apt の source.list の正当性はどのようにチェックされている？
ubuntu 14.04 `/etc/apt/sources.list`
# See for how to upgrade to
# newer versions of the distribution.
deb trusty main
deb-src trusty main
## Major bug fix updates produced after the final release of the
## distribution.
deb trusty-updates main
deb-src trusty-updates main
apt http http MitM
apt