Because RHN configuration files rely exclusively on fully qualified domain names (FQDN), it is imperative key applications are able to resolve the name of the RHN Proxy Server into an IP address. Red Hat Update Agent, Red Hat Network Registration Client, and the Apache HTTP Server are particularly prone to this problem with the RHN applications issuing errors of "host not found" and the Web server stating "Could not determine the server's fully qualified domain name" upon failing to start.
This problem typically originates from the /etc/hosts file. You may confirm this by examining /etc/nsswitch.conf, which defines the methods and the order by which domain names are resolved. Usually, the /etc/hosts file is checked first, followed by Network Information Service (NIS) if used, followed by DNS. One of these has to succeed for the Apache HTTP Server to start and the RHN client applications to work.
To resolve this problem, identify the contents of the /etc/hosts file. It may look like this:
127.0.0.1 this_machine.example.com this_machine localhost.localdomain.com localhost
First, in a text editor, remove the offending machine information, like so:
127.0.0.1 localhost.localdomain.com localhost
Then, save the file and attempt to re-run the RHN client applications or the Apache HTTP Server. If they still fail, explicitly identify the IP address of the Proxy in the file, such as:
127.0.0.1 localhost.localdomain.com localhost 188.8.131.52 this_machine.example.com this_machine
Replace the value here with the actual IP address of the Proxy. This should resolve the problem. Keep in mind, if the specific IP address is stipulated, the file will need to be updated when the machine obtains a new address.