Week 3, Day 2 — DHCP Troubleshooting & Reservations

Lab 3B — Error diagnosis, DORA tcpdump capture, MAC-based reservation

Part 1 — Deliberate configuration errors (35 min)

• Introduce each error below one at a time. After each error: restart the daemon, check the journal for the error message, record what the message says, then fix it before introducing the next. The pattern to learn: the error message always includes the file and line number.
• Error 1 — Missing semicolon: Remove the semicolon after option routers 192.168.50.1. Restart. Record the exact journal error message. Fix and restart. Expected: "parse error at /etc/dhcp/dhcpd.conf line X near 'option'"
• Error 2 — Invalid range: Change the range to range 10.14.6.45 192.168.50.125; (a range spanning two different networks). Restart. Record the error. Expected: the daemon will refuse to start because the first address is not in the declared subnet.
• Error 3 — Missing subnet declaration: Remove the entire subnet block (keep the global options). Restart. Record the error. Expected: the server starts but complains about not being configured to serve any subnets on eth1.
• Restore the working dhcpd.conf after each error is documented. Final state must be a fully functional configuration.

Part 2 — DORA capture with tcpdump (25 min)

• On S1, start a capture on the LAN1 interface filtering DHCP traffic:
  sudo tcpdump -i eth1 -n -v port 67 or port 68
• Switch S2's Netplan back to DHCP (dhcp4: true, remove static config). Apply: sudo netplan apply. Watch the four DORA packets appear in the capture on S1.
• In the capture output, identify and label each packet:
  · DHCPDISCOVER — source 0.0.0.0, destination 255.255.255.255
  · DHCPOFFER — source S1, destination 255.255.255.255 (or unicast to S2's MAC)
  · DHCPREQUEST — source 0.0.0.0 (or newly offered IP), destination 255.255.255.255
  · DHCPACK — source S1, destination assigned IP
• Record the tcpdump command and a sample of the output in the lab sheet. Answer: are any DORA steps unicast? Which ones and why?

Part 3 — MAC-based reservation (30 min)

• Find S2's MAC address: on S2, run ip link show eth0. Record the MAC address exactly (format: xx:xx:xx:xx:xx:xx).
• Add a host block to dhcpd.conf on S1. Add it inside the subnet block or globally — outside the subnet block also works:
  host s2 {
hardware ethernet [S2-MAC];
fixed-address 192.168.50.2;
}
  The fixed-address must be within the declared subnet but does not need to be inside the range — reservations are separate from the pool.
• Restart isc-dhcp-server. On S2, release and renew the lease:
  sudo dhclient -r eth0 && sudo dhclient eth0
  Verify S2 now receives 192.168.50.2 instead of a pool address.
• Check the lease database: cat /var/lib/dhcp/dhcpd.leases. Find S2's entry — it should show the reserved address. Note the structure: binding state, lease expiry, MAC address, client hostname.

Part 4 — Restore S2 static IP (10 min)

• Before ending the session, restore S2 to a static IP. The DNS labs (3C and 3D) need a predictable address for zone file records:
  Edit S2's Netplan back to: dhcp4: false, addresses: [192.168.50.2/24], gateway: 192.168.50.1. Apply: sudo netplan apply.
• Verify S2 is reachable from S1: ping 192.168.50.2. The DHCP reservation is still in dhcpd.conf — if S2 is ever put back on DHCP, it will still receive .2.