preseed & security updates protocol

[Jaime Ventura <jaimeventura@ipp.pt>]

Hello
   Im trying to create a preseed file to network install some machines 
using a FTP local mirror of debian & debian-security(mirrored on 
192.168.2.1 and available via FTP only).
   How do i choose the protocol for updates as being ftp?
   I've tried
   apt-setup/uri_type   select   ftp
   apt-setup/security_host       192.168.2.1

   but install complains about not reaching http://192.168.2.1
   Thanks,
   Jaime