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I have admin privilege on my office computer (running RHEL 6) which is connected in a network. I was trying to install development tools using the command: sudo yum groupinstall "Development Tools" but I got an error message saying:

Updating certificate-based repositories.
There was an error communicating with RHN.
RHN Satellite or RHN Classic support will be disabled.

Error Message:
 Please run rhn_register as root on this client
 Error Class Code: 9
 Error Class Info: Invalid System Credentials.

I tried to run rhn_register, it said "It appears this system has already been registered for software updates".

Scrolling down, I found following lines:

Setting up Group Process
Warning: Group development does not have any packages.  
No packages in any requested group available to install or update

Are these two things connected? Or the installation process of development tools have changed?

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It already told you There was an error communicating with RHN. – warl0ck Mar 8 '13 at 2:50

So, two things...

One: Certificate based repositories are a function of "subscription_manager" while "RHN Classic" is the traditional RHN. First you need to know which type your system is using. If it's "certificate-based repositories" (aka subscription_manager) then you'll see the file /etc/yum.repos.d/redhat.repo with content like:

name = Red Hat Enterprise Linux 6 Server (RPMs)
baseurl = https://cdn.redhat.com/content/rhel/server/6/$releasever/$basearch/os
enabled = 0
gpgcheck = 1
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-beta,file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
sslverify = 1
sslcacert = /etc/rhsm/ca/redhat-uep.pem
sslclientkey = /etc/pki/entitlement/1111111111111111111-key.pem
sslclientcert = /etc/pki/entitlement/1111111111111111111.pem
metadata_expire = 86400

If this is the case you're likely using subscription_manager, if on the other hand you have a file located at: /etc/sysconfig/rhn/systemid then the system has been registered via "RHN Classic". Regardless, whichever method you have used, the other one should be turned off. This can be done by configuring the corresponding plugin's config located in: /etc/yum/pluginconf.d/

For "RHN Classic" this will be the file "rhnplugin.conf" and for "subscription_manager" it will be "subscription-manager.conf". These are enabled/disabled by setting the boolean value "enabled=[0|1]" in the corresponding configuration file. If you have neither of these configurations then your system is not registered via RHN/subscription_manager and despite having local administration privileges you should contact your sysadmin who is in control of your RHN entitlements. If you are registered, let's keep going.

Once you have disabled the plugin you have chosen not to use, verify that you can see the groups available and no error messages via the command:

yum grouplist

or, even better:

yum grouplist | grep -i development 

this should yield output similar to:

redbeard@yum01 [~]# yum grouplist | grep -i development
   Additional Development
   Desktop Platform Development
   Development tools
   Server Platform Development

You'll notice that the name "Development tools" has a lowercase initial character in the second word. I did validate that this should work regardless of case.

redbeard@yum01 [~]# sudo yum groupinstall "Development Tools"
Loaded plugins: product-id, security, subscription-manager
This system is receiving updates from Red Hat Subscription Management.
Setting up Group Process
Package 1:pkgconfig-0.23-9.1.el6.x86_64 already installed and latest version
Package binutils- already installed and latest version
Package elfutils-0.152-1.el6.x86_64 already installed and latest version
Resolving Dependencies
--> Running transaction check
---> Package autoconf.noarch 0:2.63-5.1.el6 will be installed
---> Package automake.noarch 0:1.11.1-4.el6 will be installed
---> Package bison.x86_64 0:2.4.1-5.el6 will be installed
---> Package byacc.x86_64 0:1.9.20070509-7.el6 will be installed
---> Package cscope.x86_64 0:15.6-6.el6 will be installed
---> Package ctags.x86_64 0:5.8-2.el6 will be installed
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