Quick hack for https://github.com/NixOS/nixpkgs/pull/76542

hostname-test.c

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

// gcc -Wall hostname-test.c -o hostname-test

// Example for /etc/hosts:
// 127.0.0.1 localhost
// ::1 localhost
// 127.0.0.1 nixos-unstable.test.tld nixos-unstable
// ::1 nixos-unstable.test.tld nixos-unstable

enum allowedAfs {IPv4, IPv6};

void showhostnameaf(const char* hostname, enum allowedAfs af) {
	char *afname;
	struct hostent *he;
	if (af == IPv4) {
		he = gethostbyname(hostname);
		afname = "IPv4";
	} else if (af == IPv6) {
		he = gethostbyname2(hostname, PF_INET6);
		afname = "IPv6";
	} else {
		return;
	}
	printf("Hostname (%s): %s\n", afname, hostname);
	printf("- Canonical name: %s\n", he->h_name);
	printf("- Aliases:");
	for (char **h_aliases = he->h_aliases; *h_aliases != NULL; h_aliases++) {
		printf(" %s", *h_aliases);
	}
	printf("\n");
}

void showhostname(const char* hostname) {
	showhostnameaf(hostname, IPv4);
	showhostnameaf(hostname, IPv6);
}

void showhostnameaddr(const char* host, int type) {
	struct hostent *he;
	if (type == AF_INET) {
		struct in_addr addr;
        	inet_pton(type, host, &addr);
		he = gethostbyaddr(&addr, sizeof(addr), type);
	} else if (type == AF_INET6) {
		struct in6_addr addr;
        	inet_pton(type, host, &addr);
		he = gethostbyaddr(&addr, sizeof(addr), type);
	} else {
		return;
	}
	printf("Address: %s\n", host);
	printf("- Canonical name: %s\n", he->h_name);
	printf("- Aliases:");
	for (char **h_aliases = he->h_aliases; *h_aliases != NULL; h_aliases++) {
		printf(" %s", *h_aliases);
	}
	printf("\n");
}

int main() {
	char hostname[256];
	gethostname(hostname, sizeof(hostname)/sizeof(hostname[0]));
	showhostname(hostname);

	showhostnameaddr("127.0.0.1", AF_INET);
	showhostnameaddr("::1", AF_INET6);
	
	return 0;
}