If you receive an error message like ‘unable to resolve host’, check, if your /etc/hosts file contains an alias for your hostname.
127.0.0.1 localhost hostname 127.0.1.1 hostname:domain hostname
If your /etc/hosts file does not contain such entries, become root and make the appropriate entries.
The Debian Reference (sub 10.4 Domain Name Service (DNS) as of 2010 December) says:
“/…/ Some software (e.g., GNOME) expects the system hostname to be resolvable to an IP address with a canonical fully qualified domain name. /…/ If your system has a permanent IP address then use that; otherwise use the address 127.0.1.1. /…/”.