Default to localhost if no host given.

index.js

@@ -514,6 +514,9 @@ function wrap(protocols) {
         maxBodyLength: exports.maxBodyLength,
       }, input, options);
       options.nativeProtocols = nativeProtocols;
+      if (!isString(options.host) && !isString(options.hostname)) {
+        options.hostname = "::1";
+      }
 
       assert.equal(options.protocol, protocol, "protocol mismatch");
       debug("options", options);
@@ -596,7 +599,8 @@ function abortRequest(request) {
 }
 
 function isSubdomain(subdomain, domain) {
-  const dot = subdomain.length - domain.length - 1;
+  assert(isString(subdomain) && isString(domain));
+  var dot = subdomain.length - domain.length - 1;
   return dot > 0 && subdomain[dot] === "." && subdomain.endsWith(domain);
 }
 

test/test.js

@@ -1332,6 +1332,29 @@ describe("follow-redirects", function () {
           assert.equal(body.host, "localhost:3600");
         });
     });
+
+    it("defaults to localhost for relative redirect if no host given", function () {
+      app.get("/a", redirectsTo(302, "/b"));
+      app.get("/b", function (req, res) {
+        res.write(JSON.stringify(req.headers));
+        req.pipe(res); // will invalidate JSON if non-empty
+      });
+
+      return server.start(app)
+        .then(asPromise(function (resolve, reject) {
+          var opts = { port: 3600, path: "/a" };
+          http.get(opts, resolve).on("error", reject);
+        }))
+        .then(asPromise(function (resolve, reject, res) {
+          assert.deepEqual(res.statusCode, 200);
+          assert(res.responseUrl.endsWith("/b"));
+          res.pipe(concat({ encoding: "string" }, resolve)).on("error", reject);
+        }))
+        .then(function (str) {
+          var body = JSON.parse(str);
+          assert(body.host.endsWith(":3600"));
+        });
+    });
   });
 
   [