d-i mirror/country string manual
d-i mirror/http/hostname string ca.archive.ubuntu.com
d-i mirror/http/directory string /ubuntu
-d-i mirror/http/proxy string http://artifacts.internal.efficios.com:3128
# Alternatively: by default, the installer uses CC.archive.ubuntu.com where
# CC is the ISO-3166-2 code for the selected country. You can preseed this