I’ve found that whenever I dial up PPPoE
the /etc/resolv.conf will be rewritten
so that name server is always set as ISP’s default DNS.
However, it’s affected by the configuration of PPPoE.
Now we’ll try to solve this.

$ sudoedit /etc/ppp/peers/dsl-provider

find:

usepeerdns

change it to:

#usepeerdns

Then feel free to edit /etc/resolv.conf.
PPPoE will not overwrite your settings anymore.