DNF (and yum) can have variables configured under /etc/dnf/vars (or /etc/yum/vars) which can be used in the rest of the configuration. Our integration of dnf in tl-setup doesn't read these so any configuration relying on them will fail to work properly.
We've had a customer report that this is apparently the case with a default installation of CentOS 8.
We have gotten hints that this issue only happens when tl-setup actually tries to install packages.
I have tested the issue on CentOS 8 and I can reproduce it, but it does not seem to be the default behaviour.
Tips for reproducing:
1. Edit one of the files that uses a url and replace the domain with a variable (e.g. $domain).
2. Create a file in /etc/dnf/vars/ with the same name as the variable (here 'domain').
3. The file should contain the variable value.
I tested the provided patch and it seems to be working.
I can verify that issue is fixed. Tested on CentOS 8 and could reproduce when adding my own variables in /etc/yum/vars.