proxy: don't append some predefined headers

fix issue #1086
Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/proxy/proxy_test.go

@@ -450,6 +450,8 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 		w.Header().Add("Merge-Me", "Initial")
 		w.Header().Add("Remove-Me", "Remove-Value")
 		w.Header().Add("Replace-Me", "Replace-Value")
+		w.Header().Add("Content-Type", "text/html")
+		w.Header().Add("Overwrite-Me", "Overwrite-Value")
 		w.Write([]byte("Hello, client"))
 	}))
 	defer backend.Close()
@@ -473,6 +475,10 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 		t.Fatalf("Failed to create request: %v", err)
 	}
 	w := httptest.NewRecorder()
+	// set a predefined skip header
+	w.Header().Set("Content-Type", "text/css")
+	// set a predefined overwritten header
+	w.Header().Set("Overwrite-Me", "Initial")
 
 	p.ServeHTTP(w, r)
 
@@ -507,6 +513,22 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
 			headerKey, expect, got)
 	}
+
+	headerKey = "Content-Type"
+	got = actualHeaders[headerKey]
+	expect = []string{"text/css"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
+	}
+
+	headerKey = "Overwrite-Me"
+	got = actualHeaders[headerKey]
+	expect = []string{"Overwrite-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
+	}
 }
 
 var (

caddyhttp/proxy/reverseproxy.go

@@ -252,8 +252,28 @@ func (rp *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 	io.CopyBuffer(dst, src, buf.([]byte))
 }
 
+// skip these headers if they already exist.
+// see https://github.com/mholt/caddy/pull/1112#discussion_r80092582
+var skipHeaders = map[string]struct{}{
+	"Content-Type":        {},
+	"Content-Disposition": {},
+	"Accept-Ranges":       {},
+	"Set-Cookie":          {},
+	"Cache-Control":       {},
+	"Expires":             {},
+}
+
 func copyHeader(dst, src http.Header) {
 	for k, vv := range src {
+		if _, ok := dst[k]; ok {
+			// skip some predefined headers
+			// see https://github.com/mholt/caddy/issues/1086
+			if _, shouldSkip := skipHeaders[k]; shouldSkip {
+				continue
+			}
+			// otherwise, overwrite
+			dst.Del(k)
+		}
 		for _, v := range vv {
 			dst.Add(k, v)
 		}