postinst modifies the permissions of 2 configuration files, pap-secrets and chap-secrets:
# install /etc/ppp/pap-secrets and /etc/ppp/chap-secrets from templates,
# if necessary
for file in pap-secrets chap-secrets; do
if [ -f /etc/ppp/$file ]; then
TAG=$(head --lines=1 /etc/ppp/$file)
if [ "$TAG" = "#GENERATED-BY-DEBIAN-INSTALLER#" ]; then
(sed -e "s/-HOSTNAME-/$HOST/g" /usr/share/ppp/$file; \
cat /etc/ppp/$file) > /etc/ppp/$file.tmp
mv /etc/ppp/$file.tmp /etc/ppp/$file
fi
else
sed -e "s/-HOSTNAME-/$HOST/g" < /usr/share/ppp/$file > /etc/ppp/$file
fi
chmod 600 /etc/ppp/$file
done
The permission change at the last line is always performed, even on upgrade. This causes /etc/ppp/chap-secrets to go back to 0600 even if the administrator has set different permissions.