Freebsd, dhcp, and an epiphany

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So I’ve been experimenting with the BSDs lately, and one of the things I’ve adopted here is pfSense as a DHCP server and firewall. The other day when I was setting up static reservations, I was trying to do what your home router ordinarily will let you do, which is say your DHCP range is 192.168.1.2 through 192.168.1.50 … I can assign a device a reservation of 192.168.1.2 and it’ll just hand me that IP every time… I also have the option of putting an IP outside that range, and then I have to go in and set it manually, because DHCP won’t touch it to avoid collision.

pfSense doesn’t let you do that, it’ll error every single time, and the reason is because your home router/adguard/pihole kind of “covers for the fallacy” for the consumer. What you’re really doing is setting a static IP, there is no such thing as a “DHCP reservation” technically. That blew my mind. If you don’t want DHCP handing you what IT thinks you should get, you better be setting addresses outside the range .. which is basically a static IP.

I had always wondered about that .. what is the true difference between a static IP and a DHCP reservation? In theory, DHCP reservations should be the default method of assigning addresses, because it’s all benefits with no loss right? You don’t have to configure IPs at the device level anymore, and it hands you the same thing every time so no dynamic change. But this made me realize I’ve been drinking the koolaid with these consumer grade routers/DHCP servers for the last idk how long. Really took a minute for me to wrap my head around that one.

Any professional network administrator that configures Cisco and Broadcom and Brocade is probably scratching his head reading this going “way to state the obvious, asshole” … but I guess I just graduated into big boy networking. so go me.

Just figured I’d share.

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The reservation , to my mind, simply stops DHCP from allocating an IP at random from a range for a particular machine. For a reservation , the range is one address.

You are asking how is that different from allocating a static address from outside the dhcp range?
When a network manager (ie a human) allocates a static IP, from the machine’s point of view the effect is the same, but from outside the machine it is a matter of who/what is originating the allocation.

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That DHCP reservation feature sure seems useful to me, even if it isn’t an official thing. I use it all the time. If you want to expose one of your computers on a port, you don’t want the internal IP changing on you. Yes, you could assign one statically, but it sure is nice to have them all managed in one spot and have all clients receive their IP via DHCP.

Not sure why this isn’t a “real” feature.

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