net/http/httputil: don't append to X-Forwarded-For in ReverseProxy when nil

Fixes #38079

Change-Id: Iac02d7f9574061bb26d1d9a41bb6ee6cc38934e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/230937
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.15.html

@@ -200,7 +200,7 @@ TODO
 
 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
   <dd>
-    <p><!-- CL -->
+    <p><!-- CL 228645 -->
       If an I/O operation exceeds a deadline set by
       the <a href="/pkg/net/#Conn"><code>Conn.SetDeadline</code></a>,
       <code>Conn.SetReadDeadline</code>,
@@ -217,12 +217,23 @@ TODO
   </dd>
 </dl>
 
+<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
+  <dd>
+    <p><!-- CL 230937 -->
+      <a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a>
+      now supports not modifying the <code>X-Forwarded-For</code>
+      header when the incoming <code>Request.Header</code> map entry
+      for that field is <code>nil</code>.
+    </p>
+  </dd>
+</dl>
+
 <dl id="net/http/pprof"><dt><a href="/pkg/net/http/pprof/">net/http/pprof</a></dt>
   <dd>
-    <p><!-- CL 147598, 229537 -->
-      All profile endpoints now support a "seconds" parameter. When present,
+    <p><!-- CL 147598, CL 229537 -->
+      All profile endpoints now support a "<code>seconds</code>" parameter. When present,
       the endpoint profiles for the specified number of seconds and reports the difference.
-      The meaning of the "seconds" parameter in the <code>cpu</code> profile and
+      The meaning of the "<code>seconds</code>" parameter in the <code>cpu</code> profile and
       the trace endpoints is unchanged.
     </p>
   </dd>

src/net/http/httputil/reverseproxy.go

@@ -25,10 +25,15 @@ import (
 // sends it to another server, proxying the response back to the
 // client.
 //
-// ReverseProxy automatically sets the client IP as the value of the
+// ReverseProxy by default sets the client IP as the value of the
 // X-Forwarded-For header.
+//
 // If an X-Forwarded-For header already exists, the client IP is
-// appended to the existing values.
+// appended to the existing values. As a special case, if the header
+// exists in the Request.Header map but has a nil value (such as when
+// set by the Director func), the X-Forwarded-For header is
+// not modified.
+//
 // To prevent IP spoofing, be sure to delete any pre-existing
 // X-Forwarded-For header coming from the client or
 // an untrusted proxy.
@@ -248,10 +253,14 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		// If we aren't the first proxy retain prior
 		// X-Forwarded-For information as a comma+space
 		// separated list and fold multiple headers into one.
-		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
+		prior, ok := outreq.Header["X-Forwarded-For"]
+		omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
+		if len(prior) > 0 {
 			clientIP = strings.Join(prior, ", ") + ", " + clientIP
 		}
-		outreq.Header.Set("X-Forwarded-For", clientIP)
+		if !omit {
+			outreq.Header.Set("X-Forwarded-For", clientIP)
+		}
 	}
 
 	res, err := transport.RoundTrip(outreq)

src/net/http/httputil/reverseproxy_test.go

@@ -277,6 +277,39 @@ func TestXForwardedFor(t *testing.T) {
 	}
 }
 
+// Issue 38079: don't append to X-Forwarded-For if it's present but nil
+func TestXForwardedFor_Omit(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if v := r.Header.Get("X-Forwarded-For"); v != "" {
+			t.Errorf("got X-Forwarded-For header: %q", v)
+		}
+		w.Write([]byte("hi"))
+	}))
+	defer backend.Close()
+	backendURL, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	proxyHandler := NewSingleHostReverseProxy(backendURL)
+	frontend := httptest.NewServer(proxyHandler)
+	defer frontend.Close()
+
+	oldDirector := proxyHandler.Director
+	proxyHandler.Director = func(r *http.Request) {
+		r.Header["X-Forwarded-For"] = nil
+		oldDirector(r)
+	}
+
+	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
+	getReq.Host = "some-name"
+	getReq.Close = true
+	res, err := frontend.Client().Do(getReq)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	res.Body.Close()
+}
+
 var proxyQueryTests = []struct {
 	baseSuffix string // suffix to add to backend URL
 	reqSuffix  string // suffix to add to frontend's request URL