How best to secure admin directory?

[Leonardo Crespo]

Morning guys!

Can anyone help with best practice to secure an /portal/admin
directory? A link to documentation will do just fine.

Our website has and /admin directory which don't want to be accessible
via www.domain.com/portal/admin. Instead, we want hackoff.domain.com
to point to /portal/admin directory. If we could add authentication to
hackoff.domain.com would be great too.

The point is some files on www.domain.com/* include (php include)
files on /portal/admin, so this has to work internally.

I searched the wiki and this website https://calomel.org/nginx.html
(which is great too) but I couldn't figure out a way to do this.

Any ideas are very welcomed.

Cheers!