There is an eight year old issue (at the point of writing this) with pfSense DHCPd that somehow did not restrict DHCPd IP “handouts” despite the chosen setting to “Deny unknown clients”… Which, after some digging, turns out more to be of a misunderstanding than what the “common people” would think.
Despite the “Deny unknown clients” setting, certain clients requesting an IP from a pool/interface that does not explicitly list its MAC address will still get an IP address. It turns out that said client is considered “known” if the MAC is listed anywhere else (i.e. in some other MAC address list)…
Anyway, I got fed up with this seemingly insecure behaviour and managed to hack a fix… some 8+ months ago… Just that I never got around to posting the details for people willing to hack their own pfSense fix (unlike my other SSHd configuration fix which was documented in full)…
Well, to cut the long story short, the pull request (merged with another upstream fix) has now been accepted and merged (actual changes)… You will see this fix some-time-soon-now in some upcoming pfSense release… Enjoy!
2021/02/28 Update: A year later and only now is the DHCPd fixes released with a new stable release (2.5.0), instead of the expected 2.4.x! Well, it’s “finally out there”…
2021/06/01 Update: As of time of writing, it appears that 2.5.0 and 2.5.1 are, unfortunately, bugged and I do not recommend upgrading to 2.5.0/2.5.1…
2021/07/07 Update: pfSense 2.5.2 is now released… YMMV…
