DHCP Configuration

DHCP Configuration

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses, subnet masks, default gateways, and DNS servers to clients on a network. For Ubuntu servers, DHCP is typically used in development/test environments while production servers use static IPs. However, understanding how DHCP works and how to set up a DHCP server is an essential networking skill for managing lab environments, infrastructure networks, and bare-metal provisioning.

How DHCP works

DHCP DORA process:
  Client ──[DISCOVER]──────────────→ Broadcast (255.255.255.255)
  Server ──[OFFER: 192.168.1.50]──→ Client
  Client ──[REQUEST: I want .50]──→ Broadcast
  Server ──[ACK: .50 is yours]────→ Client

  Client now has:
    IP: 192.168.1.50
    Mask: 255.255.255.0
    Gateway: 192.168.1.1
    DNS: 8.8.8.8
    Lease time: 86400 seconds (24 hours)

Configuring an interface for DHCP

# In Netplan, DHCP is the simplest configuration:
# /etc/netplan/00-installer-config.yaml
network:
  version: 2
  ethernets:
    ens3:
      dhcp4: true           # Enable IPv4 DHCP
      dhcp6: false          # Disable IPv6 DHCP (optional)
      dhcp4-overrides:
        use-dns: true        # Accept DNS from DHCP
        use-routes: true     # Accept routes from DHCP

sudo netplan apply

DHCP lease files

# systemd-networkd stores DHCP lease info here:
ls /run/systemd/netif/leases/

# View the current lease
cat /run/systemd/netif/leases/2    # 2 = interface index for ens3

# dhclient stores leases here (if using dhclient):
cat /var/lib/dhcp/dhclient.leases

Lease file contents (key fields)

ADDRESS=192.168.1.50
NETMASK=255.255.255.0
ROUTER=192.168.1.1
DNS=8.8.8.8 1.1.1.1
LEASETIME=86400

Renewing and releasing leases

# Force DHCP renewal via networkctl (systemd-networkd)
sudo networkctl renew ens3

# Via dhclient (if using dhclient):
sudo dhclient -r ens3    # Release current lease
sudo dhclient ens3        # Request new lease

# If you changed DHCP server configuration:
sudo networkctl down ens3
sudo networkctl up ens3

Setting up isc-dhcp-server

sudo apt install -y isc-dhcp-server

# Configure the DHCP server
sudo nano /etc/dhcp/dhcpd.conf

/etc/dhcp/dhcpd.conf — basic configuration

option domain-name "lab.local";
option domain-name-servers 8.8.8.8, 1.1.1.1;

default-lease-time 86400;     # 24 hours
max-lease-time 604800;         # 7 days

# Which interface to listen on:
# /etc/default/isc-dhcp-server: INTERFACESv4="ens3"

subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.50 192.168.1.200;    # IP pool
  option routers 192.168.1.1;          # Default gateway
  option subnet-mask 255.255.255.0;
}

# Specify which interface to listen on
sudo nano /etc/default/isc-dhcp-server
# Set: INTERFACESv4="ens3"

sudo systemctl enable --now isc-dhcp-server
sudo systemctl status isc-dhcp-server

# View current leases
cat /var/lib/dhcpd/dhcpd.leases

Static DHCP reservations

# Assign a fixed IP to a specific MAC address
# Add inside /etc/dhcp/dhcpd.conf:
host server01 {
  hardware ethernet 52:54:00:12:34:56;    # Client's MAC address
  fixed-address 192.168.1.10;             # Always gets this IP
  option host-name "server01";
}

host printer01 {
  hardware ethernet 00:11:22:33:44:55;
  fixed-address 192.168.1.20;
}

# Reload DHCP server after config changes
sudo systemctl reload isc-dhcp-server    # If supported
sudo systemctl restart isc-dhcp-server  # Or full restart

Conclusion

For Ubuntu servers, configure DHCP in Netplan with dhcp4: true. For running a DHCP server on a local network, install isc-dhcp-server and configure the subnet block in /etc/dhcp/dhcpd.conf. Static DHCP reservations (fixed IP by MAC address) are better than hardcoding IPs on clients because the DHCP server remains the single source of truth for address assignments. Check /var/lib/dhcpd/dhcpd.leases to see which MAC addresses have received leases.