I created a simple script to build a live image inside mock. When running it in a Fedora 25 mock environment this works fine (building a Fedora 25 image). The same thing running against Fedora 26 (building a 26 image) is failing however because the metalink URLs are not parsed correctly.

It seems that in both cases the generated kickstart files look almost identical, i.e. a repo defined like:
> repo --name="fedora" --mirrorlist=http://mirrors.fedoraproject.org/metalink?repo=fedora-$releasever&arch=$basearch

This works fine on Fedora 25 but for Fedora 26 the installer interprets the metalink file as mirrorlist and uses the lines of the XML document to build URLs. Simply replacing "metalink" with "mirrorlist" in the kickstart file works around the issue.


In the mock environment I installed the following packages:
 * lorax-lmc-novirt
 * git (to grab https://pagure.io/fedora-kickstarts.git)
 * vim-minimal
 * pykickstart