On DHCP the wrong subnet is distributed. How to solve the problem?

Question

linux

There is a Linux computer that works as a DHCP server. It is configured to distribute both fixed IP (by MAC address) and random IP from three subnets: 192.168.11, 192.168.12 and 192.168.13. No other subnets are configured on the DHCP server. But somehow, sometimes the 192.168.30 subnet is distributed to mobile phones via Wi-Fi and computers, of course there is no Internet in this case. What could be the problem? Could it be that someone brought his wi-fi router with DHCP enabled from home and it starts to send subnet 30 to our network? The problem is definitely not in the DHCP server.

Answer 1

Yes, if someone stuck their dhcp into your network, then it will be so.
The client, when turned on, sends a broadcast request, who is the dhcp server here. Whose server is the first to respond, he will distribute addresses to this client.

Answer 2

The problem is now to find the culprit. Through Wireshark, I determined who is distributing DHCP, there is information about the MAC address, but this did not make it any easier for me, since there is no information anywhere about what kind of device it is. The same Wireshark writes, let's say that under such a MAC address it is TP-LINK, etc., with a MAC address that distributes 30 subnets - deaf.
Through the iptables firewall on the DHCP server, this MAC address was completely blocked, even if someone connected the router to the network, then the Internet should not work for him.