dns: deprecate passing falsy hostname to dns.lookup

We can `dns.lookup` a falsy `hostname` like `dns.lookup(false)`
for the reason of backwards compatibility long before(see #13119
for detail). This behavior is undocumented and seems useless in
real world apps.

We could also make invalid `hostname` throw in the future and the
change might be semver-major.

Fixes: https://github.com/nodejs/node/issues/13119

PR-URL: https://github.com/nodejs/node/pull/23173
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

2 files changed, 21 insertions, 3 deletions

diff --git a/lib/internal/dns/promises.js b/lib/internal/dns/promises.js
index 6d2fa3b170..8baa0fa0de 100644
--- a/lib/internal/dns/promises.js
+++ b/lib/internal/dns/promises.js
@@ -2,7 +2,8 @@
 const {
   bindDefaultResolver,
   Resolver: CallbackResolver,
-  validateHints
+  validateHints,
+  emitInvalidHostnameWarning,
 } = require('internal/dns/utils');
 const { codes, dnsException } = require('internal/errors');
 const { isIP, isIPv4, isLegalPort } = require('internal/net');
@@ -55,6 +56,7 @@ function onlookupall(err, addresses) {
 function createLookupPromise(family, hostname, all, hints, verbatim) {
   return new Promise((resolve, reject) => {
     if (!hostname) {
+      emitInvalidHostnameWarning(hostname);
       if (all)
         resolve([]);
       else
@@ -99,7 +101,7 @@ function lookup(hostname, options) {
 
   // Parse arguments
   if (hostname && typeof hostname !== 'string') {
-    throw new ERR_INVALID_ARG_TYPE('hostname', ['string', 'falsy'], hostname);
+    throw new ERR_INVALID_ARG_TYPE('hostname', 'string', hostname);
   } else if (options !== null && typeof options === 'object') {
     hints = options.hints >>> 0;
     family = options.family >>> 0;
diff --git a/lib/internal/dns/utils.js b/lib/internal/dns/utils.js
index 143d36193a..d279a58840 100644
--- a/lib/internal/dns/utils.js
+++ b/lib/internal/dns/utils.js
@@ -140,10 +140,26 @@ function validateHints(hints) {
   }
 }
 
+let invalidHostnameWarningEmitted = false;
+
+function emitInvalidHostnameWarning(hostname) {
+  if (invalidHostnameWarningEmitted) {
+    return;
+  }
+  invalidHostnameWarningEmitted = true;
+  process.emitWarning(
+    `The provided hostname "${hostname}" is not a valid ` +
+    'hostname, and is supported in the dns module solely for compatibility.',
+    'DeprecationWarning',
+    'DEP0118'
+  );
+}
+
 module.exports = {
   bindDefaultResolver,
   getDefaultResolver,
   setDefaultResolver,
   validateHints,
-  Resolver
+  Resolver,
+  emitInvalidHostnameWarning,
 };