Нужен аналог опции always-broadcast в isc-dhcp. Которая позволяет вставлять во все dhcp пакеты, бродкаст флаг. Заставляя клиента общаться только через релей агент.

Ткните пальцем, возможно уже реализовано и я не нашел.

ways-broadcast flag;

The DHCP and BOOTP protocols both require DHCP and BOOTP clients to
set the broadcast bit in the flags field of the BOOTP message header.
Unfortunately, some DHCP and BOOTP clients do not do this, and there-
fore may not receive responses from the DHCP server. The DHCP
server can be made to always broadcast its responses to clients by
setting this flag to 'on' for the relevant scope; relevant scopes
would be inside a conditional statement, as a parameter for a class,
or as a parameter for a host declaration. To avoid creating excess
broadcast traffic on your network, we recommend that you restrict the
use of this option to as few clients as possible. For example, the
Microsoft DHCP client is known not to have this problem, as are the
OpenTransport and ISC DHCP clients.