Unable To Determine The Host Or Domain Name Of Your Computer

Problem

Updating Plesk to 11.0.9 fails because the installer can't find the main IP or hostname.

Example of Error Output

Warning!
Unable to determine the host or domain name of your computer.
Setting it to localhost.localdomain

ERROR while trying to check the hostname
Check the error reason(see log file: /tmp/plesk_11.0.9_installation.log), fix and try again

***** problem report *****
ERROR while trying to check the hostname
Check the error reason(see log file: /tmp/plesk_11.0.9_installation.log), fix and try again

Solution

Update the Full hostname in Server > Server Settings to a fully qualified domain name (FQDN) like example.com.

There may already be an entry there created by your host when the machine was provisioned. It will probably have the form of x0123456.pubip.webhost.com. I don't know why, but these won't work for the Plesk 11.0.9 upgrade. You have to update the hostname to use one of the sites you're hosting.