DHCP Server Setup Guide Using ISC DHCP Server

This guide explains how to configure a DHCP server on Ubuntu or Debian using ISC DHCP Server.

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1. What Is DHCP?

DHCP, or Dynamic Host Configuration Protocol, automatically provides network settings to client devices.

A DHCP server can assign:

• IP address
• Subnet mask
• Default gateway
• DNS server
• Lease duration
• Reserved IP addresses for specific devices

For example, instead of manually configuring every computer, DHCP can automatically assign:

IP address:      192.168.1.100
Subnet mask:     255.255.255.0
Gateway:         192.168.1.1
DNS server:      192.168.1.10

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2. Important DHCP Terms

Term	Definition
DHCP server	Provides IP addresses and network configuration
DHCP client	A device requesting network settings
DHCP lease	Temporary assignment of an IP address
DHCP pool/range	Range of addresses available for clients
Reservation	A fixed IP assigned to a particular device
Subnet mask	Defines the size of the local network
Default gateway	Router used to access other networks
DNS server	Converts domain names into IP addresses
MAC address	Hardware address identifying a network device
DORA	DHCP Discover, Offer, Request, and Acknowledgment

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3. Example Network

This guide uses the following network:

Network:             192.168.1.0/24
DHCP server IP:      192.168.1.10
Router/gateway:      192.168.1.1
DNS server:          192.168.1.10
DHCP address range:  192.168.1.100–192.168.1.200
Interface:           eth0
Domain:              example.local

Your values may be different.

Only one DHCP server should normally operate on the same network. Disable DHCP on your router if this Linux server will provide DHCP.

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4. Requirements

You need:

• Ubuntu or Debian server
• Root or sudo access
• A fixed IP address for the DHCP server
• Network interface connected to the client network
• A network router or gateway
• At least one DHCP client for testing

The DHCP server must have a static IP address. Do not allow the DHCP server itself to receive its address from DHCP.

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5. Identify the Network Interface

List network interfaces:

ip link

Example output:

1: lo: ...
2: eth0: ...
3: ens18: ...

The connected interface may be named:

eth0
ens18
enp0s3

Find its current IP address:

ip addr

For this guide, we use:

eth0

Replace eth0 with your actual interface name.

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6. Configure a Static IP Address

Ubuntu commonly uses Netplan.

List Netplan files:

ls /etc/netplan/

Open the configuration file:

sudo nano /etc/netplan/00-installer-config.yaml

Example configuration:

network:
  version: 2
  ethernets:
    eth0:
      dhcp4: false
      addresses:
        - 192.168.1.10/24
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses:
          - 192.168.1.10
          - 1.1.1.1

Apply the configuration:

sudo netplan apply

Check the address:

ip addr show eth0

Test the gateway:

ping -c 4 192.168.1.1

Test Internet connectivity:

ping -c 4 1.1.1.1

Static configuration definitions

• dhcp4: false: Prevents the server from receiving an IPv4 address through DHCP.
• addresses: Sets the server’s fixed IP address.
• /24: Equivalent to subnet mask 255.255.255.0.
• routes: Defines the default gateway.
• nameservers: Defines DNS servers used by the DHCP server itself.

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7. Install the DHCP Server

Update the package list:

sudo apt update

Install ISC DHCP Server:

sudo apt install isc-dhcp-server -y

Check the service:

sudo systemctl status isc-dhcp-server

It may show an error initially because it has not yet been configured. This is normal.

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8. Configure the DHCP Interface

Open the DHCP server defaults file:

sudo nano /etc/default/isc-dhcp-server

Find:

INTERFACESv4=""

Change it to:

INTERFACESv4="eth0"

If you use IPv6 DHCP, you can configure:

INTERFACESv6="eth0"

For a basic IPv4 setup, leave it empty:

INTERFACESv6=""

Save the file.

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9. Configure the DHCP Scope

A DHCP scope defines the network and addresses that the server can assign.

