DNS Relay on Cisco?

Hi,

I tried to use a cisco router as a dns relay for my LAN, but without success. I've set

# ppp ipcp dns request

to learn the name server from the ppp session, and a

# ping

succeeds.

I read about a "ip dns server" command, but the IP ADSL PLUS IOSse I've tried (c1700-sy7-mz.124-3b.bin, c1700-sy7-mz.123-7.XR6.bin) don't accept this command in conf t. It's a Cisco 1721.

Any hints?

Thanks in advance, Jens