for ubuntu, nameserver must be configuration both in interfaces and resolv conf
1. /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 192.168.0.159
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1
dns-nameservers 202.106.0.20 202.106.196.115
2. /etc/resolv.conf
nameserver 202.106.0.20
nameserver 202.106.196.115