DHCP Client and Server Basics

IP and Routing Tools showed how to add an IP address by hand with ip address add — but on most networks, servers and devices get their address automatically instead. That process is managed by DHCP (Dynamic Host Configuration Protocol); its protocol-level exchange (discover-offer-request-acknowledge) is already covered in DHCP and Leases. This article isn't about the protocol itself — it covers how the DHCP client behaves on the Linux side, how to inspect lease information, and how to stand up a simple DHCP server when one is needed.

How the DHCP Client Works on Linux

On modern distributions, the DHCP request is usually sent not by a standalone dhclient program but by the network manager itself — NetworkManager or systemd-networkd — each of which has its own built-in DHCP client implementation. On older or minimal systems, ISC's standalone dhclient still shows up. Which source an interface got its IP from can be seen in the proto field of ip route show:

ip route show

default via 192.168.1.1 dev enp0s3 proto dhcp metric 100

proto dhcp means this route was obtained via DHCP — the interface is dynamically configured, not static.

Checking the Current Lease State

Depending on which tool is in use, lease information lives in different places.

If systemd-networkd is in use:

networkctl status enp0s3

●   2: enp0s3
                     Link File: /usr/lib/systemd/network/99-default.link
                  Network File: /run/systemd/network/10-netplan-enp0s3.network
                          Type: ether
                         State: routable (configured)
        Address: 192.168.1.122 (DHCP4)
                                fe80::a00:27ff:fe4a:1c9e
           Gateway: 192.168.1.1
               DNS: 192.168.1.1

If NetworkManager is in use:

nmcli device show enp0s3 | grep -E 'IP4|DHCP'

On older systems running ISC dhclient, the lease file is typically stored here:

cat /var/lib/dhcp/dhclient.leases

lease {
  interface "enp0s3";
  fixed-address 192.168.1.122;
  option subnet-mask 255.255.255.0;
  option routers 192.168.1.1;
  option domain-name-servers 192.168.1.1,1.1.1.1;
  renew 3 2026/07/29 06:00:00;
  expire 3 2026/07/29 18:00:00;
}

The fields that matter:

• fixed-address — the IP currently assigned to this interface;
• routers — the default gateway handed out by the DHCP server;
• domain-name-servers — the DNS servers distributed via DHCP (the next article, DNS Tools, covers how these values get used);
• renew/expire — when the lease needs to be renewed, and when it expires entirely. The client normally sends an automatic renewal request at the halfway point (renew time); if the server doesn't respond, it keeps retrying until expiration, after which the address returns to the pool.

Renewing or Releasing a Lease by Hand

With systemd-networkd:

sudo networkctl renew enp0s3

With NetworkManager, reactivating the interface also renews the lease:

sudo nmcli device reapply enp0s3

With standalone dhclient:

sudo dhclient -r enp0s3
sudo dhclient enp0s3

-r releases the current lease back to the server and drops it; the second command sends a fresh request. Running this on a remote server where the interface is the only connection path can lead to an IP change or a temporary disconnection.

Setting Up a Simple DHCP Server (isc-dhcp-server)

In most production environments, the DHCP server is handled by network hardware (a router or firewall) or a cloud provider, but on a lab or isolated internal network it may be necessary to configure a Linux server as the DHCP server itself. This example uses isc-dhcp-server from the Ubuntu/Debian family.

Installation:

sudo apt update
sudo apt install isc-dhcp-server

Specifying which interface the service should listen on (on Debian/Ubuntu):

sudo tee /etc/default/isc-dhcp-server > /dev/null <<'EOF'
INTERFACESv4="enp0s8"
EOF

The main configuration lives in /etc/dhcp/dhcpd.conf:

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

subnet 10.20.0.0 netmask 255.255.255.0 {
    range 10.20.0.100 10.20.0.200;
    option routers 10.20.0.1;
    option domain-name-servers 10.20.0.1, 1.1.1.1;
    option broadcast-address 10.20.0.255;
}

What each part means:

• default-lease-time/max-lease-time — the lease time applied when a client doesn't request a specific duration, and the longest lease permitted, in seconds;
• range — the block of IP addresses handed out to clients (the pool); this range must never overlap with the server's own static addresses;
• option routers — the default gateway distributed to clients;
• option domain-name-servers — the DNS servers distributed to clients.

Checking the configuration's syntax and starting the service:

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

dhcpd -t is a check mode that validates the configuration file without restarting the service; don't reload the service if it reports an error.

