Configuring the DHCP Server on Debian

In this article, you will learn how to configure the isc-dhcp-server. Additionally, we will configure an IP reservation and set it up to operate on two different scopes.

Here is my DHCP server’s network configuration:

# Router external network
auto ens4
iface ens4 inet dhcp

# Internal network 1
auto ens5 
iface ens5 inet static 
        address 192.168.10.1
        netmask 255.255.255.0
        # SNAT for ens5 network , ensure that bit of forwarding is activated.
        post-up iptables -t nat -A POSTROUTING -o ens4 -j MASQUERADE
        post-down iptables -t nat -D POSTROUTING -o ens4 -j MASQUERADE


# Internal network 2
auto ens6 
iface ens6 inet static 
        address 192.168.20.1
        netmask 255.255.255.0

Installing the isc-dhcp-server

To install the DHCP server on Debian, run the following command:

sudo apt update && sudo apt install isc-dhcp-server

When APT finishes, it will return an error indicating that the service is misconfigured. You can ignore this for now as we will configure it next.

Configuring the isc-dhcp-server service

First, we need to configure the network interface that the DHCP server will operate on. Edit the interface configuration file:

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

Add the interface that will distribute IP addresses, for example for network1:

INTERFACESv4="ens5"

Then we configure the DHCP server with the following characteristics:

  • IP address range: 192.168.10.100 - 192.168.0.110
  • Subnet mask: 255.255.255.0
  • Lease duration: 1 hour
  • Gateway: 192.168.10.1
  • DNS servers: 8.8.8.8

Edit the isc-dhcp-server configuration file:

sudo nano /etc/dhcp/dhcpd.conf

The most important parameters inside this file are:

  • max-lease-time: IP address lease time
  • default-lease-time: Lease renewal time
  • option routers: Gateway IP address
  • option domain-name-server: IP addresses of the DNS servers the client will use
  • option domain-name: Domain name sent to the client
  • option subnet-mask: Subnet sent to the clients
  • option broadcast-address: Network broadcast address

Here is the configuration I will add for network1:

subnet 192.168.10.0 netmask 255.255.255.0 {
  range 192.168.10.100 192.168.10.110;
  option routers 192.168.10.1;
  option domain-name-servers 8.8.8.8;
  default-lease-time 3600;
  max-lease-time 3600;
}

With this configuration, the DHCP server will provide addresses within the specified range and assign the specified gateway and DNS server.

To apply the changes, restart the DHCP service:

sudo systemctl restart isc-dhcp-server

Configuring clients to obtain dynamic addresses

To allow clients to receive IP addresses dynamically, ensure their network settings are set to use DHCP.

On a Debian client

Edit the network configuration to use DHCP. Open the /etc/network/interfaces file:

sudo nano /etc/network/interfaces

Ensure the interface is configured like this:

auto ens4 
iface ens4 inet dhcp

Restart the network interface to apply changes:

debian@client1:~$ sudo ifdown ens4 && sudo ifup ens4

You should see the DHCP client’s output as it obtains its configuration:

ifdown: interface ens4 not configured
Internet Systems Consortium DHCP Client 4.4.3-P1
...

bound to 192.168.10.100 -- renewal in 1479 seconds.

You can also verify the configuration using:

debian@client1:~$ ip -4 a
...
2: ens4: ...
    inet 192.168.10.100/24 brd 192.168.10.255 scope global dynamic ens4

💡 Tip:
If you need to modify the DHCP client configuration or simply request a new one, these commands are helpful.
On Windows: ipconfig /release and then ipconfig /renew
On Linux: dhclient -r to release and dhclient to renew.

Checking address leases

To view the IP leases on the server, open the DHCP lease file, typically located at:

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

Do not edit this file manually, as it may cause issues.

Reserving an IP address

To reserve an IP address for a specific client, edit the DHCP server configuration:

sudo nano /etc/dhcp/dhcpd.conf

Add the following to reserve based on the MAC address:

host client_name {
  hardware ethernet 00:11:22:33:44:55;
  fixed-address 192.168.10.120;
}

Restart the DHCP service:

sudo systemctl restart isc-dhcp-server

Verify on the client (Windows or Linux) that the reserved IP was correctly assigned using ipconfig or ip a.

Configuring two scopes

If you want the DHCP server to handle two networks, such as 192.168.10.0 and 192.168.20.0, add another network interface.

Edit /etc/default/isc-dhcp-server:

INTERFACESv4="ens5 ens6"

Add a second address range to the DHCP configuration file:

sudo nano /etc/dhcp/dhcpd.conf

Add:

subnet 192.168.20.0 netmask 255.255.255.0 {
  range 192.168.20.100 192.168.2.110;
  option routers 192.168.20.1;
  option domain-name-servers 8.8.8.8;
  default-lease-time 3600;
  max-lease-time 3600;
}

Restart the DHCP service:

sudo systemctl restart isc-dhcp-server

Now, clients on the second network will receive the configured DHCP settings.