00
Route between two real /26 networks
Build two bounded subnets and an isolated router, then prove exactly what breaks when one route disappears.
Tool
Predict 192.0.2.0/26 and 192.0.2.64/26.
Kernel
Read three routing tables and forwarding state.
PCAP
See one ICMP exchange enter and leave the router.
01
Draw the boundary
Left host: 192.0.2.10/26. Router: .1 and .65. Right host: 192.0.2.70/26.
PredictionThe hosts cannot exchange traffic through connected routes alone. Each needs a route through the router address in its own subnet.
02
Build two links and one router
Ubuntu
chmod +x mission-act1-labs.sh
sudo ./mission-act1-labs.sh lab03 doctor
sudo ./mission-act1-labs.sh lab03 build
sudo ./mission-act1-labs.sh lab03 verifySafety boundaryForwarding is enabled only inside
mls1-l03-router. The host default route is never changed.03
Watch the router forward
Capture on both router interfaces and compare Ethernet changes with stable IP endpoints.
Capture
sudo ./mission-act1-labs.sh lab03 capture
sudo tcpdump -nn -e -r results/*-lab03/left-link.pcap
sudo tcpdump -nn -e -r results/*-lab03/right-link.pcapPacket proofThe IP source and destination stay the same across the router. The local Ethernet addresses change at the routing boundary.
04
Remove one route
Create a controlled failure without touching any host route outside the namespace.
Break and inspect
sudo ./mission-act1-labs.sh lab03 break
sudo ip -n mls1-l03-left route
sudo ip netns exec mls1-l03-left ping -c 1 -W 1 192.0.2.70Expected failureLinux should report that no route reaches the remote /26. This is evidence, not a broken lab.
05
Restore and prove
Restore
sudo ./mission-act1-labs.sh lab03 fix
sudo ./mission-act1-labs.sh lab03 evidence06
Close cleanly
Cleanup
sudo ./mission-act1-labs.sh lab03 destroy