diff --git a/metropolis/test/e2e/preseedtest/main.go b/metropolis/test/e2e/preseedtest/main.go
index a44377f..77fcba6 100644
--- a/metropolis/test/e2e/preseedtest/main.go
+++ b/metropolis/test/e2e/preseedtest/main.go
@@ -19,6 +19,9 @@
 import (
 	"fmt"
 	"net/http"
+	"os"
+	"strconv"
+	"strings"
 )
 
 func main() {
@@ -26,6 +29,28 @@
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintf(w, "Hello world from preseeded image\n")
 	})
+	http.HandleFunc("/ready_userns", func(w http.ResponseWriter, r *http.Request) {
+		uidMapRaw, err := os.ReadFile("/proc/self/uid_map")
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		uidMapFields := strings.Fields(string(uidMapRaw))
+		if len(uidMapFields) != 3 {
+			http.Error(w, fmt.Sprintf("Bad uid_map contents, not 3 fields: %q", string(uidMapRaw)), http.StatusInternalServerError)
+			return
+		}
+		startId, err := strconv.ParseUint(uidMapFields[1], 10, 64)
+		if err != nil {
+			http.Error(w, fmt.Sprintf("while parsing start ID: %v", err), http.StatusInternalServerError)
+			return
+		}
+		if startId == 0 {
+			http.Error(w, "Not in a non-initial user namespace, UID space starts at 0", http.StatusInternalServerError)
+			return
+		}
+		fmt.Fprintf(w, "Hello world from a user namespace\n")
+	})
 	err := http.ListenAndServe(":80", nil)
 	if err != nil {
 		fmt.Printf("Serve failed: %v\n", err)