According to the man pages, yum list extras performs the following action:
$ yum list extras [glob_exp1] [...] List the packages installed on the system that are not available in any yum repository listed in the config file.
According to another web page, yum list extras performs the following:
Show all packages not available via subscribed channels or repositories i.e show packages installed via other repos:
http://www.cyberciti.biz/faq/rhel-centos-fedora-linux-yum-command-howto/
When I run yum list extras all the packages come from either base, or updates. Those seem like pretty standard repo locations to me.
[root@mongo-test3 ~]# yum list extras |tail
smartmontools.x86_64 1:5.42-2.el6 @base/6.3
sos.noarch 2.2-29.el6 @base/6.3
strace.x86_64 4.5.19-1.11.el6_3.2 @updates/6.3
systemtap-runtime.x86_64 1.7-5.el6_3.1 @updates/6.3
tar.x86_64 2:1.23-7.el6 @base/6.3
tcsh.x86_64 6.17-19.el6_2 @base/6.3
udev.x86_64 147-2.42.el6 @updates/6.3
util-linux-ng.x86_64 2.17.2-12.7.el6_3 @updates/6.3
virt-what.x86_64 1.11-1.1.el6 @base/6.3
yum.noarch 3.2.29-30.el6.centos @base/6.3
I've even tried the following. It still is not clear to me what qualifies as extras
[root@mongo-test3 ~]# yum list > /tmp/yum_list.txt
[root@mongo-test3 ~]# yum list extras > /tmp/yum_list_extras.txt
$ vimdiff /tmp/yum_list.txt /tmp/yum_list_extras.txt
Update #1:
Version
CentOS 6.4
[root@mongo-test3 tmp]# ls /etc/yum.repos.d/
10gen.repo centos-base.repo epel.repo epel-testing.repo puppetlabs.repo
[root@mongo-test3 tmp]# uname -a
Linux mongo-test3 2.6.32-279.22.1.el6.x86_64 #1 SMP Wed Feb 6 03:10:46 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
