Week 3 Day 1 — NETW-0014 Advanced Operating Systems

0:00–0:30
Lecture

0:30–1:50
Lab 3A

1:50–2:00
Debrief

0:00 – 0:30 Lecture · 30 min

The DORA process, DHCP lease lifecycle, and dhcpd.conf structure

Part 1 — Why DHCP? (5 min)

Open with: "In Weeks 1–2, every IP address was manually configured with Netplan. In a real network with 200 devices, what's the problem with that approach?" — manual config is error-prone, time-consuming, and doesn't scale. IP address tracking becomes a spreadsheet nightmare. DHCP automates assignment, ensures no duplicates, and handles address reclamation when devices leave the network.
In this lab we aren't eliminating static IPs for S1, S2, and S3 — the servers need predictable addresses for DNS and service configuration. DHCP is for clients that don't need permanent addresses: workstations, guest devices, printers. The reservation feature bridges the gap — a device can get DHCP convenience while receiving a consistent address.

Part 2 — The DORA process (12 min)

Draw the four-step exchange on the board. Students who did networking courses will have seen this, but go through it carefully — the broadcast vs. unicast distinction matters for understanding why DHCP server placement is important:
D — Discover: Client has no IP. It broadcasts a DHCPDISCOVER to 255.255.255.255 from source 0.0.0.0. Any DHCP server on the segment hears it. The client includes its MAC address so the server can identify it later.
O — Offer: The server responds with a DHCPOFFER — unicast to the client's MAC (since the client has no IP yet, the server uses the MAC address at layer 2). The Offer contains: proposed IP, subnet mask, gateway, DNS servers, lease time, and the server's own IP.
R — Request: The client broadcasts a DHCPREQUEST — it broadcasts rather than unicasts so that any other DHCP servers that also sent Offers know their offer was declined. The Request identifies which server's offer is being accepted.
A — Acknowledge: The server sends a DHCPACK confirming the lease. The client can now configure its interface with the offered parameters. If something went wrong between Offer and Ack (e.g., the IP was assigned to someone else), the server sends DHCPNAK instead.
Lease timers: T1 (50% of lease time) — client sends a unicast Renew to the original server. T2 (87.5% of lease time) — if renewal failed, client broadcasts to any DHCP server to Rebind. If lease expires without renewal — client gives up the address and starts the DORA process again.
Important: DHCP traffic uses UDP ports 67 (server) and 68 (client). DHCP servers must be on the same broadcast domain as clients, or a DHCP relay agent must be configured on the router.

Part 3 — dhcpd.conf structure (10 min)

Two config files: /etc/default/isc-dhcp-server — specifies which interfaces the daemon listens on. Critical: only bind to internal interfaces. If eth0 (External) is listed, the server broadcasts DHCP offers onto the classroom network. /etc/dhcp/dhcpd.conf — the actual scope and option configuration.
Walk through the dhcpd.conf structure:
authoritative; — declares this server as the authoritative DHCP server for the subnets it manages. Without this, it will not send DHCPNAK to clients with incorrect addresses.
default-lease-time — lease duration offered if the client doesn't request a specific time (in seconds).
max-lease-time — maximum lease a client can request.
subnet ... netmask ... { range ...; option routers ...; option domain-name-servers ...; } — the scope block. Semicolons are mandatory after every option — one missing semicolon stops the daemon cold.
Ask: "Where should we point the DNS servers in our DHCP scope right now?" — the classroom DNS (172.17.0.1). In Week 3 we will also add S1's IP once DNS is configured. Clients then resolve both internal names (via S1) and internet names (via the classroom DNS).

Part 4 — DHCP logging (3 min)

By default, DHCP events land in syslog with everything else. With the custom rsyslog template from Week 2, they're readable — but they're mixed in with all other events. We can separate them by adding log-facility local0; to dhcpd.conf. Then a rsyslog rule routes local0.* to /var/log/dhcpd.log. This is a direct application of the rsyslog skills from Week 2.

0:30 – 1:50 Lab 3A · 80 min

Lab 3A — Install and configure isc-dhcp-server, DHCP logging, verify S2 lease

Part 1 — Install isc-dhcp-server (15 min)

Install on S1: sudo apt install isc-dhcp-server. Check status immediately: sudo systemctl status isc-dhcp-server. It will show failed — this is expected. Read the error carefully: it says it is "not configured to listen on any interfaces." This is the most important lesson of the install: the package installs but doesn't run until we configure it.
Check whether it is enabled to start on boot: sudo systemctl is-enabled isc-dhcp-server. Note the result — students record this in the lab sheet.

Part 2 — Configure the listening interface (10 min)

Edit /etc/default/isc-dhcp-server. Find the line INTERFACESv4="" and add the LAN1 interface: INTERFACESv4="eth1". Use S1's actual LAN1 interface name — confirm with ip link show first. Do not add the External interface (eth0) — that would broadcast DHCP onto the classroom network.

