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I noticed that on Ubuntu 18.04.1 DHCP server (isc-dhcp-server) always responds (DHCPOFFER) directly to the IP it offers to the host having sent DHCPDISCOVER.

The DHCPDISCOVER packet has its Broadcast flag set, yet DHCP server does not respond with broadcast. As a result, none of hosts receive their network settings via DHCP.

Is it possible to make the DHCP server to respond via broadcast (eitehr by using some setting of it I don't know about, or by forwarding all DHCP responses to broadcast address?

Update: the issue is definitely related to Ubuntu 18.04.1 - in all cases I tried to use its DHCPd server (mentioned above), both on physical computer and from a virtual machine running under Ubuntu 18.04.1 KVM host, there were the mentioned DHCP issues.

However, when I installed a CentOS guest on ESXi host in the same network, the same DHCP configuration started to workat once, and DHCP clients got their dynamic IPs assigned at first attempt.

So the question remains, how do I run DHCP server from Ubuntu 18.04.1 without entering the mentioned DHCPDISCOVER/DHCPOFFER loop. Note: the hosts on the intranet are a mix of various Windows and Linux systems.

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Well the DHCPOFFER is a Layer 2 unicast packet, so it uses mac-adresses to find the correct destination 1. It should not use IP at all.

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  • The fact is that traffic I see via tcpdump running on the mentioned DHCP sever system is a constant flow of DHCPDISCOVER / DHCPOFFER sequences, not a single pair of DHCPREQ/DHCPACK. I tried several other Ubuntu 18.04-based computers within the same network. Same result. iptables doesn't block any traffic. Dec 1, 2018 at 23:34

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