Bug#836598:

[Tom H <tomh0665@gmail.com>]

I installed ldnsutils with

# cat /etc/apt/apt.conf.d/90test
Binary::apt-get::APT::Keep-Downloaded-Packages "0";

and I got

# ls /var/cache/apt/archives/
lock  partial
#

I installed dnsutils with

# cat /etc/apt/apt.conf.d/90test
Binary::apt-get::APT::Keep-Downloaded-Packages "1";

and I got

# ls /var/cache/apt/archives/
bind9-host_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
libbind9-140_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
libisc160_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
liblwres141_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
dnsutils_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
libdns162_1%3a9.10.3.dfsg.P4-10.1_amd64.deb
libisccc140_1%3a9.10.3.dfsg.P4-10.1_amd64.deb   lock
geoip-database_20160812-1_all.deb      libgeoip1_1.6.9-2_amd64.deb
 libisccfg140_1%3a9.10.3.dfsg.P4-10.1_amd64.deb  partial
#

So it was a question of knowing that
"Binary::apt::APT::Keep-Downloaded-Packages" controls the behavior of
apt and "Binary::apt-get::APT::Keep-Downloaded-Packages" controls the
behavior of apt-get.