Subject: DHCP server/Client behavior when options change while lease is valid

From: Peter Memishian <[email-masked]>
Date: Thu, 18 Nov 1999 19:55:37 -0500 (EST)

> If a client has a valid lease of an IP address but some of 
 > the options change (the subnet mask, for example), what 
 > should the server do when the client attempts to renew the 
 > lease. Should the server renew the lease returning 
 > the options requested (some of which may have changed) or 
 > cause the client to reinitialize.

there are a few controversial issues here.  first, while the rfc
permits the server to send a NAK to a renewing client, and permits the
renewing client to restart lease negotiation upon receipt of such a
NAK, i do not see the rfc state that the client is required to
restart.  at the very least, the NAK may have been lost in transit.
perhaps, however, the client has chosen to ignore the NAK and continue
to use the remainder of its existing lease.  so i do not see how the
server can "cause the client to reinitialize".

also, i'm of the opinion that a change in the subnet mask is similar
to a change in ip address, since the subnet mask directly affects the
client's ability to communicate using its ip address.  since it seems
to me that by granting the initial lease, the server has given the
client the right to use its ip address (which includes the related
broadcast and network addresses) for the duration of the lease, the
client's subnet mask should not be (technically) allowed to change
during that lease.  in practice, i'd question how often the subnet
mask could really change without being forced to renumber the client's 
ip addresses as well.

however, like i said, this is one man's opinion ;-)

 > Should the client check the set of options received each time a
 > lease is renewed and take appropriate action if the some of the
 > options have changed?

for portability, i'd say so.

-- 
meem

---

From: Stuart Cheshire <[email-masked]>
Date: Mon, 29 Nov 1999 21:30:44 -0500 (EST)

>Should the client check the set of options received
>each time a lease is renewed and take appropriate action
>if the some of the options have changed?

I believe that if the server administrator changes some parameter like 
the DNS server address, then it would be desirable for all client hosts, 
eventually, to start using that new DNS server. The change doesn't need 
to happen immediately, but it would be good to know that it will happen 
eventually, in some bounded about of time. If the DHCP address renewal 
time is set to 24 hours, then it would be good to know that any changed 
options will also take effect within at most 24 hours.

Given that it might be quite reasonable for DHCP client hosts to run for 
months at a time without reboots, renewing their address leases hundreds 
or thousands of times, this implies that they need to be checking the 
options returned in their renewal packets to see if anything has changed.

The Mac OS DHCP client currently does not do this. It is one of the 
things on our list of work to do.

Stuart Cheshire <[email-masked]>
 * <A HREF="http://ResComp.Stanford.EDU/~cheshire/">Web Page</A>
 * Wizard Without Portfolio, Apple Computer