net/http/httputil: don't insert default User-Agent header in proxied requests

When ReverseProxy forwards a request with no User-Agent header, leave
the header in the forwarded request blank rather than inserting the
default Go HTTP clent User-Agent.

We already did this for NewSingleHostReverseProxy; generalize it to
every ReverseProxy.

Change-Id: Id81a230cb8d384acdfae190b78a4265d80720388
Reviewed-on: https://go-review.googlesource.com/c/go/+/407375
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

2 files changed, 25 insertions, 31 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index a5a3900fb3..0c52497177 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -156,10 +156,6 @@ func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
 		} else {
 			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
 		}
-		if _, ok := req.Header["User-Agent"]; !ok {
-			// explicitly disable User-Agent so it's not set to default value
-			req.Header.Set("User-Agent", "")
-		}
 	}
 	return &ReverseProxy{Director: director}
 }
@@ -321,6 +317,12 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	if _, ok := outreq.Header["User-Agent"]; !ok {
+		// If the outbound request doesn't have a User-Agent header set,
+		// don't send the default Go HTTP client User-Agent.
+		outreq.Header.Set("User-Agent", "")
+	}
+
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		p.getErrorHandler()(rw, outreq, err)
diff --git a/src/net/http/httputil/reverseproxy_test.go b/src/net/http/httputil/reverseproxy_test.go
index 23453c8bdc..3090e37582 100644
--- a/src/net/http/httputil/reverseproxy_test.go
+++ b/src/net/http/httputil/reverseproxy_test.go
@@ -614,46 +614,38 @@ func TestNilBody(t *testing.T) {
 
 // Issue 15524
 func TestUserAgentHeader(t *testing.T) {
-	const explicitUA = "explicit UA"
+	var gotUA string
 	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		if r.URL.Path == "/noua" {
-			if c := r.Header.Get("User-Agent"); c != "" {
-				t.Errorf("handler got non-empty User-Agent header %q", c)
-			}
-			return
-		}
-		if c := r.Header.Get("User-Agent"); c != explicitUA {
-			t.Errorf("handler got unexpected User-Agent header %q", c)
-		}
+		gotUA = r.Header.Get("User-Agent")
 	}))
 	defer backend.Close()
 	backendURL, err := url.Parse(backend.URL)
 	if err != nil {
 		t.Fatal(err)
 	}
-	proxyHandler := NewSingleHostReverseProxy(backendURL)
+
+	proxyHandler := new(ReverseProxy)
 	proxyHandler.ErrorLog = log.New(io.Discard, "", 0) // quiet for tests
+	proxyHandler.Director = func(req *http.Request) {
+		req.URL = backendURL
+	}
 	frontend := httptest.NewServer(proxyHandler)
 	defer frontend.Close()
 	frontendClient := frontend.Client()
 
-	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
-	getReq.Header.Set("User-Agent", explicitUA)
-	getReq.Close = true
-	res, err := frontendClient.Do(getReq)
-	if err != nil {
-		t.Fatalf("Get: %v", err)
-	}
-	res.Body.Close()
-
-	getReq, _ = http.NewRequest("GET", frontend.URL+"/noua", nil)
-	getReq.Header.Set("User-Agent", "")
-	getReq.Close = true
-	res, err = frontendClient.Do(getReq)
-	if err != nil {
-		t.Fatalf("Get: %v", err)
+	for _, sentUA := range []string{"explicit UA", ""} {
+		getReq, _ := http.NewRequest("GET", frontend.URL, nil)
+		getReq.Header.Set("User-Agent", sentUA)
+		getReq.Close = true
+		res, err := frontendClient.Do(getReq)
+		if err != nil {
+			t.Fatalf("Get: %v", err)
+		}
+		res.Body.Close()
+		if got, want := gotUA, sentUA; got != want {
+			t.Errorf("got forwarded User-Agent %q, want %q", got, want)
+		}
 	}
-	res.Body.Close()
 }
 
 type bufferPool struct {