Exercises with static routing between networks

In these exercises, we will observe how routing principles apply when packets are forwarded by a router from one network segment to the next.

Exercise - static routes for forwarding between networks

In this exercise, we will focus on routing traffic between two hosts in different network segments: romeo and othello. You will need to open:

Reserve four terminal windows - one on each host - for running interactive commands. Then, the remaining terminal windows will be used to observe network traffic with tcpdump. Arrange the six terminal windows in order and run a tcpdump on

  1. the experiment interface of romeo (on the red network)
  2. the interface of router-1 that is on the red network
  3. the interface of router-1 that is on the blue network
  4. the interface of router-2 that is on the blue network
  5. the interface of router-2 that is on the green network
  6. the interface of othello that is on the green network

(Use the ifconfig output to identify which interface is on which network, for hosts and routers that have more than one interface.)

Use the -n argument to tcpdump so that addresses will be shown in dotted decimal notation and not resolved to hostnames, and use icmp to filter the traffic so you only capture ICMP traffic. For example:

sudo tcpdump -n -i eth1 icmp

(if you are capturing on eth1).

Part 1: No added rules

First, get all the routing table rules. On romeo, othello, router-1 and router-2, run

route -n

Save these outputs for your lab report.

On romeo, run

ping -c 5 10.10.1.104

to send an ICMP echo request to othello. Save the output for your lab report.

Stop the tcpdump processes and save the output for your lab report.

Part 2: Add a route on romeo

Because the routing table on romeo has no entry for othello’s subnet, you will get a “Network is unreachable” message in the previous step. Let’s add a rule for othello’s subnet.

There are two Linux utilities that are widely used to add static routes: route and ip. The route utility is a bit simpler to use, and is older. The ip utility is newer and more powerful. I will show you how to use both of these to add a static route.

Method A: For the route command, the syntax is as follows, but you will have to substitute the correct values for all of the words in capital letters:

sudo route add -net NETADDR netmask NETMASK gw GW dev IFACE

where,

This rule says that: “All traffic for destinations in the network defined by NETADDR and NETMASK should be forwarded to GW (a “local” address) through IFACE.”

Method B: Note that instead of the netmask notation, you can alternatively use prefix length notation:

sudo route add -net NETADDR/PREFIX gw GW dev IFACE

where,

This rule says that: “All traffic for destination addresses where the first PREFIX bits match NETADDR should be forwarded to GW (a “local” address) through IFACE.”

Method C: Alternatively, you can use the ip command as follows:

sudo ip route add NETADDR/PREFIX via GW dev IFACE

Use method A, B, or C to add a route on romeo that will forward traffic to the green network through router-1. Save the command and output for your lab report.

Then, get all the routing table rules. On romeo, othello, router-1 and router-2, run

route -n

(you should see the new rule you added on romeo!) Save these outputs for your lab report.

Restart the six tcpdump processes.

On romeo, run

ping -c 5 10.10.1.104

to send an ICMP echo request to othello. Save the output for your lab report.

Stop the tcpdump processes and save the output for your lab report.

Part 3: Add a route on router-1

After adding a rule on romeo, you should see that the ICMP echo request is forwarded to router-1. However, router-1 has no rule in its routing table that applies to the destination address, so it sends an ICMP Destination Net Unreachable message back to romeo.

Use method A, B, or C to add a route on router-1 that will forward traffic to the green network through router-2. Save the command and output for your lab report.

Then, get all the routing table rules. On romeo, othello, router-1 and router-2, run

route -n

(you should see the new rule you added on router-1!) Save these outputs for your lab report.

Restart the six tcpdump processes.

On romeo, run

ping -c 5 10.10.1.104

to send an ICMP echo request to othello. Save the command d output for your lab report.

Stop the tcpdump processes and save the output for your lab report.

Part 4: set up the reverse path

After following the previous steps, you should see that the ICMP echo request from romeo arrives at othello via router-1 and router-2. However, othello does not respond because it has no route to romeo! You will need to set up the reverse path from othello to romeo in a similar way.

Use method A, B, or C to add a route on othello that will forward traffic to the red network through router-2. Save the command and output for your lab report.

Use method A, B, or C to add a route on router-2 that will forward traffic to the red network through router-1. Save the command and output for your lab report.

Then, get all the routing table rules. On romeo, othello, router-1 and router-2, run

route -n

and note the two new rules you added. Save these outputs for your lab report.

