ICMP: network unreachable

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Capture the ICMP error for a missing route.

Sign in and launch

First: Routing between two subnets

Network

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PC1 at 192.168.1.1/24 connects directly to R1 at 192.168.1.100/24.
R1 has no route to 8.8.4.4.
Open a console

1. Test the gateway

PC1

ping -n -c 3 -W 1 192.168.1.100

R1

ip -j route show

The gateway replies, but no route to 8.8.4.4 exists.

2. Capture the rejection

PC1

timeout 120 tcpdump -U --immediate-mode -i eth0 -w /tmp/lab-icmp-network.pcap "icmp or tcp port 5555" &
capture_pid=$!
sleep 1

PC1

printf 'network-unreachable\n' | nc -N -w 5 8.8.4.4 5555

PC1

sleep 2
kill -INT "$capture_pid"
wait "$capture_pid" || true
tcpdump -nn -e -r /tmp/lab-icmp-network.pcap "icmp[0] == 3 and icmp[1] == 0"

Find ICMP 3/0 from R1 — network unreachable. Do not add a default route.

Validation

Click "Check". Once all steps pass, click "End".