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Re: [libvirt] Plan A or Plan B?
- From: Gene Czarcinski <gene czarc net>
- To: libvir-list redhat com
- Cc: Laine Stump <laine laine org>
- Subject: Re: [libvirt] Plan A or Plan B?
- Date: Mon, 19 Nov 2012 13:50:18 -0500
SInce I wrote this, I have "Plan B" about a half to two-thirds complete
as far as the coding goes. I have this "feeling" that postponing the
conf-file changes might be better.
On 11/19/2012 01:06 PM, Laine Stump wrote:
On 11/19/2012 08:31 AM, Gene Czarcinski wrote:
This consists of the set of patches I have submitted for conf-file,
DHCPv6, etc. The response has been a resounding SILENCE. Thus, I am
assuming that there is some reluctance to adopting these changes in
their current form. This especially includes the conf-file changes.
While I believe that changing dnsmasq parameters from the command-line
to a configuration file makes a great deal of sense and might have
been the approach if this was being done today, there is resistance to
At least on my part, you're misinterpreting the lack of response as
reluctance to change, when it really is just due to a lack of time to
draft a response.
Understood and I expected that this might be a big part of it.
Since you first brought this up, I have dropped the conf-dir= because
what you said made sense.
I have no problems with switching to using a conf file (although, as
I've said a couple of times, pointing to a conf *directory* which could
contain admin-supplied additions to the configuration is unlikely to be
accepted - we do need an alternative to that, though; I think the method
most likely to succeed would be to add a private dnsmasq:commandline
namespace, as we've already done for qemu).
Yes, this does look like a much better approach. I am not sure how to
do it yet but this makes a lot more sense that the conf-dir or a
Here is an open BZ that mentions the idea of adding similar
functionality for dnsmasq (see comment 8, in particular):
I took a look at what comes out of --help and there is going to be
nothing pretty about extracting info from it.
This is "the right way" to do this, since it makes sure that all
configuration information (including for knobs that we don't support) is
available in one place.
One of the things that's actually taken up some of my time in the last
few days is that I'm looking at an open CVE about dnsmasq and its use in
Fixing this also requires changing the dnsmasq commandline dependent
upon the version of dnsmasq running on the host. In this case, though,
we can't rely on the version number, but need to check for presence of a
particular option in the output of "dnsmasq --help" (this is because
there will be backports of the new dnsmasq option to older versions of
dnsmasq on various distros that can't/won't upgrade to a new release,
but still require the CVE fix, and yet we can't completely refuse to run
if someone hasn't upgraded their dnsmasq, because the problem doesn't
occur in the majority of configurations).
I wonder if Simon Kelley would be receptive to adding a "special"
dnsmasq interface which would provide more information and in a
structured manner to make it easy to be processed by other software.
Done. But, if you want to "reverse the order," it appears that it will
not be that difficult.
Since there will be conflicts with your changes, and the libvirt fixes
also will need to be backported to older versions of libvirt, the CVE
fix will need to be pushed before your patches (in order to make the
backport patch as similar to the upstream patch as possible)
In the meantime, this bug has shown that we really need a
general-purpose "capabilities" mechanism for dnsmasq similar to what we
have for qemu, so I'm making a trimmed down version of that.
This is my top priority right now, so hang on for a couple days and I'll
see how much conflict it creates with your patches (one thing is that
the mechanism for getting the dnsmasq version will be different).
So before you go re-arranging all your patches, give me a couple days to
work out the patch for the CVE and see how it sits with your current
I have looked over the code as well as done some testing (the code is
all in network/bridge_driver.c). Unless there really is an IPv6 address
OK, so here is Plan B:
I am removing the conf-file and related changes. They will be
repackaged and resubmitted at some later time. The new multi-file
patch will focus on "IPV6 Enhanced Support" which will consist of the
1. In a manner similar to what is done for IPV6, add ip6tables rules
to permit virtual systems to communicate via a defined virtual
interface which has no gateway addresses defined. This does mean that
virtual systems will not be able to communicate with the host via this
interface ... only with each other. Also, the following must be:
net.ipv6.conf.virbr19.disable_ipv6 = 1
so that the kernel does not start anything.
This discussion was left open at the end - Dan, do you see any problem
with adding the rules permitting IPv6 traffic between the guests as long
as the host has disable_ipv6 set? Or will we still need to add an
"ipv6='yes'" attribute to the toplevel <network> element?
This implements IPv6 functionality currently available for IPv4.
Documentation will be added to explain the functionality for both IPv4
[BTW, the only place I have found to add documentaiotn is in the
"docs/formatnetwork.html.in" file. If there are other files I should
be updating, please enlighten me.]
If you can find a good place in the wiki to expand on what's in
formatnetwork, that would be good, but it's a separate process (since
the wiki isn't stored in git).
2. Add code to get the dnsmasq version and save that information. The
added code described below will require a dnsmasq version greater than
or equal to 2.64. Documentation will be updated to state that dnsmasq
= 2.64 is required for DHCPv6.
3. Implement support for DHCPv6. Most of this is already done with
the existing patch. However, this refits the code to work with
command-line parameters AND adds a check for dnsmasq >= 2.64.
Naturally, tests and documentation will be updated.
4. If dnsmasq >= 2.64, do not use radvd but instead use dnsmasq to
support the RA service for both state-full and state-less IPV6.
a) If dnsmasq < 2.64, just ignore dhcp-range or dhcp-host definitions
No. It must log an UNSUPPORTED error and fail. This should be done at
the time the network is started/created though, not when it is defined,
since the dnsmasq version may change in the interim.
b) issue error message and stop dnsmasq startup if dhcp-range or
dhcp-host is specified.
I have been assuming this but wanted to mention the alternative.
The network object was "handy." ;) This version stuff should be done
once when libvirtd starts up.
c) Currently, tests for valid dhcp specifications is only done when a
network is started (all significant changes are to
"network/bridge_driver.c". This situation could continue. Thus,
virsh could be used to specify dhcp-range and dhcp-host but it would
not work if the network was started.
Yes. Although I'm creating a small "dnsmasqCapabilities" API where you
will be able to get this information (btw, I don't think the version
should be stored in the network object, but instead be retrieved
directly from the API (we may want to cache the info somewhere globally
in situations such as a mass-restart of all networks).
I prefer to do the check when the network is started because there just
might be something else using the code in conf/network.c. However, I did
need to add some code the conf/network.c just to make things work. For
example, with dhcp-host definitions, the only "reasonable" identifier
IPv6 is the system name since the mac-address is not defined in IPv6 ...
and it does work.
d) Move the dnsmasq version checks back into when the network is
defined. This would be a bit "trickier" to implement properly since
the same code is used by multiple network types and not just that
supported by "network/bridge_driver.c". If this is the approach, I
will need some guidance as to modifying "conf/network.c".
No. If something requires support in libvirt itself that is lacking
(e.g. different code needs to be compiled in, or required code isn't yet
written), you can fail during network define, but if it requires support
in an external binary that may or may not have the support, you need to
wait until you're actually going to start it, since that support may be
added or removed at any time.
OK folks. I need some input here. I realize that all of you are very
busy working your own interests but I need a little time from someone
with "commit" authority to say "go ahead" or "get lost".
Again, I would say wait for a few days. Hopefully before we take off for
the holiday on Thursday I'll have a patch that adds dnsmasqCapabilities
functions and uses that to conditionally enable --bind-dynamic (as
required for the CVE mentioned above) and we can see how disruptive that
is to your current patches.
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