linuxexpert-practice-labs/labs/linux-plus/5-3-1-interface-and-link-issues/README.md

Linux+ 5.3.1: Interface and link problems

A link that's down, an MTU mismatch, a dummy interface, and an address conflict.

Before you start

The peer, at 10.99.0.2 (name peer), is another machine on this VM's second network, 10.99.0.0/24; this VM is 10.99.0.1 there, on interface le-host. Get a shell there with sudo ip netns exec lepeer bash. The lab has broken a few things on that network. None of it lasts past a reboot: do the tasks and check without rebooting. Leave your main interface alone. Write answers to the files in /root/lab-answers/.

Tasks

1. Read your links' details and counters. This VM can't reach the peer (ping 10.99.0.2): find out why at the link level and fix it.
2. Small pings to the peer work, but big packets never arrive. Find the largest packet (the MTU, IP header included) that gets from this VM to the peer with "don't fragment" set, and write it to /root/lab-answers/2.
3. Create a dummy interface lab0 with address 192.0.2.10/24 and MTU 1400, bring it up, then take its link down again. Leave it there.
4. You want to add 10.99.0.5 to le-host. First check whether another host on that network already uses it, and write the MAC address of the host that answers to /root/lab-answers/4.

Hints

• ip -br link, ip -br addr, ip -s link show, ethtool; ip link set DEV up.
• ping -c1 -M do -s SIZE (SIZE + 28 is the packet size), tracepath -n.
• ip link add NAME type dummy, ip addr add, ip link set NAME up mtu 1400, ip link set NAME down.
• arping -D -c 2 -I le-host ADDRESS (on Ubuntu, install iputils-arping); exit 0 means nobody answered.