pfSense

What I'm trying to do is actually to relay DHCP requests received in multicast by a switch to pfSense using unicast.
So pfSense doesn't need to have an interface configured in the subnet to which the DHCP served IP belongs.

I was able to do this by configuring an empty statement in /var/dhcpd/etc/dhcpd.conf :

subnet 172.24.0.0 netmask 255.255.255.0 { ### The network to which belongs the IP configured on the DHCP enabled interface.
}

subnet 172.24.3.0 netmask 255.255.255.0 { ### No pfSense interface is configured with an IP in this network.
pool {
range 172.24.3.10 172.24.3.0;
}
option routers 172.24.3.3;
option domain-name-servers 172.24.3.3;
}