Allow X-Forwarded-For delete operation

Co-authored-by: landrybe <lbenguigui@gmail.com>

pkg/middlewares/headers/header.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/traefik/traefik/v2/pkg/config/dynamic"
 	"github.com/traefik/traefik/v2/pkg/log"
+	"github.com/vulcand/oxy/v2/forward"
 )
 
 // Header is a middleware that helps setup a few basic security features.
@@ -70,6 +71,10 @@ func (s *Header) modifyCustomRequestHeaders(req *http.Request) {
 	// Loop through Custom request headers
 	for header, value := range s.headers.CustomRequestHeaders {
 		switch {
+		// Handling https://github.com/golang/go/commit/ecdbffd4ec68b509998792f120868fec319de59b.
+		case value == "" && header == forward.XForwardedFor:
+			req.Header[header] = nil
+
 		case value == "":
 			req.Header.Del(header)
 

pkg/middlewares/headers/header_test.go

@@ -29,11 +29,14 @@ func TestNewHeader_customRequestHeader(t *testing.T) {
 			desc: "delete a header",
 			cfg: dynamic.Headers{
 				CustomRequestHeaders: map[string]string{
+					"X-Forwarded-For":         "",
 					"X-Custom-Request-Header": "",
 					"Foo":                     "",
 				},
 			},
-			expected: http.Header{},
+			expected: http.Header{
+				"X-Forwarded-For": nil,
+			},
 		},
 		{
 			desc: "override a header",