The entire mirror build process can be performed from behind a non-transparent proxy.
This mode requires configuring the various build tools that don't observe the system's proxy environment configuration.
To configure the various build tools -
sudo su
export http_proxy=http://<proxy_host>:<proxy_port>
export https_proxy=http://<proxy_host>:<proxy_port>
. set-proxy-env.sh
The script tool automatically detects the system proxy settings and generates the configurations needed for the various build tools.