Added sshd example, which starts up sshd on each host, allowing host access via ssh.

(It also adds a route from the root namespace to the data network via
the first OpenFlow switch, so when you ssh in you go through OpenFlow!)

Modified Network() to optionally decouple starting the network, running
tests, and stopping the network. This allows multiple tests to be run
(or additional modifications to be made to the network before actually
starting the switches and controller.)

Still tweaking xterms - it sort of works but the screen sessions are not
cleaned up, which leads to nasty garbage.

examples/sshd.py

@@ -0,0 +1,55 @@
+#!/usr/bin/python
+
+"Create a network and start sshd on the hosts."
+
+from mininet import init, Node, createLink, TreeNet, Cli
+
+def nets( hosts ):
+   "Return list of networks (/24) for hosts."
+   nets = {}
+   for host in hosts:
+      net = host.IP().split( '.' )[ : -1 ]
+      net = '.'.join ( net ) + '.0/24'
+      nets[ net ] = True
+   return nets.keys()
+   
+def addRoutes( node, nets, intf ):
+   """Add routes from node to nets through intf, assuming
+      a 24-bit netmask."""
+   for net in nets:
+      node.cmdPrint( 'route add -net ' + net + ' dev ' + intf )
+
+def removeRoutes( node, nets ):
+   "Remove routes to nets from node."
+   for net in nets:
+      node.cmdPrint( 'route del -net ' + net )
+   
+def sshd( network ):
+   "Start sshd up on each host, routing appropriately."
+   controllers, switches, hosts = (
+      network.controllers, network.switches, network.hosts )
+   # Create a node in root ns and link to switch 0
+   root = Node( 'root', inNamespace=False )
+   createLink( root, switches[ 0 ] )
+   ip = '10.0.123.1'
+   root.setIP( root.intfs[ 0 ], ip, '/24' )
+   network.start()
+   # Add routes
+   routes = nets( hosts )
+   addRoutes( root, routes, root.intfs[ 0 ] )
+   # Start up sshd on each host
+   for host in hosts: host.cmdPrint( '/usr/sbin/sshd' )
+   # Dump out IP addresses and run CLI
+   print
+   print "*** Hosts are running sshd at the following addresses:"
+   for host in hosts: print host.name, host.IP()
+   print
+   print "*** Starting Mininet CLI - type 'exit' or ^D to exit"
+   network.runTest( Cli )
+   network.stop()
+   removeRoutes( root, routes )
+   
+if __name__ == '__main__':
+   init()
+   network = TreeNet( depth=1, fanout=2, kernel=True )
+   sshd( network )