postinst is in libpam-winbind 2:3.6.25-0ubuntu0.12.04.10.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #!/bin/sh
set -e
pam-auth-update --package
|