Watching Leases Handed Out by the Server

sudo tail -n 20 /var/lib/dhcp/dhcpd.leases

This file shows the server's side of the story — which MAC address got which IP, and until when — and is the first place to look when investigating IP conflicts on the network.

Assigning a Permanent IP to a Specific Device (Static Lease)

If a device — a printer or an internal service, for example — should always get the same IP, this can be done through host reservation on the DHCP server instead of static configuration on the network side:

host printer-1st-floor {
    hardware ethernet 08:00:27:aa:bb:cc;
    fixed-address 10.20.0.50;
}

This entry can sit inside or outside the subnet block in dhcpd.conf, depending on the implementation; when the device requests an address with that specific MAC, the server always hands out that exact IP — but this is still delivered via DHCP, meaning the device gets no IP at all if the server is down. For services that must always be reachable, such as a database server, static configuration on the interface itself is the recommended approach instead.

Practical Scenario: A Server Is Getting the Wrong IP Instead of Its Reservation

Problem: a newly installed server gets a random address from the pool on every reboot, even though it was reserved for 10.20.0.50.

Step 1 — check:

ip -brief address show
cat /var/lib/dhcp/dhclient.leases 2>/dev/null | tail -n 10

Step 2 — hypothesis: the server's MAC address doesn't match the value in the host block on the DHCP server, or the reservation was written in the wrong subnet block.

Step 3 — check on the DHCP server side (without changing anything yet):

ip link show enp0s8 | grep link/ether
sudo grep -A2 'host printer-1st-floor' /etc/dhcp/dhcpd.conf
sudo grep 10.20.0.50 /var/lib/dhcp/dhcpd.leases

Step 4 — one change: if the MAC address in dhcpd.conf is wrong, fix it, re-check the configuration, and reload the service:

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

Step 5 — confirm: without restarting the server, renew the lease from the client side to verify:

sudo dhclient -r enp0s8 && sudo dhclient enp0s8
ip -brief address show

Step 6 — documentation: record the MAC address error and how it was fixed, for the next administrator.

Common Mistakes

Accidentally Running Two DHCP Servers on the Same Network

An isc-dhcp-server set up for testing can end up left running on an interface connected to the production network. Always confirm INTERFACESv4 (or the equivalent configuration) points only to the intended, isolated interface.

Interpreting Lease Expiration as "the Network Went Down"

An IP address changing is sometimes just a lease expiring and a new one being handed out, especially when renewal failed. Check the renew/expire times in the lease file before drawing any other conclusion.

Not Noticing the Pool Range Has Run Out

If the range is small and the number of devices on the network is large, a new device may simply fail to get an address. Compare the number of active leases in dhcpd.leases against the total size of the range.

Confusing a Static Lease with a Static IP at the Interface Level

A DHCP host reservation still depends on the DHCP server — if the server is down, the device gets no address at all. This isn't sufficient for servers that must always be available; those need static configuration on the interface itself.

Exercises

1. Confirm your server is getting its IP via DHCP by checking for proto dhcp in ip route show, then find the current lease file or the output of networkctl status/nmcli device show and explain every field in it.
2. On a test VM, manually release a lease (dhclient -r or networkctl renew) and confirm a new IP was assigned.
3. On an isolated lab network (a virtual host-only network, for example), install isc-dhcp-server, write a configuration with a small range, connect another test VM to that network, and watch it get an address.
4. Repeat the practical scenario above: deliberately write a host block with the wrong MAC address, find the problem through the checks shown, then fix it.

Verification criterion: for exercise 4, your fix must be confirmed by the client actually receiving 10.20.0.50 (or your equivalent reserved address) after a lease renewal — not merely by the configuration file looking correct.

References

• man7.org: dhclient(8): https://man7.org/linux/man-pages/man8/dhclient.8.html
• man7.org: dhcpd(8): https://man7.org/linux/man-pages/man8/dhcpd.8.html
• man7.org: dhcpd.conf(5): https://man7.org/linux/man-pages/man5/dhcpd.conf.5.html
• man7.org: dhclient.leases(5): https://man7.org/linux/man-pages/man5/dhclient.leases.5.html
• freedesktop.org: networkctl(1): https://www.freedesktop.org/software/systemd/man/latest/networkctl.html
• Ubuntu Server — Networking: DHCP: https://ubuntu.com/server/docs/network-configuration
• ISC Kea/DHCP documentation: https://www.isc.org/dhcp/