sources.list:

deb http://mirror.ox.ac.uk/debian stable main contrib non-free

#deb http://emma:9999/debian stable main contrib non-free
deb http://security.debian.org/debian-security stable/updates main contrib non-free


for example:

emma:/home/phlax# apt-get install shorewall
Reading package lists... Done
Building dependency tree... Done
Package shorewall is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package shorewall has no installation candidate