Review comments.

index.js

@@ -361,19 +361,24 @@ RedirectableRequest.prototype._processResponse = function (response) {
     }
 
     // Prefer previous host from Host header
-    var previousHost = getMatchingHeader(/^host$/i, this._options.headers) ||
-      url.parse(this._currentUrl).host;
+    var currentUrlParts = url.parse(this._currentUrl);
+    var previousHost = removeMatchingHeaders(/^host$/i, this._options.headers) ||
+      currentUrlParts.host;
+
+    // If the redirect is relative, carry over the host of the last request from host header
+    var previousHostUrl = !url.parse(location).host ?
+      url.format(Object.assign({}, currentUrlParts, { host: previousHost })) :
+      this._currentUrl;
 
     // Create the redirected request
-    var redirectUrl = url.resolve(this._currentUrl, location);
+    var redirectUrl = url.resolve(previousHostUrl, location);
     debug("redirecting to", redirectUrl);
     this._isRedirect = true;
     var redirectUrlParts = url.parse(redirectUrl);
     Object.assign(this._options, redirectUrlParts);
 
-    // Drop the Host & Authorization header if redirecting to another host
-    if (redirectUrlParts.host !== previousHost && url.parse(location).host) {
-      removeMatchingHeaders(/^host$/i, this._options.headers);
+    // Drop the Authorization header if redirecting to another host
+    if (redirectUrlParts.host !== previousHost) {
       removeMatchingHeaders(/^authorization$/i, this._options.headers);
     }
 
@@ -503,16 +508,6 @@ function urlToOptions(urlObject) {
   return options;
 }
 
-function getMatchingHeader(regex, headers) {
-  var lastValue;
-  for (var header in headers) {
-    if (regex.test(header)) {
-      lastValue = headers[header];
-    }
-  }
-  return lastValue;
-}
-
 function removeMatchingHeaders(regex, headers) {
   var lastValue;
   for (var header in headers) {

test/test.js

@@ -1243,7 +1243,7 @@ describe("follow-redirects", function () {
         });
     });
 
-    it("uses the existing host header if redirect host is the same", function () {
+    it("uses the location host if redirect host is the same", function () {
       app.get("/a", redirectsTo(302, "http://localhost:3600/b"));
       app.get("/b", function (req, res) {
         res.write(JSON.stringify(req.headers));
@@ -1282,7 +1282,7 @@ describe("follow-redirects", function () {
         }))
         .then(asPromise(function (resolve, reject, res) {
           assert.deepEqual(res.statusCode, 200);
-          assert.deepEqual(res.responseUrl, "http://127.0.0.1:3600/b");
+          assert.deepEqual(res.responseUrl, "http://localhost:3600/b");
           res.pipe(concat({ encoding: "string" }, resolve)).on("error", reject);
         }))
         .then(function (str) {
@@ -1344,6 +1344,32 @@ describe("follow-redirects", function () {
         });
     });
 
+    it("drops the header when redirected to a different host (same hostname and different port)", function () {
+      app.get("/a", redirectsTo(302, "http://localhost:3600/b"));
+      app.get("/b", function (req, res) {
+        res.end(JSON.stringify(req.headers));
+      });
+
+      var opts = url.parse("http://127.0.0.1:3600/a");
+      opts.headers = {
+        host: "localhost",
+        authorization: "bearer my-token-1234",
+      };
+
+      return server.start(app)
+        .then(asPromise(function (resolve, reject) {
+          http.get(opts, resolve).on("error", reject);
+        }))
+        .then(asPromise(function (resolve, reject, res) {
+          res.pipe(concat({ encoding: "string" }, resolve)).on("error", reject);
+        }))
+        .then(function (str) {
+          var body = JSON.parse(str);
+          assert.equal(body.host, "localhost:3600");
+          assert.equal(body.authorization, undefined);
+        });
+    });
+
     it("drops the header when redirected to a different host", function () {
       app.get("/a", redirectsTo(302, "http://127.0.0.1:3600/b"));
       app.get("/b", function (req, res) {