Restart the six tcpdump processes.

On romeo, run

ping -c 5 10.10.1.104

to send an ICMP echo request to othello. Save the command and output for your lab report.

Stop the tcpdump processes and save the output for your lab report.

Lab report: For each of the four parts of this experiment,

Exercise - packet headers

On romeo, othello, router-1, and router-2, run

ifconfig

and save the output for your lab report.

Start a tcpdump on romeo with

sudo tcpdump -i eth1 -w $(hostname -s)-static-headers.pcap icmp

and on othello, router-1, and router-2, with

sudo tcpdump -i eth1 -w $(hostname -s)-1-static-headers.pcap icmp

in one terminal window and

sudo tcpdump -i eth2 -w $(hostname -s)-2-static-headers.pcap icmp

in a second terminal window.

Then, on romeo, send an ICMP echo request to othello with

ping -c 1 10.10.1.104

Observe the response. Stop the tcpdump processes. You can transfer the packet captures to your laptop with scp, or you can play them back on the remote host with

tcpdump -r $(hostname -s)-static-headers.pcap -nev

on romeo, or on othello and the routers with

tcpdump -r $(hostname -s)-1-static-headers.pcap -nev

and

tcpdump -r $(hostname -s)-2-static-headers.pcap -nev

Lab report: When a packet is forwarded by a router, explain how the source and destination Ethernet addresses were changed. Use evidence from your packet captures and from the ifconfig output on each host to support your answers.

Answer the same questions, but now for the echo reply that was returned from the remote host.

In a previous lesson, you answered a similar question for traffic traversing a bridge. Compare the behavior of the bridge in last week’s experiment (with respect to the packet headers) to the behavior of the router in this week’s experiment.

Lab report: For the ICMP echo request, find the TTL value in the IP header on each network segment: red, blue, and green. What happens to the value in this field when a packet is forwarded by a router? Why?

Exercise - longest prefix matching

In the previous exercise, you added a rule to the routing table on router-1 that matches all destination addresses in the green network (10.10.1.0/24), and that uses router-2 as the next hop. (This rule should still be in router-1’s routing table!)

Now, we will add more routes so that multiple rules in router-1’s routing table match the destination address 10.10.1.104, and we’ll see which rule is actually applied.

First, let’s make sure that router-3 can also forward packets to the destination address 10.10.1.104. Add a rule on router-3 that uses othello’s interface on the purple network as the next hop toward the green network:

sudo route add -net 10.10.1.0/24 gw 10.10.2.104

Part 1: 10.10.0.0/16

Add the following rule on router-1:

sudo route add -net 10.10.0.0/16 gw 10.10.100.3

This rule says that for all destination addresses matching 10.10.0.0/16, use router-3 as the next hop. Note that this rule matches othello’s address, 10.10.1.104!

On router-1, run

route -n

and save the output. Note that there are now two rules that match othello’s address, 10.10.1.104. One rule says to use router-2 as the next hop and one rule says to use router-3 as the next hop.

Start a tcpdump on both interfaces of router-2 and on both interfaces of router-3:

sudo tcpdump -env -i eth1 icmp

and

sudo tcpdump -env -i eth2 icmp

Make sure you know which tcpdump shows the interface on the blue network and which shows the interface on the green network or the purple network.

Now, on romeo, ping othello:

ping -c 1 10.10.1.104

Stop the tcpdump processes and save the output for your lab report.

Part 2: 10.10.1.96/28

Add the following rule on router-1:

sudo route add -net 10.10.1.96/28 gw 10.10.100.3

This rule says that for all destination addresses matching 10.10.1.96/28, use router-3 as the next hop. Note that this rule also matches othello’s address, 10.10.1.104!

On router-1, run

route -n

and save the output. Note that there are now three rules that match othello’s address, 10.10.1.104.

Start a tcpdump on both interfaces of router-2 and on both interfaces of router-3:

sudo tcpdump -env -i eth1 icmp

and

sudo tcpdump -env -i eth2 icmp

Make sure you know which tcpdump shows the interface on the blue network and which shows the interface on the green network or the purple network.

Now, on romeo, ping othello:

ping -c 1 10.10.1.104

Stop the tcpdump processes and save the output for your lab report.

Lab report: Use your tcpdump output to answer the following questions about Part 1 and Part 2 of this exercise: