[libvirt] [PATCH] network: Add support for configuring dhcp lease time

[Nehal J Wani]

On Sat, Sep 24, 2016 at 3:01 AM, Laine Stump <laine at laine.org> wrote:
> On 09/23/2016 02:43 PM, Martin Wilck wrote:
>>
>> On Fri, 2016-09-23 at 11:56 -0400, Laine Stump wrote:
>>
>>> Martin's patch tries to solve the problem with "dhcp-authoritative"
>>> which, as far as I understand, tells dnsmasq "you are the keeper of
>>> *all* lease information on this network, so if you think the address
>>> is
>>> unused, it really is unused"; dnsmasq uses this information to
>>> freely
>>> grant a guest any address it asks for, as long as there is no
>>> current
>>> lease for it. This sounds troublesome to me - If client A's lease
>>> has
>>> expired (because it's turned off for a bit), is it okay for client B
>>> to
>>> come barging in and insist on grabbing the address that client A has
>>> just lost even though there are many other addresses still
>>> available?
>>> Sure, technically it's legal, but it seems unnecessarily disruptive.
>>
>> I am not sure what you mean. What scenario would be "disrupted" by this
>> approach?
>
>
> Disruptive may not have been the best word. What I mean is that an
> unfriendly guest with proper information could take advantage of this "give
> any currently-unused address to anyone who asks for it" policy by
> purposefully taking over an IP address that it knew had previously been
> leased to a different guest (and who was expecting on getting the address
> back the next time it booted). Of course 1) it would take a lot of inside
> knowledge / good timing / luck to make this happen, and 2) you can argue
> that any guest who relied on a dhcp address remaining unchanged deserves
> what it gets (and the same for any other machine trying to contact that
> guest via the IP address).
>
>>
>> Without dhcp_authoritative, client A will not get its lease back,
>
>
> Is that true even if dnsmasq isn't restarted, and even if leasefile-ro isn't
> used? The Openstack discussion on this topic implies that the behavior only
> exists because they aren't saving a leasefile, not just because the lease is
> expired.
>
>> whether or not B applied for it in the meantime, and B wouldn't get the
>> lease, either. The user wouldn't be able to ping either one. If
>> dhcp_authoritative is used, at least one of them will get what it needs
>> (and serving both is impossible - it's hard to tell what's the Right
>> Thing in the situation you describe!).
>
>
> Well, I guess it's a policy decision, but I would tend toward *not* allowing
> B to demand a particular IP address that it has never before been given.
>
>>
>> Anyway, unless the virtual network is really crowded (in which case it
>> might make sense for the admin to use a class B network instead) the
>> probability of such clashes should be rather minimal in the real world.
>> It's much more likely that A itself tries to reacquire the lease, and
>> that situation is fixed by dhcp_authoritative.
>
>
>
> Yeah, as long as the guests are all friendly. My concern is with unfriendly
> guests.
>
>
>>
>>> I think the *real* solution is to fix the lease handling so that
>>> dnsmasq
>>> remembers leases after they've expired (assuming that can be done
>>> once
>>> leasefile-ro is set). They would be marked as "expired", and
>>> probably
>>> not even reported externally, but all their info would still be
>>> there
>>> internally for dnsmasq's use when considering what to do with
>>> incoming
>>> requests for specific IP addresses.
>>
>> Is that possible at all with the current leasehelper setup?
>
>
> "maybe"? See Nehal's recent email - we weren't notifying dnsmasq of expired
> leases, and that could be the source of the problem (or not, I didn't
> understand if he's verified whether doing so actually causes dnsmasq to
> accept the requests for expired addresses). If that works, then  the problem
> is solved. If not, then we may need to consider using dhcp-authoritative.
>

I did a small test. I compiled the SHA b55c064f of libvirt with a
small modification. I enforced the lease time to be 2mins. Then I
spawned a VM, and did the following:

$ sudo journalctl -f | grep dnsmasq-dhcp
Sep 24 19:31:13 lenovo dnsmasq-dhcp[9093]: DHCPDISCOVER(virbr1)
52:54:00:12:34:56
Sep 24 19:31:13 lenovo dnsmasq-dhcp[9093]: DHCPOFFER(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:31:13 lenovo dnsmasq-dhcp[9093]: DHCPREQUEST(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:31:13 lenovo dnsmasq-dhcp[9093]: DHCPACK(virbr1)
192.168.123.76 52:54:00:12:34:56

--- Now I pause the VM by sending 'stop' on the qemu monitor and wait.
--- As soon as lease expires, I go back to the qemu monitor and
un-pause the VM by sending 'cont' on the qemu monitor.

Sep 24 19:34:06 lenovo dnsmasq-dhcp[9093]: DHCPREQUEST(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:34:06 lenovo dnsmasq-dhcp[9093]: DHCPNAK(virbr1)
192.168.123.76 52:54:00:12:34:56 lease not found

--- I haven't restarted dnsmasq in between, so clearly, it knows about
the expired lease. But it still sends a DHCPNAK.

Sep 24 19:34:22 lenovo dnsmasq-dhcp[9093]: DHCPDISCOVER(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:34:22 lenovo dnsmasq-dhcp[9093]: DHCPOFFER(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:34:22 lenovo dnsmasq-dhcp[9093]: DHCPREQUEST(virbr1)
192.168.123.76 52:54:00:12:34:56
Sep 24 19:34:22 lenovo dnsmasq-dhcp[9093]: DHCPACK(virbr1)
192.168.123.76 52:54:00:12:34:56

--- And the VM has to send a DHCPDISCOVER again :'(

Inside the VM, the version of dhclient used by the Network Manager
was: isc-dhclient-4.3.3-P1 Gentoo-r0

So this begs the question. Does dnsmasq really do anything about the
expired lease? Or does it simply forget about it no matter what?

>>   I have to
>> admit that, being new to this discussion, I haven't understood the
>> purpose of the leasehelper yet. What benefit does it have for the
>> operation of the virtual network, compared to dnsmasq alone? Why is the
>> "custom leases file" needed at all? I tried to find a rationale on the
>> web, but I didn't.
>
>
>
> libvirt previously allowed dnsmasq to handle the lease files, but apparently
> they didn't have enough information for the APIs that report guest IP info,
> so Nehal replaced dnsmasq's automatic leasefile handling with custom
> handling that stores the extra data in addition to what dnsmasq needs. He
> can provide more info if you need it.

There was quite an amount of discussion on this dating back to 2013,
involving IRC as well as mailing list. All the discussion on the
mailing list is available at:
https://www.mail-archive.com/search?l=libvir-list%40redhat.com&q=from%3A%22Nehal%22&a=1&o=oldest
. If there are any specific queries, I am available on email/IRC.


-- 
Nehal J Wani