diff --git a/.gcloudignore b/.gcloudignore
new file mode 100644
index 00000000..199e6d9f
--- /dev/null
+++ b/.gcloudignore
@@ -0,0 +1,25 @@
+# This file specifies files that are *not* uploaded to Google Cloud Platform
+# using gcloud. It follows the same syntax as .gitignore, with the addition of
+# "#!include" directives (which insert the entries of the given .gitignore-style
+# file at that point).
+#
+# For more information, run:
+#   $ gcloud topic gcloudignore
+#
+.gcloudignore
+# If you would like to upload your .git directory, .gitignore file or files
+# from your .gitignore file, remove the corresponding line
+# below:
+.git
+.gitignore
+
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+# Test binary, build with `go test -c`
+*.test
+# Output of the go coverage tool, specifically when used with LiteIDE
+*.out
\ No newline at end of file
diff --git a/.gitignore b/.gitignore
index 252f74e8..9bebe3fe 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,4 @@ _cgo_*
 _test*
 *.out
 _obj
+ga-beacon
\ No newline at end of file
diff --git a/ga-beacon.go b/ga-beacon.go
index 51508bdf..928e7cb4 100644
--- a/ga-beacon.go
+++ b/ga-beacon.go
@@ -13,11 +13,9 @@ import (
 	"os"
 	"strings"
 	"time"
-
-	"google.golang.org/appengine/delay"
 )
 
-const beaconURL = "http://www.google-analytics.com/collect"
+var beaconURL = "http://www.google-analytics.com/collect"
 
 var (
 	pixel        = mustReadFile("static/pixel.gif")
@@ -64,26 +62,26 @@ func generateUUID(cid *string) error {
 	return nil
 }
 
-var delayHit = delay.Func("collect", logHit)
+// var delayHit = delay.Func("collect", logHit)
 
-func sendToGA(c context.Context, ua string, ip string, cid string, values url.Values) error {
+func sendToGA(c context.Context, ua string, ip string, cid string, values url.Values) {
 	client := &http.Client{}
 
 	req, _ := http.NewRequest("POST", beaconURL, strings.NewReader(values.Encode()))
-	req.Header.Add("User-Agent", ua)
+	if ua != "" {
+		req.Header.Add("User-Agent", ua)
+	}
 	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
 
 	if resp, err := client.Do(req); err != nil {
 		log.Printf("GA collector POST error: %s", err.Error())
-		return err
 	} else {
 		log.Printf("GA collector status: %v, cid: %v, ip: %s", resp.Status, cid, ip)
 		log.Printf("Reported payload: %v", values)
 	}
-	return nil
 }
 
-func logHit(c context.Context, params []string, query url.Values, ua string, ip string, cid string) error {
+func logHit(c context.Context, params []string, query url.Values, ua string, ip string, cid string) {
 	// 1) Initialize default values from path structure
 	// 2) Allow query param override to report arbitrary values to GA
 	//
@@ -95,14 +93,17 @@ func logHit(c context.Context, params []string, query url.Values, ua string, ip
 		"tid": {params[0]},  // tracking / property ID
 		"cid": {cid},        // unique client ID (server generated UUID)
 		"dp":  {params[1]},  // page path
-		"uip": {ip},         // IP address of the user
+	}
+
+	if ip != "" {
+		payload["uip"] = []string{ip} // IP address of the user
 	}
 
 	for key, val := range query {
 		payload[key] = val
 	}
 
-	return sendToGA(c, ua, ip, cid, payload)
+	sendToGA(c, ua, ip, cid, payload)
 }
 
 func handler(w http.ResponseWriter, r *http.Request) {
@@ -119,14 +120,16 @@ func handler(w http.ResponseWriter, r *http.Request) {
 
 	// activate referrer path if ?useReferer is used and if referer exists
 	if _, ok := query["useReferer"]; ok {
-		if len(refOrg) != 0 {
-			referer := strings.Replace(strings.Replace(refOrg, "http://", "", 1), "https://", "", 1)
-			if len(referer) != 0 {
-				// if the useReferer is present and the referer information exists
-				//  the path is ignored and the beacon referer information is used instead.
-				params = strings.SplitN(strings.Trim(r.URL.Path, "/")+"/"+referer, "/", 2)
-			}
+		referer := strings.Replace(strings.Replace(refOrg, "http://", "", 1), "https://", "", 1)
+
+		if referer == "" {
+			http.Error(w, "could not extract referer from headers", http.StatusBadRequest)
+			return
 		}
+
+		// if the useReferer is present and the referer information exists
+		//  the path is ignored and the beacon referer information is used instead.
+		params = strings.SplitN(strings.Trim(r.URL.Path, "/")+"/"+referer, "/", 2)
 	}
 	// /account -> account template
 	if len(params) == 1 {
@@ -138,7 +141,7 @@ func handler(w http.ResponseWriter, r *http.Request) {
 			Referer: refOrg,
 		}
 		if err := pageTemplate.ExecuteTemplate(w, "page.html", templateParams); err != nil {
-			http.Error(w, "could not show account page", 500)
+			http.Error(w, "could not show account page", http.StatusInternalServerError)
 			log.Printf("Cannot execute template: %v", err)
 		}
 		return
@@ -168,21 +171,27 @@ func handler(w http.ResponseWriter, r *http.Request) {
 		// delayHit.Call(c, params, r.Header.Get("User-Agent"), cid)
 	}
 
