Fix #36

README.md

@@ -94,7 +94,7 @@ Default: `3600` (1 hour)
 
 When the DNS server responds with `ENOTFOUND` or `ENODATA` and the OS reports that the entry is available, it will use `dns.lookup(...)` directly for the requested hostnames for the specified amount of time (in seconds).
 
-If you don't query internal hostnames (such as `localhost`, `database.local` etc.), it is strongly recommended to set this value to `0`.
+**Note**: You should avoid setting this to `0` unless the provided DNS servers' database is limited to few domains.
 
 ##### options.errorTtl
 
@@ -121,7 +121,7 @@ Default: [`dns.lookup`](https://nodejs.org/api/dns.html#dns_dns_lookup_hostname_
 
 The fallback function to use when the DNS server responds with `ENOTFOUND` or `ENODATA`.
 
-**Note**: This has no effect if the `fallbackDuration` option is less than `1`.
+If you don't query internal hostnames (such as `localhost`, `database.local` etc.), it is strongly recommended to set this to `false`.
 
 ### Entry object
 

index.d.ts

@@ -52,7 +52,7 @@ export interface Options {
 	 * **Note**: This has no effect if the `fallbackDuration` option is less than `1`.
 	 * @default dns.lookup
 	 */
-	lookup?: typeof lookup;
+	lookup?: typeof lookup | false;
 }
 
 export interface EntryObject {

index.test-d.ts

@@ -26,6 +26,10 @@ import CacheableLookup, {EntryObject} from '.';
 		lookup
 	});
 
+	new CacheableLookup({
+		lookup: false
+	});
+
 	expectType<string[]>(cacheable.servers);
 
 	expectType<EntryObject>(await cacheable.lookupAsync('localhost', 4));

source/index.js

@@ -80,7 +80,7 @@ class CacheableLookup {
 
 		this._cache = cache;
 		this._resolver = resolver;
-		this._dnsLookup = promisify(lookup);
+		this._dnsLookup = lookup && promisify(lookup);
 
 		if (this._resolver instanceof AsyncResolver) {
 			this._resolve4 = this._resolver.resolve4.bind(this._resolver);
@@ -96,11 +96,9 @@ class CacheableLookup {
 		this._nextRemovalTime = false;
 		this._hostnamesToFallback = new Set();
 
-		if (fallbackDuration < 1) {
-			this._fallback = false;
-		} else {
-			this._fallback = true;
+		this.fallbackDuration = fallbackDuration;
 
+		if (fallbackDuration > 0) {
 			const interval = setInterval(() => {
 				this._hostnamesToFallback.clear();
 			}, fallbackDuration * 1000);
@@ -109,6 +107,8 @@ class CacheableLookup {
 			if (interval.unref) {
 				interval.unref();
 			}
+
+			this._fallbackInterval = interval;
 		}
 
 		this.lookup = this.lookup.bind(this);
@@ -326,10 +326,10 @@ class CacheableLookup {
 
 		let query = await this._resolve(hostname);
 
-		if (query.entries.length === 0 && this._fallback) {
+		if (query.entries.length === 0 && this._dnsLookup) {
 			query = await this._lookup(hostname);
 
-			if (query.entries.length !== 0) {
+			if (query.entries.length !== 0 && this.fallbackDuration > 0) {
 				// Use `dns.lookup(...)` for that particular hostname
 				this._hostnamesToFallback.add(hostname);
 			}

tests/test.js

@@ -769,7 +769,7 @@ test('custom cache support', async t => {
 });
 
 test.serial('fallback works', async t => {
-	const cacheable = new CacheableLookup({resolver, fallbackDuration: 3600});
+	const cacheable = new CacheableLookup({resolver, fallbackDuration: 0.1});
 	resolver.resetCounter();
 
 	const entries = await cacheable.lookupAsync('osHostname', {all: true});
@@ -790,6 +790,10 @@ test.serial('fallback works', async t => {
 		4: 1,
 		lookup: 2
 	});
+
+	await sleep(100);
+
+	t.is(cacheable._hostnamesToFallback.size, 0);
 });
 
 test.serial('fallback works if ip change', async t => {
@@ -865,7 +869,7 @@ test('real DNS queries first', async t => {
 });
 
 test('fallback can be turned off', async t => {
-	const cacheable = new CacheableLookup({resolver, fallbackDuration: 0});
+	const cacheable = new CacheableLookup({resolver, lookup: false});
 
 	await t.throwsAsync(cacheable.lookupAsync('osHostname', {all: true}), {
 		message: 'cacheableLookup ENOTFOUND osHostname'