diff --git a/test/backchannel b/test/backchannel
index 7187880..9e6d34e 100644
--- a/test/backchannel
+++ b/test/backchannel
@@ -1,4 +1,9 @@
 #!/bin/sh
+
+# Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
 # Backchannel control script - sets up and tears down backchannel network
 # interfaces. Backchannel interfaces are hidden from flimflam and will never be
 # the default route.
@@ -8,10 +13,6 @@
 # pseudo-modem0
 #
 
-# Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
 test_if=eth_test
 
 usage () {
@@ -30,6 +31,11 @@
                       | awk -F ':' '{ print $2 }'
 }
 
+netmask () {
+	ifconfig "$1" | awk '/Mask:/ { print $4 }' \
+                      | awk -F ':' '{ print $2 }'
+}
+
 # We need to down the interface (and therefore stop flimflam) across the
 # invocation of nameif, according to nameif(1).
 renameif () {
@@ -44,11 +50,12 @@
 
 setup () {
 	oldip=$(ipaddr "$1")
+	oldnetmask=$(netmask "$1")
         if [ ! -z $2 ] ; then
 		test_if="$2"
 	fi
 	renameif "$1" "$test_if"
-	ifconfig "$test_if" "$oldip"
+	ifconfig "$test_if" "$oldip" netmask "$oldnetmask"
 }
 
 teardown () {