Ubuntu 12.04 use resolvconf(a set of scripts and hooks) instead of resolv.conf to manage DNS server. The most notable difference for the user is that any change manually done to /etc/resolv.conf will be lost as it gets overwritten next time something triggers resolvconf.

To add the static dns entries in resolv.conf file,edit /etc/resolvconf/resolv.conf.d/base:

# vi /etc/resolvconf/resolv.conf.d/base

Add these lines:

nameserver xxx.xxx.xxx.xxx
nameserver xxx.xxx.xxx.xxx

Run this command to refresh the resolvconf setting:

#service resolvconf restart