fix(provider): drop the relay's capabilities to the strict minimum

The relay only dials an upstream and forwards bytes: it needs none of
Docker's default Linux capabilities. Run it with CapDrop: ["ALL"],
keeping only NET_BIND_SERVICE back via CapAdd since a route commonly
targets a privileged port (e.g. 80, 443) that the relay -- running
unprivileged as UID 65532 -- must still be able to listen on inside
its own container.

Validated end-to-end against a real provider project: a container on
the relay's network reaches the provider through its published, low
port with the hardened capability set in place.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Signed-off-by: Nicolas De Loof <[email protected]>

pkg/compose/relay.go

@@ -332,6 +332,13 @@ func (s *composeService) createRelayContainer(ctx context.Context, project *type
 		RestartPolicy: container.RestartPolicy{
 			Name: container.RestartPolicyUnlessStopped,
 		},
+		// The relay only dials out and forwards bytes: it needs none of
+		// Docker's default capabilities. NET_BIND_SERVICE is kept because a
+		// route commonly targets a privileged port (e.g. 80, 443) that the
+		// relay — running unprivileged as UID 65532 — must still be able to
+		// listen on inside its own container.
+		CapDrop: []string{"ALL"},
+		CapAdd:  []string{"NET_BIND_SERVICE"},
 	}
 
 	// First network at creation, remaining ones connected afterwards — the

pkg/compose/relay_test.go

@@ -17,6 +17,7 @@
 package compose
 
 import (
+	"context"
 	"testing"
 
 	"github.com/compose-spec/compose-go/v2/types"
@@ -276,6 +277,43 @@ func TestRemoveServiceRelayRemovesExisting(t *testing.T) {
 	assert.NilError(t, svc.removeServiceRelay(t.Context(), "p", "db"))
 }
 
+// The relay only dials out and forwards bytes: it gets none of Docker's
+// default capabilities except NET_BIND_SERVICE, kept because a route
+// commonly targets a privileged port the relay must still listen on.
+func TestCreateRelayContainerDropsCapabilities(t *testing.T) {
+	mockCtrl := gomock.NewController(t)
+	defer mockCtrl.Finish()
+	apiMock, cli := prepareMocks(mockCtrl)
+	tested, err := NewComposeService(cli)
+	assert.NilError(t, err)
+	svc := tested.(*composeService)
+
+	project := &types.Project{
+		Name:     "p",
+		Networks: types.Networks{"default": {Name: "p_default"}},
+	}
+	db := types.ServiceConfig{Name: "db", Provider: &types.ServiceProviderConfig{Type: "test"}}
+
+	apiMock.EXPECT().ContainerList(gomock.Any(), gomock.Any()).
+		Return(client.ContainerListResult{}, nil)
+
+	var got client.ContainerCreateOptions
+	apiMock.EXPECT().ContainerCreate(gomock.Any(), gomock.Any()).
+		DoAndReturn(func(_ context.Context, opts client.ContainerCreateOptions) (client.ContainerCreateResult, error) {
+			got = opts
+			return client.ContainerCreateResult{ID: "relay-1"}, nil
+		})
+	apiMock.EXPECT().ContainerStart(gomock.Any(), "relay-1", gomock.Any()).
+		Return(client.ContainerStartResult{}, nil)
+
+	endpoints := map[int]string{80: "host.docker.internal:49152"}
+	err = svc.ensureServiceRelay(t.Context(), project, db, endpoints, []string{"default"})
+	assert.NilError(t, err)
+
+	assert.DeepEqual(t, got.HostConfig.CapDrop, []string{"ALL"})
+	assert.DeepEqual(t, got.HostConfig.CapAdd, []string{"NET_BIND_SERVICE"})
+}
+
 // No relay ever existed for the service: nothing to do, and nothing calls
 // ContainerRemove (mockCtrl.Finish would fail an unexpected call anyway).
 func TestRemoveServiceRelayNoopWhenNoneExists(t *testing.T) {