Re: I never thought of this scenario

On 4/8/24 22:56, Lawrence D'Oliveiro wrote:

It works specifically on 169.254 addresses, because those are the only
kind you can use without a proper IP configuration.

The DHCP protocol doesn't operate any differently for 169.254.0.0/16 vs 192.168.0.0/16 vs 203.0.113.0/24.
169.254.0.0/16 is actually not part of the DHCP protocol. 169.254.0.0/16 is an IPv4 network reserved for link-local addresses, often called Automatic Private IP Addressing (APIPA).  This is a convention of DHCP /client/ implementations when the DHCP protocol fails to successfully obtain a lease.
But the DHCP protocol operates the same independent of the prefix it's being used for.

They are both non-routable.

Nope.  DHCP *IS* routable.  When you use DHCP relay agents, they use the DHCP protocol to talk to the DHCP server.  They do so from known / configured IP addresses and they are PERFECTLY HAPPY to send DHCP requests through a routed IP network.

You might say that DHCP is technically layer 3, but it is restricted to the domain of layer 2.

No it is not.
The 0.0.0.0 source IP address and 255.255.255.255 destination IP address are what's supposed to be not routed.  The DHCP protocol has no problem using other IPs and is perfectly happy to be routed.
In fact, I believe that a DHCP client that used a DHCP relay helper to obtain it's release originally is perfectly capable of communicating with a remote DHCP server itself w/o the use of the helper.
DHCP uses the UDP protocol which uses the IP protocol and is routable.
I agree that DHCP is almost never routed.  But lack of doing something doesn't mean that it's not possible to do it.
--
Grant. . . .