Ciao nolo, se il tuo file di configurazione si è corrotto questo è il mio perfettamente funzionante :
#------------------------------------------------------------
# !!DO NOT MODIFY THIS FILE!!
#
# Manual changes will be lost when this file is regenerated.
#
# Please read the developer's guide, which is available
# at
http://wiki.contribs.org/development/#
# Copyright (C) 1999-2006 Mitel Networks Corporation
#------------------------------------------------------------
[main]
cachedir=/var/cache/yum
debuglevel=2
exactarch=1
installonlypkgs=kernel kernel-smp kernel-hugemem kernel-enterprise kernel-debug kernel-unsupported kernel-smp-unsupported kernel-hugemem-unsupported
logfile=/var/log/yum/yum.log
obsoletes=1
tolerant=1
[addons]
enabled=0
name=CentOS - addons
baseurl=http://mirror.centos.org/centos/4/addons/$basearch
gpgcheck=1
enablegroups=0
[base]
enabled=0
name=CentOS - os
baseurl=http://mirror.centos.org/centos/4/os/$basearch
gpgcheck=1
enablegroups=0
[centosplus]
enabled=0
name=CentOS - centosplus
baseurl=http://mirror.centos.org/centos/4/centosplus/$basearch
gpgcheck=1
enablegroups=0
[contrib]
enabled=0
name=CentOS - contrib
baseurl=http://mirror.centos.org/centos/4/contrib/$basearch
gpgcheck=1
enablegroups=0
[extras]
enabled=0
name=CentOS - extras
baseurl=http://mirror.centos.org/centos/4/extras/$basearch
gpgcheck=1
enablegroups=0
[smeaddons]
enabled=1
name=SME Server - addons
baseurl=http://mirror.contribs.org/smeserver/releases/7/smeaddons/$basearch
gpgcheck=1
enablegroups=1
[smedev]
enabled=0
name=SME Server - dev
baseurl=http://mirror.contribs.org/smeserver/releases/7/smedev/$basearch
gpgcheck=1
enablegroups=1
[smeos]
enabled=1
name=SME Server - os
baseurl=http://mirror.contribs.org/smeserver/releases/7/smeos/$basearch
gpgcheck=1
enablegroups=1
[smetest]
enabled=0
name=SME Server - test
baseurl=http://mirror.contribs.org/smeserver/releases/7/smetest/$basearch
gpgcheck=1
enablegroups=1
[smeupdates]
enabled=1
name=SME Server - updates
baseurl=http://mirror.contribs.org/smeserver/releases/7/smeupdates/$basearch
gpgcheck=1
enablegroups=1
[smeupdates-testing]
enabled=0
name=SME Server - updates testing
baseurl=http://mirror.contribs.org/smeserver/releases/7/smeupdates-testing/$basearch
gpgcheck=1
enablegroups=1
[testing]
enabled=0
name=CentOS - testing
baseurl=http://mirror.centos.org/centos/4/testing/$basearch
gpgcheck=1
enablegroups=0
[updates]
enabled=0
name=CentOS - updates
baseurl=http://mirror.centos.org/centos/4/updates/$basearch
gpgcheck=1
enablegroups=0
PS : il file è /etc/yum.conf