DNS from ISP on Cisco router

Hi,
I would like to obtain the DNS from ISP automatically.
I have the following DHCP configuration in my router :
ip dhcp pool guest
import all
network 192.168.123.0 255.255.255.0
default-router 192.168.123.1

and I added the following command on my Dialer interface :
ppp ipcp dns request

but still I do not get DNS. I can connect to Internet and ping a public address but can not browse.
Any idea please?
Thanks