When attempting to install nethserver-backup, I get the following error.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: centos-base. Please verify its path and try again
The install/update may have failed due to metadata caching issues. Please clean the cache by clicking the button below and retry the install/update operation.
Screenshot: http://puu.sh/q79hc/afdf22c5d7.png
Which version of CentOS and NethServer are in use?
cat /etc/centos-release
cat /etc/nethserver-release
Can you try running:
yum clean all && yum update
report any error message it throws 
CentOS release 6.7 (Final)
NethServer release 6.7 (final)
Loaded plugins: changelog, fastestmirror, nethserver_events, presto
Cleaning repos: centos-base centos-updates nethserver-base nethserver-updates
Cleaning up Everything
0 delta-package files removed, by presto
Still Erroring:
Cannot retrieve repository metadata (repomd.xml) for repository: centos-base. Please verify its path and try again
The install/update may have failed due to metadata caching issues. Please clean the cache by clicking the button below and retry the install/update operation
If NethServer can ping to external domains but yum -y update still fails after the clean, you can report the output of:
yum -v repolist enabled[root@msdp ~]# yum -v repolist enabled
Loading “changelog” plugin
Loading “fastestmirror” plugin
Loading “nethserver_events” plugin
Loading “presto” plugin
Config time: 0.040
Yum Version: 3.2.29
Determining fastest mirrors
You can manually download and install a package to update from 6.7 to 6.8 (release notes):
yum install http://packages.nethserver.org/nethserver/6/updates/x86_64/Packages/nethserver-release-6.8-2.ns6.noarch.rpm
yum clean all
yum update
Unless there is some proxy (filtering/caching) or a firewall blocking the connection, or a broken repository, it should work.
Not yet, I have to wait for our scheduled maintenance days to attempt it, but I will post back as soon as I attempt it! =D
CentOS server seems to deny access to your client (yum)! Crazy, I can’t reproduce it. The following command works as expected on my host, by retrieving an XML document:
curl -H 'Cache-Control: no-cache' -v -A 'urlgrabber/3.9.1 yum/3.2.29' http://vault.centos.org/6.7/os/x86_64/repodata/repomd.xml
Could you paste the output of the command above? Are you behind a web proxy?
BTW: I suggest to update your system to v6.8!
[root@msdp ~]# curl -H ‘Cache-Control: no-cache’ -v -A ‘urlgrabber/3.9.1 yum/3.2 .29’ http://vault.centos.org/6.7/os/x86_64/repodata/repomd.xml
GET /6.7/os/x86_64/repodata/repomd.xml HTTP/1.1
User-Agent: urlgrabber/3.9.1 yum/3.2.29
Host: vault.centos.org
Accept: /
Cache-Control: no-cache
< HTTP/1.1 403 Forbidden
< Server: openresty/1.9.7.3
< Date: Thu, 25 Aug 2016 22:03:26 GMT
< Content-Type: text/html
< Transfer-Encoding: chunked
< Connection: keep-alive
<
Looks like it was OpenDNS blocking it!
I’d rather say someone is trying to redirect your client to an opendns address. I can’t say if this is wanted or not: it depends on your network setup!
Im setting up a brand new NS7 - and no matter where or what I try change I keep on ending up with:
Loading “changelog” plugin
Loading “fastestmirror” plugin
Loading “nethserver_events” plugin
Config time: 0.044
Yum version: 3.4.3
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
_ One of the configured repositories failed (Unknown),_
_ and yum doesn’t have enough cached data to continue. At this point the only_
_ safe thing yum can do is fail. There are a few ways to work “fix” this:_
_ 1. Contact the upstream for the repository and get them to fix the problem._
_ 2. Reconfigure the baseurl/etc. for the repository, to point to a working_
_ upstream. This is most often useful if you are using a newer_
_ distribution release than is supported by the repository (and the_
_ packages for the previous distribution release still work)._
_ 3. Disable the repository, so yum won’t use it by default. Yum will then_
_ just ignore the repository until you permanently enable it again or use_
_ --enablerepo for temporary usage:_
_ yum-config-manager --disable _
_ 4. Configure the failing repository to be skipped, if it is unavailable._
_ Note that yum will try to contact the repo. when it runs most commands,_
_ so will have to try and fail each time (and thus. yum will be be much_
_ slower). If it is a very temporary problem though, this is often a nice_
_ compromise:_
_ yum-config-manager --save --setopt=.skip_if_unavailable=true_
Cannot find a valid baseurl for repo: base/7/x86_64
[root@atlantis ~]# yum clean all && yum update
Loaded plugins: changelog, fastestmirror, nethserver_events
Cleaning repos: base centos-sclo-rh centos-sclo-sclo epel extras nethforge
_ : nethserver-base nethserver-updates updates_
Cleaning up everything
Loaded plugins: changelog, fastestmirror, nethserver_events
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - “Could not resolve host: mirrorlist.centos.org; Unknown error” "
I thought it was the Service provide, Thought it was the CD, USB, DVD, even the User (PEBCAK - Problem exists between chair and keyboard.but no.
I tried to change the repository, download the ISO 20 Million times…
Any Suggestions
thanks
Clint 
ahem… is your server connected to wan? can you ping an ipaddress? if so, you have a dns issue
That it is, DNS I have , aimed to google.
I have pinged ip addresses.
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=55 time=85.3 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=55 time=83.3 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=55 time=50.2 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=55 time=77.3 ms
64 bytes from 8.8.8.8: icmp_seq=5 ttl=55 time=54.4 ms
^[64 bytes from 8.8.8.8: icmp_seq=6 ttl=55 time=36.3 ms
64 bytes from 8.8.8.8: icmp_seq=7 ttl=55 time=35.3 ms
64 bytes from 8.8.8.8: icmp_seq=8 ttl=55 time=88.6 ms
^C
— 8.8.8.8 ping statistics —
8 packets transmitted, 8 received, 0% packet loss, time 7006ms
rtt min/avg/max/mdev = 35.371/63.868/88.603/20.840 ms
And I only have one network connection.
This is the first time have encounted this on Nethserver. 7. THis is the 26th aNS7 server I am setting up for someone and its the FIRST time I have encounted this.
For 4 days, struggling fighting breaking stuff and Then 
@Stefano_Zamboni Just says its a DNS issue. I log back into the server and Suddenly its got updates.
Must have been a upstream provider issue. SOrry.
Thank you @Stefano_Zamboni
Hi @clinton 
if you believe that the problem is solved, please marks “solved” the answer that helped you.
This helps anyone looking for the same problem and who responds to the support 