Ensure IPv6 hostname is enclosed within square brackets in Host header

lib/_http_client.js

@@ -87,6 +87,17 @@ function ClientRequest(options, cb) {
     }
     if (host && !this.getHeader('host') && setHost) {
       var hostHeader = host;
+      var posColon = -1;
+
+      // For the Host header, ensure that IPv6 addresses are enclosed
+      // in square brackets, as defined by URI formatting
+      // https://tools.ietf.org/html/rfc3986#section-3.2.2
+      if (-1 !== (posColon = hostHeader.indexOf(':')) &&
+          -1 !== (posColon = hostHeader.indexOf(':', posColon)) &&
+          '[' !== hostHeader[0]) {
+        hostHeader = `[${hostHeader}]`;
+      }
+
       if (port && +port !== defaultPort) {
         hostHeader += ':' + port;
       }

test/parallel/test-http-host-header-ipv6-fail.js

@@ -0,0 +1,41 @@
+'use strict';
+/*
+ * When using the object form of http.request and using an IPv6 address
+ * as a hostname, and using a non-standard port, the Host header
+ * is improperly formatted.
+ * Issue: https://github.com/nodejs/node/issues/5308
+ * As per https://tools.ietf.org/html/rfc7230#section-5.4 and
+ * https://tools.ietf.org/html/rfc3986#section-3.2.2
+ * the IPv6 address should be enclosed in square brackets
+ */
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const hostname = '::1';
+const port = common.PORT;
+
+function httpreq() {
+  var req = http.request({
+    host: hostname,
+    port: port,
+    path: '/',
+    method: 'GET'
+  });
+  req.end();
+}
+
+if (!common.hasIPv6) {
+  console.error('Skipping test, no IPv6 support');
+  return;
+}
+
+const server = http.createServer(common.mustCall(function(req, res) {
+  assert.ok(req.headers.host, `[${hostname}]`);
+  res.end();
+  server.close(true);
+}));
+
+server.listen(port, hostname, () => httpreq());
+