Ubuntu 18.04では問題なく動作しましたが、Ubuntu 20.04ではapt-cacher-ngの問題は次のようになります。
$ sudo apt update
Get:1 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Hit:2 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:4 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Ign:5 http://archive.ubuntu.com/ubuntu focal/main amd64 c-n-f Metadata
Ign:6 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
Ign:7 http://archive.ubuntu.com/ubuntu focal/universe i386 Packages
Ign:8 http://archive.ubuntu.com/ubuntu focal/universe Translation-en
Ign:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 c-n-f Metadata
Ign:10 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 c-n-f Metadata
Ign:5 http://archive.ubuntu.com/ubuntu focal/main amd64 c-n-f Metadata
Get:6 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages [8,600 kB]
Get:7 http://archive.ubuntu.com/ubuntu focal/universe i386 Packages [4,624 kB]
Get:8 http://archive.ubuntu.com/ubuntu focal/universe Translation-en [5,131 kB]
Get:8 http://archive.ubuntu.com/ubuntu focal/universe Translation-en [5,131 kB]
Ign:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 c-n-f Metadata
Ign:10 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 c-n-f Metadata
Ign:5 http://archive.ubuntu.com/ubuntu focal/main amd64 c-n-f Metadata
Ign:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 c-n-f Metadata
Ign:10 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 c-n-f Metadata
Err:5 http://archive.ubuntu.com/ubuntu focal/main amd64 c-n-f Metadata
403 Forbidden file type or location [IP: 192.168.17.34 3142]
Ign:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 c-n-f Metadata
Ign:10 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 c-n-f Metadata
Fetched 15.2 MB in 1min 15s (203 kB/s)
Reading package lists... Done
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/main/cnf/Commands-amd64 403 Forbidden file type or location [IP: 192.168.17.34 3142]
E: Some index files failed to download. They have been ignored, or old ones used instead.
しかし、プロキシがなければsudo apt update、
$ sudo apt update
Hit:1 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:2 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Get:4 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Get:5 http://archive.ubuntu.com/ubuntu focal/main amd64 c-n-f Metadata [29.2 kB]
Get:6 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages [8,600 kB]
Get:7 http://archive.ubuntu.com/ubuntu focal/universe i386 Packages [4,624 kB]
Get:8 http://archive.ubuntu.com/ubuntu focal/universe Translation-en [5,131 kB]
Get:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 c-n-f Metadata [264 kB]
Get:10 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 c-n-f Metadata [9,164 B]
Fetched 567 kB in 31s (18.4 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
そして、私がサポートしているファイルタイプのパターン/etc/apt-cacher-ng/acng.confは
PfilePattern = .(.d?deb|.db|.rpm|.drpm|.dsc|.tar(.gz|.bz2|.lzma|.xz|.zst)(.gpg)?|.diff(.gz|.bz2|.lzma|.zst|.xz)|.jigdo|.template|changelog|copyright|.udeb|.debdelta|.diff/..gz|(Devel)?ReleaseAnnouncement(\?.)?|[a-f0-9]+-(susedata|updateinfo|primary|deltainfo).xml.gz|fonts/(final/)?[a-z]+32.exe(\?download.)?|/dists/./installer-[^/]+/[0-9][^/]+/images/.)$
そしてここに/etc/apt/sources.list
# deb cdrom:[Ubuntu-MATE 20.04 LTS _Focal Fossa_ - Beta amd64 (20200402)]/ focal main multiverse restricted universe
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu focal main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu focal-updates main restricted
# deb-src http://bd.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu focal universe
deb http://archive.ubuntu.com/ubuntu focal-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu focal multiverse
deb http://archive.ubuntu.com/ubuntu focal-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb-src http://bd.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu focal partner
deb http://archive.ubuntu.com/ubuntu focal-security main restricted
# deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://archive.ubuntu.com/ubuntu focal-security universe
deb http://archive.ubuntu.com/ubuntu focal-security multiverse
deb http://archive.ubuntu.com/ubuntu focal-backports multiverse restricted main universe
#deb http://archive.ubuntu.com/ubuntu focal-proposed multiverse restricted main universe #Not for humans during development stage of release focal
また、この問題は20.04でのみ発生し、18.04では発生しません（実機とVMでも同様でした）。また、sudo apt updateリポジトリを直接更新して使用すると、問題は でのみ発生します。apt-chacher-ngとしてプロキシパッケージをダウンロードするには、問題はありません。では、具体的にどこが間違っているのでしょうか?
答え1
apt update20.04 でも apt-cacher-ng 経由でこのエラーが発生していますが、apt upgradeまだ動作しているようです。サーバー上のファイルに があることに気付きました。xz拡張機能、それが手がかりになるかどうか。
http://archive.ubuntu.com/ubuntu/dists/focal-security/main/cnf/
Index of /ubuntu/dists/focal-security/main/cnf
[ICO] Name Last modified Size
[PARENTDIR] Parent Directory -
[ ] Commands-amd64.xz 2020-06-03 12:29 2.6K
[ ] Commands-i386.xz 2020-06-03 12:29 1.7K
[DIR] by-hash/ 2019-10-25 05:33 -
まだ修正方法を特定できていません...これをacng.cnf（「揮発性」としてマークするために）追加しようとしましたが、問題が解決したかどうかはわかりません。apt proxyをコメントアウトして実行した後、クライアントで問題がなくなったためです。apt update
#PfilePatternEx: .*(\.xz)
VfilePatternEx: .*(\.xz)$
SVfilePatternEx: .*(\.xz)$