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We were trying to install our software on an Ubuntu machine. To do so, we needed root privileges. Basically, all we needed to do was run a runnable jar like: sudo java -jar runnableJar.jar.
All such commands would return: Unable to resolve host xxxxx.

The /etc/hosts file had the incorrect hostname listed against the loopback interface which was causing this error. All commands which did not require sudo ran well.

I have been reading up on the loopback interface and my understanding is that it sets up localhost and is a virtual network interface. However, why does sudo need it at all?

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Since the sudoers file permits the specifying of hostnames in the rules, sudo needs to know what the name of your Ubuntu machine is.

Because of this, sudo collects a list of all interfaces on your Ubuntu machine (loopback and "real"). See relevant section from sudo source code for interfaces.c, at the link below.

http://www.sudo.ws/repos/sudo/file/d8150a3fd577/interfaces.c

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