[openstack-dev] [neutron] high dhcp lease times in neutron deployments considered harmful (or not???)

Salvatore Orlando sorlando at nicira.com
Wed Jan 28 20:41:50 UTC 2015

On 28 January 2015 at 20:19, Brian Haley <brian.haley at hp.com> wrote:

> Hi Kevin,
> On 01/28/2015 03:50 AM, Kevin Benton wrote:
> > Hi,
> >
> > Approximately a year and a half ago, the default DHCP lease time in
> Neutron was
> > increased from 120 seconds to 86400 seconds.[1] This was done with the
> goal of
> > reducing DHCP traffic with very little discussion (based on what I can
> see in
> > the review and bug report). While it it does indeed reduce DHCP traffic,
> I don't
> > think any bug reports were filed showing that a 120 second lease time
> resulted
> > in too much traffic or that a jump all of the way to 86400 seconds was
> required
> > instead of a value in the same order of magnitude.
> >
> > Why does this matter?
> >
> > Neutron ports can be updated with a new IP address from the same subnet
> or
> > another subnet on the same network. The port update will result in
> anti-spoofing
> > iptables rule changes that immediately stop the old IP address from
> working on
> > the host. This means the host is unreachable for 0-12 hours based on the
> current
> > default lease time without manual intervention[2] (assuming half-lease
> length
> > DHCP renewal attempts).
> So I'll first comment on the problem.  You're essentially "pulling the
> rug" out
> from under these VMs by changing their IP (and that of their router and
> server), but you expect they should fail quickly and come right back
> online.  In
> a non-Neutron environment wouldn't the IT person that did this need some
> pretty
> good heat-resistant pants for all the flames from pissed-off users?  Sure,
> the
> guy on his laptop will just bounce the connection, but servers (aka VMs)
> should
> stay pretty static.  VMs are servers (and cows according to some).

I actually expect this kind operation to not be one Neutron users will do
very often, mostly because regardless of whether you're in the cloud or
not, you'd still need to wear those heat resistant pants.

> The correct solution is to be able to renumber the network so there is no
> issue
> with the anti-spoofing rules dropping packets, or the VMs having an
> unreachable
> IP address, but that's a much bigger nut to crack.

Indeed. In my opinion the "update IP" operation sets false expectations in
users. I have considered disallowing PUT on fixed_ips in the past but that
did not go ahead because there were users leveraging it.

> > Why is this on the mailing list?
> >
> > In an attempt to make the VMs usable in a much shorter timeframe
> following a
> > Neutron port address change, I submitted a patch to reduce the default
> > lease time to 8 minutes.[3] However, this was upsetting to several
> people,[4] so
> > it was suggested I bring this discussion to the mailing list. The
> following are
> > the high-level concerns followed by my responses:
> >
> >   * 8 minutes is arbitrary
> >       o Yes, but it's no more arbitrary than 1440 minutes. I picked it
> as an
> >         interval because it is still 4 times larger than the last short
> value,
> >         but it still allows VMs to regain connectivity in <5 minutes in
> the
> >         event their IP is changed. If someone has a good suggestion for
> another
> >         interval based on known dnsmasq QPS limits or some other
> quantitative
> >         reason, please chime in here.
> We run 48 hours as the default in our public cloud, and I did some digging
> to
> remind myself of the multiple reasons:
> 1. Too much DHCP traffic.  Sure, only that initial request is broadcast,
> but
> dnsmasq is very verbose and loves writing to syslog for everything it does
> -
> less is more.  Do a scale test with 10K VMs and you'll quickly find out a
> large
> portion of traffic is DHCP RENEWs, and syslog is huge.

This is correct, and something I overlooked in my previous post.
Nevertheless I still think that it is really impossible to find an optimal
default which is regarded as such by every user. The current default has
been chosen mostly for the reason you explain below, and I don't see a
strong reason for changing it.

> 2. During a control-plane upgrade or outage, having a short DHCP lease
> time will
> take all your VMs offline.  The old value of 2 minutes is not a realistic
> value
> for an upgrade, and I don't think 8 minutes is much better.  Yes, when
> DHCP is
> down you can't boot a new VM, but as long as customers can get to their
> existing
> VMs they're pretty happy and won't scream bloody murder.

In our cloud we were continuously hit bit this. We could not take our dhcp
agents out, otherwise all VMs would lose their leases, unless the downtime
of the agent was very brief.

> There's probably more, but those were the top two, with #2 being most
> important.

Summarizing, I think that Kevin is exposing a real, albeit well-know
problem (sorry about my dhcp release faux pas - I can use jet lag as a
justification!), and he's proposing a mitigation to it. On the other hand,
this mitigation, as Brian explains, is going to cause real operational
issues. Still, we're arguing on the a default value for a configuration
parameter. I therefore think the best thing that we can do is explicitly
stating what happens when setting long or short lease times.
I expected this to be documented in [1], but it's not. I think that place
and neutron.conf might contain this kind of documentation, such as:

# DHCP Lease duration (in seconds).
# Use -1 to tell dnsmasq to use infinite lease times.
# dhcp_lease_duration = 86400
# Note that long DHCP leases will result in delays
# in instances acquiring updated IP addresses. This
# may result in downtime for those instance as anti
# spoof policy will then block all traffic in and out of
# them. In order to minimise this downtime window
# the lease time should be shorter, for example
# dhcp_lease_duration = 480

However, I would not change the current system default, as this might
affect operational systems.

Apologies again for my stupid dhcp-release note,

[1] http://developer.openstack.org/api-ref-networking-v2.html

> >   * other datacenters use long lease times
> >       o This is true, but it's not really a valid comparison. In most
> regular
> >         datacenters, updating a static DHCP lease has no effect on the
> data
> >         plane so it doesn't matter that the client doesn't react for
> hours/days
> >         (even with DHCP snooping enabled). However, in Neutron's case,
> the
> >         security groups are immediately updated so all traffic using the
> old
> >         address is blocked.
> Yes, and choosing the lease time is a deployment decision that needs to
> take a
> lot of things into account.  Like I said, we don't even use the default.
> The
> default should just be a good guess for a standard deployment, not a value
> that
> caters towards the edge cases, especially when the value is tunable in
> neutron.conf.
> >   * dhcp traffic is scary because it's broadcast
> >       o ARP traffic is also broadcast and many clients will expire
> entries every
> >         5-10 minutes and re-ARP. L2population may be used to prevent ARP
> >         propagation, so the comparison between DHCP and ARP isn't always
> >         relevant here.
> I don't recall anyone being scared of broadcast, and can't find any
> comments
> regarding it in https://review.openstack.org/#/c/150595/
> > Please reply back with your opinions/anecdotes/data related to short
> DHCP lease
> > times.
> I can only speculate on why 24 hours was chosen as the default back in
> 2013,
> possibly because a lot of wireless router firmware defaults are set as
> such?
> > 1.
> https://github.com/openstack/neutron/commit/d9832282cf656b162c51afdefb830dacab72defe
> > 2. Manual intervention could be an instance reboot, a dhcp client
> invocation via
> > the console, or a delayed invocation right before the update. (all
> significantly
> > more difficult to script than a simple update of a port's IP via the
> API).
> > 3. https://review.openstack.org/#/c/150595/
> > 4. http://i.imgur.com/xtvatkP.jpg
> I was a much bigger baby than that :)
> -Brian
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