Since older versions of Debian Linux are supported for a limited time only, we are forced to update all Linux systems at regular intervals.
Debian Releases (→ DebianMirror):
Debian 10 Buster
Debian 9 Stretch Upstream release in preparation (frozen since 5th Feb 2017, under evaluation)
Debian 8 Jessie Upstream release 25/4/2015, currently GSI default
Debian 7 Wheezy (Regular support until May 2016, long term support until 31 May 2018)