topic Re: ACL for DNS Service in Network Security

ACL for DNS Service

argnetworking — Mon, 11 Mar 2019 18:46:50 GMT

Hello, I would like to know what is the recommended ACL to configure in a router, to give a public DNS service.

I know the basic, permit eq domain, deny special use IP address (10.0.0.0/8, 192.168.0.0/16, ...), block everything else....

Do I need to open upper ports?, any ACL with established?

I would like suggestions to configure a solid ACL.

Thanks,

Gonzalo

Re: ACL for DNS Service

Federico Coto Fajardo — Tue, 28 Sep 2010 21:04:00 GMT

Hi,

Do you mean allowing the internal network to contact an external DNS server?

If so, just need to open UDP domain on the ACL applied to the inside interface.

i.e

Internal network 10.1.1.0/24

External DNS 4.2.2.2

access-list INSIDE permit udp 10.1.1.0/24 host 4.2.2.2 eq 53

access-group INSIDE in interface INSIDE

The above ACL will only allow outbound DNS requests to port 53 on UDP to 4.2.2.2 from the internal LAN.

Remember that every other outbound traffic that needs to get out should be permitted on that ACL as well.

Federico.

Re: ACL for DNS Service

argnetworking — Tue, 28 Sep 2010 21:38:08 GMT

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

Re: ACL for DNS Service

Federico Coto Fajardo — Tue, 28 Sep 2010 21:50:15 GMT

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.

Re: ACL for DNS Service

argnetworking — Wed, 29 Sep 2010 02:31:55 GMT

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

Re: ACL for DNS Service

Collin Clark — Wed, 29 Sep 2010 14:23:57 GMT

Check this link for the most current public facing ACL we use. It matches the DISA standard so it should be pretty secure. You will have to tweak a couple of things like allowing BGP.

http://www.packetpros.com/cisco_kb/DIACAP_ACL.html

Hope it helps.

Re: ACL for DNS Service

argnetworking — Wed, 29 Sep 2010 21:30:58 GMT

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

Re: ACL for DNS Service

Collin Clark — Thu, 30 Sep 2010 13:37:39 GMT

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

Re: ACL for DNS Service

argnetworking — Thu, 30 Sep 2010 14:09:50 GMT

Both, transfers and look ups?

Re: ACL for DNS Service

Collin Clark — Thu, 30 Sep 2010 14:22:15 GMT

For transfers from another DNS server to yours (make sure this is correct, can be very dangerous) -

permit tcp host [remote dns server] host [your dns servers public IP] eq 53

For external people querying your DNS servers for dns lookups -

permit udp any host [your dns server public IP] eq 53

Hope that helps.

Re: ACL for DNS Service

argnetworking — Sun, 03 Oct 2010 04:44:33 GMT

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.

Re: ACL for DNS Service

Collin Clark — Tue, 05 Oct 2010 13:21:55 GMT

It's call Tigra Tree Menu-

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

Re: ACL for DNS Service

RoeeM — Thu, 01 Dec 2022 18:13:55 GMT

hello i have similar Q, i need all users/PC int the net int diffrent site and vlans have access only to the internal DNS server

i have 3 routers one main the other 2 connect to the main router.

what i need to do?