linuxexpert-practice-labs/labs/ubuntu-desktop/1-2-layer-2

Ubuntu Desktop 1.2: Layer 2: MAC addresses, ARP and broadcast

Finding MAC addresses and the maker's prefix, watching an ARP exchange, and checking that an address is free before you use it.

Before you start

The lab sets up a second machine, the peer, at 10.99.0.2 (name peer). It's on the same network as this VM's interface le-host (10.99.0.1), so the two can talk directly over Ethernet. Two of the addresses 10.99.0.21, 10.99.0.22 and 10.99.0.23 are also in use on that network; one is free. iputils-arping isn't installed. Write answers to the files in /root/lab-answers/.

Tasks

1. Find your machine's MAC address and the maker's prefix in it, and your default gateway's MAC address. Use the interface your default route goes out of. Write your MAC on line 1 of /root/lab-answers/1, the prefix (its first three bytes) on line 2, and the gateway's MAC on line 3.
2. Empty the neighbor table, then watch the ARP exchange as you ping the peer. Write the peer's MAC address, from its reply, to /root/lab-answers/2.
3. Check which of 10.99.0.21, 10.99.0.22 and 10.99.0.23 is free before giving it to a new machine (install iputils-arping first). Write the free address to /root/lab-answers/3.

Hints

• ip -br link; ip route | grep default gives the gateway's IP; after ping -c1 to it, ip neigh show lists it with lladdr and its MAC.
• sudo ip neigh flush dev le-host, then sudo tcpdump -i le-host -e -n arp in one terminal and ping -c1 10.99.0.2 in another: look for Request who-has to ff:ff:ff:ff:ff:ff and Reply ... is-at.
• sudo arping -D -c 2 -I le-host ADDRESS; echo $? prints Received 0 response(s) and 0 for a free address, and a reply and 1 for one in use.