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From: Grant Taylor <gtaylor@tnetconsulting.net>
Newsgroups: comp.os.linux.misc
Subject: Re: I never thought of this scenario
Date: Mon, 8 Apr 2024 23:27:28 -0500
Organization: TNet Consulting
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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. . . .