diff --git a/lib/build-vms.nix b/lib/build-vms.nix
index a84f55435c8..a32e1448939 100644
--- a/lib/build-vms.nix
+++ b/lib/build-vms.nix
@@ -45,6 +45,7 @@ rec {
           EOF
           chmod +x $out/bin/run-vms
         ''; # */
+      passthru = { nodes = nodes_; };
     };
 
 
diff --git a/lib/testing.nix b/lib/testing.nix
index 3a33ca69f08..f2cc29a33eb 100644
--- a/lib/testing.nix
+++ b/lib/testing.nix
@@ -95,17 +95,19 @@ rec {
       if t ? machine then { machine = t.machine; }
       else { };
     vms = buildVirtualNetwork { inherit nodes; };
-    test = runTests vms t.testScript;
+    test = runTests vms
+      # Call the test script with the computed nodes.
+      (if builtins.isFunction t.testScript then t.testScript { inherit (vms) nodes; } else t.testScript);
     report = makeReport test;
   };
 
-  runInMachine = {
-    drv
-  , machine
-  , preBuild ? ""
-  , postBuild ? ""
-  , ...
-  }:
+  runInMachine =
+    { drv
+    , machine
+    , preBuild ? ""
+    , postBuild ? ""
+    , ...
+    }:
     let
       vms =
         buildVirtualNetwork { nodes = { client = machine; } ; };
diff --git a/tests/bittorrent.nix b/tests/bittorrent.nix
index ac42fb33a85..051454da5cc 100644
--- a/tests/bittorrent.nix
+++ b/tests/bittorrent.nix
@@ -13,10 +13,10 @@ let
   # Some random file to serve.
   file = pkgs.nixUnstable.src;
 
-  miniupnpdConf = pkgs.writeText "miniupnpd.conf"
+  miniupnpdConf = nodes: pkgs.writeText "miniupnpd.conf"
     ''
       ext_ifname=eth1
-      listening_ip=192.168.2.3/24
+      listening_ip=${nodes.router.config.networking.ifaces.eth2.ipAddress}/24
       allow 1024-65535 192.168.2.0/24 1024-65535
     '';
   
@@ -56,6 +56,7 @@ in
     };
 
   testScript =
+    { nodes, ... }:
     ''
       startAll;
 
@@ -63,11 +64,12 @@ in
       $router->mustSucceed(
           "iptables -t nat -F",
           "iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -d 192.168.2.0/24 -j ACCEPT",
-          "iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -j SNAT --to-source 192.168.1.3", # !!! ugly
+          "iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -j SNAT "
+          . "--to-source ${nodes.router.config.networking.ifaces.eth1.ipAddress}",
           "iptables -t nat -N MINIUPNPD",
           "iptables -t nat -A PREROUTING -i eth1 -j MINIUPNPD",
           "echo 1 > /proc/sys/net/ipv4/ip_forward",
-          "miniupnpd -f ${miniupnpdConf}"
+          "miniupnpd -f ${miniupnpdConf nodes}"
       );
 
       # Create the torrent.
diff --git a/tests/nat.nix b/tests/nat.nix
index 7f2d8c6432f..88aa609774f 100644
--- a/tests/nat.nix
+++ b/tests/nat.nix
@@ -31,6 +31,7 @@
     };
 
   testScript =
+    { nodes, ... }:
     ''
       startAll;
 
@@ -45,7 +46,8 @@
       $router->mustSucceed(
           "iptables -t nat -F",
           "iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 192.168.1.0/24 -j ACCEPT",
-          "iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j SNAT --to-source 192.168.2.2", # !!! ugly
+          "iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j SNAT "
+          . "--to-source ${nodes.router.config.networking.ifaces.eth1.ipAddress}",
           "echo 1 > /proc/sys/net/ipv4/ip_forward"
       );