To do this properly, you'll really want to learn iptables so that you can troubleshoot any problems that may arise. Otherwise, iptables may become a 'black box' to you because you don't understand how to use it and prove to yourself that the problem isn't with iptables. Don't be put off though, learning the basics of iptables really isn't that hard and if you're interested, there are many different tutorials howtos and cookbooks on the wonderful things you can do with iptables under your control (Google "upside down internet" for a comical example).
What you want to do comes down to 3 simple steps.
1) Append/Insert a rule to the
FORWARD chain to accept TCP port 1688. If you require UDP access as well, add a similar rule but for UDP.
2) Append/Insert rules to accept TCP and UDP port 53 to the
3) Set the default policy on the
FORWARD chain to DROP.
After a very basic crash course in iptables this should be fairly easy, don't be put off by the potential complexity because you won't need to know any of it for this, this is basic stuff.