Hello,
I need to change some machines IP settings from static IPs to auto/DHCP.
In order to do that, DHCP must serve to specific clients with fixed leases different DNS settings and different Gateway (all clients on the same subnet).
Is that possible?
(These clients are communicating with IPFire on the blue interface.)
nice question @panosst
there seems to be 
on some dhcp options:
https://community.ipfire.org/t/add-possibility-to-edit-router-gateway-directly-in-the-web-page-cgi-bin-dhcp-cgi/8559
and
https://community.ipfire.org/t/how-can-i-point-dhcp-clients-to-other-gateway/5738/12
and your question is even more
demanding with client-specific
dhcp options 
i also have a dream of ipfire
alble to send different dhcp
options based on client mac 