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From: The Natural Philosopher <tnp@invalid.invalid>
Newsgroups: comp.os.linux.misc
Subject: Re: I never thought of this scenario
Date: Sun, 21 Apr 2024 11:10:49 +0100
Organization: A little, after lunch
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On 20/04/2024 21:16, Grant Taylor wrote:
> On 4/20/24 14:19, Rich wrote:
>> DHCP the protocol is itself is not routable -- because DHCP the 
>> protocol is not a transport layer protocol.  It relies upon UDP for 
>> its transport.
> 
> Given that logic, HTTP(S) and NNTP(S), both of which are dependent on 
> TCP, which is dependent on IP, aren't routable either.
> 
>> And whether a given DHCP message is routed or not is wholly dependent 
>> upon whether the UDP packet carrying the DHCP message is itself routable.
> 
> I believe you want to go another layer and say that the UDP datagram is 
> dependent on the IP packet carrying it.  And the IP packet's routability 
> is dependent on it's source IP and if there is a route to the 
> destination IP or not.
> 
> }:-)
> 
Is a destination address of 255.255.255.255 routable?
Of course it is, if a router is configured to do an 'all stations 
broadcast' across the internet!

What changes is that the client which has no IP address at this stage, 
instead has to be given one by the first router it encounters. When the 
router receives a response it has to translate that back to the MAC 
address of the sender  on its local port.

It's not much different from address translation, in that the router 
needs to exercise intelligence about some packet contents, rather  than 
juts their source and destination  and next hop addresses.

AS far as intelligence goes its nothing like as complex as running BGP 
or OSPF or  other routing protocols.

Calling this action a 'relay agent' makes it all into something it is 
not - a separate addition to routers in general.  DHCP can be and is 
routed by routers.

The rest is semantics


-- 
"What do you think about Gay Marriage?"
"I don't."
"Don't what?"
"Think about Gay Marriage."