no, I have to allow external users to get to the DNS (public DNS) behind a cisco router.

To allow external users to access an internal DNS, you do something like this:

ip access-list extended OUTSIDE

  permit udp any host x.x.x.x eq 53

interface fasx/x

  ip access-group OUTSIDE in

The above ACL only permits inbound DNS traffic on port 53 to host x.x.x.x (which is going to be the public IP assigned to the DNS server).

Now,

Referring to the ACL, you should specify all other traffic that should be permitted.

Normally what you do on an IOS router is to configure some sort of stateful behavior (like an ASA), to avoid having to open all ports in that ACL.

Easiest way is to use CBAC to permit the return traffic of the outside connections (and you only worry about permitting traffic that it's initiated from the outside world coming in).

Recommended way is Zone-Based Firewall.

Federico.

Federico,

Thanks for the replay, I have a working ACL but is too open. I would like to do it again and that is why I’m asking for recommendations. I inherited the configuration and I’m trying to do it the right way.

I can not apply any sort of stateful firewall because it is a DNS that has a lot of hits , and the CPU goes to the roof. So, my intention is to open only the necessary ports on the ACL.

Thanks,

Gonzalo

Collin, thanks a lot, it is great, that it is exactly one of the parts I was looking for. Now I only need to know if there are any other recommendations on specific ACLs for DNS service.

Thanks,

Gonzalo

Are you doing any DNS transfers or are they simply look ups?

Both, transfers and look ups?

I have both of them already working like that.

Thanks

PS: What did you use to buid your site? I liked the organization like folders.

It's call Tigra Tree Menu-

http://www.softcomplex.com/products/tigra_tree_menu/