Add NAT script to examples.

examples/README

@@ -68,6 +68,12 @@ multitest.py:
 
 This example creates a network and runs multiple tests on it.
 
+nat.py:
+
+This example shows how to connect a Mininet network to the Internet
+using NAT. It also answers the eternal question "why can't I ping
+google?"
+
 popen.py:
 
 This example monitors a number of hosts using host.popen() and

examples/nat.py

+#!/usr/bin/python
+
+"""
+Example to create a Mininet topology and connect it to the internet via NAT
+through eth0 on the host.
+
+Glen Gibb, February 2011
+
+(slight modifications by BL, 5/13)
+"""
+
+from mininet.cli import CLI
+from mininet.log import lg, info
+from mininet.node import Node
+from mininet.topolib import TreeNet
+from mininet.util import quietRun
+
+#################################
+def startNAT( root, inetIntf='eth0', subnet='10.0/8' ):
+    """Start NAT/forwarding between Mininet and external network
+    root: node to access iptables from
+    inetIntf: interface for internet access
+    subnet: Mininet subnet (default 10.0/8)="""
+
+    # Identify the interface connecting to the mininet network
+    localIntf =  root.defaultIntf()
+
+    # Flush any currently active rules
+    root.cmd( 'iptables -F' )
+    root.cmd( 'iptables -t nat -F' )
+
+    # Create default entries for unmatched traffic
+    root.cmd( 'iptables -P INPUT ACCEPT' )
+    root.cmd( 'iptables -P OUTPUT ACCEPT' )
+    root.cmd( 'iptables -P FORWARD DROP' )
+
+    # Configure NAT
+    root.cmd( 'iptables -I FORWARD -i', localIntf, '-d', subnet, '-j DROP' )
+    root.cmd( 'iptables -A FORWARD -i', localIntf, '-s', subnet, '-j ACCEPT' )
+    root.cmd( 'iptables -A FORWARD -i', inetIntf, '-d', subnet, '-j ACCEPT' )
+    root.cmd( 'iptables -t nat -A POSTROUTING -o ', inetIntf, '-j MASQUERADE' )
+
+    # Instruct the kernel to perform forwarding
+    root.cmd( 'sysctl net.ipv4.ip_forward=1' )
+
+def stopNAT( root ):
+    """Stop NAT/forwarding between Mininet and external network"""
+    # Flush any currently active rules
+    root.cmd( 'iptables -F' )
+    root.cmd( 'iptables -t nat -F' )
+
+    # Instruct the kernel to stop forwarding
+    root.cmd( 'sysctl net.ipv4.ip_forward=0' )
+
+def fixNetworkManager( root, intf ):
+    """Prevent network-manager from messing with our interface,
+       by specifying manual configuration in /etc/network/interfaces
+       root: a node in the root namespace (for running commands)
+       intf: interface name"""
+    cfile = '/etc/network/interfaces'
+    line = '\niface %s inet manual\n' % intf
+    config = open( cfile ).read()
+    if ( line ) not in config:
+        print '*** Adding', line.strip(), 'to', cfile
+        with open( cfile, 'a' ) as f:
+            f.write( line )
+    # Probably need to restart network-manager to be safe -
+    # hopefully this won't disconnect you
+    root.cmd( 'service network-manager restart' )
+
+def connectToInternet( network, switch='s1', rootip='10.254', subnet='10.0/8'):
+    """Connect the network to the internet
+       switch: switch to connect to root namespace
+       rootip: address for interface in root namespace
+       subnet: Mininet subnet"""
+    switch = network.get( switch )
+    prefixLen = subnet.split( '/' )[ 1 ]
+    routes = [ subnet ]  # host networks to route to
+
+    # Create a node in root namespace
+    root = Node( 'root', inNamespace=False )
+
+    # Prevent network-manager from interfering with our interface
+    fixNetworkManager( root, 'root-eth0' )
+
+    # Create link between root NS and switch
+    link = network.addLink( root, switch )
+    link.intf1.setIP( rootip, prefixLen )
+
+    # Start network that now includes link to root namespace
+    network.start()
+
+    # Start NAT and establish forwarding
+    startNAT( root )
+
+    # Establish routes from end hosts
+    for host in network.hosts:
+        host.cmd( 'ip route flush root 0/0' )
+        host.cmd( 'route add -net', subnet, 'dev', host.defaultIntf() )
+        host.cmd( 'route add default gw', rootip )
+
+    return root
+
+if __name__ == '__main__':
+    lg.setLogLevel( 'info')
+    net = TreeNet( depth=1, fanout=4 )
+    # Configure and start NATted connectivity
+    rootnode = connectToInternet( net )
+    print "*** Hosts are running and should have internet connectivity"
+    print "*** Type 'exit' or control-D to shut down network"
+    CLI( net )
+    # Shut down NAT
+    stopNAT( rootnode )
+    net.stop()