+	var err error
+
 	// Write out GIF pixel or badge, based on presence of "pixel" param.
 	if _, ok := query["pixel"]; ok {
 		w.Header().Set("Content-Type", "image/gif")
-		w.Write(pixel)
+		_, err = w.Write(pixel)
 	} else if _, ok := query["gif"]; ok {
 		w.Header().Set("Content-Type", "image/gif")
-		w.Write(badgeGif)
+		_, err = w.Write(badgeGif)
 	} else if _, ok := query["flat"]; ok {
 		w.Header().Set("Content-Type", "image/svg+xml")
-		w.Write(badgeFlat)
+		_, err = w.Write(badgeFlat)
 	} else if _, ok := query["flat-gif"]; ok {
 		w.Header().Set("Content-Type", "image/gif")
-		w.Write(badgeFlatGif)
+		_, err = w.Write(badgeFlatGif)
 	} else {
 		w.Header().Set("Content-Type", "image/svg+xml")
-		w.Write(badge)
+		_, err = w.Write(badge)
+	}
+
+	if err != nil {
+		log.Print(err)
 	}
 }
diff --git a/ga-beacon_test.go b/ga-beacon_test.go
new file mode 100644
index 00000000..8bebc420
--- /dev/null
+++ b/ga-beacon_test.go
@@ -0,0 +1,171 @@
+package main
+
+import (
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+const (
+	defaultTID = "UA-XXXXX-X"
+	defaultDP  = "homepage"
+	defaultURL = "/" + defaultTID + "/" + defaultDP
+)
+
+// Record the response from the handler function
+func record(req *http.Request) *httptest.ResponseRecorder {
+	rec := httptest.NewRecorder()
+	handler(rec, req)
+	return rec
+}
+
+// Create a request for a URL
+func newRequest(t *testing.T, url string) *http.Request {
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return req
+}
+
+// Record the response for a URL
+func recordURL(t *testing.T, url string) *httptest.ResponseRecorder {
+	return record(newRequest(t, url))
+}
+
+// Read a file
+func readFile(t *testing.T, path string) []byte {
+	b, err := ioutil.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
+// Read an input stream
+func readAll(t *testing.T, r io.Reader) []byte {
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
+// Test the correct data is sent to Google Analytics
+func testBeaconRequest(t *testing.T, r *http.Request, tid string, dp string) {
+	assert.Equal(t, "POST", r.Method)
+	assert.Equal(t, "application/x-www-form-urlencoded", r.Header.Get("Content-Type"))
+	err := r.ParseForm()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, "1", r.PostForm.Get("v"))
+	assert.Equal(t, "pageview", r.PostForm.Get("t"))
+	assert.Equal(t, tid, r.PostForm.Get("tid"))
+	assert.Equal(t, dp, r.PostForm.Get("dp"))
+	// The requests are not actually sent, so should not have an IP address
+	assert.Empty(t, r.PostForm.Get("uip"))
+}
+
+// Test the tracking request
+func testTrackRequest(t *testing.T, tid string, dp string, cid string, req *http.Request) *http.Response {
+	if req == nil {
+		req = newRequest(t, "/"+tid+"/"+dp)
+	}
+	var cidf string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		testBeaconRequest(t, r, tid, dp)
+		cidf = r.PostForm.Get("cid")
+		if cid != "" {
+			assert.Equal(t, cid, cidf)
+		}
+	}))
+	defer server.Close()
+	beaconURL = server.URL
+	res := record(req).Result()
+	assert.Equal(t, http.StatusOK, res.StatusCode)
+	assert.Equal(t, "no-cache, no-store, must-revalidate, private", res.Header.Get("Cache-Control"))
+	expires, err := time.Parse(http.TimeFormat, res.Header.Get("Expires"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.True(t, expires.Before(time.Now()))
+	if cid == "" {
+		// ensure cid cookie has been set
+		var cidc string
+		for _, c := range res.Cookies() {
+			if c.Name == "cid" {
+				cidc = c.Value
+				break
+			}
+		}
+		assert.Equal(t, cidf, cidc)
+	}
+	return res
+}
+
+func TestBeacon(t *testing.T) {
+	t.Run("redirect on no params", func(t *testing.T) {
+		rec := recordURL(t, "/")
+		assert.Equal(t, http.StatusFound, rec.Code)
+		assert.Equal(t, "https://github.com/igrigorik/ga-beacon", rec.Header().Get("Location"))
+	})
+	t.Run("account page", func(t *testing.T) {
+		req := newRequest(t, "/UA-XXXXX-X")
+		req.Header.Set("Referer", "https://example.com")
+		rec := record(req)
