After the centos 6.2 update I noticed libvirt was not running on some servers. Looking at the logs I found
Starting libvirtd daemon: 10:35:16.697: 6933: info : libvirt version: 0.9.4, package: 23.el6_2.1 (CentOS BuildSystem <http://bugs.centos.org>, 2011-12-17-16:39:59, c6b4.bsys.dev.centos.org)
10:35:16.697: 6933: error : virNetServerMDNSStart:460 : internal error Failed to create mDNS client: Daemon not running
Further investigation found avahi was needed for this to work. The final fix was running
yum -y install avahi /etc/init.d/messagebus restart /etc/init.d/avahi-daemon restart /etc/init.d/libvirtd restart /sbin/chkconfig messagebus on /sbin/chkconfig avahi-daemon on
In centos 6.2 restarting libvirt will not restart the vm’s. Once done libvirt was running again.