diff --git a/internal/pkg/exec_test.go b/internal/pkg/exec_test.go
index e692858..ef293b6 100644
--- a/internal/pkg/exec_test.go
+++ b/internal/pkg/exec_test.go
@@ -4,7 +4,9 @@ package pkg_test
 
 import (
 	_ "embed"
+	"encoding/gob"
 	"errors"
+	"net"
 	"os"
 	"os/exec"
 	"slices"
@@ -263,6 +265,27 @@ func newTesttool() (
 			); err != nil {
 				return err
 			}
+
+			if ift, err := net.Interfaces(); err != nil {
+				return err
+			} else {
+				var f *os.File
+				if f, err = os.Create(t.GetWorkDir().Append(
+					"ift",
+				).String()); err != nil {
+					return err
+				} else {
+					err = gob.NewEncoder(f).Encode(ift)
+					closeErr := f.Close()
+					if err != nil {
+						return err
+					}
+					if closeErr != nil {
+						return closeErr
+					}
+				}
+			}
+
 			return os.WriteFile(t.GetWorkDir().Append(
 				"bin",
 				"testtool",
diff --git a/internal/pkg/testdata/main.go b/internal/pkg/testdata/main.go
index f883aec..a49fb37 100644
--- a/internal/pkg/testdata/main.go
+++ b/internal/pkg/testdata/main.go
@@ -3,14 +3,14 @@
 package main
 
 import (
-	"errors"
+	"encoding/gob"
 	"log"
 	"net"
 	"os"
 	"path"
+	"reflect"
 	"slices"
 	"strings"
-	"syscall"
 
 	"hakurei.app/container/check"
 	"hakurei.app/container/fhs"
@@ -60,16 +60,21 @@ func main() {
 		wantExec     = "/opt/bin/testtool"
 		wantExecWork = "/work/bin/testtool"
 	)
+	var iftPath string
 	if got, err := os.Executable(); err != nil {
 		log.Fatalf("Executable: error = %v", err)
-	} else if got != wantExec {
-		switch got {
-		case wantExecWork:
-			overlayWork = true
-			log.SetPrefix("testtool(overlay work): ")
+	} else {
+		iftPath = path.Join(path.Dir(path.Dir(got)), "ift")
 
-		default:
-			log.Fatalf("Executable: %q, want %q", got, wantExec)
+		if got != wantExec {
+			switch got {
+			case wantExecWork:
+				overlayWork = true
+				log.SetPrefix("testtool(overlay work): ")
+
+			default:
+				log.Fatalf("Executable: %q, want %q", got, wantExec)
+			}
 		}
 	}
 
@@ -98,15 +103,37 @@ func main() {
 		}
 	}
 
-	if _, err := net.Dial("tcp", "127.0.0.1:0"); err == nil {
-		log.Fatal("Dial unexpectedly succeeded")
-	} else if hostNet {
-		if !errors.Is(err, syscall.ECONNREFUSED) {
-			log.Fatalf("Dial: error = %v", err)
+	if ift, err := net.Interfaces(); err != nil {
+		log.Fatal(err)
+	} else if !hostNet {
+		if len(ift) != 1 || ift[0].Name != "lo" {
+			log.Fatalln("got interfaces", strings.Join(slices.Collect(func(yield func(ifn string) bool) {
+				for _, ifi := range ift {
+					if !yield(ifi.Name) {
+						break
+					}
+				}
+			}), ", "))
 		}
 	} else {
-		if !errors.Is(err, syscall.ENETUNREACH) {
-			log.Fatalf("Dial: error = %v", err)
+		var iftParent []net.Interface
+
+		var r *os.File
+		if r, err = os.Open(iftPath); err != nil {
+			log.Fatal(err)
+		} else {
+			err = gob.NewDecoder(r).Decode(&iftParent)
+			closeErr := r.Close()
+			if err != nil {
+				log.Fatal(err)
+			}
+			if closeErr != nil {
+				log.Fatal(closeErr)
+			}
+		}
+
+		if !reflect.DeepEqual(ift, iftParent) {
+			log.Fatalf("Interfaces: %#v, want %#v", ift, iftParent)
 		}
 	}