...

If these settings are correct, move to checking configuration of the server and networking functions. To run diagnostics, it is necessary to know a MAC address of a client's device and have an opportunity to initiate providing a client with an address via DHCP.

Checking a DHCP server

To check DHCP server operation, analyse its logs.
The three main problems can be seen from logs:

• Discover is absent.
• Discover is present, Offer is absent.
• Discover and offer are present, Request is absent.

1. Discover from a client's device is absent in logs.

...

In the file /etc/default/isc-dhcp-server all listened interfaces should be listed, for example:
INTERFACES="eth0 eth1"

2. Discover is present in DHCP server logs, but Offer is absent.

...

• one of the subnetworks used is not specified
• classes are specified incorrectly
• pool vacant addresses exhaustion
• failover problems

2.1 One of the subnetworks used is not specified

Subnetworks containing addresses of listened interfaces and a DHCP-relay (if used) should be specified in the file /etc/dhcp/dhcpd.conf. Even if addresses from these subnetworks are not allocated by the server, for example:

subnet 192.168.1.0 netmask 255.255.255.0 {}

2.2 Pool vacant addresses exhaustion

One of the common causes is a case when a server runs out of vacant addresses. To check it, use a guide: ISC DHCP server pool range usage monitoring

...