Ubuntu: 解决 “Require Installation Of Untrusted Packages”

1. Write in a terminal:

   sudo apt-get update && sudo apt-get upgrade

     or edit Update Manager settings (try this first if you have already configured Update Manager).

   

2. Launch Update Manager and try again.

   

Running update through a terminal gives option whether to allow or disallow untrusted packages.

WARNING: The following packages cannot be authenticated!
  playonlinux
Install these packages without verification [y/N]?

If you trust the package source, type y.

原文：http://askubuntu.com/a/117097