Better IP address selection for controllers

Previously we looked for 'eth0', but linux has renamed interfaces.

Instead of looking by name, we now look for an interface which
is the appropriate gateway to a remote server address.

fixes #831

examples/cluster.py

@@ -843,11 +843,26 @@ class MininetCluster( Mininet ):
     def addController( self, *args, **kwargs ):
         "Patch to update IP address to global IP address"
         controller = Mininet.addController( self, *args, **kwargs )
-        # Update IP address for controller that may not be local
-        if ( isinstance( controller, Controller)
-             and controller.IP() == '127.0.0.1'
-             and ' eth0:' in controller.cmd( 'ip link show' ) ):
-            Intf( 'eth0', node=controller ).updateIP()
+        loopback = '127.0.0.1'
+        if ( not isinstance( controller, Controller ) or
+             controller.IP() != loopback ):
+            return
+        # Find route to a different server IP address
+        serverIPs = [ ip for ip in self.serverIP.values()
+                      if ip is not controller.IP() ]
+        if not serverIPs:
+            return  # no remote servers - loopback is fine
+        remoteIP = serverIPs[ 0 ]
+        # Route should contain 'dev <intfname>'
+        route = controller.cmd( 'ip route get', remoteIP,
+                        '| egrep -o "dev\s[^[:space:]]+"' )
+        if not route:
+            raise Exception('addController: no route from', c0, 'to',
+                             remoteIP )
+        intf = route.split()[ 1 ].strip()
+        debug( 'adding', intf, 'to', controller )
+        Intf( intf, node=controller ).updateIP()
+        debug( controller, 'IP address updated to', controller.IP() )
         return controller
 
     def buildFromTopo( self, *args, **kwargs ):