Part 3 — Configure dhcpd.conf (25 min)

Rename the default config file to preserve it as a reference: sudo mv /etc/dhcp/dhcpd.conf /etc/dhcp/dhcpd.conf.original. Create a clean new file: sudo nano /etc/dhcp/dhcpd.conf.
Enter the following configuration (adjusting the subnet to match the desk number):
authoritative;
log-facility local0;

default-lease-time 86400; # 1 day in seconds
max-lease-time 604800; # 1 week in seconds

subnet 192.168.50.0 netmask 255.255.255.0 {
range 192.168.50.10 192.168.50.110;
option routers 192.168.50.1;
option domain-name-servers 172.17.0.1, 1.1.1.1;
option domain-name "yourname.net";
}
Try to start the daemon: sudo systemctl start isc-dhcp-server. If it fails, check the journal: sudo journalctl -u isc-dhcp-server -n 20. Read the error — it will point to the specific line with the problem.

Part 4 — Configure DHCP-specific logging (15 min)

The log-facility local0; directive in dhcpd.conf makes DHCP events use the local0 syslog facility. Now add an rsyslog rule to route those events to a dedicated file. Edit /etc/rsyslog.d/50-default.conf on S1 and add:
local0.* /var/log/dhcpd.log
Create the log file and set correct ownership:
sudo touch /var/log/dhcpd.log
sudo chown syslog:adm /var/log/dhcpd.log
Restart rsyslog: sudo systemctl restart rsyslog
Restart isc-dhcp-server and watch the log: sudo tail -f /var/log/dhcpd.log. The server startup events should appear there rather than in syslog.

Part 5 — Verify S2 lease (15 min)

S2 currently has a static IP via Netplan. To test DHCP without losing SSH access, temporarily reconfigure S2's Netplan to use DHCP:
In S2's Netplan file, change the eth0 section to: dhcp4: true (remove the static address and gateway lines).
Apply: sudo netplan apply.
On S1, watch the DHCP log: sudo tail -f /var/log/dhcpd.log. Within a few seconds you should see the DORA exchange: DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK. Record the severity and facility that appear in the log for each event.
On S2, verify the assigned IP: ip addr show eth0. It should be in the 192.168.50.10–110 range. Also check: ip route show — the default gateway should be 192.168.50.1 (S1). Try pinging 8.8.8.8 — internet access should still work through S1's NAT.
On S1, find the lease database: cat /var/lib/dhcp/dhcpd.leases. Record the structure of S2's lease entry — the MAC address, the assigned IP, the lease start and end times.

Lab 3A complete when: isc-dhcp-server running and enabled on S1. dhcpd.conf has a valid scope with correct range, gateway, DNS, and lease times. DHCP events appear in /var/log/dhcpd.log (not in syslog). S2 has received a DHCP lease in the 192.168.50.10–110 range. Lease entry visible in dhcpd.leases.

1:50 – 2:00 Debrief · 10 min

Run a quick tcpdump on S1: sudo tcpdump -i eth1 -n port 67 or port 68. Release and renew S2's lease: on S2, run sudo dhclient -r eth0 then sudo dhclient eth0. Walk through the four packets together on the projector — identify each packet type by the DHCP option field in the verbose output.
Before leaving: restore S2 to a static IP in Netplan. Students who leave S2 on DHCP will have a different IP tomorrow, which will break the Lab 3B reservation exercise and cause problems in Labs 3C and 3D. Preview Tuesday: deliberate errors in dhcpd.conf and creating a MAC-based reservation.

Learning outcomes — by end of Day 1, students can…

Explain the DORA processDescribe all four steps including source/destination addresses, broadcast vs unicast, and what each packet contains

Configure isc-dhcp-serverSet the listening interface in /etc/default/isc-dhcp-server and write a dhcpd.conf scope with range, gateway, DNS, and lease times

Route DHCP logs to a dedicated fileUse log-facility local0 in dhcpd.conf and a matching rsyslog rule to separate DHCP events from syslog

Read the DHCP lease databaseLocate /var/lib/dhcp/dhcpd.leases and interpret the fields in a lease entry

Common issues and fixes

Issue	Likely cause	Fix
isc-dhcp-server fails to start after configuring interface	Syntax error in dhcpd.conf — most commonly a missing semicolon	Check journal: `sudo journalctl -u isc-dhcp-server -n 30`. The error message includes the file name and line number. Every line in dhcpd.conf must end with a semicolon except the closing braces
S2 doesn't get a lease after switching to DHCP	Wrong interface name in /etc/default/isc-dhcp-server, or firewall blocking UDP 67/68	Verify interface name matches exactly: `ip link show`. Check UFW is not blocking DHCP ports. Check tcpdump on S1 — is S2's Discover arriving?
DHCP log entries appear in syslog instead of dhcpd.log	log-facility local0 not in dhcpd.conf, or rsyslog rule not added / rsyslog not restarted	Verify dhcpd.conf contains `log-facility local0;`. Check 50-default.conf has `local0.* /var/log/dhcpd.log`. Restart both services
dhcpd.leases file is empty	No leases have been issued yet	The file exists but starts empty. It populates as leases are granted. Trigger a lease by running dhclient on S2

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