[geeks] Airport Express arrived yesterday!

[Mike Meredith]

On Sun, 18 Jul 2004 17:52:08 +1000, Scott Howard wrote:
> On Sat, Jul 17, 2004 at 08:12:15PM +0100, Mike Meredith wrote:
> > Some ISPs operate 'DHCP' servers out of spec to avoid customers
> > thinking they own a particular IP address. Strictly speaking these
> > aren't DHCP servers at all.
> 
> Pretty much wrong on both accounts.

Could be; however ...

> than a DHCPREQUEST/INIT-REBOOT request. With a DHCPDISCOVER the server
> is free to give the client whatever address it wants - even if there
> is an existing lease for the client. By default, most servers will
> attempt to give the same address as the client previously had, but
> there's no requirement for it to.

Actually the RFC states (for replying to a DHCPDISCOVER request) :-

'If an address is available, the new address SHOULD be chosen as
 follows:

      o The client's current address as recorded in the client's current
        binding, ELSE

      o The client's previous address as recorded in the client's (now
        expired or released) binding, if that address is in the server's
        pool of available addresses and not already allocated, ELSE' 

I've left out the other two options which correspond to assigning a
new address. This is section 4.3.1. And an RFC 'SHOULD' means you'd
better have a damm good reason for doing anything else. An earlier
section implies that re-assigning addresses that have been used is to be
avoided unless IP addresses are a scarce resource.  

> eg, the section of the RFC on renewing leases says :
> " A client MAY choose to renew or extend its lease prior to T1. The
> server MAY choose to extend the client's lease according to policy set
> by the network administrator. The server SHOULD return T1 and T2, and
> their values SHOULD be adjusted from their original values to take
> account of the time remaining on the lease."

A server might choose not to renew a lease because it has discovered
that the IP address associated with the lease is in use, or because the
client is requesting a now invalid lease (if for example the client has
moved to a different subnet). I don't believe a server is supposed to
NAK a lease renewal for trivial reasons.

> So even if the ADSl/cable router is trying to renew an existing lease
> (and like I said, most of them dont) then the server isn't really

A DHCP client that won't do renewals is severely broken ... a lease
length could be extremely short and a DHCP client has to expect to renew
a lease whilst network traffic is taking place.