dns: handle array holes in setServers()

This commit adds better handling of exceptional array formats
passed to dns.setServers(). Prior to this commit, the input
array was validated using map(), which preserves holes, allowing
them to be passed to c-ares, crashing Node. This commit replaces
map() with forEach(), which skips holes.

Fixes: https://github.com/nodejs/node/issues/8538
PR-URL: https://github.com/nodejs/node/pull/8567
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/dns.js

@@ -286,25 +286,26 @@ exports.setServers = function(servers) {
   // cache the original servers because in the event of an error setting the
   // servers cares won't have any servers available for resolution
   const orig = cares.getServers();
+  const newSet = [];
 
-  const newSet = servers.map((serv) => {
+  servers.forEach((serv) => {
     var ipVersion = isIP(serv);
     if (ipVersion !== 0)
-      return [ipVersion, serv];
+      return newSet.push([ipVersion, serv]);
 
     const match = serv.match(/\[(.*)\](:\d+)?/);
     // we have an IPv6 in brackets
     if (match) {
       ipVersion = isIP(match[1]);
       if (ipVersion !== 0)
-        return [ipVersion, match[1]];
+        return newSet.push([ipVersion, match[1]]);
     }
 
     const s = serv.split(/:\d+$/)[0];
     ipVersion = isIP(s);
 
     if (ipVersion !== 0)
-      return [ipVersion, s];
+      return newSet.push([ipVersion, s]);
 
     throw new Error(`IP address is not properly formatted: ${serv}`);
   });

test/parallel/test-dns.js

@@ -7,6 +7,33 @@ const dns = require('dns');
 var existing = dns.getServers();
 assert(existing.length);
 
+// Verify that setServers() handles arrays with holes and other oddities
+assert.doesNotThrow(() => {
+  const servers = [];
+
+  servers[0] = '127.0.0.1';
+  servers[2] = '0.0.0.0';
+  dns.setServers(servers);
+});
+
+assert.doesNotThrow(() => {
+  const servers = ['127.0.0.1', '192.168.1.1'];
+
+  servers[3] = '127.1.0.1';
+  servers[4] = '127.1.0.1';
+  servers[5] = '127.1.1.1';
+
+  Object.defineProperty(servers, 2, {
+    enumerable: true,
+    get: () => {
+      servers.length = 3;
+      return '0.0.0.0';
+    }
+  });
+
+  dns.setServers(servers);
+});
+
 function noop() {}
 
 var goog = [