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



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.

 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
-- 
1.9.3


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