[libvirt] [PATCH RFC] network: try to eliminate default network conflict during package install

[Cole Robinson]

On 09/10/2014 01:54 PM, Laine Stump wrote:
> Sometimes libvirt is installed on a host that is already using the
> network 192.168.122.0/24. If the libvirt-daemon-config-network package
> is installed, this creates a conflict, since that package has been
> hard-coded to create a virtual network that also uses
> 192.168.122.0/24. In the past libvirt has attempted to warn of /
> remediate this situation by checking for conflicting routes when the
> network is started, but it turns out that isn't always useful (for
> example in the case that the *other* interface/network creating the
> conflict hasn't yet been started at the time libvirtd start its owm
> networks).
> 
> This patch attempts to catch the problem earlier - at install
> time. During the %post install for libvirt-daemon-config-network, we
> look through the output of "ip route show" for a route that exactly
> matches 192.1 68.122.0/24, and if found we search for a similar route
> that *doesn't* match (e.g. 192.168.123.0/24). When we find an
> available route, we just replace all occurences of "122" in the
> default.xml that is being created with ${new_sub}. This could
> obviously be made more complicated - automatically determine the
> existing network address and mask from examining the template
> default.xml, etc, but this scripting is simpler and gets the job done
> as long as we continue to use 192.168.122.0/24 in the template. (If
> anyone with mad bash skillz wants to suggest something to do that, by
> all means please do).
> 
> This is intended to at least "further reduce" the problems detailed in:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=811967
> 
> I acknowledge that it doesn't help for cases of pre-built cloud images
> (or live images that are created on real hardware and then run in a
> virtual machine), but it should at least eliminate the troubles
> encountered by individuals doing one-off installs (and could be used
> to stifle complaints for live images, as long as libvirtd was running
> on the system where the live image compose took place (or the compose
> was itself done in a virtual machine that had a 192.168.122.0/24
> interface address).
> ---
> 
> The question here is: "Will this help some people's situation without
> causing new problems for anyone else?" I wouldn't mind pushing this
> patch, but also wouldn't mind if it was just the catalyst for
> discussion that leads to a better solution. We do need *some kind* of
> solution though, as more and more people are installing OSes that
> include the libvirt package in virtual machines, and are running into
> this problem with increasing frequency.
> 

Agree with all the above FWIW. This isn't the perfect 'fix' but no one has
come up with one yet. So I say we give this a spin in rawhide/f21 and see how
it works out.

>  libvirt.spec.in | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/libvirt.spec.in b/libvirt.spec.in
> index a6a58cf..539d9ef 100644
> --- a/libvirt.spec.in
> +++ b/libvirt.spec.in
> @@ -1728,8 +1728,32 @@ fi
>      %if %{with_network}
>  %post daemon-config-network
>  if test $1 -eq 1 && test ! -f %{_sysconfdir}/libvirt/qemu/networks/default.xml ; then
> +    # see if the network used by default network creates a conflict,
> +    # and try to resolve it
> +    orig_sub=122
> +    sub=${orig_sub}
> +    net=192.168.${sub}.0/24
> +    routes=$(ip route show | cut -d' ' -f1)
> +    for route in $routes; do
> +      if [ "${net}" = "${route}" ]; then
> +        # there was a match, so we need to look for an unused subnet
> +        for new_sub in $(seq 123 254); do
> +          new_net="192.168.${new_sub}.0/24"
> +          usable=yes
> +          for route in ${routes}; do
> +            [ "${new_net}" = "${route}" ] && usable=no
> +          done
> +          if [ "${usable}" = "yes" ]; then
> +            sub=${new_sub}
> +            break;
> +          fi
> +        done
> +      fi
> +    done
> +
>      UUID=`/usr/bin/uuidgen`
> -    sed -e "s,</name>,</name>\n  <uuid>$UUID</uuid>," \
> +    sed -e "s/${orig_sub}/${sub}/g" \
> +        -e "s,</name>,</name>\n  <uuid>$UUID</uuid>," \
>           < %{_datadir}/libvirt/networks/default.xml \
>           > %{_sysconfdir}/libvirt/qemu/networks/default.xml
>      ln -s ../default.xml %{_sysconfdir}/libvirt/qemu/networks/autostart/default.xml
> 

My shell is pretty weak, but that looks okay to me. ACK. But I'd feel more
comfortable if Eric checked it out :)

Thanks,
Cole