Chapter 3: Basics and structure

DHCP Server Configuration (dhcpd.conf)

Where to Find It

The main configuration file for the ISC DHCP server is:

/etc/dhcp/dhcpd.conf

Configuration Hierarchy

The file is hierarchical:

1. Global Settings – Defaults that apply everywhere (lease times, logging, DNS domain).
2. Shared Networks – Group multiple subnets on the same broadcast domain.
3. Subnets – Define address ranges, gateways, and subnet-specific options.
4. Pools – Split a subnet into smaller groups based on policy (e.g., guest vs corporate).
5. Hosts – Define static mappings for specific devices (by MAC address or client ID).

Tip: Always document why a setting is global vs subnet-specific to avoid conflicts.

Key Directives

• authoritative; – Tells clients this server is the definitive DHCP authority for these networks. This reduces client timeouts when they hold invalid leases.
• default-lease-time / max-lease-time – Controls how long clients keep their IPs.
• log-facility – Sends logs to a specific syslog facility for easier filtering.
• option domain-name / option domain-name-servers – Provides DNS configuration to clients.

Maintainability Best Practices

• Use include files to split configs by site or VLAN (keeps the main file clean).
• Add comments to explain why options are set (important for multi-admin environments).
• Keep the config file under version control (e.g., Git) so changes are tracked and rollbacks are easy.

Minimal Example

A basic authoritative DHCP config might look like this:

authoritative;                 # This server is authoritative
default-lease-time 600;        # 10 minutes
max-lease-time 7200;           # 2 hours
log-facility local7;           # Send logs to syslog facility 'local7'

option domain-name "corp.example";
option domain-name-servers 10.0.0.53, 10.0.1.53;   # Primary and secondary DNS

subnet 10.0.10.0 netmask 255.255.255.0 {
  option routers 10.0.10.1;
  range 10.0.10.50 10.0.10.199;   # Address pool for clients
}

Why These Choices?

• authoritative; ensures clients quickly discard invalid leases.
• Global options (like DNS servers) apply to all subnets unless overridden.
• Placing defaults at the top makes it easy to override settings in subnet, pool, or host blocks when necessary.

Key Takeaway: Think in layers (global → shared-network → subnet → pool → host). Keep configs clean, documented, and versioned for long-term reliability.