I have a Cisco 1841 Router. I am attempting to take an internal DNS server and expose it to the Internet via port UDP 53. I have successfully wrote the NAT rules for connections inbound and have properly routed them to the internal DNS. Additionally, I have created rules in the firewall to allow the traffic both inbound and outbound.
DNS functions properly internally, but when someone attempts to access it externally, it does not work. I can see the packets coming in and all looks well. However, performing a packet capture on the machine thats external (user trying DNS), it says that "incorrect UDP checksum". To ensure it was not the DNS server, I set the server up with an external static address (directly on the Internet) and it worked fine. I am thinking it has something to do with the router and maybe specifically how the ISR is sending it back out. Any help would be greatly appreciated. I am completely stumped. Thanks in advance.