Open the main DHCP configuration file:

sudo nano /etc/dhcp/dhcpd.conf

You can replace the contents with this example:

authoritative;

default-lease-time 600;
max-lease-time 7200;

option domain-name "example.local";
option domain-name-servers 192.168.1.10, 1.1.1.1;

subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.100 192.168.1.200;

    option subnet-mask 255.255.255.0;
    option routers 192.168.1.1;
    option broadcast-address 192.168.1.255;
}

Configuration definitions

authoritative

authoritative;

Declares that this is the official DHCP server for the network.

Use this only if you control the network.

default-lease-time

default-lease-time 600;

Sets the default lease duration in seconds.

600 seconds = 10 minutes

max-lease-time

max-lease-time 7200;

Sets the maximum lease duration.

7200 seconds = 2 hours

option domain-name

option domain-name "example.local";

Provides a local DNS domain name to clients.

option domain-name-servers

option domain-name-servers 192.168.1.10, 1.1.1.1;

Tells clients which DNS servers to use.

If your DNS server is configured correctly, you can use:

option domain-name-servers 192.168.1.10;

subnet

subnet 192.168.1.0 netmask 255.255.255.0

Defines the network served by DHCP.

range

range 192.168.1.100 192.168.1.200;

Defines the addresses that may be dynamically assigned.

option routers

option routers 192.168.1.1;

Sets the default gateway for clients.

option broadcast-address

option broadcast-address 192.168.1.255;

Sets the broadcast address for the subnet.

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10. Validate the DHCP Configuration

Before starting the service, check the configuration:

sudo dhcpd -t -cf /etc/dhcp/dhcpd.conf

If successful, you should see no serious errors.

You can also check the service configuration:

sudo systemctl status isc-dhcp-server

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11. Start and Enable the DHCP Service

Start the service:

sudo systemctl start isc-dhcp-server

Enable automatic startup:

sudo systemctl enable isc-dhcp-server

Restart the service after configuration changes:

sudo systemctl restart isc-dhcp-server

Check its status:

sudo systemctl status isc-dhcp-server

View recent DHCP logs:

sudo journalctl -u isc-dhcp-server --no-pager -n 50

You can also monitor logs in real time:

sudo journalctl -u isc-dhcp-server -f

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12. Configure the Firewall

DHCP uses:

UDP port 67: DHCP server
UDP port 68: DHCP client

Allow DHCP through UFW:

sudo ufw allow 67/udp
sudo ufw reload

Check the firewall:

sudo ufw status

If the server is inside a virtual machine or cloud environment, also check the external firewall or security group.

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13. Test from a Linux Client

On a Linux client, identify its interface:

ip link

Release its current DHCP lease:

sudo dhclient -r eth0

Request a new lease:

sudo dhclient -v eth0

Replace eth0 with the client’s actual interface.

Display the assigned address:

ip addr show eth0

Check the routing table:

ip route

Check DNS configuration:

resolvectl status

The client should receive an address in this range:

192.168.1.100–192.168.1.200

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14. Test from a Windows Client

Open Command Prompt as Administrator and run:

ipconfig /release
ipconfig /renew
ipconfig /all

Check that the client received:

IPv4 Address:        192.168.1.x
Subnet Mask:         255.255.255.0
Default Gateway:     192.168.1.1
DHCP Server:         192.168.1.10
DNS Servers:         192.168.1.10

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15. Create a Reserved IP Address

A DHCP reservation assigns the same IP address to a specific device.

You need the device’s MAC address.

On Linux:

ip link

Example:

link/ether 00:11:22:33:44:55

On Windows:

ipconfig /all

Add this reservation to /etc/dhcp/dhcpd.conf:

host office-pc {
    hardware ethernet 00:11:22:33:44:55;
    fixed-address 192.168.1.50;
    option host-name "office-pc";
}

The reserved address should be outside the dynamic range:

Dynamic range:   192.168.1.100–192.168.1.200
Reserved IP:     192.168.1.50

Validate and restart:

sudo dhcpd -t -cf /etc/dhcp/dhcpd.conf
sudo systemctl restart isc-dhcp-server

Then renew the client’s lease.

