Dynamic DNS server update in DHCP

[~David~]

Hi all, first time post in these NGs

Running a linux server (all software fully updated) with ISC's dhcpd giving ips
to the clients; the external net connection (sangomas dsl modem using rp-ppoe)
gets the dns server ip addresses from the ISP and updates resolv.conf correctly.
 When I set the 'option domain-name-servers' line in my dhcpd.conf to the
internal ip address of the internal ethernet interface, my clients get their ip
but cannot resolve domains correctly although they can ping ip address so I know
the connection and NAT works.  In other words they can't use the box as a DNS
server  - is there a way to setup dhcpd to dynamically get the dns ips and hand
them out to the clients.

I've googled this but to no avial.

TIA,
David