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DHCP Server Setup on Linux

Overview​

Note: The NetFoundry zLAN firewall does not manage DHCP services. Customers must install and configure their own DHCP server using standard Linux tools. This guide provides step-by-step instructions for installing and setting up a DHCP server with isc-dhcp-server.

What is DHCP?​

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and network configuration to devices on your network. A DHCP server manages a pool of IP addresses and hands them out to clients as they join the network.

Installing DHCP Server​

sudo apt update
sudo apt install isc-dhcp-server

Configuring DHCP Server​

Main Configuration File​

  • Ubuntu/Debian: /etc/dhcp/dhcpd.conf
  • Red Hat/CentOS: /etc/dhcp/dhcpd.conf

Edit the configuration file to define your DHCP settings:

subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.200;
  option routers 192.168.1.1;
  option domain-name-servers 8.8.8.8, 8.8.4.4;
  option domain-name "example.local";
  default-lease-time 600;
  max-lease-time 7200;
}
  • subnet: The network served by DHCP
  • range: The pool of IP addresses to assign
  • option routers: Default gateway
  • option domain-name-servers: DNS servers
  • option domain-name: Local domain name
  • default-lease-time: Lease duration in seconds
  • max-lease-time: Maximum lease duration

Specify Interfaces​

Edit /etc/default/isc-dhcp-server (Ubuntu/Debian) or /etc/sysconfig/dhcpd (Red Hat/CentOS) to specify which network interface(s) DHCP should listen on:

Ubuntu/Debian:

INTERFACESv4="eth0"

Red Hat/CentOS:

DHCPDARGS="eth0"

Starting and Enabling DHCP Server​

Ubuntu/Debian​

sudo systemctl enable isc-dhcp-server
sudo systemctl start isc-dhcp-server

Red Hat/CentOS​

sudo systemctl enable dhcpd
sudo systemctl start dhcpd

Verifying DHCP Server​

  • Check service status: 
    sudo systemctl status isc-dhcp-server   # Ubuntu/Debian
    sudo systemctl status dhcpd             # Red Hat/CentOS
  • View logs: 
    sudo journalctl -u isc-dhcp-server      # Ubuntu/Debian
    sudo journalctl -u dhcpd                # Red Hat/CentOS
  • Test DHCP assignment:
    • Connect a client device to the network and verify it receives an IP address from the configured range.

Summary of Commands​

ActionCommand Example
Install DHCP serversudo apt install isc-dhcp-server / sudo dnf install dhcp-server
Edit configsudo nano /etc/dhcp/dhcpd.conf
Specify interfaceEdit /etc/default/isc-dhcp-server or /etc/sysconfig/dhcpd
Enable servicesudo systemctl enable isc-dhcp-server / sudo systemctl enable dhcpd
Start servicesudo systemctl start isc-dhcp-server / sudo systemctl start dhcpd
Check statussudo systemctl status isc-dhcp-server / sudo systemctl status dhcpd
View logssudo journalctl -u isc-dhcp-server / sudo journalctl -u dhcpd

Opening DHCP Ports in the Firewall UI​

Important: The NetFoundry zLAN firewall manages all firewall functions. Do not use ufw or firewall-cmd to open ports. Use the Add/Remove Rule UI in the Console to allow traffic.

To allow DHCP traffic (UDP port 67), add a rule using the UI as described in the Adding & Removing Rules guide:

  • Type: Custom
  • Protocol: UDP
  • Direction: INBOUND
  • Action: Allow
  • Port Range: 67-67
  • Source: (as needed, e.g., 0.0.0.0/0)
  • Destination: (your DHCP server IP or subnet)

Repeat for each interface where DHCP traffic should be allowed.

Troubleshooting Tips​

  • Ensure the DHCP server is listening on the correct interface.
  • Check for syntax errors in /etc/dhcp/dhcpd.conf: 
    sudo dhcpd -t
  • Ensure the required ports are open in zfw for DHCP and any dynamic routing protocols in use.
  • Review logs for errors or denied requests.
  • Confirm that no other DHCP server is running on the same network.

References​