IP Routers in a Line|Mininet

IP Routers in a Line

In the next example we build a Mininet example involving a router rather than a switch. A router here is simply a multi-interface Mininet host that has IP forwarding enabled in its Linux kernel. Mininet support for multi-interface hosts is somewhat fragile; interfaces may need to be initialized in a specific order, and IP addresses often cannot be assigned at the point when the link is created. In the code presented below we

An Introduction to Computer Networks, Release 2.0.4

assign IP addresses using calls to Node.cmd() used to invoke the Linux command ifconfig (Mininet containers do not fully support the use of the alternative ip addr command).

Our first router topology has only two hosts, one at each end, and N routers in between; below is the diagram with N=3. All subnets are /24. The program to set this up is routerline.py, here invoked as python routerline.py -N 3. We will use N=3 in most of the examples below. A somewhat simpler version of the program, which sets up the topology specifically for N=3, is routerline3.py.



In both versions of the program, routing entries are created to route traffic from h1 to h2, but not back again. That is, every router has a route to, but only r1 knows how to reach (to which r1 is directly connected). We can verify the “one-way” connectedness by running WireShark or tcpdump on h2 (perhaps first starting an xterm on h2), and then running ping on h1 (perhaps using the Mininet command h1 ping h2). WireShark or tcpdump should show the arriving ICMP ping packets from h1, and also the arriving ICMP Destination Network Unreachable packets from r3 as h2 tries to reply (see 10.4 Internet Control Message Protocol).

It turns out that one-way routing is considered to be suspicious; one interpretation is that the packets involved have a source address that shouldn’t be possible, perhaps spoofed. Linux provides the interface configuration option rp_filter – reverse-path filter – to block the forwarding of packets for which the router does not have a route back to the packet’s source. This must be disabled for the one-way example to work; see the notes on the code below.

Despite the lack of connectivity, we can reach h2 from h1 via a hop-by-hop sequence of ssh connections (the program enables sshd on each host and router):

h1: slogin r1:
slogin r2:
slogin r3:
slogin (that is, h3)

To get the one-way routing to work from h1 to h2, we needed to tell r1 and r2 how to reach destination This can be done with the following commands (which are executed automatically if we set ENABLE_LEFT_TO_RIGHT_ROUTING = True in the program):

r1: ip route add to via r2:
ip route add to via

To get full, bidirectional connectivity, we can create the following routes to

r2: ip route add to via
r3: ip route add to via


When building the network topology, the single-interface hosts can have all their attributes set at once (the code below is from routerline3.py:

h1 = self.addHost( 'h1', ip='', defaultRoute='via' )
h2 = self.addHost( 'h2', ip='', defaultRoute='via' )

An Introduction to Computer Networks, Release 2.0.4

The routers are also created with addHost(), but with separate steps:

r1 = self.addHost( 'r1' )
r2 = self.addHost( 'r2' )


self.addLink( h1, r1, intfName1 = 'h1-eth0', intfName2 = 'r1-eth0')
self.addLink( r1, r2, inftname1 = 'r1-eth1', inftname2 = 'r2-eth0')

Later on the routers get their IPv4 addresses:

r1 = net['r1']
r1.cmd('ifconfig r1-eth0')
r1.cmd('ifconfig r1-eth1')
r1.cmd('sysctl net.ipv4.ip_forward=1')


The sysctl command here enables forwarding in r1. The rp_disable(r1) call disables Linux’s default refusal to forward packets if the router does not have a route back to the packet’s source; this is often what is wanted in the real world but not necessarily in routing demonstrations. It too is ultimately implemented via sysctl commands.



























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