I had same issue a while back, and it was because the
resolvconf package was not install. try to install it using
sudo apt-get install resolvconf
then edit resolvconf file and restart the network via
sudo /etc/init.d/networking restart
also what does your /etc/network/interfaces file look like?
I also got a tip from this answer in a previous thread similar to yours. http://askubuntu.com/a/58802/49237
A possible trigger for the seemingly spontaneous updates to
/etc/resolv.conf is when your DHCP lease is renewed. Check how long
you get DHCP leases for (this should appear in the system logs, I
think in /var/log/syslog).
You can use auditd to find out what modifies the file. Start the
daemon (sudo service auditd start) and tell it to watch for
modifications to that file:
sudo auditctl -w /etc/resolv.conf -p w
Audit logs are in
/var/log/audit/audit.log. You'll see the time the file was modified
and the name of the program that modified it.
If you have the resolvconf package installed, Network Manager may be
stepping on its toes. Try bringing all network interfaces down, then
stop Network Manager (sudo service network-manager stop), then restart