+		assert.Equal(t, http.StatusOK, rec.Code)
+		assert.Equal(t, readFile(t, "page_test.html"), rec.Body.Bytes())
+	})
+	t.Run("badge", func(t *testing.T) {
+		res := testTrackRequest(t, defaultTID, defaultDP, "", nil)
+		assert.Equal(t, "image/svg+xml", res.Header.Get("Content-Type"))
+		assert.Equal(t, readFile(t, "static/badge.svg"), readAll(t, res.Body))
+	})
+	t.Run("pixel", func(t *testing.T) {
+		res := testTrackRequest(t, defaultTID, defaultDP, "", newRequest(t, defaultURL+"?pixel"))
+		assert.Equal(t, "image/gif", res.Header.Get("Content-Type"))
+		assert.Equal(t, readFile(t, "static/pixel.gif"), readAll(t, res.Body))
+	})
+	t.Run("badge gif", func(t *testing.T) {
+		res := testTrackRequest(t, defaultTID, defaultDP, "", newRequest(t, defaultURL+"?gif"))
+		assert.Equal(t, "image/gif", res.Header.Get("Content-Type"))
+		assert.Equal(t, readFile(t, "static/badge.gif"), readAll(t, res.Body))
+	})
+	t.Run("badge flat", func(t *testing.T) {
+		res := testTrackRequest(t, defaultTID, defaultDP, "", newRequest(t, defaultURL+"?flat"))
+		assert.Equal(t, "image/svg+xml", res.Header.Get("Content-Type"))
+		assert.Equal(t, readFile(t, "static/badge-flat.svg"), readAll(t, res.Body))
+	})
+	t.Run("badge flat gif", func(t *testing.T) {
+		res := testTrackRequest(t, defaultTID, defaultDP, "", newRequest(t, defaultURL+"?flat-gif"))
+		assert.Equal(t, "image/gif", res.Header.Get("Content-Type"))
+		assert.Equal(t, readFile(t, "static/badge-flat.gif"), readAll(t, res.Body))
+	})
+	t.Run("referer as path", func(t *testing.T) {
+		req := newRequest(t, "/"+defaultTID+"?useReferer")
+		t.Run("warn on missing referer", func(t *testing.T) {
+			rec := record(req)
+			assert.Equal(t, http.StatusBadRequest, rec.Code)
+			assert.Contains(t, rec.Body.String(), "could not extract referer from headers")
+		})
+		dp := "example.com"
+		req.Header.Set("Referer", "https://"+dp)
+		testTrackRequest(t, defaultTID, dp, "", req)
+	})
+	t.Run("existing cid", func(t *testing.T) {
+		req := newRequest(t, defaultURL)
+		cid := "5d7b632fef264b76a7938362e5aba2c8"
+		req.AddCookie(&http.Cookie{
+			Name:  "cid",
+			Value: cid,
+			Path:  "/" + defaultTID,
+		})
+		testTrackRequest(t, defaultTID, defaultDP, cid, req)
+	})
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 00000000..aae8925b
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module github.com/igrigorik/ga-beacon
+
+go 1.15
+
+require (
+	github.com/stretchr/testify v1.7.0
+	google.golang.org/appengine v1.6.7
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 00000000..cb1b3698
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,22 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/golang/protobuf v1.3.1 h1:YF8+flBXS5eO826T4nzqPrxfhQThhXl0YzfuUPu4SBg=
+github.com/golang/protobuf v1.3.1/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190603091049-60506f45cf65 h1:+rhAzEzT3f4JtomfC371qB+0Ola2caSKcY69NUBZrRQ=
+golang.org/x/net v0.0.0-20190603091049-60506f45cf65/go.mod h1:HSz+uSET+XFnRR8LxR5pz3Of3rY3CfYBVs4xY44aLks=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+google.golang.org/appengine v1.6.7 h1:FZR1q0exgwxzPzp/aF+VccGrSfxfPpkBqjIIEq3ru6c=
+google.golang.org/appengine v1.6.7/go.mod h1:8WjMMxjGQR8xUklV/ARdw2HLXBOI7O7uCIDZVag1xfc=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/page_test.html b/page_test.html
new file mode 100644
index 00000000..9f024041
--- /dev/null
+++ b/page_test.html
@@ -0,0 +1,23 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+  <title>GA account: UA-XXXXX-X</title>
+
+  <script>
+  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+  })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+
+  ga('create', 'UA-XXXXX-X', 'ga-beacon.appspot.com');
+  ga('send', 'pageview');
+  </script>
+</head>
+
+<body>
+<p>GA account: UA-XXXXX-X</p>
+<p>Beacon Referrer: https://example.com</p>
+<p>Setup instructions: <a href="https://github.com/igrigorik/ga-beacon">https://github.com/igrigorik/ga-beacon</a></p>
+</body>
+</html>