What does “The following packages have been kept back” mean?

Question

When doing an apt-get upgrade I sometimes get a message saying "The following packages have been kept back". For example:

$ sudo apt-get upgrade                                                                                                
Reading package lists... Done                                                                                                          
Building dependency tree                                                                                                               
Reading state information... Done                                                                                                      
The following packages have been kept back:                                                                                            
  linux-headers-server linux-image-server linux-server                                                                                 
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.

What does this mean exactly? Obviously the packages have been held back and not installed, but why?

The follow-on question would be: how does one upgrade these kept back packages?

Answer 1

Basic report is that "apt-get upgrade" will not upgrade packages for-which the package-manager would like to delete and reinstall the package. This happens in security patches a lot because, often, it was actually libraries precursor to the final distributable compilation which were patched. The "go ahead and delete things," functionality lives in "apt-get dist-upgrade" but-be-ware, be sure to run "apt-get update" first just in case the upstream guys figured something out since you last tried. Sometimes they miss things and fix them that fast.