diff --git a/gnu/build/marionette.scm b/gnu/build/marionette.scm
index 173a67cef9..bb018fc9c1 100644
--- a/gnu/build/marionette.scm
+++ b/gnu/build/marionette.scm
@@ -26,6 +26,7 @@ (define-module (gnu build marionette)
             make-marionette
             marionette-eval
             wait-for-file
+            wait-for-tcp-port
             marionette-control
             marionette-screen-text
             wait-for-screen-text
@@ -187,6 +188,32 @@ (define* (wait-for-file file marionette
     ('failure
      (error "file didn't show up" file))))
 
+(define* (wait-for-tcp-port port marionette
+                            #:key (timeout 20))
+  "Wait for up to TIMEOUT seconds for PORT to accept connections in
+MARIONETTE.  Raise an error on failure."
+  ;; Note: The 'connect' loop has to run within the guest because, when we
+  ;; forward ports to the host, connecting to the host never raises
+  ;; ECONNREFUSED.
+  (match (marionette-eval
+          `(begin
+             (let ((sock (socket PF_INET SOCK_STREAM 0)))
+               (let loop ((i 0))
+                 (catch 'system-error
+                   (lambda ()
+                     (connect sock AF_INET INADDR_LOOPBACK ,port)
+                     'success)
+                   (lambda args
+                     (if (< i ,timeout)
+                         (begin
+                           (sleep 1)
+                           (loop (+ 1 i)))
+                         'failure))))))
+          marionette)
+    ('success #t)
+    ('failure
+     (error "nobody's listening on port" port))))
+
 (define (marionette-control command marionette)
   "Run COMMAND in the QEMU monitor of MARIONETTE.  COMMAND is a string such as
 \"sendkey ctrl-alt-f1\" or \"screendump foo.ppm\" (info \"(qemu-doc)
diff --git a/gnu/tests/dict.scm b/gnu/tests/dict.scm
index b9c741e3e0..4431e37dc1 100644
--- a/gnu/tests/dict.scm
+++ b/gnu/tests/dict.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -96,22 +96,7 @@ (define %dico-socket
           ;; Wait until dicod is actually listening.
           ;; TODO: Use a PID file instead.
           (test-assert "connect inside"
-            (marionette-eval
-             '(begin
-                (use-modules (ice-9 rdelim))
-                (let ((sock (socket PF_INET SOCK_STREAM 0)))
-                  (let loop ((i 0))
-                    (pk 'try i)
-                    (catch 'system-error
-                      (lambda ()
-                        (connect sock AF_INET INADDR_LOOPBACK 2628))
-                      (lambda args
-                        (pk 'connection-error args)
-                        (when (< i 20)
-                          (sleep 1)
-                          (loop (+ 1 i))))))
-                  (read-line sock 'concat)))
-             marionette))
+            (wait-for-tcp-port 2628 marionette))
 
           (test-assert "connect"
             (let ((addr (make-socket-address AF_INET INADDR_LOOPBACK 8000)))