[CentOS] "installation source" specification for netinstall

Trying to specify the "installation source" in the configuration of 
netinstall for centos 7 (7.1).  Three places on the web said

mirror.centos.org/centos/7/os/x86_64/

But that configuration page probes, then it says, "Error setting up base 
repository".

What's the magic needed?

Also, if anyone knows specs for epel and others, they might help too.

Thanks.