nat.py

#!/usr/bin/python2
"""
Connect Mininet to the Internet, based on
https://github.com/mininet/mininet/blob/7a3159c9afec461b6133c0fd3f415439e07340a9/examples/nat.py

Basically, just removed NetworkManager stuff and added passing in the interface
on which to get access to the Internet since no longer is it just eth0 on most
Linux distributions.

Even when I'm using NetworkManager it apparently sets the root-eth0 ipv4.mode
to be manual by default. And, since on Arch Linux there is no
/etc/network/interfaces file, /usr/lib/python2.7/site-packages/mininet/nodelib.py
errors if you try to create a NAT with net.addNat().configDefault().
"""

from mininet.cli import CLI
from mininet.log import lg
from mininet.node import Node
from mininet.topolib import TreeNet

#################################
def startNAT( root, inetIntf='eth0', subnet='10.0.0.0/8' ):
    """Start NAT/forwarding between Mininet and external network
    root: node to access iptables from
    inetIntf: interface for internet access
    subnet: Mininet subnet (default 10.0.0.0/8)="""

    # 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 connectToInternet( network, inetIntf, switch='s1', rootip='10.0.0.254', subnet='10.0.0.0/8' ):
    """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 ]

    # Create a node in root namespace
    root = Node( 'root', inNamespace=False )

    # 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, inetIntf )

    # 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