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Aditya
Asked: 2015-02-20 10:12:34 +0800 CST

How does the fallback mechanism of apt update work in case of connection problems?

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While installing the updates today, I noticed this:

Get:15 http://in.archive.ubuntu.com/ubuntu/ trusty-updates/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2 [626 kB]
Err http://in.archive.ubuntu.com/ubuntu/ trusty-updates/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2
  Connection failed [IP: 91.189.92.200 80]
Get:16 http://security.ubuntu.com/ubuntu/ trusty-security/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2 [626 kB]

There was some connection problem in downloading the package from my regular update mirror http://in.archive.ubuntu.com/ubuntu/ and the updater chose http://security.ubuntu.com/ubuntu/ as fallback to download the same package and the update succeeded thereafter. This is the first time that I have noticed such fallback mechanism when downloading the updates.

However, the following is the content of my /etc/apt/sources.list file:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

# newer versions of the distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-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://in.archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty universe
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-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://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-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 http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security 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 trusty partner
# deb-src http://archive.canonical.com/ubuntu trusty partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main

As one can see, only trusty-security repository is enabled for security.ubuntu.com. However, the package that had problems was from trusty-updates. So, how does this fallback mechanism work? Moreover, what are the scenarios when this mechanism is triggered?

apt

1 Answers

  1. Best Answer

    If multiple repositories provide the exact same version of a package, then apt uses the repository that comes first in the sources.list as the first choice and the others for fallback.

    If use apt-cache policy, you'll get results of the form:

    $ apt-cache policy libdns100
libdns100:
  Installed: 1:9.9.5.dfsg-3ubuntu0.1
  Candidate: 1:9.9.5.dfsg-3ubuntu0.1
  Version table:
 *** 1:9.9.5.dfsg-3ubuntu0.1 0
        500 http://security.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
        500 http://in.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:9.9.5.dfsg-3 0
        500 http://in.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

    As you can see, the exact same version of the package is provided by two repositories - even though they are in different channels, they provide the same package and version.

    From man apt_preferences:

       Several instances of the same version of a package may be available
   when the sources.list(5) file contains references to more than one
   source. In this case apt-get downloads the instance listed earliest in
   the sources.list(5) file. The APT preferences do not affect the choice
   of instance, only the choice of version.
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