Add an option to preserve the ForwardAuth Server Location header

pkg/config/dynamic/middlewares.go

@@ -258,6 +258,8 @@ type ForwardAuth struct {
 	ForwardBody bool `json:"forwardBody,omitempty" toml:"forwardBody,omitempty" yaml:"forwardBody,omitempty" export:"true"`
 	// MaxBodySize defines the maximum body size in bytes allowed to be forwarded to the authentication server.
 	MaxBodySize *int64 `json:"maxBodySize,omitempty" toml:"maxBodySize,omitempty" yaml:"maxBodySize,omitempty" export:"true"`
+	// PreserveLocationHeader defines whether to forward the Location header to the client as is or prefix it with the domain name of the authentication server.
+	PreserveLocationHeader bool `json:"preserveLocationHeader,omitempty" toml:"preserveLocationHeader,omitempty" yaml:"preserveLocationHeader,omitempty" export:"true"`
 }
 
 func (f *ForwardAuth) SetDefaults() {

pkg/config/label/label_test.go

@@ -1329,6 +1329,7 @@ func TestEncodeConfiguration(t *testing.T) {
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TLS.InsecureSkipVerify":                  "true",
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TLS.Key":                                 "foobar",
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TrustForwardHeader":                      "true",
+		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.PreserveLocationHeader":                  "false",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowCredentials":               "true",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowHeaders":                   "X-foobar, X-fiibar",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowMethods":                   "GET, PUT",

pkg/middlewares/auth/forward.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"net/url"
 	"regexp"
 	"strings"
 	"time"
@@ -55,6 +56,7 @@ type forwardAuth struct {
 	headerField              string
 	forwardBody              bool
 	maxBodySize              int64
+	preserveLocationHeader   bool
 }
 
 // NewForward creates a forward auth middleware.
@@ -78,6 +80,7 @@ func NewForward(ctx context.Context, next http.Handler, config dynamic.ForwardAu
 		headerField:              config.HeaderField,
 		forwardBody:              config.ForwardBody,
 		maxBodySize:              dynamic.ForwardAuthDefaultMaxBodySize,
+		preserveLocationHeader:   config.PreserveLocationHeader,
 	}
 
 	if config.MaxBodySize != nil {
@@ -222,9 +225,7 @@ func (fa *forwardAuth) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		utils.CopyHeaders(rw.Header(), forwardResponse.Header)
 		utils.RemoveHeaders(rw.Header(), hopHeaders...)
 
-		// Grab the location header, if any.
-		redirectURL, err := forwardResponse.Location()
-
+		redirectURL, err := fa.redirectURL(forwardResponse)
 		if err != nil {
 			if !errors.Is(err, http.ErrNoLocation) {
 				logger.Debug().Err(err).Msgf("Error reading response location header %s", fa.address)
@@ -282,6 +283,18 @@ func (fa *forwardAuth) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	fa.next.ServeHTTP(middlewares.NewResponseModifier(rw, req, fa.buildModifier(authCookies)), req)
 }
 
+func (fa *forwardAuth) redirectURL(forwardResponse *http.Response) (*url.URL, error) {
+	if !fa.preserveLocationHeader {
+		return forwardResponse.Location()
+	}
+
+	// Preserve the Location header if it exists.
+	if lv := forwardResponse.Header.Get("Location"); lv != "" {
+		return url.Parse(lv)
+	}
+	return nil, http.ErrNoLocation
+}
+
 func (fa *forwardAuth) buildModifier(authCookies []*http.Cookie) func(res *http.Response) error {
 	return func(res *http.Response) error {
 		cookies := res.Cookies()

pkg/middlewares/auth/forward_test.go

@@ -711,6 +711,34 @@ func TestForwardAuthTracing(t *testing.T) {
 	}
 }
 
+func TestForwardAuthPreserveLocationHeader(t *testing.T) {
+	relativeURL := "/index.html"
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Location", relativeURL)
+		http.Error(w, "Unauthorized", http.StatusUnauthorized)
+	}))
+	t.Cleanup(server.Close)
+
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+	auth := dynamic.ForwardAuth{
+		Address:                server.URL,
+		PreserveLocationHeader: true,
+	}
+	middleware, err := NewForward(context.Background(), next, auth, "authTest")
+	require.NoError(t, err)
+
+	ts := httptest.NewServer(middleware)
+	t.Cleanup(ts.Close)
+
+	req := testhelpers.MustNewRequest(http.MethodGet, ts.URL, nil)
+	res, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+
+	assert.Equal(t, http.StatusUnauthorized, res.StatusCode)
+	assert.Equal(t, relativeURL, res.Header.Get("Location"))
+}
+
 type mockTracer struct {
 	embedded.Tracer
 

pkg/provider/kubernetes/crd/kubernetes.go

@@ -790,6 +790,7 @@ func createForwardAuthMiddleware(k8sClient Client, namespace string, auth *traef
 		AuthRequestHeaders:       auth.AuthRequestHeaders,
 		AddAuthCookiesToResponse: auth.AddAuthCookiesToResponse,
 		ForwardBody:              auth.ForwardBody,
+		PreserveLocationHeader:   auth.PreserveLocationHeader,
 	}
 	forwardAuth.SetDefaults()
 

pkg/provider/kubernetes/crd/traefikio/v1alpha1/middleware.go

@@ -165,6 +165,8 @@ type ForwardAuth struct {
 	ForwardBody bool `json:"forwardBody,omitempty"`
 	// MaxBodySize defines the maximum body size in bytes allowed to be forwarded to the authentication server.
 	MaxBodySize *int64 `json:"maxBodySize,omitempty"`
+	// PreserveLocationHeader defines whether to forward the Location header to the client as is or prefix it with the domain name of the authentication server.
+	PreserveLocationHeader bool `json:"preserveLocationHeader,omitempty"`
 }
 
 // ClientTLS holds the client TLS configuration.

pkg/provider/kv/kv_test.go

@@ -90,6 +90,7 @@ func Test_buildConfiguration(t *testing.T) {
 		"traefik/http/middlewares/Middleware08/forwardAuth/trustForwardHeader":                       "true",
 		"traefik/http/middlewares/Middleware08/forwardAuth/forwardBody":                              "true",
 		"traefik/http/middlewares/Middleware08/forwardAuth/maxBodySize":                              "42",
+		"traefik/http/middlewares/Middleware08/forwardAuth/preserveLocationHeader":                   "true",
 		"traefik/http/middlewares/Middleware15/redirectScheme/scheme":                                "foobar",
 		"traefik/http/middlewares/Middleware15/redirectScheme/port":                                  "foobar",
 		"traefik/http/middlewares/Middleware15/redirectScheme/permanent":                             "true",
@@ -442,8 +443,9 @@ func Test_buildConfiguration(t *testing.T) {
 							"foobar",
 							"foobar",
 						},
-						ForwardBody: true,
-						MaxBodySize: pointer(int64(42)),
+						ForwardBody:            true,
+						MaxBodySize:            pointer(int64(42)),
+						PreserveLocationHeader: true,
 					},
 				},
 				"Middleware06": {