I noticed once in the syslog:
dhcpd self-test failed. Please fix the config file
Isc-dhcp-server was installed on the server.
To check the correctness of the configuration file, use the command:
dhcpd -t dhcpd -t -cf /dir/dhcpd.conf /usr/sbin/dhcpd -t
The command should tell which line the error is, but noted that if it is not critical, it may not.
The key “t” executes the configuration test, and “cf” allows you to specify the path to the configuration file if it is not standard.
In my case, in the configuration file /etc/dhcp/dhcpd.conf, someone made a typo, in the line below (there was an extra letter):
Because of this, an error occurred, but despite the error dhcp worked.
Although there were also some critical errors, such as an incorrectly written mac address, DHCP did not start because of this error.