[Bug 1759118] Re: apt attempts to connect to IPv6 addresses even when there is no IPv6 route

[Paul]

There is one notable misconfiguration: the crappy ISP (which doesn't
support IPv6) routinely sends Martian packets in through the ADSL modem.
This includes ARP, which the modem helpfully broadcasts onto the LAN. So
although the modem doesn't support IPv6, Ubuntu is seeing IPv6 ARP
packets and may be deciding that there is IPv6 on the network.

However, since there is only a link-local IPv6 address configured, that
would be an incorrect assumption. No other application exhibits this
fault (though they may be performing a similar fallback in software,
disguising the issue).

Whatever the cause, it's a real-world scenario that users have no
control over, and we need to code for it. So I'm happy to hear this has
been patched. Thanks! If the problem recurs, I'll file another bug.

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https://bugs.launchpad.net/bugs/1759118

Title:
  apt attempts to connect to IPv6 addresses even when there is no IPv6
  route

Status in apt package in Ubuntu:
  Invalid

Bug description:
  Configure internet connectivity without IPv6, e.g. via DHCP on a
  router that supports only IPv4. The active interface should have only
  a link-local IPv6 address.  (Link-local addresses are not routable.)

  Run apt update.  You may see output like the following:

  Hit:1 http://security.ubuntu.com/ubuntu bionic-security InRelease              
  Ign:2 http://ddebs.ubuntu.com bionic InRelease                                 
  Ign:3 http://ddebs.ubuntu.com bionic-updates InRelease                         
  Ign:4 http://ddebs.ubuntu.com bionic-proposed InRelease            
  Get:5 http://ddebs.ubuntu.com bionic Release [34.7 kB]                         
  Hit:6 http://ddebs.ubuntu.com bionic-updates Release                           
  Get:8 http://ddebs.ubuntu.com bionic-proposed Release [34.7 kB]                
  Get:9 http://ddebs.ubuntu.com bionic Release.gpg [819 B]                       
  Get:10 http://ddebs.ubuntu.com bionic-proposed Release.gpg [819 B]             
  Get:11 http://ddebs.ubuntu.com bionic/main amd64 Packages [528 kB]             
  Get:12 http://ddebs.ubuntu.com bionic/main i386 Packages [525 kB]              
  Get:13 http://ddebs.ubuntu.com bionic/universe amd64 Packages [4,372 kB]       
  0% [Connecting to au.archive.ubuntu.com (2001:388:30bc:cafe::beef)] [13 Package^C

  Here, apt has correctly used IPv4 for ddebs and security, but
  continues to try IPv6 for au.archive, despite having no valid IPv6
  address from which to reach the internet.

  apt should be more clever about this, and only attempt to use a
  protocol if there is a non-zero chance of success.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: apt 1.6~beta1
  ProcVersionSignature: Ubuntu 4.15.0-12.13-generic 4.15.7
  Uname: Linux 4.15.0-12-generic x86_64
  ApportVersion: 2.20.9-0ubuntu1
  Architecture: amd64
  CurrentDesktop: KDE
  Date: Tue Mar 27 14:52:03 2018
  InstallationDate: Installed on 2018-03-12 (15 days ago)
  InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180310)
  SourcePackage: apt
  UpgradeStatus: No upgrade log present (probably fresh install)

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