I have found that since upgrading mock on my sole builder for doing livemedia tasks that these tasks no longer work. Previously, I had mock-1.4.6-1.fc26 with no problems, but mock-1.4.8-1 causes issues.
Digging around the logs for a failed livemedia task is always fun because lots of errors are normal, but not fatal. However, I see in anaconda-anaconda.log messages to the effect that "The group 'core' is required for this installation. This group does not exist. This is a fatal error and installation will be aborted." I think the real cause though is revealed in anaconda-dnf.librepo.log where I see "Curl error (6): Couldn't resolve host name for http://hostname..." which stems from the "url --url=http://hostname..." directive in my kickstart.
I see in the release notes for 1.4.7 and 1.4.8 mentions about changes related to how network connectivity is affected in the build roots, but those seem relevant only to systemd-nspawn. Since koji-1.15 hasn't made it's way to CentOS or Fedora repos yet, I'm still on 1.14 and so necessarily have a tag option "mock.new_chroot : False" set. To overcome the issues with upgrading to mock-1.4.8, I've also tried adding "mock.rpmbuild_networking : True" and "mock.use_host_resolv : True" but still the build root is preventing name resolution.
Any ideas?