test: add unit test for UseLocationHost fix in HandlerWithErrorResponder

Author: udayakp

pkg/server/handler/handler_test.go

@@ -0,0 +1,53 @@
+package handler
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"gotest.tools/assert"
+	"k8s.io/client-go/rest"
+)
+
+// fakeResponder satisfies the ErrorResponder interface for tests.
+type fakeResponder struct{}
+
+func (f *fakeResponder) Error(w http.ResponseWriter, _ *http.Request, err error) {
+	http.Error(w, err.Error(), http.StatusInternalServerError)
+}
+
+// TestHandlerWithErrorResponder_UseLocationHost verifies that UseLocationHost=true causes
+// the UpgradeAwareHandler to send the backend hostname as the Host header, not the
+// original client Host header.
+//
+// On clusters where the API server is fronted by an Istio proxy (e.g. Gardener shoots),
+// Istio enforces virtual host routing and returns 404 for any Host value it does not
+// recognise. Without this fix, kubectl logs/exec/port-forward fail because the handler
+// forwards the vcluster LB hostname instead of the actual API server hostname.
+func TestHandlerWithErrorResponder_UseLocationHost(t *testing.T) {
+	var receivedHost string
+	backend := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		receivedHost = r.Host
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer backend.Close()
+
+	cfg := &rest.Config{
+		Host:            backend.URL,
+		TLSClientConfig: rest.TLSClientConfig{Insecure: true},
+	}
+
+	h, err := HandlerWithErrorResponder("", cfg, nil, &fakeResponder{})
+	assert.NilError(t, err)
+
+	// Simulate a client request with a vcluster LB hostname as Host —
+	// this is what the UpgradeAwareHandler used to forward before the fix.
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/namespaces/default/pods/test/log", nil)
+	req.Host = "vcluster-lb.us-west-2.elb.amazonaws.com"
+
+	h.ServeHTTP(httptest.NewRecorder(), req)
+
+	// The backend must receive its own hostname, not the client's LB address.
+	assert.Equal(t, receivedHost, backend.Listener.Addr().String(),
+		"UpgradeAwareHandler must send Host: <backend hostname> (UseLocationHost=true), not the original client Host header")
+}