fix(NODE-3109): prevent servername from being IP

servername should only ever be a hostname. Add a condition
to check if the host is an IP and skip auto setting the servername.

lib/core/connection/connect.js

@@ -243,7 +243,7 @@ function parseSslOptions(family, options) {
   }
 
   // Set default sni servername to be the same as host
-  if (result.servername == null) {
+  if (result.servername == null && !net.isIP(result.host)) {
     result.servername = result.host;
   }
 

test/manual/tls_support.test.js

@@ -1,5 +1,9 @@
 'use strict';
 const MongoClient = require('../..').MongoClient;
+const expect = require('chai').expect;
+const util = require('util');
+const dns = require('dns');
+const dnsLookup = util.promisify(dns.lookup);
 
 const REQUIRED_ENV = ['MONGODB_URI', 'SSL_KEY_FILE', 'SSL_CA_FILE'];
 
@@ -28,6 +32,24 @@ describe('TLS Support', function() {
         .join('&')}`
     )
   );
+
+  it('should ignore ip addresses in servername', function() {
+    const hostname = connectionString.match(/mongodb:\/\/(.+):/)[1];
+    return dnsLookup(hostname)
+      .then(res => {
+        const client = new MongoClient(connectionString.replace(hostname, res[0].address), {
+          tls: true
+        });
+        return client.connect();
+      })
+      .then(client => {
+        const connections = client.topology.connections();
+
+        for (const connection of connections) {
+          expect(connection.options.servername).to.not.exist;
+        }
+      });
+  });
 });
 
 function makeConnectionTest(connectionString, clientOptions) {