internal-ip/index.js

import {networkInterfaces} from 'node:os';
import defaultGateway from 'default-gateway';
import ip from 'ipaddr.js';

function findIp(gateway) {
	const gatewayIp = ip.parse(gateway);

	// Look for the matching interface in all local interfaces.
	for (const addresses of Object.values(networkInterfaces())) {
		for (const {cidr} of addresses) {
			const net = ip.parseCIDR(cidr);

			// eslint-disable-next-line unicorn/prefer-regexp-test
			if (net[0] && net[0].kind() === gatewayIp.kind() && gatewayIp.match(net)) {
				return net[0].toString();
			}
		}
	}
}

async function async(family) {
	try {
		const {gateway} = await defaultGateway[family]();
		return findIp(gateway);
	} catch {}
}

function sync(family) {
	try {
		const {gateway} = defaultGateway[family].sync();
		return findIp(gateway);
	} catch {}
}

export async function internalIpV6() {
	return async('v6');
}

export async function internalIpV4() {
	return async('v4');
}

export function internalIpV6Sync() {
	return sync('v6');
}

export function internalIpV4Sync() {
	return sync('v4');
}