Re: [dhcwg] DDNS-DHCP [6]: Relationship between DNS TTL and DHCP lease length

On Thursday 19 June 2003 06:19, Robert Elz wrote:
> That is, reworded, to make sure my understanding is correct, the
> problem to be avoided is having a name referring to an address that
> is now to be assigned to a different name ?
>
> Who cares?

Let's say machine X has an SMTP listener, and machine Y also has an SMTP 
listener.   Machine X gets an IP address, Z, from the DHCP server.   Then the 
owner of Machine X wanders away, leaving the lease active.   The lease 
expires, and then machine Y gets the address.   But there is still an A 
record for machine X pointing at IP address Z.   So now, machine Q connects 
to Z because of that A record, and tries to drop mail for X on Y.   Y will 
either bounce it immediately, or bounce it after it notices that the A record 
for X is pointing at it.   So we'd really like it if the time that the A 
record goes away and the time that the lease goes away are fairly close 
together, so that the chances of this happening are slim.

Of course, I would say that this is a broken configuration anyway - you really 
want to use protocols that verify who they're talking to if you have a mobile 
computer.   But that's the basis for caring about this sort of thing, and 
while I don't think we can completely solve the problem, it's worth setting 
things up to minimize the damage that occurs in cases like this.

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