apt update is trying to use IPv6 for some reason, which does not work

[Marius Gedminas]

Today I've discovered that I cannot use apt-get any more:

    apt update
    Hit:1 http://ppa.launchpad.net/mgedmin/ppa/ubuntu disco InRelease
    Ign:2 http://ddebs.ubuntu.com disco InRelease
    Ign:3 http://ddebs.ubuntu.com disco-updates InRelease
    Hit:4 http://ppa.launchpad.net/pov/ppa/ubuntu disco InRelease
    Hit:5 http://ddebs.ubuntu.com disco Release
    Hit:6 http://security.ubuntu.com/ubuntu disco-security InRelease
    Hit:7 http://ddebs.ubuntu.com disco-updates Release
    0% [Connecting to lt.archive.ubuntu.com (2001:778:1::d)]

and there it stalls for about 30 seconds, after which

    Err:10 http://lt.archive.ubuntu.com/ubuntu disco InRelease
      Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable) Could not connect to lt.archive.ubuntu.com:80 (83.171.20.4), connection timed out
    Err:11 http://lt.archive.ubuntu.com/ubuntu disco-updates InRelease
      Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable)
    Err:12 http://lt.archive.ubuntu.com/ubuntu disco-backports InRelease
      Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable)
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    16 packages can be upgraded. Run 'apt list --upgradable' to see them.
    W: Failed to fetch http://lt.archive.ubuntu.com/ubuntu/dists/disco/InRelease Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable) Could not connect to lt.archive.ubuntu.com:80 (83.171.20.4), connection timed out
    W: Failed to fetch http://lt.archive.ubuntu.com/ubuntu/dists/disco-updates/InRelease Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable)
    W: Failed to fetch http://lt.archive.ubuntu.com/ubuntu/dists/disco-backports/InRelease Cannot initiate the connection to lt.archive.ubuntu.com:80 (2001:778:1::d). - connect (101: Network is unreachable)
    W: Some index files failed to download. They have been ignored, or old ones used instead.

I don't know why it's trying to connect to my Ubuntu mirror over IPv6. There's no IPv6 in Lithuania (I'm exaggerating a bit, but my ISP -- the largest ISP in the country -- does not support IPv6). I've never had working IPv6 connectivity in my life. apt-get used to work before today. I've no idea what changed.

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: apt 1.8.1
ProcVersionSignature: Ubuntu 5.0.0-27.28-generic 5.0.21
Uname: Linux 5.0.0-27-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.10-0ubuntu27.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Sep 11 11:46:28 2019
EcryptfsInUse: Yes
InstallationDate: Installed on 2019-06-12 (90 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)

[Julian Andres Klode (juliank)]

It tried both IPv4 and IPv6; the IPv4 mirror is down.

[Marius Gedminas (mgedmin)]

Oops, I didn't read the error messages closely enough, sorry!

(Also the mirror came back up right when I applied the Acquire::ForceIPv4 "true"; workaround, making me think my initial analysis was right. What a coincidence!)