直接接続を使用する場合
root@cc90861af3e4:/etc/apt/apt.conf.d# apt-get update
Hit http://security.debian.org jessie/updates InRelease
Ign http://deb.debian.org jessie InRelease
Hit http://deb.debian.org jessie-updates InRelease
Hit http://deb.debian.org jessie Release.gpg
Hit http://deb.debian.org jessie Release
Get:1 http://security.debian.org jessie/updates/main amd64 Packages [641 kB]
Get:2 http://deb.debian.org jessie-updates/main amd64 Packages [23.1 kB]
Get:3 http://deb.debian.org jessie/main amd64 Packages [9064 kB]
Fetched 9729 kB in 21s (448 kB/s)
Reading package lists... Done
httpプロキシを使用している場合
root@cc90861af3e4:/etc/apt/apt.conf.d# apt-get update
Hit http://security.debian.org jessie/updates InRelease
Ign http://deb.debian.org jessie InRelease
Hit http://deb.debian.org jessie-updates InRelease
Get:1 http://security.debian.org jessie/updates/main amd64 Packages [641 kB]
Hit http://deb.debian.org jessie Release.gpg
Get:2 http://deb.debian.org jessie-updates/main amd64 Packages [23.1 kB]
Hit http://deb.debian.org jessie Release
Get:3 http://deb.debian.org jessie/main amd64 Packages [9064 kB]
Fetched 9729 kB in 31s (305 kB/s)
Reading package lists... Done
しかし使用するとき apt-cacher-ng プロキシそれは失敗します。
root@cc90861af3e4:/etc/apt/apt.conf.d# apt-get update
Ign http://security.debian.org jessie/updates InRelease
Ign http://security.debian.org jessie/updates Release.gpg
Ign http://security.debian.org jessie/updates Release
Err http://security.debian.org jessie/updates/main amd64 Packages
Err http://security.debian.org jessie/updates/main amd64 Packages
Err http://security.debian.org jessie/updates/main amd64 Packages
Err http://security.debian.org jessie/updates/main amd64 Packages
Err http://security.debian.org jessie/updates/main amd64 Packages
500 Missing Content-Length
Ign http://deb.debian.org jessie InRelease
Hit http://deb.debian.org jessie-updates InRelease
Hit http://deb.debian.org jessie Release.gpg
Hit http://deb.debian.org jessie Release
Get:1 http://deb.debian.org jessie-updates/main amd64 Packages [23.1 kB]
Get:2 http://deb.debian.org jessie/main amd64 Packages [9064 kB]
Fetched 9087 kB in 16s (567 kB/s)
W: Failed to fetch http://security.debian.org/dists/jessie/updates/main/binary-amd64/Packages 500 Missing Content-Length
E: Some index files failed to download. They have been ignored, or old ones used instead.
同じapt-cacherが他のシステムでもうまく機能しています（Ubuntu Trusty、Xenial、ArtfulそしてBionicでテスト済み）。
なぜこれがdebianに失敗するのですか？他に何を確認できますか？