Dnsmasq: Redirect entire domain to localhost only for specific MAC Addresses

The dnsmasq manpage says:

--cname=<cname>,<target>

Return a CNAME record which indicates that <cname> is really <target>. There are significant limitations on the target; it must be a DNS name which is known to dnsmasq from /etc/hosts (or additional hosts files) or from DHCP. If the target does not satisfy this criteria, the whole cname is ignored. The cname must be unique, but it is permissable to have more than one cname pointing to the same target.

I think the "known target" criteria will make it unsuitable for your requirements.

I do know that Unbound does support this feature, though.

or are they stoerd in separate zones?

Separate zones, one per old C network (last byte in the octet).

What stops me from adding a PTR record saying that resolves to say gmail.com?

Nothing. But as this is not used exceptt for nice pings or some email validity checks, you achieved nothing. people will still go to gmail when they type in gmail.com. All people now see is gmail.com in a traceroute, nothing else.

The one real use for this is smtp - the HELO string given in SMTP should match the PTR record name given. Basically the server must say it is who the ptr record says it is. Note that it can still accept emails for other domains.