There's clearly not going to be any amount of time that satisfies both
concerns here.

Just to get some other options on the table, here's some things that would
allow a non-zero dhcp lease timeout _and_ address Kevin's original bug

- Just don't allow users to change their IPs without a reboot.

- Bounce the link under the VM when the IP is changed, to force the guest
to re-request a DHCP lease immediately.

- Remove the IP spoofing firewall feature  (<- my favourite, for what it's
worth. I've never liked presenting a layer2 abstraction but then forcing
specific layer3 addressing choices by default)

- Make the IP spoofing firewall allow an overlap of both old and new
addresses until the DHCP lease time is up (or the instance reboots).  Adds
some additional async tasks, but this is clearly the required solution if
we want to keep all our existing features.

On Wed Feb 04 2015 at 4:28:11 PM Aaron Rosen <> wrote:

> I believe I was the one who changed the default value of this. When we
> upgraded our internal cloud ~6k networks back then from folsom to grizzly
> we didn't account that if the dhcp-agents went offline that instances would
> give up their lease and unconfigure themselves causing an outage. Setting a
> larger value for this helps to avoid this downtime (as Brian pointed out as
> well). Personally, I wouldn't really expect my instance to automatically
> change it's ip  - I think requiring the user to reboot the instance or use
> the console to correct the ip should be good enough. Especially since this
> will help buy you shorter down time if an agent fails for a little while
> which is probably more important than having the instance change it's ip.
> Aaron
> On Tue, Feb 3, 2015 at 5:25 PM, Kevin Benton <> wrote:
>> I definitely understand the use-case of having updatable stuff and I
>> don't intend to support any proposals to strip away that functionality.
>> Brian was suggesting was to block port IP changes since it depended on DHCP
>> to deliver that information to the hosts. I was just pointing out that we
>> would need to block any API operations that resulted in different
>> information being delivered via DHCP for that approach to make sense.
>> On Tue, Feb 3, 2015 at 5:01 PM, Robert Collins <
>> > wrote:
>>> On 3 February 2015 at 00:48, Kevin Benton <> wrote:
>>> >>The only thing this discussion has convinced me of is that allowing
>>> users
>>> >> to change the fixed IP address on a neutron port leads to a bad
>>> >> user-experience.
>>> ...
>>> >>Documenting a VM reboot is necessary, or even deprecating this (you
>>> won't
>>> >> like that) are sounding better to me by the minute.
>>> >
>>> > If this is an approach you really want to go with, then we should at
>>> least
>>> > be consistent and deprecate the extra dhcp options extension (or at
>>> least
>>> > the ability to update ports' dhcp options). Updating subnet attributes
>>> like
>>> > gateway_ip, dns_nameserves, and host_routes should be thrown out as
>>> well.
>>> > All of these things depend on the DHCP server to deliver updated
>>> information
>>> > and are hindered by renewal times. Why discriminate against IP updates
>>> on a
>>> > port? A failure to receive many of those other types of changes could
>>> result
>>> > in just as severe of a connection disruption.
>>> So the reason we added the extra dhcp options extension was to support
>>> PXE booting physical machines for Nova baremetal, and then Ironic. It
>>> wasn't added for end users to use on the port, but as a generic way of
>>> supporting the specific PXE options needed - and that was done that
>>> way after discussing w/Neutron devs.
>>> We update ports for two reasons. Primarily, Ironic is HA and will move
>>> the TFTPd that boots are happening from if an Ironic node has failed.
>>> Secondly, because a non uncommon operation on physical machines is to
>>> replace broken NICs, and forcing a redeploy seemed unreasonable. The
>>> former case doesn't affect running nodes since its only consulted on
>>> reboot. The second case is by definition only possible when the NIC in
>>> question is offline (whether hotplug hardware or not).
>>> -Rob
>>> --
>>> Robert Collins <>
>>> Distinguished Technologist
>>> HP Converged Cloud
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>> --
>> Kevin Benton
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