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16. Create Multiple DHCP Networks

If the server has multiple network interfaces, you can configure separate subnets.

Example:

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 subnet-mask 255.255.255.0;
    option domain-name-servers 192.168.1.10;
}

subnet 192.168.2.0 netmask 255.255.255.0 {
    range 192.168.2.100 192.168.2.200;
    option routers 192.168.2.1;
    option subnet-mask 255.255.255.0;
    option domain-name-servers 192.168.1.10;
}

Each subnet must be connected to the correct interface or VLAN.

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17. DHCP Relay for Different Networks

A DHCP broadcast normally does not cross routers. A DHCP relay forwards DHCP requests from another network to the DHCP server.

Install the relay:

sudo apt install isc-dhcp-relay -y

During installation, specify the DHCP server address:

192.168.1.10

You can edit the relay configuration later:

sudo nano /etc/default/isc-dhcp-relay

Example:

SERVERS="192.168.1.10"
INTERFACES="eth1 eth2"
OPTIONS=""

Restart the relay:

sudo systemctl restart isc-dhcp-relay

The DHCP server also needs a subnet declaration for the remote network:

subnet 192.168.2.0 netmask 255.255.255.0 {
    range 192.168.2.100 192.168.2.200;
    option routers 192.168.2.1;
    option subnet-mask 255.255.255.0;
    option domain-name-servers 192.168.1.10;
}

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18. View DHCP Leases

Active leases are stored in:

/var/lib/dhcp/dhcpd.leases

View the file:

sudo cat /var/lib/dhcp/dhcpd.leases

Look for entries such as:

lease 192.168.1.100 {
  starts 2026/08/31 10:00:00;
  ends 2026/08/31 10:10:00;
  hardware ethernet 00:11:22:33:44:55;
  client-hostname "office-pc";
}

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19. Troubleshooting

DHCP service does not start

Check the configuration:

sudo dhcpd -t -cf /etc/dhcp/dhcpd.conf

Check service logs:

sudo journalctl -u isc-dhcp-server -n 100

Incorrect interface configured

Check the interface name:

ip link

Edit:

sudo nano /etc/default/isc-dhcp-server

Set the correct interface:

INTERFACESv4="ens18"

Then restart:

sudo systemctl restart isc-dhcp-server

Clients do not receive IP addresses

Check the following:

• The client is connected to the correct network.
• The DHCP server interface is connected.
• Another DHCP server is not running.
• UDP port 67 is allowed.
• The DHCP range is not exhausted.
• The server and clients use the same subnet.
• VLAN configuration is correct.
• A DHCP relay is configured if the client is on another subnet.

Monitor DHCP traffic:

sudo tcpdump -i eth0 -n port 67 or port 68

You should see messages such as:

DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
DHCPACK

These four messages are known as DORA:

• Discover — Client searches for a DHCP server.
• Offer — Server offers an IP address.
• Request — Client requests the offered address.
• Acknowledgment — Server confirms the lease.

Clients receive addresses from the wrong server

Disable DHCP on the router or other DHCP server. Multiple DHCP servers on one network can cause incorrect gateways, DNS settings, and IP addresses.

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20. Complete Example Configuration

The final /etc/dhcp/dhcpd.conf may look like this:

authoritative;

default-lease-time 600;
max-lease-time 7200;

option domain-name "example.local";
option domain-name-servers 192.168.1.10;

subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.100 192.168.1.200;

    option subnet-mask 255.255.255.0;
    option routers 192.168.1.1;
    option broadcast-address 192.168.1.255;
}

host office-pc {
    hardware ethernet 00:11:22:33:44:55;
    fixed-address 192.168.1.50;
    option host-name "office-pc";
}

Validate and restart:

sudo dhcpd -t -cf /etc/dhcp/dhcpd.conf
sudo systemctl restart isc-dhcp-server
sudo systemctl enable isc-dhcp-server

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Conclusion

Your DHCP server should now automatically assign IP addresses and network settings to devices on the configured network. Clients simply connect and lease their configuration automatically — no manual IP setup required.