct sockaddr *) &sa, sizeof(sa), buf, sizeof(buf), NULL, 0, NI_NAMEREQD);
	if (ret != 0) { 
		printf("[-]Error %s\n", gai_strerror(ret));
		exit(-1);
	}

	printf("%s -> %s\n", target, buf);

}

static void
host2ip(const char *target)
{
	struct addrinfo *result = NULL;
	struct addrinfo hints = { 0 };
	struct addrinfo *info = NULL;

	int ret = 0;
	
	hints.ai_socktype = SOCK_STREAM;

	ret = getaddrinfo(target, NULL, &hints, &result);
	if (ret != 0) {
		printf("[-]Error %s\n", gai_strerror(ret));
		exit(-1);
	}

	for (info = result; info != NULL; info = info->ai_next) {
		char buf[INET_ADDRSTRLEN] = { 0 };
		struct in_addr ia;
		ia.s_addr = ((struct sockaddr_in *)(info->ai_addr))->sin_addr.s_addr;
		inet_ntop(info->ai_family, &ia, buf, sizeof(buf));
		printf("%s -> %s\n", target, buf);
	}

	freeaddrinfo(result);

}

static void
usage(const char *progname)
{
	fprintf(stderr, "usage %s [-ih] host/addr\n", progname);
	exit(-1);
}

int
main(int argc, char **argv)
{
	int flag = Ip2Addr;
	char *target = NULL;

	if (argc != 3)
		usage(argv[0]);

	if (!strcmp(argv[1], "-i")) 
		flag = Ip2Addr;
	else if (!strcmp(argv[1], "-h"))
		flag = Addr2Ip;
	else
		usage(argv[0]);

	target = argv[2];

	if (flag == Ip2Addr)
		ip2host(target);
	else
		host2ip(target);

	return 0;
}