I (think) that I've configured a static IP address in /etc/network/interfaces:

# The primary network interface 

auto eth0 

iface eth0 inet static

    address 10.1.1.2

    netmask 255.255.255.0

    network 10.1.1.0

    broadcast 10.1.1.255

    gateway 10.1.1.1

And I change the file /etc/resolv.conf to include the desired nameservers.

Then I restart networking sudo /etc/init.d/networking restart

But after some hours resolv.conf always reverts back to the DHCP nameserver.

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I should also say that this is Ubuntu Server with only l0 and eth0 interfaces.