To install xdm just follow these instructions.
sudo apt-get install xdm
Which will install xdm and any other packages on which it depends.
|Maintainer||Debian X Strike Force <firstname.lastname@example.org>|
|Description||X display manager remote machines. It provides services similar to those provided by init, getty, and login on character-based terminals: prompting for login name and password, authenticating the user, and running a session. xdm supports XDMCP|