403 Erro Proibido ao tentar se conectar a repositórios do Ubuntu

Question

Estou executando o Ubuntu 14.04 e não execute sudo apt-get update por muitos meses.
Quando eu tento atualizar local /etc/apt/sources.list apt-get pode se conectar a muitos repos não há problema, mas muitos (talvez 15% do total) retornam erros como este

W: Failed to fetch http://nz.archive.ubuntu.com/ubuntu/dists/trusty-backports/multiverse/binary-i386/Packages  403  Forbidden

W: Some index files failed to download. They have been ignored, or old ones used instead.

Gostaria de saber se os endereços ou as configurações do repositório foram alterados desde que meu computador fez o último contato e, portanto, preciso passar por algum processo para liberar completamente e restabelecer o /etc/apt/sources.list ? Estou pensando em algo parecido com

sudo apt-cache clean

Mas eu não quero bork meu sistema mais do que já é.
Como posso resolver isso 403 Forbidden error ?

Estado atual do /etc/apt/sources.list

# deb cdrom:[Ubuntu 14.04 LTS _Trusty Tahr_ - Release amd64 (20140417)]/ trusty main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://nz.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://nz.archive.ubuntu.com/ubuntu/ trusty main restricted

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

    

Answer 1

Parece haver um problema de chave GPG para o repositório http://nz.archive.ubuntu.com :

W: GPG error: http://nz.archive.ubuntu.com trusty-backports Release: The following signatures were invalid: BADSIG 40976EAF437D05B5

Para resolvê-lo, você precisa obter a chave pública da impressão digital 40976EAF437D05B5 :

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5

Agora, sudo apt-get update deve funcionar normalmente.

Como uma solução alternativa, você pode usar o repositório principal ou um espelho diferente:

deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse

EDITAR:

Como @muru apontou, há alguns problemas com o nz mirror, então você deve usar um repositório diferente como a solução.

    

Answer 2

Acabei de gerar uma nova lista no link , tentando incluir o mesmo conjunto de repos. Você pode gerá-lo e ver que é quase idêntico.

Eu tentei instalar o ubuntu com o seu sources.list e este, e ambos funcionaram depois do apt-get clean e do apt-update. Mesmo aquele em que você recebeu um erro funcionou bem para mim:

Get:79 http://nz.archive.ubuntu.com trusty-backports/multiverse i386 Packages [1552 B]   

Houve alguns sem chaves públicas, mas sem erros 403. Quantas vezes você já tentou? foram alguns dos repositórios off-line por um curto período de tempo?