Name: The Foundation of Iron — Week 20 — DHCP
Author: Amine RAITI

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THE FOUNDATION OF IRON · COURSE MATERIAL · WEEK 20

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DHCP
ON THE ACQUIRED NETWORK BASE
Week 20 of 26 · Block 7 — DHCP and DNS
8h theory · 27h practice

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◆ WEEKLY LEARNING OBJECTIVES

1. Understand the DHCP protocol operation (DORA)
2. Install and configure a DHCP server on Linux
3. Manage address leases and exclusion ranges
4. Configure inter-VLAN DHCP relay
5. Diagnose common address assignment problems

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⚠ WARNING — SHELF LIFE OF VERSIONS REFERENCED HERE

This material references isc-dhcp-server. Other DHCP implementations exist (dnsmasq, Kea DHCP). The instructor adapts the configuration to the implementation actually available and maintained at the time of delivery.

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Amine RAITI · Infrastructure Architect & SRE
Public document · CC BY-NC-SA 4.0 · AI Powered by Amine
Opération Dindon

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COURSE OUTLINE · 8H

THEORY GUIDING THREAD — DHCP

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20.1 · DHCP protocol — DORA exchange3h

— DORA: Discover (client broadcast looking for a server), Offer (server proposes an address), Request (client confirms), Acknowledge (server confirms the assignment)
— Lease concept: the duration for which the address is reserved for the client
— Parameters distributed with the address: mask, gateway, DNS servers (direct link with Week 19 and Week 21)

20.2 · DHCP server configuration2h

— Structure of a DHCP configuration file: subnet, range, options
— Static reservations: always assign the same address to a machine identified by its MAC address
— Exclusion ranges: exclude addresses from the pool (for fixed-IP devices)

20.3 · Inter-VLAN DHCP relay3h

— Problem: DHCP Discover is a broadcast that does not cross VLAN/router boundaries
— Solution: the DHCP relay (DHCP helper) on the router/layer-3 switch forwards the Discover as unicast to the centralised DHCP server
— Relay configuration: the DHCP server's IP address is configured on each VLAN interface of the router — direct link with Week 18 (VLANs) and Week 19 (routing)

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EXERCISE 1 · DHCP SERVER INSTALLATION AND CONFIGURATION · 14H

Equipment: already-configured Linux VM (DHCP server), client VMs (Linux and Windows), the Week 18 VLAN topology.

(2h) Installing the DHCP server on the Linux VM, configuring a first simple pool for a single subnet (address range, mask, gateway, lease duration).

(3h) Testing automatic assignment on a client VM: verify the VM receives an address from the configured range, check the DHCP server logs to see the DORA exchange.

(3h) Configuring a static reservation for a VM identified by its MAC address, verifying this VM always receives the same address.

(3h) Extending to 3 DHCP pools corresponding to the 3 VLANs from Week 18, configuring ranges and VLAN-specific options (different gateways per VLAN).

(3h) Configuring the DHCP relay on the router (each VLAN interface pointing to the centralised DHCP server), testing assignment from a client in each VLAN.

SOLUTION — EXERCISE 1

DHCP relay verification: a client in VLAN 20 must receive an address from the VLAN 20 pool configured on the DHCP server, even if the DHCP server is physically in VLAN 10 — the relay forwards the Discover across the VLAN boundary.

Common mistake: forgetting to configure the gateway (routers option) in the DHCP pool — the client receives an address but cannot communicate beyond its subnet.

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EXERCISE 2 · DIAGNOSIS AND ADVANCED ADMINISTRATION · 13H

Equipment: the DHCP system configured in Exercise 1, Wireshark.

(3h) Capturing the full DORA exchange with Wireshark: identify each packet (Discover, Offer, Request, Ack), read the key fields (CHADDR, siaddr, yiaddr, options).

(3h) DHCP failure simulation: pool saturation (all addresses assigned), client receiving a wrong configuration (wrong gateway), DHCP server unreachable from a VLAN — diagnosis and fix.

(4h) Lease administration: view the list of active leases, manually revoke a lease, observe the client's re-request.

(3h) Writing a DHCP configuration reference document covering the 3 pools, reservations, and diagnostic procedures.

SOLUTION — EXERCISE 2

DHCP fields to identify in Wireshark: CHADDR = client's MAC address, yiaddr = proposed/assigned IP address, siaddr = server's IP address, option 3 = gateway, option 6 = DNS servers, option 51 = lease duration.

◆ SUMMARY SHEET — WEEK 20 SELF-ASSESSMENT

1. I can describe the DORA exchange and the role of each message.

2. I can configure a DHCP server with pool, mask and options.

3. I can configure a static reservation by MAC address.

4. I can explain why the Discover does not cross VLAN boundaries.

5. I can configure an inter-VLAN DHCP relay.

6. I can capture and analyse a DORA exchange with Wireshark.

7. I can diagnose a DHCP address assignment failure.

8. I can administer the active leases of a DHCP server.