Re: I never thought of this scenario

On 20.04.2024 um 15:13 Uhr Grant Taylor wrote:

On 4/20/24 14:07, Rich wrote:

Yes, correct.  However, that is not "DHCP" the protocol itself
specifying such.  That is the IP layer specifying that certian
addresses used in UDP packets are not routed. 

 
Nit-pick:  That's the IP layer saying that certain addresses can't be
routed.  It doesn't matter what transport is on top of those IP
packets.
 

The reason it impacts DHCP is that the "bootstrap an IP address
configuration" portion of DHCP means that those addresses are all
the client can make use of until after it has been configured with
a valid IP address for the local subnet. 

 
Nit-pick:  I believe I've read about DHCP implementations that
remember what they used last time and will start to use them instead
of 0.0.0.0. If that remembered IP fails / is rejected for some reason
then it falls back to a discover.
 
RFC 951 - Bootstrap Protocol - the precursor to DHCP - section 3
paragraph 2 says:  "In the IP header of a bootrequest, the client
fills in its own IP source address if known, otherwise zero.  When
the server address is unknown, the IP destination address will be the
'broadcast address' 255.255.255.255."
 
So I'm not sure that a client /must/ use 0.0.0.0 -> 255.255.255.255
when doing a BOOTP / DHCP request.

How can a DHCP client assume that a remembered address can be still
used?
If the lease is still valid, it can use it and contact the server via
unicast, if there is no valid lease, it must not use the IP anymore
because it could be assigned to some other node.

IIRC IP unicast src addr can only be used if the lease is still valid
and the client wants to extend the lease.
Of course asking the DHCP for additional information after the IP has
been assigned is another situation where unicast can be used.

Does anybody know other situations?

--
kind regards
Marco

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